At about six o'clock in the morning on September 14, 2015,
2015年9月14日清晨6点左右，
scientists witnessed something no human had ever seen: two black holes colliding.
科学家观测到了一个人类从未看到的现象：两个黑洞相撞。
Both about 30 times as massive as our Sun, they had been orbiting each other for millions of years.
这两个黑洞的质量均相当于太阳的30倍，它们已经围绕对方旋转了数百万年。
As they got closer together, they circled each other faster and faster.
它们逐渐靠近，旋转速度也越来越快。
Finally, they collided and merged into a single, even bigger, black hole.
最终，它们撞了在一起，并融合为一个更大的黑洞。
A fraction of a second before their crash, they sent a vibration across the universe at the speed of light.
就在他们即将相撞之前，它们以光速，向宇宙发出了振动。
And on Earth, billions of years later,
而数十亿年后，在地球上，
a detector called the Laser Interferometer Gravitational Wave Observatory, or LIGO for short, picked it up.
一个简写为“LIGO”的激光干涉引力波探测器检测到了这次振动。
The signal only lasted a fifth of a second and was the detector's first observation of gravitational waves.
振动信号仅持续了0.2秒，也是LIGO首次观测到的引力波。
What are these ripples in space? The answer starts with gravity, the force that pulls any two objects together.
这些宇宙中的涟漪到底是什么？让我们从引力说起，这种拉近两个物体的力量。
That's the case for everything in the observable universe.
在可观测的宇宙中，它无处不在。
You're pulling on the Earth, the Moon, the Sun, and every single star, and they're pulling on you.
你正在拉动地球、月球、太阳以及宇宙中每颗星星，它们同时也在拉动你。
The more mass something has, the stronger its gravitational pull.
物体的质量越大，它产生的引力就越大。
The farther away the object, the lower its pull.
物体间的距离越大，引力就越小。
If every mass has an effect on every other mass in the universe, no matter how small,
假设在宇宙中，任何物质都对其他物质均有引力，无论这物质多么微小，
then changes in gravity can tell us about what those objects are doing.
引力的变化都能告诉我们，这些物质在做什么。
Fluctuations in the gravity coming from the universe are called gravitational waves.
宇宙中传来的引力的波动，就是所谓的引力波。
Gravitational waves move out from what caused them, like ripples on a pond,
引力波从碰撞点向外扩散，就像水面的涟漪，
getting smaller as they travel farther from their center. But what are they ripples on?
远离涟漪中心的同时，强度也会逐步衰减。但是传递引力波的介质是什么呢？
When Einstein devised his Theory of Relativity, he imagined gravity as a curve in a surface called space-time.
当爱因斯坦提出广义相对论时，他将引力比拟为时空面上的一条曲线。
A mass in space creates a depression in space-time,
空间中有质量的物体，让时空产生了弯曲，
and a ball rolling across a depression will curve like it's being attracted to the other mass.
另一物体穿过弯曲区域时，便会改变自身原有轨迹，就像被其他物体吸引过去一样。

The bigger the mass, the deeper the depression and the stronger the gravity.
物体的质量越大，时空的弯曲也就越明显，该物体产生的引力也就越强。
When the mass making the depression moves, that sends out ripples in space-time.
当该物体发生运动时，便在时空中产生了“涟漪”。
These are gravitationl waves. What would a gravitational wave feel like?
这些“涟漪”就是引力波。引力波会让人产生什么感觉呢？
If our bodies were sensitive enough to detect them,
如果人体足够敏感，能够感知引力波的话，
we'd feel like we were being stretched sideways while being compressed vertically.
我们会感觉身体被水平拉伸，同时被竖向挤压。
And in the next instant, stretched up and down while being compressed horizontally, sideways, then up and down.
接下来，身体会被竖向拉伸、并水平挤压；水平拉伸、竖向挤压，再是竖向拉伸、水平挤压。
This back and forth would happen over and over as the gravitational wave passed right through you.
这个过程周而复始，这就是引力波穿过人体的感觉。
But this happens on such a minute scale that we can't feel any of it.
不过，这个效应十分微弱，人体根本无法察觉。
So we've built detectors that can feel it for us. That's what the LIGO detectors do.
所以，我们建造了探测器，以感知引力波。这就是LIGO的功能。
And they're not the only ones. There are gravitational wave detectors spread across the world.
LIGO并不是唯一的探测器。全球各地都有引力波探测器。
These L-shaped instruments have long arms, whose exact length is measured with lasers.
它们呈L形，两侧有很长的臂状结构，臂状结构的准确长度由激光测量。
If the length changes, it could be because gravitational waves are stretching and compressing the arms.
如果测量值发生变化，很可能就是因为引力波正在拉伸、挤压这些臂状结构。
Once the detectors feel a gravitational wave, scientists can extract information about the wave's source.
一旦探测器感知到了引力波，科学家就能够提取关于引力波来源的信息。
In a way, detectors like LIGO are big gravitational wave radios.
某种意义上说，LIGO这类探测器，如同一台大型引力波收音机。
Radio waves are traveling all around you, but you can't feel them or hear the music they carry.
普通的无线电信号穿梭在我们周围，但我们却无法感知它们，也听不到它们携带的音乐。
It takes the right kind of detector to extract the music.
我们需要正确的探测器，才能提取信号中的音乐。
LIGO detects a gravitational wave signal, which scientists then study for data about the object that generated it.
科学家借助LIGO探测到的引力波信号，研究产生引力波的物体。
They can derive information, like its mass and the shape of its orbit.
他们能够从中提取物体质量、运行轨道等信息。
We can also hear gravitational waves by playing their signals through speakers,
这些信号也可通过扬声器播放，这样我们就能听到引力波，
just like the music a radio extracts from radio waves.
就像收音机从无线电波中提取音乐那样。
So those two black holes colliding sounds like this.
因此，两个黑洞相撞，听起来是这样的。
Scientists call this slide whistle-like noise a chirp, and it's the signature of any two objects orbiting into each other.
科学家将这种口哨一样的声音称作啾声，这是两个物体互相环绕时发出的标志性的声音。
The black hole collision was just one example of what gravitational waves can tell us.
黑洞相撞只是一个例子，引力波还能传达很多信息。
Other high-energy astronomical events will leave gravitational echoes, too.
其他高能量的天文现象也会向外传递引力波。
The collapse of a star before it explodes in a supernova, or a very dense neutron stars colliding.
比如爆炸成为超新星前恒星的坍缩，或是高密度中子星的相撞。
Every time we create a new tool to look at space,
每当人类发明了探索太空的新工具，
we discover something we didn't expect, something that might revolutionize our understanding of the universe.
我们总能获得意外发现，这些发现或许能彻底改变我们对宇宙的认知。
LIGO's no different. In the short time it's been on, LIGO's already revealed surprises,
LIGO也不例外。即使仅仅被投入使用了很短时间，LIGO已经为我们带来了惊喜，
like that black holes collide more often than we ever expected.
例如，黑洞相撞的发生比我们想象得更为频繁。
It's impossible to say, but exciting to imagine what revelations may now be propagating across space
尽管现在还很难确定，但我们可以很兴奋地想象还有什么新发现正在跨越宇宙，
towards our tiny blue planet and its new way of perceiving the universe.
走向我们这颗小小的蓝色星球，让我们对宇宙产生新的看法。