Reconsidering Acceleration and Non-Inertial Frames in Quantum Field Theory

[tzimie]

When a changed body is accelerated in EM field, it is just a macroscopic net effect of multiple interactions of charged particles in that body with the other changed particles outside. When we say "particle accelerator", it is still a simplification, because particle is not being accelerated in a sense like macroscopic body does, we just ignore individual interactions which increase the particle's energy.

So I suspect that QFT shouldn't have a concept of "acceleration" of a particle on a fundamental level (not as an approximation).

So my question is: if an "accelerating particle" (not accelerated particle) is something unphysical, why "non-inertial frames" should be considered something physical? Accelerated frame is a result of macroscopic averaging. Then could weird things like the Unruh effect be just an artifact of asking incorrect questions, like famous "what an observer riding a photon would see"?

 

[atyy]

The Unruh effect is not an artifact of asking an incorrect question because it is possible to have an accelerated detector, whereas it is not possible to have a detector travel at the speed of light.

http://arxiv.org/abs/1108.0320
Unruh effect without trans-horizon entanglement
Carlo Rovelli, Matteo Smerlak

 

[tzimie]

As had I mentioned earlier, "accelerated detector" is a macroscopic concept. Which means that you must have something different on the fundamental level, like when you try to split "measurement devices" into elementary particles, Copenhagen Interpretation loses any sense, so you have to use decoherence approach.

 

[atyy]

No, the Copenhagen interpretation allows the detector to be quantum. The Heisenberg cut between macroscopic and quantum realms can be put at different places. Decoherence by itself does not produce any outcomes, and does not solve the "measurement problem", and needs to be considered with an interpretation to make sense.

 

[Demystifier]

Tzimie, Unruh effect can be viewed as a macroscopic effect in a macroscopically accelerated measuring apparatus. The acceleration of the macroscopic apparatus turns out to be the cause of decoherence in a "weird" Rindler basis. For more details see
- B.L. Hu, A. Matacz, Phys. Rev. D 49, 6612 (1994).
- J. Audretsch, M. Mensky, R. Muller, Phys. Rev. D 51, 1716 (1995).
- P. Kok, U. Yurtsever, Phys. Rev. D 68, 085006 (2003).

 

[tzimie]

Tzimie, Unruh effect can be viewed as a macroscopic effect in a macroscopically accelerated measuring apparatus.

I have to admit, it is quite smart, because declaring this effect "macroscopic" you get rid of most of problems... but not all...

Below is the correspondence between macroscopic and microscopic worlds:

Macro <-- Micro
Macroscopic body <-- consists of particles
Force <-- QFT
Acceleration as result of force <-- QFT (averaged over many particles)
...
Unruh effect <-- ?

Or, rephrasing my question (on purpose) in extremely naive manner, "would a single proton in accelerator "see" Unruh radiation"?

So you have to provide microscopic "ingredients" for the Unruh effect (filling my "?") or to claim that effect is emerging on macroscopic level only (like decoherence, for example) - but in such case there are other problems.

 

[Demystifier]

Or, rephrasing my question (on purpose) in extremely naive manner, "would a single proton in accelerator "see" Unruh radiation"?

It would not.

or to claim that effect is emerging on macroscopic level only (like decoherence, for example) - but in such case there are other problems.

What problems do you have in mind? Are they specific to the Unruh effect, or can they be reduced to the general problem of measurement in QM?

 

[tzimie]

What problems do you have in mind? Are they specific to the Unruh effect, or can they be reduced to the general problem of measurement in QM?

There are phenomena which exist on macroscopic level only and which can't be traced back to individual particles. However, in case of Unruh effect we can have a body heated and melted because of intense Unruh radiation, and we should be able to trace this extra energy on microscopic level.

 

[Demystifier]

However, in case of Unruh effect we can have a body heated and melted because of intense Unruh radiation, and we should be able to trace this extra energy on microscopic level.

As you said, there is a body involved, which is a macroscopic concept. Can you rephrase your claim without using any macroscopic concept such as a "body"? (I think you cannot.)

Without a body/detector, there is no any "extra energy" in the Unruh effect, because energy of the Minkowski vacuum (or of any other quantum state) does not depend on acceleration. Indeed, it has been shown that energy responsible for a detectable Unruh effect originates from the force which accelerates the detector:
http://lanl.arxiv.org/abs/gr-qc/9905024

 

[Demystifier]

There are phenomena which exist on macroscopic level only and which can't be traced back to individual particles

The real question is: Is there any phenomenon which does not exist on a macroscopic level only?

For instance, the Copenhagen interpretation maintains that there is no such a phenomenon. As Wheeler said, "No phenomenon is a real phenomenon until it is an observed phenomenon." Or to quote A. Peres, "A quantum system is a useful abstraction, which frequently appears in the literature, but does not really exist in nature. In general, a quantum system is defined by an equivalence class of preparations."

Of course, you do not need to be an adherent of Copenhagen interpretation, but then you must specify which interpretation of QM do you use. Without specifying it, you cannot coherently speak about the relation between micro and macro in QM.

 

[tzimie]

As you said, there is a body involved, which is a macroscopic concept. Can you rephrase your claim without using any macroscopic concept such as a "body"? (I think you cannot.)

Without a body/detector, there is no any "extra energy" in the Unruh effect, because energy of the Minkowski vacuum (or of any other quantum state) does not depend on acceleration. Indeed, it has been shown that energy responsible for a detectable Unruh effect originates from the force which accelerates the detector:
http://lanl.arxiv.org/abs/gr-qc/9905024

I admit defeat - your explanation is perfect. And getting energy from the force also makes sense (I had suspected that, my intuition is not bad!)
Thank you!

 

[tzimie]

The real question is: Is there any phenomenon which does not exist on a macroscopic level only?

For instance, the Copenhagen interpretation maintains that there is no such a phenomenon. As Wheeler said, "No phenomenon is a real phenomenon until it is an observed phenomenon." Or to quote A. Peres, "A quantum system is a useful abstraction, which frequently appears in the literature, but does not really exist in nature. In general, a quantum system is defined by an equivalence class of preparations."

Of course, you do not need to be an adherent of Copenhagen interpretation, but then you must specify which interpretation of QM do you use. Without specifying it, you cannot coherently speak about the relation between micro and macro in QM.

I was trying to stay interpretation-neutral as long as possible (in this topic, not in my mind). Of course, at some point you have to chose one

 

[Demystifier]

I was trying to stay interpretation-neutral as long as possible (in this topic, not in my mind). Of course, at some point you have to chose one

Exactly!

 

[Demystifier]

I admit defeat - your explanation is perfect. And getting energy from the force also makes sense (I had suspected that, my intuition is not bad!)
Thank you!

See also the analogy with the measurement of spin:
https://www.physicsforums.com/threads/regarding-the-hawking-radiation.760337/#post-4794897