## Main Question or Discussion Point

(sorry for quality of the drawing)
There are 2 accelerating observers, A and B in the infinite and asymptotically flat universe. When accelerating both A and B observe Unruh radiation from their Rindler horizon. If there is a cold screen between observer B and his horizon, B detects less Unruh radiation.

Now the question.

Before they accelerate, observer B in unaware of the screen (as universe is dark, there is no light). However, when they start accelerating, B from having less radiation can instantly deduce the presence of the screen. Even more, varying the acceleration B can even calculate the distance to the screen. How B can instantly obtain the information about the screen far away?

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WannabeNewton
How B can instantly obtain the information about the screen far away?
What exactly is the issue here? This is just an example of a correlation. There is no transmission of information between two different observers. The entire setup with the Unruh radiation and Rindler observers is entirely superfluous. There are a myriad examples of correlation measurements like the one you described.

For example if we have an observer in uniform circular motion and an inertial observer at the center, the inertial observer can use light signals to measure the angular velocity of the other observer; having done this the inertial observer can simply count one period on their clock from some initial time at which a light signal arrives from the other observer to instantly know the location of said observer.

The inertial observer is just making use of correlation; they are not instantly transmitting the information about the location of the other observer to some receiver-such a transmission must of course be constrained by causality. But correlations are not.

atyy
I don't think this addresses the question exactly, but it is similar in that even if one puts a boundary with reflecting conditions, one gets Unruh radiation, so we don't have to think of the Unruh radiation as necesssarily coming from the horizon. (I may have garbled that, there are some subtleties I didn't understand when I read this paper.)

http://arxiv.org/abs/1108.0320
Phys. Rev. D 85, 124055
Unruh effect without trans-horizon entanglement
Carlo Rovelli, Matteo Smerlak
(Submitted on 1 Aug 2011 (v1), last revised 20 Jun 2012 (this version, v3))
We estimate the transition rates of a uniformly accelerated Unruh-DeWitt detector coupled to a quantum field with reflecting conditions on a boundary plane (a "mirror"). We find that these are essentially indistinguishable from the usual Unruh rates, viz. that the Unruh effect persists in the presence of the mirror. This shows that the Unruh effect is not merely a consequence of the entanglement between left and right Rindler quanta in the Minkowski vacuum. Since in this setup the state of the field in the Rindler wedge is pure, we argue furthermore that the relevant entropy in the Unruh effect cannot be the von Neumann entanglement entropy. We suggest, in alternative, that it is the Shannon entropy associated with Heisenberg uncertainty.

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Demystifier
View attachment 85090

(sorry for quality of the drawing)
There are 2 accelerating observers, A and B in the infinite and asymptotically flat universe. When accelerating both A and B observe Unruh radiation from their Rindler horizon. If there is a cold screen between observer B and his horizon, B detects less Unruh radiation.

Now the question.

Before they accelerate, observer B in unaware of the screen (as universe is dark, there is no light). However, when they start accelerating, B from having less radiation can instantly deduce the presence of the screen. Even more, varying the acceleration B can even calculate the distance to the screen. How B can instantly obtain the information about the screen far away?
It is not correct to think of Unruh effect as radiation created at the horizon. Instead, the Unruh effect is a particle-detector response created in the detector itself. Both A and B will observe it, and the screen does not influence it.

It is not correct to think of Unruh effect as radiation created at the horizon. Instead, the Unruh effect is a particle-detector response created in the detector itself. Both A and B will observe it, and the screen does not influence it.
Now it makes sense.
Thank you.

Demystifier

Yes, I remember that conversation
Based on your earlier reply in the old thread, there is no delay - time for photons to travel from the Rindler horizon to the observer, instead, an accelerated frame is already 'filled' with Unruh radiation. However, I had assumed (and that was wrong) that these photons really came from Rindler horizon, and hence there were shadows.

It is not correct to think of Unruh effect as radiation created at the horizon. Instead, the Unruh effect is a particle-detector response created in the detector itself. Both A and B will observe it, and the screen does not influence it.
Wait...
It means that in an accelerating body ALL atoms receive Unruh radiation!
Even INSIDE the body.
And solid object can't be accelerated without heating it, in other words, it is not possible, in principle, to accelerate cold (t->0) object without giving it additional entropy.
Is it correct?

Demystifier