M-theory and the Big Bang

In an earlier thread of mine one of the greatest advancements of learning derived from M-theory (the latest incarnation of Relativity/Quantum Mechanics unification) was its prediction of the internal structure of Black Holes.

It is said that, like a black hole, the early universe was a quantum singularity.

I also remember that due to the limited understanding of the separate theories of relativity and quantum mechanics, only a few nanoseconds after the big bang can be determined, since before then, gravity was still united with EM and the nuclear forces.

How much farther back can we see using the quantum gravity found in M-theory?

If by “see”, you mean directly, quantum gravity doesn’t really help. The earliest time from which we can directly detect light (of any frequency, from radio through gamma) is about 300,000 years after the Big Bang, far after the Universe was governed by quantum gravity. With gravitational waves, we could see much earlier, to around the end of Inflation, but even that’s probably after the quantum gravity era (though there’s the possibility that Inflation was a quantum-gravitational phenomenon, and its end would therefore require a theory of quantum gravity to properly understand the data).

If you mean theoretical extrapolation, a working theory of quantum gravity would presumably take us all the way back to t = 0. Or, at least, if there’s some other limitation, we don’t know yet what would cause it: Doubtless, someone will eventually come along and point out shortcomings in quantum gravity, just as a bunch of guys with German accents pointed out shortcomings in Newton’s theory, a century ago.

Note that I referred to a “working theory of quantum gravity”, there. That’s still a hypothetical, since we don’t have such a theory yet. M-theory (aka string theory) shows some promise, but it still has a ton of free parameters in it. Basically, anything you might want to predict, there’s some variation of M-theory out there which will predict it for you. So the problem right now is not that it can’t make predictions, but rather, that it makes too many of them.

Is there an M-theory yet? Witten sez there should be M-theory behind the superstring theories (and 11-d supergravity), but I wasn’t under the impression we’d found an M-theory, at least as of a few years ago.

I thought M-theory was defined as the 11-dimensional system Witten found that shows all the five major 10-dimensional systems to be mathematically equivalent.

No, 11-d supergravity is about as well worked out as the five superstring theories, but jsut as incompatible with any of them as they are with each other. M-theory is a hypothesized “unification” of them, that in various limits reduces to each of those six, and maybe others as yet undreamt of. Yes, it also takes place in 11 (or more) spacetime dimensions, but it’s characterized by the use of branes, while 11-d supergravity uses a supersymmetric quantum field theory that still assumes point particles like QED and QCD.

Go argue with the world:

Yes, there are known relationships between each theory, but that doesn’t mean they’re completely compatible. IIA and heterotic SO(32) translate into each other under certain circumstances, but neither works right everywhere, so they sort of patch holes in each other. Similarly with each other pair of theories. What Witten put forth (as the article states) is the idea that there’s one “master theory” sitting behind all of them, and in special circumstances it reduces to each of them just like GR becomes Newtonian gravitation in the low-curvature limit.

Also as stated in the article: “The original formulation of M-theory was in terms of a (relatively) low-energy effective field theory, called 11-dimensional Supergravity.” That is, just as each string theory is in some way a limiting case of M-theory (whatever that may turn out to be), so is 11-d supergravity. When energy is low, the equations of the original proposed formulation of M-theory become those of 11-d supergravity.

Most important, though, is the paragraph of the header that you omitted.

(bolding mine)

There is no single coherent “M-theory” as it stands. There are a list of properties such a theory should satisfy, but no explicit formulation.

Nothing in that article contradicts anything I’ve said. Please point out where you think it does.

I don’t see any conflict between saying that M-theory was named when Witten showed the underlying equivalency of 10d theories, but that M-theory is not yet completely worked out. Just two different connotations of what M-theory means. There’s already much too much heat generated by differing connotations of words around here, so I’ll gladly let this one go.