Manny's Reviews > The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory

[Original review, written December 2008]

When I read this book, I remember thinking it was pretty interesting, but I am surprised how few insights I have retained... to be honest, hardly any. Smolin's The Trouble with Physics, which I read much more recently, suggests that string theory is in big trouble, and right now I am more tempted to side with Smolin.

There's this old Nasrudin story, where he's somehow ended up as judge in a court case. The D.A. really makes a good case, and Nasrudin can't restrain himself. "Yes, you're right!" he shouts. Then the defense lawyer gets up and makes his pitch, and Nasrudin is equally impressed. "Yes, you're right!" he shouts again. The court recorder clears his throat and leans over towards Nasrudin. "Your honor," he says respectfully, "they can't both be right!". Nasrudin shakes his head. "Yes, you're right!" he agrees.

Well, between Greene and Smolin I feel a bit like Nasrudin, but luckily I am not the judge here. Am I just agreeing with Smolin because I heard him most recently? Maybe. But trying to correct for that, I still think that there is a reason why Smolin seems more convincing and memorable, and why very little of what Greene says has stuck. String theory has become so divorced from experimental reality that it rarely if ever gives you that feeling you get from good science, of suddenly grasping a real physical phenomenon that you have known about for a while, but not understood.

I guess the example that makes me least happy is supersymmetry, according to which every particle has a supersymmetric partner. Compare this with the discovery of the periodic table in the late 19th century, or the development of the Standard Theory in the 60s and 70s. There, insightful people gradually realized that objects (atoms in the first case, subatomic particles in the second) were related in a complicated pattern. Most of the time the pattern fit, but there were a few holes, and they were later able to find the things (new elements, new particles) that filled in the holes! I was astonished to read that there is not one single particle which has a known supersymmetric partner - so far, it's all hypothesis, and perhaps none of these "selectrons", "photinos" etc actually exist. I'm not saying that this means supersymmetry is wrong; I'm just saying it means I don't find it exciting.

Maybe next year they will get the LHC working, discover a whole slew of supersymmetric partners (even one would be a lot), and put string theory on a proper experimental footing. If that happens, I'm sure I'll go back to reading books on this subject; I won't be able to stop myself. But until then, well, it may be beautiful math, but I feel no emotional connection to it. I'd love to hear from people who disagree, and can explain to me just what it is I'm missing out on.
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[Update, May 2011]

We had another particle physicist over for dinner last night. He'd come mainly to play chess, but when I found out that he was involved in looking for supersymmetric particles I took the opportunity to ask how it was going. Well: assuming he's to be trusted, and he sounded pretty knowledgeable on the subject, we should know pretty soon. The LHC is now up to high enough energies. They're collecting data. If supersymmetric particles exist, there is every reason to suppose that we'll have clear evidence of them within a year or two.

I wondered what would happen if they didn't find any supersymmetric particles? Would the theoreticians just retreat into saying that they needed a more powerful collider? Not so, said my informant; if the particles can't be found at the current range of energies, the predictions were wrong. Sounds like we're finally getting a straight up-or-down vote.

String theory, you can run but you can't hide!
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[Update, September 2011]

I knew it was too good to be true. We had yet another particle physicist over, whose PhD topic had been something to do with searching for a supersymmetric quark. I asked her if it really was the case that we'd soon know if supersymmetric particles existed.

Alas, it turns out that, although the energies they're now reaching in the LHC are indeed sufficient to find supersymmetric particle according to the mainstream versions of string theory, there are other versions which predict higher energies - energies which are outside the LHC's range.

"Of course," she added, "the mainstream version is the one that contains the original motivation for supersymmetry. If they retreat to one of the other versions, then most of the rationale disappears. But people have a lot riding on string theory."

"That's terrible!" I said indignantly. She just shrugged her shoulders.
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[Update, May 2015]

Browsing the physics section at the South Australian State Library earlier this week, I picked up a copy of Becker, Becker and Schwarz's String Theory and M-Theory (2007). The introduction says clearly that supersymmetry is essential to string theory/M-theory, and moreover that the LHC should be able to reach high enough energies to produce supersymmetric particles, if they do in fact exist. Consulting Google Scholar, my impression is that the book is highly respected: I see 661 citations.

Eight years later, no supersymmetric particles have been observed. But no doubt string theorists have an explanation for this inconvenient fact.
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[Update, Dec 2015]

Hey, if you think I'm being mean to those poor string theorists, just look at what Randall Munroe said the other day!


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[Update, August 2017]

It struck me today that the people who are criticising CERN for spending so much money finding the Higgs boson are wrong on at least two counts. First, $13B isn't actually such a large price tag for making a fundamental discovery about the laws of the universe, the truth of which is obvious only in retrospect; many physicists were unsure that the Higgs existed. Second, and perhaps even more importantly, there's the dog that didn't bark in the night. Many physicists were also expecting to find supersymmetric particles, but none have been detected. This greatly weakens the plausibility of string theory and shifts attention to competing theories for unifying quantum mechanics and gravity, of which by far the most attractive is Loop Quantum Gravity.

Speaking as someone who used to work for NASA and was involved with the International Space Station project ($150B and counting), I would say CERN has given the taxpayer value for money and then some. It's a pity that all research funding isn't allocated in such a responsible manner.
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[Update, March 2018]

On pages 368-9 of Leonard Susskind's 2008 book The Black Hole War, I find the following passage:

... there is a whole collection of particles whose existence is only conjectural, but a lot of physicists (including me) think they may exist*. For reasons that are not important to us here, these hypothetical particles are called superpartners.

* We will know within a few years, when the European accelerator called the LHC (Large Hadron Collider) starts operating.

Well, there it is again. Susskind, one of the foremost proponents of string theory and a world-renowned expert on fundamental physics in general, said ten years ago that the LHC would soon find the superpartners/supersymmetric particles if they were there. It hasn't found them. Ergo...