Entropy is turning out to be more subtle than ever. As a measure of the number of microstates accessible to a system, entropy is clearly Lorentz-invariant, but now Donald Marolf of the University of California in Santa Barbara, Djordje Minic of Virginia Tech, and Simon Ross of Durham University have had a close look at what happens in transformations to non-inertial (accelerated) frames, and have found that things can become very tricky.
As with Hawking radiation, the Unruh effect predicts that the vacuum of an inertial observer will have particles in it as seen by an accelerated observer. It is not immediately obvious then what can be said about the corresponding entropies, and a great deal of careful rethinking about statistical thermodynamics may be in order. With luck, when the dust settles, the results may be comparable to those after it was realized that the second law had to be revised if black holes were to be accounted for properly.