Unruh effect and lessons regarding reality

tom.stoer

(sometimes) even philosophers use strict definitions - and ''perceived reality" misses the essential point. You have observer dependent perceptions (obviously) and you may have or believe in one single underlying observer-independent (!) reality. Observer dependent realities or perceived realities are a contradictio in eo ipso. You should call them simply perceptions or phenomena, not reality. Perhaps these philosophical terms may not map directly to our mathematical structures we use in QFT, but we should be clear about them.

This is about perceptions and physical predictions; that's fine and nothing is missed - except for a definition of reality. I would not translate "accessable event set" as reality.

The basic question is whether the concept "event" or "one particle" can be an element of an underlying reality, or if it is an element of oberver dependent perceptions. If the latter one is assumed, what is the underlying reality? or do we want to give it up completely and stick to positivism? Does the particle exist w/o any observation or detection? In QM the answer is either "yes" (for all observers) or "no" (for all observers) and we only need to consider different perceptions (momentum, color, ...) of the same particle. In QFT this fades away and it seems that we cannot say that the particle exists independent from any observation. Not even the concept of "average number of particles in a given volume" cannot be used as an element of reality w/o contradiction.

Demystifier

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

Ilmrak

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

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 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

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:

Demystifier

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]