Florian Girelli: I decided to study the fundamental nature of space-time when finishing high-school. My university/research cursus: Master of Mathematics in Toulouse, France PhD theoretical physics in Marseille, France 3 years postdoc at the Perimeter Institute, in Waterloo , Canada 3 years postdoc at SISSA, in Trieste , Italy Starting 3 years of postdoc in Sydney, Australia Doing some research: Main good points : Love to do research! I do what I like! Free to do my research and get organized how I wish Travel all around the world (conferences/jobs/collaborations) Difficult points: Have to be committed 100% of your time Hard to get a permanent job (money is often not very good) Travel too much sometimes….

Can we measure Quantum Gravity effects? Florian Girelli School of Physics USyd Can we measure Quantum Gravity effects?

Plan Quantum Gravity: An important issue: From Special and General Relativity to Quantum mechanics An important issue: QG effects are too weak… Gamma-rays Bursts and quantum space-time

Special Relativity (1905) Time and space are unified. There is no more an absolute time. There is an universal (maximum) speed: the speed of light c. We have contraction of length and dilatation of time.

General Relativity (1915) Space-time becomes dynamical. Gravity is curvature of space-time. Newton’s law is some approximation of General Relativity.

General Relativity (1915) Gravity is curvature of space-time. Matter tells space-time how to curve. Space-time tells matter how to propagate. Special Relativity is a specific solution of the equations.

Quantum mechanics Microphysics is given in terms of « quanta »: discrete quantities. Uncertainty principle: In particular , we have a minimum on the precision we can measure a particle position: the Compton length. Quantize a theory: consider a classical theory, identify the right fundamental variables, quantize them: introduce a new mathematical formalism such that the variables are quantized , ie quanta are introduced. « We put a hat on the variables »

Quantum Gravity Why? Interaction between quantum systems. Like all the other interactions : electromagnetism photon strong force gluon Weak force W+, W- and Z bosons Gravity graviton

Quantum Gravity Why? Interaction between quantum systems. Solving singularties issues in General Relativity.

Quantum Gravity Why? Interaction between quantum systems. Solving singularties issues in Gravity. Look at when quantum effects (Compton length) are comparable to gravitational effects (Schwarschild length). Very small length!

Quantum Gravity Why? Interaction between quantum systems. Solving singularties issues in General Relativity.

Quantum Gravity How to construct it? Add extra dimensions to space-time (4+7=11), more symmetries (supersymmetry), consider strings and branes as the right degrees of freedom, and quantize them: String theory.

Quantum Gravity How to construct it? Add extra dimensions to space-time (4+7=11), more symmetries (supersymmetry), and quantize strings and branes: String theory. Start from General Relativity and quantize it without adding anything extra, you get Loop Quantum Gravity.

LooP Quantum Gravity Quantum space has discrete structure, just like the quantum Hydrogen atom. It is encoded into spin networks A quantum spacetime is a spinfoam.

A big issue… Quantum gravitational effect are extremely tiny! Fine, we can constrcut a nice theory, but physics is more than about theory! Quantum gravitational effect are extremely tiny! Grav. Interaction << electomag. Interaction for atoms Planck length is just incredibly small…

A possible solution Use astrophysical mega events, such as the gamma rays bursts Their origin is not clear: possibly collision of neutron stars, formation of black holes... Possible origin of a massive extinction on Earth, 450 millions years ago

A possible solution Being so energetic, ie high frequency, they can feel the « granular » nature of space-time. If they travel long enough, the high frequency gamma ray could « interact » with the « atoms of space-time » and arrive later/earlier than the low er frequency ones.

QG phenomenology (Consider flat space-time) Rays with different energy travel at different speed due to the quantum gravitational background. What is the distance L that the rays with different energy E1 and E2 need to travel to get 1 second of difference in the time of arrival?

A bit of help would be appreciated!! Conclusion Help guys!! A bit of help would be appreciated!! It is therefore possible to find astrophysical effects to enhance the QG effects. We need more smart ideas to find QG imprints. Very recently such delay in a gamma ray burst was measured! But we need much more data to really relate it to QG…

Further reading FOR QUANTUM GRAVITY Lee Smolin: Three roads to quantum gravity Lee Smolin: The trouble with physics Fermi Observations of High-Energy Gamma-Ray Emission from GRB 080916C Science 27 March 2009, Vol. 323. no. 5922, pp. 1688 - 1693 FOR QUANTUM GRAVITY

Gravity and extra dimensions In a n dimensional space-time, the Newton force becomes: For n=5 dimension, and one is compactified, with radius R: Lower Planck scale…

End of the XIX century: Lord Kelvin said “There is nothing new to be discovered in physics now. All that remains is more and more precise measurements. “ He mentions however some “Clouds over the Dynamical Theory of Heat and Light” due to the Michelson-Morley experiment and the black body radiation. General Relativity Quantum mechanics