A case for Strings (or whatever may bridge relativity to quarks)

[rrw4rusty]

Hi,

 

What follows may be complete BS and, if so, I’m sure all of you will tell me about it.

 

Newton described gravity with mathematics. Einstein described relativity with mathematics. Quantum Mechanics was described with mathematics. String Theory is described with mathematics… elegant, balanced, anomaly free (at one time anyway) mathematics that ties relativity, quantum physics and gravity together. It’s the first theory that does so. If you only look at mathematics… relativity, quantum mechanics and M-Theory are all mathematically correct (or, if not ‘completely’ correct, then to a significant degree).

 

However, we’re told that strings do not – indeed cannot make predictions that can be tested.

 

But… is that really true?

 

Isn’t it true that String Theory by its very nature should predict everything around us… predict Quantum Mechanics, predict relativity and everything else. However, if it does any of this, none of it counts. It doesn’t count because all these things have already been predicted and tested during the verification of relativity or quantum physics or other sciences or theories.

 

Lets toss out strings and say instead ‘the big question mark’ (B?). Hasn’t science piled up right against the B? on all sides? No matter what the B? turns out to be… if we’ve predicted and tested everything right up to the edges of B?... there may not be much left to predict or test.

 

I wonder – and this is perhaps an impossible hypothetical example – but if String Theory had come first and relativity and quantum physics had come afterwards… if String Theory would have had lots to predict and test while one of the other theories or, at least parts of another theory, would have been left out in the cold with nothing left to predict and test? That perhaps puts the chicken before the egg but you see what I mean.

 

If we took away that rule that says that ‘the things that have already been predicted and tested by other sciences can’t be used’, what would we see?

 

Well, probably we’d see lots to confirm string theory but… that is because these ‘already predicted and tested’ items were used to construct string theory in the first place.

Perhaps no matter what theory is found to fill in that last blank between relativity and the quantum world, it will find itself in the position where there is nothing left to predict and test – a classic ‘catch 22’ position.

 

Cheers,

Rusty

[ajb]

As it stands string theory makes only generic predictions as no-one has found the correct string vacua that applies to our world. Indeed, assuming it exists.

 

If it is found, and this would also require some explanation as why this one is chosen in nature then one should (mod calculation issues) make predictions.

 

I do not think you should think of relativity and quantum theory as separate from string theory as string theory (first quantised version) is a quantum theory that is obeys Einsteinian relativity.

 

The generic predictions are

 

1) 10 dimensions - to be anomaly free, that is quantum mechanically consistent.

2) gravity - the graviton is necessarily in the closed string spectrum. Also, string theory predicts small scale modifications to general relativity which may be testable.

3) supersymmetry - needed to include fermions and overall consistency.

4) whole host of scalar particles (string moduli) - arise due to various compactifications. (could be the inflaton?)

 

The biggest thing string theory has going for it, as a possible unification scheme are

 

1) It appears to be the only consistent theory of quantum gravity, in perturbation theory at least.

 

2) The symmetries are "large enough" to include the standard model. Yang-mills theories with SU(3)xSU(2)xSU(1) gauge groups can easily be incorporated. Chiral gauge couplings are also allowed, this has been a stumbling block for other unification schemes.

 

 

Hope that is of some help.

[Mr Skeptic]

The problems is that it's not String Theory, it is String Theories -- there are lots of them. Once you define the various parameters then you can say you have a string theory. However if that particular one is wrong it says nothing of the other ones. I think there were about 10⁵⁰⁰ different string theories, which would be far more than the number of atoms in the universe.

[ajb]

The problems is that it's not String Theory, it is String Theories -- there are lots of them. Once you define the various parameters then you can say you have a string theory. However if that particular one is wrong it says nothing of the other ones. I think there were about 10⁵⁰⁰ different string theories, which would be far more than the number of atoms in the universe.

 

This is the string landscape problem in M-theory.

 

The 5 consistent (defined via perturbation theory) string theories were understood as perturbations about different vacua of some 11-d encompassing theory called M-theory. This placed string theories in a non-perturbative context.

 

It turns out that you can interpolate between these theories smoothly.

 

The space of "string theories" is called the moduli space of vacua. The parameters you vary are called moduli, for example they can come from parameters like size and shape of the compactification.

 

In the low energy theory these moduli appear as scalar fields.

 

There is loads more to say about the landscape, but I am no expert.

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Consecutive posts merged

I should say that the landscape problem is not really any different to say the number of solutions in general relativity.

 

There are many many of them, but for most it is not clear if they have any physical significance.

[swansont]

Newton described gravity with mathematics. Einstein described relativity with mathematics. Quantum Mechanics was described with mathematics. String Theory is described with mathematics… elegant, balanced, anomaly free (at one time anyway) mathematics that ties relativity, quantum physics and gravity together. It’s the first theory that does so. If you only look at mathematics… relativity, quantum mechanics and M-Theory are all mathematically correct (or, if not ‘completely’ correct, then to a significant degree).

 

However, we’re told that strings do not – indeed cannot make predictions that can be tested.

 

And that last part is very important, because there are a lot of hypotheses out there that were described with mathematics and were wrong — they didn't describe how nature actually behaves.

[rrw4rusty]

Hi,

 

Thank you for all the informative replies. It sounds like a level of stability had been reached when there were 5 string theories but when M-Theory attempted to tie them together, strings slipped back into roller coaster mode -- string theory was again 'all over the place'. 10\500 theories sounds like the total number of values and combinations of values for a lot of variable parameters (moduli?). It sounds like string theory is floundering once again and, perhaps, that there is a mass exodus of scientists from the once popular field.

 

Perhaps I'm over or under stating the current situation.

 

Rusty

[ajb]

It sounds like string theory is floundering once again and, perhaps, that there is a mass exodus of scientists from the once popular field.

 

I don't know if there really has ever been a mass exodus.

 

What might be more correct is that the view that string theory may the correct framework to describe a unification scheme has waned.

 

String and M-theory seems to have small jumps and false starts.

 

There was the first string revolution in 1984 when Green and Schwarz showed that string theory is free of anomalies (for certain gauge groups). This meant that string theory appeared to be a possible unification scheme and did not suffer from the quantum inconsistencies that plague other schemes.

 

The second string revolution was in 1995 when Witten and others realised that the 5 superstring theories were all perturbative expansions about different vacua in M-theory. Fundamental to this was the role of p-branes and Dp-branes in string theory. This has lead to many new discoveries and deep links with mathematics. A good example of this is mirror symmetry and the AdS-CFT correspondence. Also of interest to me is the relation between string field theory and homotopy algebras. Though, I need to do some more reading on that.

 

 

M-theory although some things are known has until recently defied any Lagrangian description. What is know is that M-theory is 11d and consists on M2 and M5 branes (not D-branes). In 2006 Bagger and Lambert constructed a Lagrangian theory multiple M2 branes. The novel feature is that the fields involved take values in a Lie 3-algebra. (Brackets with three not two entries)

 

So, right now we are in the third string theory revolution. I expect to hear lots of work based on Bagger and Lambert's theory. (In fact there are already plenty of papers both physical as well as mathematical.)