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  • Have you ever daydreamed

  • about traveling through time,

  • perhaps fast forward in the centuries

  • and see the distant future?

  • Well, time travel is possible,

  • and what's more,

  • it's already been done.

  • Meet Sergei Krikalev,

  • the greatest time traveler in human history.

  • This Russian cosmonaut holds the record

  • for the most amount of time

  • spent orbiting our planet,

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

  • During his stay in space,

  • he time traveled into his own future

  • by 0.02 seconds.

  • Traveling at 17,500 miles an hour,

  • he experienced an effect

  • known as time dilation,

  • and one day the same effect

  • might make significant time travel

  • to the future commonplace.

  • To see why moving faster through space

  • affects passage of time,

  • we need to go back to the 1880s,

  • when two American scientists,

  • Albert Michelson and Edward Morley,

  • were trying to measure the effect

  • of the Earth's movement around the Sun

  • on the speed of light.

  • When a beam of light was moving

  • in the same direction as the Earth,

  • they expected the light to travel faster.

  • And when the Earth was moving in the opposite direction,

  • they expected it to go slower.

  • But they found something very curious.

  • The speed of light remained the same

  • no matter what the Earth was doing.

  • Two decades later, Albert Einstein was thinking

  • about the consequences

  • of that never-changing speed of light.

  • And it was his conclusions,

  • formulated in the theory of special relativity,

  • that opened the door

  • into the world of time travel.

  • Imagine a man named Jack,

  • standing in the middle of a train carriage,

  • traveling at a steady speed.

  • Jack's bored

  • and starts bouncing a ball up and down.

  • What would Jill, standing on the platform,

  • see through the window

  • as the train whistles through.

  • Well, between Jack dropping the ball

  • and catching it again,

  • Jill would have seen him move

  • slightly further down the track,

  • resulting in her seeing the ball

  • follow a triangular path.

  • This means Jill sees the ball

  • travel further than Jack does

  • in the same time period.

  • And because speed is distance divided by time,

  • Jill actually sees the ball move faster.

  • But what if Jack's bouncing ball

  • is replaced with two mirrors

  • which bounce a beam of light between them?

  • Jack still sees the beam dropping down

  • and Jill still sees the light beam

  • travel a longer distance,

  • except this time Jack and Jill

  • cannot disagree on the speed

  • because the speed of light

  • remains the same no matter what.

  • And if the speed is the same

  • while the distance is different,

  • this means the time taken will be different as well.

  • Thus, time must tick at different rates

  • for people moving relative to each other.

  • Imagine that Jack and Jill have highly accurate watches

  • that they synchronize before Jack boarded the train.

  • During the experiment, Jack and Jill would each see

  • their own watch ticking normally.

  • But if they meet up again later

  • to compare watches,

  • less time would have elapsed on Jack's watch,

  • balancing the fact

  • that Jill saw the light move further.

  • This idea may sound crazy,

  • but like any good scientific theory,

  • it can be tested.

  • In the 1970s, scientists boarded a plane

  • with some super accurate atomic clocks

  • that were synchronized

  • with some others left on the ground.

  • After the plane had flown around the world,

  • the clocks on board showed a different time

  • from those left behind.

  • Of course, at the speed of trains and planes,

  • the effect is minuscule.

  • But the faster you go,

  • the more time dilates.

  • For astronauts orbiting the Earth for 800 days,

  • it starts to add up.

  • But what affects humans also affects machines.

  • Satellites of the global positioning system

  • are also hurdling around the Earth

  • at thousands of miles an hour.

  • So, time dilation kicks in here, too.

  • In fact, their speed causes

  • the atomic clocks on board

  • to disagree with clocks on the ground

  • by seven millionths of a second daily.

  • Left uncorrected,

  • this would cause GPS to lose accuracy

  • by a few kilometers each day.

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

  • to the far, distant future?

  • Well, the faster you go,

  • the greater the effect of time dilation.

  • If you could travel really close

  • to the speed of light, say 99.9999%,

  • on a round-trip through space

  • for what seemed to you like ten years,

  • you'd actually return to Earth

  • around the year 9000.

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

  • Humanity merged with machines,

  • extinct due to climate change

  • or asteroid impact,

  • or inhabiting a permanent colony on Mars.

  • But the trouble is,

  • getting heavy things like people,

  • not to mention space ships,

  • up to such speeds requires

  • unimaginable amounts of energy.

  • It already takes enormous particle accelerators

  • like the Large Hadron Collider

  • to accelerate tiny subatomic particles

  • to close to light speed.

  • But one day, if we can develop the tools

  • to accelerate ourselves to similar speeds,

  • then we may regularly send time travelers

  • into the future,

  • bringing with them tales of a long, forgotten past.

Have you ever daydreamed

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B1 TED-Ed jill speed travel time travel time dilation

【TED-Ed】Is time travel possible? - Colin Stuart

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    Kevin Lu posted on 2014/05/03
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