Yesterday at the KITP Wati Taylor gave a talk entitled Freedom and Constraints in the Landscape of Intersecting/Magnetized Branes. During the talk he explained in detail the problem of lack of predictivity caused by the landscape. As far as anyone knows, to the extent you can calculate anything, you can get whatever you want: “Anything Goes”, and string theory is useless for ever predicting anything. He was looking at some particular classes of vacua, chosen for their computational tractability, and hoping to find some constraints among the quantities one can compute. There’s no known reason to expect this, but one can compute anyway and hope. The end result was the expected one: you can get whatever you want. Here are some quotes from the talk:
So, We’re really in a very challenging situation where we don’t really know how to define the theory, we don’t know what the set of solutions are, and even if we did we would have a very hard time making a sensible statement about what that means for predictions…
Every piece of data we have so far I would say is consistent with the notion that everything is pretty much uniformly and randomly distributed in the landscape.
There was extensive discussion of the predictivity problems and overwhelming evidence string theory can’t ever predict anything below the Planck scale (this wasn’t discussed, but I don’t see how it predicts much above the Planck scale either). For some reason there was no drawing of the obvious conclusion that one should just give up on the idea and try something else.
The counsel of dispair is too bitter a drink to swallow in one gulp..
If “everything is pretty much uniformly and randomly distributed in the landscape” I wonder which space is larger:
1) landscape space of possible string theories
2) space of possible random text-strings with a given maximum length, corresponding to the possible TOE Lagrangians one could type on a piece of paper.
Monkeys and Shakespeare and keyboards come to mind.
er, despair.
I think they’re taking Margaret Thatcher’s old TINA position–there is no alternative.
Did we move away from 10^500 string theories to infinite number?
If we did not, the problem is not that string theory is useless for ever predicting anything, it’s that the structure is too big and complex. But it should still be theoretically possible to answer the question whether there is at least one in 10^500 that has the right particle content and predicts low-energy constants in agreement with observations. Maybe not at this stage (we need 500*ln(10) = 1150 bits of information), but eventualy. We have 19 constants just in SM, some, such as electron mass, are known to 10 digits. Neutrino oscillation means that we get at least 7 there (three masses, four MNS parameters). If LHC finds supersymmetry, the number of parameters will blow up. Sooner or later we will get to the point where the number of string theory false vacua within the boundaries of observed constants is either 1 or 0. And that will be the definitive test. In the mean time it’s a valid avenue of research to analyse the landscape.
Does anyone have a reference to an article that initially computed the 10^500 number?
Odd. This doesn’t quite seem to agree with what Dienes and Lenneck have found. See arXiv:0804.4718 or PIRSA:08120000.
Vintage Woit, well said. My namesake tells me you’re in Dublin next month – I’ll see if I can get a day off in the big smoke…regards Cormac
Who cares whether string theory can ever predict anything below the Planck scale? It’s supposed to be quantum gravity, so the real test is what it predicts ABOVE the Planck scale!
Do you throw out the Standard Model because it does not predict biology? If we ever discover life on other planets, it will likely have very different biology from our own. All these possible different biologies will of course be consistent with the Standard Model, just like many different low energy field theories are consistent with string theory, but the Standard Model does not select out a unique possible biology, just like string theory does not select out a unique low energy physics!
Bee,
What Dienes is doing is a bit different. He’s counting vacua and hoping for correlations. Taylor explicitly says he doesn’t think counting is relevant: if you don’t know the details of what cosmological process is producing our universe, you don’t know what the right probability measure on vacua will be, no reason to believe it’s the simple counting measure.
Nameless,
I don’t have a specific reference at hand, but look up Michael Douglas’s papers on the landscape and either he’s the first to come up with this number, or he refers to the paper that did. People (e.g. Denef) have come up with constructions of vacua that produce much larger numbers of vacua than 10^500, effectively the number you would have to examine is infinite.
Hi Cormac,
I’ll be in Edinburgh April 21-23 (for the conference in honor of Atiyah), then the 24th will stop for the day in Dublin on the way back to NY. I’m hoping to extend that and stay for another day or so. Once I’ve got definite plans I’ll contact you and maybe we can get together.
Yesterday, Renata Kallosh finaly proved (a physicist proof) that
N=8 Sugra is renormalizable at all loops and for all legs. It seems
that a certain the E7 symmetry is never broken, and delays the firt
non renormalizable singularity up to infinity and rendering the theory
renormalizable.
Here is the article:
http://arxiv.org/abs/0903.4630
Yes–I have been talking with Feyman about how ridiculous the string theory folly is getting. You know how Feyman once said that science is like creating in a straight jacket. And just the other dya he said string theory is like taking the jacket off, saying and doing anything in the landscape, calling it science, and then putting the straight jacket on your critics.
Remember that one band which tried to get famous by just removing all the frets, putting their fingers anywhere on the guitar neck, and proclaiming it to be music? I don’t remember them either. And I’m pretty sure they weren’t state funded nor tenured either.
Dissing fretless guitars now? You’re still bitter about that whole Fleetwood Mac thing, aren’t you?
We were all for innovation, rebellion, and rock’n’roll, but even we had our rules honoring the mathematical vibrations of strings. It just wouldn’t be fun walking out there on stage with strings vibrating in an infinite array of possibilities, all over the landscape, and then shouting at the press, “THIS IS BEAUTIFUL, ELEGANT MUSIC!” Sure, some of the press would write wahatever we said just so they could join us on the bus later, but would we really want them there? I mean they don’t even trust their own ears so how can we trust them? Would you want them on your bus around your groupies?
Peter wrote, “For some reason there was no drawing of the obvious conclusion that one should just give up on the idea and try something else,” and my good friened Einstein stated, “Insanity: doing the same thing over and over again and expecting different results.” Maybe Feynman was more right about those straight jackets than he even knew.
Well, you shouldn’t stop thinking about tomorrow. It’ll soon be here.
Yes–tomorrow will be better day as string tehory fades away.
I have also had the opportunity to converse with a lot of other classical economists–folks like Adam Smith and Ludwig von Mises. They see a lot of parallels between the snarky math of String Theory and Wall Street. Smith and von Mises dealt in ideals, ideas, integrity and *words* that meant things. String Theorists and Quants deal in math that never quite proves their fundamental postualte taht they are smarter than us, but rather which proves the exact opposite. They have given math a bad name and this makes me angry. Good Pythagoreas comes up with the theory of harmonies on vibrating strings and then they corrupt and convolute it all into a discordant landscape, while bankrupting a nation and currency with their silly little equations which have absolutely no predictive power, robbing them of math’s higher beauty, which is married to the power of prediction. Riding on Einstein’s, Smith’s, et als. coattails, the modern snarkyalarks levelethey pay d physics and finance. Newton stood upon the shoulders of giants to see further, and they stood upon the shoulders of giants to cut the giants’ heads off. And now we’re supposed to buy their “Elegant Universe” DVDs and bail them out, while they pay Lubos to call us names.
Speaking of Lubos, not only did he inspire that character on The Big Bang (perhaps String Theory’s greatest lasting contribution), but back in the eighties we were able to hop on Michio Kaku’s time machine and travel to 2012 and then back again with Lubos so that he could play the cop at 50 seconds in this 80’s video for Rock’n’Roll High School:
http://www.youtube.com/watch?v=c5vh0QHUA1w
Hi Peter,
about predictivity of string theory:
Imagine we had one million string vacua, whose low energy physics content look exactly like the standard model. (We can discuss later how realistic this is.) Now, we could say, that predictivity is totally lost, when we want to go to high energy physics. On the other hand, let’s compare the status of string theory in this situation with quantum field theory. In quantum field energy there is simply no way at all to go to high energy physics. QFTs are even understood today as the low energy effective theories of some unknown high energy theory. So computational issues aside we can use one of our one million string vacua and it is just as good as the standard modell. One could argue that it is even better because it is a theory of gravity as well and you can calculate general features of gravity like the black hole entropy.
Questions:
1. Is there an error in this line of thought? Is the error that in the assumed standard model string vacuum you just had to many parameters to fit?
2. Do you think my assumption is realistic to have string vacua, which look exactly like the standard model in the low energy range?
Mikael,
One problem with this sort of argument is that it’s just not true that string theories with a choice of one of these vacua gives you a consistent theory at any energies. You don’t even in principle know what the theory is outside of perturbation theory, so you don’t know that you have a consistent unified theory at arbitrarily high energies. Even if you did, the freedom of choices presumably would allow you to get all sorts of different physics at very high energies.
The Standard Model is a gauge theory with one of the simplest possible choices of groups and representations, and it is this simplicity that makes it highly predictive and gives an extensive array of very strong tests that it has passed. In “string vacua” constructions, simple choices don’t look like the real world, so people have to make constructions more and more complicated in order to evade making a prediction that can be compared to existing data.
There’s all the difference in the world between these two situations, and going to high energy changes nothing. String theory is no more predictive there than at low energies. The only difference is that at high enough energies you can make sure your predictions can’t be checked.
Peter,
let’s forget about high energies. Let’s just compare string theory and QFT in energy regions explored by today’s colliders. Is it conceivable that we arrive at a situation, where we can say, string theory did not keep its promise but it is not worse than QFT?
Mikael,
Again, at collider energies, quantum gauge theory is a huge success. You pick one of the simplest ones, it predicts all sorts of things, they all come out true on the nose. In string theory instead, you get no predictions, because you have to make your model more and more complicated in order to evade getting a wrong answer. The difference between these is the difference between dramatic success and dramatic failure.
Peter (and other QG enthusiasts), what do you people think of
a href=”http://arxiv.org/abs/0806.0665″>this
Hogan has given talks on this which are available
at here?
This paper however as been refuted
here
Have you seen any papers by string theorists claiming this as a prediction
of string theory?
Shantanu,
I haven’t seen this claimed as a “prediction” of string theory. For one thing, just being “holographic” doesn’t mean you’re doing string theory. Personally, I find it hard to believe that you can get any serious prediction just by invoking “holography”, which is a very general concept. Because of that and my general lack of interest in quantum gravity, I haven’t looked into exactly what Hogan is doing.
Right, this is just like how classical mechanics is totally worthless to predict anything, because a lagrangian can be anything I want it to. A lagrangian can be any function at all! How can we expect to ever find a lagrangian that describes reality when they can be anything we want!
Not quite. Not only, once the Lagrangian is chosen, the predictions of the classical mechanics are known, but the useful Lagrangians are simple. If one had to use a completely different Lagrangian to predict the fall of an apple compared to the behavour of the Moon, or even the fall of different apples, then we would have a “stringy” classical mechanics.
Peter, from listening to one of his talks, the kind of holography is a
prediction of Bank/Susskind et al’s Matrix theory models .
(see the audio of his talk at
here
Anyhow do u know of any string theorists who are citing this tentative result from GEO-600 as an “evidence” for string theory?
Shantanu,
I haven’t seen any string theorists claiming this.
In any case, this can’t be claimed as a prediction of such matrix models, since they haven’t been made to work at all in realistic cases (four large dimensions).
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