# Why Does E=mc2?

PUBLISHED : Sunday, 20 December, 2009, 12:00am
UPDATED : Sunday, 20 December, 2009, 12:00am

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Why Does E=mc2?
by Brian Cox and Jeff Forshaw
Da Capo, HK\$194

Mariah Carey named her last CD after Albert Einstein's equation, and if she is aware of it then most people probably are. E=mc2 is the most famous equation in the world - but what does it mean?

This book, written by two young British particle physicists, promises to explain the equation to non-scientists with a minimum of mathematics. Unfortunately, the nature of Einstein's work means the maths is still too complex for the general reader. But it is an enthusiastic attempt to popularise science, and physics undergraduates will find the detailed arguments very useful.

The equation stands for 'energy is equal to mass multiplied by the speed of light squared'. Einstein worked out that an object grows heavier the faster it moves. From this he concluded that motion energy is being converted into mass. Mass is a measure of how much 'stuff' an object contains. Einstein discovered that energy and mass are interchangeable by using the speed of light squared as the 'exchange rate'. This equation underpins the theory of relativity, which is the basis for our understanding of the workings of the universe.

'The process of converting mass into energy and energy into mass ... is absolutely fundamental to the workings of nature; it really is an everyday occurrence,' write Cox and Forshaw. 'For anything to happen in the universe, energy and mass must be continually sloshing back and forth.'

Readers of the above paragraph may be wondering why it is the speed of light that acts as the exchange rate between energy and mass. That discovery is at the heart of Einstein's special theory of relativity, and that is what the book tries to explain. The authors take a digressive route through three main concepts: invariance (the idea that the laws of nature should be universal), causality and distance. The tool they use is mathematics, which is the best fit for the task: 'Perhaps we will never understand the true nature of the relationships between mathematics and nature, but history has shown that mathematics allows us to organise our thinking in a way that proves to be a reliable guide to a deeper understanding,' they write.

The book is ambitious - perhaps a little too ambitious. The writers not only seek to explain the reasoning that led to the equation in an accessible manner, they try to do it using contemporary scientific ideas.

The special theory of relativity has been around since 1905 and the general theory of relativity since 1915, and there are established ways of teaching them. Most textbooks begin with the discovery that light always travels at the same speed in a vacuum, then explain the strange things that happen when objects travel close to the speed of light. Cox and Forshaw do all this, but in a roundabout way that focuses on geometry. Relativity neophytes will probably become lost in all the numbers and equations.

This book may not succeed in explaining E=mc2 for the layperson, but the authors' enthusiasm for their subject is still highly infectious. That can only be a good thing.