Name: 最も神秘的な性質「時間」を5段階のレベルで解説 | 5 Levels | WIRED Japan
Uploaded: 2025-01-22T09:46:13.000Z
Duration: 31 min 26 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

I'm Brian Green, and today I have been challenged to explain a topic in five levels of difficulty.

Future continuous

We're going to be talking about the nature of time, the most familiar and the most mysterious quality of the physical universe.

There is nothing that we experience that does not take place in some duration of time.

So if you can understand time, you're on your way to understanding reality.

Hello.  What's your name?  How old are you, Kayla?

So if you're nine years old, what does that mean about the earth?

How many times has it gone around the sun?

So there's a relationship between motion through space, the earth is going through space, and the passage of time.

If I asked you to move through space, you could do it freely, right?

And let's see how easy it is to move through space.

If I were to ask you to sit perfectly still in space, can you do that?

But if I ask you to hold still in time, to not go to the next second or the next second, can you do that?

So there's definitely this difference between space and time, some fundamental quality that distinguishes how freely we can move through space versus how freely we can move through time.

He does have crazy hair, and I think I may be heading in that direction, actually.

He showed us an approach to travel to the future.

You turn around and you come back to planet Earth.

And he showed us that when you're on that ship, your clock will tick off time more slowly.

So that journey may only take you, say, a year, six months out and six months back.

When you step out of the ship, it'll be 100 years into the future or 1,000 years, a million years into the future.

Everybody else would be dead who stayed on Earth, but your body would only age one year, and yet it would be 1,000 years into the future.

The question, though, is could you get back?

We don't know if you can travel back, but we certainly know that you can travel forward.

Has anyone ever tried to go forward and back?

That same guy with the crazy hair, Albert Einstein, showed that there's actually a limit to how fast things can go.

The limit is the speed of light, because light travels 671 million miles per hour.

That is fast enough to go around the entire Earth seven times in one second.

So if we could build a spaceship that would go as fast as light, we'd be able to do what Einstein noted.

There's something else that's really curious about time.

Things tend to go in one direction, and we call it the arrow of time.

It sort of points from the past into what we call the future.

If you were to ask me why is there an arrow to time, ask me.

I have some ideas, but I'd say we've still not completely nailed it down.

Kayla, what have you learned about time from talking about it here?

That you can't really travel back through time.

I think it's unlikely we'll learn how to travel to the past, but it's not been ruled out.

That's kind of exciting, that it's still at least an open possibility.

If I was to ask you what is time, what would you say?

Time is kind of strange, because it's almost a man-made idea.

There is the tangible of, you know, how the earth revolves around the sun, or how we orbit around ourselves.

It's almost in a way, does it exist if the way that we measure it is man-made?

Before there was any life on planet Earth, I think we all agree the universe existed, did things change before there was life on Earth?

And how would you talk about that change without invoking this concept of time?

It's difficult to talk about something without adding time into it.

Even if it is a human-made concept, that the universe evolved, developed, changed through time, ultimately giving rise to galaxies, stars, planets, and on this particular planet, life.

That conception of time gives a feel that it's like universal, that it's out there, it's the same for everybody, it's independent of our actions or activities.

Do you know that Albert Einstein shattered that view of time?

He found that if you and I, say, have identical wristwatches, I'm sitting still and I'm watching you move, I will find that your clock is ticking off time more slowly than my clock.

You can figure out this quality of time if you know one fact, that the speed of light is constant.

Yeah, if you're clever, you can use that with high school algebra, maybe even a little high school trigonometry to make it even easier, to derive that clocks tick off time at different rate.

Do you want me to show you how that goes?

All right, so to figure out the effect of motion on time, I'm gonna use a really simple clock.

It's two mirrors that are facing each other, and what we do is we have a little ball of light called a photon, right, that goes up, hits the top mirror, then comes back down and hits the bottom mirror.

And every time it does that, we go tick-tock, that's one unit of time.

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最も神秘的な性質「時間」を5段階のレベルで解説 | 5 Levels | WIRED Japan

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