String theory’ takes a hit in latest experiments

The latest news in the particle physics media is that string theory is in trouble. That might not mean much to you if you don’t know what string theory is.
Suppose all forces between fundamental particles — say a proton and electron, for example — could be described by a set of mathematical equations, a kind of “theory of everything.” Einstein spent much of the latter part of his life looking for such a theory but never found it.
Jump ahead about 50 years. Now, we have a candidate for this “theory of everything,” and it is called string theory. If string theory is correct, then it would explain how all forces between particles, including gravity, can be derived from a single set of equations.
Whether string theory is correct depends, to some extent, on experiments. Unless string theory can predict something that can be measured, then it’s just a set of intriguing but useless equations.
For example, the so-called standard model of particle physics predicted many years ago that a particle called the Higgs boson should exist. The Higgs was discovered in experiments just last year.
String theory also predicts that there should be new particles, which many physicists hoped to see in the same experiments that found the Higgs boson. These new particles are called super-symmetric, or SUSY for short.
Last month, scientists working on those same experiments released new data that were expected to show whether SUSY particles exist.The experiments use the Large Hadron Collider in Switzerland to accelerate protons to extremely high speeds and smash them together. Huge detectors surrounding the collision point measure hundreds of particles propelled by the energy of the collision.The experiment has been likened to firing two pocket-watches at each other, then photographing the springs and cogs that fly out. This analogy is not quite apt — the proton collisions create new particles — but does give at least a hint of the complexity of the experiments.
While SUSY particles have not been directly observed, these new particles were expected to modify the properties of a particle called the “strange B-meson.” Although this particle lasts a short time — it breaks into several lighter particles — its lifetime is measurable.
If SUSY particles don’t exist, the strange B-meson should have one lifetime. Conversely, if SUSY particles do exist, B-mesons should have different lifetimes. The new results are consistent with no SUSY.
So does this result debunk string theory? The answer is no, but it does make it much more difficult for people who believe that string theory is the answer. In other words, the simplest form of string theory predicted that SUSY particles would have been seen in the recent experiments. But other variations of string theory are still possible.
Why should an astronomer care about string theory? One explanation for the makeup of dark matter is that it is composed of SUSY particles. Dark matter makes up about 25 percent of the mass of the universe, and it’s known that dark matter is not made from regular stuff such as protons, electrons etc.
It would be a triumph of particle physics if SUSY particles were discovered and these particles had the properties of dark matter. It would answer two unexplained mysteries of the universe in one fell swoop.
Unfortunately, nature is not so kind. Or perhaps string theory simply is incorrect. After all, there is no fundamental principle that says string theory is the only mathematical solution to describe nature.
So until string theory comes up with a prediction that can be verified experimentally, we will continue to wonder if a “theory of everything” is really possible.
Kenneth Hicks is a professor of physics and astronomy at Ohio University in Athens.
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