Nuclear Physics
Time Dilation of Mu-Mesons
Earth is constantly bombarded by cosmic rays. A delicious variety of cosmic events produces these particles (mostly protons). FACT: The cosmos is full of things that go BOOM. Examples are supernova explosions, which release tons of cosmic-rays.
Once they reach Earth, cosmic rays interact with the atmosphere and produce secondary cosmic-rays, dominated by particles called mu-mesons (µ-mesons), or simply muons. Yes, more Greek. Scientists ran out of English letters long ago.
As µ-mesons enter the Earth's atmosphere, they decay into other particles.
Depending on the charge, a µ-meson will decay into an electron or anti-electron, along with a neutrino and an anti-neutrino. Note: these two neutrinos are actually of different kinds, but that's not our topic of interest at the moment.
The goal today isn't to review particle physics, though, but to point out that these mu mesons travel at speeds very close to the speed of light as they zoom into Earth's atmosphere. What happens at relativistic speeds?
If "time dilation" is one of your answers, give yourself a pat on the back.
Mesons decay into their products at a certain rate, just like radioactive isotopes have half-lives. However, we can count the number of mesons at the top of a mountain, based on the height L of a mountain and on their decay rate of mesons per second, and we can predict the number of mesons at the bottom of the mountain after they travel for 
where v is nearly c.
See mesons shouldn't have time to travel all the way down to earth because they decay so quickly, but they do. If you suspect wizardry is involved, good guess, but guess again.
We haven't taken time dilation of the muon in its own frame of reference into account. Since their decay rate is based on time, a lot more mesons will survive the trip than non-relativistic calculations predict.
In 1941, B. Rossi and D. B. Hall conducted a muon experiment10. With the use of a sophisticated particle detector, Rossi and Hall detected a very large number of μ-mesons at the bottom of Mount Washington in New Hampshire. Instead of the predicted rate of 25 mesons per hour, the rate was 400 mesons per hour.
This is because according to Earth, the trip lasted about t ~ 6.5 μsec, assuming the velocity of the mesons was near the speed of light. Taking into account time dilation, the trip actually lasted only about 0.7 μsec according to the muons.
Special relativity strikes again.