Despite having a physics degree and some notion of the stretchiness of space and time in Einstein’s theory of special relativity, I’ve never felt comfortable with these ideas. In particular, I have never really had a good feel for why – or how — energy and mass might be interchangeable, as dictated by his most famous equation: E = mc2.

Apparently, this is something that I had in common with Brian Cox’s wife, Gia Milinovich.

Which is fortunate because when she asked him “Why does E = mc2?”, Cox’s attempt to answer his wife’s query ultimately resulted in a quite wonderful book, co-written with his University of Manchester colleague, Jeff Forshaw.

The book is an unusual piece of popular science in that it is almost entirely conceptual, going straight to the core of the theories with only the briefest of diversions into pen portraits of some of the figures mentioned along the way. The outlines of Einstein’s own life are not even sketched, such is the authors’ devotion to the task of exposing his space, time and mind-bending theory.

Why does E equal mc squared?

Look at the creases – I really did read it.


Why does E=mc2? is therefore a courageous and ambitious book and one that, for me, succeeded brilliantly. Cox and Forshaw are solicitous and engaging guides through the observations and logic that drove Einstein to his far-reaching conclusions. They are apologetic at times about some of the mathematics encountered along the way but at the same time agreeably enthusiastic about the need to do to in order to appreciate one of the true wonders of twentieth century science.

The book is not long but plots a steady, clear route through the various conundrums that led Einstein forward. It starts with the necessary abandonment of the notion of absolute rest, continues through Maxwell’s uncovering the speed of light, c, as a universal constant and, by leaning primarily on the notion of invariance (the idea that all observers should agree in their measurements of the same objects, events or phenomena), takes us into the inescapable heart of Einstein’s four-dimensional space-time.

It’s a breath-taking, epiphanic journey. In the first stage alone our everyday experience of the world is revealed as a mere shadow of nature’s interweaving of space and time. And we are discovered to all move within space-time at the speed of light. Even when we are sitting down.

But there is more. Beginning from the simple notion that no-one has yet observed a infraction of the law of conservation of momentum (the masses of objects multiplied by their velocities are maintained in all collisions or interactions) and by shifting that law into 4D space-time, the authors inexorably track Einstein’s E = mc2 equation to its source to deliver another instant of sweet revelation.

Mass and energy are found to be equivalent. And not just in the exotic way that sub-atomic particles and energy are interconverted in particle accelerators. It’s happening every day – step into your car and accelerate away from the kerb and your mass inexorably, though almost imperceptibly, increases. The world is not what is seems to be. It is far more _interesting_.

The latter part of the book is perhaps the most challenging. Cox and Forshaw don’t hesitate to throw down the ‘central’ equation that describes all the known interactions between matter and the forces that bind it together (except gravity).  Despite its formidable complexity, they walk the reader with a sure and steady step through its meaning and beauty. You don’t get a complete explanation – the details are simply beyond the reach of a popular science book (that’s why they call it ‘physics’, to paraphrase Mamet) — but there is enough meat for a satisfying feed.

Cox talks science

Brian Cox talks to my daughter and my nose (Photo by Richard Grant)


What I loved about this book was that it re-ignited my love for physics. I found that I ‘got’ relativity for the first time. I had mastered the equations during my degree but not properly absorbed their weight and significance. Perhaps there _is_ something to the old adage that education is wasted on the young? At the Geek Calendar launch last week Alok Jha, another physics graduate (who has reviewed the book for The Guardian), told me he’d had felt a similar sense of discovery. And Brian Cox himself confessed to me the same revelation in the process of writing the book, as the morass of maths yielded up its load of meaning.

This is why questions from the uninitiated are so important for scientists.

What is more, the book made physics seem important again. My degree had left me with the impression that the 20th Century has been one of spectacular progress leaving today’s physicists mostly to fiddle at the edges of our most fundamental theories of how the universe was put together. Perhaps that partly explains my shift into the life sciences, but physics now seems to be back on centre stage. I find myself eagerly awaiting the results from the Large Hadron Collider which is set to deliver the next seismic shift in our universe of understanding.

It is a mark of the success of the book that I am hungry for more. There were glimpses of complexity beyond the reach of the present narrative and I have questions for the authors. What exactly are the rules governing the interconversions between the particles produced in the high-energy collisions within particle accelerators? And how does the Higgs mechanism – which is treated in more detail in Ian Sample’s book, Massive – fit with the idea of the equivalence of mass and energy? There wasn’t the space-time to delve into this sufficiently but perhaps Cox and Forshaw can be persuaded to write a sequel. Maybe once the LHC has pronounced on the matter?

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5 Responses to Ask

  1. Ken Doyle says:

     Added to my Goodreads "to read" list…

  2. Cath Ennis says:

     Sounds good!
    I’ve read David Bodanis’ "E=mc2 - a biography of the world’s most famous equation", and enjoyed it greatly, despite having dropped physics as soon as I possibly could! I may look up this one too though, as it sounds like a very different treatment of the subject matter. The book I read included some of the subjects you mentioned, but in less depth. However, it also went into a history of the US nuclear weapons programme during WWII, and discussed how the equation fits in with what we know of the formation of matter after the Big Bang. Very interesting stuff!

  3. Stephen Curry says:

    Thanks Ken and Cath – hope you might both enjoy the book.
    And thanks Cath for pointing out the Bodanis book – looks very interesting, If you want to know more about the Manhattan project (and the key developments in 20th Century physics), you might look at Richard Rhodes’ ‘The Making of the Atomic Bomb’ – a Pulitzer prize winner (and it’s not hard to see why).

  4. As I can see from description of the book, it is not free from scientific myths. Among other things:
    - Space-time doesn’t exist. It’s just space. Time is an attribute of matter, sponaneously doesn’t appear and is not feasible to marry space.
    - Time cannot be regarded as 4th dimension, as all the three dimensions are time ones, as you need time to move in any of them. It’s the fifth wheel to the cart.
    I agree energy must have it’s mass, because energy is the most primitive and primordial form of matter and could be named as pre-matter in other words.

  5. Stephen Curry says:

     Utterly baffling I’m afraid, Andrzej.  Unlike Cox and Forshaw’s book.