Ideas on M-Theory: Credibility & Evidence

[Benjamin113]

What do thoughts do you have on M-theory? It's credibility?

Also, is there evidence that these supposed membranes exist?

 

[Physics Monkey]

Right now there is no evidence for strings or membranes or anything else. I think this is hard to argue with. But that isn't really very surprising, there just isn't any evidence period. There's a decent chance we will never be able to probe the energy scales of quantum gravity.

Of course, string theory or M theory seem consistent and reduce to general relativity at long distances and low energies, so they aren't ruled out. But we haven't yet seen membranes, or strings, or extra dimensions, or even supersymmetry.

Note that this is logically unrelated to the consistency of M-theory. M-theory has been useful theoretically, and it's possible that it describes physics in our world through a kind of back door called holographic duality, but even this remains to be seen.

So to answer your original question, I think the theory is credible. It's certainly beautiful to many people, including me. At the very least it offers some interesting and potentially useful insights of a theoretical nature. Still, I'm often doubtful that we'll ever be able to really test whether there are M2 branes or whatever.

 

[tom.stoer]

First of all I would like to learn whether there is a definition of M-theory and how it looks like.

As far as I know there are only hints (in terms of string dualities) making it likely that M-theory exists. But to call it a theory there should be (a set of) equations plus calculational tools beyond string theory (you would not call the large-N limit of QCD an own theory but merely a calculation method only)

As far as I know the dualities in string theory have been established in certain limits only. That means that limit X of ST model A is dual to limit Y of ST model B. This is certainly impressive but M-theory shall provide something beyond these limits.

Compare it to water: you start with water, steam and ice and call X. But in order to do that you must a) calculate the phase transitions (which is something ST does) and b) identify the fundamental entity X=H₂O (which I expect from M-theory but which is missing).

Are there some steps into that direction?

 

[Demystifier]

It's certainly beautiful to many people, including me.

M-theory is not beautiful to me for the simple reason that nobody knows what the theory actually is. We know quite well what is perturbative string theory or what is 11-dimensional supergravity, and these are beautiful to me, but we don't really know what is M-theory.

 

[Fra]

Ideally a mature theory is either corroborated, or wrong.

Until there is a mature theory, I think all one can judge is the methodology of the research, they way questions are asked and so on.

IMO, I have no problem to see possible beauty in a possible remote undrestanding, but I've always felt that the questions generated by string thinking are not the right ones. In that sense I have low hopes that string thinkers are to solve their own puzzle. I find it' more likely that outsides solve it for them, by providing new angles.

That's not to say I don't see possible visions, that might resemble what string theory has, but I think the "string-path" is unattractive.

/Fredrik

 

[Physics Monkey]

M-theory is not beautiful to me for the simple reason that nobody knows what the theory actually is. We know quite well what is perturbative string theory or what is 11-dimensional supergravity, and these are beautiful to me, but we don't really know what is M-theory.

Interesting. I think M-theory is beautiful partly because it is mysterious and unexpected. Haha, not that perturbative string theory isn't beautiful too.

 

[Physics Monkey]

First of all I would like to learn whether there is a definition of M-theory and how it looks like.

As far as I know there are only hints (in terms of string dualities) making it likely that M-theory exists. But to call it a theory there should be (a set of) equations plus calculational tools beyond string theory (you would not call the large-N limit of QCD an own theory but merely a calculation method only)

As far as I know the dualities in string theory have been established in certain limits only. That means that limit X of ST model A is dual to limit Y of ST model B. This is certainly impressive but M-theory shall provide something beyond these limits.

Compare it to water: you start with water, steam and ice and call X. But in order to do that you must a) calculate the phase transitions (which is something ST does) and b) identify the fundamental entity X=H₂O (which I expect from M-theory but which is missing).

Are there some steps into that direction?

I wouldn't say we know a full definition of M-theory, or at least we're not sure if we do. But we know quite a lot about it. There are the dualities you mentioned. In particular, the unification of objects in IIA coming from M-theory on a circle is pretty dramatic. There is also the low energy limit, 11 dimensional supergravity, which is relatively well understood. We also have the matrix model approach coming from D0 branes. Then there is holographic duality giving us some understanding of the world-volume physics of M2 branes. And the work goes on, now with this 3-algebras business that some people have been excited about.

So I would agree that we're not there yet, but I don't think its that bad. We don't have an overall picture, but many corners of the theory are quite well formulated. In particular, I think the theory provides well defined calculational tools beyond string theory. And we have some hints about possible interpretations of the microscopic degrees of freedom i.e. matrices, etc.

 

[Demystifier]

Interesting. I think M-theory is beautiful partly because it is mysterious and unexpected.

Unlike you, I don't like the mysterious. But that should be obvious from my name, wouldn't it?

 

[atyy]

Unlike you, I don't like the mysterious. But that should be obvious from my name, wouldn't it?

But would you exist if there were nothing mysterious?

 

[tom.stoer]

There are the dualities you mentioned ... the unification of objects in IIA on a circle ... low energy limit, 11 dimensional supergravity ... matrix model approach coming from D0 branes ... holographic duality - world-volume physics of M2 branes ... 3-algebras business.

We don't have an overall picture, but many corners of the theory are quite well formulated. ... And we have some hints about possible interpretations of the microscopic degrees of freedom i.e. matrices, etc.

The first collection of hints is exactly what I mean; they are just hints (not more, not less). I have to admit that I do not know anything regarding this 3-algebra stuff.

Regarding the microscopic degrees of freedom: is there a review paper from which the basic ideas can be understood? I would like to understand the fundamental ideas and then see how to derive all the other stuff (what I currently know is just the other way round)

 

[Demystifier]

But would you exist if there were nothing mysterious?

Certainly not under the present name.

 

[atyy]

Certainly not under the present name.

Since you exist, M-theory also exists

 

[tom.stoer]

Cogito ergo M

 

[Demystifier]

Cogito ergo M