Saturday, June 24, 2006

String Theory

An recent article in the Wall Street Journal wonders whether string theory--the now 2-plus decades-old idea in theoretical physics that the fundamental components of the universe are strings (and more generally, membranes)--isn't getting in the way of what could be more productive uses of physicist's time. In other words, is string theory even knocking on the right door?

The article highlights two physicists (Peter Woit and Lee Smolin) who have books coming out in September, and who both believe that string theory--which everyone admits has yet to produce a prediction that can be tested by experiment--is taking physics in a very wrong direction. The WSJ article only interviews one pro-string physicist, and if you want to fault the article you can use this ratio as evidence.

All of the cool crowd in physics works on string theory. But you'd never know it by this WSJ article or by the blog of the most prominent string theorist on the Web, Lubos Motl of Harvard. Motl is a different sort of physicist in that he's not reserved about expressing his opinion about string theory, or anything else for that matter, including your manhood and your mother's manhood.

Lubos goes to bat for string theory's case and against the WSJ's point of view, but it's not a very strong turn at bat because mostly instead of making counterarguments against the points raised in the article he goes about questioning people's sanity and, seemingly, their very right to walk upon this earth while holding the opinions they do. He would have made a better case if he'd stuck to the physics. But such is not his way.

This is all just a way for me to say that I've been reading Leonard Susskind's new book, The Cosmic Landscape: String Theory and the Illusion of Intelligent Design. The picture of the universe that string theorists are painting lately is a strange and bizarre one that, if only it were true, would be perhaps the most amazing thing you could imagine: that we live in a universe that is a mere bubble in a much more immense sea of bubbles, in each of which the laws of physics are profoundly different than they are here.

Why do we live here? Because the laws of physics allow it. That's not the complete answer, but it's close.

The problem is, as I understand it, that string theory (or, more generally, the theory that physicist call "M-theory") seems to allow a very large number of possible solutions, or as the physicists call them, vacuua (as in the plural of "vacuum"). In fact, there are roughly 10-to-the-500th-power vacuua. That's an immense number that I don't even know how to describe except by using scientific notation. It's much more than a googol, or even a googol of googols. But it's less than a googolplex.

It's about 10^109! (10-to-the-109-factorial). (Oops. Obviously not.)

So rather than asking "how do we manipulate the mathematics to choose the one vacuum that represents our universe?" string theoriests like Susskind are saying, all these vacuums are allowed and all describe possible universes. In one or two of them the photon mass is zero and the electron has a mass of 0.511 MeV (and...), but in others the graviton is massive and quarks can be light-years apart and atoms can't even form and nothing is the way it seems here. And in still others..., well, you get the picture, times 10^500.

The universe in which we happen to find ourselves is distinguished as the one whose fundamental constants and particle masses are such that everything combines to produce life (at least, as we know it).

It's pretty weird, wild stuff, and if you want to know more you should read Susskind's book as best you can. Despite the WSJ article, string theory doesn't appear to be going away anytime soon, though maybe a few more people will start jumping on the Woit/Smolin bandwagon. If two people can comprise a bandwagon. I'm not sure.

13 comments:

NYC TAXI SHOTS said...

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Anonymous said...

... 10-to-the-500th-power vacuua. That's an immense number ... It's about 10^109! (10-to-the-109-factorial).

10^500 is approximately 253 factorial
(253! = 5.17^499)

On the other hand, 109! ~ 10^176,

so, 10^(109!) ~ 10^(10^176),

which is far bigger than a googleplex.

Anonymous said...

I should say you have presented the landscape argument quite correctly. Just to be fair, you should also point out that the vast majority of string theorists do not work in landscape-- in fact probably a majority of string theorists are as opposed to landscape as Peter Woit and Lee Smolin are.
I think it is somewhat unfair to reduce all of string theory to mere landscapology-- there is much more, in fact much much more to it. That said, I should add that pretty much everything else suffers from the same handicap, i.e lack of experimental evidence. Whether that means string theory is not science, or not even wrong as some self declared acolytes of Pauli would choose to describe, is a matter of philosophical debate.
At the end of the day what will stand are results-- so may be string theory will be swept away by a new approach that some clever theorist will come up with-- or may be string theory will finally live upto it's admittedly somewhat overhyped claims and find some new physics. Instead of writing entirely negative diatribes, people should instead try to come up with their own theories if they do not like string theory.

fh said...

The landscape would be an intriguing picture... if there was any sort of mechanism or theoretical framework for it that goes beyond the vaguest sort of handwaving.
In the context of stringtheory the first question you have to ask then is "why compactification?" and there simply is no mechanism that, starting from a quantum theory that has a natural low energy description in 11 dimensions, explains how we end up with only 3 spatial macroscopic ones.
In the absence of any such mechanism there is no reason to assume any particular distribution function over the vacua and the theory is vacuous (pun not intended).
To reduce the critizism of those within and without string theory that are opposed to the landscape ideas to "they are not cool" (or: They don't make for sensational headlines) is seriously unfair.

fh said...

"people should instead try to come up with their own theories if they do not like string theory."

As of course they are: find a Smolin

Not that these are better/less controversial then String Theory.

L. Riofrio said...

My little research got drawn into the controversy, and I say: Bring it on. I prefer to work on real theories with testable results.

Anonymous said...

fth, of course I have absolutely nothing against Smolin doing LQG and advertising it. That's exactly what everyone should get down to doing-- we need more people to work on theories instead of writing books against string theory, as if it were a philosophical war against fascism! What I object to are a) somehow boiling down everything to landscape b) dark hints that string theory suppreses alternatives or whatever. Also as you point out all the alternatives are equally fantastic and have the same drawback of not having any experimental predictions.
It is rather unfortunate that Lubos Motl chooses to launch into apalling drivel to defend string theory, but I do not see how that is relevant to any discussion on the merits or demerits of string theory, LQG or anything.

Peter Woit said...

Anonymous,

"entirely negative diatribe"

I'm wondering what book you're talking about, you clearly haven't read mine.

"a majority of string theorists are as opposed to landscape as Peter Woit"

It's not a matter of being "opposed to landscape". Either string theory implies the landscape or it doesn't. Witten is quite explicit that while he doesn't like it, he doesn't have any good arguments against the landscape. The argument that string theory implies the landscape is quit solid, and a devastating one to the whole string theory program

"It is rather unfortunate that Lubos Motl chooses to launch into apalling drivel to defend string theory, but I do not see how that is relevant to any discussion on the merits or demerits of string theory, LQG or anything"

Lubos Motl was the Harvard string theory group's choice as the most talented and brilliant young physicist around. His behavior now is not that different than what it was when they hired him as an assistant professor. Kind of makes you wonder about the judgement of some of the most prominent string theorists in the country, doesn't it? Have they changeed their mind about him? I haven't seen any evidence of this.

Anonymous said...

"Lubos Motl was the Harvard string theory group's choice as the most talented and brilliant young physicist around. His behavior now is not that different than what it was when they hired him as an assistant professor. Kind of makes you wonder about the judgement of some of the most prominent string theorists in the country, doesn't it?"

I still fail to see why Lubos's manners-- or lack of them-- should have anything to do with a scientific argument about string theory. Are you proposing that people with awfully inadequate social skills should not be hired as physicists?
In case this needs clarification, I am in no way supporting Lubos's attacks directed at you personally-- I think they are quite disgusting and mostly pretty hilariously stupid-- but that does not add/ subtract anything to/ from his scientific abilities.

That string theory invariably implies landscape is a statement that needs quite a few technical qualifiers, doesn't it?

Peter Woit said...

Anonymous,

Since we don't know what "string theory" is, we don't know whether it implies the landscape. My only point was that this is an issue the theory is supposed to give an answer to, independent of who likes what it is.

The problem with Lubos is not his lack of social skills, it's that he has no interest in logic or normal standards of scientific discussion. With him, everything is just ideology. And yes, this very much does subtract from his scientific abilities. It's a remarkable comment on the string theory community that they think very highly of someone like this as a scientist.

Knotted String said...

I read somewhere (I'm sorry I didn't save the hyperlink) that many years ago, at a Czech Republic university, a then obscure Lubos Motl uploaded a paper on string theory to arXix.org.

Edward Witten emailed Lubos encouragement a few hours later!

This is exactly what the arXiv is supposed to be about, speeding up the exchange of information and allowing researchers to cut through the traditional red tape.

It's just ironic that string theory is not actual science, and that arXiv.org now censors stuff on the basis of academic affiliation.

The Calabi-Yau 6-dimensional manifold added to 4 or 5 dimensional spacetime to achieve SUSY, is very abstract and intellectually Platonic mathematics.

Can I just ask how is it that it has a finite rather than infinite number of solutions? Surely the parameters of the Calabi-Yau manifold affecting the supersymmetry unification energy and thereby the vacuum energy and are not quantized, and a continuously variable parameter implies an infinite number of exact solutions for the vacuum energy?

I know the string is supposed to be able to resonate at a set of discrete frequencies to represent different particles (without being able to predict particle masses or anything), but I don't see how this can lead to 10^100 - 10^1000 vacuum states rather than any other range.

It all depends on what you want from a theory. If you think a purely abject, uncheckable, extra dimensional mathematical model is elegant then why choose M-theory?

Why not move back to the epicycles of Ptolemy. Every time the theory disagrees with observation, modify the theory by adding an extra epicycle. (Then defend it by saying that if the earth rotated, we'd feel it moving under our feet.)

In 1989 there was 12 part TV series "The Nuclear Age" made by Channel 4 in the UK in conjunction with American and Japanese TV companies.

One of Kennedy's advisers is interviewed and complained that science was becoming a "substitute for religion."

I thought that was over the top. But it makes more sense now. People do need some kind of faith and belief system in their lives, even scientists.

They have to believe in some kind of scientific method or ultimate objective. String theory is just more of Plato's beautiful and elegant theorizing. It is the most ideal type of religion you can possibly believe in, containing all the vital vagueness and lack of falsification which is vital to inspire widespread confidence.

When people genuinely believe in a religion, they are not sincere seekers of fact, but are in need of comfort. The key comfort provided by a religion like string is the lack of falsifiability.

If string theory was in danger of being falsified from one day to the next, it would not inspire any more confidence that that of any other "crackpot" theory.

The reason why it has shot to the top is precisely that it is secure, providing a community of researchers with the security they need in order to remain confident, hopeful and optimistic.

Religion is a collective paranoia and superstition which serves the utility function of insulating human life from the harsh realities of the universe. So it is entirely natural that religion should infiltrate high energy theoretical physics at its most vulnerable frontier point.

Best Tech Gadgets said...

There is also a very interesting string theory with 2 time dimensions.

Anonymous said...

Is anyone going to see a comment about a two year old article?

First, the comment that no other theory makes a testable prediction is absolutely pertinent-- the LHC is our best hope to date of getting physics beyond the standard model, and physicists have mused that the worst possible result is they'll find the Higgs boson and nothing else. Science is driven by data, and we simply don't have data.

Also, whenever the word "paradigm" pops up in this sort of context, it's necessary and unavoidable to explore pretty thoroughly various dead-ends. For instance, between Bohr's theory and quantum mechanics proper, attempts were made to match predictions to spectra by imagining elliptical orbits, and so on. I had a prof who complained that it had set quantum mechanics back ten years. But science couldn't be ready to accept the fundamental change of quantum mechanics until it'd become satisfied that the results really don't fit into the old and comfortable Newtonian paradigm. And the same is true of string theory-- strings are a logical extension of points, and until it's explored, scientists will always wonder if it's worth exploring. And it still might be. We can look back with hindsight on elliptical Bohr orbits, but anyone who says right now that string theory is a dead end really is guessing.