WEBVTT 00:00:06.634 --> 00:00:10.947 In 1977, the physicist Edward Purcell 00:00:10.947 --> 00:00:12.900 calculated that if you push a bacteria 00:00:12.900 --> 00:00:14.198 and then let go, 00:00:14.198 --> 00:00:16.947 it will stop in about a millionth of a second. 00:00:16.947 --> 00:00:18.992 In that time, it will have traveled less 00:00:18.992 --> 00:00:21.173 than the width of a single atom. 00:00:21.173 --> 00:00:23.203 The same holds true for a sperm 00:00:23.203 --> 00:00:24.664 and many other microbes. 00:00:24.664 --> 00:00:27.615 It all has to do with being really small. 00:00:27.615 --> 00:00:31.340 Microscopic creatures inhabit a world alien to us, 00:00:31.340 --> 00:00:32.704 where making it through an inch of water 00:00:32.704 --> 00:00:34.723 is an incredible endeavor. 00:00:34.723 --> 00:00:37.741 But why does size matter so much for a swimmer? 00:00:37.741 --> 00:00:39.365 What makes the world of a sperm 00:00:39.365 --> 00:00:40.765 so fundamentally different 00:00:40.765 --> 00:00:42.621 from that of a sperm whale? 00:00:42.621 --> 00:00:44.056 To find out, we need to dive in 00:00:44.056 --> 00:00:46.197 to the physics of fluids. 00:00:46.197 --> 00:00:47.955 Here's a way to think about it. 00:00:47.955 --> 00:00:49.855 Imagine you are swimming in a pool. 00:00:49.855 --> 00:00:52.618 It's you and a whole bunch of water molecules. 00:00:52.618 --> 00:00:54.356 Water molecules outnumber you 00:00:54.356 --> 00:00:57.152 a thousand trillion trillion to one. 00:00:57.152 --> 00:00:58.439 So, pushing past them 00:00:58.439 --> 00:01:00.526 with your gigantic body is easy, 00:01:00.526 --> 00:01:02.910 but if you were really small, 00:01:02.910 --> 00:01:04.626 say you were about the size of a water molecule, 00:01:04.626 --> 00:01:06.380 all of a sudden, it's like you're swimming 00:01:06.380 --> 00:01:07.965 in a pool of people. 00:01:07.965 --> 00:01:09.792 Rather than simply swishing by 00:01:09.792 --> 00:01:11.599 all the teeny, tiny molecules, 00:01:11.599 --> 00:01:13.557 now every single water molecule 00:01:13.557 --> 00:01:16.069 is like another person you have to push past 00:01:16.069 --> 00:01:17.736 to get anywhere. 00:01:17.736 --> 00:01:20.979 In 1883, the physicist Osborne Reynolds 00:01:20.979 --> 00:01:23.102 figured out that there is one simple number 00:01:23.102 --> 00:01:26.016 that can predict how a fluid will behave. 00:01:26.016 --> 00:01:27.483 It's called the Reynolds number, 00:01:27.483 --> 00:01:29.505 and it depends on simple properties 00:01:29.505 --> 00:01:31.238 like the size of the swimmer, 00:01:31.238 --> 00:01:32.529 its speed, 00:01:32.529 --> 00:01:33.598 the density of the fluid, 00:01:33.598 --> 00:01:35.649 and the stickiness, or the viscosity, 00:01:35.649 --> 00:01:36.905 of the fluid. 00:01:36.905 --> 00:01:38.744 What this means is that creatures 00:01:38.744 --> 00:01:40.742 of very different sizes inhabit 00:01:40.742 --> 00:01:42.739 vastly different worlds. 00:01:42.739 --> 00:01:44.708 For example, because of its huge size, 00:01:44.708 --> 00:01:46.114 a sperm whale inhabits 00:01:46.114 --> 00:01:48.385 the large Reynolds number world. 00:01:48.385 --> 00:01:50.129 If it flaps its tail once, 00:01:50.129 --> 00:01:52.510 it can coast ahead for an incredible distance. 00:01:52.510 --> 00:01:54.251 Meanwhile, sperm live 00:01:54.251 --> 00:01:56.208 in a low Reynolds number world. 00:01:56.208 --> 00:01:58.381 If a sperm were to stop flapping its tail, 00:01:58.381 --> 00:02:00.936 it wouldn't even coast past a single atom. 00:02:00.936 --> 00:02:03.102 To imagine what it would feel like to be a sperm, 00:02:03.102 --> 00:02:04.560 you need to bring yourself down 00:02:04.560 --> 00:02:06.010 to its Reynolds number. 00:02:06.010 --> 00:02:08.151 Picture yourself in a tub of molasses 00:02:08.151 --> 00:02:09.178 with your arms moving 00:02:09.178 --> 00:02:12.142 about as slow as the minute hand of a clock, 00:02:12.142 --> 00:02:13.596 and you'd have a pretty good idea 00:02:13.596 --> 00:02:15.614 of what a sperm is up against. 00:02:15.614 --> 00:02:17.849 So, how do microbes manage to get anywhere? 00:02:17.849 --> 00:02:20.186 Well, many don't bother swimming at all. 00:02:20.186 --> 00:02:22.796 They just let the food drift to them. 00:02:22.796 --> 00:02:24.023 This is somewhat like a lazy cow 00:02:24.023 --> 00:02:27.073 that waits for the grass under its mouth to grow back. 00:02:27.073 --> 00:02:29.041 But many microbes do swim, 00:02:29.041 --> 00:02:32.009 and this is where those incredible adaptations come in. 00:02:32.009 --> 00:02:33.469 One trick they can use 00:02:33.469 --> 00:02:35.827 is to deform the shape of their paddle. 00:02:35.827 --> 00:02:37.550 By cleverly flexing their paddle 00:02:37.550 --> 00:02:39.992 to create more drag on the power stroke 00:02:39.992 --> 00:02:41.425 than on the recovery stroke, 00:02:41.425 --> 00:02:44.940 single-celled organisms like paramecia 00:02:44.940 --> 00:02:45.522 manage to inch their way 00:02:45.522 --> 00:02:47.715 through the crowd of water molecules. 00:02:47.715 --> 00:02:49.777 But there's an even more ingenious solution 00:02:49.777 --> 00:02:52.496 arrived at by bacteria and sperm. 00:02:52.496 --> 00:02:54.897 Instead of wagging their paddles back and forth, 00:02:54.897 --> 00:02:57.359 they wind them like a cork screw. 00:02:57.359 --> 00:02:59.206 Just as a cork screw on a wine bottle 00:02:59.206 --> 00:03:01.992 converts winding motion into forward motion, 00:03:01.992 --> 00:03:04.829 these tiny creatures spin their helical tails 00:03:04.829 --> 00:03:06.627 to push themselves forward 00:03:06.627 --> 00:03:10.255 in a world where water feels as thick as cork. 00:03:10.255 --> 00:03:12.509 Other strategies are even stranger. 00:03:12.509 --> 00:03:14.531 Some bacteria take Batman's approach. 00:03:14.531 --> 00:03:17.251 They use grappling hooks to pull themselves along. 00:03:17.251 --> 00:03:18.808 They can even use this grappling hook 00:03:18.808 --> 00:03:21.767 like a sling shot and fling themselves forward. 00:03:21.767 --> 00:03:24.219 Others use chemical engineering. 00:03:24.219 --> 00:03:27.508 H. pylori lives only in the slimy, acidic mucus 00:03:27.508 --> 00:03:29.990 inside our stomachs. 00:03:29.990 --> 00:03:30.399 It releases a chemical 00:03:30.399 --> 00:03:32.721 that thins out the surrounding mucus, 00:03:32.721 --> 00:03:34.674 allowing it to glide through slime. 00:03:34.674 --> 00:03:35.678 Maybe it's no surprise 00:03:35.678 --> 00:03:37.147 that these guys are also responsible 00:03:37.147 --> 00:03:39.058 for stomach ulcers. 00:03:39.058 --> 00:03:41.280 So, when you look really, really closely 00:03:41.280 --> 00:03:42.984 at our bodies and the world around us, 00:03:42.984 --> 00:03:45.112 you can see all sorts of tiny creatures 00:03:45.112 --> 00:03:47.121 finding clever ways to get around 00:03:47.121 --> 00:03:48.777 in a sticky situation. 00:03:48.777 --> 00:03:50.124 Without these adaptations, 00:03:50.124 --> 00:03:52.626 bacteria would never find their hosts, 00:03:52.626 --> 00:03:55.383 and sperms would never make it to their eggs, 00:03:55.383 --> 00:03:57.546 which means you would never get stomach ulcers, 00:03:57.546 --> 00:04:00.711 but you would also never be born in the first place.