Over the past few months, I have been asked a number of questions about String Theory and the Universe, including from readers Benhead and Mastery Mistery. But now Jamie, whom I’m going to marry later this year, has been asking me about it, and so it’s time to write something about the scientific topic of String Theory. (Send in your questions now, because I’ll answer them all this week if there’s enough interest.) Let’s start with this pair of questions:
String theory has been around for over 20 years, and so far, there is not one shred of experimental or observational evidence in support of this theory. Is it even a scientific theory at this point, and why do people still care about this at all?
First off, this hypothesis is that all the particles of matter that we now call “fundamental,” such as quarks, electrons, neutrinos, and photons, are actually different vibrational modes of the one truly fundamental thing: a string.
The theory says that some strings are open like a jumprope, and some strings are closed like a loop, and the different vibrations make up everything that we see today.
Let’s get to the first part: is it even a scientific theory at this point? Well, part of a scientific theory is that you have a hypothesis, and we’ve got that. The next part is that you have to either devise a way to experimentally test the theory or observationally test the theory. This is where the alarm bells go off. String theory has so many free, unconstrained parameters (literally, hundreds) that as far as being able to make scientific predictions as to the outcomes of various experiments or observational tests, it has never been able to definitively make one. Why not? Because for every value of these parameters (known as string vacua) that I can choose that predicts one thing definitively, there are other values I can choose that will predict the opposite. Since we don’t know what rules these string vacua follow, we can’t make predictions. All we can do is rule out some range of values for some parameters. So at best, what we’ve got with string theory right now is an untestable hypothesis, but nobody’s going to fund you if they ask you what you work on and you say “I work on the Untestable Hypothesis of Strings.” But that’s what it is. Or, to quote xkcd:
So this brings us to the second part: why do people still care? Well, as far as understanding how the Universe works, we’ve got the quantum world on one hand,
and gravity on the other.
We don’t know how gravity works on quantum scales. I’ll say it again in a different way, because that is the really important reason behind all of this. We know how gravity works on terrestrial scales and up, but experimentally, we only know how gravity works on scales down to about a tenth of a millimeter. What happens to gravity on atomic or subatomic scales? Not only don’t we know experimentally, but we don’t have a theory for it, either. String theory, it is argued, is the only way we know of to approach this problem. And it is a self-consistent mathematical framework for approaching this problem. And that’s its value, and that’s why people care.
But is that enough? What do you think?