Is Hawking Radiation a Kinematic Effect or a Real Phenomenon?

[ChrisVer]

Has the Hawking radiation ever been observed from bodies like black holes or in laboratory? From what I saw, it hasn't...
1. What are the difficulties?
2. Why are we using Hawking Radiation in some reasonings, since we haven't observed it really happening?

 

[bapowell]

Hawking radiation is much colder than the CMB. For a few stellar mass black hole, the temperature of the Hawking radiation is around [itex]10^{-8}[/itex] K. If the Hawking effect is to be observed at all, it is expected that laboratory experiments involving analog condensed matter systems are the way to go. For example, see: http://www.technologyreview.com/view/420940/first-observation-of-hawking-radiation/. According to wikipedia, though, whether this constitutes "genuine confirmation" (whatever that is) remains in doubt.

As for why we reason about it, it's a striking prediction of semiclassical gravity. Of course, we shouldn't be basing further science on it until it's observed...do you get the sense that this is happening? My personal suspicion is that much of the paradoxical and frenzied discourse of late regarding black hole firewalls and the information paradox has arisen because at bottom we are using a spliced, semiclassical, and ultimately incomplete theory.

 

[wabbit]

My impression perhaps wrongly is that the case is even stronger, i.e, it is an unavoidable consequence of the symmetries of QFT and the effect of horizons and acceleration - so that, while it could be wrong of course, it seems that in the absence of contrary evidence, the most reasonable course is to tentatively assume it is correct at least broadly - not only for black holes but for all horizons actually, including cosmological.

 

[bapowell]

My impression perhaps wrongly is that the case is even stronger, i.e, it is an unavoidable consequence of the symmetries of QFT and the effect of horizons.

True, but the horizons (and apparent horizons in the case of accelerated observers) are still classical gravity.

 

[wabbit]

True, although in the case of a Rindler horizon, the Unruh radiation doesn't depend on a theory of gravity as far as I can tell - or even a de Sitter horizon, which can be viewed as purely geometrical (at least this is how I read it in this presentation : http://ned.ipac.caltech.edu/level5/Sept02/Padmanabhan/Pad9.html) and occurs in the absence of gravitating mass - both can be interpreted as gravity though but it seems to me mainly because geometry can be read as gravity in the classical theory.

It is a matter of perspective though, at the end this isprobably more about how psychologically convincing the argument is from one or the other viewpoint, obviously there isno concrete evidence.

 

[Vanadium 50]

Has the Hawking radiation ever been observed from bodies like black holes or in laboratory?

Let's start with the basics. How many black holes do you think are in laboratories?

 

[bapowell]

Let's start with the basics. How many black holes do you think are in laboratories?

Emphasis on the word "or" I believe.

 

[Vanadium 50]

Dadgummit, I give up.

The point is to try and get the OP to think clearly, critically, and to provide him enough information and hints to try and work it out for himself. Instead, it's off to the races - who can post a message that's over the head of the OP first?

 

[PeterDonis]

in the case of a Rindler horizon, the Unruh radiation doesn't depend on a theory of gravity

It does in the sense that it is derived in flat spacetime.

or even a de Sitter horizon, which can be viewed as purely geometrical

This is true of any horizon: it's a geometrical feature of the spacetime (basically, a null surface that is picked out as causally separating two regions of interest). The only difference with a Rindler horizon, as opposed to a de Sitter horizon or a Schwarzschild horizon, is that the two regions of interest are determined by the state of motion of some particular observer, instead of by an overall symmetry of the spacetime geometry.

 

[PeterDonis]

or in laboratory?

Trying to observe Hawking radiation itself in a laboratory would basically require creating a black hole in the laboratory. As V50's question in post #6 hinted, that's not likely to happen any time soon.

The laboratory observations that bapowell referred to aren't observations of Hawking radiation, IMO; they're observations of something that (assuming their interpretation of what is happening is true) has some of the same properties as Hawking radiation. (The "event horizon" they talk about is not a true event horizon, because it is based on the speed of light in the material, not in vacuum.)

Why are we using Hawking Radiation in some reasonings, since we haven't observed it really happening?

Because we are trying to work out the implications of various possible ways that GR and quantum field theory could be combined. This is important theoretically even if we can't test many of the theoretical models directly by observation.

 

[ChrisVer]

Let's start with the basics. How many black holes do you think are in laboratories?

Quoting from wiki article on Hawking radiation:

In September 2010, however, an experimental set-up created a laboratory "white hole event horizon" that the experimenters claimed was shown to radiate Hawking radiation

http://en.wikipedia.org/wiki/Hawking_radiation#Experimental_observation_of_Hawking_radiation

And that's why i added the laboratory scheme into the question for discussion. Of course in the article it states that this claim is still in doubt...

Because we are trying to work out the implications of various possible ways that GR and quantum field theory could be combined. This is important theoretically even if we can't test many of the theoretical models directly by observation.

Well that question was for the statement we use to say that possible black holes produced in the LHC (although I'm not a supporter of the idea of BH creations, but Oh well, I have read answers about it here and there), will evaporate through Hawking radiation almost instantly... This reasoning didn't sound right to me, since we take for granted that Hawking Radiation is a physical phenomenon that can "save" the day.

 

[PeterDonis]

that question was for the statement we use to say that possible black holes produced in the LHC (although I'm not a supporter of the idea of BH creations, but Oh well, I have read answers about it here and there), will evaporate through Hawking radiation almost instantly

Well, the hypothesis that the LHC could form black holes in the first place is also based on combining quantum field theory with GR; any process in the LHC that could theoretically form black holes is not a purely classical process. So it's no more of a theoretical stretch to claim that any such holes that were formed would evaporate by Hawking radiation, than it is to claim that they could be formed in the first place. Neither process has been observed, and the same theoretical foundation predicts both.

 

[haael]

Hawking radiation analog has been observed in laboratory in sonic black holes.

 

[PeterDonis]

Hawking radiation analog

The bolded word is crucial. Whether this is even relevant to actual Hawking radiation depends on how valid you think the analogy is. Opinions on that are, to say the least, varied.

 

[haael]

The OPs question was: Has the Hawking radiation ever been observed from bodies like black holes or in laboratory?
Sonic black hole is a "body like a black hole". Hawking radiation has been observed from it in laboratory. I guess this answers the OPs question.

 

[PeterDonis]

Sonic black hole is a "body like a black hole".

That depends, once again, on how valid you believe the analogy is between "sonic black holes" and real black holes.

Hawking radiation has been observed from it in laboratory.

Same comment here; the analogue of Hawking radiation has been observed, but that's only relevant to actual Hawking radiation to the extent you think the analogy is valid.

I guess this answers the OPs question.

That depends on whether the OP is interested in analogues to black holes, or just actual black holes.

 

[ChrisVer]

What is the actual analogy by the way?

 

[PeterDonis]

What is the actual analogy by the way?

Wikipedia has a good brief summary, along with some good links to references:

http://en.wikipedia.org/wiki/Sonic_black_hole

 

[haael]

In short, fluid is a model of spacetime, speed of sound is analogous to the speed of light and the relative speed of the fluid models the spacetime geometry. If you make the fluid moving (accelerating) in such a way so it passes the speed of sound at some moment, then you have a model of event horizon.

Once I thought that thermal fluctuations of the fluid are a model of quantum fluctuations of spacetime. Now I know it is not. To have the proper analogy you have to use very cold superfluid and the analog of quantum fluctuations are the actual quantum fluctuations from the fluid.

Why does it work? Mainly because the equations that govern gravity are very similar to the equations describing propagation of sound in a moving fluid.

 

[ShayanJ]

If we define Hawking radiation as the radiation from a black hole, then yes, those laboratory experiments are just showing analogs.
But in one of the papers I was reading, the author argued that Hawking radiation is a(I would use "can be defined as" instead of "is") kinematical effect. Meaning that we can have it anywhere that there is a one-way-barrier. I think its better to accept the more general definition.Of course if there is something in the math of the Hawking radiation that makes it inseparable from gravity, then we can't use the more general definition but as far as I know its not like that. Is it?

 

[PeterDonis]

in one of the papers I was reading

Which one? Can you give a link?

Hawking radiation is a(I would use "can be defined as" instead of "is") kinematical effect. Meaning that we can have it anywhere that there is a one-way-barrier.

But there's a difference between a "barrier" that's one-way for some things but not for others (which is what the "barrier" in the "sonic black hole" is) and a "barrier" that's one-way for everything. Only the latter is a causal boundary. So if the barrier being a causal boundary makes a difference, then Hawking radiation from such a boundary is different from its analogue from boundaries that are not causal.

 

[ShayanJ]

Which one? Can you give a link?
But there's a difference between a "barrier" that's one-way for some things but not for others (which is what the "barrier" in the "sonic black hole" is) and a "barrier" that's one-way for everything. Only the latter is a causal boundary. So if the barrier being a causal boundary makes a difference, then Hawking radiation from such a boundary is different from its analogue from boundaries that are not causal.

I'm really sorry. It wasn't about this. It was another paper which said Hawking radiation is a kinematic effect because it can arise in any Lorentzian geometry with a Horizon even if its not a solution to Einstein's equations. I just mixed everything up in mind!