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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" ?

    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.

  • Water might be wet, updates to follow as this story develops. Over to you, Pat.

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