Words

Abbas at 3 Quarks reminds us that next year is W.H. Auden’s centenary (and that Britain is curiously unenthusiastic about celebrating the event). The BBC allows you to hear Auden read this poem at a 1965 festival; his father was a physicist.

If all a top physicist knows
About the Truth be true,
Then, for all the so-and-so’s,
Futility and grime,
Our common world contains,
We have a better time
Than the Greater Nebulae do,
Or the atoms in our brains.

Marriage is rarely bliss
But, surely it would be worse
As particles to pelt
At thousands of miles per sec
About a universe
Wherein a lover’s kiss
Would either not be felt
Or break the loved one’s neck.

Though the face at which I stare
While shaving it be cruel
For, year after year, it repels
An ageing suitor, it has,
Thank God, sufficient mass
To be altogether there,
Not an indeterminate gruel
Which is partly somewhere else.

Our eyes prefer to suppose
That a habitable place
Has a geocentric view,
That architects enclose
A quiet Euclidian space:
Exploded myths – but who
Could feel at home astraddle
An ever expanding saddle?

This passion of our kind
For the process of finding out
Is a fact one can hardly doubt,
But I would rejoice in it more
If I knew more clearly what
We wanted the knowledge for,
Felt certain still that the mind
Is free to know or not.

It has chosen once, it seems,
And whether our concern
For magnitude’s extremes
Really become a creature
Who comes in a median size,
Or politicizing Nature
Be altogether wise,
Is something we shall learn.

Ol’ Wystan is right; we do have a better time than most of the universe. It would be no fun to constantly worry that “a lover’s kiss / Would either not be felt / Or break the loved one’s neck.” And in a sense, it’s surprising (one might almost say unnatural) that our local conditions allow for the build-up of the delicate complexity necessary to nurture passion and poetry among we creatures of median size.

In most physical systems, we can get a pretty good idea of the relevant scales of length and time just by using dimensional analysis. If you have some fundamental timescale governing the behavior of a system, you naturally expect most processes characteristic of that system to happen on approximately that timescale, give or take an order of magnitude here or there. But our universe doesn’t work that way at all — there are dramatic balancing acts that stretch the relevant timescales far past their natural values. In the absence of any fine-tunings, the relevant timescale for the universe would be the Planck time, 10^-44 seconds, whereas the actual age of the universe is more like 10¹⁸ seconds. This is actually two problems in one: why doesn’t the vacuum energy rapidly dominate over the energy density in matter and radiation — the cosmological constant problem — and, imagining that we’ve solved that one, why doesn’t spatial curvature dominate over all the energy density — the flatness problem. It would be much more “natural,” in other words, to live in either a cold and empty universe, or one that recollapsed in a jiffy.

But given that the universe does linger around, it’s still a surprise that the matter within it exhibits interesting dynamics on timescales much longer than the Planck time. A human lifespan, for example, is about 10⁹ seconds. The human/Planck hierarchy actually owes its existence to a multi-layered series of hierarchies. First, the characteristic energy scale of particle physics is set by electroweak symmetry breaking to be about 10¹¹ electron volts, far below the Planck energy at 10²⁷ electron volts. (That’s known to particle physicists as “the” hierarchy problem.) And then the mass of the electron (m_e ~ 5 x 10⁵ electron volts) is smaller than it really should be, as it is suppressed with respect to the electroweak scale by a Yukawa coupling of about 10^-6. But then the weakness of the electromagnetic interaction, as manifested in the small value of the fine-structure constant α = 1/137, implies that the Rydberg (which sets the scales for atomic physics) is even lower than that:

Ry ~ α² m_e ~ 10 electron volts.

This energy corresponds to timescales (by inserting appropriate factors of Planck’s constant and the speed of light) of about 10^-18 seconds; much longer than the Planck time, but still much shorter than a human lifetime. The cascade of hierarchies continues; molecular binding energies are typically much smaller than a Rydberg, the timescales characteristic of mesocopic collections of slowly-moving molecules are correspondingly longer still, etc.

Because we don’t yet fully understand the origin of these fantastic hierarchies, we can conclude that God exists. Okay, no we can’t. Really we can conclude that we live in a multiverse in which all of the constants of nature take on different values in different places. Okay, we can’t actually conclude that either. What we can do is keep thinking about it, not jumping to too many conclusions while we try to fill one of those pesky “gaps” in our understanding that people like to insist must be evidence for their personal favorite story of reality.

But “politicizing Nature,” now that’s just bad. Not altogether wise at all.

I was asked to review Lee Smolin’s The Trouble With Physics by New Scientist. The review has now appeared, although with a couple of drawbacks. Most obviously, only subscribers can read it. But more importantly, they have some antiquated print-journal notion of a “word limit,” which in my case was about 1000 words. When I started writing the review, I kind of went over the limit. By a factor of about three. This is why the Intelligent Designer invented blogs; here’s the review I would have written, if the Man hadn’t tried to stifle my creativity. (Other reviews at Backreaction and Not Even Wrong; see also Bee’s interview with Lee, or his appearance with Brian Greene on Science Friday.)

——————————————————————

It was only after re-reading and considerable head-scratching that I figured out why Lee Smolin’s The Trouble With Physics is such a frustrating book: it’s really two books, with intertwined but ultimately independent arguments. One argument is big and abstract and likely to be ignored by most of the book’s audience; the other is narrow and specific and part of a wide-ranging and heated discussion carried out between scientists, in the popular press, and on the internet. The abstract argument — about academic culture and the need to nurture speculative ideas — is, in my opinion, important and largely correct, while the specific one — about the best way to set about quantizing gravity — is overstated and undersupported. It’s too bad that vociferous debate over the latter seems likely to suck all the oxygen away from the former.

Fundamental physics (for want of a better term) is concerned with the ultimate microscopic laws of nature. In our current understanding, these laws describe gravity according to Einstein’s general theory of relativity, and everything else according to the Standard Model of particle physics. The good news is that, with just a few exceptions (dark matter and dark energy, neutrino masses), these two theories are consistent with all the experimental data we have. The bad news is that they are mutually inconsistent. The Standard Model is a quantum field theory, a direct outgrowth of the quantum-mechanical revolution of the 1920’s. General relativity (GR), meanwhile, remains a classical theory, very much in the tradition of Newtonian mechanics. The program of “quantum gravity” is to invent a quantum-mechanical theory that reduces to GR in the classical limit.

This is obviously a crucially important problem, but one that has traditionally been a sidelight in the world of theoretical physics. For one thing, coming up with good models of quantum gravity has turned out to be extremely difficult; for another, the weakness of gravity implies that quantum effects don’t become important in any realistic experiment. There is a severe conceptual divide between GR and the Standard Model, but as a practical matter there is no pressing empirical question that one or the other of them cannot answer.

Quantum gravity moved to the forefront of research in the 1980’s, for two very different reasons. One was the success of the Standard Model itself; its triumph was so complete that there weren’t any nagging experimental puzzles left to resolve (a frustrating situation that persisted for twenty years). The other was the appearance of a promising new approach: string theory, the simple idea of replacing elementary point particles by one-dimensional loops and segments of “string.” (You’re not supposed to ask what the strings are made of; they’re made of string stuff, and there are no deeper layers.) In fact the theory had been around since the late 1960’s, originally investigated as an approach to the strong interactions. But problems arose, including the unavoidable appearance of string states that had all the characteristics one would expect of gravitons, particles of gravity. Whereas most attempts to quantize gravity ran quickly aground, here was a theory that insisted on the existence of gravity even when we didn’t ask for it! In 1984, Michael Green and John Schwarz demonstrated that certain potentially worrisome anomalies in the theory could be successfully canceled, and string mania swept the particle-theory community.

In the heady days of the “first superstring revolution,” triumphalism was everywhere. String theory wasn’t just a way to quantize gravity, it was a Theory of Everything, from which we could potentially derive all of particle physics. Sadly, that hasn’t worked out, or at least not yet. (String theorists remain quite confident that the theory is compatible with everything we know about particle physics, but optimism that it will uniquely predict the low-energy world is at a low ebb.) But on the theoretical front, there have been impressive advances, including a “second revolution” in the mid-nineties. Among the most astonishing results was the discovery by Juan Maldacena of gauge/gravity duality, according to which quantum gravity in a particular background is precisely equivalent to a completely distinct field theory, without gravity, in a different number of dimensions! String theory and quantum field theory, it turns out, aren’t really separate disciplines; there is a web of dualities that reveal various different-looking string theories as simply different manifestations of the same underlying theory, and some of those manifestations are ordinary field theories. Results such as this convince string theorists that they are on the right track, even in the absence of experimental tests. (Although all but the most fervent will readily agree that experimental tests are always the ultimate arbiter.)

But it’s been a long time since the last revolution, and contact with data seems no closer. Indeed, the hope that string theory would uniquely predict a model of particle physics appears increasingly utopian; these days, it seems more likely that there is a huge number (10⁵⁰⁰ or more) phases in which string theory can find itself, each featuring different particles and forces. This embarrassment of riches has opened a possible explanation for apparent fine-tunings in nature — perhaps every phase of string theory exists somewhere, and we only find ourselves in those that are hospitable to life. But this particular prediction is not experimentally testable; if there is to be contact with data, it seems that it won’t be through predicting the details of particle physics.

It is perhaps not surprising that there has been a backlash against string theory. Lee Smolin’s The Trouble With Physics is a paradigmatic example, along with Peter Woit’s new book Not Even Wrong. Both books were foreshadowed by Roger Penrose’s massive work, The Road to Reality. But string theorists have not been silent; several years ago, Brian Greene’s The Elegant Universe was a surprise bestseller, and more recently Leonard Susskind’s The Cosmic Landscape has focused on the opportunities presented by a theory with 10⁵⁰⁰ different phases. Alex Vilenkin’s Many Worlds in One also discusses the multiverse, and Lisa Randall’s Warped Passages enthuses over the possibility of extra dimensions of spacetime — while Lawrence Krauss’s Hiding in the Mirror strikes a skeptical note. Perhaps surprisingly, these books have not been published by vanity presses — there is apparently a huge market for popular discussions of the problems and prospects of string theory and related subjects.

…

The Trouble With PhysicsRead More »

I was recently asked to recommend a good popular-level book on quantum mechanics. I don’t think I know of any, at least not first hand. We had a whole thread on the Greatest Popular Science Book, filled with good suggestions, but none specifically about quantum mechanics. A quick glance through amazon.com reveals plenty of books on particle physics, or even specific notions like quantum computing, but not one book that I could recommend in good conscience to someone who just wants to know what quantum mechanics is all about. It is the greatest intellectual achievement of the twentieth century, after all.

There are some books that either come close, or might very well be perfect but I’m not familiar with them. In the latter category we have The Quantum World by Ken Ford, and David Lindley’s Where Does the Weirdness Go? These might be great, I just haven’t read them. I’m sure that the Mr. Tompkins books by George Gamow are good, since I love One, Two, Three… Infinity (and Gamow was a genius), but I haven’t actually read them. Feynman’s QED is another classic, but focuses more on quantum electrodynamics (duh) than on QM more generally. David Deutsch’s The Fabric of Reality is a fantastic book, especially if you are curious about the Many-Worlds Interpretation of quantum mechanics; but I’m not sure if it’s the best first introduction (I haven’t looked at it closely in years). And David Albert’s Quantum Mechanics and Experience is great for a careful philosophical account of what QM is all about, but again maybe not the best first exposure.

Any suggestions? Not for a good book that is related to quantum mechanics or perhaps mentions it in a chapter or two, but for something whose major goal is to provide a clear account of QM. Surely there is something?

Can’t … stop … blogging … must … resist …

So you may have heard that Pluto is still a planet, and indeed we have a few new ones as well! Phil Plait, Rob Knop, Clifford, and Steinn have all weighed in. Hey, it’s on the front page of the New York Times, above the fold!

The problem is that Pluto is kind of small, and far away. Those aren’t problems by themselves, but there are lots of similar-sized objects that are also out beyond Neptune, in the Kuiper Belt. As we discover more and more, should they all count as planets? And if not, shouldn’t Pluto be demoted? Nobody wants to lose Pluto among the family of planets — rumors to that effect were previously enough to inspire classrooms around the globe to write pleading letters to the astronomical powers that be, begging them not to discard the plucky ninth planet. But it’s really hard to come up with some objective criteria of planet-ness that would include the canonical nine but not open the doors to all sorts of unwanted interlopers. Now the Planet Definition Committee of the International Astronomical Union has proposed a new definition:

1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

It turns out that, by this proposed definition, there are twelve planets — not just the usual nine, but also Ceres (the largest asteroid, between Mars and Jupiter), and also Charon (Pluto’s moon, but far enough away that apparently it doesn’t count as a “satellite,” but as a double-planet), and UB313, a faraway rock that is even bigger than Pluto. I’m not sure why anyone thinks this is an improvement.

The thing is, it doesn’t matter. Most everyone who writes about it admits that it doesn’t matter, before launching into a passionate defense of what they think the real definition should be. But, seriously: it really doesn’t matter. We are not doing science, or learning anything about the universe here. We’re just making up a definition, and we’re doing so solely for our own convenience. There is no pre-existing Platonic nature of “planet-ness” located out there in the world, which we are trying to discover so that we may bring our nomenclature in line with it. We are not discovering anything new about nature, nor even bringing any reality into existence by our choices.

The Pragmatists figured this out long ago: we get to choose the definition to be whatever we want, and the best criterion by which to make that choice is whatever is most useful and convenient for our purposes. But people have some deep-seated desire to believe that our words should be brought in line with objective criteria, even if it’s dramatically inconvenient. (These are the same people, presumably, who think that spelling reform would be really cool.) But as Rob says, there is no physically reasonable definition that would let us stick with nine planets. That’s okay! We have every right to define “planet” to mean “Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, plus whatever other large rocky bodies we find orbiting other stars.” Or whatever else we want. It’s completely up to us.

So we really shouldn’t have to tear up a century’s worth of textbooks and illustrations, and start trying to figure out when the shape of some particular body is governed by hydrostatic equilibrium, just to pat ourselves on the back for obeying “physically reasonable” definitions. But it looks like that’s what the IAU Planet Definition Committee wants us to do. Of course that’s what you’d expect a Planet Definition Committee to suggest; otherwise why would we need a Planet Definition Committee?

Now if you’ll excuse me, I have change-of-address forms to fill out.

[And don’t even contemplate accusing me of hypocrisy for dragging myself away from a much-deserved blog-vacation to carry on about something that I claim doesn’t matter. The definition of “planet” doesn’t matter; but appreciating that the choice of definition is a matter of our own convenience, not a matter of necessarily conforming to some objective criteria about the physical world, matters a lot.]

Update: Chris Clarke for the opposition.