Some time ago, it was suggested that expeiments at the Large Hadron Collider could create extremely tiny black holes that would instantly decay. Researchers now suggest that if mini black holes are not just theoretical objects, they may gravitationally bind with matter, without significant absorption. Some might even already be present on Earth.

If they exist, mini black holes form in a completely different way than “usual” black holes. Astrophysical black holes are formed by the collapse of massive stars, under their own gravity: they have a mass of at least 1030kg. Mini black holes are thought to have formed during the Big Bang; they are also called primordial black holes. The mass of laboratory produced mini black holes is expected to be less than 10-23kg, whereas the mass of primordial mini black holes is assumed to be less than 106kg.

Searching for a way to test quantum evaporation (thermal radiation emitted by a black hole), which remains an open question, Aaron P. VanDevender from Halcyon Molecular in Redwood City, California, and J. Pace VanDevender from Sandia National Laboratories in Albuquerque, New Mexico, suggested that mini black holes with masses below 1012kg should have evaporated by now. In other words, if primordial black holes ever existed, they do not anymore.

On the other hand, if quantum evaporation does not exist, mini black holes would have a peculiar behavior. Stellar (and supermassive) black holes are so dense that any object crossing their event horizon cannot escape their gravity, not even light. In the absence of quantum evaporation, mini black holes would gravitationally bind matter, without absorbing it: matter orbits the black hole at a certain distance. The researchers name it the Gravitational Equivalent of an Atom (GEA). If these GEAs exist and can be detected, it would provide a way to test quantum evaporation.

In the process of quantum evaporation, mini black holes produce X-rays while losing mass, until they eventually disappear. Although there has been many attempts to observe these X-ray signatures, they have never been detected, suggesting that mini black holes were not created in large numbers as expected, or that they do not evaporate.

This is where comes the idea of the VanDevenders: instead of looking for this X-ray signature, scientists should look for evidence of the existence of mini black holes. If their model is correct, GEAs should produce emissions that could be detected with current detectors, even though the probability of detection would be rather small.

Could a GEA collapse and absorb the Earth? Rather not. Black holes with a mass of 1012kg have a Schwarzschild radius (the radius of the event horizon) equal to the ground state radius: this is the smallest distance at which matter particles orbit, thus GEAs cannot be heavier than this limit. The researchers compare the probability of a terrestrial GEA absorbing the Earth to that of an electron being captured by the nucleus of the atom it is orbiting: it is “vanishingly small”, they write in their paper. Particles orbiting the black hole at the center of a GEA are also unlikely to be absorbed. However, a few of them could fall into the black hole, and would then provide energy for observable emissions.

Mini black holes could also be interesting candidates for dark matter. The researchers calculated that if dark matter is primarily composed of mini black holes and evenly distributed throughout our galaxy, millions of kilograms of them should hit the Earth each and every year. They also determined that approximately 400 mini black holes per year could, in principle, be detectable through their strong electromagnetic emissions.

Because mini black holes would have a very high velocity, the researchers note that the search for electromagnetic signals from a GEA should be done in the space surrounding the Earth. Indeed, a fast-moving GEA would be quickly stripped of all mass as it passes through the Earth.

Finally, mini black holes that might be created at the LHC would be much too small to bind any matter and produce a detectable radiation. And there is no way any mini black hole created there could swallow our world: for a black hole with a mass of 1kg, which is high above what the LHC could produce, it would take 1033 years to swallow the Earth. That is more than 70 sextillion times (7 followed by 22 zeros!) the current age of the Universe. So, even though many experiments are still running at the French-Swiss border, you can sleep like a log.