Unruh effect and lessons regarding reality

[Demystifier]

Tom.stoer, have you seen my post #8? I would like to see your comments on it.

 

[Ilmrak]

Tom.stoer, it is possible to retain objective status of particles in the Unruh effect, provided that a preferred time exists as in Horava gravity. See
http://xxx.lanl.gov/abs/0904.3412

Very interesting aticle, I missed that post.
I'm a bit sceptical cause I'd really like to have Lorentz invariance at small distances, but I'll have to read more about this.

Ilm

 

[Vandam]

Sorry to pop in a bit late, but I would like to give my point of view.

How can an event be observer dependent without any underlaying reality?

In SR this seems impossible.Because how can the event -f.ex. a lightning hitting a tree'- only be observer dependent without any underlaying reality of a tree and a lightning? That's seems impossible to me. I do not see how a tree can be 'created' purely and only by our perception, measuring machines etc. That looks more like dreaming (a PF thread was closed because of this dreaming scenario). If you want to stick to that, you are positivist/solipsist.

In QM different but same story. Do we fabricate/measure particles out of nothing, a 'creatio ex nihilo'? Maybe there are no particles out there, but at least 'something' (call it a field if you like) must be out there to help 'make' the particles.

If the simple fact of observing creates a reality, then we really have to go into philosophy.

Unless you are a positivist/solipsist we have to accept that physics forces us to believe there is an observer independend reality. In QM this might be a non-observable reality. In SR it is an observable reality, through our different 3D space cuts (relativity of simultaneity) through 4D Spacetime. But positivists/solipsists will of course call all this only mathematical working models in their dream state of being :-)

 

[TrickyDicky]

Vandam, I agree with you and that was the point of my post about science requiring the "unique underlying reality" to be logically consistent.
Apparently this is not well understood by most here, it looks like the solipsist school of thought is more fashionable nowadays.

 

[ZapperZ]

This thread has been re-opened.

Please ensure that your post has actual physics content, and it is not JUST a philosophical discussion devoid of physics (i.e. it is based on clear, unadulterated physics theory and/or experiment). A point made simply based on a matter of "tastes" will result in this thread suffering from the same fate.

Zz.

 

[TrickyDicky]

It would be important to make the distinction between the Unruh effect proper and Unruh radiation, the latter is not fully accepted by the physics community according to wikipedia.
The answer given in #2 applies to the former, however Tom's questions seemed to refer to Unruh radiation
.

 

[tom.stoer]

I am not sure if I correctly understand the difference between Unruh effect and Unruh radiation. The Unruh effect means that an accelerating observer will se a thermal state. But what else but a "gas or particles" can this thermal state be?

 

[TrickyDicky]

what one observer characterizes as absorbtion, the other characterizes as emission.

Wikipedia entry:
"While the skeptics accept that an accelerating object thermalises at the Unruh temperature, they do not believe that this leads to the emission of photons, arguing that the emission and absorption rates of the accelerating particle are balanced."

 

[Darwin123]

The Unruh effect claims that a quantum state which can be defined as an "empty vacuum state" by an inertial observer will become a "thermal state containing particles" for an observer with constant acceleration a; the temperature observed is T ~ a.

In contrast to other oberver dependent effects this is not just a different interpretation of one frame-independent reality but seems to be a challange for physical reality at all. Let me explain why.

Suppose we are detecting particles in different references frames, i.e. with different detectors. Each time we are detecting a particle we agree that there is a particle, but we are not in agreement regarding its energy. We are not worried about this fact b/c we can use a Lorentz transformation to get the relation between energy and momentum.

For the Unruh effect it becomes much more disturbing b/c we cannot even agree whether there there is a particle or not. So it seems that we are no longer talking about one unique quantum state with frame dependent interpretations but that we have two truly different quantum states, two different "realities". There is not one event "a particle" with different interpretations like "energies E, E', E'', ...", but there are different realities some with an event "a particle", some with "no particle".

Suppose an accelerating observer and a goup of stationary observers at rest collect the information regarding "their quantum states" over a couple minutes. Once the accelerating observer passes a stationary one they make a simultaneous measurement whether there is a particle or not. After a while the obervers at rest will not have counted any particle, but the accelerating observer will have counted many. So they disagree on the fact "whether there are particles at all".

b/c every particle can be detected only once one could get rid of the problem via the idea that the particles detected by the accelerated observer cannot be detected by the obsevers at rest, so there is no logical contradiction. This is OK, but of course the accelerating observer "knows" that there are other particles that he could detect in principle, but that they escape from his detector by whatever reason. These particles are not detected by the observers at rest, not even in principle.

So the particles detected by the accelerated observer are not and cannot be detected by the observers at rest (b/c they can be detected only once). And the particles not detected by the accelerated observer are not and need not be detected by the observers at rest. So it really seems that there is no logical contradiction - but you may understand that when talking about "reality" one may be bothered by these ideas.

Any thoughts?

The following is a conjecture. I am speculating on the importance of the event horizon in the accelerated observers frame.
The observer in the accelerating frame vehicle sees an "event horizon" in the direction opposite the direction of the acceleration vector. The event horizon is analogous to a very big black hole. The observer in an inertial frame does not see this event horizon. However, he sees that light emitted from behind the accelerated observer vehicle can never catch up to the accelerated observer vehicle. I think that it is the event horizon that resolves the Unruh conundrum.
In order to emit Bremstrahlung radiation, there has to be a charged particle that is being accelerated. An observer traveling with this charged particle sees a stationary electric charge, and an event horizon.
The accelerated observer sees a sky emitting thermal radiation. In the eyes of the accelerated observer, the electric charge is an antennae for electromagnetic radiation. Some of the thermal radiation is absorbed by the electric charge antennae. All thermal radiation not absorbed by the antennae is swallowed by the event horizon that is a finite distance from the antennae. The energy swallowed by the event horizon is lost forever to both observers.
The inertial observer does not see the thermal photons as thermal photons. He sees the thermal photons as virtual photons. In other words, the thermal distribution of photons in the accelerated frame is seen as a Lorentz invariant distribution of photons in the inertial frame. Because the Lorentz invariant distribution is a zero point radiation, it is not seen in the inertial frame. The Lorentz invariant distribution is sometimes called the virtual photons.
The photons that are absorbed by the "antennae" in the accelerated reference frame reappear as photons in the inertial reference frame. The gravitational gradient seen in the accelerated frame provides enough energy to the virtual photons to become real photons in the inertial frame.
Thus, the electric charge is an emitting antennae in the inertial frame. The accelerating electric charge emits Bremstrahlung radiation in the inertial frame.
Thus, one can look at it this way. A virtual photon falls down in the accelerating frame, acquiring enough energy to become a real photon. If it is absorbed by an electric charge, then it appears to the inertial observer as an emitted photon. If the virtual photon misses the electric charge, it is absorbed by the event horizon and never seen again by either observer.

 

[tom.stoer]

@Demystifier: regarding post #8

Tom.stoer, it is possible to retain objective status of particles in the Unruh effect, provided that a preferred time exists as in Horava gravity. See
http://xxx.lanl.gov/abs/0904.3412

This is a rather interesting attempt. However one should not overburden Horava gravity here. Isn't it possible to 'derive' a similar result simply by forbidding t-r-mixing coordinate transformations? That would mean that in any theory of gravity "similar" to GR, Horava, ... there is an objective notion of "vacuum" for an "equivalence class" of geometries related by "allowed transformations"?

The problem I see is the following: both Unruh and Hawking radiation are "global" effects; they rely on a global definition of space-time (used to define the frequencies and creation / annihilation operators) and they rely on (asymptotic) plane wave or plane-wave-like states. But the observation of an observer is alaways a local one!

In addition the relation between an asymptotic observer (observing Hawking radiation) and a free-falling observer crossing the event horizon must not be restricted by excluding certain transformations. I mean the fact that the observer is in free fall is a physical statement. So it is irrelevant for the observers whether there is a transformation relating an asymptotic observer and a free-falling observer. The observers are observing something and this 'something' is of course not affected by such a transformation.

My conclusion is therefore a bit different: ruling out certain transformations in Horava gravity seems to indicate that a definition of vacua using global rules is in conflict with GR (and similar theories) excluding global entities (like energy as a volume integral). Therefore the idea should be to get rid of a definition of vacua relying on such global entities (as has been done for EHs in the meantime which can be characterized locally w/o referring to lightlike infinity).

---------

Let me ask the following question again:

I am not sure if I correctly understand the difference between Unruh effect and Unruh radiation. The Unruh effect means that an accelerating observer will see a thermal state. But what else but a "gas or particles" can this thermal state be?

 

[Demystifier]

The problem I see is the following: both Unruh and Hawking radiation are "global" effects; they rely on a global definition of space-time (used to define the frequencies and creation / annihilation operators) and they rely on (asymptotic) plane wave or plane-wave-like states. But the observation of an observer is alaways a local one!

My conclusion is therefore a bit different: ruling out certain transformations in Horava gravity seems to indicate that a definition of vacua using global rules is in conflict with GR (and similar theories) excluding global entities (like energy as a volume integral). Therefore the idea should be to get rid of a definition of vacua relying on such global entities (as has been done for EHs in the meantime which can be characterized locally w/o referring to lightlike infinity).

Your idea that particles should be defined locally is developed in more detail here:
http://xxx.lanl.gov/abs/gr-qc/0409054 [Class.Quant.Grav.26:025002,2009]