Posted on: 3rd September 2005

“The laws by which the states of physical systems alter are independent of the alternative, to which of two systems of coordinates, in uniform motion of parallel translation relatively to each other, these alterations of state are referred (principle of relativity). With these principles (footnote: The principle of the constancy of the velocity of light is of course contained in Maxwell’s equations) as my basis I deduced inter alia the following result:” (…)”If a body gives off the energy L in the form of radiation, its mass diminishes by L/V2. The fact that the energy withdrawn from the body becomes energy of radiation evidently makes no difference, so that we are led to the more general conclusion that: The mass of a body is a measure of its energy-content; if the energy changes by L, the mass changes in the same sense by L/9 x 1020, the energy being measured in ergs, and the mass in grammes. 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 successfully put to the test. If the theory corresponds to the facts, radiation conveys inertia between the emitting and absorbing bodies. From the paper “Does the Inertia of a Body Depend on its Energy Content?” by Albert Einstein, published (in German) in Annalen der Physik 18 (1905) page 639, article submitted on 27th September 1905.

Albert Einstein

Albert Einstein was only 26 when he published the brief, 3-page article that announced the equivalence between mass and energy, known today as E=mc2 (see e.g. Wikipedia, or listen to experts). This article appeared as the last in the series of Einstein’s four 1905 breakthrough papers. The current World Year of Physics actually celebrates the 100th Anniversary of Einstein’s “Annus Mirabilis”.

Notice that in the original paper, Einstein uses V instead of c for the speed of light, and L instead of E for energy. Today’s world famous formula is simply explained in words. Anyway, the message is fascinating: as the speed of light is constant, the energy inherent to a body is proportional to its mass, with a huge constant of proportionality (c2 = 90 billion joules in each kilogram of mass). Remarkably, Einstein proposed an experiment to test his daring theory – and this is where good science can be instantly recognised. Credit must also to be given to the journal Annalen der Physik, for the courage to publish all the four revolutionary articles.

The discovery opened vast new horizons for physics, although it took quite a few years before physicists fully recognised the consequences. As a striking example, Arthur Eddington – a forefront supporter of Einstein’s theories – realised in 1920 that the mass difference between four hydrogen atoms and one helium atom would provide enough energy to power our Sun, thus solving one of the major physics puzzles of that time (see August’s Article of the Month).

Einstein’s most intriguing masterpiece, his General Theory of Relativity that explained equivalence between weight and mass (inertia), was published in 1915. In 1921, Einstein was awarded the Nobel Prize for his 1905 explanation of photoelectric effect – but
gave his Nobel Lecture a year later on a different subject, on his Theory of Relativity. The genius of Einstein was not just in his ability to derive formulas – some of the relativity equations were known even before Einstein – but mainly in his capability to correctly interpret the meaning of the results.

## Picture of the Month

When fuels are burned, rest mass is always lost but the loss is generally barely discernible. However, in a fusion reaction the difference in mass between the fuel (deuterium and tritium, left side of scales in the image) and the products (helium and neutron, right side of scales) is clearly evident as it is almost 3%. Given the huge c2 multiplier, very little fuel is therefore needed to produce a lot of energy. The unique efficiency of fusion power is one of the key motivations for our research at JET.