國(guó)家地理:送給外星人的地圖(4)

Our galactic neighborhood has no obvious street signs, and crafting a map pointing to one planet among the billions (and billions) of worlds populating the Milky Way is no simple feat.

我們的銀河系中并沒有明顯的標(biāo)識(shí)，要想繪制一張地圖，指向銀河系中數(shù)十億顆星球中的一顆，這絕非易事。

Finding Earth means finding the solar system, and the sun is rather unremarkable. There's really no way to distinguish it from the other several hundred billion stars in the galaxy, each of which is tracing its own path around the galactic center and slowly shifting in location relative to its neighbors. That stellar jostling means the constellations spangling Earth's skies won't be the same in our near future. In fact, in about 2,000 years, Polaris will no longer be the North Star, just as it was not the polestar for ancient Egyptian, Babylonian, and Chinese sky-watchers.

找到地球就意味著找到了太陽(yáng)系，而太陽(yáng)是相當(dāng)不起眼的。我們根本沒有辦法將它與銀河系其他數(shù)千億顆恒星中分辨出來(lái)，每一顆都沿著自己的軌道環(huán)繞銀河系的中心運(yùn)行，與其鄰近星球的相對(duì)位置在緩慢地變化著。恒星的這種相對(duì)位移意味著，在不久的將來(lái)，遍布在地球天空中的星座將不再是原來(lái)的樣子。事實(shí)上，在大約2000年后，北極星將不再是指北星，就像它當(dāng)初不再是古埃及、古巴比倫和古中國(guó)天文觀測(cè)者的指北星一樣。

So, what to do? Though normal stars with churning nuclear engines in their cores might not have distinctive fingerprints, Dad realized that pulsars -- the corpses of stars that once were much larger than the sun -- are potentially uniquely identifiable. Discovered in 1967, pulsars spin very rapidly, often hundreds of times per second. Using powerful radio telescopes, astronomers can measure with extreme precision how quickly pulsars rotate, meaning that each of these spinning stellar relics writes its own signature in space. Dad selected 14 pulsars that could triangulate Earth's position, and he coded information about their rotation rates into the map.

那么，該怎么辦呢？雖然正常恒星的中心有核物質(zhì)在運(yùn)轉(zhuǎn)，但它們可能沒有明顯的特征，爸爸意識(shí)到脈沖星--曾經(jīng)比太陽(yáng)大得多的恒星的“尸體”--有可能是唯一可識(shí)別的。脈沖星于1967年被發(fā)現(xiàn)，它們的轉(zhuǎn)速極快，往往每秒鐘能旋轉(zhuǎn)數(shù)百周。利用強(qiáng)大的射電望遠(yuǎn)鏡，天文學(xué)家可以極其精確地測(cè)量脈沖星旋轉(zhuǎn)的速度，這意味著每一顆旋轉(zhuǎn)的恒星殘骸都在太空中留下了自己的印跡。爸爸挑選出14顆能夠?qū)Φ厍虻奈恢眠M(jìn)行三角測(cè)量的脈沖星，并將它們的旋轉(zhuǎn)速度轉(zhuǎn)換成編碼，記錄在地圖上。