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Around nineteen-hundred, all of physics, and particularly Einstein, was in trouble: they
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,
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.