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One of the most revolutionary things we learned in the 20th century was that time is not absolute
–\hthe rate at which time passes for you (or anything else in the universe) is different
depending on how fast you're moving and how much you're accelerating.
In particular: time passes slower the more you're moving.
This fact has been confirmed experimentally countless times –\hfast-moving muons take
longer to decay, light emitted from a moving source has a lower frequency , and so on.
But relativity of time can at first glance seem somewhat contradictory.
Suppose we're flying past each other; from my perspective it seems like you're moving
(and so time should go more slowly for you), and from your perspective it seems like I'm
moving (and so time should go more slowly for me).
It seems crazy that we can both think time is going slower for the other person – someone's
time must actually be slower, right?
Well, no - take a look at my giraffe.
It's 3 meters tall.
And your giraffe is 3 meters tall, according to you.
But you're rotated relative to me, so you only measure my giraffe to be 2 meters high.
And I'm rotated relative to you, so I only measure your giraffe to be 2 meters high.
So we each think the other is measuring distances in space as longer, but it's not a contradiction.
It's just that we've rotated height and width relative to each other.
And similarly, when you change your speed, you rotate the direction of time – I have
another video explaining why.
But it looks like this: if every passing second I move to the left, then my clock will tick
like this.
And if every passing second you move to the right, then your clock will tike like this.
So when three seconds have passed on my clock, I'll measure only two seconds having passed
for you.
And yet when three seconds have passed on your clock, you'll measure only two seconds
having passed for me.
So we each think the other is measuring distances in time as shorter, but it's not a contradiction,
it's just how time behaves when it's rotated\h– it affects not just the passage of time, but
also our notions of "the same time" .
However, there's still an unanswered question: what if I stay on earth and you go off into
space and then come back?
Will one of us have actually aged more, or will we both have aged the same amount despite
constantly thinking the other was aging less?
This conundrum is called the Twins Paradox, and I'll explain the solution to it in my
next video.
But in the meanwhile, can you use rotating time to figure out why the twins paradox isn't
a paradox?