# Proof That Einstein Was Right

02 April 2006

## Interview with

Dr Simon Rainville from Laval University, Canada

## NS_Logo_Pod_Teaser_120.png

Chris - E=mc^2 is probably the most important equation in the whole of science and it's one scientific equation that most people will have heard. But how do we know that Einstein was actually right? He was a theoretician and didn't do any experiments. For years people have been assuming that he was right, but now they've gone and tested it. Simon Rainville from the University of Laval in Canada along with his colleagues at MIT have actually done the experiments to prove that E really does equal mc^2.

Simon - One of the most famous equations in all of science is E=mc^2. We've all heard of it. This formula was derived by Einstein about 100 years ago. This formula says that mass and energy are equivalent. What we've done is directly tested this relationship. We independently measured mass 'm' and energy 'E', and 'c' which is the speed of light is constant in our system so we don't have to measure it. We compared these measurements of 'e' and 'm' and we actually found that Einstein is correct.

Chris - So in his grave he's breathing a sigh of relief that he was right. But getting down to the nuts and bolts of it, how did you actually do this? What was the experimental protocol that you followed?

Simon - The idea is that the nuclei of atoms are made of protons and neutrons and these are the building blocks of nuclei. If you shoot neutrons at atoms, sometimes the nucleus will absorb one of these neutrons and become a little bigger. When that happens, there is some energy that binds the neutron to its new nucleus in order to hold it together. Because of the relationship that mass is the same as energy, we know that energy has to come from somewhere. This is because energy is conserved and so it comes from the mass. In other words, the mass of the big nucleus is slightly smaller than the mass of the original nucleus plus the neutron when we conserve them separately.

Chris - How did you actually make those measurements? The kinds of tolerance in your experiments shows to the extent of 0.00004% that E=mc^2 is correct. But how did you actually do it?

Simon - For that we had to measure the energy to that level of precision. That was done by measuring the wavelength of the little gamma rays. When the nucleus relaxes from its excited state after absorbing a neutron, the energy is emitted in the form of gamma rays. These have been measured with a very precise spectrometer and that gives you 'E'. Independently out team at MIT measured the small difference in mass. The way we did that was by isolating a single atom or molecule of the two species or nuclei we were interested in. We put them in the heart of our apparatus, which is a big magnet. Believe it or not, this apparatus allowed us to hold on to these single molecules for weeks and then measure their motion in the trap very precisely, which is proportional to their mass. In fact, the measurements that we're presenting in this paper are the world's most precise mass measurements. That's equivalent to measuring the distance between Boston and Los Angeles with an error less than the width of a human hair.