The day is finally here! My new book, The Universe in the Rearview Mirror, is being released today! Many of the topics in the book were prompted by all of the great questions from the io9 community, and I wanted to thank you again by offering you this free excerpt.

Also, starting right now (2pm EDT, July 11), I'm doing an "Ask Me Anything" on Reddit. Come over and participate!



Top Image Credit: Herb Thornby

Introduction: In which I set everything up, so it's probably best not to skip ahead

Why is there something rather than nothing?


Why is the future different from the past?

Why are these questions a serious person should even ask?


There is a gleeful skepticism of the orthodox in popular discussion of science. Reading some of the twittering, blogging chatter out there, you might suppose that relativity is nothing more than the ramblings of some dude at a party instead of one of the most successful physical theories ever, and one that has passed every observational and experimental test thrown at it for a century.

To the uninitiated, physics can seem littered with a ridiculous number of rules and equations. Does it have to be so complicated?


Physicists themselves sometimes bask in the aloof complexity of it all. A century ago when asked if it was true that only three people in the world understood Einstein’s Theory of General Relativity, Sir Arthur Eddington thought for a few moments and casually replied, “I’m trying to think who the third person is.” These days, relativity is considered part of the standard physicist toolkit, the sort of thing taught every day to students barely out of short pants. So let’s put aside the highfalutin idea that you have to be a genius to understand the mysteries of the universe.

The deep insights into our world have almost never been the result of simply coming up with a new equation, whether you are Eddington or Einstein. Instead, breakthroughs almost always come in realizing that things that appear different are, in fact, the same. To understand how things work, we need to understand symmetry.


Symmetries show up just about everywhere in nature, even though they may seem unremarkable or even obvious. The wings of a butterfly are perfect reflections of one another. Their function is identical, but I would feel extremely sad for a butterfly with two right wings or two left ones as he pathetically flew around in circles. In nature, symmetry, and asymmetry generally need to play off one another. Symmetry, ultimately, is a tool that lets us not only figure out the rules but figure out why those rules work.

Space and time, for instance, aren’t as different from one another as you might suppose. They are a bit like the left and right wings of a butterfly. The similarity between the two forms the basis of Special Relativity and gives rise to the most famous equation in all of physics. The laws of physics seem to be unchanging over time—a symmetry that gives rise to conservation of energy. It’s a good thing too; it’s thanks to the conservation of energy that the giant battery that is the sun manages to power all life here on earth.

Our minds enjoy the challenge of symmetries. In American-​style crosswords, typically the pattern of white and black squares look identical whether you rotate the entire puzzle a half a turn or view it in a mirror. Great works of art and architecture: the pyramids, the Eiffel
tower, the Taj Mahal, are all built around symmetries.

Image Credit: Herb Thornby



Search the deepest recesses of your brain, and you may be able to summon the five Platonic solids. The only regular three-​dimensional figures with identical sides are the tetrahedron (four sides), cube (six), octahedron (eight), dodecahedron (twelve), and the icosahedron
(twenty). A nerd (e. g., me) will think back fondly to his early years and recognize these as the shapes of the main dice in a Dungeons & Dragons set.*

If you run in particularly geeky circles, you may have heard a scientist refer to a theory as natural or elegant. What this normally means is that an idea is based on assumptions so simple that they absolutely must be correct. Or to put it another way, that if you start with a very simple rule you could derive all sorts of complicated behavior like behavior of black holes or the fundamental laws of nature. This is a book about symmetry: how it shows up in nature, how it guides our intuition, and how it shows up in unexpected ways. The Nobel laureate Phil Anderson put it most succinctly:

It is only slightly overstating the case to say that physics is the study of symmetry.

Some symmetries will be so obvious as to seem to be completely trivial, but will produce some incredibly nonintuitive results. When you ride on a roller coaster, your body can’t distinguish between being pushed into your seat by gravity or by the acceleration of your car; the two feel
the same. When Einstein supposed that “feels the same” really means “is the same,” he derived how gravity really works, eventually leading to the proposal of black holes.

The flow of time, on the other hand, seems to be just as obviously not symmetric. The past is most definitely distinct from the future. Oddly, however, no one seems to have informed the laws of physics about the arrow of time. On the microscopic level, almost every experiment you can do looks equally good forward and backward.

Image Credit: Herb Thornby


It’s easy to overstate the case and assume that everything is symmetric.

Without having met you, I’m willing to make some outrageous assumptions. Back in college you had at least one stoner conversation along the lines of, “What if our whole universe is just an atom in a way bigger universe, man?”


Have you grown up any since then? Admit it, you saw the perfectly decent Men in Black, or think back fondly to your childhood reading Horton Hears a Who!, and even now, you can’t help but wonder if there is a miniature universe far beyond our perceptions.

The answer, Smoky, is no, but the why is a somewhat deeper question.


For those of you who’ve read Gulliver’s Travels, you may recall that when we meet the Lilliputians, Jonathan Swift goes into excruciating detail explaining the consequence of the difference in height between Gulliver and the Lilliputians and, later, between Gulliver and the giant Brobdingnags. If you’ve never read the book, Swift really belabors the point, describing the ratios of everything from the size of a man’s step to the number of local animals required to feed Gulliver.

But even in Swift’s time, it was pretty well established that the story wouldn’t make physical sense (to say nothing of talking horses).

A hundred years earlier, Galileo wrote his Two New Sciences, in which he probes the scientific plausibility of giants.** After much deliberation, he concludes against the proposition, basically ruining everyone’s fun forever. The problem is that a bone that doubles in length becomes eight times heavier but has only four times the surface area. Eventually
it would collapse under its own weight. As he puts it:


An oak two hundred cubits high would not be able to sustain its own branches if they were distributed as in a tree of ordinary size;and that nature cannot produce a horse as large as twenty ordinary horses or a giant ten times taller than an ordinary man unless by miracle or by greatly altering the proportions of his limbs and especially his bones, which would have to be considerably enlarged over the ordinary.

He obligingly sketches a giant’s bones for the benefit of the reader.

Image Credit: Galileo



and concludes with the adorably disturbing imagery:

Thus a small dog could probably carry on his back two or three dogs of his own size; but I believe that a horse could not carry even one of his own size.

This is why Spider-​Man is such an ill-​conceived premise.*** Spidey wouldn’t have the proportional strength of a spider. He’d be of such bulky construction that he wouldn’t even need squashing. Gravity would do the trick for you.

The problem goes much deeper than just the tensile strength of the bones of giants and the proportional strength of spiders. Once you get down to atomic scales all bets are off. The
atomic world is also the quantum mechanical world, and that means that the concreteness of our macroscopic experience is suddenly replaced with uncertainty.


I trust I’ve made the point. Some changes matter and some don’t. My approach in this book is to focus each chapter on a specific question that will turn out to be answered, however indirectly, by fundamental symmetries in the universe.

On the other hand, that other hand isn’t perfectly symmetric. One of the most important puzzles humans can ever ponder is that, in some sense, the universe isn’t symmetric. Your heart is on the left side of your chest; the future is different from the past; you are made of matter and not antimatter.

So this is also, or perhaps more fundamentally, a book about broken and imperfect symmetries. There’s a proverb: A Persian rug is perfectly imperfect, and precisely imprecise. Traditional rugs have a small imperfection, a break in the symmetry that gives the whole thing more character.

So too will it be with the laws of nature, and a good thing because a perfectly symmetric universe would be staggeringly boring. Our universe is anything but. The universe in the rearview mirror is closer than it appears— and that makes all of the difference in the world. But let’s not look back; we’re on a tour of the universe. Symmetry will guide our way, but symmetry breaking will make our tour something to write home about.

* Black belt level nerds will notice that I’ve somehow omitted the ten-​sided die. Well, I’ll have you know that the D10 is not a Platonic solid. It’s one of a class of objects known as antidipyramids and goes by the charmingly ridiculous name of Bimbo’s lozenge.



** Yeah. That seems like a good use of his time and talents.

*** It is a well-​established fact that if you talk to a science nerd for long enough, they will ruin everything by looking into it too deeply. This is why we spend so, so many nights alone.

Dave Goldberg normally brings you the "Ask a Physicist" column. He is a Physics Professor at Drexel University. His newest book, The Universe in the Rearview Mirror is coming out today! You should definitely become a fan on facebook, or better yet, send a question about the universe.