# Discussing Gravity with Erik Verlinde

We have just returned from a talk by the Bethe colloquium. Erik Verlinde from the university of Amsterdam spoke about “Dark Matter, Dark Energy and the Emergence of Gravity ”.

Verlinde is a dutch theoretical physicist working on string theory and gravity. He became very famous for his theory of entropic gravity and was awarded the Spinoza Prize for his work.

In his talk, he showed that his approach can not only reproduce the MONDian behavior of the different kinds of galaxies. He even gave an explanation on why the centers of galaxy clusters deviate from the baryonic Tully-Fisher relation by a factor of four. The reason, he says, lies in the distribution of matter. Very roughly, galaxies can be approximated by a point mass if we look at their outskirts only. In galaxy clusters, however, the matter is more evenly distributed. Assuming a spatially constant matter density, he can even motivate the amount of the deviation

All in all, his results look very promising. After the talk, Pavel and I discussed with him for about an hour, explaining some of the failures of the LCDM model, but mostly asking about details and implications of his approach. He explained that his intention is to understand gravity by starting from scratch. So not only change and modify the formula used so far, but basing our understanding of gravity on a more fundamental basis. To do so, he looks at the observational evidence unbiased. We agreed that this is not always easy because especially cosmological results are usually analyzed and expressed in a model-dependent form. He does not aim at reproducing MOND, which even its adherents usually describe rather as a phenomenological effect than a new fundamental law. But his model naturally contains MONDian behavior, it seems to explain/give a reason for MOND's free parameter (the acceleration a0) and he also showed that his approach can predict the ratio of baryonic to (phantom) dark matter correctly: 4% to 22.5%.

What he told was impressive and looks like it could be a major step forward in our understanding of gravity and the universe. Unfortunately, we have to wait some more until he will publish a paper on this topic. But there is good reason to look forward to it.

By Pavel Kroupa and Marcel Pawlowski (28.06.2012): "Discussing Gravity with Erik Verlinde" on SciLogs. See the overview of topics in *The Dark Matter Crisis*.

Micro and macroscopic phase changeAbout the microscopic phase space volume related to entropy and gravity, as a nice example for Erik Verlinde.

For people interested in entropy, look for Slinky drop physics at the Bad astronomy blog.

If I am allowed to post a link to explain what I am talking about:

http://blogs.discovermagazine.com/...drop-physics/

Tension in a hanging slinky equals the gravitational force.

The gravitational force represents a positive entropy change.

And the tension therefore represents negative entropy, negentropy.

It is interesting to see that the microscopic phase space volume

is related to the atomic/molecular bondings in the slinky.

The atoms are slightly further away from each other when they are under tension. Thus there is a small change in phase space volume involved.This microscopic change in phase space volume is representing negentropy, in above example.

We can see that when the slinky is dropped (in macro space) the 3D information behind the slinky is rapidly decreasing.

It is an a way showing us that the microscopic phase changes have there visible counterpart on a macroscale.

Can't wait to see the paper, when its publishedGeneral Relativity fixed Newtonian Physics in high velocity regime, and Entropic gravity is promising to fix General Relativity in Low acceleration regime.

I am hoping after this it will become easier to marry Quantum physics with Entropic gravity.