## Quantum gravity

Quantum gravity appears today as the Holy Grail of physics. This is so far detached from any possible experimental result but with a lot of attentions from truly remarkable people anyway. In some sense, if a physicist would like to know in her lifetime if her speculations are worth a Nobel prize, better to work elsewhere. Anyhow, we are curious people and we would like to know how does the machinery of space-time work this because to have an engineering of space-time would make do to our civilization a significant leap beyond.

A fine recount of the current theoretical proposals has been rapidly presented by Ethan Siegel in his blog. It is interesting to notice that the two most prominent proposals, string theory and loop quantum gravity, share the same difficulty: They are not able to recover the low-energy limit. For string theory this is a severe drawback as here people ask for a fully unified theory of all the interactions. Loop quantum gravity is more limited in scope and so, one can think to fix the problem in a near future. But of all the proposals Siegel is considering, he is missing the most promising one: Non-commutative geometry. This mathematical idea is due to Alain Connes and earned him a Fields medal. So far, this is the only mathematical framework from which one can rederive the full Standard Model with all its particle content properly coupled to the Einstein’s general relativity. This formulation works with a classical gravitational field and so, one can possibly ask where quantized gravity could come out. Indeed, quite recently, Connes, Chamseddine and Mukhanov (see here and here), were able to show that, in the context of non-commutative geometry, a Riemannian manifold results quantized in unitary volumes of two kind of spheres. The reason why there are two kind of unitary volumes is due to the need to have a charge conjugation operator and this implies that these volumes yield the units $(1,i)$ in the spectrum. This provides the foundations for a future quantum gravity that is fully consistent from the start: The reason is that non-commutative geometry generates renormalizable theories!

The reason for my interest in non-commutative geometry arises exactly from this. Two years ago, I, Alfonso Farina and Matteo Sedehi obtained a publication about the possibility that a complex stochastic process is at the foundations of quantum mechanics (see here and here). We described such a process like the square root of a Brownian motion and so, a Bernoulli process appeared producing the factor 1 or i depending on the sign of the steps of the Brownian motion. This seemed to generate some deep understanding about space-time. Indeed, the work by Connes, Chamseddine and Mukhanov has that understanding and what appeared like a square root process of a Brownian motion today is just the motion of a particle on a non-commutative manifold. Here one has simply a combination of a Clifford algebra, that of Dirac’s matrices, a Wiener process and the Bernoulli process representing the scattering between these randomly distributed quantized volumes. Quantum mechanics is so fundamental that its derivation from a geometrical structure with added some mathematics from stochastic processes makes a case for non-commutative geometry as a serious proposal for quantum gravity.

I hope to give an account of this deep connection in a near future. This appears a rather exciting new avenue to pursue.

Ali H. Chamseddine, Alain Connes, & Viatcheslav Mukhanov (2014). Quanta of Geometry: Noncommutative Aspects Phys. Rev. Lett. 114 (2015) 9, 091302 arXiv: 1409.2471v4

Ali H. Chamseddine, Alain Connes, & Viatcheslav Mukhanov (2014). Geometry and the Quantum: Basics JHEP 12 (2014) 098 arXiv: 1411.0977v1

Farina, A., Frasca, M., & Sedehi, M. (2013). Solving Schrödinger equation via Tartaglia/Pascal triangle: a possible link between stochastic processing and quantum mechanics Signal, Image and Video Processing, 8 (1), 27-37 DOI: 10.1007/s11760-013-0473-y

### 4 Responses to Quantum gravity

1. OM says:

Perhaps you will find this interesting for you:

http://arxiv.org/abs/hep-th/0610090

2. Niclas says:

Does noncommutative geometry make any predictions about dark matter or the geometry of the universe?

I am not a physicist but interested in the fundamental questions. Most of NCG is beyond my skills but I have tried to understand parts of it. I agree that the work by Connes is unique in that it seems to capture fundamental aspects of the SM. I wonder why this work does not get more attention. Maybe the failure of string theory will make alternative approaches more popular.

• mfrasca says:

Dear Niclas,

Connes’ approach has an important application in physics as the author and his collaborators were able to show that the Standard Model of elementary particles can be completely derived in terms of noncommutative geometry. There has been some tension due to the observed mass of the Higgs particle that did not seem to fit the prevision by Connes et al. but so far, as I heard directly from him, this point seems to be fixed. Here would help some expert comments from people well aware of this matter.

I used the Connes’ ideas mostly in the realm of non-relativistic quantum mechanics as you can judge by my paper but the insights they contain are wide-ranging and I hope more success to them. Surely, they represent a sound starting point to approach quantum gravity but most as to be said yet.

Cheers,

Marco

3. […] Een aantal van deze theorieën heb ik hier al eens eerder genoemd, in een rijtje van de zogeheten Theorieën van Alles, theories of everything (TOE’s). Die proberen niet alleen de zwaartekracht te kwantificeren, maar eigenlijk alles te verklaren, dus ook de donkere materie en energie en het verschil in massa en antimaterie. Dat is nu nog een raadsel, dus er is nog een lange weg te gaan voor de dames en heren natuurkundigen. Voor deze blog heb ik talloze bronnen gebruikt, waarvan er een aantal al maanden liggen in het vakje ‘interessant, nog te lezen’ en waarvoor ik nu in de kerstvakantie eindelijk de tijd heb gevonden ze te bekijken. Zoals: Koberlein + The Reference Frame + Backreaction + The Reference Frame + Backreaction + Starts with a Bang + The Gauge Connection. […]

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