Transcript Can we measure Quantum Gravity effects?

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….
Florian Girelli
School of Physics
USyd
Can we measure Quantum
Gravity effects?
Plan
 Quantum Gravity:
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…
 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?
Conclusion
• 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…
Help guys!!
A bit of help would be
appreciated!!
• Lee Smolin: Three roads to quantum gravity
• Lee Smolin: The trouble with physics
• Fermi Observations of High-Energy GammaRay Emission from GRB 080916C
Science 27 March 2009, Vol. 323. no. 5922, pp.
1688 - 1693
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.