How can Unruh radiation ever be real? It is speculated that Unruh radiation exists for accelerating observers in flat spacetime. A thermometer in the frame will point at a higher value than for someone in a stationary frame. In the spacetime of the accelerating observer, creation operators exist which create real particles that can affect the thermometer (assume the observer is shielded from the CMBR).
But how can this be? If the acceleration is constant and a pair of particles is created, won't these particles get the same acceleration both? I can imagine that if the particles are accelerated differently, like in the spacetime of a black hole, they become real, but not if they are accelerated exactly the same. Not even if you let a frame accelerate in the most strange ways. Where does the energy come from to make them real? Aren't tidal forces required to make them real?
 A: The Unruh effect is the hypothetical prediction that an accelerating observer will observe a thermal bath, like blackbody radiation, there where an inertial observer sees the vacuum (no particle). In other words, the inertial observer sees no particle, while the non-inertial observer sees a thermal distribution of particle with temperature related to her/his acceleration. The concept of particle is Lorentz invariant, but it is not generally covariant. If you work with non inertial frames you cannot talk about particles in an absolute way: you have to say to which observer you are referring to. This is like time in special relativity (versus time in Newtonian theory). In Newtonian theory time is absolute, you don't have to say to which observer you are referring to, but in special reativity the time coordinate depends on the observer.
Coming back to the particles of the bath, I don't think we can say that they are real: as soon as the non-inertial observer stops he would see exactly the same vacuum (no particle) as any other inertial observer. This is in some sense similar to the length contraction in special relativity: as soon as the observer in motion stops she/he will see the same length as the one measured in the LAB frame. Notice that length contraction is very different than time dilation, which is a real effect: if you move with very high velocity with respect to the LAB frame, then you stop and compare your watch with the one of the LAB frame you will see that your watch slowed down!
