Name: 我一直不明白為什麼速度不能超過光速，直到現在我才明白！ (I never understood why you can't go faster than light - until now!)
Uploaded: 2024-11-19T05:02:20.000Z
Duration: 16 min 40 s

Why can't you ever accelerate any object to the speed of light?

為什麼不能將任何物體加速到光速？

I mean, we can go very close, but we can never ever reach the speed of light.

我的意思是，我們可以非常接近光速，但永遠不可能達到光速。

The most common explanation I got is if you take the graph of the kinetic energy versus the speed, look, as the speed approaches C, kinetic energy goes to infinity, so it takes infinite energy.

我得到的最常見的解釋是，如果用動能與速度的關係圖來表示，那麼當速度接近 C 時，動能就會達到無窮大，所以需要的能量是無限的。

But my problem is that that's not an explanation.

但我的問題是，這不是一種解釋。

I don't have an intuition behind why that is true.

Another explanation was mass goes to infinity, but again, there's no evidence for that.

另一種解釋是品質無窮大，但同樣沒有證據證明這一點。

What's the physically happening that's limiting that ship from going towards the speed of light?

是什麼物理現象限制了飛船向光速前進？

That's what we're gonna try and answer in this video.

這就是我們要在本視頻中嘗試回答的問題。

And so if you're ready for it, let's begin.

如果你準備好了，那就開始吧。

那麼，愛因斯坦，我們從哪裡開始呢？

Einstein says, imagine you are inside a very fast-moving ship and you have a photon clock with you.

愛因斯坦說，想象一下，你在一艘高速行駛的飛船裡，身邊有一個光子鍾。

Now at this point, I say, wait a second, Einstein.

此時此刻，我要說，等一下，愛因斯坦。

I don't wanna deal with hypothetical clocks.

愛因斯坦說，好吧，夥計，耐心點。

If you understand photon clocks, you'll be able to understand everything else.

如果理解了光子鍾，就能理解其他一切。

It's a clock where you have two mirrors and photon bounces between the two.

這是一個有兩面鏡子的時鐘，光子在兩面鏡子之間反彈。

And we can say that, hey, when the photon hits the bottom mirror, it hits a tick.

我們可以說，嘿，當光子擊中底部鏡面時，它擊中了一個刻度。

So we get tick, tick, tick, tick, tick, and so on and so forth.

於是，我們就有了嘀嗒、嘀嗒、嘀嗒、嘀嗒、嘀嗒，等等等等。

Now Einstein asks, what would the same clock look like if you are seeing the whole thing from outside the ship?

現在愛因斯坦要問，如果你從船外看到整個過程，同樣的時鐘會是什麼樣子？

At first, I'm like, hey, it looks the same to me.

一開始，我想，嘿，我看都一樣。

The photon is bouncing between the two mirrors.

But Einstein says, if you look carefully, you see the photon is now traveling a diagonal path.

但愛因斯坦說，如果你仔細觀察，就會發現光子現在正沿著對角線軌跡移動。

This means it's gonna travel a longer distance.

這意味著它的飛行距離會更長。

But remember, the photon always travels with the speed of light.

但請記住，光子始終以光速傳播。

See, the speed of light is the same in all reference frames, which means the photon now is gonna take a longer time between the ticks.

你看，光速在所有參照系中都是一樣的，這就意味著現在的光子在 "嘀 "和 "嘀 "之間需要更長的時間。

In other words, we will see that photon clock ticks slower.

換句話說，我們會看到光子時鐘的滴答聲變慢了。

Here's what the photon clock would look like from inside the ship.

And now what happens if the ship moves even faster?

現在，如果飛船移動得更快，會發生什麼呢？

Well, it will travel even longer path, as you will see.

那麼，它將走更長的路，你會看到的。

And therefore, the ticks will become even slower.

So the photon clock shows that clocks slow down when they are moving.

是以，光子時鐘表明，時鐘在運動時會變慢。

And at this point, I say, cool, photon clocks tick slower.

這時候，我說，酷，光子鐘的滴答聲變慢了。

Now what about real physical clocks, Einstein?

那麼真正的物理時鐘呢，愛因斯坦？

Well, Einstein says, if you look at this clock, and if you zoom in to the second hand, what would the animation look like from the atomic scale?

愛因斯坦說，如果你看這個時鐘，如果你放大秒針，從原子尺度上看，動畫會是什麼樣子？

And I say, well, if the second hand is just ticking like this, then the animation would look something like this.

我說，好吧，如果秒針像這樣滴答作響，那麼動畫就會像這樣。

但愛因斯坦說，這不可能發生。

And the reason is when, say, this atom accelerates, this atom in particular accelerates, all the other atoms cannot accelerate at the same time.

原因在於，當這個原子加速時，尤其是這個原子加速時，其他所有原子都無法同時加速。

These atoms are bonded electromagnetically.

這些原子以電磁方式結合在一起。

So when one atom accelerates, we need to wait for an electromagnetic wave to go from one atom to another, and only then the other atom can accelerate.

是以，當一個原子加速時，我們需要等待電磁波從一個原子傳到另一個原子，只有這樣，另一個原子才能加速。

An electromagnetic wave is basically a photon.

So the better way to think about this is, when this atom accelerates, it actually sends a photon, and only then the next atom accelerates, and then the next one, and the next one, and so on, and so forth.

是以，更好的思考方式是，當這個原子加速時，它實際上會發出一個光子，然後下一個原子才會加速，再下一個，再下一個，以此類推。

So you see, the whole thing is not instantly going, but it's so fast, it feels like it's instantly going.

所以，你看，整個過程並不是瞬間完成的，但它是如此之快，感覺就像是瞬間完成的。

Mahesh, what if now this set of atoms were moving?

馬赫什，如果現在這組原子在移動呢？

This also is very similar to the photon clock, which means if this was moving, the photons would end up taking a diagonal path again, and therefore the whole thing slows down.

這也與光子鍾非常相似，也就是說，如果它在移動，光子最終會再次走對角線路徑，是以整個速度會變慢。

The photon is not slowing down, the transfer is slowing down, because the photons are taking a diagonal path just like before, and therefore, real physical clocks actually slow down.

光子並沒有變慢，而是傳輸變慢了，因為光子走的是對角線路徑，就像以前一樣，是以，真正的物理時鐘實際上變慢了。

Look at this, this one is ticking slower than this one.

看看這個，這個的滴答聲比這個慢。

Real physical clocks slow down when they're moving.

真正的物理時鐘在移動時會變慢。

But at this point, we could ask Einstein, is there evidence for this?

但此時此刻，我們可以問愛因斯坦，有證據證明這一點嗎？

So Einstein says, well, probably not in his time, but today we do.

所以愛因斯坦說，也許在他那個時代不會，但今天我們會。

You see, we've actually taken atomic clocks, which are synced on Earth, and then we put them on planes, and then we flew around the world, and then we brought them back, they went out of sync.

你看，我們實際上是把地球上同步的原子鐘放在飛機上，然後飛到世界各地，再把它們帶回來，它們就不同步了。

And we did this multiple times, and every time we checked, we saw that the amount they went out of sync perfectly predicts, is perfectly predicted by special relativity.

我們這樣做了多次，每次檢查都發現，它們脫節的程度完全符合狹義相對論的預測。

Now, of course, gravity also affects it, which is dealt in general relativity, so there is this additional effect that's happening, but when you put all of that together, there is evidence that suggests that time dilation is real.

當然，萬有引力也會對其產生影響，這在廣義相對論中也有論述，是以還有額外的影響正在發生，但當你把所有這些放在一起時，有證據表明時間膨脹是真實存在的。

So with this, we know that time itself is slowing down, or is it?

是以，我們知道時間本身正在變慢，還是這樣？

You see, clocks are slowing down, but how does it tell us that the time itself is slowing down, Einstein?

愛因斯坦，你看，時鐘在變慢，但它怎麼能告訴我們時間本身在變慢呢？

Einstein says, for that, let's look at radioactivity.

愛因斯坦說，為此，讓我們來看看放射性。

Radioactive decay is where you have an unstable atom that spontaneously splits.

放射性衰變是指不穩定原子自發分裂。

The cool thing about radioactivity is that they have something called the half-life.

放射性的神奇之處在於它們有一種叫做半衰期的東西。

For example, if you take a particular radium isotope, it has a half-life of about 1,600 years, which means that if you take a billion radium isotopes, wait for 1,600 years, half of them would have decayed, and the rest half would have stayed.

例如，如果你使用一種特定的鐳同位素，它的半衰期約為 1600 年，這意味著如果你使用 10 億個鐳同位素，等待 1600 年，其中一半會衰變，剩下的一半會保持不變。

Wait for another 1,600 years, another half would have decayed, and so on and so forth.

再等 1600 年，又有一半會腐爛，如此循環往復。

Now, it turns out that if you take elementary particles like muons, they have a half-life of about 1 1⁄2 microseconds.

現在，事實證明，像μ介子這樣的基本粒子，它們的半衰期約為 1 1⁄2 微秒。

We did that in lab, we have tested this in labs, but we also get muons in our atmosphere due to the cosmic rays colliding with our atmosphere.

我們在實驗室裡做過，我們在實驗室裡測試過，但由於宇宙射線與我們的大氣層相撞，我們的大氣層中也會出現μ介子。

Turns out that these muons, when we looked at them, they have a half-life of 10 times more.

結果發現，當我們觀察這些μ介子時，它們的半衰期是原來的10倍。

At first, I'm like, Einstein, why is that a problem?

一開始，我想，愛因斯坦，這怎麼會是個問題呢？

Like, these are maybe different muons, and Einstein says, no.

比如，這些可能是不同的μ介子，而愛因斯坦說，不。

Just like how the charge of an electron stays the same, regardless of where the electrons come from, or when you do the experiment, or where you do it, the half-life of radioactive sample of a particular radioactive isotope would be the same, regardless of where that isotope comes from.

就像電子的電荷如何保持不變一樣，無論電子來自哪裡、何時做實驗、在哪裡做實驗，某種放射性同位素的放射性樣本的半衰期都是一樣的，無論該同位素來自哪裡。

So, how is it possible that these muons would have 10 times more half-life?

那麼，這些μ介子的半衰期怎麼可能多出 10 倍呢？

Well, it turns out that's because they're traveling close to speed of light.

原來，這是因為它們的速度接近光速。

And so, if you, again, plug in the numbers according to special relativity, you get the exact same result.

是以，如果你再次根據狹義相對論輸入數字，就會得到完全相同的結果。

Time dilation is literally making these muons age slower.

時間膨脹確實讓這些μ介子的老化速度變慢了。

Something very similar to what we saw in the photon clock.

與我們在光子鍾中看到的非常相似。

The fact that these muons are moving, this means that photons are taking longer, or whatever that force carrier, photons are not the force carrier, this is a weak nuclear force, but whatever the force carriers are, they will now take a longer time, and therefore the radioactive process gets slowed down.

事實上，這些μ介子在移動，這意味著光子需要更長的時間，或者說，不管是什麼力載體，光子不是力載體，這是一種弱核力，但不管是什麼力載體，它們現在都需要更長的時間，是以放射性過程會變慢。

And that's why muons are literally aging slower because they're moving.

這就是為什麼μ介子的老化速度會變慢，因為它們在移動。

And if muons can age slower, if radioactivity process can slow down, all biochemical processes can also slow down.

如果μ介子可以減緩衰老，如果放射性過程可以減緩，那麼所有的生化過程也可以減緩。

And that means living beings will age slower.

這意味著生物衰老的速度會變慢。

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我一直不明白為什麼速度不能超過光速，直到現在我才明白！ (I never understood why you can't go faster than light - until now!)

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