This might be one of the worst, least-descriptive titles I’ve ever seen
I generally always have an issue with headlines that say something could happen, too.
“New research could bring about the total destruction of earth.” Could have been a headline talking about the LHC making microscopic black holes.
Steffen Gielen, Lucía Menéndez-Pidal.
Black Hole Singularity Resolution in Unimodular Gravity from Unitarity
Physical Review Letters, 2025; 134 (10)
DOI: 10.1103/PhysRevLett.134.101501i can’t even understand the very beginning of the discussion : why are physicists so obsessed with “information loss” ?
It has long been stated that a quantum theory of black hole dynamics that is required to be unitary must deviate strongly from semiclassical expectations. Usually this is discussed in the context of unitarity of black hole formation and evaporation, leading to the famous issue of information loss.[24]*
[24]* : S. W. Hawking, Breakdown of predictability in gravitational collapse
Phys. Rev. D 14, 2460 (1976).I get why this feels confusing—“information loss” isn’t exactly an everyday concept! Let me break it down:
Physicists are fascinated by the idea of “information loss” because it challenges one of the core principles of quantum mechanics: unitarity. In simple terms, unitarity means that the total information about a system (like the state of particles in the universe) must always be preserved, even if the system changes over time. You should, theoretically, be able to trace backward and recover all information about the system’s past, no matter what has happened.
Now, here’s where black holes come into play: when something falls into a black hole, classical physics tells us that the information about it seems to disappear forever. This creates a tension between general relativity (which governs black holes) and quantum mechanics, which insists that information can’t just vanish. This mystery is called the black hole information paradox.
The “information loss” problem specifically arises during the process of black hole evaporation through Hawking radiation. Stephen Hawking proposed that black holes emit radiation over time due to quantum effects at their event horizons. Eventually, they can shrink and vanish completely. But here’s the kicker: Hawking radiation is seemingly random and doesn’t carry information about what originally fell into the black hole. So, when the black hole disappears, does the information just… go poof? That would violate unitarity!
This paradox has huge implications for how we understand the universe and its laws. If information is lost, it means we need to rethink some foundational ideas in physics. But if information isn’t
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?
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If there is no hard evidence yet I have no reason to believe. Go find the evidence, then I will believe it.
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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.
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.
My buddy is a PhD candidate and he’s heavily involved in black hole research, I’m not even going to try to summarize it because when he talks to me it’s like me watching a finger puppet play.
One of the things he talks about is how The jury is still out on if energy is even real.
(…) if energy is even real.
at my level of comprehension : https://libquotes.com/aristotle/quote/lbd3b5p
To say of what is that it is not, or of what is not that it is, is false, (…)
→ Aristotleif you want to get serious, read @[email protected] in this post.
Yes, that’s how science usually works
