WEBVTT - Ep143 "How do things last?" Part 1: neurons to civilizations 0:00:05.120 --> 0:00:09.320 What makes things last and what do very different lasting 0:00:09.480 --> 0:00:13.160 things have in common from the brain's point of view. 0:00:13.200 --> 0:00:17.279 What is the secret behind how music works? And why 0:00:17.360 --> 0:00:20.880 might a space alien not be able to understand the 0:00:21.040 --> 0:00:26.599 concept of music. Why does glass in medieval cathedrals look 0:00:26.720 --> 0:00:29.280 thicker at the bottom and what does this tell us 0:00:29.320 --> 0:00:33.280 about the world's religions. What was the most important weapon 0:00:33.440 --> 0:00:37.960 in ancient history and how did it completely disappear? Today 0:00:38.000 --> 0:00:41.560 we're going to talk about the concept of persistence, from 0:00:41.880 --> 0:00:45.559 sharks to Roman concrete to DNA. So get ready for 0:00:45.680 --> 0:00:52.559 a great brain stretch. Welcome to Inner Cosmos with me 0:00:52.720 --> 0:00:56.200 David Eagelman. I'm a neuroscientist and author at Stanford and 0:00:56.280 --> 0:00:59.560 in these episodes we dive deeply into our three pound 0:00:59.680 --> 0:01:03.680 universe to understand some of the most surprising aspects of 0:01:03.720 --> 0:01:20.039 our world. Today, we're talking about how things persist. I 0:01:20.080 --> 0:01:24.280 want to start today's episode by talking about Greek fire, 0:01:24.480 --> 0:01:28.560 which I'm obsessed with. This story begins with a refugee 0:01:28.760 --> 0:01:33.720 named Kalanikos who arrived in Constantinople in the late six 0:01:33.840 --> 0:01:38.520 hundreds and he handed the emperors a weapon that he'd invented, 0:01:38.560 --> 0:01:43.600 and this completely changed warfare in the Mediterranean. His weapon 0:01:43.680 --> 0:01:48.320 was known as Greek fire. Think of it like a flamethrower, 0:01:48.760 --> 0:01:52.600 a thousand years before the invention of the modern flamethrower. 0:01:52.960 --> 0:01:56.480 Greek fire was a thick liquid. It was petroleum based, 0:01:56.800 --> 0:01:59.920 and you'd hurl this in pots or you'd blast it 0:02:00.040 --> 0:02:03.480 from siphons and it would burn on the water. And 0:02:03.520 --> 0:02:07.240 the key is you could incinerate the enemy's ships this way, 0:02:07.520 --> 0:02:10.200 and no one had ever seen anything like this, So 0:02:10.400 --> 0:02:14.919 the Byzantine army immediately leveraged this as a defense that 0:02:15.040 --> 0:02:19.680 broke the Arab sieges that were attacking Constantinople, and Greek 0:02:19.720 --> 0:02:23.280 fire quickly became the most important weapon of the age. 0:02:23.720 --> 0:02:28.160 The Byzantines were able to turn naval battles completely on 0:02:28.200 --> 0:02:31.840 their head and control the waters. Now here's the thing. 0:02:32.639 --> 0:02:38.119 The recipe for how to make Greek fire was ferociously secretive. 0:02:38.800 --> 0:02:42.960 The recipe was compartmentalized, and it was known only to 0:02:43.080 --> 0:02:47.880 the emperor's circle and Klinikos's genetic line. And this worked 0:02:47.880 --> 0:02:50.360 out well. No one else knew how to make it, 0:02:50.840 --> 0:02:55.959 and over centuries the Byzantine chronicles credit Greek fire for 0:02:56.080 --> 0:03:00.400 saving the capitol, and by extension, the entire empire. But 0:03:01.240 --> 0:03:06.919 the price of extreme secrecy is fragility. They had a 0:03:07.080 --> 0:03:10.480 small bus factor. Now I'm not sure how Cammon this 0:03:10.600 --> 0:03:13.000 term is. We use this in Silicon Valley a lot. 0:03:13.320 --> 0:03:17.280 The idea of a bus factor is how many people 0:03:17.280 --> 0:03:20.280 in your company would have to get hit and killed 0:03:20.320 --> 0:03:23.679 by a bus before no one knows how to do 0:03:23.760 --> 0:03:26.760 something anymore. You want to make sure that you always 0:03:26.800 --> 0:03:31.440 have a bus factor bigger than one like only Bob 0:03:31.639 --> 0:03:35.520 knows the password to our servers, because if Bob gets 0:03:35.560 --> 0:03:39.080 hit by a bus, then your company grinds to a halt. 0:03:39.640 --> 0:03:43.440 The smaller the bus factor, the more likely the company 0:03:43.480 --> 0:03:45.840 is not going to make it. So you always need 0:03:45.880 --> 0:03:50.480 to make sure you have redundancy. So in the case 0:03:50.600 --> 0:03:54.280 of Greek Fire, you've got a very small bus factor 0:03:54.360 --> 0:03:58.640 for secrecy reasons, and all you need are some political 0:03:58.720 --> 0:04:03.160 convulsions or the failure of a single lineage, and suddenly 0:04:03.520 --> 0:04:07.080 no one knows how to make Greek fire anymore. And 0:04:07.080 --> 0:04:11.680 that's what happened after the Fourth Crusades sack of Constantinople. 0:04:11.960 --> 0:04:16.880 After that, the Byzantines all remembered the weapon, but nobody 0:04:16.920 --> 0:04:20.919 remembered how to make it anymore. So the lesson of 0:04:20.960 --> 0:04:25.799 Greek fire is that critical knowledge can easily evaporate. And really, 0:04:25.839 --> 0:04:30.240 when we look across history, we find that's a common story. 0:04:30.400 --> 0:04:35.839 Things or organizations get started or invented, and eventually they 0:04:36.040 --> 0:04:40.120 die out. So this kind of story arc how we 0:04:40.520 --> 0:04:43.839 once knew something and then totally lost it has always 0:04:43.880 --> 0:04:48.120 made me wonder how anything survives for a long time, 0:04:48.200 --> 0:04:53.760 how anything persists because the universe is slipping away moment 0:04:53.839 --> 0:04:57.839 by moment. But everywhere we look we do find systems 0:04:58.120 --> 0:05:01.680 that come into existence that we fuse to let things go. 0:05:02.400 --> 0:05:07.360 Brains hold on to a continuous sense of self, cultures 0:05:07.760 --> 0:05:13.160 hold on to very old traditions, religion's last hundreds of generations. 0:05:13.520 --> 0:05:18.080 The body plans of sharks has continued essentially unchanged for 0:05:18.480 --> 0:05:23.680 hundreds of millions of years. So today's episode is about persistence, 0:05:24.160 --> 0:05:27.920 about things lasting through time. This is something I've always 0:05:27.960 --> 0:05:30.920 been fascinated with. So today we're going to weave together 0:05:31.040 --> 0:05:34.040 several paths to get to the bottom of a question. 0:05:34.680 --> 0:05:38.480 What makes things last from neurons to civilizations, and what 0:05:38.520 --> 0:05:43.600 do very different lasting things have in common? And this 0:05:43.640 --> 0:05:46.400 is part one. Next week, in part two, we're going 0:05:46.440 --> 0:05:50.520 to talk about thinking about our current moment on a 0:05:50.600 --> 0:05:55.080 ten thousand year timescale, we'll talk with Alexander Rose, the 0:05:55.160 --> 0:05:59.480 former executive director of the Long Now Foundation, where we'll 0:05:59.480 --> 0:06:05.160 hear about what organizations last through generations and why. For today, 0:06:05.200 --> 0:06:08.000 we're going to start in the brain with very brief 0:06:08.040 --> 0:06:11.960 windows of persistence, and then we'll move out to see 0:06:11.960 --> 0:06:17.200 some of these same principles across decades and centuries and millennia. 0:06:17.720 --> 0:06:22.599 So let's dive into the inner cosmos. Every sensory system 0:06:22.720 --> 0:06:25.800 faces a fundamental problem, which is that the world arrives 0:06:26.240 --> 0:06:31.240 in small brief pieces. The brain solves this by holding on, 0:06:31.400 --> 0:06:36.120 by making the signals persist longer than they would otherwise. 0:06:36.520 --> 0:06:40.680 For example, when you're hearing my voice, every sound I 0:06:40.760 --> 0:06:44.560 make is only in the outside world between us for 0:06:44.680 --> 0:06:49.040 some tens of milliseconds. But your brain holds onto those sounds, 0:06:49.279 --> 0:06:53.719 it retains them internally, so they're around long enough to 0:06:53.720 --> 0:06:57.520 be combined with whatever follows. So, in other words, all 0:06:57.560 --> 0:07:00.760 that comes through your speakers at any moment is a 0:07:00.800 --> 0:07:05.040 single phoneme, a little unit of speech. So you hear 0:07:05.480 --> 0:07:15.960 one phoneme and then another, and then another and then another. 0:07:17.120 --> 0:07:19.480 And the key is that it wouldn't mean anything to you, 0:07:19.560 --> 0:07:22.480 except they are able to hold them in your brain 0:07:22.560 --> 0:07:25.800 and replay them so that each little moment is heard 0:07:25.880 --> 0:07:30.400 with the others kept in mind, so you hear speech. 0:07:31.480 --> 0:07:34.880 So with these little sounds of speech, these phonemes, the 0:07:34.920 --> 0:07:37.440 brain preserves each of them long enough to be able 0:07:37.440 --> 0:07:42.080 to understand sounds in context. If the sound appeared and 0:07:42.200 --> 0:07:45.239 disappeared as quickly as it does from the world, nothing 0:07:45.320 --> 0:07:48.600 would have any meaning. And I'll give you another example 0:07:48.600 --> 0:07:53.080 which will make this even clearer. Music Music also depends 0:07:53.160 --> 0:07:57.360 on persistence in the brain. You can understand a melody 0:07:57.800 --> 0:08:02.120 only because earlier notes remain present in your brain when 0:08:02.160 --> 0:08:06.600 the later notes arrive. Without that, music would collapse into 0:08:06.760 --> 0:08:11.440 isolated blips with no rhythm and meaning. Why Because a 0:08:11.560 --> 0:08:18.600 single pitch is just an air compression wave. It arrives, 0:08:18.680 --> 0:08:23.360 it vibrates the ear drum, and then it physically disappears 0:08:23.800 --> 0:08:26.760 on its own. It doesn't carry much meaning. But when 0:08:26.880 --> 0:08:40.960 notes arrive in sequence, a melody appears. But the melody 0:08:41.080 --> 0:08:45.840 only exists because of the brain's ability to keep information alive. 0:08:46.440 --> 0:08:49.720 As each note reaches your ears, the previous notes are 0:08:49.800 --> 0:08:54.560 still present in neural activity. They remain in there shooting 0:08:54.600 --> 0:08:57.960 around in the neurons, and they do this long enough 0:08:58.040 --> 0:09:01.760 to overlap with what comes next. This allows your brain 0:09:01.840 --> 0:09:07.200 to create relationships across time. What you hear at any 0:09:07.280 --> 0:09:10.920 moment is shaped by what you heard in the moments before. So, 0:09:11.120 --> 0:09:16.360 just like speech, music depends on this internal persistence. A 0:09:16.480 --> 0:09:21.480 melody requires comparison, just like a word or sentence does. 0:09:21.840 --> 0:09:25.640 When we talk about resolving tension in music, this is 0:09:25.679 --> 0:09:31.559 only possible because the tension has been sustained. Even understanding 0:09:31.640 --> 0:09:36.239 a simple beat depends on your brain's capacity to predict 0:09:36.360 --> 0:09:39.920 the next thump by preserving the timing of at least 0:09:40.000 --> 0:09:43.679 the last two thumps. The stuff of the outside world 0:09:44.040 --> 0:09:46.960 has to be held onto and maintained. So let me 0:09:47.040 --> 0:09:49.800 drive this point home with an example that I love. 0:09:50.280 --> 0:09:53.800 A tune that you recognize has nothing to do with 0:09:53.960 --> 0:09:59.719 the individual notes. Instead, it's all about the relationship between 0:09:59.760 --> 0:10:04.040 the ne notes. And this is why you can transpose 0:10:04.160 --> 0:10:07.080 a tune to any key, so that all the notes 0:10:07.120 --> 0:10:11.120 are different, but their relationship is the same. So here's 0:10:11.160 --> 0:10:20.880 happy Birthday and the key of C. But if I 0:10:21.000 --> 0:10:24.679 transpose this to another key, you'll see that the individual 0:10:24.720 --> 0:10:29.160 notes are different, but you have no trouble recognizing the melody. 0:10:36.760 --> 0:10:39.720 Why can you still recognize the tune? Because it's not 0:10:39.960 --> 0:10:43.400 about the individual notes. A piece of music is about 0:10:43.720 --> 0:10:49.200 the relationship between the notes, and that relationship depends on 0:10:49.679 --> 0:10:53.440 holding in mind what just came before. So here's my 0:10:53.520 --> 0:10:57.400 assertion for today. I suggest that music would not be 0:10:57.720 --> 0:11:03.480 understood by any the alien species that doesn't keep signals 0:11:03.640 --> 0:11:08.080 alive internally in their nervous systems for a little while. 0:11:08.240 --> 0:11:11.600 If some space alien didn't have a way to make 0:11:11.640 --> 0:11:15.800 signals persist, but instead they were just reactive to the moment, 0:11:16.440 --> 0:11:20.440 then they would have an immediate sensory experience for each note, 0:11:20.679 --> 0:11:23.920 but they wouldn't be able to understand the notes as 0:11:23.960 --> 0:11:28.640 having a relationship. Music only makes sense when you think 0:11:28.640 --> 0:11:31.960 about it as the tension between what is being held 0:11:32.000 --> 0:11:35.200 and what arrives next. Same with language, of course, if 0:11:35.280 --> 0:11:39.160 the alien heard each of our phonemes and then forgot 0:11:39.200 --> 0:11:43.440 it before the next phoneme arrived, language couldn't mean anything. 0:11:43.840 --> 0:11:50.720 Everything would just be disconnected. Sounds. Meaning depends on holding 0:11:50.960 --> 0:11:56.120 earlier signals in mind while later ones unfold, so the 0:11:56.200 --> 0:11:59.960 brain is constantly retaining signals and giving them a law 0:12:00.040 --> 0:12:03.600 longer life than they actually have in the outside world, 0:12:03.880 --> 0:12:07.400 and we see this across all the senses. Take vision. 0:12:07.679 --> 0:12:10.400 Imagine you're sitting in a dark room and I flash 0:12:10.440 --> 0:12:14.080 an LED light on and off. It's so brief, let's 0:12:14.080 --> 0:12:17.840 say twenty five milliseconds, that it's already gone before you 0:12:17.880 --> 0:12:20.360 can register it. But here's the wacky part that my 0:12:20.440 --> 0:12:23.280 lab has done a lot of experiments on. The flash 0:12:23.320 --> 0:12:26.960 seems to you to last a little while, like one 0:12:27.040 --> 0:12:31.240 hundred milliseconds. It's not existing in the world anymore, but 0:12:31.320 --> 0:12:36.199 it exists for even longer directly in your conscious experience. 0:12:37.080 --> 0:12:42.560 The neurons that represented in awareness continue firing. That brief 0:12:42.600 --> 0:12:48.600 flash is stretched in time. This phenomenon is called visual persistence, 0:12:48.960 --> 0:12:51.040 and it's a version of the same idea, but this 0:12:51.080 --> 0:12:56.320 time directly in conscious awareness. That perception makes things last 0:12:56.400 --> 0:13:00.439 longer than they exist in the world. And visual persistence 0:13:00.559 --> 0:13:04.160 is the only reason that movies work, because we're able 0:13:04.160 --> 0:13:09.679 to perceive continuity and motion from looking at still frames 0:13:10.120 --> 0:13:15.040 flashed one after the other. In other words, because of persistence, 0:13:15.320 --> 0:13:21.280 successive images overlap in the mind. The brain integrates pictures 0:13:21.360 --> 0:13:25.680 over time, blending what just happened with what is happening now, 0:13:25.720 --> 0:13:30.840 and the result is motion from flashed still frames. You 0:13:30.880 --> 0:13:34.800 can see this visual persistence with a simple demonstration. If 0:13:34.840 --> 0:13:36.840 you flash a light on and off, and on and 0:13:36.840 --> 0:13:39.560 off and on and off really quickly, it will fuse 0:13:39.800 --> 0:13:43.320 into what looks like a steady light. The speed at 0:13:43.320 --> 0:13:47.200 which this happens is called the flicker fusion threshold, and 0:13:47.240 --> 0:13:50.719 it's normally about fifty five hurts, meaning you do this 0:13:50.800 --> 0:13:54.320 on off, on off fifty five times every second. So 0:13:54.520 --> 0:13:58.679 if a light flickers fast enough, the flicker goes away 0:13:58.800 --> 0:14:02.200 and it seems like a solid light now. One of 0:14:02.200 --> 0:14:04.200 the things my lab has studied over the years is 0:14:04.240 --> 0:14:07.760 the way we can leverage that as a diagnostic tool, 0:14:07.840 --> 0:14:10.880 because it turns out that not all brains make things 0:14:10.960 --> 0:14:14.800 persist for the same amount of time, and specifically, people 0:14:14.840 --> 0:14:20.160 who are suffering with schizophrenia have a longer window of persistence. 0:14:20.880 --> 0:14:25.160 A brief flash lingers in their brain for a longer time. 0:14:25.520 --> 0:14:29.960 What this means is that the visual world has more overlap, So, 0:14:30.080 --> 0:14:34.560 for example, a person with schizophrenia has a lower flicker 0:14:34.640 --> 0:14:37.840 fusion threshold. What that means is that I can flash 0:14:37.880 --> 0:14:41.160 a light on and off more slowly and the person 0:14:41.200 --> 0:14:44.880 will see that as a steady light. In other words, 0:14:44.920 --> 0:14:48.400 persistence lasts longer. Now I'm telling you that little detail 0:14:48.400 --> 0:14:52.360 about schizophrenia because this allowed me to build a purely 0:14:52.640 --> 0:14:56.640 visual test for diagnosing schizophrenia, and this is something I'm 0:14:56.680 --> 0:14:59.960 currently running tests on at Stanford. If you're interested, I'm 0:15:00.120 --> 0:15:17.160 linking my papers on this to the show notes. So 0:15:17.720 --> 0:15:20.520 let's zoom out to what we've seen so far about 0:15:20.640 --> 0:15:25.040 how the brain makes very short signals persist. The brain 0:15:25.120 --> 0:15:29.080 constructs what the psychologist William James a century ago called 0:15:29.520 --> 0:15:34.320 this specious present, by which he meant that the present 0:15:34.440 --> 0:15:38.360 moment is not actually an instant, but it's a short 0:15:38.560 --> 0:15:44.120 time window. This little sliding window of now causes the 0:15:44.160 --> 0:15:48.880 present moment to have a certain thickness. Everything sticks around 0:15:48.920 --> 0:15:51.560 for a bit in the brain so that events in 0:15:51.760 --> 0:15:56.320 time can get compared. In other words, your neural circuits 0:15:56.400 --> 0:16:00.600 refuse to let the world disappear too quickly. And we 0:16:00.640 --> 0:16:05.120 see this kind of temporal integration in every sensory system, 0:16:05.120 --> 0:16:09.080 which allows the immediate past to remain active in the present, 0:16:09.360 --> 0:16:12.960 whether the brain is stitching notes into music or still 0:16:13.040 --> 0:16:17.640 frames into motion. It relies on short lived memory to 0:16:17.880 --> 0:16:24.080 transform fragments into meaning. So persistence is something we talk 0:16:24.160 --> 0:16:27.160 about in neuroscience, but today I want to explore the 0:16:27.200 --> 0:16:32.640 notion of persistence from animals to nations to civilizations. And 0:16:32.680 --> 0:16:35.720 in thinking about this, I now think there are five 0:16:36.040 --> 0:16:40.160 main ways that things persist. So let's dive in. The 0:16:40.240 --> 0:16:44.800 First reason something persists is that it's optimized on some 0:16:44.960 --> 0:16:49.960 fitness landscape, meaning that it can't really get better. So generally, 0:16:50.040 --> 0:16:54.960 when we think about evolution, we picture a constant transformation 0:16:55.080 --> 0:16:58.720 of species, with new forms replacing old ones, like the 0:16:59.080 --> 0:17:03.600 cartoon of Homo sapiens evolving from a common ancestor with 0:17:03.680 --> 0:17:07.720 great apes until he's walking upright. But what's interesting is 0:17:07.760 --> 0:17:11.800 that some animals haven't changed in hundreds of millions of years, 0:17:11.920 --> 0:17:16.240 like sharks. Little details have shifted, but the overall body 0:17:16.280 --> 0:17:21.639 plan has persisted. Why Well, because they work, they're streamlined, 0:17:21.720 --> 0:17:26.480 they have very efficient propulsion, and their sensory systems are 0:17:26.520 --> 0:17:31.320 perfectly tunfoer hunting, so the design continues to succeed. So 0:17:31.480 --> 0:17:35.960 this first reason is obvious. Some things persist because they 0:17:35.960 --> 0:17:39.600 are at or near a local optimum, and if they 0:17:39.600 --> 0:17:43.240 were to change, that would probably introduce risk, as in, 0:17:43.320 --> 0:17:45.679 you're more likely to muck up the system rather than 0:17:45.720 --> 0:17:50.720 to improve it. So stability becomes the winning strategy, and 0:17:50.840 --> 0:17:54.639 persistence here is just a signal of success. And of 0:17:54.640 --> 0:17:58.840 course we see this with everything. Technologies persist when they 0:17:58.920 --> 0:18:03.359 meet needs efficiently. We can see these kind of technological 0:18:03.720 --> 0:18:08.680 sharks everywhere. The bicycle rolling down the street still has 0:18:08.720 --> 0:18:12.400 the triangular frame that was designed over a century ago. 0:18:13.000 --> 0:18:16.240 It's a design that balances strength and weight and efficiency 0:18:16.640 --> 0:18:21.400 so well that most attempts to reinvent the bicycle these 0:18:21.440 --> 0:18:27.240 remain curiosities. Or look at steel shipping containers. These stack 0:18:27.320 --> 0:18:31.080 and ports around the world. They have standardized dimensions and 0:18:31.119 --> 0:18:34.680 that locks ships and cranes and trucks and warehouses into 0:18:34.720 --> 0:18:37.960 a single, coordinated system that would only get messed up 0:18:38.000 --> 0:18:40.080 if you tried to change it. Or look at this 0:18:40.240 --> 0:18:46.679 screw with its spiral thread which translates rotation into linear force. 0:18:47.240 --> 0:18:51.800 This solves its little mechanical problems so cleanly that its 0:18:51.880 --> 0:18:56.480 basic form has persisted for centuries. Or take the four 0:18:56.600 --> 0:19:01.760 tined fork. This is centuries. There are variations, with the 0:19:01.800 --> 0:19:05.479 basic form of a fork stays the same because it 0:19:05.520 --> 0:19:11.120 accomplishes spearing and lifting efficiently across lots of food types. 0:19:11.600 --> 0:19:15.600 So these technologies persist because they reached a place of 0:19:15.640 --> 0:19:20.760 alignment with performance and costs and usability. Just like this shark, 0:19:21.040 --> 0:19:26.359 they solve their ecological problems efficiently, and once a form 0:19:26.480 --> 0:19:30.000 settles into a valley where it works well, staying there 0:19:30.600 --> 0:19:35.800 often proves a better decision than constant reinvention. So, like 0:19:35.840 --> 0:19:38.399 I said, I think that's the obvious reason why some 0:19:38.440 --> 0:19:41.359 things persist. But I was thinking about this, and I 0:19:41.400 --> 0:19:45.760 think there are four more interesting angles that we could 0:19:45.760 --> 0:19:51.080 take on this about other reasons why things last. For 0:19:51.160 --> 0:19:53.679 the first one, let's turn to something I think about often. 0:19:54.119 --> 0:19:57.440 Why you can recognize a friend that you haven't seen 0:19:57.480 --> 0:20:00.159 in like ten years. This happened to me at my 0:20:00.400 --> 0:20:03.880 high school reunion, where I saw people whose hair had 0:20:03.960 --> 0:20:07.359 changed entirely, maybe they didn't have hair anymore, Their face 0:20:07.400 --> 0:20:10.639 had changed shape a bit, their body had changed shape, 0:20:11.040 --> 0:20:15.240 and yet I was able to mostly recognize them, so 0:20:15.359 --> 0:20:20.280 something about them remained unmistakable. How do our brains allow 0:20:20.400 --> 0:20:24.760 someone's identity to persist for so long? And, by the way, 0:20:24.800 --> 0:20:27.160 I've wondered the same thing when I look at Google 0:20:27.359 --> 0:20:31.520 or Apple photo albums. In these photo albums, they recognize 0:20:31.560 --> 0:20:34.720 you or your kids through decades of time, even though 0:20:34.760 --> 0:20:37.920 you can see that everyone's faces have changed a lot, 0:20:38.040 --> 0:20:44.119 everyone's gotten older. So what's going on with brains and algorithms? Okay, 0:20:44.160 --> 0:20:46.480 for both of these, it's simply that they're figuring out 0:20:47.040 --> 0:20:51.879 which aspects of a face are worth carrying forward. They're 0:20:51.920 --> 0:20:56.719 sensitive to deep structure rather than the surface detail, so 0:20:57.080 --> 0:21:00.639 things like the spacing of features, how how far apart 0:21:00.680 --> 0:21:03.320 your eyes are, and how that relates to your nose, 0:21:03.480 --> 0:21:06.560 and the details of your mouth and your smile and 0:21:06.840 --> 0:21:10.119 your filterum, that's the vertical groove that connects to the 0:21:10.119 --> 0:21:12.520 bottom of the nose to your upper lip. All that 0:21:12.640 --> 0:21:18.399 geometry remains stable enough to anchor down your identity even 0:21:18.440 --> 0:21:22.960 while everything else changes. This is why airport security cameras 0:21:23.000 --> 0:21:26.240 can recognize the face even if you dye your hair 0:21:26.680 --> 0:21:31.600 or wear colored contacts or whatever. Recognition works because certain 0:21:31.640 --> 0:21:36.480 information persists, and the algorithms can ignore certain kinds of change, 0:21:36.560 --> 0:21:39.560 like shifts in the lighting or changes in the hairstyle. 0:21:39.840 --> 0:21:42.639 They filter all that stuff away. And for your brain, 0:21:42.680 --> 0:21:47.320 in particular, it's training on faces from infancy onward. It 0:21:47.400 --> 0:21:51.280 sees them again and again, and it learns which features 0:21:51.720 --> 0:21:56.960 persist across situations. Over time, it builds an internal model 0:21:57.359 --> 0:22:03.000 that figures out what matters enough to preserve. This question 0:22:03.119 --> 0:22:07.960 of what information to keep is asked by memory. More generally, 0:22:08.400 --> 0:22:12.040 most of your world passes through you and you don't 0:22:12.040 --> 0:22:17.400 remember it. Your memory only retains some experiences and decides 0:22:17.440 --> 0:22:21.359 the rest just doesn't matter. And in fact, culture works 0:22:21.359 --> 0:22:25.240 this way too, It repeats practices that continue to matter 0:22:25.760 --> 0:22:30.560 while letting most others drift into obscurity. What these all 0:22:30.600 --> 0:22:35.439 have in common is that persistence depends on abstraction. What 0:22:35.560 --> 0:22:39.080 survives is something a little removed from the details. So 0:22:39.240 --> 0:22:43.240 the brain's ability to recognize a familiar face after decades 0:22:43.800 --> 0:22:48.240 is one example of a universal strategy, which is preserving 0:22:48.440 --> 0:22:54.200 identity by learning what to ignore. In other words, persistence 0:22:54.440 --> 0:22:58.080 is about knowing what to hold on to. And nowhere 0:22:58.160 --> 0:23:00.360 does this apply more than to the notion of your 0:23:00.440 --> 0:23:05.040 own self. You consider you you, even though you have 0:23:05.160 --> 0:23:08.560 changed enormously over the years. You're not the same person 0:23:08.920 --> 0:23:11.800 that you were five years ago, and certainly all your 0:23:11.960 --> 0:23:15.800 cells have changed. I refer you to Inner Cosmos episode 0:23:15.840 --> 0:23:19.040 eighty two, about the continuity of the Self and the 0:23:19.080 --> 0:23:21.560 ship of theseus for more on this front. But The 0:23:21.600 --> 0:23:24.800 point I want to emphasize now is simply that you 0:23:25.240 --> 0:23:29.320 change through time, but you have certain things in common, 0:23:29.440 --> 0:23:32.000 like your name and the stories you like to tell, 0:23:32.040 --> 0:23:34.160 and where did you go to college and so on, 0:23:34.600 --> 0:23:38.280 and you end up with the illusion of a stable 0:23:38.440 --> 0:23:42.800 self that persists through time. Now here's the thing that 0:23:42.880 --> 0:23:47.120 might come as a surprise. The persistence of your self 0:23:47.720 --> 0:23:50.719 is something that you have to learn. This is what 0:23:50.760 --> 0:23:55.360 we call a cognitive development. So look at very young children. 0:23:55.840 --> 0:23:59.600 A young child can recognize her reflection in a mirror 0:24:00.040 --> 0:24:03.000 pretty early in life. She notices that when she smiles, 0:24:03.040 --> 0:24:06.040 the mirror image smiles. She notices that when she touches 0:24:06.080 --> 0:24:09.080 her face, so does the mirror image. So that's good. 0:24:09.560 --> 0:24:15.560 But this kind of self recognition doesn't yet work across time. 0:24:16.119 --> 0:24:19.280 If you show that same child a video of herself 0:24:19.600 --> 0:24:23.960 from a year earlier, something surprising happens. She might point 0:24:24.000 --> 0:24:26.879 to the screen and say that's a girl, or she 0:24:27.000 --> 0:24:31.080 calls the child by another name. The idea that this 0:24:31.359 --> 0:24:37.000 earlier version of her belongs to herself hasn't yet taken hold. 0:24:37.760 --> 0:24:42.200 So selfhood across time is something we have to figure out. 0:24:42.560 --> 0:24:45.359 What happens over the first few years of life is 0:24:45.359 --> 0:24:50.920 that children gradually assemble a sense of persistence. They come 0:24:50.920 --> 0:24:55.000 to understand that the person who fell down yesterday is 0:24:55.040 --> 0:24:58.600 the same one standing here right now. This depends on 0:24:58.840 --> 0:25:03.800 memory and languag whig and narrative all developing together, so 0:25:03.920 --> 0:25:07.720 the self becomes a story that can be updated while 0:25:07.800 --> 0:25:11.840 retaining a central thread. Now, one consequence of this is 0:25:11.880 --> 0:25:16.119 we all have what's called childhood amnesia, meaning we don't 0:25:16.359 --> 0:25:20.000 remember anything from the first few years of our lives. Now, 0:25:20.040 --> 0:25:23.000 it's not that you didn't have memory during those first 0:25:23.040 --> 0:25:25.639 few years, because you were remembering a ton of stuff 0:25:25.640 --> 0:25:29.280 and learning from it. But what was missing was a 0:25:29.440 --> 0:25:35.600 durable narrative to scaffold everything. You had experiences, but there 0:25:35.680 --> 0:25:40.439 was no persistent identity that you could stitch them into. 0:25:41.280 --> 0:25:46.480 As you grew, you began to tag experiences as belonging 0:25:46.800 --> 0:25:50.800 to you. You learned which details madded enough to carry 0:25:50.920 --> 0:25:55.320 forward and which didn't really matter. So your identity emerged 0:25:55.400 --> 0:26:01.040 as a stable pattern extracted from constant chain, just like 0:26:01.240 --> 0:26:05.159 with face recognition algorithms. And now on to the next 0:26:05.200 --> 0:26:08.439 way that some things persist, and that has to do 0:26:08.520 --> 0:26:12.800 with self repair. So, for example, if you walk along 0:26:12.880 --> 0:26:17.520 an ancient Roman Harbor, you can still find concrete structures 0:26:17.960 --> 0:26:23.000 that have endured wave after wave for two thousand years. 0:26:23.480 --> 0:26:27.840 These blocks were sitting in seawater, exposed to salt and 0:26:27.880 --> 0:26:32.680 stress and erosion, but they only got better with time. Now, 0:26:32.720 --> 0:26:36.920 how in the world does Roman concrete shrug off centuries 0:26:37.320 --> 0:26:42.760 of battering while our modern concrete would spawl and corrode 0:26:42.880 --> 0:26:49.600 under these circumstances, Roman concrete persists because it can self repair. 0:26:50.040 --> 0:26:53.560 This is probably accidental, but it's baked into the chemistry. 0:26:54.040 --> 0:26:57.320 In twenty twenty three, a team at MIT performed all 0:26:57.440 --> 0:27:01.320 kinds of imaging and found that Roman builder hot mixed 0:27:01.400 --> 0:27:07.119 their concrete with quicklime, which left tiny lime clasts scattered 0:27:07.160 --> 0:27:11.600 through the mortar. And these act like self healing capsules. 0:27:11.960 --> 0:27:18.040 So when micro cracks form and water seeps in, calcium 0:27:18.080 --> 0:27:24.280 dissolves and reprecipitates this calcium carbonate, which seals the crack. So, 0:27:24.400 --> 0:27:29.440 whether purposeful or not, the Romans achieved persistence of their structures. 0:27:29.680 --> 0:27:33.960 So even though the concrete lasts, how did this knowledge fade? Well, 0:27:33.960 --> 0:27:39.680 it's partly because history re optimizes, so various other recipes 0:27:39.720 --> 0:27:43.760 for cement took over, most notably Portland cement by the 0:27:43.800 --> 0:27:48.240 eighteen hundreds, mostly because Portland cement cures quickly and it 0:27:48.400 --> 0:27:52.680 pairs well with steel. In other words, we switched goals 0:27:52.720 --> 0:27:57.120 to speed and early strength and skyscrapers and so on, 0:27:57.440 --> 0:28:01.080 and so the recipe changed. But just go stand in 0:28:01.119 --> 0:28:04.280 an aqueduct and run your fingers over the mortar and 0:28:04.320 --> 0:28:08.840 you'll see a material that knows how to persist across 0:28:09.240 --> 0:28:12.800 centuries by self repair. And now I want to move 0:28:12.840 --> 0:28:15.800 on to a fourth way that things persist. And by 0:28:15.800 --> 0:28:19.199 the way, none of these methods are mutually exclusive. To 0:28:19.400 --> 0:28:23.080 understand this, one walk into an old cathedral and look 0:28:23.119 --> 0:28:27.280 closely at the windows. The glass often appears thicker at 0:28:27.320 --> 0:28:30.560 the bottom than at the top. Why well, what was 0:28:30.560 --> 0:28:32.960 explained to me when I was touring a cathedral in 0:28:33.040 --> 0:28:36.480 Europe a while ago, was that the glass behaves like 0:28:36.560 --> 0:28:40.200 a very slow liquid. You give it enough centuries and 0:28:40.240 --> 0:28:44.280 it gradually flows downward under its own weight. What's cool 0:28:44.280 --> 0:28:47.360 about this idea is that it invites you to imagine 0:28:47.440 --> 0:28:51.360 time as a force so powerful that even solid matter 0:28:51.520 --> 0:28:54.720 eventually yields. And by the way, I've heard this explanation 0:28:54.800 --> 0:28:59.640 of cathedral glass a dozen times since then, But that 0:28:59.720 --> 0:29:05.880 explanation is totally wrong. Glass at room temperature behaves as 0:29:05.920 --> 0:29:09.840 a solid, and its viscosity is so high that any 0:29:10.000 --> 0:29:14.160 flow would require time scales far longer than the age 0:29:14.200 --> 0:29:18.160 of the universe. As it turns out, the uneven thickness 0:29:18.240 --> 0:29:24.240