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In the past several major advances in physics have been associated with the discovery that something wasn't absolute, but depended on the observer.
Padmanabhan just posted A Dialogue on the Nature of Gravity, which is remarkably readable, and which argues among other things that entropy is observer dependent.
See section 3, pages 7 and 8.
One could conclude that this gets rid of Penrose's objection to cosmological bounce scenarios. In 2003 Penrose expressed the opinion that a Big Bounce would violate the Second Law, because the collapsing region would have high entropy (as seen by an observer with the bounce in his future), and the subsequent expanding region would have low entropy (as seen by an observer with the bounce in his past). Entropy seems to have been reduced, a violation of the Second Law. But since they are two different observers, and since entropy, instead of being absolute, is relative to the observer, the Second Law is not violated. Mr. Before and Mr. After do not define/measure the same entropy. In any case that is one possible conclusion one might draw from Padmanabhan's Dialogue.
Apart from a few isolated tough spots, the first third or so of the Dialogue is fun reading. Thought experiments and general arguments without many equations. Dialogue can be an effective vehicle for communicating science.
The latter half gets more technical because he is trying a tour de force---he attempts to derive the Einstein equation of General Relativity from thermodynamics. Curiously enough Ted Jacobson tried something similar, perhaps not quite so ambitious, in a 1995 paper which Padma cites. There may be something to it. Both Jacobson and Padmanabhan are recognized world class. The fact that they both had a similar intuition could be significant. As I understand it, TJ made stronger assumptions, so his result is limited but still suggestive. I have the impression that TP assumes less and derives more, so in this case he may have done more heavy lifting. This comparison could be in error. Also parts of Padma's paper refer to work still in progress.
Here's Padmanabhan's Dialogue
http://arxiv.org/abs/0910.0839
Here's Jacobson's 1995 paper
http://arxiv.org/abs/gr-qc/9504004
Padmanabhan just posted A Dialogue on the Nature of Gravity, which is remarkably readable, and which argues among other things that entropy is observer dependent.
See section 3, pages 7 and 8.
One could conclude that this gets rid of Penrose's objection to cosmological bounce scenarios. In 2003 Penrose expressed the opinion that a Big Bounce would violate the Second Law, because the collapsing region would have high entropy (as seen by an observer with the bounce in his future), and the subsequent expanding region would have low entropy (as seen by an observer with the bounce in his past). Entropy seems to have been reduced, a violation of the Second Law. But since they are two different observers, and since entropy, instead of being absolute, is relative to the observer, the Second Law is not violated. Mr. Before and Mr. After do not define/measure the same entropy. In any case that is one possible conclusion one might draw from Padmanabhan's Dialogue.
Apart from a few isolated tough spots, the first third or so of the Dialogue is fun reading. Thought experiments and general arguments without many equations. Dialogue can be an effective vehicle for communicating science.
The latter half gets more technical because he is trying a tour de force---he attempts to derive the Einstein equation of General Relativity from thermodynamics. Curiously enough Ted Jacobson tried something similar, perhaps not quite so ambitious, in a 1995 paper which Padma cites. There may be something to it. Both Jacobson and Padmanabhan are recognized world class. The fact that they both had a similar intuition could be significant. As I understand it, TJ made stronger assumptions, so his result is limited but still suggestive. I have the impression that TP assumes less and derives more, so in this case he may have done more heavy lifting. This comparison could be in error. Also parts of Padma's paper refer to work still in progress.
Here's Padmanabhan's Dialogue
http://arxiv.org/abs/0910.0839
Here's Jacobson's 1995 paper
http://arxiv.org/abs/gr-qc/9504004
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