String Theory?

[ydoaPs]

many people have said that it is impossible to make any predictions or test them using string theory, but, in his second book, The Fabric Of The Cosmos, Brian Greene says the opposite.

 

Of course, the lack of evidence for extra dimensions might also mean they don't exist and that superstring theory is wrong. However, drawing that conclusion would be extremely hasty. Even decades before superstrs discovery, visionary scientists, including Einstein, pondered the idea of spatial dimensions beyond the ones we see, and suggested possibilities for where they might be hiding. String theorists have substantially refined these ideas and have found that extra dimensions might be so tightly crumpled that they're too small for us or any of our existing equipment to see (Chapter 12), or they might be lare but invisible to the ways we probe the universe (Chapter 13). Either scenario comes with profound implications.Through their impact on string vibrations,the geometrical shapes of tiny crumpled dimensions might hold answers to some of the most basic questions, like why our universe has stars and planets. And the room provided by large extra space dimensions might allow for something even more remarkable: other, nearby worlds-not nearby in ordinary space, but nearby in the extra dimensions- of which we've so far been completely unaware.

 

Although a bold idea, the existance of extra dimensions is not just theoretical pie in the sky. It may shortly be testible. If they exist, extra dimensions may lead to spectacular results with the next generation of atom smashers, like the first human synthesis of a microscopic black hole, or the production of a huge variety of new, never before discovered species of particles (Chapter 13). These and other exotic results may provide the first evidence for dimensions beyond those directly visible, taking us one step closer to establishing superstring theory as the long-sought unified theory.

 

Ill check out chapters 12 and thirteen tommarrow and post what he was talking about.

[Severian]

I posted about this somewhere else on the forums. Let me summarize here.

 

In order to have extra dimensions other than our usual 4 we have to hid them somehow, otherwise we would see them in every day life. Normally this is done by curling them up really small. Imagine a very long straw: if we move a long way away from it, it looks like a line (a one dimensional object) - we can't see that it is really a 2d surface with one dimension curled up. Similarly one proposal is that extra dimensions are curled up, so that we can't see them.

 

It was realised recently (a few years ago) that the experimental limits on this are not as strong as was assumed. If they are curled up really tightly them we have no hope of seeing them in current experiments, but it may be that they are not so tightly curled up after all. It may be that we can see them at the Large Hadron Collider (LHC). This would be very exciting because it could mean that string theories were relevant at much much lower energies than previously thought, and may be able to be investigated experimentally in our lifetimes!

 

Don't hold your breath though. The interest is because the experimental limits are in the ball park of the LHC experiment. (And secondly that the compactification could break electroweak symmetry, removing the need for the higgs boson.) Most likely the LHC will just say 'no extra dimensions here', and move on. There is still another 16 orders of magnitude in energy before the Planck scale - and they could be sitting anywhere in that.

[Martin]

Hi Severian, Yourda, the issue of prediction and testability of scientific theories is interesting.

 

Do either of you like to make the distinction between these two?

 

A. a theory bets its life on the outcome of some specific experiment that one can reasonably expect to do in the foreseeable.

 

B. a theory implies something indefinite---not connected to a concrete measurement at some specified scale or energy level that one can expect to attain. It implies some idea, but the idea does not lead to a prediction about some practical quantitative experimental outcome.

 

------

As an example of a prediction. Einstein published Gen Rel in 1915 and it predicted bending of light by a certain angle, something one could expect to see by checking positions of stars during the darkness of a total eclipse of the sun.

 

In 1919 Eddington's team purportedly measured the bending angle. They may have used bad equipment or made errors or fudged----we dont know, it is the kind of thing that is appropriate to repeat----but they thought they measured accurately and the world trusted them. It came out according to the prediction.

 

If it had not, it would have refuted the theory. In other words the theory bet its life on a definite prediction about a definite measurement by some concrete experiment that was practical to do.

 

I think that is an example of what it means for a theory to be falsifiable.

Some people, when they say a theory is "predictive" or "empirically testable" what they mean is that it is falsifiable. In 1915 when it was proposed, Gen Rel was falsifiable----it risked being refuted by the outcome of an experiment which had not been done but could be done (and in fact was).

 

I feel pretty sure that one or both of you are familiar with this distinction, so this is hardly news! The question for me is whether or not we want to make that distinction here at SFN forum in our discussion right here. there are no rights or wrongs---it concerns a voluntary choice about the use of language.

[Severian]

String theory is not really predictive or falsifiable. It is too general a term to be predictive: there are a lot of different string theories out there which behave very differently. And it is probably not falsifiable become the required energy scalaes are too high.

 

Large extra dimensions on the other hand are predictive and are falsifiable. They predict Kaluza-Klein states which would be visible at the LHC (at least to precision tests) and their phenomenology is rather constrained. They are definitely falsifiable because ther motivation for being at a low energy scale is to solve the hierarchy problem; if they don't do that there is no point in having them, and the theory goes away.

[Martin]

Hi Severian, I remember there was considerable talk of Large Extra Dimensions (people abbreviated it LED, although that seems a bit confusing) in past years.

 

But I dont know of any string theorist who is currently on record as saying to expect evidence of LED. Do people still expect to find Large Extra Dimensions, say, at the LHC when it goes into operation?

 

If you can, it would be great if you could say what predictions have been made that stand to be tested at LHC.

The predictions I hear being discussed are not specific to string, but are instead things like

electroweak symmetry breaking.

Michael Peskin, IIRC, did a rundown of what's to look for at LHC when he spoke at KITP25 conference a few days ago and i dont recall any string stuff at all. I guess that confirms your main point!

 

 

It was 7 October, here is the link to the KITP25 program.

http://online.itp.ucsb.edu/online/kitp25/

there are slides and audio

Michael Peskin is on the panel that comes right after Steven Weinberg and Frank Wilczek.

[Severian]

Yes, pretty much all of the LED predictions are basic phenomenology type stuff. No string theory at all. But (and this was Brian Greene's point) LED's imply that the Planck scale is much lower than we previously supposed, so you might expect that string theory would be probable. I haven't seen any phenomenology on this though, probably because the string theorists don't know much about expeiment and how to work out real phenomenology.

 

Take a look at this paper. It is not the most recent or anything particularly groundbreaking but you can use their bibliography to browse the literature. (I know the authors, which is mainly why I know this paper.)

 

This paper by Joe Lykken looks interesting too, and is surprisingly old. (Ironically enough, I think I was using his office during a visit to Fermilab round about the time he wrote this...)

[ydoaPs]

String theory is not really predictive or falsifiable. It is too general a term to be predictive: there are a lot of different string theories out there which behave very differently. And it is probably not falsifiable become the required energy scalaes are too high.

 

i would have thought that someone such as yourself would have known that the different strign theories are just part of the same theory. it is akin to the blind men describing an elephant. if each man is holing a different part of it, their descriptions will not match, but they will still be describing the same elephant.

[1veedo]

That's only in the case of 5 theories that in addition to 11D supergravity make up M-Theory. There are still other less known string theories that are not part of the M-Theroy map.

[Severian]

i would have thought that someone such as yourself would have known that the different strign theories are just part of the same theory.

 

Yes and no. Yes, in the sense that that M-theory, if anyone could actually calculate things with it, has limiting cases which are string theories. No, in the sense that the string theories themselves are actually not linked: they are different limits of the M-theory. So before you can make predictions you have to know which limit you are in (if any at all). So they are not very predictive.

 

If you are in a dark room with an elephant, can you predict which part of it you will touch if you stretch out your arm?

 

We would also have to know the correct pattern of symmetry breaking down to our energy scales...

[ydoaPs]

yes, they are linked. when i get home from school, in like 12 hours, i will check and post how they are linked without the use of m-theory