How can time run at different speeds?

And reality still be sane.

We've learned from Einstein's special theory of relativity that moving clocks run slow.

The faster you go in space, the slower you go in time.

This is no trick of perception, not just clocks, but all physical processes run slow.

If you were to see me blazing by you at nearly the speed of light and you were to look at the watch

on my wrist, it will tick

slower than the watch on your wrist. I will age slower than you. It takes forever for me

to do literally anything from your perspective. We've done episodes in the past about time dilation,

the twin paradox and all that, the age of the universe,

and hopefully, after all those episodes and after a century since Einstein's relativity,

while we may not fully understand time dilation, we can at least get used to it, which is common

in physics, much to the horror of my students. Moving clocks run slow. Okay, let's just take that as a bare fact of

existence. Einstein said so, and he seems like a smart guy, so we'll go with it. But let me illustrate

an example to show just how absurd the universe becomes once you allow for time dilation.

There's this little subatomic particle called a muon.

It's like an electron's more massive cousin.

Because it's massive, it doesn't live all that long,

right around 2.2 microseconds.

You make a muon and you blink and it's gone.

Now we know that muons are created in the upper atmosphere when high-energy cosmic rays

strike various insundry air molecules.

...