Having focused for many years on the giant black holes that form when stars collapse and the supermassive black holes at the centre of galaxies, physicists have more recently begun to study microscopic black holes, with tiny masses.

One reason to think about these objects is that they may have been formed during the Big Bang and may still permeate the universe today. The existence of so-called primordial black holes is one possible explanation for the universe’s missing mass.

Another reason physicists are interested in micro black holes is that some theorists predict that the Large Hadron Collider will produce them.

So the work of Gia Dvali and pals at the Ludwig-Maximilians-Universitat in Munich, Germany, will be of great interest. These guys say that if black holes form on this tiny quantum scale, then their masses must be quantised.

Their reasoning is simple. If black hole mass is not quantised, then the mass could take essentially any value. And if that were the case, the rate of production of micro black holes would be infinite: they could form in any collision, at any energy.

Since that’s clearly not the case, the masses of micro black holes must be quantised.

That immediately raises a number of important questions, not least of which is what governs black hole quantisation. Dvali and co reasonably argue that black holes must be quantised in units of the fundamental Planck length. But exactly how this would affect the way they spring in and out of existence isn’t clear.

Dvali and co suggest that micro black holes would first appear in their lowest quantum state at the LHC in the form of some kind of quantum resonance, what particle physicists call a hump in their data. This would initially be hard to distinguish from an ordinary particle, but higher energy experiments ought to reveal black holes in higher states too.

For the moment, there’s no way to work out at exactly what energy we should expect to see them. “To uncover the precise form of the quantization rule for lowest black hole resonances, we need more experimental input,” says Dvali and co. Quite!

Of course, the question of this kind of black hole production at the LHC once again raises the thorny question of whether the safety assurances we’ve been given about these experiments are valid.

We’ve looked at the arguments before. One important question is whether state-of-the-art theoretical physics is up to the task of making a trustworthy prediction that the LHC is safe.

Today’s paper makes clear that our understanding of micro black hole physics is rapidly changing. So it would be entirely reasonable to ask on what basis physicists are able to make safety assurances.

(Let’s put aside for a moment the question of whether particle physicists are in any position to make safety assessments in the first place, given that they have the most to gain from running these experiments.)

This is a debate that particle physicists are strangely reluctant to engage in, having ignored most of the questions marks over safety.

So this is a good opportunity to raise the issue again. Sit back and enjoy the fireworks (or puzzle over the deafening silence)!

Ref: arxiv.org/abs/1106.5894: Black Hole Masses Are Quantized