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Ruth’s Reviews: Why does E=mc2? (and why should we care) – Brian Cox and Jeff Forshaw

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Ruth Francis, Nature’s Head of Press, is reviewing all the entries shortlisted for the Royal Society’s science book prize. She’ll be reading one per week and posting her thoughts on The Great Beyond every Friday between now and the prize ceremony on 21 October.

Mark Haddon’s The Curious Incident of the Dog in the Night Time is a work of fiction that features an awful lot of complicated mathematics. Understanding the maths isn’t crucial in enjoying the book, fortunately, and it was a runaway hit back in 2003.

Brian Cox and Jeff Forshaw pull off a similar trick in their explanation of Einstein’s famous equation, tackling maths that could intimidate some readers. The authors are gentle from the off, and reassure that following the sums isn’t crucial to following the book, but do urge the reader to give it a try.

Why does E=mc2? takes the reader on a journey from space and time, via spacetime to the warping of spacetime – black holes. At each step maths underpins the theory, and each step becomes more complicated. Many readers will begin to skim the maths as they go, as I did in the later chapters, but will experience some of its beauty.

The penultimate chapter about the Standard Model of Particle Physics may not bring the so called ‘Ionian enchantment’ described by mathematicians upon realising that the world is orderly and can be explained by a small number of laws but It does break down the equation for the lay reader and this brings a certain satisfaction.

Equations and symbols aside the authors’ explanations throughout are clear, accessible and enjoyable. Although I gave up on the maths somewhere along the way I did learn a lot, and enjoyed the tone of the authors. In places they could have been more brief, and acknowledged our 21st Century desire for speedy gratification.

It may not achieve the popularity of Haddon’s novel, but Why does E=mc2? will be well received by non-specialist readers. Although it can be tough going it is an enjoyable and educational read.

Previously on Ruth’s Reviews:

Ruth’s Reviews: Life Ascending

Ruth’s Reviews: We need to talk about Kelvin

Ruth’s Reviews: Everyday Practice of Science


  1. Report this comment

    Steve K said:


    I’m desperate for someone to explain / clarify further several aspects of this book – I’ve tried comments/forums etc but no response. I love this book, but am unable to grasp the whole of it until the final pieces of the jigsaw are put in place. In particular,

    On p.77 of ‘Why does E=mc²’, I fail to grasp why the minus-sign version of the Pythagorean distance equation is considered as a possible solution to calculating the distance in spacetime (the book mentions “hyperbolic”, but not the reason for the positive-sign turning to a negative-sign when changing from three dimensions to four.

    Also, ref momentum vector in time direction, p.131-133, top p.133, “Don’t be confused by the fact that we multiplied by c…….included term ½mv² rather than ½mv²/c²”. If we take gamma = 1 + ½(v²/c²) from top p.132 and put into gamma x mc from bottom of p.131, we would get conserved energy E = mc + ½mv²/c, the second bit being divided by c (not c²) – please explain why it’s ½mv²/c². So if mass is stationary, E would equal mc (not mc²), which would mean a factor of c less energy produced !? A monumental difference from simply choosing to not multiply by c, ref bottom p.131, which is apparently done pure and simply to give the kinetic energy term ½mv²) Please help with my confusion !

    Is there a forum for Q & A on this book somewhere on the internet or anywhere ?

    Can the authors themselves be contacted (unlikely, I realise) ?

    Please forward this on to anyone who might be able to help.

    Many many thanks.

    Steve Kirby

    07871 154461

  2. Report this comment

    Bob Pierce said:

    Steve Kirby is not alone! I read this book twice last year and I’m reading it again at the moment. As previously, I’ve found p.77 such a stumbling block that this time around I searched for forums which might have an explanation.

    The authors go to great lengths earlier in the book (perhaps unnecessarily given the likely readership) to explain Pythagoras’ theorem. Yet at this crucial point they briefly assert that the derivation of the hypotenuse in space time can only be given by the standard Pythagorian method or by a similar equation but with a minus sign.

    They’re correct, I’m sure, but it’s a big disappointment in an otherwise brilliant book.

  3. Report this comment

    Simon Martin said:

    Not sure about the pythagoras problem, but i’ve been thinking for weeks about Steve’s other problem regarding the unjustified multiplication by c on p.131 and the apparent calculation error on page 133 where he says that the formula would contain 1/2mv^2/C^2 if the multiplication by c hadn’t been used. As far as i can work out, if we stick with ymc, we reach mc+1/2mcv^2/c. Is this a typo, or am i missing something? Even more worrying is the unexplained multiplication by c, which i admit neatly makes the KE formula appear, but there seems to be no mathematical reason to do so. Anyone got any ideas? 🙂

  4. Report this comment

    Mike Mackay said:

    Im here to say I have the exact same problem with page 77, reading it today, and, a year on from Steve Kirbys original observation on this, I havent seen anything online to explain why the authors think that you can swap out elements of pythagorus and still consider that its a valid equation to use. Or why you can explain pythagoras at length in one part of the book and select not to explain inverting that explanation in a subsequent chapter. Surely if this is possible then its also possible to simply decide that distance is negative vt. Or even that distance is v/t. Or you could happily simply select to also put a minus sign behind the other elements in pythagorus since it appears from p77 that its perfectly acceptable to edit fundamental aspects to accepted equations and still consider them equally valid. Confused …

  5. Report this comment

    Paul Johnson said:

    Fellow Readers, I am stuck in the same place, although since I am reading Why Does E = mc2 on Kindle, it it is on page 76. I understand why it is reasonable to try Pythagorus to calculate the hypotenuse in spacetime and I follow the elegant reasoning as why it does not work. I seems perfectly reasonable to me to try Pythagorus with a negative operand but I do not understand why all other functions of ct and x are excluded. Why not x2 – (ct)2 or ct/x or any of the “infinite number of ways that we might imagine calculating distance.” (pg 75) Has anyone been able to make any headway in understanding this important point? If I cannot get beyond this, I think it will be pointless for me to continue with this otherwise excellent book.

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