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By Edwin Cartlidge in Rome
Whether string theory can tell us anything about reality is a moot point. In the last two or three years this purported “theory of everything” – in principle unifying gravity with the three other forces in nature – has been given a kicking by certain scientists who see it as a kind of intellectual play thing that makes no testable predictions. See What Gina says.. for a taste of that debate.
Certainly the names of some of the talks at the world’s leading string-theory conference, Strings 2009, held in Rome this past week, were on the abstract side. “Holography and the S-Matrix”, “Superconducting black holes”, and “Stringy instantons and duality” give some flavour of the discussions held among the roughly 500 participants at the five-day conference. The fact that the meeting was held at the Pontifical University of Saint Thomas Aquinas only seemed to reinforce its other-worldliness.
But help was on hand for outsiders wanting to try and understand what on Earth this all means. Earlier today, particle physicist grandee Nicola Cabibbo introduced the curious of Rome to two of the big names of string theory – Edward Witten and Brian Greene. Witten, widely regarded as the leading figure in string theory, introduced himself with a few words of Italian and then told the audience what physicists hope to discover when they finally, hopefully, switch on the Large Hadron Collider at the CERN laboratory in Geneva this autumn.
In addition to the expected Higgs boson, the endower of all mass, Witten said that within the debris of particle collisions at the LHC might also be evidence of dark matter and of supersymmetry, which says that a whole slew of new fundamental particles must exist for there to be balance in the subatomic world. And one of the intriguing things about supersymmetry is that it could provide some kind of evidence for string theory.
It was at this point that Witten handed the baton to Greene. Greene is well known for his popularization of science, and with good reason. With some snazzy graphics and his flair for performing, he told us why it is so hard to come up with a theory of quantum gravity, explaining that the smooth variation of space-time as described by general relativity “runs headlong” into the turbulent, chaotic world of quantum mechanics. Postulating that the ultimate constituents of matter are tiny lengths of string, whose different modes of vibration correspond to different fundamental particles, is one way of resolving this problem, he went on, because such strings are like spread-out points that smooth the wild undulations at the smallest scale.
This model, however, has some very odd implications. Greene pointed out that string theory requires an extra 6 (or 7) dimensions of space in addition to the three that we are aware of. Helpfully, these dimensions are so small that we can’t see them, but unhelpfully there are rather a lot of ways of curling these extra dimensions up – some 10500 different ways as it turns out. And we would have to study all 10500 if we want to find out whether or not string theory describes the real world.
For Greene, all is not lost, however. He pointed out that 10500 is somewhat bigger than 10120, and that’s a measure of how much we don’t understand dark energy. In a nutshell he argued that if we happen to live in one of the few of the 10500 universes where conditions are just right for us to exist then there’s a damn good chance that we could have such an apparently statistically unlikely dark energy. For Greene, this suggests we might be on the right lines with string theory. Others may be less convinced.
In the article above, there’s a typo if I understand this at all.
10500 should be rendered as 10^500 — a much more impressive and daunting number. Or 10e500, I note this character set supports either.
String theory is far from being real for now. Somehow those hidden dimensions have to be found in order to establish the mathematical framework for string theory to be testable science. General Relativity as well as Quantum Mechanics have long been established and surpassed every single test. We have to be careful on how to unify them or can it really be possible? It’s easy to toss things around to make some sense of it but we need solid evidence.
Paraphrasing “But is it art?”, we could say about string theory: “But is it physics?”
If string theory is wrong, it was not a waste of time or effort. String theory has taught us lots about how to work with quantum gravity and the Theory Of Everything.
The point is that, as Peter Woit has put it, it’s not even wrong. Both him and Lee Smolin have discussed this extensively from different points of view. The original idea was worth pursuing, but the problem now is that it lacks all observational support and is being justified on quite unscientific concepts such as the so called anthropic principle. If this goes on it will simply give a bad name to physics, specially because these people pretend they are the forefront of the field.
Some physicists have their own doubts about the notion of ‘objective physical reality’ without anthropic principle ingredients. And indeed, following Einstein’s revolutionary requirement of fundamental physical quantities to be principally observable, that ‘reality’ would be unobservable and therefore not physical. A consistent definition of ‘objective reality’ is given by the great Irish philosopher of 18 century (and religious man) G. Berkeley; he stated that objective reality is that observed by god. The last term is not appropriate in physics. So I would like to rephrase it as: objective reality is indispensably united with the existence of an emergent community of intelligent observers at some places at some times; they will necessarily develop the idea of objective reality that ‘exist’ independent of their minds. As an inference, there cannot be a physical idea of objective physical reality without the anthropic ingredient.
‘These people’? ‘pretend to be at the forefront’? Sigh. As an experimentalist I find the level of feeling generated for and against string theory somewhat bemusing. Like everybody else (I guess) I have no idea whether string theory is correct or not. But the ‘not even wrong’ dismissal has always struck me as weird. I would have thought the distinction between something that makes no predictions at all and is not provable in principle (like ‘There is a god’) and something that makes predictions that are tough to test is pretty clear.
Why get upset about 10^500? Number theorists would consider it puny. That it’s finite at all is quite impressive and I don’t suppose the theory’s complete yet. Why so impatient? Bit early to give up. Where I do find common ground with string theory’s detractors however is that it can’t be good to have one set of ideas so totally dominating the field.
Finally, finding supersymmetry or even evidence for extra dimensions is well within the bounds of experimental possiblity and the LHC may give us that in just a year or two.
Probably, the practically infinite number of basic string theory solutions is an advantage of that theory. It is a needed background for an extreme theory describing objective physical reality by anthropic selection from a reach manifold of conceptual realities. From that point of view, anthropic selection is the new ‘crazy’ fundamental physical idea (in the original Nils Bohr meaning) of that intellectual ‘string theory’ achievement.
I don’t have a problem with anthropic reasoning in string theory. It makes perfect sense that, while trying to find the correct Calabi-Yau shape out of the 10^500 possibilites, we should throw out the ones that would obviously lead to an incorrect universe. For example, culling out those Calabi-Yau shapes that lead to a universe with more than 3 spatial dimensions is perfectly reasonable. Isn’t that itself an example of anthropic reasoning?
As long as we leave the concept of a deity out of the picture (an essentially unproveable concept), anthropic reasoning may well help us out of the “much more than a googol” hole in which we currently find ourselves.