Is the Large Hadron Collider safe?

The Large Hadron Collider, usually referred to by scientists as the LHC, had its first preliminary test today.  All went well.  But what does the LHC do, and is it safe?

What is the LHC?

The LHC is a "particle collider".  It has two main parts: beams and detectors.  Two beams of protons will be channeled at near the speed of light around a tunnel 27 km in circumference, one clockwise, one counterclockwise, by a pair of rings made of 9000 superconducting magnets.  The beams will cross in several places, allowing the particles within them to collide (hence the term "collider").  The by-products of those collisions will be observed by two enormous detectors (as well as two somewhat smaller ones).  It short, it collides beams of particles and detects what happens.

What is the LHC for?

Physicists have learned a lot about the fundamental constituents of matter by bashing particles together.  The higher the energy scale of the collisions, the deeper, in a sense, one can probe.  We now understand what particles make up all the matter we can see, and what particles are responsible for forces.  For example, as I said in a previous post, we understand about electrons and their siblings (yes, I know I haven't gotten around to doing the followup posts yet), and we understand that the electromagnetic force comes from the particle of light, the photon.  In fact, we have understood how these particles and forces behave in terms of some rather beautiful symmetries.  A symmetry is an invariance, as in "looks the same in a mirror", or "runs the same if you switch all the red and black cables for one another".   A key point is that a symmetry can be broken.  For example, you don't look the same in a mirror.  Even if you part your hair down the middle, there is always some freckle to give away that it is a mirror image.

Our theory of particle physics using symmetries works great, except for understanding why most of the particles have mass.  If the symmetries of the theory were not broken,  these particles would have to be massless.  We need to understand how the symmetries are broken—we have to find the freckles.   The main freckle is called the Higgs boson (please, can we stop using the awful term "God particle"!).  It has never been seen.  We think that is how the electron gets its mass, but we don't know for sure.  And we don't understand how the Higgs boson might fit into a more complete theory.

The LHC is designed to find the Higgs boson, and we hope it will point us to a more complete theory of matter and energy.  It may also shed light on the dark matter, but that is a post for another day.

Is the LHC Safe?

Sometimes the LHC is described as "recreating the Big Bang".  This sort of language is colorful, and conveys the grand nature of the endeavor, but it also makes it sound scary, and, more to the point, is completely inaccurate.  The LHC will probe a new frontier for humans, but the kinds of collisions that will take place in it happen in and around the Earth all the time.  Cosmic ray protons hit protons in the atmosphere and create sprays of particles just like in the LHC.  If you were to wait in one location, it would be quite rare that you would see a collision at the same energies as the LHC, but across the whole atmosphere they happen all the time.  If these collisions were dangerous, they would have done their damage long ago.  

One worry that has been stated in the press is that the LHC might produce mini black holes.  Well, that is a possibility if there are extra dimensions of space that become visible just at the LHC energy scale, but that is unlikely (not quite as crazy as it sounds though).  But such mini black holes would not be like the monsters you may have seen in Sci Fi.  They would be tiny (way smaller than protons) and would decay in a fraction of a second.

Could these mini black holes be stable?  First of all, even if they were, a mini black hole would take hundreds of millions of years to grow appreciably in size in the Earth, so it could not be the doomsday machine some have feared.  But everything we know about the theory says that such mini black holes must decay very rapidly due to quantum processes.   Mini black holes are essentially  just another kind of particle that decays. 

If all of that is not enough to convince you that the LHC is safe, here is a final comfort:  we have seen pulsars.  Comforting eh?  You see, pulsars are like canaries in the coal mine.  They are spinning neutron stars.  Neutron stars are dense cinders of dying stars that just barely avoided collapsing on themselves into black holes (the large kind).  They would feel the effects of a mini black hole much much faster than the Earth would.  They too are bombarded by cosmic rays all the time.  They recreate the LHC experiments each second.  If particle collisions created mini black holes that somehow were stable, all neutron stars would quickly be triggered into collapsing.  We see pulsars, so that can't have happened.

So the LHC is not a threat.  It is just a tool to look for freckles.

An engineer leans on a magnet in the 27km-long tunnel that houses the Large Hadron Collider (BBC News; Image: Cern/Maximilien Brice)