There has been a lot of debate lately about string theory and testability (see for instance Hanging on by a Thread) . It seems to be one of the only theories in the modern time that is taken seriously by physicists in spite of the fact that it cannot be empirically tested, and in fact it is hard to see how it could be so tested. This gets a lot of people upset as it looks like string theory can’t be falsified and so shouldn’t count as a scientific theory.
Ed Witten is famous for, among other things, arguing that string theory does make one prediction. It predicts gravity. It is concievable, he argues, that some alien society discovered string theory before they discovered gravity. In that world gravity is a prediction of string theory and not of quantuum mechanics. So it merely an historical accident that in our actual world we discovered gravity first and string theory second. So there is a sense in which string theory has already made one bery important ‘prediction’ albeit one that is already established.
Is this all the evidence for string theory that we can muster? I have been thinking lately that perhaps we can get some evidence for something that is at least more string-ish than particle physics if we think about special relativity….So perhaps the most famous result of the special theory of relativity is the equivelance of mass and energy captured by E=MC^2. There have been two philosophical interpretations of this equivelance (see the Stanford Encyclopedia entry linked to above). There are those who take it to show that the properties picked out by ‘energy’ and ‘mass’ turn out to be the same property, and those who take it to show that there is no ontological distinction between fields and matter, or in other words that there is just one kind of stuff out there. It seems to me that either way one interprets the equivelance of mass and energy it provides some reason for thinking that a theory like string thoery will turn out to be correct (as opposed to a theory like particle physics).
So, say that you take it to show that there is only one kind of fundamental stuff out there that we can describe as either energy or mass. This is evidence for something string-ish in that string theory posits only on fundamental entity, viz. the string. Particle physics, on the other hand, posits a zoo of particles that are all made from different stuff. Electrons are made from one kind of stuff quarks from another kind of stuff. Thus particle physics as standardly construed seems fundamentally at odds with the ‘one stuff’ interpretation of E=MC^2.
On the other hand, let’s say that you take the equivelance to show that energy and mass are the same property, though that property may be had by several different kinds of stuff. This is evidence for something string-ish in that string theory posits that the one property is actually string vibration and so can explain what the property is as well as how it will seem to us that one converts into the other (i.e. the pattern of vibration changes). What is the candidate property that particle physics offers? It’s only answer is ‘a property, we know not what’. Thus the two fundamental properties of physics are rendered completely mysterious.
Now, I don’t think this second argument is as decisive as the first, and I also think that the property interpretation is more likely to be true, so I tentatively conclude that special relativity does give us some reason to prefer a string-ish theory over particle physics…but I will have to think about it some more…