Now, I'm a little confused here. From what I've read of Ed Witten's work--the guy who took the five aspects of string theory and unified them into one coherent theory, known as M-theory--the 11th dimension is the string which makes up the membranes (branes for short) of the 11-dimensional construct that makes of the mutlitverse. On the other hand, I have heard other respected theoretical physicists refer to the 11th dimension as the room needed in hyperspace for the vibrations--of what probably aren't even strings--but rather of different frequencies, with each specific frequency of vibrations determining the properties of things such as baryonic matter.

Now, given that Ed Witten was the one who formulated M-theory in the first place, I would tend to go with what he says. But, the thing is, I'm not certain that I have correctly understood what he has written. And this doubt is further fueled by the fact that other reputable theorists refer to it as that room needed to vibrate. So my question is, have I misunderstood what Witten has written (pardon the rhyme) or do the other theorists have it wrong?

Eric

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You have misunderstood. There is no M-theory. It is a conjecture of Witten's. They have some bits of what it might be like, but haven't actually constructed a theory. "String theory" has no theory either, despite decades of effort to find one.

But don't take it from me, here's what Weinberg (one of the true greats, Nobel Prize for electro-weak unification, worked on string theory in the 80s) said to Science News last month at the New Horizons in Science conference at Austin, Texas.

Of course the physicists, especially those who made sure that nearly all the fundamental research money went into this area, are bit coy about this, apart from a few "heretics" like Smolin and Woit: you have to read between the lines. But for example in the Wikipedia article on M-theory http://en.wikipedia.org/wiki/M-theory we eventually read "M-theory would implement the notion that all of the different string theories are different special cases." Notice that "would", ie, that is what it would do if it existed.

The only reason that people talk of 10 dimensions (string theory) or 11 dimensions (M-theory) is because these are the only numbers of dimensions there is any hope for constructing such a theory. In any other number of dimensions, they just won't work. (There was an earlier version of string theory that required precisely 28 dimensions or something.) No one has ever given a plausible, theoretical reason why a 10-dimensional string theory should result in precisely 4 macroscopic dimensions and 6 tiny rolled-up ones, other than the anthropic reason that 4 is what we have, so far as we can tell.

Einstein did all this wonderful early work, Special Relativity, General Relativity, and loads of other stuff. But he wasted the rest of his life trying to unify gravity and electromagnetism as a classical field theory because he didn't think quantum theory was the answer ("God doesn't play dice..."). Even though he got his Nobel Prize for work in quantum mechanics. So the leading genius can spend most of his life barking up the wrong tree.

Theoretical physicists have put all this huge effort into string theory and spent 25 years getting not very far (though some rather wonderful mathematics has come out of it), mainly because our present-day Einstein decided that was the way to go, after he had done his early ground-breaking work in quantum field theory. And, sheep-like, loads of people worshipped the cult of Witten, and followed him, despite the warnings of people like Feynmann (though he was getting on and aware of risk of being as imprescient as Einstein was in his later career). If that effort had been rather more balanced across alternative approaches, we might have had a better chance of getting somewhere. The senior adherents of the cult of Witten have largely suppressed any heretics wanting to do fundamental work outside string theory, so the heretics have had difficulty getting jobs and money. But I think that people are finally waking up to the fact that string theory hasn't got very far despite the efforts of the world's finest, and the others will slowly get a better crack of the whip now.

Ed Witten did not formulate M-theory. In fact no one has formulated M-theory.

What Witten did was to give a talk and write a paper in 1995 giving a plausibility argument for why it is possible that there is a single theory, which he called M-theory, that includes the 5 competing string theories at that time. However, neither Ed Witten nor anyone else has every clearly defined what M-theory is or provided the "dictionary" that translates the various string theories into M-theory.

M-theory remains a conjecture.

The 11-dimensions do not make the multiverse. It is simply the case that the formulations of some string theories require 11 dimensions for mathematical consistency. Those dimensions are required for the basic string theories themselves.

The "multiverse" that is advocated by some string theory proponents (see Leonard Susskind's book The Cosmic Landscape arises out of the failure of the string theory program to identify a single unique string theory the mathematical consistency of which would determine all of the necessary physical constants required for the laws of physics. It was anticipated that such a unique theory would be the ultimate result of string theory research. But instead it appears that there are a plethora of mathematically consistent string theories (to the extent that there are any, since no one has yet rigorously defined any single string theory), at least 10^500. String theorists find the mathematical beauty of these theories sufficiently compelling that they believe that all of them must be true, in the sense that they represent some universe with the associated physical laws. The multiverse is simply a construct that includes all such possible sets of physical laws.

There is zero evidence for the existence of alternate universes, and given that the universe in which we find ourself is sufficiently large that parts of it are causally disconnected from our little piece of it, alternate pocket universes at greater distance are also causally disconnected. Thus, even if such pockets did exist, there is no way, even in principle for them to be detected or to affect us. In short, the multiverse theory is untestable.

So, the situation is that no one has rigorously defined any string theory. No string theory has ever produced a testable prediction. M-theory remains a conjecture (as does the other often quoted pillar of string theory, the AdS/CFT correspondence of Maldacena). If one of the string theories is correct, then there may well by additional compactified dimensions tied up in some Calabi-Yau manifold. But to make such a conclusion one might reasonably ask for a clear and consistent definition of the string theory and some experimental justification to believe in it.

Imagine the noive of some people....asking a string theorist for evidence from the real world. Hmmpff.! pete

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ROFL, trinitree88!!! And touché!

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