WEBVTT

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Have you ever daydreamed
about traveling through time,

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perhaps fast forward in the centuries
and seeing the distant future?

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Well, time travel is possible,

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and what's more, it's already been done.

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Meet Sergei Krikalev,

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the greatest time traveler
in human history.

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This Russian cosmonaut holds the record

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for the most amount of time
spent orbiting our planet,

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a total of 803 days,
9 hours, and 39 minutes.

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During his stay in space,

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he time traveled into his own future

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by 0.02 seconds.

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Traveling at 17,500 miles an hour,

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he experienced an effect
known as time dilation,

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and one day the same effect

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might make significant time travel
to the future commonplace.

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To see why moving faster through space
affects passage of time,

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we need to go back to the 1880s,

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when two American scientists,

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Albert Michelson and Edward Morley,

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were trying to measure the effect
of the Earth's movement around the Sun

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on the speed of light.

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When a beam of light was moving
in the same direction as the Earth,

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they expected the light to travel faster.

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And when the Earth was moving
in the opposite direction,

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they expected it to go slower.

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But they found something very curious.

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The speed of light remained the same
no matter what the Earth was doing.

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Two decades later,
Albert Einstein was thinking

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about the consequences
of that never-changing speed of light.

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And it was his conclusions,

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formulated in the theory
of special relativity,

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that opened the door
into the world of time travel.

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Imagine a man named Jack,

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standing in the middle
of a train carriage,

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traveling at a steady speed.

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Jack's bored and starts bouncing
a ball up and down.

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What would Jill, standing on the platform,
see through the window

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as the train whistles through?

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Well, between Jack dropping the ball
and catching it again,

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Jill would have seen him move
slightly further down the track,

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resulting in her seeing the ball
follow a triangular path.

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This means Jill sees the ball
travel further than Jack does

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in the same time period.

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And because speed
is distance divided by time,

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Jill actually sees the ball move faster.

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But what if Jack's bouncing ball
is replaced with two mirrors

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which bounce a beam of light between them?

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Jack still sees the beam dropping down

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and Jill still sees the light beam
travel a longer distance,

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except this time Jack and Jill
cannot disagree on the speed

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because the speed of light
remains the same no matter what.

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And if the speed is the same
while the distance is different,

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this means the time taken
will be different as well.

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Thus, time must tick at different rates
for people moving relative to each other.

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Imagine that Jack and Jill
have highly accurate watches

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that they synchronize
before Jack boards the train.

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During the experiment,
Jack and Jill would each see

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their own watch ticking normally.

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But if they meet up again later
to compare watches,

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less time would have
elapsed on Jack's watch,

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balancing the fact that Jill saw
the light move further.

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This idea may sound crazy,

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but like any good scientific theory,

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it can be tested.

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In the 1970s, scientists boarded a plane

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with some super-accurate atomic clocks

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that were synchronized
with some others left on the ground.

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After the plane had
flown around the world,

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the clocks on board
showed a different time

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from those left behind.

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Of course, at the speed
of trains and planes,

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the effect is minuscule.

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But the faster you go,
the more time dilates.

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For astronauts orbiting
the Earth for 800 days,

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it starts to add up.

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But what affects humans
also affects machines.

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Satellites of the global
positioning system

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are also hurdling around the Earth

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at thousands of miles an hour.

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So, time dilation kicks in here, too.

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In fact, their speed causes
the atomic clocks on board

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to disagree with clocks on the ground

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by seven millionths of a second daily.

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Left uncorrected,

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this would cause GPS to lose accuracy

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by a few kilometers each day.

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So, what does all this have
to do with time travel

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to the far, distant future?

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Well, the faster you go,
the greater the effect of time dilation.

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If you could travel really close

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to the speed of light, say 99.9999%,

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on a round-trip through space

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for what seemed to you like ten years,

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you'd actually return to Earth

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around the year 9000.

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Who knows what you'd see
when you returned?!

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Humanity merged with machines,

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extinct due to climate change
or asteroid impact,

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or inhabiting a permanent colony on Mars.

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But the trouble is,

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getting heavy things like people,
not to mention space ships,

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up to such speeds requires
unimaginable amounts of energy.

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It already takes enormous
particle accelerators

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like the Large Hadron Collider

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to accelerate tiny subatomic particles
to close to light speed.

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But one day, if we can develop the tools
to accelerate ourselves to similar speeds,

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then we may regularly send time travelers

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into the future,

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bringing with them tales
of a long, forgotten past.