Emergent Gravity

Sabine Hossenfelder's latest post at Backreaction on "Emergent Gravity" is one of her best educated layman oriented posts explaining concepts in fundamental physics yet.

Notably, she emphasizes the connection, lacking in many other explanations of this approach, to the similarities between thermodynamics (which is know is emergent via statistical mechanics from the mechanics of atoms and molecules) and gravity.

Emergent gravity has been in the news lately because of a new paper by Erik Verlinde. 

. . . 

Almost all such attempts to have gravity emerge from some underlying “stuff” run into trouble because the “stuff” defines a preferred frame which shouldn’t exist in general relativity. They violate Lorentz-invariance, which we know observationally is fulfilled to very high precision. 

An exception to this is entropic gravity, an idea pioneered by Ted Jacobson 20 years ago. Jacobson pointed out that there are very close relations between gravity and thermodynamics, and this research direction has since gained a lot of momentum. 

The relation between general relativity and thermodynamics in itself doesn’t make gravity emergent, it’s merely a reformulation of gravity. But thermodynamics itself is an emergent theory – it describes the behavior of very large numbers of some kind of small things. Hence, that gravity looks a lot like thermodynamics makes one think that maybe it’s emergent from the interaction of a lot of small things. . . . as long as you’re not looking at very short distances, it might not matter much exactly what gravity emerges from. Like thermodynamics was developed before it could be derived from statistical mechanics, we might be able to develop emergent gravity before we know what to derive it from.

This is only interesting, however, if the gravity that “emerges” is only approximately identical to general relativity, and differs from it in specific ways. For example, if gravity is emergent, then the cosmological constant and/or dark matter might emerge with it, whereas in our current formulation, these have to be added as sources for general relativity. 

So, in summary “emergent gravity” is a rather vague umbrella term that encompasses a large number of models in which gravity isn’t a fundamental interaction. The specific theory of emergent gravity which has recently made headlines is better known as “entropic gravity” and is, I would say, the currently most promising candidate for emergent gravity. It’s believed to be related to, or maybe even be part of string theory, but if there are such links they aren’t presently well understood.

She references the following article for a more technical description of many of the leading theories.

We give a critical overview of various attempts to describe gravity as an emergent phenomenon, starting from examples of condensed matter physics, to arrive to more sophisticated pregeometric models. The common line of thought is to view the graviton as a composite particle/collective mode. However, we will describe many different ways in which this idea is realized in practice.

Lorenzo Sindoni, Emergent Models for Gravity: an Overview of Microscopic Models (May 12, 2012).

The notion that you might be able to derive gravity from first principles through a clever macro-level understanding of particle physics is very exciting indeed. It would be miraculous enough for dark matter and dark energy to emerge naturally from a quantum gravity theory. But, it would be even more amazing if quantum gravity itself could be derived from and emerged naturally from fundamental particle physics. 

Certainly, that hasn't been established yet, but it does seem like a very plausible possibility.