Right now, dozens of subatomic particles called muons are passing through your body every second. They were created roughly 15 kilometers above your head, when high-energy cosmic rays from deep space struck atoms in the upper atmosphere. Muons are heavier cousins of the electron, produced abundantly in those collisions. They reach the ground in measurable numbers — around one through every square centimeter of you per minute. Yet according to classical physics, none of them should ever arrive.

A muon at rest has a mean lifetime of about 2.2 microseconds. Even moving at very close to the speed of light, it would only have time to travel about 660 meters before decaying. The atmosphere is more than twenty times thicker than that. By the standards of pre-1905 physics, every muon should be gone long before it reaches the ground.

The resolution comes from Einstein's special relativity. From the muon's frame, time runs at its ordinary rate, but the distance to the ground is dramatically contracted, from 15 kilometers down to a few hundred meters. From your frame on the ground, the muon's clock runs slow due to time dilation. Both descriptions are equivalent, and both predict the same answer: the muon survives.

Carl Anderson detected muons in cloud chamber experiments in the mid-1930s, before fully understanding what kind of particle they were. They became one of the first relativistic effects to be measured at the level of individual particles. They are still routinely measured today in cosmic ray detectors, university physics labs, and small-scale demonstrations on mountaintops, where the count goes up because there is less atmosphere above to absorb them.

The particles arrive only because spacetime, at high speeds, behaves differently than common sense expects.