1 00:00:08,007 --> 00:00:10,118 Say two people are walking down the street, 2 00:00:10,118 --> 00:00:11,662 and they bump into each other. 3 00:00:11,662 --> 00:00:14,139 They'll just shake it off and walk on. 4 00:00:14,139 --> 00:00:16,549 Sometimes that happens with molecules too. 5 00:00:16,549 --> 00:00:19,000 They just bounce off each other, and that's that. 6 00:00:19,000 --> 00:00:21,458 But what if two people were to bump into each other, 7 00:00:21,458 --> 00:00:22,625 and during that collision, 8 00:00:22,625 --> 00:00:24,380 one person's arm got severed 9 00:00:24,380 --> 00:00:27,838 and reattached to the other person's face? 10 00:00:27,838 --> 00:00:29,418 Now that sounds really weird, 11 00:00:29,418 --> 00:00:31,298 but it's similar to one of the many ways 12 00:00:31,298 --> 00:00:33,757 that molecules can react with each other. 13 00:00:34,726 --> 00:00:36,928 Two molecules can join and become one. 14 00:00:36,928 --> 00:00:39,040 One can split apart and become two. 15 00:00:39,040 --> 00:00:40,871 Molecules can switch parts. 16 00:00:41,624 --> 00:00:43,760 All these changes are chemical reactions, 17 00:00:43,760 --> 00:00:45,964 and we can see them happening around us. 18 00:00:45,964 --> 00:00:48,140 For example, when fireworks explode, 19 00:00:48,140 --> 00:00:49,520 or iron rusts, 20 00:00:49,520 --> 00:00:50,211 or milk goes bad, 21 00:00:50,211 --> 00:00:51,304 or people are born, 22 00:00:51,304 --> 00:00:52,186 grow old, 23 00:00:52,186 --> 00:00:52,816 die, 24 00:00:52,816 --> 00:00:54,139 and then decompose. 25 00:00:54,754 --> 00:00:57,622 But chemical reactions don't just happen willy nilly! 26 00:00:57,622 --> 00:00:59,537 Everything has to be right. 27 00:00:59,537 --> 00:01:01,748 First, the molecules have to hit each other 28 00:01:01,748 --> 00:01:03,591 in the right orientation. 29 00:01:03,591 --> 00:01:05,759 And second, they have to hit each other hard enough, 30 00:01:05,759 --> 00:01:07,637 in other words, with enough energy. 31 00:01:08,067 --> 00:01:09,096 Now you're probably thinking 32 00:01:09,096 --> 00:01:11,818 that a reaction just happens in one direction and that's it. 33 00:01:12,463 --> 00:01:13,484 Sometimes that's true. 34 00:01:13,484 --> 00:01:15,845 For example, things can't unburn 35 00:01:15,845 --> 00:01:17,305 or unexplode. 36 00:01:17,935 --> 00:01:20,973 But most reactions can happen in both directions, 37 00:01:20,973 --> 00:01:22,893 forward and reverse. 38 00:01:23,415 --> 00:01:25,340 There's no reason that our face-arm guy 39 00:01:25,340 --> 00:01:27,071 can't bump into armless girl, 40 00:01:27,071 --> 00:01:30,371 reattaching that arm back to its original socket. 41 00:01:30,371 --> 00:01:31,944 Now let's zoom out a bit. 42 00:01:31,944 --> 00:01:32,830 Now let's say that you've got 43 00:01:32,830 --> 00:01:34,506 a thousand people on the street, 44 00:01:34,506 --> 00:01:35,864 and all of them start with their limbs 45 00:01:35,864 --> 00:01:37,530 normally attached. 46 00:01:37,530 --> 00:01:39,825 At the beginning, every collision is a chance 47 00:01:39,825 --> 00:01:43,861 for Person A to transfer an arm to Person B's face. 48 00:01:43,861 --> 00:01:44,841 And so at the beginning, 49 00:01:44,841 --> 00:01:45,939 more and more people end up 50 00:01:45,939 --> 00:01:48,963 with arms attached to their faces or arms missing. 51 00:01:48,963 --> 00:01:51,431 But as the number of people with arm-faces 52 00:01:51,431 --> 00:01:53,271 and missing arms grows, 53 00:01:53,271 --> 00:01:56,725 collisions between those people become more likely. 54 00:01:56,725 --> 00:01:58,586 And when they bump into each other, 55 00:01:58,586 --> 00:01:59,605 guess what? 56 00:01:59,605 --> 00:02:02,564 Normal-appendage people are reproduced. 57 00:02:02,564 --> 00:02:05,531 Now the number of limb transfers per second forward 58 00:02:05,531 --> 00:02:07,530 will start high and then fall, 59 00:02:07,530 --> 00:02:10,279 and the number of limb transfers per second backward 60 00:02:10,279 --> 00:02:12,368 will start at zero and then rise. 61 00:02:12,368 --> 00:02:13,788 Eventually they'll meet, 62 00:02:13,788 --> 00:02:14,877 they'll be the same. 63 00:02:14,877 --> 00:02:15,993 And when that happens, 64 00:02:15,993 --> 00:02:19,247 the number of people in each state stops changing, 65 00:02:19,247 --> 00:02:21,338 even though people are still bumping into each other 66 00:02:21,338 --> 00:02:23,051 and exchanging limbs. 67 00:02:23,603 --> 00:02:25,087 Now how many people do you think 68 00:02:25,087 --> 00:02:26,724 there are in each state? 69 00:02:26,724 --> 00:02:27,880 Half and half, right? 70 00:02:27,880 --> 00:02:29,674 No, well, maybe. 71 00:02:29,674 --> 00:02:30,768 It depends. 72 00:02:30,768 --> 00:02:32,138 It could be 50/50, 73 00:02:32,138 --> 00:02:33,302 but it could be 60/40 74 00:02:33,302 --> 00:02:34,177 or 15/85, 75 00:02:34,177 --> 00:02:35,230 or anything. 76 00:02:35,230 --> 00:02:38,672 We chemists have to get our little, gloved hands dirty 77 00:02:38,672 --> 00:02:41,146 - ah, well, we're in a lab so not really dirty - 78 00:02:41,146 --> 00:02:43,156 to figure out what the actual distribution 79 00:02:43,156 --> 00:02:44,901 of molecules is. 80 00:02:44,901 --> 00:02:46,409 Even though each of limb transfers 81 00:02:46,409 --> 00:02:49,510 is a pretty dramatic event for the people involved, 82 00:02:49,510 --> 00:02:50,443 if we zoom out, 83 00:02:50,443 --> 00:02:53,003 we see population numbers that don't change. 84 00:02:53,003 --> 00:02:55,443 We call this nirvana equilibrium, 85 00:02:55,443 --> 00:02:58,014 and it doesn't just happen with chemical reactions. 86 00:02:58,568 --> 00:02:59,828 Things like gene pools 87 00:02:59,828 --> 00:03:02,487 and highway traffic show the same pattern. 88 00:03:02,487 --> 00:03:04,736 It looks pretty still from 30,000 feet, 89 00:03:04,736 --> 00:03:06,335 but there is lots of crazy stuff 90 00:03:06,335 --> 00:03:07,595 happening on the ground, 91 00:03:07,595 --> 00:03:10,440 you just need to zoom in to see it.