Before you ask yourself, “what kind of incendiary title is that,” let me put this in perspective. In 2001, I started graduate school at the University of Florida, and in 2002, I took one of the most difficult year-long courses a physics student can take: Quantum Field Theory. This was both the best and worst course I’ve ever taken. I worked harder for it than I ever have for any other course, I learned more for doing it than I had at any other time, it was the most difficult time I’ve ever had in a course, and it was superbly taught by one Richard Woodard. Quantum field theory is possibly the best-tested physical theory ever, with every single particle physics experiment ever performed confirming its validity.
Now, Richard’s a pretty interesting and unusual guy. He worked under Sidney Coleman at Harvard, researched String Theory for a few years after graduating, and after about 4 years, came to the conclusion that String Theory was a blind alley, publishing a paper in 1989 detailing why. Since then, he’s pursued interests in field theory, quantum gravity, and quantum issues within cosmology. A couple of months ago, he wrote a paper entitled, How far are we from the Quantum Theory of Gravity, and the answer, not surprisingly, is very far. We simply don’t know what goes on at the center of a black hole, for example: at very high energies or at very small distances.
But reading Richard’s paper, I’m struck by the simplicity and frankness of his arguments against the entire approach of String Theory.
A central point to understanding string theory is that it cannot be formulated the way all other fundamental theories are, by giving the dynamical variables and the equations they obey. We do not know what the fundamental dynamical variables of string theory are, nor the equations they obey.
For example, in Newton’s gravity, you give it the variables of masses and distances, and the equations it gives you tells you how force, position, velocity, and acceleration will change over time.
If I understand Richard’s paper correctly, then in string theory, we don’t know both the variables and the equations. In fact, unless another theory (and M-theory is this possibility) comes along that encompasses and expands upon string theory, string theory isn’t a fundamental theory at all, due to instabilities.
Richard gives a brief history of string theory from pages 49-59 of this paper, and it is completely correct, as far as I can tell. After relating the fundamental idea behind superstring theory (originally conceived to explain the strong interactions, which QCD now does very well), he talks about successes and setbacks for the theory.
The major setbacks started happening in this decade, where it was realized that when you turn this high-dimensional theory into our three spatial dimensions and one time dimension, there are far more choices available to you than there are particles in the Universe. The big problem with this is that there is no principle known to help choose which one is right.
(And note, if you use anthropic arguments to help you, which I personally don’t buy, you still have more choices than you do particles in the Universe.)
But this isn’t about me. What’s Richard’s conclusion about all of this?
A personal anecdote might best convey the current state of affairs. Early in
the spring 2007 semester my University of Florida colleague, Charles Thorn,
began a seminar by announcing his belief that:
String theory is just a technique for summing the leading terms
in the 1/N expansion of QCD.
After years of hearing more ambitious assessments this was so shocking that
I checked to be sure I had understood correctly. Charles confirmed that I
had; in his current view, the effort to regard superstrings as a fundamental
theory of everything was a blind alley. Later that year I related Charles’
pronouncement to string theory colleagues on three continents and solicited
their own opinions. About half of them agreed with him, more often the
So it looks like the last 40+ years of work on this topic may be nothing more than a mathematical exercise, with no physically interesting relevance at all. This is one of the great fears I have as a theoretical physicist: that my work will turn out to not be physically relevant to the Universe we live in. It’s a tough thing to wrestle with, as even the best among us have demons to wrestle with.
And at this point, we don’t know. We don’t know whether string theory is a more fundamental theory describing our Universe than the one (Quantum Field Theory) we’re using now, or whether it is a blind alley. I don’t know enough about it to tell you definitively what the answer is, but I do know that those 11 pages of Richard’s article are very interesting and thought-provoking. Peter wrote a little bit about this, too, and I’m curious as to what your opinions are on this.