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Black Hole Information - What's the Problem?


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A summary of the black hole information loss problem and solution attempts, aimed at an audience mostly of philosophers of physics.

Published in: Science
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Black Hole Information - What's the Problem?

  1. 1. Black Hole Information What’s the Problem? Sabine Hossenfelder
  2. 2. What is a black hole • Mathematically: It’s a solution to the field equations of general relativity with an event horizon. • But that’s an unphysical definition because the event horizon must be eternal. We will never be able to establish its existence! • Physically, therefore, a black hole is what looks like a mathematical black hole for some while. • This means it’ll have an “apparent horizon” or “trapped horizon” which isn’t necessarily eternal
  3. 3. Black Holes are Real • Black holes were originally thought to be mathematical curiosity, a solution that can’t be created with physically realistic initial conditions. • Singularity theorems: Turned out the opposite is the case! Black holes are hard to avoid with physically realistic initial conditions. • Observational evidence confirms their existence: Very compact dim objects that don’t seem to have a hard surface • We are presently waiting from data from the EHT that’s supposed to show the shadow of Sag A*
  4. 4. What’s so interesting about black holes? • For the experimentalist: An extreme environment that allows precision tests of general relativity and particle physics at high energies/densities • For the theorist: Black holes bring together many different areas of physics: gravity, particle physics, thermodynamics, stat mech, quantum gravity, quantum information… • For the public?
  5. 5. The Singularity • At the center of the black hole is a singularity • At that point, curvature and energy-density is infinitely large • This is widely believed to be unphysical and a mathematical artifact • When the curvature/density reaches the Planck scale, quantum gravity should become important. GR breaks down. • In a fully consistent theory, the singularity should be absent
  6. 6. Black Hole Thermodynamics • Black holes have an entropy proportional to the surface area • They have a temperature inversely proportional to the radius • A body with a temperature must be able to radiate • Hawking showed that indeed black holes emit particles • This Hawking-effect is due to the quantum effects of matter, gravity is not quantized The black hole temperature for solar-mass and supermassive black holes is tiny, below even the CMB temperature. It is unobservable and will remain so for the foreseeable future.
  7. 7. The Black Hole Information Loss Problem • Hawking radiation carries energy away from the black hole • The black hole shrinks. As it shrinks it heats up. Eventually it’s gone • Hawking radiation does not carry information besides the temperature • This means the endstate of the evaporation is always the same (for black holes of the same initial mass) • Black hole evaporation is fundamentally irreversible. • It is the only such process that physicists know of and it’s incompatible with quantum theory.
  8. 8. Is it a paradox? • It’s not a paradox in the sense that we know what destroys information: The singularity • The singularity spoils the irreversibility (pretty much by definition) because it’s the same infinity regardless of the initial state • The horizon is not the problem. The horizon is merely where information becomes practically unavailable. That’s inconvenient but nothing paradoxical about that. • If the singularity is removed, this means that information can’t be destroyed. But that in itself doesn’t help: The problem is to find out what happens with the information.
  9. 9. Solution Attempts 1. Denial: Nothing falls in/black holes don’t form 2. Hope: Information comes out with radiation 3. Desperation: Remnants, stable or quasi-stable 4. Acceptance: Non-unitarity
  10. 10. 1. Denial: Black holes don’t form • Requires strong deviations from general relativity in regimes we have tested. • Extremely implausible. • Most papers on this “solution” are wrong.
  11. 11. 2. Hope: Information comes out • Information starts leaking out long before the Planckian quantum gravitational phase • Unclear how that can happen. Requires some kind of non-locality or causality violation • Presently the most popular solution because supported by the gauge/gravity duality
  12. 12. 3. Depression: Remnants • Information just stays in the black hole • Either eternally (stable) or for a very long time (quasi-stable) • This requires that the black hole entropy does not count microstates of the black hole but merely what’s accessible from the outside • Has been criticized on the grounds of enabling infinite pair production but these complaints are unfounded: In this regime quantum gravity actually is strong • Remains unpopular because nothing can be calculated
  13. 13. 4. Acceptance: Non-unitarity • Black holes can be created in virtual processes. If their decay violates unitarity, in principle all processes could • Unitarity is an assumption to quantum field theory that we use, and it would no longer be justified. Then what? • But it’s unclear how bad violations of unitarity would be • This solution has never been ruled out but is even more unpopular than remnants
  14. 14. 5. – 3.320 • Loads of other solution attempts
  15. 15. What’s the Black Hole Firewall • Equivalence principle requires infalling observer doesn’t notice anything at the horizon • Paper in 2012 claimed that if information is in the outgoing radiation (early, long before Planck phase), then observer must notice because the state can no longer be vacuum • Instead of being empty, there’s a “firewall” at the horizon that burns the observer • Big headache for string theorists, hence the attention • Imo, the claim is plainly wrong
  16. 16. My Conclusion Math alone will not solve the problem