## Quantum gravity

27/12/2015

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

## DICE 2014

21/09/2014

I have spent this week in Castiglioncello participating to the Conference DICE 2014. This Conference is organized with a cadence of two years with the main efforts due to Thomas Elze.

Castello Pasquini at Castiglioncello  (DICE 2014)

I have been a participant to the 2006 edition where I gave a talk about decoherence and thermodynamic limit (see here and here). This is one of the main conferences where foundational questions can be discussed with the intervention of some of the major physicists. This year there have been 5 keynote lectures from famous researchers. The opening lecture was held by Tom Kibble, one of the founding fathers of the Higgs mechanism. I met him at the registration desk and I have had the luck of a handshake and a few words with him. It was a recollection of the epic of the Standard Model. The second notable lecturer was Mario Rasetti. Rasetti is working on the question of big data that is, the huge number of information that is currently exchanged on the web having the property to be difficult to be managed and not only for a matter of quantity. What Rasetti and his group showed is that topological field theory yields striking results when applied to such a case. An application to NMRI for the brain exemplified this in a blatant manner.

The third day there were the lectures by Avshalom Elitzur and Alain Connes, the Fields medallist. Elitzur is widely known for the concept of weak measurement that is a key idea of quantum optics. Connes presented his recent introduction of the quanta of geometry that should make happy loop quantum gravity researchers. You can find the main concepts here. Connes explained how the question of the mass of the Higgs got fixed and said that, since his proposal for the geometry of the Standard Model, he was able to overcome all the setbacks that appeared on the way. This was just another one. From my side, his approach appears really interesting as the Brownian motion I introduced in quantum mechanics could be understood through the quanta of volumes that Connes and collaborators uncovered.

Gerard ‘t Hooft talked on Thursday. The question he exposed was about cellular automaton and quantum mechanics (see here). It is several years that ‘t Hoof t is looking for a classical substrate to quantum mechanics and this was also the point of other speakers at the Conference. Indeed, he has had some clashes with people working on quantum computation as ‘t Hooft, following his views, is somewhat sceptical about it. I intervened on this question based on the theorem of Lieb and Simon, generally overlooked in such discussions, defending ‘t Hoof ideas and so, generating some fuss (see here and the discussion I have had with Peter Shor and Aram Harrow). Indeed, we finally stipulated that some configurations can evade Lieb and Simon theorem granting a quantum behaviour at macroscopic level.

This is my talk at DICE 2014 and was given the same day as that of  ‘t Hooft (he was there listening). I was able to prove the existence of fractional powers of Brownian motion and presented new results with the derivation of the Dirac equation from a stochastic process.

The Conference was excellent and I really enjoyed it. I have to thank the organizers for the beautiful atmosphere and the really pleasant stay with a full immersion in wonderful science. All the speakers yielded stimulating and enjoyable talks. For my side, I will keep on working on foundational questions and look forward for the next edition.

Marco Frasca (2006). Thermodynamic Limit and Decoherence: Rigorous Results Journal of Physics: Conference Series 67 (2007) 012026 arXiv: quant-ph/0611024v1

Ali H. Chamseddine, Alain Connes, & Viatcheslav Mukhanov (2014). Quanta of Geometry arXiv arXiv: 1409.2471v3

Gerard ‘t Hooft (2014). The Cellular Automaton Interpretation of Quantum Mechanics. A View on the Quantum Nature of our Universe, Compulsory or Impossible? arXiv arXiv: 1405.1548v2