One of the most striking predictions of Einstein's special theory of relativity is also perhaps the best known formula in all of science: E=mc². If this equation were found to be even slightly incorrect, the impact would be enormous — given the degree to which special relativity is woven into the theoretical fabric of modern physics and into everyday applications such as global positioning systems. Here we test this mass–energy relationship directly by combining very accurate measurements of atomic-mass difference, m, and of -ray wavelengths to determine E, the nuclear binding energy, for isotopes of silicon and sulphur. Einstein's relationship is separately confirmed in two tests, which yield a combined result of 1-mc²/E=(-1.44.4)10^-7, indicating that it holds to a level of at least 0.00004%. To our knowledge, this is the most precise direct test of the famous equation yet described.

1. Research Laboratory of Electronics, MIT–Harvard Center for Ultracold Atoms, and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2. Present address: Département de Physique, Université Laval, Québec G1K 7P4, Canada
3. Department of Physics, Florida State University, Tallahassee, Florida 32306-4350, USA
4. The Physical and Theoretical Chemistry Laboratory, Department of Chemistry, South Parks Road, Oxford OX1 3QZ, UK
5. National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
6. Institut Laue-Langevin, 38042 Grenoble Cedex, France

Correspondence to: Simon Rainville^1,2 Email: rainville@alum.mit.edu

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