Posted: Dec 16, 2011 7:52 am
by twistor59
andrewk wrote:
I've never read any loop quantum gravity, but Smolin is a reputable physicist, so I can only assume that the reason he writes about the possible existence of a minimum volume is that perhaps loop quantum gravity overrides quantum mechanics under some circumstances.


No, it doesn't really override quantum mechanics, it applies quantum mechanical principles (in particular the commutation relations between conjugate variables) to a particular abstract system in such a way that the elements of classical spacetime (volumes, areas etc) "emerge" as a result of measurements performed on that system.

andrewk wrote:
My understanding is that loop quantum gravity is a highly speculative and entirely hypothetical field at present, whereas QM is one of the most highly experimentally verified theories around. That is not to say that LQG may not be right, just that it's speculative.


Yes, it is speculative. If you want a bit of amusement, have a look for Lubos Motl's opinions on loop quantum gravity. LQG is much narrower in scope than string theory, i.e. it's "just" gravity, you have to shovel the matter in by hand. But who knows, it may bear some fruit eventually....