Relativity in brief... or in detail..

E = mc2: is it true?

How precisely does E = mc2? We saw in the introductory film clip and also in What do those energy equations mean, and where did they come from? the reasons why the mass-energy equivalence is difficult to measure. In terms of normal quantites of energy and mass, if one converts even a modest amount of mass, one obtains a huge energy. Alternatively, a reasonable amount of energy corresponds to the conversion of minuscule mass. Difficul: the presence of large quantities of energy (objects travelling at very high speed or hard radiation) makes it difficult to measure a tiny change in mass. In E = mc2 and binding energies in the nucleus (and in molecules.), we saw that moderately precise measurement and calculation is necessary even to notice E = mc2 when doing the accounting for nuclear reactions, and that it is virtually impossible to measure in chemistry.

In But is it true? Is the speed of light really independent of the motion of the observer?, we saw that the uniformity of the speed of light in different frames of reference had been tested to a precision of 6 parts in 1016. Measurement of such precision seems impossible for E = mc2, but just how precisely can this famous equation be measured?

The word "difficult" is a challenge to physicists. I am writing this text on the summer solstice (in Australia - the winter solstice for Northerners) and this morning the journal Nature carries a short report entitled A direct test of E = mc2, from a Canadian, American, English and French team, whose lead author is Simon Rainville. Their various measurements give a precision of better than one part in a million. How fitting that in the last month of 2005, the International Year of Physics, the year that celebrates the hudredth anniversary of relativity, the precision of this equation has been improved by a factor of 55. Congratulations to M. Rainville and his team. Oh, and congratulations to Einstein.

Home | Summary | Quiz | Credits
School of Physics - The University of New South Wales
Sydney, Australia 2052 © School of Physics UNSW 2052
Disclaimer
feedback