This made me wonder how far we are from being able to create and detect gravitons. The Wikipedia page on gravitons [0] addresses this question:
Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. [...]
However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are underway (such as LIGO and VIRGO). Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass [...].
Fascinating! I take it that the question of whether the graviton could have mass is now considered to be well answered in the negative.
"Gravity is a weak force, in the sense that the gravitational force between two protons is about 10^33 times weaker than the electric force between them. And I'm using protons rather than electrons here to make the gravity stronger - with electrons gravity would be almost 10^40 times weaker.
This has various consequences, but one is that gravitational waves are absorbed by matter much less than electromagnetic waves. It would be fun to estimate the amount of energy absorbed by the Earth as this particular gravitational wave came through, but it would be absurdly small. Gravitational waves make neutrinos look like rampaging gorillas."
Rephrasing, and assuming waving commutes with rampaging, gravitons make neutrinos look like WAVES of rampaging gorillas. I'm no physicist but to answer the original question I'd hazard a guess: quite far!
If gravitons have mass, then the universe is too strange to exist. Gravity is an interaction that defines the presence of matter (see dark matter). For the object that transmits that force between masses to itself have mass ... how can a black hole then project gravity?
Imho whatever is carrying gravity between masses cannot itself have a mass.
Force carrying particles in general don't have mass. Except that some of them seem to do, which was rather puzzling for some time, but was solved using the Higgs mechanism.
I can't think of an obvious reason the Higgs mechanism wouldn't work for gravitons, but I could be mistaken, it's not exactly the most intuitive area of physics.
Also, keep in mind that the strong force transmits the force between colour charges while also having a colour charge itself, so it isn't entirely inconceivable for the force transmitting the attraction between masses to have a mass.
No clue if a massive graviton would allow for black holes, but it's not entirely sure what black holes even are (especially quantum mechanically). At the very least it's presumably possible for some particles to escape it (e.g. as Hawking radiation).
If the massive gravitron was leaving a black hole it would be slowed by the black hole's gravity.
(1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons, pulling more in.
(2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves?
(3) If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron? This would require a new set of particles be created by non-gravitron massive objects (ie black holes) alongside the gravitrons. Like I said, too strange to exist.
(None of my points below say the graviton is massless, just that it's not crazy. As another post says, this new observation probably confines the graviton mass to be less than 10^-55 grams)
> If the massive gravitron was leaving a black hole it would be slowed by the black hole's gravity.
A graviton wouldn't be able to escape a black hole. A photon can't, and it's massless. The gravity of a black hole is actually a self-sustaining effect of the curvature of the spacetime around the black hole.
> (1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons.
We don't know details of the gravitational field around black holes and the mass that created it, because none have been observed close up. To an extent, the mass of a black hole is defined by its gravity.
> (2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves?
Again, photons are massless and subject to the doppler effect. Gravitons, massless or not, will be too.
> (3) If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron? This would require a new set of particles be created by non-gravitron massive objects (ie black holes) alongside the gravitrons. Like I said, too strange to exist.
Force carying particles can interact with themselves, c.f. gluons in QCD. In fact, GR is a non-linear theory so there will be non-linear interactions (as far as you can describe them in the weak limit).
> Is this where the translation from GR -> QM breaks down?
Sort of, to get a graviton you introduce perturbations on a background metric. (Basically small wiggles of spacetime around an 'average.') You don't do anything like that when you solve the Einstein equations. Consequently, the background spacetime ( that is, the black hole) is not really made of gravitons. (At least in some sense.)
So in GR the metric itself is warping, which is a problem if you need a background reference/stable metric to define your graviton field? Is that close? Anyway thanks.
> We should see this as some inconsistency in how gravity scales with the mass of a black hole.
This inconsistency is a part of general relativity (even though gravitons themselves aren't). A black hole does not follow the GM/r^2 gravity law.
> If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves?
Yes, there is a doppler effect.
> If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron? This would require a new set of particles be created by non-gravitron massive objects (ie black holes) alongside the gravitrons. Like I said, too strange to exist.
Gravitons. Every now and then a pair of gravitons may exchange more gravitons (these are also virtual particles, so it's fine). This is why Feynman diagrams exist, so that you can not only calculate the interaction between two particles through a graviton, but also calculate the contribution of the lower-probability situations where more gravitons magically appear to transmit gravity between gravitons.
This is nothing new. Gluons (the carrier for the color/strong force) do basically the same thing. They are also bound by the force they carry, and thus gluons can interact with each other with more gluons.
Because these are virtual particles and only exist as a probability, this doesn't lead to infinite recursion. The secondary gravitons are improbable, and the tertiary gravitons more so, and so on, and the final series converges.
>(1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons.
Perhaps, no clue how a quantum mechanical gravity would interact with a black hole.
>(2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves?
The Doppler effect happens even for light, which isn't massive at all (that we know of).
>(3) If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron?
There's no reason they couldn't interact with themselves, in fact I guess that's probably the most likely case.
If they interact with each other, then wouldn't any "wave" collapse?
The particles leave the event in a smooth wave. Then they run into other waves, or each other, or just the background gravity fields. This perturbation should cause them to clump together. So in short order the smooth wave would become large blobs of gravitrons more akin to raindrops than waves. And without anything holding them apart, might not some of these clumps condense into some sort of ... I don't have the words for such an object. I wouldn't want to get in its way.
I think what you're trying to describe is something like 'confinement'. I'm not confident that a massive graviton will necessarily lead to confinement, neither gravitons nor confinement are that well understood.
It's a converging series; instead of getting a clumping you may get a slight mass increase. These new gravitons that modulate gravity between existing gravitons are not 100% there, so they do not contribute that much.
Sure, and in several theories of gravitation where there are gravitons as an uncharged massless spin-2 gauge boson (General Relativity isn't one of these; it doesn't have any gravitons at all, although the non-quantized classical gravitational waves have spin-2 symmetry) then gravitons are their own anti-particles, just as photons (uncharged massless spin-1 gauge bosons) are their own anti-particles in the Standard Model.
(i.e., anti-gravitons and gravitons are the same thing, just as anti-photons and photons are the same thing).
There are a variety of other theories of gravitation with gravitons, but as far as I know, there are none in which gravitons are not their own antiparticles. (There may be such theories available in universes with a very different cosmological constant or with different numbers of dimensions than the one we are in).
I love groups who, while considering something as crazy as time turning into a physical direction as it does in a black hole, can still manage voting down someone down thinking outside the box.
I like the idea of anti-gravitons being the inside of a graviton, or inside of a black hole. Given the inside of a black hole is essentially the end of time, coming out of a black hole or coming out of an anti-graviton, could equal going back to the beginning of time.
> Force carrying particles in general don't have mass.
Massless particles don't have energy. Massless and energyless particles have no speed. I have no interest in massless and energyless particles that stand still.
Protons are said to be massless, but a proton may have mass that is so small that we cannot measure it easily. We can't currently say with 100% certainty that it is massless- only that it is at most very, very small: < 1×10−18 eV/c2
First, I'll assume you meant 'photon'. With that in mind, is it your assertion that if photons have no mass, they necessarily possess no energy?
Because if so, and assuming you have some reason for believing this i.e. you can prove it, I would urge you to forward these findings to a physics journal of your choice posthaste, as this basically represents a total refutation of much of physics of the last century or so. You will easily win a Nobel Prize.
> Imho whatever is carrying gravity between masses cannot itself have a mass.
Forget gravitons, this already exists in pure general relativity. Spacetime is curved around a massive object, and that curvature contains energy. That's just another word for mass, so the curvature itself exerts gravity. This creates more curvature (...etc etc). This is one of the reasons as to why Einstein's equations are nonlinear.
Note that the concept of having mass is separate from the gravity force. Interaction with the Higgs field gives rise to mass, whereas gravitons are the force carrying particle for gravity.
Only electrons get mass from the Higgs mechanism. Most of your mass comes from your protons and neutrons (or rather the energy "stored" in the bonds between the quarks that make them up).
All _fundamental_ particles gets their mass from the Higgs (up to some issues with the neutrinos). Composite particles, say the Proton, is a strongly coupled system (that is, can not be described by perturbation theory) and it does not have the mass being the sum of its constituents (not even most of it). Hence, it is not known what gives most of the mass of particles such as the Proton.
Er, sorry, implicitly was talking about fundamental particles.
IIRC we do know where protons get their mass. The internal color field has some energy, thus some mass, which comes from that field interacting with Higgs.
This is true for fundamental particles. However, more than 99% of the protons mass doesn't come from its constituent parts. I couldn't find any papers on this question in the 10 minutes I spent googling (and I don't have my QCD textbook handy), but here's a few links:
What do you think of the strong nuclear force? Gluons are the force carriers between color-charged particles. And gluons themselves have a color charge. So gluons transmit force between each other. Does that mean the universe is already too strange to exist?
By the way, this is why the strong nuclear force has such a short range. Gravity has infinite range as far as we can tell, so that makes it unlikely that the graviton, if it exists, has mass.
Photon's and gravitons have to be masses because Gravity and Light and interact with matter at finite distances.
If your 1 billion light years from earth, you are still effected by the Earth's gravity (well its likely smaller then experimental error but never mind that it still exist).
So there needs to be gravitons from Earth flooding the entire sphere of space for 1 billion light years around the Earth. All these gravitons have mass, and are emitting their own gravitons. Which all have mass and energy! Where is this mass and energy coming from? It really can't, it violates the laws of thermodynamics. But so does Dark Energy so who knows.
There are two aspects to it. And I'll be very hand-wavey because QCD is not my field.
Suppose you have two quarks and you start to pull them apart. The gluons that transmit the force between the two quarks tend to "bunch" together because they have their own charge. You can think of it roughly like a rope of gluons trying to pull the quarks back together.
If you keep pulling on the quarks, you might expect the gluons to eventually "break". But this doesn't happen, because the gluons act on each other. If there were ever a break, more gluons would join in, tugging the break back together. Eventually, you end up with so much energy density in all these gluons that they start forming new quarks and other particles. These new particles will bind with your quarks and each other to form color-neutral particles.
So I spoke a little imprecisely. Gluons, being massless, have infinite range. But you won't ever see the strong force acting over any large distance because anytime you try to get color-charged particles far enough apart, you'll end up making more particles.
Gravitation does not technically interact with light either, but rather bends the spacetime the light travels through. So question is, what makes gravitons different?
I'm a physicist, but in a different field, but my (possibly incorrect) impression is what a quantized particle is is a bit mysterious when the field is strong, for example, with lensing.
Best example: the Hydrogen atom is supposedly quantum, but if it is quantum, where are the photons? the q^2/r potential is a mean field that one finds from classical electrodynamics, it isn't formed by the summation of photons. [Another mental poker, photons are momentum eigenstates, so how can potential be described in position space? You'd need to sum up an infinite number of them! (For EM students, recall how to represent 1/r in spherical harmonics or in terms of sines and cosines)]
What happens, as I understand it, is with strong fields, one tends to use a semi-classical description because in the strong field limit, one deals with many photons, which should approach the classical limit.
Basically, quanta are like "pertubations" of the fields from their "free" solutions, as they are in GR (linearization of the GR field eqns) and as they are in EM. Free essentially means in the absence of sources, like charges, or masses for GR. So trying to explain general phenomena in terms of "pertubations", which are basically the solutions for "free" fields, is not always fair.
One doesn't always face this in high energy physics because in HEP, most of the incoming and outgoing states in a problem are these "free" solutions. For example when doing scattering off a hydrogen atom, the incoming states are "free" (a free nuclei, a free electron), so one can use photons for that phenomena, and one finds that the scattering is like scattering against a (mean) 1/r potential.
But in the case where the strong fields don't turn off, like when you are bound to a Hydrogen atom, or when considering nucleons in nuclei in the low energy limit, one turns away from the pertubative, photon/gluon model and either solving the problem numerically or treats the fields as semi-classical, as with the Hydrogen atom. For my field of laser-plasma physics, this shows up in the so-called "Volker-state", rather than treating the strong laser field as a sum of innumerable (ie., not-simulatable) photons, one treats the Laser field as a semi-classical background for the quantum guys (electrons, ions).
I think lensing is like strong static fields in EM. One wouldn't really think of them in terms of quanta of the field.
>how far we are from being able to create and detect gravitons //
Surely we create gravitons whenever we move a mass just like we create photons whenever we move a body that interacts electromagnetically? Isn't the point that we're constantly exchanging gravitons with all matter as they mediate gravitational attraction.
I think you must be right -- but maybe there's some other way to create them, without needing to move a mass. The ability to manipulate gravity itself might be the key to space travel.
First we need to find out how to create repulsion. Right now I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs.
Maybe we could make orbital graviton beam generator that could literally suck an object off the face of the Earth.
I'm not terribly knowledgeable about relativity, but I don't think that gravitational repulsion is a very meaningful concept in GR. I would appreciate being corrected on this matter if that is not true.
Inside a charged black hole there is a second horizon. Beyond this point the black hole is gravitationally repulsive. http://casa.colorado.edu/~ajsh/rn.html
So it's not explicitly ruled out by relativity. However, that link says:
> The Universe at large appears to be electrically neutral, or close to it. Thus real black holes are unlikely to be charged. If a black hole did somehow become charged, it would quickly neutralize itself by accreting charge of the opposite sign.
> It is not clear how a gravitationally repulsive, negative-mass singularity could form.
So it falls under the same sort of category as negative- or imaginary-mass 'exotic matter': not ruled out, but there's nothing suggesting that it actually exists.
I wasn't ever arguing that it occurs in nature, or even that we will one day engineer it. I was just answering your question that GR does indeed allow for such a thing.
I vaguely recall other weird edge cases where gravity is repulsive, but I can't find any links right now.
Of course, and I do appreciate your reply. I was purposefully vague in my original wording because there is that niggling difference between "unphysical" and "not proved impossible" that I didn't want to get on the wrong side of. You have enabled me to speak more precisely about the subject in the future -- many thanks. My last reply was merely trying to place this information in context; that is to say not unphysical, but in the same category as stable wormholes, Alcubierre drives, FTL/time travel, and other such theories. And ultimately it seems like either the guy talking about graviton beams was either talking about something extremely far-fetched, or a great way to destroy large parts of the planet, or both.
> a novelty device that weighs more than what it's mass would lead you to think it weighs
This is an interesting concept. As far as I'm aware, we have ways of measuring weight, but no way of measuring mass. How would you know whether something weighed more than it "should", based on its "mass"?
You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. Or you throw it at something of known mass and measure the speed it imparts onto the known object. Or you hang the known mass and the unknown mass on strings and measure the force of gravity between them, which may seem hard but can be done with stuff from Home Depo.
> You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight.
I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one.
> Or you throw it at something of known mass and measure the speed it imparts onto the known object.
Blind application of the principle of conservation of momentum does indeed tell us that we can measure the mass of one object by colliding it at known velocity with another object of known mass and measuring the resulting velocities. But I tend to worry that the mechanism for transferring velocity from one object to another object in a collision is the force it exerts during the collision, and that that force might be determined by the object's weight (also a force) rather than mass (a platonic concept). But, I'm not sure here either.
This ties in to the "fun factoid" that physics has no explanation for inertial mass and gravitational mass being the same quantity. If they in fact aren't necessarily the same thing, momentum transfer, measuring inertial mass, would solve this problem. If there is a reason they coincide, this approach will be confounded by that reason.
> Or you hang the a known mass and the unknown mass on strings and measure the force of gravity between them
I'm absolutely certain this wouldn't work to distinguish the mass and weight of an object that has extra weight because it's emitting extra gravity. The measured force of gravity is going to include the extra gravity you're trying to ignore.
>>As far as I'm aware, we have ways of measuring weight, but no way of measuring mass.
Then we are speaking of different things. I understand 'weight' as how heavy something is within particular gravity field (ie on a bathroom scale on earth) whereas mass is independent of local gravity. The schemes I suggest measure mass without resort to weight.
>>I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one.
The motion of the more massive pair will describe a smaller circle than the lighter one. The ratios of the two circles/motions allows you to calculate the unknown mass from the known.
We should be speaking of "a novelty device that weighs more than its mass would lead you to predict", that is to say, an object the measured weight of which does not correspond to what a different object of the same mass would weigh in the same location.
Measuring the force of gravitation between two objects definitely doesn't measure the mass of those objects without resorting to weight; the weight is the quantity you're measuring. Similarly, the fact that the center of gravity for a two-objects-attached-by-a-string system will lie closer to the massier object relies on the massier object also being heavier. If the massier object weighs less, why do you believe the center of gravity would still be closer to it?
> I understand 'weight' as how heavy something is within particular gravity field (ie on a bathroom scale on earth) whereas mass is independent of local gravity. The schemes I suggest measure mass without resort to weight.
Yes, those are the definitions of weight and mass. We can measure weight directly, because it's a force and we have tools to measure those. All of our methods of determining the mass of something, as far as I know, rely on the assumption that if you know the gravitational field at a point, all objects with the same mass would, if located at that point, have the same weight. The most common method of determining an object's mass involves measuring the gravitational attraction between the object and the earth (colloquially known as the object's "weight"), and then imputing a mass to it based on that weight.
In the spinning example, I could say that two objects attached by a string and set spinning around each other will spin around a point that balances the torque from each object (this might not be, strictly speaking, correct, but it's close enough that I think it's suggestive). But torque is defined by force, not mass -- if one of the objects gets heavier without becoming massier, that should draw the center of rotation closer to that object, shouldn't it?
Or phrased yet another way: if one of the two attached objects is heavier than it "should be" according to its "true mass", then the two-objects-and-a-string system will have the center of mass and the center of gravity in different places. Those terms are currently synonymous, but if we had a novelty object such as undersuit described they would be distinct. Is there any reason to believe that the two-objects-and-a-string system would, if spinning, rotate around the center of mass rather than the center of gravity?
Could such a device be used to increase the reaction mass of your fuel when it exits your engine? A sort of way to cheat F=MA by artificially boosting M, but only after you are in orbit?
No, you're confusing mass and weight. Mass is the amount of matter in a thing. Weight measures gravity's pull on the thing.
This theoretical device could make things weight more than with just Earth's gravity... but it wouldn't help your spaceship. Your engine is still pushing out the same amount of matter, so thrust remains unchanged.
I, the spaceship, would like to accelerate through space. I take up some fuel and hurl it in the opposite direction, which requires me to apply force to the fuel I'm ejecting. It goes off into space at some rate determined by the impulse I applied and the mass of fuel I applied it to.
Newton's third law means that when I hurl the fuel, it applies a symmetrical impulse to me, accelerating me in the opposite direction.
In this model, the acceleration I get from the fuel doesn't depend in any way on the mass of the fuel I eject, only on the force I apply to it. What's wrong with the model?
Related to this... According to Kip Thorne, this recent discovery alone places an upper bound of maximum mass for a graviton, if it exists and has mass, at no more than 10^-55 grams. Since otherwise the observed waves would have been distorted. It's cool that we are already seeing conclusions drawn from this observation. This single obsrrvation. https://starguyspeaks.wordpress.com/2016/02/11/on-gravitatio...
Gravitational waves are a prediction of classical linearlized gravity - gravitational waves are to (classical) gravitons what light waves are to photons.
But we have direct experimental confirmation of light behaving light both a wave and as a particle. Do we have any such evidence to show gravity exhibiting particle like behavior?
I remember learning about the LIGO experiment back when it was being built, a decade ago, and at the time it seemed so amazing: a giant tube of vacuum, sealed underground and so sensitive that it could detect animals walking nearby, listening to the moving and twisting of space itself… I guess we're finally seeing that with immense human ingenuity and the most careful of engineering, the universe will offer its secrets up to us.
This also means that between LIGO and ATLAS/CMS, the last few years have screwed in the final screws on two of the big physics advances of the 20th century: quantum field theory and general relativity are now both experimentally complete, and both look nearly unassailed in their correctness. The next steps for physics look increasingly abstruse: understanding the exceptional cases, like black holes, holography, and the fundamentally computational form of the universe. It's an exciting time, and it looks more and more like we're close to the very bottom, since we have to look so far now to find anything outside our models.
Well we've accounted for about 5% of the universe--the stuff we know about.
Dark matter (about 25%) seems to only interact gravitationally, which means that we've just, today, proven that we have an instrument that could possibly observe it directly. To date, all our evidence for dark matter is indirect--observing the otherwise unexplained behavior of normal matter. Today is the gravitational equivalent to Galileo pointing his first telescope at the night sky.
Dark energy (about 70%) still seems to be a total mystery.
And of course there is our inability to reconcile quantum mechanics with gravity. With each further proof of the correctness of each of those theories, the mystery of their apparent incompatibility deepens.
All of these factors lead me to believe that we may still have a long way to go in our understanding of the physical universe. I hope I'm right.
This is also why I believe it is so important to pursue nuclear energy. If we do invent further theories and experiments, it's likely that they will require even greater energy levels than we can create now, and potentially imply even greater dangers. If we can't learn to manage nuclear physics in a practical, routine way, we'll never have a hope of going beyond it (if indeed there is a "beyond.")
How do we know dark matter is some mysterious form of matter and not just small distributed particles (gas or solid) that are beyond our ability to detect? Do we have proof of a specific, exotic, non-atomic matter?
Scientists are pretty sure that dark matter is not just regular gas and dust because the amount required to create the gravity we see, would be visible. It would block or reflect a lot of the nearby starlight.
Just on the back of an envelope: If we assume the percentages in my post above apply to an individual galaxy, then there has to be 5x as much dark matter mass as lit mass. There's no way you could have 5x as much gas and dust in a galaxy as stars, and not see it.
For comparison, the sun makes up about 99.8% of the Solar System mass (500x as much mass as all the planets, dust, etc. combined).
That always confused me. We have an Oort cloud, whose members we cannot resolve very well/at all. Why do we assume only our star has such a thing? If all stars did, that isn't enough mass to explain dark matter?
The total mass of the Oort cloud is guessed at (3×10^25 kg), or about five Earth masses. With dark matter, we are talking about roughly 5.6x the amount of the total solar system mass. The Oort could would need to be about 371,691x more massive than it is.
Could be an anthropic explanation for that. In a solar system with a hypothetically-"normal" Oort cloud, comets and debris from the cloud might wipe out life on the habitable inner planets every few hundred million years, never allowing it to advance to human-like levels.
So we might be here only because our solar system is surrounded by an unusual amount of nothing.
But we also look at a lot of other stars in the sky. If every single one (or almost every single one) had a massive 5x mass Oort cloud around it, it would affect the light we see from that star.
Consider that we can currently detect differences in luminosity small enough to tell whether an Earth-size planet is passing between us and the star. A 5x mass Oort cloud would be thousands of times more mass than that. It would have noticeable effect on luminosity.
And, while our sun has an Oort cloud, there are a lot of stars out there that probably don't--too small, too big, too hot, too young, too old, etc.
Well for that explanation to scale up, the Oort Cloud would have to total about 5x the mass of the sun. That would have a pretty good chance of perturbing the orbits of all the planets, and vice versa.
A bit of Googling tells me that the current estimate of its mass is in the order of 5-10 Earth masses--not nearly enough to explain dark matter.
Google "sun percentage mass solar system" and the highlighted answer is "By far most of the solar system's mass is in the Sun itself: somewhere between 99.8 and 99.9 percent."
Please don't just disagree when you don't know what you are talking about.
I think you've misinterpreted my post. The "No" was in response to this:
> That always confused me. We have an Oort cloud, whose members we cannot resolve very well/at all. Why do we assume only our star has such a thing? If all stars did, that isn't enough mass to explain dark matter?
No, that isn't enough mass to explain dark matter, since it's only 0.1% to 0.2% of the mass of the solar system.
The text I quoted was in complete agreement with what you and others have posted. I was pointing out that the questioner's point had already been answered.
Ok, that's just a really confusing way of communicating, nobody is going to puzzle that out when the obvious way of looking at your response is disagreement with the grandparent.
Scientists are not prone to falling back on explaining observations via postulating a new kind of matter we can scarcely observe. Ever since the first indications of "dark matter" scientists have been attempting to explain it as something more familiar to us, some kind of atomic matter or some-such, maybe gas or dust or lots of planets or dark stars or something. At every single turn they've been stymied, and instead of eliminating the idea of dark matter as an ethereal particle they've instead eliminated other possibilities.
Don't look at the current theory of dark matter (weakly interacting massive particles) as some hare-brained scheme that scientists thought up, instead look at it as the hard-fought victor of numerous observational challenges. Dark matter is the theory that survived. We tried explaining things a zillion other ways (gas clouds, compact objects, neutrinos) and those theories just didn't match the observations. There are also a few exceptional circumstances (such as the bullet cluster) that indicate very strongly that dark matter is something different than either gas clouds or stuff like stars and planets, because in the bullet cluster we can observe the gas and the stars and planets and the mass, and each of them are in different places because each of them follow different rules when it comes to interacting during a galactic cluster collision.
Dark matter was "invented" because there wasn't enough observable mass in galactic-scale objects to account for their behavior. In other words, they acted like they had more mass than we could observe. Dark matter is basically characterized by not responding on the electromagnetic spectrum, which is what we use to do these observations. Since all the matter we know of generally does respond on this spectrum, that's why dark matter is considered to be "exotic".
https://en.wikipedia.org/wiki/Bullet_Cluster is fairly good evidence that dark matter isn't just unobserved regular matter. In these massive cluster wide collisions the dark matter seems to have "kept going" (you see very strong gravitational lensing where there appears to be nothing) while regular matter that we know is subject to forces besides gravity collided together, slowed down, and became very hot.
>It's an exciting time, and it looks more and more like we're close to the very bottom, since we have to look so far now to find anything outside our models.
For what it's worth we thought the same thing a little over 100 years ago. We just had to figure out a few pesky things like blackbody radiation and physics would be all wrapped up.
This is a popular thinking, but actually there were people like Kelvin, Jeans, Rayleigh, Planck and many others who did not get famous who knew there were problems with the theory. In no point in time of modern science there was widespread opinion that "it's mostly done".
"fundamentally computational form of the universe", you must be a seth lloyd guy :)
But yes, it's the fringes that we'll find new physics. It's not unlike the late 19th century when newtonian + E&M seemed to account for all there was to know.
There hardest thing in fundamental physics right now is to know what questions to ask. We've got answers that work for a lot of the biggest ones that the last 100 years have been spent developing and exploring.
> quantum field theory and general relativity are now both experimentally complete, and both look nearly unassailed in their correctness
Well, we know that both theories are "wrong" in the sense that they give nonsense answers if you ask them the wrong questions. It's just that all of those questions are well beyond our ability to test experimentally.
An underground device sensitive enough to detect animals walking around could be useful for other things... (from ecology research to large-scale surveillance)
A conceptual issue that some of the commenters may have missed is that part of the detection is done by matched filtering (https://en.wikipedia.org/wiki/Matched_filter), in which it is necessary to have a good idea of the signal you're looking for. This detection has built upon analytical and numerical advances in relativity. While people may not know about the prevalence of e.g. binary black hole collisions, they have a pretty good idea of the signal that would result if such a collision were to occur. Similarly with other potential sources like binary neutron star collisions.
Yeah, too many LHC reports have primed people to expect counting experiments where the scientists struggle to get to 5 sigma. The waveforms we're talking about here have a signal to noise ratio over 20.
I'm assuming that the same rules apply as do in straight RF detection. A signal becomes a decent signal at 6db above noise and gets exponentially better every 6db above that. Something 20db above noise is rock solid reliable.
A signal 20db above the noise, you could put your eye out with it.
db is confusing, when you're talking voltage it's 20log(Vs/Vw) And in absolute terms engineers talk about the power over 1mW.
Myself I get miffed a bit because people have been conditioned to think in terms of trying to pull facts out of crappy data sets using poorly thought out statistics. However in a lot of engineering and physics fields the data is often really good. Often good enough that you can work off a single measurement.
> And then the ringing stopped as the two holes coalesced into a single black hole, a trapdoor in space with the equivalent mass of 62 suns. All in a fifth of a second, Earth time.
Am I reading this correctly, that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided?
Was that extremely coincidental? Or do these events happen all the time, and so if it wasn't those two black holes it would be two others?
The predictions for the LIGO detection rate are very poor. They're based on a sample of just a handful of binary pulsars observed in our Galaxy, which would produce NS-NS mergers. The BH-BH merger rate is almost totally unconstrained, although it is generally thought to be less than the NS-NS merger rate. So the fact that a BH-BH merger was the first detection, and the fact that it was detected so soon after the sensitivity increases is evidence that the BH-BH merger rate is probably somewhat higher than expected. But we won't know for sure until LIGO detects more events and the rate can be better constrained. Sometimes you do just get lucky.
I should add that there are lots of selection biases and educated guesses in all of this, too. The signal from BH-BH mergers is louder and easier to detect from larger distances. At the same time, NSs are probably more common than BHs, but it's not really clear whether there are more NS-NS binaries than BH-BH binaries because NSs receive kicks from the supernova when they are born but BHs (probably) do not. This may have the effect of blowing apart many nascent NS-NS binaries but leaving the BH-BH binaries intact.
From the paper: "To account for the search background noise varying across the target signal space, candidate and background events are divided into three search classes based on template length. The right panel of Fig. 4 shows the background for the search class of GW150914. The GW150914 detection- statistic value of ρˆ_c = 23.6 is larger than any background event, so only an upper bound can be placed on its false alarm rate. Across the three search classes this bound is 1 in 203 000 years. This translates to a false alarm probability < 2 × 10^−7, corresponding to 5.1σ. A second, independent matched-filter analysis that uses a different method for estimating the significance of its events [85,86], also detected GW150914 with identical signal parameters and consistent significance" (https://dcc.ligo.org/LIGO-P150914/public). Take a look at Figure 4 as well.
In case you'd like to dig deeper, the 85 and 86 mentioned are:
[85] K. Cannon et al., Astrophys. J. 748, 136 (2012).
[86] S. Privitera, S. R. P. Mohapatra, P. Ajith, K. Cannon, N. Fotopoulos, M. A. Frei, C. Hanna, A. J. Weinstein, and J. T. Whelan, Phys. Rev. D 89, 024003 (2014),
It's not a counting experiment, which makes the calculation of a false positive rate somewhat harder. The key for LIGO is certainly that they saw the signal coincident at two stations, far apart.
Isn't the point though that the gravitational wave observatories are looking specifically for "black swans" rather than just observing swans generally. So when a swan with a lower reflectivity is observed then it now fits the "black swan" profile. Could be just a swan covered in soot; you need more data to show that this swan is always black or that the lower reflectivity wasn't caused by a measuring anomaly, etc.
This comment is making the page formatting gross. Those special characters with the strike-throughs make the entire page over-wide, thus requiring horizontal scrolling to read comments.
The browser layout engine should break on the spaces (Chrome does). They are just normal spaces, the combining character should have no effect. You have a bug somewhere.
Also, I cannot edit nor delete it now, so tough luck!
" " SPACE Basic Latin
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"̶" COMBINING LONG STROKE OVERLAY Combining Diacritical Marks
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Because of the shape of the event, detection in two places, and more importantly, it matching the signature of the theoretical event extremely closely (especially the ringing at the end)
The detection uncertainty is a separate matter from the predicted rate. Sure, if you had a strong prior that GWs should be detected once in a billion years, then you would want a better detection. But as it is the priors on the detection are pretty weak and this is totally consistent with what is expected.
Others have answered other aspects of this, but as I understand it, it is not the case that we don't know how rare they (BH-BH events) are because they are so rare, we don't know how rare they are, because we don't have a really good model for them. So, we don't know how often we'd expect to detect them, once we had a detector.
I recall reading some years ago that gravitational wave would be used to prove multiverse theory. How would that scale compared to bh-bh or ns-ns mergers?
Also, have read today that this discovery backs inflationary theory, how so?
It seems highly unlikely that they could say a specific bh-bh merger was the cause. It seems implied they are triangulating the source, with two detectors?
AFAIK no multiverse theory has yet been put forth that is experimentally testable (even in theory given infinite time, energy etc.) So it's not a proper (falsifiable) scientific theory at present, merely a (in my opinion wild) conjecture.
> no multiverse theory has yet been put forth that is experimentally testable
Just to be clear here; that's because there is no theory for a multiverse. Not yet, anyways. Nobody has put one forth yet. When you hear "multiverse" come out of physicist's mouth, it's because it's a concept indirectly related to other theories. The current popular theory which involves a multiverse is string theory. When string theorists do the math, there is some evidence that a multiverse is possible.
However, that doesn't mean much. Even if string theory was correct and little strings are really the fundamental component of everything in the universe, the multiverse part of string theory could still be wrong. The theory isn't reliant on it, it just doesn't forbid it.
The source isn't the greatest, but it shows that we can look at the CMB for indirect evidence. With higher resolution scanning years in the future, such a theory may be testable. I only mention this because the way your comment reads, it sounds like you're saying a multiverse would be inherently untestable.
You are suggesting possible future events that would provide evidence for a multiverse. That does not make something a falsifiable theory.
Analogy: it could happen that tomorrow Jesus Christ descends from the heavens and brings the day of reckoning. That would prove Christianity to be true, but the fact that this could happen does not make Christianity a falsifiable theory.
A falsifiable theory is a theory that predicts something that we can (in theory) measure today (possibly requiring infinite resources etc).
Your second example (which is not a multiverse theory at all) is actually a good example of a falsifiable theory. People have calculated [1] that if spacetime was folded back onto itself at even just a single point, it would leave a distinct signature in the cosmic microwave background. We do not observe this signature, so we are pretty sure spacetime does not fold back onto itself.
Ok. What if spacetime folds back onto itself over a very long distance? Wouldn't that be (viewed locally, with our limited instruments) as if another version of spacetime touches upon our version of spacetime?
Physics will always be based on observations. Consciousness is fundamentally hinged TO observation. I'd argue that the way your brain works is more fundamental to reality than the physics causing a Mhz of conduction throughout your synapses. In summary, all bullshit theories are possible in spirit of deceit. For why should good senses be wasted on a cohesive system when the mind is simply a slave to its own devices?
TFA says "they had heard and recorded the sound of two black holes colliding a billion light-years away" and "1.2 billion years ago".
And from the paper: "The source lies at a luminosity distance of 410+160-180 Mpcc corresponding to a redshift z=0.09+0.03-0.04.". (https://dcc.ligo.org/LIGO-P150914/public) Which corresponds to 1.337+0.522-0.587 billion ly (or between 750.2 million and 1.859 billion ly).
Looks like there are roughly three million galaxies within a billion light years. Seems like lots of space for black hole pairs to live in. I suppose over the coming years, these gravity wave observatories will nail down just how common they are.
So that's wayyyyyyy outside our galaxy? Any idea how many galaxies fit into a 1 billion ly sphere around the milky way? I'm guessing a shit ton, which makes the detection of a bh merger seem more realistic to me.
It was mentioned during the press conference today that gravitational waves are not affected by interstellar/intergalactic dust the same way light is. In theory, once our detectors are good enough we should be able to use gravitational wave astronomy to peer all the way back to the big bang!
This would be true in a static universe, but, during the 1.2 billion years the waves have been traveling, the universe was experiencing accelerating expansion.
For example, the edge of the observable universe is about 46.5 lightyears away, while the universe is thought to be 13.8 billion years.
Well, the number of solar-mass black holes in our galaxy is about 10^8. Since black holes form from stars, you can assume the probablity of having binaries is probably related to the probablity of having binary systems in stars, which is high. And the distance to the event is several megaparsecs (much bigger than our galaxy). The fact that they detected two 30 solar mass black holes coalescing 2 days after their sensitivity upgrades says that they almost certainly have had other, less pretty, detections in the few months they've been running their detectors for. Or they should go buy some lottery tickets.
>> that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided?
Counterintuitive, but yes. Because it happened billions of years ago, it happened a long long way away. The sphere of objects billions of years away/ago is far larger than those closer to us. So such a detector should be detecting exponentially more very old objects than new ones. Given the rarity, I would expect nearly all detected events to have happened long long ago in galaxies far far away.
Also, models point to such events being more common in the distant past where there were more black holes (primordials) floating around than there are now.
If one event happens 1B years ago 1B light years away and another event happens at .5B years ago .5B light years away... how would we know there are two events?
The wave may contain the information necessary to describe the event that created it. If you know, perhaps by frequency, that the wave came from two super-massive black holes spinning around each other at a given rate, then you know how massive each hole should be. From that you can predict the energy of the wave. And from that you know how far away the event must be for the energy behind the wave to have spread so thin. It's basically the same process as calculating distance via cepheid variable stars.
I am not a physicist, and dont have a good mental model of gravitational waves (or general relativity at all), so maybe someone can answer my laymans question: do these waves behave like ripples in water, so a single event generates multiple repetitive, concentric waves permeating through space? That would make me think that a single massive event would be easier to detect because it would leave many repeated "echoes", ringing space for a long time after the actual event.
The article makes it sound like the detection of these waves is just a quick one-time blip though. I'd expect something as big as black holes merging to generate more longer lasting waves than just a quick blip. What is the period of these waves?
https://www.black-holes.org/gw150914 has some visualization of the event. There is the initial inspiral, and then there is a ringing afterwards. However, the entire event is over in a fraction of a second, which may be a "blip" to humans, but is very long when things happen at the speed of light.
Thanks, that's helpful. It's hard to get my head around the idea that an event so massive can be over so "quickly", without any residual longer-lasting effects.
Well it sounds like a massive ripple in the fabric of reality which passes by us in a fraction of a second, never to be seen again.
So from my non-physicist point of view, no it doesnt seem like a very long lasting effect, relative to us at least. Thanks for the snark though.
There's a theorem that black holes have "no hair": two black holes of the same mass, charge and angular momentum are indistinguishable. So the merge must happen instantaneously: if the combined black hole were "sloshing" afterwards that would violate that theorem.
"do these waves behave like ripples in water, so a single event generates multiple repetitive, concentric waves permeating through space?"
No they don't. There is a law known as [Huygens' principle](https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_princi...) which says that when a disturbance at a particular point creates a wave, that wave only propagates on an outwards-expanding sphere that is centered at that point of disturbance, and does not produce any effect on the interior of that sphere. This was originally formulated for light waves, but it also holds for other kinds of waves, such as sound waves or, in this case, gravitational waves. What this means is that when you look at something that's far away you see a sharp image of exactly what happened there a short time ago (the time it had taken the light to reach you), whereas if the principle did not hold, each light source would have a small "echo" after it which would blur the image.
However, one of the reasons Huygens' principle holds is that the waves are propagating over three dimensions. In contrast, water waves only propagate over two dimensions, so Huygens' principle fails. That is why ripples continue to emanate from a spot even long after the disturbance there is over. More generally, Huygens' principle holds whenever the number of dimensions is odd and fails whenever the number of dimensions is even.
[Note: I may be wrong on why Huygens' principle fails for water waves. Water waves are actually pretty complicated compared to other kinds of waves and I am not knowledgeable in all the subtleties.]
An echo is a reflection. I don't know whether gravitational waves can be reflected even in principle, but even if they can, space is so empty that in practice there's nothing to reflect them. So even a single massive event, like the one described in the article, will just send out a single expanding spherical wavefront; if you're not listening at the right moment, you'll miss it.
Sorry, "echo" wasnt the right term (hence my quotes).
What I am trying to ask is if these behave like concentric water ripples, where from a single event you get first one peak of a wave, followed by many more repeated concentric peaks gradually getting smaller in amplitude? It sounds like there is just a single momentary wavefront without any residual secondary waves? Why is that?
So if you look at the waveform of the signal, there are in fact smaller ripples after the main event. However, how long these ripples take to settle afterwards to equilibrium is related to how quickly the waves propagate. In the case of ripples on a pond, those travel at about 1 m/s; these gravitational waves travel at the speed of light, roughly 300,000,000 m/s, so we should expect it to settle to equilibrium about 300,000,000 times faster. If it takes 60 seconds on a pond, we would expect the gravitational waves to settle in about 0.0000002 s, or 200 nanoseconds.
Note that this is a _very_ rough estimate, but it should give you an idea of the order of magnitude for the settling time.
> Was that extremely coincidental? Or do these events happen all the time, and so if it wasn't those two black holes it would be two others?
From the article, no one knows: "Black holes, the even-more-extreme remains of dead stars, could be expected to do the same, but nobody knew if they existed in pairs or how often they might collide. If they did, however, the waves from the collision would be far louder and lower pitched than those from neutron stars."
The coalescing and ring-down takes a fraction of a second. The fraction of a second refers to the duration of the event. It does not mean they caught an event a fraction of a second after they turned it on.
But he neglected to mention the error bars on this, which AFAIK are huge at least for BH-BH mergers. Every time we built a new instrument, we saw something new, whether in astrophysics, or nuclear physics, or particle physics. Maybe the BH-BH rate is much higher than expected.
Typically one would assume that it was not coincidental and then adjust the bounds for how often we expect the event occurs based on observations, or lack thereof.
From the abstract of the paper, energy equivalent to three solar masses were radiated away in gravitational waves. That's a simply incredible amount!
Possibly stupid question: Given how far away it was, and that the inverse square law applies, would the effect of these waves be visible on the human scale if we were closer? We can see the effects of the compression of spacetime with LIGO after all, so presumably we could?
This thing was a billion light years away. Say it were closer; let's put it at a single light year away.
LIGO measures wave amplitude, as far as I can tell, which goes down linearly with distance (unlike wave energy, which goes down quadratically, since it's proportional to square of the amplitude). So we could expect to see an effect about a billion times bigger.
The detected effect was a change in metric of one part in 6e20 if I'm not mistaken: (4e-3 * (diameter of proton))/4km based on the article's claim of "four one-thousandths of the diameter of a proton". So at one light year distance we could expect an effect of one part in 6e11.
Not really visible on the human scale, seems to me. You could detect it easily with something like the Mössbauer effect, I expect. Your typical lab bench laser interferometer has errors on the order of 1 in 1e6 as far as I can tell, so probably wouldn't be able to pick this up.
Disclaimer: I could be totally off on what a lab bench laser interferometer can do. I'm pretty confident in the rest of the numbers above.
On the other hand, we may well detect the 3 solar masses radiated away as energy. That decreases as an inverse square law, so as one solar mass is about 10^30kg, and 1kg gives off about 10^17 J, we're talking about an explosion releasing something around 10^47 J. For comparison, a 1 kiloton nuclear bomb gives off about 10^15 J.
So, inverse square that explosion... 1 light year is about 10^16m, so we square that and get 10^32m, so we're now talking about ... 10^15 J.
So, unless my maths is all off (which is possible), if this happened about a light year away, whoever's on the side facing towards the blast wouldn't get to observe very much because they'd feel as if a 1kt nuke just went off above their head. Not a great way to start the day.
Chances are it would wipe out life on Earth too, through the ensuing side-effects like lighting the atmosphere on fire, sterilising half the planet, significantly heating up the oceans, possibly even stripping part of the atmosphere away, etc.
For a great novel based around a strikingly similar premise to what was just observed (and the main reason I even bothered to calculate this), Diaspora by Greg Egan is a fantastic book.
The big question is how much of the energy would get transferred in practice.
I agree that 3 solar masses worth of electromagnetic radiation at 1 light year distance would feel like a nuke going off. What I don't know is to what extent the energy of the equivalent gravitational waves (which _would_ have a lot of energy I agree) would actually get transferred to things we care about, like the atmosphere and us. If it's a few percent, say, we'd clearly be in trouble. If it's more like what neutrinos do, it would probably be detectable but probably not by unaided human senses.
I tried doing some quick looking around for estimates of gravitational wave coupling and energy transfer and didn't find anything so far...
I would like to understand why a gravitational wave distorts length in relation to normal gravity wells; specifically is this particular to waves? Why don't lengths get distorted in a normal gravity well, or do they? In essence, what is different between a gravity wave and a gravity well, which i understand both distort space, but only the wave distorts it in a way we can measure? Does the gravity well change lengths proportionally in all directions and thus isnt measurable?
A gravity well also distorts lengths, as best I understand (which is not very well, to be honest; take everything I'm saying here with a big grain of salt).
The difference in terms of detection is that the wave does this in a time-varying, periodic fashion.
For something like LIGO, we're trying to measure length changes on the order of 1e-18 meters. We're not actually measuring the lengths of LIGO's arms to that accuracy, though. What we're measuring is the difference between the times light takes to travel down those arms. And even that's hard to measure on an absolute scale, so what we really measure is how that difference changes in time.
Or put another way, the effect of Earth's gravitational well is not really distinguishable from inaccuracies in making the two legs of the interferometer equal length to start with, and is a much smaller effect than those inaccuracies. Again, if I understand this right...
Actually, we have ample proof of the distortion of spacetime in a gravity well - gravitational lensing. It's an observed effect around very massive objects and we have been able to see it at work very well. Also, arguably, the fact that we're not falling towards the sky is itself evidence of a spacetime gradient near the Earth, but that was also explained by Newton's Law of Gravitation.
But back in 1916, Einstein also theorised, as part of his general theory of gravitation, that there would be such things as gravity waves, caused by very massive objects moving through spacetime making 4-dimensional ripples appear in spacetime. Until today, that was just an unproven theory, though everyone believed it was likely to be true. There is now solid evidence to back it.
Agree... my question, though poorly worded, is less about proof of spacetime gradients (they do in the ways you describe).
It's more about understanding what the measurable effects of a gravitational well on earth has on the LIGO experimental setup (or a similar one with infinite precision), in the absence of gravitational waves.
Well, something like LIGO can only measure gravitational waves, because it looks for changes in the geometry of spacetime. If you were to move the LIGO in and out of Earth's gravitational well, I guess then it would record a shift.
That is a good point - perhaps it would all come out as neutrinos or gravitons... then we'd be fine. I doubt such an event would result in no electromagnetic radiation at all however... Why would it? The creation of a new black hole typically releases enormous amounts of all kinds of energy, electromagnetic as well as in the form of neutrinos.
The energy was all dumped into the gravitational waves we detected, not into electromagnetic radiation: Gravity waves don’t interact with matter very much (the cross section of the graviton is believed to be extremely small) so the quantity of energy transferred to matter as the wave passes through is likewise extremely small. I haven’t run the numbers, but I’m not sure you’d notice this even from a light year away without fairly sensitive detectors.
What I didn't understand after reading the article is how do they separate out a set of waves for one specific thing vs. the many other objects sending out waves. Is it just that the set of blackholes are the strongest set of waves and thus the ones we can detect?
That, and the high frequency. For example, the Earth orbiting the Sun produces gravitational waves at a frequency that's about (factors of 2 and pi here and there) the orbital frequency; order of 1e-8 Hz. The black holes were producing 250Hz waves if I read the article right.
The longer the interferometer arms, the better you can do in sensitivity. The reason LIGO has 4000 m long arms is that it makes the experiment 4000x more sensitive than something you can do on a bench. (and their laser stabilization is excellent, improving things further)
Sure, but LIGO is sensitive at something like 1 part in 1e20, which is a lot more than 4000x better than 1 part in 1e6. I agree that their laser stabilization is likely much better, their vacuum is likely a lot better, etc. I was just surprised by how much better, I guess; 10 orders of magnitude is a lot.
Part of the reason for that is that LIGO isn't exactly a Michaelson interferometer in that it has an extra pair of mirrors in each arm. If you look at this schematic [1] then in a traditional Michaelson interferometer you would only have the mirrors that are at the end of both arms.
With LIGO there is an extra set of mirrors within the arms this allows the light from the laser to bounce between them ~100 times or so increasing the effective path length greatly.
Yeah,I got to the point mentioning the masses of the black holes before and after collision and said, "What, they didn't just lose three solar masses..." But, they did.
Which was the order of predictions I'd read, years back, but egads. Considering how much larger that is than a supernova, I'd be concerned to have such an event happen in this galaxy...
The energy is dumped into gravitational waves rather than electromagnetic radiation & they don’t interact with matter much. I’m not sure you’d notice it happening in the same galaxy unless you were looking for it.
Yup. Possibly one of the most energetic events in the Universe. Fascinating in many ways when you think about it. That mass was once matter, and somehow it got converted into gravitons.
According to this paper ( https://dcc.ligo.org/LIGO-P150914/public ) they detected the signal first at Livingston, Louisiana and 6.9ms later in Hanford, Washington. The distance between them according to wikipedia ( https://en.wikipedia.org/wiki/LIGO ) is 3002km (Ok, the 3002 km distance is on the Earth). If the gravity wave travel at the speed of light they should detect 10ms later (300 000/3002 sec = 1/100 sec = 10ms ). From these data the gravity travels at 434 000km/sec instead of 300 000km/sec. Almost 50% faster then light... Is there any error in my calc?
10ms is the absolute maximum difference in time, if the source was located on a line running through the two detectors. If the source was located on a perpendicular bisector of the line running through the two detectors, the difference in time between detections at the two detectors would be zero. Any value between the two is possible depending on the geometry.
I think your calculation assumes that the waves are traveling parallel to the line connecting Livingston/Hanford. In the diagram below, 's' is the source of the waves.
H-----L-------s
If instead the waves are traveling perpendicularly to the line between those two cities, they should be detected at the same time.
s
/|\
/ | \
L-----H
Since the measured time difference is between 0ms and 10ms, the reality is probably somewhere in between these two extremes.
Even ignoring the curvature of the Earth, the signal source was not necessary located on the straight line between two LIGO locations, but rather at some angle to this line. For example if the signal origin was on the line that is exactly between the two LIGO detectors, the time delay would be zero.
Consider that the black holes merged about 1.3 billion years ago. If gravitational waves travelled 50% faster than the speed of light, they would've passed by earth long before our species came around, unless the effect of the collision went on for, say, a few million years after the event?
How do the detectors work? In my mind they don't make physical sense. They're saying the distance between the mirrors changes, but I don't understand how that's possible in this context.
Let's say a gravitational wave compresses space. To someone inside that compressed space, there should be no noticeable difference. Light will still flow the same way through the compressed space at the same speed relative to the compression. Matter will behave identically, because both light and matter are part of the fabric of that space. As I understand it, the only way the mirror lengths could change is if space is created or destroyed.
If that doesn't make sense, consider the 2d analogy of drawings living on paper. Assume also that light moves only along the surface of the paper. If you bend the paper, the light will bend with it. But when you bend the paper, the creatures living on the paper can't know it's bent. The fabric of the paper is still identical. Even if some of the paper gets compressed in one direction, it will still have the same amount of particles, so any light travelling through there will hit the same amount of resistance. And stretching the paper, even if you're a drawing on the part being stretched, would have no effect. A 2d creature looking at something 1 foot away, even if the paper is stretched to 10 feet, won't see any difference, because the fabric light travels through is also stretched.
The only way I can see this making sense is if light travels independent of the fabric of space, but it's my understanding that light travels through it, not independent of it?
I think the crucial detail you are missing from the article is this:
"According to the equations physicists have settled on, gravitational waves would compress space in one direction and stretch it in another as they traveled outward."
LIGO is two sets of 2 L-shaped antennas spread far apart on the globe, so that we can compare the compression of space in orthogonal directions and measure the very short delay between the gravitational wave hitting the first detector followed by the second. In this case, that difference was 7 milliseconds, which is also consistent with the speed of gravitational waves (also the speed of light)
I still don't understand. It doesn't matter where the compression happens, because it should be undetectable to any light/matter that's fundamentally a part of that space? If one of the arms gets compressed - the matter will be compressed too, so light still has the same density and amount of space to travel through?
Check the comic posted by AdrianN, it explains what you're missing. Basically light takes longer to travel stretched space (but matter does not, as you correctly said).
I see. If that's true, then light travels through a higher dimension, and this is definitive proof of at least a fourth spatial dimension. Otherwise there would be nothing for the 3d space to ripple through, or for light to travel through.
I'm surprised I haven't heard that light travels independent of 3d space compression before. That would also imply that if you enter a black hole with your feet at the bottom, you would see them visibly stretched far away from you (noticeably? I'm not sure) because light would take longer in the distortion to reach your eyes.
Doubts about higher dimensions and general relativity is common and a crucial point, so I dont think you should get downvoted.
Some points which might be helpful. We have a way, using the concept of a manifold, for ants on a surface like a sphere or a dougnut to figure this out without appealing to a third dimension. One could imagining say ant geographers making maps of portions of the surface, and noting how common regions covered in two different maps have different labels/coordinates. One can then figure out a definition of when two collections of maps(called an atlas) are equivalent and then show that an atlas for a plane, sphere, doughnut are mutually nonequivalent.
But all this is topology and involves global considerations. What is relevant here is local curvature. We can also do this appealing to an extra dimensions. Now, you used the example of a folded paper and you are correct that for an ant on the surface, the curvature is indetectable. The curvature of the paper is extrinsic and not intrinsic. We say that is isometric to flat space, and its curvature tensor is 0.
On the other hand, if the ant was on the surface of a ball, it could figure out this curvature intrinsically, for instance, by measuring sum of the angles of a triangle or the distance between parallel lines keeps shrinking. Not only is this intrinsic, but it is locally measurable. One cant have maps, even for a small area of the earth's surface, without some kind of distortion because of this intrinsic curvature.
An additional complexity - in GR, spacetime is curved rather that just space. Also, dont take 'curvature' too literally, it is just a way of measuring deviation from numbers that you would get in the flat scenario.
For more read up on manifolds, riemann curvature. John Baez had some essays on the geometric meaning of the curvature tensor in terms of the volume of a ball relative to the usual flat Euclidean case.
Thanks - still not sure I get it. The "fabric" is stretched in x, squeezed in y, sure. Is it that the wavelength of the light is -not- stretched? Guess I need to go back and study physics again :/
Still not explaining it. If spacetime is stretched why doesn't light "speed up" to accommodate for it is what he's asking. I'm assuming it's because the speed of light is invariant to that.
Exactly. The speed of light in a vacuum is constant no matter how quickly/slowly your frame of reference is (Special Relativity). You can think of it as the space "stretching/shrinking" because of the gravitational wave, as that is how it looks from the point of view outside the experiment.
Another way to look at it is to change your reference to be internal to the experiment. You can imagine the experiment moving through space-time at a constant rate of speed. When the gravitational wave hits the experiment its movement through space-time changes an infinitesimal amount (faster than slower as the wave passes, or vice-versa). Since the speed of the light passing through space-time has not changed (due to special relativity), the difference between its start and end points can be used to measure the amount of change caused by the gravitational wave, essentially in the same way you can use a laser to measure the speed of a moving object relative to a stationary object. Only you are basically "bouncing" the laser off of the experiment itself to determine the change.
Here's my re-statement of this confusion, isn't everything we can experience embedded in time-space, including the LIGO experiment itself? So how is there any relative shift allowed to be detected when everything we know is fundamentally intrinsic to time-space? That is, I too would appreciate having this mis-conceptualizing, of mine, cleared away.
Another re-phrase: how can we detect that space has stretched out if all of our rulers also get stretched by exactly the same amount?
The answer is that we have a ruler that doesn't get stretched in this way: light. The speed of light is a constant dictated by the laws of physics; stretching out our flashlight to twice its normal size wouldn't make the light it emits go twice as fast. So if you just measure the time it takes for a light ray to go from one point to another, you can compute the distance that it must have traveled, and if that distance changes, then you know that the space in between must have been stretched.
Thank you. i think that settles my confusion. (it's somewhat like we witnessed a length/Lorentz–FitzGerald contraction in a situation where there was apparently no reason to witness one). Now i can go back to being simply amazed by it all.
Thanks for the nice explanation! But another question: since the expansion rate of the universe has been different at different times, does this mean that the measured speed of light would be different at different times as well?
Because if the ruler you are using to measure is expanding at rate x, the measured speed of light would be different than when the ruler is expanding at rate y?
So, during the deflationary period of the universe, the speed of light would be significantly smaller, correct? In fact, would it be "negative" due to the universe expanding faster than light?
Does this mean that in earth's gravity well, there is an absolute difference in the time light takes to travel compared with light travelling in the void of space?
Can we compute the strength of a static gravity field we are inside, by measuring the time that light takes to propagate through it?
The light is constant, which means it moves at the same speed in both cases (assuming the light is in a vacuum). It won't move faster or slower based on the gravity field (other than in the case of black hole where it can't escape at all).
What happens instead is that the speed that an object moves through space-time changes dependent upon gravity. Using an atomic clock, we've actually measured the effect of gravity to show that time moves more slowly down on earth than it does in an orbiting satellite.
Are there any potential competing theories this detection could also support? I'm wondering how much room there is here for confirmation bias, but I suppose that's a pretty hard thing to measure without the benefit of hindsight.
Even before this discovery, it's been pretty solidly established that any alternative theory to General Relativity would need to behave essentially identically to GR in the limits where we've been able to test it. So, for example, the "low energy limit" of string theory is general relativity (plus other content, in most cases). I'm not sure whether the loop quantum gravity folks have a working low-curvature limit yet (I'm out of touch), but that would be a requirement for them, too.
At first glance, I'd guess that this discovery only strengthens that conclusion: even a small deviation from GR might well change the detailed behavior of an immensely high curvature situation like a black hole merger, and what we saw seems to have been a spot on match for the GR-based models.
Well, they extracted a lot from the waveform: Distance, the two masses, the resulting mass. I could imagine that a competing theory gives the same waveform maybe with different values for these parameters.
A "competing theory" would first have to match the GR predictions in all the other regimes where it's already been tested. But doing that is an extremely strong constraint on a theory, to the point where the only theory that can meet it is GR itself. Physicists know this because alternative theories to GR have been constructed and tested, and they have all failed. See, for example, here:
That's a very common sentiment, but a mistaken one. Theories do not get accepted only if they match the predictions of the previous theories. People value other features besides accuracy of predictions, like simplicity and explanatory power. Just recall how Kopernik's theory of solar system got accepted. It had worse predictions than Ptolemy's scheme at the time it was introduced; Ptolemy's scheme was way better in accuracy, but utterly complex and explained little.
You're missing the point. I agree that matching the predictions of experiments (not previous theories--I'm talking about experimental results that match the predictions of GR, not just those predictions themselves) is not a sufficient condition for a theory to be accepted (which is what you are saying); but it is certainly a necessary condition (which is what I was saying).
> Just recall how Kopernik's theory of solar system got accepted. It had worse predictions than Ptolemy's scheme at the time it was introduced
Yes, and it wasn't accepted at the time it was introduced. Actually, Copernicus' theory in its original form was never really "accepted"; what was accepted was Kepler's reformulation using elliptical orbits, based on Brahe's more accurate observations. Kepler's model was more accurate than Ptolemy's, and that was a key factor in its acceptance.
I agree with you that if a new theory was to replace the old one for making specific set of predictions, it should give predictions of similar or better accuracy. But I do not think that replacement is necessary for the new theory to compete or be accepted; it is the new benefit it brings, whatever its nature may be, that is crucial. The two can temporarily both be accepted to coexist, if both have their strengths. For example, quantum theory does not make the same predictions as classical theory when it comes to classical experiments (mechanics, basic EM phenomena) and is largely useless in that domain. It only gives probabilities of results of specified experiments of certain kinds; it does not reproduce the old predictions (like definite trajectories, Moon phases or solar eclipses), but provides new results (like resonance frequencies of atoms and molecules and their bond energies). Similar thing can happen with a new theory of gravity; it may not give the same prediction for Mercury perihelion precession, but it may be able to explain other things, like why the inverse square law, why no repulsive gravity or why the mutual gravity force between electrons is so much lower than the mutual EM force. Explanation for oddities in Mercury motion could then wait for further data and repetition of calculations. It is natural to expect of any new theory to bring new results, but demanding that it reproduces all the old ones along is too much. That happens rarely and such expectation only prevents any new ideas from being considered.
> quantum theory does not make the same predictions as classical theory when it comes to classical experiments (mechanics, basic EM phenomena)
Yes, it does. Do you know how the classical limit of quantum theory works? That limit is what allows us to use classical physics in the domain where it works. If that limit didn't work, we would have a serious problem with consistency.
> It only gives probabilities of results of specified experiments of certain kinds; it does not reproduce the old predictions (like definite trajectories, Moon phases or solar eclipses)
Are you aware that all of those "old predictions" can indeed be derived from quantum theory, using the classical limit I described above? The reason that works is that, in the classical limit, quantum theory predicts a probability of 1 for one result--the classical result.
> It is natural to expect of any new theory to bring new results, but demanding that it reproduces all the old ones along is too much.
You appear to have a mistaken understanding of how new theories get accepted. New theories that don't reproduce all of the predictions of the theory they replace, in the domains where the old theory is verified by experiment, are not accepted. If general relativity had not reproduced all of the predictions of Newtonian gravity in the weak field, slow motion limit, it would not have been accepted. And if quantum theory had not reproduced all of the predictions of classical physics in the classical limit, it would not have been accepted.
As I wrote above, I agree with you on the requirements for replacement theory. My point is that a new theory of a phenomenon does not need to replace and reproduce all the results of the old theory to be considered worthwile, competing, acceptable.
Can you give an example of a new theory that was considered worthwhile even though it didn't replace and reproduce all the results of the old theory? I'm not aware of any. (The Copernicus example given upthread is not a valid example, as I said in response to that post.)
Schroedinger's theory of hydrogen atom and his wave mechanics (1926). It explained positions of emission lines of excited hydrogen, but it didn't explain how the atoms lose excitation energy as there is no c and no spontaneous emission in that theory. Larmor's older theory (1897) explained how the energy is lost - by EM radiation - and gave formula connecting acceleration and losses that is used to this day.
Joseph Larmor, LXIII, On the theory of the Magnetic Influence on Spectra ;
and on the Radiation from moving Ions, Philosophical Magazine Series 5
Vol. 44, Iss. 271, 1897
Erwin Schrodinger, Quantisierung als Eigenwertproblem. Annalen der Phys. 384 (4) (1926)
> Schroedinger's theory of hydrogen atom and his wave mechanics (1926).
This was not a "new theory" that was competing with any "old theories". It was a tentative model in a regime where no previous theory existed, and it was never claimed to cover anything outside that limited regime. It wasn't competing with any other theories, because there were no other theories to compete with. The question of whether or not Schrodinger's model reproduced the predictions of the "old" theory never arose, because there was no "old" theory. (Technically, there was a sort of "old" theory of the hydrogen atom--Bohr's model--but Schrodinger's model did reproduce all of its correct predictions, plus it added more correct predictions of things that the Bohr model got wrong.)
The position with regard to gravitational waves is very different; we already have a comprehensive, fundamental theory--General Relativity--that explains them. Any alternative theory that only explained GWs, and didn't also explain all the other experimental results that GR explains, would be a nonstarter.
> Larmor's older theory (1897)
This wasn't a separate "theory" at all; it was just a derivation of a particular formula using an already known theory, Maxwell's Equations.
Schroedinger theory certainly was a new theory of the atom and later of molecules at that time, successfully competing and largely replacing classical EM models of atoms and molecules such as Larmor's theory of molecules, although it didn't cover the EM radiation aspect and EM theory needs to be used in parallel with Schroedinger's to get, say, intensities of emission lines. I think this is a good example of what I was saying in the first post. It is the new benefit that the theory brings, not reproduction of every single result of the previous theories, that makes the new theory interesting and helps its adoption. Cases where the new theory completely replaces the old theory and reproduces all of its positive results happen too, but are not the only way how new knowledge is adopted.
> I think this is a good example of what I was saying in the first post. It is the new benefit that the theory brings, not reproduction of every single result of the previous theories
Of course Schrodinger's model didn't reproduce the results of classical EM with regard to the atom. It wasn't supposed to, because those results of classical EM were wrong. In other words, there wasn't a correct "old theory" that covered the regime the Schrodinger model covered (the atom)--there was only a wrong "old theory".
As far as using Schrodinger's model plus classical EM theory to get results like emission line intensities, there also there was no correct "old theory"; there was only a wrong "old theory" (classical EM by itself, which did not predict emission lines at all, let alone their intensities--it predicted a continuous emission spectrum). Also, this hybrid classical-quantum model was known to be incomplete at the time; it was only used because nobody had yet figured out how to quantize the EM field.
> It is the new benefit that the theory brings, not reproduction of every single result of the previous theories
Once again, this is not the situation under discussion in this thread (gravitational waves). In the case you describe, the results of the previous theories were wrong in the regime the new model covered, so there was nothing to reproduce; there was no correct "old theory" for the new theory to compete with.
In the case of gravitational waves, we have a correct "old theory"--General Relativity--so any new theory that did not match that correct old theory would be a nonstarter. I am not aware of any case where a new theory was accepted as interesting when there was a correct old theory covering the same regime and the new theory did not reproduce its results.
> It wasn't supposed to, because those results of classical EM were wrong.
You're badly mistaken. Although nobody succeeded in obtaining the emission line frequencies of gases out of the classical EM theory, the theory did correctly give other results consistent with observations. One of them is the formula for emission intensity that connects energy radiated with second derivative of electric moment; it goes back to Larmor's work. This was the result the new theory would preferably reproduce or at least be consistent with. Wave mechanics wasn't consistent with it - the hydrogen atom oscillates indefinitely in wave mechanics. Schroedinger himself viewed this as a deficiency and planned to get back to it - check the ending part of his seminal papers on wave mechanics. The classical formula is taught to this day both in macroscopic EM theory and quantum optics courses, although there are some deficiencies and problems about the formula that Larmor did not know.
> In the case of gravitational waves, we have a correct "old theory"--General Relativity--so any new theory that did not match that correct old theory would be a nonstarter.
I do not think any physics theory could even be "correct" in the sense of Platonic ideals, but I do not know what you mean by "correct". I do not claim a new theory could completely replace the old one before it could deliver the same or better results. I claim theory has value and is accepted based on its new benefits, not its superiority in every aspect the old theory was superior before. Calling incomplete theory non-starter makes no sense to me, as all theories, including General Relativity, are incomplete.
No, I'm not; you're just mistaken about which classical results I was referring to. I meant the results of classical EM that predicted that atoms could not exist--because the electrons would radiate until they fell into the nucleus. And what classical formula tells you how much the electrons will radiate because of their acceleration due to responding to the electric field of the nucleus? Larmor's formula.
In other words, Larmor's formula was not a "theory"--it was a particular result derived within a theory. The particular result happened to be correct, within a particular limited domain; but the underlying theory that was used to derive it could not explain why it was correct--because the same theory, and indeed the same particular result--the same formula--made other predictions that were obviously egregiously wrong (like predicting that atoms would collapse).
> nobody succeeded in obtaining the emission line frequencies of gases out of the classical EM theory
You're drastically understating the failure of classical EM here. It's not that classical EM couldn't predict the particular frequencies of emission lines. It's that classical EM couldn't predict the existence of emission lines at all. Classical EM predicted that atoms would emit a continuous spectrum of radiation--not radiation sharply peaked at particular frequencies.
> The classical formula is taught to this day both in macroscopic EM theory and quantum optics courses
Sure, because within its domain of validity, it works fine as an approximation. But that's all it is--an approximation. And we explain why the approximation works, and why it works only within a particular domain of validity, by reference to the more complete underlying theory--quantum electrodynamics.
> I do not know what you mean by "correct".
I mean "makes predictions that match the results of experiments".
> all theories, including General Relativity, are incomplete.
I agree; but there's a big difference between:
- A theory that is incomplete because it doesn't cover absolutely everything, including where we haven't tested yet and won't be able to for the foreseeable future, but makes correct predictions everywhere we can actually test it; and
- A theory that is incomplete because it makes predictions about some things that are obviously at variance with observation, even though it makes correct predictions about others.
GR is an incomplete theory in the former sense; and theories that are incomplete in that sense can still be used to safely rule out competing theories that don't match their predictions in regimes where those predictions have been extensively confirmed.
However, classical electromagnetism is an incomplete theory in the latter sense; it made obviously wrong predictions, like the ultraviolet catastrophe and the instability of atoms. And even the correct predictions it made, like using the Larmor formula to predict radiative properties of atoms, were only obtained by using the theory inconsistently: by first assuming, contrary to the classical EM prediction, that atoms could be stable at all, and then working out what classical EM said about how these impossible objects (impossible according to classical EM) could radiate.
In a situation like that, you can't safely use the theory to rule out other theories, because the theory contradicts itself, and you can prove anything from contradictory assumptions. That's why classical EM physicists couldn't say "well, the Schrodinger theory can't be right, because I can't use it to derive the Larmor formula". You can't consistently use classical EM to derive the Larmor formula either; you have to sweep certain things under the rug and wave your hands that somehow or other it's ok.
In a situation like the latter, yes, you're right that anything that can give some handle on making predictions is going to be at least tried. But that's a very different situation from the former situation, where we have a correct theory that, within its domain of validity, doesn't have any of those issues. The only issue with GR is that it's not a quantum theory, which means, in the eyes of many physicists, that it's incomplete; but that incompleteness has no practical consequences whatsoever. It certainly is not a reason to entertain alternative theories of gravitational waves that get other predictions wrong that GR gets right.
I sometimes wonder why tech people like space-related stuff so much. It is a major news indeed and a feat of science and technology, but why is space so popular? Because it's otherworldly, large-scale and kind of making you feel empowered or united? I'm probably more interested in mundane, obscure and humble stuff, so this disproportionate popularity of space-related news is always baffling to me.
In grappling with this question myself -- it's a good question -- I've concluded the interest is in understanding and predicting the behavior of a system based on a few laws.
The system is quite complex and full of exotic objects, so ordinary real world intuition is a poor guide. And the laws are couched in a mathematical language that is also foreign to our everyday world.
Yet, predictions can be made and tested. It's an intellectual puzzle like "what does this very tight loop do?", or "how does the Y combinator work?" -- but in a different arena.
I believe it's all about the thrill of discovery. We know Earth comparatively well and there's simply not many areas left where a major discovery can be made. Sure, there's still much to learn, but these advances are made in small, incremental steps rather than major leaps. Space, on the other hand, is still largely a mystery, just like the oceans were for our ancestors.
When I was younger, I loved physics because it bridged the gap between pure maths ("when are we ever going to need this stuff?") and the physical world. I didn't pursue it beyond high school
Now, as a full-time software engineer and part time jack of all trades, I appreciate stuff like this experiment and the work of Space X and others much as I appreciate good engineering. It's a difficult problem to solve. So many disciplines had to cooperate to grant us some small insight into the inner workings of our universe. It's marvelous, and makes me feel like a kid again.
We (as in, species) just observed a phenomenon that's related on a fundamental way to any form of matter, doesn't matter(no pun intended) if it is space or not.
The mechanical and software engineering underlying these research endeavors is breathtaking. The laser apparatus, LISA pathfinder, ELISA - how on earth do they calibrate/debug/test such complex systems?
... and I shudder to think that more often than not, anything I code in C/C++ will segfault on first run.
My intuition is that this is unlikely, but I'd love to see someone do the math. Given the scale of interstellar distances, any locations on our planet (and even in our solar system) are going to effectively function as a single point. Given arbitrarily-accurate measurement, it could work, but I'd bet physical limitations will prevent that from being a possibility.
To my mind, it'd be roughly like trying to triangulate an earthquake in France with three sensors in a 1mm^3 cube in NYC (scale is probably way off, I definitely didn't do the math).
It definitely would work. The distance to the event is irrelevant, it's the light travel time between the detectors compared to the accuracy with which you can pin the event down in time that matters. The light travel time across the Earth is of order a hundredth of a second, which is a significant fraction of an event that takes ~ a tenth of a second.
However, the error ellipse will probably be quite larg, and given that they come from cosmological distances it's unlikely that they would be anything but isotropically distributed (like gamma ray bursts are).
This gives you the direction, but not distance which is an important component of triangulation. I should have been more clear in my original post. Is it plausible that we'll ever be able to determine distance through this method?
They'd have to build 4, actually. Imagine 3 intersecting spheres, there would be 2 possible positions remaining (just as with 2d triangulation, having 2 circles would leave 2 points remaining and we'd need a 3rd to find out which was correct
Even with just two LIGO locations, they were able to give a very rough estimate of the source location during the livestream this morning. (It was somewhere kinda in the direction of the Magellanic clouds. Probably.) Once advanced VIRGO comes online (later this year?) we'll have three, which will significantly improve directional accuracy for much of the sky. And when KAGRA turns on in Japan sometime around 2018, we'll have four. During the livestream this morning, someone said that would lead to source locations accurate to within 5-10 degrees on the sky.
One would think so. I'm not an expert in this area, but from the descriptions it seems like they would have at least two options for finding the source direction:
One is to compare the arrival time at each of the detectors and infer direction from the speed-of-light delay.
Another option is to measure the difference in relative strength between each of the detectors. I'm assuming the detectors aren't uniformly sensitive; perhaps they're most sensitive to waves travelling in a direction parallel to one arm of the detector and perpendicular to the other, and completely insensitive to waves that are perpendicular to both (i.e. it would affect them both the same and cancel out, or perhaps not affect either of them at all). With multiple detectors at right angles to each other, you can get a pretty good idea which way it came from.
Combining the two methods could give you greater accuracy, and also help to rule out spurious signals that are not gravity waves.
This makes me wonder what you would see if you had a sufficiently accurate directional gravity wave detector and let it run for a long time. Sooner or later, you'd get an actual image, like a telescope.
I wonder of this means the space version of these antennas, eLISA, will get more funding. Using space seems like a much better way to access long distance laser conduits in a vacuum needed to detect gravitational waves.
Since the LISA has longer baselines, it measures gravitational waves of different frequencies, from different phenomena--supermassive black hole coalescence, not these (mere!) stellar mass black holes. So the experiments are complimentary
In all honesty I do think Einstein is getting too much credit today. I'd paste the list of co-authors here to congratulate them but HN doesn't allow comments that large. The list is available here for reference, and I think every one of them deserves credit for this.
On another note, I feel like the importance of this finding is less in proving Einstein's theory; having taken a formal relativity class and an degree in Physics, I think GR itself is an astounding mathematical framework for describing spacetime, to which Einstein deserves credit, but the existence of gravitational waves is completely natural consequence of the equations within. It's not very different from the existence of light being a natural consequence of Maxwell's equations.
I'd say the true importance of this discovery is in successfully creating an experimental apparatus to detect what was previously almost universally agreed to probably exist but thought to be nearly impossible to detect. What's truly exciting isn't proving Einstein right, but the possibilities of what we'll be able to to detect with this apparatus in the future. So it's the team that built the apparatus which truly deserves the credit today.
Pedanticly, it'd have to be while he was alive since the Nobel Committee won't nominate anyone who's deceased. Rosalind Franklin would've surely received one for her work on DNA (among others who passed before their work was recognized).
It was mentioned that during this event, three sun's mass equivalents were turned into gravity waves, I guess that means that matter particles were turned into gravitons.
But what happens to them? Is there any way to turn them back into matter? If not, then at some point, will all matter in the universe end up as gravitons?
Also, if an object moving through space creates gravitational waves, doesn't that violate the law that states that a non-accelerating object will not lose/gain any energy? Because if you have to emit gravitons as you move in space, and emitting them requires energy or matter expenditure, then an object moving through space will slowly lose all it's mass?
Does anyone know if G-Waves are effected by velocity, like EM-Waves are?
In other words, if two bodies are moving relative to one another, one emits G-Waves, and one detects them. Are the waves at the detector doppler shifted in frequency by the relative velocities?
I don't see how they could not be. I think that shift arises from transformations between different coordinate frames, it doesn't matter what you're observing.
As they're supposed to be traveling at the speed of light I'd jump to the conclusion that you'd find them doppler shifted. In this case though we don't yet have anything like the spectral lines of hydrogen to be able to measure that shift so I don't think we can do anything with it yet.
Lost in the transformation was three solar masses’ worth of energy, vaporized into gravitational waves in an unseen and barely felt apocalypse. As visible light, that energy would be equivalent to the brightness of a billion trillion suns.
Note this is a stellar black hole merger of several tens of solar masses.
Imagine the disturbance of a galactic core black hole mergers of millions of stellar masses. These are probably much rarer, but do occur when galaxies merge.
If this happened in the centre of the Milky Way, we're about 25k light years away.
Let's say 2 1 million solar masses black holes merged there... and they also gave off about 3/60 of their mass as radiation, that's about 100'000 solar masses being radiated 25k light years away.
Using my calculation in the other post, we're talking 10^52 Joules. Across a distance of 25'000 light years, or about 10^20 metres, that's then decreased by 10^40 (inverse square) so we're left with about 10^12 Joules...
Which is good news! If that happened in the Milky Way, we would probably survive it - though we'd definitely notice some strange atmospheric effects...
Maybe it's the psychology of how we (fail to) deal with different scales. Discovering new, larger, more wonderful places in the Universe doesn't make the Earth any smaller or less wonderful than it is. Our brains might "zoom out" our mental map to fit these new places in, which makes us appear smaller, but in fact it's our horizons that have grown.
Most of those very subtle gravitational waves are also rather insignificant. Normally their effects would be swamped by other forces and interactions. At least as far as we know, we are the first agents in the universe to build structures that isolate and amplify those effects in a way that allows them to be perceived and appreciated. That seems pretty significant. We may also be the only means the universe has to feel proud of this little accomplishment.
On the other hand, it's nice to know the world really does appear to be boundless. I mean in terms of the possible.
We aren't much, but we are here and it's a pretty awesome experience.
Maybe we need to be here, otherwise what is the point?
I like to think there are others too, thinking thoughts like we are. Maybe that is necessary too. Maybe nobody has reached a point in their development for more, or contact to make sense.
I find our time here and now bittersweet. So much is yet to be experienced and understood. But, then again, here and now isn't all bad. We have great science, new frontiers opening up all the time. Our stories of the future are fantastic, and there is still a lot of magic and wonder about us, the world, reality...
We may not see the best. In fact, I say none of us will, but right now is never dull.
I feel like we are just beginning to get a real grasp on reality. That seems powerful and exciting. We could have lived in much darker, harder, ignorant times.
These times may be seen that way too, or they may be a peak, with a decline to come. Nobody knows, and I like it that way.
Does this discovery make us insignificant or does it increase our understanding of the universe?
It depends on how you look at it. You are taking a pessimist's view on things that the universe is so large and we are so small that we don't matter. If you take an optimists's view on things you'll discover that we do matter and learning new things increases our significance.
I have a question: what does this mean for theoretical physics? (except for Einstein was right) Does it settle any major debates? Does it make any competing theory more or less likely?
It's by far the most explicit verification we've ever had that black holes exist in pretty nearly the exact form predicted by Einstein's equations of general relativity, which is pretty cool. It provides the tightest limits on any possible mass for the graviton (the presumed particle carrying the gravitational force, which is generally believed to be massless but you always have to wonder about more exotic possibilities). It gives a stunningly clear confirmation that modern numerical simulations of relativistic dynamics are an accurate reflection of nature. (And by the same token, it presumably puts limits on the strength of any potential deviation in the laws of physics from the equations used in designing those simulations.) And it probably does something to give preference to models of astrophysics in which binary systems with these characteristics are common.
Beyond that, I guess I'd say that this particular signal doesn't feel like that much of a surprise: we were already pretty confident that if a black hole binary were to merge, a signal more or less like this would be an expected result. The scientists were evidently surprised that their very first signal was so strong (this one was even borderline detectable by the previous version of LIGO), which may teach us something, but it's not revolutionary.
On the webcast, they described how this let us listen in on massive disruptions of space-time, environments we could never create to test on earth, and that could help us better compare our models to events seen in extreme cases.
This is the result the theories predicted, and there's nothing for the theorists to do except gloat. If people had kept building more sensitive detectors, and kept failing to detect gravity waves, then eventually that would have been a very big deal for theoretical physics. But it didn't happen.
On the other hand, there is now a way to see dark matter. That could enable a lot of new astronomy.
One thing I don't quite understand - how can the "chirp" from LIGO be unambiguously categorized as extraterrestrial in origin? The waveform shown onscreen in the NYtimes video looks like an extremely noisy signal - not sure if that's the actual sampled data or just an artistic rendition. Couldn't there be a variety of physical disturbances that explain a sine-tone sweep like that, given how sensitive the instrument is to physical vibrations?
There are two independent sites, 2000 miles apart. Additionally, each site has two perpendicular experiments: one shows contraction when the perpendicular one shows expansion. Orientations at both sites are aligned I presume.
In a very real way it was. There were two completely geographically separate instruments which recorded the same signal with a delay that is consistent with the light travel time between them.
In the future this will get better when VIRGO in Italy and KAGRA [2] in Japan come online. Then we will have 4 independent detectors which will be able to verify that same signal is observed at the same time.
Obviously of course given the transient nature of what is being observed once the merger has occurred it will very rapidly stop producing gravitational waves so we will not be able to measure the same event again.
On November 25, 1915 (at the time of WWI) Einstein presented the actual Einstein field equations to the Prussian Academy of Sciences.
Almost exactly 100 years later on September 14, 2015 LIGO observed the first gravitational-wave signal.
Is that a coincidence?
ok, so some of the best minds on Earth can build a machine to detect gravitational waves from an event 1.3B light years away. This is an incredible motivation for those of us on what is possible with technology in simple terrestrial projects.
Honest question: there is any example of Einstein being proved wrong?
Was he indeed always right on his theories for phenomenons before they could be proved by experiments; or is that the case that we only hear about when he is proved right?
He (kind of) had to add the cosmological constant to GR just so the universe wouldn't collapse onto itself and instead be static. The idea of the unchanging universe was maybe partly due to personal or religious preconceptions. So at least in his mind the universe was static, which of course was proven "wrong" by the discovery that the universe is expanding. This was not a proper scientific theory of his, rather a preconception, like his aversion to the randomness of Quantum Physics (again, religion it appears).
Of course, being Einstein, he was again on the right side of the argument when the universe was much later discovered to be accelerating, again requiring a cosmological constant (or some similar fudge factor).
I don't know if you can say he was proven wrong, but the Einstein-Podolsky-Rosen paradox was put forward to demonstrate that quantum mechanics must be incomplete, because to accept it implied "spooky action at a distance".
Today, most physicists accept the spooky action at a distance rather than the idea that QM is incomplete.
He was a proponent of hidden variable theory, which tried to reconcile quantum mechanics with determinism, famously saying "God does not play dice". People often say that hidden variable theories were proven impossible, and thus Einstein was proven wrong. That's not quite true, and only local hidden variables have been ruled out.
It also disparages his contribution to the scientific discussion to just state that he was "proven wrong".
Bohr's argument in the discussion was a bit of a mess and I couldn't pull anything out of his rebuttal to EPR other than an assertion that QM behaves the way it does and not to pay any attention to the man behind the curtain. Its a very philosophical argument with very little scientific content and he just proposes that the QM math is correct because its correct, as far as I can tell.
EPR made a logical cogent argument. It was based on the philosophical principle of the locality of physics. They translated that into the mathematics of Quantum Mechanics and proposed a simple experimental test. Later that was refined by Bell and tested experimentally by Aspect and others. It was the Einstein-Podolsky-Rosen paper that laid the groundwork of how to test the non-locality/hidden-variables of QM though.
EPR moved the scientific discussion forwards much more than Bohr did, but it turns out the test they proposed showed that the position they favored was incorrect.
Also Einstein was arguing first and foremost that physics must be _local_. That's in opposition to the "spooky action at a distance" bit that he didn't like. Since local hidden variables are ruled out then he really was proven "wrong".
TL;DR I think Bohr's argument is rubbish, and Einstein's is solid, but the Universe is a bitch and doesn't care...
No they haven't proven Einstein right, they've failed to prove him wrong; that distinction is the essence of science. Theories are never proven right, they can only be proven wrong.
Any observer will observe the same speed of light in their location.
Any effect of gravitational fluctuations in spacetime on the speed of light is a bit like a car driving on a race track that has treadmills scattered around it pointing in various directions and speeds. The car's speedometer will always read the same value because it's measuring the speed of its tires on whatever it's driving on.
I am not sure why but I am really hung up on the quote “Finally, astronomy grew ears. We never had ears before.” They are detecting gravitational waves not sound waves.
It's perhaps not the perfect metaphor. But up till now all we have really directly observed from outside[1] our solar system are electromagnetic waves(of various wavelengths including visible light), and this is something quite different.
Any vibration is just a signal. We don't actually have a direct experience with a sound, hearing is our brains interpreting the nerve signals generated by tiny organelles jiggling in our ears. What is concrete then, is only the shape of the signal, but not the medium through which it propagates.
This device just acts like a gigantic hearing device. Except it's not pressure waves, but the fabric of the universe which reverbates.
Note that the frequency of the signal is indeed in the audible range.
Anyway, I was a bit irritated of this same phrase, but because I tought radio astronomers had been listening to skies for quite some time now.
I think it bothers me because it is focusing on the sound that some small piece of this operation creates when the amazing thing is to be able to measure gravitational waves. Who cares if you hook the output of some piece of LIGO up to a speaker or not and make a noise? Unplug that speaker and these results are just as amazing.
Interference patterns from two lasers do not make any sound. You can take data describing the interference pattern and convert it to a signal that is run through a speaker but that doesn't mean the interference pattern is audible.
The amazing thing is they detected gravitational waves, not that they hooked up a speaker to the data.
I think a proper analogy would be if back when someone created the first function generator they connected it to a speaker. Then in an interview the main message they delivered was that you could now "hear electricity."
It just seems like they are focusing on an afterthought.
I also have no idea why I am so fixated on this. :-p
It is a bit awkward, but, I kinda like it: it's not common in physics for signals to come at frequencies that are actually audible, and these ones definitely are.
" On 14 September 2015, while Drago was on the phone with a LIGO colleague in Italy, his pipeline sent him an email alert—of which he receives about one each day—telling him that both LIGO detectors had registered an “event” (a nonroutine reading) 3 minutes earlier, at 11:50:45 a.m. local time. It was a big one. “The signal-to-noise ratio was quite high—24 as opposed to [the more typical] 10,” he says."
A hole that is less than the sum of its parts. Three suns’ worth of mass has been turned into energy, in the form of gravitational waves;
The coalescing holes pumped 50 times more energy into space this way than the whole of the rest of the universe emitted in light, radio waves, X-rays and gamma rays combined.
Not sure why this received downvotes. There was significant debate until the 1950s about whether gravitational waves were real phenomena or just coordinate artifacts. The sticky bead argument played a big role in settling the issue.
Tangential question. With blackholes merging, can more and more merge some time in future, creating a net gravitational pull to slow down the expansion of the universe and then may be eventually cause the universe to collapse into that continually merging mega black hole.
The gravity pull of the new black hole is essentially equal to the sum of the gravity pull of the two isolated black holes, and it's essentially equal to the gravity pull of the two stars before they transformed into black holes. So merging black holes don't increase the pull.
Actually, in each transformation there is an explosion and part of the mass goes away (try to not be very close). So the total mass in the final black hole is smaller than the mass in the initial stars, the rest are debris forming a nebula or something around the black hole.
(And, as the other commenter said, gravity is too weak.)
I'm interested to know how they can be so sure that the change in distance between the two arms of LIGO is attributed to gravitational waves? I would of thought that miniscule movements in the Earth's crust would be a more likely culprit.
Ahh, just realised that there are two instruments located on either side of the states, so if both instruments register the same event then it is unlikely to be tectonic movement...
The LIGO mirrors and beam splitter are attached to Earth very loosely. They hang from towers of pendulums; when Earth moves, LIGO catches up eventually, but over the time it takes a gravity wave to oscillate the components might as well be in orbit.
That is one of the more routine feats of engineering involved.
I understood how LIGO works but one question I have is, how did scientists conclude the gravitational waves they detected are from a collision of 2 black holes that happened before 1.3 Billion/Million years ago.
Are gravitational waves supposed to be that weak or is it because of the distance between us and those black holes? Do they lose power as they travel through space?
For comparison, the wave that was detected is claimed to be "four one-thousandths of the diameter of a proton". That's about 7e-18 meters, on a baseline arm length of 4 km, so about one part in 6e20 -- about 175,000 times stronger than the waves Earth's orbit produces. And that was about 40x as strong as minimal sensitivity on LIGO, according to the article ("can detect changes in the length of one of those arms as small as one ten-thousandth the diameter of a proton").
Obviously if we were closer to the black hole collision we'd see much stronger waves. But you really do need very massive bodies accelerating very much (or equivalently orbiting very fast) to produce something that's detectable by LIGO over interstellar distances at all. The key part from this article is that the orbital period was about 1/250 of a second at the end; compare to Earth's orbital period. Going back to the formula given in the above Wikipedia entry, the frequency dependence is hiding in the "1/r" factor for the amplitude. 1/r is proportional to w^{2/3} (though it's not clear to me whether that's still true in a general-relativistic treatment; it's true enough for the Earth's orbit), which tells you how the wave amplitude scales with frequency...
Only that in space, it drops even faster: The wavefront carries the same energy, but spread out over ever increasing length/area. For gravitational waves (or radio waves) they form spheres, not circles, and the surface size scales with r^2, not r. (Also water waves are /complicated/)
As others have said, intensity (power per unit area) decreases according to the same inverse square law that governs most effects due to localized sources in three dimensions of space. In this case, you're looking at a distance of over a billion light years, and then squaring it: that's a pretty enormous "per unit area"!
But gravity itself is also a tremendously weak force compared to the others. That may seem surprising at first, but it becomes pretty clear when I point out that a cheap little refrigerator magnet exerts enough force to overcome the gravitational pull of an entire planet right beneath it. Gravitational waves are pretty much just ripples on the top of that already tiny force.
Huh. That sounds entirely sensible and correct, and at the same time it's bugging my physical intuition a bit. I guess they're not trying to do this measurement by absorbing energy, but much more directly by just watching the change in length. I think I believe you: thanks!
Near as I can tell, they're intrinsically extremely weak, so much so that the only thing we've been able to see it all is extreme events like these black hole mergers. In theory, pretty much any time anything moves, some level of gravitational waves should be generated, though no telling if we'd ever be able to detect them.
So does this verification of gravitational waves help with Physics theory-building? Have people really doubted Einstein : the existence of gravitational waves?
The amount of energy needed to transmit via a gravitational wave is INSANE. It would involve very rapidly accelerating and decelerating a black hole / neutron star. While it might be possible to do this, it's not within the realm of something we could accomplish without several orders of magnitude technology improvements, and possibly may not be physically possible at all (moving object that heavy that quickly might require creating a black hole)--though I don't have the skill to prove or disprove that.
Even if it were possible, what sort of crazy alien would it take to burn 3 solar masses of negentropy to basically run a ping, compared to the amount of data that could be transmitted with electromagnetic waves with 3 solar masses of negentropy? It's literally dozens of orders of magnitude in difference. Any aliens that are that bad at engineering probably aren't going to grow to the point that they can shake neutron stars several times per second.
Our power generation stations are the black hole equivalent to the caveman with only access to generating fire through rubbing stones.
Based on what level of civilization you are. Rubbing two black holes in for a ping, might be the same as rubbing two stones for a spark. Advanced civilization go really advanced, to a point their activities would be undetectable to us or would appear to us the nature of reality itself.
"Advanced civilization go really advanced, to a point their activities would be undetectable to us or would appear to us the nature of reality itself."
This is science fiction, not an argument. We have no rational reason at the moment to believe this is the case, or even possible.
What we do in fact have is an increasing trend towards efficiency. Projecting that out along crazy growth curves suggests that advanced aliens are likely to be more horrified by such a waste of negentropy than we are. What can we do with that much negentropy? Nothing, basically. What can they do? Simulate many millions/billions/whoknows of human-level civilizations?
They're not more likely to be indifferent about such waste, they're more likely to prosecute you, for mass civilizational murder.
I've often thought that if civilization could advance to that point in the future, that I'd have a difficult time explaining to my great-great-great-X grandchildren that when ol' great-great-great-X-grandpa was young, you know, pouring a tank of gasoline into the car got me from point A to point B and that was it, despite it being enough energy in that one tank of gas to, say, simulate an entire human's life time. Well, kids, we didn't have that option! The tech didn't exist. So stop trying to put ol' Greats on trial for things he couldn't control, OK?
>>This is science fiction, not an argument. We have no rational reason at the moment to believe this is the case, or even possible.
There are not only rational reasons, but even evidences to support what I'm trying to say.
Look at any insect colony or bacteria, they don't even recognize our presence, let alone our technology.
>>What we do in fact have is an increasing trend towards efficiency. Projecting that out along crazy growth curves suggests that advanced aliens are likely to be more horrified by such a waste of negentropy than we are.
We the advanced aliens to ants, are indulging waste and plastic pollution like never before. And ants the aliens to bacteria might appear the same.
Efficiency and waste are very relative terms based on what level of abundance or austerity on is supposed to live on.
approach to fighting spam. Your idea will not work. Here is why it won't work. (One or more of the following may apply to your particular idea, and it may have other flaws which used to vary from state to state before a bad federal law was passed.)
(X) The amount of energy involved would likely destroy the planet.
(X) Many email users cannot afford to lose business or alienate potential employers
Specifically, your plan fails to account for
(X) The relative sparseness of non-dark energy in our vicinity
(X) Huge existing software investment in SMTP
and the following philosophical objections may also apply:
(X) Incompatiblity with open source or open source licenses
> Would it be possible to listen for information transmitted via gravitational waves? Would there be any benefit over radio?
Well, if you observe a meaningful, non-natural gravity wave signal, you know that you've discovered not merely another technical situation (which you'd know if you detected the same thing in radio waves), but a phenomenally advanced one.
So, if not an advantage, there is at least a meaningful difference.
AFAIK that doesn't seem at all feasible yet. Currently you need an enormous facility, which struggles to detect anything but the most powerful gravitational waves.
You can't transmit information through entangled pairs. What is instantaneous is the change of the state for the whole system (the pair) after you measure one of particles. However the result of that measurement (if it's non-trivial, i.e. if the measurement actually changes the state) is fundamentally random so the only thing you would be seeing is perfectly and instantaneously correlated noise on both ends.
I'm sorry but no, you cannot transfer information with quantum entanglement. What entanglement says is that if you have a photon and I have a photon and they are entangled and you make a measurement on some attribute of your photon, my photon will assume the complimentary state. However, the state your photon assumes when you measure it is random and once you measure it, you lose the entanglement. So, there's no way for you to encode any information in your entangled photon. Yes, I can infer what state your photon was in as soon as you measure it, this is useful for encryption as we can then compare notes after making a measurement and make sure nobody tampered with our entangled photons.
As with most physics experiments for the last 40 years, nothing new was discovered that we didn’t already predict. Confirming something widely believed to be true isn't nearly as valuable as finding out we don't understand something. This is actually one of the reasons I dropped out of my physics phd program.
Because this is astrophysics and not particle physics, this discovery is just the beginning! We don't know the rates of these mergers, the distribution of the masses of the binary components, what electromagnetic signature accompanies the events (if any)...
We've known gravity waves existed since the Hulse-Taylor pulsar, so just observing them for the first time is not nearly as interesting as the science to come in the next decade. Advanced LIGO is a powerful new tool that will open up exciting new observations.
It's super early, and detection seems at the limits of our technology.
This is just understanding at present. That, in itself is worth it.
As that understanding develops, and our tech advances, engineering may be able to apply it in useful ways, maybe object detection above a specified mass? New ways to visualize things?
One "application" is to serve as a ruler to measure out tech with. The limits are there, putting these observations just within reach.
Now that we have some confirmation, we also have the metrics as well as the compelling new science that may arise from all of this as a strong motivation to advance.
It's like being able to detect color for the first time. At first we understand what color is, then we refine, and after iterations, engineering, experiments, we get to a place where we see it all in color.
Applications will follow.
These waves being confirmed are like a new sense. Crude, but real. We can now follow this new perception to its conclusion, just as we have many other things.
We don't always know what that conclusion will be, or the form an application may take, but we do know we won't actualize any of it if we don't do the basic, hard, expensive work needed first.
1.1. billion and 40 years vs. pencil and paper and a few years. That should tell us something about education, mode of thinking and research. And a few other things.
Maybe we will get lucky and literally see two black holes merge somewhere nearby. But I think these detectors will be the last of our concerns with all the praying and bunker building going on. Bruce Willis won't save us from that one.
I swear a big bang theory rerun about this was on last night. Sheldon detected waves at the north pole, but they were actually a blender turned on by the rest of the gang. He's embarrassed and goes home. Leonard beds Penny. Decent episode.
I am a bit skeptical of the conclusion given the methods. Here, there's no observable phenomena independent of the test apparatus that corresponds to the proposed cause. The conclusion is circular.
1. Theory predicts gravitational waves when massive objects collide and that the gravitational waves would have an effect that could be measured by the experimental instruments.
2. The experimental instruments measure something.
3. This is considered proof that massive objects collided.
4. Therefore gravitational waves exist.
To reframe my skepticism, the experiment measures something. The conclusion as to what it measures, however, is unsupported by statistical inference or direct experience of a causal phenomenon. That's not to say that what the phenomena measured -- the earth resonating -- is uninteresting or unimportant or even inconsistent with the theory of gravitational waves.
Yet, I don't find the possibility of a geophysical cause -- i.e. that the earth maintains consistent dimensions at a sub-atomic scale -- the many orders of magnitude less likely than gravitational waves necessary to reach a conclusion. In particular, I find natural fluctuation to be more likely because the experiment acknowledges its existence.
For a point of comparison, consider the Perihelion precession of Mercury that provides evidence in support of general relativity. The theory was used to predict the results of an observable event. The experimenters trained their telescopes at a particular location and particular time and observed phenomena consistent with a prediction based on the theory. The same is true of the Higgs. In both cases the experiment is of the form "when X, I will observe Y."
The reasoning here is:
If X, then Y.
Y, therefore X.
It treats an ordinary implication as mutual implication.
(1) It simply fails to understand the scientific method, which is empiricism, not mathematical/logical proof. Scientific evidence is essentially failed disproof, not logical proof.
(2) It mischaracterizes the nature of the prediction, which includes not merely that something will be measured, but that a particular pattern will be measured.
(3) It proposes unspecified "geophysical causes" as an alternate explanation, but there was no pre-existing geophysical model which predicted the pattern observed. (Any after-the-fact geophysical -- or other -- alternative explanation which explains the observed pattern would also need to perform differently on some other test to be verifiably different, and then we could do the test to distinguish the source.)
(4) It misstates the reasoning to contrast it with other experiments, this is exactly "when X, I will observe Y" (where X is "I construct detectors of a particular type in more than one location" and Y is "I will periodically detect particular patterns of signals on those detectors -- not just one of them alone -- which the model predicts will be produced by collisions of massive, distant objects.")
Under the theory of science as the study of what is falsifiable, there's nothing here to falsify because there is no way to disprove that a conjectured but unobserved collision of two massive bodies was something else or didn't occur. Which is to say that it is impossible to falsify that a conjecture is a conjecture.
That there is not a geophysical theory, doesn't have a bearing on the correctness of the gravitational wave theory one way or the other...anymore than the absence of a helio-centric model for the solar system made the geocentric model more correct or the absence of a theory of oxygen made the theory of pholgiston more correct. More importantly, both these incorrect theories had reasonable explanatory power to the point that they were useful.
The reason they were useful theories is because they were predictive, pholgiston allowed a person to calculate the weight of ashes after burning and the geocentric solar model made the prediction of the location of stars possible with reasonable precision. On the other hand, theories that offer conclusions about unfalsifiable propositions are what Carnap and the Vienna circle termed "metaphysics".
The conclusion that the experiment justifies is that the Earth resonates. There is no external event to which the measurements can be correlated to establish causality. There's no confidence interval. It's a case where the observations confirm a pre-existing world view under the same human cognitive structures by which seashells on mountain tops confirm a world-wide flood. It assumes that because we live on the Earth we know everything about it.
Anyway, it's a case of over-reaching with the conclusions. It's an argument from design.
> Under the theory of science as the study of what is falsifiable, there's nothing here to falsify because there is no way to disprove that a conjectured but unobserved collision of two massive bodies was something else or didn't occur.
Yes, there is: if the predicted kind of observations did not occur, it would imply one of two things:
(1) the model of gravity waves and their generation and propagation on which the prediction was based was incorrect, or
(2) the expectation of large-object collisions on which the prediction was also based is incorrect.
Now, were that the case, distinguishing which of those assumptions was false would require coming up with a new set of experiments that would have different results if the first was correct and the second false than if those were flipped, and yet a different set of results if both were false.
> That there is not a geophysical theory, doesn't have a bearing on the correctness of the gravitational wave theory one way or the other
Science isn't about correctness, its about continuous refinement of models which better predict observations. The absence of a better alternative model doesn't "prove" that a given model is "correct", but science deals with neither proof (except in the negative sense) nor correctness. (Further, the model of gravity waves being tested here is an implication of broader models whose other implications have also withstood attempts to falsify them.)
Lower two graphs are predicted waveform in event of binary system coalescence. Upper waveform left and right are observed events at each LIGO facility.
Consider furthermore that the event was observed at each facility with the appropriate lightspeed time offset, and that the wave directionality of the event was lateral and not radial from the center of the earth or some other point as would be expected from a "geophysical" cause.
Furthermore I find it highly unlikely that multiple theoretical and applied physicists would come to apparently total agreement on the significance of these findings and simply overlook the gaping logical fallacy you imply.
It's not a gaping logical fallacy. It's human nature to tell interesting stories. Black holes colliding is sexy. "We measured a 10^-18 variation in the diameter of the Earth and we are uncertain regarding its cause" is not sexy.
As Jack Friday used to say, "Just the facts, mam."
Unfortunately not-entirely-unreasonable skepticism can be mistaken for hostility by people excited about a potentially groundbreaking result.
That said, it sounds like they did a lot more work to eliminate sources of error than you may be aware of. OTOH if seismic resonance was causing the correlation, there would probably be more time between the events at the two facilities.
Scientists will try to poke holes in these results while further experiments will try to corroborate them. Meanwhile laypeople will be introduced to more oversimplified and counterproductive ways to think about this stuff. Business as usual.
I'm sure I was not clear, since that was the first pass.
What the experiment indicates is that the Earth varies in size. Roughly:
measured distance of 10^-20 meters
4km is 10^-5 of earth circumference.
delta Earth's circumference 10^-25
total distance change in earth's diameter = 10^-18 meters
Given the non-intuitive nature of geology[1], I am saying that the possibility that the Earth varies that much in dimension due to it's internal structure is not so vanishingly remote as to be left unaddressed. Saying it's ten or a thousand times less likely doesn't move the needle much at that scale...if such a thing were said.
[1]: I'm old enough to have been introduced to platetechonics as a distuptive theory.
The LIGO detectors were very carefully constructed to rule out this sort of noise; they are the result of a lot of human engineering ingenuity. It turns out that before making this investment in instrumentation specifically for the purpose of detecting gravitational waves, the scientists in question also came up with the objections that you spent presumably less than fifteen minutes thinking of. They then spent time and substantial engineering effort to avoid those objections.
My tone here is rather curt as it's clear your skepticism is unfounded and that you didn't look at even the layman explanations put out by the scientists who made these claims. For example, the video announcement talks about the construction of the equipment in question: https://www.youtube.com/watch?v=aEPIwEJmZyE&feature=youtu.be... and specifically t=3310 talks directly about how they worked to avoid detecting the motion of the earth.
I'm old enough to remember Pons-Fleischmann. But not so old as to forget the more recent faster than light neutrinos social media storm. I've been thinking about the nature of scientific claims for more than twenty years. Few people are intentionally wrong: that doesn't mean astronomers as men of science didn't sware by the Ptolemaic cosmology.
To be clear, I am not denying the possibility of the earth expanding in and contracting in what passes for space time. I am skeptical of the proposition that the instruments are measuring the collisions of black holes is methodologically sound.
The careful preparation and engineering that occurred before LIGO was constructed resulted in an instrument that ran from 2002 to 2010 without detecting gravitational waves. The consequence of this $400 million experiment was not reexamining the theory, but sinking another $200 million into an instrument that created good tweetable data. That's the way careers and politics and human nature works.
There are three components to the theory. Spatial change, gravitational waves, and colliding massive bodies. The reason I am skeptical is that the scientists are inferring two of them: gravitational waves and colliding black holes from the component most likely to have other causes. If I had the mountaintop shells and the flood, god would be more plausible. If I have the shells and god, the flood is. Two outta three is my threshold for reasonable scientific inference in this case.
If you're not denying the possibility, then what exactly are you trying to say here? That you're skeptical that the experiment detected anything, or that the "anything" it detected is what they are saying it is?
Clearly the first can and will be found out over time as other detectors are being built to replicate these findings. However, I believe there is enough scrutiny of their claims that this kind of skepticism can likely be ruled out.
The second, that their story doesn't fit the data, is kind of odd to me. Gravity is so weak and the detectors they are creating are still so new that it seems likely that colliding massive bodies would be the first kinds of things they would pick up. Just as with early telescopes where objects that were very close or very bright were the first ones to yield useful data.
If you have another explanation that you believe fits the data better, put it forward and try and find a way to test it. That's how science works. But this is not the kind of thing that is going to collapse in a heap of logic. Data and the scientific method doesn't work like that.
Maybe it would help to re-frame the relevant part of the (apparent) argument:
Oscillations show up everywhere in nature, and even a pattern as specific as a frequency sweep with ringdown could be the result of many different phenomena. Even if in this case many possible sources have been ruled out, there must be others that we do not know about. Since the only observation we have to go on is the signal (so far), we should remember that the cause implied by the model is contingent on the signal not being one of these unknown sources.
Some commentary: Imagining alternative explanations is only half of the work that comes next. Once more of these alternatives are found, we also need new experiments that will be designed to rule them out. It sounds like the space-based interferometers will go a long way toward ruling out potential "terrestrial" factors. And if the same signal is detected in both ground-based and space-based systems that is an even greater step forward.
I was also wondering how they successfully rule out other possibilities. Many people seem very eager to believe. Some skepticism here can help aid a more thorough understanding.
I suppose the prediction didn't include a scale (it was determined from the observation+model) so there is a little bit of room for model fitting there. And sure, other stuff could have generated a frequency sweep. Maybe if this is replicated the newer methods will narrow down the possibilities.
[0. A long history of theoretical and experimental discoveries lead to a specific, well-motivated theory (General Relativity) describing gravitational phenomena.]
1. Theory predicts a very specific and recognizable class of gravitational wave patterns for likely sources, whose details are determined by a small number of parameters that correspond to meaningful physical quantities (specifically, two masses and a distance; I don't know if there are any more than that).
2. Two independent experimental instruments measure a wave pattern that (apart from some low-level noise) is an excellent match to one of the predictions. Fitting the model to match the specific data, the resulting parameters have entirely plausible astrophysical values.
3. This is considered an example of a theory making a specific, novel prediction that is later confirmed by experiment: pretty close to the classic definition of the scientific method.
4. Therefore, the experimental data has provided strong support for the premises of the theory being tested: both its prediction of gravitational waves and its detailed dynamical predictions for the source scenario matching the observations.
Maybe I'm misunderstanding something in your argument, but I don't see any circularity here. What about this process isn't precisely the way that science is supposed to work?
Except that if two detectors, preferably three, detect the wave at the appropriate times then it can be said that whatever phenomena is the cause exists at a particular vector. A bunch of waves all coming from a direction pointing to the center of a huge galaxy would fit the wave model more than any earthbound cause.
Someone comes up with a theory that's mathematically simple, beautiful, and consistent. That's great, but we don't consider it "correct" until it actually predicts something novel and we verify the prediction holds experimentally.
The core idea is plausible, if circular at present. That's OK. Now, we take that and run with it to expand on what we think we understand and hopefully, confirm and even more hopefully, crack open some new physics.
Seems like Einstein nailed it.
But, questions remain. Any of those could yield insights of similar potency and, ahem... gravity.
In this case I don't know if your skepticism is warranted.
1st: Paper said that the waves are redshifted to a certain degree (it's in the abstract) [1], ergo 1.3 billion light-years away in space-time. I'm not thinking you want to be so fundamentally skeptical.
Fine, but I find comments like "I don't really grasp what's going on, but I can show it's crap using high school logic" to be extremely low-quality, and I'm sure I'm not the only one.
I read the article. I am in the habit of calling periodic changes in dimension "resonance." I consider it rather consistent with the use of "wave" in the discussion and hence a handy way of describing what the instruments measured rather than what the theory suggests as the first cause.
Resonance is quite a bad term if that's what you use it for. Resonance would require some amount of positive feedback. Vibration or oscillation would be a much better terms for simple periodic changes.
It's consistent with vibration caused by a wave which is why it seemed appropriate to me in this context...even if I think the claim of waves is an over-reach, I am not being deliberately argumentative.
A detector of this sensitivity seems like a boon for spying. Rather difficult to relocate, fortunately, but it makes me wonder about the future of the technology. No one would have looked at the first computer and envisioned an iPhone.
Not before they were solid state. Computers had their own buildings and relied on glass tubes for operation. Screens weren't on the radar. Operators were highly educated. But technology did bring us to a point where the vision became more focused, and led to what we have today.
There are many possible paths to rebutting my statement, which to be clear is idle morning musing, but your objection doesn't hold water.
I agree that you do make a point. There is really no way for us to know what is to come. Sometimes we think we are looking at a square, but it really is a cube and we are just unable to look beyond are current perspective for whatever reason.
I would say one obstacle that stands in the way of "spying" on objects moving on the Earth's surface is that the gravitational wave energy emitted by accelerating objects on Earth would be "too small" for current detectors. Not to mention that there would be the issue of how to filter gravitational wave noise, and/or isolate frequencies. However, if it possible to build an amplifier or filter to resolve these issues, that remains to be seen - or maybe somebody else could chime in.
ok, here's an idea that someone might either build on or refute: would it be possible to build an amplifier of gravitational waves using some arrangement of microscopic and/or macroscopic objects having a "known" defined 3D physical relationship (say in a lattice), and under known interactional forces (including EM). You would have to take into account the uncertainty principle in system parameter measurement, though the propagating gravitational wave should have a deterministic effect on the potential well and thus the quantum wave(s) of the system(s). Thus, amplification through propagation in the space-time of the system might be feasible. This is of course all hand-waving, and very rough.
Gravity for navigation, perhaps ... but I believe they use magnetic fields, not gravitational fields, to detect other subs (and also some rather sophisticated real-time degaussing to prevent said detection): https://news.ycombinator.com/item?id=10979452
I'm wondering no-one mentioned Electric Universe which is the greatest opponent of this gravitational hocus-pocus religion. It would be much more beneficial to the humanity to focus smart peoples' attention to Birkeland currents or plasma or to the recent experiments of the SAFIRE project. They have several series on their youtube channel:
Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. [...]
However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are underway (such as LIGO and VIRGO). Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass [...].
Fascinating! I take it that the question of whether the graviton could have mass is now considered to be well answered in the negative.
[0] https://en.wikipedia.org/wiki/Graviton