15 Jun '10 20:03>
The equivalence principle is one of the more fascinating ideas in modern science. It asserts that gravitational mass and inertial mass are identical. Einstein put it like this: the gravitational force we experience on Earth is identical to the force we would experience were we sitting in a spaceship accelerating at 1g. Newton might have said that the m in F=ma is the same as the ms in F=Gm1m2/r^2.
This seems eminently sensible. And yet it is no more than an assertion. Sure, we can measure the equivalence with ever increasing accuracy but there is nothing to stop us thinking that at some point the relationship will break down. Indeed several modifications to relativity predict that it will.
One important question is what quantum mechanics has to say on the matter. But physicists have so far been unable to use quantum theory as a lever to tease apart the behaviour of inertial and gravitational mass.
All that changes today with the extraordinary work of Endre Kajari at the University of Ulm in Germany and a few buddies. They show how it is possible to create situations in the quantum world in which the effects of inertial and gravitational mass must be different. In fact, they show that these differences can be arbitrarily large.