WEBVTT 00:00:06.659 --> 00:00:12.099 Could human civilization eventually spread across the whole Milky Way galaxy? 00:00:12.099 --> 00:00:15.249 Could we move beyond our small blue planet 00:00:15.249 --> 00:00:19.699 to establish colonies in the multitude of star systems out there? 00:00:19.699 --> 00:00:21.957 This question's a pretty daunting one. 00:00:21.957 --> 00:00:25.939 There are around 300 billion stars in the galaxy, 00:00:25.939 --> 00:00:29.488 which is about 160,000 light-years across. 00:00:29.488 --> 00:00:34.016 So far we've sent a single spacecraft outside our solar system, 00:00:34.016 --> 00:00:39.209 trudging along at 0.006% of the speed of light. 00:00:39.209 --> 00:00:42.615 At that rate, it would take over 2.5 billion years 00:00:42.615 --> 00:00:45.759 just to get from one end of the galaxy to the other. 00:00:45.759 --> 00:00:48.329 And then there's the question of human survival. 00:00:48.329 --> 00:00:51.460 The gulf between stars is simply enormous. 00:00:51.460 --> 00:00:54.169 We couldn't live sustainably on most planets, 00:00:54.169 --> 00:00:57.880 and we require a lot of resources to stay alive. 00:00:57.880 --> 00:01:01.880 And yet, decades ago, scholars found that it's theoretically possible 00:01:01.880 --> 00:01:05.181 to not just spread human civilization across the galaxy, 00:01:05.181 --> 00:01:07.610 but to do so quite quickly, 00:01:07.610 --> 00:01:10.980 without breaking any known laws of physics. 00:01:10.980 --> 00:01:15.139 Their idea is based on the work of a mathematician named John von Neumann, 00:01:15.139 --> 00:01:18.890 who designed on paper machines that could self-replicate 00:01:18.890 --> 00:01:21.679 and create new generations of themselves. 00:01:21.679 --> 00:01:25.570 These would later come to be known as von Neumann machines. 00:01:25.570 --> 00:01:27.532 In the context of space exploration, 00:01:27.532 --> 00:01:30.041 von Neumann machines could be built on Earth 00:01:30.041 --> 00:01:32.182 and launched into space. 00:01:32.182 --> 00:01:36.242 There, the self-sufficient machines would land on distant planets. 00:01:36.242 --> 00:01:40.442 They would then mine the available resources and harvest energy, 00:01:40.442 --> 00:01:42.691 build replicas of themselves, 00:01:42.691 --> 00:01:44.632 launch those to the nearest planets, 00:01:44.632 --> 00:01:46.451 and continue the cycle. 00:01:46.451 --> 00:01:49.492 The result is the creation of millions of probes 00:01:49.492 --> 00:01:54.112 spreading outwards into the universe like a drop of ink in a fishbowl. 00:01:54.112 --> 00:01:57.722 Scholars crunched the numbers and found that a single von Neumann machine 00:01:57.722 --> 00:02:00.401 traveling at 5% of the speed of light 00:02:00.401 --> 00:02:06.123 should be able to replicate throughout our galaxy in 4 million years or less. 00:02:06.123 --> 00:02:07.972 That may sound like a long time, 00:02:07.972 --> 00:02:11.554 but when you consider that our universe is 14 billion years old, 00:02:11.554 --> 00:02:14.323 on a cosmic scale, it's incredibly fast - 00:02:14.323 --> 00:02:19.763 the equivalent of about 2.5 hours in an entire year. 00:02:19.763 --> 00:02:23.253 Creating von Neumann machines would require a few technologies 00:02:23.253 --> 00:02:24.893 we don't have yet, 00:02:24.893 --> 00:02:27.054 including advanced artificial intelligence, 00:02:27.054 --> 00:02:28.554 miniaturization, 00:02:28.554 --> 00:02:30.793 and better propulsion systems. 00:02:30.793 --> 00:02:34.253 If we wanted to use them to spread actual humans throughout the galaxy, 00:02:34.253 --> 00:02:37.013 we would need yet another technological leap - 00:02:37.013 --> 00:02:41.599 the ability to artificially grow biological organisms and bodies 00:02:41.599 --> 00:02:44.933 using raw elements and genetic information. 00:02:44.933 --> 00:02:47.432 Regardless, if in the last billion years 00:02:47.432 --> 00:02:50.221 an alien civilization created such a machine 00:02:50.221 --> 00:02:53.124 and set it multiplying its way toward us, 00:02:53.124 --> 00:02:56.373 our galaxy would be swarming with them by now. 00:02:56.373 --> 00:02:58.978 So then where are all these machines? 00:02:58.978 --> 00:03:01.075 Some astronomers, like Carl Sagan, 00:03:01.075 --> 00:03:06.032 say that intelligent aliens wouldn't build self-replicating machines at all. 00:03:06.032 --> 00:03:07.770 They might hurtle out of control, 00:03:07.770 --> 00:03:11.521 scavaging planets to their cores in order to keep replicating. 00:03:11.521 --> 00:03:13.730 Others take the machines absence as proof 00:03:13.730 --> 00:03:17.115 that intelligent alien civilizations don't exist, 00:03:17.115 --> 00:03:21.684 or that they go extinct before they can develop the necessary technologies. 00:03:21.684 --> 00:03:25.056 But all this hasn't stopped people from imagining what it would be like 00:03:25.056 --> 00:03:27.097 if they were out there. 00:03:27.097 --> 00:03:28.865 Science fiction author David Brin 00:03:28.865 --> 00:03:33.006 writes about a universe in which many different von Neumann machines exist 00:03:33.006 --> 00:03:35.193 and proliferate simultaneously. 00:03:35.193 --> 00:03:38.274 Some are designed to greet young civilizations, 00:03:38.274 --> 00:03:41.855 others to locate and destroy them before they become a threat. 00:03:41.855 --> 00:03:44.825 In fact, in Brin's story "Lungfish," 00:03:44.825 --> 00:03:50.375 some von Neumann machines are keeping a close watch over the Earth right now, 00:03:50.375 --> 00:03:53.198 waiting for us to reach a certain level of sophistication 00:03:53.198 --> 00:03:55.739 before they make their move. 00:03:55.739 --> 00:03:59.026 For now, all we have is curiosity and theory. 00:03:59.026 --> 00:04:01.140 But the next time you look at the night sky, 00:04:01.140 --> 00:04:04.085 consider that billions of self-replicating machines 00:04:04.085 --> 00:04:09.515 could be advancing between stars in our galaxy right now. 00:04:09.515 --> 00:04:13.295 If they exist, one of them will eventually land on Earth, 00:04:13.295 --> 00:04:17.115 or maybe, just maybe, they're already here.