Search Results for: hawking

The God Particle

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Dennis Overbye does us all a huge favor by coming clean about “the God Particle.” The phrase refers to the hypothetical Higgs boson, long-time target of particle physics experiments. It was coined by Leon Lederman as a shameless ploy to sell books, and ever since has managed to appear in every single mention of the Higgs in the popular media — for example, in the headline of Dennis’s article from a couple of weeks ago.

Physicists, regardless of their stance toward timeless theological questions, hate this phrase. For one thing, it puts this particular boson on a much higher pedestal than it deserves, without conveying anything helpful about what makes it important. But more importantly, it loads an interesting but thoroughly materialist idea with absolutely useless religious overtones. Even harmful overtones — as Lederman himself notes, his coinage came about just around the time when creationism began to (once again) become a big problem, and this confusion was the last thing that anyone needed.

Furthermore, everyone knows that “the God particle” is misleading — even all of the journalists and headline writers who keep trotting it out. It’s just too damn irresistible. Particle physics is fascinating, but it takes some effort to convey the real excitement felt by experts to people who are watching from the sidelines, and a hook is a hook, shameless or not. If my job were writing about particle physics for a general audience, I doubt I’d be able to resist the temptation.

But, as Dennis notes, this God-talk is part of a venerable tradition on the part of physicists. We use “God” all the time to refer the workings of Nature, without meaning anything religious by it. Or at least, we used to; the nefarious encroachment of Intelligent Design and the religious right on our national discourse has given some of us pause. In the past I could have given a talk and said “Either you need a dynamical origin for the primordial cosmological perturbations, or you just have to accept that this is how God made the universe,” without any worry whatsoever that the physicists in the audience would have been confused. They would have known perfectly well that I was just using a colorful metaphor for “that’s just how the universe is,” in a purely cold-hearted and materialistic fashion. Nowadays I find myself avoiding such language, or substituting “Stephen Hawking” for “God” in a desperate attempt to preserve some of the humor.

All of which is to say: religion is impoverishing our language. I want God back, dammit.

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String Theory: Not Dead Yet

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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.

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Guest Post: Joe Polchinski on Science or Sociology?

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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 »

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

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How Did the Universe Start?

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I’m on record as predicting that we’ll understand what happened at the Big Bang within fifty years. Not just the “Big Bang model” — the paradigm of a nearly-homogeneous universe expanding from an early hot, dense, state, which has been established beyond reasonable doubt — but the Bang itself, that moment at the very beginning. So now is as good a time as any to contemplate what we already think we do and do not understand. (Also, I’ll be talking about it Saturday night on Coast to Coast AM, so it’s good practice.)

There is something of a paradox in the way that cosmologists traditionally talk about the Big Bang. They will go to great effort to explain how the Bang was the beginning of space and time, that there is no “before” or “outside,” and that the universe was (conceivably) infinitely big the very moment it came into existence, so that the pasts of distant points in our current universe are strictly non-overlapping. All of which, of course, is pure moonshine. When they choose to be more careful, these cosmologists might say “Of course we don’t know for sure, but…” Which is true, but it’s stronger than that: the truth is, we have no good reasons to believe that those statements are actually true, and some pretty good reasons to doubt them.

I’m not saying anything avant-garde here. Just pointing out that all of these traditional statements about the Big Bang are made within the framework of classical general relativity, and we know that this framework isn’t right. Classical GR convincingly predicts the existence of singularities, and our universe seems to satisfy the appropriate conditions to imply that there is a singularity in our past. But singularities are just signs that the theory is breaking down, and has to be replaced by something better. The obvious choice for “something better” is a sensible theory of quantum gravity; but even if novel classical effects kick in to get rid of the purported singularity, we know that something must be going on other than the straightforward GR story.

There are two tacks you can take here. You can be specific, by offering a particular model of what might replace the purported singularity. Or you can be general, trying to reason via broad principles to argue about what kinds of scenarios might ultimately make sense.

Many scenarios have been put forward among the “specific” category. We have of course the “quantum cosmology” program, that tries to write down a wavefunction of the universe; the classic example is the paper by Hartle and Hawking. There have been many others, including recent investigations within loop quantum gravity. Although this program has led to some intriguing results, the silent majority or physicists seems to believe that there are too many unanswered questions about quantum gravity to take seriously any sort of head-on assault on this problem. There are conceptual puzzles: at what point does spacetime make the transition from quantum to classical? And there are technical issues: do we really think we can accurately model the universe with only a handful of degrees of freedom, crossing our fingers and hoping that unknown ultraviolet effects don’t completely change the picture? It’s certainly worth pursuing, but very few people (who are not zero-gravity tourists) think that we already understand the basic features of the wavefunction of the universe.

At a slightly less ambitious level (although still pretty darn ambitious, as things go), we have attempts to “smooth out” the singularity in some semi-classical way. Aguirre and Gratton have presented a proof by construction that such a universe is conceivable; essentially, they demonstrate how to take an inflating spacetime, cut it near the beginning, and glue it to an identical spacetime that is expanding the opposite direction of time. This can either be thought of as a universe in which the arrow of time reverses at some special midpoint, or (by identifying events on opposite sides of the cut) as a one-way spacetime with no beginning boundary. In a similar spirit, Gott and Li suggest that the universe could “create itself,” springing to life out of an endless loop of closed timelike curves. More colorfully, “an inflationary universe gives rise to baby universes, one of which turns out to be itself.”

And of course, you know that there are going to be ideas based on string theory. For a long time Veneziano and collaborators have been studying what they dub the pre-Big-Bang scenario. This takes advantage of the scale-factor duality of the stringy cosmological field equations: for every cosmological solution with a certain scale factor, there is another one with the inverse scale factor, where certain fields are evolving in the opposite direction. Taken literally, this means that very early times, when the scale factor is nominally small, are equivalent to very late times, when the scale factor is large! I’m skeptical that this duality survives to low-energy physics, but the early universe is at high energy, so maybe that’s irrelevant. A related set of ideas have been advanced by Steinhardt, Turok, and collaborators, first as the ekpyrotic scenario and later as the cyclic universe scenario. Both take advantage of branes and extra dimensions to try to follow cosmological evolution right through the purported Big Bang singularity; in the ekpyrotic case, there is a unique turnaround point, whereas in the cyclic case there are an infinite number of bounces stretching endlessly into the past and the future.

Personally, I think that the looming flaw in all of these ideas is that they take the homogeneity and isotropy of our universe too seriously. Our observable patch of space is pretty uniform on large scales, it’s true. But to simply extrapolate that smoothness infinitely far beyond what we can observe is completely unwarranted by the data. It might be true, but it might equally well be hopelessly parochial. We should certainly entertain the possibility that our observable patch is dramatically unrepresentative of the entire universe, and see where that leads us.

Landscape

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How Did the Universe Start? Read More »

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Humankind’s Basic Picture of the Universe

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Scott Aaronson has thown down a gauntlet by claiming that theoretical computer science, “by any objective standard, has contributed at least as much over the last 30 years as (say) particle physics or cosmology to humankind’s basic picture of the universe.” Obviously the truth-value of such a statement will depend on what counts as our “basic picture of the universe,” but Scott was good enough to provide an explanation of the most important things that TCS has taught us, which is quite fascinating. (More here.) Apparently, if super-intelligent aliens landed and were able to pack boxes in our car trunks very efficiently, they could also prove the Riemann hypothesis. Although the car-packing might be more useful.

There are important issues of empiricism vs. idealism here. The kinds of questions addressed by “theoretical computer science” are in fact logical questions, addressable on the basis of pure mathematics. They are true of any conceivable world, not just the actual world in which we happen to live. What physics teaches us about, on the other hand, are empirical features of the contingent world in which we find ourselves — features that didn’t have to be true a priori. Spacetime didn’t have to be curved, after all; for that matter, the Earth didn’t have to go around the Sun (to the extent that it does). Those are just things that appear to be true of our universe, at least locally.

But let’s grant the hypothesis that our “picture of the universe” consists both of logical truths and empirical ones. Can we defend the honor of particle physics and cosmology here? What have we really contributed over the last 30 years to our basic picture of the universe? It’s not fair to include great insights that are part of some specific theory, but not yet established as true things about reality — so I wouldn’t include, for example, anomalies canceling in string theory, or the Strominger-Vafa explanation for microstates in black holes, or inflationary cosmology. And I wouldn’t include experimental findings that are important but not quite foundation-shaking — so neutrino masses don’t qualify.

With these very tough standards, I think there are two achievements that I would put up against anything in terms of contributions to our basic picture of the universe:

  1. An inventory of what the universe is made of. That’s pretty important, no? In units of energy density, it’s about 5% ordinary matter, 25% dark matter, 70% dark energy. We didn’t know that 30 years ago, and now we do. We can’t claim to fully understand it, but the evidence in favor of the basic picture is extremely strong. I’m including within this item things like “it’s been 14 billion years since the Big Bang,” which is pretty important in its own right. I thought of a separate item referring to the need for primordial scale-free perturbations and the growth of structure via gravitational instability — I think that one is arguably at the proper level of importance, but it’s a close call.
  2. The holographic principle. I’m using this as a catch-all for a number of insights, some of which are in the context of string theory, but they are robust enough to be pretty much guaranteed to be part of the final picture whether it involves string theory or not. The germ of the holographic principle is the idea that the number of degrees of freedom inside some region is not proportional to the volume of the region, but rather to the area of its boundary — an insight originally suggested by the behavior of Hawking radiation from black holes. But it goes way beyond that; for example, there can be dualities that establish the equivalence of two different theories defined in different numbers of dimensions (ala AdS/CFT). This establishes once and for all that spacetime is emergent — the underlying notion of a spacetime manifold is not a fundamental feature of reality, but just a good approximation in a certain part of parameter space. People have speculated about this for years, but now it’s actually been established in certain well-defined circumstances.

A short list, but we have every reason to be proud of it. These are insights, I would wager, that will still be part of our basic picture of reality two hundred years from now. Any other suggestions?

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Unsolicited advice, 1: How to get into graduate school

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Your humble bloggers here at Cosmic Variance have spent quite a bit of accumulated time in academic and research settings — in fact, my guess is that none of us have spent an entire year away from such a setting since the age of about six or so. That’s a lot of accumulated wisdom right there, and it’s about time we started sharing it. Since it’s that time of year when applications are being sent off to graduate schools, I thought I would start off by letting everyone in on the secret to how to get accepted everywhere you apply. Of course I can only speak for physics/astronomy departments, but the basic lessons should be widely applicable. [Update: see also Choosing a Graduate School and How to Be a Good Graduate Student.]

So, here goes: have great grades, perfect GRE scores, significant research experience, and off-scale letters of recommendation. Any questions?

If, perhaps, it’s a bit too late to put that plan into action, here are some personal answers to questions that come up during the process. Co-bloggers (and anyone else) are free to chime in with their own take on these complicated issues. Keep in mind that every person is different, as is every grad school — in fact, specific schools might behave quite differently from year to year as different people serve on the admissions committee. Don’t sink your sense of self-worth into how you do on these applications; there’s a strong random component in the decisions, and there are a very large number of good schools where you can have a fun and successful graduate career.

  • What do graduate school admissions committees look at?
    Everything. Keep in mind that, unlike being admitted to college (undergrad), at the grad school level the admissions are done by individual departments, with committees comprised of faculty members with different kinds of expertise, and often students as well. They’ll look at your whole application, and in my experience they really take the responsibility seriously, poring over a huge number of applications to make some hard decisions. Still, it’s well-known that careful examination of a thick file of papers is no substitute for five minutes of talking to someone, which schools usually don’t have the luxury of doing, so decisions are always somewhat fickle.
  • Even my personal essay?
    Well, okay. I wouldn’t sweat the personal essay; in my experience it doesn’t have too much impact. Let’s put it this way: an incredibly good essay could help you, but a bad essay won’t do too much harm (unless it’s really bad). To a good approximation, all these essays sound alike after a while; it’s quite difficult to be original and inspiring in that format.
  • Are GRE scores important?
    Yes. At least, in the following sense: while bad GRE’s won’t kill your chances, good GRE’s make it much easier to admit you. (We’re speaking of the Physics GRE, of course; the general tests are completely irrelevant.) It stands to reason: given two applicants from similar schools with similar grades and interests, there’s no reason for a department to choose the student with lower GRE scores. At the same time, you can certainly overcome sub-par GRE’s by being outstanding in other areas; this is particularly true for students who want to do experiment. I know at Chicago that we let in students with quite a range of scores.
  • What about research experience?
    Research can be a big help, although it’s by no means absolutely necessary. These days it seems that more and more undergrads are doing research, to the point where it begins to look unusual when people haven’t done any. There is some danger that people think you must want to keep on doing the kind of research that you did as an undergrad, although I wouldn’t worry too much about that. Mostly it shows some initiative and passion for the field. It can be very difficult to do theoretical research as an undergrad, but that’s okay; even if you eventually want to be a string theorist, it’s still great experience to do some experimental work as an undergrad (in fact, perhaps it’s especially useful).
  • How do I get good letters of recommendation?
    It’s more important to have letters from people who know you well than from people who are well-known themselves. One of the best side benefits of doing research is that you can get your supervisor (who hopefully has interacted with you quite a bit) to write letters for you. It’s really hard to write a good letter for a student who you only know because they took one class from you a year or two ago. Over the course of your undergrad career, you should find some way to strike up a personal relationship with one or more faculty members, if only to sit in their office now and then and ask some physics questions. Then they can write a much more personal and effective letter. Of course, if you are just a bad person who annoys everyone, it would be just as well to stay hidden. (Kidding!)
  • Is it true that the standards are different for theorists and experimenters?
    Typically, yes, although it might be different from place to place. Because a lot of undergrads haven’t been exposed to a wide range of physics research, a large number of them want to be Richard Feynman or Stephen Hawking or Ed Witten. Which is great, since we need more people like that. But even more, we need really good experimenters. Generally the ratio of applicants to available slots is appreciably larger for theorists than for experimenters, and schools do take this into account. Also, of course, the standards are a little different: GRE’s count more for prospective theorists, and research experience counts more for prospective experimenters. And let’s be honest: many schools will accept more prospective theorists than they can possible find advisors for, in the hopes of steering them into experiment once they arrive.
  • So should I claim to be interested in experiment, even if I’m not?
    No. Think about it: given that schools already tend to accept more students who want to do theory than they can take care of, what are your chances of getting a good advisor if you sneak into a department under false pretenses and have to compete with others who came in with better preparation? It makes much more sense to go someplace where they really want you for who you are, and work hard to flourish once you get there.
  • Do I need to know exactly what I will specialize in?
    Not really, although in certain circumstances it can help. Professors like to know that someone is interested in their own area of research, and might push a little harder to accept someone whose interest overlaps with their work; on the other hand, most people understand that you don’t know everything after three and a half years of being an undergraduate, and it can take time to choose a specialization. In particular, at most American physics departments (unlike other countries and some other disciplines), it is generally not expected that you need to know ahead of time who your advisor will be when you arrive, or which “group” you will work in.
  • Should I contact faculty members individually if I’m interested in their research?
    That depends, mostly on whether the person you are contemplating contacting is desperate for more grad students, or is overwhelmed with too many requests as it is. In popular areas (ahem, like theoretical particle physics, string theory, and cosmology), there are generally more applicants than departments have advisors for. In that case, most people who receive random emails from undergraduates will just urge them to wait for the admissions process to take its course; remember that it’s a zero-sum game, and for everyone who gets in there’s someone else who doesn’t, and it would be a little unfair to penalize those applicants who didn’t contact faculty members personally. On the other hand, if you have reason to believe that someone you’re interested in working with is trying to get more students, or if you think your case is somehow unique and requires a bit of attention, feel free to email the appropriate faculty member with a polite inquiry. The worst that can happen is that you get a brush-off; I can’t imagine it would actually hurt your chances.
  • Is my life over if I don’t get into my top grad school?
    Yes. Well, only if you let it be. The truth is, how you do in grad school and beyond (including how you do on the postdoc and faculty job market) depends much more on you than it does on where you go to school. In the next episode of “unsolicited advice,” we’ll think about how to actually choose where to go, including how to get the most out of visiting different schools.

Actually this episode was not completely unsolicited; thanks to Philip Tanedo for suggesting we share some of our invaluable insight. See, sometimes we really do listen.

Unsolicited advice, 1: How to get into graduate school Read More »

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Two cheers for string theory

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I am often surprised at the level of disdain and resentment with which string theory is viewed by non-string-theorists. I’m thinking not so much of people on the street, but of physicists, other scientists, and even other academics. As a physicist who is not personally identified as a string theorist, I get to hear all sorts of disparaging remarks about the field from experimental particle physicists, condensed matter physicists, astrophysicists, chemists, philosophers, and so on. I sometimes wonder whether most string theorists understand all the suspicion directed against them.

It shouldn’t be like this. String theory, with all of its difficulties, is by far the most promising route to one of the most long-lasting and ambitious goals of natural science: a complete understanding of the microscopic laws of nature. In particular, it is by far the most promising way to reconcile gravity and quantum mechanics, the most important unsolved problem in fundamental physics. At the moment, it’s a notably incomplete and frustrating theory, but not without genuinely astonishing successes to its credit.

The basic idea is incredibly simple: instead of imagining that elementary particles are really fundamentally pointlike, imagine that they are one-dimensional loops or line segments — strings. Now just take that idea and try to make it consistent with the rules of relativity and quantum mechanics. Once you set off down this road, you are are inevitably led to a remarkably rich structure: extra dimensions, gauge theories, supersymmetry, new extended objects, dualities, holography, and who knows what else. Most impressively of all, you are led to gravity: one of the modes of a vibrating string corresponds to a massless spin-two particle, whose properties turn out to be that of a graviton. It’s really this feature that separates string theory from any other route to quantum gravity. In other approaches, you generally start with some way of representing curved spacetime and try to quantize it, soon getting more or less stuck. In string theory, you just say the word “strings,” and gravity leaps out at you whether you like it or not.

…

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Two cheers for string theory Read More »

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Crackpots today… brain cells tomorrow?

We all know that Fafblog! is one of the funniest sites you can find on the internets, and in a somewhat darker vein Girls Are Pretty is extremely amusing. But if you’re looking for sure-fire guaranteed entertainment, it’s hard to beat Intelligent Design the Future, the new creationist website. It’s always good for a laugh, especially when they start talking about physics.

A recent post finds contributor Paul Nelson rubbing his hands together in undisguised glee — the physicists are talking about design!

Teapots today…cells tomorrow?
Paul Nelson

That sound you hear is Jerry Coyne’s head exploding. A few weeks after he organized an all-star team of evolutionary biologists and Nobel laureates to slap down design in Nature, that journal goes and publishes an essay by the cosmologist George Ellis, arguing the following:

I have to admit that I was a little worried upon reading this. George Ellis is a respected cosmologist, and co-author with Stephen Hawking of one of the best books on general relativity you can find, but he has been known to skate along the ragged edges of, shall we say, overly enthusiastic speculation. (Well, so have I.) And he is himself religious, indeed a Templeton Prize winner. (Well, so is Freeman Dyson — nobody’s perfect.)

Here is the quote from Ellis’s essay:

Our environment is dominated by objects that embody the outcomes of intentional design (buildings, books, computers, teaspoons). Today’s physics has nothing to say about the intentionality that has resulted in the existence of such objects, even though this intentionality is clearly causally effective.

A simple statement of fact: there is no physics theory that explains the nature of, or even the existence of, football matches, teapots, or jumbo-jet aircraft. The human mind is physically based, but there is no hope whatever of predicting the behavior it controls from the underlying physical laws. Even if we had a satisfactory fundamental physics ‘theory of everything’, this situation would remain unchanged: physics would still fail to explain the outcomes of human purpose, and so would provide an incomplete description of the real world around us.

Well, okay. Ellis is talking about design, but in the context of things that we know perfectly well are designed — football matches, teapots, or jumbo-jet aircraft. The fact that teapots are indeed designed is not worthy of media attention. Ellis’s point in the essay is simply the old chestnut that reductionistic laws of physics are of little help if we want to understand many of the complex phenomena that we see in the macroscopic world — even if every particle in your car is happily obeying the rules of the Standard Model, being a well-trained particle physicist won’t help you when you muffler dies (as mine did yesterday). Read the essay for yourself; there’s nothing in there about cells or higher purpose.

So how does Nelson take anything hopeful from Ellis? Let’s see what he says.

Irreducible higher-level causation? From there it’s a day’s walk down an English country lane to current hypotheses of intelligent design. More resources here for the hard work of assembling a robust theory of design.

Aha. Nelson turns Ellis’s essay to his advantage via the venerable technique of “making shit up.” The notion of “irreducible” complexity is much beloved by creationists; it’s an advanced version of the standard argumentative fallacy of “if I can’t see how it would happen, it must be impossible.” Michael Behe uses the example of a mousetrap to illustrate a mechanism that would be useless if you removed any one of its component parts, and is therefore irreducibly complex. One problem with this notion is that nobody knows what it means, since no sensible definition of “irreducible” has ever been given. And of course, you can patiently explain to the creationists how mousetraps are not irreducibly complex, but they are strangely unmoved.

So it would be strange to find a real scientist talk about “irreducible higher-level causation.” But then you look at Ellis’s essay and — he doesn’t! The word “irreducible” doesn’t appear anywhere in the article. Nelson kind of insinuated it into the text. And then it’s “a day’s walk down an English country lane to current hypotheses of intelligent design.” The day’s walk appears to take you from “teapots are clearly designed” to “cells are clearly designed.” That’s quite a long walk! I encourage Mr. Nelson and his colleagues to start walking, and report back to us when they reach their destination.

Crackpots today… brain cells tomorrow? Read More »

Information retrieval

We’re in the midst of the University of Chicago’s famed Scav Hunt, in which teams of energetic, creative, and sleep-deprived undergraduates scour the universe for all sorts of unusual objects. (The link is correct, trust me.) This year the hunt is being blogged by Connor Cyone, and the list of items can be found here (pdf). And yes, as Will Baude mentions, I do make an appearance in item 48: “Retrieve information from a black hole. Must be Sean Carroll certified. [4 points]” Consequently, I have been asked by three different teams to certify that they have indeed extracted information from a black hole, just like Hawking says we can. Accommodating soul that I am, I readily signed my name to various pieces of paper that can now be used to discredit me in the future. All for a good cause.

No more teams are likely to find me, as I am off to Geneva to spend the week at CERN giving lectures on cosmology. Do they have the Internets in Europe? Perhaps I will check in.

Information retrieval Read More »

We get letters

Not everyone is appreciative of our efforts to explain dark energy to a wider audience. 

From: [redacted]@aol.com

Date: Mon, 24 Jan 2005 14:19:00 EST

Subject: dark junk hoax

To: carroll@theory.uchicago.edu



i am amazed you morons are trying to keep this crap alive...along  with 

'inflation'....hawking as i do thinks it is junk.....why cant you  acknowledge that 

hoyle, narlikar, and burbridge are right with their ideas  published 3-4 yrs 

ago...i know that none of you can admit that halton arp is  dead right with 

his postulate that redshift is not solely distance  related.....the whole of us 

and british scientific punditry have so much  reputations at stake that they 

will never accord hoyle, arp and others the  recognition due......and you fools 

have wasted 30 yrs and billions of dollars in  an obscece 'cover-your-ass'  

endeavor...you make me sick

If anyone’s interested, Ned Wright has done a thorough job of explaining why some “alternatives” to the Big Bang are all miserable failures. My own intemperate thoughts are here. Amazing to me how emotional people get about this.

We get letters Read More »

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