Miscellany

Advice

As a Labor Day special here at Preposterous, we offer some advice for anyone out there who might be thinking of becoming a professional academic physicist. Fortunately, since the spirit of Labor Day is that you’re not supposed to do any work, I can just link to other people who have already written various pieces of good advice.

  • Starting at the top, Nobel Laureate Gerard ‘t Hooft offers a crash course (that would only take a few years) on how to become a good theoretical physicist. ‘t Hooft, for those who don’t know, is one of the startlingly smart physicists of the modern era. I interacted with him a little when we were thinking about time travel in three dimensions. He would make some sort of claim that we didn’t believe, and give a thoroughly unconvincing explanation for why it was true, and almost always turn out to be right in the end. My hypothesis at the time was that he was actually a marginally-talented time-traveling physicist from the future, who knew all sorts of true things but had trouble justifying them. But he recommends my general relativity lecture notes, so I have to compliment his taste. (Although he insists on misspelling my name, which you would think he’d be more careful about, given his own struggles to get people to punctuate his name correctly.) His web page is also very charming, well worth checking out.
  • Amanda Peet, a string theorist at the University of Toronto (not the actress), has two very useful advice pages: one for high school students deciding what to major in, and another for undergraduates contemplating graduate school. Both are aimed at students who are particularly interested in string theory, but much of the advice is pretty universal. (Amanda is also using my book for a course she’s teaching this year, so she also gets points for taste. You can see my criteria for deciding whom to link to — it’s all about me me me, baby.)
  • John Baez is a mathematical physicist working on quantum gravity, who has become well-known for his wonderful expository articles on all sorts of physics topics. He has a page of advice for young scientists that covers both philosophical issues and very practical matters.
  • Just because you’ve arrived at graduate school (or become a professor, for that matter) doesn’t mean you have it all figured out. Michael Nielsen has written a thoughtful series of blog posts on the principles of effective research, something we’re all constantly trying to figure out but rarely making explicit. (At the moment the site appears to be down, but I hope I have the url right.)
  • As a more specialized skill, my colleague Bob Geroch has written some suggestions on giving talks. Very few people will successfully implement his advice, but if more people at least tried the quality of talks in the field would be immeasurably higher.

These are the pieces of worthy advice that I know about; let me know if there are any good ones I’ve missed. I should say that I only point at all these well-intentioned articles with some trepidation, as reading them all at once could give someone the idea that become a physicist is an incredibly exhausting grind. The impression by no means inaccurate; but the rewards are more than commensurate!

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Sunday song lyric

With apologies to Juan Non-Volokh, it’s time for another Sunday song lyric. This time from progressive-rock supergroup Emerson, Lake & Palmer, with bonus theological commentary!

The Only Way (Hymn)

Music by Emerson; Lyrics by Lake

People are stirred

Moved by the Word

Kneel at the shrine

Deceived by the wine

How was the Earth conceived?

Infinite space

Is there such a place?

You must believe in the human race

Can you believe

God makes you breathe

Why did he lose

Six million Jews?

Touched by the wings

Fear’s angel brings

Sad winter storm

Grey autumn dawn

Who looks on life itself

Who lights your way?

Only you can say

How can you just obey?

Don’t need the word

Now that you’ve heard

Don’t be afraid

Man is man-made

And when the hour comes

Don’t turn away

Face the light of day

And do it your way

It’s the only way

Okay, not the most sophisticated statement of the problem of evil (and Lake is trying to sing way outside his range). But when I was younger, hearing this song was what made me realize it might actually be okay to not believe in God.

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Fred Whipple

I’m late learning this, but Fred Whipple passed away on August 30th at the age of 97. He was best-known for his work on comets, and played an important role in U.S. astronomy as director of the Smithsonian Astrophysical Observatory (now merged with the Harvard College Observatory to form the Harvard-Smithsonian Center for Astrophysics). I met him a couple of times as a graduate student in the Harvard Astronomy Department; he was a warm and friendly spirit as well as a major figure in astrophysics.

Update: My Ph.D. advisor, George Field, was close friends with Whipple. George mentions with pride how the speakers at the funeral stressed Whipple’s atheism.

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Probability of discovery

I’m always on the lookout for ways to make money off of my esoteric physics knowledge. Here’s one: you can bet on the chance that a big physics experiment will discover something by a certain date. Inspired by an article in New Scientist on large physics experiments, Ladbrokes betting agency is placing odds on various possibilities, and taking cash bets up until Sept. 6th. The four bets up there now are: LIGO discovering gravitational waves by 2010 at 2-1; understanding the origin of cosmic rays (presumably they mean ultra-high-energy cosmic rays) by 2010 also at 2-1; discovery of the Higgs boson by the ATLAS experiment at CERN’s Large Hadron Collider by 2010 at 3-1; and completion of a working fusion power station by 2010 at 40-1. Personally I would happily wager 100 pounds to win 300 on the Higgs being discovered, although I’m not sure what happens if the CMS experiment finds it rather than ATLAS. A fusion power plant is very unlikely. I don’t know what counts as “understanding” cosmic rays, so I’d be a little leery of that one. LIGO finding gravitational waves by 2010 is a trickier one — everybody things gravitational waves are out there, so the bet depends more on the technological progress (and funding) of the observatory, which is hard to predict.

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Blogs are the best thing ever

As someone with his finger firmly on the erratic pulse of the American body politic, I might at some point in the past have thought of it as my patriotic duty to watch at least a tiny portion of the Republican convention. But in these enlightened and blogospherical times, ordinary folks are saved from the need to torture themselves by the generous sacrifice of intrepid souls with a modem and a sense of humor; between Michael Bérubé and Fafblog!, it’s better than being there.

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Extraterrestrials?

Since this seems to be Provocative Day here at Preposterous, I might as well throw this one out there: the Search for Extraterrestrial Intelligence (SETI) project has found a signal they can’t explain, possibly originating from an extraterrestrial civilization. This is something I’m truly not an expert on, so I’ll just point you to some commentary from Simon DeDeo, who brought this to my attention. (Original data here.) The basic idea is that they’ve found a signal at 1420 megahertz (a very noisy part of the electromagnetic spectrum) that has appeared a few times, starting at a fixed frequency and slowly drifting (as if Doppler-shifted from a moving source). Certainly the most likely explanation is some perfectly mundane, although interesting, source of emission in atomic hydrogen. I would put the chance that we’ve actually found some intelligent life to be about one in a million. But that’s a chance worth exploring.

Interestingly, the signal was first identified via the SETI@Home project, running as a screensaver on countless home computers. I have it on mine. We physicists are introducing Einstein@Home, a way you can help search for signals of gravitational waves from LIGO data, but if we’ve found aliens that will never catch on.

Now, what I want to know is, do these aliens feel emotions?

Update: Ooops! Almost before I finished typing, Simon points me to this disclaimer. Now I know how Matt Drudge must feel all the time.

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Emotional states

At a big multi-disciplinary conference like the European Forum at Alpbach, the real fun is not giving your own presentations but sitting down over some Hefeweizen and chatting with some of the other participants. One of the students in our seminar was a Dutch psychologist who explained to us what an emotion really is. Unfortunately I don’t remember his name, and also unfortunately I am not at all an expert in talking about these things, but let me try to explain his major idea.

The claim was essentially that true “emotional states” are not distinguished by the kind of response a creature gives, but the timing: emotions are distinguished by persisting long after the stimulus that has caused them has been removed, or by being prompted by conditions that merely recall the original stimulus, rather than duplicating it. The example given was that we get chewed out at work by our boss, get angry, but rather than actually taking it out on our boss (which might be maladaptive behavior) we go home and act cross to our family. In contrast, most animals (chimpanzees perhaps being the only counterexample) are “machines,” reacting simply to the stimulus of the moment. They might remember previous stimuli, and react appropriately with fear or joy if it looks like the stimuli might return, but they don’t nurse emotions that cause apparently-inappropriate responses long after the stimuli have disappeared.

The empirical support that was adduced for this position was the role of the frontal lobe in the feeling of emotions. Lobotomized patients, whose frontal lobes have disconnected from the rest of their brains, have IQs that are essentially unchanged, but become completely different people as their emotional responses are dramatically altered to the point of almost disappearing. Interestingly, these patients also lose the ability to plan future events, even something as simple as a dinner party. The claim is thus that it’s the frontal lobe, which is much more developed in humans than in other animals, that provides us with the ability to experience emotional states. (In 1949 the Nobel Prize was awarded to Egas Moniz for the development of the frontal lobotomy technique for treating patients with schizophrenia; this has often been called the biggest mistake in the history of the Prizes, although the official Nobel website seems less than completely contrite.)

It seems clear that there is some complicated relationship between emotions, persistence, and the frontal lobes, although it’s not perfectly clear to me that the idea of maintaining a response even after stimuli are removed is really the most important aspect of emotions. But there are clearly consequences for the question of animal rights, namely that we should not attribute true emotional import to signs of animals’ “suffering”; when the lobster is struggling to get out of the pot of boiling water, this is merely a robotic reaction, not analogous to a true human emotion (so the reasoning goes). In fact, our Alpbach friend related amusing stories about how he had been invited to speak at gatherings of animal-rights activists, who had apparently noticed that he had done research on animals and emotions without looking closely at was his conclusions were. Happily, he managed to escape the meetings in one piece.

So if emotions are what separates us and the chimps from the rest of the animal kingdom, what is it that separates us from the chimps? In one sense, not much; I just finished reading Will Self’s novel Great Apes, which features a London artist who wakes up one morning to find that the roles of chimps and humans have been miraculously interchanged, complete with horrible puns (“going humanshit” and worse). I tend to agree with Steven Pinker that grammar is what separates us from other animals; the subjunctive mood is what makes us human. This fits in well with my social-contractarian outlook; human beings can get together and make up rules about how we agree to act in certain situations, something other species just can’t do.

But I better quit roaming outside areas I know anything about before I completely destroy my credibility in those areas in which I’m supposed to be an expert.

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Testing general relativity

Some slightly-recycled content. (But it’s new to you, right?) Science writer Amanda Gefter is working on an article for Sky and Telescope about testing general relativity. (See other articles by Amanda here and here.) She emailed me to ask some general questions about the state of GR and its experimental tests; here are the questions and my answers, just off the top of my head.

What are scalar-tensor theories of gravity? In these theories, where does the extra field come from (in other words, what is it, and why is it there?) How do these theories modify GR? If a scalar-tensor theory is found to explain experimental results, does that necessarily mean that there are extra dimensions? How viable do you think these theories are?

Scalar-tensor theories are simply generalizations of GR that add a new scalar field that interacts directly with gravity (i.e., couples directly to the curvature of spacetime). A scalar field is like the electromagnetic field, except that it only has a magnitude and not a direction; it simply takes on a single numerical value at each point in spacetime. The first scalar-tensor theories proposed that the gravitational constant, which fixes the strength of gravity, could have a variable strength that depended on some scalar field; but current theories are more general.

Scalar fields can arise in different ways. Often, they are simply put there. They can also arise from extra dimensions of spacetime, or from superstring theory. But if we find a scalar field, it certainly doesn’t imply the existence of extra dimensions, as there are many other ways to get such scalars.

Scalar-tensor theories are simple and natural generalizations of GR, and it wouldn’t be surprising if one of them were true. However, many theories that are studied in the literature assume that the scalar field is very light, and therefore leads to (potentially detectable) effects at large distances. It’s much more likely that any such scalar has a significant mass, perhaps near the Planck scale, and so would remain undetectable in any conceivable experiment. However, we know little for sure, so it pays to keep an open mind.

Are there other alternatives to GR that are being explored? Any that you find particularly promising?

There are many alternatives being explored, too many to list or even catalogue. The most straightforward, and perhaps most promising, imagine that something like GR is true in extra dimensions, and lead to a modified theory at the level of our observed four-dimensional spacetime. The ways in which this theory can be modified will depend in the specific model of extra dimensions; it has been proposed that such theories can help explain the value of the cosmological constant, or explain the acceleration of the universe without any cosmological constant, or affect cosmology at very early times. It is also possible to modify GR directly in four dimensions to help do away with the need for dark energy.

Other models try to do away with the need for dark matter, by modifying gravity on the scale of galaxies. A famous example is MOND (Modification Of Newtonian Dynamics) by Milgrom, although that is more of an “idea” than a “theory” (although Bekenstein has recently tried to put it on a more sound footing). The biggest problem with such models is that they have a very hard time reproducing the many successes of the dark matter idea, for example in accounting for the perturbation spectrum of the cosmic microwave background.

Finally, there are models that don’t try to explain some specific feature of astrophysical observations, but instead simply try to see how far we can go in modifying GR. For example, there are models which violate Lorentz invariance at a fundamental level. These are interesting to explore, if only to help us understand the extent to which GR can be trusted.

Why is it important for us to test GR? Has it become more imperative in recent years?

Gravity is an important force, and GR is our best theory of gravity, so it should be tested as well as we possibly can. More specifically, cosmological observations (dark matter, dark energy, and primordial perturbations) have revealed a universe that seems very surprising to us, and our interpretations of these observations rely on extrapolating ordinary GR to scales of time and distance that are far larger than where it has been directly tested. So any new tests can give us more confidence that we have the right to make such extrapolations.

Why is our understanding of gravity so important?

See above. On the large scales characteristic of cosmology, gravity is by far the most important force. In addition, it is the only force that has thus far evaded a quantum-mechanical understanding; reconciling GR with quantum mechanics is the greatest single quest in contemporary fundamental physics, and any information we have about gravity itself could be an invaluable clue along the way.

Up to this point, is GR a well-tested theory?

It is extremely well-tested in certain regimes, less so in others. Three regimes have been especially well-tested: the Solar System, where precision measurements have tightly constrained deviations from GR; the binary pulsar, whose orbit implies exactly the amount of gravitational radiation predicted by GR; and the early universe, where observations of light elements produced by nucleosynthesis and the anisotropies of the cosmic microwave background provide good evidence for the validity of GR when the universe was seconds old and hundreds of thousands of years old, respectively.

There is still a lot we don’t know. For example, are the predictions of GR for gravitational lensing and dynamical measures of mass consistent with each other? Are there deviations at very strong curvatures, or for that matter very weak curvatures? Are there deviations at very small distances that may be probed in the laboratory? (Current best limits go down to about one tenth of a millimeter.) Are there long-range but subtle effects that still may show up in the Solar System?

As I understand it, GR has been inadequately tested in the strong field regime. Why is it important to test GR in such extreme circumstances? What kinds of tests will be helpful? In particular, how can we use black holes to test GR?

I wouldn’t say “inadequately”, but we can always do better. To be honest, I think that testing GR with black holes is interesting, but somewhat overrated. If GR is going to be modified, there are two likely ways it can happen: subtle long-range effects, and deviations that become important when the curvature of spacetime reaches a certain fixed value. In the first case, Solar System tests will usually do better than astrophysical tests, just because the precision is higher. In the second case, we would have to get extremely lucky indeed to notice any effects in black holes. The curvature outside astrophysical-sized black holes is actually not that great; the curvature radius would be measured in kilometers, while we would probably need to go to much smaller scales to observe any deviations from general relativity.

On the other hand, as already mentioned it’s important to keep an open mind. Many of our tests of GR thus far have either been in cosmology or in the quasi-static, weak-field regime of the Solar System, with the binary pulsar being the notable exception. Even if our most respectable alternative theories wouldn’t necessarily show up first in a dynamical, strong-field situation, we should certainly do as many tests in such regimes as we can, if only to make sure there are no surprises.

Is it strange to use black holes as testing grounds for GR when they them selves are consequences of the theory of GR? (in other words, if GR were wrong, would there even be black holes?)

Most respectable theories of gravity (all that I know of, to be honest) predict that there should be black holes, although their properties might be different in different models. So they are well worth investigating, keeping in mind the previous answer.

I understand you worked with Ed Guinan on DI Herculis — what are your thoughts on that problem? Do you think it points to a gap in GR, or is it an experimental anomaly? Are physicists worried about it?

It’s a very interesting system, and I don’t know what is going on. I became skeptical that gravity is to blame when I worked out that the stars in DI Her are well within the weak-field regime where Solar System tests have already tightly constrained any possible deviations from GR; it seems very hard indeed to find a theory that could explain the motion of DI Her yet remain consistent with Solar-System tests. So I suspect that some astrophysical phenomenon is causing the discrepancy, but I’m by no means certain.

Have there been any other observations that seem to violate GR?

It’s hard to say that any given observation violates GR, since there are always other assumptions that come into play. For example, the anomalous acceleration of the Pioneer spacecraft may be due to some extremely unexpected gravitational effect; more likely, however, there is some much more mundane explanation involving the spacecraft themselves. So far, there is certainly nothing we have observed that gives anything like a good reason to doubt GR.

In your opinion, what have been the most significant tests of GR?

Historically: precession of Mercury, deflection of light, gravitational redshift, and gravitational time delay. More recently: the binary pulsar and cosmological nucleosynthesis.

Any thoughts on the importance of Gravity Probe-B? Lunar laser ranging?

GPB is in a somewhat awkward position; it will either confirm the GR prediction for frame-dragging, or it will find a discrepancy and very few people will believe it. I’m not an expert, but my understanding is that the regime it is testing (in the “parameterized post-Newtonian” sense) has already been ruled out by other observations.

Lunar laser ranging is a very different story, well worth doing. There is an opportunity to greatly improve the precision of constraints on long-range deviations from GR, which is always interesting, even if there is no firm prediction from a specific model.

Will the detection of gravity waves be an important confirmation of GR?

Yes, absolutely. At this point, however, very few people doubt that gravitational waves exist, with essentially the properties predicted by GR, so they are more looking forward to learning about the astrophysical sources of the waves. If the observations are somehow inconsistent with GR, that would be an even more spectacular finding than anyone expects.

What makes general relativity such a beautiful theory?

It is extremely powerful (accounting for all gravitational phenomena ever observed), mathematically compelling (applying elegant results from differential geometry), and remarkably simple and robust (unlike, say, the Standard Model of particle physics). GR is simply the statement that “Gravitation is the curvature of spacetime”, made precise and mathematical; few theories in science are simultaneously so simple, elegant, and comprehensive.

Finally, do you think that GR will ultimately prove to be wrong (or incomplete) at some level?

Yes. Everybody (in their right mind) does. GR is a classical theory, fundamentally inconsistent with the quantum world in which we live. At the very least we will have to find a quantum version of GR; more likely, we will have to find some more profound theory that is intrinsically quantum-mechanical and reduces to GR in the appropriate circumstances. If experiments reveal deviations from GR at even the classical level, so much the better.

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Cosmically Considered

The nice thing about jet lag after a long trip westward (say, from Austria to the U.S.) is that you wake up early enough to enjoy the morning, something I tend not normally to do. Which is why it’s too bad I am not still in my beloved Chicago enjoying a cup of coffee while watching the sun rise over Lake Michigan, but instead sitting in a hotel room in Riverside, California, site (Riverside, not my hotel room) of the annual meeting of the Division of Particles and Fields of the American Physical Society. Tuesday afternoon I’ll be giving a review talk on “Theoretical Cosmology,” which sounds a bit too comprehensive to be coherent, but we’ll give it a go. Since it’s the last talk of the conference, likely nobody will be in the audience; if I remember I’ll post a link to the slides (once, you know, they’ve been written).

In the meantime, I’ll share this. Poet Richard O’Connell was kind enough to email me a copy of this poem he wrote in 1976:

To Beta, Cosmically Considered

If relic radiation bathes the spheres

Isotropically, as water is to fish,

To an observer here or on Andromeda,

Time has an arrow sharp as Cupid’s kiss.

If all is that primeval fireball

Exploding yet beyond the verge of sight,

We’re genesis and apocalypse ourselves

Galactic cousins, catastrophic flesh.

Let us junk tyrannical cyclopean clocks

Geared to the wormwork of industrious forebears

Who added pittance by the pendulum

Only to leave their wealth to wastrel heirs.

Let us accept that arrow in our hearts

Transfixing us, targets of joy and tears;

The stars may see how in our spendthrift love

We keep a better time by keeping theirs.

Maybe I can figure out some way to work it into the talk.

Also, read about a Big Bang in your bedroom. (No, it’s not what you think.) I will explain later.

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