Will We Ever Prove String Theory?

No.

If you want to hear Cumrun Vafa’s latest “predictions of string theory”, there’s a podcast at Quanta magazine you could listen to. Over the years, Vafa has promoted various proposals for “predictions” of string theory. For instance, back in 2009 the Harvard Gazette reported on Cumrun Vafa briefs Large Hadron Collider scientists on string theory’s predictions. This was about some complicated, ugly scenario with no evidence for it that somehow could be sold as an LHC-testable “prediction of string theory”. Of course the LHC saw no such thing.

Here’s the podcast explanation of the latest complicated, ugly scenario with no evidence for it that is supposed to be a “prediction of string theory”:

Okay, so that’s the tiny dark energy. Now, as I just was telling you about, whenever there’s a tiny number or fine-tuned parameter in your theory, you should be asking what’s happening to these extra dimensions? Are they getting large or is there light string somewhere?

So already we are saying that having a dark energy, which is so extreme, must necessitate having new particles, where are they? On the other hand, we say there’s dark matter. So now we are saying that two facts, the fact that dark energy extreme and there’s some extra light particles around could naturally play the role of dark matter. So that’s the idea that already automatically comes from the swampland principles.

Now you can say, “Can we make it more quantitative?” It turns out that the dark energy predicts actually a length scale. And it turns out that length scale is about a micron. Micron is 1000th of a millimeter. And it suggests that exactly one of the extra dimensions is of roughly that size.

Now, you could ask then, what about the dark matter? Well, the dark matter would be the graviton waves, which were created in this extra dimension, what we call the dark dimension. So we have three spatial dimensions that we know, which are huge. One more dimension, which is this micron scale. And then the rest of them we think are much, much smaller.

So, therefore this one-micron dimension space will potentially carry in it some long gravity waves which would play the role of dark matter. So, in this context, we have a unification of dark energy and dark matter, just from this simple principle that when you have extreme values in your physical theory, there are light particles…

Because this tower of particles I was telling you about, which comes from these light particles has to be weakly interacting, which is the smoking gun of dark matter. It’s weakly interacting not only with us, but even with themselves.

So that is a property, it’s a prediction, I would say. So, we are making a prediction that whenever you have this dark energy being so extreme, you better look for light particles, which are very weakly interacting, just like our universe has it. So, this is a prediction for our universe.

And in fact, it makes another prediction: You cannot directly detect them because their interaction strength is gravitational. So, these direct dark matter detections will not succeed based on this study. So, we are making very specific prediction.

But actually you can make it even a stronger prediction. If you have two objects, two masses at the distance are, Newton taught us that there’s a gravitational force between them, which attracts them. And this force falls off with the inverse square of the distance between them. That is a property of three-dimensional space and one time. If you increase the number of dimension, each time you add one dimension, the power of the distance in the force law increases by one.

So instead of distance squared in three spatial dimension, if you have one extra dimension, it becomes distance cubed. And if you have more, it becomes distance fourth and so on. So, if we have one larger dimension, it should have been distance cubed. So, we are making a prediction that if you bring these objects together and put them at a distance roughly of a micron or so, you should find the stronger gravitational force between them.

This experiment to detect this is actually being undertaken now to bring it down from 30 micron perhaps to 10 micron and below to try to see if the force law changes as we are making a prediction. So that’s a very concrete prediction that we are making based on this link.

If you want to know what this is actually about, you could look at this, where you will read about a large number of different “predictions”, which somehow are derived from a grotesquely complicated scenario with no evidence for it which somehow is motivated by the swampland program, which somehow is motivated by string theory. As back in 2009, Vafa has a very idiosyncratic understanding of what the word “prediction” means.