couldn't figure out how anything could move… Now before you complain that I'm exaggerating,

check out this cat! You can clearly see that the cat is moving away from Einstein at a

constant velocity… but do a little sliding switcheroo, and suddenly it looks like Einstein's

the one moving. This is the "old-fashioned principle of relativity," so of course it's

the one we teach in schools… but the point is that the switcheroo changes relative things,

like position and velocity, and not absolute ones, like the separation of Einstein from

his cat.

Now for the problem: before Einstein was even born, physicists showed that the speed of

light was one of those absolute things which can't be changed by a switcheroo, so any switcheroo

we do has to keep light moving at the same speed. But then it's obvious that we can't

do our sliding switcheroo at all, which means we can't explain how anything other than light

can move!

Ok, I spoke too soon… there is one solution – do you see it? We were assuming that our

switcheroo had to keep every slice of time at the same, well, time. But there's no law

of physics that says time is an absolute thing that can't be changed by switcheroos… so

if we just rotate the slices of time while sliding them, then we can keep the speed of

light the same, and explain how things can move, too.

Of course, Einstein didn't figure out this "special principle of relativity" in 1905

- it was already done by a guy named Lorentz ten years earlier. But Lorentz just thought

this time-rotation was a mathematical trick… and it took Einstein to step in, and, you

guessed it, propose that "time-rotation" is real, that time really is relative, and that

consequently, simultaneous events relative to one observer aren't simultaneous relative

to another who's moving. Now that's a real switcheroo of perspective.