I Guess It’s a Deuteron

Seed has just offered the world a “Cribsheet” on string theory. It looks pretty slick, although their portrayal of a “hydrogen atom” seems to have an extra nucleon (as Wolfgang notes in the Cosmic Variance thread). I’m inclined to forgive the multiple electron orbits, since they only show one actual electron — and besides, ellipses aren’t that great a way of drawing orbitals anyway.

(Incidentally, if you want to see orbitals in video, check out episode 51 of The Mechanical Universe, available for free online via Annenberg Media.)

They do cite Barton Zwiebach’s First Course in String Theory (2004), which gives me a slight tinge of pride. I mean, somebody had to work the problems in the last five chapters to see if they were solvable by students and not just professors.

The portion of this post below the fold is a rough draft of several different rants, developed in embryonic form and smushed together. Read only if you’re exceptionally curious.

As exactly everybody was expecting, Peter Woit jumped into the fray right off. It looks like he got peeved by the following bit, down in the gray box at the Cribsheet’s lower edge.

Directly observing strings is far beyond our capabilities now and for the foreseeable future. Additionally, string theory’s rich diversity makes it difficult to derive any clear predictions that apply to all its versions. Still, particle physics experiments being performed with collisions of very heavy ions at the Brookhaven National Laboratory and with proton collisions at CERN could connect string theory with reality. In particular, two discoveries, which are supersymmetry and the existence of extra dimensions, would suggest that string theory is on the right track.

About which Woit has the following to say:

Not mentioning that the “string theory” in question that may or may not get connected to reality at BNL is a different one than the one described in the rest of the document might accidentally mislead some readers. Of course I’m sure no one intends that to happen.

If I were in a sufficiently Puckish mood, and were I granted sufficient caffeine, I could argue with some verve that the “Newtonian mechanics” one can connect to reality inside a high-school physics laboratory is a different one than that described in a celestial mechanics textbook. Really! What do they have in common, except a certain similarity of equations?

Like many other exciting aspects of string theory, the AdS/CFT correspondence is not yet sufficiently well understood for us to have a clear picture of its implications. We don’t yet know whether this exotic, out-of-the-blue duality between gravity and gauge theories can tell us meaningful things about quark-gluon plasmas, yet perhaps it is already more than we had any right to expect! Furthermore, the subject is abstrusely mathematical: not only do the individual statements we make require sophisticated jargon, but also the reasoning which connects one statement to another is mathematical in character. It seems to me that a duality between theories is among the hardest of physical notions to translate into “layman’s terms,” because one can’t appreciate a duality using words alone.

In The Character of Physical Law, Feynman gives an example which I’d like to appropriate. I can say that when a planet travels in its orbit, a line from the planet to the Sun sweeps out equal areas in equal times. I can also say that the force pulling on the planet is always directed toward the Sun. Both of these statements require a little math — “equal areas,” “equal times” — but it’s not really math, not a kind to give the layman heebie-jeebies. Given some time for elaboration, one could translate both of these statements into “layman language.” However, one cannot explain in lay terminology why the two statements are equivalent.

If planets looping around the Sun can give us so much trouble, imagine having to explain the relation between gravity in anti-de Sitter spacetime and conformal field theory. Even if you could cook up a lay explanation of either one, you’re hamstrung when you try to connect them.

At a philosophical level, I think my joke about Newtonian mechanics has a certain validity. If you’d only read philosophy-of-science books and blogo-polemics about string theory, you wouldn’t have the knowledge to tell those situations apart. I think we find ourselves in the unhappy situation where no matter how informed the lay audience feels they are, they can’t render a reasonable judgment.

Woit has a few other things to say. About string theory’s “rich diversity”, he writes,

I guess that’s one way of saying it is an ugly mess that can’t predict anything.

It is not difficult to guess that this subject has come up before. Jacques Distler recently wrote,

It is far from clear that you will be able to “tune all of the knobs” (corresponding to the 100+ couplings of the MSSM) independently. I rather doubt that you will. Every knob that “can’t be tuned independently” is a prediction, and there’s no more reason to believe that number is zero than there is to believe that all 100+ parameters are uniquely determined.

But the real point is that I (and everyone else who opines on this question) is, at this point, simply speculating. We just don’t know, and expressing our personal prejudices on how this will work out is not a scientific argument.


I don’t see the relevance of the existence of a large number of string vacua that look nothing like the real world, particularly since the Standard Model itself has a landscape of vacua that look nothing like the real world. […] “objective statements about what the problems with the ‘Landscape’” (#) are nothing more than a guess at an answer to a question that is still very much open.

Perhaps your guess is correct. Most likely, it isn’t. No one knows (though everyone seems to have a strong opinion).

More of Distler’s remarks with additional technical details can be found here. He also makes an interesting point which could be raised much more often:

there is a huge degeneracy of vacua whose particle physics (at energies probeable at the LHC and beyond) which differ in the values of the cosmological constant and Newton’s constant, but whose particle physics is absolutely identical.

For the purposes of making particle physics predictions, it doesn’t matter which of this exponentially large number of vacua you calculate in. You will get the same answers for all of them.

I’d also like to quote Mark Srednicki, who has developed an interesting metric over the space of untested theories:

Yes, string theory has so far failed to make a definite prediction. But so has every other approach to the problem of quantum gravity. Lee has pushed the idea that violations of Lorentz invariance might turn up in one particular flavor of LQG [loop quantum gravity]. But there are versions of LQG that are Lorentz invariant from the start, so this cannot be said to be a prediction of LQG. It seems to me that this is exactly analogous to statements that certain classes of compactifications in string theory make certain predictions, but when all possible classes are considered, no prediction is common to all. Similarly, LQG makes no predictions that are common to all versions of it that are currently under investigation.

There is then a subjective issue of which framework is closer to making a prediction. Peter evaluates this in terms of the number of bong hits needed to believe any particular scenario, which is as good a measure as any. For me, string theory currently needs about 2 bong hits, the Lorentz-invariant flavors of LQG maybe 4 (though if one of them works I would expect it to also have a string interpretation), and I’m passed out before we get to anything else.

I’m about to head out for lunch (yes, it is almost 17 o’clock here in Cambridge, Mass.) so I figure I should bring this quote-heavy ramble to a close. My one real point is this: it’s all well and good to complain that physicists invest themselves in mathematical constructions which are difficult to tie in with observable reality. However, if you want to talk like that, you should hold yourself to the same standard. Making statements about the philosophy or sociology of science which have ideological support but cannot be tied to observation is, well, bad form. So what if the AdS/CFT work doesn’t probe the most grandiose claims about string theory and quantum gravity? Even if that turns out to be a blind alley, it’s got to be significant for understanding the social behavior of string theorists. Leave it out of your description, and you’re painting a caricature. (I recall that last July, physicist blogger Chad Orzel didn’t even know that accelerator experiments on quark-gluon plasmas could test any facet of string theory! So much for getting an informed opinion from polemical books.) Divorce your arguments from observation, even of the social kind, and they become beneath regard.

One thought on “I Guess It’s a Deuteron”

Comments are closed.