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marcus

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## Main Question or Discussion Point

It made a considerable difference to read

http://prime-spot.de/Physics/notes6.pdf

which gives Hossenfelder's explanation of Verlinde's paper, on Newton gravity being an entropic force.

It took away a vague sense of mystery. The experience was like opening the window to full daylight. The notes seem to have been posted today 3 March, or else I just didn't see them until today.

She summarized her comments on her blog, in a few words without the math.

http://backreaction.blogspot.com/2010/03/gravity-is-entropy-is-gravity-is.html

===sample excerpts from blog===

Here is a short summary: With a suitable definition of quantities, describing gravity by a Newtonian potential or describing it as an entropic force in terms of an "entropy," "temperature" and "holographic screens" is equivalent. One can do it back and forth. The direction Verlinde has shown in his paper is the more difficult and more surprising one. That it works both ways relies on the particularly nice properties that harmonic functions have...

Some assumptions made in the paper are actually not necessary. For example,...

The biggest problem is that Verlinde's argument to show ...

... It does not seem entirely impossible to actually do this derivation, but there are some gaps in his argument.

In any case, let us consider for a moment these gaps can be filled in.

==endquote==

The notes do not give intuition about how entropic forces work. Verlinde already did a good job with the intution, both in his paper and in his two blog posts at the Amsterdam faculty website. More intuition was not what we needed. What was needed was critical hard-headed math derivation making sure it all worked. To the extent possible this is what H. provides---some gaps are pointed out which presumably are minor and can be filled in.

http://prime-spot.de/Physics/notes6.pdf

which gives Hossenfelder's explanation of Verlinde's paper, on Newton gravity being an entropic force.

It took away a vague sense of mystery. The experience was like opening the window to full daylight. The notes seem to have been posted today 3 March, or else I just didn't see them until today.

She summarized her comments on her blog, in a few words without the math.

http://backreaction.blogspot.com/2010/03/gravity-is-entropy-is-gravity-is.html

===sample excerpts from blog===

Here is a short summary: With a suitable definition of quantities, describing gravity by a Newtonian potential or describing it as an entropic force in terms of an "entropy," "temperature" and "holographic screens" is equivalent. One can do it back and forth. The direction Verlinde has shown in his paper is the more difficult and more surprising one. That it works both ways relies on the particularly nice properties that harmonic functions have...

Some assumptions made in the paper are actually not necessary. For example,...

The biggest problem is that Verlinde's argument to show ...

... It does not seem entirely impossible to actually do this derivation, but there are some gaps in his argument.

In any case, let us consider for a moment these gaps can be filled in.

**Then the interesting aspect clearly is not the equivalence.**The interesting aspect is to consider the thermodynamical description of gravity would continue to hold where we cannot use classical gravity, that it might provide a bridge to a statistical mechanics description of a possibly underlying more fundamental theory...==endquote==

The notes do not give intuition about how entropic forces work. Verlinde already did a good job with the intution, both in his paper and in his two blog posts at the Amsterdam faculty website. More intuition was not what we needed. What was needed was critical hard-headed math derivation making sure it all worked. To the extent possible this is what H. provides---some gaps are pointed out which presumably are minor and can be filled in.