1 00:00:00,000 --> 00:00:04,411 Okay. So, I tried to make GR comprehensible and in words we didn't 2 00:00:04,411 --> 00:00:08,264 have the equations that are the meat of the subject available. 3 00:00:08,264 --> 00:00:12,925 And the question at this point is, okay, have you made all this effort? Is it 4 00:00:12,925 --> 00:00:14,851 worth it? What do we get for it? 5 00:00:14,851 --> 00:00:19,822 And gravitation is what drives astronomy, we know that. So, we expect GR to be 6 00:00:19,822 --> 00:00:23,364 important in astronomical circumstances, and indeed it is. 7 00:00:23,364 --> 00:00:28,397 And the first check of Einstein's theory, performed by Einstein, immediately in 8 00:00:28,397 --> 00:00:33,979 1916 when he figured out the general theory has to do with the orbit of 9 00:00:33,979 --> 00:00:37,511 Mercury. Remember, that what we saw was that 10 00:00:37,511 --> 00:00:43,249 Newtonian results are with a -GmM/R potential 11 00:00:43,249 --> 00:00:50,479 energy are modified by general relativity adding this 1 - v^2/c^2 term. 12 00:00:50,479 --> 00:00:54,616 And the fastest moving planet in the solar system, of course, is Mercury. 13 00:00:54,616 --> 00:00:57,296 And so you'd, might expect to see this effect. 14 00:00:57,296 --> 00:01:01,666 And what does this do? Well, any deviation from -GmM/R will ruin 15 00:01:01,666 --> 00:01:05,802 this beautiful property of Kepler orbits which is that they are closed. 16 00:01:05,802 --> 00:01:10,289 In other worlds, Keplerian orbit is this red ellipse over here in which every 17 00:01:10,289 --> 00:01:14,775 period the planet returns to exactly the same point, and the orbit is exactly 18 00:01:14,775 --> 00:01:17,392 periodic. This is not a property of orbits in 19 00:01:17,392 --> 00:01:20,926 general, and indeed, it's not a property of Mercury's orbit. 20 00:01:20,926 --> 00:01:25,733 Mercury orbits the sun in an ellipse, but that ellipse is precessing like this blue 21 00:01:25,733 --> 00:01:29,588 ellipse so that Mercury does not return after a full 22 00:01:29,588 --> 00:01:33,103 orbit to exactly the same point, it actually deviates likely. 23 00:01:33,103 --> 00:01:36,619 Now, the deviation is very slight, this is highly exaggerated. 24 00:01:36,619 --> 00:01:41,482 In fact, the deviation had been measured long before Einstein think about a one 25 00:01:41,482 --> 00:01:45,701 and a half degrees per century. So imagine, the axis of this blue ellipse 26 00:01:45,701 --> 00:01:48,280 rotating one and a half degrees per century. 27 00:01:48,280 --> 00:01:53,401 And most of this is due to what perturbs the pure Newtonian potential in which 28 00:01:53,401 --> 00:01:56,654 Mercury orbits. Well, most of the deviation is to do, of 29 00:01:56,654 --> 00:02:01,654 course, with the gravitational influences of Jupiter and to smaller extent other 30 00:02:01,654 --> 00:02:04,667 planets. And to an even smaller extent, the fact 31 00:02:04,667 --> 00:02:08,161 that the sun is not exactly a sphere, it's slightly oblate. 32 00:02:08,161 --> 00:02:11,053 It's fat around the middle due to it's rotation. 33 00:02:11,053 --> 00:02:17,425 All of these effects were in the precinct of Newtonian physics, had been taken into 34 00:02:17,425 --> 00:02:23,725 account and tallied carefully famously by [UNKNOWN] in 1859. And he came up with a 35 00:02:23,725 --> 00:02:27,723 result that was almost a degree and a half, per century, 36 00:02:27,723 --> 00:02:30,639 but was short by 43 arc seconds per century. 37 00:02:30,639 --> 00:02:35,808 This gives you a sense of the precision with which the calculations were done. 38 00:02:35,808 --> 00:02:41,308 And what Einstein does is he computes the effect of this term on the procession of 39 00:02:41,308 --> 00:02:45,351 the [UNKNOWN] of Mercury. And putting in the parameters for 40 00:02:45,351 --> 00:02:48,599 Mercury, he gets exactly 43 arc seconds. And so 41 00:02:48,599 --> 00:02:53,581 some part of the failure of Mercury's orbit to be closed is due to general 42 00:02:53,581 --> 00:02:59,095 relativistic corrections, and this is the first validated prediction. If you want 43 00:02:59,095 --> 00:03:05,054 the first validation of his theory clearly given this v^2/c^2 term when you 44 00:03:05,054 --> 00:03:09,851 have close binary stars like the Algol system, two things with masses of order 45 00:03:09,851 --> 00:03:14,766 in the mass of the sun orbiting each other with a an orbit that is an eighth 46 00:03:14,766 --> 00:03:18,674 of the orbit of Mercury. Now, the velocities are suitably higher, 47 00:03:18,674 --> 00:03:22,820 and there general relativistic effects are going to be more important. 48 00:03:22,820 --> 00:03:27,202 So, whenever we're dealing with orbits and using Newtonian physics, we were 49 00:03:27,202 --> 00:03:31,762 making approximations which we will by and large continue to make, by the way. 50 00:03:31,762 --> 00:03:36,160 We will not be doing, but, but we know now that we were missing something. 51 00:03:36,160 --> 00:03:41,630 since then, precision tests of Einstein's theory have been done in various waves. 52 00:03:41,630 --> 00:03:46,021 Famously, I just have to mention this, by the gravity Probe B Mission which 53 00:03:46,021 --> 00:03:51,326 recently finished a measurement of the deviations from Newtonian predictions due 54 00:03:51,326 --> 00:03:55,838 to the curved geometry around Earth. As well as the tiny effect of Earth's 55 00:03:55,838 --> 00:04:00,778 rotation turns out to change the gravitational behavior in the vicinity of 56 00:04:00,778 --> 00:04:03,034 Earth due to general relativistic corrections. 57 00:04:03,034 --> 00:04:07,303 And both of those were validated brilliantly by the, the gravity Probe B 58 00:04:07,303 --> 00:04:10,352 mission. We won't get into too many of the details 59 00:04:10,352 --> 00:04:12,931 of this. There's this famous second validation of 60 00:04:12,931 --> 00:04:16,123 Einstein's theory. Remember, that one of the predictions of 61 00:04:16,123 --> 00:04:18,820 general relativity is that gravity acts on light. 62 00:04:18,820 --> 00:04:21,462 We found that through the equivalence principle, 63 00:04:21,462 --> 00:04:25,920 you can argue rather easily through the mass energy equivalence that gravity has 64 00:04:25,920 --> 00:04:29,113 to have an effect on light. This settles an old dispute 65 00:04:29,113 --> 00:04:31,810 there was always the question on the one hand, 66 00:04:31,810 --> 00:04:37,872 the force applied by gravity is GMm/R^2 and light beam has a vanishing little m 67 00:04:37,872 --> 00:04:42,128 so the force should be zero. On the other hand, the acceleration due 68 00:04:42,128 --> 00:04:46,127 to gravity is GM/R^2, and here the little m is gone. 69 00:04:46,127 --> 00:04:49,416 And so, would light be accelerated in a similar way? 70 00:04:49,416 --> 00:04:53,866 Would light or would it not be affected by gravity was an old debate. 71 00:04:53,866 --> 00:04:58,680 GR settles it, and in fact you can make explicit calculation, and 72 00:04:58,680 --> 00:05:03,578 this is a very important calculation for us later, of the degree to which, 73 00:05:03,578 --> 00:05:09,092 so, so what this means, in fact, is that a light beam traveling near a massive 74 00:05:09,092 --> 00:05:15,213 object will like any massive thing be deflected so that it's straight line 75 00:05:15,213 --> 00:05:21,144 motion in empty space will be curved. And, I've drawn the curve clumsily, but 76 00:05:21,144 --> 00:05:25,858 the net result is that the light beam will be deflected. 77 00:05:25,858 --> 00:05:31,257 And the closer to the planet, to the star, the closer to the gravitating 78 00:05:31,257 --> 00:05:36,546 source, the light beam travels, the more it grazes it the larger this deflection 79 00:05:36,546 --> 00:05:41,007 angle, of the light beam will be. The deflection angle is related to the 80 00:05:41,007 --> 00:05:45,348 mass and the distance by this formula which shows you that it's clearly 81 00:05:45,348 --> 00:05:49,206 relativistic but it's to do with light so that's to be expected. 82 00:05:49,206 --> 00:05:53,968 And the effect of this is that if you are studying, if you are observing a star 83 00:05:53,968 --> 00:05:56,440 which is lined up, let's say, with the sun. 84 00:05:56,440 --> 00:06:00,442 This is a difficult thing to do. But if you could imagine viewing a star 85 00:06:00,442 --> 00:06:04,779 that if you viewed it correctly would look as though it were right behind the 86 00:06:04,779 --> 00:06:07,225 sun, then that star would actually appear to 87 00:06:07,225 --> 00:06:11,561 you because of the deflection of the light to be in the wrong part of the sky. 88 00:06:11,561 --> 00:06:15,953 So you can measure this deflection all you need to do is look up at the, a star 89 00:06:15,953 --> 00:06:19,164 right near the sun. But you can't see a star near the sun 90 00:06:19,164 --> 00:06:23,416 except during a total solar eclipse. Fortunately, three years after Einstein 91 00:06:23,416 --> 00:06:26,761 publishes his paper, there is a total solar eclipse in 1919. 92 00:06:26,761 --> 00:06:31,014 This is the middle of World War I. Sir Arthur Eddington, who realizes the 93 00:06:31,014 --> 00:06:35,096 significance of Einstein's result, organizes a big expedition to various 94 00:06:35,096 --> 00:06:39,461 places in the Southern Hemisphere where the eclipse will be visible. And they 95 00:06:39,461 --> 00:06:42,013 measure, here's an, a picture from Eddington's 96 00:06:42,013 --> 00:06:44,847 original expedition of the total solar eclipse. 97 00:06:44,847 --> 00:06:48,760 This is, of course, a negative. You can see the dark being the light 98 00:06:48,760 --> 00:06:53,672 cromoline, cromosphere and corona that they could photograph around the sun and 99 00:06:53,672 --> 00:06:58,464 they do take plates and notice measure the positions of stars that are very near 100 00:06:58,464 --> 00:07:01,400 the sun during the eclipse. And indeed, they are 101 00:07:01,400 --> 00:07:05,537 deviating from their positions by precisely the amount predicted by 102 00:07:05,537 --> 00:07:08,580 Einstein. There were doubts about the precision of 103 00:07:08,580 --> 00:07:11,988 the measurement. It's since been reproduced in many ways 104 00:07:11,988 --> 00:07:17,038 and at high precision, and its deflection of light by massive objects has become a 105 00:07:17,038 --> 00:07:20,994 topic of research in itself. It goes under the main gravitational 106 00:07:20,994 --> 00:07:24,219 lensing, and it's a very important tool in modern 107 00:07:24,219 --> 00:07:28,540 astronomy. And this famous photo of the Abell Cluster is 108 00:07:28,540 --> 00:07:33,174 one of the the best examples I know of, this is the Hubble photo. 109 00:07:33,174 --> 00:07:38,338 And what we see here in the foreground of the picture, we're looking deep into 110 00:07:38,338 --> 00:07:41,648 space. This is the Hubble shot. And these yellow 111 00:07:41,648 --> 00:07:46,893 objects are cluster of galaxies. Now, this weird red shaped thing over on 112 00:07:46,893 --> 00:07:50,035 the right hand side, I hope you can see it. 113 00:07:50,035 --> 00:07:54,824 this is a galaxy which, in fact, is many, many, many, many megap, 114 00:07:54,824 --> 00:08:00,885 many, many, many, light years behind this cluster of galaxies that we're looking 115 00:08:00,885 --> 00:08:04,165 at. And the reason this galaxy looks so weird 116 00:08:04,165 --> 00:08:09,529 is that, the light coming from the galaxy has been deflected by the mass of the 117 00:08:09,529 --> 00:08:14,730 galaxy cluster in the foreground. And we see a deformed image of the galaxy 118 00:08:14,730 --> 00:08:19,533 spread out, this is not a perfect spherical lens. The distribution of mass 119 00:08:19,533 --> 00:08:24,533 here is somewhat interesting. And in fact, this this, this spread out 120 00:08:24,533 --> 00:08:29,533 thing, if you look at it carefully in a magnified, in a larger version of the 121 00:08:29,533 --> 00:08:35,187 image, you'll see that it is in fact two distinct images of the same galaxy both 122 00:08:35,187 --> 00:08:39,224 lens by this. And then once you realize how to look for 123 00:08:39,224 --> 00:08:44,656 them, you find more lensed background objects here, and here, they show this 124 00:08:44,656 --> 00:08:48,400 sort of streakiness like images through bad optics. 125 00:08:48,400 --> 00:08:52,574 And so we have in this image, picture, multiple 126 00:08:52,574 --> 00:08:57,608 images often of the same background galaxy as seen through the lensing of 127 00:08:57,608 --> 00:09:01,400 this galaxy cluster. This has been used by astronomers for 128 00:09:01,400 --> 00:09:04,211 many purposes. one is you have a lens. 129 00:09:04,211 --> 00:09:09,245 This means that the light coming from behind this cluster, if you, things are 130 00:09:09,245 --> 00:09:12,579 lined up correctly, will be focused and intensified. 131 00:09:12,579 --> 00:09:17,743 And the dimmest, most distant objects we've been able to image have been imaged 132 00:09:17,743 --> 00:09:22,516 through gravitational lenses. So, you can basically use this as another 133 00:09:22,516 --> 00:09:29,648 lens in your telescope. even more excitingly this expression that 134 00:09:29,648 --> 00:09:37,660 I wrote, theta is G, 4GM/rc^2. this equation characterizes the 135 00:09:37,660 --> 00:09:43,076 deflection of light by the mass that is deflecting it and the distance at which R 136 00:09:43,076 --> 00:09:46,305 here is the what I called B in the previous picture. 137 00:09:46,305 --> 00:09:50,004 The distance of closest approach of the light beam to the mass. 138 00:09:50,004 --> 00:09:54,936 you can imagine adding up the deflections of a given beam of light due to all of 139 00:09:54,936 --> 00:09:59,852 the sources of mass in this cluster and figuring out how light will be deflected. 140 00:09:59,852 --> 00:10:03,463 You have here, in the image, the lensed image of the galaxy, 141 00:10:03,463 --> 00:10:07,511 a lot of information on how different light beams were deflected. 142 00:10:07,511 --> 00:10:11,247 You could put them together to learn, in fact, about the lens, 143 00:10:11,247 --> 00:10:13,987 about the mass distribution in this cluster. 144 00:10:13,987 --> 00:10:18,284 And this is very important. it has been used, for example, to detect 145 00:10:18,284 --> 00:10:21,833 exoplanets. When an exoplanet passes in front of its 146 00:10:21,833 --> 00:10:24,760 star, the lensing by the planet intensifies the 147 00:10:24,760 --> 00:10:27,938 light of the star. And that light curve peak that is 148 00:10:27,938 --> 00:10:32,153 characteristic of what's called micro lensing events, has been used to detect 149 00:10:32,153 --> 00:10:34,781 exto planets, which are otherwise too dim to see. 150 00:10:34,781 --> 00:10:38,888 So, you're learning about the lens. Another famous example about, of learning 151 00:10:38,888 --> 00:10:42,771 about the lens is we'll talk in the last week or next week about dark 152 00:10:42,771 --> 00:10:45,839 matter, and one of the methods we use to detect dark matter. 153 00:10:45,839 --> 00:10:50,102 Dark matter does not produce light in any wavelength, and does not interact in any 154 00:10:50,102 --> 00:10:54,261 way other than gravitationally, you know. So we need to measure its gravitational 155 00:10:54,261 --> 00:10:56,185 field. One of the ways we measure its 156 00:10:56,185 --> 00:11:00,344 gravitational field is by observing the lensing of light and we can reconstruct 157 00:11:00,344 --> 00:11:04,400 the mass distribution, and I'll show you a brilliant example of that next week. 158 00:11:04,400 --> 00:11:08,715 So, gravitational lensing is a field of study all of its own and its very 159 00:11:08,715 --> 00:11:13,090 important to an astronomy and is, of course intrinsically relativistic. 160 00:11:13,090 --> 00:11:17,266 And then there's the beautiful case of the binary pulsar PSR1913+16.16. 161 00:11:17,266 --> 00:11:22,582 So, this is a pulsar in the constellation Aquila about 6,400 parsecs from Earth. 162 00:11:22,582 --> 00:11:27,581 It's a millisecond pulsar, we didn't get to talk about millisecond pulsar as much. 163 00:11:27,581 --> 00:11:32,896 But sometimes in binary systems pulsar, neutron stars speed up as they accrete 164 00:11:32,896 --> 00:11:37,136 matter from their companion. So that rather than slowing down their 165 00:11:37,136 --> 00:11:41,756 period for awhile, they speed up. And the period of this particular pulsar 166 00:11:41,756 --> 00:11:45,300 is 59 milliseconds, that's a very rapid, rotation rate. 167 00:11:45,300 --> 00:11:50,709 And, we notice when housian Hulse and Taylor careful measurements of the pulses 168 00:11:50,709 --> 00:11:53,849 from the timing of the pulses from this pulsar. 169 00:11:53,849 --> 00:11:59,259 They found that some of the pulses were delayed and some were advanced and there 170 00:11:59,259 --> 00:12:03,400 was a periodic pulse delay with a period of about eight hours. 171 00:12:03,400 --> 00:12:08,287 Turns out after further study that the reason for this is that this pulsar is 172 00:12:08,287 --> 00:12:13,113 actually a member of a binary system, where both members are neutron stars of 173 00:12:13,113 --> 00:12:17,812 masses about one and a half solar masses. And the semi-major axis is 2.8 174 00:12:17,812 --> 00:12:20,319 astronomical units, that's correct there. 175 00:12:20,319 --> 00:12:23,264 The semi-major axis is about three solar radii. 176 00:12:23,264 --> 00:12:28,152 So, I have two solar mass objects, but because they're neutron stars with radii 177 00:12:28,152 --> 00:12:32,416 of only ten kilometers or so. Orbiting each other at a radius about 178 00:12:32,416 --> 00:12:37,221 three times the sun's radius. Moreover, this system is special because 179 00:12:37,221 --> 00:12:42,506 the elliptical orbits that these objects follow are very eccentric. And so, the 180 00:12:42,506 --> 00:12:47,204 distance of nearest approach at periastron, they are only at ten percent 181 00:12:47,204 --> 00:12:50,205 more than a solar radius away from each other. 182 00:12:50,205 --> 00:12:55,425 They get very near, and then they move off to about 4.8 or something solar radii 183 00:12:55,425 --> 00:12:58,626 at their farthest. And this means we have very 184 00:12:58,626 --> 00:13:03,594 relativilistic velocities for sure when we have two solar mass objects at a 185 00:13:03,594 --> 00:13:07,124 distance of the solar radius from each other in orbit. 186 00:13:07,124 --> 00:13:12,485 And so, this is a great lab to study GR. And what do Hulse and Taylor extract from 187 00:13:12,485 --> 00:13:17,257 this study of GR in this beautiful lab? Well, first of all the pulse delay 188 00:13:17,257 --> 00:13:21,310 structure is interesting. It exhibits the usual Doppler effect. 189 00:13:21,310 --> 00:13:23,977 Pulses are delayed when the pulsar is moving away. 190 00:13:23,977 --> 00:13:28,245 Of course, we don't know if both neutron stars are pulsars and once just doesn't 191 00:13:28,245 --> 00:13:32,353 happen to aim at us or maybe the other one does not have jets, we do not know 192 00:13:32,353 --> 00:13:34,843 this. but we only see pulses from one of the 193 00:13:34,843 --> 00:13:37,595 neutron stars. There are other systems that are even 194 00:13:37,595 --> 00:13:41,353 richer, where both members are pulsars, but this is the famous first one. 195 00:13:41,353 --> 00:13:44,159 And so the pulse display exhibits the Doppler effect. 196 00:13:44,159 --> 00:13:48,616 We see that when the pulsar is retreating from us, the pulses are seperated by a 197 00:13:48,616 --> 00:13:52,060 longer time. But that does not explain all of the, pulse delay. 198 00:13:52,060 --> 00:13:56,351 I will post a link to the, to, to, review by Taylor of what it was they did. 199 00:13:56,351 --> 00:14:00,868 The calculation is very complicated and take into account many, many effects, but 200 00:14:00,868 --> 00:14:04,538 at the end of the day, Doppler Effect does not explain everything. 201 00:14:04,538 --> 00:14:08,603 It turns out that the pulses are more delayed near periostron than near 202 00:14:08,603 --> 00:14:12,330 apostron when the two when the pulsar is very near its partner. 203 00:14:12,330 --> 00:14:16,995 The pulses are delayed by more and the reason for this is a gravitational 204 00:14:16,995 --> 00:14:20,211 redshift. The pulses are delayed because the pulsar 205 00:14:20,211 --> 00:14:23,363 is spinning at it's accustomed rate and it's free. 206 00:14:23,363 --> 00:14:28,344 But as we see it from far away, when it's low down in the bottom floor close to 207 00:14:28,344 --> 00:14:33,199 it's partner, there is an additional gravitational redshift and an additional 208 00:14:33,199 --> 00:14:36,665 delay in the pulses. Furthermore, they can measure the 209 00:14:36,665 --> 00:14:42,013 procession of the perihelion by, because they can locate where in these eight hour 210 00:14:42,013 --> 00:14:46,225 periods periastron lies. And the perihelion procession agrees 211 00:14:46,225 --> 00:14:50,704 completely with GR predictions. So they have a whole bunch of 212 00:14:50,704 --> 00:14:53,845 measurements that in this binary pulsar system, 213 00:14:53,845 --> 00:14:59,060 reproduce brilliantly the predictions of GR but it gets much better than this. 214 00:14:59,060 --> 00:15:03,146 You may recall that a long time ago, I talked about quantum mechanics and the 215 00:15:03,146 --> 00:15:06,118 problem of stability of atoms. And I said something like. 216 00:15:06,118 --> 00:15:10,205 well, if you have a nucleus here, and you have an electron that is orbiting it, 217 00:15:10,205 --> 00:15:13,920 that's an accelerating charge. And that should radiate electromagnetic 218 00:15:13,920 --> 00:15:16,521 radiation. And that would mean it's losing energy. 219 00:15:16,521 --> 00:15:19,705 And the electron would spiral down, and an atom wouldn't last. 220 00:15:19,705 --> 00:15:23,077 How are atoms stable? Well, now that we have a field theory of 221 00:15:23,077 --> 00:15:27,084 gravity, you can ask the same question about the Earth orbiting the sun. 222 00:15:27,084 --> 00:15:30,188 The Earth is orbiting the sun, the mass is accelerating. 223 00:15:30,188 --> 00:15:34,533 Remember, that means that there is a ripple in the gravitational field of the 224 00:15:34,533 --> 00:15:37,863 Earth-Sun system. That ripple is broadcast out as something 225 00:15:37,863 --> 00:15:41,700 called gravitational waves. And those carry off energy Shouldn't the 226 00:15:41,700 --> 00:15:45,651 Earth be spiraling down towards the sun? Well, yes, but only very slowly. 227 00:15:45,651 --> 00:15:48,642 Gravity is a much weaker force than electromagnetism. 228 00:15:48,642 --> 00:15:53,503 And therefore the Earth is in the Earth's orbit is spiraling down due to gravity. 229 00:15:53,503 --> 00:15:57,680 But that effect is not only negligible, but completely impossible to measure. 230 00:15:57,680 --> 00:16:02,128 On the other hand, if you have a system that is accelerating as violently as two 231 00:16:02,128 --> 00:16:06,967 neutron stars a solar radius from each other there the emission of gravitational 232 00:16:06,967 --> 00:16:09,914 waves carries off a non-trivial fraction of the energy. 233 00:16:09,914 --> 00:16:13,473 And what do you expect? Well, you expect the two neutron stars as 234 00:16:13,473 --> 00:16:16,310 they lose energy to spiral down towards each other. 235 00:16:16,310 --> 00:16:19,932 Which means, since they're in gravitational orbits, 236 00:16:19,932 --> 00:16:23,145 the closer they are, the faster they will orbit. 237 00:16:23,145 --> 00:16:27,519 You expect the period, that 7.8 hours, to be decreasing with time. 238 00:16:27,519 --> 00:16:32,099 the pulsar was discovered in the 70s' and has been studied since. 239 00:16:32,099 --> 00:16:37,294 And what you see here is a plot of the decline in the period of this binary 240 00:16:37,294 --> 00:16:42,394 pulsar over the past 30 years plotted against a theoretical prediction from GR 241 00:16:42,394 --> 00:16:46,258 of the rate at which the system will lose energy to gravitational waves. 242 00:16:46,258 --> 00:16:50,498 And so, this is not just evidence for general relativity as well as a beautiful 243 00:16:50,498 --> 00:16:53,450 confirmation of our understanding of the binary pulsar. 244 00:16:53,450 --> 00:16:58,009 But, it's also telling us that this system is emitting gravitational waves, 245 00:16:58,009 --> 00:17:02,804 we'll talk about that in a second. And that gravitational energy travels through 246 00:17:02,804 --> 00:17:07,600 space in the form of waves similar, analogous to electromagnetic waves. what 247 00:17:07,600 --> 00:17:11,923 will be the end of this system? Well, as the pulsars, as the two neutron 248 00:17:11,923 --> 00:17:16,659 stars spiral closer and closer to each other, they're accelerating more and more 249 00:17:16,659 --> 00:17:21,277 violently. The motion becomes faster, the rate of energy loss becomes higher. 250 00:17:21,277 --> 00:17:26,407 Hence, the decay and the period becomes more and more precipitously, precipitous. 251 00:17:26,407 --> 00:17:31,141 And within as little as 250 million years, they will actually come close 252 00:17:31,141 --> 00:17:35,868 enough to violently collide, and presumably merge to form well, a bigger 253 00:17:35,868 --> 00:17:41,812 neutron star if most of the material is ejected in the collision or possibly a 254 00:17:41,812 --> 00:17:46,757 black hole, the subject of our next clip. So, there's these gravitational waves 255 00:17:46,757 --> 00:17:50,169 being emitted. Can we see those? Well, the answer is 256 00:17:50,169 --> 00:17:53,563 almost. There are a collection of gravitational 257 00:17:53,563 --> 00:17:56,889 wave detectors like this one, this is LIGO, Laser Interferometer Gravitational 258 00:17:56,889 --> 00:18:05,045 Wave Observatory and what this is is two three miles laser tubes. a gravitational 259 00:18:05,045 --> 00:18:11,039 wave passing through will cause the large and well insulated masses at the tips of 260 00:18:11,039 --> 00:18:14,782 these to move by about the size of an atomic nucleus. 261 00:18:14,782 --> 00:18:18,252 And because, again, a very sensitive interferometry, 262 00:18:18,252 --> 00:18:24,083 this machine is supposed to be able to detect this sort of fluctuation in the 263 00:18:24,083 --> 00:18:29,011 relative length of the two arms. And it has not detected, as far as I 264 00:18:29,011 --> 00:18:33,731 know, gravitational radiation. It's at the end of its calibration run. 265 00:18:33,731 --> 00:18:39,422 And sensitivities and noise detection and background reduction are continually 266 00:18:39,422 --> 00:18:40,950 being improved. And 267 00:18:40,950 --> 00:18:45,521 maybe any day we will hear the discovery of gravitational waves and it won't be 268 00:18:45,521 --> 00:18:51,350 from the binary, Hulse-Taylor binary pulsar because the Hulse-Taylor binary 269 00:18:51,350 --> 00:18:55,807 pulsar does not emit nearly intense enough gravitational waves for this 270 00:18:55,807 --> 00:18:59,235 detector to detect them. Although, it is capable of detecting 271 00:18:59,235 --> 00:19:03,007 waves, there's a detector in Louisiana, one of the two detectors. 272 00:19:03,007 --> 00:19:07,407 This one I think is in Washington. The detector in Louisiana detects waves 273 00:19:07,407 --> 00:19:11,635 breaking on the Gulf Coast which they have to subtract. But it won't be 274 00:19:11,635 --> 00:19:14,550 sensitive enough to detect the Hulse-Taylor pulsar. 275 00:19:14,550 --> 00:19:18,749 But if something very violent occurs, like the merger of neutron stars, 276 00:19:18,749 --> 00:19:23,470 those last three orbits before a merger are sufficiently violent and sufficiently 277 00:19:23,470 --> 00:19:26,637 energetic. That enough energy will be emitted in the 278 00:19:26,637 --> 00:19:31,070 gravitational radiation that we think we will be able to observe such events. 279 00:19:31,070 --> 00:19:34,767 This will be the second time in this class that we're talking about 280 00:19:34,767 --> 00:19:38,520 non-electromagnetic telescopes. We talked about neutrino astronomy 281 00:19:38,520 --> 00:19:42,435 opening up the field of gravitational wave astronomy, which is what this 282 00:19:42,435 --> 00:19:45,970 detector is hoping to do. Would give us a whole new window on the 283 00:19:45,970 --> 00:19:49,722 universe and one focused on the most dramatic and cataclysmic events. 284 00:19:49,722 --> 00:19:52,985 So, we're all hoping that they do detect gravitational waves.