# What experiments could be done *in principle* to help down-select between different versions of string theory?

I understand that string theory (broadly defined) is a solution to quantum gravity. That is, it is a unified theory the explains both quantum phenomena (such as the particles of the standard model observed in particle colliders) and gravitational phenomena (such as gravitational waves, black holes, curved spacetime etc.).

But I also understand that people are unhappy with string theory because of something like: "there are way to many versions of it and we don't know how to select the right version".

This somehow reminds me of the case regular modern day physics. Take for example the mass of a certain flavor of neutrino. We don't know the mass exactly, but based on certain experiments we can constrain the mass to be within some certain range that is compatible with existing experiments.

It sounds to me like the zoo of string theories are all consistent with all current experiments (if there is a theory that is inconsistent with current experiments then I would say the study of that particular theory is more mathematics than physics). But, if there are many different theories they must make different predictions about something, and those predictions could be tested.

What are these predictions? In other words, what experiments could we perform in principle to select between different versions of string theory?

I'm personally imagining things like:

• Take a massive particle and put it in a large spatial superposition. Examine it's gravitational effects on a test mass.
• Make an EPR pair, send one into one black hole and the other into another black hole. Monitor All the Hawking radiation in $$4 \pi$$ angle for both black holes until they both evaporate, draw some conclusion.
• Particle collider experiments that can detect gravitons or something in addition to the regular standard model particles.
• Entangle two particles and move them through regions of large gravitational curvature, observe something.

These are just random things I thought of that (1) involve a quantum gravity theory to make a prediction and (2) haven't been done yet to my knowledge. They're also things I just made up, so I'm curious if some string theorist could point me towards actual predictions/future experiment proposals that could in principle help us rule out certain flavors of string theory in favor of others.

Perhaps it's the case that we don't even need quantum gravity experiments to downselect between existing solutions to string theory. Perhaps for many solutions to string theory we don't even know if they are consistent or not with existing quantum and gravitational experiments. If this is the case I'd like to know this as well.

• Suggestion to post (v7): Replace the phrase versions of ST with the phrase solutions of ST. May 8 at 1:51
• The answer to this had some comment on your question. How does an isolated electron in deep space 'know' it is spinning? May 8 at 2:44
• "there are way to many solutions of it and we don't know how to select the right solution". This is not correct. there are thousands of string theory versions , not solutions. May 8 at 5:59
• @annav haha I had it called "versions" in the first version of the post but I was asked to change it to "solutions" so I did in an edit. I didn't really know why "solutions" was requested over "versions". May 8 at 6:03
• "string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings." en.wikipedia.org/wiki/String_theory .frameworks do not hav esolutions. May 8 at 6:06

Here are the tough problems related to that.

First, string theory is claimed to contain quantum gravity because it posits a massless, spin-2 particle which string theorists associate with the graviton. But that same theory does not automatically contain or produce the Standard Model, which string theorists instead assert is a low-energy approximation to the real, ultrahigh-energy string theory- an assertion which no one yet knows how to prove, or even to test.

This is because there are almost no testable predictions made by string theory in general which are in the energy scale accessible to us with our current accelerator technology.

• I see, so it sounds like the correct thing to say is that we don't know if string theory reproduces the standard model or not. Of course it sounds like string theorists hope it does. If that last statement is true then I would say that it is also not known is string theory is a solution to quantum gravity. May 8 at 3:30
• That said, implicit in your answer is a partial answer to my question. It sounds like "ultrahigh energy" collider/accelerator experiments would help us select between different string theory solutions. Is that true? May 8 at 3:39
• yes- but those experiments would require an accelerator several light-years long! May 8 at 3:54
• Do the different string theory solutions make concrete predictions at ultrahigh energy scales at least? Like can a string theorist make a concrete statement like: accelerate one string to $(1-10^{24}) c$, accelerate the other to the same speed in the opposite direction, collide the strings. We expect to see X, Y, Z type strings come out whereas a different string theory solution predicts A, B, C type strings? May 8 at 4:55
• "But that same theory does not produce the Standard Model" this is not correct. Sring theories can embed the standard model SU(3)xSU(2)XU(1) in the vibrational modes, have a mode for gravitons too. It is common calculational methods in particle physics to separate high energy /momentum behavior from the low energy one. Note they are the only theories that can naturally embed the symmetries. May 8 at 6:03

In order for a physics theory to be validated, it has to make predictions for measurable quantities, not only fit existing ones. To do that one must have a unique mathematical proposal , which can be used to calculate the various quantities.

As far as I know, string theories that might fulfill the job are a big multitude and no way known how to pick up the one that could be the theory of everything, at the moment. It is the objective of part of the research in string theories to do so.

Why are theorists working on string theories?

1. Because all versions can embed the standard model, i.e. the mathematical repository of all experimental measurements up to now. The standard model is full of symmetries that define the behavior of particle interactions. String theories with their vibrational states have the group structure that can accommodate all the symmetries of the standard model.

2. Because all versions can model quantized gravity , the holy grail of the search for a theory of everything.

Once a specific string theory is picked from the plethora, one would be able to calculate and predict data to validate the theory.

So experiments are looking for effects that are common to all string theories possible , particularly at supersymmetry, to point out if the string theory path for the theory of everything is viable.

Supersymmetry is an integral part of string theory, a possible theory of everything.

At present, if the experiments find supersymmetry, it will be a positive indication to go on with the search for the specific string theory model. Here is an article on the search for supersymmetry at CERN.

• your answer seems to say the opposite of what I concluded from niels nielsen's answer. You seem to imply that some (greater than one but maybe as many as all?) versions of string theory DO in fact reproduce the standard model. Would you agree with that statement? If so do we know which ones? Does any experimental work need to be done, or do we ONLY need to do theoretical work to determine which versions of string theory produce the standard model? May 8 at 17:39
• @Jagerber48 They do not reproduce , all string theory versions can embed the standard model particles and their symmetries in the string vibrations . The string vibrations have the group structure of SU(3)xSU(2)xU(1), plus the possibility of spin 2 vibrationa for the graviton. All have supersymmetry, that is why we are looking for it at the LHC. If supersymmetry is found in LHC it will be the first experimental validation of going for string theories for the theory of everything. May 8 at 18:02
• See for the complexity of the theoretical work needed en.wikipedia.org/wiki/String_theory_landscape May 8 at 18:02
• I wish I knew what you meant by embed vs. reproduce. The question is simple: does string theory (or "does any version of string theory") predict the results we have seen at the LHC up until this point? yes, no, or we don't know? May 8 at 18:45
• @Jagerber48 if you read the wiki link I gave, you will see that in order to get numbers to compare with data one has to chose a specific theory and axiomatically connect some of the particle masses , or various constants, in order to get specific predictions in numbers to check, and to make sure for the rest of the numbers coming out of the standard model to be correctly calculated . It is a long road. Embed means that there exist the location of particles, and their quantum numbers, but the masses are not predictable up to now , until the string theory is found. May 8 at 18:56