Could human civilization eventually spread across the whole Milky Way galaxy? Could we move beyond our small blue planet to establish colonies in the multitude of star systems out there? This question's a pretty daunting one. There are around 300 billion stars in the galaxy, which is about 160,000 light-years across. So far we've sent a single spacecraft outside our solar system, trudging along at 0.006% of the speed of light. At that rate, it would take over 2.5 billion years just to get from one end of the galaxy to the other. And then there's the question of human survival. The gulf between stars is simply enormous. We couldn't live sustainably on most planets, and we require a lot of resources to stay alive. And yet, decades ago, scholars found that it's theoretically possible to not just spread human civilization across the galaxy, but to do so quite quickly, without breaking any known laws of physics. Their idea is based on the work of a mathematician named John von Neumann, who designed on paper machines that could self-replicate and create new generations of themselves. These would later come to be known as von Neumann machines. In the context of space exploration, von Neumann machines could be built on Earth and launched into space. There, the self-sufficient machines would land on distant planets. They would then mine the available resources and harvest energy, build replicas of themselves, launch those to the nearest planets, and continue the cycle. The result is the creation of millions of probes spreading outwards into the universe like a drop of ink in a fishbowl. Scholars crunched the numbers and found that a single von Neumann machine traveling at 5% of the speed of light should be able to replicate throughout our galaxy in 4 million years or less. That may sound like a long time, but when you consider that our universe is 14 billion years old, on a cosmic scale, it's incredibly fast - the equivalent of about 2.5 hours in an entire year. Creating von Neumann machines would require a few technologies we don't have yet, including advanced artificial intelligence, miniaturization, and better propulsion systems. If we wanted to use them to spread actual humans throughout the galaxy, we would need yet another technological leap - the ability to artificially grow biological organisms and bodies using raw elements and genetic information. Regardless, if in the last billion years an alien civilization created such a machine and set it multiplying its way toward us, our galaxy would be swarming with them by now. So then where are all these machines? Some astronomers, like Carl Sagan, say that intelligent aliens wouldn't build self-replicating machines at all. They might hurtle out of control, scavenging planets to their cores in order to keep replicating. Others take the machines absence as proof that intelligent alien civilizations don't exist, or that they go extinct before they can develop the necessary technologies. But all this hasn't stopped people from imagining what it would be like if they were out there. Science fiction author David Brin writes about a universe in which many different von Neumann machines exist and proliferate simultaneously. Some are designed to greet young civilizations, others to locate and destroy them before they become a threat. In fact, in Brin's story "Lungfish," some von Neumann machines are keeping a close watch over the Earth right now, waiting for us to reach a certain level of sophistication before they make their move. For now, all we have is curiosity and theory. But the next time you look at the night sky, consider that billions of self-replicating machines could be advancing between stars in our galaxy right now. If they exist, one of them will eventually land on Earth, or maybe, just maybe, they're already here.
人類文明能散播到 整個銀河系嗎? 我們能夠離開地球, 到眾多的星系中 建立殖民地嗎? 這個問題其實有點令人氣餒。 銀河系中有約3000億個星體, 距離橫跨16萬光年。 而目前我們只能將一個飛行器 送出太陽系, 而它只能以 光速的0.006%倍速度旅行。 依照這樣的速度, 得花費超過25億年 才能到達銀河系的另一端。 而且人類如何生存 也是個問題。 每個星體的距離都非常遙遠, 況且大多數的行星 都不宜人居, 但人類需要很多資源才能生活。 然而,幾十年前有學者發現 在理論上這是可行的, 不只是將人類文明 散播到整個銀河系 而且速度又很快, 也不需要打破任何物理定律。 這樣的想法是根據數學家 約翰·馮·諾伊曼 設計的一種理論機器, 不但具有自我複製能力, 而且能不斷繁殖後代。 這個想法被後人稱之為 馮·諾伊曼飛行器。 進行太空旅行時, 馮·諾伊曼飛行器 可於地球上製造, 再發射到外太空。 這些能自給的機器 會登陸遙遠的行星。 然後鑽探可取得的資源 並獲取能量, 複製更多個體, 再發射到附近的行星, 不斷重複以上的循環。 最終便會產出 幾百萬台飛行器, 如同滴入水缸中的墨水般 不斷向宇宙擴散。 學者們經過不斷計算之後發現, 如果每台飛行器 以5%的光速飛行, 大概400萬年以內 就能遍布整個銀河系。 聽起來或許很久, 但想到宇宙已經有140億年的歷史時, 以宏觀的角度而言是非常快的, 只相當於一年中的2.5個小時, 製造馮·諾伊曼飛行器 需要幾項關鍵技術, 目前還沒研發出來, 包含更先進的人工智慧, 縮小技術, 還有改良的推進系統。 如果我們想傳送真正的"人類", 我們必須再發生一次科技大躍進, 發展出以人工方式培養器官, 還有使用原料跟基因資訊 培養人體的技術。 除此之外, 如果過去數十億年當中, 已經有外星文明 製造出這樣的機器, 並且不斷在宇宙中擴散的話, 現在銀河系當中 早就到處都有那些機器了。 可是那些機器在哪裡呢? 卡爾賽根等天文學家認為, 有智慧的外星生物 是不會製造這種機器的, 因為有可能會失控, 為了持續繁殖 而侵蝕星球的核心。 其他人則認為 這些機器沒有出現, 證明了高度智慧的外星文明 仍然不存在, 又或者是這些文明 在研發出必要的科技前就滅絕了。 但這並不會停止人們去想像 存在宇宙中的外星生命。 科幻小說作家大衛布林 曾描述過一個宇宙 存在著各種馮·諾伊曼飛行器, 同時他們也不斷地繁殖擴散。 有些是去拜訪剛發展出來的文明, 有些則是會在 文明成為威脅前摧毀他們。 事實上,在布林所寫的 《肺魚》(Lungfish)故事中提到, 有些馮·諾伊曼飛行器 正密切監視著地球, 等待我們的文明發展到某種程度 再做出行動。 目前為止我們所說的 還只是理論和好奇而已。 下次當你凝望夜空的時候,想像一下 有數十億台能自我繁殖的機器 正在銀河系中行進。 如果真的有, 其中一台一定會來到地球。 但也有可能,只是可能而已, 其實它們已經到了。