String Theory: Not Dead Yet

I know that everyone is waiting breathlessly for more opinionmongering about the String Wars. After Joe’s guest post, filled with physics and insight and all that stuff, it’s time for a punchy little polemic.

The folks at New Scientist noticed a comment of mine to the effect that, contrary to the impression one might get from the popular media, most string theorists were going about their research basically as they always have, solving equations and writing papers — curious about, but undeterred by, the surrounding furor. This surprised them, as their readers seemed to be of the opinion that string theory was “dead and buried” (actual quote). So they asked me to write a short op-ed piece, which appeared last week, and which they’ve allowed me to reprint here. Nothing deep about the substance of what physicists should be thinking about; just pointing out that string theory is still alive and kicking.

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A philandering string theorist is caught by his wife with another woman. “But darling,” he pleads, “I can explain everything!”

I didn’t invent the joke; it appeared in the satirical magazine The Onion. The amazing thing is that people got it! Apparently the person on the street is sufficiently caught up with current thinking in high-energy physics to know that string theory — the idea that the ultimate building blocks of nature are quantized loops of string, not pointlike elementary particles — is our leading candidate for a theory that would, indeed, “explain everything.”

But, despite capturing the popular imagination, string theory has fallen on hard times lately, at least in the public-relations arena. We read articles such as “Hanging on by a Thread” (USA Today), “Theorists snap over string pieces” (Nature) and “The Unraveling of String Theory” (Time). Much of the attention given to string skepticism can be traced to books by Lee Smolin and Peter Woit that appeared last year. But those aren’t the only sources; increasingly, professional physicists as well as fearless pundits outside the academy are ready to pronounce the failure of string theory’s ambitious project of uniting all of the forces of nature.

So is the jig up? Is string theory in its last throes? No, not at all. At least, not if we measure the health of the field by more strictly academic criteria. String theorists are still being hired by universities in substantial numbers; new graduate students are still flocking to string theory to do their Ph.D. work; and, most importantly, the theory continues to be our most promising idea for bridging the gap between quantum mechanics and gravity.

String theory is unique; never has so much effort been devoted to exploring an idea in physics without the benefit of any direct experimental tests. One important reason for this has been the absence of experimental surprises in all of high-energy physics; for thirty years, the Standard Model of particle physics has resisted all challenges. But even that would not have been enough to coax theorists into thinking about the famously difficult problem of quantum gravity if string theory hadn’t come along to present a surprisingly promising approach.

It was realized in the 1970’s that string theory was a theory of quantum gravity, whether we liked it or not — certain vibrating strings have the right properties to represent gravitons, carriers of the gravitational force. Already, this feature distinguished string theory from other approaches; whereas head-on assaults on quantum gravity tended to run into dead ends, here was a quantum theory that insisted on gravity!

In the 1980’s the triumph of the Standard Model became complete, and work by Michael Green and John Schwarz demonstrated that string theory was a consistent framework. Physicists who would never have though of devoting themselves to quantum gravity quickly dived into string theory. It was a heady time, when promises to compute the mass of the electron any day now were thrown back and forth. True, there were five different versions of string theory, and they all lived in ten dimensions. The trick would be to find the right way to compactify those extra dimensions down to the four we know and love, and the connection to observation would be established.

That didn’t happen, but the 1990’s were nevertheless a boom time. It was realized that those five versions of the theory were different manifestations of a single underlying structure, M-theory. Tools were developed, in certain special circumstances, to tackle a famous problem introduced by Stephen Hawking in the 1970’s — calculating the entropy of black holes. Amazingly, string theory gave precisely the right answer. More and more people became convinced that there must be something right about this theory, even if we didn’t understand it very well, and even if connection to experiments remained elusive.

Since 2000, progress has slowed. In the mid-90’s it seemed as if there was a revolution every month, and — perhaps unsurprisingly — that’s no longer the case. Instead of finding a unique way to go from ten dimensions down to four, current ideas suggest that we may be faced with 10500 or more possibilities, which is pretty non-unique. It might be — maybe — that only a tiny number of those possibilities are anywhere close to the world we observe, so that there are still concrete predictions to be made. We don’t know, and it may be wishful thinking.

The truth remains — the miracles that got people excited about string theory in the first place haven’t gone away. The biggest obstacle to progress is that we don’t understand string theory very well; it’s a collection of bits and pieces that show tantalizing promise, but don’t yet fit together into a coherent whole. But it is a theory of quantum gravity, it is compatible with everything we know about particle physics, and it continues to provide startling new ways to think about space and time.

Meanwhile, spinoffs from string theory continue to proliferate. Ideas about higher-dimensional branes have re-invigorated model-building in more conventional particle physics. The theory has provided numerous deep insights into pure mathematics. Cosmologists thinking about the early universe increasingly turn to ideas from string theory. And a promising new approach has connected string theory to the dynamics of the quark-gluon plasma observed at particle accelerators.

Ultimately, of course, string theory must make contact with data in order to remain relevant and interesting. But profound ideas don’t come with expiration dates; that contact might come next year, ten years from now, or a century from now. In the meantime, the relative importance of string theory within the high-energy physics community is bound to take a hit, as results from the Large Hadron Collider promise to bring us firmly beyond the Standard Model and present theorists with new experimental puzzles to solve. A resurgent interest in more phenomenological particle physics is already easy to discern in hiring patterns and graduate-student interests.

But string theory isn’t going to disappear. Gravity exists, and quantum mechanics exists, and the two are going to have to be reconciled. Ambitious theoretical physicists will continue to pursue string theory, at least until an even better idea comes along — and even then, the odds are good that something stringy will be part of the ultimate story.

  1. I’m just an ignorant telescope-monkey, so here’s a question I have that may be facile:

    I hear this “string theory demands the graviton” thing a lot, but the only explanation I’ve seen is that it predicts a spin-2 particle.

    Is there more to it than that? If not, why are we so sure that this must be the graviton? After all, you can find spin-2 composite particles that don’t couple to the curvature of spacetime in particularly special way. Is there something about a fundamental spin-2 particle that requires it to be the graviton? Or is there more to it than that?

    I also had the impression that coupling to spacetime curvature wasn’t something that was really worked out yet in string theory; is this wrong?

    -Rob

  2. A massless spin 2 particle is pretty much required to be a graviton by some results that go back to Feynman, I think. In addition, the Weinberg-Witten theorem tells you that a massless spin 2 particle can’t be composite.

    One can also look at the effect of a coherent state of these spin two particles and see how they modify the action. In fact, they exactly modify the metric as one expects. Finally, one can ask about the backgrounds on which one can do string perturbation theory. These backgrounds turn out to be precisely those which satisfy Einstein’s equations + matter + corrections. Thus, string perturbation theory satisfies every test one can think of (or at least that I can think of) to be the perturbation theory for a theory of quantum gravity.

  3. Dear Rob:

    Just to be more precise than Aaron, masless spin two particles have fewer degrees of freedom than massive ones. This means that the extra degrees of freedom that could have beeen there have been removed by a (gauge) symmetry. This in turns implies that the masless spin two particle has to couple to a conserved current of the right spin. The only option here is the stress energy tensor, so one finds that these particles have to be interpreted as gravity.

    This is very similar to what happens to the photon: with a spin one particle, you reproduce electromagnetism and Maxwell’s equations instead, and the conserved current is the electromagnetic current. If you have a lot of different spin one particles you might get a nonabelian gauge theory.

  4. Pingback: The philandering string theorist « Perfectly Reasonable Deviations

  5. Is there a nontechnical, intuitive explanation why the theory demands a spin-2 particle?
    George

  6. Someday, in a development that is possibly be more important than quantizing gravity, someone is going to solve the problem of print news sources rendering 10^500 as 10500. I suspect this would be counted as a major success by essentially every scientist in every discipline.

  7. Rob, in addition to all the excellent reasons why a massless spin 2 particle must be the graviton, there are also explicit calculations demonstrating that forgone conclusion, namely that the effective action of string theory is that of Einstein gravity coupled to matter (with known small corrections). I think that is what you meant by “coupling to spacetime curvature”.

  8. A tiny question wrt to the claim that it’s gravity: if the effective action is GR + matter the original action must include nonlocal effects with respect to the background metric as distances spacelike wrt the background metric can turn spacelike with respect to the physical metric in the effective action, right?
    Yet String Theory supposedly insists on Lorentz invariance, correct? And a Lorentz invariant causal theory can’t have non locality, correct?

    So how is this done in String Theory?

  9. Dear Sean

    You wrote, some time ago:

    “I have a long-percolating post that I hope to finish soon (when everything else is finished!) on `Why String Theory Must Be Right.’ ”

    Is this it with a slightly different title? (If so, congratulations for finishing everything else!)

  10. No, this wasn’t it! The hypothetical post will be filled with physics arguments. If it ever happens; I have many long-percolating posts.

  11. Pingback: Is there more to gravity than gravitons? « Theorema Egregium

  12. George, it’s just a matter of how strings vibrate.

    There’s no nontechnical explanation that can take the layperson one level deeper? Do strings oscillate in all possible ways, producing particles of all possible spins (including spin > 2)? If not, what effects screen out the untoward spins?

    George

  13. I’m not the one to ask. Strings do vibrate in all possible ways, but the different vibrational modes are associated with different energies, and therefore different masses; it’s the fact that you have a massless spin-two mode that gives you a graviton. But I don’t know a layperson’s explanation for why the spin-two mode is massless.

  14. I do not understand why some people hate string theory so much, with some rather stupid fights with string theorists. Why?

  15. Hi Rob #1:
    I was about to say roughly the same as David (#3). The spin 2 particle can only couple to the energy-momentum tensor – as gravity does.

    My related question to the experts: does that spin 2 particle appear in the volume element?

    Hi Sean,
    nice piece. Here’s my prediction for the next twist in this story: string theory is becoming increasingly attractive for the ‘outsiders’ and ‘rebels’ who like to see themselves as those ‘independent thinkers’ that ‘don’t fit into the system’ – right? Isn’t it ironic…

    btw,

    But it is a theory of quantum gravity […]

    Thanks for a careful a instead of the

    Best,

    B.

  16. Particles of higher spin are, IIRC, given by string states excited to higher modes (i.e., with more creation operators acting on their ground state). Because the square of the mass scales linearly with the level of excitation, the states beyond the graviton are heavier and therefore harder (perhaps impossible) to detect.

    Other massless states, produced by strings vibrating “just as strongly” but in a slightly different way, give you more exotic fields: Kalb-Ramond tensors and dilaton scalars. These are also massless but do not behave like gravity. The Kalb-Ramond field is a kind of one-dimension-up generalization of electromagnetism, and the dilaton mediates how strongly strings couple to one another.

  17. In order for string theory to remain a viable theory, is it true that the existence of super-symmetric particles be a necessary but not sufficient condition?
    If they do not find them in the LHC in a few years, will this send string theorists to the drawing board or are there modifications to M-Theory that circumvent the need for supersymmetry?

  18. Sean,

    Forgive me for popping up with a question perhaps only tangentially related to the subject at hand, but as a layperson only just now reading Susskind’s The Cosmic Landscape, I couldn’t think of a better place to ask the following question re Feynman’s Two-Slit experiment: if a photon is a particle that behaves like a wave (as per the experiment with single photons producing the interference pattern), could it be that the waveform is inherent in the (a?) brane and the photon is simply following it when two possible avenues are presented?

    Please forgive my ignorance if it is too readily apparent here. I am utterly fascinated by quantum mechanics and relativity. Thank you in advance for your time and patience.

  19. Moody, I’m afraid I just don’t know what that means. When it’s not being observed, the electron really is described as a wave, not a particle; we don’t need any additional explanation above and beyond that. Quantum mechanics works really well even without branes and extra dimensions!

    Khurram, string theory tends to predict supersymmetry, but not necessarily at energies accessible to the LHC. If the LHC discovers supersymmetry, string theorists will be happy, but if it doesn’t there’s no reason to give up on string theory — the superpartners might just be too heavy.

  20. I like this Monty Python clip better–describing the LQG whine “He’s repressing me”.

  21. Sean wrote:

    One important reason for this has been the absence of experimental surprises in all of high-energy physics; for thirty years, the Standard Model of particle physics has resisted all challenges.

    Why does the discovery of neutrino oscillation not count? Because it can be accomodated by changes to the SM? Or is not considered a “high-energy” surprise?