The validity of string theory.

Obviously this topic is going to step on some toes. But I want people to have a good, honest think about string theory, about whether it actually qualifies as science.

Consider. It has arseloads of perfectly functioning equations and models. A huge number of people around the world either believe in it or say they do. A huge number of people around the world work with string theory, and their research grants and income depend on it. Yet it has produced not one shred of evidence of strings, and has not produced one real thing which is not part of other already functioning models.

But it has mathematical models which fit together nicely. Is that all it takes for it to be considered science?

 

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Yes, if you don't care about if it has anything to do with reality. I'm not trained in math but my sh*t detector goes off if the correspondence between math and reality isn't discussed in the popular science media. I look for any description of the physical picture. What can we relate it to in the real world. What is there about it that keeps people going down the path. I see nothing so maybe someone will show me something that makes any sense to a layman.

 

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So your **** detector goes into overload when physicists come up with quantitative models derived from a small number of statements which covers quantum field theory and general relativity but it doesn't go off when you admit you simply make up stuff without justification or predictive ability?

Good one.

Actually the money would be there irrespective of what people actually researched. If string theory were proved wrong tomorrow the people researching it would not lose their jobs, they would simply change research area. Some would go into QCD, others the MSSM or cosmology or LQG or whatever. The amount of money put into theoretical physics research is not dependent on the existence of string theory. If string theory were stopped the money would just be given for other research topics.

The existence of gravity. It's the only model of gravity which has not had gravity put in by hand.

But the major problem in theoretical physics is unifying things. Yes, we have seperate models for 4 forces of nature. But we want one model for all forces. And string theory is the ONLY known model which naturally incorporates gravity, is a quantum field theory compatible with gravity and which naturally includes gauge groups, cosmology and a number of technical things which are put in 'by hand' within usual quantum field theory.

The nearest competitor is loop quantum gravity and despite it being a theory whose initial idea is prompted by GR it cannot even recover the governing equations of GR. String theory can.

 

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I'm one of those who doesn't believe string theory is going anywhere - but at the same time I'm NOT opposed to working on it and the funding involved.

Even if it doesn't work out, there's always a chance that it might provide just enough to splinter off in a direction that does. Point being to keep on working and see where/if it eventually leads to something.

 

Precisely. In the 1960s people tries all kinds of things which were attempts to describe the interaction of what now are known as quarks. One of them involves modelling the interaction via a string, and became the founding of string theory, and another become QCD. An attempt to model the strong force became a model of gravity.

Pretty much any area of mathematics as application to physics. As an undergrad I wonders what the point of group theory, over and above mathematics, was. Turns up everywhere in physics.

String theory has a different perspective on just about every area of physics than the standard formulation. Some of these are extremely non-trivial and novel. We now examine QCD confinement and asymptotic freedom in terms of gravity, which we understand a lot better. Or we examine black holes in terms of gauge theory, which we understand a lot better. None of that is possible without string theory and even if we were to find those theories aren't models of nature they have told us a great deal which me might then find is applied to nature.

We know pretty much all pre 1900s physics is wrong in some way but the insight it provides makes it essential learning to anyone doing physics.

 

Even if string theory were proven wrong tomorrow, which I highly doubt, scientists would continue to use it to do research into strongly coupled quantum field theory which has been accepted and is supported by experimental evidence. That works via the AdS/CFT correspondence and it's generalisations. FYI QCD is strongly coupled at low energies and there is no other way to analytically study strongly coupled field theories except AdS/CFT.

 

Yes, but I did qualify it by saying, "my sh*t detector goes off if the correspondence between math and reality isn't discussed in the popular science media. I look for any description of the physical picture. What can we relate it to in the real world. What is there about it that keeps people going down the path. I see nothing so maybe someone will show me something that makes any sense to a layman."

Can you address that part for me. I'd like someone even if it is in the popular media to give me a physical picture if there is one. Just a description of what physical evidence there is for it or what its relationship is to the standard particle model (the particles we can observe anyway), or how the presence of mass is established and how gravity and mass are linked in string theory.

 

It is an attempt to unify two quite different areas of physics. We have good separate theories for gravity and sub-atomic forces which are within our ability to detect and anyone wanting to further examine something which we can measure will almost certainly end up working in one of those particular theories.

Those particular theories do not overlap in any experiment we can currently do. If sub-atomic forces are important, gravity is not. If gravity is important quantum theory is not. At least in experiments we can currently do. Hence any theory which makes an attempt to meld them all together will have to make statements which cannot be tested, as it is specifically aiming to merge things in regimes which we currently cannot examine. Small black holes are the canonical example. A black hole needs both quantum gravity and gauge theory but we can't make them and examine them so despite string theory saying a hell of a lot about them none of it can be tested.

The energy scales of such things are vastly beyond anything we can currently reach and 99.99% of string theory physics and predictions are at those scales. The 0.01% of predictions at reachable scales are unfortunately at the end of a great deal of complicated work. If you want to construct the standard model in string theory you need to compactify M theory down onto a particular 7 dimensional compact space (the right kind of 7d compact spaces were practically unknown pre 1990), solve all the equations of motion for all the fields (which is a bitch), work out your effective theory of point particles viewed in terms of M2 and M5 branes (which is a bitch), make sure all your configurations match comsological requirements and then run your resultant GUT theory down to the electroweak scale.

Each of those steps is a HUGE problem. Each of them is an entire area of research in itself. You need a full formulation of M theory. Not known. You need to know the properties of G2 structure manifolds. Not known. You need to know how to compactify such fields down. Difficult but mostly known. You need to solve all equations of motion, strong and weak couplings. Not known. You need to turn your membranes into effective points. Not known. You need to run your GUT down to get the standard model. Not known. Each of those currently have lots of people working on them. It's a huge group effort which is being chipped away at from all different sides.

But the point is it is possible. It's not that its impossible its that we don't know the specifics yet. You can't quantise gravity like you do electromagnetism. We know its impossible and thus we don't try. This is the only known approach which isn't killed by problems with gravity.

You're asking questions which I know, from experience with you, you throw around when talking about qwc. The relationship with the SM particles depends entirely on GUT constructions and is model dependent at present. As for 'presence of mass', that's your qwc phrase. String and M theory fields are not SM fields. You run your string or M theory fields down into an effective theory to give a theory which looks like a GUT of point particles. Then you do as any normal GUT does, you run your theory down from the GUT scale to the electroweak scale, get the SM particles and if its right you get the Higgs mechanism via the Higgs boson, which would be some effective viewpoint of a string or M theory mode.

Gravity in M or string theory is as it is in GR, it's the curvature of a space-time metric (at least on an effective level). Contributions to the energy-momentum tensor alter gravity. Masses of fields are determined by the eigenvalue of the fields under the Laplacian. Fields contribute to the energy-momentum tensor. On an effective level gravity, mass, equivalence principle etc is in string theory as it is in GR. It must be as that's the effective theory of gravity. At quantum levels string theory has things to say about space-time which have absolutely no analogy within normal quantum field theory and thus do not have a nice picture for the popular media to latch onto. But then it would naive to think everything could be summed up in a nice soundbite.

 

Thanks.

 

The funny thing about string theory is that it hasn't been proven wrong, but it hasn't ever been right either. All it has, the only thing it has, is an arseload of mathematical models which fit and agree with each other. That's it. Nothing more.

 

If you have a better idea to unify QM and GR...

The scientific community awaits.

 

Original Post:

But your goal in starting this thread is clear:

You don't want an honest assessment of string theory. You want to start a flame war.

Thread closed.