Wednesday, August 11, 2004

Quantum gravity

More quantum blogging: Over the past few days, I've finally bitten the bullet and starting reading (big pdf) about loop quantum gravity, the major alternative to string theory in the quest to unify quantum physics and general relativity. I've been a fan of string theory for over a year now, but now I think I've found something better.

Back when I was taking a string theory class in 2003, I learned that the way to quantize gravity was to treat the curvature of spacetime it causes as a pertubation of flat spacetime and then quantize that. In the context of string theory, this procedure nicely gives you gravitons and such. (In the conventional quantum field theory, trying this gives all sorts of nasty infinities, hence the need for something like string theory or loop quantum gravity.) The problem with this, which hadn't really registered with me until now, is that this doesn't take general relativity seriously. GR says there is no preferred coordinate system, flat or curved. Period.

Loop quantum gravity takes what I think is the right approach. Coordinate invariance is assumed from the start. The cost is that a lot of mathematical tools from conventional quantum field theory no longer work; one is left with a "bare-bones" version of the quantum formalism. Although I've got a lot more reading to do and limited leisure time for such reading, I've already learned some of the interesting things that the loop quantum gravity researchers have managed to predict after slogging through the math. For instance, the spacetime is a quantum superposition of discrete objects at the Planck scale. Just as molecules have discrete emissions spectra, "regions" of space have discrete volume spectra. In contrast, string theory and conventional quantum field theory assume a continuous spacetime at all scales.

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