WEBVTT - How were pulsars discovered? 0:00:08.920 --> 0:00:14.040 What is the moment of scientific discovery actually? Like I mean, 0:00:14.080 --> 0:00:17.960 in the movies, it always seems so crisp. Scientists find 0:00:18.120 --> 0:00:22.200 something in her data or an experiment suddenly dramatically works. 0:00:22.640 --> 0:00:25.480 We go from ignorance to knowledge in a moment, from 0:00:25.560 --> 0:00:29.400 failure to success. That kind of drama works for the 0:00:29.440 --> 0:00:32.520 movie screen, But how does it happen in real life? 0:00:33.080 --> 0:00:36.000 Is it a slow and steady march rather than a 0:00:36.040 --> 0:00:40.879 sudden leap, or are there actually real moments of insight 0:00:41.200 --> 0:00:44.640 where all of a sudden light penetrates the darkness and 0:00:44.640 --> 0:00:48.559 the scientists learned something new about the universe that no 0:00:48.880 --> 0:01:07.920 human has ever known before. Hi, I'm Daniel. I'm a 0:01:07.959 --> 0:01:11.760 particle physicist and I've been doing particle physics experiments for 0:01:11.959 --> 0:01:16.440 decades but never discovered a new particle. And Welcome to 0:01:16.480 --> 0:01:20.640 the podcast. Daniel and Jorge explain the Universe, in which 0:01:20.680 --> 0:01:24.679 we examine everything about the universe, from its origins to 0:01:24.800 --> 0:01:28.000 its ends, from its biggest things to its smallest things, 0:01:28.160 --> 0:01:31.240 from all of its mysteries and all of our discoveries. 0:01:31.360 --> 0:01:34.319 Our goal in this podcast is to open our minds 0:01:34.360 --> 0:01:38.920 to all of the craziest, biggest, deepest, most important questions 0:01:39.120 --> 0:01:42.160 the one that frame the context of being human, the 0:01:42.200 --> 0:01:44.800 ones that tell us what it means to be in 0:01:44.840 --> 0:01:48.520 this universe and how this universe works. We tackle all 0:01:48.560 --> 0:01:51.160 of those questions and we go right to the forefront 0:01:51.240 --> 0:01:53.800 of scientific knowledge. We take you right to the edge 0:01:54.040 --> 0:01:57.520 where scientists are currently working, and we explain all of 0:01:57.560 --> 0:01:59.560 it to you in a way that we hope it 0:01:59.560 --> 0:02:03.440 makes and maybe even occasionally makes you laugh. My co 0:02:03.560 --> 0:02:06.840 host Orgy him the Creative PhD Comics, can't be here today, 0:02:06.960 --> 0:02:08.919 so I'm gonna share with you one of my favorite 0:02:08.919 --> 0:02:13.080 stories of scientific discovery. And I mentioned earlier on that 0:02:13.200 --> 0:02:17.560 I have never discovered a new particle. That's not a true. 0:02:17.800 --> 0:02:20.919 My career and particle physics spans from the mid nineteen 0:02:21.040 --> 0:02:24.240 nineties till today, and in the mid nineteen nineties was 0:02:24.280 --> 0:02:27.040 the discovery of the top cork or and I did 0:02:27.040 --> 0:02:30.800 a really fun episode about that whole, amazing, hilarious, dramatic story. 0:02:31.080 --> 0:02:33.280 But I just would have joined the field right when 0:02:33.280 --> 0:02:36.160 that had already happened, So I wasn't around when the 0:02:36.160 --> 0:02:38.919 top cork was discovered. I didn't get to participate in 0:02:38.960 --> 0:02:42.400 that moment of discovery. I was, however, part of the 0:02:42.400 --> 0:02:45.720 team that discovered the Higgs boson. But you have to understand, 0:02:45.919 --> 0:02:48.680 this was a really big group of people, thousands of 0:02:48.760 --> 0:02:52.160 thousands of people who all contributed little bits here and there, 0:02:52.440 --> 0:02:55.760 and there wasn't really a dramatic moment when we said, 0:02:55.800 --> 0:02:59.720 ah ha, the Higgs is there. It's solely emerged out 0:02:59.720 --> 0:03:01.280 of the a a sort of the way a treasure 0:03:01.360 --> 0:03:03.560 chest might be revealed in the sand of a beach, 0:03:03.840 --> 0:03:06.799 as a tide pulls out inch by inch, showing you 0:03:06.880 --> 0:03:08.640 more and more of it. That was sort of the 0:03:08.639 --> 0:03:11.600 way the Higgs boson discovery went. We saw a little peak, 0:03:11.680 --> 0:03:13.360 we thought it might be it. It got bigger and 0:03:13.440 --> 0:03:16.280 bigger and bigger, and there was never really a moment 0:03:16.440 --> 0:03:19.600 of than the official announcement when we could say, now 0:03:19.639 --> 0:03:21.760 we have discovered the Higgs. But that sort of was 0:03:21.800 --> 0:03:25.320 a bureaucratic choice and artificial choice. There was no single 0:03:25.400 --> 0:03:28.800 aha moment. And part of that is because we knew 0:03:28.840 --> 0:03:31.360 what we were looking for. We suspect that the Higgs 0:03:31.440 --> 0:03:33.519 was there, we knew how to find it, we knew 0:03:33.520 --> 0:03:35.920 how to look for it, we knew what to expect, 0:03:35.960 --> 0:03:37.720 and so when we saw it, it was just sort 0:03:37.720 --> 0:03:40.880 of this slow creeping realization that we had found what 0:03:41.040 --> 0:03:44.040 we had been hunting. But that doesn't mean it's always 0:03:44.080 --> 0:03:48.120 like that. There are moments of discovery in science. Usually 0:03:48.200 --> 0:03:51.960 they happen when we're more surprised, when we see something 0:03:52.120 --> 0:03:54.680 we didn't expect, when you go looking for one thing 0:03:54.760 --> 0:03:57.960 and you find something else. Moments, for example, like the 0:03:58.000 --> 0:04:00.640 discovery of the cosmic microwave back round that we talked 0:04:00.640 --> 0:04:03.120 about a few episodes ago. Today, we're going to tell 0:04:03.160 --> 0:04:06.880 the story of one of those moments when discovery came quickly, 0:04:07.200 --> 0:04:10.480 when someone went looking for one thing and found something else, 0:04:10.600 --> 0:04:16.040 something alarming and astonishing, a moment of insight about the universe. Actually, 0:04:16.320 --> 0:04:19.000 we're gonna tell a story of two of those moments, 0:04:19.200 --> 0:04:23.280 because this discovery has multiple parts, and for one of 0:04:23.320 --> 0:04:28.120 those parts, we happen to have real historical audio of 0:04:28.160 --> 0:04:33.560 those scientists realizing their discovery in real time as it happens, 0:04:33.600 --> 0:04:36.360 so you'll get to hear what it actually sounds like 0:04:36.520 --> 0:04:40.880 when scientists are astonished when they make a real life discovery. 0:04:41.000 --> 0:04:43.800 So that's super fun, and for me, it's always really 0:04:43.839 --> 0:04:47.120 interesting to try to understand what it was like to 0:04:47.279 --> 0:04:51.120 make that discovery. You know, it's easy in hindsight to say, 0:04:51.360 --> 0:04:53.920 all these things exist, here's how you look for them 0:04:54.000 --> 0:04:56.080 that when did it bad? A boom but being done. 0:04:56.520 --> 0:04:58.200 But you have to go back to what it was 0:04:58.240 --> 0:05:01.200 like before we knew it was there, to put yourself 0:05:01.240 --> 0:05:04.719 back in that mental position of ignorance, not knowing whether 0:05:04.760 --> 0:05:07.320 something is out there, not understanding whether you live in 0:05:07.320 --> 0:05:10.520 the universe where it's real or where it's just an idea, 0:05:10.839 --> 0:05:14.720 not knowing which direction human knowledge and science will take. 0:05:14.960 --> 0:05:19.279 Science is so easy in hindsight and so difficult in foresight. 0:05:19.560 --> 0:05:22.000 When you stand in the forefront of human ignorance, you 0:05:22.080 --> 0:05:25.440 don't know necessarily which way to go. So it's really 0:05:25.520 --> 0:05:28.839 valuable to revisit these moments when we took a step forward, 0:05:29.080 --> 0:05:32.880 when we went from ignorance to knowledge, and understand what 0:05:33.000 --> 0:05:35.799 was required, how it happened, and the bravery it took 0:05:36.000 --> 0:05:39.400 to make that claim to say I have found something new. 0:05:39.760 --> 0:05:42.520 I now know something about the universe that no human 0:05:42.680 --> 0:05:45.240 ever knew before. And so today we're gonna be telling 0:05:45.440 --> 0:05:48.359 one of my favorite stories of discovery, one about a 0:05:48.440 --> 0:05:52.679 really weird kind of star, a very fast, very dense, 0:05:53.120 --> 0:05:55.680 very bizarre kind of star that we've talked about on 0:05:55.720 --> 0:05:59.040 the podcast before and so today's episode we'll be answering 0:05:59.080 --> 0:06:08.559 the question how we're pulsars discovered? And so, as usual, 0:06:08.640 --> 0:06:10.680 before we dig into the topic and tell you the 0:06:10.680 --> 0:06:14.040 story today, I wanted to know how much people already 0:06:14.120 --> 0:06:17.360 knew about this sort of famous story. So I went 0:06:17.360 --> 0:06:20.440 out and solicited volunteers from the internet to tell us 0:06:20.480 --> 0:06:23.280 what they knew about various questions and science of this 0:06:23.480 --> 0:06:25.360 being one of them. So thank you to all of 0:06:25.360 --> 0:06:28.520 those who participated and give us their speculation without the 0:06:28.520 --> 0:06:32.760 opportunity to look into any reference material whatsoever on the 0:06:32.760 --> 0:06:36.040 honor system. Of course, if you'd like to participate and 0:06:36.160 --> 0:06:39.120 hear your voice on the podcast in the future, please 0:06:39.279 --> 0:06:42.440 don't be shy. I promise you it's fun. Send me 0:06:42.480 --> 0:06:46.240 an email to questions at Daniel and Jorge dot com. 0:06:46.279 --> 0:06:48.480 But in the meantime, think to yourself, do you know 0:06:48.560 --> 0:06:52.200 the story of how pulsars were discovered? Here's what people 0:06:52.400 --> 0:06:56.039 had to say. I am a d percent sure that 0:06:56.320 --> 0:07:00.440 pulsels were discovered when they stuck a stepiscope pots to 0:07:00.480 --> 0:07:04.960 the whole space telescope. I'm guessing pulsars were discovered by 0:07:05.000 --> 0:07:10.120 scientists who observed these stars that were kind of flashing, 0:07:10.560 --> 0:07:17.640 so dimming and brightening in these regular pulses. Hence the 0:07:17.720 --> 0:07:23.760 name pulsar. I realized I just described what a pulsar is, 0:07:23.880 --> 0:07:28.400 not how they were discovered, so sorry about that. UM. 0:07:28.400 --> 0:07:31.040 For what a pulsar is, I would say it was 0:07:31.080 --> 0:07:36.280 discovered as a rapidly blinking source of light in the sky. Um. 0:07:36.400 --> 0:07:41.920 They were discovered by I think she was a graduate 0:07:42.040 --> 0:07:49.960 student ince in the sixties. Something they were They discovered 0:07:50.160 --> 0:07:58.520 through listening to some radio signals, and first they thought 0:07:58.520 --> 0:08:02.840 there was extracted see in life, because they called that 0:08:03.520 --> 0:08:07.560 little Green Men l g M. But I always confused 0:08:07.760 --> 0:08:14.920 pulsars and quasars. I'm going to guess that someone saw 0:08:15.880 --> 0:08:19.440 repetition of light in some part of the sky over 0:08:19.520 --> 0:08:23.800 and over and that led to an investigation that found 0:08:23.800 --> 0:08:28.320 the pulsars. Pulsars were discovered by a woman, and I 0:08:28.360 --> 0:08:31.160 believe it was in the nineties seventies, but I'm not 0:08:31.240 --> 0:08:36.640 sure how or why or where even There was a 0:08:36.640 --> 0:08:42.200 woman astronomer radio astronomer whose name, unfortunately I cannot remember, 0:08:42.480 --> 0:08:45.560 was doing some sort of sky survey when she noticed 0:08:45.960 --> 0:08:52.680 a set of pulses that were incredibly regularly spaced. She 0:08:52.800 --> 0:08:56.439 actually annotated them as l g M for Little Green Men. 0:08:56.520 --> 0:08:59.599 That one time they thought it might have been discovery 0:08:59.640 --> 0:09:04.679 of aliens, but later they discovered that it was actually 0:09:05.520 --> 0:09:10.280 a rotating uh neutron star and the magnetic field was 0:09:11.000 --> 0:09:14.720 exciting the gas molecules molecules around it and giving off 0:09:15.520 --> 0:09:21.760 radio energy. Alright, so congratulations to our excellently informed listeners. Together, 0:09:21.840 --> 0:09:24.600 they really do have most of the story there. There's 0:09:24.600 --> 0:09:26.440 a lot of really insightful stuff and a lot of 0:09:26.480 --> 0:09:29.800 bits of the story are there in pieces here and there. 0:09:29.960 --> 0:09:32.360 So let's dig into it and to really understand how 0:09:32.400 --> 0:09:35.240 pulsars were discovered, we have to understand, of course, first 0:09:35.400 --> 0:09:38.880 what a pulsar is, how we came to the idea 0:09:39.160 --> 0:09:41.320 of it existing in the universe, and that will help 0:09:41.360 --> 0:09:43.760 us understand how it was seen and how we knew 0:09:43.880 --> 0:09:46.840 what we were seeing, all right, So first of all, 0:09:47.040 --> 0:09:50.760 what is a pulsar. A pulsar is a very very 0:09:50.800 --> 0:09:55.520 compact object. Neutron stars and white dwarfs are more famous 0:09:55.760 --> 0:09:58.600 as the sort of like densest things in the universe, 0:09:58.679 --> 0:10:02.040 and a pulsar is a version of these. It's most 0:10:02.080 --> 0:10:05.120 commonly considered to be a version of a neutron star, 0:10:05.200 --> 0:10:07.920 but it can also be a white dwarf, but both 0:10:07.920 --> 0:10:10.960 of them are essentially are the end points of stars. 0:10:11.360 --> 0:10:14.520 Stars have these incredible life cycles where you start out 0:10:14.520 --> 0:10:18.440 as a big molecular cloud, huge blob of gas and 0:10:18.520 --> 0:10:21.800 dust that somehow shocked to collapse into a hot and 0:10:21.880 --> 0:10:25.520 dense object a star, which burns for billions and billions 0:10:25.600 --> 0:10:29.880 of years in this incredible, incredible balance between gravity that's 0:10:29.880 --> 0:10:32.160 pulling it together, trying to turn it into a black 0:10:32.200 --> 0:10:35.320 hole or something very very dense, and fusion which is 0:10:35.480 --> 0:10:38.959 erupting and sending radiation out to prevent the collapse of 0:10:39.040 --> 0:10:41.319 that star. And it always amazes me that these things 0:10:41.360 --> 0:10:45.000 go on for so long, these two cosmic forces so different, 0:10:45.280 --> 0:10:48.920 both so powerful, can be so balanced for so many 0:10:49.040 --> 0:10:52.079 billions of years. Well, at some point the star gives 0:10:52.120 --> 0:10:54.400 up because it's burned most of its fuel and its 0:10:54.480 --> 0:10:57.080 core has become very very heavy, and it's filled with 0:10:57.160 --> 0:11:00.120 things that it can no longer fuse. When the war 0:11:00.200 --> 0:11:02.679 of the star is filled with iron, for example, I 0:11:02.880 --> 0:11:06.360 using iron doesn't generate heat, it actually costs energy, so 0:11:06.400 --> 0:11:08.960 it cools the star. So now the star no longer 0:11:09.040 --> 0:11:12.760 has that power from fusion to resist gravity, and it collapses. 0:11:13.040 --> 0:11:15.560 There's some intermediate stages in there will skip over, such 0:11:15.600 --> 0:11:17.840 as it becoming a red giant, but depending on the 0:11:17.880 --> 0:11:21.200 size of the star, this collapse generally triggers a supernova. 0:11:21.280 --> 0:11:24.119 So you have this collapse where the materials racing inwards, 0:11:24.240 --> 0:11:28.720 which then causes that back reaction outwards, a massive explosion 0:11:29.040 --> 0:11:31.280 where a huge chunk of the stuff that used to 0:11:31.280 --> 0:11:34.040 be the star is now spread out into a new nebula, 0:11:34.160 --> 0:11:37.600 like a big sprawling cloud of gas and dust. At 0:11:37.640 --> 0:11:41.080 the core of it, however, is a very dense, very 0:11:41.200 --> 0:11:44.680 hot remnant, and that remnant can either be a white 0:11:44.760 --> 0:11:49.720 dwarf or a neutron star or a black hole, depending 0:11:49.760 --> 0:11:52.800 on the mass of the original star. So smaller stars 0:11:52.880 --> 0:11:55.440 end up as white dwarfs, which are basically just like 0:11:55.800 --> 0:11:59.920 huge hot chunks of metal that are resisting collapsing because 0:12:00.240 --> 0:12:02.560 their fermions and they don't like to overlap too much, 0:12:03.200 --> 0:12:06.360 or if they are larger, they become neutron stars, where 0:12:06.559 --> 0:12:09.679 gravity now pushes them together and forces all of the 0:12:09.679 --> 0:12:13.760 protons and the electrons together into forming new neutrons, and 0:12:13.800 --> 0:12:15.960 you have this really weird material that's sort of like 0:12:16.160 --> 0:12:19.400 the nucleus of an atom, but the size of a mountain. 0:12:19.559 --> 0:12:22.960 So it's incredibly dense, incredibly weird stuff, something we even 0:12:23.000 --> 0:12:25.920 still today do not understand in detail. And then, of course, 0:12:25.960 --> 0:12:28.240 if the star is more massive, it would become a 0:12:28.320 --> 0:12:31.959 black hole. So the gravity totally wins and nothing prevents 0:12:32.000 --> 0:12:34.320 the collapse and it becomes a black hole. But it's 0:12:34.360 --> 0:12:36.440 the first two categories that are more interested in. And 0:12:36.520 --> 0:12:38.960 let's focus on the neutron star category because that's the 0:12:39.000 --> 0:12:42.520 majority of pulsars. So you have this very dense object, right, 0:12:42.640 --> 0:12:44.640 and the object is a huge chunk of the material 0:12:44.720 --> 0:12:46.560 that used to be a star, not all of it. 0:12:46.880 --> 0:12:49.320 Some of the material is lost in the supernova and 0:12:49.360 --> 0:12:51.880 some of it remains in this cloud that surrounds the 0:12:51.920 --> 0:12:54.520 neutron star. But this neutron star is a very very 0:12:54.559 --> 0:12:58.480 dense object and very very small because gravity is really 0:12:58.520 --> 0:13:00.760 pulled it together. And with that means is that it's 0:13:00.800 --> 0:13:04.640 spinning really fast. Why is it spinning fast, Well, the 0:13:04.679 --> 0:13:08.760 star itself was spinning because everything in the universe is spinning. 0:13:08.840 --> 0:13:12.760 And the reason is simple is because angular momentum is conserved. 0:13:13.320 --> 0:13:16.200 You know how momentum is conserved. If you push on something, 0:13:16.600 --> 0:13:19.720 it stays in motion until something else pushes on it, 0:13:19.840 --> 0:13:22.199 or if you don't push on something, it stays still 0:13:22.559 --> 0:13:25.920 until something does push on it. That's conservation of momentum. 0:13:25.960 --> 0:13:29.080 Those are Newton's laws. Well, there are similar laws for 0:13:29.240 --> 0:13:33.079 angular momentum. That is that something spinning tends to keep spinning, 0:13:33.200 --> 0:13:35.520 and to make something spin, you've gotta give it a push. 0:13:35.920 --> 0:13:38.680 So if you leave something alone, it will keep spinning 0:13:38.800 --> 0:13:42.040 the way it's always been spinning. Right, that's conservation of 0:13:42.080 --> 0:13:45.680 angular momentum. And so the original gas cloud that formed 0:13:45.760 --> 0:13:48.760 that star had some spin to it, and that spin 0:13:48.920 --> 0:13:52.240 can't go away. It needs to stick around. And as 0:13:52.280 --> 0:13:54.840 the gas cloud gets smaller and smaller and turns into 0:13:54.840 --> 0:13:58.240 a star, the star spins faster. Now that might sound 0:13:58.280 --> 0:14:01.040 like it violates conservation of because the momentum because it's 0:14:01.040 --> 0:14:04.199 spinning faster. Right, Well, the velocity of the star's spin 0:14:04.320 --> 0:14:07.400 is not what's conserved. It's the angular momentum, which is 0:14:07.440 --> 0:14:11.040 the product of the velocity and their radius. So things 0:14:11.040 --> 0:14:15.160 that are larger spin slower with the same angular momentum 0:14:15.360 --> 0:14:18.480 as things that are smaller than spin faster. You know 0:14:18.559 --> 0:14:20.880 this because if you're a figure skater and you pull 0:14:20.960 --> 0:14:24.120 your arms in, you spin faster. You have the same 0:14:24.160 --> 0:14:27.440 angular momentum. You're not pushing against anything to spin faster, 0:14:27.640 --> 0:14:30.400 but you spin faster because your radius is smaller. So 0:14:30.480 --> 0:14:33.120 to have the same angular momentum, you've got to go faster. 0:14:33.560 --> 0:14:37.040 That's why the star spins faster than the original gas cloud. 0:14:37.200 --> 0:14:40.840 And that's why the super compact, dense, little neutron star 0:14:41.120 --> 0:14:43.280 that has a huge chunk of the star's mass but 0:14:43.440 --> 0:14:46.800 is much much smaller. We're talking about something only kilometers 0:14:46.880 --> 0:14:50.360 in size, you know, maybe ten fifteen kilometers, has to 0:14:50.360 --> 0:14:53.520 be spinning really really fast to have the same angular 0:14:53.560 --> 0:14:56.320 momentum as most of the original star. So that's why 0:14:56.360 --> 0:14:59.360 these things are spinning so fast because they are so 0:14:59.440 --> 0:15:02.720 small as they are so dense. In addition, some of 0:15:02.720 --> 0:15:05.600 these things are highly magnetic. There's a magnetic field of 0:15:05.640 --> 0:15:08.320 these stars, just like every star and most planets have 0:15:08.400 --> 0:15:11.200 a magnetic field, and that's because the motion of charged 0:15:11.240 --> 0:15:14.760 particles inside it. A neutron star is mostly neutrons, but 0:15:14.800 --> 0:15:17.440 there are protons and there are electrons, and they are 0:15:17.480 --> 0:15:19.840 moving around sometimes on the surface, and the flux of 0:15:19.840 --> 0:15:23.080 the particles on the inside can create these magnetic fields. 0:15:23.080 --> 0:15:25.800 So you have this object that's spinning really really fast, 0:15:26.320 --> 0:15:30.239 and it has a magnetic field. In addition, it's generating 0:15:30.280 --> 0:15:34.000 a huge amount of radiation. The magnetic field of the 0:15:34.000 --> 0:15:37.520 thing is rotating, which generates an electric field which accelerates 0:15:37.520 --> 0:15:39.960 the protons and the electrons on the surface of the 0:15:39.960 --> 0:15:43.120 neutron star, and that creates a bunch of radiation because 0:15:43.120 --> 0:15:46.920 when you accelerate particles, they radiate photons. So you have 0:15:46.960 --> 0:15:50.480 this magnetic fields on this neutron star that's rotating and 0:15:50.600 --> 0:15:53.160 is generating an electric field which pushes the electrons and 0:15:53.200 --> 0:15:55.440 protons on the surface of the star, creating a lot 0:15:55.480 --> 0:15:58.440 of radiation. And that radiation doesn't go in every direction 0:15:58.800 --> 0:16:02.400 because there's a strong magnetic field. That radiation tends to 0:16:02.400 --> 0:16:06.360 go along the magnetic north and the magnetic south because 0:16:06.400 --> 0:16:09.920 magnetic fields are really good at bending the path of 0:16:10.040 --> 0:16:12.640 charged particles. The reason that we don't get a lot 0:16:12.640 --> 0:16:15.840 of radiation from space is because we have a magnetic 0:16:15.880 --> 0:16:19.120 field here on Earth, and when particles come from space, 0:16:19.440 --> 0:16:23.000 they are bent around those magnetic field lines. The magnetic 0:16:23.000 --> 0:16:24.920 field lines are sort of like the lines on a 0:16:24.920 --> 0:16:27.400 basketball right they run from north to south, and if 0:16:27.400 --> 0:16:30.360 a particle comes from space, it gets bent by those 0:16:30.400 --> 0:16:33.360 magnetic fields and goes out in another direction where sometimes 0:16:33.520 --> 0:16:36.560 they loop around those magnetic field lines all the way 0:16:36.640 --> 0:16:38.640 up to the north or the south pole, and then 0:16:38.680 --> 0:16:41.840 they can slip in between the magnetic field lines. And 0:16:41.880 --> 0:16:44.560 that's for example, why we have the Northern lights and 0:16:44.640 --> 0:16:49.320 the Southern lights, because magnetic fields guide charged particles in 0:16:49.360 --> 0:16:52.520 the same way. If you generate radiation on the surface 0:16:52.520 --> 0:16:56.400 of the planet, it's also bound by those magnetic fields, 0:16:56.480 --> 0:16:58.560 and so in this case, the magnetic fields are even 0:16:58.640 --> 0:17:01.920 much more powerful, and essentially all of the radiation from 0:17:01.920 --> 0:17:05.719 the neutron star gets guided towards the north or the 0:17:05.760 --> 0:17:08.880 south pole of the magnetic field. So you get these 0:17:08.960 --> 0:17:12.800 beams of radiation shooting off of this crazy neutron star. 0:17:12.960 --> 0:17:16.080 Right like it's not crazy enough, it's already super hot, 0:17:16.320 --> 0:17:21.280 super dense, super small, spinning, super fast, really magnetized, and 0:17:21.280 --> 0:17:24.400 now on top of that, it's shining these two crazy 0:17:24.440 --> 0:17:27.960 flashlights out into the universe, one from its magnetic north 0:17:28.000 --> 0:17:30.800 pole and the other from its magnetic south pole. And 0:17:30.840 --> 0:17:33.600 these beams don't come for free. They are very bright, 0:17:33.720 --> 0:17:36.359 they cost a lot of energy, and this energy comes 0:17:36.400 --> 0:17:39.440 from the spinning of the neutron star. Because that's what's 0:17:39.480 --> 0:17:43.000 generating this electric field, the rotation of the magnetic field, 0:17:43.080 --> 0:17:46.560 and eventually it's going to slow it down. Like these pulsars, 0:17:46.560 --> 0:17:49.560 they generate these beams and they last for maybe ten 0:17:49.760 --> 0:17:52.520 or a hundred million years, but they don't last for 0:17:52.560 --> 0:17:55.040 their whole lifetime. At some point the beams turned off 0:17:55.320 --> 0:17:58.080 because the neutron star has slowed down, it's not generating 0:17:58.080 --> 0:18:01.200 that radiation anymore. What that means is that for most 0:18:01.240 --> 0:18:04.280 of a lifetime, the pulsar is actually quiet. They don't 0:18:04.280 --> 0:18:08.119 emit these beams, and so something like nine percent of 0:18:08.200 --> 0:18:12.000 the pulsars out there aren't actually emitting any radiation anymore. 0:18:12.040 --> 0:18:16.400 They are quiet. The universe is filled with dead pulsars, 0:18:16.440 --> 0:18:19.760 pulsars that have gone quiet. So we've explained what a 0:18:19.800 --> 0:18:22.880 pulsar is and how it emits these beams. But why 0:18:22.880 --> 0:18:26.199 do we call it a pulsar. Are these beams themselves 0:18:26.240 --> 0:18:29.879 like pulsing? Do they turn on and off? The beams 0:18:29.920 --> 0:18:31.800 don't turn on and off. I mean, they last for 0:18:31.920 --> 0:18:34.320 millions of years and they eventually fade, but they don't 0:18:34.359 --> 0:18:36.800 like flicker on and off. The reason we call it 0:18:36.840 --> 0:18:40.360 a pulsar is because we only see those beams as 0:18:40.400 --> 0:18:43.479 they pass by the Earth, because the beam is shooting 0:18:43.560 --> 0:18:46.680 up and down along the magnetic field lines. But that's 0:18:46.720 --> 0:18:49.800 not necessarily the same as the axis that the pulsar 0:18:49.960 --> 0:18:53.119 is spinning around. So if it were, if the magnetic 0:18:53.240 --> 0:18:56.760 north and the magnetic south were the same as the 0:18:56.840 --> 0:18:59.119 north and south of the actual stars, so it was 0:18:59.160 --> 0:19:01.919 spinning around on the north pole, then it would always 0:19:01.920 --> 0:19:04.960 be shooting the beam north and the beam south. However, 0:19:05.400 --> 0:19:08.560 if instead the magnetic field is tilted so that it's 0:19:08.600 --> 0:19:11.960 like spinning along one axis, but it's beams are shooting 0:19:11.960 --> 0:19:15.360 off a little bit skewed, then when it spins around, 0:19:15.440 --> 0:19:18.760 the direction of that beam changes right. It's like if 0:19:18.800 --> 0:19:21.520 you're holding a flashlight and you point it straight up 0:19:21.560 --> 0:19:24.520 and then spin, the direction of the flashlight doesn't change. 0:19:24.560 --> 0:19:26.760 But if you hold a flashlight straight out and then 0:19:26.840 --> 0:19:30.479 spin right, then what happens. Then Your flashlight's gonna sweep 0:19:30.520 --> 0:19:34.679 around three sixty degrees every time you rotate. And what 0:19:34.760 --> 0:19:37.040 does somebody see if they're standing in front of you 0:19:37.119 --> 0:19:40.080 watching you spin, they see a flash. They see a 0:19:40.119 --> 0:19:43.479 pulse of light only when the flashlight is pointed in 0:19:43.560 --> 0:19:47.040 your direction. So it's this difference between the direction of 0:19:47.080 --> 0:19:52.160 the pulsars magnetic field and it's actual spin axis, which 0:19:52.240 --> 0:19:54.720 makes it a pulse. Are right, that's what makes it 0:19:54.760 --> 0:19:58.680 appear to pulse. They don't actually pulse. They're sending bright 0:19:58.800 --> 0:20:01.720 streams of light contain usually out into the universe until 0:20:01.760 --> 0:20:05.119 they fade. But we see them pulsing because that beam 0:20:05.400 --> 0:20:08.640 sweeps across Earth and that's what we see. So that's 0:20:08.640 --> 0:20:11.359 what a pulsar is. An introduction to these weird things 0:20:11.400 --> 0:20:14.080 in the universe. Next, we're gonna talk about why we 0:20:14.160 --> 0:20:17.399 suspect that they might exist and how they were actually found. 0:20:17.480 --> 0:20:33.400 But first let's take a quick break. All right, we're 0:20:33.440 --> 0:20:35.760 back and we're talking about the incredible story of the 0:20:35.800 --> 0:20:39.640 discovery of pulsars. And we reminded ourselves that pulsars are 0:20:39.640 --> 0:20:44.480 a tiny, very hot, very dense, very quickly spinning stars 0:20:44.680 --> 0:20:47.639 the left over heart of a supernova. They're shooting a 0:20:47.720 --> 0:20:49.880 beam of light out into the universe, and they are 0:20:49.920 --> 0:20:52.800 also spinning, and so that beam of light passes over 0:20:52.840 --> 0:20:56.520 the Earth and looks like pulsations. It looks like pulses 0:20:56.880 --> 0:21:00.159 from something out there in the universe. And before we 0:21:00.240 --> 0:21:03.800 discovered these things, we had a suspicion that they existed. 0:21:04.040 --> 0:21:06.680 People have been thinking about the life cycle of stars, 0:21:06.920 --> 0:21:10.480 and in nineteen thirty four people suggested that when you 0:21:10.520 --> 0:21:13.600 had a supernova, it might not all blow out into 0:21:13.640 --> 0:21:17.440 the universe, that you might get this small, dense core 0:21:17.640 --> 0:21:20.440 left over, and if so, it would have this really 0:21:20.480 --> 0:21:23.760 weird state of matter. These neutrons would form, that would 0:21:23.760 --> 0:21:25.960 be in a really dense state, this thing that's sort 0:21:25.960 --> 0:21:29.040 of like nuclear matter, the things in the heart of atoms, 0:21:29.080 --> 0:21:31.720 but now on the sides of like a mountain, something 0:21:31.840 --> 0:21:35.480 kilometers wide. Imagine that the nucleus of an atom, but 0:21:35.640 --> 0:21:38.560 kilometers wide. So this was a novelty, but nobody had 0:21:38.600 --> 0:21:41.040 ever seen one before. We didn't know if neutron stars 0:21:41.080 --> 0:21:43.840 existed in nineteen thirty four, and they would be difficult 0:21:43.880 --> 0:21:47.080 to detect because these things don't have fusion anymore. They 0:21:47.080 --> 0:21:50.240 don't glow the same way that very bright stars do. 0:21:50.520 --> 0:21:53.159 So to see neutron stars seemed like a puzzle. But 0:21:53.280 --> 0:21:55.600 then decades later people said, well, you know, they might 0:21:55.640 --> 0:21:59.160 have very strong magnetic fields, and if so, they might 0:21:59.200 --> 0:22:03.560 be rotating, and if so, then they might be pulsing. 0:22:03.920 --> 0:22:07.320 And so this idea sort of came into existence in 0:22:07.359 --> 0:22:13.320 the sixties idea the pulsars as weird, spinning, magnetized, beaming 0:22:13.480 --> 0:22:16.760 light neutron stars might be out there. But you have 0:22:16.840 --> 0:22:19.760 to remember that there are lots of crazy ideas for 0:22:19.800 --> 0:22:22.959 what might be out there. The astronomy literature is filled 0:22:22.960 --> 0:22:26.639 with people speculating maybe these things exist, maybe boson stars exist, 0:22:26.680 --> 0:22:29.720 maybe these other things exist. Now, with a hindsight of history, 0:22:29.760 --> 0:22:32.040 we can go back and trace the development of this 0:22:32.200 --> 0:22:34.880 one thread of an idea that turned out to describe 0:22:34.920 --> 0:22:37.840 something in the actual universe. But don't forget it was 0:22:37.960 --> 0:22:41.360 buried at the time in a forest of other crazy, 0:22:41.400 --> 0:22:44.320 wrong ideas about what might be out there in the universe. 0:22:44.800 --> 0:22:47.080 You know, pulsars exist, and if you took a time 0:22:47.080 --> 0:22:49.240 machine back to the sixties, you might say, I know 0:22:49.359 --> 0:22:51.040 these things exist, and I know how to find them. 0:22:51.080 --> 0:22:53.679 It's not actually that hard. But without the hindsight of 0:22:53.680 --> 0:22:56.159 that history, of course, it's hard to pick the wheat 0:22:56.240 --> 0:22:58.320 from the chaff. So let's get to the story of 0:22:58.359 --> 0:23:00.800 how they were actually discovered. They were found by a 0:23:00.920 --> 0:23:04.200 graduate student at the University of Cambridge, a woman named 0:23:04.280 --> 0:23:07.800 Joscelynn Bell, and she was not looking for pulsars. In fact, 0:23:07.840 --> 0:23:11.040 she wasn't looking for stars at all. She was trying 0:23:11.080 --> 0:23:15.040