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 accelarating 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?