String Theory and M Theory

[dstebbins]

I've heard a lot of string theory and it's descendant, M Theory, but what little I have heard leaves me with more questions than answers.

 

Where are these strings and membranes located? What is their nature? Are they made of energy or matter?

 

Also, I hear about eleven dimensions. I'm pretty sure I can understand this if I'm taught it, but I've heard no answers. All I hear is that there are eleven dimensions in the universe; ten spacial dimensions and one time. Where are these seven other spacial dimensions located and what's their nature? What exists in these seven extra dimensions?

 

I'm confused as hell. Can someone explain this to me?

[MolotovCocktail]

Well, it is supposed that these extra dimensions are wrapped up in the strings themselves. The idea is that on macroscopic and even atomic scales, things only appear to be 4 dimensional because the extra dimensions are so small that they cannot be seen. However, at Plank sizes, these extra dimensions can be observed. An analogy would be that of a strand of hair, in that from a distance it looks like a one dimensional line but close up it is 3 dimensional.

 

As far as I understand, these strings are supposed to be responsible for all of the physical laws that govern our universe, and it is theorized that particles are in reality specific resonant vibrations of these strings.

 

Here is some layman's article on the subject:

http://en.wikipedia.org/wiki/String_theory

[Martin]

string/M theory is in trouble

 

I'd advise you to take string popularization with a grain of salt

(or even a lot of salt )

[dstebbins]

Well, it is supposed that these extra dimensions are wrapped up in the strings themselves. The idea is that on macroscopic and even atomic scales, things only appear to be 4 dimensional because the extra dimensions are so small that they cannot be seen. However, at Plank sizes, these extra dimensions can be observed. An analogy would be that of a strand of hair, in that from a distance it looks like a one dimensional line but close up it is 3 dimensional.

 

As far as I understand, these strings are supposed to be responsible for all of the physical laws that govern our universe, and it is theorized that particles are in reality specific resonant vibrations of these strings.

 

Here is some layman's article on the subject:

http://en.wikipedia.org/wiki/String_theory

 

I checked out that wikipedia article, and I'm a little skeptical because it mentions 10^-35 m as the size for which these strings can be observed, yet I understand that atoms are around 10^-50 m, and you state that these extra dimensions are only visible below atomic levels.

[dstebbins]

string/M theory is in trouble

 

I'd advise you to take string popularization with a grain of salt

(or even a lot of salt )

 

???

[MolotovCocktail]

???

 

It's in trouble because there we haven't yet found away to test the theory through experiment. We need an enormous amount of energy in order to probe the plank length, and we don't have any idea on how to test for it indirectly either.

[Martin]

I checked out that wikipedia article, and I'm a little skeptical because it mentions 10^-35 m as the size for which these strings can be observed,

 

Yes, do be skeptical. It's not clear why there should be strings to observe at any scale.

the idea has come upon hard times

 

yet I understand that atoms are around 10^-50 m, and you state that these extra dimensions are only visible below atomic levels.

 

you seem off in your understanding by a factor of 10^40

 

10,000,000,000,000,000,000,000,000,000,000,000,000,000

 

atomic scale is 10^-10 m

[dstebbins]

Yes, do be skeptical. It's not clear why there should be strings to observe at any scale.

the idea has come upon hard times

 

 

 

you seem off in your understanding by a factor of 10^40

 

10,000,000,000,000,000,000,000,000,000,000,000,000,000

 

atomic scale is 10^-10 m

 

That wasn't the point. I was questioning the article's credibility.

 

And just because current technology doesn't allow us to study quark-like sizes shouldn't be a reason to discard it completely. In a little while, maybe in our own lifetime, technology might be able to magnify things 10^35 times. In fact, it could happen in just fifty years with the way things are going now.

[ajb]

The questions you pose are real research questions.

 

On the extra dimensions there are two solutions to this. Either they are compactifed, that is curled up so small that our space-time looks 4 dimensional or we live on some 4-d extended object known as a brane. (or maybe some combination of the two ideas).

 

As of yet, there is no good explanation as to why we live in 4 dimensions.

 

Depending on your mathematical background I suggest you read some introductory books on the subject. I suggest "An Introduction to String Theory and D-Brane Dynamics" by Richard J. Szabo or "A First Course in String Theory" by Barton Zwiebach.

 

Szabo has an online version of his book. Search on SPIRES to find it.

[dstebbins]

The questions you pose are real research questions.

 

On the extra dimensions there are two solutions to this. Either they are compactifed, that is curled up so small that our space-time looks 4 dimensional or we live on some 4-d extended object known as a brane. (or maybe some combination of the two ideas).

 

As of yet, there is no good explanation as to why we live in 4 dimensions.

 

Depending on your mathematical background I suggest you read some introductory books on the subject. I suggest "An Introduction to String Theory and D-Brane Dynamics" by Richard J. Szabo or "A First Course in String Theory" by Barton Zwiebach.

 

Szabo has an online version of his book. Search on SPIRES to find it.

 

Does it cost money? I'm kind of broke right now.

 

and what exactly do you mean by "mathematical background?"

[fredrik]

I checked out that wikipedia article, and I'm a little skeptical because it mentions 10^-35 m as the size for which these strings can be observed

 

A general (non-string specific) reason for this particular scale is that, according to what we is currently thought about unfication of general relativity and quantum Mechanics, beyond this scale identities themselves start to blur. The idea is that if you are to focus a sufficiently high energy that would required by QM for such "resolution" into a smaller object than that, the energy density is high enough to - according to GR - create a microscopic black hole. And it is not thought to make sense to probe the interior of a black hole, because focusing energy into it will only enlarge it, blurring things even more.

 

This is one of the basis for the expectations that whatever tomorrows theory will be, we suspect that there are logical reasons why we cannot probe infinitely deep into matter. It's not *merely* a technical or financial problem, there are thought to be more profound reasons.

 

/Fredrik

[ajb]

Does it cost money? I'm kind of broke right now.

 

and what exactly do you mean by "mathematical background?"

 

If you know a little quantum mechanics and special relativity then you should be able to cope with the books I suggest.

 

If you look at http://arxiv.org/abs/hep-th/0207142 you can get a free version of Szabo's book.

[Severian]

string/M theory is in trouble

 

I don't think that is true. String Theory is alive and well. There is plenty of research being done on it and I see no move by the funding agencies to curb it.

 

If you look at http://arxiv.org/abs/hep-th/0207142 you can get a free version of Szabo's book.

 

That is the 2001 BUSSTEPP course. I was asked to tutor at that one, but had other commitments, so couldn't go.

[ajb]

I don't think that is true. String Theory is alive and well. There is plenty of research being done on it and I see no move by the funding agencies to curb it.

 

I agree with this. Not that I work on string theory directly, I know plenty of people that do. What I would say is that is is a little frustrating that very little phenomenology has come from string theory.

 

That is the 2001 BUSSTEPP course. I was asked to tutor at that one, but had other commitments, so couldn't go.

 

I have met Szabo before, he seems a nice guy. I have also sopken to one of his students before.

[fredrik]

I'd personally encourage anyone paying interest in physics to think themselves and evaluate information on their own. If you think string theory is the radical change we all need, go ahead and blow the critics away.

 

But I think new students in particular (that I think tend to have an innocent mind and make the mistake that the teacher is always right) should do themselves and everyone else the favour and don't take anyone elses word for what is the future of physics. Use your own judgement, there is no acceptable replacement.

 

Considering the fact that to a certain extent at least some string physicists has discouraged other thinking among students and forced them either to align or to pursue other carriers - making them think that unless you like string theory this business isn't for you. I think it's in place that alternatives becomes visible unless someone missed it.

 

As someone beeing philosophically inclinced (which btw doesn't equal ignorant to math) it's beyond me how so many people can settle with the almost nonexistent improvement on the philosophical and logical foundations? That's why string theory smells like a mathematical game, lacking depth.

 

This is a line that I find amusing

 

"String theory is a rather radical generalization of quantum field theory whereby the fundamental objects are extended, one-dimensional lines or loops."

-- http://arxiv.org/PS_cache/hep-th/pdf/0207/0207142v1.pdf

 

/Fredrik

[ajb]

 

This is a line that I find amusing

 

"String theory is a rather radical generalization of quantum field theory whereby the fundamental objects are extended, one-dimensional lines or loops."

-- http://arxiv.org/PS_cache/hep-th/pdf/0207/0207142v1.pdf

 

/Fredrik

 

 

 

Why do you find that amusing?

[GutZ]

String Theory states that if you make a theory more interesting you will get more funding.

 

take that String Theory!!!!!!!!

 

I am just bitter because I don't understand it. Regardless though I doubt it's going to find proof for or against it anytime soon, even with CERN "almost" ready, or rather be ready and testing.

[fredrik]

Why do you find that amusing?

 

Because going from point to string is described as "radical"

 

I think from a pure mathematical point of view, the idea may be worth some exploration (as would other things) but to me I can't see how it possibly can qualify as fundamental radicalism.

 

Point to string, and string to brane. And then imagine that all previously known particles are various states of this extended object. Clearly any model based on higher dimensional objects are expected to have more adaptive power, but I fail to see the radical nature of this starting point - it is not fundamental, not in my eyes. A string is not much of an improvement to an answer to my questions than a point is, except from some extra mathematical features. I see no fundamental motivation for this, it seems to be an attempt at a mathematical exploit only, resting on the same old foundations of physical reality and spacetime as before.

 

It is clear from the beginning that the old concept of a point is a mathematical abstraction and idealisation due to limited resolution and simplified theories. In reality we could probably not distinguish a point from a neighbouring point anyway. And I am not sure I find the string much more sensible. From a philosophical point of view I see similar problems as with points.

 

I'd like to see a logical prescription on howto identify a string (in principle), as opposed to identify the fuzzball that could be a string to only a low level of confidence, but could also be something else within a reasonable level of confidence. Unless the string can be identified, string theory seems to me to be some sort of "hidden variable" approach, that uses some unobservable constructed objects to come up with a mathematical model of what we see. I for one will not accept such a fundamental solution. I'll keep looking because I am convinced that there is something much better that describes the nature of reality in a context of observations.

 

Instead of starting from a ad hoc starting point and hope to reproduce what we know, and then suggest that as a fundamental explanation, would it not be more natural to start from what we know, and prescribe how to move forward by analyzing real data and allow the data to suggest what new constructs that we need?

 

Anyway, I think the most important thing is for everyone to try to think for themselves. The one reason why I feel motivated to express my opinion (which is no more valid than anyonese else) is to counterbalance what in my personal experience has been dominating beyond motivation.

 

/Fredrik

[ajb]

Because going from point to string is described as "radical"

 

I think from a pure mathematical point of view, the idea may be worth some exploration (as would other things) but to me I can't see how it possibly can qualify as fundamental radicalism.

 

It is radical in the sence that it is different to quantum theory of point like particles.

 

 

 

Point to string, and string to brane. And then imagine that all previously known particles are various states of this extended object. Clearly any model based on higher dimensional objects are expected to have more adaptive power, but I fail to see the radical nature of this starting point - it is not fundamental, not in my eyes. A string is not much of an improvement to an answer to my questions than a point is, except from some extra mathematical features. I see no fundamental motivation for this, it seems to be an attempt at a mathematical exploit only, resting on the same old foundations of physical reality and spacetime as before.

 

 

The one thing it does seem to is include gravity. In the lower lying states spin-2 gravitons are present, so string theory naturally includes gravity. This I think is one of it's biggest selling points. That and the fact that one can also include gauge theories and potentially the standard model.

 

 

I'd like to see a logical prescription on howto identify a string (in principle), as opposed to identify the fuzzball that could be a string to only a low level of confidence, but could also be something else within a reasonable level of confidence. Unless the string can be identified, string theory seems to me to be some sort of "hidden variable" approach, that uses some unobservable constructed objects to come up with a mathematical model of what we see. I for one will not accept such a fundamental solution. I'll keep looking because I am convinced that there is something much better that describes the nature of reality in a context of observations.

 

It is possible that during inflation fundamental strings were "blown up" to the macroscopic level. These strings could be detected astronomically. However, I don't think it is clear how one could tell the difference between a cosmic string and a fundamental string.

 

So, in principle maybe one day we will see a string!

 

Instead of starting from a ad hoc starting point and hope to reproduce what we know, and then suggest that as a fundamental explanation, would it not be more natural to start from what we know, and prescribe how to move forward by analyzing real data and allow the data to suggest what new constructs that we need?

 

Well, so far there is no experimental observations that go against the standard model. So in that respect we are "finished".

 

However, there are problems with the standard model from a theoretical and philosphical point of view. It is these that need attention. (Also not to mention calculational difficulties).

 

 

Anyway, I think the most important thing is for everyone to try to think for themselves. The one reason why I feel motivated to express my opinion (which is no more valid than anyonese else) is to counterbalance what in my personal experience has been dominating beyond motivation.

 

/Fredrik

 

Yes, people should think for themselves, but science works by adding to current knowlegde. You would be very foolish if you went back to the "very begining" every time you tried to understand something. By this I mean one always works on top of ideas and constructions that other scientists have developed.

 

Currently, string theory looks like the best candidate for a unified theory. Also, to my knowledge there are no theoretical results from other approaches that go against string theory calculations.

 

Things like black hole entropy and discrete space-time on small scales all agree (maybe up to some numerical factors ect.).

 

Also of verry recient interests are gravity-gauge dualities such as the AdS-CFT correspondence. It is now possible to do calculations in QCD using gravity and string theory!

 

What I would say, is don't knock string theory untill you have had a go at string theory. Although I do not work on string theory directly, I know about it's sucesses and failures. It is these failures that allow research on the subject to continue.

 

On a personal note, I think that we will undertand a lot more about string theory when we have a good knowledge of string field theory. I think this is partly why string theory is difficult at the moment. Most approaches are based on first quantisation and not second quantisation. If string field theory can be developed then one might be able to use geometric ideas to get at string interactions. This I think would be a break through. At the moment adding interaction to first quantised strings is done "by hand". It is this that I feel makes things unclear.

[fredrik]

...but science works by adding to current knowlegde.

You would be very foolish if you went back to the "very begining" every time you tried to understand something. By this I mean one always works on top of ideas and constructions that other scientists have developed.

 

I'm glad you said this, this is exactly what it should ideally be like, and why I think we need a good fundamental framework, that can support this.

 

A framework that is *designed* for intelligent dataprocessing to start with and do just what you suggest. Trying to manually "add information" to exisiting knowledge based on static models, will result in a gigantic manual patchwork and allow for arbitration, that might not even be possible to finish. The model itself should be able to tell us how the optimum modificiation should be done. This has traditionally been done by hand, and human intellect. I feel that this is a primitive and old faishoned, and I definitely see a room for improvement.

 

A new framework doesn't mean we need to redo history or all experiments, a modern model should be readily trained to adapt to current knowledge at an estimated confidence level.

 

The thing is that such kind of model, places fairly high demands on logical consistency and founding all concepts ultimately in terms of data. And the current physics models simply wont do. Their philosophy and structure is old style that seems unsuitable for such things. QM and general relativity certain have elements that will live, but there is not yet a consistent unified logic - set aside the lack of experimental data in the relevant domains. This is what we need solved. And in that sense the suggestion that world is made out of excited strings simply isn't sufficient. In fact it's no improvement at all from my the described point of view, and is I think a wrong focus.

 

I expect a fundamental change to in detail, analyse the concept and nature of measurement, information and knowledge. The foundations of physics need to be more firmly attached to reality. Reality means handling uncertainties, noise. If we hide too distant mathematical abstractions without clearly defining it's observable connection I think we risk missing the whole point.

 

/Fredrik

[velo]

 

It is clear from the beginning that the old concept of a point is a mathematical abstraction and idealisation due to limited resolution and simplified theories. In reality we could probably not distinguish a point from a neighbouring point anyway. And I am not sure I find the string much more sensible. From a philosophical point of view I see similar problems as with points.

 

I'd like to see a logical prescription on howto identify a string (in principle), as opposed to identify the fuzzball that could be a string to only a low level of confidence, but could also be something else within a reasonable level of confidence. Unless the string can be identified, string theory seems to me to be some sort of "hidden variable" approach, that uses some unobservable constructed objects to come up with a mathematical model of what we see. I for one will not accept such a fundamental solution. I'll keep looking because I am convinced that there is something much better that describes the nature of reality in a context of observations.

 

Instead of starting from a ad hoc starting point and hope to reproduce what we know, and then suggest that as a fundamental explanation, would it not be more natural to start from what we know, and prescribe how to move forward by analyzing real data and allow the data to suggest what new constructs that we need?

 

Anyway, I think the most important thing is for everyone to try to think for themselves. The one reason why I feel motivated to express my opinion (which is no more valid than anyonese else) is to counterbalance what in my personal experience has been dominating beyond motivation.

 

/Fredrik

 

I am sure others are with you on this

[Severian]

To me, the most surprising thing about string theory was how long it took anyone to generalize it to branes. I would have though that the 'radical leap' was points to strings. But once your number of dimensions for the object is >0, I would have thought it very natural to increase the dimensionality.

[theCPE]

As someone has already posted, string theory and all of its derivatives are mathematical games and not physics at all. It really is a shame so many physicists get roped into following such a silly "theory".

[ajb]

As someone has already posted, string theory and all of its derivatives are mathematical games and not physics at all. It really is a shame so many physicists get roped into following such a silly "theory".

 

Spoken as someone who has much experience of string theory?

[theCPE]

I don't think anyone really has much experience of string theory. They may have much experience of advanced mathematics and balancing equations with extra dimensions though.

 

This is of course my opinion and therefore could be entirely wrong and in 50 years when a powerful enough accelerator is built maybe all these strings will come vibrating out...