Sorry if this question is naive. It is just a curiosity that I have.
Are there theoretical or experimental reasons why gravity should not be an emergent property of nature?
Assume a standard model view of the world in the very small. Is it possible that gravity only applies to systems with a scale large enough to encompass very large numbers of particles as an emergent property?
After all: the standard model works very well without gravity; general relativity (and gravity in general) has only been measured at distances on the millimeter scale.
How could gravity emerge? For example, it could be that space-time only gets curved by systems which have measurable properties, or only gets curved by average values. In other words that the stress-energy tensor has a minimum scale by which it varies.
Edit to explain a bit better what I'm thinking of.
- We would not have a proper quantum gravity as such. I.e. no unified theory that contains QM and GR at the same time.
- We could have a "small" (possibly semi-classical) glue theory that only needs to explain how the two theories cross over:
- the conditions and mechanism of wave packet reduction (or the other corresponding phenomena in other QM interpretations, like universe branching or decoherence or whatnot)
- how this is correlated to curvature – how GM phenomena arise at this transition point.
Are there theoretical or experimental reasons why such a reasoning is fundamentally incorrect?
Best Answer
Despite all I wrote in my other answer, there's a very interesting attempt by Xiao-Gang Wen to come up with emergent models of gravity starting from quantum lattice models with no gravity, and only nearest neighbor interactions. His work can be found at gr-qc/0606100 and arXiv:0907.1203. He managed to show that quasiparticles with no energy gap and a helicity of $\pm 2$ can emerge without being accompanied by helicity $\pm 1$ or $0$ quasiparticles. Whether or not this model can be considered a model of gravity though is another matter.