String Theory

[needimprovement]

What is really a string theory? What are the fundamental principles and how the final theory will look like (in terms of strings or other fundamental degrees of freedom)? what are the major obstacles (inherent to string theory) preventing us from identifying these underlying principles and constructing this unique framework or theory?

[ajb]

What is really a string theory?

 

Classically it is the dynamics of 1-dimensional extended objects.

 

One can then apply the methods of first quantisation to a single sting to get the quantum theory.

 

What are the fundamental principles and how the final theory will look like (in terms of strings or other fundamental degrees of freedom)?

 

The fundamental principle seems to be superconformal symmetry and in particular in two dimensions.

 

what are the major obstacles (inherent to string theory) preventing us from identifying these underlying principles and constructing this unique framework or theory?

 

The main difficulties, as I understand them really rooted in the problem of describing interacting strings quantum mechanically. To my mind the best way to approach this is via string field theory. This has had various revivals over the years, but I think has not really been as useful as maybe once hoped.

 

 

The other problem for creating phenomenological models is the so called string landscape. Within string theory there appears to be many "universes" one of which (presumably) is ours.

 

If you are serious about learning some string theory have a look at BUSSTEPP Lectures on String Theory, R. Szabo.

[needimprovement]

Classically it is the dynamics of 1-dimensional extended objects.

 

One can then apply the methods of first quantisation to a single sting to get the quantum theory.

 

 

 

The fundamental principle seems to be superconformal symmetry and in particular in two dimensions.

 

 

 

The main difficulties, as I understand them really rooted in the problem of describing interacting strings quantum mechanically. To my mind the best way to approach this is via string field theory. This has had various revivals over the years, but I think has not really been as useful as maybe once hoped.

 

 

The other problem for creating phenomenological models is the so called string landscape. Within string theory there appears to be many "universes" one of which (presumably) is ours.

 

If you are serious about learning some string theory have a look at BUSSTEPP Lectures on String Theory, R. Szabo.

Thanks ajb for the incredible information. What is a multiverse? I don't think that multiverse theory is a necessary part of string or M-theory. In some cases, it seems the multiverse hypothesis is used as a way to escape the "anthropic" problem--i.e. that the basic constants of the universe we observe, seem to be fine tuned to support human life.

[ajb]

The multiverse is really an interpretation of quantum mechanics. It states that whenever a wave function collapse all the possible outcomes are real, that is another actual worlds exist realising the other outcomes we do not see.

 

It is not special to quantum string of M-theory. Rather any quantum theory can be interpreted in this way. I have no idea if such worlds really exist and if we can contact them.

[needimprovement]

The multiverse is really an interpretation of quantum mechanics. It states that whenever a wave function collapse all the possible outcomes are real, that is another actual worlds exist realising the other outcomes we do not see.

 

It is not special to quantum string of M-theory. Rather any quantum theory can be interpreted in this way. I have no idea if such worlds really exist and if we can contact them.

M-theory is interesting, but I don't think it has a particular connection to proving the existence of God.

 

Personally, I do not agree that a universe with a temporal beginning requires God but an temporally infinte universe doesn't. Both require a creator, because both sorts of universes are contingent, not necessary, realities.

 

There are all sorts of infinities. The series of even integers: 2, 4, 6, 8, etc, is an infinite series. And it is exactly equal to the infinte series of all integers: 1, 2, 3, 4, etc. Because they can always be matched up one for one, with none left over.

 

Sorry, I didn't mean to get into that......

[ajb]

I don't see how string and/or M-theory says anything about the existence or not of god(s).

[needimprovement]

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Edited September 14, 2010 by needimprovement

[needimprovement]

What exactly does M-theory say about the Big Bang and Planck scale physics? I understand that M-theory’s beautiful symmetry visible on the Planck scale must be broken at lower scale. Do cosmologists use M-theory to model Planck-scale Big Bang? What does M-theory say about the Planck era where GR becomes a singularity?

 

Does M-theory do what Hawking wants it to do, makes a falsifiable prediction of a multiverse, where other universes have different values of fundamental constants?

 

What does M-theory tell us was the physics of the Big Bang in this energy scale?

 

Can we take an M-theory prediction of planck scale physics in the Big Bang era, follow it forward in time into the future until it re-reproduces current Big Bang CDM model?

 

How do the predictions of M-theory differ or similar to Loop quantum cosmology?

[ajb]

Very little is known about M-theory in general.

 

I don't know much about stringy and brane cosmology, but yes people do consider such things.

[Severian]

I don't see how string and/or M-theory says anything about the existence or not of god(s).

 

Steven Hawking disagrees with you!

[needimprovement]

The multiverse is really an interpretation of quantum mechanics. It states that whenever a wave function collapse all the possible outcomes are real, that is another actual worlds exist realising the other outcomes we do not see.

 

It is not special to quantum string of M-theory. Rather any quantum theory can be interpreted in this way. I have no idea if such worlds really exist and if we can contact them.

Isn't it the "Many Worlds Interpretation" (often just "MWI"), not the "multiverse", in the M-Theory lingo? Isn't it M-Theory multiverse a different concept, and one that does proceed from M-Theory? Do you think the universes in the that multiverse model are not "forked copies" that cleave apart for every quantum decoherence, as is supposed to be the case in the Many Worlds Interpretation? In the multiverse model (Susskind calls this the "cosmic landscape"), a multitude of universe exist as completely independent universes, generated with different cosmological parameters according to the mathematical landscape that proceeds from M-Theory.

 

Do you think universes in the multiverse, as its used in physics are not the products of quantum decoherence?

 

To clear things up, do you believe there is a classification system being adopted where a "Level II" metaverse is what proceeds from M-Theory, and a "Level III" metaverse is what proceeds from the Many Worlds Interpretation? Ostensibly there is some metaverse that is implied by the combination of these two ideas, where a multitude of discrete universes are each forking of distinct new permutations for every wave function collapse. That's a fairly mind boggling production, and it's probably got a "class" name, but I don't know what it would be, and a simple Google to recall it didn't show anything obvious.

[ajb]

So, although string theory is probably a enqueue theory it has many vacua , each of one representing a "potential universe". This is the string landscape.

 

Taking the multivese in QM idea seriously, then I suppose each of these "string vacua universes" would then split-off into there own multiverses. Assuming that every string vacua is realised in nature somehow.

 

Honestly, I have not thought much along these lines.