Why the hell are we making micro black holes?

[Q Classic]

ecth, i've read cosm. not too bad, and yes, this did make me sorta think of the book.

[Rogan Josh]

OK - I feel kind of obliged to answer this, since I just had lunch with someone who has just written a paper on this very topic.

First of all, in order to create a black hole of any kind you need to have energies of the order of the Planck mass. In most theories this is something like 100000000000000000000 times the energy of current collider technology. This mass is set by the scale of gravity - it is so large because gravity is so weak.

However, there are theories around which claim that there are extra space dimensions. In fact all(?) string theories need extra dimensions, but usually they are wound up really tightly (ie. if you travel in that extra dimension for just a tiny amount you come back to where you started - a bit like being on the surface of a really really small cylinder) so we don't notice them. But it has not been experimentally ruled out that the extra dimension(s) might be quite big ater all.

If they were, then gravity would be weakened by being allowed to exist in the extra dimension and the Planck scale would be much lower. Then black holes could be made with the energy of soon to be built colliders.

So it seems pretty unlikely that they will be there.

Now, even if they could be produced, the theory tells us that they will evapourate away pretty quickly via Hawking radiation. For example, an electron-positron pair is created just next to one - the electron wanders off ond the positron falls into the hole. Since a positron is anti-matter, this 'empties' the black hole. It evapourates away. (All black holes give off Hawking radiation - whether or not they evapourate completely is down to their mass.)

To check this, CERN comissioned a study on this very subject and it was confirmed that there was no danger. In fact, the problem is more how to see them in the very messy LHC environment (they look like lots of other things).

I don't know why journalists don't research these things a bit better. All they need to do it write an email to a physicist working on the subject and they will get as much help as they want. Physicists are always happy to publicise their work (it's good for funding).