1 00:00:00 --> 00:00:01,333 2 00:00:01,333 --> 00:00:06,374 Today I would like to talk to you about some of the research 3 00:00:06,374 --> 00:00:09,622 that I did during my early days at MIT. 4 00:00:09,622 --> 00:00:12,442 It's a long time ago. I got my Ph. 5 00:00:12,442 --> 0. 6 0. --> 00:00:15,006 in the Netherlands on nuclear work and so the tension during 7 00:00:15,006 --> 00:00:19,449 physics and I came over to MIT in nineteen sixty-six. 8 00:00:19,449 --> 00:00:23,124 I was supposed to be here only for one year. 9 00:00:23,124 --> 00:00:25,944 I had a one-year postdoc position. 10 00:00:25,944 --> 00:00:28,935 But I loved it so much I never left. 11 00:00:28,935 --> 00:00:33,88 I changed fields. I joined the research group of 12 00:00:33,88 --> 00:00:37,143 Professor Bruno Rossi. Here at MIT I changed from 13 00:00:37,143 --> 00:00:39,454 nuclear physics to x-ray astronomy. 14 00:00:39,454 --> 00:00:41,698 X-ray astronomy speaks for itself. 15 00:00:41,698 --> 00:00:44,349 You're trying to do astronomy in x-rays. 16 00:00:44,349 --> 00:00:48,292 You cannot see any x-rays from the ground because the earth 17 00:00:48,292 --> 00:00:50,603 atmosphere absorbs them completely. 18 00:00:50,603 --> 00:00:54,275 So you have to go outside the atmosphere unlike optical 19 00:00:54,275 --> 00:00:57,946 astronomy and radio astronomy which you can do from the 20 00:00:57,946 --> 00:01:01,753 ground. When I use the word x-rays I'm 21 00:01:01,753 --> 00:01:06,111 thinking of the kind of x-rays that your dentist would be 22 00:01:06,111 --> 00:01:09,77 using, medical purposes, about one to fifty kilo 23 00:01:09,77 --> 00:01:13,35 electron-volts. And since all of you took eight 24 00:01:13,35 --> 00:01:17,709 oh two you should know by now what a kilo electron-volts. 25 00:01:17,709 --> 00:01:20,667 Uh optical light is two electron volts, 26 00:01:20,667 --> 00:01:24,092 where x-rays, way more energetic than optical 27 00:01:24,092 --> 00:01:26,894 light. Uh during the Second World War 28 00:01:26,894 --> 00:01:31,564 Werner von Braun in Peenemunde developed under Hitler Germany 29 00:01:31,564 --> 00:01:36,939 destructive rockets. They were used to destroy the 30 00:01:36,939 --> 00:01:39,433 Allies. To destroy you and me. 31 00:01:39,433 --> 00:01:43,39 And after the war, around nineteen forty-eight, 32 00:01:43,39 --> 00:01:47,346 the Americans used these rockets to do science. 33 00:01:47,346 --> 00:01:52,335 They also got Werner von Braun over to this country and for 34 00:01:52,335 --> 00:01:55,603 reasons unknown to me he became a hero. 35 00:01:55,603 --> 00:01:59,904 They tried to observe x-rays from the solar system. 36 00:01:59,904 --> 00:02:04,033 And they found indeed that the sun 37 00:02:04,033 --> 00:02:06,653 emits x-rays. The sun is very close. 38 00:02:06,653 --> 00:02:09,722 So you may say well that's not a surprise. 39 00:02:09,722 --> 00:02:13,914 But it's really very unusual because to create x-rays you 40 00:02:13,914 --> 00:02:16,31 need extremely high temperatures, 41 00:02:16,31 --> 00:02:19,304 which we didn't think existed on the sun. 42 00:02:19,304 --> 00:02:23,571 And if you take the power that the sun puts out in x-rays, 43 00:02:23,571 --> 00:02:27,763 this is joules per second now, and you divide that by the 44 00:02:27,763 --> 00:02:31,806 power in the optical and the infrared light of the sun, 45 00:02:31,806 --> 00:02:36,073 this symbol stands for sun, that ratio is about ten to the 46 00:02:36,073 --> 00:02:38,312 minus seven. 47 00:02:38,312 --> 00:02:42,741 So you must conclude that the sun emits largely optical light 48 00:02:42,741 --> 00:02:47,465 and infrared and that the amount of x-rays is a modest byproduct. 49 00:02:47,465 --> 00:02:49,975 Interesting as it is all by itself. 50 00:02:49,975 --> 00:02:54,256 In nineteen sixty-two several scientists here in Cambridge, 51 00:02:54,256 --> 00:02:57,209 Massachusetts, among them Bruno Rossi and 52 00:02:57,209 --> 00:03:01,047 Riccardo Giacconi and Herb Gursky, made an attempt to 53 00:03:01,047 --> 00:03:04,738 observe x-rays from stars outside our solar system. 54 00:03:04,738 --> 00:03:08,576 The odds were strongly against them. 55 00:03:08,576 --> 00:03:11,424 The detectors were not sensitive enough. 56 00:03:11,424 --> 00:03:15,439 If you take the sun and you bring the sun to the nearest 57 00:03:15,439 --> 00:03:19,017 stars, which is a distance say of ten light-years, 58 00:03:19,017 --> 00:03:23,324 then there would be no hope that you would be able to detect 59 00:03:23,324 --> 00:03:25,807 x-rays from an object like the sun. 60 00:03:25,807 --> 00:03:30,333 In fact the detectors were too insensitive by about a factor of 61 00:03:30,333 --> 00:03:33,473 one billion. They tried anyhow and they were 62 00:03:33,473 --> 00:03:35,955 successful. They did indeed find to 63 00:03:35,955 --> 00:03:40,555 everyone's surprise and joy, they found x-rays 64 00:03:40,555 --> 00:03:44,768 from at least one object outside the solar system. 65 00:03:44,768 --> 00:03:48,036 This object was later called SCO X one. 66 00:03:48,036 --> 00:03:52,336 SCO stands for the constellation Scorpio in the sky 67 00:03:52,336 --> 00:03:57,582 and X for x-rays and one for the first source observed in that 68 00:03:57,582 --> 00:04:01,452 constellation. We now know that this object is 69 00:04:01,452 --> 00:04:05,494 a faint blue star. And what is extremely special 70 00:04:05,494 --> 00:04:10,74 about the object SCO X one is that if you take the ratio x-ray 71 00:04:10,74 --> 00:04:15,728 power over optical power then that ratio is 72 00:04:15,728 --> 00:04:19,65 about ten to the third. Compare that with the sun. 73 00:04:19,65 --> 00:04:22,932 This object, we had no clue what it was in 74 00:04:22,932 --> 00:04:27,575 those days, primarily emit x-rays, and the optical emission 75 00:04:27,575 --> 00:04:30,857 is a byproduct. Whereas with the sun it is 76 00:04:30,857 --> 00:04:34,059 reversed. And so the burning question was 77 00:04:34,059 --> 00:04:37,342 in those days what kind of animal is this? 78 00:04:37,342 --> 00:04:40,223 It must be a totally different beast. 79 00:04:40,223 --> 00:04:43,185 Something very different from our sun. 80 00:04:43,185 --> 00:04:47,727 And when I came to MIT in nineteen sixty-six, 81 00:04:47,727 --> 00:04:51,015 there were six sources known outside our solar system. 82 00:04:51,015 --> 00:04:53,56 And they were all discovered with rockets. 83 00:04:53,56 --> 00:04:57,345 The rockets in those days could spend about five minutes above 84 00:04:57,345 --> 00:05:01,006 the earth atmosphere and they would quickly make a scan over 85 00:05:01,006 --> 00:05:03,613 the sky, five minutes, that's all they had. 86 00:05:03,613 --> 00:05:06,157 And I joined the group of uh George Clark, 87 00:05:06,157 --> 00:05:09,322 who is still at MIT, uh he was doing x-ray astronomy 88 00:05:09,322 --> 00:05:12,983 from very high-flying balloons, very close to the top of the 89 00:05:12,983 --> 00:05:16,644 atmosphere, and the advantage of balloons was that you could 90 00:05:16,644 --> 00:05:20,055 observe the sky for many many hours, 91 00:05:20,055 --> 00:05:22,779 if you're lucky sometimes even a day or more. 92 00:05:22,779 --> 00:05:25,998 But on the other hand, since there is always a little 93 00:05:25,998 --> 00:05:29,527 bit of atmosphere left above you, even though there's very 94 00:05:29,527 --> 00:05:33,056 little, there is still some left, the x-rays are absorbed, 95 00:05:33,056 --> 00:05:36,647 almost all x-rays below twenty kilo electron-volts would be 96 00:05:36,647 --> 00:05:39,495 absorbed, and we would not be able to see them. 97 00:05:39,495 --> 00:05:43,147 But of course the compensation was that we could look at the 98 00:05:43,147 --> 00:05:46,8 sky for many many many hours. Nowadays no one is doing these 99 00:05:46,8 --> 00:05:51,721 balloon observations anymore. No more rocket observations. 100 00:05:51,721 --> 00:05:54,883 Everything is done of course from satellites. 101 00:05:54,883 --> 00:05:58,403 So when I came to MIT, together with George Clark, 102 00:05:58,403 --> 00:06:02,858 I developed new x-ray detectors for these balloon observations. 103 00:06:02,858 --> 00:06:05,444 Many graduate students were involved. 104 00:06:05,444 --> 00:06:08,965 Many undergraduates. It would take about two years 105 00:06:08,965 --> 00:06:12,413 to build a telescope. To give you a rough idea it 106 00:06:12,413 --> 00:06:16,58 would take a million dollars in terms of nineteen sixty-six 107 00:06:16,58 --> 00:06:20,819 dollars, and the weight of such a telescope 108 00:06:20,819 --> 00:06:23,13 would be roughly a thousand kilograms. 109 00:06:23,13 --> 00:06:26,377 The balloons in those days would cost about a hundred 110 00:06:26,377 --> 00:06:29,686 thousand dollars to get us up to these high altitudes, 111 00:06:29,686 --> 00:06:33,245 and we would need about eighty thousand dollars of helium, 112 00:06:33,245 --> 00:06:35,493 and you will see some slides of that. 113 00:06:35,493 --> 00:06:39,177 We have to go to altitudes of about a hundred forty thousand 114 00:06:39,177 --> 00:06:41,362 feet. We had huge balloons for that. 115 00:06:41,362 --> 00:06:44,172 You will see one. They have diameters of about 116 00:06:44,172 --> 00:06:46,483 six hundred feet. And the material was 117 00:06:46,483 --> 00:06:48,98 polyethylene. Extremely thin to make them 118 00:06:48,98 --> 00:06:52,572 light-weight so that they can go high. 119 00:06:52,572 --> 00:06:55,454 The thickness of that polyethylene was about half of 120 00:06:55,454 --> 00:06:58,675 one-thousandth of an inch. Which is thinner than the saran 121 00:06:58,675 --> 00:07:00,539 wrap that you have in the kitchen. 122 00:07:00,539 --> 00:07:02,46 It is thinner than cigarette paper. 123 00:07:02,46 --> 00:07:04,89 A very risky business to fly these balloons. 124 00:07:04,89 --> 00:07:07,32 No guarantee of course that they would work. 125 00:07:07,32 --> 00:07:09,863 You pay your money. If they work that's great. 126 00:07:09,863 --> 00:07:12,179 If they don't work that's just tough luck. 127 00:07:12,179 --> 00:07:15,626 There is a good chance that you have a failure when you launch 128 00:07:15,626 --> 00:07:17,491 the balloon. They're very fragile. 129 00:07:17,491 --> 00:07:19,808 There could be damage right at the launch. 130 00:07:19,808 --> 00:07:23,198 But even if they make it up in the atmosphere they have to go 131 00:07:23,198 --> 00:07:26,08 through the tropopause, 132 00:07:26,08 --> 00:07:30,148 near about a hundred thousand feet where it's very very cold, 133 00:07:30,148 --> 00:07:33,471 the balloons get brittle, and then they can burst. 134 00:07:33,471 --> 00:07:37,743 And that of course would be the end then of that balloon flight. 135 00:07:37,743 --> 00:07:40,456 And that could also be the end of a Ph.D. 136 00:07:40,456 --> 00:07:42,965 thesis. Because that all these flights 137 00:07:42,965 --> 00:07:47,033 of course were connected with research and therefore with Ph. 138 00:07:47,033 --> 0. 139 0. --> 00:07:49,135 in the Netherlands on nuclear work and so the tension during 140 00:07:49,135 --> 00:07:53,136 these early phases of the launch were always extremely high. 141 00:07:53,136 --> 00:07:57,002 Sometimes even unbearable. So now I would like to show you 142 00:07:57,002 --> 00:08:04,132 some slides. Which will give you a good idea 143 00:08:04,132 --> 00:08:09,732 of what these expeditions were like. 144 00:08:09,732 --> 00:08:19,012 Oh, yeah, a classic problem. This is nice that these -- ah, 145 00:08:19,012 --> 00:08:26,212 now they work. All right, so if I can have the 146 00:08:26,212 --> 00:08:32,369 first slide, you see here Jim and Pat who at 147 00:08:32,369 --> 00:08:37,519 the time were undergraduates, they are now both Ph.D.s, 148 00:08:37,519 --> 00:08:41,811 and they are working there, very tedious work, 149 00:08:41,811 --> 00:08:45,435 trying to put the electronics together. 150 00:08:45,435 --> 00:08:49,918 You may think that science is not very romantic. 151 00:08:49,918 --> 00:08:54,21 But I can assure you it is. They fell in love. 152 00:08:54,21 --> 00:08:56,88 They married. They have kids. 153 00:08:56,88 --> 00:09:02,508 And that's the way it sometimes goes in life. 154 00:09:02,508 --> 00:09:06,288 And so here you see the plant in Texas where these huge 155 00:09:06,288 --> 00:09:09,579 balloons were made. Uh balloons are put together 156 00:09:09,579 --> 00:09:13,92 sort of like the -- the way that the tangerine is put together. 157 00:09:13,92 --> 00:09:17,351 At the surface you see these gores of the balloon. 158 00:09:17,351 --> 00:09:21,553 And the sealing of these gores to make up the balloon were -- 159 00:09:21,553 --> 00:09:25,333 was only done by women. Only women were allowed to work 160 00:09:25,333 --> 00:09:27,574 there. Has nothing to do with sex 161 00:09:27,574 --> 00:09:31,495 discrimination of any kind. It just turned out that women 162 00:09:31,495 --> 00:09:33,973 were more patient. 163 00:09:33,973 --> 00:09:38,081 They did the work better. They make way fewer mistakes 164 00:09:38,081 --> 00:09:40,793 than men did. That's the way it goes 165 00:09:40,793 --> 00:09:44,591 sometimes in life. Here you see balloon coming out 166 00:09:44,591 --> 00:09:46,76 of the box. Nicely protected. 167 00:09:46,76 --> 00:09:50,171 Plastic cover. And we also have here cloth on 168 00:09:50,171 --> 00:09:55,053 the -- on the grass because the balloon is so thin that it would 169 00:09:55,053 --> 00:09:58,541 certainly get damaged if it touches the grass, 170 00:09:58,541 --> 00:10:03,346 it's enormously thin. This was not my balloon. 171 00:10:03,346 --> 00:10:06,881 Uh we were worried that there was something wrong with it. 172 00:10:06,881 --> 00:10:10,54 You can see here the concern. They thought it was a -- there 173 00:10:10,54 --> 00:10:14,013 was a hole in the balloon. And that if there is a hole in 174 00:10:14,013 --> 00:10:17,735 the balloon there's just nothing you can do about it anymore. 175 00:10:17,735 --> 00:10:21,394 You can't patch it because the hole is almost always through 176 00:10:21,394 --> 00:10:24,309 many many layers. What you're looking at here is 177 00:10:24,309 --> 00:10:27,658 hundreds of layers of balloon that are folded together. 178 00:10:27,658 --> 00:10:30,263 But as I said, since it wasn't my balloon I 179 00:10:30,263 --> 00:10:33,24 wasn't too worried, but of course it's never nice 180 00:10:33,24 --> 00:10:36,899 if you see a failure of your colleagues. 181 00:10:36,899 --> 00:10:40,323 Now I bring you to the desert town Alice Springs in uh 182 00:10:40,323 --> 00:10:42,326 Australia. Right at the heart of 183 00:10:42,326 --> 00:10:44,845 Australia. And now you get a pretty good 184 00:10:44,845 --> 00:10:48,268 idea of what it's like. Here you see the launch truck. 185 00:10:48,268 --> 00:10:51,111 The telescope is there. And then you see this 186 00:10:51,111 --> 00:10:54,599 enormously big balloon. All of it is empty now and most 187 00:10:54,599 --> 00:10:57,958 of this will stay empty. This is the roller arm which 188 00:10:57,958 --> 00:11:01,317 holds this part down. This is the only part that will 189 00:11:01,317 --> 00:11:03,836 be inflated. And here you see the helium 190 00:11:03,836 --> 00:11:07,453 truck. And here you see the inflation 191 00:11:07,453 --> 00:11:09,671 tubes. And we will let helium in from 192 00:11:09,671 --> 00:11:13,243 both sides which will then gradually begin to fill this top 193 00:11:13,243 --> 00:11:16,384 part of the balloon. And you see here the roller arm 194 00:11:16,384 --> 00:11:18,354 in detail. The roller arm is very 195 00:11:18,354 --> 00:11:22,173 important because when this part of the balloon is being filled 196 00:11:22,173 --> 00:11:24,328 it wants to lift, it wants to go up, 197 00:11:24,328 --> 00:11:27,9 and of course you have to keep it down, you have to keep it 198 00:11:27,9 --> 00:11:30,672 under control. And so this roller arm and this 199 00:11:30,672 --> 00:11:33,135 -- this car is loaded down with concrete. 200 00:11:33,135 --> 00:11:37,261 It's very heavy. And then just before the launch 201 00:11:37,261 --> 00:11:41,19 this roller arm by command is fweet flipped over and then as 202 00:11:41,19 --> 00:11:44,586 you will see later then the balloon will make it up. 203 00:11:44,586 --> 00:11:47,782 And here you see the early part of the inflation. 204 00:11:47,782 --> 00:11:49,846 Helium comes in from both sides. 205 00:11:49,846 --> 00:11:53,375 And so we -- we fly these balloons almost always early 206 00:11:53,375 --> 00:11:56,305 morning because then the winds are very calm. 207 00:11:56,305 --> 00:11:58,503 You need extremely reliable winds. 208 00:11:58,503 --> 00:12:01,166 You need to know the direction very well. 209 00:12:01,166 --> 00:12:05,095 And the winds should be uh no more than something like three 210 00:12:05,095 --> 00:12:08,624 or four miles per hour. If they are stronger you would 211 00:12:08,624 --> 00:12:11,239 lose the balloon. 212 00:12:11,239 --> 00:12:15,677 You see here these gores that I mentioned to you earlier. 213 00:12:15,677 --> 00:12:18,451 Where the sun is behind the balloon. 214 00:12:18,451 --> 00:12:21,937 Here the bubble is nearly fully inflated now. 215 00:12:21,937 --> 00:12:25,107 Here it's still going on. Still going on, 216 00:12:25,107 --> 00:12:28,435 the inflation. But we are very close to the 217 00:12:28,435 --> 00:12:32,556 end of the inflation. Here is the roller arm and then 218 00:12:32,556 --> 00:12:37,073 in this direction here five hundred feet or so down is the 219 00:12:37,073 --> 00:12:41,738 payload with the truck. We're now very close to a 220 00:12:41,738 --> 00:12:44,005 launch. We're still in Alice Springs. 221 00:12:44,005 --> 00:12:47,532 This is -- was my graduate student Jeff McClintock at the 222 00:12:47,532 --> 00:12:49,548 time. He's now Doctor McClintock. 223 00:12:49,548 --> 00:12:53,201 Here you see radar reflectors which allows us to follow the 224 00:12:53,201 --> 00:12:55,909 balloon -- radar. Here you see the telescope 225 00:12:55,909 --> 00:12:59,121 hanging on the launch truck. Here is the roller arm. 226 00:12:59,121 --> 00:13:02,207 All this is empty. And here you see the parachute. 227 00:13:02,207 --> 00:13:05,608 We have here a connection between the parachute and the 228 00:13:05,608 --> 00:13:08,631 bottom of the balloon. And we can control that on 229 00:13:08,631 --> 00:13:12,831 radio command. We can separate that so that 230 00:13:12,831 --> 00:13:15,773 the telescope safely comes back to earth. 231 00:13:15,773 --> 00:13:18,199 At least that's the idea on paper. 232 00:13:18,199 --> 00:13:21,435 And so now you see the release of the bubble. 233 00:13:21,435 --> 00:13:25,332 So the roller arm is up and this bubble now takes off. 234 00:13:25,332 --> 00:13:28,126 This is an incredibly fantastic moment. 235 00:13:28,126 --> 00:13:32,391 This is really butterflies in your stomach and ants in your 236 00:13:32,391 --> 00:13:35,112 pants. This is the moment that balloon 237 00:13:35,112 --> 00:13:37,686 can easily fail. Very thin material, 238 00:13:37,686 --> 00:13:41,991 the helium goes up, reflects against the top, 239 00:13:41,991 --> 00:13:45,222 is pushed back again, you get this peculiar mushroom 240 00:13:45,222 --> 00:13:48,137 shape, it makes an enormous sound like a storm. 241 00:13:48,137 --> 00:13:51,369 The idea now is that this balloon will go higher and 242 00:13:51,369 --> 00:13:54,22 higher in the sky. Will pick up all this empty 243 00:13:54,22 --> 00:13:55,868 part. This is not inflated. 244 00:13:55,868 --> 00:13:59,353 As the balloon goes up in the atmosphere the atmospheric 245 00:13:59,353 --> 00:14:02,648 pressure will go down. And the helium will expand and 246 00:14:02,648 --> 00:14:06,007 will fill the balloon. And the -- the trick now is for 247 00:14:06,007 --> 00:14:09,428 this truck to manipulate, to maneuver itself under what 248 00:14:09,428 --> 00:14:11,977 we call the bubble. 249 00:14:11,977 --> 00:14:16,379 And therefore the wind has to be in this direction so that the 250 00:14:16,379 --> 00:14:20,564 balloon comes to the truck. And then the truck tries to get 251 00:14:20,564 --> 00:14:24,532 straight under the balloon. And then the payload will be 252 00:14:24,532 --> 00:14:27,78 released here. Here you see a close-up of this 253 00:14:27,78 --> 00:14:30,233 mushroom. You can actually see this 254 00:14:30,233 --> 00:14:33,769 reflection of the helium going up and coming back. 255 00:14:33,769 --> 00:14:36,727 You can also see these gores very clearly. 256 00:14:36,727 --> 00:14:41,49 It's tedious work. By these women who have to seal 257 00:14:41,49 --> 00:14:44,678 these balloons. Enormous amount of labor goes 258 00:14:44,678 --> 00:14:47,141 into it. Amount of helium as I said 259 00:14:47,141 --> 00:14:50,04 earlier is about eighty thousand dollars. 260 00:14:50,04 --> 00:14:52,576 About the same price of the balloon. 261 00:14:52,576 --> 00:14:55,981 And here it goes higher. We're in Alice Springs. 262 00:14:55,981 --> 00:14:59,242 The cover is falling off. Balloon is going up. 263 00:14:59,242 --> 00:15:01,633 See the engine is already running. 264 00:15:01,633 --> 00:15:05,546 The truck cannot move yet because if it started to move 265 00:15:05,546 --> 00:15:09,241 this part of the balloon would slide over the cloth. 266 00:15:09,241 --> 00:15:13,154 There would be friction and there would be holes in the 267 00:15:13,154 --> 00:15:17,716 balloon. So this truck has to wait until 268 00:15:17,716 --> 00:15:21,128 all of this is off the ground. Going higher. 269 00:15:21,128 --> 00:15:25,811 And I'm now so close to the balloon that I couldn't continue 270 00:15:25,811 --> 00:15:28,668 my picture-taking from Alice Springs. 271 00:15:28,668 --> 00:15:32,874 So I will jump back to an earlier flight in the United 272 00:15:32,874 --> 00:15:35,731 States. We flew these balloons in the 273 00:15:35,731 --> 00:15:39,62 United States from a town called Palestine, Texas. 274 00:15:39,62 --> 00:15:44,064 And so you will see then the remaining part of the flight 275 00:15:44,064 --> 00:15:46,923 from Palestine, Texas. 276 00:15:46,923 --> 00:15:50,044 So the balloon is now completely off the ground. 277 00:15:50,044 --> 00:15:53,232 See a little bit of gas, well it's not so little, 278 00:15:53,232 --> 00:15:56,685 but it looks very little compared to the size of this 279 00:15:56,685 --> 00:15:59,209 balloon. You see the parachute here and 280 00:15:59,209 --> 00:16:02,927 here then is the connection which on radio command we can 281 00:16:02,927 --> 00:16:05,451 separate. So now this is a very crucial 282 00:16:05,451 --> 00:16:07,775 moment. The person in charge on this 283 00:16:07,775 --> 00:16:11,959 launch truck has probably driven the truck to get straight under 284 00:16:11,959 --> 00:16:14,615 the balloon. And when it's straight under 285 00:16:14,615 --> 00:16:18,313 there they will allow the payload to 286 00:16:18,313 --> 00:16:20,963 go free. The payload is attached to this 287 00:16:20,963 --> 00:16:23,476 truck. If the balloon is too far ahead 288 00:16:23,476 --> 00:16:27,552 and the payload is released it will pendulum into the ground. 289 00:16:27,552 --> 00:16:31,764 And if you release it too early then of course the payload will 290 00:16:31,764 --> 00:16:34,142 pendulum back into the launch truck. 291 00:16:34,142 --> 00:16:37,81 Both would be a disaster. If the pull of the balloon is 292 00:16:37,81 --> 00:16:41,479 not enough, for instance if a hole developed during the 293 00:16:41,479 --> 00:16:44,604 launch, so if the tension is not strong enough, 294 00:16:44,604 --> 00:16:49,241 you would release the payload, it would go bang, 295 00:16:49,241 --> 00:16:53,289 back to the ground. So all these factors have to be 296 00:16:53,289 --> 00:16:57,257 taken into account, and then finally the person in 297 00:16:57,257 --> 00:17:01,305 charge commits to a launch. And then there it goes. 298 00:17:01,305 --> 00:17:04,706 All the way empty. Here you see the helium. 299 00:17:04,706 --> 00:17:08,35 The parachute. And you see the -- the payload. 300 00:17:08,35 --> 00:17:13,289 And here you see the balloon at a hundred fifty thousand feet, 301 00:17:13,289 --> 00:17:16,285 forty-five kilometers high in the sky. 302 00:17:16,285 --> 00:17:21,887 The helium has now expanded. The balloon is fully inflated. 303 00:17:21,887 --> 00:17:24,335 And you can look straight through it. 304 00:17:24,335 --> 00:17:27,395 It's only half of one-thousandth of an inch of 305 00:17:27,395 --> 00:17:30,319 polyethylene. And these are huge ducts which 306 00:17:30,319 --> 00:17:32,903 have openings of about ten meters each. 307 00:17:32,903 --> 00:17:36,575 And they are there because the balloon cannot stand any 308 00:17:36,575 --> 00:17:39,499 over-pressure. If there is any over-pressure 309 00:17:39,499 --> 00:17:43,375 the balloon would pop and so when the balloon keeps rising 310 00:17:43,375 --> 00:17:47,387 and rising and rising when it reaches -- reaches its maximum 311 00:17:47,387 --> 00:17:51,671 volume the helium would escape at the bottom. 312 00:17:51,671 --> 00:17:55,343 That's the idea of these ducts. Here you see George Ricker who 313 00:17:55,343 --> 00:17:58,173 was graduate student, my graduate student at the 314 00:17:58,173 --> 00:17:59,738 time. This is in Australia. 315 00:17:59,738 --> 00:18:02,267 He is now Doctor Ricker. He's still at MIT. 316 00:18:02,267 --> 00:18:04,855 He's a staff member. And this is the kind of 317 00:18:04,855 --> 00:18:07,444 equipment that we built, at least partially. 318 00:18:07,444 --> 00:18:11,057 And he is checking the early results during the ascent of the 319 00:18:11,057 --> 00:18:13,164 balloon. The balloon will go up with 320 00:18:13,164 --> 00:18:16,054 about a thousand feet a minute. If all goes well, 321 00:18:16,054 --> 00:18:18,101 there's no leak, it will take about 322 00:18:18,101 --> 00:18:20,509 two-and-a-half, three hours to make it to 323 00:18:20,509 --> 00:18:24,288 altitude. You see me here sitting on the 324 00:18:24,288 --> 00:18:26,757 plane that we used to follow the balloon. 325 00:18:26,757 --> 00:18:29,226 We fly of course at much lower altitudes. 326 00:18:29,226 --> 00:18:31,202 Five thousand, ten thousand feet. 327 00:18:31,202 --> 00:18:34,288 We stay as close to the balloon as we possibly can. 328 00:18:34,288 --> 00:18:37,622 It's not always so -- not always easy, certainly not in 329 00:18:37,622 --> 00:18:40,153 Australia. And so we keep an eye on things 330 00:18:40,153 --> 00:18:43,487 and if necessary we can terminate the balloon flight by 331 00:18:43,487 --> 00:18:46,882 giving a radio command so that the parachute comes down. 332 00:18:46,882 --> 00:18:50,401 Certainly when we get close to the ocean of course that is 333 00:18:50,401 --> 00:18:54,135 necessary if you don't want to lose the 334 00:18:54,135 --> 00:18:56,706 payload. The data -- data come back via 335 00:18:56,706 --> 00:18:59,547 radio, so we wouldn't lose the -- the data. 336 00:18:59,547 --> 00:19:02,591 You get very sick, by the way in these planes. 337 00:19:02,591 --> 00:19:06,786 If you're sitting for eight or ten or twelve hours or longer in 338 00:19:06,786 --> 00:19:10,439 these planes as I have, I learned actually a little bit 339 00:19:10,439 --> 00:19:13,956 of flying which is quite easy with a plane like this. 340 00:19:13,956 --> 00:19:16,053 Here you see a map of Australia. 341 00:19:16,053 --> 00:19:19,503 Here's Alice Springs. Uh we fly pro-balloons weather 342 00:19:19,503 --> 00:19:23,901 balloons at a hundred forty thousand feet every 343 00:19:23,901 --> 00:19:26,9 day to find out in what direction the balloon would be 344 00:19:26,9 --> 00:19:28,768 drifting. And we had all reason to 345 00:19:28,768 --> 00:19:31,767 believe that the balloon would drift somewhere in this 346 00:19:31,767 --> 00:19:33,804 direction. And we alerted these radar 347 00:19:33,804 --> 00:19:35,898 stations. These circles here are radar 348 00:19:35,898 --> 00:19:38,105 stations. Because there are no airfields 349 00:19:38,105 --> 00:19:40,596 here in Australia. And so we knew we couldn't 350 00:19:40,596 --> 00:19:43,142 follow the balloon. We would probably lose it. 351 00:19:43,142 --> 00:19:45,689 And therefore we alerted these radar stations. 352 00:19:45,689 --> 00:19:48,688 They could tell us then when the balloon was in sight. 353 00:19:48,688 --> 00:19:51,348 And that would allow us then to cut the balloon, 354 00:19:51,348 --> 00:19:55,543 cut the payload loose and -- and make the recovery. 355 00:19:55,543 --> 00:19:57,923 Instead, the balloon went straight south. 356 00:19:57,923 --> 00:20:01,136 So the predictions by the weather balloons were not too 357 00:20:01,136 --> 00:20:03,159 accurate. The balloon went straight 358 00:20:03,159 --> 00:20:05,242 south. And here there was sunset and 359 00:20:05,242 --> 00:20:08,455 then we don't know too precisely where the balloon was. 360 00:20:08,455 --> 00:20:11,013 Remember this was in the nineteen seventies. 361 00:20:11,013 --> 00:20:14,345 And so we were uncertain. But here about twenty-six hours 362 00:20:14,345 --> 00:20:18,034 later when we were getting close to Melbourne which we were not 363 00:20:18,034 --> 00:20:21,187 allowed to enter the air space here between Sydney and 364 00:20:21,187 --> 00:20:24,46 Melbourne we cut the balloon loose. 365 00:20:24,46 --> 00:20:27,746 That means we separate the payload from the balloon. 366 00:20:27,746 --> 00:20:31,419 The balloon is very brittle. It's extremely cold up there. 367 00:20:31,419 --> 00:20:35,414 The balloon then fractures and comes down in many pieces and if 368 00:20:35,414 --> 00:20:39,087 everything worked well then parachute opens and brings the 369 00:20:39,087 --> 00:20:42,889 payload safely back to earth. And then comes the big problem 370 00:20:42,889 --> 00:20:45,466 how are you going to recover the payload. 371 00:20:45,466 --> 00:20:47,464 You're in the middle of nowhere. 372 00:20:47,464 --> 00:20:50,815 This balloon came down, this payload came down in the 373 00:20:50,815 --> 00:20:52,941 desert. And there are no airports. 374 00:20:52,941 --> 00:20:56,192 At least the chances are that you 375 00:20:56,192 --> 00:20:59,796 are a few hundred miles away from the nearest airport with 376 00:20:59,796 --> 00:21:02,262 your payload. So what you do then is the 377 00:21:02,262 --> 00:21:04,728 following. You try to find a house close 378 00:21:04,728 --> 00:21:08,143 to where the payload is located. We locate the payload. 379 00:21:08,143 --> 00:21:11,684 We see the stuff come down. Radio beacons on the payload. 380 00:21:11,684 --> 00:21:15,288 And then you fly over that house many many times in a very 381 00:21:15,288 --> 00:21:18,007 obnoxious way. You make a -- a lot of noise. 382 00:21:18,007 --> 00:21:21,295 You fly over very low. And so the people who -- whose 383 00:21:21,295 --> 00:21:26,228 next neigh- neighbor is probably se- seventy miles away from 384 00:21:26,228 --> 00:21:28,531 them know what you're trying to tell them. 385 00:21:28,531 --> 00:21:30,609 You're trying to draw their attention. 386 00:21:30,609 --> 00:21:33,867 And they know what that means is that they want you -- they 387 00:21:33,867 --> 00:21:35,889 want you to meet them at the airport. 388 00:21:35,889 --> 00:21:38,867 Whatever airport means. It's sometimes just a strip in 389 00:21:38,867 --> 00:21:41,114 the desert. You can't land there at night 390 00:21:41,114 --> 00:21:43,192 but you can land there during the day. 391 00:21:43,192 --> 00:21:45,046 And so that's exactly what we did. 392 00:21:45,046 --> 00:21:47,686 We drew attention to the house of this guy Jack. 393 00:21:47,686 --> 00:21:50,607 He was -- he was a complete nut, he was always drunk, 394 00:21:50,607 --> 00:21:53,191 was a crazy guy. And so we went to the airstrip 395 00:21:53,191 --> 00:21:56,617 and we waited and indeed after fifteen 396 00:21:56,617 --> 00:21:59,135 hours he showed up with this truck. 397 00:21:59,135 --> 00:22:03,059 Uh there is no windshield here in the -- in the truck. 398 00:22:03,059 --> 00:22:05,725 And he used to shoot kangaroos there. 399 00:22:05,725 --> 00:22:10,019 He would go sixty miles per hour on the desert floor and he 400 00:22:10,019 --> 00:22:12,463 would -- he would shoot kangaroos. 401 00:22:12,463 --> 00:22:16,979 And he gave me a demonstration. Uh he put his dog on the roof. 402 00:22:16,979 --> 00:22:19,275 He would go sixty miles an hour. 403 00:22:19,275 --> 00:22:23,717 And he would slam the brakes. And then the dog would catapult 404 00:22:23,717 --> 00:22:27,113 through the air, the poor dog. 405 00:22:27,113 --> 00:22:31,363 And then all he would say is oh, you can't teach an old dog 406 00:22:31,363 --> 00:22:34,515 any new tricks. And he seemed to enjoy that. 407 00:22:34,515 --> 00:22:38,765 When we go after the payload the plane, the recovery plane, 408 00:22:38,765 --> 00:22:41,843 is in the air. Takes off from that airstrip 409 00:22:41,843 --> 00:22:44,994 and we have contact with the recovery plane. 410 00:22:44,994 --> 00:22:49,098 They and only they can see and know where the payload is. 411 00:22:49,098 --> 00:22:52,029 From the ground of course you can't tell. 412 00:22:52,029 --> 00:22:56,133 And so they maneuver you to the payload. 413 00:22:56,133 --> 00:22:59,433 And so of course Jack's help was invaluable. 414 00:22:59,433 --> 00:23:02,733 We needed him. That was independent from the 415 00:23:02,733 --> 00:23:05,65 fact that the man was a little strange. 416 00:23:05,65 --> 00:23:09,18 On these recoveries you encounter many animals. 417 00:23:09,18 --> 00:23:13,018 You see a koala bear here. In a l- eucalyptus tree. 418 00:23:13,018 --> 00:23:15,781 Very lazy animal. Unlike most of you. 419 00:23:15,781 --> 00:23:20,079 And then when we came close to the payload there was this 420 00:23:20,079 --> 00:23:22,996 animal, a goanna, six-feet tall goanna. 421 00:23:22,996 --> 00:23:27,678 I'll tell you, it scared the hell out of me. 422 00:23:27,678 --> 00:23:31,492 And uh I didn't want to show that to my graduate student who 423 00:23:31,492 --> 00:23:35,177 was with me and I said to him look you know these a- these 424 00:23:35,177 --> 00:23:38,733 animals are completely harmless, why don't you go first. 425 00:23:38,733 --> 00:23:42,483 This animal was no farther than four feet from the payload. 426 00:23:42,483 --> 00:23:46,426 And so my graduate student went first and the amazing thing is 427 00:23:46,426 --> 00:23:50,176 during the ten hours that it took us to recover the payload 428 00:23:50,176 --> 00:23:53,732 and put it back on Jack's truck this animal never moved. 429 00:23:53,732 --> 00:23:57,481 It was just sitting there completely still. 430 00:23:57,481 --> 00:24:00,473 This is their way of trying not to be noticed. 431 00:24:00,473 --> 00:24:02,533 And so here you see the payload. 432 00:24:02,533 --> 00:24:04,593 This was Alice, was Jack's wife, 433 00:24:04,593 --> 00:24:07,917 this is Tom Brooks, he came from the United States, 434 00:24:07,917 --> 00:24:11,705 he was an electronic expert. And you see here the payload. 435 00:24:11,705 --> 00:24:14,63 It looks heavily damaged but it really isn't. 436 00:24:14,63 --> 00:24:18,02 This is crash pad which protects the payload when it 437 00:24:18,02 --> 00:24:20,546 impacts. And an impact crash pad worked 438 00:24:20,546 --> 00:24:23,537 very well, very little damage to this payload. 439 00:24:23,537 --> 00:24:27,392 And then you come back after several days to Alice Springs. 440 00:24:27,392 --> 00:24:31,261 Alice Springs is a hole in the ground. 441 00:24:31,261 --> 00:24:35,233 Nothing ever happens there. And so obviously you make it to 442 00:24:35,233 --> 00:24:38,041 the front page of the Centralian Advocate. 443 00:24:38,041 --> 00:24:41,602 Perfect balloon launch, thousand watch start of space 444 00:24:41,602 --> 00:24:43,999 probe. They think of this as a space 445 00:24:43,999 --> 00:24:47,286 probe which is fine. Balloon professor is back in 446 00:24:47,286 --> 00:24:49,341 Alice. They called me there the 447 00:24:49,341 --> 00:24:52,628 balloon professor. I gave -- I gave several talks 448 00:24:52,628 --> 00:24:55,984 there for high schools and for uh the Rotary Club. 449 00:24:55,984 --> 00:24:58,449 So I was a sort of a local celebrity. 450 00:24:58,449 --> 00:25:02,285 I talked to the uh news reporter for 451 00:25:02,285 --> 00:25:06,95 several hours. And when you read this story 452 00:25:06,95 --> 00:25:13,17 you won't believe the nonsense but that's all a detail of 453 00:25:13,17 --> 00:25:16,725 course. OK, that's enough for the 454 00:25:16,725 --> 00:25:20,835 slides for now. I had si- about twenty 455 00:25:20,835 --> 00:25:26,611 successful s- flights. Between nineteen sixty-six and 456 00:25:26,611 --> 00:25:32,832 nineteen eighty. Many from the United States. 457 00:25:32,832 --> 00:25:35,405 From Canada and also from Australia. 458 00:25:35,405 --> 00:25:39,595 Where that's a s- southern hemisphere which covers part of 459 00:25:39,595 --> 00:25:43,198 the sky that we cannot see from the United States. 460 00:25:43,198 --> 00:25:46,947 I had two free falls. Two of my balloon burst on the 461 00:25:46,947 --> 00:25:50,697 way up in the tropopause. We were unable to separate 462 00:25:50,697 --> 00:25:55,034 payload fast enough and then the whole thing, parachute gets 463 00:25:55,034 --> 00:25:57,24 entangled. You get a free fall. 464 00:25:57,24 --> 00:26:02,681 Big hole in the ground and that's the end of the telescope. 465 00:26:02,681 --> 00:26:05,28 And it was. Twice did I lose the telescope 466 00:26:05,28 --> 00:26:07,627 completely. But I was lucky and I made 467 00:26:07,627 --> 00:26:11,622 several interesting discoveries during those successful flights. 468 00:26:11,622 --> 00:26:14,792 We discovered very early on five new x-ray sources. 469 00:26:14,792 --> 00:26:17,836 None of them had ever been seen with the rockets. 470 00:26:17,836 --> 00:26:21,577 And several of these sources, that was the really new thing, 471 00:26:21,577 --> 00:26:24,431 were highly variable. They changed their x-ray 472 00:26:24,431 --> 00:26:26,714 intensity on a very short time scale. 473 00:26:26,714 --> 00:26:30,708 We noticed even one source went up by a factor of three in about 474 00:26:30,708 --> 00:26:32,61 ten minutes. 475 00:26:32,61 --> 00:26:35,965 And that of course could not have been discovered with 476 00:26:35,965 --> 00:26:38,308 rockets. Because the rockets were only 477 00:26:38,308 --> 00:26:40,65 above the atmosphere for five minutes. 478 00:26:40,65 --> 00:26:44,638 They would quickly scan the sky and so there's no way they could 479 00:26:44,638 --> 00:26:48,12 discover variability on a time scale of tens of minutes. 480 00:26:48,12 --> 00:26:50,21 But with balloons you can do that. 481 00:26:50,21 --> 00:26:54,008 So it did pay off that we were watching the sky sometimes for 482 00:26:54,008 --> 00:26:56,73 ten, twenty hours. Our -- my longest balloon 483 00:26:56,73 --> 00:26:59,009 flight was actually twenty-six hours. 484 00:26:59,009 --> 00:27:02,744 We also observed x-rays from one source 485 00:27:02,744 --> 00:27:04,906 which we named GX one plus four. 486 00:27:04,906 --> 00:27:09,09 Which stands the one plus four stands for the position in the 487 00:27:09,09 --> 00:27:11,391 sky. And um this showed a periodic 488 00:27:11,391 --> 00:27:14,599 signal in x-rays. About two point three minutes 489 00:27:14,599 --> 00:27:17,598 periodicity. At the time we had no clue what 490 00:27:17,598 --> 00:27:21,781 that meant but later of course we understood the significance 491 00:27:21,781 --> 00:27:24,292 of that. And you will understand that 492 00:27:24,292 --> 00:27:27,29 also very shortly. How significant that was. 493 00:27:27,29 --> 00:27:29,522 So what kind of objects are they? 494 00:27:29,522 --> 00:27:32,381 They are very very different from the sun. 495 00:27:32,381 --> 00:27:35,923 And we now know what they are. 496 00:27:35,923 --> 00:27:39,269 These objects are binaries. Binary stars. 497 00:27:39,269 --> 00:27:44,287 One star is not unlike the sun, it's a normal nuclear burning 498 00:27:44,287 --> 00:27:46,797 star. And it is in orbit with a 499 00:27:46,797 --> 00:27:50,31 neutron star or in some cases a black hole. 500 00:27:50,31 --> 00:27:54,826 They go around each other. And if they are close enough 501 00:27:54,826 --> 00:28:00,179 together it is possible that the matter of this star is attracted 502 00:28:00,179 --> 00:28:04,361 by this neutron star with a larger force than it is 503 00:28:04,361 --> 00:28:07,998 attracted by the star itself. 504 00:28:07,998 --> 00:28:12,392 And if that's the case this matter doesn't want to stay 505 00:28:12,392 --> 00:28:15,24 here. But wants to go to the neutron 506 00:28:15,24 --> 00:28:17,925 star. Now of course the matter has 507 00:28:17,925 --> 00:28:21,261 angular momentum because they goes around. 508 00:28:21,261 --> 00:28:24,678 So it cannot free fall to the neutron star. 509 00:28:24,678 --> 00:28:28,909 But it would spiral in and slowly make its way to the 510 00:28:28,909 --> 00:28:32,327 neutron star. And we call this an accretion 511 00:28:32,327 --> 00:28:36,314 disk. And we call this the donor, 512 00:28:36,314 --> 00:28:40,591 provides the fuel for the transfer of mass onto the 513 00:28:40,591 --> 00:28:43,841 neutron star. And let's assume that the 514 00:28:43,841 --> 00:28:48,289 neutron star has a mass capital M and has a radius R. 515 00:28:48,289 --> 00:28:52,652 And let's assume that we dump some matter, little M, 516 00:28:52,652 --> 00:28:57,186 onto the neutron star. Well all of you should remember 517 00:28:57,186 --> 00:29:02,404 from eight oh one that you can calculate very easily the speed 518 00:29:02,404 --> 00:29:06,425 with which this matter reaches the neutron star. 519 00:29:06,425 --> 00:29:10,359 The kinetic energy, one-half MV 520 00:29:10,359 --> 00:29:12,885 squared, must be equal to MMG over R. 521 00:29:12,885 --> 00:29:16,953 We had this equation on the blackboard last lecture when we 522 00:29:16,953 --> 00:29:20,11 discussed cosmology. It was the same equation. 523 00:29:20,11 --> 00:29:24,109 This is the speed with which the matter will fall onto the 524 00:29:24,109 --> 00:29:26,844 neutron star. If this is the mass of the 525 00:29:26,844 --> 00:29:30,703 neutron star and this is the radius of the neutron star. 526 00:29:30,703 --> 00:29:33,579 You lose the -- the mass as you always do. 527 00:29:33,579 --> 00:29:36,034 And so you can calculate this speed. 528 00:29:36,034 --> 00:29:39,892 This speed is horrendous because the radius of a neutron 529 00:29:39,892 --> 00:29:43,166 star is so ridiculously small. 530 00:29:43,166 --> 00:29:47,084 The mass of a neutron star is very comparable to the mass of 531 00:29:47,084 --> 00:29:48,678 the sun. A little larger. 532 00:29:48,678 --> 00:29:51,8 But not much larger. But the radius is a hundred 533 00:29:51,8 --> 00:29:54,656 thousand times smaller than that of the sun. 534 00:29:54,656 --> 00:29:58,109 It's only ten kilometers. And as a result of that the 535 00:29:58,109 --> 00:30:01,895 speed with which the matter hits the neutron star is about 536 00:30:01,895 --> 00:30:05,946 one-third of the speed of light. When it hits the neutron star 537 00:30:05,946 --> 00:30:08,603 this kinetic energy is converted to heat. 538 00:30:08,603 --> 00:30:12,388 It will heat up the surface layers of the neutron star and 539 00:30:12,388 --> 00:30:16,322 increases the temperature to about um ten 540 00:30:16,322 --> 00:30:19,625 million, a hundred million degrees, and at such high 541 00:30:19,625 --> 00:30:22,41 temperatures, the star would emit almost all 542 00:30:22,41 --> 00:30:25,389 its radiation in x-rays and not in the optical. 543 00:30:25,389 --> 00:30:28,757 Our sun is relatively cold. It's only six hundred six 544 00:30:28,757 --> 00:30:31,801 thousand degrees. And so the sun has most of its 545 00:30:31,801 --> 00:30:35,687 radiation in the optical but when the temperature becomes ten 546 00:30:35,687 --> 00:30:39,12 million, a hundred million degrees, the maximum of the 547 00:30:39,12 --> 00:30:42,099 emission is in x-rays. To give you a little bit 548 00:30:42,099 --> 00:30:45,467 respect, a little bit of insight, for this incredible 549 00:30:45,467 --> 00:30:48,641 power, for this incredible 550 00:30:48,641 --> 00:30:51,623 gravitational pull of the neutron star, 551 00:30:51,623 --> 00:30:55,625 because R is so small, if you took a marshmallow and 552 00:30:55,625 --> 00:31:00,412 you threw a marshmallow from a large distance onto the surface 553 00:31:00,412 --> 00:31:04,021 of a neutron star, at impact the energy that is 554 00:31:04,021 --> 00:31:08,18 going to be released is comparable to the energy of an 555 00:31:08,18 --> 00:31:12,809 atomic bomb as was thrown on Hiroshima and Nagasaki near the 556 00:31:12,809 --> 00:31:17,361 end of the Second World War. Neutron stars have very strong 557 00:31:17,361 --> 00:31:22,065 magnetic fields. And the matter that flows from 558 00:31:22,065 --> 00:31:25,136 the donor onto the neutron star is ionized. 559 00:31:25,136 --> 00:31:26,964 It's plasma. It's charged. 560 00:31:26,964 --> 00:31:31,351 And as you remember from eight oh two when you have a charged 561 00:31:31,351 --> 00:31:35,445 particle in a magnetic field there is the V cross B term. 562 00:31:35,445 --> 00:31:38,955 The V cross B force. And the V cross B force will 563 00:31:38,955 --> 00:31:42,903 then spiral these charged particles around the magnetic 564 00:31:42,903 --> 00:31:47,071 field lines and they would end up near the magnetic poles, 565 00:31:47,071 --> 00:31:52,042 not unlike the solar wind when it reaches the earth, 566 00:31:52,042 --> 00:31:57,161 these charged particles enter the earth atmosphere near the 567 00:31:57,161 --> 00:32:00,867 magnetic poles, giving rise to aurora as we 568 00:32:00,867 --> 00:32:04,573 discussed earlier. And so you end up on the 569 00:32:04,573 --> 00:32:10,133 neutron star with two hot spots where this matter slams into the 570 00:32:10,133 --> 00:32:13,221 neutron star. At the magnetic poles. 571 00:32:13,221 --> 00:32:18,692 And if the axis of rotation of the neutron star is not the same 572 00:32:18,692 --> 00:32:24,163 as the -- [Audience noise] Q: Tall male physicist. 573 00:32:24,163 --> 00:32:29,991 [noise] Hi, Miss Peltier. [laughter] I remember your 574 00:32:29,991 --> 00:32:34,105 name. [laughter] You were in my eight 575 00:32:34,105 --> 00:32:38,104 oh one class. Q: [unintelligible] Q: 576 00:32:38,104 --> 00:32:44,274 [noise] And [unintelligible] physics, is this a physics 577 00:32:44,274 --> 00:32:47,131 lecture? Ask the students. 578 00:32:47,131 --> 00:32:50,102 I don't know. [laughter] Q: 579 00:32:50,102 --> 00:32:53,53 Very handsome and charming. OK. 580 00:32:53,53 --> 00:32:57,529 [laughter] So are you. [laughter] Q: 581 00:32:57,529 --> 00:27:25,486 [unintelligible] I think you know the answer to 582 00:27:25,486 --> 00:20:53,703 that one. Q: OK, I think we have the 583 00:20:53,703 --> 00:14:33,115 right person. [background noise] Q: 584 00:14:33,115 --> 00:04:06,263 We have a song to sing to you. Q: [unintelligible] [tone] 585 00:04:06,263 --> 00:00:00 Singing: When I 586 00:00:00 --> 00:33:31 587 00:33:31 --> 00:33:37,103 wisdom, times have changed. Singing: Maxwell's equations 588 00:33:37,103 --> 00:33:41,874 are too hard for me, so please just help me, 589 00:33:41,874 --> 00:33:44,315 help me. Singing: Help. 590 00:33:44,315 --> 00:33:46,645 I need somebody. Help. 591 00:33:46,645 --> 00:33:49,087 Not just anybody. Help. 592 00:33:49,087 --> 00:33:52,527 You know I need someone. Walter. 593 00:33:52,527 --> 00:33:58,63 Singing: When I was younger, so much younger than today. 594 00:33:58,63 --> 00:34:04,844 I never needed anybody's help in any way. 595 00:34:04,844 --> 00:34:09,481 But now these days are gone, I'm not so self-assured, 596 00:34:09,481 --> 00:34:14,209 now I've found my homework's hard, no nothing anymore. 597 00:34:14,209 --> 00:34:18,044 Singing: Help me if you can I'm failing now. 598 00:34:18,044 --> 00:34:20,72 Walter Lewin, can't do physics, 599 00:34:20,72 --> 00:34:23,841 I don't know how. I am on pass-fail, 600 00:34:23,841 --> 00:34:27,677 don't let me down. Singing: Won't you please 601 00:34:27,677 --> 00:34:30,62 please pass me. When I was dumber, 602 00:34:30,62 --> 00:34:35,258 so much dumber than today, I didn't know about Lentz, 603 00:34:35,258 --> 00:34:38,402 Maxwell or Faraday. 604 00:34:38,402 --> 00:34:43,968 Singing: But now those days are gone, I'm not so dumb no more. 605 00:34:43,968 --> 00:34:48,257 Thanks to eight oh two and our daring professor. 606 00:34:48,257 --> 00:34:51,359 Help me if you can I'm failing now. 607 00:34:51,359 --> 00:34:54,918 Singing: Walter Lewin, can't do physics, 608 00:34:54,918 --> 00:34:59,206 I don't know how. I am on pass-fail don't let me 609 00:34:59,206 --> 00:35:01,944 down. Singing: Won't you please 610 00:35:01,944 --> 00:35:05,411 please pass me. Walter Lewin thanks for 611 00:35:05,411 --> 00:35:11,798 teaching eight oh two. Now it's summer and we will all 612 00:35:11,798 --> 00:35:16,228 miss you. Singing: You've been crazy and 613 00:35:16,228 --> 00:35:22,361 we've all had a good time. So in your praise the Muses' 614 00:35:22,361 --> 00:35:26,904 voices chime. Singing: Eight days a week. 615 00:35:26,904 --> 00:35:30,198 I love you. Eight days a week. 616 00:35:30,198 --> 00:35:36,899 Is not enough to show I care. Singing: Thanks for all you've 617 00:35:36,899 --> 00:35:42,918 done, I'll pass somehow. E and M can't get 618 00:35:42,918 --> 00:35:52,86 me down, I know this now. I'll miss pass-fail next year, 619 00:35:52,86 --> 00:35:59,367 this I vow. Singing: Won't you please 620 00:35:59,367 --> 00:36:05,151 please pass me, pass me, pass me, 621 00:36:05,151 --> 00:36:10,212 ooh. I have tears in my eyes. 622 00:36:10,212 --> 00:36:17,804 [applause] So that was very nice. 623 00:36:17,804 --> 00:36:22,121 So now you won't have any time to fill out your evaluation 624 00:36:22,121 --> 00:36:24,62 form. So I was going to um talk to 625 00:36:24,62 --> 00:36:28,786 you about the um show you some evidence for these binary 626 00:36:28,786 --> 00:36:31,588 systems. So we have these hot spots on 627 00:36:31,588 --> 00:36:35,829 the neutron star and as the neutron star rotates then and 628 00:36:35,829 --> 00:36:39,844 the axis of rotation doesn't coincide with the dipole, 629 00:36:39,844 --> 00:36:43,631 magnetic dipole axis, you're going to see hot spot, 630 00:36:43,631 --> 00:36:47,115 hot spot, hot spot, hot spot, and that explains 631 00:36:47,115 --> 00:36:50,144 then the x-ray pulsations. 632 00:36:50,144 --> 00:36:53,12 Uh you can also see in some cases x-ray eclipses. 633 00:36:53,12 --> 00:36:56,716 If the neutron star as seen from the earth hides behind the 634 00:36:56,716 --> 00:37:00,621 donor star which is much bigger then all the x-rays are absorbed 635 00:37:00,621 --> 00:37:02,667 and so the x-rays stop completely. 636 00:37:02,667 --> 00:37:06,263 You go into an x-ray eclipse and a few hours later you come 637 00:37:06,263 --> 00:37:10,107 out of the x-ray eclipse again. And that's what I would like to 638 00:37:10,107 --> 00:37:13,144 show you the evidence for, which came in the early 639 00:37:13,144 --> 00:37:15,314 seventies. With the satellite Uhuru, 640 00:37:15,314 --> 00:37:17,36 the first slide is the basic idea. 641 00:37:17,36 --> 00:37:19,406 No, that was right, uh John, John, 642 00:37:19,406 --> 00:37:24,077 go back to that picture, yeah, so you see here this is 643 00:37:24,077 --> 00:37:26,963 of course a sketch, this is not the real thing. 644 00:37:26,963 --> 00:37:29,41 Here you see a star. Not unlike the sun. 645 00:37:29,41 --> 00:37:31,794 And then here you see the neutron star. 646 00:37:31,794 --> 00:37:35,245 In some cases a black hole. And then the matter is being 647 00:37:35,245 --> 00:37:38,947 sucked off this star because the gravitational force in this 648 00:37:38,947 --> 00:37:41,959 direction is larger. Forms the accretion disk and 649 00:37:41,959 --> 00:37:45,598 ends up on the neutron star. And the next slide is then the 650 00:37:45,598 --> 00:37:48,735 discovery of the early -- convincing discovery of a 651 00:37:48,735 --> 00:37:51,496 pulsating system. The rotation of the neutron 652 00:37:51,496 --> 00:37:54,947 star. This time scale here is about 653 00:37:54,947 --> 00:37:58,303 one-and-a-quarter second. And the data are here, 654 00:37:58,303 --> 00:38:01,017 this is the data, unfortunately in this 655 00:38:01,017 --> 00:38:05,231 publication this -- this very bold line dominates almost the 656 00:38:05,231 --> 00:38:09,302 data, but the idea being very clear that the x-ray signal, 657 00:38:09,302 --> 00:38:12,159 this is the strength of the x-ray signal, 658 00:38:12,159 --> 00:38:14,945 this is time, one-and-a-quarter seconds, 659 00:38:14,945 --> 00:38:17,659 is oscillating one-and-a-quarter second 660 00:38:17,659 --> 00:38:20,373 periods. And that's the rotation of the 661 00:38:20,373 --> 00:38:23,515 neutron star. And the next slide shows you of 662 00:38:23,515 --> 00:38:25,86 the same object, 663 00:38:25,86 --> 00:38:29,34 it's called Hercules X one. You see on a very different 664 00:38:29,34 --> 00:38:32,305 time scale this is days. You see that the x-ray 665 00:38:32,305 --> 00:38:35,786 eclipses, that the x-rays disappear completely when the 666 00:38:35,786 --> 00:38:37,977 neutron star goes behind the donor. 667 00:38:37,977 --> 00:38:40,942 And the orbital period is one point seven days. 668 00:38:40,942 --> 00:38:42,94 The x-rays disappear completely. 669 00:38:42,94 --> 00:38:45,453 And so this picture is well-established. 670 00:38:45,453 --> 00:38:49,32 It's beyond a shadow of a doubt we do know what these objects 671 00:38:49,32 --> 00:38:51,512 are. With no one is flying balloons 672 00:38:51,512 --> 00:38:56,603 anymore I make all my reserv- o- observations nowadays from 673 00:38:56,603 --> 00:39:00,538 satellites using European satellites, Japanese satellites 674 00:39:00,538 --> 00:39:04,612 and American observatories. Lately we have the Rossi timing 675 00:39:04,612 --> 00:39:08,405 explorer in orbit and also Chandra which is the biggest 676 00:39:08,405 --> 00:39:10,794 thing in town. Now between nineteen 677 00:39:10,794 --> 00:39:14,166 seventy-five and nineteen seventy-nine we were so 678 00:39:14,166 --> 00:39:17,96 fortunate here at MIT that we had our own private x-ray 679 00:39:17,96 --> 00:39:20,489 observatory. It was called Sas three. 680 00:39:20,489 --> 00:39:24,352 It was an all-MIT operation. We uh maneuvered it from my 681 00:39:24,352 --> 00:39:28,076 building, Center for Space Research, 682 00:39:28,076 --> 00:39:31,064 Building thirty-seven, twenty-four hours a day, 683 00:39:31,064 --> 00:39:33,533 three hundred sixty-five days per year. 684 00:39:33,533 --> 00:39:37,171 It was at that time that Josh Grindlay and John Heise had 685 00:39:37,171 --> 00:39:40,679 discovered using a Dutch satellite believe it or not -- 686 00:39:40,679 --> 00:39:44,447 they had discovered that some of these x-ray sources showed 687 00:39:44,447 --> 00:39:47,695 sudden x-ray bursts. The x-ray intensity would rise 688 00:39:47,695 --> 00:39:50,294 in about a few seconds, would become ten, 689 00:39:50,294 --> 00:39:54,127 twenty, thirty times stronger, and then over a time scale of 690 00:39:54,127 --> 00:39:57,44 maybe a few minutes, the x-ray intensity would peter 691 00:39:57,44 --> 00:39:59,302 out again. 692 00:39:59,302 --> 00:40:03,416 And we had this Sas three observatory which ideally suited 693 00:40:03,416 --> 00:40:06,014 to do research in these x-ray bursts. 694 00:40:06,014 --> 00:40:10,199 And we discovered within two years eight new burst sources. 695 00:40:10,199 --> 00:40:14,24 And I think it's fair to say that it's largely due to our 696 00:40:14,24 --> 00:40:18,065 observational work and also to the theoretical work by 697 00:40:18,065 --> 00:40:22,394 Professor Joss who was at MIT and still is at MIT that we now 698 00:40:22,394 --> 00:40:25,137 understand what these x-ray bursts are. 699 00:40:25,137 --> 00:40:30,682 They are huge nuclear bomb explosions on the surface of a 700 00:40:30,682 --> 00:40:34,287 neutron star. The matter that falls onto the 701 00:40:34,287 --> 00:40:37,892 neutron star is largely hydrogen and helium. 702 00:40:37,892 --> 00:40:41,078 Because that's the matter of this star. 703 00:40:41,078 --> 00:40:45,437 And the density and the temperature on the surface of 704 00:40:45,437 --> 00:40:50,299 the neutron star is so high that you get nuclear reactions. 705 00:40:50,299 --> 00:40:54,826 And three helium four nuclei can fuse to carbon twelve. 706 00:40:54,826 --> 00:40:59,521 And then energy is released. And this nuclear reaction is 707 00:40:59,521 --> 00:41:03,824 very unstable, is extremely sensitive to the 708 00:41:03,824 --> 00:41:06,414 temperature. If the temperature goes up the 709 00:41:06,414 --> 00:41:09,373 reaction rate is higher, more energy is released. 710 00:41:09,373 --> 00:41:12,333 Temperature goes up. When the temperature goes up 711 00:41:12,333 --> 00:41:16,033 the reaction rate goes up and so on, more energy is released. 712 00:41:16,033 --> 00:41:18,253 And the whole thing gets out of hand. 713 00:41:18,253 --> 00:41:21,027 You get a thermonuclear runaway as we call it. 714 00:41:21,027 --> 00:41:24,604 You see a thermonuclear flash. A gigantic bomb explosion on 715 00:41:24,604 --> 00:41:28,489 the surface of the neutron star. These bomb explosions are about 716 00:41:28,489 --> 00:41:31,88 eighteen orders of magnitude more powerful than the most 717 00:41:31,88 --> 00:41:35,754 powerful hydrogen bombs that we can 718 00:41:35,754 --> 00:41:38,81 build on earth. So this -- this layer, 719 00:41:38,81 --> 00:41:42,94 this fresh accreted layer, goes up in one huge bomb 720 00:41:42,94 --> 00:41:47,731 explosion and then new material is accreted and a few hours 721 00:41:47,731 --> 00:41:51,118 later you will see another bomb explosion. 722 00:41:51,118 --> 00:41:55,496 So you can see several of these x-rays bursts per day. 723 00:41:55,496 --> 00:41:59,791 The optical counterparts of these stars, these binary 724 00:41:59,791 --> 00:42:04,417 systems, uh are very faint, but you can see them from the 725 00:42:04,417 --> 00:42:08,06 -- the ground with uh optical 726 00:42:08,06 --> 00:42:12,954 observatories from the ground. And we had reasons to believe 727 00:42:12,954 --> 00:42:17,93 at the time that simultaneously with an x-ray burst you might 728 00:42:17,93 --> 00:42:20,668 actually observe an optical burst. 729 00:42:20,668 --> 00:42:24,815 And I'll give you the reasons why we believed that. 730 00:42:24,815 --> 00:42:29,791 You see here the neutron star and you see the accretion disk. 731 00:42:29,791 --> 00:42:35,764 And let us assume that there is an x-ray burst going on now. 732 00:42:35,764 --> 00:42:39,432 These red wiggles is uh x-rays from the x-ray burst. 733 00:42:39,432 --> 00:42:43,891 The earth is in this direction. So these x-rays reach the earth 734 00:42:43,891 --> 00:42:46,409 first. But there are x-rays which go 735 00:42:46,409 --> 00:42:49,861 in this direction. They are absorbed by the disk. 736 00:42:49,861 --> 00:42:53,601 And then the disk heats up. To thirty, forty thousand 737 00:42:53,601 --> 00:42:56,047 degrees. And starts to emit optical 738 00:42:56,047 --> 00:42:58,708 light. Some of that optical light will 739 00:42:58,708 --> 00:43:01,01 go in the direction of the earth. 740 00:43:01,01 --> 00:43:06,476 And so what this means now is that there is a delay between 741 00:43:06,476 --> 00:43:09,713 these x-rays, these -- this optical light and 742 00:43:09,713 --> 00:43:11,626 these x-rays. Because look. 743 00:43:11,626 --> 00:43:15,598 This path length is longer to us than this path length. 744 00:43:15,598 --> 00:43:19,791 And therefore we were hoping not only to be able to see an 745 00:43:19,791 --> 00:43:22,734 optical flash, which would mean the x-ray 746 00:43:22,734 --> 00:43:26,853 heating of the disk with the optical light from the disk, 747 00:43:26,853 --> 00:43:30,09 but we were al- also hoping to see the delay. 748 00:43:30,09 --> 00:43:34,43 If you can measure the delay, if you see a one-second delay, 749 00:43:34,43 --> 00:43:38,329 it would mean that this geometry was 750 00:43:38,329 --> 00:43:41,107 roughly one light-second from here to here. 751 00:43:41,107 --> 00:43:44,679 And so we were very ambitious. We organized a worldwide 752 00:43:44,679 --> 00:43:47,855 campaign in the summer of nineteen seventy-seven. 753 00:43:47,855 --> 00:43:51,758 With Sas three we were going to observe one particular x-ray 754 00:43:51,758 --> 00:43:54,404 burst source in the sky for two weeks on. 755 00:43:54,404 --> 00:43:57,314 And we were asking observers from the ground, 756 00:43:57,314 --> 00:43:59,564 optical, radio, infrared observers, 757 00:43:59,564 --> 00:44:02,937 to keep an eye on that object and also observe that, 758 00:44:02,937 --> 00:44:05,253 all the time, as long as they could. 759 00:44:05,253 --> 00:44:07,303 Seventeen countries contributed. 760 00:44:07,303 --> 00:44:09,685 Forty-four observatories contributed. 761 00:44:09,685 --> 00:44:13,889 Participated. And during these two weeks we 762 00:44:13,889 --> 00:44:17,786 saw one hundred and ten x-rays bursts from this object with Sas 763 00:44:17,786 --> 00:44:20,741 three, none was -- were observed in the optical, 764 00:44:20,741 --> 00:44:22,689 none were observed in the radio. 765 00:44:22,689 --> 00:44:26,083 In nineteen seventy-eight we tried it again and we were 766 00:44:26,083 --> 00:44:28,598 successful. This was a collaboration with 767 00:44:28,598 --> 00:44:32,306 Joss Grindlay who was at the time at Harvard and my graduate 768 00:44:32,306 --> 00:44:35,952 student Jeff McClintock who was at that time already Doctor 769 00:44:35,952 --> 00:44:38,466 McClintock. He also was at Harvard and he 770 00:44:38,466 --> 00:44:42,237 still is there. It was a smashing success. 771 00:44:42,237 --> 00:44:45,642 We made it to the cover of Nature, which is a rather 772 00:44:45,642 --> 00:44:47,845 prestigious uh scientific journal. 773 00:44:47,845 --> 00:44:51,918 It was covered by the New York Times and by many newspapers in 774 00:44:51,918 --> 00:44:54,655 the world. So we were the first to be able 775 00:44:54,655 --> 00:44:58,594 to detect simultaneously an x-ray burst and an optical burst 776 00:44:58,594 --> 00:45:02,266 from this binary system. Uh the data that I want to show 777 00:45:02,266 --> 00:45:05,137 you are not the nineteen seventy-eight data. 778 00:45:05,137 --> 00:45:09,076 But they are data from a year later because they have better 779 00:45:09,076 --> 00:45:11,345 quality. We learned how to do it of 780 00:45:11,345 --> 00:45:13,682 course. These are the x-ray data not 781 00:45:13,682 --> 00:45:18,114 from Sas three. Because Sas three was no longer 782 00:45:18,114 --> 00:45:20,274 in orbit in nineteen seventy-nine. 783 00:45:20,274 --> 00:45:22,696 This was a Japanese satellite Hakucho. 784 00:45:22,696 --> 00:45:26,688 And so here you see the times in seconds, and here you see the 785 00:45:26,688 --> 00:45:30,746 x-ray intensity and you see here the x-ray burst as we observed 786 00:45:30,746 --> 00:45:33,363 it with the Japanese observatory Hakucho. 787 00:45:33,363 --> 00:45:36,963 And here you see the optical data which were taken by my 788 00:45:36,963 --> 00:45:39,319 friend and colleague Holger Pedersen. 789 00:45:39,319 --> 00:45:42,853 He used a European southern observatory in -- in Chile. 790 00:45:42,853 --> 00:45:46,715 And he observed clearly an optical flash. 791 00:45:46,715 --> 00:45:50,722 This is the intensity of the light before the x-ray burst and 792 00:45:50,722 --> 00:45:54,262 then you see an incredible increase and then you see a 793 00:45:54,262 --> 00:45:56,533 decay. I have plotted these so that 794 00:45:56,533 --> 00:46:00,54 they both have the same height. That of course is artificial. 795 00:46:00,54 --> 00:46:03,279 But it makes it easier to compare the two. 796 00:46:03,279 --> 00:46:07,086 And so now comes the acid test. If now I put one on top of 797 00:46:07,086 --> 00:46:10,826 another then you see very clearly that as we expected all 798 00:46:10,826 --> 00:46:14,967 along that the optical signal is indeed delayed relative to the 799 00:46:14,967 --> 00:46:17,305 x-ray signal. And you can now do the 800 00:46:17,305 --> 00:46:21,161 measurement that we hoped we could do. 801 00:46:21,161 --> 00:46:25,364 You shift one curve on top of another and if you shift it by 802 00:46:25,364 --> 00:46:29,496 about two seconds then they're almost carbon copies of each 803 00:46:29,496 --> 00:46:31,918 other. And so we succeeded then for 804 00:46:31,918 --> 00:46:36,122 the first time to measure the geometry of the accretion disk 805 00:46:36,122 --> 00:46:40,254 around these neutron stars. They have radii of very roughly 806 00:46:40,254 --> 00:46:43,744 two light-seconds. Light had to travel two seconds 807 00:46:43,744 --> 00:46:46,95 more, first x-rays, x-ray heating and then the 808 00:46:46,95 --> 00:46:49,372 optical. Two light-seconds is about 809 00:46:49,372 --> 00:46:53,505 twice the distance from the earth to the 810 00:46:53,505 --> 00:46:56,097 moon. So these systems are amazingly 811 00:46:56,097 --> 00:47:00,394 compact, very small indeed. Needless to say that during the 812 00:47:00,394 --> 00:47:03,95 past term I haven't been able to do any research, 813 00:47:03,95 --> 00:47:07,431 no science at all, eight oh two has swallowed up 814 00:47:07,431 --> 00:47:10,32 everything that I had to offer and more. 815 00:47:10,32 --> 00:47:14,617 And I think you should feel sorry for my graduate students. 816 00:47:14,617 --> 00:47:18,84 And I think you have all the right to feel guilty as well. 817 00:47:18,84 --> 00:47:23,136 Uh you were on my mind all the time and not only at MIT but 818 00:47:23,136 --> 00:47:26,895 also at home, in my living room, 819 00:47:26,895 --> 00:47:31,247 my kitchen, when I took showers, and even very often 820 00:47:31,247 --> 00:47:35,685 appeared in my dreams. Believe me, it was hell for my 821 00:47:35,685 --> 00:47:40,037 -- for my significant other, who is in the audience, 822 00:47:40,037 --> 00:47:42,939 Susan, it wasn't very nice for her. 823 00:47:42,939 --> 00:47:46,353 Frankly, my life will change after today. 824 00:47:46,353 --> 00:47:49,169 I make myself no illusion however. 825 00:47:49,169 --> 00:47:54,546 Most of you will quickly forget all four Maxwell's equations and 826 00:47:54,546 --> 00:47:59,751 you will forget all about induction and about 827 00:47:59,751 --> 00:48:04,882 nonconservative fields. I hope for you though that it 828 00:48:04,882 --> 00:48:10,703 will not be before next week. But surely when you will see a 829 00:48:10,703 --> 00:48:16,524 rainbow you won't be able to resist to check that the red is 830 00:48:16,524 --> 00:48:20,865 on the outside and the blue is on the inside. 831 00:48:20,865 --> 00:48:25,008 It is a disease for which there is no cure. 832 00:48:25,008 --> 00:48:32,229 And if you carry your personal polarizers with you, 833 00:48:32,229 --> 00:48:40,487 then you will want to verify that indeed the bows are 834 00:48:40,487 --> 00:48:48,427 strongly polarized, and if you do that I'll be very 835 00:48:48,427 --> 00:48:55,255 proud of you. And I hope those will be happy 836 00:48:55,255 --> 00:49:00,178 memories. Thank you for 837 00:49:00,178 --> 49:05 attending my lecture. [applause]