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0:00- What's the farthest
manmade object from Earth?
0:02It's a space probe called Voyager 1
0:05launched by NASA in 1977.
0:07It's zooming away at 17
kilometers per second
0:10and is now more than 25
billion kilometers away.
0:13But what's weird is that
the clock on Voyager 1
0:15reads a different time than ours.
0:18That's thanks to something
our friend Albert Einstein
0:21His theory of special relativity
0:22showed that time passes slower for clocks
0:25It sounds crazy, but scientists
have demonstrated this
0:28effect over and over.
0:29We actually have to account
for how time passes differently
0:32on GPS satellites orbiting the Earth
0:34in order to track our location accurately.
0:36Okay, so Voyager 1's been traveling
0:38for a little over 48 years
0:40at around 17 kilometers per second.
0:42According to Einstein's
equations for special relativity,
0:45Voyager's clock has lost about two
0:48and a half seconds since it launched,
0:49but Einstein gave us
another mind bending lesson
0:52to consider, his theory
of general relativity.
0:54In addition to time being
stretched for moving objects,
0:57time is also warped by gravity.
0:59When you're close to a
massive object like a planet
1:01or a black hole, your clock
will tick a tiny bit slower
1:05than if you're far away from it.
1:06Our GPS satellites have to
account for this effect too.
1:09And since Voyager 1 is now
more than 25 billion kilometers
1:12from Earth, it obviously
feels the gravitational pull
1:14of our planet much less than we do.
1:16That difference from Earth's
gravity means Voyager's clock
1:18will actually tick about a second
1:20faster than an Earth clock.
1:21And if you combine both of
these time dilation effects,
1:24Voyager's clock should be about a second
1:26and a half behind ours.
1:27But it gets even weirder
1:29because Earth is moving really fast too.
1:31In fact, the Earth's velocity
1:32around the sun is even faster than Voyager
1:35zooming through space.
1:36Plus, we're much closer
to the giant heavy sun
1:39So our clock is slowed even
further by its gravity.
1:42If you do all these calculations relative
1:43to a point at the center
of our solar system,
1:45factoring in the speed of Voyager,
1:47the differences in gravity
1:48and the speed of spaceship Earth,
1:50the clock on Voyager is actually somewhere
1:52around 20 seconds ahead of ours on Earth.
1:55Woo. So what time is
it really on Voyager 1?
1:59Well, it could be the future or the past.
2:01Just depends on where you sit.
2:03Follow me for more cool science videos.
