1 00:00:00,060 --> 00:00:02,430 The following content is provided under a Creative 2 00:00:02,430 --> 00:00:03,820 Commons license. 3 00:00:03,820 --> 00:00:06,030 Your support will help MIT OpenCourseWare 4 00:00:06,030 --> 00:00:10,120 continue to offer high-quality educational resources for free. 5 00:00:10,120 --> 00:00:12,660 To make a donation, or to view additional materials 6 00:00:12,660 --> 00:00:16,620 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,620 --> 00:00:17,500 at ocw.mit.edu. 8 00:00:20,466 --> 00:00:23,010 PROFESSOR: So I'd like to begin today 9 00:00:23,010 --> 00:00:26,850 by clarifying something I talked about at the end of last class. 10 00:00:34,590 --> 00:00:38,019 OK, so last week I was talking about time travel in general. 11 00:00:38,019 --> 00:00:40,560 And at the end of last class, I was talking about time travel 12 00:00:40,560 --> 00:00:42,540 to the past. 13 00:00:42,540 --> 00:00:44,520 I talked about this method that some people 14 00:00:44,520 --> 00:00:48,360 have thought of that might work, possibly, 15 00:00:48,360 --> 00:00:49,830 using what's called wormholes. 16 00:00:55,520 --> 00:00:59,780 And remember a wormhole is a path, a shortcut between points 17 00:00:59,780 --> 00:01:02,840 in space time, that the equations of general relativity 18 00:01:02,840 --> 00:01:03,920 just allowed to exist. 19 00:01:03,920 --> 00:01:07,910 It's this weird thing that the equations just allow to exist. 20 00:01:07,910 --> 00:01:11,316 So a supposed space is two-dimensional. 21 00:01:14,209 --> 00:01:16,000 It's not really two-dimensional, but I just 22 00:01:16,000 --> 00:01:19,100 want to draw some kind of picture. 23 00:01:19,100 --> 00:01:22,010 Let's suppose you're looking at it from this view. 24 00:01:22,010 --> 00:01:25,037 Oh, attendance, OK. 25 00:01:25,037 --> 00:01:26,870 Suppose you're looking at it from this view, 26 00:01:26,870 --> 00:01:29,450 then it would look like that. 27 00:01:29,450 --> 00:01:35,040 Now a wormhole would connect two regions like this. 28 00:01:35,040 --> 00:01:37,580 Now they can just travel from one region to the next 29 00:01:37,580 --> 00:01:39,190 by traveling through a wormhole. 30 00:01:39,190 --> 00:01:41,280 It would be a shortcut. 31 00:01:41,280 --> 00:01:44,720 Now a student asked after class last week. 32 00:01:44,720 --> 00:01:47,180 So, OK, this is space, and this is 33 00:01:47,180 --> 00:01:49,420 the wormhole, but what's this? 34 00:01:49,420 --> 00:01:51,650 What is this in-between space? 35 00:01:51,650 --> 00:01:53,720 Is it some kind of extra dimension, 36 00:01:53,720 --> 00:01:55,490 like a fourth dimension of space, 37 00:01:55,490 --> 00:01:58,490 or some kind of a hyperspace thing? 38 00:01:58,490 --> 00:01:59,729 The answer is no. 39 00:01:59,729 --> 00:02:00,770 This is actually nothing. 40 00:02:05,750 --> 00:02:07,640 I just needed a way to draw space 41 00:02:07,640 --> 00:02:09,699 being curved on the board. 42 00:02:09,699 --> 00:02:11,834 I needed a way to connect two points in space, 43 00:02:11,834 --> 00:02:14,000 and this was the only way I could think of doing it. 44 00:02:14,000 --> 00:02:17,690 So as a result, I got this extra stuff that looks like 45 00:02:17,690 --> 00:02:19,850 it's actually something, which isn't. 46 00:02:19,850 --> 00:02:21,650 It's not an actual dimension of space. 47 00:02:21,650 --> 00:02:25,310 It's just an artifact of the way that I drew things. 48 00:02:25,310 --> 00:02:26,992 It's not actually anything. 49 00:02:26,992 --> 00:02:29,450 General relativity doesn't need an extra dimension of space 50 00:02:29,450 --> 00:02:31,850 to have wormholes. 51 00:02:31,850 --> 00:02:37,160 It's weird enough to get the job done in three dimensions. 52 00:02:37,160 --> 00:02:41,210 Does anybody have any questions about wormholes or time travel? 53 00:02:41,210 --> 00:02:43,614 Any questions from last week? 54 00:02:43,614 --> 00:02:45,048 Yeah. 55 00:02:45,048 --> 00:02:47,797 AUDIENCE: Is space [INAUDIBLE]? 56 00:02:47,797 --> 00:02:49,630 PROFESSOR: Well, space is three-dimensional, 57 00:02:49,630 --> 00:02:50,690 time is one-dimensional. 58 00:02:50,690 --> 00:02:54,924 So spacetime is three plus one, four-dimensional. 59 00:02:54,924 --> 00:02:57,017 AUDIENCE: How do you [? draw that? ?] 60 00:02:57,017 --> 00:02:58,350 PROFESSOR: How do you draw that? 61 00:02:58,350 --> 00:02:59,320 You can't. 62 00:02:59,320 --> 00:03:01,410 [LAUGHS] I mean, I can draw something 63 00:03:01,410 --> 00:03:05,550 that's supposed to give you an idea of what it could be. 64 00:03:05,550 --> 00:03:09,160 I mean, I don't know of any way of drawing it 65 00:03:09,160 --> 00:03:10,099 in an adequate way. 66 00:03:10,099 --> 00:03:11,640 Any way that you draw it, it's always 67 00:03:11,640 --> 00:03:12,764 going to have some defects. 68 00:03:12,764 --> 00:03:15,000 Like here you have this extra stuff, 69 00:03:15,000 --> 00:03:16,740 which isn't really anything. 70 00:03:16,740 --> 00:03:18,600 There's no good way of drawing it. 71 00:03:18,600 --> 00:03:20,558 I mean ultimately you have to sit down and look 72 00:03:20,558 --> 00:03:21,690 at the equations. 73 00:03:21,690 --> 00:03:24,062 I'm not going to show you the equations. 74 00:03:24,062 --> 00:03:25,520 And if you want, you can completely 75 00:03:25,520 --> 00:03:32,250 disregard this equation on my shirt, if it distracts you. 76 00:03:32,250 --> 00:03:36,762 But yeah, this is just a defect of the way that I drew things. 77 00:03:36,762 --> 00:03:38,220 It's actually similar it's actually 78 00:03:38,220 --> 00:03:41,520 similar to what happened when I talked about the expanding 79 00:03:41,520 --> 00:03:43,380 universe. 80 00:03:43,380 --> 00:03:45,900 Remember the way I described it was 81 00:03:45,900 --> 00:03:51,330 to imagine that the universe is like the surface 82 00:03:51,330 --> 00:03:52,506 of the balloon. 83 00:03:52,506 --> 00:03:53,880 And then you inflate the balloon, 84 00:03:53,880 --> 00:03:55,750 you blow up the balloon. 85 00:03:55,750 --> 00:03:58,957 So the balloon becomes bigger. 86 00:03:58,957 --> 00:04:00,540 As a result, the points on the balloon 87 00:04:00,540 --> 00:04:02,250 move away from each other. 88 00:04:02,250 --> 00:04:04,920 The distances between the points increase. 89 00:04:04,920 --> 00:04:08,130 And that's what we mean by the expansion of space. 90 00:04:11,130 --> 00:04:14,454 The universe isn't expanding into anything. 91 00:04:14,454 --> 00:04:15,120 There's nothing. 92 00:04:15,120 --> 00:04:15,930 The universe just is. 93 00:04:15,930 --> 00:04:17,339 It's not expanding into anything. 94 00:04:17,339 --> 00:04:19,200 When I say that the universe is expanding, all that I mean 95 00:04:19,200 --> 00:04:21,269 is that the distances between points increases. 96 00:04:21,269 --> 00:04:22,750 That's all that expanding means. 97 00:04:22,750 --> 00:04:25,590 But we want to have some way of visualizing it, 98 00:04:25,590 --> 00:04:27,874 and this is an easy way of visualizing it. 99 00:04:27,874 --> 00:04:29,790 But it makes you think that space is expanding 100 00:04:29,790 --> 00:04:31,310 into something, which it isn't. 101 00:04:31,310 --> 00:04:34,521 That's just a defect of the way that we drew things. 102 00:04:34,521 --> 00:04:35,020 Question. 103 00:04:35,020 --> 00:04:36,853 AUDIENCE: If the universe is like a balloon, 104 00:04:36,853 --> 00:04:38,335 will it blow up? 105 00:04:38,335 --> 00:04:40,210 PROFESSOR: If the universe is like a balloon, 106 00:04:40,210 --> 00:04:40,930 will it blow up? 107 00:04:40,930 --> 00:04:41,740 No. 108 00:04:41,740 --> 00:04:46,102 See, that's another defect of the way that I describe things. 109 00:04:46,102 --> 00:04:47,560 The only way it's like a balloon is 110 00:04:47,560 --> 00:04:50,642 that points on the surface of the balloon 111 00:04:50,642 --> 00:04:52,850 move away from each other, and points in the universe 112 00:04:52,850 --> 00:04:55,470 move away from each other. 113 00:04:55,470 --> 00:04:56,845 It could also be the case that it 114 00:04:56,845 --> 00:04:59,080 has a geometry like a balloon. 115 00:04:59,080 --> 00:05:01,250 That's another way, it could be closed. 116 00:05:01,250 --> 00:05:02,740 It could be finite and closed. 117 00:05:02,740 --> 00:05:03,531 That's another way. 118 00:05:03,531 --> 00:05:08,269 But the notion of blowing up or popping, 119 00:05:08,269 --> 00:05:10,810 those are just defects of the way that we use to describe it. 120 00:05:15,160 --> 00:05:17,500 Although, remember, it doesn't even 121 00:05:17,500 --> 00:05:20,280 look like the universe is like a balloon. 122 00:05:20,280 --> 00:05:22,880 It looks like it's actually infinite and flat 123 00:05:22,880 --> 00:05:24,750 in a certain sense. 124 00:05:24,750 --> 00:05:29,807 AUDIENCE: [INAUDIBLE] 125 00:05:29,807 --> 00:05:31,390 PROFESSOR: Just like the way that they 126 00:05:31,390 --> 00:05:33,670 thought the Earth was flat? 127 00:05:33,670 --> 00:05:36,920 Yeah, kind of like that. 128 00:05:36,920 --> 00:05:41,770 OK, now, so general relativity doesn't 129 00:05:41,770 --> 00:05:44,560 require there to be extra dimensions of space. 130 00:05:44,560 --> 00:05:45,060 Right? 131 00:05:45,060 --> 00:05:47,284 It can get the job done in three dimensions of space. 132 00:05:47,284 --> 00:05:49,450 But that's not to say that extra dimensions of space 133 00:05:49,450 --> 00:05:50,286 don't exist. 134 00:05:50,286 --> 00:05:51,910 I'm just saying that general relativity 135 00:05:51,910 --> 00:05:53,810 doesn't require them. 136 00:05:53,810 --> 00:05:56,341 And actually later, later, I'll talk about the very real 137 00:05:56,341 --> 00:05:58,716 possibility that there are extra dimensions in space when 138 00:05:58,716 --> 00:06:01,040 I talk about string theory. 139 00:06:01,040 --> 00:06:04,480 But the general relativity itself 140 00:06:04,480 --> 00:06:06,510 doesn't require there to be extra dimensions. 141 00:06:06,510 --> 00:06:08,530 And so if you add another dimension, 142 00:06:08,530 --> 00:06:10,210 you just add extra stuff to the theory. 143 00:06:10,210 --> 00:06:13,180 You make it more complicated. 144 00:06:13,180 --> 00:06:14,740 So why should you add extra stuff? 145 00:06:14,740 --> 00:06:18,762 Why should you make the theory more complicated? 146 00:06:18,762 --> 00:06:20,470 This actually raises an interesting point 147 00:06:20,470 --> 00:06:23,770 in the philosophy of scientific theories. 148 00:06:23,770 --> 00:06:26,850 Before I get to that are there any questions about this? 149 00:06:30,690 --> 00:06:35,990 OK, the profound point that I was alluding to 150 00:06:35,990 --> 00:06:39,530 in the philosophy of science is it 151 00:06:39,530 --> 00:06:46,100 has to do with the simplicity of theories. 152 00:06:46,100 --> 00:06:49,580 In general, when scientists go about making theories, 153 00:06:49,580 --> 00:06:52,760 they generally want the one that's the simplest. 154 00:06:52,760 --> 00:06:54,550 The simpler the theory, the better. 155 00:06:59,630 --> 00:07:01,520 And this is called Occam's Razor. 156 00:07:32,960 --> 00:07:36,352 Occam was just a philosopher that was around 700 years ago 157 00:07:36,352 --> 00:07:37,310 or something like that. 158 00:07:37,310 --> 00:07:42,530 [LAUGHS] So the simpler the theory, the better. 159 00:07:42,530 --> 00:07:44,909 And this sounds reasonable, right? 160 00:07:44,909 --> 00:07:46,700 It's something that we use in everyday life 161 00:07:46,700 --> 00:07:49,950 when we try to come up with an explanation of something. 162 00:07:49,950 --> 00:07:53,060 I mean suppose somebody just walked in and was trying 163 00:07:53,060 --> 00:07:57,020 to explain the observation that there are a bunch of students 164 00:07:57,020 --> 00:07:58,790 in this room. 165 00:07:58,790 --> 00:08:02,060 Why are there these students in this room? 166 00:08:02,060 --> 00:08:07,100 A very simple explanation is that about six weeks ago, 167 00:08:07,100 --> 00:08:10,100 or a little bit more than that, looked at the class list 168 00:08:10,100 --> 00:08:13,050 for this program at MIT called HSSP, 169 00:08:13,050 --> 00:08:16,640 saw an interesting class at 1:30 to 3:30 PM every Sunday, 170 00:08:16,640 --> 00:08:18,344 and so you decided to take it. 171 00:08:18,344 --> 00:08:20,510 Then you've been coming here for the past six weeks. 172 00:08:20,510 --> 00:08:22,220 And today's no different from all those other Sundays, 173 00:08:22,220 --> 00:08:23,095 and so you came here. 174 00:08:23,095 --> 00:08:27,220 That's a very simple explanation for why you're here. 175 00:08:27,220 --> 00:08:29,710 It's probably the most natural one that we would think of. 176 00:08:29,710 --> 00:08:31,376 But you can think of other explanations. 177 00:08:31,376 --> 00:08:34,340 You can think of crazier explanations. 178 00:08:34,340 --> 00:08:37,159 Here's another one. 179 00:08:37,159 --> 00:08:41,270 It could be that all of you guys are 180 00:08:41,270 --> 00:08:44,570 very special in a certain way. 181 00:08:44,570 --> 00:08:47,690 And you're here because-- 182 00:08:47,690 --> 00:08:48,500 [LAUGHS] Sorry. 183 00:08:54,350 --> 00:08:58,580 You're here because wizards thought 184 00:08:58,580 --> 00:09:03,770 that you should be here, because in the very far future, 185 00:09:03,770 --> 00:09:07,100 something drastic will happen to the world. 186 00:09:07,100 --> 00:09:11,590 And you guys are special in a certain way 187 00:09:11,590 --> 00:09:14,040 that you can save the world. 188 00:09:14,040 --> 00:09:15,710 And having knowledge of big questions 189 00:09:15,710 --> 00:09:19,560 will help you save the world, something like that. 190 00:09:19,560 --> 00:09:22,640 And so you're here, because the wizards manipulated your mind 191 00:09:22,640 --> 00:09:26,000 in giving you the illusion of wanting to be here, 192 00:09:26,000 --> 00:09:28,040 and then you're just here for all the Sundays, 193 00:09:28,040 --> 00:09:29,456 because the wizards made you here. 194 00:09:29,456 --> 00:09:31,700 Something like that. 195 00:09:31,700 --> 00:09:35,590 Just to think of a random explanation. 196 00:09:35,590 --> 00:09:38,850 And sure that sounds crazy, or silly, or whatever, 197 00:09:38,850 --> 00:09:41,309 because it's more complicated than the explanation 198 00:09:41,309 --> 00:09:43,850 that I gave just before, which is that you're, because you're 199 00:09:43,850 --> 00:09:44,990 excited to be here. 200 00:09:44,990 --> 00:09:46,790 And you've come here every Sunday, 201 00:09:46,790 --> 00:09:48,700 and so you're here today. 202 00:09:48,700 --> 00:09:52,520 So this rule that we use is pretty reasonable. 203 00:09:52,520 --> 00:09:55,142 We use it in everyday life. 204 00:09:55,142 --> 00:09:57,350 Simpler explanations are better than more complicated 205 00:09:57,350 --> 00:10:00,377 explanations. 206 00:10:00,377 --> 00:10:01,710 But why should this be the case? 207 00:10:01,710 --> 00:10:03,860 Why should simpler theories be better? 208 00:10:03,860 --> 00:10:07,317 Why should the best theory be the simplest theory? 209 00:10:07,317 --> 00:10:09,400 Can anybody actually give an explanation for that? 210 00:10:09,400 --> 00:10:11,510 A justification for this Occam's razor? 211 00:10:16,880 --> 00:10:17,954 Yeah? 212 00:10:17,954 --> 00:10:21,650 AUDIENCE: Because it was simpler and easier to prove. 213 00:10:21,650 --> 00:10:24,292 PROFESSOR: It was simpler and maybe easier to prove? 214 00:10:29,260 --> 00:10:33,234 Why might it be easier to prove if it's simpler? 215 00:10:33,234 --> 00:10:41,090 AUDIENCE: Because [INAUDIBLE] really complicated, 216 00:10:41,090 --> 00:10:43,661 then it's easier to make a mistake with it. 217 00:10:43,661 --> 00:10:45,910 PROFESSOR: So, yeah, if you have a complicated theory, 218 00:10:45,910 --> 00:10:50,490 then you have to have everything just right. 219 00:10:50,490 --> 00:10:53,400 There might be a lot of steps in the explanation. 220 00:10:53,400 --> 00:10:55,650 If one goes wrong, then the whole explanation's wrong. 221 00:10:55,650 --> 00:10:57,066 Whereas with a simple explanation, 222 00:10:57,066 --> 00:10:59,250 you only have a few number of steps. 223 00:10:59,250 --> 00:11:01,430 You just need a few them to come right. 224 00:11:01,430 --> 00:11:04,200 You just need a few of them to be right. 225 00:11:04,200 --> 00:11:08,864 Yeah, I think that's kind of the gist of the explanation 226 00:11:08,864 --> 00:11:11,280 that I've heard, at least the best explanation I've heard. 227 00:11:14,100 --> 00:11:17,154 Any other explanations for this? 228 00:11:17,154 --> 00:11:20,190 For the explanation? 229 00:11:20,190 --> 00:11:23,570 So probably the best justification I've heard 230 00:11:23,570 --> 00:11:25,950 goes as follows. 231 00:11:25,950 --> 00:11:29,370 So every physical theory has some fundamental-- 232 00:11:29,370 --> 00:11:32,790 I'm just going to make your argument a little more precise. 233 00:11:32,790 --> 00:11:36,450 So every physical theory has some fundamental principles, 234 00:11:36,450 --> 00:11:38,940 some fundamental postulates. 235 00:11:38,940 --> 00:11:41,320 Now remember last week I talked about relativity. 236 00:11:41,320 --> 00:11:43,920 Special relativity has these two postulates. 237 00:11:43,920 --> 00:11:45,960 The first is that the laws of physics 238 00:11:45,960 --> 00:11:48,060 are the same for all observers that are 239 00:11:48,060 --> 00:11:49,424 moving at a constant velocity. 240 00:11:49,424 --> 00:11:51,090 And at the speed of light, it's the same 241 00:11:51,090 --> 00:11:53,006 for all observers moving at constant velocity. 242 00:11:53,006 --> 00:11:54,690 Those are the two fundamental postulates 243 00:11:54,690 --> 00:11:55,700 of special relativity. 244 00:11:55,700 --> 00:11:56,658 They can't be provable. 245 00:11:56,658 --> 00:11:57,990 They're just fundamental. 246 00:11:57,990 --> 00:12:00,030 And all the consequences of the theory 247 00:12:00,030 --> 00:12:03,240 derive from those postulates. 248 00:12:03,240 --> 00:12:06,410 Every theory has some fundamental postulates. 249 00:12:06,410 --> 00:12:10,810 But it's impossible to know if they're actually true. 250 00:12:10,810 --> 00:12:13,050 You can perform a number of experiments, 251 00:12:13,050 --> 00:12:14,550 and we've certainly done experiments 252 00:12:14,550 --> 00:12:16,390 around special relativity, for example. 253 00:12:16,390 --> 00:12:18,210 And experiments might be consistent 254 00:12:18,210 --> 00:12:20,580 with those postulates every single time. 255 00:12:20,580 --> 00:12:25,170 But it could be that, deep down, the postulates-- it's not 256 00:12:25,170 --> 00:12:27,210 that they're wrong, per se. 257 00:12:27,210 --> 00:12:29,046 It could be that the postulates are 258 00:12:29,046 --> 00:12:30,420 incomplete in some kind of a way. 259 00:12:30,420 --> 00:12:33,120 It could be that nature is just fooling us. 260 00:12:33,120 --> 00:12:37,860 That we're not really seeing the true nature of nature. 261 00:12:37,860 --> 00:12:41,010 It could be that the postulates are just incomplete. 262 00:12:41,010 --> 00:12:43,860 So every physical theory has postulates, 263 00:12:43,860 --> 00:12:46,050 and every postulate has some amount of uncertainty 264 00:12:46,050 --> 00:12:47,760 associated with it. 265 00:12:47,760 --> 00:12:51,940 I mean, you could be wrong about the postulate. 266 00:12:51,940 --> 00:12:54,050 Now the justification for Occam's razor, 267 00:12:54,050 --> 00:12:57,630 at least the best one that I've heard, 268 00:12:57,630 --> 00:13:01,410 says, let's suppose that every postulate has 269 00:13:01,410 --> 00:13:03,234 a set amount of uncertainty. 270 00:13:09,104 --> 00:13:10,770 Every postulate of every physical theory 271 00:13:10,770 --> 00:13:12,340 has the same amount of uncertainty. 272 00:13:12,340 --> 00:13:14,760 So the more postulates that you have in a theory, the more 273 00:13:14,760 --> 00:13:16,062 uncertainty. 274 00:13:16,062 --> 00:13:19,050 Now the more postulates that you have, the more complicated 275 00:13:19,050 --> 00:13:22,140 you make your theory, the less simple you make your theory. 276 00:13:22,140 --> 00:13:28,080 So therefore, the simplest theory 277 00:13:28,080 --> 00:13:29,580 has the fewest number of postulates, 278 00:13:29,580 --> 00:13:32,500 and, therefore, has the least amount of uncertainty. 279 00:13:32,500 --> 00:13:35,610 So the less uncertain you are in the theory, the more likely 280 00:13:35,610 --> 00:13:37,440 the theory is of being right. 281 00:13:37,440 --> 00:13:40,980 And so the simpler the theory, the better. 282 00:13:40,980 --> 00:13:43,710 Where better means having the greatest chance of being right. 283 00:13:46,380 --> 00:13:48,480 So I think that's kind of a reasonable argument. 284 00:13:48,480 --> 00:13:51,961 I mean it's not perfect, but it's kind of reasonable. 285 00:13:54,350 --> 00:13:56,100 There are other justifications that people 286 00:13:56,100 --> 00:13:59,010 give for this Occam's razor. 287 00:13:59,010 --> 00:14:01,940 For example, I mean nature-- 288 00:14:01,940 --> 00:14:03,480 I don't really like the other ones. 289 00:14:03,480 --> 00:14:05,210 I mean a lot of-- 290 00:14:05,210 --> 00:14:06,930 one of them is an aesthetic reason. 291 00:14:06,930 --> 00:14:09,145 I mean nature has to be simple. 292 00:14:09,145 --> 00:14:10,020 It has to be elegant. 293 00:14:10,020 --> 00:14:13,710 It has to be beautiful. 294 00:14:13,710 --> 00:14:16,183 I think the one that I gave was probably the best one. 295 00:14:16,183 --> 00:14:16,970 [LAUGHS] 296 00:14:16,970 --> 00:14:22,196 OK, are there any questions about Occam's razor, 297 00:14:22,196 --> 00:14:23,070 about how that works? 298 00:14:31,310 --> 00:14:33,830 OK, so I'd like to continue along 299 00:14:33,830 --> 00:14:37,910 these lines of thinking about theories, the theory that 300 00:14:37,910 --> 00:14:40,430 describes nature. 301 00:14:40,430 --> 00:14:42,364 Last week I talked about relativity. 302 00:14:42,364 --> 00:14:44,780 A few weeks before when I talked about parallel universes, 303 00:14:44,780 --> 00:14:46,370 I mentioned quantum mechanics. 304 00:14:46,370 --> 00:14:49,804 And today I'd like to talk about the theory of everything. 305 00:14:49,804 --> 00:14:51,470 I mean we want to understand everything. 306 00:14:51,470 --> 00:14:54,031 We want to have some kind of a picture 307 00:14:54,031 --> 00:14:55,280 of the way the universe works. 308 00:14:55,280 --> 00:14:56,470 We want to have a theory. 309 00:14:56,470 --> 00:14:58,803 We want to be able to predict things and then manipulate 310 00:14:58,803 --> 00:14:59,854 nature. 311 00:14:59,854 --> 00:15:01,270 So we need a theory of everything. 312 00:15:01,270 --> 00:15:03,353 I mean if we really want to understand everything, 313 00:15:03,353 --> 00:15:05,510 we need a theory of everything. 314 00:15:05,510 --> 00:15:08,720 Now before I talk about the theory of everything 315 00:15:08,720 --> 00:15:12,440 [MUSIC ECHOING IN THE DISTANCE] 316 00:15:12,440 --> 00:15:16,000 OK, that was interesting. 317 00:15:16,000 --> 00:15:19,442 The theory of everything is beautiful as you can see. 318 00:15:19,442 --> 00:15:20,900 Before I talk about everything, I'd 319 00:15:20,900 --> 00:15:25,040 like to give just a brief history of mankind's quest 320 00:15:25,040 --> 00:15:27,290 in coming up with the theory of everything. 321 00:15:27,290 --> 00:15:30,530 It's essentially a brief history of physics. 322 00:15:30,530 --> 00:15:36,560 Because then we'll be able to better appreciate, 323 00:15:36,560 --> 00:15:39,530 you'll be able to better appreciate the things 324 00:15:39,530 --> 00:15:43,900 that people are working on today and just how 325 00:15:43,900 --> 00:15:47,960 strange the theory of everything would have to be, and to better 326 00:15:47,960 --> 00:15:48,980 understand it. 327 00:15:48,980 --> 00:15:51,890 I could start by saying, OK, this 328 00:15:51,890 --> 00:15:54,120 is what people think the theory of everything is. 329 00:15:54,120 --> 00:15:55,619 I could start like that, but I think 330 00:15:55,619 --> 00:15:57,830 I think it would be more enjoyable if I just give 331 00:15:57,830 --> 00:15:59,010 a brief history of physics. 332 00:16:01,520 --> 00:16:03,110 It won't take incredibly long. 333 00:16:08,370 --> 00:16:16,100 For thousands of years, people have looked around, 334 00:16:16,100 --> 00:16:18,170 and we've observed things. 335 00:16:18,170 --> 00:16:19,310 We've asked why? 336 00:16:19,310 --> 00:16:20,210 Why this? 337 00:16:20,210 --> 00:16:23,940 Why does an apple fall down? 338 00:16:23,940 --> 00:16:25,160 Why is the sky dark? 339 00:16:27,690 --> 00:16:31,340 Why does it hurt when somebody punches me in the face? 340 00:16:31,340 --> 00:16:33,260 Why does it hurt a lot? 341 00:16:33,260 --> 00:16:35,352 And why do people have to do that? 342 00:16:35,352 --> 00:16:36,690 [LAUGHS] 343 00:16:38,030 --> 00:16:43,130 So for a long time people weren't really making 344 00:16:43,130 --> 00:16:45,860 a whole lot of progress on these sorts of questions. 345 00:16:45,860 --> 00:16:48,980 The types of answers that people would give 346 00:16:48,980 --> 00:16:52,430 would be pretty philosophical answers 347 00:16:52,430 --> 00:16:55,364 or religious answers that are not scientific answers. 348 00:16:55,364 --> 00:16:56,780 I mean a scientist today would not 349 00:16:56,780 --> 00:16:58,488 be satisfied with the answers that people 350 00:16:58,488 --> 00:17:01,430 gave for a very long time. 351 00:17:01,430 --> 00:17:04,220 But finally this guy, Sir Isaac Newton 352 00:17:04,220 --> 00:17:13,000 comes along in the 1600s. 353 00:17:13,000 --> 00:17:15,670 And he explains a lot of things. 354 00:17:15,670 --> 00:17:25,420 Newton came along, gave his three laws of motion, 355 00:17:25,420 --> 00:17:27,940 that you've all probably heard. 356 00:17:27,940 --> 00:17:29,500 He also gave his law of gravity. 357 00:17:39,390 --> 00:17:43,060 And with these laws, he was able to explain a lot. 358 00:17:43,060 --> 00:17:45,780 He was able to explain why the apple falls down and hits you 359 00:17:45,780 --> 00:17:47,490 in the head. 360 00:17:47,490 --> 00:17:49,140 Why the Earth revolves around the Sun, 361 00:17:49,140 --> 00:17:51,780 why the Moon revolves around the Earth, why all the planets 362 00:17:51,780 --> 00:17:55,140 revolve around suns. 363 00:17:55,140 --> 00:17:56,040 A lot of stuff. 364 00:18:01,020 --> 00:18:06,710 The next revolution in physics happened in about the 1800s. 365 00:18:06,710 --> 00:18:11,290 Newton was probably-- physics was probably defined by Newton. 366 00:18:11,290 --> 00:18:15,520 Or put into its modern form by Newton. 367 00:18:15,520 --> 00:18:18,330 It was completely remarkable. 368 00:18:18,330 --> 00:18:22,350 Probably the next revolution in physics came about 369 00:18:22,350 --> 00:18:24,363 in the 1800s with Maxwell 370 00:18:24,363 --> 00:18:27,261 [CHANTING ENTHUSIASTICALLY] 371 00:18:29,680 --> 00:18:33,260 Yeah, Circle of Life, circle of physics. 372 00:18:40,930 --> 00:18:46,290 In the 1800s, somebody called James Clerk Maxwell 373 00:18:46,290 --> 00:18:49,740 came along and explained electricity and magnetism. 374 00:19:02,480 --> 00:19:05,000 We're all familiar with electricity, magnets, 375 00:19:05,000 --> 00:19:06,730 and that sort of thing. 376 00:19:06,730 --> 00:19:09,620 Maxwell actually showed that electricity and magnetism 377 00:19:09,620 --> 00:19:11,240 are unified. 378 00:19:11,240 --> 00:19:14,150 They're actually two sides of the same coin 379 00:19:14,150 --> 00:19:15,350 of electromagnetism. 380 00:19:26,630 --> 00:19:30,302 Turns out that an electric, as an electric field changes, 381 00:19:30,302 --> 00:19:32,510 a magnetic field is produced, and as a magnetic field 382 00:19:32,510 --> 00:19:34,177 changed, an electric field was produced. 383 00:19:34,177 --> 00:19:36,343 And there's this deep connection between electricity 384 00:19:36,343 --> 00:19:37,230 and magnetism. 385 00:19:37,230 --> 00:19:39,040 And the best way of thinking of them 386 00:19:39,040 --> 00:19:44,900 is thinking of them as a unified object called electromagnetism. 387 00:19:44,900 --> 00:19:48,380 And so by 1900, people had explained pretty 388 00:19:48,380 --> 00:19:51,590 much all everyday phenomena. 389 00:19:51,590 --> 00:19:53,760 You can use Newton's laws of motion 390 00:19:53,760 --> 00:19:59,660 to describe how things, how everyday objects move around, 391 00:19:59,660 --> 00:20:02,210 how things fall, how things push each other, 392 00:20:02,210 --> 00:20:03,770 how things pull each other. 393 00:20:03,770 --> 00:20:07,490 You can explain the stars, how they orbit through gravity. 394 00:20:07,490 --> 00:20:11,045 You can also explain a wide variety of electrical phenomena 395 00:20:11,045 --> 00:20:13,550 and magnetic phenomena. 396 00:20:13,550 --> 00:20:17,300 So pretty much all everyday phenomena 397 00:20:17,300 --> 00:20:19,519 were figured out by 1900s. 398 00:20:19,519 --> 00:20:21,560 People thought, people actually thought back then 399 00:20:21,560 --> 00:20:24,500 that they had finally come up with the theory of everything. 400 00:20:24,500 --> 00:20:27,890 And that the next task, the remaining task for physicists 401 00:20:27,890 --> 00:20:32,054 was just to figure out some of the constants a little better. 402 00:20:32,054 --> 00:20:33,470 For example, there's this constant 403 00:20:33,470 --> 00:20:35,180 called the gravitational constant 404 00:20:35,180 --> 00:20:38,480 that determines how strong gravity is. 405 00:20:38,480 --> 00:20:42,442 It's a number, and physicists thought 406 00:20:42,442 --> 00:20:44,900 at the time, that number's like the gravitational constants 407 00:20:44,900 --> 00:20:46,966 and other constants just needed to be 408 00:20:46,966 --> 00:20:48,090 determined more accurately. 409 00:20:48,090 --> 00:20:50,190 And that's all that was remaining for physics. 410 00:20:50,190 --> 00:20:54,880 So physics was solved by the 1900s. 411 00:20:54,880 --> 00:20:58,670 But it turns out that Newton's theory and Maxwell's theory 412 00:20:58,670 --> 00:21:01,610 actually are at odds with each other. 413 00:21:01,610 --> 00:21:04,460 They're actually inconsistent with each other. 414 00:21:04,460 --> 00:21:09,890 So either one of them, or both of them had to be false. 415 00:21:09,890 --> 00:21:13,840 Can anybody guess which one turns out winning, or if 416 00:21:13,840 --> 00:21:16,970 or if any of them turned out winning? 417 00:21:16,970 --> 00:21:18,230 Who thinks Newton was right? 418 00:21:21,420 --> 00:21:22,752 One for Newton. 419 00:21:22,752 --> 00:21:23,960 Who thinks Maxwell was right? 420 00:21:26,786 --> 00:21:33,980 One point, one, two, three, four, five, 421 00:21:33,980 --> 00:21:36,360 5.5-ish for Maxwell. 422 00:21:36,360 --> 00:21:40,640 Yeah, it turned Maxwell's electromagnetism 423 00:21:40,640 --> 00:21:43,650 was the right theory. 424 00:21:43,650 --> 00:21:45,140 It's not that Newton was wrong. 425 00:21:45,140 --> 00:21:47,470 He wasn't wrong. 426 00:21:47,470 --> 00:21:49,860 It was just incomplete. 427 00:21:49,860 --> 00:21:51,650 It turns out that Newton's laws just 428 00:21:51,650 --> 00:21:54,335 worked for things moving at small speeds. 429 00:21:57,394 --> 00:21:59,060 Once things start moving at high speeds, 430 00:21:59,060 --> 00:22:01,400 you need Einstein's theory of relativity. 431 00:22:01,400 --> 00:22:08,850 So in the early 1900s, Einstein essentially 432 00:22:08,850 --> 00:22:10,350 showed how to fix Newton's laws. 433 00:22:10,350 --> 00:22:14,910 I mean Newton's laws actually, I mean Newton's theory actually 434 00:22:14,910 --> 00:22:17,620 comes out as a limiting case of relativity. 435 00:22:17,620 --> 00:22:19,050 And Einstein showed that. 436 00:22:19,050 --> 00:22:28,242 So 1905, Einstein proposed the special theory of relativity. 437 00:22:28,242 --> 00:22:30,166 [SCRIBBLES] 438 00:22:38,343 --> 00:22:42,830 The theory with two postulates, and it basically 439 00:22:42,830 --> 00:22:46,429 solved this discrepancy between Newton and Maxwell. 440 00:22:46,429 --> 00:22:48,470 Last week I talked about some of the consequences 441 00:22:48,470 --> 00:22:50,066 of special relativity. 442 00:22:50,066 --> 00:22:52,190 For example, I talked about time dilation, the fact 443 00:22:52,190 --> 00:22:56,160 that clocks that are in motion go slow. 444 00:22:56,160 --> 00:23:00,090 I also mentioned length contraction. 445 00:23:00,090 --> 00:23:05,420 A moving stick is slower than one standing still. 446 00:23:05,420 --> 00:23:09,390 I also mentioned that simultaneity is relative. 447 00:23:09,390 --> 00:23:11,900 The fact that if I observe two things happening 448 00:23:11,900 --> 00:23:15,560 at the same time like [SNAPS] both of my fingers snapping 449 00:23:15,560 --> 00:23:16,700 at the same time. 450 00:23:16,700 --> 00:23:19,104 That doesn't necessarily mean that you will. 451 00:23:19,104 --> 00:23:21,020 If you're moving at some speed relative to me, 452 00:23:21,020 --> 00:23:22,970 it might not happen at the same time. 453 00:23:22,970 --> 00:23:27,110 Simultaneity is also relative, so everything is relative. 454 00:23:27,110 --> 00:23:29,030 Well, a lot of things are relative. 455 00:23:29,030 --> 00:23:32,050 But the speed of light, you remember, is not relative. 456 00:23:32,050 --> 00:23:34,070 The speed of light is the same for everybody. 457 00:23:34,070 --> 00:23:36,720 But anyway that was the special theory of relativity, 458 00:23:36,720 --> 00:23:39,230 which I talked about last time. 459 00:23:39,230 --> 00:23:42,950 Then about 10 years later, 10 years or so later, 460 00:23:42,950 --> 00:23:47,466 Einstein came up with his general theory of relativity. 461 00:23:47,466 --> 00:23:50,418 [CHANTING LOUDLY] 462 00:23:53,370 --> 00:23:55,830 [SCRIBBLING ON CHALK BOARD] 463 00:23:58,790 --> 00:24:00,890 And that was the theory of gravity remember. 464 00:24:08,190 --> 00:24:10,890 Special relativity does not take into account things 465 00:24:10,890 --> 00:24:11,940 that accelerate. 466 00:24:11,940 --> 00:24:14,820 It doesn't work for accelerated observers, 467 00:24:14,820 --> 00:24:16,950 and it can't explain gravity. 468 00:24:16,950 --> 00:24:19,800 But general relativity can explain gravity. 469 00:24:19,800 --> 00:24:21,420 And, in fact, it's a better theory 470 00:24:21,420 --> 00:24:23,790 of gravity than Newton's law of gravity. 471 00:24:23,790 --> 00:24:27,000 And you remember in relativity, space and time 472 00:24:27,000 --> 00:24:30,500 are unified as a fundamental object called spacetime. 473 00:24:30,500 --> 00:24:32,650 They're linked to each other. 474 00:24:32,650 --> 00:24:35,130 Whereas according to Newton, time and space 475 00:24:35,130 --> 00:24:36,480 are separate things. 476 00:24:36,480 --> 00:24:38,190 They're absolute. 477 00:24:38,190 --> 00:24:45,750 According to Newton, if I measure the duration of a song 478 00:24:45,750 --> 00:24:48,060 to be three minutes, then everybody 479 00:24:48,060 --> 00:24:50,280 will measure that song to be three minutes long. 480 00:24:50,280 --> 00:24:54,060 But according to Einstein, if you're moving at some speed 481 00:24:54,060 --> 00:24:56,070 relative to me, then maybe you'll 482 00:24:56,070 --> 00:25:04,183 measure it to be shorter, or perhaps, longer. 483 00:25:08,430 --> 00:25:11,550 I imagined it to be three minutes, yeah, 484 00:25:11,550 --> 00:25:12,715 then it would be longer. 485 00:25:12,715 --> 00:25:14,340 That song would be longer, because time 486 00:25:14,340 --> 00:25:15,490 would be slowed for me. 487 00:25:15,490 --> 00:25:18,480 Remember my clock will tick slower to a moving observer. 488 00:25:18,480 --> 00:25:19,980 And so, therefore, it takes it takes 489 00:25:19,980 --> 00:25:23,760 longer for the stationary observer for the whole song 490 00:25:23,760 --> 00:25:25,200 to finish. 491 00:25:25,200 --> 00:25:28,309 So time is relative-- 492 00:25:28,309 --> 00:25:30,600 time is not absolute in Einstein's theory of relativity 493 00:25:30,600 --> 00:25:33,820 and neither is space. 494 00:25:33,820 --> 00:25:35,190 So that's relativity. 495 00:25:35,190 --> 00:25:37,260 That was the first half of the physics revolution 496 00:25:37,260 --> 00:25:41,380 that happened in the 20th century. 497 00:25:41,380 --> 00:25:48,990 In the 1920s, there was another revolution 498 00:25:48,990 --> 00:25:50,430 in physics that happened. 499 00:25:50,430 --> 00:25:53,020 And this was the development of quantum mechanics, 500 00:25:53,020 --> 00:25:55,747 which is the theory that describes the very small. 501 00:25:55,747 --> 00:25:57,330 It's the theory that's been enormously 502 00:25:57,330 --> 00:26:00,480 successful in explaining the very small, atoms, 503 00:26:00,480 --> 00:26:05,160 protons, neutrons, electrons, very small. 504 00:26:05,160 --> 00:26:08,314 This was developed in 1920S, so, there wasn't just one guy 505 00:26:08,314 --> 00:26:09,730 associated with quantum mechanics. 506 00:26:09,730 --> 00:26:14,910 It was a number of people, people like Erwin Schrodinger, 507 00:26:14,910 --> 00:26:17,880 [WRITING] 508 00:26:21,260 --> 00:26:29,230 Werner Heisenberg, and other than other people. 509 00:26:29,230 --> 00:26:35,180 And this was in the 1920s. 510 00:26:35,180 --> 00:26:37,240 Schrodinger from Schrodinger's cat, 511 00:26:37,240 --> 00:26:38,820 remember you put the cat in the box 512 00:26:38,820 --> 00:26:42,599 and put the radioactive poison in there. 513 00:26:42,599 --> 00:26:44,140 They developed the quantum mechanics. 514 00:26:49,972 --> 00:26:52,888 [SQUEAKING] 515 00:27:00,536 --> 00:27:03,390 I mentioned quantum mechanics briefly 516 00:27:03,390 --> 00:27:05,670 when I talked about parallel universes. 517 00:27:05,670 --> 00:27:09,120 In particular, the level three multiverse 518 00:27:09,120 --> 00:27:11,965 where the parallel universe, in that case, 519 00:27:11,965 --> 00:27:14,340 corresponded to the many worlds interpretation of quantum 520 00:27:14,340 --> 00:27:17,680 mechanics. 521 00:27:17,680 --> 00:27:20,474 Now I'll just I'll just briefly remind you 522 00:27:20,474 --> 00:27:22,390 of some of the things about quantum mechanics. 523 00:27:22,390 --> 00:27:26,100 I'm not going to try to do justice to quantum mechanics, 524 00:27:26,100 --> 00:27:29,480 but I'll just say a few essential things. 525 00:27:29,480 --> 00:27:32,660 So according to quantum mechanics, 526 00:27:32,660 --> 00:27:34,830 there are objects in the universe, 527 00:27:34,830 --> 00:27:37,140 and each object has its states. 528 00:27:37,140 --> 00:27:38,490 Each object has quantum states. 529 00:27:41,430 --> 00:27:45,385 And this states they describe objects with 530 00:27:45,385 --> 00:27:47,010 is completely different from the states 531 00:27:47,010 --> 00:27:50,110 that you use in Newton's theory or Einstein's theory. 532 00:27:50,110 --> 00:27:52,470 It's completely different. 533 00:27:52,470 --> 00:27:54,870 According to pre-quantum physics, 534 00:27:54,870 --> 00:27:57,774 the physics before quantum mechanics, 535 00:27:57,774 --> 00:27:59,190 the way you can describe an object 536 00:27:59,190 --> 00:28:03,120 is by using what's called the classical states. 537 00:28:03,120 --> 00:28:13,430 The classical states is just an object's speed and position. 538 00:28:17,080 --> 00:28:19,220 If you know an object's speed and location, 539 00:28:19,220 --> 00:28:20,850 then you know everything about it. 540 00:28:20,850 --> 00:28:21,710 That's the classical state. 541 00:28:21,710 --> 00:28:22,668 The object has a speed. 542 00:28:22,668 --> 00:28:24,260 It has a location. 543 00:28:24,260 --> 00:28:28,039 Then you can learn it, and then you learn the classical state. 544 00:28:28,039 --> 00:28:30,080 It might be that you have a hard time determining 545 00:28:30,080 --> 00:28:31,430 what that state is. 546 00:28:31,430 --> 00:28:32,750 You can have a hard time figuring out the speed, 547 00:28:32,750 --> 00:28:34,874 maybe it moves very fast, or maybe it's very small, 548 00:28:34,874 --> 00:28:37,450 you have a hard time figuring out the location. 549 00:28:37,450 --> 00:28:39,800 But you can rest assured that it definitely 550 00:28:39,800 --> 00:28:41,630 has a speed and position. 551 00:28:41,630 --> 00:28:44,415 That's something that we can all be happy with. 552 00:28:44,415 --> 00:28:45,540 That's the classical state. 553 00:28:48,610 --> 00:28:49,962 The quantum state 554 00:28:49,962 --> 00:28:52,914 [WRITING] 555 00:28:58,340 --> 00:29:00,790 Is completely different from the classical state. 556 00:29:00,790 --> 00:29:02,840 Objects no longer have speeds and they no longer 557 00:29:02,840 --> 00:29:05,198 have positions. 558 00:29:05,198 --> 00:29:07,134 [WRITING] 559 00:29:13,940 --> 00:29:19,550 An object doesn't have a speed, it doesn't have a position. 560 00:29:19,550 --> 00:29:22,130 What it does have is some kind of a mixture between positions 561 00:29:22,130 --> 00:29:24,645 and speeds, and remember this is called the superposition. 562 00:29:24,645 --> 00:29:26,420 It does have superpositions. 563 00:29:33,800 --> 00:29:36,590 And remember these were mixtures of states, 564 00:29:36,590 --> 00:29:38,960 mixtures of definite states. 565 00:29:38,960 --> 00:29:41,390 So while an electron might not have a speed. 566 00:29:41,390 --> 00:29:43,880 Just doesn't have a speed, doesn't have that property, 567 00:29:43,880 --> 00:29:46,400 or have a location. 568 00:29:46,400 --> 00:29:48,100 It does have a mixture of those. 569 00:29:48,100 --> 00:29:50,558 It has a mixture of definite speeds and definite locations, 570 00:29:50,558 --> 00:29:53,000 and that's called a superposition. 571 00:29:53,000 --> 00:29:57,380 Now this is only true if you're not measuring the object. 572 00:29:57,380 --> 00:30:00,050 Once you measure the object, you give it a definite speed, 573 00:30:00,050 --> 00:30:02,024 or you give it a definite location. 574 00:30:02,024 --> 00:30:03,440 So an object doesn't have a speed, 575 00:30:03,440 --> 00:30:07,760 and it doesn't have a location until you measure it. 576 00:30:07,760 --> 00:30:09,462 And the weird thing is that you never 577 00:30:09,462 --> 00:30:11,420 know what that speed or position is going to be 578 00:30:11,420 --> 00:30:14,960 until you make a measurement. 579 00:30:14,960 --> 00:30:16,790 You have no idea what it's going to be. 580 00:30:16,790 --> 00:30:19,340 You have an uncertainty in the future. 581 00:30:19,340 --> 00:30:22,790 There's a fundamental random aspect to nature 582 00:30:22,790 --> 00:30:24,772 that quantum mechanics gives the world. 583 00:30:24,772 --> 00:30:26,480 You don't know with certainty what you're 584 00:30:26,480 --> 00:30:27,170 going to get in a measurement. 585 00:30:27,170 --> 00:30:29,670 You only know that you might get something or something else 586 00:30:29,670 --> 00:30:31,250 with some probability, and that's it. 587 00:30:34,610 --> 00:30:36,650 So that's the catch. 588 00:30:36,650 --> 00:30:37,760 It's very weird. 589 00:30:37,760 --> 00:30:39,470 Yeah, I know it's very weird, but that's 590 00:30:39,470 --> 00:30:44,330 the way that the world seems. 591 00:30:44,330 --> 00:30:49,460 Quantum mechanics and relativity are both 592 00:30:49,460 --> 00:30:51,170 the two most successful physical theories 593 00:30:51,170 --> 00:30:53,840 that have ever been proposed. 594 00:30:53,840 --> 00:30:57,629 And we just have to live with their weirdness. 595 00:30:57,629 --> 00:30:58,920 We just have to live with them. 596 00:31:04,480 --> 00:31:11,200 OK, now while every experiment and every observation 597 00:31:11,200 --> 00:31:14,620 has confirmed predictions of quantum mechanics or relativity 598 00:31:14,620 --> 00:31:15,880 with flying colors. 599 00:31:15,880 --> 00:31:18,220 I mean we can test these theories 600 00:31:18,220 --> 00:31:20,200 past the 10th decimal point. 601 00:31:20,200 --> 00:31:23,240 I mean we know that they're accurate-- 602 00:31:23,240 --> 00:31:31,180 we can make a prediction of something within 0.00000000-- 603 00:31:31,180 --> 00:31:34,110 approximately that many zeros and then a 1. 604 00:31:34,110 --> 00:31:37,100 That's how good it is. 605 00:31:37,100 --> 00:31:39,030 It's super accurate, super precise. 606 00:31:39,030 --> 00:31:42,370 They're pretty good theories, the best theories you've ever 607 00:31:42,370 --> 00:31:44,840 had to explain the world. 608 00:31:44,840 --> 00:31:49,910 Unfortunately, we know that they can't possibly be right. 609 00:31:49,910 --> 00:31:51,040 They can't be right. 610 00:31:51,040 --> 00:31:53,610 They can't be completely right. 611 00:31:53,610 --> 00:31:57,226 And that's because when you look at the two theories, 612 00:31:57,226 --> 00:32:00,360 when you look at the equations that describe the two theories, 613 00:32:00,360 --> 00:32:04,440 you find that they contradict each other. 614 00:32:04,440 --> 00:32:06,850 I mean deep down at a conceptual level, 615 00:32:06,850 --> 00:32:10,210 everything in quantum mechanics fluctuates. 616 00:32:13,090 --> 00:32:17,010 Remember there's a random aspect to the way that things changed. 617 00:32:17,010 --> 00:32:17,920 So things fluctuate. 618 00:32:17,920 --> 00:32:19,060 Things randomly fluctuate. 619 00:32:19,060 --> 00:32:22,430 They're called quantum fluctuations. 620 00:32:22,430 --> 00:32:24,730 Now relativity is a theory of space and time. 621 00:32:24,730 --> 00:32:27,460 It's the theory of spacetime. 622 00:32:27,460 --> 00:32:30,100 Now if you want to make a quantum theory of spacetime, 623 00:32:30,100 --> 00:32:31,990 then spacetime should also fluctuate. 624 00:32:31,990 --> 00:32:35,710 That's a straightforward way of turning general relativity 625 00:32:35,710 --> 00:32:38,050 into a quantum theory. 626 00:32:38,050 --> 00:32:40,230 In a straightforward way of combining the theory, 627 00:32:40,230 --> 00:32:42,280 spacetime should also fluctuate. 628 00:32:42,280 --> 00:32:44,260 But according to general relativity 629 00:32:44,260 --> 00:32:46,880 spacetime is a fixed, rigid thing. 630 00:32:46,880 --> 00:32:49,090 I mean it can change as-- spacetime can certainly 631 00:32:49,090 --> 00:32:52,450 change as with matter and energy, 632 00:32:52,450 --> 00:32:56,536 cause different geometries of the universe. 633 00:32:56,536 --> 00:32:59,350 Remember I talked about the curvature of spacetime. 634 00:32:59,350 --> 00:33:02,320 The more mass there is some place, the more curved it is. 635 00:33:02,320 --> 00:33:05,770 So spacetime can change that way. 636 00:33:05,770 --> 00:33:08,320 But there's an intrinsic way that spacetime 637 00:33:08,320 --> 00:33:10,910 can change that quantum mechanics says it must change. 638 00:33:10,910 --> 00:33:13,270 But general relativity says it can't change like that. 639 00:33:13,270 --> 00:33:14,770 So you're bound to run into problems 640 00:33:14,770 --> 00:33:17,114 when you try to combine the two theories, and you do. 641 00:33:17,114 --> 00:33:18,280 That's exactly what happens. 642 00:33:18,280 --> 00:33:19,696 You run into problems when you try 643 00:33:19,696 --> 00:33:21,460 to combine the two theories. 644 00:33:21,460 --> 00:33:26,410 For example, if you ask, OK, suppose 645 00:33:26,410 --> 00:33:28,360 I try to measure something. 646 00:33:28,360 --> 00:33:30,370 Remember, quantum mechanics, you only 647 00:33:30,370 --> 00:33:32,454 have a probability that you measure something. 648 00:33:32,454 --> 00:33:34,620 You don't know for certainty that you'll measure it. 649 00:33:34,620 --> 00:33:36,970 So you calculate the probability. 650 00:33:36,970 --> 00:33:39,130 Probabilities are between zero and 100%, 651 00:33:39,130 --> 00:33:41,239 or between zero and one, so you should 652 00:33:41,239 --> 00:33:43,030 expect to get a number between zero and one 653 00:33:43,030 --> 00:33:45,452 when you calculate a probability. 654 00:33:45,452 --> 00:33:47,410 Well, when you try to combine quantum mechanics 655 00:33:47,410 --> 00:33:52,070 and general relativity, then you get infinite probabilities. 656 00:33:52,070 --> 00:33:55,830 What's an infinite possibility? 657 00:33:55,830 --> 00:33:57,430 You get nonsensical answers. 658 00:33:57,430 --> 00:34:00,514 I mean a probability has to be-- it has to be between 0 and 1. 659 00:34:00,514 --> 00:34:01,930 So what's an infinite probability? 660 00:34:01,930 --> 00:34:04,280 It just doesn't make any sense. 661 00:34:04,280 --> 00:34:09,280 So our two theories, when you combine the two, 662 00:34:09,280 --> 00:34:12,989 they give nonsensical answers, even though by themselves, they 663 00:34:12,989 --> 00:34:14,050 make sense. 664 00:34:14,050 --> 00:34:17,800 And they describe experiments and observations well. 665 00:34:17,800 --> 00:34:22,000 So we have a big problem here. 666 00:34:22,000 --> 00:34:24,920 We have this fundamental conflict. 667 00:34:24,920 --> 00:34:30,480 So it seems we need some kind of a new theory 668 00:34:30,480 --> 00:34:36,370 to describe nature, some kind of theory that 669 00:34:36,370 --> 00:34:38,290 combines elements of quantum theory 670 00:34:38,290 --> 00:34:40,250 and elements of general relativity. 671 00:34:40,250 --> 00:34:44,634 So we need a theory of quantum gravity. 672 00:34:44,634 --> 00:34:47,604 [WRITING] 673 00:35:00,480 --> 00:35:02,400 A quantum theory of gravity. 674 00:35:05,710 --> 00:35:09,150 Now I should say from the outset that nobody knows, 675 00:35:09,150 --> 00:35:11,490 nobody in the world knows what the correct theory 676 00:35:11,490 --> 00:35:13,650 of quantum gravity is. 677 00:35:13,650 --> 00:35:15,960 Nobody knows. 678 00:35:15,960 --> 00:35:18,750 But people have over the years tried to come up with theories. 679 00:35:18,750 --> 00:35:21,930 Theoretical physicists have worked on theories. 680 00:35:21,930 --> 00:35:23,971 And probably the most promising theory 681 00:35:23,971 --> 00:35:25,470 that people are currently working on 682 00:35:25,470 --> 00:35:27,984 is called string theory. 683 00:35:27,984 --> 00:35:29,400 Maybe it's not the most promising, 684 00:35:29,400 --> 00:35:31,108 but at least it's the most popular theory 685 00:35:31,108 --> 00:35:32,850 that people are working on. 686 00:35:32,850 --> 00:35:37,080 And so for the rest of today I'll talk about string theory, 687 00:35:37,080 --> 00:35:40,305 and then I'll briefly talk a bit about some other theories 688 00:35:40,305 --> 00:35:41,430 that people are working on. 689 00:35:41,430 --> 00:35:43,620 Because its not the only theory that people are working on. 690 00:35:43,620 --> 00:35:44,994 But it's the most popular theory, 691 00:35:44,994 --> 00:35:47,130 and people think it's cool. 692 00:35:47,130 --> 00:35:49,170 It's fun to talk about. 693 00:35:49,170 --> 00:35:51,450 I mean you've probably heard about it 694 00:35:51,450 --> 00:35:57,660 from TV specials, or newspapers, or whatever, 695 00:35:57,660 --> 00:35:59,940 because it's very strange. 696 00:35:59,940 --> 00:36:02,560 So I'll talk about that after the break. 697 00:36:02,560 --> 00:36:06,250 So we'll take an early break today for five minutes, 698 00:36:06,250 --> 00:36:09,550 and then we'll reconvene. 699 00:36:09,550 --> 00:36:13,530 OK so string theory is a theory of quantum gravity 700 00:36:13,530 --> 00:36:14,750 that people have proposed. 701 00:36:17,659 --> 00:36:19,200 It's probably the most popular theory 702 00:36:19,200 --> 00:36:20,575 that people are working on today. 703 00:36:23,842 --> 00:36:25,425 It seems like it's the most promising, 704 00:36:25,425 --> 00:36:27,050 but that's not for me to say. 705 00:36:27,050 --> 00:36:29,490 I'm not an expert. 706 00:36:29,490 --> 00:36:31,680 So according to string theory, everything 707 00:36:31,680 --> 00:36:40,960 is made up of extremely tiny objects called strings. 708 00:36:40,960 --> 00:36:44,830 So calling it string theory, every object in the universe, 709 00:36:44,830 --> 00:36:50,860 every person, every atom, every proton, every electron, 710 00:36:50,860 --> 00:36:52,780 every quark, every photon, they're 711 00:36:52,780 --> 00:36:57,580 all made up of extremely tiny objects called strings. 712 00:36:57,580 --> 00:37:02,296 These strings are many, many times smaller than atoms, 713 00:37:02,296 --> 00:37:03,670 but they have properties that are 714 00:37:03,670 --> 00:37:08,910 similar to the types of strings that we all know, 715 00:37:08,910 --> 00:37:10,570 everyday strings. 716 00:37:10,570 --> 00:37:13,110 For example, I can have an open string, 717 00:37:13,110 --> 00:37:15,560 like an open rubber band. 718 00:37:15,560 --> 00:37:17,130 I can stretch it. 719 00:37:17,130 --> 00:37:18,700 I can have-- 720 00:37:18,700 --> 00:37:22,420 I have, sorry, this is a closed string. 721 00:37:22,420 --> 00:37:23,890 Because it closes on itself. 722 00:37:23,890 --> 00:37:25,630 It's closed. 723 00:37:25,630 --> 00:37:29,316 I can also have an open string like in everyday life. 724 00:37:29,316 --> 00:37:30,940 And in string theory, there are objects 725 00:37:30,940 --> 00:37:32,560 that are similar to everyday strings. 726 00:37:32,560 --> 00:37:35,320 They can be closed or open, similar to the sense 727 00:37:35,320 --> 00:37:36,292 that I just showed you. 728 00:37:40,080 --> 00:37:43,930 You can also stretch everyday strings. 729 00:37:43,930 --> 00:37:47,120 And once you stretch a string, it will pull back on itself. 730 00:37:47,120 --> 00:37:50,440 There's a tension. 731 00:37:50,440 --> 00:37:51,940 It pulls back, pulls back on itself. 732 00:37:51,940 --> 00:37:52,930 There's a tension. 733 00:37:52,930 --> 00:37:56,920 The strings of string theory also have a tension. 734 00:37:56,920 --> 00:38:01,150 Now here's the remarkable thing about string theory. 735 00:38:01,150 --> 00:38:04,330 So for everyday strings, if I take it and I stretch in 736 00:38:04,330 --> 00:38:07,540 a certain way to produce a sound, 737 00:38:07,540 --> 00:38:11,714 depending on how I pluck it, I can get a different sound. 738 00:38:11,714 --> 00:38:13,630 So depending on how I make the string vibrate, 739 00:38:13,630 --> 00:38:15,260 I can get a different musical note. 740 00:38:15,260 --> 00:38:18,640 That's just a simple thing that we've all observed before. 741 00:38:21,280 --> 00:38:24,580 If you haven't, then I feel sorry for those of you 742 00:38:24,580 --> 00:38:26,760 who have had childhoods deprived of rubber bands. 743 00:38:26,760 --> 00:38:28,510 Because they're a lot of fun to play with. 744 00:38:28,510 --> 00:38:30,760 You can learn a lot about the nature of reality 745 00:38:30,760 --> 00:38:32,310 by playing with rubber bands 746 00:38:32,310 --> 00:38:33,450 [THRUMS] 747 00:38:33,450 --> 00:38:37,012 Now with rubber bands and everyday strings, 748 00:38:37,012 --> 00:38:38,470 you can get different musical notes 749 00:38:38,470 --> 00:38:40,053 by stretching it and making the string 750 00:38:40,053 --> 00:38:41,180 vibrate in a certain way. 751 00:38:41,180 --> 00:38:44,842 With string theory, you can also have 752 00:38:44,842 --> 00:38:46,050 different kinds of vibration. 753 00:38:46,050 --> 00:38:47,864 You can have different modes of vibration. 754 00:38:47,864 --> 00:38:49,780 And in string theory, the different vibrations 755 00:38:49,780 --> 00:38:51,613 don't correspond to different musical notes, 756 00:38:51,613 --> 00:38:53,620 they correspond to different particles 757 00:38:53,620 --> 00:38:56,270 that we observe in the universe. 758 00:38:56,270 --> 00:39:00,490 So one kind of vibration could lead to a proton. 759 00:39:00,490 --> 00:39:03,410 Another kind of vibration can lead to an electron, 760 00:39:03,410 --> 00:39:09,322 or another kind of vibration can lead to her-- 761 00:39:09,322 --> 00:39:11,010 [LAUGHS] 762 00:39:11,010 --> 00:39:16,410 Well, many strings vibrating in a certain way, not just one. 763 00:39:16,410 --> 00:39:19,260 So string theory actually explains where 764 00:39:19,260 --> 00:39:20,920 all these particles come from. 765 00:39:20,920 --> 00:39:23,970 It explains a wide diversity of particles. 766 00:39:23,970 --> 00:39:26,970 Particle physicists have long observed a number 767 00:39:26,970 --> 00:39:28,590 of fundamental particles. 768 00:39:28,590 --> 00:39:31,070 There are about 60 of them. 769 00:39:31,070 --> 00:39:32,700 Or about half of them are anti-matter, 770 00:39:32,700 --> 00:39:37,110 so 30 fundamental particles in the universe. 771 00:39:37,110 --> 00:39:39,330 And we just we just observe the stuff. 772 00:39:39,330 --> 00:39:41,820 We don't really know now where they come from. 773 00:39:41,820 --> 00:39:44,790 String theory has a very elegant answer to that question. 774 00:39:44,790 --> 00:39:47,910 String theory says that the particles are just 775 00:39:47,910 --> 00:39:51,010 different ways that fundamental strings vibrate. 776 00:39:51,010 --> 00:39:53,670 That's it. 777 00:39:53,670 --> 00:39:58,440 So how does string theory solve the problem of quantum gravity? 778 00:39:58,440 --> 00:40:04,210 How does it reconcile quantum mechanics and relativity? 779 00:40:04,210 --> 00:40:10,960 Well, it turns out that the way some of these strings vibrate 780 00:40:10,960 --> 00:40:11,580 is the key. 781 00:40:14,760 --> 00:40:16,590 So if a string vibrates in a certain way, 782 00:40:16,590 --> 00:40:19,860 you can actually get a graviton, which the particle that 783 00:40:19,860 --> 00:40:22,520 transmits the force of gravity. 784 00:40:22,520 --> 00:40:25,740 So let me let me just backtrack a little bit. 785 00:40:25,740 --> 00:40:29,520 So in particle physics, in modern particle physics, 786 00:40:29,520 --> 00:40:32,638 we have things called messenger particles. 787 00:40:42,636 --> 00:40:43,385 Or force carriers. 788 00:40:52,600 --> 00:40:55,870 Gravity is a force that we've all heard of. 789 00:40:55,870 --> 00:41:00,400 Every object that has mass attracts every other object. 790 00:41:00,400 --> 00:41:02,542 You've all heard of electricity and magnetism, 791 00:41:02,542 --> 00:41:03,750 or the electromagnetic force. 792 00:41:03,750 --> 00:41:05,500 That's another force that we've heard of. 793 00:41:05,500 --> 00:41:09,500 So those are two forces in nature. 794 00:41:09,500 --> 00:41:11,800 But it turns out if you try to explain everything 795 00:41:11,800 --> 00:41:15,190 that we've ever observed, you need two more forces 796 00:41:15,190 --> 00:41:18,140 in addition to those. 797 00:41:18,140 --> 00:41:20,770 So if you look inside the atomic nucleus, 798 00:41:20,770 --> 00:41:22,810 then you see protons and neutrons. 799 00:41:22,810 --> 00:41:25,840 And you might wonder why is the nucleus held together. 800 00:41:25,840 --> 00:41:28,690 After all protons are positively charged 801 00:41:28,690 --> 00:41:30,880 and positive charges repel each other. 802 00:41:30,880 --> 00:41:33,700 That's something that's true. 803 00:41:33,700 --> 00:41:37,060 That's something that we learned from electricity and magnetism. 804 00:41:37,060 --> 00:41:39,700 You might wonder why are nuclei held together? 805 00:41:39,700 --> 00:41:41,440 They should repel from each other. 806 00:41:41,440 --> 00:41:44,410 Protons repel other protons. 807 00:41:44,410 --> 00:41:47,000 They're surely not attracted to neutrons. 808 00:41:47,000 --> 00:41:48,920 Neutrons don't have any charge at all. 809 00:41:48,920 --> 00:41:51,010 So why are nuclei held together? 810 00:41:51,010 --> 00:41:53,950 Well, it turns out that there's another force, completely 811 00:41:53,950 --> 00:41:55,840 different from electromagnetism, completely 812 00:41:55,840 --> 00:41:57,220 different from gravity. 813 00:41:57,220 --> 00:42:01,840 I mean, yeah, sure, gravitationally, the protons 814 00:42:01,840 --> 00:42:04,480 and neutrons in the nuclei attract each other, 815 00:42:04,480 --> 00:42:05,830 but it's so small. 816 00:42:05,830 --> 00:42:07,960 Gravity is this is the weakest force. 817 00:42:07,960 --> 00:42:11,030 It's much weaker than electromagnetism. 818 00:42:11,030 --> 00:42:12,820 So gravity alone can't account for why 819 00:42:12,820 --> 00:42:13,917 nuclei are held together. 820 00:42:13,917 --> 00:42:16,000 But it turns out that there's another force called 821 00:42:16,000 --> 00:42:18,970 the strong nuclear force that explains 822 00:42:18,970 --> 00:42:21,986 why nuclei are held together. 823 00:42:21,986 --> 00:42:24,110 There's this force called the strong nuclear force, 824 00:42:24,110 --> 00:42:26,800 and that's what it does. 825 00:42:26,800 --> 00:42:29,560 It turns out there's another nuclear force called 826 00:42:29,560 --> 00:42:31,380 the weak nuclear force. 827 00:42:31,380 --> 00:42:34,120 And that's responsible for radioactivity. 828 00:42:34,120 --> 00:42:37,160 It explains certain types of decay. 829 00:42:37,160 --> 00:42:40,670 So there are four fundamental forces that we know about. 830 00:42:40,670 --> 00:42:56,030 There is gravity, electromagnetism, 831 00:42:56,030 --> 00:43:00,105 the strong nuclear force, actually-- 832 00:43:00,105 --> 00:43:01,460 OK, the strong nuclear force. 833 00:43:08,436 --> 00:43:09,560 And the weak nuclear force. 834 00:43:18,050 --> 00:43:20,010 I guess this is kind of arranged in order 835 00:43:20,010 --> 00:43:22,620 of historical discovery. 836 00:43:22,620 --> 00:43:25,860 It turns out that the strong nuclear force is the strongest 837 00:43:25,860 --> 00:43:27,240 force of all these. 838 00:43:27,240 --> 00:43:30,120 Strong force is the strongest one. 839 00:43:30,120 --> 00:43:32,190 The second strongest is electromagnetism. 840 00:43:32,190 --> 00:43:34,500 The third strongest is the weak nuclear force, 841 00:43:34,500 --> 00:43:38,070 and the weakest, the fourth strongest, is gravity. 842 00:43:38,070 --> 00:43:42,670 These are the four fundamental forces of nature. 843 00:43:42,670 --> 00:43:44,880 Now according to particle physics, 844 00:43:44,880 --> 00:43:47,190 each one of these forces is carried 845 00:43:47,190 --> 00:43:48,990 by a messenger particle. 846 00:43:48,990 --> 00:43:53,230 It's carried by some kind of messenger particle. 847 00:43:53,230 --> 00:43:57,620 So, for example, you can have an electron here. 848 00:43:57,620 --> 00:43:59,490 And a proton here. 849 00:43:59,490 --> 00:44:02,715 So electron and proton. 850 00:44:06,350 --> 00:44:10,010 And they're opposite charges, so they should attract each other. 851 00:44:10,010 --> 00:44:11,870 Why do they attract each other? 852 00:44:11,870 --> 00:44:18,470 Well, it's because there's a messenger particle transmitted 853 00:44:18,470 --> 00:44:19,970 by one of them to another one. 854 00:44:19,970 --> 00:44:24,820 And essentially, it tells them to attract. 855 00:44:24,820 --> 00:44:26,810 It just tells them to attract. 856 00:44:26,810 --> 00:44:29,660 It gets sent from one to the other and says, suppose-- 857 00:44:29,660 --> 00:44:31,640 I mean one way of visualizing it, 858 00:44:31,640 --> 00:44:35,180 like a cartoonish way of visualizing it, for example, is 859 00:44:35,180 --> 00:44:38,290 that the proton sends a messenger 860 00:44:38,290 --> 00:44:41,180 particle to the electron and then 861 00:44:41,180 --> 00:44:44,000 tells the electron, hey, come this way. 862 00:44:44,000 --> 00:44:48,020 It just tells it the instruction. 863 00:44:48,020 --> 00:44:51,470 It turns out that the messenger particle for electromagnetism 864 00:44:51,470 --> 00:44:53,300 is the photon. 865 00:44:53,300 --> 00:44:55,524 And this is the symbol that we give for a photon. 866 00:44:55,524 --> 00:44:57,440 So the messenger particle for electromagnetism 867 00:44:57,440 --> 00:44:58,315 is called the photon. 868 00:45:01,400 --> 00:45:04,620 It just tells charged particles what to do. 869 00:45:04,620 --> 00:45:06,230 Attract or repel. 870 00:45:06,230 --> 00:45:08,780 If this was an electron, the messenger particle 871 00:45:08,780 --> 00:45:10,856 would say repel. 872 00:45:10,856 --> 00:45:12,230 There's also a messenger particle 873 00:45:12,230 --> 00:45:13,970 for the strong nuclear force. 874 00:45:13,970 --> 00:45:16,352 It's called a gluon. 875 00:45:16,352 --> 00:45:17,810 There are three messenger particles 876 00:45:17,810 --> 00:45:19,210 for the weak nuclear force. 877 00:45:19,210 --> 00:45:22,750 They're called W and Z bosons. 878 00:45:26,980 --> 00:45:30,100 And theoretically, there's a messenger particle for gravity, 879 00:45:30,100 --> 00:45:31,422 and it's called a graviton. 880 00:45:35,760 --> 00:45:38,370 We've observed all of these. 881 00:45:38,370 --> 00:45:40,610 We observed all of these messenger particles. 882 00:45:40,610 --> 00:45:43,142 We've never actually observed a graviton. 883 00:45:43,142 --> 00:45:45,100 But in principle we'd like to think that they-- 884 00:45:45,100 --> 00:45:47,140 I mean they should exist. 885 00:45:47,140 --> 00:45:49,460 All these other forces transmit forces-- 886 00:45:49,460 --> 00:45:52,717 all these other forces transmit messenger particles, 887 00:45:52,717 --> 00:45:55,300 and we've observed them, but we haven't observed the graviton, 888 00:45:55,300 --> 00:45:57,310 simply because gravity is just so weak. 889 00:45:57,310 --> 00:45:58,870 It's just so weak, so much weaker 890 00:45:58,870 --> 00:46:00,380 than all these other forces. 891 00:46:00,380 --> 00:46:02,680 But in principle it should exist. 892 00:46:02,680 --> 00:46:08,260 So that's the way that forces are transmitted 893 00:46:08,260 --> 00:46:10,780 between particles in our modern particle physics 894 00:46:10,780 --> 00:46:14,110 way of looking at forces. 895 00:46:14,110 --> 00:46:16,384 Now according to string theory, if you 896 00:46:16,384 --> 00:46:17,800 work through the equations, if you 897 00:46:17,800 --> 00:46:19,990 work through the different ways that things can 898 00:46:19,990 --> 00:46:27,100 vibrate, then it turns out that you get a graviton. 899 00:46:27,100 --> 00:46:29,020 It just pops out. 900 00:46:29,020 --> 00:46:31,120 One mode of vibration corresponds 901 00:46:31,120 --> 00:46:32,580 to the graviton particle. 902 00:46:32,580 --> 00:46:34,750 And it has all the properties of being a graviton. 903 00:46:34,750 --> 00:46:35,791 It looks like a graviton. 904 00:46:35,791 --> 00:46:37,240 It feels like a graviton. 905 00:46:37,240 --> 00:46:38,940 Maybe it really is a graviton. 906 00:46:38,940 --> 00:46:39,815 We also get a photon. 907 00:46:39,815 --> 00:46:41,731 And you get all the other messenger particles. 908 00:46:41,731 --> 00:46:43,390 It just pops out of the equations. 909 00:46:43,390 --> 00:46:47,530 So string theory is not only a quantum theory of gravity, 910 00:46:47,530 --> 00:46:50,530 not only does the gravitation pop out. 911 00:46:50,530 --> 00:46:53,770 But all these other forces pop out. 912 00:46:53,770 --> 00:46:59,230 So what, so what is there in the universe besides matter 913 00:46:59,230 --> 00:47:00,850 and forces? 914 00:47:00,850 --> 00:47:03,640 I mean that's pretty much it. 915 00:47:03,640 --> 00:47:06,130 Are we thinking, in principle, to be described in terms 916 00:47:06,130 --> 00:47:08,014 of the two, matter and forces. 917 00:47:08,014 --> 00:47:09,930 Matter interacts in a certain way with forces. 918 00:47:09,930 --> 00:47:10,990 We're all made of matter. 919 00:47:10,990 --> 00:47:12,700 Right, we think things matter. 920 00:47:12,700 --> 00:47:19,760 So string theory is a theory of everything, essentially. 921 00:47:25,020 --> 00:47:25,975 Matter, forces. 922 00:47:35,431 --> 00:47:39,690 So string theory is one candidate Theory of Everything 923 00:47:39,690 --> 00:47:40,773 that people have proposed. 924 00:47:54,823 --> 00:47:58,530 [INAUDIBLE] 925 00:47:58,530 --> 00:48:00,310 Does anybody have any questions about how 926 00:48:00,310 --> 00:48:01,684 this is supposed to work out, how 927 00:48:01,684 --> 00:48:03,620 all these all these different forces just 928 00:48:03,620 --> 00:48:05,638 pop out of the equations? 929 00:48:05,638 --> 00:48:08,991 AUDIENCE: [INAUDIBLE] 930 00:48:11,390 --> 00:48:16,946 PROFESSOR: You can see them in a particle physics experiments. 931 00:48:16,946 --> 00:48:19,700 [LAUGHS] You just see them. 932 00:48:23,120 --> 00:48:28,580 I mean, usually, I mean, well, you see evidence of them. 933 00:48:28,580 --> 00:48:30,070 You form your theories. 934 00:48:30,070 --> 00:48:32,030 You form your particle physics theory. 935 00:48:32,030 --> 00:48:36,291 And if messenger particles exist, then you get this. 936 00:48:36,291 --> 00:48:37,165 You get this results. 937 00:48:41,380 --> 00:48:44,030 And you can measure the results of your particle physics 938 00:48:44,030 --> 00:48:46,667 experiments, and if it's consistent with the theory 939 00:48:46,667 --> 00:48:48,500 where messenger particles exist, then that's 940 00:48:48,500 --> 00:48:49,458 good evidence for that. 941 00:48:49,458 --> 00:48:51,050 But we've actually we've actually 942 00:48:51,050 --> 00:48:52,870 directly observed these. 943 00:48:52,870 --> 00:48:56,492 We've directly observed those messenger particles. 944 00:48:56,492 --> 00:48:58,700 But often the way that they work is in a virtual way. 945 00:48:58,700 --> 00:49:00,700 They only exist for a very short amount of time. 946 00:49:00,700 --> 00:49:02,780 Then they disappear from existence. 947 00:49:02,780 --> 00:49:07,742 But we can actually observe them directly, too. 948 00:49:07,742 --> 00:49:09,450 But often we don't observe them directly. 949 00:49:09,450 --> 00:49:11,510 Usually we don't observe them directly. 950 00:49:11,510 --> 00:49:16,070 But we've seen them, so they exist. 951 00:49:16,070 --> 00:49:21,590 We actually didn't see the W and Z bosons until the 1980s. 952 00:49:21,590 --> 00:49:24,140 So I suppose, until the 1980s it could 953 00:49:24,140 --> 00:49:26,810 have been that, it could have been that the theory was wrong, 954 00:49:26,810 --> 00:49:28,610 and there weren't messenger particles. 955 00:49:28,610 --> 00:49:31,065 But finally the in the late 80s, we saw it. 956 00:49:31,065 --> 00:49:32,310 Yeah? 957 00:49:32,310 --> 00:49:35,450 AUDIENCE: What's the Higgs boson? 958 00:49:35,450 --> 00:49:36,790 PROFESSOR: The Higgs boson-- 959 00:49:36,790 --> 00:49:39,410 so a boson is a type of particle. 960 00:49:39,410 --> 00:49:44,450 I'll say more about bosons and fermions in five minutes or so. 961 00:49:44,450 --> 00:49:47,210 Higgs boson is not a messenger particle. 962 00:49:47,210 --> 00:49:48,920 It doesn't transmit these forces. 963 00:49:48,920 --> 00:49:53,330 But it's actually a particle required by the Standard Model 964 00:49:53,330 --> 00:49:57,140 of particle physics that's required for the theory 965 00:49:57,140 --> 00:49:58,690 to make any sense at all. 966 00:49:58,690 --> 00:50:01,890 And it's actually what gives particles mass 967 00:50:01,890 --> 00:50:04,790 in a weird, complicated way. 968 00:50:04,790 --> 00:50:08,240 But there's a Higgs field that permeates all the universe, 969 00:50:08,240 --> 00:50:13,970 and there are Higgs particles that should exist. 970 00:50:13,970 --> 00:50:17,267 But we've never observed the Higgs particle. 971 00:50:17,267 --> 00:50:19,100 But all of our theory rests on the existence 972 00:50:19,100 --> 00:50:20,384 of the Higgs particle. 973 00:50:20,384 --> 00:50:23,050 AUDIENCE: [INAUDIBLE] 974 00:50:23,050 --> 00:50:25,970 PROFESSOR: Yeah, so the Large Hadron Collider, 975 00:50:25,970 --> 00:50:29,570 which is the most powerful particle accelerator 976 00:50:29,570 --> 00:50:34,220 ever, well, will be the most powerful particle accelerator 977 00:50:34,220 --> 00:50:38,829 ever and will hopefully start be operating maybe 978 00:50:38,829 --> 00:50:40,620 in a month, or two months, or three months. 979 00:50:40,620 --> 00:50:43,080 It's very soon. 980 00:50:43,080 --> 00:50:44,700 It's going to search for the Higgs. 981 00:50:44,700 --> 00:50:47,940 It's also going to search for a bunch of other stuff. 982 00:50:47,940 --> 00:50:49,670 In particular, the search for the Higgs 983 00:50:49,670 --> 00:50:52,050 and hopefully it will find it. 984 00:50:52,050 --> 00:50:55,550 If it finds it, then that's exactly the missing piece 985 00:50:55,550 --> 00:50:56,900 of the Standard Model. 986 00:50:56,900 --> 00:51:00,170 The standard model is basically our accumulation 987 00:51:00,170 --> 00:51:04,160 of all of our particle physics knowledge ever. 988 00:51:04,160 --> 00:51:08,060 And actually, it doesn't actually 989 00:51:08,060 --> 00:51:10,280 take into account gravity. 990 00:51:10,280 --> 00:51:15,490 The Standard Model explains these three forces 991 00:51:15,490 --> 00:51:18,880 and all the particles that interact through these forces 992 00:51:18,880 --> 00:51:20,250 doesn't explain gravity. 993 00:51:20,250 --> 00:51:22,416 So the Standard Model is not a theory of everything. 994 00:51:22,416 --> 00:51:25,250 But it needs the Higgs boson. 995 00:51:25,250 --> 00:51:27,170 And hopefully the LHC will find it. 996 00:51:31,940 --> 00:51:33,350 So what was I saying? 997 00:51:37,909 --> 00:51:39,950 Oh yeah, I was asking if there was any questions. 998 00:51:39,950 --> 00:51:47,430 OK, so string theory could be a theory of everything, 999 00:51:47,430 --> 00:51:50,490 because it explains all the particles and all the forces. 1000 00:51:50,490 --> 00:51:53,160 Now it turns out, it turns out that for the theory 1001 00:51:53,160 --> 00:51:56,395 to make sense at all, similar to the way for the Standard Model 1002 00:51:56,395 --> 00:51:58,560 to make sense at all, you need a Higgs boson. 1003 00:51:58,560 --> 00:52:01,830 For string theory to make any sense at all, for the equations 1004 00:52:01,830 --> 00:52:04,240 to be consistent with each other, 1005 00:52:04,240 --> 00:52:07,440 as a mathematical necessity to avoid contradiction, 1006 00:52:07,440 --> 00:52:09,804 you need 10 dimensions of space. 1007 00:52:09,804 --> 00:52:10,700 [LAUGHTER] 1008 00:52:10,700 --> 00:52:14,940 You need 10 dimensions of space for the theory 1009 00:52:14,940 --> 00:52:16,920 to make any sense at all. 1010 00:52:16,920 --> 00:52:21,030 Now we're all familiar with three dimensions 1011 00:52:21,030 --> 00:52:24,510 that we perceive in everyday life, 1012 00:52:24,510 --> 00:52:28,290 forward, backward, left, right, up, down. 1013 00:52:28,290 --> 00:52:29,550 We're all familiar with those. 1014 00:52:29,550 --> 00:52:32,955 But string theory says that seven more should exist. 1015 00:52:36,600 --> 00:52:38,820 So that sounds kind of weird. 1016 00:52:38,820 --> 00:52:41,310 So you have to make sense of it in some kind of way. 1017 00:52:41,310 --> 00:52:44,107 The equations just say that this number pops up, 10. 1018 00:52:44,107 --> 00:52:46,440 The number 10 pops up for the number of space dimensions 1019 00:52:46,440 --> 00:52:47,670 you need. 1020 00:52:47,670 --> 00:52:52,770 Now one way of having a universe with 10 spatial dimensions 1021 00:52:52,770 --> 00:52:56,100 is by having three that are similar, 1022 00:52:56,100 --> 00:52:58,710 the three that we ordinarily observe, 1023 00:52:58,710 --> 00:53:07,000 and the other seven could be curled up. 1024 00:53:07,000 --> 00:53:12,910 So there are three that are normal. 1025 00:53:12,910 --> 00:53:14,410 There are three normal dimensions. 1026 00:53:14,410 --> 00:53:16,520 And the others are curled up in a sense. 1027 00:53:16,520 --> 00:53:20,140 So to explain this. 1028 00:53:20,140 --> 00:53:28,070 So imagine I have this long water bottle or this long tube. 1029 00:53:28,070 --> 00:53:32,590 I mean suppose it's, just look at it, this long tube. 1030 00:53:32,590 --> 00:53:33,550 It's curled up. 1031 00:53:33,550 --> 00:53:36,040 The surface of it is curled up. 1032 00:53:36,040 --> 00:53:40,020 So the cross-section of it looks like this. 1033 00:53:40,020 --> 00:53:43,010 That's closed. 1034 00:53:43,010 --> 00:53:47,140 And when you look at it, when you look at it far away, 1035 00:53:47,140 --> 00:53:49,040 you don't really see the extra dimension. 1036 00:53:49,040 --> 00:53:52,770 You look at the tube from far away, it kind of looks flat. 1037 00:53:52,770 --> 00:53:55,034 So you don't really notice the extra dimension. 1038 00:53:55,034 --> 00:53:56,950 It could be that the extra dimensions of space 1039 00:53:56,950 --> 00:53:57,790 are similar to this. 1040 00:53:57,790 --> 00:54:00,730 They're curled up like this. 1041 00:54:00,730 --> 00:54:02,440 They would have to be enormously small, 1042 00:54:02,440 --> 00:54:04,000 because we don't observe them. 1043 00:54:04,000 --> 00:54:06,090 They'd have to be enormously small. 1044 00:54:06,090 --> 00:54:07,090 This is one possibility. 1045 00:54:07,090 --> 00:54:10,510 They could be curled up, or compactified, 1046 00:54:10,510 --> 00:54:14,990 is the term that the theorists use. 1047 00:54:14,990 --> 00:54:17,470 But in principle they can be very small and curled up, 1048 00:54:17,470 --> 00:54:19,240 and we would never observe them, and then 1049 00:54:19,240 --> 00:54:21,640 that would be totally fine. 1050 00:54:21,640 --> 00:54:26,530 Another way is that the other dimensions are 1051 00:54:26,530 --> 00:54:29,550 like the dimensions that we know, 1052 00:54:29,550 --> 00:54:33,610 but our existence is confined to a small portion 1053 00:54:33,610 --> 00:54:35,290 of all of space. 1054 00:54:35,290 --> 00:54:37,900 So it could be that we live on a brane, it's called. 1055 00:54:37,900 --> 00:54:42,237 So that's possibility one, curled up dimensions. 1056 00:54:49,700 --> 00:54:51,650 Possibility two is called D-branes. 1057 00:54:54,640 --> 00:54:56,630 So it's similar to this. 1058 00:54:56,630 --> 00:54:59,390 So imagine that you're a two-dimensional creature, 1059 00:54:59,390 --> 00:55:02,930 and you're living on a sheet of paper. 1060 00:55:02,930 --> 00:55:04,550 Your existence is completely confined 1061 00:55:04,550 --> 00:55:06,650 to the two-dimensional sheet of paper. 1062 00:55:06,650 --> 00:55:09,470 But we the omniscient observer can look and say, 1063 00:55:09,470 --> 00:55:11,990 aha, the sheet of paper actually lives inside a three 1064 00:55:11,990 --> 00:55:13,350 dimensional space. 1065 00:55:13,350 --> 00:55:16,460 So the observer isn't actually seeing all of reality. 1066 00:55:16,460 --> 00:55:20,630 String theory says that well, maybe 1067 00:55:20,630 --> 00:55:22,140 our existence is like this. 1068 00:55:22,140 --> 00:55:23,870 Maybe we live on D-branes. 1069 00:55:23,870 --> 00:55:27,110 So that's what these things are called. 1070 00:55:27,110 --> 00:55:28,940 Maybe we live on things called D-branes, 1071 00:55:28,940 --> 00:55:32,060 which are similar to sheets of paper living 1072 00:55:32,060 --> 00:55:33,877 in three-dimensional space. 1073 00:55:33,877 --> 00:55:36,168 So these are Sheets living in higher dimensional space. 1074 00:55:48,380 --> 00:55:50,890 Brane, by the way, is an abbreviation of membrane. 1075 00:55:58,340 --> 00:56:02,420 And D is just the first letter of our guy's name. 1076 00:56:02,420 --> 00:56:04,410 Dirichlet was his name. 1077 00:56:04,410 --> 00:56:05,672 What's his name? 1078 00:56:05,672 --> 00:56:06,950 [LAUGHS] 1079 00:56:06,950 --> 00:56:11,480 So it could be that we're just living on a sheet of, 1080 00:56:11,480 --> 00:56:13,250 essentially, like a sheet of paper. 1081 00:56:13,250 --> 00:56:15,860 And there's a whole lot of universe 1082 00:56:15,860 --> 00:56:18,700 out there that we never see. 1083 00:56:18,700 --> 00:56:21,620 Or maybe the other dimensions are curled up. 1084 00:56:21,620 --> 00:56:24,070 Or maybe there's a combination of both of them. 1085 00:56:24,070 --> 00:56:25,680 Nobody really knows today. 1086 00:56:25,680 --> 00:56:27,654 It could be both of them. 1087 00:56:27,654 --> 00:56:30,070 Are there any questions about the way the dimensions work? 1088 00:56:34,860 --> 00:56:38,330 OK, actually there-- 1089 00:56:38,330 --> 00:56:42,260 I mean there's a lot of enthusiasm for string theory 1090 00:56:42,260 --> 00:56:43,220 these days. 1091 00:56:43,220 --> 00:56:45,050 But there are a lot of basic questions about string theory 1092 00:56:45,050 --> 00:56:46,380 that nobody knows the answers to. 1093 00:56:46,380 --> 00:56:48,530 I mean one of them is how do the dimensions work out, 1094 00:56:48,530 --> 00:56:49,738 how does that all make sense. 1095 00:56:52,220 --> 00:56:55,980 Another is just what really is the theory? 1096 00:56:55,980 --> 00:57:00,410 So by the mid-1990s, people had essentially 1097 00:57:00,410 --> 00:57:03,660 discovered five different versions of string theory. 1098 00:57:23,470 --> 00:57:26,360 I mean they were all types of string theory, 1099 00:57:26,360 --> 00:57:28,990 but they looked like they were fundamentally different 1100 00:57:28,990 --> 00:57:30,490 from each other. 1101 00:57:30,490 --> 00:57:32,657 But then it was noticed that they're all 1102 00:57:32,657 --> 00:57:34,240 related to each other in a certain way 1103 00:57:34,240 --> 00:57:36,422 by some kind of a transformation. 1104 00:57:36,422 --> 00:57:38,380 You can turn one of the theories into the other 1105 00:57:38,380 --> 00:57:41,210 by applying some kind of transformation. 1106 00:57:41,210 --> 00:57:43,480 So then the idea emerged that perhaps these five 1107 00:57:43,480 --> 00:57:49,450 different versions of the theory are one aspect of an underlying 1108 00:57:49,450 --> 00:57:50,980 fundamental theory. 1109 00:57:50,980 --> 00:57:57,360 So you have string theory one, two, three, four, five. 1110 00:57:57,360 --> 00:57:59,950 Maybe they're just different manifestations 1111 00:57:59,950 --> 00:58:04,907 of some underlying structure. 1112 00:58:08,650 --> 00:58:13,690 Some underlying structure, yeah, similar to the way space 1113 00:58:13,690 --> 00:58:16,030 and time, they look fundamentally different 1114 00:58:16,030 --> 00:58:16,690 at first sight. 1115 00:58:16,690 --> 00:58:18,190 But then when you think harder about it, 1116 00:58:18,190 --> 00:58:20,106 they're actually just different manifestations 1117 00:58:20,106 --> 00:58:22,040 of an underlying structure. 1118 00:58:22,040 --> 00:58:24,610 And similar to the way that electricity and magnetism, 1119 00:58:24,610 --> 00:58:26,290 they look different from first sight. 1120 00:58:26,290 --> 00:58:27,748 But once you think harder about it, 1121 00:58:27,748 --> 00:58:31,240 they're actually two different of the same coin. 1122 00:58:31,240 --> 00:58:35,447 So often in physics, we see two things that, 1123 00:58:35,447 --> 00:58:38,030 or we see multiple things that look different from each other. 1124 00:58:38,030 --> 00:58:40,000 But when you actually think harder about it, 1125 00:58:40,000 --> 00:58:41,080 you realize they're not really different. 1126 00:58:41,080 --> 00:58:42,413 They're actually the same thing. 1127 00:58:42,413 --> 00:58:45,880 They're just different sides of the same coin. 1128 00:58:45,880 --> 00:58:48,490 So often in physics, unification occurs. 1129 00:58:48,490 --> 00:58:50,140 Things get unified. 1130 00:58:50,140 --> 00:58:52,000 Electricity and magnetism were unified, 1131 00:58:52,000 --> 00:58:53,964 space and time were unified. 1132 00:58:53,964 --> 00:58:56,380 Well, maybe these five different versions of string theory 1133 00:58:56,380 --> 00:58:57,760 are unified. 1134 00:58:57,760 --> 00:59:00,820 And so this question mark indicates 1135 00:59:00,820 --> 00:59:04,290 that fundamental theory, that foundational theory, 1136 00:59:04,290 --> 00:59:06,040 that perhaps these five different versions 1137 00:59:06,040 --> 00:59:09,000 of string theory are different manifestations of. 1138 00:59:09,000 --> 00:59:12,720 And that theory has been come to known as M-theory. 1139 00:59:22,620 --> 00:59:31,770 Where M stands for magical, or matrix, or master, or mystery, 1140 00:59:31,770 --> 00:59:34,980 depending on your preference. 1141 00:59:34,980 --> 00:59:38,100 Nobody knows all the details about M-theory, 1142 00:59:38,100 --> 00:59:42,630 but many people are working on it today. 1143 00:59:42,630 --> 00:59:46,770 But the real test of any theory, the test that 1144 00:59:46,770 --> 00:59:48,720 determines whether it's any good at all, 1145 00:59:48,720 --> 00:59:51,030 is if it confirms with experiments. 1146 00:59:51,030 --> 00:59:54,212 Quantum mechanics, and special relativity, and general theory, 1147 00:59:54,212 --> 00:59:55,920 they've all been confirmed by experiment. 1148 00:59:55,920 --> 00:59:58,200 They've passed all tests with flying colors. 1149 00:59:58,200 --> 01:00:01,116 But string theory has yet to be confirmed by experiments. 1150 01:00:01,116 --> 01:00:02,490 Right now it's completely theory. 1151 01:00:02,490 --> 01:00:04,720 It's purely mathematical. 1152 01:00:04,720 --> 01:00:08,250 In fact, even the mathematical status of the string theory 1153 01:00:08,250 --> 01:00:10,600 is questionable. 1154 01:00:10,600 --> 01:00:12,180 So the critics of string theory like 1155 01:00:12,180 --> 01:00:15,270 to point out that it's not even wrong, because it hasn't even 1156 01:00:15,270 --> 01:00:17,014 been tested yet. 1157 01:00:17,014 --> 01:00:18,930 And the reason for that is that these strings, 1158 01:00:18,930 --> 01:00:21,750 these fundamental strings, are just so small. 1159 01:00:21,750 --> 01:00:22,899 You can't see them at all. 1160 01:00:22,899 --> 01:00:24,690 You can't see them with today's technology. 1161 01:00:24,690 --> 01:00:26,481 You probably won't see them with technology 1162 01:00:26,481 --> 01:00:28,630 of 100 years from now or 200 years from now, 1163 01:00:28,630 --> 01:00:30,960 maybe even 1,000 years from now. 1164 01:00:30,960 --> 01:00:32,970 So there's a big technological problem 1165 01:00:32,970 --> 01:00:35,880 in testing string theory, which has caused a lot of people 1166 01:00:35,880 --> 01:00:36,810 to criticize it. 1167 01:00:36,810 --> 01:00:38,906 I mean people say, well, a physical theory, 1168 01:00:38,906 --> 01:00:40,780 a scientific theory should be testable, which 1169 01:00:40,780 --> 01:00:42,029 string theory isn't right now. 1170 01:00:42,029 --> 01:00:43,539 At least it's not directly testable, 1171 01:00:43,539 --> 01:00:44,580 because we can't see the. 1172 01:00:44,580 --> 01:00:47,880 Strings but there is hope in testing string theory. 1173 01:00:47,880 --> 01:00:55,780 And the hope comes from the LHC which you mentioned before. 1174 01:00:55,780 --> 01:00:58,920 So the LHC, which stands for Large Hadron Collider-- 1175 01:00:58,920 --> 01:01:00,620 don't worry about what Hadron means-- 1176 01:01:00,620 --> 01:01:06,960 will collide protons with each other at enormous speeds, 1177 01:01:06,960 --> 01:01:09,670 with higher energies than has ever been done before. 1178 01:01:09,670 --> 01:01:14,580 And so we'll finally have very, very energetic particles, 1179 01:01:14,580 --> 01:01:17,820 very, very high energies to observe that 1180 01:01:17,820 --> 01:01:21,270 have never been observed before. 1181 01:01:21,270 --> 01:01:24,930 And there's hope that we'll discover some new physics, 1182 01:01:24,930 --> 01:01:28,370 and perhaps, even test string theory. 1183 01:01:28,370 --> 01:01:32,310 So string theory, let's see. 1184 01:01:32,310 --> 01:01:33,340 I'll erase this guy. 1185 01:01:38,600 --> 01:01:42,830 String theory is called a supersymmetric theory, 1186 01:01:42,830 --> 01:01:45,840 because it has the symmetry called supersymmetry. 1187 01:01:53,680 --> 01:01:58,580 There are two kinds of particles. 1188 01:01:58,580 --> 01:02:01,150 fermions and bosons. 1189 01:02:04,110 --> 01:02:05,860 Every particle in nature can be classified 1190 01:02:05,860 --> 01:02:08,180 as being a fermion or a boson. 1191 01:02:11,030 --> 01:02:13,480 And it turns out that the distinguishing feature 1192 01:02:13,480 --> 01:02:16,330 is something called spin. 1193 01:02:16,330 --> 01:02:22,150 So these are half, I'll just-- 1194 01:02:22,150 --> 01:02:24,517 how do I want to say it? 1195 01:02:24,517 --> 01:02:25,600 I'll just a say half spin. 1196 01:02:28,725 --> 01:02:30,850 I'll have to leave you in confusion for the next 30 1197 01:02:30,850 --> 01:02:32,610 seconds, sorry. 1198 01:02:32,610 --> 01:02:33,445 Half-integer spin. 1199 01:02:37,900 --> 01:02:38,890 Integer spin. 1200 01:02:42,860 --> 01:02:45,987 So particles have many properties. 1201 01:02:45,987 --> 01:02:47,570 Particles have the property of a mass. 1202 01:02:47,570 --> 01:02:49,190 All particles have some mass. 1203 01:02:49,190 --> 01:02:51,557 They all have some charge, some electric charge. 1204 01:02:51,557 --> 01:02:53,390 And they all have something called the spin. 1205 01:03:00,510 --> 01:03:04,710 You know I can spin around and have a certain spin. 1206 01:03:04,710 --> 01:03:09,270 The spin the particles have is an intrinsic kind of spin. 1207 01:03:09,270 --> 01:03:11,540 The particles aren't actually spinning around 1208 01:03:11,540 --> 01:03:13,030 on some kind of an axis. 1209 01:03:13,030 --> 01:03:17,760 But they still have this fundamental, intrinsic angular 1210 01:03:17,760 --> 01:03:21,870 momentum, it's called, which it's just 1211 01:03:21,870 --> 01:03:25,880 impossible to understand on a classical, everyday level. 1212 01:03:25,880 --> 01:03:30,580 Classical just refers to pre-quantum physics. 1213 01:03:30,580 --> 01:03:32,490 So it's a kind of intrinsic spin. 1214 01:03:32,490 --> 01:03:34,200 Spin is a kind of intrinsic. 1215 01:03:37,440 --> 01:03:39,420 Kind of intrinsic spin that all particles have. 1216 01:03:52,697 --> 01:03:54,530 And it turns out that this spin could either 1217 01:03:54,530 --> 01:03:58,390 be a half-integer, like 1/2, or 3/2, or 5/2, 1218 01:03:58,390 --> 01:04:00,890 or it could be an integer like zero, one, two, three. 1219 01:04:00,890 --> 01:04:04,160 And all particles fall into one of two categories, fermions 1220 01:04:04,160 --> 01:04:05,870 and bosons. 1221 01:04:05,870 --> 01:04:10,280 Now a supersymmetric theory, a supersymmetric physical theory 1222 01:04:10,280 --> 01:04:13,540 says that for every fermion, there's a boson. 1223 01:04:23,900 --> 01:04:26,772 Which is to say that there is, for every given fermion, 1224 01:04:26,772 --> 01:04:28,730 there is a boson that has every single property 1225 01:04:28,730 --> 01:04:31,090 that the fermion has except for the spin, same mass, 1226 01:04:31,090 --> 01:04:32,530 same electric charge, et cetera. 1227 01:04:32,530 --> 01:04:34,342 But the spin is the opposite. 1228 01:04:34,342 --> 01:04:36,050 So instead of half-integer, it's integer. 1229 01:04:36,050 --> 01:04:38,330 Instead of 1/2, maybe it's zero. 1230 01:04:38,330 --> 01:04:40,890 So that's what a supersymmetric theory is like. 1231 01:04:40,890 --> 01:04:43,460 It just called supersymmetry. 1232 01:04:43,460 --> 01:04:46,100 String theory is an example of a supersymmetric theory. 1233 01:04:46,100 --> 01:04:48,200 And so string theory predicts the existence 1234 01:04:48,200 --> 01:04:49,580 of supersymmetric partners. 1235 01:04:52,250 --> 01:04:56,330 To date we haven't observed supersymmetric partners, 1236 01:04:56,330 --> 01:04:58,370 but string theory says they should exist. 1237 01:04:58,370 --> 01:05:00,500 Now at the LHC we'll finally have the opportunity 1238 01:05:00,500 --> 01:05:03,200 to measure, perhaps, we will have the opportunity 1239 01:05:03,200 --> 01:05:05,840 to see some supersymmetric partners. 1240 01:05:05,840 --> 01:05:08,540 And therefore, we'll finally have a way of testing string 1241 01:05:08,540 --> 01:05:10,970 theory, at least indirectly. 1242 01:05:10,970 --> 01:05:12,860 So if the LHC see supersymmetry, then that 1243 01:05:12,860 --> 01:05:14,780 would be good news for string theory. 1244 01:05:14,780 --> 01:05:17,969 If it doesn't see supersymmetry, then that 1245 01:05:17,969 --> 01:05:19,760 wouldn't be so good news for string theory. 1246 01:05:19,760 --> 01:05:21,290 And then the string theorists would 1247 01:05:21,290 --> 01:05:22,498 have to work a little harder. 1248 01:05:26,540 --> 01:05:30,362 OK, any questions about string theory? 1249 01:05:34,808 --> 01:05:43,510 OK, how string theory isn't the only theory of quantum gravity 1250 01:05:43,510 --> 01:05:44,900 that people are working on. 1251 01:05:44,900 --> 01:05:46,870 I mean it's certainly the most popular. 1252 01:05:46,870 --> 01:05:48,280 Maybe it's the most promising. 1253 01:05:48,280 --> 01:05:48,821 I don't know. 1254 01:05:48,821 --> 01:05:49,863 That's not for me to say. 1255 01:05:49,863 --> 01:05:51,529 But there are other theories that people 1256 01:05:51,529 --> 01:05:54,130 are working on that I'll just briefly mention today. 1257 01:05:54,130 --> 01:05:56,890 So probably the biggest competitor of the string theory 1258 01:05:56,890 --> 01:05:58,480 is called loop quantum gravity. 1259 01:06:08,940 --> 01:06:10,666 Now to explain loop quantum gravity, 1260 01:06:10,666 --> 01:06:12,040 I should probably finally explain 1261 01:06:12,040 --> 01:06:13,396 what the word quantum means. 1262 01:06:13,396 --> 01:06:16,060 [LAUGHS] I haven't used this word many times. 1263 01:06:16,060 --> 01:06:19,010 You might be wondering what does quantum mean? 1264 01:06:19,010 --> 01:06:25,214 Quantum just means the smallest unit of something. 1265 01:06:30,400 --> 01:06:34,230 So, for example, matter has quanta. 1266 01:06:34,230 --> 01:06:35,520 Matter comes in quanta. 1267 01:06:35,520 --> 01:06:37,140 There's a quantum of matter. 1268 01:06:37,140 --> 01:06:40,929 If we take a piece of matter, you cut it in half, 1269 01:06:40,929 --> 01:06:42,970 then you take the half, and you cut that in half, 1270 01:06:42,970 --> 01:06:46,200 and then cut that half in half, and so forth. 1271 01:06:46,200 --> 01:06:48,060 Can you do this process indefinitely? 1272 01:06:48,060 --> 01:06:50,820 Can you do it infinitely many times? 1273 01:06:50,820 --> 01:06:53,910 Well you've all heard about atoms. 1274 01:06:53,910 --> 01:06:56,610 Our modern scientific answer is that no, 1275 01:06:56,610 --> 01:06:59,960 eventually you'll get to a quantum of matter. 1276 01:06:59,960 --> 01:07:01,660 Eventually you get to an atom, then 1277 01:07:01,660 --> 01:07:04,896 you can divide the atom into protons, neutrons, electrons. 1278 01:07:04,896 --> 01:07:07,020 It turns out that protons and neutrons can actually 1279 01:07:07,020 --> 01:07:10,500 be divided even further into quarks. 1280 01:07:10,500 --> 01:07:12,510 But then it ends there. 1281 01:07:12,510 --> 01:07:14,190 So quarks are fundamental. 1282 01:07:14,190 --> 01:07:16,041 Actually electrons are also fundamental. 1283 01:07:16,041 --> 01:07:17,040 Photons are fundamental. 1284 01:07:17,040 --> 01:07:18,840 These guys are all fundamental. 1285 01:07:18,840 --> 01:07:24,120 So those are all the quanta of matter. 1286 01:07:24,120 --> 01:07:25,500 The plural is quanta. 1287 01:07:28,650 --> 01:07:30,500 That's the plural. 1288 01:07:30,500 --> 01:07:35,240 Loop quantum gravity not only is matter quantized, not only 1289 01:07:35,240 --> 01:07:39,200 does matter come in quanta, but also spacetime is quantized. 1290 01:07:39,200 --> 01:07:45,677 So space and time come in fundamental chunks. 1291 01:07:55,230 --> 01:07:58,470 So if you have a region of space, 1292 01:07:58,470 --> 01:08:00,920 and you keep probing deeper, and deeper, and deeper, 1293 01:08:00,920 --> 01:08:02,450 eventually you would reach a point 1294 01:08:02,450 --> 01:08:06,242 where you just don't have smaller lengths anymore. 1295 01:08:06,242 --> 01:08:07,450 You don't have smaller areas. 1296 01:08:07,450 --> 01:08:08,741 You don't have smaller volumes. 1297 01:08:08,741 --> 01:08:12,150 Eventually you would, you would reach a basic grain, 1298 01:08:12,150 --> 01:08:15,660 a basic quantum of space. 1299 01:08:15,660 --> 01:08:17,899 So maybe it looks like this. 1300 01:08:17,899 --> 01:08:21,470 Maybe this is how, something like this. 1301 01:08:21,470 --> 01:08:26,510 This is how space is granular. 1302 01:08:26,510 --> 01:08:31,460 Now it doesn't look like space is quantized. 1303 01:08:31,460 --> 01:08:34,567 I mean it certainly looks like we can move continuously 1304 01:08:34,567 --> 01:08:35,150 through space. 1305 01:08:35,150 --> 01:08:36,657 It certainly looks that way. 1306 01:08:36,657 --> 01:08:38,240 But according to loop quantum gravity, 1307 01:08:38,240 --> 01:08:40,609 if you go deep down further enough, eventually, 1308 01:08:40,609 --> 01:08:43,590 you reach a point where you just can't 1309 01:08:43,590 --> 01:08:48,830 move in smaller steps than this fundamental quantum of space. 1310 01:08:48,830 --> 01:08:54,470 So the quantum of space turns out to be something around 10 1311 01:08:54,470 --> 01:08:59,219 to the minus 99 cubic centimeters. 1312 01:08:59,219 --> 01:09:00,760 That's how small the quantum of space 1313 01:09:00,760 --> 01:09:03,140 is in loop quantum gravity. 1314 01:09:03,140 --> 01:09:10,945 The quantum of time is about 10 to the minus 43 seconds. 1315 01:09:14,170 --> 01:09:17,380 So not only is space quantized, time is also quantized. 1316 01:09:17,380 --> 01:09:20,200 So time isn't analog. 1317 01:09:20,200 --> 01:09:24,069 It's more like a digital thing. 1318 01:09:24,069 --> 01:09:28,240 Time comes in steps, similar to the way a computer calculates 1319 01:09:28,240 --> 01:09:29,319 things. 1320 01:09:29,319 --> 01:09:32,560 If you run a program, it will run in discrete steps. 1321 01:09:32,560 --> 01:09:35,220 So time is quantized, and space is quantized. 1322 01:09:35,220 --> 01:09:36,819 And it turns out that when you apply 1323 01:09:36,819 --> 01:09:42,120 this program of quantizing spacetime, 1324 01:09:42,120 --> 01:09:44,970 you can actually reconcile, it looks 1325 01:09:44,970 --> 01:09:47,260 like you can actually reconcile quantum mechanics 1326 01:09:47,260 --> 01:09:48,460 and general relativity. 1327 01:09:48,460 --> 01:09:50,710 It looks like you can actually get a consistent theory 1328 01:09:50,710 --> 01:09:52,740 of gravity by doing this. 1329 01:09:52,740 --> 01:09:55,000 But there are a lot of problems. 1330 01:09:55,000 --> 01:09:57,280 As with string theory, it hasn't been tested. 1331 01:09:57,280 --> 01:10:00,040 And it also looks like the maybe loop quantum gravity is only 1332 01:10:00,040 --> 01:10:02,830 a theory of gravity, only a quantum theory gravity. 1333 01:10:02,830 --> 01:10:04,630 You don't get the other forces. 1334 01:10:04,630 --> 01:10:06,310 So there are a lot of the problems 1335 01:10:06,310 --> 01:10:10,500 with every proposed theory of quantum gravity 1336 01:10:10,500 --> 01:10:13,090 that people have thought of. 1337 01:10:13,090 --> 01:10:16,030 There are other approaches that people have thought of. 1338 01:10:16,030 --> 01:10:16,800 I made a list. 1339 01:10:16,800 --> 01:10:19,051 Some include semi-classical gravity, twitch re-theory, 1340 01:10:19,051 --> 01:10:20,758 Boolean approaches, and causal sets, just 1341 01:10:20,758 --> 01:10:22,600 to mention some words that I talked about. 1342 01:10:22,600 --> 01:10:24,160 I don't know anything about them. 1343 01:10:24,160 --> 01:10:28,330 But those are other approaches that people have thought of. 1344 01:10:28,330 --> 01:10:31,850 But I guess the important point is to keep an open mind. 1345 01:10:31,850 --> 01:10:33,730 Remember that ultimately experiments 1346 01:10:33,730 --> 01:10:35,230 will determine the truth. 1347 01:10:35,230 --> 01:10:37,390 The theory doesn't agree with the experiments, 1348 01:10:37,390 --> 01:10:39,400 then it's no good. 1349 01:10:39,400 --> 01:10:42,100 We don't know what the theory of quantum gravity is today, 1350 01:10:42,100 --> 01:10:44,035 but many people actually believe that we're 1351 01:10:44,035 --> 01:10:44,910 getting pretty close. 1352 01:10:44,910 --> 01:10:46,951 The string theorists believe we're getting close, 1353 01:10:46,951 --> 01:10:49,240 and they think we're getting close. 1354 01:10:49,240 --> 01:10:51,580 So it's quite possible that within own lifetime, 1355 01:10:51,580 --> 01:10:54,160 we'll finally have the Theory of Everything. 1356 01:10:54,160 --> 01:10:55,960 And perhaps that theory of everything 1357 01:10:55,960 --> 01:10:58,120 will be simple enough to fit on a t-shirt. 1358 01:11:01,120 --> 01:11:03,270 And I'll end there today.