1 00:00:08,000 --> 00:00:10,000 Say two people are walking down the street 2 00:00:10,000 --> 00:00:11,000 and they bump into each other. 3 00:00:11,000 --> 00:00:14,000 They'll just shake it off and walk on. 4 00:00:14,000 --> 00:00:16,000 Sometimes that happens with molecules too. 5 00:00:16,000 --> 00:00:19,000 They just bounce off each other and that's that. 6 00:00:19,000 --> 00:00:21,000 But what if two people were to bump into each other, 7 00:00:21,000 --> 00:00:22,000 and during that collision, 8 00:00:22,000 --> 00:00:24,000 one person's arm got severed 9 00:00:24,000 --> 00:00:27,000 and reattached to the other person's face? 10 00:00:27,000 --> 00:00:29,000 Now that sounds really weird, 11 00:00:29,000 --> 00:00:31,000 but it's similar to one of the many ways 12 00:00:31,000 --> 00:00:33,000 that molecules can react with each other. 13 00:00:34,000 --> 00:00:36,000 Two molecules can join and become one. 14 00:00:36,000 --> 00:00:39,000 One can split apart and become two. 15 00:00:39,000 --> 00:00:40,000 Molecules can switch parts. 16 00:00:41,000 --> 00:00:43,000 All these changes are chemical reactions, 17 00:00:43,000 --> 00:00:45,000 and we can see them happening around us. 18 00:00:45,000 --> 00:00:48,000 For example, when fireworks explode, 19 00:00:48,000 --> 00:00:49,000 or iron rusts, 20 00:00:49,000 --> 00:00:50,000 or milk goes bad, 21 00:00:50,000 --> 00:00:51,000 or people are born, 22 00:00:51,000 --> 00:00:52,000 grow old, 23 00:00:52,000 --> 00:00:52,000 die, 24 00:00:52,000 --> 00:00:54,000 and then decompose. 25 00:00:54,000 --> 00:00:57,000 But chemical reactions don't just happen willy nilly! 26 00:00:57,000 --> 00:00:59,000 Everything has to be right. 27 00:00:59,000 --> 00:01:01,000 First, the molecules have to hit each other 28 00:01:01,000 --> 00:01:03,000 in the right orientation. 29 00:01:03,000 --> 00:01:05,000 And second, they have to hit each other hard enough, 30 00:01:05,000 --> 00:01:07,000 in other words, with enough energy. 31 00:01:08,000 --> 00:01:09,000 Now you're probably thinking 32 00:01:09,000 --> 00:01:11,000 that a reaction just happens in one direction and that's it. 33 00:01:12,000 --> 00:01:13,000 Sometimes that's true. 34 00:01:13,000 --> 00:01:15,000 For example, things can't unburn 35 00:01:15,000 --> 00:01:17,000 or unexplode. 36 00:01:17,000 --> 00:01:20,000 But most reactions can happen in both directions, 37 00:01:20,000 --> 00:01:22,000 forward and reverse. 38 00:01:23,000 --> 00:01:25,000 There's no reason that our face-arm guy 39 00:01:25,000 --> 00:01:27,000 can't bump into armless girl, 40 00:01:27,000 --> 00:01:30,000 reattaching that arm back to its original socket. 41 00:01:30,000 --> 00:01:31,000 Now let's zoom out a bit. 42 00:01:31,000 --> 00:01:32,000 Now let's say that you've got 43 00:01:32,000 --> 00:01:34,000 a thousand people on the street, 44 00:01:34,000 --> 00:01:35,000 and all of them start with their limbs 45 00:01:35,000 --> 00:01:37,000 normally attached. 46 00:01:37,000 --> 00:01:39,000 At the beginning, every collision is a chance 47 00:01:39,000 --> 00:01:43,000 for Person A to transfer an arm to Person B's face. 48 00:01:43,000 --> 00:01:44,000 And so at the beginning, 49 00:01:44,000 --> 00:01:45,000 more and more people end up 50 00:01:45,000 --> 00:01:48,000 with arms attached to their faces or arms missing. 51 00:01:48,000 --> 00:01:51,000 But as the number of people with arm-faces 52 00:01:51,000 --> 00:01:53,000 and missing arms grows, 53 00:01:53,000 --> 00:01:56,000 collisions between those people become more likely. 54 00:01:56,000 --> 00:01:58,000 And when they bump into each other, 55 00:01:58,000 --> 00:01:59,000 guess what? 56 00:01:59,000 --> 00:02:02,000 Normal-appendage people are reproduced. 57 00:02:02,000 --> 00:02:05,000 Now the number of limb transfers per second forward 58 00:02:05,000 --> 00:02:07,000 will start high and then fall, 59 00:02:07,000 --> 00:02:10,000 and the number of limb transfers per second backward 60 00:02:10,000 --> 00:02:12,000 will start at zero and then rise. 61 00:02:12,000 --> 00:02:13,000 Eventually they'll meet, 62 00:02:13,000 --> 00:02:14,000 they'll be the same. 63 00:02:14,000 --> 00:02:15,000 And when that happens, 64 00:02:15,000 --> 00:02:19,000 the number of people in each state stops changing, 65 00:02:19,000 --> 00:02:21,000 even though people are still bumping into each other 66 00:02:21,000 --> 00:02:23,000 and exchanging limbs. 67 00:02:23,000 --> 00:02:25,000 Now how many people do you think 68 00:02:25,000 --> 00:02:26,000 there are in each state? 69 00:02:26,000 --> 00:02:27,000 Half and half, right? 70 00:02:27,000 --> 00:02:29,000 No, well, maybe. 71 00:02:29,000 --> 00:02:30,000 It depends. 72 00:02:30,000 --> 00:02:32,000 It could be 50/50, 73 00:02:32,000 --> 00:02:33,000 but it could be 60/40 74 00:02:33,000 --> 00:02:34,000 or 15/85, 75 00:02:34,000 --> 00:02:35,000 or anything. 76 00:02:35,000 --> 00:02:38,000 We chemists have to get our little, gloved hands dirty 77 00:02:38,000 --> 00:02:41,000 - ah, well, we're in a lab so not really dirty - 78 00:02:41,000 --> 00:02:43,000 to figure out what the actually distribution 79 00:02:43,000 --> 00:02:44,000 of molecules is. 80 00:02:44,000 --> 00:02:46,000 Even though each of limb transfers 81 00:02:46,000 --> 00:02:49,000 is a pretty dramatic event for the people involved, 82 00:02:49,000 --> 00:02:50,000 if we zoom out, 83 00:02:50,000 --> 00:02:53,000 we see population numbers that don't change. 84 00:02:53,000 --> 00:02:55,000 We can this nirvana equilibrium, 85 00:02:55,000 --> 00:02:58,000 and it doesn't just happen with chemical reactions. 86 00:02:58,000 --> 00:02:59,000 Things like gene pools 87 00:02:59,000 --> 00:03:02,000 and highway traffic show the same pattern. 88 00:03:02,000 --> 00:03:04,000 It looks pretty still from 30,000 feet, 89 00:03:04,000 --> 00:03:06,000 but there is lots of crazy stuff 90 00:03:06,000 --> 00:03:07,000 happening on the ground, 91 00:03:07,000 --> 00:03:10,000 you just need to zoom in to see it.