The n-Category Café

John Baez wrote:

The stuff you’re talking about is just a clever new way of doing calculations in Standard Model physics. Even if it works, it doesn’t mean the universe is made of strings.

When my lawyer and artist friends ask me (“that physics guy”) about string theory, this is one of the points I try to get across, and it seems to transmit well. It’s just string theory going back to its roots, after all. So, leaving aside the point raised by Polchinski, the idea appears to be pretty easily grokkable.

Polchinski wrote:

String-theory skeptics could take the point of view that it is just a mathematical spinoff. However, one of the repeated lessons of physics is unity — nature uses a small number of principles in diverse ways. And so the quantum gravity that is manifesting itself in dual form at Brookhaven is likely to be the same one that operates everywhere else in the universe.

One could easily take this idea too far. In undergrad quantum mechanics, I was taught to solve the hydrogen atom with a method which is essentially the grandchild of superstrings, but that doesn’t mean the universe is “stringy”!

My biggest concern with the AdS/CFT business is not that it shows string theory is correct, or anything like that. Instead, it worries me that a discussion which purports to concern the sociology of science seems to sidestep the question of what a significant fraction of the scientists are actually doing. If a large group of people are not being seduced by the “Theory of Everything” grail-shaped beacon, but instead choosing to work in a mathematically related field with direct ties to experiment, then doesn’t that have incredible importance for the psychological and sociological parts of the argument? This holds true, I think, even if the QGP calculations never really bear fruit — say, if the whole thing doesn’t give much more precise answers than dimensional analysis.

To steal the Vegas analogy, it’s as if a group of gamblers had decided to play the odds a better way: they put computers in their shoes to predict where roulette balls will fall, or they make side bets with people around the craps table who have superstitious ideas about lucky numbers. They’re not playing the same game as the tourists, but they’re seeing real money. Can any study of the gambling world rightfully ignore them?

Conflict of Interest Disclaimer: my only stake in the String Wars is a small one. I took Barton Zwiebach’s String Theory for Undergraduates (8.251) and helped proofread the textbook, when it was only a stack of LaTeX documents. I worked the exercises in the last few chapters to make sure that an actual undergraduate with no intellectual superpowers had a chance of solving them. In my mind, this was a good thing to do even if the whole thing goes kaput and the M in M-theory turns out to stand for “mud pie”. After all, we’ll only be able to tell if the ideas are good or not if enough brains can gather around them. CITOKATE — “criticism is the only known antidote to error”.

I’m all in favor of string theorists using the technology they’ve developed to tackle real-world problems like the study of quark-gluon plasma. I just wanted to make sure Mr. Serg271 here understood the difference between the supersymetric quark-gluon plasmas these folks are studying, the actual quark-gluon plasmas folks are creating at Brookhaven, and string theory as a theory of fundamental physics.

I’m not surprised that a bunch of string theorists, starved for contact with experiment, would enjoy working on this stuff. Grail-shaped beacons are all very well and good, but physicists only bring home the bacon when they predict the results of experiment.

So, the big question is: how much reliable information can we obtain about real-world quark-gluon plasmas from studying their supersymmetric analogues? I’d like to know… but I guess this is very controversial, since I’ve seen diametrically opposite claims.

I would also enjoy knowing how many string theorists are working on this stuff. Clifford Johnson claims it’s “a huge percentage”. Any idea what percentage that is, or how many people it amounts to?

I’m less interested in grinding some sociological axe than getting some data. I’m sick of the String Wars — I only wrote about these books because I felt some kind of duty to do so. What I really want to talk about is Schur functors, Littlewood–Richardson rules, cohomology of Grassmannians, and groupoidification! But that’ll be next Week’s Finds.

In TWF 246, John wrote:

Once I drove through Las Vegas, where there really is just one game in town: gambling. I stopped and took a look. I saw the big fancy casinos. I saw the glazed-eyed grannies feeding quarters into slot machines, hoping to strike it rich someday. It was clear: the odds were stacked against me. But, I didn’t respond by saying “Oh well - it’s the only game in town” and starting to play.

Instead, I left that town.

Earlier on this blog page, he also wrote:

It’s as if Einstein figured out a way to use math from general relativity to solve problems in hydrodynamics. Suppose it turned out that these methods could correctly predict what happens when you flush your toilet. This would not mean general relativity is correct! To test general relativity you need to look at bending starlight or black holes, not flush toilets.

I laughed out loud when I read these. Arguments about the “String Wars” aside, I feel that these two passages demonstrate John’s amazing ability for a turn of phrase! He gets the Oscar “Lifetime Achievement Award for Explaining the Complicated in Simple Terms.”

It is true that N=4 and QCD are quite different. However, AdS/CFT can be deformed to cases which are non-SUSY and or non-conformal, and many view this as an existence proof that a string theory dual to QCD is possible. I really yearn for it because it will put to rest a lot of absurd criticisms of string theory. For example:

“Even if it works, it doesn’t mean the universe is made of strings.”

I imagine a conversation like this many years ago.

“Even if this so-called `wave-particle duality’ you propose works, it doesn’t mean the particles in our universe are actually waves. It’s just some new-fangled mathematical mumbo-jumbo.”

Gauge/gravity duality is no different. If you prefer to say the world is “made of gluons”, which in a certain limit behave like coherent states of gravitons, that’s fine. If I prefer to say the world is “made of gravitons, which move around in some higher dimensional curved space”, but in certain limits behave like coherent states of gluons in 4-dimensional Minkowski space, that’s equally fine. Each description is useful in different regimes, but none is more “true” than the other.

To put it another way: there are hundreds or thousands of people around the world who are working on QCD. In fact, they are working on string theory; they just don’t know it yet.

Perhaps Atiyah would object to his “extremely complicated mathematics” becoming “rich mathematics”. I would guess that he thought it wasn’t rich.

A lot of the math that appeared in string theory was his math: K-theory, index theory.

If string theory turns out to have nothing to do with physics, it will remain a pool of rich mathematics.

The dynamics of string backgrounds is that of Ricci flow (or the other way around).

If you like to put it that way: points in that infamous “string landscape” are fixed points of a generalized Ricci flow.

I’d call that “rich mathematics”. Though it is certainly complicated, too.

There are mathematicians working on a mathematical field called topological T-duality who don’t know the first thing about string theory. Their field originates in string theory and is being pursued as a mathematical entity in its own right.

As you know, the latest in that direction is geom. Langlands. Rich and complicated.

Some argue that it’s not the temporal order of the devising of a theory and the observation of data that matters, but rather a question of the extent to which to explain the data you need to fix certain parameters, i.e., that there is a timeless relation of support between evidence and theory.

Yes, exactly.

The Schrödinger equation (plus some extra data) still predicts the diameter of the hydrogen atom, doesn’t it?

We don’t teach students that Schrödinger’s equation “retrodicts” or “postdicts” the size of the hydrogen atom, just because the comparison with experiment had been done long time ago.

I would not think that the term “predictions of a theory” is usually used in the sense of “gives us a vision of the future” (though that may be a special case), but in the sense of “these facts are derivable from the axioms of the theory plus given extra data”.

In math we say “axioms” and “implications/theorems/corollaries”. In physics we say “theory” and “predictions”.

The fight about whether string theory pre- posts- or retrodicts something is hence like many of those fights in the String Wars: it is not so much about the theory itself, but about its sociological consequences.

Technically I think it is quite right to say that string theory predicts gravity. But when the discussion is all about whether or not we, as a society, are taking great risks by spending so many resources on this theory, we may tend to dislike stating this technically correct statement this way, because we may feel that it does not sufficiently amplify the point that this prediction is rather useless, for our practical purposes.

David Corfield wrote:

Many historians of physics reckoned that general relativity’s accounting for the anomalous precession of Mercury’s perihelion was hugely influential in getting physicists to sign up to it. Now what kind of ‘-diction’ was involved there? The anomaly had been known for several years.

You already said yourself what it was: an explanation. That’s valuable, but that’s all; it’s intellectually dishonest to try to turn this into a prediction. (And GR had its own important prediction at the time: the bending of light by massive objects).

Philosophers of science usually call it a retrodiction.

Do they? This doesn’t fit my own understanding of the word’s meaning (which is well represented by the definition that I quoted from Wikipedia). Perhaps people do not use the word consistently?

Did it matter whether the data had been used in the construction of the theory, etc. Indeed, Einstein had rejected an earlier theory because it was incompatible with this data.

In my opinion, this is exactly what is important. If Einstein hadn’t known about Mercury, then this should have counted as a prediction (or retrodiction, but as I said the distinction is vague and unimportant) for him, increasing his own confidence in the theory. As it was, Einstein was still rightly confident, as only his theory provided an explanation, but not as confident as a confirmed prediction should make him. In any case, astronomers at large would count this only as an explanation, not a prediction. But if they too had not known about Mercury’s anomaly (say, if they were just beginning to get such precise measurements), then they too would count it as a prediction, and it would have been a more stunning success.

String theorists may argue that string theory explains gravity. (I don’t think so, since the explanation is more complicated and less clear than Einstein’s theory, which is to be explained. But at least it is consistent with quantum physics, so I can certainly accept a difference of opinion here.) But unless they’re using words differently from the way that I understand them, it certainly does not predict (or retrodict, whatever) gravity.

Science is more than merely a search for explanations of known facts. It relies also on tests against new facts. This is where prediction comes in.

Hi Christine,

There is plenty of room for all of them. If there’s only one point in which I agree with Lee 110% then it’s that theoretical physicists aren’t expensive. Compare that to the money that goes into experimental physics, or worse - military applications. Sometimes I want to scream at the funding agencies to just add .1% to the total and give it to me, gee, how many people could be hired with that?! It’s so totally stupid that positions are so rare because it’s totally unnecessary. The problem is (I think) that most positions still are at universities and tied to teaching, of which there is limited demand. What we really need are more pure-research positions. But these are currently mostly provided by private institutions. Best,

B.

The idea of evolution is a great distillation of the concept which Urs mentions: beautiful laws and messy data. As Dawkins put it, life develops through the non-random survival of randomly varying replicators. The complexity of a living thing is the reflection of the environments in which its ancestors lived, the messiness of the past environments borne out in the intricate genome.

So, if we consider the Universe as an evolved thing, what are the “replicators” and what forces affect their “survival”? What is the analog of genetic information, and how is it expressed? Are there counterparts to the “selfish gene” model, group selection, viruses?

I had a wonderful thought about Smolin’s reproduction-by-black-hole idea combined with the Hsu–Zee CMB message, but this blog comment window is too narrow to contain it.

Hi B,

What exactly do you mean with ‘universe’ and are you sure you use the word in the same meaning as Lee does?

No, I’m not. And you’re right I should read his book about that, in fact I’m really excited to learn of its existence and will definitely check it out.

See, we know that in our observable part of the universe the parameters of the standard model can’t have varied very much. So where did the ‘unlikely things’ happen and where did they ‘propagate themselves’?

I was referring to the Big Bang and what happened right after that. I guess the picture I have in my mind is of some set of contingencies very early on that could have determined those parameters in ways we have no hope of reconstructing, at this late date. And once they were set, then they could have just remained fixed as the particles (or their precursors)exploded out beyond the point that any similar types of contingency could affect them. Does this resemble Smolin’s (or anyone else’s) idea? I hope so, because it seems very logical to me.

What’s been on my mind is a question stemming from the “Out of Africa” theory of human evolution. I.e., if we are all descended from the same “Ur” homo sapien, or small group of homo sapiens, then what is the source of all our genotypic and phenotypic differences? In very crude terms, why do Chinese people look Chinese and Europeans look European, etc.?

In his book “The Real Eve,” Stephen Oppenheimer offers an amazing answer in the form of a contingent event that we actually do happen to know a great deal about: the explosion of Mt. Toba, circa 72,000 years ago. This event, or something like it, happening in the early stages of the Out of Africa migration, could have led to a whole set of different population bottlenecks, and consequent “founder effects” that might possibly have been the source of such differences.

It’s an interesting idea for physicists to contemplate because there may be a parallel between “Out of Africa” and the Big Bang, from an evolutionary standpoint.

So, soon after the Big Bang, maybe something drastic happened, like Mt. Toba, that could have led to something like a particle or protoparticle “bottleneck,” with subsequent “founder effects” producing the particle zoo we contemplate today.

Geneticists can actually use the DNA of currently existing humans to make inferences about such bottlenecks.
So I guess the question here is whether physicists have some analogous historical tool(s) that would enable them to look back and make inferences about what happened just after the Big Bang. And I guess they do, because they have in fact made such inferences. But as with genetics, it may only be possible to take the research so far and no farther.

Thanks a lot for that link, John. For me this is probably the “missing link” in my understanding of cosmic evolution. I’m printing it out right now.

Smolin claims that his Darwinian cosmology has already delivered an event of type 2!

Or another example: The theory called MOND produces lots of events of type 2 and 3.

Still, as it says in TWF 206,

MOND should instantly make any decent physicist cringe.

Apparently, just making lots of predictions is also not quite what we demand of a theory.

Next logical question, then: can we have a D2.7-brane?

I tried once, but all I heard was an echo.

Frankly, solipsism is such a convenient perspective that I’m surprised more people aren’t.

Have I ever tried to communicate with a solipsist? Heh.

Well, for one thing, a true solipsist would have no reason to communicate with me – or anyone else. That would be pointless, no? Since from that perspective no one else actually exists, only the solipsist.

And yes, it IS a “convenient perspective,” in fact it may well be irrefutable. But there is no way to attempt to communicate such a conviction without immediately contradicting oneself. So while it may be a perfectly logical position it is also indefensible.

Now I have a question for all you physicists out there. If, in the context of the measurement problem, the waveform collapses when a measurement takes place, and a measurement can be meaningful only when it registers in someone’s brain, and the question then is “whose brain,” is it possible to see this situation also as a black hole leading to the same singularity?

Victor Grauer wrote in part:

a true solipsist would have no reason to communicate with me

This is like saying that a true mathematical formalist (or other sort of mathematical fictionalist or anti-Platonist) would have no reason to think about mathematics. I communicate with you because it is interesting; what other reason should there be?

But there is no way to attempt to communicate such a conviction without immediately contradicting oneself.

I don’t understand why you say this. Have you never mused about philosophy within your own mind? Why shouldn’t a solipsist do the same?

Now I have a question for all you physicists out there.

A very good question! It was thinking about this very question that led me on the path to becoming a solipsist.