WEBVTT - Why is momentum conserved? 0:00:08.840 --> 0:00:13.200 Hey, Dado, do you think podcasts obey the laws of physics? Oh? 0:00:13.280 --> 0:00:14.800 I sure, hope. So I don't want to get some 0:00:14.840 --> 0:00:17.880 sort of fine. What makes you worry about it? Well, 0:00:18.239 --> 0:00:21.479 I've heard some people who use podcasts to fall asleep. Well, 0:00:21.600 --> 0:00:23.400 I don't know. That sounds pretty harmless. How is that 0:00:23.520 --> 0:00:26.200 violating the laws of physics? Well, isn't there something about 0:00:26.239 --> 0:00:29.120 how bodies and motions stay in motion? I see where 0:00:29.120 --> 0:00:31.319 you're going with this. That could be a problem if 0:00:31.360 --> 0:00:34.559 the podcast puts bodies at rest that used to be 0:00:34.640 --> 0:00:36.880 in motion. Yeah, yeah, that might be a problem, like 0:00:36.920 --> 0:00:40.360 if they're driving or something. Maybe if you get in 0:00:40.440 --> 0:00:42.479 bed and get comfortable before they, you know, put on 0:00:42.520 --> 0:00:45.560 a podcast, then they could just continue being at the rest. 0:00:46.800 --> 0:00:48.519 Then there might be asleep before we get to the 0:00:48.520 --> 0:00:50.800 main topic of the podcast. Maybe we just put a 0:00:50.800 --> 0:00:54.960 bunch of people to sleep. Wake up? Wake up? What 0:00:55.320 --> 0:00:58.600 I put you to sleep? You're the physicist. I take 0:00:58.720 --> 0:01:16.440 naps wherever I can find them. Hi am or handmake 0:01:16.480 --> 0:01:19.760 cartoonist and the creator of PhD comics. Hi. I'm Daniel. 0:01:19.760 --> 0:01:22.640 I'm a particle physicist and a professor at U c Irvine, 0:01:22.760 --> 0:01:27.200 and I try to stay in motion is that hard 0:01:27.240 --> 0:01:29.319 as a physicist, don't you sit in your couch or 0:01:29.319 --> 0:01:31.840 your desk all the time and just think of solutions 0:01:31.840 --> 0:01:35.199 to the questions about the universe. Yeah, but I find 0:01:35.200 --> 0:01:37.279 that as I get older, a body at rest tends 0:01:37.319 --> 0:01:39.360 to stay at rest, so it's harder and harder to 0:01:39.360 --> 0:01:41.959 get out of that couch. I think a body at 0:01:42.000 --> 0:01:45.919 rest also tends to get bigger. Unfortunately, with our age, 0:01:47.280 --> 0:01:50.760 that's true, and we gravitationally attract more physicists onto the couch. 0:01:51.360 --> 0:01:55.720 That's right, We expand spacetime or on our waist. It's 0:01:55.720 --> 0:01:59.640 a weird law of the universe, the no diet theorem, 0:01:59.680 --> 0:02:02.760 the law of general snack ativity. But anyways, welcome to 0:02:02.800 --> 0:02:05.640 our podcast, Daniel and Jorge Explain the Universe, a production 0:02:05.680 --> 0:02:08.320 of My Heart Radio in which we put your mind 0:02:08.560 --> 0:02:12.760 into motion to understand the fundamental nature of the universe 0:02:12.840 --> 0:02:16.120 around us, or at least to ask the deep questions 0:02:16.160 --> 0:02:19.320 about how it works and try to observe the patterns, 0:02:19.400 --> 0:02:23.359 the trends, the symmetries, the conservation laws, the fundamental rules 0:02:23.360 --> 0:02:27.200 that seem to be organizing our universe. On this podcast, 0:02:27.240 --> 0:02:29.840 we ask all of those big questions, and we don't 0:02:29.880 --> 0:02:32.960 shy away from trying to find answers. Yeah, because the 0:02:33.040 --> 0:02:35.679 universe is full of things to ask questions about, lots 0:02:35.720 --> 0:02:38.200 of questions that we still haven't figured out despite hundreds 0:02:38.200 --> 0:02:41.280 and maybe thousands of years of science and observing the universe, 0:02:41.720 --> 0:02:44.880 and we try to talk about it and to increase 0:02:44.919 --> 0:02:47.880 the gravity I guess in your brain, right, like information 0:02:48.160 --> 0:02:51.720 causes gravity to increase, right, that's true. Yeah, Eventually our 0:02:51.760 --> 0:02:53.720 goal is to turn your brain into a black hole. 0:02:54.120 --> 0:02:58.720 Oh no, no, we want stuff to get out. That's true. 0:02:59.480 --> 0:03:02.320 That would be undermining the fundamental purpose of our podcast. 0:03:02.520 --> 0:03:04.480 That's right. It wouldn't go viral if I don't want 0:03:04.480 --> 0:03:07.760 to turn into a black hole. But we do want 0:03:07.880 --> 0:03:10.880 your brain to absorb information or to feel like you 0:03:10.919 --> 0:03:14.280 are part of this centuries or maybe even millennial long 0:03:14.639 --> 0:03:18.560 progress towards understanding the universe. When people look back on 0:03:18.600 --> 0:03:21.480 the path of theoretical physics in a hundred years or 0:03:21.520 --> 0:03:24.359 a thousand years, we will wonder how far along that 0:03:24.400 --> 0:03:27.200 path we are. Have we just gotten started. Are we 0:03:27.280 --> 0:03:31.040 around the corner from revealing the deepest secrets of the universe? 0:03:31.280 --> 0:03:33.440 Only time will tell. Yeah, because it's amazing that the 0:03:33.520 --> 0:03:36.560 universe is even understandable. Right, Like, we look at it, 0:03:36.560 --> 0:03:38.760 it seems kind of chaotic, But the closer we look, 0:03:38.840 --> 0:03:41.640 we start to notice patterns and trends that seems to 0:03:41.640 --> 0:03:44.200 sort of govern how it works and what's going to happen. 0:03:44.400 --> 0:03:47.600 It is amazing that the universe can be described by 0:03:47.720 --> 0:03:52.160 sets of physical laws that don't seem to change in time. Right. 0:03:52.200 --> 0:03:54.640 We take that for granted. I can do an experiment 0:03:54.720 --> 0:03:58.200 and measure something about the universe, like the gravitational constant, 0:03:58.440 --> 0:04:00.400 and then I can do that same experiment in fifty 0:04:00.480 --> 0:04:03.320 years and get the same number. Why is that right? 0:04:03.320 --> 0:04:06.520 Why do repeated experiments get the same answer. That's not 0:04:06.680 --> 0:04:09.040 something we know, It's just something we've seen. It's just 0:04:09.080 --> 0:04:12.360 something we basically assume as one of the foundational principles 0:04:12.440 --> 0:04:14.960 of science. We don't know why it's true, but we 0:04:15.000 --> 0:04:16.960 certainly do rely on it. Yeah, and you don't even 0:04:17.000 --> 0:04:18.800 have to get that fancy to see how the universe 0:04:18.839 --> 0:04:21.000 has these laws. Right, You can just toss an apple 0:04:21.000 --> 0:04:23.279 in the air over and over and it will always 0:04:23.400 --> 0:04:25.040 sort of come back to your head. But you can 0:04:25.120 --> 0:04:26.800 also do it while being fancy. You can wear a 0:04:26.800 --> 0:04:29.520 tuxedo and toss an apple into the air, nothing stopping 0:04:29.560 --> 0:04:32.280 you from getting fancy? Are you anti fancy? Now? That's way. 0:04:32.279 --> 0:04:34.440 I guess you could be tossing a kind of caviar 0:04:34.600 --> 0:04:37.960 instead of an apple too. I mean, personally, I'm always 0:04:37.960 --> 0:04:39.960 wearing a tuxedo while doing these podcasts. I thought we 0:04:40.000 --> 0:04:42.200 had a dress code on this podcast. What are you wearing? 0:04:42.880 --> 0:04:46.440 I'm wearing a pajama with a tuxedo printed on it. O. Man, 0:04:46.880 --> 0:04:51.560 standards are sliding everywhere, folks, It's hard, that's right. We 0:04:51.600 --> 0:04:53.640 don't have a dress code law here in the podcast. 0:04:54.440 --> 0:04:56.280 That's right. And you're also allowed to wear whatever you 0:04:56.400 --> 0:04:58.320 like when you're listening to this podcast. So if you've 0:04:58.360 --> 0:05:02.080 been dutifully getting your txedo dry cleaned before you listen 0:05:02.120 --> 0:05:04.679 to this podcast, you can now just wear pajama pants. 0:05:05.279 --> 0:05:08.400 I'm pretty sure nobody was wearing a tuxedo while listening 0:05:08.440 --> 0:05:10.880 to us, and that's maybe they're like a waiter maybe 0:05:10.880 --> 0:05:13.840 at a fancy restaurant that you know, tunes out the 0:05:13.880 --> 0:05:16.520 customers by listening to our podcast. I hope that we 0:05:16.560 --> 0:05:19.400 have a pretty broad variety of what folks are wearing, 0:05:19.640 --> 0:05:22.160 you know, all the way from athletic gear to tuxedos 0:05:22.160 --> 0:05:24.200 with tails. And top hats. I'd just like to imagine 0:05:24.240 --> 0:05:27.080 we're sampling all of the human experience the same way 0:05:27.240 --> 0:05:30.279 we are trying to explore the entire range of physical 0:05:30.320 --> 0:05:33.159 phenomena out there in the universe. Yeah, but it is 0:05:33.200 --> 0:05:35.600 interesting that the universe has lost right, Like you can 0:05:35.640 --> 0:05:38.400 imagine maybe a universe without laws? Is that even sort 0:05:38.440 --> 0:05:41.279 of like a possible to a physicist. It's possible to 0:05:41.360 --> 0:05:44.680 imagine that that universe exists, but it's hard to understand 0:05:44.760 --> 0:05:48.000 how you would understand it. You know, the idea that 0:05:48.120 --> 0:05:50.920 there are laws and that we can reveal them through experiment, 0:05:51.000 --> 0:05:52.920 and that we can try to simplify them and use 0:05:52.920 --> 0:05:55.680 them to predict the future. It's pretty basic to our 0:05:55.760 --> 0:05:58.000 notion of understanding. It's sort of like goes to the 0:05:58.040 --> 0:06:01.360 heart of storytelling, even well before like what we call 0:06:01.480 --> 0:06:05.880 modern science. Indigenous culture is just learning about their environment 0:06:05.920 --> 0:06:08.720 as they experience it, are telling stories, you know, like 0:06:08.960 --> 0:06:10.920 you take this tree bark, you make a tea out 0:06:10.920 --> 0:06:12.680 of it, you drink it, you feel better, and it's 0:06:12.720 --> 0:06:14.720 a story, and it's sort of fundamental to the way 0:06:14.760 --> 0:06:18.680 I think humans think. Yeah, that that is science as well, right, Absolutely, 0:06:18.720 --> 0:06:21.680 it's accumulation of knowledge through experience. Yeah, Well, it's a 0:06:21.680 --> 0:06:23.640 good thing that the universe does seem to have laws 0:06:23.680 --> 0:06:25.800 because it allows us to kind of predict what's going 0:06:25.839 --> 0:06:28.520 on and to build things to make our lives better, 0:06:28.640 --> 0:06:31.200 and to have a little bit of context about where 0:06:31.320 --> 0:06:33.680 we sit in the universe and why we're here. That's right, 0:06:33.720 --> 0:06:36.360 and the patterns and trends that we notice in the universe, 0:06:36.600 --> 0:06:38.200 they give us a lot of clues as to the 0:06:38.240 --> 0:06:41.640 fundamental nature of the universe. Though we assume the universe 0:06:41.680 --> 0:06:43.719 has laws, we don't make a lot of assumptions for 0:06:43.800 --> 0:06:46.360 what those laws are. So we'd like to look around 0:06:46.360 --> 0:06:48.720 and notice, like, what are the patterns that happened in 0:06:48.720 --> 0:06:50.719 the universe, what are those laws that it seems to 0:06:50.720 --> 0:06:52.839 follow in, what do those laws mean? And why do 0:06:52.880 --> 0:06:55.839 we have those laws and not other laws. Yeah, And 0:06:55.880 --> 0:06:58.440 probably one of the most important laws, or at least 0:06:58.440 --> 0:07:02.120 the most useful laws we found and understanding the universe 0:07:02.120 --> 0:07:04.440 and predicting what's going to happen, is the idea that 0:07:04.560 --> 0:07:08.839 momentum is conserved. That's a law, right, that's like written 0:07:08.839 --> 0:07:12.240 in the Statute of the universe. That is still a law. 0:07:12.440 --> 0:07:14.360 A lot of things that you learned about in grade 0:07:14.400 --> 0:07:16.680 school that you found were conserved in the universe, are 0:07:16.760 --> 0:07:20.960 not mass, energy, energy plus mass, all of this stuff. 0:07:21.000 --> 0:07:23.160 You thought that stuff was conserving universe. It turns out 0:07:23.160 --> 0:07:26.640 it's not. But conservation and momentum still holds as far 0:07:26.680 --> 0:07:29.640 as we know. Oh interesting, some laws have been repealed 0:07:30.000 --> 0:07:31.760 that they made it all the way up to the 0:07:32.320 --> 0:07:35.920 spraying cooard of the universen't they They got shut down? Well, 0:07:35.960 --> 0:07:38.640 like Newtonian physics, some of these laws turned out to 0:07:38.680 --> 0:07:42.600 be almost true, true in many many cases, but not 0:07:42.800 --> 0:07:46.720 fundamentally true, not actually written in stone at the foundation 0:07:46.800 --> 0:07:49.320 of the universe, just sort of like mostly working in 0:07:49.360 --> 0:07:52.240 the scenarios we had tested them in so far, which 0:07:52.280 --> 0:07:55.160 is a cool testament to how science progresses. You find 0:07:55.160 --> 0:07:57.240 a pattern, it seems to be true everywhere, and then 0:07:57.240 --> 0:08:00.960 a hundred years later people find exceptions. Exceptions reveal a 0:08:01.040 --> 0:08:03.880 deeper truth. So when you say that momentum is conserved 0:08:03.960 --> 0:08:05.560 is still a lot, it's it just means that it 0:08:05.600 --> 0:08:08.640 hasn't been revealed yet kind of right, like, as far 0:08:08.680 --> 0:08:11.120 as we know, that's the one that's still true. Yeah, 0:08:11.120 --> 0:08:13.600 that's why we say it's still a law as of 0:08:13.680 --> 0:08:15.800 the date of this podcast. But you know, in a 0:08:15.840 --> 0:08:18.520 thousand years, when people are listening to this episode, they'll 0:08:18.600 --> 0:08:21.440 laugh into their hands at our naivete right before they 0:08:21.440 --> 0:08:26.280 fall asleep. But there is something really deep and fundamental 0:08:26.320 --> 0:08:29.360 that we're doing when we find these conservation laws. You know, 0:08:29.400 --> 0:08:31.160 we are looking around to trying to figure out, like 0:08:31.440 --> 0:08:35.240 what is important in the universe, what's a meaningful thing? 0:08:35.440 --> 0:08:37.800 Like when we think, oh, maybe energy is conserved in 0:08:37.840 --> 0:08:41.400 the universe, we just look around. We notice that. We say, like, okay, 0:08:41.440 --> 0:08:43.600 you have a bunch of energy in this configuration. You 0:08:43.679 --> 0:08:46.160 let the universe do its thing, and you notice, oh, 0:08:46.160 --> 0:08:49.199 there's the same amount of total energy. That suggests that 0:08:49.240 --> 0:08:53.439 maybe energy is like important, it's fundamental, it's interesting, more 0:08:53.480 --> 0:08:55.600 important than like, you know, the number of ice cream 0:08:55.640 --> 0:08:58.600 cones in the universe, which changes a lot with time. 0:08:58.640 --> 0:09:00.960 There's billions of years there was no ice cream and 0:09:01.000 --> 0:09:03.320 then a brief flash of ice cream, and who knows 0:09:03.559 --> 0:09:05.880 whether they'll be ice cream in the future. But nobody 0:09:05.920 --> 0:09:09.360 expects the number of ice cream cones to be conserved 0:09:09.400 --> 0:09:12.080 because nobody thinks that that's an important thing in the universe. 0:09:12.360 --> 0:09:15.040 So if something is conserved, that suggests it is probably 0:09:15.080 --> 0:09:17.960 important somehow to the universe. Well, Daniel, I think ice 0:09:18.000 --> 0:09:21.719 cream is important about you, but it's an important part 0:09:21.720 --> 0:09:24.680 of my diet for sure. Well, if only the things 0:09:24.679 --> 0:09:27.520 that are important to you are also important to the universe, 0:09:29.160 --> 0:09:30.760 I think ice cream is pretty important to a lot 0:09:30.760 --> 0:09:35.880 of people. It's still around, it's it survived several lost 0:09:35.880 --> 0:09:38.480 trying to ban But if ice cream was conserved, then 0:09:38.480 --> 0:09:41.240 you could ask questions like, well, where did this ice 0:09:41.280 --> 0:09:43.520 cream come from? More? Where does the ice cream go? 0:09:43.880 --> 0:09:46.280 When you eat it? Right? It turns into something else 0:09:46.280 --> 0:09:48.920 which is not ice cream. So that suggests that what's 0:09:48.920 --> 0:09:51.840 conserved is not ice cream, but like some larger category 0:09:51.920 --> 0:09:54.480 of things anyway, it's the same thing with energy and 0:09:54.480 --> 0:09:57.080 with momentum. We discover whether these things are conserved by 0:09:57.080 --> 0:09:59.000 the universe, and then we get to ask what does 0:09:59.040 --> 0:10:02.079 that mean about the nature of the universe, right, because 0:10:02.080 --> 0:10:05.280 I guess asking these questions is how we understand the universe, right, Like, 0:10:05.480 --> 0:10:07.560 it's one thing to notice trends in it, but it's 0:10:07.600 --> 0:10:10.680 another to sort of understand why the universe has these trends, 0:10:10.679 --> 0:10:14.480 and like why do certain things that conserved and others don't? 0:10:14.640 --> 0:10:16.559 You know, what does it mean about the universe? Can 0:10:16.600 --> 0:10:19.160 we find some fundamental principle that tells us about all 0:10:19.200 --> 0:10:22.040 these conservation laws where they come from. The wonderful thing 0:10:22.040 --> 0:10:24.560 about science is that every answer leads to more questions. 0:10:24.679 --> 0:10:26.760 You know, the question number one could be what's conserved 0:10:26.760 --> 0:10:28.920 in the universe? Question number two is said, all right, well, 0:10:28.920 --> 0:10:31.640 why these things and not other things? Always leads to 0:10:31.679 --> 0:10:34.480 more questions which reveal a deeper truth of the universe. 0:10:34.520 --> 0:10:37.439 That's the joy of science that the questions never do end. Well, 0:10:37.520 --> 0:10:40.520 let's reveal some deeper truths today. So today on the podcast, 0:10:40.559 --> 0:10:50.360 we'll be asking the question why is momentum conserved? Okay, so, Daniel, 0:10:50.400 --> 0:10:52.920 you saying that energy is not conserved in the universe. 0:10:53.000 --> 0:10:55.960 We have a podcast discussing that. But momentum is conserved. 0:10:55.960 --> 0:10:58.959 Does that mean momentum is more important than energy? That's 0:10:59.000 --> 0:11:02.360 a good question. Yeah. I would say that momentum conservation 0:11:02.480 --> 0:11:05.520 tells us something fundamental about the nature of the universe 0:11:05.559 --> 0:11:08.560 and the nature of space, which we'll get into. The 0:11:08.600 --> 0:11:11.839 fact that energy is not conserved actually also tells us 0:11:11.960 --> 0:11:15.240 something about the universe and the nature of time. I 0:11:15.280 --> 0:11:17.760 think both of those facts, that momentum is conserved and 0:11:17.800 --> 0:11:20.920 that energy is not do tell us something deep about 0:11:20.920 --> 0:11:23.160 the nature of the universe. But yeah, I think momentum 0:11:23.200 --> 0:11:26.359 fundamentally is more important. If you had like a competition 0:11:26.760 --> 0:11:30.480 between quantities and physics, I would vote for momentum over energy. 0:11:30.880 --> 0:11:34.600 Who would win in a fight, momentum or energy. Momentum 0:11:34.679 --> 0:11:37.280 is a vector also, right, so it has multiple components. 0:11:37.520 --> 0:11:39.520 It would definitely defeat energy, which is just a scale. 0:11:39.559 --> 0:11:42.240 It's just a number. So momentum is a bigger army. 0:11:42.520 --> 0:11:45.680 I see. Yeah. Also momentum has more momentum going for it. 0:11:46.040 --> 0:11:48.679 You know that's important in a fight. Yeah, it's momentous, right, 0:11:48.800 --> 0:11:51.280 but only in the moment. Maybe they should have called 0:11:51.320 --> 0:11:55.280 it importum instead of momentum. Yeah, I'm not sure renaming 0:11:55.320 --> 0:11:58.839 and would help there. But this is a fascinating question, 0:11:58.840 --> 0:12:01.760 like why is momentum conserved? Because, as you were saying earlier, 0:12:01.800 --> 0:12:05.040 does something you learn you know early on, like high school, 0:12:05.040 --> 0:12:08.000 middle school, maybe even before, like the idea that if 0:12:08.040 --> 0:12:11.480 something is in motion, it stays in motion, and if 0:12:11.520 --> 0:12:13.240 something is address it it will stay in the rest 0:12:13.280 --> 0:12:16.839 unless something changes m And I think that when people 0:12:16.920 --> 0:12:20.040 learn about these conservation laws, the one that's most intuitive 0:12:20.240 --> 0:12:23.520 is the one that's actually least true. You know, conservation 0:12:23.559 --> 0:12:26.400 of mass, it sort of feels like it should make sense. 0:12:26.720 --> 0:12:29.079 Like you have a chemical reaction. You start out with 0:12:29.080 --> 0:12:32.160 a bunch of little lego brick chemical atoms and molecules, 0:12:32.240 --> 0:12:34.200 and all you're doing is rearranging them, so of course 0:12:34.200 --> 0:12:35.839 you should end up with the same amount of stuff 0:12:35.920 --> 0:12:37.960 at the end. And that's the one that seems to 0:12:38.200 --> 0:12:40.000 make the most sense to people. And I think that's 0:12:40.000 --> 0:12:42.679 a revealing example because it makes sense because you think 0:12:42.679 --> 0:12:46.000 of like stuff as being basic and fundamental to the universe. 0:12:46.120 --> 0:12:48.480 Is that can be like created or destroyed, And you 0:12:48.520 --> 0:12:50.680 hear that a lot in science fiction. Of course, now 0:12:50.720 --> 0:12:53.120 we know that it's not and you can destroy mass 0:12:53.160 --> 0:12:55.200 and turn into energy, and you can turn energy into 0:12:55.240 --> 0:12:56.800 mass and all that kind of stuff. But you know, 0:12:56.880 --> 0:12:59.240 the idea that a conservation law tells you about what's 0:12:59.240 --> 0:13:02.240 important in the verse is sort of underlying all of this. 0:13:02.400 --> 0:13:05.120 And I remember learning about conservation momentum and wondering, like, well, 0:13:05.160 --> 0:13:07.679 what does that mean is conserved? Is like what does 0:13:07.679 --> 0:13:10.559 that mean about the nature of the universe. Well, and 0:13:10.600 --> 0:13:12.720 in the case of mass, I mean, you just kind 0:13:12.720 --> 0:13:15.880 of have to figure that all mass is really energy, right, 0:13:15.920 --> 0:13:18.319 And so then you're just it's like it's embedded in 0:13:18.360 --> 0:13:21.680 the question of is energy conserved in the universe, which 0:13:21.760 --> 0:13:25.480 it generally is, just to be clear, but we've recently 0:13:25.520 --> 0:13:28.439 found that sometimes energy is not conserved. Yeah, it's a 0:13:28.559 --> 0:13:31.800 nice idea to generalize the conservation and mass into the 0:13:31.800 --> 0:13:34.120 conservation of energy and say mass is just one form 0:13:34.160 --> 0:13:36.920 of energy. So when it disappears in the energy, that's okay, 0:13:36.960 --> 0:13:40.080 because it's not true that mass is the fundamental stuff 0:13:40.080 --> 0:13:42.600 in the universe. Energy is right. But then, as you say, 0:13:42.640 --> 0:13:46.680 we discovered, actually the universe doesn't really care if energy 0:13:46.760 --> 0:13:49.360 is conserved. It can just go away and we can 0:13:49.400 --> 0:13:52.040 just sort of increase it. So dig into that podcast 0:13:52.080 --> 0:13:54.080 if you're curious about that. But what that tells us 0:13:54.480 --> 0:13:57.080 is that energy is not like the fundamental element of 0:13:57.080 --> 0:13:59.640 the universe. It's more like ice cream cones. It's just 0:13:59.679 --> 0:14:02.080 like something we came up with that makes sense to 0:14:02.200 --> 0:14:05.040 us or is important to us. It's not deeply true. 0:14:05.120 --> 0:14:07.600 It's not a deep feature of the universe. It's not 0:14:07.679 --> 0:14:10.760 the chocolate ice cream of the universe. Maybe or are 0:14:10.760 --> 0:14:12.920 the same momentum is a vanilla ice cream of the universe. 0:14:14.280 --> 0:14:15.920 First of all, ice cream is not important to the 0:14:16.000 --> 0:14:19.520 universe only and humans. But I guess we would be 0:14:19.520 --> 0:14:21.880 saying that, you know, momentum is the dark chocolate at 0:14:21.880 --> 0:14:27.160 the universe and energy is the white chocolate, well chocolate aside, 0:14:27.240 --> 0:14:30.200 this is an interesting question why is momentum conserved? Because 0:14:30.200 --> 0:14:32.840 we we all the field like it's true, and it 0:14:33.520 --> 0:14:35.560 does seem to be true so far, But the question 0:14:35.640 --> 0:14:38.320 is why does it get conserved in the universe. So, 0:14:38.360 --> 0:14:40.320 as usual, we were wondering how many people out there 0:14:40.480 --> 0:14:43.360 think they have an answer to this interesting and deep 0:14:43.440 --> 0:14:46.560 question about how everything works. And thank you to our 0:14:46.640 --> 0:14:50.240 volunteers who are willing to answer deep, difficult questions of 0:14:50.360 --> 0:14:53.040 physics without any chance to prepare, so that we can 0:14:53.040 --> 0:14:55.840 get a senswer what people out there are thinking. If 0:14:55.880 --> 0:14:58.880 you'd like to participate, please you are very welcome right 0:14:58.920 --> 0:15:01.800 to us to questions at Daniel and Jorge dot com. 0:15:01.920 --> 0:15:04.280 So Daniel asked this question on the internet, why is 0:15:04.360 --> 0:15:07.600 momentum conserved? And here's what people had to say. I'm 0:15:07.600 --> 0:15:13.080 not really sure that momentum is preserved because momentum massive 0:15:13.160 --> 0:15:17.240 velocity a product of mass and velocity and velocities relatives. 0:15:17.280 --> 0:15:20.400 So if some velocities relative, I don't know how it 0:15:20.400 --> 0:15:24.160 could be preserved, because if somebody else measures it, it's 0:15:24.200 --> 0:15:26.760 going to be a different velocity. I was just took 0:15:26.800 --> 0:15:29.200 that kind of as a given and never really questioned 0:15:29.240 --> 0:15:32.680 why why. It's just because they tell us it's always conserved. 0:15:33.200 --> 0:15:37.240 But it's related to, you know, a closed system of 0:15:37.520 --> 0:15:43.600 matter and energy. None of that matter energy is created 0:15:43.680 --> 0:15:45.560 or loss. It's just converted one way or the other. 0:15:45.600 --> 0:15:49.840 And since momentum is in a way of function of 0:15:49.880 --> 0:15:53.160 your mass and your energy, then it's just always there. 0:15:53.160 --> 0:15:56.440 It can't it can't be bled off into another dimension 0:15:56.760 --> 0:16:00.000 or something as far as I know, well, I guess 0:16:00.200 --> 0:16:05.280 the reason momentum is conserved is because of Mutant said so, 0:16:05.600 --> 0:16:10.560 and Einstein said so, maybe both of them. I think 0:16:11.280 --> 0:16:16.239 all that we've been able to observe so far indicates 0:16:16.320 --> 0:16:20.600 that momentum is conserved in terms of interactions that we've 0:16:20.640 --> 0:16:26.760 measured those kinds of things. In terms of a conceptual 0:16:27.040 --> 0:16:31.680 reason behind why momentum should be something that is conserved, 0:16:32.400 --> 0:16:36.280 I don't know if there's any good explanation for that. 0:16:37.040 --> 0:16:41.880 Thinking about a collision right now between let's say two objects, 0:16:43.200 --> 0:16:49.240 and why is consu so an energy to another object 0:16:49.400 --> 0:16:57.760 is transferred to the other one. So um is conserved 0:16:57.760 --> 0:17:04.520 because nothing the energy, it's constantly nothing, It's wasted. Everything 0:17:04.560 --> 0:17:11.399 gets transferred or transformed. Momentum may be described as mass 0:17:11.440 --> 0:17:17.359 times velocity. It may further be described as the second 0:17:17.400 --> 0:17:22.639 law of thermodynamics, or is it Newton's second law of motion? 0:17:22.760 --> 0:17:26.439 I forget college was a long time ago. However, the 0:17:26.560 --> 0:17:30.240 question of why is it preserved, I think is something 0:17:30.960 --> 0:17:33.600 nobody knows yet. I think it just all has to 0:17:33.640 --> 0:17:36.240 do with the way energy is transferred. So if I 0:17:36.280 --> 0:17:39.200 were to be pushing something, for me to be pushing 0:17:39.240 --> 0:17:42.520 that thing so it moves, I need to be adding 0:17:42.600 --> 0:17:44.960 energy to the system. I need to be pushing it 0:17:45.000 --> 0:17:48.240 and adding a force and therefore transferring energy from me 0:17:48.359 --> 0:17:52.760 into it. But just due to the way Newton's laws work, 0:17:53.200 --> 0:17:56.000 it too has to be pushing back on me. All right, 0:17:56.280 --> 0:17:59.159 what do you think of the answer? I like the 0:17:59.359 --> 0:18:02.240 because new And said so, and Einstein agreed. They're like 0:18:02.280 --> 0:18:06.679 a council of physics, and they decide what's true. Oh, 0:18:06.800 --> 0:18:09.359 maybe my kids should try that the next time. Their 0:18:09.400 --> 0:18:13.199 parents don't know. But Newton said so, and Einstein agreed 0:18:13.560 --> 0:18:16.440 that you should go to bed two out of two 0:18:16.480 --> 0:18:21.440 seminal physicists agreed as if physicists have any voice and 0:18:21.520 --> 0:18:23.959 anything relevant at all. You know, do you think physics 0:18:24.000 --> 0:18:26.720 and general is sort of done by by polling and 0:18:27.040 --> 0:18:29.680 kind of right, like, there's a little bit of sense 0:18:29.720 --> 0:18:32.640 that in science it's about what the majority thinks is true. Right, 0:18:33.040 --> 0:18:36.760 that's true. The consensus view is important, although you know, 0:18:36.880 --> 0:18:39.480 one person against the world doesn't have to be wrong, 0:18:39.720 --> 0:18:43.600 and in some cases, like one seminal physicists can persuade 0:18:43.600 --> 0:18:45.320 a lot of folks. You know, if you say, well, 0:18:45.359 --> 0:18:48.879 Einstein thought this, or Murray Gelman said that, or Feineman 0:18:48.920 --> 0:18:51.080 put it this way, that can be pretty persuasive. But 0:18:51.119 --> 0:18:53.800 I guess in general, none of the answers questioned that 0:18:53.920 --> 0:18:56.480 idea that momentum is conserved, right, Nobody said, like, who 0:18:56.480 --> 0:18:59.480 said momentum is conserved? Yeah, though some people thought that 0:18:59.520 --> 0:19:02.119 we don't know the answer that we have no idea 0:19:02.200 --> 0:19:04.800 why momentum is conserved, sort of like why is the 0:19:04.840 --> 0:19:06.359 speed of light? What it is? We don't know, we 0:19:06.440 --> 0:19:08.560 just measure it. Some people put it in that category. 0:19:09.040 --> 0:19:11.200 I think they've been listening to our podcast and reading 0:19:11.200 --> 0:19:14.720 our books too much. You know. Sometimes yes, is we 0:19:14.800 --> 0:19:17.359 have no idea. You think they've become persuaded that physics 0:19:17.400 --> 0:19:20.600 actually doesn't really know very much about the universe. Oh, 0:19:20.800 --> 0:19:26.280 mission accomplished. We spoiled the ice cream. It just means 0:19:26.320 --> 0:19:29.640 there's more ice cream of discovery left to eat for everybody. Well, 0:19:29.720 --> 0:19:33.040 let's just recap it for people. So, momentum conservation means 0:19:33.119 --> 0:19:36.520 that momentum is conservad Like, momentum is defined as what 0:19:36.680 --> 0:19:41.120 mass times your velocity. Yes, so momentum for slow moving objects, 0:19:41.200 --> 0:19:45.320 it's just mass times velocity. And when we say momentum 0:19:45.400 --> 0:19:48.120 is conserved, we mean that if you have the momentum 0:19:48.119 --> 0:19:50.280 of a bunch of stuff and then you let it 0:19:50.320 --> 0:19:52.680 do its thing, follow the laws of physics, banging into 0:19:52.760 --> 0:19:55.919 each other, bounce off of stuff, or just float through space, 0:19:56.280 --> 0:19:59.000 and then later on you add up the momentum again 0:19:59.200 --> 0:20:02.520 you should get the same number. And interestingly, momentum is 0:20:02.560 --> 0:20:05.639 actually three different quantities. Is momentum and X momentum And 0:20:05.680 --> 0:20:08.120 why momentum and z because we live in three dimensional 0:20:08.160 --> 0:20:12.560 space and those are all independently conserved. The momentum conservation 0:20:12.680 --> 0:20:17.440 is kind of three laws in one to deal Yeah, exactly. 0:20:17.480 --> 0:20:19.399 For those of you who do like classical mechanics or 0:20:19.440 --> 0:20:22.000 freshman physics. You know, that makes it very powerful because 0:20:22.040 --> 0:20:24.880 you get three equations to constrain your answer rather than 0:20:24.920 --> 0:20:28.040 just one from conservation of energy. Right, But you were 0:20:28.080 --> 0:20:30.720 saying that it's only for slow moving things, like it's 0:20:30.760 --> 0:20:33.960 different for other things. It turns out in relativity, the 0:20:34.040 --> 0:20:37.960 definition of momentum is different from just mass times velocity. 0:20:38.320 --> 0:20:41.479 There's this boost factor we call it the Gamma factor 0:20:41.600 --> 0:20:44.720 or the Lorentz factor, which is one for slow moving 0:20:44.760 --> 0:20:47.960 objects and approaches infinity as things get towards the speed 0:20:47.960 --> 0:20:51.159 of light. So the real equation for momentum is mass 0:20:51.560 --> 0:20:55.480 times velocity times this Gamma factor. We never noticed because 0:20:55.480 --> 0:20:57.479 the Gama factor is close to one, so you can 0:20:57.520 --> 0:20:59.919 ignore it for stuff that's less than you know, like 0:21:00.040 --> 0:21:01.600 half the speed of light or a third at the 0:21:01.600 --> 0:21:04.520 speed of light. But it becomes important as you get 0:21:04.520 --> 0:21:06.399 near the speed of light. Oh, I said, you have 0:21:06.440 --> 0:21:08.320