Q & A: On String Theory

Posted by Ethan on April 14, 2008
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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?

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  1. #1 Bret
    April 17, 2008

    I don’t understand your argument above for why string theory is untestable. In fact, it seems to me you just outlined the best possible case for string theory! What you said above is that if I have a string theory construction of a phenomenon (say, the Standard Model or Inflation), which uses a set of parameters X, and makes some predictions, then I can find another set of parameters Y that gives a different set of predictions. Wonderful! This means I can determine the parameters (either X or Y) by making measurements, and rule out models and parameter sets!

    Perhaps what you meant to say is that the inverse of any observation always exists: for *any* observation, there *always* exists a matching string theory model which itself does not predict a unique future observation. Thus, string theory models are in one-to-one correspondence with all possible observations. It’s almost as if they parameterize all possible observations. Experience with building string theory models tells us that this is absolutely not true.

    In fact, it’s so incredibly hard to get a controllable string theory model of inflation (for example) that even matches current CMB observations – there’s not a whole lot of numbers to match there – let alone get anything I want. The reason is that string theory comes with very stringent consistency conditions. Satisfying these consistency conditions in realistic models rules out large classes of parameter values, and the parameters that remain seem to affect observational parameters. As an example, it is very difficult, if not impossible, to get an observable primoridal tensor wave signal from string theory inflation models (although there is recent progress in building potential models which do) – consistency conditions from string theory rule this out in large classes of models. So it’s simple – observe large tensors, falsify large classes (if not all) of models of inflation in string theory. Bing!

    The nightmare scenario for me is the following: suppose I describe a phenomenon (Standard Model or Inflation) in string theory with a set of parameters X, which gives some predictions. Then I consider another different set of parameters Y that give the same predictions. Thus, predictions are not unique, so I cannot distinguish models!

    It’s also not actually clear that my nightmare scenario is really the case in string theory – again, the few concrete (consistent) models of Inflation we have in string theory seem to be quite sensitive to the microphysical parameters used to describe them. So, parameter set X and set Y do seem to give rise to largely distinguishable models. As for the LHC and particle physics…more realistic models are needed before we can say much here, I think.

  2. #2 ethan
    April 17, 2008

    This sounds like Bret Underwood to me. Hi, Bret! I’ll write a separate post on this for you tomorrow.

  3. #3 revyaj
    November 4, 2008

    wonderful!!!!!!!!!

  4. #4 Widhye
    November 29, 2008

    thank you very much that have helped me to finish my homework