Universe Calder





MAGIC UNIVERSE

A Grand Tour of Modern Science

NIGEL CALDER

in the accounts, from your point of view. The extra light due to the Doppler effect must be supplied from the star’s energy of motion. But how can the star spare some of its energy of motion, without slowing down? Only by losing mass—which means that light energy possesses mass.

Then come the quick-fire masterstrokes of Einstein’s intuition. It can’t be just the extra light energy needed to account for the Doppler effect that has mass, but all of the light given off by the star. And the fact that the star can shed mass in the form of radiant energy implies that all of its mass is a kind of energy.

‘We are led to the more general conclusion that the mass of a body is a measure of its energy-content,’ Einstein wrote in 1905. He added, ‘It is not impossible that with bodies whose energy-content is variable to a high degree (e.g. with radium salts) the theory may be put successfully to the test.’