国家地理:送给外星人的地图(4)（在线收听）

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.

我们的银河系中并没有明显的标识，要想绘制一张地图，指向银河系中数十亿颗星球中的一颗，这绝非易事。

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.

找到地球就意味着找到了太阳系，而太阳是相当不起眼的。我们根本没有办法将它与银河系其他数千亿颗恒星中分辨出来，每一颗都沿着自己的轨道环绕银河系的中心运行，与其邻近星球的相对位置在缓慢地变化着。恒星的这种相对位移意味着，在不久的将来，遍布在地球天空中的星座将不再是原来的样子。事实上，在大约2000年后，北极星将不再是指北星，就像它当初不再是古埃及、古巴比伦和古中国天文观测者的指北星一样。

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，该怎么办呢？虽然正常恒星的中心有核物质在运转，但它们可能没有明显的特征，爸爸意识到脉冲星--曾经比太阳大得多的恒星的“尸体”--有可能是唯一可识别的。脉冲星于1967年被发现，它们的转速极快，往往每秒钟能旋转数百周。利用强大的射电望远镜，天文学家可以极其精确地测量脉冲星旋转的速度，这意味着每一颗旋转的恒星残骸都在太空中留下了自己的印迹。爸爸挑选出14颗能够对地球的位置进行三角测量的脉冲星，并将它们的旋转速度转换成编码，记录在地图上。