Strange theory of quantum gravity

Posted April 27, 2006

Yesterday I trekked up to Harvard to, ironically, attend a talk by an
MIT professor, Seth Lloyd. The topic was
his theory of quantum gravity,
which I found to be pretty novel. I've no idea if it is *true*,
but it's one of those theories which is worth thinking hard about even in
spite of being false. Plus it may well be right.

The problem is how to merge quantum theory with general relativity. These two theories are highly successful in their own domains, but unfortunately they picture the world in (what seem to be) fundamentally incompatible ways. Ideally, a fully-merged theory would allow us to speak of spacetime itself, the fabric of general relativity, in the language of quantum mechanics, with probabilities of having one configuration or another. The problem is that quantum mechanics treats spacetime as a fixed background within which all of the different particles and fields do their probabilistic dances. And GR, on the flip side, treats the particles and fields classically, without the full quantum treatment.

Lloyd's theory is interesting in that he takes a new approach, not
even treating space and time in the normal "coordinate" fashion, as a backdrop
against which other events happen. Instead, distances in space and time are
*derived* quantities, constructed (if we so choose) after the fact
from the known interactions of particles. That is, "if this photon hit this
electron here, we know they were in the same place at the same time, and if
it hits another electron over there, we know that it had to travel so far
for so long at the speed of light to get there, so the electron must have
been..." and so on. Do this for lots and lots of interactions, and you
build up a self-consistent picture of where and when things had to have been,
and how spacetime must have been curved to allow it.

Why do this? Well, now, because we *don't* know precisely what
events took place among all the particles, due to quantum uncertainties,
our picture of spacetime is *also* uncertain. We're looking at the
fabric of spacetime in a quantum, probabilistic sense, which is precisely
what we wanted to have a decent theory of quantum gravity!

At this point, my hand-waving understanding of his results starts to diverge from their actual contents, so I'll leave you to investigate the paper linked above for more information. Oh, but one last teaser: you can argue that spacetime naturally should be four-dimensional (3D plus time) from the fact that interactions in physics are between two particles... two incoming and two outgoing paths translates to four fundamental directions. If this is true, I can only say, "Whoa," in my best Keanu impression.