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Why Does Time Stop at the SPEED OF LIGHT? Feynman's Mind-Blowing Truth — Transcript

by Imagine the Physics · 2,345 words · 456 segments · language en · Watch on YouTube

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0:00Simple question. You've heard it before.
0:02Someone says if you travel fast enough,
0:05you age slower.
0:07Maybe you've seen it in a movie.
0:09Maybe someone explained it to you once
0:11and you nodded along.
0:13But do you actually know why?
0:15Not the Hollywood version, not the
0:18hand-wavy Einstein said so version.
0:21The real version.
0:23The version that when you actually
0:24understand it, makes you sit back and
0:26stare at the ceiling for a while.
0:29Because the answer is not what you
0:30think. It is not that space does
0:33something strange to your body. It is
0:35not radiation or zero gravity or some
0:37exotic effect that only happens far from
0:39Earth.
0:41It is something far more fundamental,
0:43far more unsettling.
0:46It is about what time actually is. Start
0:49with something simple.
0:51You are sitting in a chair right now,
0:53completely still,
0:55not moving. Except you are moving.
0:59Right now, sitting perfectly still, you
1:02are hurtling through space at roughly 30
1:04km per second because Earth is orbiting
1:06the Sun.
1:08The Sun itself is moving around the
1:09center of the Milky Way at about 230 km
1:12per second.
1:14The galaxy is moving through the
1:15universe at hundreds of kilometers per
1:17second more.
1:19You are never still,
1:21not for a single moment in your entire
1:23life.
1:24But here is the thing.
1:26None of that motion is what we are
1:28talking about. Because in physics,
1:31motion is relative.
1:33There is no fixed background against
1:35which to measure speed.
1:37You can only ask how fast something
1:39moves relative to something else.
1:43So forget the galaxy, forget the solar
1:45system. Let's get simple. Imagine you
1:48and a friend, empty space, nothing else.
1:52You are moving apart from each other at,
1:54say, half the speed of light.
1:58Who is moving?
1:59You might say you are both moving.
2:02Or you might say you are still and your
2:04friend is moving.
2:05Or your friend is still and you are
2:07moving.
2:08And here is the shocking part.
2:10You are all correct.
2:13Every single one of those answers is
2:15equally valid.
2:17There is no experiment you can perform,
2:20no measurement you can take that will
2:22tell you which one of you is really
2:24moving.
2:26Motion is not absolute. It is relative,
2:29always.
2:30This sounds like a philosophical nicety,
2:33a technicality, something that does not
2:35really matter in practice.
2:37It matters more than anything else in
2:39this story.
2:42Now let's talk about light.
2:44>> [clears throat]
2:44>> In the late 1800s, physicists were
2:47trying to measure the speed of light
2:49relative to Earth's motion.
2:51The idea was simple.
2:53If you are on a moving train and you
2:55throw a ball forward, the ball moves
2:58faster relative to the ground than it
3:00would if you threw it while standing
3:02still.
3:03The ball's speed plus the train's speed,
3:07basic addition.
3:09So light should work the same way,
3:11right?
3:12If Earth is moving through space and you
3:15measure the speed of light going in the
3:16direction of Earth's motion, it should
3:19be a little faster.
3:20And if you measure it going against
3:22Earth's motion, it should be a little
3:24slower.
3:26They measured it
3:27very carefully,
3:29multiple times, different directions,
3:32different times of year as Earth moved
3:34around the Sun.
3:36The speed of light was identical every
3:39single time in every single direction.
3:43It did not matter how fast Earth was
3:45moving.
3:46It did not matter which direction the
3:48light was going.
3:49The answer was always the same number.
3:52Always. 299,792,458
3:58m/s.
4:00Exactly.
4:02This broke physics.
4:04Not metaphorically.
4:06Literally broke the existing
4:08understanding of how reality works.
4:11Because if the speed of light is the
4:13same for everyone, regardless of their
4:15motion,
4:17then the universe is doing something
4:18deeply strange to make that happen.
4:22And Einstein sitting in a patent office
4:24in Switzerland in 1905
4:27figured out what that strange thing was.
4:30If the speed of light is always the
4:31same,
4:33and if motion is relative, then
4:35something else has to give.
4:37Something else has to change to keep
4:39that number constant.
4:41That something else is time.
4:44Here is the key idea.
4:46And I want you to sit with this because
4:48it is genuinely one of the most
4:50beautiful and disturbing things in all
4:53of science.
4:55You are always moving through space-time
4:59at the speed of light.
5:01Not space.
5:02Space-time.
5:04Einstein realized that space and time
5:07are not separate things. They are woven
5:10together into a single four-dimensional
5:12fabric.
5:14And everything in the universe is always
5:16moving through this fabric at exactly
5:19the speed of light.
5:20Always. No exceptions.
5:24But here is the twist.
5:27That motion is divided between space and
5:30time.
5:31If you are sitting completely still in
5:34space,
5:35all of your motion through space-time is
5:38going through time.
5:41You are aging at the maximum possible
5:43rate. Time is ticking as fast as it can
5:46for you.
5:48But if you start moving through space,
5:51some of that motion gets redirected.
5:54You are now spending some of your
5:56space-time speed on moving through
5:58space,
6:00which means you have less speed left
6:02over for moving through time.
6:05So, time slows down for you.
6:08The faster you move through space,
6:11the slower you move through time.
6:14These two things are directly,
6:16mathematically, unavoidably connected.
6:20It is not a side effect. It is not an
6:23approximation.
6:24It is the fundamental structure of
6:27reality.
6:28Your total speed through space-time is
6:32always exactly the speed of light.
6:35You are always spending it somewhere.
6:38The question is just how you divide it
6:41between space and time.
6:45Let's make this concrete, because
6:47abstract is fine, but numbers make it
6:49real.
6:51Imagine a spaceship.
6:53It leaves Earth and travels to a star 10
6:55light-years away at 99% of the speed of
6:59light.
7:00From Earth's perspective, that journey
7:02takes a little over 10 years.
7:04You watch the ship leave. You wait 10
7:07years. It arrives.
7:09But on the ship, the clocks tell a
7:11completely different story.
7:14Due to time dilation, the crew on that
7:17ship ages only about 1.4 years during
7:20the journey.
7:22While 10 years pass on Earth,
7:24less than a year and a half passes for
7:26the people on the ship.
7:28They did not cheat.
7:30They did not sleep through it.
7:33They did not experience anything
7:34unusual.
7:36Every second on that ship felt like a
7:39normal second to them.
7:41Their hearts beat normally.
7:43Their coffee got cold normally.
7:46Their hair grew normally.
7:49But fewer of those seconds passed
7:52because they were moving through space
7:54very fast. They were moving through time
7:57very slowly.
7:59Now push it further.
8:01Imagine the ship travels at 99.9999%
8:05of the speed of light.
8:07The same 10-light-year journey now takes
8:10the crew only about 5 days.
8:135 days of normal lived experience.
8:17Meanwhile, over 10 years pass on Earth.
8:21You leave Earth on a Monday.
8:23You arrive at a star 10 light-years away
8:26the following Saturday.
8:28You turn around, come back at the same
8:31speed. You walk off the ship 2 weeks
8:34older.
8:36Everyone you left behind is 20 years
8:39older.
8:40This is not science fiction.
8:42This is not speculation.
8:44This is measured, tested, confirmed
8:48physics.
8:49We have done it with atomic clocks on
8:51airplanes. We have done it with
8:53particles in accelerators.
8:55GPS satellites have to correct for time
8:58dilation every single day or the
9:01navigation system would drift by
9:03kilometers within hours.
9:06Time dilation is not theoretical. It is
9:09engineering.
9:10It is built into the technology you use
9:13right now.
9:16But now comes the question you should be
9:17asking.
9:19What happens at exactly the speed of
9:22light?
9:23If moving fast through space slows down
9:25your time, what happens when you reach
9:28maximum speed? When all of your
9:30space-time motion is going through space
9:33and none of it is going through time?
9:36Time stops.
9:39Not slows down a little.
9:41Not almost stops.
9:43Stops.
9:45Completely.
9:47A photon of light traveling at exactly
9:49the speed of light experiences zero time
9:53between the moment it is emitted and the
9:55moment it is absorbed.
9:57From the photon's perspective, if it had
10:00one,
10:01no time passes at all.
10:04Ever.
10:05A photon leaving a star a billion light
10:08years away and arriving at your eye
10:10right now experienced the journey as
10:13instantaneous.
10:15It did not wait a billion years. It did
10:18not age. It did not experience anything.
10:21It simply was at one place and then at
10:24another place with no time in between.
10:28This is why traveling at the speed of
10:30light does not age you.
10:32Because at the speed of light, there is
10:34no aging. There is no time.
10:37Time itself has been completely traded
10:40away for motion through space.
10:44You would not age because the very thing
10:46that drives aging, the flow of time,
10:49would have been redirected entirely into
10:51your motion.
10:53But of course you cannot actually reach
10:55the speed of light.
10:56Because you have mass.
10:58And as we discussed elsewhere, getting
11:01mass to light speed requires infinite
11:03energy.
11:05So this particular trick of timelessness
11:07is reserved for massless particles like
11:10photons.
11:12For everything with mass, time merely
11:14slows.
11:16It never stops.
11:19Now here is where it gets truly strange.
11:22>> [clears throat]
11:23>> Everything we have discussed so far,
11:26the time dilation, the way fast motion
11:29slows your clock,
11:30all of that has an equally bizarre
11:32partner.
11:34Space itself compresses.
11:38When you move very fast [music] through
11:39space, space in the direction of your
11:42motion shrinks.
11:45The faster [music] you go, the more it
11:47shrinks.
11:48At the speed of light, space in your
11:50direction of travel would compress to
11:53zero.
11:54This is called length contraction.
11:58And it is just as real as time dilation.
12:02So, from the crew's perspective on that
12:04fast spaceship, they are not just
12:06experiencing time slowly,
12:09the distance between Earth and the star
12:11has physically shrunk.
12:14The journey is not 10 light-years long
12:16for them. It is much shorter.
12:18That is why it takes less time.
12:21Not just because their time runs slow,
12:23but because the distance they have to
12:25cover has genuinely decreased.
12:28The universe literally rearranges its
12:31geometry around a fast-moving object.
12:35From outside watching the ship, you see
12:37it moving very fast and its clocks
12:40running slow.
12:42From inside the ship, the crew sees
12:44themselves as stationary and the
12:46universe compressed in the direction of
12:48travel.
12:49Both perspectives are equally valid.
12:52Both are equally real. Neither is wrong.
12:56This is what Einstein meant by
12:58relativity.
13:00Not that everything is subjective. Not
13:02that there is no truth.
13:05But that different observers moving at
13:06different speeds genuinely experience
13:09different versions of reality. And all
13:12of those versions are equally correct
13:14descriptions of the universe.
13:17Let's pause and think about what this
13:19means for your life right now.
13:22Right now, in this moment, there are
13:25billions of other observers in the
13:27universe moving at different speeds
13:29relative to you.
13:31And each of them is experiencing time at
13:33a slightly different rate.
13:36Each of them has a slightly different
13:39now.
13:40The concept of a single universal now, a
13:44single moment that everyone shares
13:46simultaneously, does not exist.
13:50It is an illusion created by the fact
13:52that we are all moving at roughly the
13:54same slow speeds on the surface of one
13:57planet.
13:59Two observers moving at different
14:00velocities will disagree about whether
14:02two events happened at the same time.
14:05Not because one of them made a mistake.
14:08Not because their instruments were
14:09faulty. But because simultaneity
14:13is relative.
14:14There is no absolute before and after in
14:18the universe. There is only before and
14:20after from a particular reference frame.
14:25This is not philosophy.
14:27This is not interpretation.
14:29This is what the equations say and what
14:32experiments confirm.
14:35Your intuition insists there is a single
14:37universal clock ticking away measuring
14:40out moments that everyone shares.
14:43There is not.
14:45There never was.
14:47The universe does not have one clock.
14:50Every object moving through it carries
14:53its own clock. And all those clocks run
14:56at different rates depending on how fast
14:58they are moving.
15:01So, why does none of this feel real to
15:03us?
15:05Because we are moving so slowly.
15:08At everyday human speeds, time dilation
15:11exists.
15:13But it is so tiny, it is completely
15:15undetectable to human senses.
15:18A commercial airplane moves at about 900
15:21km/h.
15:23At that speed, time dilation amounts to
15:25a few nanoseconds per hour.
15:28Your body cannot feel nanoseconds.
15:31Your brain cannot perceive nanoseconds.
15:34So, you never notice.
15:37But, the universe does not care whether
15:38you notice.
15:40It is happening
15:41right now.
15:43Your feet, slightly lower in Earth's
15:45gravitational field than your head,
15:48are aging very slightly more slowly than
15:50your head.
15:52This is real,
15:53measured,
15:55confirmed.
15:57At human scales, these effects are
15:59invisible.
16:00At relativistic speeds, they become
16:02impossible to ignore.
16:05At the speed of light, they become
16:06everything.
16:09Here is a thing that I want you to carry
16:10with you.
16:12Time is not a river flowing at a
16:14constant rate
16:15that everyone shares equally.
16:18Time is a dimension, like space.
16:22And you move through it the same way you
16:24move through space.
16:26And just like you can slow down your
16:28movement through space
16:30by redirecting your energy elsewhere,
16:33you can slow down your movement through
16:35time
16:36by redirecting it into motion through
16:39space.
16:41The universe gave you a fixed budget,
16:44a speed limit that is always exactly
16:47299 million 792,458
16:53m per second through space-time.
16:58You are always spending it.
17:00Every moment of your life, you are using
17:03it up.
17:04The only question is whether you are
17:06spending it on staying still and aging,
17:11or on moving fast
17:13and aging slowly.
17:15A photon spends all of it on motion,
17:19none on time.
17:21It does not age because it has traded
17:23every last bit of its space-time budget
17:26for speed.
17:28You spend almost all of it on time.
17:32You are nearly stationary on cosmic
17:34scales.
17:36You age at close to the maximum possible
17:38rate.
17:41And somewhere in between,
17:43there is a dial.
17:45Turn it toward speed and you age slower.
17:50Turn it all the way to the limit
17:52and you stop aging entirely.
17:56The universe is not keeping this from
17:58you.
17:59It is just showing you the price.

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