One of the most fundamental scientific facts is that quantum theory (the most precisely tested theory in human history) and general relativity are incomplete. There must be a bridge. And we have no idea what that bridge is. It's been this way for over a century; the lifespan of string theory is not so long in comparison. Until we find a way to falsify it, we have to keep trying, don't we?
Opportunity costs. The real debate has been whether it makes sense for string theory (whatever the prevailing definition is) to dominate funding for theoretical research of the "bridge". There are alternatives besides strings for the bridge, and there should be even more, in theory...
I don't know enough about the history of funding theoretical physics research to comment on that one way or another. However, neither did the comment I was replying to reference any actual facts about the distribution of funding that might suggest any of them had been wasted.
The fact is, we have no falsifiable theories that can unite GR and QM. Should every theory be abandoned that doesn't quickly lead to a resolution? No, clearly not. So the question is what kind of criteria we could use to determine that string theory is a dead end or is otherwise stifling true progress.
And that's pretty much what I was trying to ask previously... is ST actually sucking all the air out of the room? I'm a layperson and not just going to assume that hundreds of experts have blown their careers doing pointless calculations on a theory that "obviously" isn't worth the resources put into it. But the comment I replied to seemed to be making that assumtpion,
Yep, or at least until we find something else that is explaining things better and testing better and looks like a better path forward. As much as I dislike ST because of the ambiguity/vagueness that means it's so hard to nail down as a single concrete theory, the alternatives so far still also have a lot of those same issues (e.g. too many parameters, effectively impossible to test, etc.) so there's no reason to drop ST just yet as useless or an invalid theory.
> Until we find a way to falsify it, we have to keep trying, don't we?
By that same token, we should keep looking for the fountain of eternal youth in El Dorado until it can be conclusively proven it doesn't exist.
A theory which is not falsifiable is not a scientific theory, at least in principle, and it is hard to tell why it should be entertained for quite so long.
While I agree with the general premise, don't you think that if we were to find a simple explaination that elegantly explains many things at once, it would be at least worth a try to find ways of falsifying that explaination?
Not at any cost of course, but the falsifications gathered e.g. in trying to disprove the string theory might also help us figure out what is actually going on.
Sure, but once it keeps failing for a few decades, and given that ST is significantly more complex than either QM or GR, not simpler, there has to come a time where it simply is abandoned, even if it hasn't been disproven.
Principia was proven right at every turn, and explained all of the observed mechanics experiments, all the way up until Maxwell's equations were formalized and their effects tested out, some 200 years after it was published. It's also important the theories it replaced were massively more complex.
Maxwell's equations were first formulated in 1865, the "patch" to Newton's theories was formulated soon after (the luminiferous aether), the Michelson-Morley experiment proving the patch did not work was run in 1887, and the theory of special relativity was proposed in 1905 - so it took about 20 years at most between Newtonian mechanics being conclusively proven to contradict experiments and a new theory becoming proeminent. And special relativity was quickly replaced with general relativity (just 10 years later), because, despite its success, its limitations were immediately apparent.
As far as I understand it, the ether was supposed to be matter, ultimately made out of some kind of particles that obey laws of motion.
Conversely, the electric field is just a potential. It's not made up of anything, it just describes how charged particles interact if they are in a particular place in space-time.
Yes, but we shouldn’t pull that theory out our mathematical asses. We should always be falsifying things not trying to make an unfalsifiable theory work because the math is interesting.
Why do you assume there is a bridge? Or even if there is one, why do you assume that descriptions of that bridge in a mathematical language is at all possible?
We do know that mathematical frameworks cannot be at the same time totally complete and internally consistent. Would it be a stretch to assume descriptions of our physical reality could have the same restriction? General relativity and Quantum mechanics are relatively complete in describing our physical reality, however they are not consistent with each other. Perhaps if we ever find a description that is consistent, it won’t be complete. Perhaps grand unified theories are simply a mathematical impossibility.
The problem with this line of thinking comes from the way the problem is posed. The reality is worse for both GR and QM.
QM is not a theory about small particles: it is a theory that describes the evolution of any object whatsoever. It just happens to be completely wrong for large objects. And even if we accepted that there is some objective cutoff points between "small objects" and "large objects" (the "objective collapse" interpretation), it would still be wrong, because it predicts effects like gravitational lensing don't exist.
Conversely, GR is also a theory about any size of object; and it is also completely wrong when tested on small objects, as it predicts effects like the self-interference of particles don't exist.
And again, even if some kind of objective boundary existed between the domains of GR and QM existed, that would need to be incorporated into the maths of both of them, and questions about behaviors close to that margin would arise.
I see. So me and my parent are both wrong. The problem isn’t about a lack of bridge, or inconsistencies/incompleteness, it is about a lack of well defined scopes for each theory.
In a sibling post I talk about how this is not a problem in psychology, and ask what the difference is. This answers that question kind of well, the difference is that in psychology/sociology the scopes are well defined. We know what a population is and apply sociology to it, and we know what an individual human being is, and apply psychology to it.
With quantum mechanics and general relativity the scope is supposed to be the cosmos, but both theories fail on some places in the cosmos. So either the theories are wrong, or the scopes are wrong. I’m leaning towards the latter.
The problem is that scale is completely smooth. IF there were some strict scale at which each theory applies, as I said, you'd then have problems for phenomena that straddle that scope. You can always add an extra electron or atom to a clump, so you'd have objects oscillating between obeying one set of equations vs the other, which seems very very very hard to actually believe.
A similar problem actually exists in psychology vs sociology, though it seems you choose to ignore it. When you want to study the behavior of two human beings (say, a married childless couple), do you apply sociology or psychology? How about a married couple with children? An extended family?
Also, psychology has to explain how an individual human behaves inside of a population, and sociology has to explain how the behavior of groups emerges out of the individual psychologies of their constituents. If they don't do this, then either one or both theories must be wrong. To take an extreme silly example, if psychology predicted that no person would never choose to kill themselves for the sake of another; but sociology predicted groups of people regularly have members sacrificing themselves for the group - then one of the theories must be wrong, you can't just say "we apply one theory when talking to a man and another when talking about the group".
> So either the theories are wrong, or the scopes are wrong. I’m leaning towards the latter.
Well, no. That's not really possible here. The scopes can't be adjusted. The theories are wrong. They both must explain all physical phenomena, but neither does, therefore a more accurate theory exists which we have not found yet.
I just don't know if there's much value in trying to make an analogy to psychology here. Currently it seems to be getting in the way of understanding.
No no; I'm saying that if you apply QM as it exists today to describe the motion of light beams around the sun, you will not get any effect similar to gravitational lensing. Since gravitational lensing is a real effect that we have clearly seen, it means that QM as it exists today is just wrong (in its predictions about how light moves around the sun).
GR does of course predict gravitational lensing. However, if you used the formulae of GR to compute the motion of photons passing through a double-slit experiment, the solution would show two different bright spots, corresponding to the two slits; in reality, we see an interference pattern. So GR is also wrong.
By definition, a (correct) theory of quantum gravity would predict both gravitational lensing and the way photons behave in a dobule-slit experiment (otherwise, we would say that either the theory is wrong, or it is not a theory of quantum gravity). However, no such theory exists today, at least none that doesn't contradict other observations.
There are already experiments (and simulations) which show Raychaudhuri focusing and Einstein lensing in purely-quantum analog gravity (see e.g. <https://scitechdaily.com/bridging-quantum-theory-and-relativ...> for something moderately accessible that allows for attaching a causal cone at each point in a relevant analog spacetime), so
> no; I'm saying that if you apply QM as it exists today to describe the motion of light beams around the sun, you will not get any effect similar to gravitational lensing
falls down because we can describe such an effect purely quantum mechanically.
Also, I think you should be put to proof with respect to a claim against quantum perturbative or canonical methods in the solar-mass lensing regime in which perturbation theory works great for classical GR, taking into account all sorts of beyond-leading-order (classical) effects like noncircularity, backreaction, helicity, you name it. The sun is a fixed enough background that it's a linearized gravity problem. What exactly in "QM as it exists today" breaks (or is broken by) this?
> ... if you used the formulae of GR to compute the motion of photons passing through a double-slit experiment ... GR is also wrong.
How exactly does taking the fully Lorentz-invariant QED or Standard Model to local Lorentz-invariance (with the radius of curvature significantly larger than the laboratory experiment) break the picture? We are nowhere near needing to consult Birrell & Davies.
What do you think needs doing here if "you used the formulae of GR", beyond solving the EFEs and the geodesic equations for the whole (region of) spacetime, and then wondering what geodesic any given photon will couple to? What do you think the scale of the correction from Minkowskian geodesics will be? And how much of that do you impute to the apparatus?
It’s sort of incomprehensible that there wouldn’t be, even if that bridge is forever incomprehensible to us. There are boundaries between where GR and QM are predictive, so presumably, unless there’s some strange smooth transition of equations between those two regimes (which would itself lend itself to a mathematical theory), then there must be a consistent set of equations which explain both regimes consistently and simultaneously.
Coming from psychology this feels alien to me. In psychology there is a definite boundary between individual behavioral dynamics and population behavioral dynamics. There is no smooth transition between the two, either you describe the individual or you describe a group, you cannot do both (even thought debates about IQ here on HN will have you believe otherwise) and there is certainly no smooth transition.
How is the boundary between GR and QM different from the boundary of psychology and sociology?
I would say it is completely different. In physics, quantum mechanics gives extremely precise and verifiable predictions of the outcomes of experiments within a certain range of physical conditions. So does general relativity. In contrast, psychology has no mathematical model that will, for example, accurately predict what I am going to eat for lunch, nor is there a model that predicts the exact outcomes of elections. Since physics has two models which are exquisitely precise in different size regimes, but which are mutually incompatible, you have a definite puzzle about what happens in situations where the effects of both theories should be important. There are no exquisitely precise and accurate mathematical models of anything in the social sciences, as far as I am aware.
Im neither a psychologist nor a physicist, but I think one can analogize population vs individual behaviour to condensed matter physics versus particle physics. Condensed matter physics finds emergent behaviour in large clumps of stuff that would, in principle, be totally predictable from first principles (the standard model), but which in practice are quite difficult to guess a priori. Different scales lend themselves to different tools, since nonlinear dynamics (chaos) makes it intractable to apply bedrock reductionist formulae to large systems. The higher order behaviour of complex systems is in no way less interesting or true, I would argue, than the seemingly simpler behaviour of very small systems.
In contrast to condensed matter vs the standard theory (QM basically), QM vs GR has fundamental incongruities, since both theories make claims about what happens at the same scale. Only one (or most likely neither) can be correct at the event horizon and center of a black hole.
The difference would be that the laws of physics are much more rigorous than the 'laws' of psychology/sociology.
Outliers in the latter aren't necessarily indicative of anything wrong with theory in general, while even a single outlier in anything in the former is indicative of an incomplete model.
There are various results in physics which should be predictable via both GR and QM independently. The results should be the same as both models are supposed to be describing the same thing, so it follows that there should be some sort of gradual transition as one set of effects gradually comes to dominate over the other. Otherwise we'd see a single point in the data where QM stops being accurate and GR takes over, but despite investigating so many different scales, we have not seen any such cutoff point.
> In psychology there is a definite boundary between individual behavioral dynamics and population behavioral dynamics.
I'm aware of my own behavior as an individual being influenced by social context, is that not the kind of bleed over you might look for? Maybe you're referring to specific concepts I'm not actually even understanding.
You still use theories from psychology to describe the interaction. This is precisely what social-psychology does (admittedly spectacularly often without replication). And a good social psychology theory is consistent with other fields of psychology, like cognitive, or—more often—behavioral psychology. You don’t use population statistics to predict how you as an individual will behave in a certain situation.
If you could precisely and reliably describe and predict individual behavior for any individual, then population behavior would follow directly from those laws.
> In psychology there is a definite boundary between individual behavioral dynamics and population behavioral dynamics.
> either you describe the individual or you describe a group
In gravitational physics, with respect to a flow in a dynamical system (an example is galaxies in an expanding universe) we can use a Lagrangian observer (e.g., one galaxy, drifting along with the flow, tracing out a pathline/worldtube that depends on features like its mass-evolution and proper motion within a cluster of galaxies) or a Eulerian observer (e.g. a notional observer with no spatial motion at all, watching alllll the galaxy clusters jiggle, swirl, turn, and age a little differently in relation to her). One can convert observations of each type of observer to the other in a rigorous mathematical procedure, since they are just two (families of) the infinity of different observers allowed by even just Special Relativity. See e.g. <https://en.wikipedia.org/wiki/Lagrangian_and_Eulerian_specif...> for more detail.
>> There are boundaries between where GR and QM are predictive
> this feels alien to me
You can do both quantum matter and classical General Relativity in one of several effective field theories, which I'll return to below.
GR and relativistic quantum field theories (QFT) purport to make accurate predictions in strong gravity, which one only finds deep within black holes (i.e., not on our side of any horizon), but they make very different predictions in that regime pretty generically. Generically in the sense that choosing different behaviours of particle-particle interactions (and self-interactions) do not really move the needle on GR's prediction of a collapse to a core of infinite density. However, in various approaches which convert GR's classical gravitational waves into large number of gravitons, one can write down a matter QFT in which charged particles' self-interaction can lead to a degeneracy pressure (a repulsive force) that increases at higher particle energies such that they overwhelm gravitational collapse at very high but finite density in black holes of arbitrary mass.
In weak gravity, like we have in our solar system, QFTs allow us to prepare significant masses in superpositions of (spatial) position. General relativity does not allow for such superpositions. We are approaching lab-testability, with results from sensitive accelerometers allowed to point at tiny superposed masses.
However, in regimes far from (non-negligibly) gravitating superpositons and strong gravity, GR and QFT are usefully (and possibly fully) compatible. We get good results in astrophysics from semi-clasical gravity, where the classical curved spacetime of General Relativity couples with the expectation value of QFT matter (i.e., we average out some quantum weirdness and justify this by the weak gravitational effects of the "lumpy" weirdness being practically impossible to measure; superpositions and ultra-high-energy/ultra-high-denisty systems might be too lumpy).
We also get good results from perturbative quantum gravity and canonical quantum gravity, for example. Neither of these latter two is really classical General Relativity so they can deal with the gravitation of superposed matter (otherwise they give for all practical purposes the same answers as semiclassical gravity). These approaches do not work in strong gravity, however. Essentially they become calculationally intractable or they crash into unresolved problems splitting spacetime into space and time (in order to do time-dependent quantum mechanics).
I can't say anything about promising. The hard part seems to be building an apparatus that works, and I don't know how to do that. I hear of short-lived superpositions with increasing amus or daltons but I can't deal with those units without the newest SI prefixes (fun facts, 666 Yamu is a bit more than 1 kg; and there is maybe about 0.666 YMsun locked up in observable galaxy clusters).
> We do know that mathematical frameworks cannot be at the same time totally complete and internally consistent.
I take it you're referencing Godel's theorems here, but "consistent" and "complete" have rather technical (and somewhat limited) meanings within that context, so it's not clear to me how they'd usefully map onto the potential relationship between QM and GR?
That's a very good point. In particular, "complete" refers to the ability of the mathematical-logical system to prove every statement that is true within that system, in terms of the system.
This property is completely irrelevant to a theory like QM or GR - it is only relevant for a system that aims to be a universal foundation for mathematics (a formal language in which any mathematical statement whatsoever could be precisely formally encoded, and then proven or disproven).
Mathematical completeness is a mathematical concern and is irrelevant for physics, because physics and mathematics have different notions of existence.
One of the most fundamental scientific facts is that quantum theory (the most precisely tested theory in human history) and general relativity are incomplete. There must be a bridge. And we have no idea what that bridge is. It's been this way for over a century; the lifespan of string theory is not so long in comparison. Until we find a way to falsify it, we have to keep trying, don't we?