This week I’ve been busy, attending a workshop here at Perimeter on Superstring Perturbation Theory.
Superstrings are the supersymmetric strings that string theorists use to describe fundamental particles, while perturbation theory is the trick, common in almost every area of physics, of solving a problem by a series of increasingly precise approximations.
Based on that description, you’d think that superstring perturbation theory would be a central topic in string theory research. You wouldn’t expect it to be the sort of thing only a few people at the top of the field dabble in. You definitely wouldn’t expect one of the speakers at the workshop to mention that this might be the first conference on superstring perturbation theory he’s been to since the 1980’s.
String perturbation theory is an important subject, but it’s not one many string theorists use. And the reason why is that, oddly enough, very few string theorists actually use strings.
Looking at arXiv as I’m writing this, I can see only one paper in the theoretical physics section that directly uses strings. Most of them use something else: either older concepts like black holes, quantum field theory, and supergravity, or newer ones like d-branes. If you talked to the people who wrote those papers, though, most of them would describe themselves as string theorists.
The reason for the disconnect is that string theory as a field is much more than just the study of strings. String theory is a ten-dimensional universe (or eleven with M theory), where different ways of twisting up some of the dimensions result in different apparent physics in the remaining ones. It’s got strings, but also higher-dimensional membranes (and in the eleven dimensions of M theory it only has membranes, not strings). It’s the recipe for a long list of exotic quantum field theories, and a list of possible relations between them. It’s a new way to look at geometry, to think about the intersection of the nature of space and the dynamics of what inhabits it.
If string theory were really just about strings, it likely wouldn’t have grown any bigger than its quantum gravity rivals, like Loop Quantum Gravity. String theory grew because it inspired research directions that went far afield, and far beyond its conceptual core.
That’s part of why most string theorists will be baffled if you insist that string theory needs proof, or that it’s not the right approach to quantum gravity. For most string theorists, it doesn’t matter whether we live in a stringy world, whether gravity might eventually be described by another model. For most string theorists, string theory is a tool, one that opened up fields of inquiry that don’t have much to do with predicting the output of the LHC or describing the early universe. Or, in many cases, actually using strings.
4 gravitons 
“Looking at arXiv as I’m writing this, I can see only one paper in the theoretical physics section that directly uses strings. … and in the eleven dimensions of M theory it only has membranes, not strings.”
Excellent article.
Matt Strassler (a great Theoretical Physicist, at Harvard) said that M-theory is not a theory at all, but a hodgepodge (see http://profmattstrassler.com/2014/02/26/quantum-field-theory-string-theory-and-predictions-part-9/ ).
The mission for the string-theory is for the string-unification {describing quarks and leptons with a theoretical language}. Yet, only G-string (see http://putnamphil.blogspot.com/2014/06/a-final-post-for-now-on-whether-quine.html?showComment=1403375810880#c249913231636084948 ) is a LANGUAGE which provides string-unification with a set of theoretical lexicons. After all, string theory must use strings.
“… String theory grew because it inspired research directions that went far afield, and far beyond its conceptual core. … For most string theorists, string theory is a tool, one that opened up fields of inquiry that don’t have much to do with predicting the output of the LHC or describing the early universe.”
How sad this is!
A true useful theory must DESCRIBE this universe which is now organized as dark energy, dark mass and visible mass. A theory which cannot describe these is not a PHYSICS theory.
Matt Strassler (a great Theoretical Physicist, at Harvard) had a few great posts on Dark matter, and there was a very nice debate between Alain and emptyspacewaves. The debate starts at http://profmattstrassler.com/2015/04/20/completed-final-section-of-article-on-dark-matter-and-lhc/#comment-322571 .
The most interesting and significant point is that Matt Strassler (the blog owner) keeps silent in those exchanges while he is always eagerly refuting other comments in his blog.
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“Hodgepodge” is a rather unfair interpretation of what Matt said. There’s certainly a hodgepodge of methods used to address M theory, but M theory itself is one unique theory, which is kind of the opposite of a hodgepodge.
The mission of string theory isn’t unification, that was one project people were working on in the 80’s. It got a lot of press, which is why people think it’s the main thrust of string theory, but it’s really not.
The “G-String” in physics is derived from a Warren Siegel parody paper.
“A true useful theory must DESCRIBE this universe which is now organized as dark energy, dark mass and visible mass.”
So the theory of superconductors must describe dark matter? The thermodynamics of protein folding should describe dark energy?
Different concepts in physics serve different roles. String theory provides a few new tools for the study of dark matter and dark energy, but those questions are usually best addressed by older methods, from quantum field theory.
Again, I think you’re being misled by old attempts at physics popularization. The idea that string theory would generate every parameter of the universe was one particular research program, but it died out. If Matt ever finishes his series on Quantum Field Theory, String Theory, and Predictions, he’ll give an account of why, in the meantime you might find my account helpful.
I wouldn’t read too much into Matt’s absence from that particular conversation. He can’t get to every discussion, and in this case both sides have a relatively high density of nonsense. (Note in particular how many times emptyspacewaves repeats almost the exact same words, in some cases without anyone else commenting in between!)
Speaking of that, not to be offensive, but do you have Tourette’s? I ask because your tic of repeating “(a great Theoretical Physicist, at Harvard)” makes it a bit hard to take you seriously, but I get if it’s a Tourette’s-based compulsion then it’s not really something you have control over.
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