Browsing through the "New Springerlink", I came across(adsbygoogle = window.adsbygoogle || []).push({});

Gravity, the Inside Story

It's got some interesting, though non-classical, points about gravity. The author starts out with an argument about the inevitability of horizons.

This is sort of amusing considering the amount of angst we appear to be having currently at PF about the mere existence of event horizons. But that's not the main point of the paper. But—conceptually—strange things happen as soon as: (i) we let the metric to be

dynamical and (ii) allow for arbitrary coordinate transformations or, equivalently,

observers on any timelike curve examining physics. Horizons are inevitable in such a

theory and they are always observer dependent.

and This raises the famous

question first posed by Wheeler to Bekenstein: what happens if you mix cold and hot

tea and pour it down a horizon, erasing all traces of “crime” in increasing the entropy

of the world? The answer to such thought experiments demands that horizons should

have an entropy which should increase when energy flows across it.

With hindsight, this is obvious.

The author argues that it's not possible for horizons to have a temperature without some sort of microstructure in spacetime, even if we don't yet have a theory that gives said microstructure. This historical sequence raises a some serious issues for which there is no satisfactory

answer in the conventional approach:

1. How can horizons have temperature without the spacetime having a microstructure?

The really interesting stuff happens when the author writes expressions for the entorpy S_matter and S_grav, the entropy of the matter fields and the horizons - and derives Einstein's equations as a low order approximation to the maxmization of entropy,

There's also some higher order terms which make the theory depart from the simple action of GR.

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# Gravity - the inside story

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