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**Chapter 5: Why Does E=mc ^{2}?**

At the end of day two, I’m exhausted. I recall something I read in another science book, one about neuroscience: Apparently, the prefrontal cortex is very ineffective when processing a completely new concept. And all the brainpower I’m using to understand relativity is exhausting my body’s supply of brain-nourishing glucose, which explains why I keep getting so tired even though I’m not bored.

I get a jolt of energy when I see the title of this chapter, and sense the finish line is near. I already assume why “c” is the speed of light, as it is seemingly the only invariant quantity left after the dismantling of 20th century physics, which would be useful in this equation. But I’m very curious now how “m,” mass, came into the picture. The authors attempt to enlighten me: Einstein knew that momentum and the ability to measure kinetic energy had to be adjusted according to the new principles of spacetime.

What I manage to understand is that Einstein built an updated version of momentum using the remaining laws of physics that everyone still agreed upon—distance within spacetime, a universal speed limit, and mass. The resulting equation is E=mc^{2} (with “e” representing energy, “m” representing momentum, and “c” equal to the speed of light). It’s now clear to me why the authors pointed out early on that equations often reveal unintended deep connections: Einstein’s equation also revealed an unexpected relationship between mass at rest and energy.

The authors note, “You may well be juggling a lot of mental balls as you read this sentence.” I’ve lost a few of those balls, but I am still feeling like I’ve glimpsed what it is like to think like a physicist.

**Chapter 6: And Why Should We Care?**

One of the chief reasons that Einstein’s E=mc^{2} is so famous is simple timing—the turn of the 20th century saw the spread of the coal-fueled steam engine and the height of the industrial revolution. Harnessing energy was crucial. Einstein’s equation clearly showed that vast, practically limitless amounts of energy were everywhere, locked inside mass, and that atoms could produce energy far more efficiently than fossil fuels. This realization, of course, led to the development of nuclear energy, as well as to nuclear weapons.

But I end up caring about E=mc^{2} mostly because it reveals the lives of stars, without which life as we know it could not even exist. Using this and other equations from relativity, Cox and Forshaw are able to trace the history of the universe. In the “cosmic dark ages,” the universe was filled with hydrogen, helium, and a sprinkling of other lighter chemical elements. Gravity forced the gases to fall into each other, picking up speed (and heat) as they rushed inward to a central locus. At 10 million&#deg;, nuclear fusion happens inside of stars, giving them power and creating the precursor atomic elements that form all the structures in the universe. Everything on this planet is made from stardust, and the very fact that the Sun shines is a direct consequence of the mass-energy relationship Einstein clarified with E=mc^{2}. Thanks for that, Albert, and thanks to Cox and Forshaw for helping me grasp these concepts.

**Chapter 7: The Origin of Mass**

It’s day three, and I’m starting to think like the high school physics geek I never was, asking myself surprising questions. At the gym I wonder what happens to transform my food into energy for motion on the treadmill? Thanks to the number-crunching authors, I now know the human body can turn a kilogram of mass into one watt of power, whereas the Sun can only produce 1/5,0000th of a watt of power from one kilogram. The Sun is not very efficient. Our bodies are.

What we haven’t covered yet is what gives objects mass (an important component of Einstein’s equation). Unfortunately, the answer isn’t entirely clear. The authors introduce the “master equation” of the Standard Model of particle physics and delve deeply into the interaction of elementary particles. At first blush, it’s an extravagantly large equation, until you realize what it explains—the rules of interaction for every kind of particle in the entire universe. But the Standard Model isn’t exactly complete. This chapter discusses one of its most substantial missing pieces, the Higgs mechanism, the elusive entity that physicists believe gives particles their mass. There is no experimental evidence of a Higgs particle, and it is the great hope of physicists that the LHC will finally confirm the existence of the Higgs—or else the the Standard Model will have to be scrapped.

**Chapter 8: Warping Spacetime**

Thinking on the subatomic scale for almost 40 pages makes me glad to be back to grappling with things like space and time, and knowing the book is coming to an end makes me feel like I can handle anything this chapter throws at me. A decade after Einstein’s theory of special relativity, he devised his theory of general relativity, which is a geometric description of gravity that combines special relativity with Newton’s observation that all objects fall to the ground at the same rate. General relativity states that gravity is a byproduct of curved spacetime, and the book provides a clever analogy: Think of general relativity as two friends starting out at the equator hundreds of miles apart, both walking towards the North Pole. They walk in a straight line, yet because they’re walking on curved space, their paths get closer together until they meet at the North Pole. In a similar fashion, we see the Earth tracing an ellipse as it orbits the Sun, but in actuality it’s traveling in a straight line through curved spacetime.

**Epilogue**

I’ve come to the end, it’s Day 4, and I will miss my daily dose of “a-ha” moments. I grasp that the seemingly separate laws of mass, energy, time, and space are all tied together. I don’t fully understand the strange world of particle physics, but that’s okay. I am not alone: Modern physicists can’t reconcile gravity with the Standard Model after all. Finding answers to these great questions remains one of the most active areas of physics research today. As the authors write in the final pages, “Taking delight in observing and considering the smallest and seemingly most insignificant details of nature has led time and again to the most majestic of conclusions.” It just takes small steps, meaning I’ll probably need to reread this book in a few months at a much slower pace.

Originally published August 25, 2009

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