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Every second, thousands of cosmic rays - mostly hydrogen and helium nuclei - strike every square meter of the earth’s upper atmosphere.
每一秒钟，成千上万的宇宙射线——主要是氢和氦核——撞击地球上层大气的每一平方米。
We don’t really know where they come from, but we do know that when cosmic rays crash into air molecules in the atmosphere, they create a shower of other fundamental particles:
我们并不知道它们从何而来，但我们确实知道，当宇宙射线撞击到大气中的空气分子时，它们会产生一系列其他基本粒子:
pions, kaons, positrons, electrons, neutrons, neutrinos, gamma and X rays, and muons.
介子，K中介子 ，正电子，电子，中子，中微子，伽马射线和X射线以及μ介子。
We know this because we have particle detectors in labs down on the surface that detect the directions and energies of the particles in these showers, and use them to study the original cosmic rays.
我们知道这一点，是因为我们在地面的实验室里有粒子探测器，可以探测到这些粒子簇中的粒子的方向和能量，并利用它们来研究原始的宇宙射线。
But there’s something fascinating about the fact that we detect a lot of the muons from cosmic rays down on the surface of the earth.
但有趣的是，我们从地球表面的宇宙射线中探测到了很多μ介子。
Because muons, if you make them in a laboratory, only have a 1.5 microsecond half life before they spontaneously decay into an electron or positron and some neutrinos.
因为μ介子，如果你在实验室里制造它们，在它们自发衰变为电子或正电子和一些中微子之前，只有1.5微秒的半衰期。
Oh yeah, the greek symbol, mu is both used for “muon” AND for “microsecond”, which can certainly be a little confusing;
哦，对了，希腊符号mu既代表“μ介子”也代表“微秒”，这肯定会让人有点困惑;
but the lifetime of muons is really close to a microsecond, so it’s also kind of beautifully appropriate/fitting.
但是介子的寿命非常接近一微秒，所以这也是非常合适的。
Anyway, the point is that if you have a bunch of muons, more specifically, if you have a bunch of muons,
总之，关键是如果你有一堆μ介子，具体地说，如果你有一堆μ介子，
you’ll only be left with about 50% after 1.5 microseconds, and 25% after 3 microseconds, and after 10 microseconds there will only be 0.1% of the muons left.
1.5微秒后你只剩下50%，3微秒后只剩下25%，10微秒后只剩下0.1%的μ介子。
Muons don’t live very long -2.2 microseconds on average!
μ介子的寿命不长——平均2.2微秒!
To put that into perspective, light, which travels fast enough that it could go around the earth 7 times in a second, only travels 660 meters, or less than half a mile, in 2.2 microseconds.
从这个角度来看，光的速度足够快，每秒可以绕地球7圈，但在2.2微秒内只走了660米，或不到半英里。
So even muons traveling at essentially the speed of lighta , wouldn’t make it more than a kilometer or two before the vast majority of them decayed.
所以即使μ介子以光速运动，在绝大多数衰变前也不会超过一到两公里。
Which is far less than the 10 or 20 or 30 kilometers that muons do regularly travel from the upper atmosphere to the ground.
远低于μ介子从高层大气到地面的10公里、20公里或30公里。
So how do muons travel dozens of kilometers through the atmosphere without spontaneously decaying, when in fact they should only be able to travel less than one kilometer?
那么，μ介子是如何在大气层中传播几十公里而不自然衰变的呢?而事实上，它们应该只能传播不到一公里。
Time dilation.
时间膨胀。
Yes - because the muons are traveling close to the speed of light, their time literally passes more slowly
是的，因为μ介子的速度接近光速，所以它们的时间确实过得更慢
- at a speed of 99.5% the speed of light, 2.2 microseconds for them would be 22 microseconds for us , enough time for the average muon to travel at least 6km (instead of half of a kilometer) before decaying.
在99.5%光速的速度下，2.2微秒对它们来说是我们的22微秒，这足够让介子在衰变之前平均行进至少6公里(而不是半公里)。
And even higher-energy muons going even faster would even more easily reach our detectors on the earth’s surface before they decayed- at 99.995% the speed of light,
即使是能量更高、速度更快的μ介子，也更容易在衰变前到达地球表面的探测器——衰变速度为光速的99.995%，
the average muon would live for 220 microseconds and travel at least 66 kilometers before decaying.
μ介子的平均寿命为220微秒，衰变前至少能移动66公里。
So from our perspective, the fact that so many cosmic ray muons reach our detectors on the earth’s surface is direct evidence for special relativity and time dilation!
所以从我们的角度来看，如此多的宇宙射线介子到达地球表面的探测器是狭义相对论和时间膨胀的直接证据!
But what about from the muons’ perspectives, where they DO only live on average 2.2 microseconds?
但从μ介子的角度来看，它们的平均寿命只有2.2微秒呢?
Well, for them the answer to the apparent paradox is also relativistic - relativistic length contraction.
对他们来说，这个明显的悖论的答案也是相对论性的——相对论长度收缩。
From the muon’s perspective, it’s the earth and the atmosphere which are moving - at 99.995% the speed of light - towards the muon.
从μ介子的角度来看，是地球和大气层以99.995%的光速向介子移动。
And the lengths of moving objects are literally contracted by a factor dependent on their speed - in this case, 50km of our atmosphere is, to the muon, literally only half a kilometer - aka 500 meters - thick.
运动物体的长度实际上是由它们的速度决定的——在这种情况下，我们的大气层50公里，对于μ介子来说，实际上只有半公里——也就是500米深。
Which is thin enough for even a muon with a lifetime of 2.2 microseconds to traverse
哪一个足够薄，哪怕是一个μ介子的生命周期为2.2微秒
- well, actually from this perspective the atmosphere moves past the muon
嗯，实际上，从这个角度来看，大气穿过了μ介子
- but at a speed of 300 meters per microsecond and at a distance of only 500 meters, the ground has no problem reaching the muon before the muon decays.
—但以每微秒300米的速度和仅500米的距离，在μ介子衰变之前，地面没有问题可以到达μ介子。
This, in my mind, is one of the most awesome experimental verifications of special relativity:
在我看来，这是狭义相对论最令人敬畏的实验验证之一:
the unequivocal time dilation (or length contraction, depending on your perspective) for objects moving close to the speed of light.
速度接近光速的物体的时间膨胀(或长度收缩，取决于你的观点)。
The specific time dilation and length contraction factors I talked about can be calculated using the time dilation and length contraction formulas
我所说的具体的时间膨胀和长度收缩因子可以用时间膨胀和长度收缩公式来计算
- once you know how to use them, you can plug in any speed you want and see how much distances and time intervals will be distorted.
一旦你知道如何使用它们，你可以计算任何你想要的速度，看看多少距离和时间间隔将被扭曲。
