1 00:00:00,060 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,790 Commons license. 3 00:00:03,790 --> 00:00:06,000 Your support will help MIT OpenCourseWare 4 00:00:06,000 --> 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 --> 00:00:16,590 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,590 --> 00:00:17,830 at ocw.mit.edu. 8 00:00:21,080 --> 00:00:23,060 NICHOLAS DIBELLA: So, last class, 9 00:00:23,060 --> 00:00:26,030 I began to introduce the mathematical universe 10 00:00:26,030 --> 00:00:31,110 hypothesis, which was proposed by a cosmologist here at MIT 11 00:00:31,110 --> 00:00:32,850 named Max Tegmark. 12 00:00:32,850 --> 00:00:35,840 And briefly, the hypothesis goes, 13 00:00:35,840 --> 00:00:39,260 while the universe is mathematical, more precisely, 14 00:00:39,260 --> 00:00:41,330 it's a mathematical structure. 15 00:00:41,330 --> 00:00:45,370 So, just to briefly, just to briefly review the arguments, 16 00:00:45,370 --> 00:00:47,810 this is the mathematical universe hypothesis. 17 00:01:07,580 --> 00:01:12,000 Just to briefly go through the arguments, pretty simple. 18 00:01:12,000 --> 00:01:31,185 You start out by making the external reality hypothesis, 19 00:01:31,185 --> 00:01:32,560 which is simply that there exists 20 00:01:32,560 --> 00:01:36,260 an external world in addition to me, in addition to you. 21 00:01:36,260 --> 00:01:39,850 There is not just me in my head, in my mind. 22 00:01:39,850 --> 00:01:42,280 There's a whole external world out there. 23 00:01:42,280 --> 00:01:47,620 So this is just, this is the symbol for there exists. 24 00:01:47,620 --> 00:01:58,230 There exists an external world, exactly how it sounds. 25 00:01:58,230 --> 00:02:00,080 You might choose not to believe this. 26 00:02:00,080 --> 00:02:01,670 You might choose to believe that your 27 00:02:01,670 --> 00:02:03,600 are the only thing that exists. 28 00:02:03,600 --> 00:02:06,090 Your mind is the only thing that exists. 29 00:02:06,090 --> 00:02:07,590 And that's actually, that's actually 30 00:02:07,590 --> 00:02:10,229 a very conservative viewpoint. 31 00:02:10,229 --> 00:02:13,426 I mean, it's quite a leap to say that other things exist. 32 00:02:13,426 --> 00:02:16,635 The only thing we know for sure exists is our heads. 33 00:02:16,635 --> 00:02:18,930 The only thing I know for sure exists is me. 34 00:02:18,930 --> 00:02:23,550 The last class, we saw that rejecting this external reality 35 00:02:23,550 --> 00:02:25,260 hypothesis-- 36 00:02:25,260 --> 00:02:29,760 [INAUDIBLE]---- rejecting this hypothesis actually leads 37 00:02:29,760 --> 00:02:32,190 to kind of a complicated story. 38 00:02:32,190 --> 00:02:38,280 And so, last class we saw some arguments for them. 39 00:02:38,280 --> 00:02:40,421 And we generally like the simplest theory. 40 00:02:40,421 --> 00:02:42,670 The simplest theory is the one most likely to be true. 41 00:02:42,670 --> 00:02:45,600 So we rejected the rejection of the external reality 42 00:02:45,600 --> 00:02:46,685 hypothesis. 43 00:02:46,685 --> 00:02:49,950 We rejected solipsism, it was called. 44 00:02:49,950 --> 00:02:53,340 And we saw that this external reality hypothesis 45 00:02:53,340 --> 00:02:57,510 is a reasonable hypothesis to make. 46 00:02:57,510 --> 00:03:01,450 Any questions about the external reality hypothesis? 47 00:03:01,450 --> 00:03:02,250 OK. 48 00:03:02,250 --> 00:03:04,270 So, if there is an external reality, 49 00:03:04,270 --> 00:03:08,890 then this external world should be 50 00:03:08,890 --> 00:03:12,880 intelligible to things other than us humans. 51 00:03:12,880 --> 00:03:15,370 If there exists intelligent extraterrestrials, 52 00:03:15,370 --> 00:03:18,140 then they would be able to learn about it. 53 00:03:18,140 --> 00:03:21,130 If it's possible to create artificial intelligence, 54 00:03:21,130 --> 00:03:24,790 then AI would be able to learn about this external world. 55 00:03:27,690 --> 00:03:29,740 So this, so this external world is 56 00:03:29,740 --> 00:03:34,360 something that should be comprehensible to things that 57 00:03:34,360 --> 00:03:36,940 aren't necessarily humans, and therefore should 58 00:03:36,940 --> 00:03:41,020 be describable in a language that isn't distinctly human. 59 00:03:41,020 --> 00:03:43,090 I mean, it shouldn't depend on any concepts 60 00:03:43,090 --> 00:03:48,880 that humans have invented concepts, concepts like round 61 00:03:48,880 --> 00:03:51,350 or energy, and so forth. 62 00:03:51,350 --> 00:03:53,220 There might be some fundamental things, 63 00:03:53,220 --> 00:03:58,100 but it shouldn't depend on any baggage, we call it. 64 00:03:58,100 --> 00:04:01,780 Baggage is just the distinctly human or distinctly alien 65 00:04:01,780 --> 00:04:03,370 language. 66 00:04:03,370 --> 00:04:05,890 So, the external reality hypothesis 67 00:04:05,890 --> 00:04:13,132 implies that external reality should be free from baggage. 68 00:04:13,132 --> 00:04:14,965 The description should be free from baggage. 69 00:04:30,596 --> 00:04:34,040 Now, something that is completely, 70 00:04:34,040 --> 00:04:38,870 something that can be, something that is a description of-- 71 00:04:44,558 --> 00:04:46,470 now, something that can be described 72 00:04:46,470 --> 00:04:48,790 in a completely baggage-free language, 73 00:04:48,790 --> 00:04:52,090 it's precisely what's called the mathematical structure. 74 00:04:52,090 --> 00:04:54,450 That's just what it is. 75 00:04:54,450 --> 00:04:58,000 And such a thing, such a thing is 76 00:04:58,000 --> 00:05:01,720 called a mathematical structure, i.e., 77 00:05:01,720 --> 00:05:07,475 the external reality is a mathematical structure. 78 00:05:16,690 --> 00:05:20,950 Now, the more precise definition of mathematical structure 79 00:05:20,950 --> 00:05:23,920 is a set of abstract objects that are 80 00:05:23,920 --> 00:05:25,400 related in some kind of way. 81 00:05:25,400 --> 00:05:28,090 So, a set of abstract objects with relations. 82 00:05:46,018 --> 00:05:48,150 So, the external reality hypothesis 83 00:05:48,150 --> 00:05:51,600 implies that the external reality 84 00:05:51,600 --> 00:05:52,890 is a mathematical thing. 85 00:05:52,890 --> 00:05:55,080 It's a mathematical structure. 86 00:05:55,080 --> 00:05:57,990 So the universe is mathematical. 87 00:05:57,990 --> 00:06:01,710 So that's what the mathematical universe hypothesis is. 88 00:06:01,710 --> 00:06:05,070 And so last class, I gave a simple example, 89 00:06:05,070 --> 00:06:07,710 a very simple example of a mathematical structure. 90 00:06:07,710 --> 00:06:12,800 I drew this thing, square. 91 00:06:12,800 --> 00:06:16,080 But what is a square, really? 92 00:06:16,080 --> 00:06:18,180 So, I've drawn something here, which 93 00:06:18,180 --> 00:06:20,400 is supposed to represent the idea, the abstract idea 94 00:06:20,400 --> 00:06:21,300 of a square. 95 00:06:21,300 --> 00:06:26,070 So what a square really is is it's a set of four things, 96 00:06:26,070 --> 00:06:27,490 we call them line segments. 97 00:06:27,490 --> 00:06:28,752 That's just what we call them. 98 00:06:28,752 --> 00:06:30,210 And these line segments are related 99 00:06:30,210 --> 00:06:32,970 to each other in a certain way. 100 00:06:32,970 --> 00:06:34,470 One kind of way that they're related 101 00:06:34,470 --> 00:06:40,020 is that, well, each line segment is only adjacent to two 102 00:06:40,020 --> 00:06:40,890 other line segments. 103 00:06:40,890 --> 00:06:43,940 There's one line segment that isn't adjacent to it. 104 00:06:43,940 --> 00:06:45,690 Another property is that the way that they 105 00:06:45,690 --> 00:06:49,035 are adjacent to each other is by being adjacent in right angles. 106 00:06:51,570 --> 00:06:54,390 And the way that we represent this abstract idea 107 00:06:54,390 --> 00:06:55,500 is by drawing this. 108 00:06:55,500 --> 00:06:57,930 So this is a geometrical representation 109 00:06:57,930 --> 00:07:00,630 of the abstract mathematical structure 110 00:07:00,630 --> 00:07:02,970 that we know as square. 111 00:07:02,970 --> 00:07:09,195 So that's a simple example of a mathematical structure. 112 00:07:09,195 --> 00:07:10,740 That's much more abstract. 113 00:07:10,740 --> 00:07:14,250 A square is much more abstract than we naively 114 00:07:14,250 --> 00:07:17,230 think of it as being. 115 00:07:17,230 --> 00:07:20,070 So, there are a lot of other mathematical structures. 116 00:07:20,070 --> 00:07:21,775 I mean, this is a very simple one. 117 00:07:21,775 --> 00:07:23,220 There are a lot more complex ones. 118 00:07:23,220 --> 00:07:26,190 There are also simpler ones. 119 00:07:26,190 --> 00:07:29,580 Last class, I talked about the fact 120 00:07:29,580 --> 00:07:32,700 that all of our physical theories-- 121 00:07:32,700 --> 00:07:37,170 relativity, quantum theory, string theory, and so forth-- 122 00:07:37,170 --> 00:07:39,746 all these theories, they're, deep down, 123 00:07:39,746 --> 00:07:40,620 they're mathematical. 124 00:07:40,620 --> 00:07:42,600 They're mathematical things. 125 00:07:42,600 --> 00:07:45,220 And strictly speaking, what we do, strictly speaking, 126 00:07:45,220 --> 00:07:46,954 they're just a bunch of equations. 127 00:07:46,954 --> 00:07:49,620 I mean, relativity, for example, it's just a bunch of equations. 128 00:07:49,620 --> 00:07:51,240 That's all it is. 129 00:07:51,240 --> 00:07:54,410 But what we do is we interpret those equations 130 00:07:54,410 --> 00:07:56,190 and we say, aha, they're actually, 131 00:07:56,190 --> 00:07:59,390 they actually represent physical reality. 132 00:07:59,390 --> 00:08:01,800 Now, more precisely, the way that we interpret them 133 00:08:01,800 --> 00:08:06,240 is by saying, oh, well, we model the physical world 134 00:08:06,240 --> 00:08:08,040 as a mathematical structure. 135 00:08:08,040 --> 00:08:11,569 And the mathematical structure that we use is this. 136 00:08:11,569 --> 00:08:13,110 And that mathematical structure might 137 00:08:13,110 --> 00:08:16,410 be the mathematical structure of relativity or quantum mechanics 138 00:08:16,410 --> 00:08:19,330 or string theory or so forth. 139 00:08:19,330 --> 00:08:21,450 And so the view that physicists generally 140 00:08:21,450 --> 00:08:26,340 take of the relationship between math and physics, 141 00:08:26,340 --> 00:08:28,980 the relationship between math and the physical world, 142 00:08:28,980 --> 00:08:32,610 is that the physical world can be described by math. 143 00:08:32,610 --> 00:08:34,260 The physical world can be described 144 00:08:34,260 --> 00:08:36,919 by a mathematical structure. 145 00:08:36,919 --> 00:08:39,210 But the mathematical universe hypothesis 146 00:08:39,210 --> 00:08:41,520 actually says much more. 147 00:08:41,520 --> 00:08:44,130 The mathematical universe hypothesis 148 00:08:44,130 --> 00:08:47,150 actually says that the universe is a mathematical structure. 149 00:08:47,150 --> 00:08:49,320 Not only is it described by it, it actually 150 00:08:49,320 --> 00:08:51,112 is a mathematical structure. 151 00:08:56,200 --> 00:09:00,238 Universe is mathematical. 152 00:09:09,185 --> 00:09:11,310 Now, we don't know what that mathematical structure 153 00:09:11,310 --> 00:09:13,282 is today. 154 00:09:13,282 --> 00:09:15,240 Physicists are trying to figure out what it is. 155 00:09:15,240 --> 00:09:17,000 And we're trying to figure out what the theory of everything 156 00:09:17,000 --> 00:09:17,780 is. 157 00:09:17,780 --> 00:09:20,155 We're trying to figure out the theory of quantum gravity. 158 00:09:20,155 --> 00:09:23,370 But that theory corresponds to some mathematical structure. 159 00:09:23,370 --> 00:09:28,457 So we don't know what that theory is but people are surely 160 00:09:28,457 --> 00:09:29,040 working on it. 161 00:09:29,040 --> 00:09:31,890 Maybe we just haven't been clever enough or smart enough 162 00:09:31,890 --> 00:09:34,184 to figure out what it is. 163 00:09:34,184 --> 00:09:35,850 But the mathematical universe hypothesis 164 00:09:35,850 --> 00:09:40,440 actually gives us hope in truly understanding reality. 165 00:09:40,440 --> 00:09:42,480 I mean, it could certainly be the case 166 00:09:42,480 --> 00:09:45,200 that reality is something that isn't mathematical. 167 00:09:45,200 --> 00:09:47,420 I mean, it could just be some weird kind of a thing. 168 00:09:47,420 --> 00:09:47,920 Who knows? 169 00:09:47,920 --> 00:09:50,430 Who knows what it would be? 170 00:09:50,430 --> 00:09:55,500 So, it could be the case that the true reality is just 171 00:09:55,500 --> 00:09:57,340 something that's not mathematical 172 00:09:57,340 --> 00:09:59,840 and it's just something that we can't understand. 173 00:09:59,840 --> 00:10:03,211 Well, if it is mathematical, then, well, math 174 00:10:03,211 --> 00:10:04,710 is something that we know how to do. 175 00:10:04,710 --> 00:10:06,297 We've been doing it for a long time. 176 00:10:06,297 --> 00:10:08,130 And so we can just keep working on the math. 177 00:10:08,130 --> 00:10:09,588 You just keep trying to work to see 178 00:10:09,588 --> 00:10:13,150 what the right mathematical description of the world is. 179 00:10:13,150 --> 00:10:15,570 And then maybe one day we'll find it. 180 00:10:15,570 --> 00:10:18,780 One day, hopefully, we'll be smart enough or lucky enough 181 00:10:18,780 --> 00:10:19,800 to find it. 182 00:10:19,800 --> 00:10:23,070 And once we have that structure, then we'll 183 00:10:23,070 --> 00:10:26,690 really understand the true nature of reality. 184 00:10:33,880 --> 00:10:38,620 Now, you may say, Nick, I'm looking at the universe. 185 00:10:38,620 --> 00:10:40,330 I'm just looking at the universe. 186 00:10:40,330 --> 00:10:42,440 It doesn't look mathematical. 187 00:10:42,440 --> 00:10:45,160 I mean, I mean, what's mathematical 188 00:10:45,160 --> 00:10:48,184 about a chalkboard or feelings? 189 00:10:48,184 --> 00:10:49,600 I mean, it doesn't seem like math. 190 00:10:49,600 --> 00:10:52,660 It doesn't seem like an abstract sort of thing. 191 00:10:52,660 --> 00:10:53,160 Right? 192 00:10:53,160 --> 00:10:55,930 I mean, that's the initial objection. 193 00:10:55,930 --> 00:10:59,300 That's an initial reaction that we would take. 194 00:10:59,300 --> 00:11:05,400 Well, so it's actually, so to kind of answer this reaction, 195 00:11:05,400 --> 00:11:08,200 it's important to distinguish between two types of ways 196 00:11:08,200 --> 00:11:11,260 of looking at the worlds. 197 00:11:11,260 --> 00:11:13,090 So there's a bird perspective. 198 00:11:19,421 --> 00:11:21,670 There's a bird perspective of viewing the world, which 199 00:11:21,670 --> 00:11:23,590 is kind of like looking at-- 200 00:11:23,590 --> 00:11:25,540 you look at the mathematical structure 201 00:11:25,540 --> 00:11:28,730 outside of the structure, like a mathematician studying it. 202 00:11:28,730 --> 00:11:32,270 It's as though you're looking at the universe from above, 203 00:11:32,270 --> 00:11:34,360 like a bird. 204 00:11:34,360 --> 00:11:36,700 So, there's a bird perspective. 205 00:11:41,410 --> 00:11:44,690 And there's also a frog perspective. 206 00:11:44,690 --> 00:11:47,000 These are Tegmark's terms. 207 00:11:47,000 --> 00:11:48,333 There's a frog perspective. 208 00:11:51,790 --> 00:11:54,370 In the frog perspective, you're an observer inside 209 00:11:54,370 --> 00:11:55,395 of the universe. 210 00:11:55,395 --> 00:11:57,700 And so, you're inside of it like a frog, 211 00:11:57,700 --> 00:12:00,670 being looked at by the bird. 212 00:12:00,670 --> 00:12:04,510 And so, an important question is how are these two, 213 00:12:04,510 --> 00:12:08,870 how are these two views related to each other? 214 00:12:08,870 --> 00:12:11,300 So in the birth view, you have a mathematical structure 215 00:12:11,300 --> 00:12:13,030 and that's the universe. 216 00:12:13,030 --> 00:12:14,710 And so, in this structure, so you just 217 00:12:14,710 --> 00:12:16,000 have a mathematical structure. 218 00:12:16,000 --> 00:12:16,500 That's it. 219 00:12:16,500 --> 00:12:18,230 It's just math. 220 00:12:18,230 --> 00:12:22,430 But in this structure, there will be substructures. 221 00:12:22,430 --> 00:12:26,110 There'll be things like, things like trees, 222 00:12:26,110 --> 00:12:31,810 things like doors, chalkboards, and even humans. 223 00:12:31,810 --> 00:12:35,470 And these substructures have the property that they 224 00:12:35,470 --> 00:12:37,021 change in time and so forth. 225 00:12:37,021 --> 00:12:38,770 And there's a precise mathematical meaning 226 00:12:38,770 --> 00:12:42,440 to all these, to all these different properties. 227 00:12:42,440 --> 00:12:46,480 Now, the special thing about the substructure that we are, 228 00:12:46,480 --> 00:12:48,670 the special thing about the human substructure, 229 00:12:48,670 --> 00:12:50,410 is that it possesses the property 230 00:12:50,410 --> 00:12:54,100 of being a self-aware, conscious substructure that 231 00:12:54,100 --> 00:12:56,440 actually perceives, that actually 232 00:12:56,440 --> 00:12:58,810 perceives other substructures. 233 00:12:58,810 --> 00:13:01,870 So it's because of this self-awareness property 234 00:13:01,870 --> 00:13:06,010 that we perceive the world as being physically real. 235 00:13:06,010 --> 00:13:08,740 Now, other less complex substructures, 236 00:13:08,740 --> 00:13:12,690 like a door or table, they just don't have perceptions. 237 00:13:12,690 --> 00:13:13,790 They're just too simple. 238 00:13:13,790 --> 00:13:16,665 They're not going to have any perceptions. 239 00:13:16,665 --> 00:13:18,040 So you could imagine our universe 240 00:13:18,040 --> 00:13:20,080 being a less complex structure than it is. 241 00:13:20,080 --> 00:13:23,470 You can imagine it being simple enough that there are just 242 00:13:23,470 --> 00:13:26,550 no self-aware substructures. 243 00:13:26,550 --> 00:13:28,660 There are no conscious things inside of it, 244 00:13:28,660 --> 00:13:33,190 no conscious subsystems inside of it. 245 00:13:33,190 --> 00:13:36,280 In that case, the universe would be mathematically real. 246 00:13:36,280 --> 00:13:38,410 I mean, that's the only kind of existence there is, 247 00:13:38,410 --> 00:13:40,900 according to the mathematical universe hypothesis. 248 00:13:40,900 --> 00:13:42,880 The universe would be mathematically real, 249 00:13:42,880 --> 00:13:45,400 but there would be nothing there to call it physically real. 250 00:13:45,400 --> 00:13:49,670 There would be no appearance of physical reality. 251 00:13:49,670 --> 00:13:52,960 So, in the mathematical universe hypothesis, 252 00:13:52,960 --> 00:13:55,618 mathematical existence is fundamental. 253 00:14:13,070 --> 00:14:15,690 Only when you have a complex enough universe, 254 00:14:15,690 --> 00:14:18,920 only when you have a complex enough mathematical structure 255 00:14:18,920 --> 00:14:22,220 that is the universe will you get any frog perspective 256 00:14:22,220 --> 00:14:23,270 at all. 257 00:14:23,270 --> 00:14:26,176 So complexity leads to a frog perspective. 258 00:14:40,070 --> 00:14:44,050 In other words, complexity leads to the physical, 259 00:14:44,050 --> 00:14:46,367 to the appearance, the subjective appearance 260 00:14:46,367 --> 00:14:47,200 of physical reality. 261 00:14:56,280 --> 00:14:59,240 So physical existence isn't a new, 262 00:14:59,240 --> 00:15:00,990 isn't a different kind of existence, 263 00:15:00,990 --> 00:15:03,720 according to the mathematical universe hypothesis. 264 00:15:03,720 --> 00:15:07,260 It's really a kind of existence that emerges from math. 265 00:15:07,260 --> 00:15:10,350 Now, last class I talked about, I 266 00:15:10,350 --> 00:15:13,320 talk about the relation between math 267 00:15:13,320 --> 00:15:17,360 and matter, physical existence and mind, mental existence. 268 00:15:17,360 --> 00:15:22,320 And I briefly looked at the idea that math is actually 269 00:15:22,320 --> 00:15:25,140 a product of the human mind. 270 00:15:25,140 --> 00:15:27,250 And the human mind is a physical thing. 271 00:15:27,250 --> 00:15:30,360 And so therefore, physical existence 272 00:15:30,360 --> 00:15:32,655 would give rise to mathematical existence. 273 00:15:32,655 --> 00:15:33,780 Well, this is the opposite. 274 00:15:33,780 --> 00:15:37,440 Mathematical existence gives rise to physical existence. 275 00:15:37,440 --> 00:15:42,210 And also, mathematical existence gives rise to mental existence. 276 00:15:42,210 --> 00:15:44,910 So, in the mathematical universe hypothesis, 277 00:15:44,910 --> 00:15:47,760 the way that these three kinds of existences 278 00:15:47,760 --> 00:15:54,210 are related to each other is, so math is fundamental, 279 00:15:54,210 --> 00:16:01,380 leads the physical, leads to physical and mental. 280 00:16:13,000 --> 00:16:13,500 OK. 281 00:16:13,500 --> 00:16:17,470 Are there any, are there any questions 282 00:16:17,470 --> 00:16:19,890 about what the mathematical universe hypothesis says 283 00:16:19,890 --> 00:16:22,560 and what physical reality is, according 284 00:16:22,560 --> 00:16:24,870 to the mathematical universe hypothesis? 285 00:16:24,870 --> 00:16:26,140 Related questions. 286 00:16:28,900 --> 00:16:32,780 Now, I know it's very abstract and it's a lot to take in. 287 00:16:32,780 --> 00:16:35,320 But, I mean, we're looking at really deep questions here 288 00:16:35,320 --> 00:16:37,930 and we should expect that the kind of answers that we would 289 00:16:37,930 --> 00:16:40,450 get would be weird or odd or make 290 00:16:40,450 --> 00:16:44,530 us feel a funny sort of way. 291 00:16:44,530 --> 00:16:45,430 OK. 292 00:16:45,430 --> 00:16:48,820 So now, so if you believe, if you 293 00:16:48,820 --> 00:16:51,160 believe that mathematics is, in some sense, 294 00:16:51,160 --> 00:16:54,310 out there, if it exists independently 295 00:16:54,310 --> 00:16:57,360 of our own existence, if you believe that math isn't just 296 00:16:57,360 --> 00:17:00,850 the product of human mind but it, in fact, exists out there, 297 00:17:00,850 --> 00:17:03,370 the whole mathematical world exists out there, 298 00:17:03,370 --> 00:17:05,800 then you actually arrive at a new level 299 00:17:05,800 --> 00:17:07,790 of parallel universes. 300 00:17:07,790 --> 00:17:11,170 I talked about it briefly at the end of the class 301 00:17:11,170 --> 00:17:13,599 where I talked about parallel universes. 302 00:17:13,599 --> 00:17:16,900 I guess that was five weeks ago. 303 00:17:16,900 --> 00:17:31,680 And this is called the Level IV multiverse, 304 00:17:31,680 --> 00:17:35,981 which is just that all mathematical structures are 305 00:17:35,981 --> 00:17:36,480 real. 306 00:17:36,480 --> 00:17:37,060 They're all real. 307 00:17:37,060 --> 00:17:38,934 They're all different universes on their own. 308 00:17:58,560 --> 00:18:01,740 Actually, there's a slight technicality. 309 00:18:01,740 --> 00:18:04,110 Because you run into some paradoxes 310 00:18:04,110 --> 00:18:06,160 if you have all mathematical structures. 311 00:18:06,160 --> 00:18:08,751 So, it actually turns out, all computable 312 00:18:08,751 --> 00:18:09,750 mathematical structures. 313 00:18:09,750 --> 00:18:10,690 But that's a detail. 314 00:18:10,690 --> 00:18:11,860 Don't worry about that. 315 00:18:11,860 --> 00:18:13,235 So we'll just leave it like this, 316 00:18:13,235 --> 00:18:15,630 all mathematical structures are real. 317 00:18:15,630 --> 00:18:18,014 So this is actually an extra assumption. 318 00:18:18,014 --> 00:18:20,430 This is actually an extension to the mathematical universe 319 00:18:20,430 --> 00:18:21,560 hypothesis. 320 00:18:21,560 --> 00:18:24,222 I mean, our universe is one mathematical structure. 321 00:18:24,222 --> 00:18:26,430 But this says that there are other universes that are 322 00:18:26,430 --> 00:18:28,500 other mathematical structures. 323 00:18:28,500 --> 00:18:31,521 So this extension is actually called the ultimate ensemble 324 00:18:31,521 --> 00:18:32,020 theory. 325 00:18:41,900 --> 00:18:43,940 And this was also proposed by Tegmark. 326 00:18:53,580 --> 00:18:57,690 So this Level IV multiverse is actually 327 00:18:57,690 --> 00:19:00,270 the highest level in the hierarchy 328 00:19:00,270 --> 00:19:01,950 of parallel universes. 329 00:19:01,950 --> 00:19:04,537 So, in a Level IV multiverse, you 330 00:19:04,537 --> 00:19:06,120 have different mathematical structures 331 00:19:06,120 --> 00:19:08,340 that correspond to different universes. 332 00:19:08,340 --> 00:19:10,710 And each mathematical structure corresponds 333 00:19:10,710 --> 00:19:14,520 to different physics, different laws of physics. 334 00:19:18,100 --> 00:19:25,150 So you start out at the Level IV multiverse. 335 00:19:25,150 --> 00:19:27,870 So this gives you your Level IV multiverse. 336 00:19:27,870 --> 00:19:30,292 You just have all these different universes, 337 00:19:30,292 --> 00:19:32,000 all these different Level IV multiverses. 338 00:19:32,000 --> 00:19:35,830 I'm sorry, all these different Level IV universes. 339 00:19:35,830 --> 00:19:38,460 I'm just, these circles aren't really meant to mean anything. 340 00:19:38,460 --> 00:19:39,810 I'm just drawing them. 341 00:19:39,810 --> 00:19:42,060 I'm just trying to represent the set. 342 00:19:42,060 --> 00:19:45,340 So maybe we're living in this one. 343 00:19:45,340 --> 00:19:49,140 This is the one that has our set of laws of physics. 344 00:19:49,140 --> 00:19:51,420 This is the Level IV multiverse. 345 00:19:51,420 --> 00:19:53,100 And we're living in one of the members. 346 00:19:53,100 --> 00:19:55,100 We're living in one of the abstract mathematical 347 00:19:55,100 --> 00:19:56,900 structures. 348 00:19:56,900 --> 00:19:59,010 Last class, I also talked-- not last class, 349 00:19:59,010 --> 00:20:03,052 but when I talked about parallel universes-- question? 350 00:20:03,052 --> 00:20:08,730 AUDIENCE: [INAUDIBLE] 351 00:20:08,730 --> 00:20:10,980 NICHOLAS DIBELLA: Oh. 352 00:20:10,980 --> 00:20:12,200 There we go, Level IV. 353 00:20:16,800 --> 00:20:18,880 So, I talked about other levels when 354 00:20:18,880 --> 00:20:22,200 I talked about parallel universes five weeks ago. 355 00:20:22,200 --> 00:20:25,440 The top level, the highest level is Level IV, 356 00:20:25,440 --> 00:20:27,040 different laws of physics. 357 00:20:27,040 --> 00:20:29,220 So we have our own laws of physics. 358 00:20:29,220 --> 00:20:35,340 And from there, we get to the Level III multiverse-- 359 00:20:35,340 --> 00:20:36,508 I did the same thing-- 360 00:20:43,210 --> 00:20:47,200 which are, in a sense, different universes inside a given Level 361 00:20:47,200 --> 00:20:48,960 IV universe. 362 00:20:48,960 --> 00:20:53,140 So we have three universes. 363 00:20:53,140 --> 00:20:55,090 And each one of these universes is 364 00:20:55,090 --> 00:20:58,360 supposed to represent a different part of true quantum 365 00:20:58,360 --> 00:20:59,427 reality. 366 00:20:59,427 --> 00:21:01,510 And it follows from the many worlds interpretation 367 00:21:01,510 --> 00:21:04,210 of quantum mechanics. 368 00:21:04,210 --> 00:21:07,570 The technical term is that each of these different worlds 369 00:21:07,570 --> 00:21:10,120 are different parts, different branches of the wave 370 00:21:10,120 --> 00:21:11,530 function of the universe. 371 00:21:11,530 --> 00:21:16,010 But that's just a funny phrase to use. 372 00:21:16,010 --> 00:21:17,981 But you, don't worry about that. 373 00:21:17,981 --> 00:21:20,230 So maybe we're living in this, this branch of the wave 374 00:21:20,230 --> 00:21:20,730 function. 375 00:21:23,741 --> 00:21:30,856 The Level II multiverse that follows, 376 00:21:30,856 --> 00:21:32,480 and this one's the set of all universes 377 00:21:32,480 --> 00:21:36,290 with different physical constants and different space 378 00:21:36,290 --> 00:21:38,240 and time dimensionality. 379 00:21:38,240 --> 00:21:41,960 So, some of them have four dimensions of space. 380 00:21:41,960 --> 00:21:43,919 Some of them have 15 dimensions of space. 381 00:21:43,919 --> 00:21:45,710 Some of them have three dimensions of time, 382 00:21:45,710 --> 00:21:47,100 and so forth. 383 00:21:47,100 --> 00:21:51,280 So you have all these different level universes that-- 384 00:21:51,280 --> 00:21:54,480 well, I describe them as coming from inflation, 385 00:21:54,480 --> 00:21:57,009 the cosmological theory of inflation. 386 00:21:57,009 --> 00:21:58,300 Maybe we're living in this one. 387 00:21:58,300 --> 00:21:59,110 Who knows? 388 00:21:59,110 --> 00:22:01,390 I'm just trying to represent them. 389 00:22:01,390 --> 00:22:04,437 And then the lowest level, Level I multiverse, 390 00:22:04,437 --> 00:22:06,020 which was the least controversial one. 391 00:22:10,510 --> 00:22:13,090 This was the set of all universes that start out 392 00:22:13,090 --> 00:22:16,480 with different initial conditions, different starting 393 00:22:16,480 --> 00:22:18,400 distributions of matter and energy. 394 00:22:18,400 --> 00:22:22,450 And ours started out with a given distribution, 395 00:22:22,450 --> 00:22:24,550 and we're living in it. 396 00:22:24,550 --> 00:22:26,420 So you have all these different Level 397 00:22:26,420 --> 00:22:30,820 I universes, and maybe we're living in this one. 398 00:22:30,820 --> 00:22:33,250 So, we have a given law. 399 00:22:33,250 --> 00:22:35,350 We have a certain physics. 400 00:22:35,350 --> 00:22:37,330 We have a certain living in a certain branch 401 00:22:37,330 --> 00:22:40,240 of the wave function, a certain part of quantum reality. 402 00:22:40,240 --> 00:22:42,280 We have certain constants and we start out 403 00:22:42,280 --> 00:22:44,181 with certain initial conditions. 404 00:22:48,600 --> 00:22:56,455 AUDIENCE: [INAUDIBLE] 405 00:22:56,455 --> 00:22:57,830 NICHOLAS DIBELLA: Say that again. 406 00:22:57,830 --> 00:22:58,496 AUDIENCE: Sorry. 407 00:22:58,496 --> 00:23:01,130 Basically, like Level IV, is it basically 408 00:23:01,130 --> 00:23:05,300 every universe has different mathematical structures? 409 00:23:09,325 --> 00:23:11,450 NICHOLAS DIBELLA: Different mathematical structures 410 00:23:11,450 --> 00:23:13,051 correspond to laws of physics. 411 00:23:13,051 --> 00:23:13,550 Yeah 412 00:23:13,550 --> 00:23:15,562 AUDIENCE: [INAUDIBLE] 413 00:23:15,562 --> 00:23:16,520 NICHOLAS DIBELLA: Yeah. 414 00:23:16,520 --> 00:23:18,020 You can have funky laws of physics. 415 00:23:18,020 --> 00:23:21,270 Maybe you have, maybe you have a universe where-- 416 00:23:21,270 --> 00:23:24,372 yeah, like where gravity repels instead of attracts. 417 00:23:24,372 --> 00:23:25,580 AUDIENCE: Yeah. 418 00:23:25,580 --> 00:23:27,411 NICHOLAS DIBELLA: I made a horrible joke 419 00:23:27,411 --> 00:23:28,160 several weeks ago. 420 00:23:28,160 --> 00:23:31,670 I said, gravity is not responsible for people 421 00:23:31,670 --> 00:23:32,865 falling in love. 422 00:23:32,865 --> 00:23:34,597 Well, perhaps in another universe, 423 00:23:34,597 --> 00:23:36,680 gravity is responsible for people falling in love. 424 00:23:40,000 --> 00:23:43,820 That was a louder laugh than I got last time. 425 00:23:43,820 --> 00:23:45,140 OK. 426 00:23:45,140 --> 00:23:45,640 Yeah. 427 00:23:45,640 --> 00:23:46,460 So, yeah. 428 00:23:46,460 --> 00:23:50,990 Any, pretty much any conceivable physics that you can think of 429 00:23:50,990 --> 00:23:54,590 would be another world, just as real as our world, 430 00:23:54,590 --> 00:23:58,025 living in the Level IV multiverse. 431 00:23:58,025 --> 00:24:01,280 And you can think of all crazy kinds of universes. 432 00:24:01,280 --> 00:24:03,885 Maybe, I talked about four fundamental forces, 433 00:24:03,885 --> 00:24:05,510 maybe there's seven fundamental forces. 434 00:24:05,510 --> 00:24:08,750 Maybe you like number seven, lucky seven. 435 00:24:08,750 --> 00:24:11,035 Maybe there is, so in our universe, 436 00:24:11,035 --> 00:24:15,660 we have the concept of space and time and forces and matter. 437 00:24:15,660 --> 00:24:18,470 Maybe there's a new kind of concept 438 00:24:18,470 --> 00:24:21,360 like, I don't know what you want to call it, ghosts. 439 00:24:21,360 --> 00:24:23,270 Maybe there are ghosts or something. 440 00:24:23,270 --> 00:24:26,930 If you can write down a consistent mathematical 441 00:24:26,930 --> 00:24:32,870 structure that has these properties that you want, 442 00:24:32,870 --> 00:24:36,410 then that structure would correspond to a real universe 443 00:24:36,410 --> 00:24:40,820 out there, living abstractly in the Level IV multiverse. 444 00:24:40,820 --> 00:24:44,220 We're living in an abstract universe 445 00:24:44,220 --> 00:24:46,490 according to these ideas. 446 00:24:46,490 --> 00:24:50,630 And there exist others in addition to us. 447 00:24:50,630 --> 00:24:53,540 But only in those, only in those universes 448 00:24:53,540 --> 00:24:57,170 where, only in those universes that are complex enough 449 00:24:57,170 --> 00:24:59,290 to contain observers like us will there 450 00:24:59,290 --> 00:25:03,080 be the subjective perception of a physical reality. 451 00:25:03,080 --> 00:25:05,210 Otherwise, they're just devoid of observers 452 00:25:05,210 --> 00:25:06,410 and they're just math. 453 00:25:06,410 --> 00:25:08,220 They're just abstract things and there's 454 00:25:08,220 --> 00:25:09,470 nothing there to observe them. 455 00:25:09,470 --> 00:25:11,595 There's nothing there to call them physically real. 456 00:25:13,910 --> 00:25:18,024 So, the mathematical universe hypothesis explains, 457 00:25:18,024 --> 00:25:19,690 as I talked about last time, it explains 458 00:25:19,690 --> 00:25:25,086 very well why math is so good at the describing the world. 459 00:25:25,086 --> 00:25:26,585 The world is mathematical, therefore 460 00:25:26,585 --> 00:25:27,670 it should be no surprise. 461 00:25:27,670 --> 00:25:29,770 Very simple explanation. 462 00:25:29,770 --> 00:25:31,690 Well, but you might still be wondering 463 00:25:31,690 --> 00:25:33,760 why are these laws of physics the laws of physics 464 00:25:33,760 --> 00:25:35,060 that rule our world. 465 00:25:35,060 --> 00:25:37,270 In other words, why is this mathematical structure 466 00:25:37,270 --> 00:25:39,040 the one that is our universe, and not 467 00:25:39,040 --> 00:25:40,690 some other mathematical structure? 468 00:25:40,690 --> 00:25:41,880 Might still be possible. 469 00:25:41,880 --> 00:25:46,030 The mathematical universe hypothesis doesn't answer that. 470 00:25:46,030 --> 00:25:49,930 But the ultimate ensemble theory actually does, in a way. 471 00:25:49,930 --> 00:25:51,910 So, according to the ultimate ensemble theory, 472 00:25:51,910 --> 00:25:53,950 the Level IV multiverse exists. 473 00:25:53,950 --> 00:25:57,400 And there are many, there many other universes, 474 00:25:57,400 --> 00:25:59,427 as I just said. 475 00:25:59,427 --> 00:26:01,510 So, the first thing to notice is that we shouldn't 476 00:26:01,510 --> 00:26:04,270 be surprised to find ourselves living in a universe 477 00:26:04,270 --> 00:26:05,890 where we exist. 478 00:26:05,890 --> 00:26:07,540 Because we exist, obviously, so it's 479 00:26:07,540 --> 00:26:09,400 something that shouldn't be surprising. 480 00:26:09,400 --> 00:26:12,040 Now, conceivably, the universe could 481 00:26:12,040 --> 00:26:14,650 be tweaked in a number of ways and we'd still 482 00:26:14,650 --> 00:26:17,866 be able to live in such a universe. 483 00:26:17,866 --> 00:26:20,670 So, conceivably, there are a lot of universes out there 484 00:26:20,670 --> 00:26:23,980 that contain observers that are just 485 00:26:23,980 --> 00:26:27,430 like us, that contain observers that are carbon-based, 486 00:26:27,430 --> 00:26:32,020 for example, that act like us, that have emotions, 487 00:26:32,020 --> 00:26:33,940 have intelligence, and so forth. 488 00:26:33,940 --> 00:26:35,440 So conceivably, there could be a lot 489 00:26:35,440 --> 00:26:38,260 of universes out there in the Level IV multiverse 490 00:26:38,260 --> 00:26:42,720 that conceivably be our universe. 491 00:26:42,720 --> 00:26:47,270 So, the answer to the question, why this structure, not others, 492 00:26:47,270 --> 00:26:49,210 well, there's not really a good answer. 493 00:26:49,210 --> 00:26:50,710 Essentially, we live in this one. 494 00:26:50,710 --> 00:26:52,030 It was by chance. 495 00:26:52,030 --> 00:26:55,050 I mean, there are a lot of them that, there are a lot of them 496 00:26:55,050 --> 00:26:57,430 that contain observers like us. 497 00:26:57,430 --> 00:27:00,730 So the ultimate ensemble theory actually makes a prediction. 498 00:27:00,730 --> 00:27:04,240 It predicts that we should find ourselves living 499 00:27:04,240 --> 00:27:06,940 in the mathematical structure that not only is 500 00:27:06,940 --> 00:27:08,270 consistent with our existence-- 501 00:27:08,270 --> 00:27:10,811 I mean, it has to be consistent with our existence, otherwise 502 00:27:10,811 --> 00:27:12,160 we wouldn't exist in it-- 503 00:27:12,160 --> 00:27:15,730 not only that, but it should be, in a sense, 504 00:27:15,730 --> 00:27:20,320 typical among universes that we could exist inside of. 505 00:27:20,320 --> 00:27:23,110 Now, what do I mean by typical? 506 00:27:23,110 --> 00:27:25,330 Well, so you look at the set of universes 507 00:27:25,330 --> 00:27:27,970 that we could possibly live inside of. 508 00:27:27,970 --> 00:27:29,470 Then what you want to do is you want 509 00:27:29,470 --> 00:27:32,980 to give each universe some probability of being the one 510 00:27:32,980 --> 00:27:34,720 that we live inside of. 511 00:27:34,720 --> 00:27:38,060 And if we live in a typical universe, 512 00:27:38,060 --> 00:27:40,110 then the probability should be something, 513 00:27:40,110 --> 00:27:42,550 I mean, you might have some, might have some probability 514 00:27:42,550 --> 00:27:45,886 distribution-- 515 00:27:45,886 --> 00:27:49,630 you look at universes this way, in this way is probability-- 516 00:27:49,630 --> 00:27:51,992 so typical might be something in this range. 517 00:27:51,992 --> 00:27:53,700 So you have a bunch of universes the fall 518 00:27:53,700 --> 00:27:56,069 into the typical range. 519 00:27:56,069 --> 00:27:58,360 The ultimate ensemble theory predicts that our universe 520 00:27:58,360 --> 00:27:59,650 should be such a universe. 521 00:27:59,650 --> 00:28:02,710 It should be, in some sense, typical. 522 00:28:02,710 --> 00:28:05,530 So one day when we figure out, if we figure out 523 00:28:05,530 --> 00:28:07,420 the mathematical structure of our universe 524 00:28:07,420 --> 00:28:11,680 is, then we can simply say, well, let's 525 00:28:11,680 --> 00:28:13,450 look at this probability distribution 526 00:28:13,450 --> 00:28:15,160 and see if it's typical. 527 00:28:15,160 --> 00:28:18,280 If it is, then that's good evidence 528 00:28:18,280 --> 00:28:19,947 for the ultimate ensemble theory. 529 00:28:19,947 --> 00:28:22,530 If the universe turns out to be, if the mathematical structure 530 00:28:22,530 --> 00:28:25,790 turns out to be atypical or unusual in some sort of way-- 531 00:28:25,790 --> 00:28:31,420 if it follows, for example, in the tail of this probability 532 00:28:31,420 --> 00:28:32,590 distribution-- 533 00:28:32,590 --> 00:28:35,350 then that would be bad news for the ultimate ensemble theory 534 00:28:35,350 --> 00:28:36,766 because it predicts that we should 535 00:28:36,766 --> 00:28:40,790 be in a typical universe. 536 00:28:40,790 --> 00:28:45,395 Now, a difficult question is how you determine this probability 537 00:28:45,395 --> 00:28:47,770 distribution, how you determine the probability of living 538 00:28:47,770 --> 00:28:50,290 in a given universe. 539 00:28:50,290 --> 00:28:52,090 And so that's probably the biggest problem 540 00:28:52,090 --> 00:28:53,110 facing this theory. 541 00:28:56,110 --> 00:28:58,390 Nobody really has any idea. 542 00:28:58,390 --> 00:29:00,850 Nobody really has any idea what the correct way 543 00:29:00,850 --> 00:29:05,020 of defining probability over these universes is. 544 00:29:05,020 --> 00:29:07,280 But people have made suggestions. 545 00:29:07,280 --> 00:29:09,490 One suggestion that people have made 546 00:29:09,490 --> 00:29:13,285 is that maybe complexity is what defines this probability 547 00:29:13,285 --> 00:29:13,960 distribution. 548 00:29:13,960 --> 00:29:16,990 Maybe simpler structures are more likely to be the ones 549 00:29:16,990 --> 00:29:18,730 that we exist inside of, as opposed 550 00:29:18,730 --> 00:29:20,290 to more complex structures. 551 00:29:20,290 --> 00:29:24,680 It's similar to Occam's razor, where the simpler the theory, 552 00:29:24,680 --> 00:29:26,090 the more likely to be true. 553 00:29:26,090 --> 00:29:28,450 So the simpler the mathematical structure, 554 00:29:28,450 --> 00:29:31,500 the more likely that we find ourselves living inside of it. 555 00:29:31,500 --> 00:29:33,220 So maybe these are the simple ones, 556 00:29:33,220 --> 00:29:36,115 the ones in the center of this distribution 557 00:29:36,115 --> 00:29:38,560 are the simpler ones, and these are the more complex ones. 558 00:29:38,560 --> 00:29:41,380 But nobody really has any idea if that's the right standard 559 00:29:41,380 --> 00:29:42,340 to use. 560 00:29:42,340 --> 00:29:45,580 And so this is an open question, how you define this probability 561 00:29:45,580 --> 00:29:48,940 distribution over universes in the Level IV multiverse. 562 00:29:48,940 --> 00:29:50,890 It's actually also a big question 563 00:29:50,890 --> 00:29:53,500 in the Level II multiverse, how you define probability 564 00:29:53,500 --> 00:29:55,416 distributions for different physical constants 565 00:29:55,416 --> 00:29:57,770 and dimensionalities. 566 00:29:57,770 --> 00:29:59,610 So this is called the measure problem, 567 00:29:59,610 --> 00:30:01,510 defining a measure over the universes. 568 00:30:01,510 --> 00:30:04,870 And it's currently unsolved for Level 2, 569 00:30:04,870 --> 00:30:08,920 and it's really, really unsolved for Level IV. 570 00:30:08,920 --> 00:30:10,750 But if one day we have a compelling reason 571 00:30:10,750 --> 00:30:14,860 to choose one measure, one standard for performing 572 00:30:14,860 --> 00:30:17,610 this probability distribution over another, 573 00:30:17,610 --> 00:30:19,885 and then we calculate it and we find out 574 00:30:19,885 --> 00:30:21,510 one day what our mathematical structure 575 00:30:21,510 --> 00:30:25,350 it is, and we see that we're in a the tail end of it, 576 00:30:25,350 --> 00:30:28,750 we see that we're in a typical universe, 577 00:30:28,750 --> 00:30:31,440 then that would be bad news for the ultimate ensemble theory 578 00:30:31,440 --> 00:30:33,190 and we would have to go elsewhere. 579 00:30:33,190 --> 00:30:35,834 But it's important that this theory is actually falsifiable. 580 00:30:35,834 --> 00:30:37,500 I mean, you can make predictions with it 581 00:30:37,500 --> 00:30:40,770 and you can test it with things you learn about the universe. 582 00:30:40,770 --> 00:30:43,277 That's always important for any theory 583 00:30:43,277 --> 00:30:44,610 that you want to take seriously. 584 00:30:44,610 --> 00:30:46,193 I mean, if the theory made predictions 585 00:30:46,193 --> 00:30:51,150 that you couldn't test in any kind of way, 586 00:30:51,150 --> 00:30:53,290 then the theory would be essentially useless. 587 00:30:53,290 --> 00:30:56,471 I mean, what would it be, what would it be good for? 588 00:30:56,471 --> 00:30:56,970 OK. 589 00:30:56,970 --> 00:30:58,886 So that's the mathematical universe hypothesis 590 00:30:58,886 --> 00:31:00,469 and the ultimate ensemble theory. 591 00:31:00,469 --> 00:31:02,760 Mathematical universe hypothesis says that the universe 592 00:31:02,760 --> 00:31:04,410 is a mathematical structure. 593 00:31:04,410 --> 00:31:05,970 And the ultimate ensemble theory says 594 00:31:05,970 --> 00:31:08,040 that the set of all mathematical structures 595 00:31:08,040 --> 00:31:09,930 forms our complete reality. 596 00:31:09,930 --> 00:31:13,830 The complete reality is the world of mathematics. 597 00:31:13,830 --> 00:31:16,820 And the mathematical universe explains, 598 00:31:16,820 --> 00:31:18,980 this hypothesis explains why math is so effective, 599 00:31:18,980 --> 00:31:24,300 and this one explains why these particular laws of physics 600 00:31:24,300 --> 00:31:25,930 rather than others. 601 00:31:25,930 --> 00:31:30,020 Are there any questions about these ideas? 602 00:31:30,020 --> 00:31:31,740 So, I just talked about one theory 603 00:31:31,740 --> 00:31:34,920 about the nature of reality, that reality is mathematical. 604 00:31:34,920 --> 00:31:38,780 More precisely, that reality is a mathematical structure. 605 00:31:38,780 --> 00:31:41,370 So now I'd like to talk about another one, which 606 00:31:41,370 --> 00:31:44,980 is that our reality is a computer simulation. 607 00:31:44,980 --> 00:31:49,136 And this is called the simulation argument. 608 00:31:49,136 --> 00:31:52,010 AUDIENCE: Who was that proposed by? 609 00:31:52,010 --> 00:31:55,880 NICHOLAS DIBELLA: An Oxford philosopher named Nick Bostrom. 610 00:31:55,880 --> 00:31:58,410 Simulation argument. 611 00:31:58,410 --> 00:32:08,350 AUDIENCE: [INAUDIBLE] 612 00:32:08,350 --> 00:32:09,850 NICHOLAS DIBELLA: Uh, well, actually 613 00:32:09,850 --> 00:32:14,820 the idea that our world is, in some sense, simulated 614 00:32:14,820 --> 00:32:18,090 or an illusion, goes back, goes way back. 615 00:32:18,090 --> 00:32:21,390 I mean, even Descartes, in his Meditations, 616 00:32:21,390 --> 00:32:23,790 he was using that reality is just a dream. 617 00:32:26,649 --> 00:32:28,440 Is it possible to know if you're living in, 618 00:32:28,440 --> 00:32:30,670 is it possible to know if you're dreaming or not? 619 00:32:30,670 --> 00:32:32,253 Is it possible, in principle, to know? 620 00:32:32,253 --> 00:32:35,370 Maybe the dream is just so vivid, 621 00:32:35,370 --> 00:32:39,240 could you really know if you're living in a dream? 622 00:32:39,240 --> 00:32:41,009 I mean, you might say, well, we're 623 00:32:41,009 --> 00:32:42,300 probably not living in a dream. 624 00:32:42,300 --> 00:32:45,150 But could you really know for absolute certainty? 625 00:32:45,150 --> 00:32:46,020 Maybe not. 626 00:32:46,020 --> 00:32:47,720 I don't think so. 627 00:32:47,720 --> 00:32:50,340 So the idea has been around for a while. 628 00:32:50,340 --> 00:32:54,030 But this argument, the simulation argument, it's 629 00:32:54,030 --> 00:33:01,300 pretty recent and really puts things in a modern perspective. 630 00:33:01,300 --> 00:33:04,050 So the first thing, so the first thing 631 00:33:04,050 --> 00:33:06,090 to notice, the first thing to notice 632 00:33:06,090 --> 00:33:09,750 is that there are many people in the world, 633 00:33:09,750 --> 00:33:11,790 there are many researchers in the field 634 00:33:11,790 --> 00:33:13,320 of artificial intelligence. 635 00:33:13,320 --> 00:33:15,213 Why are you laughing? 636 00:33:15,213 --> 00:33:18,370 AUDIENCE: [INAUDIBLE] 637 00:33:18,370 --> 00:33:20,370 NICHOLAS DIBELLA: OK. 638 00:33:20,370 --> 00:33:21,780 OK. 639 00:33:21,780 --> 00:33:22,740 Is it my quotes? 640 00:33:25,520 --> 00:33:26,290 Never mind. 641 00:33:26,290 --> 00:33:30,000 So the first thing to notice is that there are researchers 642 00:33:30,000 --> 00:33:32,730 in the world working on artificial intelligence, 643 00:33:32,730 --> 00:33:35,940 the goal being to artificially create intelligence. 644 00:33:35,940 --> 00:33:41,650 So to build from non-biological stuff 645 00:33:41,650 --> 00:33:43,780 a program, computer program or a robot, 646 00:33:43,780 --> 00:33:48,420 some kind of machine that is in some way intelligence. 647 00:33:48,420 --> 00:33:50,460 For example, if you create an intelligent robot, 648 00:33:50,460 --> 00:33:52,140 you will be able to have an intelligent conversation 649 00:33:52,140 --> 00:33:52,640 with it. 650 00:33:52,640 --> 00:33:59,485 Maybe you talk about physics or world events or who knows. 651 00:33:59,485 --> 00:34:02,250 You could talk about a wide range of things. 652 00:34:02,250 --> 00:34:06,060 It's also conceivable that you can, eventually one day-- 653 00:34:06,060 --> 00:34:07,560 at least these researchers believe-- 654 00:34:07,560 --> 00:34:13,886 one day you could eventually program artificial emotions. 655 00:34:13,886 --> 00:34:15,719 It's conceivable that sometime in the future 656 00:34:15,719 --> 00:34:19,679 we'll have robots for that, for example, they feel sad 657 00:34:19,679 --> 00:34:24,100 when they're neglected or they feel happy and fulfilled 658 00:34:24,100 --> 00:34:26,796 once they solve a hard, challenging math problem. 659 00:34:26,796 --> 00:34:28,754 So there's the prospect of artificial emotions. 660 00:34:31,590 --> 00:34:33,429 Conceivably, you could also create 661 00:34:33,429 --> 00:34:36,510 a robot that had an artificial sense of humor, that 662 00:34:36,510 --> 00:34:40,350 would tell funny jokes that would make you laugh 663 00:34:40,350 --> 00:34:41,880 or maybe bad jokes that don't make 664 00:34:41,880 --> 00:34:43,505 you laugh but make you laugh afterwards 665 00:34:43,505 --> 00:34:45,750 because they're bad jokes. 666 00:34:45,750 --> 00:34:48,239 And so there's even, there's even the possibility 667 00:34:48,239 --> 00:34:49,739 that one day we'll be able to create 668 00:34:49,739 --> 00:34:51,570 artificial consciousness. 669 00:34:51,570 --> 00:34:56,909 And not only, not only a robot who's intelligent-- 670 00:34:56,909 --> 00:34:59,120 guys? 671 00:34:59,120 --> 00:35:00,660 Could do just turn it down a little? 672 00:35:03,144 --> 00:35:04,810 So not only robots that are intelligent, 673 00:35:04,810 --> 00:35:07,530 not only robots that have emotions, sense of humors 674 00:35:07,530 --> 00:35:09,780 and so forth, but also are conscious, 675 00:35:09,780 --> 00:35:11,970 robots that are self-aware, that realize 676 00:35:11,970 --> 00:35:15,030 that they have a unique identity in the world that's 677 00:35:15,030 --> 00:35:19,330 different from the identity of all the stuff around them. 678 00:35:19,330 --> 00:35:21,630 So these are some of the ultimate goals 679 00:35:21,630 --> 00:35:24,390 of researchers in AI. 680 00:35:24,390 --> 00:35:26,960 We call it artificial because, because it's 681 00:35:26,960 --> 00:35:29,130 a biological thing. 682 00:35:29,130 --> 00:35:30,270 It's a machine thing. 683 00:35:30,270 --> 00:35:31,590 It's not carbon-based. 684 00:35:31,590 --> 00:35:34,140 It doesn't have anything really to do with organisms. 685 00:35:36,660 --> 00:35:39,300 So the hope is, the hope is that, eventually, these things 686 00:35:39,300 --> 00:35:41,430 will be possible. 687 00:35:41,430 --> 00:35:46,310 Most people in the field, most AI researchers, most cognitive 688 00:35:46,310 --> 00:35:51,120 scientists, and most philosophers of the mind 689 00:35:51,120 --> 00:35:54,750 believe that this program, this goal 690 00:35:54,750 --> 00:35:59,284 is one day reachable, that one day, 691 00:35:59,284 --> 00:36:01,950 we'll figure out how to do this, that consciousness is something 692 00:36:01,950 --> 00:36:04,770 that can emerge from non-biological stuff, 693 00:36:04,770 --> 00:36:07,740 that a mind can emerge from non-biological biological 694 00:36:07,740 --> 00:36:08,430 stuff. 695 00:36:08,430 --> 00:36:10,650 Once you just put it together in the right way, 696 00:36:10,650 --> 00:36:12,600 once you have the right architecture, 697 00:36:12,600 --> 00:36:15,820 the mind is something that can emerge. 698 00:36:15,820 --> 00:36:20,050 So for example, just a few years ago, 699 00:36:20,050 --> 00:36:23,370 people built a computer, an intelligent computer 700 00:36:23,370 --> 00:36:28,140 that actually beat the world champion in chess. 701 00:36:28,140 --> 00:36:30,660 And chess is something that we generally associate 702 00:36:30,660 --> 00:36:31,887 with intelligence, right? 703 00:36:31,887 --> 00:36:33,970 People who are very intelligent are good at chess. 704 00:36:33,970 --> 00:36:36,720 That's something that we generally associate with chess. 705 00:36:36,720 --> 00:36:41,760 And when a computer beat the world champion in chess, 706 00:36:41,760 --> 00:36:46,290 that was a big triumph for AI research. 707 00:36:46,290 --> 00:36:50,340 Now, you might say, well, that computer that beat the champion 708 00:36:50,340 --> 00:36:51,510 wasn't really intelligent. 709 00:36:51,510 --> 00:36:53,885 I mean, it was just doing all of these crazy calculations 710 00:36:53,885 --> 00:36:56,320 and figuring out all these possibilities and stuff. 711 00:36:56,320 --> 00:36:58,267 So it raises an interesting question 712 00:36:58,267 --> 00:36:59,850 that was actually raised when I talked 713 00:36:59,850 --> 00:37:03,570 about aliens several weeks ago. 714 00:37:03,570 --> 00:37:05,790 It's a question of what is intelligence. 715 00:37:05,790 --> 00:37:08,610 What defines intelligence? 716 00:37:08,610 --> 00:37:10,680 Does anyone have an answer what intelligence is? 717 00:37:14,166 --> 00:37:17,170 AUDIENCE: [INAUDIBLE] 718 00:37:17,170 --> 00:37:18,170 NICHOLAS DIBELLA: Right. 719 00:37:18,170 --> 00:37:19,961 So we have our own version of intelligence. 720 00:37:19,961 --> 00:37:22,875 So how might you define our intelligence, for example? 721 00:37:22,875 --> 00:37:29,242 AUDIENCE: If you're trying to elevate [INAUDIBLE] 722 00:37:29,242 --> 00:37:30,200 NICHOLAS DIBELLA: Yeah. 723 00:37:30,200 --> 00:37:31,340 So let's suppose our intelligence. 724 00:37:31,340 --> 00:37:33,257 So let's forget about the aliens for now. 725 00:37:33,257 --> 00:37:35,090 I mean, how would you define, how would you, 726 00:37:35,090 --> 00:37:39,554 how would you define our intelligence? 727 00:37:39,554 --> 00:37:43,990 AUDIENCE: Probably to adapt and think, but also 728 00:37:43,990 --> 00:37:47,010 to actually pose questions. 729 00:37:47,010 --> 00:37:52,000 [INAUDIBLE] 730 00:37:52,000 --> 00:37:53,500 NICHOLAS DIBELLA: So you define, you 731 00:37:53,500 --> 00:37:55,749 say something's intelligent if it can do these things, 732 00:37:55,749 --> 00:37:58,920 if it can ask questions, if it can adapt, 733 00:37:58,920 --> 00:38:00,160 if it can figure things out. 734 00:38:00,160 --> 00:38:02,417 If it can do these things, then it's intelligent. 735 00:38:05,100 --> 00:38:06,100 That's one prerequisite. 736 00:38:06,100 --> 00:38:09,340 That's one necessary requirement. 737 00:38:09,340 --> 00:38:11,270 OK. 738 00:38:11,270 --> 00:38:11,870 Any other? 739 00:38:11,870 --> 00:38:12,614 Yeah. 740 00:38:12,614 --> 00:38:14,280 AUDIENCE: The ability to solve problems? 741 00:38:14,280 --> 00:38:14,810 NICHOLAS DIBELLA: Yeah. 742 00:38:14,810 --> 00:38:16,340 The ability to solve problems. 743 00:38:16,340 --> 00:38:18,680 So that's another thing that intelligent things 744 00:38:18,680 --> 00:38:19,860 should be able to do. 745 00:38:19,860 --> 00:38:22,830 That's the important verb. 746 00:38:22,830 --> 00:38:24,885 It should be able to do that thing. 747 00:38:24,885 --> 00:38:25,384 Yeah? 748 00:38:25,384 --> 00:38:26,300 AUDIENCE: [INAUDIBLE] 749 00:38:26,300 --> 00:38:27,420 NICHOLAS DIBELLA: The ability to learn. 750 00:38:27,420 --> 00:38:29,253 That's another thing that intelligent things 751 00:38:29,253 --> 00:38:31,560 should be able to do. 752 00:38:31,560 --> 00:38:32,382 Another one? 753 00:38:32,382 --> 00:38:33,090 AUDIENCE: Memory. 754 00:38:33,090 --> 00:38:34,950 NICHOLAS DIBELLA: It should have memory. 755 00:38:34,950 --> 00:38:37,010 It should remember things. 756 00:38:37,010 --> 00:38:40,010 Again, that's another thing that intelligent machines 757 00:38:40,010 --> 00:38:41,460 should be able to do. 758 00:38:41,460 --> 00:38:46,730 So, I keep emphasizing this word do because it emphasizes 759 00:38:46,730 --> 00:38:50,270 that an intuitive way of defining intelligence 760 00:38:50,270 --> 00:38:53,040 is simply in terms of behavior of something. 761 00:38:53,040 --> 00:38:54,620 So if a person acts intelligently, 762 00:38:54,620 --> 00:38:56,450 then we say he's intelligent. 763 00:38:56,450 --> 00:39:01,700 And so, and so there's an idea proposed about 60 years ago 764 00:39:01,700 --> 00:39:06,230 by an English mathematician named Turing, 765 00:39:06,230 --> 00:39:07,370 called the Turing test. 766 00:39:10,870 --> 00:39:14,230 And this is a test to distinguish 767 00:39:14,230 --> 00:39:16,500 between intelligent and non-intelligent. 768 00:39:16,500 --> 00:39:19,000 So if you want to figure out if your machine is intelligent, 769 00:39:19,000 --> 00:39:21,970 for example, then the way that this task goes 770 00:39:21,970 --> 00:39:27,100 is you have a judge, a human judge. 771 00:39:30,600 --> 00:39:32,640 And you have a human judge in one room 772 00:39:32,640 --> 00:39:42,610 and you have a machine and a human in another room. 773 00:39:42,610 --> 00:39:45,820 And they're both communicating with the judge. 774 00:39:45,820 --> 00:39:49,340 They're having a conversation with the judge, for example. 775 00:39:49,340 --> 00:39:51,845 If the judge, if the human judge is unable 776 00:39:51,845 --> 00:39:53,470 to tell the difference between the two, 777 00:39:53,470 --> 00:39:56,920 if he can't tell which is which, then-- 778 00:39:56,920 --> 00:40:00,114 I guess this is weird diagram, whatever-- 779 00:40:00,114 --> 00:40:02,530 if the judge can't tell the difference between the machine 780 00:40:02,530 --> 00:40:04,510 and the human, then the test says 781 00:40:04,510 --> 00:40:06,920 that, for all practical purposes, 782 00:40:06,920 --> 00:40:08,950 the machine is intelligent. 783 00:40:08,950 --> 00:40:13,990 If I talk to it and it acts like a human, it tells jokes, 784 00:40:13,990 --> 00:40:16,750 it makes insightful remarks, it figures things out, 785 00:40:16,750 --> 00:40:21,500 if it adapts to my personality, then we say it's intelligent. 786 00:40:21,500 --> 00:40:23,170 And so, if you can't tell the difference 787 00:40:23,170 --> 00:40:24,480 between the machine and the human, 788 00:40:24,480 --> 00:40:25,790 then the machine is intelligent. 789 00:40:25,790 --> 00:40:27,081 That's what the Turing test is. 790 00:40:30,302 --> 00:40:32,260 So, it's important to have them in another room 791 00:40:32,260 --> 00:40:33,801 because, I mean, obviously, if you're 792 00:40:33,801 --> 00:40:38,870 looking at the robot, that's a good indication that it's 793 00:40:38,870 --> 00:40:39,760 a machine. 794 00:40:39,760 --> 00:40:42,850 I mean, you're looking at a thing that's doing something 795 00:40:42,850 --> 00:40:45,545 like that, it's a machine. 796 00:40:45,545 --> 00:40:47,920 I mean, unless it's a person trying to do the robot dance 797 00:40:47,920 --> 00:40:49,570 or whatever. 798 00:40:49,570 --> 00:40:52,240 Also, we talk to it. 799 00:40:52,240 --> 00:40:54,700 You probably also don't want to talk to it in person. 800 00:40:54,700 --> 00:40:56,970 You probably want to just communicate by text. 801 00:40:56,970 --> 00:41:01,110 I mean, if you talk to it and it sounds something like, 802 00:41:01,110 --> 00:41:06,160 a distant future, the year 2000, if it had a distinctive robot 803 00:41:06,160 --> 00:41:10,320 voice then that would be a good indication that the machine-- 804 00:41:13,750 --> 00:41:16,060 Maybe we don't yet know how to make the human voice. 805 00:41:16,060 --> 00:41:20,410 We can't have the distinctive human voice. 806 00:41:20,410 --> 00:41:22,090 Maybe that was a bad human voice. 807 00:41:22,090 --> 00:41:23,292 I'm a bad human. 808 00:41:23,292 --> 00:41:24,750 Well, in any case, in any case, you 809 00:41:24,750 --> 00:41:26,590 want to filter out all these, filter out 810 00:41:26,590 --> 00:41:31,900 all these silly things and have kind of a text conversation, 811 00:41:31,900 --> 00:41:34,540 like an Instant Message, have them Instant Message each 812 00:41:34,540 --> 00:41:35,760 other, for example. 813 00:41:35,760 --> 00:41:37,510 And if the judge can't tell the difference 814 00:41:37,510 --> 00:41:39,400 between the machine and the human, 815 00:41:39,400 --> 00:41:43,390 then the machine is intelligent. 816 00:41:43,390 --> 00:41:45,340 I don't think Smarter Child passes this test, 817 00:41:45,340 --> 00:41:46,820 unfortunately. 818 00:41:46,820 --> 00:41:49,960 But that would be a kind of Turing test. 819 00:41:49,960 --> 00:41:53,190 Smarter Child wouldn't pass the Turing test. 820 00:41:53,190 --> 00:41:54,610 OK. 821 00:41:54,610 --> 00:41:57,192 So this test emphasizes, I mean, this is kind of an aside. 822 00:41:57,192 --> 00:41:59,650 It's not really totally related to the simulation argument. 823 00:41:59,650 --> 00:42:01,400 It's an interesting aside. 824 00:42:01,400 --> 00:42:03,550 So the Turing test defines intelligence 825 00:42:03,550 --> 00:42:04,830 in terms of behavior. 826 00:42:04,830 --> 00:42:07,690 If it acts intelligently, then it's intelligent. 827 00:42:07,690 --> 00:42:08,980 But you might object to that. 828 00:42:08,980 --> 00:42:12,877 You might say, well, I mean, you can have some, 829 00:42:12,877 --> 00:42:14,710 I mean, maybe it's just doing all this stuff 830 00:42:14,710 --> 00:42:17,110 and it doesn't really understand what it's doing. 831 00:42:17,110 --> 00:42:19,300 I mean, maybe it doesn't have concepts 832 00:42:19,300 --> 00:42:23,410 that we humans ordinarily use in figuring stuff out. 833 00:42:23,410 --> 00:42:25,390 Maybe it just tries all possibilities blindly, 834 00:42:25,390 --> 00:42:27,160 for example. 835 00:42:27,160 --> 00:42:30,340 I mean, we humans don't really think of things like that, 836 00:42:30,340 --> 00:42:34,090 like brute force, brute force types 837 00:42:34,090 --> 00:42:37,090 of solutions as being intelligent. 838 00:42:37,090 --> 00:42:39,400 I mean, to do the brute force solution is intelligent, 839 00:42:39,400 --> 00:42:41,182 but actually doing it, you just blindly go 840 00:42:41,182 --> 00:42:43,640 through all these possibilities and try all this stuff out. 841 00:42:43,640 --> 00:42:46,310 We don't usually think it's an intelligent thing. 842 00:42:46,310 --> 00:42:49,930 So you might object to this. 843 00:42:49,930 --> 00:42:52,979 And you might also say, well, it doesn't really, 844 00:42:52,979 --> 00:42:54,520 I mean, it doesn't really test to see 845 00:42:54,520 --> 00:42:55,360 if the thing is conscious. 846 00:42:55,360 --> 00:42:57,560 It doesn't really test to see if the machine is conscious. 847 00:42:57,560 --> 00:42:58,480 I mean, the machine might be able to do 848 00:42:58,480 --> 00:42:59,650 all these intelligent things, but it might not 849 00:42:59,650 --> 00:43:00,610 have a sense of self. 850 00:43:00,610 --> 00:43:02,706 It might not have a sense of I. 851 00:43:02,706 --> 00:43:07,040 And so defining consciousness is kind of a tricky business. 852 00:43:07,040 --> 00:43:10,160 But I think it's an interesting question, though, 853 00:43:10,160 --> 00:43:12,250 how you define intelligence. 854 00:43:12,250 --> 00:43:14,555 But nonetheless, most AI researchers 855 00:43:14,555 --> 00:43:16,930 and cognitive scientists believe that, in principle, it's 856 00:43:16,930 --> 00:43:20,530 possible to create a machine artificial intelligence 857 00:43:20,530 --> 00:43:22,480 and consciousness. 858 00:43:22,480 --> 00:43:26,290 You don't need a carbon-based, organism to have it. 859 00:43:26,290 --> 00:43:29,080 You could have a silicon-based system, like a computer. 860 00:43:29,080 --> 00:43:31,340 A computer is silicon-based. 861 00:43:31,340 --> 00:43:35,020 So the simulation argument assumes 862 00:43:35,020 --> 00:43:38,290 what's called a substrate independence of consciousness. 863 00:43:48,002 --> 00:43:49,210 That's the first [INAUDIBLE]. 864 00:43:49,210 --> 00:43:51,420 A substrate is just a material. 865 00:43:51,420 --> 00:43:53,370 It doesn't depend on the material. 866 00:43:53,370 --> 00:43:55,980 So that's the first thing to notice. 867 00:43:55,980 --> 00:43:57,150 We'll assume that. 868 00:43:57,150 --> 00:43:59,310 We'll assume the substrate independence-- 869 00:43:59,310 --> 00:44:01,270 I'll just be a little more precise-- 870 00:44:01,270 --> 00:44:06,655 substrate independence of life. 871 00:44:06,655 --> 00:44:07,530 So we'll assume that. 872 00:44:07,530 --> 00:44:09,440 I mean, there's a minority of people, 873 00:44:09,440 --> 00:44:11,930 minority of philosophers of mind who don't believe in this. 874 00:44:11,930 --> 00:44:15,630 But we'll take it, we'll take it for granted. 875 00:44:15,630 --> 00:44:20,130 The next thing to notice is that computer power is increasing. 876 00:44:20,130 --> 00:44:24,050 Computers are increasing in speed and memory. 877 00:44:24,050 --> 00:44:26,690 And it's been like this ever since computers were invented. 878 00:44:26,690 --> 00:44:29,010 Computers have just continued to increase 879 00:44:29,010 --> 00:44:31,800 in computational power. 880 00:44:31,800 --> 00:44:34,170 Now our brains are very complicated things. 881 00:44:34,170 --> 00:44:37,500 And our brains are doing operations, many operations 882 00:44:37,500 --> 00:44:42,660 a second, dealing with very vast amounts of information, very 883 00:44:42,660 --> 00:44:45,070 complex kinds of information. 884 00:44:45,070 --> 00:44:46,620 So our brains are really, in a sense, 885 00:44:46,620 --> 00:44:52,890 computers doing all these things simultaneously, 886 00:44:52,890 --> 00:44:55,310 at the same time. 887 00:44:55,310 --> 00:44:59,760 Now, our brain is actually a more powerful computer 888 00:44:59,760 --> 00:45:03,150 than anything that we have built for ourselves today, 889 00:45:03,150 --> 00:45:04,170 in a sense. 890 00:45:04,170 --> 00:45:05,979 I mean, even if we knew, even if we 891 00:45:05,979 --> 00:45:08,520 knew how the human brain worked, even if we knew how the mind 892 00:45:08,520 --> 00:45:10,980 and consciousness, all that stuff worked, 893 00:45:10,980 --> 00:45:15,990 we wouldn't be able to simulate the human brain today 894 00:45:15,990 --> 00:45:18,895 just because we don't have the computational power to. 895 00:45:18,895 --> 00:45:21,242 It's just so complex. 896 00:45:21,242 --> 00:45:22,950 In fact, it's been said that the brain is 897 00:45:22,950 --> 00:45:24,780 the most complex thing we know about 898 00:45:24,780 --> 00:45:27,976 in the entire observed universe. 899 00:45:32,350 --> 00:45:34,400 But computational power is increasing. 900 00:45:34,400 --> 00:45:36,140 Computing power is increasing. 901 00:45:36,140 --> 00:45:39,110 So one day, in principle, we should 902 00:45:39,110 --> 00:45:44,030 be able to simulate the human brain, but once we figure out, 903 00:45:44,030 --> 00:45:45,620 once we figure out how to simulate 904 00:45:45,620 --> 00:45:48,625 artificial intelligence. 905 00:45:48,625 --> 00:45:50,000 I mean, I suppose if we simulate, 906 00:45:50,000 --> 00:45:51,890 if we want to simulate human intelligence, 907 00:45:51,890 --> 00:45:54,160 then that would be like artificial human intelligence. 908 00:45:54,160 --> 00:45:57,340 So that would be artificial intelligence, nonetheless. 909 00:45:57,340 --> 00:46:00,170 But once we figure out how that all works, 910 00:46:00,170 --> 00:46:02,840 once we figure out how to build the right architecture so 911 00:46:02,840 --> 00:46:06,020 that consciousness would emerge, and then 912 00:46:06,020 --> 00:46:08,640 once we had the computational power 913 00:46:08,640 --> 00:46:16,250 to build a form of artificial intelligence, 914 00:46:16,250 --> 00:46:18,910 then it be possible to simulate the human mind. 915 00:46:18,910 --> 00:46:21,530 It would be possible to recreate the human mind. 916 00:46:21,530 --> 00:46:31,340 So the second assumption we make is that, one day, it's 917 00:46:31,340 --> 00:46:34,580 possible to simulate the mind. 918 00:46:45,200 --> 00:46:47,840 And this really stems from the fact 919 00:46:47,840 --> 00:46:50,090 that AI researchers believe that one day they'll 920 00:46:50,090 --> 00:46:53,392 figure out AI, and also from the observation that one, 921 00:46:53,392 --> 00:46:55,100 well, that a computer power is increasing 922 00:46:55,100 --> 00:46:57,308 and that one day we'll have sufficient computer power 923 00:46:57,308 --> 00:46:59,450 to simulate the human mind. 924 00:46:59,450 --> 00:47:03,290 So that's the second observation we make. 925 00:47:03,290 --> 00:47:04,220 Now, why stop there? 926 00:47:04,220 --> 00:47:06,240 Why stop at one mind? 927 00:47:06,240 --> 00:47:07,870 Why not simulate another, simulate 928 00:47:07,870 --> 00:47:10,790 two minds or three minds or four minds? 929 00:47:10,790 --> 00:47:13,383 Why not simulate the entire human civilization? 930 00:47:13,383 --> 00:47:17,039 I mean, conceivably, if you go far enough in the future-- 931 00:47:17,039 --> 00:47:19,580 maybe not 50 years, maybe not 100 years-- but maybe a million 932 00:47:19,580 --> 00:47:20,880 years, can you imagine that? 933 00:47:20,880 --> 00:47:22,296 Can you imagine what technology is 934 00:47:22,296 --> 00:47:25,340 going to be like in a million years from today? 935 00:47:25,340 --> 00:47:26,694 It's just mind boggling. 936 00:47:26,694 --> 00:47:28,110 Who knows what technology is going 937 00:47:28,110 --> 00:47:29,470 be like in a million years? 938 00:47:29,470 --> 00:47:32,720 So in a million years, this will probably 939 00:47:32,720 --> 00:47:34,206 be easy, to simulate a mind. 940 00:47:34,206 --> 00:47:36,080 And we'll want to do more challenging things, 941 00:47:36,080 --> 00:47:38,450 like simulating a whole society or 942 00:47:38,450 --> 00:47:41,240 the whole human civilization. 943 00:47:41,240 --> 00:47:43,880 So in fact, one day it may be possible to simulate 944 00:47:43,880 --> 00:47:46,550 the entire history of human civilization. 945 00:47:46,550 --> 00:47:49,850 I mean, and not only the history of humans, 946 00:47:49,850 --> 00:47:52,280 but also geological history and astronomical history 947 00:47:52,280 --> 00:47:54,750 and so forth. 948 00:47:54,750 --> 00:47:59,090 So one day it may be possible to simulate our entire reality 949 00:47:59,090 --> 00:48:01,640 if these two assumptions hold up and then 950 00:48:01,640 --> 00:48:04,550 we make the additional, make some further generalizations 951 00:48:04,550 --> 00:48:08,860 to this, simulate the mind, et cetera. 952 00:48:11,540 --> 00:48:13,100 So one day, it's possible that we'll 953 00:48:13,100 --> 00:48:16,070 be able to create a computer program that contains 954 00:48:16,070 --> 00:48:20,870 self-aware entities who perceive themselves in a physically 955 00:48:20,870 --> 00:48:24,440 real world, you know, with all their surroundings 956 00:48:24,440 --> 00:48:25,440 that are familiar to us. 957 00:48:25,440 --> 00:48:27,410 There will be trees, grass. 958 00:48:27,410 --> 00:48:31,490 There'll be other people, other conscious things 959 00:48:31,490 --> 00:48:33,750 that are part of the program. 960 00:48:33,750 --> 00:48:38,810 So this is kind of like, if any of you have seen The Matrix, 961 00:48:38,810 --> 00:48:45,050 The Matrix starts out in the world that 962 00:48:45,050 --> 00:48:46,730 is, in fact, an illusion. 963 00:48:46,730 --> 00:48:50,600 I mean, all, I mean the main character is actually living 964 00:48:50,600 --> 00:48:52,560 in a computer simulation. 965 00:48:52,560 --> 00:48:54,860 Now the premise is behind that simulation, 966 00:48:54,860 --> 00:48:59,420 like the super advanced real people, 967 00:48:59,420 --> 00:49:02,690 more real people need the humans to create power. 968 00:49:02,690 --> 00:49:07,120 That's a little silly. 969 00:49:07,120 --> 00:49:10,060 But nonetheless, that's the idea, 970 00:49:10,060 --> 00:49:14,630 that one day we'll be able to artificially create 971 00:49:14,630 --> 00:49:18,530 simulations of humans and human civilizations 972 00:49:18,530 --> 00:49:21,440 and all the human surroundings, trees, earthquakes, 973 00:49:21,440 --> 00:49:22,880 and the internet. 974 00:49:22,880 --> 00:49:25,354 And we can start at any point in the past. 975 00:49:25,354 --> 00:49:27,020 Maybe we'll decide to start at the point 976 00:49:27,020 --> 00:49:30,930 where man first invented the wheel, where 977 00:49:30,930 --> 00:49:33,080 man is extremely primitive. 978 00:49:33,080 --> 00:49:35,240 And then we can just let the simulation run. 979 00:49:35,240 --> 00:49:37,760 We just let the program run. 980 00:49:37,760 --> 00:49:42,320 And then we could see how humans evolve, see how they progress. 981 00:49:42,320 --> 00:49:49,700 So humans will eventually grow as a population. 982 00:49:49,700 --> 00:49:51,160 Human knowledge will grow. 983 00:49:51,160 --> 00:49:53,030 And eventually, human technology will grow. 984 00:49:56,030 --> 00:50:00,320 And so, one day these humans in our civilization, 985 00:50:00,320 --> 00:50:04,040 if we're ever able to do this, if we're ever able to-- 986 00:50:04,040 --> 00:50:09,176 if these two postulates are true, sorry, 987 00:50:09,176 --> 00:50:12,720 these two assumptions are true-- 988 00:50:12,720 --> 00:50:16,090 that one day our simulations, the simulations that we produce 989 00:50:16,090 --> 00:50:19,876 may, in fact, want to run simulations of themselves. 990 00:50:19,876 --> 00:50:22,376 They'll be advanced enough to run simulations of themselves. 991 00:50:26,010 --> 00:50:29,360 So then you have a simulated simulated reality. 992 00:50:29,360 --> 00:50:32,630 Then it can be the case that these simulated simulated 993 00:50:32,630 --> 00:50:35,270 humans are eventually progressed to the point where they want 994 00:50:35,270 --> 00:50:37,019 to run simulations, where they're able to, 995 00:50:37,019 --> 00:50:38,450 they're able to run simulations. 996 00:50:38,450 --> 00:50:42,080 So you can imagine having a hierarchy 997 00:50:42,080 --> 00:50:44,540 of different levels of reality. 998 00:50:44,540 --> 00:50:48,270 So you start out at the true reality, 999 00:50:48,270 --> 00:50:49,640 the true physical reality. 1000 00:50:54,000 --> 00:50:58,080 And then from there, the first civilization, 1001 00:50:58,080 --> 00:51:04,260 the first civilization that emerges in the real world 1002 00:51:04,260 --> 00:51:05,292 will run a simulation. 1003 00:51:05,292 --> 00:51:06,708 So you have the simulated reality. 1004 00:51:11,690 --> 00:51:14,515 And then, eventually, they'll run simulations. 1005 00:51:14,515 --> 00:51:16,370 And so you have another simulated reality. 1006 00:51:21,760 --> 00:51:23,330 And then you can have another. 1007 00:51:23,330 --> 00:51:26,350 And who knows how many you'll have? 1008 00:51:26,350 --> 00:51:36,530 This could go on indefinitely, maybe infinitely. 1009 00:51:36,530 --> 00:51:39,120 So you have this hierarchy of reality, 1010 00:51:39,120 --> 00:51:42,602 starting out with the true physical reality, 1011 00:51:42,602 --> 00:51:44,310 the top level, the non-simulated reality. 1012 00:51:44,310 --> 00:51:46,160 Then you have the first simulated reality. 1013 00:51:46,160 --> 00:51:47,560 Then you have simulations of simulations, 1014 00:51:47,560 --> 00:51:49,893 then you have simulations of simulations of simulations, 1015 00:51:49,893 --> 00:51:50,790 and so forth. 1016 00:51:50,790 --> 00:51:55,610 So you can kind of organize these 1017 00:51:55,610 --> 00:51:58,850 according to the trueness of reality 1018 00:51:58,850 --> 00:52:00,760 as it increases upwards. 1019 00:52:08,920 --> 00:52:14,060 The top level would be the most real, the truest reality. 1020 00:52:14,060 --> 00:52:16,190 The second level, the first simulated reality 1021 00:52:16,190 --> 00:52:19,370 would be not quite there, but almost there. 1022 00:52:19,370 --> 00:52:22,250 And then you have the second, which is less real, 1023 00:52:22,250 --> 00:52:28,530 has less trueness of reality, and so forth. 1024 00:52:28,530 --> 00:52:32,135 Now in principle, it could be possible, 1025 00:52:32,135 --> 00:52:34,730 to run simulations that are completely 1026 00:52:34,730 --> 00:52:37,640 indistinguishable from the true reality. 1027 00:52:37,640 --> 00:52:40,109 I mean, a sufficiently advanced civilization 1028 00:52:40,109 --> 00:52:41,150 would be able to do that. 1029 00:52:44,040 --> 00:52:46,140 I mean, right now we can't fathom it. 1030 00:52:46,140 --> 00:52:47,520 But who knows in a million years? 1031 00:52:47,520 --> 00:52:49,892 It might be easy to do. 1032 00:52:49,892 --> 00:52:52,100 And it wouldn't even have to be a perfect description 1033 00:52:52,100 --> 00:52:52,933 of the true reality. 1034 00:52:52,933 --> 00:52:55,550 It could just be accurate enough so 1035 00:52:55,550 --> 00:52:58,647 that the observers in the reality don't notice it. 1036 00:52:58,647 --> 00:53:00,230 And even early on, you could start out 1037 00:53:00,230 --> 00:53:02,010 having it not be very accurate at all. 1038 00:53:02,010 --> 00:53:04,850 But then once the civilization progresses, you make it more, 1039 00:53:04,850 --> 00:53:08,310 you increase the resolution, for example. 1040 00:53:08,310 --> 00:53:11,600 So in principle, it could be possible 1041 00:53:11,600 --> 00:53:14,660 that each of these realities are indistinguishable 1042 00:53:14,660 --> 00:53:17,360 from each other. 1043 00:53:17,360 --> 00:53:19,790 Each simulated reality could be indistinguishable 1044 00:53:19,790 --> 00:53:22,770 from the true reality. 1045 00:53:22,770 --> 00:53:25,341 So now, you can ask the question-- 1046 00:53:25,341 --> 00:53:26,840 so first, let's make the assumption. 1047 00:53:26,840 --> 00:53:28,200 Let's make these assumptions. 1048 00:53:28,200 --> 00:53:30,200 Let's assume the substrate independence of mind. 1049 00:53:30,200 --> 00:53:35,630 Let's assume that it's possible to create AI. 1050 00:53:35,630 --> 00:53:37,310 Let's assume that, one day, it will be 1051 00:53:37,310 --> 00:53:39,320 possible to simulate the mind. 1052 00:53:39,320 --> 00:53:41,950 And let's also assume that one day, we'll want to. 1053 00:53:41,950 --> 00:53:46,760 You know, one day, we'll want to. 1054 00:53:53,435 --> 00:53:56,630 If this is the case, then you can ask the question, well, 1055 00:53:56,630 --> 00:53:58,210 what's the probability that we're 1056 00:53:58,210 --> 00:54:01,280 living in a simulated reality? 1057 00:54:01,280 --> 00:54:03,620 To the best of our knowledge, I mean, 1058 00:54:03,620 --> 00:54:07,930 since all these realities would seem the same way, 1059 00:54:07,930 --> 00:54:09,680 they would be completely indistinguishable 1060 00:54:09,680 --> 00:54:11,270 from each other. 1061 00:54:11,270 --> 00:54:13,694 To the best of our knowledge, then, we 1062 00:54:13,694 --> 00:54:15,110 can be in any one of these levels. 1063 00:54:15,110 --> 00:54:17,550 There's no observable difference between any one of them. 1064 00:54:17,550 --> 00:54:22,128 So we should give each level, each one of these levels, 1065 00:54:22,128 --> 00:54:25,270 just label the levels, give each one of these levels 1066 00:54:25,270 --> 00:54:27,660 an equal probability. 1067 00:54:27,660 --> 00:54:29,390 They're all the same, observationally. 1068 00:54:29,390 --> 00:54:31,410 So give them all equal weight. 1069 00:54:31,410 --> 00:54:33,557 So then you can ask the question, you finally 1070 00:54:33,557 --> 00:54:36,140 ask the big question, what's the probability that we're living 1071 00:54:36,140 --> 00:54:37,670 in a computer simulation? 1072 00:54:37,670 --> 00:54:40,040 Now, simply based on the fact that there's so 1073 00:54:40,040 --> 00:54:43,280 many simulated realities, possibly infinitely 1074 00:54:43,280 --> 00:54:47,295 many of them, and only one true reality, 1075 00:54:47,295 --> 00:54:48,920 if you then ask what's the probability, 1076 00:54:48,920 --> 00:54:51,474 it's very, very close to 100%. 1077 00:54:51,474 --> 00:54:53,140 Because there are just so many of these. 1078 00:54:53,140 --> 00:54:55,830 These just vastly outnumber the true reality. 1079 00:54:55,830 --> 00:54:57,119 Each has an equal weight. 1080 00:54:57,119 --> 00:54:58,910 So the probability is very, very close to 1 1081 00:54:58,910 --> 00:55:02,130 that we're living in a computer simulation. 1082 00:55:02,130 --> 00:55:04,190 So that's the simulation argument. 1083 00:55:04,190 --> 00:55:07,560 You have to assume these assumptions to get 1084 00:55:07,560 --> 00:55:11,900 the conclusion that we're probably living in a computer 1085 00:55:11,900 --> 00:55:12,960 simulation. 1086 00:55:12,960 --> 00:55:16,420 So if these assumptions are true, 1087 00:55:16,420 --> 00:55:30,931 then we are probably living in The Matrix, or matrix-like 1088 00:55:30,931 --> 00:55:31,430 thing. 1089 00:55:36,130 --> 00:55:39,030 But you have to make those assumptions otherwise 1090 00:55:39,030 --> 00:55:41,550 this is just a crazy argument. 1091 00:55:41,550 --> 00:55:43,860 OK. does anyone have any questions about the argument 1092 00:55:43,860 --> 00:55:44,860 the simulation argument. 1093 00:55:44,860 --> 00:55:47,010 Not about the premises, not about the assumptions, 1094 00:55:47,010 --> 00:55:49,384 but about the way that I formed the conclusion that we're 1095 00:55:49,384 --> 00:55:51,870 probably living in The Matrix? 1096 00:55:51,870 --> 00:55:54,120 AUDIENCE: What's the odds that we're the true reality. 1097 00:55:54,120 --> 00:55:55,578 NICHOLAS DIBELLA: Very, very small. 1098 00:55:55,578 --> 00:55:58,590 It's just 1 out of however many simulated, however many levels 1099 00:55:58,590 --> 00:56:00,250 there are here. 1100 00:56:00,250 --> 00:56:00,750 Yeah. 1101 00:56:00,750 --> 00:56:02,041 So it could be 1 over infinity. 1102 00:56:02,041 --> 00:56:04,697 So it could be 0, 0 probability. 1103 00:56:08,900 --> 00:56:10,770 Yeah. 1104 00:56:10,770 --> 00:56:12,760 Any questions? 1105 00:56:12,760 --> 00:56:13,620 OK. 1106 00:56:13,620 --> 00:56:16,830 Now, you might reject the argument 1107 00:56:16,830 --> 00:56:19,469 by rejecting the premises. 1108 00:56:19,469 --> 00:56:21,510 I mentioned before that there are some people who 1109 00:56:21,510 --> 00:56:22,530 don't believe in the first one. 1110 00:56:22,530 --> 00:56:24,690 Some people believe that consciousness can only 1111 00:56:24,690 --> 00:56:29,300 exist in human form, for whatever reasons they have. 1112 00:56:29,300 --> 00:56:32,860 You can also object to the second one. 1113 00:56:32,860 --> 00:56:36,690 Maybe it just never will be possible to run 1114 00:56:36,690 --> 00:56:37,680 these simulations. 1115 00:56:37,680 --> 00:56:40,050 I mean, maybe we'll one day understand AI, 1116 00:56:40,050 --> 00:56:42,660 but we just won't ever have the computational power 1117 00:56:42,660 --> 00:56:48,000 to do so, to run these simulations. 1118 00:56:48,000 --> 00:56:50,790 Could also be the case that, well, one day we 1119 00:56:50,790 --> 00:56:54,390 might have the ability but we destroy ourselves 1120 00:56:54,390 --> 00:56:56,700 before we do it. 1121 00:56:56,700 --> 00:57:00,630 I mean, that with great power comes 1122 00:57:00,630 --> 00:57:03,120 great destructive capabilities. 1123 00:57:05,710 --> 00:57:09,120 So, maybe our civilization is just doomed 1124 00:57:09,120 --> 00:57:11,320 and it's just not possible. 1125 00:57:11,320 --> 00:57:15,750 So we can actually look at risks involved, 1126 00:57:15,750 --> 00:57:21,090 existential risks, risks that threaten our very existence. 1127 00:57:21,090 --> 00:57:24,090 And if we see that, if you do an analysis to see that we're 1128 00:57:24,090 --> 00:57:26,760 very, very probably going to blow ourselves up 1129 00:57:26,760 --> 00:57:29,490 or something like that, then that 1130 00:57:29,490 --> 00:57:34,559 would be good evidence against the second assumption. 1131 00:57:34,559 --> 00:57:37,100 So you could reject the second assumption for whatever reason 1132 00:57:37,100 --> 00:57:37,683 that you have. 1133 00:57:40,542 --> 00:57:45,579 AUDIENCE: [INAUDIBLE] 1134 00:57:45,579 --> 00:57:46,620 NICHOLAS DIBELLA: Either. 1135 00:57:46,620 --> 00:57:48,036 I mean, the people could build it. 1136 00:57:48,036 --> 00:57:49,492 Or maybe, eventually-- 1137 00:57:49,492 --> 00:57:55,590 AUDIENCE: [INAUDIBLE] 1138 00:57:55,590 --> 00:57:56,590 NICHOLAS DIBELLA: Right. 1139 00:57:56,590 --> 00:57:57,410 Right. 1140 00:57:57,410 --> 00:58:00,230 Well, I mean, we don't know all the laws of physics. 1141 00:58:00,230 --> 00:58:05,690 But I think we know, we think we know all the necessary laws 1142 00:58:05,690 --> 00:58:07,550 for consciousness to emerge. 1143 00:58:07,550 --> 00:58:09,590 I mean, when you look at the body, 1144 00:58:09,590 --> 00:58:11,570 like when you look at how the heart works, 1145 00:58:11,570 --> 00:58:14,190 you don't need to understand quantum gravity to understand 1146 00:58:14,190 --> 00:58:15,189 how the heart works. 1147 00:58:15,189 --> 00:58:17,480 So you don't need to know quantum gravity to understand 1148 00:58:17,480 --> 00:58:19,180 why an apple falls down. 1149 00:58:19,180 --> 00:58:22,670 You don't need the true, deep down, truly fundamental laws 1150 00:58:22,670 --> 00:58:24,300 of physics to understand it. 1151 00:58:24,300 --> 00:58:26,690 The laws that we have right now are probably 1152 00:58:26,690 --> 00:58:31,070 sufficient to figure out how the mind works, how the brain, 1153 00:58:31,070 --> 00:58:33,660 how the consciousness and intelligence and emotion and so 1154 00:58:33,660 --> 00:58:35,981 forth emerge from the brain. 1155 00:58:35,981 --> 00:58:37,292 Yeah? 1156 00:58:37,292 --> 00:58:40,958 AUDIENCE: Well, if we destroyed ourselves 1157 00:58:40,958 --> 00:58:44,280 before we could simulate reality, 1158 00:58:44,280 --> 00:58:49,970 then wouldn't it be impossible for it to be simulated? 1159 00:58:49,970 --> 00:58:53,420 NICHOLAS DIBELLA: Well, I mean, so if it's 1160 00:58:53,420 --> 00:58:57,170 probable for civilizations to eventually destroy themselves 1161 00:58:57,170 --> 00:58:59,570 in some way before they have the ability 1162 00:58:59,570 --> 00:59:03,200 to run these simulations, then yeah, 1163 00:59:03,200 --> 00:59:06,935 that would be evidence against the conclusion. 1164 00:59:06,935 --> 00:59:08,810 Then it would be the case that we're probably 1165 00:59:08,810 --> 00:59:09,770 not living in the simulation. 1166 00:59:09,770 --> 00:59:11,561 We're, in fact, living in the true reality. 1167 00:59:14,060 --> 00:59:19,730 But who knows how likely it is that we'll destroy ourselves? 1168 00:59:19,730 --> 00:59:22,040 It's hard to get a number like that. 1169 00:59:22,040 --> 00:59:24,480 I mean, it's really hard to mathematize, 1170 00:59:24,480 --> 00:59:27,800 it's really hard to quantify human actions 1171 00:59:27,800 --> 00:59:30,030 and human psychology. 1172 00:59:30,030 --> 00:59:31,730 But the risk could be quite high. 1173 00:59:31,730 --> 00:59:34,900 I mean, we definitely have the resources to do it. 1174 00:59:34,900 --> 00:59:37,850 And if the wrong people get their hands 1175 00:59:37,850 --> 00:59:41,870 on the right toy, the right weapon, then that's it. 1176 00:59:41,870 --> 00:59:45,740 I mean, we could blow the world up many, many times. 1177 00:59:45,740 --> 00:59:47,610 We have the weapons to do that. 1178 00:59:47,610 --> 00:59:53,870 So some people even, some people actually, I mean, 1179 00:59:53,870 --> 00:59:55,790 the probability that we'll destroy ourselves 1180 00:59:55,790 --> 00:59:58,220 is a number that hasn't really been assessed very 1181 00:59:58,220 --> 01:00:00,180 quantitatively by many people. 1182 01:00:00,180 --> 01:00:05,000 But there are some people that give it a number as high as 1/3 1183 01:00:05,000 --> 01:00:07,790 that we'll destroy ourselves in the next century or so. 1184 01:00:10,597 --> 01:00:11,430 There's a high risk. 1185 01:00:11,430 --> 01:00:13,269 There's a high risk involved. 1186 01:00:13,269 --> 01:00:15,310 But, I mean, how do you actually get that number? 1187 01:00:15,310 --> 01:00:19,631 AUDIENCE: [INAUDIBLE] 1188 01:00:19,631 --> 01:00:20,630 NICHOLAS DIBELLA: Right. 1189 01:00:20,630 --> 01:00:22,682 AUDIENCE: [INAUDIBLE] 1190 01:00:22,682 --> 01:00:23,640 NICHOLAS DIBELLA: Yeah. 1191 01:00:23,640 --> 01:00:27,330 I mean, historically, we've always 1192 01:00:27,330 --> 01:00:32,712 figured out a way out of the doomsday, somehow. 1193 01:00:32,712 --> 01:00:34,560 AUDIENCE: We'll be OK. 1194 01:00:34,560 --> 01:00:35,896 NICHOLAS DIBELLA: Maybe. 1195 01:00:35,896 --> 01:00:47,220 AUDIENCE: [INAUDIBLE] 1196 01:00:47,220 --> 01:00:48,340 NICHOLAS DIBELLA: Yeah. 1197 01:00:48,340 --> 01:00:48,910 Who knows? 1198 01:00:48,910 --> 01:00:50,470 Who knows? 1199 01:00:50,470 --> 01:00:53,029 So yeah, you might reject that second assumption. 1200 01:00:53,029 --> 01:00:54,820 You might also reject the third assumption. 1201 01:00:54,820 --> 01:00:55,865 Oh, a question. 1202 01:00:55,865 --> 01:00:57,350 Yeah. 1203 01:00:57,350 --> 01:01:29,395 AUDIENCE: [INAUDIBLE] 1204 01:01:29,395 --> 01:01:30,520 NICHOLAS DIBELLA: Oh, yeah. 1205 01:01:30,520 --> 01:01:33,137 So if it turns out that the simulated civilizations don't 1206 01:01:33,137 --> 01:01:34,720 destroy themselves, then you're saying 1207 01:01:34,720 --> 01:01:36,256 that would be good evidence for us 1208 01:01:36,256 --> 01:01:38,380 that we're probably not going to destroy ourselves. 1209 01:01:38,380 --> 01:01:38,960 Right, right. 1210 01:01:38,960 --> 01:01:42,361 But we don't even know if these simulated realities exist. 1211 01:01:42,361 --> 01:01:42,860 Yeah. 1212 01:01:42,860 --> 01:01:45,490 I mean, if they did exist, that would 1213 01:01:45,490 --> 01:01:48,481 be a good way of getting ahold of what the probability of our 1214 01:01:48,481 --> 01:01:49,480 destroying ourselves is. 1215 01:01:49,480 --> 01:01:52,029 I mean, often, scientists run simulations 1216 01:01:52,029 --> 01:01:53,320 to get probabilities of things. 1217 01:01:53,320 --> 01:01:54,820 Like how probable is this to happen? 1218 01:01:54,820 --> 01:01:56,001 So we run a simulation. 1219 01:01:56,001 --> 01:01:57,542 For example, we simulate the weather. 1220 01:01:57,542 --> 01:01:59,690 How probable is it that this is going to happen. 1221 01:01:59,690 --> 01:02:00,940 So I mean, it would be useful. 1222 01:02:00,940 --> 01:02:03,520 It would be useful to simulate the human civilization. 1223 01:02:03,520 --> 01:02:07,000 Maybe we'll run a million civilizations of ourselves 1224 01:02:07,000 --> 01:02:12,250 and we see that 250,000 of the civilizations 1225 01:02:12,250 --> 01:02:14,590 eventually destroy themselves. 1226 01:02:14,590 --> 01:02:18,100 That would be evidence that there's about a 25% chance 1227 01:02:18,100 --> 01:02:20,995 that we'll eventually destroy ourselves. 1228 01:02:20,995 --> 01:02:45,574 AUDIENCE: [INAUDIBLE] 1229 01:02:45,574 --> 01:02:47,740 NICHOLAS DIBELLA: I'm not sure exactly I understand. 1230 01:02:47,740 --> 01:02:51,397 I mean, the simulations would be completely removed. 1231 01:02:51,397 --> 01:02:53,480 I mean, they would just be like a computer program 1232 01:02:53,480 --> 01:02:58,280 that wouldn't be able to affect the world of the simulators. 1233 01:02:58,280 --> 01:02:58,890 I mean, yeah. 1234 01:02:58,890 --> 01:03:01,280 I mean, the computer program, you know, 1235 01:03:01,280 --> 01:03:04,940 would keep running even after the civilization destroys 1236 01:03:04,940 --> 01:03:06,380 itself. 1237 01:03:06,380 --> 01:03:10,100 I mean, you can even imagine having 1238 01:03:10,100 --> 01:03:12,710 time go by in the simulation twice as fast as it goes 1239 01:03:12,710 --> 01:03:14,025 by here, or 100 times as fast. 1240 01:03:14,025 --> 01:03:16,400 I mean, we want to know what's going happen to our future 1241 01:03:16,400 --> 01:03:18,004 so let's simulate it. 1242 01:03:18,004 --> 01:03:18,920 We don't want to wait. 1243 01:03:18,920 --> 01:03:20,120 We want to simulate it and figure out 1244 01:03:20,120 --> 01:03:21,226 what's going to happen. 1245 01:03:21,226 --> 01:03:23,100 Then we we can get ahold of what will happen. 1246 01:03:23,100 --> 01:03:24,683 And so, I think it would be very, very 1247 01:03:24,683 --> 01:03:27,734 cool if we're ever able to simulate human interaction. 1248 01:03:27,734 --> 01:03:29,150 Because all the time I wonder what 1249 01:03:29,150 --> 01:03:30,525 would happen if I said that. 1250 01:03:30,525 --> 01:03:31,900 What would happen if I said that? 1251 01:03:31,900 --> 01:03:34,270 What would happen, you know, if I did that? 1252 01:03:34,270 --> 01:03:36,381 All the time I wonder about this sort of thing. 1253 01:03:36,381 --> 01:03:37,880 So yeah, simulation would definitely 1254 01:03:37,880 --> 01:03:42,436 help us to learn about our own future and our own nature. 1255 01:03:42,436 --> 01:03:43,019 Yeah? 1256 01:03:43,019 --> 01:03:45,394 AUDIENCE: If we destroyed the universe that simulated us, 1257 01:03:45,394 --> 01:03:47,076 would we cease to exist. 1258 01:03:47,076 --> 01:03:49,700 NICHOLAS DIBELLA: If we destroy the universe that simulated us, 1259 01:03:49,700 --> 01:03:51,890 would we cease to exist? 1260 01:03:51,890 --> 01:03:54,390 I suppose, because we exist inside the universe. 1261 01:03:54,390 --> 01:03:58,040 So if we destroyed it, boom, that's us. 1262 01:03:58,040 --> 01:04:00,080 Where do we go? 1263 01:04:00,080 --> 01:04:03,320 But I mean, the programmers will be smart enough 1264 01:04:03,320 --> 01:04:05,212 to avoid this sort of thing. 1265 01:04:05,212 --> 01:04:06,920 Like maybe one way that that could happen 1266 01:04:06,920 --> 01:04:09,890 is to have a computer that like really super overheats. 1267 01:04:09,890 --> 01:04:12,590 And then it could eventually become a black hole 1268 01:04:12,590 --> 01:04:15,320 by the actions of the simulated civilization. 1269 01:04:15,320 --> 01:04:18,170 Maybe that's one way of the simulation 1270 01:04:18,170 --> 01:04:20,370 like destroying the universe that it's a part of. 1271 01:04:20,370 --> 01:04:23,270 But if you're a smart simulator, then you probably 1272 01:04:23,270 --> 01:04:27,560 want to program the simulation so that that sort of thing 1273 01:04:27,560 --> 01:04:30,086 just doesn't happen. 1274 01:04:30,086 --> 01:04:31,463 Yeah. 1275 01:04:31,463 --> 01:04:32,840 AUDIENCE: I have a question. 1276 01:04:32,840 --> 01:04:35,256 You said that since there are so many simulated realities, 1277 01:04:35,256 --> 01:04:38,950 that if each one made a new one. 1278 01:04:38,950 --> 01:04:42,190 So wouldn't it be more likely that it's not a straight line? 1279 01:04:42,190 --> 01:04:45,030 It's more like a branch, like maybe one civilization 1280 01:04:45,030 --> 01:04:46,350 makes two. 1281 01:04:46,350 --> 01:04:48,241 NICHOLAS DIBELLA: Right, right, right. 1282 01:04:48,241 --> 01:04:50,260 AUDIENCE: Because if we were in like one of the higher ones, 1283 01:04:50,260 --> 01:04:52,301 that means we having made another simulation yet. 1284 01:04:54,017 --> 01:04:55,350 NICHOLAS DIBELLA: Good question. 1285 01:04:55,350 --> 01:04:57,660 I was actually going to just talk about that soon. 1286 01:04:57,660 --> 01:04:58,160 Right. 1287 01:04:58,160 --> 01:05:00,540 So I drew a pretty simple picture here. 1288 01:05:00,540 --> 01:05:02,419 I mean, I said that there's a true reality, 1289 01:05:02,419 --> 01:05:03,710 a civilization of true reality. 1290 01:05:03,710 --> 01:05:04,950 Than they run a simulation. 1291 01:05:04,950 --> 01:05:07,549 Then there's one simulation that runs another simulation. 1292 01:05:07,549 --> 01:05:09,090 That runs a simulation, and so forth. 1293 01:05:09,090 --> 01:05:11,060 So you have this straight line. 1294 01:05:11,060 --> 01:05:13,980 But things will probably be more complicated than this. 1295 01:05:13,980 --> 01:05:17,120 I mean, you'd probably start out, I mean, 1296 01:05:17,120 --> 01:05:18,860 if you started with your true reality, 1297 01:05:18,860 --> 01:05:21,840 then you can imagine it being more than one branch. 1298 01:05:21,840 --> 01:05:23,810 Maybe there were three branches. 1299 01:05:23,810 --> 01:05:26,260 Then each one of these branches has its own branches. 1300 01:05:29,200 --> 01:05:30,650 And then it keeps going on. 1301 01:05:30,650 --> 01:05:34,578 Now you have a very, very complicated tree. 1302 01:05:34,578 --> 01:05:36,570 AUDIENCE: [INAUDIBLE] 1303 01:05:36,570 --> 01:05:38,900 NICHOLAS DIBELLA: That increases the odds even more. 1304 01:05:38,900 --> 01:05:42,630 AUDIENCE: [INAUDIBLE] 1305 01:05:42,630 --> 01:05:43,630 NICHOLAS DIBELLA: Right. 1306 01:05:43,630 --> 01:05:48,280 So in this case, you just count the points in the tree. 1307 01:05:48,280 --> 01:05:49,685 1, 2, 3, 4, 5, 6, 7. 1308 01:05:49,685 --> 01:05:51,310 You count them all and then you compare 1309 01:05:51,310 --> 01:05:54,610 with the one true level, the true civilization. 1310 01:05:54,610 --> 01:05:57,340 Then it becomes even more likely. 1311 01:05:57,340 --> 01:05:59,572 But if you extend this idea even further, 1312 01:05:59,572 --> 01:06:01,030 then it doesn't even work anywhere. 1313 01:06:01,030 --> 01:06:04,240 Because conceivably, you know, if the true universe is 1314 01:06:04,240 --> 01:06:05,960 infinite, for example, then there 1315 01:06:05,960 --> 01:06:08,550 will exist other civilizations. 1316 01:06:08,550 --> 01:06:11,560 There will be other roots in this tree. 1317 01:06:11,560 --> 01:06:13,910 There will be many branches and other roots, as well. 1318 01:06:13,910 --> 01:06:16,593 And this will run simulations and then 1319 01:06:16,593 --> 01:06:19,000 that will run simulations and you have another tree. 1320 01:06:19,000 --> 01:06:20,541 If the universe is infinite, then you 1321 01:06:20,541 --> 01:06:22,744 can imagine there being infinite number of roots 1322 01:06:22,744 --> 01:06:24,160 because there are so many of them. 1323 01:06:27,410 --> 01:06:29,800 Well, in fact, you have an infinite number 1324 01:06:29,800 --> 01:06:32,230 of civilizations in the true reality 1325 01:06:32,230 --> 01:06:35,020 in an infinite universe, as well as an infinite number 1326 01:06:35,020 --> 01:06:36,541 of simulated realities. 1327 01:06:36,541 --> 01:06:38,290 And so then if we want to compare the two, 1328 01:06:38,290 --> 01:06:40,990 give each civilization equal weight, each one 1329 01:06:40,990 --> 01:06:44,170 equal probability, if you want to compare the two, then 1330 01:06:44,170 --> 01:06:48,790 the probability that you get isn't something like, you know, 1331 01:06:48,790 --> 01:06:58,780 999999 over-- 1332 01:06:58,780 --> 01:07:02,830 like, if there are 100,000 total, and 99,199 1333 01:07:02,830 --> 01:07:06,490 simulated realities, then it would be something like that, 1334 01:07:06,490 --> 01:07:11,490 or that over infinity, whatever. 1335 01:07:11,490 --> 01:07:16,430 It would, in fact, be infinity over infinity. 1336 01:07:16,430 --> 01:07:18,329 That's the symbol for infinity. 1337 01:07:18,329 --> 01:07:19,120 And so what's that? 1338 01:07:19,120 --> 01:07:20,890 What's infinity over infinity? 1339 01:07:20,890 --> 01:07:26,880 So, it's no longer well-defined. 1340 01:07:26,880 --> 01:07:28,690 And so this is actually a problem 1341 01:07:28,690 --> 01:07:31,270 with the argument itself. 1342 01:07:31,270 --> 01:07:34,720 So even if the assumptions are true, if the true universe, 1343 01:07:34,720 --> 01:07:36,310 if the true non-simulated reality is 1344 01:07:36,310 --> 01:07:39,340 an infinite universe, then the probability isn't infinite. 1345 01:07:39,340 --> 01:07:41,514 It's, in fact, if infinity were infinity, 1346 01:07:41,514 --> 01:07:42,430 it's this weird thing. 1347 01:07:42,430 --> 01:07:44,099 How do you deal with that? 1348 01:07:44,099 --> 01:07:45,640 Well, it turns out there are actually 1349 01:07:45,640 --> 01:07:46,980 ways of getting around that. 1350 01:07:46,980 --> 01:07:48,563 There are actually mathematical tricks 1351 01:07:48,563 --> 01:07:51,310 of getting a meaningful number and getting 1352 01:07:51,310 --> 01:07:53,310 ways of the argument, in fact, working. 1353 01:07:53,310 --> 01:07:57,190 But it's kind of sophisticated and not really necessary 1354 01:07:57,190 --> 01:08:03,260 for the next four minutes of class. 1355 01:08:03,260 --> 01:08:04,260 OK. 1356 01:08:04,260 --> 01:08:07,290 So, that's the simulation argument. 1357 01:08:07,290 --> 01:08:10,614 It rests on these assumptions. 1358 01:08:10,614 --> 01:08:12,280 If they're all true, then we're probably 1359 01:08:12,280 --> 01:08:13,690 living in a simulation. 1360 01:08:13,690 --> 01:08:15,340 But you can actually form a general, 1361 01:08:15,340 --> 01:08:17,590 you can actually form a more general conclusion, which 1362 01:08:17,590 --> 01:08:21,460 is that either it will never be possible to run 1363 01:08:21,460 --> 01:08:26,529 these simulations for whatever reason, or we'll never want to, 1364 01:08:26,529 --> 01:08:29,608 or something like that, or we're probably living 1365 01:08:29,608 --> 01:08:30,649 in a computer simulation. 1366 01:08:30,649 --> 01:08:34,840 So there's really like a three-fold conclusion 1367 01:08:34,840 --> 01:08:37,970 that you can make from this argument. 1368 01:08:37,970 --> 01:08:39,664 OK. 1369 01:08:39,664 --> 01:08:41,080 So that's the simulation argument. 1370 01:08:41,080 --> 01:08:42,490 Is it testable? 1371 01:08:42,490 --> 01:08:44,640 Oh, I had some stuff to say about this. 1372 01:08:44,640 --> 01:08:45,640 I'm running out of time. 1373 01:08:45,640 --> 01:08:46,473 Maybe it's testable. 1374 01:08:46,473 --> 01:08:49,359 I mean, certainly if you saw a dialog box appear suddenly 1375 01:08:49,359 --> 01:08:50,859 in front of you and say, hi, you're 1376 01:08:50,859 --> 01:08:53,380 living in a computer simulation, click 1377 01:08:53,380 --> 01:08:57,065 to find out more, that would evidence for it. 1378 01:08:57,065 --> 01:08:58,689 First, you probably think you're crazy. 1379 01:08:58,689 --> 01:09:00,564 But if it happened many times to many people, 1380 01:09:00,564 --> 01:09:03,279 then that would be evidence for it. 1381 01:09:03,279 --> 01:09:05,229 Anyone who has ever programmed knows 1382 01:09:05,229 --> 01:09:08,350 that programs often have glitches 1383 01:09:08,350 --> 01:09:09,591 or bugs or things like that. 1384 01:09:09,591 --> 01:09:11,049 So if we're living in a simulation, 1385 01:09:11,049 --> 01:09:12,790 if we're living in a computer program, 1386 01:09:12,790 --> 01:09:14,966 occasional glitches could occur. 1387 01:09:14,966 --> 01:09:18,189 In fact, it's been proposed that the apparent incompatibility 1388 01:09:18,189 --> 01:09:21,189 between quantum mechanics and general relativity 1389 01:09:21,189 --> 01:09:25,895 could just be a glitch in the program. 1390 01:09:25,895 --> 01:09:27,520 And so the first time that a distinctly 1391 01:09:27,520 --> 01:09:29,830 quantum gravitational thing would happen-- 1392 01:09:29,830 --> 01:09:31,810 I didn't talk about phenomena that 1393 01:09:31,810 --> 01:09:34,120 would happen in quantum gravity, but one example 1394 01:09:34,120 --> 01:09:36,290 would be a black hole evaporating. 1395 01:09:36,290 --> 01:09:38,740 So the first time that quantum gravity 1396 01:09:38,740 --> 01:09:42,939 would have to work in the simulation, the program 1397 01:09:42,939 --> 01:09:44,240 might just crash. 1398 01:09:44,240 --> 01:09:47,229 So the universe might just crash. 1399 01:09:47,229 --> 01:09:50,870 So that's a problem. 1400 01:09:50,870 --> 01:09:54,200 So today, I looked at two theories of reality. 1401 01:09:54,200 --> 01:09:56,670 Reality is mathematical, reality is a simulation. 1402 01:09:56,670 --> 01:09:58,930 But I suppose even in the simulation argument, 1403 01:09:58,930 --> 01:09:59,770 eventually you get to the true reality. 1404 01:09:59,770 --> 01:10:01,400 Then you ask what's the nature of that. 1405 01:10:01,400 --> 01:10:02,870 And in that case, the mathematical universe 1406 01:10:02,870 --> 01:10:04,390 hypothesis would help you with that. 1407 01:10:04,390 --> 01:10:06,314 But there are other theories. 1408 01:10:06,314 --> 01:10:08,230 Maybe the universe isn't a mathematical thing. 1409 01:10:08,230 --> 01:10:12,580 I talked about the relation between math, matter, 1410 01:10:12,580 --> 01:10:13,330 and mind-- 1411 01:10:13,330 --> 01:10:18,495 math, matter, and mind. 1412 01:10:18,495 --> 01:10:21,050 They're related to each other. 1413 01:10:21,050 --> 01:10:24,460 They're related to each other in some way, 1414 01:10:24,460 --> 01:10:26,376 nobody really knows how. 1415 01:10:26,376 --> 01:10:28,750 But often in physics, when we see things that are related 1416 01:10:28,750 --> 01:10:30,760 to each other, like space and time, 1417 01:10:30,760 --> 01:10:32,842 they related to each other in some way-- in fact, 1418 01:10:32,842 --> 01:10:35,050 Einstein figured out that they're unified in a single 1419 01:10:35,050 --> 01:10:36,139 object-- 1420 01:10:36,139 --> 01:10:37,930 it's been proposed that maybe math, matter, 1421 01:10:37,930 --> 01:10:41,500 and mind are, in fact, different aspects 1422 01:10:41,500 --> 01:10:43,540 of some fundamental thing. 1423 01:10:43,540 --> 01:10:47,177 Math is some manifestation of this mystical thing. 1424 01:10:47,177 --> 01:10:48,760 Matter is another manifestation of it. 1425 01:10:48,760 --> 01:10:50,650 Mind is another manifestation of it. 1426 01:10:50,650 --> 01:10:53,470 Who knows? 1427 01:10:53,470 --> 01:10:55,520 That's another idea. 1428 01:10:55,520 --> 01:10:59,050 It's also been proposed, similar to the parallel universe stuff 1429 01:10:59,050 --> 01:11:01,960 that I talked about, is that all possible worlds exist. 1430 01:11:01,960 --> 01:11:04,120 All logically possible worlds exist-- 1431 01:11:04,120 --> 01:11:06,970 A world in which I, you know, walked out 1432 01:11:06,970 --> 01:11:09,370 of the classroom, a world in which I don't, a world 1433 01:11:09,370 --> 01:11:13,240 in which I throw this up and don't catch it-- 1434 01:11:13,240 --> 01:11:14,530 all possible worlds exist. 1435 01:11:24,250 --> 01:11:27,520 This is another theory that's been proposed, called 1436 01:11:27,520 --> 01:11:29,820 modal realism-- 1437 01:11:29,820 --> 01:11:32,590 I'm just trying to present as many interesting theories 1438 01:11:32,590 --> 01:11:34,840 as I can in the time that I have left-- 1439 01:11:34,840 --> 01:11:38,970 called modal realism. 1440 01:11:38,970 --> 01:11:41,710 And it was proposed by a philosopher called David Lewis. 1441 01:11:41,710 --> 01:11:43,630 And it's similar to these parallel universes 1442 01:11:43,630 --> 01:11:44,840 that I talked about. 1443 01:11:44,840 --> 01:11:48,850 But in fact, Lewis arrives at it through purely philosophical 1444 01:11:48,850 --> 01:11:49,810 methods. 1445 01:11:49,810 --> 01:11:51,550 He doesn't make any physical or mathematical arguments. 1446 01:11:51,550 --> 01:11:54,008 In fact, he doesn't even believe that these possible worlds 1447 01:11:54,008 --> 01:11:55,650 could be mathematical things. 1448 01:11:55,650 --> 01:11:58,320 He arrives at it from a purely philosophical argument, 1449 01:11:58,320 --> 01:11:59,771 which is interesting. 1450 01:11:59,771 --> 01:12:00,270 OK. 1451 01:12:00,270 --> 01:12:02,560 So, summarize the course. 1452 01:12:02,560 --> 01:12:04,440 Over the past eight weeks, we've talked 1453 01:12:04,440 --> 01:12:05,520 about some big questions. 1454 01:12:05,520 --> 01:12:06,770 Was there a beginning of time? 1455 01:12:06,770 --> 01:12:08,010 Will there be an end? 1456 01:12:08,010 --> 01:12:08,610 And so forth. 1457 01:12:08,610 --> 01:12:11,670 And today I talked about the nature of reality. 1458 01:12:11,670 --> 01:12:14,430 Although we're definitely a long way from knowing for sure what 1459 01:12:14,430 --> 01:12:15,945 the answer to these questions are, 1460 01:12:15,945 --> 01:12:17,820 I hope that I've been able to convince to you 1461 01:12:17,820 --> 01:12:21,060 all that we're making progress. 1462 01:12:21,060 --> 01:12:23,040 We've come a long way as a civilization, 1463 01:12:23,040 --> 01:12:25,801 particularly over the last 100 years. 1464 01:12:25,801 --> 01:12:27,300 Now, these questions, when you first 1465 01:12:27,300 --> 01:12:30,130 encounter them, they seem hopelessly difficult. 1466 01:12:30,130 --> 01:12:32,850 They seem possibly impossible to answer. 1467 01:12:32,850 --> 01:12:37,290 But with the rise of modern science and even philosophy, 1468 01:12:37,290 --> 01:12:39,490 we've finally been able to attack these questions. 1469 01:12:39,490 --> 01:12:41,615 And I think we've really been able to start to make 1470 01:12:41,615 --> 01:12:43,335 some meaningful progress. 1471 01:12:43,335 --> 01:12:45,210 Now, I don't know if we'll ever know for sure 1472 01:12:45,210 --> 01:12:47,650 what the correct answers to these questions are, 1473 01:12:47,650 --> 01:12:50,580 or even if it's possible to know in principle. 1474 01:12:50,580 --> 01:12:53,670 But I do find it uplifting that we minuscule 1475 01:12:53,670 --> 01:12:57,870 systems, minuscule subsystems in a vast, possibly infinite 1476 01:12:57,870 --> 01:13:02,250 universe, are able to even begin to unlock some of the deepest 1477 01:13:02,250 --> 01:13:05,100 secrets of reality. 1478 01:13:05,100 --> 01:13:08,130 And with that, it was a pleasure to have you all as a class. 1479 01:13:08,130 --> 01:13:10,069 And I hope you enjoyed it as much as I did. 1480 01:13:13,860 --> 01:13:16,590 If anyone has any questions about any of the big questions 1481 01:13:16,590 --> 01:13:21,270 that I've talked about, I'm happy to stay after class. 1482 01:13:21,270 --> 01:13:22,710 OK. 1483 01:13:22,710 --> 01:13:24,960 You're free to leave, but you have a question? 1484 01:13:24,960 --> 01:13:25,642 Sure. 1485 01:13:25,642 --> 01:13:29,420 AUDIENCE: [INAUDIBLE] 1486 01:13:29,420 --> 01:13:32,000 NICHOLAS DIBELLA: How can you prove it? 1487 01:13:32,000 --> 01:13:34,060 Well, the way that you prove these things 1488 01:13:34,060 --> 01:13:40,210 about the universe, they're all by indirect means. 1489 01:13:40,210 --> 01:13:43,060 I mean, in the first two weeks, I talked about the fact 1490 01:13:43,060 --> 01:13:47,110 that we observe the universe is expanding. 1491 01:13:47,110 --> 01:13:50,100 That was something that was noticed in the 1920s, 1930s. 1492 01:13:50,100 --> 01:13:51,550 The universe is expanding. 1493 01:13:51,550 --> 01:13:54,880 Distances between points in the universe is increasing. 1494 01:13:54,880 --> 01:13:58,540 Eventually, actually in 1998, it was finally 1495 01:13:58,540 --> 01:14:00,340 observed that the expansion of the universe 1496 01:14:00,340 --> 01:14:02,450 is actually accelerating. 1497 01:14:02,450 --> 01:14:05,950 So not only is it expanding, but it's expanding at an increasing 1498 01:14:05,950 --> 01:14:07,120 rate. 1499 01:14:07,120 --> 01:14:09,550 So it's never going to stop expanding 1500 01:14:09,550 --> 01:14:10,930 and eventually start contracting. 1501 01:14:10,930 --> 01:14:12,910 It's, in fact, going to expand forever. 1502 01:14:12,910 --> 01:14:16,720 So, we have observational evidence 1503 01:14:16,720 --> 01:14:18,410 that the universe will expand forever, 1504 01:14:18,410 --> 01:14:22,630 indefinitely, through acceleration 1505 01:14:22,630 --> 01:14:25,390 caused by dark energy. 1506 01:14:25,390 --> 01:14:28,566 And nobody has any idea what dark energy is. 1507 01:14:28,566 --> 01:14:30,787 AUDIENCE: [INAUDIBLE] 1508 01:14:30,787 --> 01:14:32,620 NICHOLAS DIBELLA: It's not going to run out. 1509 01:14:32,620 --> 01:14:34,360 There's no reason to think that it's 1510 01:14:34,360 --> 01:14:37,000 going to run out in any way. 1511 01:14:37,000 --> 01:14:39,370 But yeah, if there's no longer any dark energy, 1512 01:14:39,370 --> 01:14:41,650 than the universe would just be subject to the gravity 1513 01:14:41,650 --> 01:14:44,320 of normal matter and dark matter and radiation, 1514 01:14:44,320 --> 01:14:47,560 like lights, electromagnetic waves. 1515 01:14:47,560 --> 01:14:51,210 In that case, the universe would eventually contract, 1516 01:14:51,210 --> 01:14:54,280 eventually. 1517 01:14:54,280 --> 01:14:56,550 But there's no reason to think that dark matter, 1518 01:14:56,550 --> 01:14:58,400 that dark matter will run out. 1519 01:14:58,400 --> 01:15:00,690 In fact, it seems to magically keep reappearing. 1520 01:15:00,690 --> 01:15:02,530 The density of dark matter has been 1521 01:15:02,530 --> 01:15:06,580 a constant over all the history of the universe. 1522 01:15:06,580 --> 01:15:09,160 I mean, usually the density of things 1523 01:15:09,160 --> 01:15:11,530 decreases when you expand the volume. 1524 01:15:11,530 --> 01:15:14,557 Like if you have a balloon that has a fixed number of atoms, 1525 01:15:14,557 --> 01:15:17,140 then expanding the balloon will cause the density of the atoms 1526 01:15:17,140 --> 01:15:18,975 to decrease. 1527 01:15:18,975 --> 01:15:20,350 So the density of ordinary matter 1528 01:15:20,350 --> 01:15:22,109 decreases when the universe expands. 1529 01:15:22,109 --> 01:15:23,650 That's not the case with dark energy. 1530 01:15:23,650 --> 01:15:25,858 When the universe expanse, the density of dark energy 1531 01:15:25,858 --> 01:15:27,684 just stays the same. 1532 01:15:27,684 --> 01:15:30,025 AUDIENCE: [INAUDIBLE] 1533 01:15:30,025 --> 01:15:31,400 NICHOLAS DIBELLA: So it's really, 1534 01:15:31,400 --> 01:15:35,710 so some people speculate that it's actually vacuum energy. 1535 01:15:35,710 --> 01:15:37,410 There's always vacuum. 1536 01:15:37,410 --> 01:15:38,870 There's always space. 1537 01:15:38,870 --> 01:15:42,150 So there's always going to be energy from the vacuum.