《那些古怪又讓人憂心的問題》第46期:外星天文學家(2)
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.
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.
一些雷達波會泄露到外太空去，如果雷達波掃過某一片天空時，附近的地外星球上恰好有外星人，那麼他們很有可能會收到這些雷達波。但正如之前科技的進步淘汰了廣播電視塔那樣，早期預警雷達也受到了相似的影響。如今的雷達系統——還在運作的那些——更加“安靜”，最終可能會完全被新技術所取代。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.)
地球上最強大的無線電信號來自阿雷西博天文臺。這個位於波多黎各的巨型碟形天線就像雷達的發射機一樣，向附近天體（如水星和小行星帶）發射信號。它就像一個手電筒一樣照向我們想更好地觀測的其他星球（這聽起來很瘋狂，事實上也的確如此）。
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.
但它只是不定期地傳送一些信號，而且波束非常窄。如果一顆地外星球恰好被照射到，而且恰好他們又把接收天線對準我們這邊，那麼他們能收到的也只是一個短暫的無線電脈衝而已，之後便是一片寂靜。So hypothetical aliens looking at Earth probably wouldn't pick us up with radio antennas.
所以外星人應該不會通過無線電的方式觀測地球。But there's also . . .
但我們還有……Visible light
可見光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.
這種方法更有前途。太陽其實非常明亮，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，另一些則會貼近地球，穿過大氣層後繼續向其他星球傳播。這兩種效應都很有可能被地外星球上的外星人探測到。
