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Conférences du Professeur Gerard’t Hooft
prix Nobel de Physique 1999

Thursday, April 21, Séminaire Général du Département de Physique,
24 rue Lhomond, CONF.IV (2ème étage) : 13h30
"Black hole complementarity and the constants of nature"

Summary :
There is a domain of the physical world where quantum effects, relativistic effects and gravitational effects all play important roles simultaneously. This "Planck world" is hardly accessible experimentally, but the theoretical challenge is there to reconcile the apparently conflicting principles of General Relativity and Quantum Mechanics. An important obstacle is the fact that the classical General Relativity admits black holes as solutions. To realize a quantum mechanical description of such objects the notion of black hole complementarity has been coined, which is the idea that observers may either fall into a black hole or stay outside, but their descriptions of quantum mechanical events should agree. This requirement leads to new constraints regarding the interaction of matter with gravity but also the way matter interacts with other matter. In particular, all constants of Nature are constrained to obey algebraical equations. However, we also know that constants of nature show gigantic variations in size, variations without which the universe could not have been as large and complex as it is. This is known as the Hierarchy Problem.

Tuesday, May 10, Grande Conférence Grand Public ,
29 rue d’Ulm, Salle Jules Ferry, 17h :
"The Extremes in Physics"

Summary :
The laws of nature, as we experience them in our daily lives, are known extremely precisely. We have mechanical laws and forces such as gravity and electromagnetism. If we study more extreme conditions these laws become more exotic, such as at extreme velocities, extremely large distances or extremely tiny objects. These are also well understood, but theoretically one can also combine the extremes, and then real challenges are encountered. To combine extreme velocities, extremely tiny distances and extremely heavy masses one needs a theory called "quantum gravity" and this is not yet understood. Conceivably, the theory that does it right will require the combination of all existing types of particles and forces : the "Theory of Everything".
> But then another question presents itself : why are there extremes ? Where do all these very large numbers in the universe come from ? The number of particles has something like 80 digits, and, in terms of the tiniest existing distance scales, the size of the universe is a similar large number. How can a "Theory of Everything" ever explain the existence of such numbers ?

Friday, May 13, Séminaire du LPTENS,
24 rue Lhomond, CONF.IV (2ème étage) - 16h00
"Conformal symmetry in quantum gravity"br>

Summary :
Reconciling the existence of black hole solutions with the laws of quantum mechanics forces us to look again at the canonical functional integral that one usually encounters in quantum gravity. It can be split in two parts : the integral over the conformal factor and whatever remains after that. The conformal factor can be handled just like any other scalar field, and is renormalizable, but then one encounters a surprise : conformal anomalies. We cannot handle all of these, but the simplest one can be cancelled out ; this then leads to the surprising observation that all constants of nature, including the cosmological constant, must be fixed and computable, in terms of algebraic coefficients that are not yet known. Then comes the rest of the functional integral, which now must feature conformal invariance. Problems and challenges encountered at this stage are discussed.