If you're like the rest of us, you're almost certainly made of matter. But where did all that delicious, gooey matter come from? In this In this week's "Ask a Physicist" we'll find out.
I get a lot of questions pondering the general nature of existence, so for this week's column, I decided to get a bit philosophical and give a primer on:
Where did I come from?
I don't mean in an immediate sense; that's a discussion you need to have with your parents. I also don't want to talk about the origin of heavy atoms like carbon, oxygen, and calcium, that make up most of your good parts. That's an interesting story as well, but one I'll have to save for a later date.
Instead, I'd like you all to fire up your bongs for the ultimate in stoner questions: Why is there something rather than nothing? To explain why this is even an issue, I need to say a few words about antimatter, our often misunderstood friend.
Matter and Antimatter
As an avid io9 reader, you probably already have more than a passing familiarity with antimatter. The basic drill is that antimatter behaves almost exactly the same as ordinary matter — same mass, same amount of spin, etc. — but has the opposite charge and other quantum numbers. We'll see in a bit that the "almost" carries with it some important implications for the universe. A positron, for instance, behaves just like an electron, but has a positive charge rather than a negative one. An anti-proton has a negative charge, and so on.
And then there's the exciting part: If an electron and a positron (or any other pair) come into contact with one another, they'll completely annihilate, and in the process, the magic of E=mc^2 turns their mass into a huge amount of energy. This is why Geordi always gets concerned when the antimatter containment field's been breached.
There's nothing special about which particle we choose to call the "antiparticle" and which one is "normal." We decided our universe is normal, so we name it that way. There are some particles, like photons, the particles of light, which are their own antiparticles. This begs the question, if you were suddenly transported to a parallel universe with particles replaced with antiparticles (including in your own body), could you even notice? We'll get to that shortly, but first I need to address the central mystery with matter and antimatter.
Everything's in pairs.
Experimentally, you can't create or destroy particles without doing the same for the same number of antiparticles at the same time. Stir an electron and a positron into a pot, and you destroy both in the process. On the flip side of the coin, if I throw enough energy into the mix — in a particle accelerator, or in the very early universe — I can create a particle-antiparticle pair out of thin air (and energy). As I've discussed in a couple of previous columns this sort of thing happens in the vacuum of space all the time. Particles and antiparticles get created and destroyed in, as near as we can tell, perfect concert.
At least that's what happens in the lab, and what all of our currently agreed upon physical laws say.
But there's a problem with this happy symmetry. The early universe was very hot, and in those early days, there was plenty of energy to go around, and particle-antiparticle pairs of all sorts got constantly created. But the universe cooled, eventually to the point where new pairs couldn't be created anymore. For instance, when the universe was about 5 seconds old, it was about 6 billion degrees Kelvin, just slightly too cool to make electrons and positrons anymore (but still a lot hotter than the interior of our sun now). Unable to make any new material, all of the particles and antiparticles should have eventually found one another and annihilated.
This is the key to the big mystery: If matter and antimatter are always created and destroyed in equal quantity, then there shouldn't be any of either around today, flaunting your matter, apparently in direct contradiction to everything we've seen in the lab.
So where did you come from?
How matter and antimatter are different
Everything we encounter seems to be made of ordinary matter. Sure, we get a few antiparticles, here and there from cosmic rays or in labs, but for the most part, we're all matter.
If you're of a particularly contrary nature, you might propose that maybe half the galaxies in the universe are made of matter and the other half antimatter and that we just happen to live in a matter one. Perhaps. Except that galaxies collide with each other all the time, and if a matter galaxy hit an antimatter galaxy, the eruption of X-rays would be huge, and visible across the universe. We're looking for that sort of thing, and we should be able to see those sorts of events. We haven't seen one, at least not yet.
In short, our universe seems to be made of matter.
Having led you this far into the puzzle, I now have to make a confession. We — that is, physicists — don't really know why there's this imbalance, why the universe is made of matter. However, we do have a few hints. It seems that matter and antimatter are a bit different from one another.
- Matter knows left from right.
I made a little side comment way up above about how matter and antimatter behave almost exactly the same according to the laws of physics. They have the same mass, so they react to and create gravity the same. They have opposite charges, but that means that since two electrons repel one another, two positrons will also repel one another. In other words, if we change all particles to antiparticles, then the fundamental forces seem to react the same way. Except for one.
The weak nuclear force is, as you might expect, very weak. It involves particles like neutrinos that interact so rarely that they can travel through something like a light year's worth of lead without scattering off a single atom. The weak force is incredibly important, though, since it fundamentally governs the nuclear reactions in the sun allowing for the existence of, for example, you.
The weak force, and no other, makes a small distinction between matter and antimatter. Without going into too much detail, the basic idea is that when a particle, a neutrino, for example, gets created in a weak reaction, it is always left-handed, which is just another way of saying that if it's headed toward you, it'll look like it's spinning clockwise. Antiparticles, on the other hand, have the same amount of spin, but do so in the opposite direction. That's it. That's the difference.
We really have no idea where this fundamentally comes from and why it's not the other way around. We put this difference into the equations by hand. Also, it still doesn't explain why matter is so much better than antimatter that the entire universe should be made of it.
- Things fall apart, but not identically.
It turns out that you can learn a lot about the universe by looking at how stuff falls apart. For example, at very high energies, particles known as Kaons can be produced in particle accelerators, along with their antiparticles. For the most part, these kaons and antikaons behave the same, and produce very similar stuff when they decay, as all massive particles eventually do. However, in about one case in a thousand, kaons produce different decay products than antikaons. A tiny effect, but a demonstration that the universe does, indeed, distinguish between matter and antimatter.
I should mention that while the original kaon result has been around for more than 50 years, there have been a number of similar, and potentially even more dramatic experiments in the interim. You may recall a bunch of stories last year announcing an asymmetry in the decay of the "B meson." The results were exciting, but despite popular accounts didn't result in an excess of matter being created over antimatter. It did, however show once again that matter and antimatter decayed differently from one another.
Still, all of this doesn't ultimately tell us why there is a difference between matter and antimatter, or, more to the point, what the exact reactions were that allowed us to create more of one than the other, and the more immediate question of where we came from.
As near as we can tell, you came from a symmetry violation in the universe from very near the beginning. It was a very small effect. For every billion antiparticles that were created, there were a billion and one particles. 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.
To put it another way, you're a roundoff error from around 10^-35 seconds after the big bang. Doesn't make you feel very important, does it?
Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (follow us on twitter, facebook, twitter or our blog.) He is an Associate Professor of Physics at Drexel University and is currently working on a new book on symmetry. Feel free to send email to firstname.lastname@example.org with any questions about the universe.
Images via Michael Taylor, Spectral-Design, Shutterstock