Valentina Baccetti, Matt Visser
Is entropy objectively "real"? Or is entropy in some sense subjective and observer-dependent? These innocent questions open a Pandora's box of often inconclusive debate. A consensus opinion, though certainly not universally held, seems to be that the Clausius entropy (thermodynamic entropy, defined via a Clausius relation d S = dQ/T) should be objectively real, but that the ontological status of the statistical entropy (the Shannon or von Neumann entropy) is much more ambiguous, and much more likely to be observer-dependent. This question is particularly pressing when it comes to understanding Bekenstein entropy (black hole entropy). To perhaps further add to the confusion, we shall argue that even Clausius entropy can often be observer-dependent. We shall demonstrate that one can meaningfully assign a notion of Clausius entropy to the matter crossing arbitrary bifurcate null surfaces --- effectively defining a virtual Clausius entropy" for arbitrary virtual causal horizons".
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http://arxiv.org/abs/1303.3185
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