WEBVTT 00:00:08.007 --> 00:00:10.118 Say two people are walking down the street, 00:00:10.118 --> 00:00:11.662 and they bump into each other. 00:00:11.662 --> 00:00:14.139 They'll just shake it off and walk on. 00:00:14.139 --> 00:00:16.549 Sometimes that happens with molecules too. 00:00:16.549 --> 00:00:19.000 They just bounce off each other, and that's that. 00:00:19.000 --> 00:00:21.458 But what if two people were to bump into each other, 00:00:21.458 --> 00:00:22.625 and during that collision, 00:00:22.625 --> 00:00:24.380 one person's arm got severed 00:00:24.380 --> 00:00:27.838 and reattached to the other person's face? 00:00:27.838 --> 00:00:29.418 Now that sounds really weird, 00:00:29.418 --> 00:00:31.298 but it's similar to one of the many ways 00:00:31.298 --> 00:00:33.757 that molecules can react with each other. 00:00:34.726 --> 00:00:36.928 Two molecules can join and become one. 00:00:36.928 --> 00:00:39.040 One can split apart and become two. 00:00:39.040 --> 00:00:40.871 Molecules can switch parts. 00:00:41.624 --> 00:00:43.760 All these changes are chemical reactions, 00:00:43.760 --> 00:00:45.964 and we can see them happening around us. 00:00:45.964 --> 00:00:48.140 For example, when fireworks explode, 00:00:48.140 --> 00:00:49.520 or iron rusts, 00:00:49.520 --> 00:00:50.211 or milk goes bad, 00:00:50.211 --> 00:00:51.304 or people are born, 00:00:51.304 --> 00:00:52.186 grow old, 00:00:52.186 --> 00:00:52.816 die, 00:00:52.816 --> 00:00:54.139 and then decompose. 00:00:54.754 --> 00:00:57.622 But chemical reactions don't just happen willy nilly! 00:00:57.622 --> 00:00:59.537 Everything has to be right. 00:00:59.537 --> 00:01:01.748 First, the molecules have to hit each other 00:01:01.748 --> 00:01:03.591 in the right orientation. 00:01:03.591 --> 00:01:05.759 And second, they have to hit each other hard enough, 00:01:05.759 --> 00:01:07.637 in other words, with enough energy. 00:01:08.067 --> 00:01:09.096 Now you're probably thinking 00:01:09.096 --> 00:01:11.818 that a reaction just happens in one direction and that's it. 00:01:12.463 --> 00:01:13.484 Sometimes that's true. 00:01:13.484 --> 00:01:15.845 For example, things can't unburn 00:01:15.845 --> 00:01:17.305 or unexplode. 00:01:17.935 --> 00:01:20.973 But most reactions can happen in both directions, 00:01:20.973 --> 00:01:22.893 forward and reverse. 00:01:23.415 --> 00:01:25.340 There's no reason that our face-arm guy 00:01:25.340 --> 00:01:27.071 can't bump into armless girl, 00:01:27.071 --> 00:01:30.371 reattaching that arm back to its original socket. 00:01:30.371 --> 00:01:31.944 Now let's zoom out a bit. 00:01:31.944 --> 00:01:32.830 Now let's say that you've got 00:01:32.830 --> 00:01:34.506 a thousand people on the street, 00:01:34.506 --> 00:01:35.864 and all of them start with their limbs 00:01:35.864 --> 00:01:37.530 normally attached. 00:01:37.530 --> 00:01:39.825 At the beginning, every collision is a chance 00:01:39.825 --> 00:01:43.861 for Person A to transfer an arm to Person B's face. 00:01:43.861 --> 00:01:44.841 And so at the beginning, 00:01:44.841 --> 00:01:45.939 more and more people end up 00:01:45.939 --> 00:01:48.963 with arms attached to their faces or arms missing. 00:01:48.963 --> 00:01:51.431 But as the number of people with arm-faces 00:01:51.431 --> 00:01:53.271 and missing arms grows, 00:01:53.271 --> 00:01:56.725 collisions between those people become more likely. 00:01:56.725 --> 00:01:58.586 And when they bump into each other, 00:01:58.586 --> 00:01:59.605 guess what? 00:01:59.605 --> 00:02:02.564 Normal-appendage people are reproduced. 00:02:02.564 --> 00:02:05.531 Now the number of limb transfers per second forward 00:02:05.531 --> 00:02:07.530 will start high and then fall, 00:02:07.530 --> 00:02:10.279 and the number of limb transfers per second backward 00:02:10.279 --> 00:02:12.368 will start at zero and then rise. 00:02:12.368 --> 00:02:13.788 Eventually they'll meet, 00:02:13.788 --> 00:02:14.877 they'll be the same. 00:02:14.877 --> 00:02:15.993 And when that happens, 00:02:15.993 --> 00:02:19.247 the number of people in each state stops changing, 00:02:19.247 --> 00:02:21.338 even though people are still bumping into each other 00:02:21.338 --> 00:02:23.051 and exchanging limbs. 00:02:23.603 --> 00:02:25.087 Now how many people do you think 00:02:25.087 --> 00:02:26.724 there are in each state? 00:02:26.724 --> 00:02:27.880 Half and half, right? 00:02:27.880 --> 00:02:29.674 No, well, maybe. 00:02:29.674 --> 00:02:30.768 It depends. 00:02:30.768 --> 00:02:32.138 It could be 50/50, 00:02:32.138 --> 00:02:33.302 but it could be 60/40 00:02:33.302 --> 00:02:34.177 or 15/85, 00:02:34.177 --> 00:02:35.230 or anything. 00:02:35.230 --> 00:02:38.672 We chemists have to get our little, gloved hands dirty 00:02:38.672 --> 00:02:41.146 - ah, well, we're in a lab so not really dirty - 00:02:41.146 --> 00:02:43.156 to figure out what the actual distribution 00:02:43.156 --> 00:02:44.901 of molecules is. 00:02:44.901 --> 00:02:46.409 Even though each of limb transfers 00:02:46.409 --> 00:02:49.510 is a pretty dramatic event for the people involved, 00:02:49.510 --> 00:02:50.443 if we zoom out, 00:02:50.443 --> 00:02:53.003 we see population numbers that don't change. 00:02:53.003 --> 00:02:55.443 We call this nirvana equilibrium, 00:02:55.443 --> 00:02:58.014 and it doesn't just happen with chemical reactions. 00:02:58.568 --> 00:02:59.828 Things like gene pools 00:02:59.828 --> 00:03:02.487 and highway traffic show the same pattern. 00:03:02.487 --> 00:03:04.736 It looks pretty still from 30,000 feet, 00:03:04.736 --> 00:03:06.335 but there is lots of crazy stuff 00:03:06.335 --> 00:03:07.595 happening on the ground, 00:03:07.595 --> 00:03:10.440 you just need to zoom in to see it.