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SYFY WIRE Bad Astronomy

Muons, relatively speaking

By Phil Plait
Muons pose an interesting problem, but you can learn the solution in Minute Physics. Credit: Henry Reich

If you ask me what the most wildly successful scientific theory of all time is, that's an easy one: Relativity.

Specifically, General Relativity*. Einstein's breakthrough in physics literally rewrote how we think of space and time, gave us a grip on places in the Universe yet to be discovered at the time (like black holes and very distant galaxies), and fixed so many scientific problems that honestly we should be using it for a metaphor instead of duct tape.

One of the most interesting things that comes out of GR (as us cool science kids call it) is how it can be applied just about anywhere physics happens. Like, say, the surface of the Earth. This is one of my favorite places, as it's where all my friends live. But it's a vast experimental lab, too, if you know what to look for, how to look for it, and ask why you see what you see.

And that brings me to muons. These, of course, are the fundamental particle of cows. No! Wait! They're actually fundamental subatomic particles similar to but more massive than electrons. Lots of different processes can create them, but one way is when cosmic rays — much heavier subatomic particles that whiz around space — slam into Earth's atmosphere. This can create a shower of muons that then rain down to Earth's surface… where we shouldn't see them. They shouldn't live long enough to make it to the ground, decaying while still high above the planet's surface.

But we do see them. Why?

Well, for that I'll let my friend Henry Reich of Minute Physics explain, because he's really just so very good at it.

Aha! So you see what I mean, I hope. Muons shouldn't "live" long enough to reach the ground. They do because, as it's sometimes colorfully put, their clocks run slower because they move so close to the speed of light relative to us. That's not just some mechanical metaphor, either; it's woven into the way the Universe itself behaves. Time runs more slowly for something moving very rapidly relative to something else, which itself is a fundamental part of what GR tells us.

It's an elegant and lovely piece of evidence that GR is actually true. It's also not that hard to explain, which served me well once.

Some years ago I was doing a project with a Major Metropolitan Museum. When the project was done, they had an event for some big donors to the museum, to present it to them for a sneak peek. Some of us who worked on the project were invited as well, and acted as ambassadors. I had the pleasure of hanging out with many of the donors over hors d'oeuvres, answering any questions that might have.

I was waxing lyrical about black holes and how GR helped us explain them. One gentleman made a face when I did, and made a comment that relativity was so weird, it couldn't possibly be correct. He came right out and said he didn't think relativity was true.

Now, many times when faced with this situation, I make mumbly noises about how no, it really does work, and try to avoid a confrontation that might wind up making me just being frustrated. But this time something told me I should dig in a little.

Ah, I said. But really there's a lot of stuff going on around us all the time that GR explains really well, I continued. Then I told him about the muons, saying pretty much what Henry did above in his fun video. I could tell I had his attention, so I pressed on.

Did you use GPS to navigate your way to the museum today, I asked. He nodded. Well, that depends on GR as well! The satellites that orbit the Earth, which you use to triangulate your position, depend on GR to get it right. Their clocks run differently than ones on Earth because they are farther out in Earth's gravitational field, and also because they move so rapidly relative to those of us on Earth's surface. If you don't account for time dilation due to both gravity and velocity using GR, then GPS would be off by several kilometers after just one day!

The fact that GPS works means GR must be right.

Readers, let me tell you: This was one of those rare and wondrous moments when talking facts and science actually changed someone's mind. He was taken aback when I told him this, and said that this was pretty interesting and that he'd read more about it. Sometimes that's a blow-off line, but I could tell when he said it he meant it. He and his wife peppered me with questions for a while after that, too.

I suspect that part of the reason I could sway him was that he was, after all, a donor to a big science museum, and may have been more likely to listen. But I was also enthusiastic and passionate as well as polite and respectful. I kept him listening, and was able to get to my point without making him angry or defensive.

Huh. Funny how that works. Who could've guessed?

Oh, one more thing: Henry notes in his video that the actual source of cosmic rays is still unknown. That's true enough, but in more detail we do know that some come from the Sun, some from exploding stars, and some from distant active galaxies where huge black holes voraciously gobble down material; they're sloppy eaters and spew out all kinds of matter and radiation. The debate is over how many cosmic rays are made by each, what precise mechanisms are at work creating the vast energies needed to accelerate these particles to near light speed, what distribution of energies they have as they fly through space (some are merely really energetic, while others are at holy-crap-how-is-this-even-possible energies), and whether there are other sources of them we don't understand yet.

In science, there is always room for debate… it just depends on how much room that is. Even GR isn't the be-all and end-all of science; there are places where it firmly disagrees with other theories, like quantum mechanics. The beauty is, that doesn't mean either is wrong; it means there are gaps in our knowledge yet to be filled.

And that is my absolute all-time favorite part of science. It's fun, it's cool, it's awe-inspiring, and there will always be more of it.

*Writing that I made myself laugh.