쉐도잉 연습: Why Does Time Stop at the SPEED OF LIGHT? Feynman's Mind-Blowing Truth - 영상으로 영어 말하기 배우기
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Simple question.
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You've heard it before.
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Someone says if you travel fast enough, you age slower.
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Maybe you've seen it in a movie.
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Maybe someone explained it to you once and you nodded along.
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But do you actually know why?
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Not the Hollywood version, not the hand-wavy Einstein said-so version.
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The real version.
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The version that when you actually understand it, makes you sit back and stare at the ceiling for a while.
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Because the answer is not what you think.
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It is not that space does something strange to your body.
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It is not radiation or zero gravity or some exotic effect that only happens far from earth.
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It is something far more fundamental, far more unsettling.
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It is about what time actually is.
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Start with something simple.
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You are sitting in a chair right now, completely still, not moving, except you are moving.
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Right now, sitting perfectly still, you are hurtling through space at roughly 30 kilometers per second because Earth is orbiting the sun.
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The sun itself is moving around the center of the Milky Way at about 230 kilometers per second.
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The galaxy is moving through the universe at hundreds of kilometers per second more.
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You are never still, not for a single moment in your entire life.
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But here is the thing.
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None of that motion is what we are talking about, because in physics motion is relative.
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There is no fixed background against which to measure speed.
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You can only ask how fast something moves relative to something else.
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So forget the galaxy.
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Forget the solar system.
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Let's get simple.
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Imagine you and a friend, empty space, nothing else.
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You are moving apart from each other at, say, half the speed of light.
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Who is moving?
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You might say you are both moving.
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Or you might say you are still and your friend is moving.
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Or your friend is still and you are moving.
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And here is the shocking part.
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You are all correct.
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Every single one of those answers is equally valid.
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There is no experiment you can perform, no measurement you can take, that will tell you which one of you is really moving.
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Motion is not absolute, it is relative, always.
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This sounds like a philosophical nicety, a technicality, something that does not really matter in practice.
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It matters more than anything else in this story.
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Now let's talk about light.
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In the late 1800s, physicists were trying to measure the speed of light relative to Earth's motion.
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The idea was simple.
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If you are on a moving train and you throw a ball forward, the ball moves faster relative to the ground than it would if you threw it while standing still.
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The ball speed plus the train speed, basic addition.
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So light should work the same way, right?
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If Earth is moving through space and you measure the speed of light going in the direction of Earth's motion, it should be a little faster.
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And if you measure it going against Earth's motion, it should be a little slower.
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They measured it very carefully, multiple times, different directions, different times of year as Earth moved around the sun.
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The speed of light was identical every single time, in every single direction.
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It did not matter how fast Earth was moving.
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It did not matter which direction the light was going.
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The answer was always the same number,
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always 299,792,458 meters per second.
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Exactly.
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This broke physics.
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not metaphorically, literally broke the existing understanding of how reality works.
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Because if the speed of light is the same for everyone,
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regardless of their motion, then the universe is doing something deeply strange to make that happen.
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And Einstein, sitting in a patent office in Switzerland in 1905, figured out what that strange thing was.
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If the speed of light is always the same and if motion is relative, then something else has to give.
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Something else has to change to keep that number constant.
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That something else is time.
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Here is the key idea, and I want you to sit with this
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because it is genuinely one of the most beautiful and disturbing things in all of science.
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You are always moving through space-time at the speed of light.
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Not space, space-time.
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Einstein realized that space and time are not separate things.
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They are woven together into a single four-dimensional fabric,
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and everything in the universe is always moving through this fabric at exactly the speed of light.
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Always.
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No exceptions.
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But here is the twist.
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That motion is divided between space and time.
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If you are sitting completely still in space, all of your motion through space-time is going through time.
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You are aging at the maximum possible rate.
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Time is ticking as fast as it can for you.
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But if you start moving through space, some of that motion gets redirected.
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You are now spending some of your space-time speed on moving through space,
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which means you have less speed left over for moving through time.
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So time slows down for you.
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The faster you move through space, the slower you move through time.
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These two things are directly, mathematically, unavoidably connected.
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It is not a side effect.
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It is not an approximation.
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It is the fundamental structure of reality.
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Your total speed through space-time is always exactly the speed of light.
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You are always spending it somewhere.
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The question is just how you divide it between space and time.
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Let's make this concrete because abstract is fine, but numbers make it real.
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Imagine a spaceship.
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It leaves Earth and travels to a star 10 light years away at 99% of the speed of light.
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From Earth's perspective, that journey takes a little over 10 years.
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You watch the ship leave, you wait 10 years, it arrives.
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But on the ship, the clocks tell a completely different story.
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Due to time dilation, the crew on that ship ages only about 1.4 years during the journey.
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While 10 years pass on Earth, less than a year and a half passes for the people on the ship.
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They did not cheat.
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They did not sleep through it.
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They did not experience anything unusual.
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Every second on that ship felt like a normal second to them.
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Their hearts beat normally.
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Their coffee got cold normally.
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Their hair grew normally.
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But fewer of those seconds passed because they were moving through space very fast.
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They were moving through time very slowly.
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Now push it further.
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the ship travels at 99.9999% of the speed of light.
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The same 10 light-year journey now takes the crew only about five days, five days of normal lived experience.
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Meanwhile, over 10 years pass on Earth.
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You leave Earth on a Monday.
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You arrive at a star 10 light-years away the following Saturday.
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You turn around, come back at the same speed.
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You walk off the ship two weeks older.
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Everyone you left behind is 20 years older.
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This is not science fiction.
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This is not speculation.
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This is measured, tested, confirmed physics.
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We have done it with atomic clocks on airplanes.
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We have done it with particles and accelerators.
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GPS satellites have to correct for time dilation every single day or the navigation system would drift by kilometers within hours.
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Time dilation is not theoretical.
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It is engineering.
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It is built into the technology you use right now.
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But now comes the question you should be asking.
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What happens at exactly the speed of light?
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If moving fast through space slows down your time, what happens when you reach maximum speed?
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When all of your space-time motion is going through space and none of it is going through time?
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Time stops.
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Not slows down a little, not almost stops, stops completely.
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A photon of light traveling at exactly the speed of light experiences zero time between the moment it is emitted
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and the moment it is absorbed.
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From the photon's perspective, if it had one, no time passes at all, ever.
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A photon leaving a star a billion light years away and arriving at your eye right now experienced the journey as instantaneous.
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It did not wait a billion years.
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It did not age.
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It did not experience anything.
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It simply was at one place and then at another place with no time in between.
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This is why traveling at the speed of light does not age you, because at the speed of light there is no aging.
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There is no time.
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Time itself has been completely traded away for motion through space.
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You would not age because the very thing that drives aging, the flow of time, would have been redirected entirely into your motion.
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But of course you cannot actually reach the speed of light because you have mass.
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And as we discussed elsewhere, getting mass to light speed requires infinite energy.
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So this particular trick of timelessness is reserved for massless particles like photons.
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For everything with mass, time merely slows.
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It never stops.
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Now here is where it gets truly strange.
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Everything we have discussed so far, the time dilation, the way fast motion slows your clock,
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all of that has an equally bizarre partner.
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Space itself compresses.
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When you move very fast through space, space in the direction of your motion shrinks.
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The faster you go, the more it shrinks.
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At the speed of light, space in your direction of travel would compress to zero.
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This is called length contraction, and it is just as real as time dilation.
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So from the crew's perspective on that fast spaceship, they are not just experiencing time slowly,
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the distance between Earth and the star has physically shrunk.
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The journey is not ten light years long for them.
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It is much shorter.
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That is why it takes less time, not just because their time runs slow, but because the distance they have to cover has genuinely decreased.
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The universe literally rearranges its geometry around a fast-moving object.
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From outside, watching the ship, you see it moving very fast and its clocks running slow.
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From inside the ship, the crew sees themselves as stationary and the universe compressed in the direction of travel.
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Both perspectives are equally valid.
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Both are equally real.
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Neither is wrong.
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This is what Einstein meant by relativity.
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Not that everything is subjective, not that there is no truth,
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but that different observers moving at different speeds genuinely experience different versions of reality.
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And all of those versions are equally correct descriptions of the universe.
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Let's pause and think about what this means for your life right now.
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Right now, in this moment, there are billions of other observers in the universe moving at different speeds relative to you.
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And each of them is experiencing time at a slightly different rate.
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Each of them has a slightly different now.
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The concept of a single universal now, a single moment that everyone shares simultaneously does not exist.
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It is an illusion created by the fact
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that we are all moving at roughly the same slow speeds on the surface of one planet.
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Two observers moving at different velocities will disagree about whether two events happened at the same time, not because one of them made a mistake,
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not because their instruments were faulty, but because simultaneity is relative.
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There is no absolute before and after in the universe.
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There is only before and after from a particular reference frame.
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This is not philosophy.
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This is not interpretation.
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This is what the equations say and what experiments confirm.
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Your intuition insists there is a single universal clock ticking away, measuring out moments that everyone shares.
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There is not.
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There never was.
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The universe does not have one clock.
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Every object moving through it carries its own clock, and all those clocks run at different rates depending on how fast they are moving.
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So why does none of this feel real to us?
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Because we are moving so slowly.
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At everyday human speeds, time dilation exists.
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But it is so tiny it is completely undetectable to human senses.
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A commercial airplane moves at about 900 kilometers per hour.
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At that speed, time dilation amounts to a few nanoseconds per hour.
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Your body cannot feel nanoseconds.
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Your brain cannot perceive nanoseconds, so you never notice.
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But the universe does not care whether you notice.
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It is happening right now.
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Your feet, slightly lower in Earth's gravitational field than your head, are aging very slightly more slowly than your head.
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This is real, measured, confirmed.
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At human scales, these effects are invisible.
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At relativistic speeds, they become impossible to ignore.
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At the speed of light, they become everything.
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Here is a thing that I want you to carry with you.
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Time is not a river flowing at a constant rate that everyone shares equally.
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Time is a dimension like space, and you move through it the same way you move through space.
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And just like you can slow down your movement through space by redirecting your energy elsewhere,
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you can slow down your movement through time by redirecting it into motion through space.
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The universe gave you a fixed budget,
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a speed limit that is always exactly 299,792,458 meters per second through space-time.
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You are always spending it.
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Every moment of your life, you are using it up.
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The only question is whether you are spending it on staying still and aging, or on moving fast and aging slowly.
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A photon spends all of it on motion, none on time.
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It does not age because it has traded every last bit of its space-time budget for speed.
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You spend almost all of it on time.
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You are nearly stationary on cosmic scales.
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You age at close to the maximum possible rate.
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And somewhere in between, there is a dial.
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Turn it toward speed, and you age slower.
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Turn it all the way to the limit, and you stop aging entirely.
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The universe is not keeping this from you.
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It is just showing you the price.
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이 영상의 문법
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| 문형 | 영상 속 표현 |
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| 현재완료 have/has + 과거분사 — 과거의 일이 지금도 관련이 있을 때 | You've heard · you've seen · have done |
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