square meter of the earth's upper atmosphere . We don't really know where they come from,

地球上層大氣的平方米。我們真的不知道它們從哪裡來。

but we do know that when cosmic rays crash into air molecules in the atmosphere, they

但我們知道當宇宙射線撞擊到大氣中的空氣分子時

create a shower of other fundamental particles: pions, kaons, positrons, electrons, neutrons,

創造出其他基本粒子的淋浴： pions，kaons，positrons，電子，中子。

neutrinos, gamma and X rays, and muons.

中微子、伽馬射線和X射線以及μ子。

We know this because we have particle detectors in labs down on the surface that detect the

我們知道這一點，因為我們在實驗室裡有粒子探測器，可以探測到地表上的粒子。

directions and energies of the particles in these showers, and use them to study the original

粒子在這些雨中的方向和能量，並利用它們來研究原始的。

cosmic rays.

宇宙射線。

But there's something fascinating about the fact that we detect a lot of the muons

但有一些迷人的事實 我們檢測到很多的μ子

from cosmic rays down on the surface of the earth.

從宇宙射線照射到地球表面。

Because muons, if you make them in a laboratory, only have a 1.5 microsecond half life before

因為μ子，如果你在實驗室裡製造它們，只有1.5微秒的半衰期，然後才是

they spontaneously decay into an electron or positron and some neutrinos.

它們自發地衰變成電子或正電子和一些中微子。

Oh yeah, the greek symbol, mu is both used for “muon” AND for “microsecond”,

哦，對了，希臘符號mu既是 "μ子 "的意思，也是 "微秒 "的意思。

which can certainly be a little confusing; but the lifetime of muons is really close

這當然會讓人有些困惑；但μ子的壽命真的很接近。

to a microsecond, so it's also kind of beautifully appropriate/fitting.

到微秒，所以這也是一種美麗的合適/適合。

Anyway, the point is that if you have a bunch of muons, More specifically, if you have a

總之，重點是，如果你有一堆μ子，更具體地說，如果你有一個。

bunch of muons, you'll only be left with about 50% after 1.5 microseconds, and 25%

一堆μ子，1.5微秒後只剩下50%，25%的μ子。

after 3 microseconds, and after 10 microseconds there will only be 0.1% of the muons left.

3微秒後，10微秒後將只剩下0.1%的μ子。

Muons don't live very long -2.2 microseconds on average!

繆子的壽命不長，平均為-2.2微秒!

To put that into perspective, light, which travels fast enough that it could go around

從這個角度看，光的速度快到可以繞行的程度

the earth 7 times in a second, only travels 660 meters, or less than half a mile, in 2.2

一秒鐘走了七次，只走了660米，也就是不到半英里，在2.2秒內走完了。

microseconds.

微秒。

So even muons traveling at essentially the speed of lighta , wouldn't make it more

所以，即使μ子以光速飛行，也不會使它更加

than a kilometer or two before the vast majority of them decayed . Which is far less than the

超過一兩公里後，它們中的絕大部分才會腐爛。這遠遠低於

10 or 20 or 30 kilometers that muons DO regularly travel from the upper atmosphere to the ground.

10或20或30公里，μ子確實定期從高層大氣到地面。

So how do muons travel dozens of kilometers through the atmosphere without spontaneously

那麼，μ子是如何在大氣層中穿行幾十公里而不自發的呢？

decaying, when in fact they should only be able to travel less than one kilometer?

腐爛，而事實上它們應該只能行駛不到一公里?

Time dilation.

時間膨脹。

Yes - because the muons are traveling close to the speed of light, their time literally

是的 - 因為μ子的速度接近光速，他們的時間從字面上看

passes more slowly - at a speed of 99.5% the speed of light, 2.2 microseconds for them

通過更慢--速度為光速的99.5%，對它們來說是2.2微秒。

would be ~22 microseconds for us , enough time for the average muon to travel at least

對我們來說，將是〜22微秒，足夠時間為平均μ子旅行至少

6km (instead of half of a kilometer) before decaying.

6公里（而不是半公里）才衰減。

And even higher-energy muons going even faster would even more easily reach our detectors

而更高能量的μ子的速度更快，更容易到達我們的探測器。

on the earth's surface before they decayed - at 99.995% the speed of light, the average

在它們衰變之前，地球表面上--在99.995%的光速下，它們的平均壽命為1年。

muon would live for 220 microseconds and travel at least 66 kilometers before decaying.

μ子的壽命為220微秒，在衰變前至少要行駛66公里。

So from our perspective, the fact that so many cosmic ray muons reach our detectors

是以，從我們的角度來看，事實上，這麼多的宇宙射線μ子到達我們的探測器。

on the earth's surface is direct evidence for special relativity and time dilation!

在地球表面是狹義相對論和時間膨脹的直接證據!

But what about from the muons' perspectives, where they DO only live on average 2.2 microseconds?

但從μ子的角度來看呢，它們的平均壽命只有2.2微秒？

Well, for them the answer to the apparent paradox is also relativistic - relativistic

好吧，對他們來說，表面矛盾的答案也是相對論--相對論。

length contraction.

長度收縮。

From the muon's perspective, it's the earth and the atmosphere which are moving

從μ子的角度來看，是地球和大氣層在移動。

- at 99.995% the speed of light - towards the muon.

- 以99.995%的光速--向μ子。

And the lengths of moving objects are literally contracted by a factor dependent on their

而運動物體的長度實際上是由一個取決於它們的因素收縮的

speed - in this case, 50km of our atmosphere is, to the muon, literally only half a kilometer

在這種情況下，我們大氣層的50公里，對μ子來說，實際上只有半公里。

- aka 500 meters - thick.

- 也就是500米--厚。

Which is thin enough for even a muon with a lifetime of 2.2 microseconds to traverse

哪怕是一個壽命為2.2微秒的μ子，也足夠薄到可以穿越。

- well, actually from this perspective the atmosphere moves past the muon - but at a

- 事實上，從這個角度來看，大氣層移動經過μ子 - 但在一個很好的角度

speed of 300 meters per microsecond and at a distance of only 500 meters, the ground

的速度，每微秒300米，而在距離只有500米的時候，地

has no problem reaching the muon before the muon decays.

在μ子衰變之前，達到μ子沒有問題。

This, in my mind, is one of the most awesome experimental verifications of special relativity:

在我看來，這是狹義相對論最厲害的實驗驗證之一。

the unequivocal time dilation (or length contraction, depending on your perspective) for objects

時空膨脹

moving close to the speed of light.

以接近光速的速度運動。

The specific time dilation and length contraction

具體的時間擴張和長度收縮

factors I talked about can be calculated using the time dilation and length contraction formulas

我說過的因素可以用時間膨脹和長度收縮公式來計算。

- once you know how to use them, you can plug in any speed you want and see how much distances

- 一旦你知道如何使用它們，你可以插入任何你想要的速度，看看有多少距離。

and time intervals will be distorted.

和時間間隔將被扭曲。

And Brilliant.org, this video's sponsor, is a great place to learn about not just the

而本視頻的贊助商Brilliant.org，是一個很好的地方，不僅可以學習到關於

details of time dilation and length contraction, but many of the other amazing equations that

時間膨脹和長度收縮的細節，但許多其他驚人的方程，都是在時間膨脹和長度收縮的基礎上進行的。

describe our universe.

描述我們的宇宙。

Like, they have a course that leads you towards understanding the Schrodinger equation of

就像，他們有一個課程，引導你走向瞭解薛定諤方程的。

quantum mechanics, and one on Hubble's law in astronomy, and the famous Bayes' theorem

量子力學，以及天文學中的哈勃定律，還有著名的貝葉斯定理。

of probability and statistics.

概率和統計學；

And the first 200 people who go to brilliant.org/minutephysics will get 20% off a premium subscription to

前200名訪問 brilliant.org/minutephysics 的人將獲得高級訂閱20%的優惠

brilliant with access to all of brilliant's courses and puzzles.

輝煌與訪問所有輝煌的課程和拼圖。

Again, that's brilliant.org/minutephysics for a deeper understanding of the equations

同樣，這是輝煌.org/分鐘物理學 更深層次的理解方程。

(and not just the concepts) that underlie our universe.

(而不僅僅是概念)，是我們宇宙的基礎。