I’m going to try to blow your mind two separate times in this post. Stay with me while things get weird, ok?
Let’s have some fun with neutrinos. For this very-sophisticated physics demonstration you’ll need your hand and one second of time. I’ll wait while you collect those supplies.
Ok, first hold out your hand, palm up. Now wait for one second. Are you done? Did you notice anything freaky happening?
What if I told you that about a trillion neutrinos passed through your hand in that one second? Not figuratively, the same way you might say “I’m so hungry I could eat, like, a trillion pizzas”. I mean that several thousand billion particles — called neutrinos — pass through your body every single second of every day and night.
Don’t freak out! Well, go ahead and freak out a little if you want. A trillion is a freaky-big number. But the thing about neutrinos is that they’re guaranteed not to bother you. They basically never interact with other matter. You could shoot a neutrino through a brick of lead one light-year long, and that neutrino would only have a 50% chance of colliding with one of the atoms in that lead brick. They’re like the ghosts of the particle physics world.
Wait wait, come back! I’m sorry I made you think about trillions of ghosts whooshing silently through your body. I promise they won’t hurt you.
All those neutrinos are coming from the sun, by the way. The sun is so hot and massive that individual solar protons will squish together and fuse into helium. One of the by-products of that solar fusion is some neutrinos. And by “some” I mean “a number so big it hurts to think about it”.
Now let’s change gears slightly. Remember the Big Bang? When the universe was a crazy-hot pinprick of horrendous energy? As the infant universe expanded and cooled, little globs of matter — particles! — started to condense out of that energy. That happened the way Einstein said: some energy E would turn into some mc2, and then all of a sudden there’d be a particle (with mass m) where before there was just a wad of energy.
The thing is, Einstein’s E = mc2 doesn’t say anything about particles vs. anti-particles. A wad of Big Bang energy should be just as likely to make an anti-electron as to make an electron. (Or whatever, pick your favorite particle.) Statistically, then, you would expect equal amounts of matter and anti-matter to form after the Big Bang.
But! Any Trekkie will tell you that when matter and anti-matter collide, they annihilate in a puff of energy. (That’s how the Enterprise runs!) So in an early, hot universe with particles zooming around helter-skelter, you’d expect matter to collide with anti-matter, leaving nothing. That’s right folks, if physics was simple and things generally made sense, the universe would be filled with nothing. Instead of which, we have not-nothing! There is stuff, and all of the stuff we’ve ever found in the universe is made of matter. Nobody has ever seen an anti-matter galaxy full of anti-matter stars.
If I’ve done my job right, you’re freaking out again right about now.
Physicists have worked out a theoretical model that explains this matter/anti-matter asymmetry. Theoretically, there’s some mechanism that biased the early universe in favor of matter. In order to test that model, we need to study some pretty esoteric things about neutrinos. That brings us to the final question of this post: If neutrinos are so insubstantial, how can you possibly study them?
The answer has to do with statistics. Let’s say you build a particle detector and then you start throwing neutrinos at it. If you only throw one neutrino at a time, there’s basically no chance of that particular neutrino interacting with your detector. What if you throw a million trillion neutrinos at once? Each individual neutrino still has a vanishingly small chance of interacting with your detector, but now statistics is starting to work in your favor. This is kind of like the lottery. If you buy one lottery ticket, I promise you won’t win the million-dollar jackpot. But if you buy a million lottery tickets, you might have a decent chance of winning.
There are some experiments that manage to use only solar neutrinos to answer very specific questions. The Ice Cube experiment in Antarctica is a notably, amazingly hardcore example of this. But really big questions (“Why is there stuff?”) require way more neutrinos than the sun alone can provide. Another experimental approach is to make your own neutrinos in way, way larger quantities, and to throw them all at your particle detector as fast as you possibly can. (This is equivalent to buying all the lottery tickets.) And what’s the best way to do that? Why, with a particle accelerator of course! This is just what I was talking about in last week’s post.
I want to build a neutrino factory. Don’t you?
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