[zadler]

Isn’t that What he called Hawking radiation? Didn’t think it was news at this point.

It’s funny how we talk of information “not being lost” since it’s emitted as radiation... would we be able to decypher anything? If not, it’s still lost.

[ben_w]

Hawking Radiation, in its original form, did not allow anyone to learn the quantum information about what went into the black hole — if it was made entirely of photons; or entirely of antimatter; or made of any mix of photons, normal matter, and antimatter… all would be indistinguishable.

What’s news is that it isn’t all lost, that the starting state has any influence at all on what comes out. I am only an enthusiastic amateur at this, but I get the impression this is as surprising as the discovery of Hawking Radiation in the first place.

[cyphar]

> but I get the impression this is as surprising as the discovery of Hawking Radiation in the first place

It should be noted that we have yet to experimentally observe Hawking radiation (it is so low-power we don't have the means to observe it). People talk about Hawking radiation as though it is a discovered phenomena -- it isn't. It's a prediction based on our understanding of quantum mechanics and some thinking about how it interacts with event horizons -- and observing it would help reinforce our understanding of the underlying theories.

[Sharlin]

Indeed no existing stellar-mass-or-above black hole is a net emitter of radiation in the current cosmological era. They swallow cosmic microwave background photons at a much higher rate than they emit Hawking radiation and thus grow slowly even if infalling matter is not present. Indeed, they're the best heat sinks in existence. The universe will be billions of times its current age before the CMB has redshifted enough to become cooler than black holes.

Hypothetical primordial blackholes [1] could be much smaller and hotter, and indeed Hawking radiation could provide one way to detect those that have survived to present day, but as of now none have been found.

[1] https://en.wikipedia.org/wiki/Primordial_black_hole

[raattgift]

ETA: I don't think we disagree at all. I slightly misread your second sentence before hitting "reply". I'll keep this reply in place because I think it amplifies and adds to your point.

Hawking radiation is always present around a dynamical black hole -- it is produced by the dynamical spacetime itself[1]. (All black holes that aren't eternal -- that includes any that form by gravitational collapse of matter -- are dynamical, and thus have Hawking radiation, even while they're growing.)

In principle we should be able to detect Hawking radiation as black holes first form, since it will backreact with the black hole, and probably even interact with the collapsing matter. Studying BH-forming supernovae and the like will lead to discoveries in this difficult area of https://en.wikipedia.org/wiki/Semiclassical_gravity as hot Hawking radiation is in principle directly observable, and there will be indirect traces.

The problem is that BH formation typically happens in a bright environment. The candidate black holes we know about aren't that young and as a result the Hawking gas will be cold enough to have negligible impact: basically no interaction with nearby matter, basically no backreaction on the black hole itself, and much colder than the surrounding environment (infalling matter including the CMB gas) and thus in practice impossible to detect directly with telescopes.

- --

[1] Well, more precisely, given Einstein-Maxwell electrovacuum and general relativity with a black hole metric, Hawking radiation is inevitable. Hawking's original work dealt with a static spacetime (i.e., an eternal, unchanging black hole) and used negative energy quanta as a trick to proxy for a dynamic spacetime. Using a dynamical black hole (i.e., one that grows and shrinks), one does not need negative energy quanta at all, much less a mechanism which tosses only those halves of pairs into the BH (in order to keep the metric unchanged from pure static Schwarzschild).

[Sharlin]

> The problem is that BH formation typically happens in a bright environment.

That must be the understatement of the week. Love it! I guess observations of failed supernovae and possible direct-collapse black holes could shed some light (hah!) on the matter.

[thx4allthestuff]

I really want to shout out “infinity billion” right now, but I’m not going to do that because it would be childish and silly. (strains)

[coldtea]

>Isn’t that What he called Hawking radiation? Didn’t think it was news at this point.

No, that's emitting random particles (or antiparticles) from pairs created near the horizon, where one falls in and the other escapes.

That's not getting the information that falls in out.

[grkvlt]

No. As I understand it, it is exactly that - the Hawking radiation is how the information escapes.

EDIT - See https://en.wikipedia.org/wiki/Black_hole_information_paradox for more details, and note that I may well be wrong...!

[raattgift]

> I may well be wrong

Unfortunately, you are. The information that is lost is what went into the black hole in the first place.

It's not strictly a quantum problem: if black holes have no hair, then we cannot tell by looking at a spherically symmetric non-rotating black hole if it was formed by one spherical shell of infalling matter of mass M, or two concentric spherical shells of infalling matter of mass M/2, or three of M/3, etc. When we add electromagnetism to the picture, we get Hawking radiation inversely proportional to the black hole mass; but that mass does not encode the number of shells or their composition, just their total mass.

When we add in quantum electrodynamics, we find that Hawking radiation has a thermal spectrum (so, cold photons for a stellar-mass black hole, but when the black hole is very small you'll get electrons and positrons too; and potentially the whole zoo of particles if we use the full standard-model as the quantum field theory). But we could start with a black hole formed by squashing together neutral composites (positronium, atoms) and with some probability get out nothing but photons: no massive particles at all. With some smaller probability we get mostly photons but also electrons and positrons. The main problem is that we are stuck talking probabilistically about the spectrum Hawking quanta even if we know every single detail of what we threw into the black hole; there is no unitary evolution from known-in-every-detail state to known-in-every-detail state. The "every detail" part is the information that is lost.

There are a variety of ways one can try to deal with the conversion of "we know every detail" (a pure state, quantum mechanically) to "we can only talk probabilistically" (a mixed state, quantum mechanically), and some are listed in the wikipedia page you link to. Hawking's final paper is yet another approach, and throws away the idea that black holes have no hair; that is, a black hole cannot be described with a small number of parameters (dominated by mass and angular momentum) but rather develop an enormous number of parameters encoded as perturbations of the vacuum. Those perturbations in turn influence the spectrum of the Hawking quanta in such a way that it is fully predictable -- even though it looks like a thermal bath, the vacuum perturbations ("soft hairs") fully determine it. It an idea is worth further investigation, but is not much more compelling than several alternatives.

One problem is that when we take an exact analytical black hole solution to the Einstein Field Equations of general relativity, we have "no hair" as a mathematical theorem. If we perturb around such a solution we generate observables that closely match what we see of candidate astrophysical black holes in the sky. Hawking wants to treat astrophysical black holes as even more different than the theoretical models, and while that's not a crazy idea, it's also not very parsimonious as many many many more perturbations ("hairs") are necessary than the minimum required to match the observed systems, and it's not clear that a "no hair" black hole must be measurably different from a "soft hair" black hole.

(More detail here https://news.ycombinator.com/item?id=18327614 )