Questions

14-billion-years-later:

Muon vs. World: The Story of One Unstable Particle’s Fight for SurvivalFirst of all don’t freak out. You don’t need to know what a muon is, only that it’s small, a particle and whizzes about quite quickly. Secondly I need to explain that the image seen here is of the moon’s cosmic ray shadow and imaged using muons detected 700 metres underground. That means that all the blue bits are muons created in the upper atmosphere due to the collision of particles. The third thing I now need to tell you is that a muon has a fairly short life time of only 2.2 µs (10^-6), which is still the second longest of unstable elementary particles. Muons also typically travel at near light speed, however their short life times mean that they should only be able to travel around 600 metres before decaying. But that can’t be right! They manage it all the way from the upper atmosphere and down through at least 700 metres of rock! So how do these fellows beat the odds?
The trick lies in relativity, because the muon is traveling so fast the time it experiences is different from the time we observe. From the muon’s view point it really does only last 2.2 µs, however to us it appears to last much longer. In fact, according to relativity, any body under motion will appear to experience time much slower to a stationary observer, this phenomena is referred to as time dilation. That means that, at least to a physicist, going for a jog really will make you live longer!For those mathematically minded folks among us, here’s the equation for working out the time observed (t’) in relation to velocity: t’=t/(√1-v²/c²). Interestingly the same formula also applies to the mass of a particle when t is replaced with m.

Muon vs. World: The Story of One Unstable Particle’s Fight for Survival

First of all don’t freak out. You don’t need to know what a muon is, only that it’s small, a particle and whizzes about quite quickly. Secondly I need to explain that the image seen here is of the moon’s cosmic ray shadow and imaged using muons detected 700 metres underground. That means that all the blue bits are muons created in the upper atmosphere due to the collision of particles. The third thing I now need to tell you is that a muon has a fairly short life time of only 2.2 µs (10^-6), which is still the second longest of unstable elementary particles. Muons also typically travel at near light speed, however their short life times mean that they should only be able to travel around 600 metres before decaying. But that can’t be right! They manage it all the way from the upper atmosphere and down through at least 700 metres of rock! So how do these fellows beat the odds?

The trick lies in relativity, because the muon is traveling so fast the time it experiences is different from the time we observe. From the muon’s view point it really does only last 2.2 µs, however to us it appears to last much longer. In fact, according to relativity, any body under motion will appear to experience time much slower to a stationary observer, this phenomena is referred to as time dilation. That means that, at least to a physicist, going for a jog really will make you live longer!

For those mathematically minded folks among us, here’s the equation for working out the time observed (t’) in relation to velocity: t’=t/(√1-v²/c²). Interestingly the same formula also applies to the mass of a particle when t is replaced with m.