M-Theory and Branes

Discussion of theories involving time as a dimension, time travel, relativity, branes, and so on, usually applying to the "real" universe which we live in.

M-Theory and Branes

Postby Nick » Wed Dec 19, 2007 11:40 pm

According to M Theory (which is just an odd name for String Theory), every Universe is trapped inside something called a "Membrane", or Brane for short. This is obviously a very speculative part of the theory since it can neither be proven or disproven... or can it?

According to Quantum Mechanics, Gravity is the weakest of the four fundamental forces, and every force has a mediating particle. However, a particle for Gravity (named a Graviton) hasn't been found yet. According to M Theory, this is because Gravitons are capable of leaving our Brane, while the other particles aren't. In other words, if we see a Graviton in our Universe, but then it disappears, we have found (some) evidence for Branes.

My question is this:
a) Am I incorrect in any of the above?
b) Does proving Branes prove 4 dimensional space? In other words, are 3 dimensional universes trapped in 3 dimensional Branes floating around in 4D space, or infinite 3D space? Or higher?

Thank you in advance. :)
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Re: M-Theory and Branes

Postby Sideshow » Tue Nov 17, 2009 2:42 am

Okay, so I know this is old, and I haven't posted in forever, but I've taken up an interest in string theory and have learned a few things (I highly recommend Brian Greene's The Elegant Universe). So, here goes:

The universe is, at its most basic level, composed of strings (later theorized to be branes), of different tension. Their vibrations, and variations thereof, result in every sub-atomic particle. What makes string theory a major candidate as the "theory of everything"is that there are mathematical equations for strings (or branes) that result in the graviton. In fact, all variations of string have a potential form that resulta in the properties of the graviton. Other, far more limited, options result in the other particles.

All of this makes perfect sense, except that these proper equations require the presence of tiny, coiled dimensions at every single point in the universe. However, unlike the three-plus-one dimensions we're used to, strings would have to resonate across six additional dimensions, coiled together, and at every point in our three-dimensional, uncoiled, universe. The specific arrangement of these dimensions that fulfill the equations properly are called Calabi-Yau shapes. That's how we get M-theory: We have our three-plus-one (time) dimensions, six coiled dimensions, and an even larger dimension to contain it all. That gives us eleven: 3 + 1 +6 +1.

Phew. I may have been a little long-winded and mangled the facts somewhat. The important piece is this, though: higher dimensions aren't expanded according to string theory. It is the properties of the way they are intertwined that result in gravitons, as well as every other particle. It might help to think of these six coiled dimensions as a uniquely shaped cave that branes echo around in; the shape of the cave (Calabi-Yau space) determines the echo (particle). Gravitons can't disappear into this space (we, or at least I, think), because they require them to simply exist.

There's a lot of work to be done in this field, and I may have missed something crucial. I've only started researching string theory, so I can't give specifics beyond what Wikipedia will tell you. If I'm blatantly wrong, somebody please correct me -- I'm rather curious now, too.
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Re: M-Theory and Branes

Postby PWrong » Tue Nov 17, 2009 9:58 am

I think you've got everything about right Sideshow. The Elegant Universe is indeed a great book.

I think what you may have missed is that branes can be either very small or very large. So the fundamental particles are branes according to string theory, but also in some theories the universe itself can be a very large brane. This brane could be embedded in a larger space, and universe-sized branes could collide, causing a big bang. Greene mentions this in his book, as well as things like cosmic strings, but I'm not sure if it's part of traditional string theory or just an extension.

Unfortunately apparently very little progress has been made on string theory in the last ten years and it looks like a lot of physicists are preparing to scrap it. It's beautiful, but there's still no evidence for it or testable predictions
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Re: M-Theory and Branes

Postby Sideshow » Sat Nov 21, 2009 5:14 pm

PWrong wrote:Unfortunately apparently very little progress has been made on string theory in the last ten years and it looks like a lot of physicists are preparing to scrap it. It's beautiful, but there's still no evidence for it or testable predictions

I think many are waiting to see the results of the Large Hadron Collider before making a judgement. Many string theorists have been waiting anxiously for fifteen years for the evidence it could provide. From what I understand, it's the only thing humans have built to date that could be capable of giving experimental evidence of string theory. I'm interested in what they find, now that it's finally operating again.
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Re: M-Theory and Branes

Postby PWrong » Mon Nov 30, 2009 8:59 am

I think they're mostly looking for evidence of Higgs Bosons and supersymmetry. Apparently they could also work out extra dimensions by looking at how micro black holes decay.
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