Early-warning radar, a product of the Cold War, consisted of a bunch of ground and airborne stations scattered around the Arctic. These stations swept the atmosphere with powerful radar beams 24/7, often bouncing them off the ionosphere, and people obsessively monitored the echos for any hints of enemy movement.
作为冷战时期的产物之一，早期预警雷达由北极圈周围的一大堆路基和机载雷达站组成。这些功率巨大的雷达站全天候地发射雷达波，很多时候雷达波会被电离层反弹回来，而地面上的监听人员则像着了魔一样捕捉着敌人的任何蛛丝马迹。
These radar transmissions leaked into space, and could probably be picked up by nearby exoplanets if they happened to be listening when the beam swept over their part of the sky. But the same march of technological progress that made the TV broadcast towers obsolete has had the same effect on early-warning radar. Today's systems-where they exist at all-are much quieter, and may eventually be replaced completely by new technology.
一些雷达波会泄露到外太空去，如果雷达波扫过某一片天空时，附近的地外星球上恰好有外星人，那么他们很有可能会收到这些雷达波。但正如之前科技的进步淘汰了广播电视塔那样，早期预警雷达也受到了相似的影响。如今的雷达系统——还在运作的那些——更加“安静”，最终可能会完全被新技术所取代。
Earth's most powerful radio signal is the beam from the Arecibo telescope. This massive dish in Puerto Rico can function as a radar transmitter, bouncing a signal off nearby targets like Mercury and the asteroid belt. It's essentially a flashlight that we shine on planets to see them better. (This is just as crazy as it sounds.)
地球上最强大的无线电信号来自阿雷西博天文台。这个位于波多黎各的巨型碟形天线就像雷达的发射机一样，向附近天体（如水星和小行星带）发射信号。它就像一个手电筒一样照向我们想更好地观测的其他星球（这听起来很疯狂，事实上也的确如此）。
However, it transmits only occasionally, and in a narrow beam. If an exoplanet happened to be caught in the beam, and they were lucky enough to be pointing a receiving antenna at our corner of the sky at the time, all they would pick up would be a brief pulse of radio energy, then silence.
但它只是不定期地传送一些信号，而且波束非常窄。如果一颗地外星球恰好被照射到，而且恰好他们又把接收天线对准我们这边，那么他们能收到的也只是一个短暂的无线电脉冲而已，之后便是一片寂静。
So hypothetical aliens looking at Earth probably wouldn't pick us up with radio antennas.
所以外星人应该不会通过无线电的方式观测地球。
But there's also . . .
但我们还有……
Visible light
可见光
This is more promising. The Sun is really bright,[citation needed ] and its light illuminates the Earth.[citation needed ] Some of that light is reflected back into space as “Earthshine.” Some of it skims close to our planet and passes through our atmosphere before continuing on to the stars. Both of these effects could potentially be detected from an exoplanet.
这种方法更有前途。太阳其实非常明亮，它照耀着地球。其中的一部分光被以“地球反照”的形式反射回太空，另一些则会贴近地球，穿过大气层后继续向其他星球传播。这两种效应都很有可能被地外星球上的外星人探测到。