It is worth noting that there was no direct detection of dark matter. It comes from a measurement of gravitational lensing, which has been going on for awhile˟˟.
Indeed, it's more like observing a magnetic field by dropping iron dust on a sheet of paper, and gently hit/shake it until the field lines "appear".
It makes me wonder why we can't observe dark matter (which seems to emit only pure gravitation, and no light nor electromagnetic radiation). Could it be because there's no actual matter (i.e pure gravitational waves, like the magnet+iron+paper experiment)? Or are they massive clouds overloaded with Higgs bosons?
There's one thing I don't understand about the mystery of dark matter. I don't understand why the simple explanation for it isn't just that it's regular matter that is not stars. Maybe there are just bajillions of planets and dust clouds out there. Matter that isn't directly circling stars, thus not reflecting light. Why is the popular assumption that if the mass isn't stars or things in orbit of stars, that it must be a mystery substance? I assume there's scientific reasoning behind this, but I've never heard it explained before. If someone could fill me in, that would be awesome.
On top of the stuff other people have posted, we can (apparently) peer at the cosmic microwave background and look for patterns in its anisotropy power spectrum from baryonic acoustic oscillations in the early universe's quark-gluon plasma. The results we see demand WIMPs.
I'm not going to explain what any of that means because A. I've only recently woken up, B. you're perfectly capable of googling for yourself, and C. I'd probably get it wrong anyway. Suffice to say it's all brain-meltingly interesting.
We can tell how much dark matter is out there because we can "weigh" it through indirect measures. And then we can take different theories of dark matter (such as, say, the theory that it's all just a bunch of interstellar orphaned planets and "black dwarfs" and what-have-you made up of ordinary matter) and figure out what sorts of implications that would have, make predictions on observable effects of those different models and then test those predictions. And that is precisely what happened about 20-30 years ago. A lot of work was done to pin down what type of dark matter makes up the majority of it out there.
For example, you can point a telescope at a set of neighboring galaxies and look for brightening effects due to gravitational micro-lensing from a chance alignment of a "macho" (e.g. orphaned gas giant planet) along the line of sight. Surveys were set up and indeed found that there were orphaned "macho" objects in our galaxy, but the statistics showed that they were orders of magnitude too rare to make up the bulk of dark matter we know about from other studies. Another line of evidence involves studying the large-scale structure of the Universe (e.g. the layout of galaxies, galaxy clusters, etc.) and comparing it with various computer simulations of models with different assumptions on the composition of the mass of the Universe (e.g. 100% "ordinary" baryonic matter, various percentages of "special" dark matter such as cold and hot dark matter, WIMPs, etc.)
From this and many other lines of evidence we came up with very strong evidence that the vast majority of the mass budget of the Universe is in the form of so-called "cold dark matter" which is composed of weekly interacting massive particles other than neutrinos (neutrinos are dark matter, but we've been able to place an upper limit on how much they contribute to the dark matter budget of the Universe, because they are detectable to a degree, and it's only a fraction).
So that's it, just a simple matter of comparing the predictions of different theories with observations and eliminating the theories that do not predict what we actually see out there in the Universe.
(A bit off-topic question to physics expert on HN)
I've read that supermassive black hole accretion is the most energy-effective process of mass to energy conversion in the Universe (50% efficiency or so).
I'm just curious: Where does all that energy go? Extremely powerful jets of radiation are emitted into the intergalaxy space and then what? Does it just disappear? Isn't this energy responsible for Universe expansion? It must push galaxies away from each other, right?
The thing is, the Universe is a big place. So even if you have sagans of tonnes of mass being converted to energy every second in extremely powerful jets around a supermassive black hole (which we do, in the vast majority of galaxies) given enough distance it's still just another tiny point of light in the sky.
However, quasars are just this sort of phenomenon and are so bright that they are visible in telescopes across almost the entire extent of the visible universe.
It would be amazing if somebody could answer this too. If I turn a lamp on and off in a sealed room of mirrors, why doesn't the light just keep bouncing off the walls and illuminate the room?
Well yes, when the light hits a mirror, a certain percentage (say 95% of the energy) bounces back, which can hit another mirror and so forth. There's no limit, but after a few dozen bounces, the remaining light is virtually undetectable.
Really, the exact same thing happens in a standard white painted room, the two differences being that the mirrors reflect more of the light (so that a dimmer light source will suffice to reach the same level of illumination), and that the reflection is directed instead of diffuse (this only changes the shape of the reflected light, not its amount). Maybe you can explain what's confusing you.
Some of that energy turns into heat every time the photons hit something, but heat is a mix of kinetic energy and light so you do get a fraction of that light bouncing around indefinitely, it's just not in the visible spectrum.
Think of it like dropping a ball an a hard vs soft surface. In both cases things bounce. Even on a hard floor the ball stops bouncing after a while, and in both cases the ball / floor / air get's get's warmer from the balls energy.
Yeah... Warmer is kind of mixing metaphors since warmth is just a statistical aggregation of stuff moving around. The point is that photons result from electrons changing energy levels and disappear when they hit an electron and change its energy level. A big change makes a high energy photon which might bump into an electron, resulting in a higher energy electron and a new, less energetic photon. This keeps on happening.
Seen purely as electrons -- one electron had a lot of energy, now a lot of electrons have a little. Seen purely as photos, one photon had a lot of energy, now there are lots on very low energy photons.
Overall, the collective term for this is entropy -- over time we get fewer opportunities for big photons to get created, until it's all small changes in energy and small photons -- total entropy -- and everything is background radiation.
The problem with thought experiments is that sometimes you can create a non-physical situation by accidentally introducing magic which invalidates the whole thing.
For example, in this you have a perfect mirror, which is actually not physically possible and would mean violating several laws of physics such as thermodynamics and electromagnetism.
Another common problem is hypothesizing perfectly rigid materials or perfectly flat surfaces, which can't exist in any matter made out of atoms but which could easily beused to violate the laws of relativity.
(I posted this in response to another comment but am moving it here since you actually addressed it)
What I'm curious about it, is there a general principle that stops things from possessing a property perfectly? For example, IIRC friction dictates that many energy transformations never convert energy perfectly, leading to far-from-perfect engines and unavoidable power dissipation in electricity transmission. Is there a similar principle that stops collisions/materials from being perfectly elastic, surfaces from being perfectly reflective, etc.? Does it go against entropy never decreasing in a system?
edit: hmm, so are the laws that dictate that perfect objects cannot exist somehow more deeply connected by a general principle (just like Noether's theorem underlies laws in various domains)?
Friction is a very complex subject so I won't address it, but other things like the absence of materials that are perfectly flat, perfectly rigid, infinitely strong, etc. are easily explained by the fact that matter is made out of atoms.
For example, there's a recurring thought experiment about the limit of the speed of light that goes something like this: say you have a rigid rod that is one light-year long and you push on one end, won't the other end instantly move, thus proving that you can exceed the speed of light? The problem with that is that it's based on an approximate and intuitive understanding rather than a proper understanding of the physics involved. For a short rod if you push on one end the other end seemingly moves instantaneously, giving the illusion of rigidity, but in actuality what is happening is that you are transmitting forces through the rod at the speed of sound in the material, and if you make movements that are slow compared to the time and space involved then everything will appear instantaneous (since the speed of sound in steel is about six thousand meters per second). However, once you scale things up the intuitive approximation is no longer valid. What happens when you push a long rod from one end is that a displacement wave moves along the rod at the speed of sound until it reaches the opposite end, taking far, far longer to move than a signal travelling at the speed of light. Also, no material can be perfectly flat because at the scale of atoms there are... atoms, which are not flat.
That's generally the biggest reason why we can't have perfect anything, because stuff is made of atoms and atoms are messy. Often times people who let dreams of perfect materials lead them astray fail to take into account the underlying mechanism for the property they are considering (e.g. rigidity is due to forces being transmitted from atom to atom within a material). A good rule of thumb for whether or not an assumption of perfection is going to ruin a thought experiment is whether or not you're ignoring the underlying mechanism for that process. Another is whether or not you're assuming some arbitrarily small amount that you are omitting from the model because it introduces a "tax" that is annoying to account for but that can be easily bounded or instead you are assuming absolute 100% perfection that your whole model is completely reliant upon and even the slightest deviation from perfection would ruin the model.
Interestingly enough, there are a few examples of physically perfect things in the real Universe. For example, superconductors experience 0 electrical resistance to current, and superfluid helium does not experience friction internally, and electrons appear to be perfect point-like charges.
We know this is not the case because intersteller dust absorbs light, while the dark matter does not emit or absorb light at all. The only effect we see is the gravity.
Dust absorbs light, blocking it (creating dark spots) and getting hot (creating light spots in a different frequency range). We know what interstellar dust looks like. Dark matter doesn't look like that; it doesn't look like anything. We only know about it because its gravity affects the things we can see.
NASA has a brief page on dark matter and something else we know little about, dark energy:
If you'll notice though, there's one interaction between ALL of the particles that is missing: gravity. Gravity affects anything with energy. Photons, leptons, quarks -- they are all attracted to each other because they possess energy (negligible, unmeasurable attractions, but still extant).
Wouldn't it be interesting if the only way that dark matter interacted with the other particles was through the gravitational force? Maybe from some alien's perspective it would constitute the matter of everyday life, but because it didn't interact with any of our particles except through gravity we would be missing out on a large aspect of our universe!
Furthermore, is it that far-fetched to think there might exist particles that do not interact at all with the ones we have discovered? Gluons, for example, only interact with themselves and with quarks. Some other particle may interact with nothing we are familiar with -- and thus we could never study it. Is it even "real" then?
(Any particle physicists on here, please feel free to educate me further!)
There are a lot of people thinking about what kind of structure the model of dark matter particles could have. And there are a lot of different ways for there to be more than one dark matter particle, and even if the only way those other particles interact with normal matter is through gravity, there are still ways for us to understand things about them.
At the moment, the most popular dark matter theory comes from supersymmetry. In this case, there's only one dark matter particle, and all of the rest of the particles interact with normal matter, in pretty much the normal way, since all of the underlying structure of the model is almost identical.
Unfortunately, the idea that supersymmetry is "dead" has been propagated by journalists covering science who don't know any better and people with an agenda. Generally, it's people who just don't know any better. Unfortunately, you see comments supporting the idea even on HN.
The important thing to understand about supersymmetry (SUSY) is that in the most general case, there are approximately 105 new parameters. That's far too many to probe in a meaningful way, so most models choose between 2 and 5 to vary, and fix the rest. Then a bunch of models are chosen that hopefully cover a wide spread of different behaviors.
The true part is that several models have been excluded, basically as well as the LHC is going to be able to exclude/discover anything in the current energy regime. However, some of these models were just not chosen very well to begin with (but have historical importance) and others were chosen to have maximal signal strength.
So as time goes on, the search for SUSY turns away from "easy" models and looks at more complicated ones. With 105 parameters, there's a lot of parameter space unexplored.
> Some other particle may interact with nothing we are familiar with -- and thus we could never study it. Is it even "real" then?
Philosophically, this is equivalent to the question of whether other universes, which do not interact with ours and therefore we cannot study, exist or are "real". It is not a question that Science can answer.
I think it's very interesting that there's reasonable questions about things that science can't answer. If you can't study it, can't predict it, or can't reproduce it, then it isn't in the domain of science.
This is the one a lot of people miss out on. Standard theories predict several types of "other universes"--thus that question is answered from science's view, it's just not an answer people like. See http://arxiv.org/pdf/astro-ph/0302131v1 for a general overview. "Containing unobservable entities does clearly not per se make a theory non-testable."
That question really has far reaching implications. Because if we say Science is what we observe and describe as per our interpretations of logic(And the language of logic - 'Math') then our science is really broken. Because what we can observe doesn't often turn out to be true and what is true is not often observed.
Because look at it this way. We are now saying Dark Matter doesn't interact anyway with light nor something else. Hence observing, detecting or modeling them out through conjectures manufactured through thin air is nothing more than what religion was some centuries ago.
Anything unexplainable was attributed to some form of divinity in times before.
We know it exists, but we can't show you, can't explain you what it is, how it looks is the text book definition of god throughout centuries.
It is only "broken" if you assume incorrectly that the goal of science is to Discover Truth. Moreover it is broken in a more direct way: there exist certain models which are mathematically equivalent but which describe contradictory states of being. A reasonably good example of this is heliocentrism vs. geocentrism: classical mechanics allows you to say "the Earth is at the center of the Solar System, there are gravitational, Coriolis and centrifugal forces around it affecting all of the stuff in space", but it also allows you to say "The Sun-Jupiter barycentre is at the center of the Solar System, and the only force we need is gravity." There is no experiment which can distinguish between those two; they are mathematically equivalent.
(A slightly better example comes from quantum mechanics. In the "Schrodinger picture" there is a "wavefunction of the universe" which changes from moment to moment, while the definitions of space and momentum stay the same. In the "Heisenberg picture" the wavefunction stays the same while the definitions of space and momentum change. You would think there would be an ontological difference to the question, "is the state of the universe different from the state of the big bang?" but, in fact, on this description there is no observable difference, and science could never settle the question.)
This does not reduce science to a religion; science simply studies the observable differences and must be content with not knowing everything -- which most scientists are already content with, since they have to deal with matters of uncertainty and the distinctions between correlations and causations.
Dark Matter does interact with other things, but it does so indirectly, because it has mass and therefore warps spacetime. This is not actually the first use of gravitational lensing to observe dark matter; in fact, earlier it had been used to settle the question of whether dark matter felt any electromagnetic force at all, by looking at galaxy collisions. The prediction would be that the dark matter clouds of two galaxies would more or less "go through each other" in a collision while the luminous stuff would "bump into each other". This was observed as early as 6 years ago, see http://chandra.harvard.edu/photo/2006/1e0657/ .
We certainly can show you, and we can explain to you what it is. The only problem is the same problem that neutrinos have: it's just very hard to detect these particles because they don't have an electric charge and therefore don't care about the electrons which make all the rest of chemistry happen. Our best tool for understanding dark matter is still gravity; its a force which we know the dark matter feels.
> ... there exist certain models which are mathematically equivalent but which describe contradictory states of being. A reasonably good example of this is heliocentrism vs. geocentrism: classical mechanics allows you to say "the Earth is at the center of the Solar System, there are gravitational, Coriolis and centrifugal forces around it affecting all of the stuff in space", but it also allows you to say "The Sun-Jupiter barycentre is at the center of the Solar System, and the only force we need is gravity." There is no experiment which can distinguish between those two; they are mathematically equivalent.
How do heliocentrism and geocentrism represent "contradictory states of being"? They are trivially related to one another, and are mathematically equivalent as you point out. They represent a simple example of geometric relativity.
Consider a gravitational slingshot maneuver, a way to harvest some of a planet's orbital momentum to accelerate a passing spacecraft. If you make the planet the frame of reference, or the sun, or the spacecraft, the math and physics come out the same. No "contradictory states of being".
> A slightly better example comes from quantum mechanics. In the "Schrodinger picture" there is a "wavefunction of the universe" which changes from moment to moment, while the definitions of space and momentum stay the same. In the "Heisenberg picture" the wavefunction stays the same while the definitions of space and momentum change.
No,. this isn't really a "better example" -- Dirac demonstrated the mathematical equivalence of Schrodinger's wave mechanics and Heisenberg's matrix mechanics. Again, the difference is only apparent and superficial.
What's different in both cases is ontology, which is a fancy way of saying how things actually are. It may help to think of how you would program a computer to simulate either universe, or for that matter it may help to reduce the computation to something much simpler, like the following two functions:
def odd_sum(n):
return sum(range(1, 2*int(n), 2))
def square(n):
m = int(n)
return 0 if m <= 0 else m ** 2
Those two functions are mathematically indistinguishable if all you are doing is putting in various values. There happens to be a vast mathematical identity that the sum of the first n odd numbers is the n'th square number, allowing for that indistinguishability. But to claim that they're exactly the same realization of this function is obviously mistaken.
Similarly, there is a qualitative difference between a wavefunction of the universe which changes and a wavefunction of the universe which does not change. It does not matter that the two positions can be made mathematically equivalent; one is X and one is not-X, and science is permanently incapable of figuring out which it really is.
We are now saying Dark Matter doesn't interact anyway with light nor something else. Hence observing, detecting or modeling them out through conjectures manufactured through thin air is nothing more than what religion was some centuries ago.
It is, though: Science suggests that they don't exist because science favors simpler models to more complex ones, as long as the simpler model still accounts for all the evidence and makes correct predictions.
Gluons also interact gravitationally, like anything with energy. So would any other hypothetical particle, since it would have to have energy to exist at all.
I said earlier in the post "there's one interaction between ALL of the particles that is missing: gravity", so it was implied. Perhaps I should have made that a little clearer.
Two things, greatly oversimplified: 1) all forms of energy (including mass) are sources for gravity. 2) Interaction with the Higgs gives rise to what we observe as mass of particles; without this interaction, we'd have massless particle, with essentially the same energy, but moving at the speed of light. The net gravitational effect would be very different from what we observe though...
How do these filaments stay stable and not collape under their own gravity due to instabilities? Or, if there is 0 net force causing them to collapse, why doesn't the dark matter drift apart naturally and become less dense and more diffuse over time? Either way, filaments of high density don't seem to be a natural stable state. Can someone explain this?
The dark matter halos don't experience friction, and are incapable of emitting heat radiation to cool off and condense.
If you take a bunch of marbles and put them in a big bowl they will roll around for a while but eventually end up in the bottom of the bowl because they keep running into each other. But if you put in special marbles that just pass through each other and don't experience friction then you'll end up with marbles rolling around the bowl everywhere for ever, which is the way dark matter works.
So, interestingly enough the leading theory for the identity of the bulk of dark matter is the "weekly interacting massive particle" (or WIMP) the neutralino, and there has been recent evidence from the Fermi gamma-ray telescope that supports the theory that the neutralino is the primary component of dark matter.
Since this is Hacker News I have to point out that the "weekly interacting massive particle" could be, for example, my over-weight boss whom I have to meet every monday. ;)
This is something i thought about Dark Matter:
I believe Dark Matter to be the resultant force(and/or field) generated due to the interaction of the forces(and/or fields) of individual moving objects(matter).
consider a magnet(refer here as object) - something which has the property to attract(gravity like) and repel(field like):
Now if you were to have 2 magnets(moving objects) come close enough such that they repel(or attract); but due to forces(and/or fields) of other moving objects in their vicinity(or far enough[1]); they get locked or entangled such that their movement(and other properties) is now dependent on the strongest forces or fields of nearby objects. Over time; these other objects also get entangled and tend to form clusters and keep moving(exhibiting other properties like radiation etc). But now their movement(and other properties) seem to be the resultant effect of forces (and/or fields) of all the objects now entangled - giving an illusion of some matter that exists - now known as Dark matter.
I have used magnets as just as an example - one could think of matter having both these properties to attract and repel - such that the area affected by them could vary depending on various properties of the objects(matter).
[1](far enough) - such that their observation is neglected; but these objects tend to have forces(and/or fields) that they affect a particular system under observation.
Critical response: http://www.scilogs.eu/en/blog/the-dark-matter-crisis/2012-07...
Source: http://www.nature.com/nature/journal/v487/n7406/full/nature1...
˟˟ http://physicsworld.com/cws/article/news/2006/aug/25/gravity...