The August issue of Discover magazine is out, with a cover story entitled “Is String Theory About to Snap?”. The editors of the magazine describe how they recently became aware of the controversy over string theory when they organized a celebration of Einstein in Aspen last summer. They quote Lawrence Krauss as telling them “String theory may be in a worse position now regarding being testable than it has been at any time in the past 20 years.” To get a response to this, they asked Michio Kaku to write something for them. They refer to him as a “cofounder of string theory”, which I suspect some people might object to. Presumably they meant to repeat what is in their profile of him, which calls him a “cofounder of string field theory.”
Kaku’s article is entitled Testing String Theory, and is a thoroughly intellectually dishonest piece of writing, designed to mislead anyone without expertise in what is at issue here. He succeeded in misleading whoever wrote the blurb for the article which goes: “No experiment has ever allowed us to test whether any of the assumptions of string theory are true. That is about to change.” No it’s not. None of the experiments Kaku mentions will “allow us to test whether any of the assumptions of string theory are true”.
As I’ve explained in detail on other occasions, the simple fact of the matter is that string theory does not make any predictions, unless one adopts a definition of the word “prediction” different than that conventional among scientists. A scientific prediction is one that tells you specifically what the results of a given experiment will be. If the results of the experiment come out differently, the theory is wrong. String theory can’t do this, since it is not a well-defined theory, but rather a research program that some people hope will one day lead to a well-defined theory capable of making predictions.
At places in the article Kaku qualifies his claims of “predictions”, for instance saying near the beginning of the article that certain experiments “could provide significant evidence that would support string theory” (note all the qualifiers in this phrase: “could”, “significant evidence”, “support”) but that “the rub is that all the new evidence, no matter how compelling, will still provide only indirect proof.” He soon abandons his qualified language and starts talking about the following topics:
1. Gravitational waves: He says of gravitational waves created in the Big Bang: “String theory predicts the frequencies of such waves”, and that this prediction will be tested by LISA. I don’t know specifically what he has in mind here, but I know of no way to use string theory to make a specific prediction of the spectrum of gravitational waves that LISA will see. The only things he mentions are inflation and epkyrotic scenarios, the first of which has nothing to do with string theory, the second very little.
2. The LHC: Kaku discusses the possibility that superpartners exist, but does note that you don’t need string theory to have these. He also discusses possible Tev-scale particle physics effects of extra dimensions, without mentioning that string theory makes no predictions at all about what these extra dimensions are like, or even what their size is. There is absolutely no reason other than wishful thinking to expect extra dimensions in string theory of a size invisible until now, but visible at LHC energies.
3. Laboratory tests of the inverse-square law: Kaku claims: “according to string theory, at small scales like a millimeter, gravity might hop across higher dimensions and perhaps into other, parallel universes”. This is a load of nonsense. String theory predicts no such thing. It may be consistent with this, purely because it is consistent with anything. He does go on to say “Perhaps the additional dimensions would show up only on smaller scales — string theory is still somewhat vague about this prediction.” “Somewhat vague”??? As far as I know string theory makes no prediction about this at all, except that most string theorists expect effects to show up below 10-33cm, not 10-1cm.
4. Dark matter searches: according to Kaku “Once particles of dark matter are identified in the laboratory, their properties can be analyzed and compared with the predictions of string theory.” Only problem is string theory makes no such predictions. He’s talking about neutralinos, but in string theory the neutralino mass could be absolutely anything. After discussing these string theory”predictions” about dark matter, he goes on to speculate that maybe there is no dark matter anyway, just “huge clumps of shadow matter in a parallel universe, causing our galaxies to form in mirror-image locations”, then admits that such an idea is incapable of ever being experimentally tested.
After going through all this, he saves the real kicker for the end: “Some theorists, myself among them, believe that the final verdict on string theory will not come from experiments at all”. So he doesn’t even believe in any of the nonsense he has been spouting. He admits that “The principal reason predictions of string theory are not well-defined is that the theory is not finished.” So the earlier talk of “predictions” is now no longer operative. He goes on to invoke the pipe dream that someday someone will come up with a finished version of string theory that will predict precisely the standard model, neglecting to mention that there’s not the slightest evidence that this is a realistic possibility. On the contrary, all the evidence now points to the conclusion that, if string theory makes sense at all, it has an infinity of different vacuum states, and is probably a radically non-predictive theory. Impressive that Kaku could write a whole article about the prospects of string theory, and somehow neglect to mention the huge and very relevant controversy surrounding the idea of the landscape. Do you think he hasn’t heard about it?