Otherwise known as M Theory, Membrane Theory or the Theory of Everything or the coolest thing in modern Physics
Technical definition
M-theory is a recent extension of string theory in which the fundamental physical entities are considered as surfaces in a many-dimensional space rather than as lines or loop elements.
I think I understand that. See the problem is most of this stuff can only truly be explained by using the sort of mathematics that only about twelve people on the entire planet understand. I am not one of those twelve people. So what follows is necessarily a simplification of a complex subject.
The simple man's guide to modern theoretical physics
Now first of all there was the Standard Model of the universe that proposed that
matter was made up of lots of tiny, invisible particles, (electrons, protons and stuff like that), and we had ideas like the Theory of Relativity and Quantum Theory to explain how these particles and forces interacted, which unfortunately appeared to be inconsistent and convinced physicists that somewhere there was another theory that would resolve the inconsitencies and prove a Theory of Everything.
Then along came String Theory which said that no, particles were really tiny, invisible strings, which with the inclusion of supersymmetry became Superstring Theory. Now fundamental to Superstring Theory was the idea that reality was composed of ten dimensions. Ten? Well yes, the three (normal) spatial dimensions, plus one for time and another six amazingly tiny spatial dimensions whose existence is beyond our perception. Hence whereas matter might consist of ten dimensional strings, the string would only 'appear' to us in our four dimensions as a particle.
Now for a time String Theory/Superstring Theory seemed to be a strong candidate for Einstein's long sought Unified Field Theory or Theory of Everything. Unfortunately as physicists struggled to explain all the weird shit in the universe they somehow ended up with five different theories, which is rather problematic for a possible Theory of Everything. (There's only supposed to be one.)
The other problem with
String Theory was its failure to explain the birth of the universe, otherwise known as the
Big Bang which became known as the
singularity because that's when all the equations stopped working.
Now Supergravity Theory had been around for some time, and Supergravity Theory had always been convinced there were exactly eleven dimensions. But Supergravity Theory theory, being unfashionable, got ignored. Until Edward Witten, professor of physics at the Institute for Advanced Study in Princeton and Paul Townsend of the Department of Applied Mathematics and Theoretical Physics at Cambridge jointly asked the question; what happens to Superstring Theory if you have eleven instead of ten dimensions? Answer; the strings turn into a membrane.
Hence was born M Theory, with the central amazing postulate, that our entire universe is a membrane; one single vast structure to which all the matter in the universe is connected. Which in itself is a pretty weird and wonderful concept. But wait, there's more.
On a train journey from Cambridge to London, en-route from a conference to see the play Copenhagen, three physicists Neil Turok, Paul Steinhardt and Burt Ovrut were discussing M Theory and the origin of the universe. What would happen if there was more than one membrane? What happens if they collide?
Result, one Big Bang. No more singularity, no more broken equations. We have a (possible) explanation of how it all began.(Otherwise known as the ekpyrotic universe - see that node for a more detailed explanation.)
Welcome to the multiverse, where our universe co-exists with other membranes, other universes. In fact there could be an infinite number of membrane-universes, each with a utterly different law of physics. Big Bangs would be happening all the time.
If there is an infinite number of universes, then there will be an infinite number of occasions in which two membranes have collided to create a Big Bang exactly like the one that created our universe, with exactly the same physical laws, with exactly the same sequence of events since the moment of creation. Except that, in one of them Adolf Hitler was a talented and renowned painter and had no interest in politics; in another Cadwallon ap Cadfan won the Battle of Heavenfield; in another Mary and Joseph had a daughter and in yet another you had a mocha this morning instead of your usual latte arrived at the office two minutes later than usual and so missed bumping into the person that otherwise would have been the love of your life.
Every possible outcome that you can imagine will and has occured and will happen again. And again. (Infinity is a very big number, and turns the most improbable of events into an odds on certainty.)
The existence of multiple universes provides a possible explanation of why gravity is such a weak force in our universe. (Yes I know, gravity seemed strong enough that time you fell off that ladder, but physicists consider it weak compared to other forces.) The simple answer is that there is no gravity in our universe as such, what we have is gravity leaking in from an adjacent membrane-universe. Similarly there may be no need dark matter after all, no need for all that 'missing matter' to explain all anything
Of course all this is theory. It's a difficult thing proving the existence of eleven dimensional structures when your perceptions are limited to only four. If the concepts produce pretty equations with nice elegant solutions everybody's happy, as most physicists tend to think that there ought to be a nice elegant solution to the ultimate question.
And one more thing one. Physicist Alan Guth believes it may well be possible to create a membrane-universe in the laboratory. (If there's already an infinite number of them, one more is going to be neither here nor there.) He thinks he could create a universe.
Which if I read it correctly means that Alan Guth thinks he can become God.
Now I'm off to get myself a drink.
(By the way, has anyone got a subtle knife I can borrow.)
Sources for this write up;
Technical stuff at:
http://whatis.techtarget.com/definition/0,,sid9_gci549055,00.html
http://www.damtp.cam.ac.uk/user/gr/public/qg_ss.html
but mainly inspired by a BBC Horizon documentary, the transcript of which is at:
http://www.bbc.co.uk/science/horizon/2001/parallelunitrans.shtml