Will physics destroy the world?

Apparently a couple of guys are suing to stop the Large Hadron Collider, the new particle accelerator being built at CERN.  They’re worried about the possibility that the collisions will produce something like miniature black holes or other exotic objects that would then destroy the Earth.

This sort of worry has come up a bunch of times before.  Sometimes the worry is about the possibility that the state of matter that we know and love is only a metastable state, not the most stable state.  The idea then would be that, if you produce a single nugget of the true stable state, everything else would collapse into that new state.  It’d be like having a supersaturated sugar solution: as soon as you give it a nucleation point, everything crystallizes out.  Think Vonnegut’s ice-nine.

So should we be worried about the LHC destroying the world?  The short answer is no.  This sort of thing is logically possible, so it’s certainly worth considering the possibility, given the enormous downside of destroying the world.  But people have considered it very carefully and have shown quite convincingly that there is no risk.  There’s a short overview here, with links to the technical reports.

There’s one argument that dispenses with a lot of the various doomsday scenarios.  The sorts of collisions that will happen in the LHC happen regularly in the Earth’s upper atmosphere, as ultra-high-energy cosmic rays strike the Earth.  You can work out that, over the Earth’s 5-billion-year history, the number of times these events have occurred naturally is many times larger than the number of times they will occur at the LHC.   So the fact that the Earth is still around is very strong evidence that this sort of catastrophic scenario is impossible.

As the NY Times article points out, there’s a loophole in this argument.  The collisions in the upper atmosphere are fast-moving particles colliding with particles that are essentially at rest.  Because of conservation of momentum, anything produced in such a collision would be moving at close to the speed of light, so it wouldn’t stick around long enough to do any damage.  In contrast, the collisions in the collider will be of particles moving in opposite directions with essentially equal speeds, so the resulting detritus will be produced nearly at rest.  There is a big difference between a micro-black hole whizzing through the Earth at nearly the speed of light, which would have essentially no effect, and one that’s moving slow enough to stick around.   To see why you still shouldn’t worry, you have to read the technical reports.