this post was submitted on 16 Mar 2025
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Steffen Gielen, Lucía Menéndez-Pidal.
Black Hole Singularity Resolution in Unimodular Gravity from Unitarity
Physical Review Letters, 2025; 134 (10)
DOI: 10.1103/PhysRevLett.134.101501
i can't even understand the very beginning of the discussion : why are physicists so obsessed with "information loss" ?
[24]* : S. W. Hawking, Breakdown of predictability in gravitational collapse
Phys. Rev. D 14, 2460 (1976).
To oversimplify, "information" is a very specific thing in quantum physics. Classical physics has the rule that energy can change form but cannot either be created or destroyed.
Information works the same way in quantum physics, which makes black holes seem like a problem since their event horizons are inescapable and anything that falls inside is lost.
why's it a problem? why can't information just be lost at a black hole?
The problem is that it'd be like if matter and energy could just disappear. Black holes would be exclusively tiny, as soon as one formed it'd start vanishing anything that crossed it's event horizon rather than growing, so galaxies could never have formed as their cores would just shrink away as soon as they got too dense.
Black holes are regions of space where information density hits the upper limits allowed by physics. Add more information to it, and the event horizon expands proportionally to what was added. With that in hindsight, it seems rather obvious that the boundary of the event horizon could encode the information once thought to be lost to the black hole inside.
It could do that but what's the evidence that it does? Or has someone proved this is already a feature of semi-classical gravity that just wasn't noticed before? Or is it only a feature of a brand new hypothetical theory?
The other alternative is that the quantum information is somehow converted to some value of the black hole's measurable properties; charge, mass, and spin. We know that isn't the case because the values for these that we can infer from observation are consistent rather than growing faster than expected.
I still don't really understand why the information just can't be destroyed. It seems like we're starting from an assumption that it shouldn't be destroyed despite it being so in semi-classical gravity, and then trying to think of alternative theories which could preserve it such as on the boundary or in its charge/mass/spin. Maybe that's correct but it seems like speculation, and it's not speculation based on any actual contradiction between theory and practice, i.e. not because semi-classical gravity has actually made an incorrect prediction in an experiment we can go out and verify, but only because we have certain preconceptions as to how nature should work which aren't compatible with it. So it doesn't really come across to me as a scientific "problem" but more of a metaphysical one.
It's fundamentally a product of one of our most basic assumptions, that the laws of physics don't change.
When the laws of physics don't change, symmetries arise in the math used to describe them, and each of these invariant symmetries corresponds to a law of conservation we can observe experimentally and an aspect of the universe it renders un-measurable.
Conservation of Momentum is a space-translation symmetry which makes it so that absolute position is unmeasurable, we can only tell where we are in relation to other things. Conservation of angular momentum is a rotation symmetry that does the same thing for direction. There's no "center" to the universe and no "up" or "down" without something to stand on for context, and no experiment we could possibly design can prove otherwise.
Conservation of energy (and therefore mass) arises out of time-translation symmetries. There's no way we can distinguish a particular moment in time from any other without setting a relative "time zero" for comparison, and no possible clock we can build that could be 100% accurate. We have to account for the different rate of time in the atomic clocks in our GPS satellites due to their relative velocity to us on the ground, but the lack of absolute time precision means it can only ever provide an estimate with some range of error.
Exactly how the relativity of spacetime implies a universe with conservation of information would require a lot of math, and a new description of spacetime that breaks these conservation laws would have to explain why it "seems to" adhere to them in all the ways we've tested our reality so far.
If I am not mistaken, information loss inside of a black hole comes out of semi-classical gravity. If these symmetries are tied to the assumption that the laws of physics don't change and the symmetries break down in semi-classical gravity, then does that mean in semi-classical gravity the laws of physics change? Is there a particular example of that in the theory you could provide so I can understand?
I don't disagree that information is conserved in general relativity and quantum mechanics taken separately, but when you put them together it is not conserved, and my concern is that I don't understand why we must therefore conclude that this necessarily wrong and it can't just be that information conservation only holds true for limiting cases when you aren't considering how gravitational effects and interference effects operate together simultaneously. I mean, energy conservation breaks down when we consider galactic scales as well in the case of cosmic redshift.
Yes, we can experimentally verify these laws of conservation, because in practice we can only ever observe gravitational effects and interference effects separately, as a limiting case, and thus far there hasn't been an experiment demonstrating the plausibility of viewing them simultaneously and how they act upon each other. In semi-classical gravity these "weird" aspects like information loss in a black hole only arise when we actually consider them together, which is not something we have observed yet in a lab, so I don't see the basis of thinking it is wrong.
You seem to suggest that thinking it is wrong implies the laws of physics change, but I'm not really sure what is meant by this. Is semi-classical gravity not a self-consistent mathematical framework?
To oversimplify with another example from the theory, assume that planet earth was in superposition between two states with a non-zero separation. Semi-classical gravity says the distribution of the gravity field would be split evenly between the two points, but observing such a state is impossible as it must decohere into 100% of the mass being either in one point or the other. It simply doesn't make sense when we try to apply quantum maths to gravitationally-significant objects because gravity/spacetime isn't a quantum field.
So yes, the predictions made by semi-classical gravity diverge from reality when faced with extreme masses, but that theory was only ever intended to be an approximation. It is useful and consistent with reality under certain ranges of conditions, but we shouldn't jump to the conclusion that physics breaks from all known fundamentals in the presence of large masses when the simpler answer is that this is a case where the approximation is wrong. A more complete theory will be able to accurately explain physics across a wider range of conditions without requiring the untestable assumption that there are places where the rules don't apply. We've got a good reason to believe that the rules of physics don't change in the fact that no matter where we look the rules seem to always have been the same and all prior divergences from the model could be explained by better models.
The problem in physics is that we have two models that describe reality with absurd mathematical precision at different scales but which seem to be fundamentally irreconcilable. But we know they must be, because reality has to be assumed to be consistent with itself.
I understand that in semi-classical gravity the curvature of spacetime is based on the expectation value of the stress energy tensor, and so a massive object in a superposition of two possible location would curve spacetime as if the object was in the middle-point of the two locations, but when the state vector is reduced it would suddenly shift to one of those two points. While this does seem weird, no one has ever physically demonstrated that measuring this is actually possible, so until there is a demonstration that it is actually physically possible to measure this, there isn't actually a contradiction between theory and experimental practice. All we can say is "that seems weird" but that's not a scientific argument against it.
You say it diverges from reality but... how do you know that? No experiment has ever demonstrated this. It could be that this is just how reality works, or it could also be that it's just not physically possible to probe this in the first place, and so it's just a nonphysical quirk of the theory of computing something nonphysical in the first place. We can't say for certain it is wrong until someone actually conducts an experiment to probe it, and if we find it is wrong, then not only would we rule out semi-classical gravity, but we would have the data needed to actually replace it with something.
This is my issue with "fundamental physics" these days in general: they do not actually demonstrate any contradiction between theory and experimental practice. The desire to unify quantum mechanics and general relativity is just based on some preconceptions that information shouldn't be destroyed and gravity should be quantizable like every other force, but how do you know that with certainty? You did not derive this from experimental observation, because semi-classical gravity is currently compatible with all experimental observations. It is more that one begins with a preconception of how they think reality should work and says the theory is wrong because it does not fit those preconceptions. Yes, certain aspects of semi-classical gravity are "weird," but that's not a scientific argument against it.
Because of the influence of Karl Popper, people think science = falsifiability, so new theories are then constructed not based on experimental evidence but by trying to better fit into our preconceptions, but are also made to falsifiable because that is "science." When they are falsified by an experiment that just reconfirms the predictions of semi-classical gravity, they just tweak it a bit so the theory is not falsified by that experiment any longer but still technically falsifiable, and they do this ad infinitum. You end up with decades doing this and what do you have, String Theory that is only applicable to an anti-de Sitter space, a universe we don't actually live in? Or Loop Quantum Gravity which can't even reproduce Einstein's field equations?
Popper has been a detrimental influence onto the sciences. Science is not falsifiability. Science is about continually updating our models to resolve contradictions between the theory and experimental practice. If there is no contradiction between the theory and experimental practice then there is no justification to update the model. I have seen a mentality growing more popular these days which is that "fundamental physics hasn't made progress in nearly a century." But my response to this is why should it make progress? Why have not encountered a contradiction between experimental practice and theory, so all this "research" into things like String Theory is just guesswork, there is no reason to expect it to actually go anywhere.
The same is also true of the so-called "measurement problem" which as physicists like Carlo Rovelli and Francois-Igor Pris have pointed out only arise because we have certain metaphysical preconceptions about how reality should work which when applied to quantum theory lead to absurdities and so people often conclude the theory must be wrong somehow, that it's "incomplete," that it needs to be replaced by something like an objective collapse theory or a multiverse theory or something similar. Yet, this is not a scientific criticism, the theory is in no contradiction with the experimental evidence. We should just get rid of our preconceptions about how reality should work and accept how reality does. As Bohr said: stop telling God what to do.
There is no reason to assume the universe acts the way we'd like it to. Maybe the laws of physics really are just convoluted and break down at black holes. While yes, maybe one day we will discover a theory where it does not break down, it is anti-scientific to begin with an a priori assumption that this must necessarily be the case. It could be that the next breakthrough in fundamental physics even makes the mathematics more convoluted! You cannot just begin with a starting point prior to investigation that this is how nature works, you have to derive that a posteriori through investigation, and currently this is what our best theory derived from investigation states. It may be wrong, but there is no justification in claiming it is wrong without showing a contradiction between theory and experimental practice.
This is my issue here. The desire to replace semi-classical gravity with something else, the measurement problem, the desire to unify all forces of nature into a "theory of everything," trying to solve the "fine-tuning problem," these are all ultimately pseudoproblems because they do no derive from any contradiction between experimental practice and theory. They are not genuine scientific problems. I am not even against people looking into these, because who knows, maybe they will stumble across something interesting, but the issue with treating these all as genuine "problems" is that when they go "unsolved" for a century, it makes it look like there is a "crisis in fundamental physics." There just isn't. In fact, it's quite the opposite: every experimental test reconfirms our current best theories, this is the exact opposite of a "crisis." People pretend like we have a "crisis" because our current theories are too good!
On the contrary, this breaks semi-classical gravity's usage of quantum mechanics. The predictions the approximation makes are not compatible with our observations of how quantum mechanics works, and scientists are working on an experiment that can disprove the hypothesis. ( https://doi.org/10.1103/PhysRevLett.133.180201 )
I'm afraid you've got that precisely backwards. Falsifiability is the core of science, as it is the method by which factually-deficient hypotheses are discarded. If there is no contradiction between the theory and experimental practice then either all false theories have been discarded or we have overlooked an experiment that could prove otherwise.
That's distinctly false. The Higgs Boson was only proposed in 1964 and wasn't measured 'til just 13 years ago.
Because we still have falsifiable hypotheses to test.
We have, actually. The list of unsolved problems in physics on Wikipedia is like 15 pages long and we're developing new experiments to address those questions constantly.
Likewise, there's no reason to assume that the universe is not acting the way we'd like it to except where contradicted by observable evidence. If the laws of physics can "break down" then they aren't "laws", merely approximations that are only accurate under a limited range of conditions. The fact that the universe continues to exist despite the flaws in our theories proves that there must be a set of rules which are applicable in all cases.
And if the rules can change, then our theories will have to be updated to describe those changes and the conditions where they occur.
The paper is interesting and in the right direction but is just a proposal. It needs to actually be performed, because the results can finally point in the right direction rather than just guessing at what the right direction is.
No, it's a justification for pseudoscience by allowing anyone to invent anything out of whole cloth based on absolutely nothing at all and call it "science."
Except it's precisely used to justify them.
Or we just haven't conducted the experiment yet that would contradict with current theories (still talking about GR/QFT here specifically).
I am obviously not defending that position and you know for a fact that is a position that has gained a lot of steam recently, you're just trying to annoyingly turn it around on me to make it seem like I am defending a position I am not by stating something rather obvious.
And this is exactly why you're a promoter of pseudoscience: if a theory is "falsifiable" it's "science" and "needs to be tested," even if it's literally based on nothing and there is no good reason anyone should take it seriously. If I claim there is a magical teapot orbiting Jupiter that is the cause of some of its currently not well-understood weather patterns and if you just built a 20 billion dollar telescope and pointed it at specific coordinates you'd discover the proof, technically you can falsify this claim so by your logic it's "science" and therefore we should go out of our way to investigate it.
Whatever bullshit nonsense or mysticism someone makes up, as long as there is technically some way to conduct an experiment to falsify it, you will say that's "science." Popper has been complete poison to the academic discourse.
Why are you intentionally being intellectually dishonest? We have been talking about a very specific theory and a very specific field of research this whole time, and you are trying to deflect this to science generally. I am sorry I even engaged with you at all, you are not in any way intellectually honest in the slightest and intentionally trying to misrepresent everything I say to "own" me and constantly are trying to pretend my position is something that it is not.
We should just assume the universe is behaving exactly the way we observe it to based on the evidence.
Most laws work like that. Plenty of laws of physics are only applicable to certain conditions.
You want them to apply to cases that currently have not been demonstrated by physically even possible to probe, so you have not even demonstrated it is an actual "case" at all. I am not denying it isn't physically possible to probe either before you dishonestly try to turn my statement around to intentionally misrepresent me as you love to do. I am saying quite the opposite: that we should try to probe the areas that seem to not make much in our current theories. We should be trying to probe quantum effects and gravitational effects at the same time to see how they behave, because that's how we could actually make progress if semi-classical gravity is indeed wrong.
Obviously.
Thanks, your explanation is interesting and makes sense at my level of abstraction.
Eventually i would like it, if some physicist could come up with a cosmology where energy could be created and entropy of a close system could decrease ... in specific conditions and in our present day universe.
Also, in my naive understanding, chaotic pendulums creates information.
Also: The Lagrangian mathematics they use for quantum physics can be used to describe universes like the one you talk about, and if you're interested in things like that then I absolutely have to recommend some novels by the mathematician and science fiction author Greg Egan. It's way easier to start grasping how weird the physics can get when you get a story from the perspective of people who live there:
The Orthogonal Trilogy (2011-2013) is set in a 4d universe where the passage of time is dependent on the direction of travel in space, about a generation ship launched on an anti-timewise loop back around to the near future to develop a solution to an impending apocalyptic crisis of energy creation at the quantum scale.
Dichronauts (2017) is a journey to the end of the world in a universe where time has two dimensions and life evolved as a symbiosis of two creatures that could each experience only one direction in time.
Schild's Ladder (2002) is set in a distant future where an experiment gone awry creates a more stable form of vacuum, creating an event horizon that expands at half the speed of light. 600 years later, a ship studying the event horizon discovers that the complex geometry of the new space behind it harbors intelligent life at a much smaller scale, with their equivalent of microbes being built from the interactions of a veritable zoo of quantum fields rather than molecules and proteins.
Quarantine (1992) explores the copenhagen interpretation of quantum mechanics, set on a future earth where the technology to put the waveform of a human mind into superposition with reality was invented. The user could turn it on, then live all possible lives from that monent until the version of themselves that achieved the result they desired would turn it off and collapse reality back into a single state. This isn't really possible for complicated physics reasons, but if it was then it'd enable seemingly impossible things to become true. The novel explores the consequences of such a future conflicting with the existence of alien species that evolved within superpositioned reality and can't survive when it's collapsed into a single unique state.
This is very complicated, let me sleep on it, i will come back to your comment later.
"Information" in the quantum sense refers to the waveform of the quantum system as a whole, which is kind of a weird thing to get one's head around.
Even in the case of chaotic pendulums, there's no theoretical principle that keeps us from observing and accounting for every particle and quanta of energy involved and using that to prove that the waveform of the entire pendulum is consistent with itself and the expected evolution from previous states.
But the event horizons of black holes seem to break that rule, because the waveforms of black holes can be described with just three properties; mass, charge, and spin. There didn't seem to be "room" for them to encode all the waveforms of anything that falls inside until Stephen Hawking theorized that it could be saved in polarization states of the event horizon boundary and black holes would gradually radiate it away.
i like science and its immense benefits in most fields, still, i resist some theories and ideas.
Thanks again for your clear explanations ... but i will not be scientific on this ! ... and will prefer my "feelings" instead of the scientific consensus. Considering this, it's a good thing I'm not working in this field 🤣