could someone please explain the correlation between the string theory and the branes that are to be found within the 12 dimensions?

the endpoints of the string are confined to some hypersurface. this is called a D-brane.

damn the problem with this stuff is it is sli[ppery, aprticularly for someone with no science training. i undersatnd things like this when i rea dthema nd for a while after and then it slides.

I too would appreciate an expansion on string theory in simple pop physics language, aprticularly on how it is a unified theory.

well, string theory is a huge subject, and i don t know it up and down, so i don t feel as comfortable diluting it down for popular consumption, as i would, say, quantum mechanics. but i ll give it a shot.

its starts off simply, assuming that fundamental particles are actually 1 dimensional relativistic quantum strings. this means that they can not only move around as quantum waves, but they can also have internal waves.

these internal oscillation modes carry energy, so by einstein E=mc², each internal oscillation mode of the string has a different mass.

the quantum theory has various anomolies, and cancellation of these anomolies puts much tighter constraints on the theory than normal quantum theories have. for example, it restricts how many dimensions the string can propagate through. the theory is consistent only in 26 dimensions.

among the oscillation states of this string is a massless spin 2 particle, and as it turns out, any massless spin 2 particle must follow einstein s field equations, meaning we have on our hands a prediction of GR, complete with quantum scattering amplitudes. this is a wonderful thing, a theory of quantum gravity is the goal of all modern theoretical physicists.

but there are some problems. so far, the theory has only given us bosons, and most matter is made of fermions. also, one of the oscillation modes (the lowest mode) of the string is apparently unstable (the tachyon), which means that the vacuum may be unstable. that s not good. also, it is a rather odd feature for a theory to tell you that you live in 26 dimensions, when clearly we live in 4 dimensions.

to tackle the first problem, you toss supersymmetry onto the fire. for every bosonic degree of freedom, there is also a fermionic degree of freedom. this doesn t mean that there is a fermion for every boson in the theory (yet), rather, it means that in addition to propagating through spacetime, the string also propagates through superspace.

superspace is just like space, except the coordinates anticommute. this gives you fermions in your spectrum, but if you are careful about consistency and such, you discover that it also gives you supersymmetry between all the fermions and bosons in the theory.

this is why they say that string theory predicts supersymmetry, although that s only roughly true.

when you are finished with this, you discover something nice: the tachyon has disappeared from your spectrum. this is excellent, since no one wants to live in an unstable vacuum.

also, careful counting shows that now, instead of 26, the theory must live in 10 dimensions. that s less than 26, but it s still pretty large, since i only live in 4 dimensions.

so to mitigate this problem, we propose that 6 of those 10 dimensions are very small, extending in some perpendicular directions only on supersubatomic sizes. this is called compactification, and it does all kinds of crazy things to the theory. for example, now the strings can not only have internal oscillations, but they can also wind around the extra dimensions, which changes their effective mass for us observers.

there was discovered the fact that a spacetime with small dimensions of size R is physically indistinguishable from the same theory with small dimensions of size 1/R. the inversion of the size of the small dimensions switches various things about the theory, but they add up so that we cannot tell the difference.

one of the things that switched was the fact that open strings (strings with endpoints), which before had endpoints which were free to move about, are now constrained to hypersurfaces called D-branes. in one theory, the strings had no momentum flowing out the end points, but the endpoints were free to move. in the dual theory, the endpoints are constrained, but the momentum flows out the ends, and lands on the d-branes.

with arguments like these, we see that d-branes have charge and energy, and can also move around like dynamical objects.

and i think that s all i m going to say for now.

Thankyou very very much lethe.

I dont understand all, infact many, of the things you talk about in that desription but it gives me a great layout of how it is which is what i asked for and things to look up to understand it better.

That was great.

ele,

I will only add this since I only know what the following link claims.

Strings that are unattached from the D-Branes and form oscillating loops are gravitons and are responsible for gravity.


Gravity isn't actually a weaker force we simply don't feel its full brunt since they are free to move. In fact they claim they can move from universe to universe in a multiverse concept.

The following link has several excellent videos on the subject.
Have fun.



http://www.pbs.org/wgbh/nova/elegant/program.html

The problem I have with Strings and the claim of extra dimensions is that their explanation doesn't explain how a loop isn't just a form withing our current 3D or 4D view.

This may just be a technicality. What they are calling extra dimension may well be infintesimal boundries of oscillations which are actually part of our 3/4d concept and are not actual dimensions. That would be the only way I could accept String Theory as being physically real. That is the claim of more dimensions is a misnomer.

Just as the strong nuclear force has a range limit, these strings and loops may be 3D objects but with finite limits on range.

The above is my own personal view without any scientific backing.

MacM
That site is pretty good, if a little slow to load over college network!
I've heard a couple of people saying things along the lines of 'I only percieve 4D, so only 4D can exist' which is along the lines of 'I can only percieve photons, EM and gravity, therefore that's all that exists' The second statement is provably wrong, but not, I don't think, by human perception alone.

This doesn't make it any easier to visualise/understand what the ten dimensions actually correspond to, but no-one's perfect.;)

thanks everyone

If gravity spills over into the extra dimensions, shouldn't that affect the inverse square law?

AndersHermanssons,

[qoute]If gravity spills over into the extra dimensions, shouldn't that affect the inverse square law?[/quote]

ANS: They don't address anything in that technical depth. I certainly wouldn't have your answer to that one.

geodesic,


This doesn't make it any easier to visualise/understand what the ten dimensions actually correspond to, but no-one's perfect.

ANS: Just to clarify. I don't see this as an issue of our "Observability". It is a matter of claiming tightly coiled form at or below planck length and calling them other dimensions.

I see this as fuzz on a tennis ball or a peach, still part of our 3D (4D) system.

It is more like "Degrees of Freedom" than other dimensions. Does that make sense.

Anderson it shouldnt affect the inverse square law because that is what we have observed, and can observed, due to that fact that observation is relative to ones frame of reference. Thus any "spilling" that occurs would not be detectable from our current frame of reference.

In a intro that I read recently put that in bosonic string theory, closed strings can have left-moving modes and right-moving modes. What this mean? Is this also valid for superstring theory?
More questions: What's the mechanism that causes spontaneous breaking of supersymmetry in superstring theory?

Originally posted by Lucas
In a intro that I read recently put that in bosonic string theory, closed strings can have left-moving modes and right-moving modes. What this mean? Is this also valid for superstring theory?


a closed string can have oscillations that travel clockwise around the string, and oscillations that travel counterclockwise around the string. the two modes are decoupled. the two modes are called the left movers and the right movers.

closed superstrings also have left movers and right movers.