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When things move, they create waves.
当物体移动时，就会产生波。
For example, if you shake a stick back and forth in water: water waves.
例如，如果你在水中来回摇动一根棍子：会产生水波。
Vibrate a piece of metal back and forth really fast: air pressure waves.
快速地来回振动一块金属：会产生气压波。
Shake some electrons back and forth really fast: radio waves.
快速来回摇动一些电子：会产生无线电波。
And yes, shake a planet or star back and forth really fast: gravitational waves.
没错，快速地摇动行星或恒星：会产生引力波。
Gravitational waves happen because the effects of gravity don't travel outwards at infinite speed
引力波的产生是因为引力的影响不会以无限的速度向外传播，
so if the sun were to suddenly jump a few hundred thousand kilometers to the side,
所以如果太阳突然往一边移动几十万公里，
the changed gravitational field would take time to pulse outwards.
改变后的引力场将需要时间向外辐射。
And if the sun shook back and forth and back and forth,
如果太阳来回晃动，
instead of a single pulse, you'd get continuous gravitational waves.
而不是单一的移动，就会产生连续的引力波。
So what's doing the "waving"? In the case of water,
为什么会产生“波”呢？在水波现象中，
the height of the water increases and decreases at any particular location as the waves travel past.
当波浪经过时，水的高度在任何特定的位置下增加或者减少。
In the case of sound, the pressure of the air increases and decreases
在声波现象中，空气的压力在任何特定的位置
at any particular location as the waves travel past.
随着波的传播而增加和减少。
In the case of radio or cell phone signals or any other electromagnetic waves,
在你听收音机或者打电话，或者涉及到其它任何电磁波的情况下，
the electric and magnetic fields get stronger and weaker
随着电磁波经过，电场及磁场
at any particular location as the waves travel past.
在任何特定位置增强或减弱。
And in the case of gravitational waves,
在引力波的情况下，
the gravitational field gets slightly stronger or weaker as the waves travel past.
当引力波经过时，引力场会变得稍强或稍弱。
You can tell a wave has passed by looking at how nearby particles behave
你可以通过观察附近粒子的表现来判断，
a bobber on the water rises up and down,
一个浮球的上升和下降
the electrons in a radio antenna move back and forth because of the changing electric field,
收音机天线中的电子由于电场的变化而来回移动，
and free-floating people or planets or cats move back and forth
由于引力场的变化，
because of the changing gravitational field.
自由飘动的行星小行星来回移动。
though in this last case, the peculiarities of gravity mean that the free-floating things
不过，在最后一种情况下，引力的特性意味着那些自由浮动的物体
actually experiencing the gravitational wave don't feel like they're moving.
实际上已经正在经历引力波的影响，你并不觉得它们在移动。
But if you measure the space between them by sending a pulse of laser light
但是，如果你通过激光测距仪来测量它们之间的距离，
and measuring the time it takes for it to come back,
计算往返过程所花费的时间，
you'll find that the distance between them increases and decreases.
你会发现它们之间的距离会增加或减少。
In practice, physicists don't actually measure gravitational waves with free-floating cats,
实际上，物理学家并不是用自由浮动的物体来测量引力波，
they use very very fancy expensive mirrors which are effectively free-floating
他们使用非常昂贵的镜子，这种镜子可以有效地自由浮动，
because they're hung on pendulums suspended on isolation tables, suspended on isolation tables,
因为它们被悬挂在隔离台上的钟摆上，或者是隔离桌上，
or which are actually free-floating because they're attached to satellites floating in space,
或者实际上是自由漂浮，因为它们就安装在漂浮在太空中的卫星，
though this hasn't been done yet.
不过这还没有完成。
The reason physicists need fancy floating mirrors
物理学家需要这些镜子来探测引力波，
to detect gravitational waves is that the waves are very, very weak.
原因是引力波非常微弱。
For example, the electrons that vibrate back and forth in a radio antenna
例如，在收音机天线中电子来回振动，
to make electromagnetic waves, also make gravitational waves;
产生了电磁波的同时也会产生引力波；
electrons are matter moving back and forth after all.
毕竟电子是来回移动的物质。
But the waves they make are super weak: a 200 watt radio transmitter gives off something like
但是它们产生的波是非常微弱的：一个200瓦的收音机发射机释放出的引力辐射相当于
a quadrillionth of a quintillionth of a quintillionth of a quintillionth of that power as gravitational radiation.
这个功率的千万亿分之一或千亿分之一。
And that's why here on earth we can only detect the biggest baddest astronomical events
这就是为什么我们在地球上只能探测到那些最大最糟糕的天文事件,
like superfast spinning neutron stars or merging black holes or the big bang.
比如超高速旋转的中子星合并成黑洞或者宇宙大爆炸。
Though so far, we've only detected black hole collisions.
尽管到目前为止，我们只探测到了黑洞的碰撞。
