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Gerard ’t Hooft ENS Paris, October 2008 Utrecht University What is an A scientific day in memory of Philippe MEYER (1925 - 2007)

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Particles in experiments Elementary vs Composite Unstable particles Pole in propagator Dressed and bare particles Particles in GR Particles at Horizons Gravitons The disputes about QM Ontological objects Protons, Photons and Phonons

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Particles in experiments

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Elementary vs Composite In Effective Quantum Field Theory, this distinction gradually disappears. Particles are elementary at one scale, composite at another. Use Ren. Group.

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Is a black hole a composite particle ? Is a black hole a particle ? Is a particle a black hole ? Is technicolor a viable theory ? The Higgs as a shifted particle:

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Unstable particles

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Dressed and Bare Particles xx xx x x + +... + xx x

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General Relativity: Particles in curved space-time

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Particles and horizons, the hybrid picture

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Is the graviton an elementary particle? Gauge dependence: not a problem, as in Yang-Mills... Is graviton distinct from matter ? gravity – matter unification... In “crystalline gravity”, space-time is a crystal where “defects” play the role of matter and gravitons. gravitons are as photons.

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A particle is an energy quantum. Can this yet be a tangible, “ontological”, physical object? This leads to the disputes concerning the interpretation of quantum mechanics.

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The Bohr – Einstein dispute. Today’s historians of science contend that the dispute was settled in favour of Niels Bohr: Quantum Mechanics is non-deterministic. J.S. Bell is said to have settled the issue with the Bell Inequalities.

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A new variety of the same idea: the Conway – Kochen Free Will Theorem Consider two entangled massive spin 1 particles, with total spin S = 0 :

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In case of spin 2 : 1 1 0

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The 4 cubes of Conway & Kochen It is impossible to attach 0’s and 1’s to all axes at the positions of the dots, such that all orthogonal triples of axes have exactly the (1,1,0) combination.

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Source 1 2 Conclude: Free Will Theorem: If observers on the two different sites have the free will to choose which axes to pick, the spin values of the two particles cannot be pre-determined. No “hidden variables ”

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This proves (once again) that particles are NOT accompanied by “hidden variables” that dictate: < if you choose this axis, then you measure this value for the spin > But it does not disprove hidden variable models of a more delicate nature: There could be field variables at ultra short distance scales, that evolve deterministically, while non-commuting quantum operators are still needed to handle the statistics at intermediate scales.

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In such theories, what we call elementary particles today, are not exactly eigenstates of the “ontological” observables (beables). They are eigenstates of non-commuting operators, even though the evolution of the system may be deterministic: a large class of commuting operators evolve into operators that still commute with them – the beables.

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Compare: protons photons phonons

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An elementary particle is a calculational unit. Its appearance may vary, depending on cicumstances. Under some conditions it may appear to be very real, being something that can be detected in a piece of apparatus. But in other cases its ontological status is much more vague.

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