[Excerpt]
Physicist Dave Goldberg breaks down the mysteries of the cosmos in his new book The Universe in the Rearview Mirror. In an excerpt, Goldberg explores the matter-antimatter divide
Image: Courtesy Dutton Books
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The article below is excerpted from The Universe in the Rearview Mirror: How Hidden Symmetries Shape Reality, by Dave Goldberg. Dutton, 2013.
It is generally a bad idea to watch science fiction in the hopes of bolstering your understanding of science. Doing so would give you a very distorted impression of, among other things, how explosions sound in deep space (they don’t), how easy it is to blast past the speed of light (you can’t), and the prevalence of English-speaking, vaguely humanoid, but still sexy, aliens (they’re all married). But if we’ve learned one good lesson from Star Warses and Treks, it’s that no one should ever mess with antimatter.
Antimatter is not only no more exotic than ordinary matter but in almost every way that matters, it looks and acts the same. Were every particle in the universe to suddenly be replaced by its antimatter version, you wouldn’t even be able to tell the difference. To put it bluntly, there is a symmetry between how the laws of physics treat matter and antimatter, and yet they must be at least a little bit different; you and everyone you know are made of matter and not antimatter.
We like to think accidents don’t happen, that there is some ultimate cause to explain why, for instance, you’re not standing around in a room full of anti-people. To understand why that is, we’re going to delve into the past.
Origin stories are tough. Not everything can be explained as neatly as being bitten by a radioactive spider, having your home planet explode, or even by the reanimation of dead tissue (you know, for science). Our own origin story is complicated, but you’ll be pleased to know that, much like The Incredible Hulk we’re also ultimately the result of exposure to gamma radiation. It’s a long story.
Based on everything that we’ve ever seen in a lab, you should not exist. It’s nothing personal. I shouldn’t exist, either, nor should the sun, the Milky Way Galaxy, or (for many, many reasons) the Twilight movies. Let me try to put that another way.
First a summary: You are made of fundamental particles, which are almost entirely empty space, and the tiny bits that aren’t empty space aren’t all that massive. Ephemeral energy just makes them appear that way. Particles can be created from whole cloth and energy and destroyed just as quickly. You are not just much more than the sum of your parts; strictly speaking, your parts add up to a small pile of matchsticks in a tornado of pulsing, screaming energetic interactions. Yippee‑ki‑yay!
Energy can be used to make matter from whole cloth, but as a side effect, antimatter gets made as well. I’ve referred to antimatter by its effect, but haven’t really said what it is. Prepare to be underwhelmed!
Every type of particle has an antimatter version that behaves almost exactly the same—the same mass, for instance—but has the opposite charge. A positron behaves just like an electron, but has a positive charge rather than a negative one. An anti-proton has a negative charge, contrary to a proton’s positive one, and so on.
One of the craziest things about antimatter is that if you are smart enough—and apparently only the English physicist P.A.M. Dirac was—you could have actuallypredicted antimatter before it was ever discovered. In 1928, Dirac derived the equations of relativistic quantum mechanics. Yes, that’s exactly as difficult as it sounds. Plugging through the equations, he noticed that there were missing solutions. He found, for example, that electrons should pop out of the theory naturally, but other particles with the same mass and opposite charge should also be allowed.
Dirac’s equation predicted that for every particle like an electron, there was going to be an antiparticle. At first, he didn’t have it quite right. He thought of the positron as:
An electron with negative energy [that] moves in an external field as though it carries a positive charge.
Dirac didn’t know quite what his equations were saying. If his original gut reaction were correct then you’d essentially be able to generate nearly infinite energy just by producing positrons. It would be the equivalent of running a business by running up literally infinite interest-free debt.
But ultimately, Dirac hit on the truth: Positrons are just the flip side of electrons. In other words, there seemed to be a deep symmetry between matter and the then as-yet undiscovered antimatter.
And the reality of a particle’s antimatter evil twin is that while opposites may attract, it’s not always such a good idea for particles and antiparticles to act on those urges. When an electron and a positron come into contact with one another, the resulting conflagration completely annihilates them both, and in the process, the magic of E =mc2 turns their mass into a huge amount of energy.
There’s nothing special about which particle we choose to call the “antiparticle” and which one is “normal.” In a parallel universe made entirely of what we call antimatter, those anti-people would no doubt call their atoms ordinary and we’d be the anti-ones. And this is really one of those cases where both the anti-people and us are right. It’s all just a matter of semantics.
That isn’t to say that there’s no antimatter in our universe. Antimatter is made all the time in the sun, which produces positrons as a side effect of fusing hydrogen into helium. Closer to home, we’re able to make all sorts of exotic antiparticles in huge accelerators like the Large Hadron Collider in France and Switzerland.
Why is there a difference between matter and antimatter? What were the reactions that allowed the creation of more of one than the other? That, after all, is the ultimate answer to the question of where we came from and why there are no anti-people.
No one has yet figured out exactly how things played out in the early moments of the universe. All we know is that we exist because of some sort of symmetry violation in the universe from very near the beginning. The conditions in the early universe wereextremely hot—maybe that had something to do with it.
Every now and again, you’ll hear accelerators described as “re-creating the conditions of the Big Bang.” This is more or less right. The universe was hotter, and more energetic, in the past. The closer to the Big Bang that you want to explore, the hotter it is. Nothing we’ve seen so far in particle accelerators has given even the slightest inkling of producing a net matter over antimatter. The current thinking is that the small matter–antimatter accounting error occurred very, very early on, around 10−35 second after the Big Bang during which the temperatures were more than a quintillion times those at the center of the sun. Suffice it to say, we’re not able to produce those energies in a lab.
Even at those astounding energies, the asymmetry between matter and antimatter is extremely small. For every billion antiparticles that were created, there were a billion and one particles. One. Just one. We know that because there are currently about a billion times as many photons in the universe as there are protons. When the billion antiprotons annihilated with the billion protons, they left behind the billions of photons that we observe today, though greatly weakened by the expansion of the universe.
Eventually, all of antiparticles annihilated with almost all of the particles, leaving the one part in a billion to make all of the “stuff” that we now see. As Einstein put it:
I used to wonder how it comes about that the electron is negative. Negative-positive—these are perfectly symmetric in physics. There is no reason whatever to prefer one to the other. Then why is the electron negative? I thought about this for a long time and at last all I could think was “It won the fight!”
To put it another way, you’re essentially a rounding error from around 10−35 second after the Big Bang. Doesn’t make you feel very important, does it?
Of course that’s just as much a bummer for the anti-people too.
Excerpt from THE UNIVERSE IN THE REARVIEW MIRROR © 2013 by Dave Goldberg. Published by Dutton, A Member of Penguin Group (USA) Inc. Excerpted with permission from the publisher. All Rights Reserved.
ABOUT THE AUTHOR(S)
Dave Goldberg is a professor of astrophysics at Drexel University. He has a Ph.D. in astrophysics from Princeton University. Goldberg's research centers on cosmology and gravity's distorting effects on the appearance of the universe. He also writes the Ask a Physicist column at the Web site io9.
Source: Scientific American Space & Physics
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