Guest Post

A little treat for loyal CV readers: Tom Levenson is a professor of science writing at MIT, and the proprietor of the Inverse Square Blog, one of the most erudite scientifically-minded outposts in this blogosphere of ours. I’ve been enjoying how Tom writes engagingly about science while mixing in cultural and artistic references, so I asked if he would like to guest-blog a bit here at CV. This is the first of three posts he’ll be contributing; look for the other two later this week. [Here is two, and here is three.]

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Monday Isaac Newton blogging: A little light reading, Principia edition.

Update: See correction below.**

To introduce myself to the Cosmic Variance community (at Sean’s very kind invitation), let me just admit up front that I am a glutton for punishment.

Exhibit A: last year I read the Principia for pleasure.*

That’s not exactly right– it is more accurate to say that in the context of writing a book on Isaac Newton’s role as currency cop and death penalty prosecutor, I found myself reading the Principia as literature rather than the series of proofs it appears to be. Just like John Locke, who had to ask Christiaan Huygens if he could take the mathematical demonstrations on faith (Huygens said he could), I read to see what larger argument Newton was making about the ways human beings could now make sense of material experience. (This is, by the way, the only connection I can imagine that Locke and I share.)

What I got out of the exercise, more than anything else, was a reminder of how something we now mostly take for granted is in fact truly extraordinary: taken all in all, it seems genuinely remarkable that cosmology exists at all as a quantitative, empirical science.

That is: it is not obvious – or at least it wasn’t, all that long ago, that it would ever be possible to treat the universe as a whole as an object of study – especially given our very constrained vantage point from within that which we want to examine.

Most accounts of the story of modern cosmology more or less unconsciously downplay the strangeness of the claim that we can in fact make sense of the universe as a whole. They begin – mine did — with Einstein and the 1917 paper “Cosmological Considerations in the General Theory of Relativity, (to be found in English translation here.) Cosmology in this telling becomes more or less an inevitable extension of a recent advance in theoretical physics; the change in worldview precedes this extension of the apparatus of general relativity into a new calculation.

I recant: though I certainly wrote my version of this basic tale, reading Newton has reminded me of the much more radical change in the understanding of what it is possible to think about that had to precede all that cosmology (among much else) has achieved.

It certainly was not clear that the universe as a whole was subject to natural philosophical scrutiny in 1684, the year of Edmond Halley’s fortunate visit to Trinity College, Cambridge, and his more-or-less innocent question about the curve traced by a planet, assuming “the force of attraction towards the sun to be reciprocal to the square of their distance from it? that would produce an elliptical planetary orbit with the sun at one focus.

An ellipse inverse square relationship, Newton told Halley.

How did he know?

Why – he had calculated it.

By 1686, Newton had extended and revised his off-the-cuff answer into the first two books of Principia, both titled “The Motion of Bodies.” These pursued the implications of his three laws of motion through every circumstance Newton could imagine, culminating in his final demolition of Cartesian vortex physics.

But even though he had worked through a significant amount of mathematical reasoning developing the consequences of his inverse square law of gravitation, he left the ultimate demonstration of the power of these ideas for book three.

Books one and two had been “strictly mathematical,” Newton wrote. If there were any meat and meaning to his ideas, though, he must “exhibit the system of the world from these same principles.”

To make his ambitions absolutely clear Newton used the same phrase for the title of book three. There his readers would discover “The System of the World.”

This is where the literary structure of the work really comes into play, in my view. Through book three, Newton takes his audience through a carefully constructed tour of all the places within the grasp of his new physics. It begins with an analysis of the moons of Jupiter, demonstrating that inverse square relationships govern those motions. He went on, to show how the interaction between Jupiter and Saturn would pull each out of a perfect elliptical orbit; the real world, he says here, is messier than a geometer’s dream.

He worked on problems of the moon’s motion, of the issues raised by the fact that the earth is not a perfect sphere, and then, in what could have been a reasonable resting point for the book as a whole, he brought his laws of motion and gravity literally down to earth, with his famous analysis of the way the moon and the sun influence the tides.

Why not stop there? The story thus far had taken gravity from the limits of the observed solar system to the ground beneath each reader’s feet. More pragmatically – it told a story whose significance Newton’s audience would have grasped immediately: the importance of understanding the rules governing tides was obvious enough to the naval powers of the day.

No matter. Newton kept on going. The last section of his world-system turned to the celestial and seemingly impractical: the motion of comets, in an analysis of the track of the great comet of 1680.

Newton presented his findings through two different approaches: one produced by collecting all the data points he could of traveler’s observations and plotting the comet’s track against those points; and the other in which he selected just three points and calculated the path implied.

The two analyses matched almost exactly, and both showed that this comet did not complete a neat, elliptical orbit. Rather, it traced a parabola.

Newton knew what he had done. He was no accidental writer. A parabola, of course, is a curve that keeps on going – and that meant that at the end of a very long and very dense book, he lifted off again from the hard ground of daily reality and said, in effect, look: All this math and all these physical ideas govern everything we can see, out to and past the point where we can’t see anymore.

Most important, he did so with implacable rigor, a demonstration that, he argued, should leave no room for dissent. He wrote “The theory that corresponds exactly to so nonuniform a motion through the greatest part of the heavens, and that observes the same laws as the theory of the planets and that agrees exactly with exact astronomical observations cannot fail to be true.” (Italics added).

And now, finally, to get back to the point: this, I would argue, was the essential first and in some ways the most difficult step in the foundations of cosmology. With it Newton transformed the scale of the universe we inhabit, making it huge, perhaps infinite. Even more important, he demonstrated that a theory that could not fail to be true made it possible to examine one phenomenon — matter in motion under the influence of gravity — throughout all space.

That thought thrilled Newton’s contemporaries – Halley caught the mood in his dedicatory poem to the Principia, writing that “Error and doubt no longer encumber us with mist;/….We are now admitted to the banquets of the Gods;/We may deal with laws of heaven above; and we now have/The secret keys to unlock the obscure earth….” To catch a distant echo of that euphoria, just imagine what it would have been like to contemplate that ever receding comet, fifteen years into its journey towards who knew where at the time of Newton’s writing, and know that its behavior was knowable through an extraordinary act of human invention.

It’s a whole ‘nother story to ask what it would take to create a similar sense of pride and pleasure in a general audience today. But just to get the discussion going, I’d suggest that one of the oddities of contemporary cosmology as presented to the public is the degree to which the universe at large has become more homey; the very success in making the argument that there is a continuous scientific narrative to be told from the Big Bang to the present makes it harder to see just how grand a claim that is.

So, to end with an open invitation to this community: what would make current physical ideas as powerful and as intelligibly strange as Newton was able to make his story of a comet traveling from and to distances with out limit?

Last housekeeping notes: in one of the more premature bits of self-promotion in publishing history, the Newton material discussed above derives from my book, tentatively titled Newton and the Counterfeiter, coming early next year from Houghton Mifflin Harcourt (and Faber, for those of you across the pond).

Also – my thanks again to Sean Carroll for welcoming me here. If you want to see what I do when I’m at home, check out The Inverse Square Blog.

*If you are minded to pick up a copy of Principia, get this edition. Not only is it a well made book, easy to look at, well printed, with clear diagrams, it comes with the invaluable guide to reading the Principia written by I. Bernard Cohen. Accept no substitutes.

**Thanks to reader and award-winning physics teacher David Derbes for catching my inversion of the problem Halley put to Newton. Let this be a lesson to me: blog in haste; check one’s notes at leisure; repent in public.

Image: Woodcut by Jiri Daschitzsky, “The Great Comet of 1577.” Source: Wikimedia Commons.

You may have heard some of the buzz about a new result concerning the direct detection of dark matter particles in an underground laboratory. The buzz originates from a new paper by the DAMA/LIBRA collaboration; David Harris links to powerpoint slides from Rita Bernabei, leader of the experiment, from her talk at a meeting in Venice.

The new experiment is an upgrade from a previous version of DAMA, which had already been on record as having recorded a statistically significant signal of the form you would expect from the collision of weakly interacting massive particles (WIMP’s) with the detector. The experiment uses a challenging technique, in which their focus is not on eliminating all possible backgrounds so as to isolate the dark-matter signal, but to look at the annual modulation in that signal that would presumably be caused by the Earth’s orbital motion through the cloud of dark matter in the Solar System: you expect more events when we are moving with a high velocity into the dark-matter wind. Other workers in the field have not been shy about expressing skepticism, but the DAMA team has stood their ground; as Jennifer notes in her report from the recent APS Meeting, the DAMA collaboration home page currently features a quote from Kipling: “If you can bear to hear the truth you’ve spoken/ twisted by knaves to make a trap for fools,/ ……………you’ll be a Man my son!”

To help provide some insight and context, we’ve solicited the help of a true expert in the field — Juan Collar of the University of Chicago. I got to know Juan back in my days as a Midwesterner, and a trip to his bustling underground experimental empire was always a highlight of anyone’s visit to the UofC physics department. You can hear him talk about his own work in this colloquium at Fermilab; he’s agreed to post for us about his views on the new DAMA result, and more general thoughts on what it takes to search for 25% of the universe. I promise you won’t be bored.

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My dear friend Sean has me blogging: hey, I’ll try anything once. On the subject of the recent DAMA results no less, as per his request. I am normally a bit of a curmudgeon but… Sean, you really want the worst of me out there permanently on the internets, don’t you?

I’ll try to keep this to the point. A bard I am not, nor the subject invites any poetry. I have therefore chosen brief eruptions of flatulence as the metric and style for this piece. The result of indigestion, you see. I’ll start with the most negative, so as to end up on a brighter note:

• The modulation is undeniable by now. I don’t know of any colleagues who doubted these data were blatantly modulated already back in 2003, when “the lady” (DAMA) decided to keep mum for a while. However, to conclude from something this mundane that the experiment “confirms evidence of Dark Matter particles in the galactic halo with high confidence level” or that there is “an evidence for the presence of dark matter particles in the galactic halo at 8.2 sigma confidence level” is simply delusional. There is evidence for a modulation in the data at 8.2 sigma, stop. Compatible with what would be expected from some dark matter particles in some galactic halo models, full stop. Anything beyond this is wanting to believe, and it smears on the rest of us in the field. Of course, of course… there is no other observed process in nature that peaks in the summer and goes through a low in winter, so this must be dark matter, right? (Occam is turning in his grave, rusty razor still in hand. He is thinking a remake of that opening scene in “Un chien andalou“, with help from this little lady. I am channeling him loud and clear).
• Someone should take the DAMA folks aside for a beer, make them see the following. If one day soon we are all convinced that this effect was DM-induced (see below for what that will really take), they will be recognized for one of the greatest discoveries in the history of science, without them having to look desperate or foolish today. Or making the rest of us in the field do, by association: thanks DAMA, for cheapening the level of our discourse to truly imbecilic levels. (Sean, if you edit this I will scratch the paint off your car. I may not write blogs, but I do read them: I know how to hurt you).
• Deep breath. Having cleared the air some (or just made it toxic, whatever), it is not DAMA’s fault that there is a penury of signatures in this field of ours, laboratory searches for particle dark matter. The one possible exception to this is a detector with good recoil directionality and sufficient target mass to be truly competitive, but we don’t know of a good enough way to do this as of today (“good enough” folds in the price tag). People are still trying. The diurnal modulation in the DM signal that would be sensed by such a device is wickedly rich in features, extremely hard for nature to imitate with anything else. The annual modulation resides on the other side of this spectrum of complexity. It is the poor man’s smoking-gun to DM “evidence”. Inspected carefully, it is disappointingly feeble: different models of the halo can shift the phase of this modulation completely, turning expected maxima into minima and vice-versa, changing the expected amplitude as well. Add to this the fact that essentially every possible systematic effect able to pass for a “signal” can be yearly-modulated, for one reason or another. That’s the ones we can presently think of, and the ones yet to be proposed. To grow convinced that we have observed dark matter in the lab we’ll require a number of entirely different techniques, using a variety of targets, all pointing at the same WIMP (mass, cross sections), with additional back-up information from accelerator experiments and from gamma-ray satellite observations (so-called indirect searches). All of those lines crossing at one point, so to speak. This I (for one) will call “evidence”. I know of no single existing or planned DM experiment, including those I participate in, that would be able to make anything close to a bulletproof claim on its own. My advice to any overambitious individuals looking for a quick kill is to look elsewhere in physics. WIMP hunting is not it, no matter how important the discovery of these particles might be.

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Guest Post: Juan Collar on Dark Matter DetectionRead More »

Michelangelo is a grad student at Berkeley who had the fun opportunity to write a diary for the Economist that will continue through this week about his adventures doing particle physics in Antarctica. I would say more, but he does a pretty good job himself!

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First off, I’d like to thank Sean for giving me the chance to write this guest post. It’s not every day (in fact, this would be the first time) that I get to write something for a blog that I both read and enormously respect. This is an especially great opportunity since the scions of CV are graciously allowing me to do a bit of shameless self-promotion for a five-part journal, being published this week, that I got to write for the website of the Economist magazine.

Maybe I should back up and introduce myself. I’m currently a fifth year physics PhD student at UC Berkeley. My research is on the IceCube project, a neutrino physics experiment located at the South Pole. Basically, we’re building the world’s largest particle detector out of the polar icecap itself. Using hot water, we melt holes 2,500 m down into the ice and install very sensitive light detectors. This allows us to study the particle debris that results from collisions of high-energy neutrinos in the ice. Ultimately, we’re hoping to learn about the basic physics of neutrinos as well about the properties of some of the violent astrophysical objects that might produce them and send them hurtling through the universe towards our detector.

This means that I do what many physicists do. I sit in front of a computer, writing code and analyzing data. I do calculations and simulations. I drink coffee and talk and argue with colleagues. But it also means that I get to do something only a smaller subset of astrophysicists and physicists get to do. I get to travel to a really exotic location to do fieldwork.

I think this is an aspect of being a physicist that sometimes gets overlooked. It’s true that astronomers have always gone to mountaintops to build the best telescopes, and particle physicists have always traveled to underground accelerator facilities. However, fanning out to other locations to take advantage of particular natural features is something that has become increasingly important as we build bigger, deeper detectors to try to understand weak signals and/or rare and exotic phenomena. In recent years, physicists have been traveling to the vast Argentinean plains to understand the origins of the highest energy cosmic rays, particles that are constantly bombarding Earth. Folks who study the CMB and other long-wavelength radiation have been heading up to the high-altitude Atacama Desert (here, here, and here) and to the South Pole to take advantage of their thin, dry atmospheres. Selection and planning has been moving forward for a deep underground facility for doing basic neutrino and dark matter physics.

All this means that graduate students for years to come will have the exciting opportunity to go out into the field to do their work. While the research itself is exciting, traveling to these exotic locations brings us in contact with scientists from other fields doing all sorts of other great science. For those of us who get to go to Antarctica, we meet people on the cutting edge of climate and atmospheric research. For those working underground, they may encounter earth scientists or researchers study life in extreme environments. All of which make for rich and stimulating conversations and experiences.

This brings me back to the shameless self-promotion. When the Economist opportunity came up to share some of my experiences traveling, living, and doing research at the South Pole, I jumped at it. I’ve tried to squeeze in as much basic climate science and physics as I could, so if you’re interested, check it out here…

Science or Sociology?
Joseph Polchinski, 5/20/07

This is a continuation of the on-line discussion between Lee Smolin and myself, which began with my review of his book and has now continued with his response. A copy of this exchange (without the associated comment threads) is here.

Dear Lee,

Thank you for your recent response to my review. It will certainly be helpful in clarifying the issues. Let me start with your wish that I do more to address the broader issues in your book. When I accepted the offer to review these two books, I made two resolutions. The first was to stick to the physics, because this is our ultimate goal, and because it is an area where I can contribute expertise. Also, keeping my first resolution would help me to keep the second, which was to stay positive. I am happy that my review has been well-received. Your response raises some issues of physics, and these are the most interesting things to discuss, but I will also address some of the broader issues you raise, including the process of physics, ethics, and the question in the title. Let me emphasize that I have no desire to criticize you personally, but in order to present my point of view I must take serious issue both with your facts and with the way that they are presented.

Regarding your points:

The fictitious prediction of a non-positive cosmological constant. This is a key point in your book, and the explanation that you now give makes no logical sense. In your book you say (A) “… it [a non-positive cosmological constant] was widely understood to be a consequence of string theory.” You now justify this by the argument that a non-positive cosmological constant is a consequence of unbroken supersymmetry (true), so A would follow from (B) Unbroken supersymmetry was widely understood to be a consequence of string theory. But even if this were true, it would not support your story about the observation of the dark energy leading to a “genuine crisis, … a clear disagreement between observation and a prediction of string theory.” There would already have been a crisis, since supersymmetry must obviously be broken in nature; seeing the dark energy would not add to this. But in fact the true situation, as you can find in my book or in many review articles, was closer to the opposite of B than to B: (B’) Supersymmetry is broken in almost all Calabi-Yau vacua of heterotic string theory. We have no controlled examples because at least one modulus rolls off, usually to a regime where we cannot calculate. The solution to this problem may have to wait until we have a non-perturbative formulation of gravity, or even a solution to the cosmological constant problem.

In your response you largely raise issues surrounding B’, including the Witten quote, but I want to return to what you have actually written in your book. It is a compelling story, which leads into your discussion of “a group of experts doing what they can to save a cherished theory in the face of data that seem to contradict it.” It surely made a big impression on every reader; it was mentioned in several blogs, and in Peter Shor’s Amazon review. And it never happened. It is an example of something that that happens all too often in your book: you have a story that you believe, or want to believe, and you ignore the facts.

You go on to challenge the ethics of string theorists in regard to how they presented the issue of moduli stabilization in their talks and papers. I am quite sure that in every colloquium that I gave I said something that could be summarized as “We do not understand the vacuum in string theory. The cosmological constant problem is telling us that there is something that we do not understand about our own vacuum. And, we do not know the underlying principle of string theory. These various problems may be related.” The cosmological constant and the nature of string theory seemed much more critical than the moduli stabilization problem, and these are certainly what I and most other string theorists emphasized.

This scientific judgment has largely been borne out in time. In 1995-98 these incredible new nonperturbative tools were developed, and over the next few years many string theorists worked on the problem of applying them to less and less supersymmetric situations, culminating in the construction of stabilized vacua. Obviously many questions remain, and these are widely and openly debated. It seems like a successful scientific process: people knew what the important problems were, worked in various directions (a fair number did work on moduli stabilization over the years), and when the right tools became available the problem was solved. As you point out, the stabilization problem is nearly one hundred years old, and now string theorists (primarily the younger generation, I might add) have solved it. You are portraying a crisis where there is actually a major success, and you are creating an ethical issue where there is none.

AdS/CFT duality. You raise the issue of the existence of the gauge theory. There are two points here. First, Wilson’s construction of quantum field theory has been used successfully for 40 years. It is used in a controlled way by condensed matter physicists, lattice gauge theorists, constructive quantum field theorists, and many others. To argue that a technique that is so well understood does not apply to the case at hand, the scientific ethic requires that you do more than just say Not proven! Sociology! as you have done. You need to give an argument, ideally pointing to a calculation that one could do, or at least discuss, in which one would get the wrong answer.

I have given a specific argument why we are well within the domain of applicability of Wilson: there are 1+1 and 2+1 dimensional versions of AdS/CFT, which are also constructions of quantum gravity, and for which the gauge theory is super-renormalizable (and there are no chiral fermions): the counterterms needed to reach the supersymmetric continuum limit can be calculated in closed form – thus there is an algorithmic definition of the gauge theory side of the duality. You could perhaps argue that there will be a breaking of supersymmetry that will survive in the continuum limit, and we could sit down and do the calculation. But I know what this answer is, because I have done this kind of calculation many times (it is basically just dimensional analysis). Similar calculations, for rotational invariance and chiral symmetry, are routine in lattice gauge theory.

As a further ethical point, in your book you state that it is astounding that Gary Horowitz and I ignore the question of the existence of the gauge theory, and you then use this to make a point about groupthink (this is in your chapter on sociology). While you were writing your book, you and I discussed the above points in detail, so you knew that we had not ignored the issue but had thought about it deeply. You do not even acknowledge the existence of a scientific counterargument to your statement, and in saying that Gary and I ignore the issue you are omitting facts that are known to you in order to turn an issue of science into one of sociology. Again you impose your own beliefs on the facts; thus I am reluctant to accept as accurate the various statements that you attribute elsewhere to anonymous string theorists and others.

You raise again the issue of a weak form of Maldacena duality. As you know, it is very difficult to find a sensible weak form that is consistent with all the evidence and yet not the strong form. In my review I have gone through your book and papers and identified three proposals, and explained why each is wrong. Again, you have not acknowledged the existence of scientific counterarguments, but have just reasserted your original point. If your arguments had been made in a serious way, I would expect that you would have given some deep thought to them and be ready to defend them.

There are some interesting points, one of which I will turn to next.

The role of rigor and calculation. Here we disagree. Let me give some arguments in support of my point of view. A nice example is provided by your paper `The Maldacena conjecture and Rehren duality’ with Arnsdorf, hep-th/0106073.

You argue that strong forms of the Maldacena duality are ruled out because Rehren duality implies that the bulk causal structure is always the fixed causal structure of AdS_5, and so there cannot be gravitational bending of light. But this would in turn imply that there cannot be refraction in the CFT, because the causal structure in the bulk projects to the boundary: null geodesics that travel from boundary to boundary, through the AdS_5 bulk, connect points that lie on null boundary geodesics. Now, the gauge theory certainly does have refraction: there are interactions, so in any state of finite density the speed of propagation will be less than 1. (Since Rehren duality does not refer to the value of the coupling, this argument would hold even at weak coupling, where the refraction can be calculated explicitly.)

You have emphasized that Rehren duality is rigorous, so apparently the problem is that you have assumed that it implies more than it does. Generally, rigorous results have very specific assumptions and very precise consequences. In physics, which is a process of discovery, this can make them worse than useless, since one tends to assume that their assumptions, and their implications, are broader than they actually are. Further, as this example shows, a chain of reasoning is only as strong as its weakest step. Rigor generally makes the strongest steps stronger still – to prove something it is necessary to understand the physics very well first – and so it is often not the critical point where the most effort should be applied. Your paper illustrates another problem with rigor: it is hard to get it right. If one makes one error the whole thing breaks, whereas a good physical argument is more robust. Thus, your paper gives the appearance of rigor, yet reaches a conclusion that is physically nonsensical.

This question of calculation deserves further discussion, and your paper with Arnsdorf makes for an interesting case study, in comparison with mine with Susskind and Toumbas, hep-th/9903228. (I apologize for picking so much on this one paper, but it really does address many of the points at issue, and it is central to the discussion of AdS/CFT in your various reviews.) You argue that there are two difficulties with AdS/CFT: that strong forms of it are inconsistent with the bending of light by gravitational fields, and that the evidence supports a weaker relation that you call conformal induction. We also present two apparent paradoxes: that the duality seems to require acausal behavior, and negative energy densities, in the CFT. The papers differ in that yours contains a handful of very short equations, while ours contains several detailed calculations. What we do is to translate our argument from the imprecise language of words to the precise language of equations.

We then find that the amount of negative energy that must be `borrowed’ is exactly consistent with earlier bounds of Ford and Roman, gr-qc/9901074, and that a simple quantum mechanical model shows that an apparent acausality in the classical variables is in fact fully causal when one looks at the full quantum state. Along the way we learn something interesting about how AdS/CFT works.

This process of translation of an idea from words to calculation will be familiar to any theoretical physicist. It is often the hardest part of a problem, and the point where the greatest creativity enters. Many word-ideas die quickly at this point, or are transmuted or sharpened. Had you applied it to your word-ideas, you would probably have quickly recognized their falsehood. Further, over-reliance on the imprecise language of words is surely correlated with the tendency to confuse scientific arguments with sociological ones.

Finally, I have recently attended a number of talks by leading workers in LQG, at a KITP workshop and the April APS meeting. I am quite certain that the standard of rigor was not higher than in string theory or other areas of physics. In fact, there were quite a number of uncontrolled approximations. This is not necessarily bad – I will also use such approximations when this is all that is available – but it is not rigor. So your insistence on rigor does not actually describe how science is done even in your own field.

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Guest Post: Joe Polchinski on Science or Sociology?Read More »

You may have read here and there about the genteel discussions concerning the status of string theory within contemporary theoretical physics. We’ve discussed it on CV here, here, and even way back here, and Clifford has hosted a multipart discussion at Asymptotia (I, II, III, IV, V, VI).

We are now very happy to host a guest post by the man who wrote the book, as it were, on string theory — Joe Polchinski of the Kavli Institute for Theoretical Physics at UC Santa Barbara. Joe was asked by American Scientist to review Peter Woit’s Not Even Wrong and Lee Smolin’s The Trouble With Physics. Here is a slightly-modified version of the review, enhanced by footnotes that expand on some more technical points.

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This is a review/response, written some time ago, that has just appeared in American Scientist. A few notes: 1) I did not choose the title, but at least insisted on the question mark so as to invoke Hinchliffe’s rule (if the title is a question, the answer is `no’). 2) Am. Sci. edited my review for style, I have reverted figures of speech that I did not care for. 3) I have added footnotes on some key points. I look forward to comments, unfortunately I will be incommunicado on Dec. 8 and 9.

All Strung Out?

Joe Polchinski

The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next. Lee Smolin. xxiv + 392 pp. Houghton Mifflin, 2006. $26.

Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law. xxi + 291 pp. Basic Books, 2006. $26.95.

The 1970’s were an exhilarating time in particle physics. After decades of effort, theoretical physicists had come to understand the weak and strong nuclear forces and had combined them with the electromagnetic force in the so-called Standard Model. Fresh from this success, they turned to the problem of finding a unified theory, a single principle that would account for all three of these forces and the properties of the various subatomic particles. Some investigators even sought to unify gravity with the other three forces and to resolve the problems that arise when gravity is combined with quantum theory.

The Standard Model is a quantum field theory, in which particles behave as mathematical points, but a small group of theorists explored the possibility that under enough magnification, particles would prove to be oscillating loops or strands of “string.” Although this seemingly odd idea attracted little attention at first, by 1984 it had become apparent that this approach was able to solve some key problems that otherwise seemed insurmountable. Rather suddenly, the attention of many of those working on unification shifted to string theory, and there it has stayed since.

Today, after more than 20 years of concentrated effort, what has been accomplished? What has string theory predicted? Lee Smolin, in The Trouble With Physics, and Peter Woit, in Not Even Wrong, argue that string theory has largely failed. What is worse, they contend, too many theorists continue to focus their efforts on this idea, monopolizing valuable scientific resources that should be shifted in more promising directions.

Smolin presents the rise and fall of string theory as a morality play. He accurately captures the excitement that theorists felt at the discovery of this unexpected and powerful new idea. But this story, however grippingly told, is more a work of drama than of history. Even the turning point, the first crack in the facade, is based on a myth: Smolin claims that string theorists had predicted that the energy of the vacuum — something often called dark energy — could not be positive and that the surprising 1998 discovery of the accelerating expansion of the universe (which implies the existence of positive dark energy) caused a hasty retreat. There was, in fact, no such prediction [1]. Although his book is for the most part thoroughly referenced, Smolin cites no source on this point. He quotes Edward Witten, but Witten made his comments in a very different context — and three years after the discovery of accelerating expansion. Indeed, the quotation is doubly taken out of context, because at the same meeting at which Witten spoke, his former student Eva Silverstein gave a solution to the problem about which he was so pessimistic. (Contrary to another myth, young string theorists are not so intimidated by their elders.)

As Smolin charts the fall of string theory, he presents further misconceptions. For example, he asserts that a certain key idea of string theory — something called Maldacena duality, the conjectured equivalence between a string theory defined on one space and a quantum field theory defined on the boundary of that space — makes no precise mathematical statements. It certainly does. These statements have been verified by a variety of methods, including computer simulations [2]. He also asserts that the evidence supports only a weak form of this conjecture, without quantum mechanics. In fact, Juan Maldacena’s theory is fully quantum mechanical [3].

A crucial principle, according to Smolin, is background independence — roughly speaking, consistency with Einstein’s insight that the shape of spacetime is dynamical — and Smolin repeatedly criticizes string theory for not having this property. Here he is mistaking an aspect of the mathematical language being used for one of the physics being described. New physical theories are often discovered using a mathematical language that is not the most suitable for them. This mismatch is not surprising, because one is trying to describe something that is different from anything in previous experience. For example, Einstein originally formulated special relativity in language that now seems clumsy, and it was mathematician Hermann Minkowski’s introduction of four-vectors and spacetime that made further progress possible.

…

Guest Blogger: Joe Polchinski on the String DebatesRead More »

The quantum puppies post below was written in response to some excitement generated by recent work from Paul Kwiat’s group at UIUC; specifically, this paper in Nature (which is sadly only available to subscribers). Paul was nice enough to write a little clarification about what they actually did, which we’re reproducing here as a guest post.

Hi,

I’m not normally a Blogger (I also don’t have a cell phone, if you can believe that).
However, given the plethora of commentary on our article (and on articles *about* our article), I thought a few words might be useful. I’ll try to keep it short [and fail 🙁 ]

1. There has been quite a bit of confusion over what we’ve actually done (due in large part to reporters that won’t let us read their copy before it goes to print), not to mention *how* we did it. For the record—
   1. we experimentally implemented a proposal made several years ago, showing how one could sometimes get information about the answer to a quantum computation without the computer running. Specifically, we set up an optical implementation of Grover’s search algorithm, and showed how, ~25% of the time, we could *exclude* one of the answers.

      Some further remarks:

      – our implementation of the article is not “scalable”, which means that although we could pretty easily search a database with 5 or 6 elements, one with 100 elements would be unmanageable.

      – however, the techniques we discuss could equally well be applied to approaches that *are* scalable.

   2. By using the Q. Zeno effect, you can push the success probability to 100%, i.e., you can exclude one of the elements as the answer. However, if the element you are trying to exclude actually *is* the answer, then the computer *does* run.

      -The Q. Zeno effect itself is quite interesting. If you want to know more about it, you can check out this tutorial.

   3. Unless you get really lucky, and the actual answer is the last one (i.e., you’ve excluded all the others without the computer running, and so you know the right answer is the last element, without the computer running), the technique in 2. doesn’t really help too much, since if you happen to check if the answer wasn’t a particular database element, and it really was, then the computer does run.
   4. By putting the Zeno effect inside of another Zeno effect, you can work it so that even if you are looking to exclude a particular database element, and the answer *is* that element, then the computer doesn’t run (but you still get the answer). Therefore, you can now check each of the elements one by one, to find the answer without the computer running. This was the first main theoretical point of the paper. Contrary to some popular press descriptions, we did not implement this experimentally (nor do we intend to, as it’s likely to be inconveniently difficult).
   5. If you had to use the method of 4. to check each database element one-by-one, then you’d lose the entire speedup advantage of a quantum computer. Therefore, we also proposed a scheme whereby the right answer can be determined “bit-by-bit” (i.e., what’s the value of the first bit, what’s the value of the second bit, etc.). This is obviously much faster, and recovers the quantum speedup advantage.
   6. Finally, we proposed a slightly modified scheme that also seems to have some resilience to errors.

      Taken in its present form, the methods are too cumbersome to be much good for quantum error correction. However, it is our hope this article will stimulate some very bright theorists to see if some of the underlying concepts can be used to improve current ideas about quantum error correction.

2. There have been a number of questions criticisms, either about the article, or the articles about the article. Here are my thoughts on that:
   1. I guess I should disagree that our article is poorly written (no big surprise there 😉 ), though I agree 10000% that it is not at all easy to read. There are (at least) two reasons for this:

      – there is a tight length constraint for Nature, and so many more detailed explanations had to be severely shortened, put in the supplementary information, or left out entirely. Even so, the paper took over a year just to write, so that at least it was accurate, and contained all the essential information. For example, we were not allowed to include any kind of detailed explanation of how Grover’s algorithm works. [If you want more info on this, feel free to check out: P. G. Kwiat, J. R. Mitchell, P. D. D. Schwindt, and A. G. White, “Grover’s search algorithm: An optical approach”, J. Mod. Opt. 47, 257 (2000)., which is available here.

      – the concepts themselves are, in my opinion, not easy to explain. The basic scheme that we experimentally implemented is easy enough. And even the Zeno effect is not so bad (see that above tutorial). But once it becomes “chained”, the description just gets hard. (I am pointing this out, because I would reserve the criticism “poorly written” for something which *could* be easily [and correctly!] explained, but wasn’t.)

   2. I agree that some of the popular press descriptions left something to be desired, and often used very misleading wording (e.g., quantum computer answers question before it’s asked – nonsense!). Having said this, I do have rather great empathy for the writers – the phenomenon is not trivial for people in the field to understand; how should the writers (who *aren’t* in the field) explain it to readers who also aren’t in the field. Overall, the coverage was not too far off the mark.

      -To my mind, the most accurate description was in an article in Friday’s Chicago Tribune (the author kindly let us review his text for accuracy before going to print).

Okay, thanks for your attention if you made it this far.

I hope that these words (and the above web link) will clarify some of the issues in the paper.

Best wishes,
Paul Kwiat

PS Please feel free to post this response (in it’s entirely though) on any other relevant Blogs. Thanks.