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  • Every second, thousands of cosmic rays - mostly hydrogen and helium nuclei - strike every

    每一秒鐘,成千上萬的宇宙射線--大部分是氫核和氦核--都會擊中每一個。

  • 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 formuonAND formicrosecond”,

    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。

Every second, thousands of cosmic rays - mostly hydrogen and helium nuclei - strike every

每一秒鐘,成千上萬的宇宙射線--大部分是氫核和氦核--都會擊中每一個。

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不可能的μ子 (Impossible Muons)

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    Summer 發佈於 2020 年 11 月 03 日
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