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It is well know that QFT predicts that an accelerated observer would see Unruh radiation. This is related to the presence of a Rindler horizon and leads to the conclusion that an inertial and an accelerated vacuum are different.

Why can't we naively apply the equivalence principle and say that a static observer under a gravitational field? In this case there could be no horizon and the argument would fail. But I'm not sure why the equivalence principle doesn't work. Maybe the particle concept is a global one?

jinawee
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  • The Unruh radiation cannot be observed. A detection of a photon is an absolute event that does not depend on a reference frame. For example, if a high energy photon kills the observer, then he cannot possibly be dead in one frame, but alive in another. An accelerating object thermalizes at the Unruh temperature, but this does not lead to the emission of photons, because the emission and absorption rates of the accelerating particle are balanced. Secondly, the equivalence principle does hold in a uniform gravity where the Hawking radiation is equivalent to Unruh, but different if not uniform. – safesphere Jun 15 '18 at 14:27
  • @safesphere can you give a reference? Afaik the interpretation is exactly that a particle detector will detect a thermal bath of all kinds of elementary particles. Your gedanken experiment with an observer killed by a photon doesn't make sense to me – either in the observer's rest frame (and, equivalently, in all Lorentz-transformed frames) the field is in the vacuum state, in which case the observer is alive, or it is in the thermal bath, in which case the observer is dead. It really doesn't matter what state a second accelerating observer measures here. – Prof. Legolasov Jun 16 '18 at 13:57
  • @SolenodonParadoxus https://arxiv.org/abs/quant-ph/0509151 A classical observer cannot be in a superposition of quantum states. He is either dead or alive in all frames. A classical state is absolute and does not depend on the frame. – safesphere Jun 16 '18 at 23:22

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