Why Quantum Gravity is Hard

Barry McFarland will start a series of sessions on Quantum Gravity.

Most physical theories of Nature are necessarily approximate: they work very well at energy scales one typically encounters, but break down at very very high energies. Despite this fact, physics has devised a remarkably successful and consistent method of extracting the essential features of physical theory by systematically ignoring the enormous energies where the theory breaks down; a process known as re-normalization. But there is one domain where enormous energy scales cannot be ignored, and that is Quantum Gravity. The playing field of Quantum Gravity is Black Holes and cosmic singularities like the Big Bang. If physics is to make any progress in understanding what happens in those regimes, one needs something other than re-normalization.

One can trace some of the difficulty of Quantum Gravity to the fact that Einstein's Theory of Gravitation is not renormalizable. And Gravity is non-renormalizable because the high energy spectrum of Gravity is dominated by Black Holes In this week's and subsequent talks the goal will be to understand why ordinary Gravity is not renormalizable. Some groundwork will need to be laid concerning renormalization in ordinary Quantum Field Theory, so Joe Polchinski's method of integrating out UV modes will be introduced. The role of fixed points, which allow for the innocuous removal of ultraviolet cutoffs, will be emphasized. Finally, some proposed work-arounds (such as asymptotic safety and non-perturbative methods) will be presented and criticized. Why Quantum Gravity is Hard.

Cheers,
Moshe Rozenblit PhD

PS
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