Thread

Gravity seems holographic. What does that mean for reality?

300 points · 234 comments · ibobev

haitchfivee · 1790354524 · focus · HN ↗
My nitpick is with something the author said in passing about Hawking. Yes, around 1970 Hawking and the scientific community at large still used to believe nothing could escape a black hole, and between 1971 and 1973 Hawking along with Penrose and others expanded their evolving understanding and consequences of of Albert Einstein's breakthrough work.
But then Hawking argued back the opposite case, which in simple terms means that some things can, indeed, escape an event horizon. In fact, name dropping Hawking just to mention his views as they stood before his 1973 work, and his 1974 model of Hawking radiation seems dangerously incomplete journalistic reporting.
In 1974 Hawking formulated what we now know as Hawking Radiation, which is the *exact opposite* of what was previously believed about the impossibility of anything coming back from a black hole. Whilst still not conclusively proven as a physical fact, this did win him the Special Fundamental Physics Prize in 2013. specifically for his discovery of Hawking radiation from black holes and his deep contributions to quantum gravity and quantum aspects of the early universe.
1. glenstein · 1790354877 · focus · HN ↗
  I find myself not agreeing with a lot of nitpicks, but this is a good one! The article says "Contrary to what Stephen Hawking argued, black holes are not inescapable prisons." But information escaping black holes is named after him! He definitely, in the full scope of his career, was perfectly agreeable to the idea of information escaping black holes.
  1. parineum · 1790357800 · focus · HN ↗
    Those claims are backwards.
    Hawking found that energy/mass can escape but the information remains destroyed. You can't run a black hole backwards in time and reconstruct the matter that created it and hawking radiation bears no resemblance to anything that was ever beyond the horizon.
    glenstein · 1790568650 · focus · HN ↗
    Shucks, you're right. Well, mostly. I misunderstood Hawking radiation as a solution when it actually contributed to the motivation for holding that info was not recoverable. However, he did famously "concede" a bet on the question that he believed was resolved against this belief and it's still the case that his belief by the end of his life was not that it was a "prison" the way the article suggests.
2. Maxatar · 1790356614 · focus · HN ↗
  Not sure I follow your comment exactly, perhaps I am misunderstanding it but Hawking radiation doesn't involve anything escaping a black hole.
  The radiation never originates from within the event horizon. Furthermore the radiation itself does not encode any information about what fell into the black hole apart from the electric charge, angular momentum, and mass (all of which must remain conserved).
  1. pixl97 · 1790356895 · focus · HN ↗
    The only caveat that I'd ask about is the death of a black hole after it evaporates away too much mass. If that explosion does occur, is that matter/energy that was from inside the singularity itself?
    nine_k · 1790358170 · focus · HN ↗
    AFAICT nobody has a consistent theory proven by observations. Some theories offer an explosion in which the singularity vanishes, but this has trouble with information / entropy. Some say a microscopic remnant stops radiating because the original assumptions of Hawking radiation mechanism do not apply at near-Planck scale. Some say that as the curvature grows with the diameter shrinking, the black hole pops off from "our" spacetime and forms a parallel universe, without an explosion.
    The current age of the Universe is way small compared to the time needed for evaporation, so observing anything like that happening (or not) is hardly possible.
    GoblinSlayer · 1790415383 · focus · HN ↗
    At Planck scale it becomes a big particle, and such particles are known to be unstable and quickly decay.
  2. nine_k · 1790356980 · focus · HN ↗
    Indeed All Hawking radiation takes from a black hole is some mass. It's a bit like radiation resulting from annihilation of matter and antimatter: it does not "escape", it "gets born" in the process.
    empath75 · 1790357852 · focus · HN ↗
    I'm not sure that is a meaningful distinction. All a black hole _is_, is mass. If you take away the mass, you are taking away the black hole.
    nine_k · 1790358279 · focus · HN ↗
    It's also information / entropy, hence the <a href="https://en.wikipedia.org/wiki/Black_hole_information_paradox" rel="nofollow">https://en.wikipedia.org/wiki/Black_hole_information_paradox
    db48x · 1790359233 · focus · HN ↗
    And electric charge, which is conserved!
  3. empath75 · 1790357778 · focus · HN ↗
    The entire blackhole will eventually evaporate, so yes it is literally escaping from the black hole.
    Maxatar · 1790374324 · focus · HN ↗
    We can play with words all day, but in the sense of a particle entering into the event horizon of a black hole and then at some point in the future exiting from inside of the event horizon, that simply never happens and Hawking radiation does not propose that.
  4. arcticfox · 1790357896 · focus · HN ↗
    I don't know the slightest about this but... That list looks like it is supposed to be limited but that sounds like infinitely more information than zero?
    palmotea · 1790362112 · focus · HN ↗
    >> Not sure I follow your comment exactly, perhaps I am misunderstanding it but Hawking radiation doesn't involve anything escaping a black hole.
    >> The radiation never originates from within the event horizon. Furthermore the radiation itself does not encode any information about what fell into the black hole apart from the electric charge, angular momentum, and mass (all of which must remain conserved).
    > I don't know the slightest about this but... That list looks like it is supposed to be limited but that sounds like infinitely more information than zero?
    I don't know about electric charge, but can't you observe mass and angular momentum of a black hole from outside? I think the "no information" thing means the Hawking radiation won't let you tell you if the black hole was made from a dead star or just a really big pile of meat or what's going on inside of it.
    IIRC, all Hawking radiation does is provide a mechanism for a black hole to eventually decay.
    reichstein · 1790416547 · focus · HN ↗
    Hawkings radiation doesn't tell you anything about the contents of the black hole, just that it lost energy/mass. That reduces the event horizon, so there is now a (miniscule) amount of space that has become "visible". It'll be empty space, no extra mass is leaked in that way.
  5. wormius · 1790362015 · focus · HN ↗
    It's been ages since I read it and I am 99% sure I'm remember horribly wrong, but I was thinking it's something like this? Also - your "the only information" is precisely what we can measure of quantum states, so information is still preserved. That's more a limit of what we know?
    I think that with Hawking radiation you have quantum entanglement of two pairs. You have a matter-antimatter pair production near the horizon. When the antimatter falls into the blackhole without being destroyed via mutual annihilation, the matter particle at the boundary now "radiates" away. HOWEVER.
    The matter inside boundary which the anti-matter DOES interact with is destroyed, and the information of that interaction is is "passed" to the outside via the the anti-matter's entanglement with its co-created pair.
    Something like that, isn't it? Please any physicists clarify :)
    cyberax · 1790373447 · focus · HN ↗
    > I think that with Hawking radiation you have quantum entanglement of two pairs. You have a matter-antimatter pair production near the horizon. When the antimatter falls into the blackhole without being destroyed via mutual annihilation, the matter particle at the boundary now "radiates" away. HOWEVER.
    That's not how Hawking radiation actually works. It's just a "lies-to-the-children" explanation. Antiparticles have positive mass, so the black hole would be _growing_ as a result. This mechanism also violates the equivalency principle, as you shouldn't be able to detect the event horizon with only local observations.
    Reality is more interesting. In flat space, the ground state of quantum fields is actually non-zero. But black holes "bias" the fields in such a way that their ground state becomes _negative_ from the viewpoint of a remote observer.
    This in turn permits it to create new particles from just nothing. This in turn adds _negative_ energy to that space, resulting in the black hole decreasing in mass.
    But crucially, the event horizon itself is NOT involved. Particle creation can happen anywhere in the vicinity of the black hole, not just on the event horizon.
3. metalliqaz · 1790357182 · focus · HN ↗
  Please correct my understanding as I am not a scientist, but why do you assert that Hawking radiation "escapes" or "comes out of" a black hole? Nothing crosses the event horizon. Isn't it more correct to say that Hawking radiation is produced by the extreme warping of the spacetime very near the event horizon, and the black hole shrinks to conserve the energy? Black holes can also shrink as gravitational waves are produced, but we don't say that the waves "escape" the black hole.
  1. rhdunn · 1790361541 · focus · HN ↗
    My understanding is that Hawking radiation is where:
    1. a particle/anti-particle pair is created at the event horizon;
    2. the particle is on the outside edge of the event horizon, so "escapes" the black hole;
    3. the anti-particle is on the inside edge of the horizon, so decreases the size of the black hole due to particle/anti-particle annihilation (with a corresponding particle on the inside of the black hole).
    metalliqaz · 1790362861 · focus · HN ↗
    This is often used as an explanation in pop-sci materials, but it isn't particularly accurate, IIRC.
    For example, Hawking radiation is always photons, with the wavelength corresponding to the size of the black hole. That isn't explained purely by virtual particles.
    Also, for that explanation to work, it would always have to be an anti-matter particle that is selected to cross the event horizon, and I don't see how that would be the case.
    auntienomen · 1790364749 · focus · HN ↗
    Minor quibbles: Hawking radiation has a blackbody spectrum, so we get a variety of wavelengths, with a peak determined by the size of the hole. The outgoing particles should eventually include any kind of particle that can be created in the Standard Model, not just photons and gravitons.
  2. auntienomen · 1790364189 · focus · HN ↗
    Strangely, it is _not_ the instrinsic curvature of spacetime that produces Hawking radiation. There's a variation on Hawking radiation -- called Unruh radiation -- experienced by accelerated observers in flat spacetime. Any two observers will agree on the value of a field (e.g. the electric field) at a point, but an accelerated observer will experience empty spacetime as a _thermal bath of particles_, with a temperature proportional to the acceleration. Relatively accelerated observers don't agree on what the vacuum is.
4. EA-3167 · 1790359520 · focus · HN ↗
  You’re missing a big issue here… Hawking Radiation doesn’t escape from within the event horizon, and there’s no physical process preventing that. It’s radiation from the surface of the black hole, not the interior.

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