String theory is one of the more popular candidates to combine quantum mechanics and relativity into a grand unified theory. But it had remained completely untestable until recent experiments at the Large Hadron Collider. The early results don't look good.
A few years ago, a group of physicists came up with an ingenious way to test for the existence of hidden dimensions, a key aspect of many string theory models. Basically, the experiment rests upon the existence of micro black holes, objects tinier than an atomic nucleus that could theoretically be produced by smashing together a pair of protons at tremendously high velocities.
This micro black hole would be very unstable and quickly decay, releasing lots of different subatomic particles. The physicists figured out the specific combinations of particles that would be created if the universe has 10, 11, or even more dimensions. The hope was that the Large Hadron Collider would be able to produce the massive energies required to create these micro black holes.
Well, they ran the experiment, and the results are less than encouraging. The LHC has completed an extensive search for these objects in high-energy proton collisions, and no evidence at all turned up for micro black holes between 3.5 and 4.5 tera-electron-volts. That's a massive energy level and pretty much the upper limit of what we can currently test. This more or less rules out versions of string theory that includes micro black holes at those energies.
Now, let's back up a bit. This isn't good news for string theorists, but it doesn't invalidate string theory either. The original idea for this experiment was always a bit of a long shot, more an attempt to come up with something - anything - that could be used to test aspects of string theory using today's technology.
Researchers hoped to find certain exotic phenomena that would likely exist in a world governed by string theory. Even a few small things can throw this off - the micro black holes might still exist, but they might be larger than the curvature of the hidden dimensions, which would mean they remain unaffected by the extra dimensions. Or it might be even simpler: the micro black holes just can't be found at these energy levels, and we need the next generation Hadron Collider - or even the one after that - to detect them.
String theory definitely lost this particular battle. But this setback isn't the end of the line - it just means the search for a grand unified theory won't be getting any easier. We probably shouldn't have expected anything else.