I noticed this article on the news this morning, regarding the latest results from the LHC on micro-black holes. But allow me to start with a brief review of the subject...

According to Einstein's General Theory of Relativity, it is possible for an object to become so massive that not even light can escape it. This is called a black hole, and it requires a huge amount of mass and energy compressed into a small region. The energies required to produce even a tiny black hole at a particle accelerator like the LHC are enormous - almost 15 orders magnitude above the highest that the LHC will reach. We have nothing on Earth that can produce these sorts of energy densities, and no collider will reach those levels for the forseeable future.

Except for one loophole in the laws of gravity. The reason the energy levels are so high is that gravity is far weaker than any other force of nature - in fact it is also 15 orders of magnitude weaker than electromagnetic forces. But that may not be true...

In 1998 several groups of theorists independently demonstrated that if the Universe contains additional dimensions that we cannot observe, and if gravity is the only force that can cross this extra dimension, it would appear to us to be weaker. Gravity has only been studied at distances above 0.1mm and so it is possible that at shorter distances it suddenly becomes very strong - perhaps even as strong as the electromagnetic and weak nuclear forces!

And that would mean that black holes smaller than a few micrometers could be generated with the 14 TeV energy beams produced at the Large Hadron Collider. If the effects of these micro-black holes were observed in the detectors, it would prove the existence of higher dimensions and be a major scientific breakthrough.

Unfortunately the CMS detector at the LHC has finished studying the first set of billions of particle collisions, and found no trace of a black hole. Except for a few very exotic models, this effectively excludes the largest of possible extra dimensions (the exact size is model dependent, but for advanced readers they claim that the Planck mass is above M_P = 3.5 - 4.5 TeV).

But there is no surprise in that result at all, and really nothing new in spite of the press release. If the Plank mass were that low, then we would observe their effects in supernovae and in gamma-radiation from outside the Milky Way. In my own doctoral dissertation from two years ago I proved that for any reasonable model, a Planck mass that low would generate too many positrons in the center of our galaxy and the radiation produced by their annihilation would be visible in several experiments.
And for models with only two or three hidden dimensions, the energies required to produce black holes but not have an affect on the galaxy or the Universe are in the hundreds of TeV range that the LHC cannot produce.

So although it is interesting to see the LHC finally producing results and restrictions, this one is really no surprise to the astrophysics and the high energy physics community.