# What are some experimental results that support the predictions of string theory?

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I've read about a number of different experiments that support the predictions of string theory and supersymmetry lately, and I am interested in a list of some such other results. Things that are not exactly experimental results, but otherwise still interesting hints in favour of string theory and its predictions, are also welcome, but are best off in the comments.

Note that I will be answering this question myself, but I'm looking forward to seeing other responses too.

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There was also a relevant article at this URL but it's unfortunately gone now. If anyone has a saved copy or an online archive, it would be appreciated if you could share it! Thanks!

answered Oct 18, 2014 by (1,955 points)
edited Mar 28, 2015

These are hints for low energy supersymmetry. Good idea to keep a systematic (and best: updated) list of these! But since low energy supersymmetry itself is only a hint for string theory, these are maybe better thought of as just "hints for hints for" string theory. Nothing wrong with that, but maybe good to keep in mind.

As an experimentalist, I consider that the fact that a string theoretical model can embed the standard model, is the best experimental support at the moment. Think about Keplers laws about planetary motion  , the data of centuries fitted like a glove. Even if new data were hard to come by no great imagination was necessary to see them as a perfect fit to the data.I think it is the great multiplicity of possible models of string theory that confuses the issue ( no such multiplicity in classical mechanics).

On the lines of possible string signatures, I keep remembering the soft photon excess seen in hadronic experiments for years . Here is a preprint by DELPHI. The excess of soft photons over the calculations (scale of 4), is still there as I checked with a colleague who had been chasing the effect over several experiments. Here is an overview of the soft photon data and theoretical modeling.

@annav, you say:

I think it is the great multiplicity of possible models of string theory that confuses the issue

Indeed, this has led  to confusion. I sense a little bit of that also when you continue to write:

(no such multiplicity in classical mechanics).

because this is not true: the available choices of "models" in classical mechanics is almost entirely unconstrained and forms a vastly infinite-dimensional spaceOn the other hand the space of consistent string backgrounds is highly constrained. It is also much more highly constrained than models in plain QFT (without special assumptions on the Lagrangian).

So the space of models in string theory (the "landscape") is certainly much smaller than the space of possible models in quantum field theory and much much smaller than the space of possible models in classical field theory (where not even anomaly cancellation gives a constraint).

What confuses people is that this space may still be large. It's a curious psychological effect: as long as the spaces of possible models (in classical and quantum field theory) were unimaginably large, nobody wondered. As soon as the space becomes small enough, in string theory, to get any sense of it at all (such as in arguments that it may actually be finite in some corners) people marvel at how big a finite number such as the iconic $10^{500}$ is.

It's like when you tell kids that there are $\aleph_1$points in the real line, they'll shrug, it means nothing to them. But when you tell them that there are at least a "thousand times thousand times thousand" points there, they'll be impressed.

@UrsSchreiber Could you please make clear with an example how another classical theory could fit the planetary data as well as the newtonian one?

@UrsSchreiber You are telling kids the continuum hypothesis is true?? No wonder they disagree...they are very well versed in logic :-P :-P :-P

@annav, classical mechanics (classical field theory) does not predict that there is precisely a gravitational force relevant at astrophysical distances, nor that there is a sun with planets of given mass at given distances. All that is part of the model. Once you specify the model, it makes predictions. But the model is chosen such as to make the right predictions, for if it wouldn't, it would be abandonded for a different model within the same theory.

The theory (classical mechanics, classical field theory) admits many, many models. Essentially any local Lagrangian on any space of fields is one model of classical field theory. That's a humongous space of models. Quantum field theory cuts this down a bit, by admitting only those local Lagrangians which are free of quantum anomalies. String theory cuts it down much more, admitting only those local Lagrangians which give scattering amplitudes that are the low energy limit of a string perturbation series induced from a 2d SCFT of central charge -15. That's very restrictive.

@URSschreiber  My simplistic analogy/ point is that once the planetary model was presented and the constants fitted ( yes, they could be anything) there is no other  competing classical mechanics theory  because this is what the data says and the predictions of the model, within classical dimensions/sizes  are correct.

A large number of string theory models  can incorporate the existing data that are encapsulated into the standard model , but people demand more than in the classical case , that a unique ST model will predict successfully beyond the standard model. This of course is desirable, but it should not leave the impression that the embedding of the standard model is trivial,  It is a solid experimental validation for the set of possible models from  which the final model will emerge..

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