Category: Astronomy • Physics • relativity
Posted on: September 3, 2010 8:00 PM, by Ethan Siegel
"We have been forced to admit for the first time in history not only the possibility but the fact of the growth and decay of the elements of matter. With radium and with uranium we do not see anything but the decay. And yet, somewhere, somehow, it is almost certain that these elements must be continuously forming. They are probably being put together now in the laboratory of the stars. ... Can we ever learn to control the process. Why not? Only research can tell." -Robert Millikan
Ah, energy, if only you were free, limitless, and easily accessible. If you were, we could do anything we wanted, no problem. Including making those pesky, rare, unstable elements and particles.
Say hello to lead. All the way up there at element 82 on the periodic table, lead is the heaviest element that's stable, or that doesn't radioactively decay. Everything heavier, including your friends radium, uranium, and plutonium will all decay. That means that, given enough time, they spit out various particles and turn into lighter elements.
In order to make these, or any other heavy, unstable particle, nucleus, or element, you need a huge amount of energy. For the heavy nuclei, we either need a very intricate man-made fusion apparatus, like this one at Sandia Labs,
or some wonderful source of astrophysical energy, like a supernova!
Of course, there are relatively light, unstable particles, too. Some of them, like the
muon, take less energy to make than even a single proton! A muon has nearly identical properties to an electron (including charge), but it's about 200 times more massive. With a half-life of between one and two microseconds, a muon is actually one of the longest-lived unstable particles, measured very accurately by particle accelerators. But there's a much more exciting place, for me, that they come from.
There are very, very high-energy particles whizzing around through space, in all directions, known as
cosmic rays. They come from all sorts of wonderful places, like our Sun, neutron stars, black holes, and the centers of galaxies, but they also come from
remnants of supernovae! If one of these high-energy cosmic ray particles smacks into something like a proton, say, in the Earth's atmosphere, something wonderful happens.
We get a whole "shower" of unstable particles! We can detect a bunch of these decay products, including electrons, positrons, protons, anti-protons, and photons, but you might be surprised to learn that we also detect muons!
Why is that so surprising? Well, even if you assume these muons move at the
speed of light -- the speed limit governing all matter in the Universe -- you still find that your muon shouldn't make it even 1 kilometer before decaying away. Yet these muons that we find can be traced back to originating all the way in the upper atmosphere, tens of (up to even a hundred) kilometers away! In fact, there are so many of them that, if you hold your hand up to the sky, you'll get about one muon passing through it every second!
Why do so many muons
make it down to you if their lifetime is so short?
Well, one of the funny things about these muons is that they are moving at almost the speed of light. And when you move close to the speed of light,
time slows down for you! This isn't just true of
twins when one of them travels in a rocket ship and one stays home, either.
It works for subatomic particles, too! While you might think these muons are aging 10, 20, or even 100 microseconds, they can move so quickly that -- from their reference frame -- they might not even last for a single microsecond before they make it all the way from the upper atmosphere down to your hand.
And that's why the seemingly impossible -- particles living longer than their measured lifetimes -- happen all the time. Pretty amazing stuff, and a great way to get you psyched for the Labor Day weekend! Enjoy!
