After an interview with Premier magazine, the Hollywood actress Cameron Diaz was asked if there was anything she wanted to know. 'Yeah,' said Diaz, 'what does E=mc² really mean?' The journalist laughed, but Diaz reiterated that she really did want to know the meaning behind the most famous equation in physics.
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This exchange inspired science writer David Bodanis to write a short, lively book aimed at anybody who has ever wondered about Einstein's equation. It is neither a biography of Einstein, although we do learn something about him along the way, nor is it an explanation of all his work, although we do encounter a little general relativity at the end of the book. Instead, it is a highly focused introduction to the derivation and implications of E=mc².
Bodanis begins by explaining each element of the equation. For example, E represents energy, and by 1900 physicists concluded that energy is conserved. There are many types of energy, but they are all basically the same, so if you destroy some chemical energy then you must create an equal amount of energy in some other form.
As a student, Einstein had learnt that energy and mass are both conserved, one of the great axioms of physics. However, his research into the properties of light forced him into the realisation that it is the combination of mass and energy that is conserved, and that mass can be destroyed as long it is turned into energy, and vice versa. This is the essence of E=mc².The same goes for mass (m). In the eighteenth century, it was observed that burning wood in oxygen resulted in ash, smoke and carbon dioxide, but these had the same combined mass as the initial materials. Mass is conserved.
The c² is the conversion factor, effectively the exchange rate that turns one currency into another. Crucially, c is the speed of light, 1,000,000,000 kmh, so c² (c x c) is truly huge. Therefore, a tiny mass can turn into a literally massive amount of energy.
Typically, mass is happy being mass, so sheets of paper do not spontaneously explode with enough power to wipe out a city. But Einstein's equation is a reality, as demonstrated by nuclear bombs, nuclear power and nuclear sunshine.
This book is packed with the standard anecdotes relating to the equation, many of which will already be familiar to scientists. But this is not a criticism. The book is intended for a different audience, namely Cameron Diaz and anybody else with curiosity but without a degree in physics.
Finally, I rarely mention covers in reviews, but in this case it is exceptionally striking: a fluorescent Einstein sticking his tongue out. A similar image was used to promote the recent science and arts festival, Creating Sparks, and was criticised by some who thought it trivialised Einstein. I am of the opinion that the image is enticing and engaging, and I hope it will attract rather than repel potential readers.
