Unruh Radiation and Black Holes

[sbrothy]

The level may be higher that Intermediate but I use that in the hope that I'll be able to understand the answer.

I moved it here as it didn't really belong in the other thread. Feel free to move it further.

I admit I haven't yet read the paper in it's entirety but I hope you'll bear with me. I am under the impression that the Unruh effect, or at least Unruh radiation, is somewhat disputed. Looking it up on wiki (Yes I know wiki isn't really a reliable source for anything.) seems to verify my doubt. Still the authors of this paper seems to almost take it for granted (and then goes on to compare it with black hole radiation). But doesn't the Unruh effect has to do with acceleration? How does black holes enter the picture here?

Admittedly, the paper is from physics.gen-ph (General Physics), but I'm not sure if this is an indicator of less serious work.

If you think the explanation will go over my head feel free to tell me that and/or delete the post. I suspect I wont understand but I might learn a thing or two from your ascerbic comments :P


TIA,
Søren

Poincaré invariance, the Unruh effect, and black hole evaporation

"In quantum field theory, the vacuum is widely considered to be a complex medium populated with virtual particle + antiparticle pairs. To an observer experiencing uniform acceleration, it is generally held that these virtual particles become real, appearing as a gas at a temperature which grows with the acceleration. This is the Unruh effect. However, it can be shown that vacuum complexity is an artifact, produced by treating quantum field theory in a manner that does not manifestly enforce causality. Choosing a quantization approach that patently enforces causality, the quantum field theory vacuum is barren, bereft even of virtual particles. We show that acceleration has no effect on a trivial vacuum; hence, there is no Unruh effect in such a treatment of quantum field theory. Since the standard calculations suggesting an Unruh effect are formally consistent, insofar as they have been completed, there must be a cancelling contribution that is omitted in the usual analyses. We argue that it is the dynamical action of conventional Lorentz transformations on the structure of an Unruh detector. Given the equivalence principle, an Unruh effect would correspond to black hole radiation. Thus, our perspective has significant consequences for quantum gravity and black hole physics: no Unruh effect entails the absence of black hole radiation evaporation."

 

[Demystifier]

In my opinion, the relation between Unruh effect and Hawking radiation is not understood well enough in the literature. Very recently I wrote a paper in which I argue that Hawking particles are physical only far from the horizon, where the existence of Hawking particles does NOT rest on the Unruh effect.
https://arxiv.org/abs/2405.05617

 

[sbrothy]

In my opinion, the relation between Unruh effect and Hawking radiation is not understood well enough in the literature. Very recently I wrote a paper in which I argue that Hawking particles are physical only far from the horizon, where the existence of Hawking particles does NOT rest on the Unruh effect.
https://arxiv.org/abs/2405.05617

Sorry for only coming around to reading your paper now. It makes a kind of intuitive sense to me. Espescially related to the original paper. Unfortunately I’ve been scre… by my intuition before. I’ve learned that intuition makes for a bad guiding light, and to be brutally honest my level of education does not qualify me to comment either positively or negatively on your work. It was interesting nonetheless though.

I appreciate you taking time out of your schedule to educate me a little.

 

[PeterDonis]

I am under the impression that the Unruh effect, or at least Unruh radiation, is somewhat disputed.

The Wikipedia article is not saying that the theoretical prediction of Unruh radiation is not disputed. What are disputed, according to the article, are claims to have experimentally observed the effect. Since the effect is extremely tiny for accelerations we can reasonably achieve with our current technology, it is to be expected that experiments trying to detect it will be right at the edge of our capability and will not give clear-cut results.

 

[PeterDonis]

Poincaré invariance, the Unruh effect, and black hole evaporation

I note that one of the authors of this paper is Deur, who has published a number of papers arguing for the presence effects in GR that are not generally accepted or taken into account (and quite a few of them have been discussed in previous PF threads). I don't know if this line of research of his has any connection with that other research, but I wouldn't be surprised if it did.

As far as the paper itself is concerned, I'm not sure I agree with the paper's claim that standard derivations of the Unruh effect rely on artifacts of particular formalisms. The basic premise behind the Unruh effect is that the notion of which state of the quantum field is the "vacuum state" can be different for inertial vs. accelerated observers. Wald's 1993 monograph Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics develops this viewpoint in some detail in Chapter 5. An obvious consequence of this is that an accelerated detector could detect particles under conditions where an inertial detector would not, and that is the Unruh effect. That argument does not appear to me to depend on any particular formalism: it is a simple assertion about observables.

The paper appears to acknowledge (towards the end of Section 1) that the vacuum state is different for inertial ("Minkowski") vs. accelerated ("Rindler") observers, and that this leads to the standard prediction of the Unruh effect. I an unable to understand, however, on what basis the paper then claims the opposite, that accelerated "Rindler" observers would not detect particles under conditions when an inertial "Minkowski" observer would not.

 

[sbrothy]

I note that one of the authors of this paper is Deur, who has published a number of papers arguing for the presence effects in GR that are not generally accepted or taken into account (and quite a few of them have been discussed in previous PF threads). I don't know if this line of research of his has any connection with that other research, but I wouldn't be surprised if it did.

As far as the paper itself is concerned, I'm not sure I agree with the paper's claim that standard derivations of the Unruh effect rely on artifacts of particular formalisms. The basic premise behind the Unruh effect is that the notion of which state of the quantum field is the "vacuum state" can be different for inertial vs. accelerated observers. Wald's 1993 monograph Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics develops this viewpoint in some detail in Chapter 5. An obvious consequence of this is that an accelerated detector could detect particles under conditions where an inertial detector would not, and that is the Unruh effect. That argument does not appear to me to depend on any particular formalism: it is a simple assertion about observables.

The paper appears to acknowledge (towards the end of Section 1) that the vacuum state is different for inertial ("Minkowski") vs. accelerated ("Rindler") observers, and that this leads to the standard prediction of the Unruh effect. I an unable to understand, however, on what basis the paper then claims the opposite, that accelerated "Rindler" observers would not detect particles under conditions when an inertial "Minkowski" observer would not.

I obviosuly didn't read it carefully enough. I think that's a sideeffect of my missing education. I'm absolutely fascinated by these subjects but I lack the education to really have a serious discussion with you people which annoys me to no end.

 

[PeterDonis]

I an unable to understand, however, on what basis the paper then claims the opposite, that accelerated "Rindler" observers would not detect particles under conditions when an inertial "Minkowski" observer would not.

Towards the end of Section 5, a claimed explanation is given which is then repeated at the start of Section 6, the Conclusion:

"Properly, the Unruh effect is cancelled in IF dynamics by another pseudo-effect, namely, cooling of the Unruh detector owing to dynamical evolution under a succession of IF boosts."

I'm still not sure about all the argument that leads up to this.

 

[PeterDonis]

I'm still not sure about all the argument that leads up to this.

A key point in the argument seems to be that in the IF formalism, boosts are dynamical, whereas in the FF formalism, boosts are "kinematical", and the latter is a better model for the actual observable results.

However, that doesn't seem right to me, because what "boost" really means in this context is "nonzero proper acceleration of the detector". But nonzero proper acceleration is not just "kinematical"; it's a real physical, "dynamical" effect that happens for a reason. A detector doesn't have nonzero proper acceleration by magic, or just because I choose a particular set of coordinates; the nonzero proper acceleration is a direct physical observable, which has a physical cause, such as a rocket engine firing.

One could say, I suppose, that the fact I've just stated means that the Unruh effect is actually caused by the source of the proper acceleration (the rocket engine or whatever it is). But that still makes it an observable effect, an observable difference between inertial and accelerated observers. Whereas the paper is claiming that there is no observable difference in this respect.

So I'm still skeptical of the paper's argument, but I am in no position to check their math in detail. Perhaps someone will publish a response that will do so.

 

[Vanadium 50]

I suspect this will go in circles until people lose interest or it is closed.

Consider only EM for the moment. What is radiation and what is near filed is dependent on the observer. I am 99% sure this cannot be uniquely defined at a single point - so you need two observers, and the pairs will not necessarily agree.

All observers will agree the detector registered, but some will say it was an E field from radiation that triggered it, and others will say no, it was a local E field that triggered it.

This is true even in flat spacetime.

Given that, I don't think any question of the form "Isn't Effect X just a special case of Effect Y" is likely to be settled. Ever.

 

[PeterDonis]

All observers will agree the detector registered

If it does register, yes. But the question at issue in this thread is whether an accelerated detector will register in flat spacetime when the quantum field is in its Minkowski vacuum state. The standard derivation of the Unruh effect says yes; the paper the OP referenced says no.

This is not a question of definitions, or of trying to distinguish "radiation" from "near field". It's a straightforward difference in an observable prediction.

 

[sbrothy]

I suspect this will go in circles until people lose interest or it is closed.

Consider only EM for the moment. What is radiation and what is near filed is dependent on the observer. I am 99% sure this cannot be uniquely defined at a single point - so you need two observers, and the pairs will not necessarily agree.

All observers will agree the detector registered, but some will say it was an E field from radiation that triggered it, and others will say no, it was a local E field that triggered it.

This is true even in flat spacetime.

Given that, I don't think any question of the form "Isn't Effect X just a special case of Effect Y" is likely to be settled. Ever.

See? There you came. So my prediction of ascerbic comments hit very close to home. :P

And I’m learning. Amazing! :)

EDIT: just wanna make sure you know I’m joking!