## Length

A funny little particle called a "muon" provides a good example, similar to Timus Ybatu's mile run, except that because of the muon's great speed the effect is not preposterously small.

The muon is like an electron, but 200 times heavier. It is a highly unstable little thing, and dissociates spontaneously like a radioactive atom. The muons have a half-life of 2.5microseconds, meaning that in 2.5microseconds half of a population of muons will have dissociated into other particles. Physicists can make muons in a laboratory, and study them in that very short time before they fall apart.

Muons are also produced by cosmic rays in the upper atmosphere about 10 miles up. Some of these shoot downward toward earth's surface, where scientists can detect them in radiation detection instruments.

Now, like Timus (if you've forgotten about Timus click Time.) the muon carries its own clock. That is because the mechanism that makes it fall apart on schedule is part of the muon, and goes with the muon wherever it goes.

The on-board clock gives the muon 2.5 microseconds to live (in the sense of the definition of "half-life"). At the speed that these muons travel, with a half-life of 2.5 microseconds, only about 1 in a million would be expected to survive to reach the earth's surface.

The surprising thing is that actually 1 in 8 make it. That's because our clocks (like the clocks of the judges in Timus' run) give the muons a longer time (half-life) because we are watching from the sidelines. Our clocks give the muons a half-life of 18 microseconds (instead of 2.5).

With that extra time to live, 125,000 times as many muons make it to our counters.

Now if you ask the mouns how this comes about, they have a real puzzle, because they don't know about the 18 microseconds on our clocks, and they think they only have 2.5 microseconds.

What you will find out (if you read the book) is that their explanation is based on their measurement of the length that they have to travel. Because of their great speed, they measure the length that they have to travel as much less than 10 miles.

They see the length between where they are formed and the earth's surface, not as 10 miles, but as only about 1.4 miles. In their view of things, it's that shorter distance that gives them a greater chance of making it.

The phenomenon we found at work here is called a length contraction. It is a relativistic effect due to rapid motion.

CAVENDISHSCIENCE.ORG