1 00:00:00,500 --> 00:00:02,890 The following content is provided under a Creative 2 00:00:02,890 --> 00:00:04,430 Commons license. 3 00:00:04,430 --> 00:00:06,730 Your support will help MIT OpenCourseWare 4 00:00:06,730 --> 00:00:11,120 continue to offer high quality educational resources for free. 5 00:00:11,120 --> 00:00:13,720 To make a donation or view additional materials 6 00:00:13,720 --> 00:00:17,680 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,680 --> 00:00:18,850 at ocw@mit.edu. 8 00:00:21,520 --> 00:00:23,950 NICHOLAS DIBELLA: So last time, at the end of last class, 9 00:00:23,950 --> 00:00:29,380 I began to talk to you about quantum tunneling, which 10 00:00:29,380 --> 00:00:34,030 allows for a lots of crazy events to occur. 11 00:00:34,030 --> 00:00:37,280 It allows me to walk through chalkboards. 12 00:00:37,280 --> 00:00:40,000 It allows me to fly. 13 00:00:40,000 --> 00:00:45,610 It allows for various types of magic to be true. 14 00:00:45,610 --> 00:00:48,640 And it's just so wonderful that quantum mechanics 15 00:00:48,640 --> 00:00:51,700 allows for these really strange things to occur. 16 00:00:51,700 --> 00:00:55,360 It kind of gives us hope-- it gives possibility-- 17 00:00:55,360 --> 00:01:02,830 that pretty much whatever you can imagine can occur. 18 00:01:02,830 --> 00:01:07,390 And today, I'd like to expand on this theme of possibility, 19 00:01:07,390 --> 00:01:10,690 and talk to you all about parallel universes. 20 00:01:15,370 --> 00:01:21,640 So you're all here today in this room at MIT, 21 00:01:21,640 --> 00:01:24,800 watching me talk about some stuff. 22 00:01:24,800 --> 00:01:28,580 But would you believe that in a far, 23 00:01:28,580 --> 00:01:35,680 far away galaxy there is an exact copy of this classroom 24 00:01:35,680 --> 00:01:40,360 with ab exact copy of each one of you-- each person-- 25 00:01:40,360 --> 00:01:43,840 with each person having every single characteristic of you, 26 00:01:43,840 --> 00:01:49,870 having the exact DNA, the exact memories, the exact preference 27 00:01:49,870 --> 00:01:52,900 of ice cream, the exact everything, 28 00:01:52,900 --> 00:01:57,710 and an exact copy of me talking to you about stuff? 29 00:01:57,710 --> 00:02:02,260 Well, according to some parallel universe theories, 30 00:02:02,260 --> 00:02:04,590 this actually is the case. 31 00:02:04,590 --> 00:02:09,190 Whereas, in this universe, I simply mention that fact 32 00:02:09,190 --> 00:02:11,890 and continue to talk about some other things, 33 00:02:11,890 --> 00:02:17,170 in another universe I could mention that fact 34 00:02:17,170 --> 00:02:18,310 and just leave the room. 35 00:02:21,220 --> 00:02:23,160 [DOOR CREAKING] 36 00:02:23,160 --> 00:02:26,070 [MURMURS] 37 00:02:34,320 --> 00:02:35,896 Hi. 38 00:02:35,896 --> 00:02:37,500 We're not in that universe, however. 39 00:02:40,170 --> 00:02:41,010 OK. 40 00:02:41,010 --> 00:02:45,810 Now, what do I mean by parallel universes? 41 00:02:45,810 --> 00:02:48,450 Well, first I need to be a little bit more precise on what 42 00:02:48,450 --> 00:02:53,790 I mean by universe, because it's used in a few different ways. 43 00:02:53,790 --> 00:02:56,370 Now, usually when we think about the universe, 44 00:02:56,370 --> 00:02:59,550 we think about "the universe"-- the everything 45 00:02:59,550 --> 00:03:00,540 that's out there. 46 00:03:00,540 --> 00:03:03,340 We think of that as being the universe. 47 00:03:03,340 --> 00:03:07,270 Well, in this phrase, parallel universes, 48 00:03:07,270 --> 00:03:09,360 the type of universe that I'm referring to 49 00:03:09,360 --> 00:03:11,200 is a little bit different. 50 00:03:11,200 --> 00:03:14,004 And there's one type of parallel universe 51 00:03:14,004 --> 00:03:15,420 that I'd like to talk about first, 52 00:03:15,420 --> 00:03:18,850 and that has a very specific meaning of universe. 53 00:03:18,850 --> 00:03:21,870 And that's the observable universe, 54 00:03:21,870 --> 00:03:23,440 which I'll write down here. 55 00:03:33,050 --> 00:03:37,480 So as you all know, the universe is pretty big-- 56 00:03:37,480 --> 00:03:39,070 pretty big. 57 00:03:39,070 --> 00:03:42,550 But there's only a certain fraction of it that we can see. 58 00:03:42,550 --> 00:03:46,210 And that's because of the big bang. 59 00:03:46,210 --> 00:03:50,460 And let me tell you briefly about the big bang. 60 00:03:50,460 --> 00:03:54,280 You'll get a lot more about this in a couple of weeks, 61 00:03:54,280 --> 00:03:58,490 but let me just briefly go over the big bang. 62 00:03:58,490 --> 00:04:06,280 So a long time ago, all the galaxies, and all the stars, 63 00:04:06,280 --> 00:04:10,660 and all the matter were a lot closer than they were today-- 64 00:04:10,660 --> 00:04:12,040 extremely close. 65 00:04:12,040 --> 00:04:13,660 Infinitely close, in fact. 66 00:04:13,660 --> 00:04:19,725 And it's speculated that about 14 billion years ago, all 67 00:04:19,725 --> 00:04:21,850 of the matter and all of the energy in the universe 68 00:04:21,850 --> 00:04:24,590 was concentrated at a single point. 69 00:04:24,590 --> 00:04:31,750 And for some reason, these bits of matter 70 00:04:31,750 --> 00:04:34,750 just started to expand away from each other. 71 00:04:34,750 --> 00:04:39,750 And the reason for the expansion was that space itself expanded. 72 00:04:39,750 --> 00:04:42,010 And as a result, everything sitting on space 73 00:04:42,010 --> 00:04:45,700 would get farther apart from each other. 74 00:04:45,700 --> 00:04:47,560 And that might sound kind of weird, 75 00:04:47,560 --> 00:04:51,860 so let me give you an analogy. 76 00:04:51,860 --> 00:05:04,510 Suppose you're an ant and you're living on top of a volleyball. 77 00:05:04,510 --> 00:05:06,370 So this is you. 78 00:05:09,760 --> 00:05:17,500 And according to you, your universe 79 00:05:17,500 --> 00:05:19,450 consists only of two dimensions. 80 00:05:19,450 --> 00:05:22,510 You can either walk one way or you 81 00:05:22,510 --> 00:05:26,710 can walk another way, similar to how we move around 82 00:05:26,710 --> 00:05:28,720 on the surface of the Earth. 83 00:05:28,720 --> 00:05:32,380 Relative to the ground there's only, essentially, 84 00:05:32,380 --> 00:05:33,864 two directions we can move. 85 00:05:33,864 --> 00:05:36,280 Well, that's what the universe is like for this ant living 86 00:05:36,280 --> 00:05:37,810 on a volleyball. 87 00:05:37,810 --> 00:05:41,140 Of course, we, the omniscient observers, 88 00:05:41,140 --> 00:05:42,670 see the whole volleyball, and so we 89 00:05:42,670 --> 00:05:44,770 recognize that the ant's actually 90 00:05:44,770 --> 00:05:50,380 living on the surface of a three dimensional object. 91 00:05:50,380 --> 00:05:52,570 But the ant doesn't know that. 92 00:05:52,570 --> 00:05:58,524 Well, what would happen if we expanded this ant's universe? 93 00:05:58,524 --> 00:06:00,565 What would happen if we expanded this volleyball? 94 00:06:08,450 --> 00:06:10,090 We still want to see it of course. 95 00:06:10,090 --> 00:06:14,960 Let's say this is before, and this is after. 96 00:06:19,910 --> 00:06:22,130 Of course, the ant is still the same size, 97 00:06:22,130 --> 00:06:26,300 it's just that the volleyball has gotten much bigger. 98 00:06:26,300 --> 00:06:31,220 So the ant's still lower here, but something 99 00:06:31,220 --> 00:06:33,720 has changed for the ant. 100 00:06:33,720 --> 00:06:37,230 On this universe there could be, for instance, 101 00:06:37,230 --> 00:06:43,160 lots of other ants just specked all around, 102 00:06:43,160 --> 00:06:45,440 just sitting around various points. 103 00:06:48,855 --> 00:06:51,510 We didn't completely raise erase perfectly, 104 00:06:51,510 --> 00:06:53,999 so let me make this a little bit better. 105 00:06:53,999 --> 00:06:55,790 So there are these points on the volleyball 106 00:06:55,790 --> 00:06:58,160 that the ant observes, and they're all 107 00:06:58,160 --> 00:07:01,880 separated at certain distances from each other. 108 00:07:01,880 --> 00:07:04,550 And once you expand this volleyball, 109 00:07:04,550 --> 00:07:08,180 these points, as a result of the expansion of the volleyball, 110 00:07:08,180 --> 00:07:10,470 get farther apart from each other. 111 00:07:10,470 --> 00:07:11,930 So I drew five dots there. 112 00:07:11,930 --> 00:07:16,490 Now there are still five dots, but they are farther apart 113 00:07:16,490 --> 00:07:18,930 from each other. 114 00:07:18,930 --> 00:07:22,250 And then that's something that the ant would observe. 115 00:07:22,250 --> 00:07:27,470 Well, suppose right at the expansion, 116 00:07:27,470 --> 00:07:34,190 one of these other ants-- suppose these dots are ants-- 117 00:07:34,190 --> 00:07:43,080 sent a signal-- send some light beam to our friendly ant, 118 00:07:43,080 --> 00:07:44,960 the ant that we started off with. 119 00:07:44,960 --> 00:07:50,070 So right before the expansion, some ant sent a signal. 120 00:07:50,070 --> 00:07:54,640 So the signal travels from this ant to that ant. 121 00:07:54,640 --> 00:07:57,980 Well, suppose the ant sends right 122 00:07:57,980 --> 00:08:00,710 when the expansion starts. 123 00:08:00,710 --> 00:08:05,810 Well, it's going to take some time for that signal 124 00:08:05,810 --> 00:08:11,399 to reach the original ant, because light travels 125 00:08:11,399 --> 00:08:13,940 at a certain speed, and it takes a certain time to get there. 126 00:08:16,790 --> 00:08:21,440 Similarly, in our universe, and our universe's very early 127 00:08:21,440 --> 00:08:29,720 history, light from other stars and other galaxies 128 00:08:29,720 --> 00:08:35,159 has taken time to reach us since the big bang. 129 00:08:35,159 --> 00:08:41,880 And there are some stars that have since 130 00:08:41,880 --> 00:08:43,950 moved very far away from us-- 131 00:08:43,950 --> 00:08:45,630 extremely far away from us-- 132 00:08:45,630 --> 00:08:47,250 and the light from those stars simply 133 00:08:47,250 --> 00:08:49,630 hasn't had enough time to reach us yet. 134 00:08:49,630 --> 00:08:53,090 And so, there's a certain distance away from us 135 00:08:53,090 --> 00:08:56,770 that is the maximum distance that we can observe. 136 00:08:56,770 --> 00:09:00,560 And this distance defines a whole sphere 137 00:09:00,560 --> 00:09:04,050 called the Hubble sphere, which we refer 138 00:09:04,050 --> 00:09:07,390 to as the observable universe. 139 00:09:07,390 --> 00:09:12,090 So there's only a small section of the whole universe 140 00:09:12,090 --> 00:09:16,000 that we can observe, and we call that the observable universe. 141 00:09:16,000 --> 00:09:17,700 AUDIENCE: What's the radius of-- 142 00:09:17,700 --> 00:09:20,180 NICHOLAS DIBELLA: What's the radius of the Hubble sphere? 143 00:09:20,180 --> 00:09:21,840 The radius of the observable universe? 144 00:09:21,840 --> 00:09:26,260 I think it's 40 billion light years. 145 00:09:26,260 --> 00:09:28,890 And that's and that's different from the age of the universe, 146 00:09:28,890 --> 00:09:33,600 because actually the space between us 147 00:09:33,600 --> 00:09:36,000 has actually expanded. 148 00:09:36,000 --> 00:09:42,210 And there are some effects that give rise 149 00:09:42,210 --> 00:09:44,280 to that increased distance. 150 00:09:44,280 --> 00:09:47,340 And in terms of meters, I have that for you. 151 00:09:47,340 --> 00:09:50,160 In terms of meters, it's about 10 152 00:09:50,160 --> 00:09:54,270 to the 27th meters, which is pretty big. 153 00:09:54,270 --> 00:09:57,060 It would take it would take lights about 40 billion years 154 00:09:57,060 --> 00:10:00,630 to reach from here until the end of the Hubble volume-- 155 00:10:00,630 --> 00:10:02,850 to the end of the observable universe. 156 00:10:02,850 --> 00:10:07,230 Now, as each year passes by, light 157 00:10:07,230 --> 00:10:09,270 has an extra year to travel. 158 00:10:09,270 --> 00:10:11,850 And so, for each year that passes by, 159 00:10:11,850 --> 00:10:16,087 the observable universe increases by one light year. 160 00:10:16,087 --> 00:10:18,420 So the observable universe is getting bigger and bigger. 161 00:10:20,950 --> 00:10:25,290 Now, strictly speaking, we don't really 162 00:10:25,290 --> 00:10:28,980 know if there's anything outside of the observable universe, 163 00:10:28,980 --> 00:10:30,960 because we just can't observe it. 164 00:10:30,960 --> 00:10:36,840 And so, you might say that anything that's 165 00:10:36,840 --> 00:10:38,970 outside of the observable universe 166 00:10:38,970 --> 00:10:44,610 is just subject to mystical speculations 167 00:10:44,610 --> 00:10:48,180 or metaphysical speculations, and science can't do anything 168 00:10:48,180 --> 00:10:51,790 about those places. 169 00:10:51,790 --> 00:10:58,950 Well, actually, surprisingly, we can say some things 170 00:10:58,950 --> 00:11:01,950 about what's outside of the observable universe, 171 00:11:01,950 --> 00:11:05,874 although the things that we say are more indirect. 172 00:11:05,874 --> 00:11:11,130 Now, there's the question that space might be finite 173 00:11:11,130 --> 00:11:13,920 or it might be infinite. 174 00:11:13,920 --> 00:11:19,520 And we now have a kind of good answer to that question. 175 00:11:19,520 --> 00:11:22,830 But first, what does it mean for space to be finite or space 176 00:11:22,830 --> 00:11:23,880 to be infinite? 177 00:11:23,880 --> 00:11:31,830 Well, if space were finite, you might imagine that you take off 178 00:11:31,830 --> 00:11:34,660 from a rocket ship on earth, and then you travel 179 00:11:34,660 --> 00:11:37,706 and you keep traveling farther and farther away. 180 00:11:37,706 --> 00:11:43,740 And then, you eventually reach a sign that says, 181 00:11:43,740 --> 00:11:46,860 this is the end of the universe. 182 00:11:46,860 --> 00:11:48,990 Dead end. 183 00:11:48,990 --> 00:11:50,850 You might reach a sign that says that. 184 00:11:50,850 --> 00:11:55,150 Well, that's not what I mean by space being finite. 185 00:11:55,150 --> 00:11:56,790 What I mean by space being finite, 186 00:11:56,790 --> 00:12:02,910 is something like this volleyball universe of the ant. 187 00:12:02,910 --> 00:12:04,260 It's finite for-- 188 00:12:07,110 --> 00:12:09,339 I mean, we, the omniscient observer, 189 00:12:09,339 --> 00:12:11,880 look at this volleyball and we see that it has a finite size. 190 00:12:15,000 --> 00:12:17,190 Of course, this is an analogy. 191 00:12:17,190 --> 00:12:23,670 It's an analogy to the real world, where our universe might 192 00:12:23,670 --> 00:12:28,980 possibly be some four dimensional hypersphere 193 00:12:28,980 --> 00:12:30,780 that we're living on top of. 194 00:12:30,780 --> 00:12:37,310 So it's a four dimensional extension to this volleyball. 195 00:12:37,310 --> 00:12:41,340 In that sense, the universe might be finite. 196 00:12:41,340 --> 00:12:44,790 And one consequence of this is actually 197 00:12:44,790 --> 00:12:49,500 that if you started at some point in the universe-- 198 00:12:49,500 --> 00:12:51,520 supposed you started here. 199 00:12:51,520 --> 00:12:54,120 If you started at this point in the universe and you traveled, 200 00:12:54,120 --> 00:12:58,110 say, this direction, and just kept traveling straight 201 00:12:58,110 --> 00:13:01,080 forward, eventually you would return 202 00:13:01,080 --> 00:13:03,870 to where you started off. 203 00:13:03,870 --> 00:13:08,130 And that seems kind of weird, right? 204 00:13:08,130 --> 00:13:12,420 If I keep traveling this way, then ordinarily I 205 00:13:12,420 --> 00:13:14,700 wouldn't think that eventually I'd return here, 206 00:13:14,700 --> 00:13:17,550 but actually we know that the earth is earth is round, 207 00:13:17,550 --> 00:13:19,770 so eventually we're going to reach back 208 00:13:19,770 --> 00:13:21,180 at our original point. 209 00:13:21,180 --> 00:13:26,700 And that's similar to what would happen in a finite universe. 210 00:13:26,700 --> 00:13:31,140 Well, it looks like this actually isn't the case. 211 00:13:31,140 --> 00:13:35,150 There are some very interesting measurements 212 00:13:35,150 --> 00:13:39,230 that people have done about some radiation 213 00:13:39,230 --> 00:13:42,200 that we observe in the sky called the cosmic microwave 214 00:13:42,200 --> 00:13:45,290 background radiation. 215 00:13:45,290 --> 00:13:47,972 People have done measurements of the stuff. 216 00:13:47,972 --> 00:13:50,180 I'm not going to get into exactly what that stuff is, 217 00:13:50,180 --> 00:13:53,570 but in a couple of weeks you'll know more about it. 218 00:13:53,570 --> 00:13:55,525 But there is this radiation that we just 219 00:13:55,525 --> 00:13:57,650 see when we look out-- when we point our telescopes 220 00:13:57,650 --> 00:13:59,160 and look at the sky. 221 00:13:59,160 --> 00:14:00,710 There's this radiation. 222 00:14:00,710 --> 00:14:05,120 And the way that this radiation looks actually 223 00:14:05,120 --> 00:14:09,950 gives us an indication to the geometry of the universe. 224 00:14:09,950 --> 00:14:11,580 There are these spots, for instance, 225 00:14:11,580 --> 00:14:13,520 in the radiation that would indicate 226 00:14:13,520 --> 00:14:16,430 whether the universe is spherical, 227 00:14:16,430 --> 00:14:20,930 or it might be hyperbolic, or it might be flat. 228 00:14:20,930 --> 00:14:26,510 It might just be one big line-- 229 00:14:26,510 --> 00:14:30,260 one hyperplane we would have to say. 230 00:14:30,260 --> 00:14:33,590 And it looks like the universe actually is infinite. 231 00:14:33,590 --> 00:14:36,860 And it looks like the universe is flat, 232 00:14:36,860 --> 00:14:40,460 which is probably the least interesting case that's 233 00:14:40,460 --> 00:14:41,120 conceivable. 234 00:14:41,120 --> 00:14:43,230 But that's what it looks like. 235 00:14:43,230 --> 00:14:46,610 So it looks like the whole universe is infinite and flat. 236 00:14:49,330 --> 00:14:52,140 So let's assume that's the case. 237 00:14:52,140 --> 00:14:54,190 It's supported by observations. 238 00:14:54,190 --> 00:15:02,980 And I think it's actually seems-- 239 00:15:02,980 --> 00:15:06,700 it's kind of hard to reason from intuition 240 00:15:06,700 --> 00:15:08,555 that the universe should be infinite, 241 00:15:08,555 --> 00:15:10,930 so I would try to reason from intuition that the universe 242 00:15:10,930 --> 00:15:11,830 should be infinite. 243 00:15:11,830 --> 00:15:13,900 But maybe it seems natural to some of you 244 00:15:13,900 --> 00:15:19,840 that the universe should be infinite rather than finite. 245 00:15:19,840 --> 00:15:21,680 In that case, you'd probably like this. 246 00:15:21,680 --> 00:15:24,410 You'd probably like this fact. 247 00:15:24,410 --> 00:15:25,210 Question? 248 00:15:25,210 --> 00:15:27,444 AUDIENCE: What do you mean the universe is flat? 249 00:15:27,444 --> 00:15:29,860 NICHOLAS DIBELLA: Oh, what do I mean the universe is flat? 250 00:15:29,860 --> 00:15:36,200 Well, this is what I mean by the universe being curved. 251 00:15:36,200 --> 00:15:38,750 For these ants, their universe is curved. 252 00:15:38,750 --> 00:15:42,890 AUDIENCE: Oh, so it's not like it's a flat plane, 253 00:15:42,890 --> 00:15:45,492 just it doesn't curve? 254 00:15:45,492 --> 00:15:47,200 NICHOLAS DIBELLA: Yeah, it doesn't curve. 255 00:15:47,200 --> 00:15:50,350 If it went one direction, you wouldn't return 256 00:15:50,350 --> 00:15:51,780 to where you started off. 257 00:15:51,780 --> 00:15:54,220 That's what I mean by the universe being flat. 258 00:15:54,220 --> 00:15:58,168 And it looks like the universe is flat and infinite. 259 00:15:58,168 --> 00:15:59,788 AUDIENCE: In all geometries, would you 260 00:15:59,788 --> 00:16:02,060 be turned toward where you started though? 261 00:16:02,060 --> 00:16:03,020 NICHOLAS DIBELLA: In all geometries, would what? 262 00:16:03,020 --> 00:16:04,130 AUDIENCE: Like, if you're on a hypersphere, 263 00:16:04,130 --> 00:16:06,088 then unless you come back to where you started, 264 00:16:06,088 --> 00:16:08,112 it's not like that some different geometry it 265 00:16:08,112 --> 00:16:11,635 would be possible for it to be curved, but [INAUDIBLE] 266 00:16:11,635 --> 00:16:12,760 NICHOLAS DIBELLA: Oh, yeah. 267 00:16:12,760 --> 00:16:15,530 It could be possible, that the geometry is curved 268 00:16:15,530 --> 00:16:18,010 but we don't return where we started. 269 00:16:18,010 --> 00:16:19,810 You can imagine something like-- 270 00:16:19,810 --> 00:16:21,271 I don't know. 271 00:16:25,637 --> 00:16:26,470 Something like this. 272 00:16:30,400 --> 00:16:32,050 So I go from 1D to 2D, and then you 273 00:16:32,050 --> 00:16:33,550 have to use your minds to make a 3D. 274 00:16:33,550 --> 00:16:36,271 And then you have to use your minds to make it 4D. 275 00:16:36,271 --> 00:16:38,020 Yeah, you can imagine something like this. 276 00:16:38,020 --> 00:16:40,900 You travel this way, keep going, keep going, keep going, 277 00:16:40,900 --> 00:16:43,420 and then you never return to where you started. 278 00:16:43,420 --> 00:16:45,270 Yeah, that's possible. 279 00:16:45,270 --> 00:16:49,150 You could have fun thinking about possible geometries 280 00:16:49,150 --> 00:16:50,890 the universe could be. 281 00:16:50,890 --> 00:16:53,080 But that's not going to correspond to reality 282 00:16:53,080 --> 00:16:53,702 unfortunately. 283 00:16:53,702 --> 00:16:54,910 We can fun thinking about it. 284 00:16:54,910 --> 00:16:56,770 And for a long time, this is all that people were able to do, 285 00:16:56,770 --> 00:16:57,603 just think about it. 286 00:16:57,603 --> 00:16:59,200 We didn't have data long time ago. 287 00:16:59,200 --> 00:17:00,908 We don't have measurements that indicated 288 00:17:00,908 --> 00:17:03,130 that the universe is infinite. 289 00:17:03,130 --> 00:17:04,780 Question? 290 00:17:04,780 --> 00:17:06,818 AUDIENCE: How is 4D supposed to look? 291 00:17:06,818 --> 00:17:08,859 NICHOLAS DIBELLA: OK, how is 4D supposed to look. 292 00:17:08,859 --> 00:17:11,050 I can't tell you how 4D is supposed to look. 293 00:17:11,050 --> 00:17:13,089 Nobody can tell you what 4D looks like. 294 00:17:13,089 --> 00:17:15,670 You just have to have to try to visualize 295 00:17:15,670 --> 00:17:19,480 an analogy of what some properties of it would be like. 296 00:17:19,480 --> 00:17:22,930 You can't actually see it, and that's something 297 00:17:22,930 --> 00:17:25,240 that we have to live with unfortunately. 298 00:17:25,240 --> 00:17:29,050 I mean, sorry. 299 00:17:29,050 --> 00:17:31,658 Yeah, question? 300 00:17:31,658 --> 00:17:33,606 AUDIENCE: There are actually diagrams 301 00:17:33,606 --> 00:17:38,640 of what they think it might possibly look like, except not. 302 00:17:38,640 --> 00:17:40,850 They kind of explain-- 303 00:17:40,850 --> 00:17:42,620 NICHOLAS DIBELLA: Well, I know there 304 00:17:42,620 --> 00:17:47,165 are some diagrams I've heard of that, once you look at them, 305 00:17:47,165 --> 00:17:48,790 you kind of get an illusion that you're 306 00:17:48,790 --> 00:17:49,750 seeing the fourth dimension. 307 00:17:49,750 --> 00:17:51,375 Is that kind of what you're describing? 308 00:17:51,375 --> 00:17:54,330 AUDIENCE: No, like, diagrams that show hypercubes. 309 00:17:54,330 --> 00:17:55,840 NICHOLAS DIBELLA: Oh, oh, oh. 310 00:17:55,840 --> 00:17:58,940 Well, those diagrams that you're talking about are-- 311 00:17:58,940 --> 00:18:03,100 they're four dimensional objects that are embedded 312 00:18:03,100 --> 00:18:04,790 onto a two dimensional surface. 313 00:18:04,790 --> 00:18:06,570 So you're not really seeing it. 314 00:18:06,570 --> 00:18:10,992 AUDIENCE: And how would they think 315 00:18:10,992 --> 00:18:13,581 that it's flat and infinite? 316 00:18:13,581 --> 00:18:15,580 NICHOLAS DIBELLA: How, do they actually do this? 317 00:18:15,580 --> 00:18:16,930 How do they actually infer from the data 318 00:18:16,930 --> 00:18:18,130 that it's flat and infinite? 319 00:18:18,130 --> 00:18:18,976 Well, that's-- 320 00:18:18,976 --> 00:18:21,642 AUDIENCE: If you can only see as far as the observable universe, 321 00:18:21,642 --> 00:18:23,522 then that means that no matter what you see, 322 00:18:23,522 --> 00:18:26,125 you can't tell what's beyond that, and it could be. 323 00:18:26,125 --> 00:18:27,000 NICHOLAS DIBELLA: OK. 324 00:18:27,000 --> 00:18:29,200 Well, we can't directly observe what's 325 00:18:29,200 --> 00:18:31,360 outside of the observable universe, 326 00:18:31,360 --> 00:18:37,659 but we can make indirect inferences. 327 00:18:37,659 --> 00:18:39,284 AUDIENCE: But nothing comes from there. 328 00:18:39,284 --> 00:18:40,595 Like, you can't see radiation. 329 00:18:40,595 --> 00:18:41,470 You can't-- 330 00:18:41,470 --> 00:18:43,261 NICHOLAS DIBELLA: Nothing comes from there, 331 00:18:43,261 --> 00:18:45,310 but the universe-- the total universe-- 332 00:18:45,310 --> 00:18:48,410 has a certain geometrical shape. 333 00:18:48,410 --> 00:18:53,710 And there are clever ways of making these inferences. 334 00:18:53,710 --> 00:19:00,850 I don't have time to get into them now, but we'll talk later. 335 00:19:00,850 --> 00:19:04,720 So for the purpose of the rest of today's class, 336 00:19:04,720 --> 00:19:08,809 assume that the universe is infinite. 337 00:19:08,809 --> 00:19:10,600 This is the assumption we're going to make. 338 00:19:29,870 --> 00:19:35,810 This has so many wonderful consequences, the assumption 339 00:19:35,810 --> 00:19:37,490 that the universe is infinite. 340 00:19:37,490 --> 00:19:42,810 I should say "the universe" is infinite. 341 00:19:45,750 --> 00:19:48,830 there are some theories about the early-- 342 00:19:48,830 --> 00:19:51,830 well, pretty much all the theories 343 00:19:51,830 --> 00:19:55,880 about the early universe speculate 344 00:19:55,880 --> 00:20:02,900 that a long time ago after this massive expansion 345 00:20:02,900 --> 00:20:05,030 following the big bang-- 346 00:20:05,030 --> 00:20:07,520 well, during this expansion-- 347 00:20:07,520 --> 00:20:12,880 all the material that was present 348 00:20:12,880 --> 00:20:20,750 dispersed randomly and formed certain arrangements of matter. 349 00:20:20,750 --> 00:20:24,890 And not only did it form some types of arrangements 350 00:20:24,890 --> 00:20:28,550 of matter, but actually this expansion 351 00:20:28,550 --> 00:20:33,170 created all different possible arrangements of matter 352 00:20:33,170 --> 00:20:35,820 to spread out. 353 00:20:35,820 --> 00:20:39,140 So imagine a very long time ago, you've 354 00:20:39,140 --> 00:20:42,772 got all every possible arrangement of matter. 355 00:20:42,772 --> 00:20:48,650 So in this room, for instance, there are air molecules, 356 00:20:48,650 --> 00:20:51,520 there are molecules making up you, 357 00:20:51,520 --> 00:20:53,750 there are atoms and subatomic particles, 358 00:20:53,750 --> 00:20:57,630 and there's a finite number of particles in this room. 359 00:20:57,630 --> 00:21:00,860 And there is a finite number of ways 360 00:21:00,860 --> 00:21:02,728 that you can arrange those particles. 361 00:21:05,390 --> 00:21:07,700 I could, for instance-- 362 00:21:07,700 --> 00:21:09,390 I'll make an analogy. 363 00:21:09,390 --> 00:21:15,950 Suppose I've got a rectangular universe. 364 00:21:15,950 --> 00:21:17,420 Before I had a volleyball universe, 365 00:21:17,420 --> 00:21:20,320 now suppose I have a rectangular universe. 366 00:21:20,320 --> 00:21:22,070 Suppose I have a rectangular universe that 367 00:21:22,070 --> 00:21:26,020 has four different types of particles. 368 00:21:28,840 --> 00:21:32,720 No-- let's say there are two different types of particles. 369 00:21:32,720 --> 00:21:34,330 So there's particle 1 and particle 2. 370 00:21:37,495 --> 00:21:38,870 These are all the different types 371 00:21:38,870 --> 00:21:46,910 of particles in our fake universe-- our play universe. 372 00:21:46,910 --> 00:21:48,530 We have 1 and 2. 373 00:21:48,530 --> 00:21:55,220 And suppose there's only space in our universe for four 374 00:21:55,220 --> 00:21:57,220 different particles. 375 00:21:57,220 --> 00:21:59,900 Or yeah, say there's only space for four of them. 376 00:21:59,900 --> 00:22:01,670 Then what can we do? 377 00:22:01,670 --> 00:22:05,300 We can put 1 here. 378 00:22:05,300 --> 00:22:08,000 We can put 2 here. 379 00:22:08,000 --> 00:22:10,190 We can put another 2 here. 380 00:22:10,190 --> 00:22:13,165 And we can put another particle 1 there. 381 00:22:13,165 --> 00:22:14,540 I know it looks like there's more 382 00:22:14,540 --> 00:22:16,352 there's more space for more particles, 383 00:22:16,352 --> 00:22:17,810 but let's say that this is the most 384 00:22:17,810 --> 00:22:23,539 that our universe our fake universe can accommodate. 385 00:22:23,539 --> 00:22:25,830 So this is one possible arrangement of those particles. 386 00:22:25,830 --> 00:22:27,800 Another possible arrangement of those particles 387 00:22:27,800 --> 00:22:32,430 would be to have 1, 2, 1, 1. 388 00:22:32,430 --> 00:22:35,960 Another one would be 1, 2, 1, 2. 389 00:22:35,960 --> 00:22:38,195 In fact, how many different arrangements 390 00:22:38,195 --> 00:22:39,320 are there in this universe? 391 00:22:39,320 --> 00:22:39,620 AUDIENCE: 16. 392 00:22:39,620 --> 00:22:40,670 NICHOLAS DIBELLA: 16. 393 00:22:40,670 --> 00:22:41,630 Yeah, it's 16. 394 00:22:41,630 --> 00:22:44,596 There's two for this one, there's two for this one, 395 00:22:44,596 --> 00:22:46,220 two for this one, and two for this one. 396 00:22:46,220 --> 00:22:48,200 Two, times two, times two, times two, is 16. 397 00:22:48,200 --> 00:22:50,570 So there are 16 different possible arrangements 398 00:22:50,570 --> 00:22:52,250 of these particles. 399 00:22:52,250 --> 00:22:54,270 In this universe. 400 00:22:54,270 --> 00:22:56,840 And so, there are really 16 different types 401 00:22:56,840 --> 00:22:59,960 of realities in this universe. 402 00:23:02,800 --> 00:23:08,300 Reality is defined essentially by how 403 00:23:08,300 --> 00:23:10,760 various particles are arranged. 404 00:23:10,760 --> 00:23:15,350 And somehow the way these particles 405 00:23:15,350 --> 00:23:21,030 are arranged gives rise to how we perceive them and so forth. 406 00:23:21,030 --> 00:23:24,170 So the arrangement of particles gives rise to the reality 407 00:23:24,170 --> 00:23:29,450 that those particles impart to us. 408 00:23:29,450 --> 00:23:37,395 Now, our universe, of course, is not rectangular like this. 409 00:23:37,395 --> 00:23:39,020 Well, that's actually not so important. 410 00:23:39,020 --> 00:23:41,450 But our universe is much bigger than this universe 411 00:23:41,450 --> 00:23:43,046 that we have here. 412 00:23:43,046 --> 00:23:44,170 I'll go here to this board. 413 00:23:50,050 --> 00:23:54,460 Our observable universe is a sphere. 414 00:23:54,460 --> 00:23:58,900 Remember, the whole universe we suspect is flat and infinite, 415 00:23:58,900 --> 00:24:01,450 but our observable universe is some finite sphere. 416 00:24:01,450 --> 00:24:07,810 And our observable universe has a certain radius. 417 00:24:07,810 --> 00:24:09,960 And this radius happens to be-- 418 00:24:09,960 --> 00:24:10,960 I have it written down-- 419 00:24:15,670 --> 00:24:20,150 8 times 10 to 26th meters. 420 00:24:20,150 --> 00:24:26,670 Now, suppose a particle has a radius of about-- 421 00:24:26,670 --> 00:24:30,070 OK, what we want to do now is fit particles 422 00:24:30,070 --> 00:24:30,951 into this universe. 423 00:24:30,951 --> 00:24:32,950 And we're going to ask how many particles can we 424 00:24:32,950 --> 00:24:35,170 fit inside of it. 425 00:24:35,170 --> 00:24:36,670 It's going to be a lot of particles, 426 00:24:36,670 --> 00:24:39,400 because the observable universe is really big 427 00:24:39,400 --> 00:24:40,960 and the particles are really small. 428 00:24:40,960 --> 00:24:45,370 But we can get a rough estimate for how many particles 429 00:24:45,370 --> 00:24:47,750 we can put inside of it. 430 00:24:47,750 --> 00:24:52,690 So a particle-- say this is a little particle-- 431 00:24:52,690 --> 00:24:55,264 looks like a TIE fighter. 432 00:24:55,264 --> 00:24:55,930 What am I doing? 433 00:24:59,945 --> 00:25:11,880 OK, say this is a particle, and it's got a length of about 10 434 00:25:11,880 --> 00:25:14,800 to the minus 13 meters, which is significantly 435 00:25:14,800 --> 00:25:19,077 smaller than the length of the observable universe. 436 00:25:19,077 --> 00:25:21,160 Well, now you can ask, how many of these particles 437 00:25:21,160 --> 00:25:23,370 can you fit inside of this universe? 438 00:25:23,370 --> 00:25:25,480 And to do that, you can simply calculate 439 00:25:25,480 --> 00:25:28,030 the volume of this universe and divide 440 00:25:28,030 --> 00:25:31,630 by the volume of a subatomic particle, which 441 00:25:31,630 --> 00:25:36,280 might be a proton, or a neutron, or something like that. 442 00:25:36,280 --> 00:25:41,850 Simply take those two volumes and divide them. 443 00:25:41,850 --> 00:25:43,950 Take the radius, cube it. 444 00:25:43,950 --> 00:25:46,720 Use the formula for the volume of a sphere and then just 445 00:25:46,720 --> 00:25:49,990 divide by that formula applied with this radius. 446 00:25:49,990 --> 00:25:57,840 And you get about 10 to the 118th particles. 447 00:25:57,840 --> 00:25:59,347 AUDIENCE: That's more than a google. 448 00:25:59,347 --> 00:26:01,430 NICHOLAS DIBELLA: That's more than a google, yeah. 449 00:26:01,430 --> 00:26:04,615 But it's not more than a googleplex. 450 00:26:04,615 --> 00:26:05,756 Particles-- 451 00:26:05,756 --> 00:26:07,800 AUDIENCE: What's a googleple? 452 00:26:07,800 --> 00:26:12,970 NICHOLAS DIBELLA: A googleplex is 10 to the 10 to 100. 453 00:26:12,970 --> 00:26:16,040 So it's 10 to the google. 454 00:26:16,040 --> 00:26:18,410 So a google is 1 followed by 100 zero's, 455 00:26:18,410 --> 00:26:21,150 but imagine a number of followed by a Google of zeros. 456 00:26:24,280 --> 00:26:25,245 Yeah. 457 00:26:25,245 --> 00:26:26,620 There's not even there's not even 458 00:26:26,620 --> 00:26:29,350 space for that many particles in the observable universe. 459 00:26:29,350 --> 00:26:30,790 That's how big a googleplex is. 460 00:26:34,369 --> 00:26:36,660 Did you guys hear that the term googleplex was actually 461 00:26:36,660 --> 00:26:39,214 proposed by a nine-year-old or something like? 462 00:26:39,214 --> 00:26:40,380 AUDIENCE: Somebody's nephew. 463 00:26:40,380 --> 00:26:42,090 NICHOLAS DIBELLA: Yeah, some mathematician's nephew 464 00:26:42,090 --> 00:26:43,140 or something like that. 465 00:26:43,140 --> 00:26:43,620 AUDIENCE: What's a googleplex again? 466 00:26:43,620 --> 00:26:45,240 NICHOLAS DIBELLA: What's a googleplex again? 467 00:26:45,240 --> 00:26:46,156 It's 10 to the google. 468 00:26:46,156 --> 00:26:49,180 And a google is 10 to the 100. 469 00:26:49,180 --> 00:26:50,790 That's just cool stuff-- 470 00:26:50,790 --> 00:26:54,110 cool math terms to know. 471 00:26:54,110 --> 00:26:56,340 Like zenzizenzic and other stuff. 472 00:26:58,850 --> 00:26:59,730 Disregard that. 473 00:26:59,730 --> 00:27:01,530 Disregard that. 474 00:27:01,530 --> 00:27:07,058 10 to the 118th particles can fit in our universe. 475 00:27:12,890 --> 00:27:14,400 That's how many particles can fit. 476 00:27:14,400 --> 00:27:16,525 But now we can ask, how many different arrangements 477 00:27:16,525 --> 00:27:19,720 of particles can there be in this universe? 478 00:27:19,720 --> 00:27:21,930 And that's an extremely large number. 479 00:27:25,170 --> 00:27:28,646 Over here in this simple universe, 480 00:27:28,646 --> 00:27:30,270 we had space for four particles, and we 481 00:27:30,270 --> 00:27:32,550 found that the number of different arrangements 482 00:27:32,550 --> 00:27:37,210 was just 2 to the power of 4. 483 00:27:37,210 --> 00:27:38,346 And that was 16. 484 00:27:38,346 --> 00:27:39,884 AUDIENCE: [INAUDIBLE] 485 00:27:39,884 --> 00:27:41,925 NICHOLAS DIBELLA: Oh yeah, you can also have no-- 486 00:27:41,925 --> 00:27:43,020 so it would be three. 487 00:27:43,020 --> 00:27:44,490 Yeah, it would be three. 488 00:27:44,490 --> 00:27:46,170 Yeah, you could have no particles there, 489 00:27:46,170 --> 00:27:47,700 so it would be three. 490 00:27:47,700 --> 00:27:51,240 But once we apply that idea to this-- 491 00:27:51,240 --> 00:27:55,050 2, 3, raised to a huge number. 492 00:27:55,050 --> 00:27:58,830 It's not going to make a whole lot of difference. 493 00:27:58,830 --> 00:28:01,900 And oh, yeah, and here I'm just thinking about one particle. 494 00:28:01,900 --> 00:28:04,080 So I should have said 3 to the fourth, 495 00:28:04,080 --> 00:28:08,820 because you could have zero particle at a slot. 496 00:28:08,820 --> 00:28:12,240 So 10 to the 118th particles can fit in our universe. 497 00:28:12,240 --> 00:28:17,130 And that means that there are 2 to the 10, 498 00:28:17,130 --> 00:28:19,816 to 118th different arrangements. 499 00:28:31,730 --> 00:28:35,075 And that number is actually greater than a googleplex. 500 00:28:35,075 --> 00:28:35,700 AUDIENCE: Whoa. 501 00:28:35,700 --> 00:28:37,325 NICHOLAS DIBELLA: And that's actually-- 502 00:28:37,325 --> 00:28:39,310 I mean, I said 2 versus 10. 503 00:28:39,310 --> 00:28:41,393 That didn't really make a whole lot of difference. 504 00:28:41,393 --> 00:28:44,335 Now, that number's actually about 10 to the 10 505 00:28:44,335 --> 00:28:45,520 to the 118th. 506 00:28:45,520 --> 00:28:46,520 It's approximately that. 507 00:28:50,490 --> 00:28:51,510 That's how huge it is. 508 00:28:55,950 --> 00:29:03,900 Now, I can ask, if the universe started out 509 00:29:03,900 --> 00:29:08,050 with every different possible arrangements of particles-- 510 00:29:08,050 --> 00:29:11,880 If we started out with this vastly huge number 511 00:29:11,880 --> 00:29:18,340 of different ranges of particles, then by now 512 00:29:18,340 --> 00:29:21,060 you'd still expect to find every possible arrangement 513 00:29:21,060 --> 00:29:21,780 of particles. 514 00:29:21,780 --> 00:29:24,200 You'd still expect to find that. 515 00:29:24,200 --> 00:29:27,180 The laws of physics will allow that to happen. 516 00:29:27,180 --> 00:29:28,740 So now I can ask-- 517 00:29:28,740 --> 00:29:33,090 well, if you do have an infinite universe, then 518 00:29:33,090 --> 00:29:36,210 how far do I have to travel before I 519 00:29:36,210 --> 00:29:41,010 reach an exact copy of our observable universe? 520 00:29:41,010 --> 00:29:44,130 How far do I have to travel to get to that? 521 00:29:44,130 --> 00:29:48,900 Well, there are 10 to the 10 to the-- 522 00:29:48,900 --> 00:29:49,800 is that right? 523 00:29:49,800 --> 00:29:51,030 Is that approximation good? 524 00:29:51,030 --> 00:29:54,150 I think it is, right? 525 00:29:54,150 --> 00:29:56,496 Maybe I made a mistake. 526 00:29:56,496 --> 00:29:57,770 [INTERPOSING VOICES] 527 00:29:57,770 --> 00:30:00,014 NICHOLAS DIBELLA: I have things written down. 528 00:30:00,014 --> 00:30:03,353 [INTERPOSING VOICES] 529 00:30:05,477 --> 00:30:07,560 NICHOLAS DIBELLA: Well, actually, I think it's OK. 530 00:30:07,560 --> 00:30:11,620 I think it's OK Yeah, doesn't matter a whole lot. 531 00:30:11,620 --> 00:30:14,460 But now I can ask-- 532 00:30:14,460 --> 00:30:20,130 eventually, as I travel from our observable universe 533 00:30:20,130 --> 00:30:26,100 to another universe to another universe to another universe, 534 00:30:26,100 --> 00:30:31,157 eventually, you'd expect these arrangements to repeat. 535 00:30:31,157 --> 00:30:32,490 Because they have to eventually. 536 00:30:32,490 --> 00:30:38,440 If you come across 2 to 10 to 118th arrangements 537 00:30:38,440 --> 00:30:43,680 and then you get to another one, well, you have to repeat it, 538 00:30:43,680 --> 00:30:46,290 because there's only that many different arrangements. 539 00:30:46,290 --> 00:30:48,820 And so, if you travel far enough, 540 00:30:48,820 --> 00:30:51,690 then applying that reason to our own universe, 541 00:30:51,690 --> 00:30:55,320 you'd expect to find an exact copy of our own universe. 542 00:30:55,320 --> 00:30:58,140 And you'd have to travel about-- 543 00:30:58,140 --> 00:31:03,450 well, you could take the size of our observable universe 544 00:31:03,450 --> 00:31:05,067 and just multiply it by the number 545 00:31:05,067 --> 00:31:06,150 of different arrangements. 546 00:31:06,150 --> 00:31:24,660 And so, you'd expect to find an exact copy of our universe 547 00:31:24,660 --> 00:31:30,970 at about 10 to 10th to the 118th meters away. 548 00:31:30,970 --> 00:31:32,470 I mean this is a really huge number, 549 00:31:32,470 --> 00:31:37,470 so we don't have to be a whole lot precise about how 550 00:31:37,470 --> 00:31:38,800 we manipulate them. 551 00:31:38,800 --> 00:31:42,875 So you'd expect to not only find a copy of yourself, 552 00:31:42,875 --> 00:31:46,410 but you also expect to find a copy of New York. 553 00:31:46,410 --> 00:31:48,990 You expect to find a copy of the planet Mars. 554 00:31:48,990 --> 00:31:52,500 You expect to find a copy of the Andromeda galaxy. 555 00:31:52,500 --> 00:31:54,630 You'd expect to find an exact copy 556 00:31:54,630 --> 00:32:00,600 of every single piece of matter in the huge observable 557 00:32:00,600 --> 00:32:02,652 universe. 558 00:32:02,652 --> 00:32:06,570 In another you are having this exact same conversation. 559 00:32:06,570 --> 00:32:07,920 OK, OK. 560 00:32:07,920 --> 00:32:10,680 In another-- now, here years where quantum comes in. 561 00:32:10,680 --> 00:32:13,650 Here's where quantum mechanics comes in. 562 00:32:13,650 --> 00:32:21,150 According to quantum mechanics, there's a-- 563 00:32:21,150 --> 00:32:24,390 what I'll do next, my next action, 564 00:32:24,390 --> 00:32:27,990 isn't predicted with 100% certainty. 565 00:32:27,990 --> 00:32:31,440 My next action could be that I throw a chalk at you, 566 00:32:31,440 --> 00:32:36,090 or it could be that you use some sort of mental powers 567 00:32:36,090 --> 00:32:38,910 to pull the chalk from me somehow, 568 00:32:38,910 --> 00:32:42,030 through some very complicated contrived quantum tunneling 569 00:32:42,030 --> 00:32:44,850 mechanism. 570 00:32:44,850 --> 00:32:48,710 But the fact is that my future isn't 571 00:32:48,710 --> 00:32:51,370 known with complete certainty. 572 00:32:51,370 --> 00:32:54,596 And so, there are a number of possibilities 573 00:32:54,596 --> 00:32:55,720 of what my future would be. 574 00:32:55,720 --> 00:32:57,030 And there's a number of possibilities 575 00:32:57,030 --> 00:32:58,280 of what your future would be-- 576 00:32:58,280 --> 00:33:00,210 and your future, and your future, 577 00:33:00,210 --> 00:33:05,190 and the entire future of the observable universe actually. 578 00:33:05,190 --> 00:33:07,230 And actually, there's an infinite number 579 00:33:07,230 --> 00:33:10,470 of possible futures of every particle 580 00:33:10,470 --> 00:33:13,620 for the observable universe. 581 00:33:13,620 --> 00:33:17,910 And there's a non-zero probability 582 00:33:17,910 --> 00:33:21,300 associated with each one of these possible outcomes. 583 00:33:21,300 --> 00:33:24,480 And that means, that if you travel far enough, 584 00:33:24,480 --> 00:33:28,530 you'd eventually not only reach an exact duplicate of her 585 00:33:28,530 --> 00:33:31,950 of our universe at this instant, but you'd also 586 00:33:31,950 --> 00:33:34,350 find an exact duplicate of our universe 587 00:33:34,350 --> 00:33:39,190 that has the same exact future of 10 minutes from now, 588 00:33:39,190 --> 00:33:44,910 and the same exact history, and same exact everything. 589 00:33:44,910 --> 00:33:49,150 And you'd expect to find infinitely many of those. 590 00:33:49,150 --> 00:33:52,620 It's crazy, but it's true. 591 00:33:52,620 --> 00:33:56,070 And actually, this is the least controversial type 592 00:33:56,070 --> 00:33:58,050 of parallel universe. 593 00:33:58,050 --> 00:33:59,790 This is the least controversial type. 594 00:33:59,790 --> 00:34:10,090 This is only what's called a level I multiverse. 595 00:34:10,090 --> 00:34:16,130 So the multiverse is the whole collection 596 00:34:16,130 --> 00:34:19,880 of all these universes, which we say are parallel-- we call them 597 00:34:19,880 --> 00:34:21,139 parallel universes. 598 00:34:21,139 --> 00:34:26,159 The multiverse is the collection of all of these. 599 00:34:26,159 --> 00:34:28,006 And this is the simplest type of multiverse. 600 00:34:28,006 --> 00:34:29,434 AUDIENCE: So are all the different 601 00:34:29,434 --> 00:34:30,714 really possible though? 602 00:34:30,714 --> 00:34:33,215 Because, don't they have to obey laws of physics and stuff 603 00:34:33,215 --> 00:34:33,530 like that? 604 00:34:33,530 --> 00:34:34,040 NICHOLAS DIBELLA: Yeah, yeah. 605 00:34:34,040 --> 00:34:34,699 OK. 606 00:34:34,699 --> 00:34:38,810 Every arrangement that obeys the laws of physics is possible. 607 00:34:38,810 --> 00:34:42,050 But the laws of physics are pretty flexible. 608 00:34:42,050 --> 00:34:42,630 [LAUGHTER] 609 00:34:42,630 --> 00:34:43,130 No. 610 00:34:43,130 --> 00:34:46,040 I mean, flexible in the sense that if you conceive 611 00:34:46,040 --> 00:34:50,699 of something happening, then if you're smart enough, 612 00:34:50,699 --> 00:34:54,590 then you can figure out a way for it to happen. 613 00:34:54,590 --> 00:35:01,880 And actually, precisely proving that it's possible for me to, 614 00:35:01,880 --> 00:35:04,510 I don't know, as a result of lifting up 615 00:35:04,510 --> 00:35:06,440 my foot I cause one of the students 616 00:35:06,440 --> 00:35:09,410 in the back to think about Harry Potter, 617 00:35:09,410 --> 00:35:11,377 to prove that that's possible is really hard. 618 00:35:11,377 --> 00:35:12,960 And it might actually not be possible. 619 00:35:12,960 --> 00:35:15,440 But I suspect that it's possible, 620 00:35:15,440 --> 00:35:21,830 because you have all these different things that 621 00:35:21,830 --> 00:35:22,610 could happen. 622 00:35:22,610 --> 00:35:24,800 I could somehow affect the air molecules here 623 00:35:24,800 --> 00:35:27,500 and cause them to do something that will propagate, 624 00:35:27,500 --> 00:35:30,170 reaching the students in the back, 625 00:35:30,170 --> 00:35:35,100 and then affecting his neurons in some way. 626 00:35:35,100 --> 00:35:36,810 It would be it would be very complicated, 627 00:35:36,810 --> 00:35:38,360 but it's really hard to prove that. 628 00:35:38,360 --> 00:35:40,100 But I hope it's true. 629 00:35:40,100 --> 00:35:43,760 And I hope I'm not crazy for hoping 630 00:35:43,760 --> 00:35:45,710 that it's true, because I think it 631 00:35:45,710 --> 00:35:47,750 would be so cool if it's true. 632 00:35:47,750 --> 00:35:50,900 Some people object to these ideas 633 00:35:50,900 --> 00:35:52,570 because they find them weird. 634 00:35:52,570 --> 00:35:55,640 But I actually hope they're true because they're weird. 635 00:35:55,640 --> 00:35:58,850 And I think it's really cool. 636 00:35:58,850 --> 00:36:00,427 Yeah, question? 637 00:36:00,427 --> 00:36:01,010 AUDIENCE: Yes. 638 00:36:01,010 --> 00:36:04,082 So all this depends on that that the universe is infinite? 639 00:36:04,082 --> 00:36:05,040 NICHOLAS DIBELLA: Yeah. 640 00:36:05,040 --> 00:36:07,250 This all that depends on the assumption 641 00:36:07,250 --> 00:36:09,060 that the universe is infinite. 642 00:36:09,060 --> 00:36:09,560 OK. 643 00:36:09,560 --> 00:36:11,934 If the universe is big enough however, but not infinite-- 644 00:36:11,934 --> 00:36:14,000 if it's really large, but still finite-- 645 00:36:14,000 --> 00:36:16,710 then it's still possible to find an exact copy of our universe. 646 00:36:16,710 --> 00:36:18,700 But it wouldn't be guaranteed. 647 00:36:18,700 --> 00:36:20,375 It wouldn't be guaranteed. 648 00:36:24,020 --> 00:36:25,805 Question? 649 00:36:25,805 --> 00:36:28,715 AUDIENCE: Do you mean the universe or the amount 650 00:36:28,715 --> 00:36:30,922 of matter in the universe? 651 00:36:30,922 --> 00:36:32,630 NICHOLAS DIBELLA: Well, necessarily both. 652 00:36:32,630 --> 00:36:34,046 AUDIENCE: Space could be infinite, 653 00:36:34,046 --> 00:36:35,270 but there might not be-- 654 00:36:35,270 --> 00:36:36,320 NICHOLAS DIBELLA: Oh, that's true, that's true. 655 00:36:36,320 --> 00:36:38,153 Space could be infinite, but there might not 656 00:36:38,153 --> 00:36:39,980 be an infinite amount of matter. 657 00:36:39,980 --> 00:36:42,440 So you can imagine, for instance, what's 658 00:36:42,440 --> 00:36:44,780 been called an island universe. 659 00:36:44,780 --> 00:36:49,370 You have an infinite universe, but there's 660 00:36:49,370 --> 00:36:56,150 only matter and energy at a very small island in the universe. 661 00:36:56,150 --> 00:37:00,380 And if that's true, then you can make predictions 662 00:37:00,380 --> 00:37:05,360 about what you'd observe with various astronomical 663 00:37:05,360 --> 00:37:06,416 measurements. 664 00:37:06,416 --> 00:37:07,790 You can make definite predictions 665 00:37:07,790 --> 00:37:09,279 with that type of model. 666 00:37:09,279 --> 00:37:10,820 And people have made some predictions 667 00:37:10,820 --> 00:37:12,110 with that type of model. 668 00:37:12,110 --> 00:37:14,180 And it turns out that those models 669 00:37:14,180 --> 00:37:16,040 are ruled out by observation. 670 00:37:16,040 --> 00:37:18,088 The island universe is ruled out by observation. 671 00:37:18,088 --> 00:37:19,838 AUDIENCE: So there has to be enough matter 672 00:37:19,838 --> 00:37:21,052 to fill the entire universe? 673 00:37:21,052 --> 00:37:22,010 NICHOLAS DIBELLA: Yeah. 674 00:37:22,010 --> 00:37:23,610 There has to be-- 675 00:37:23,610 --> 00:37:26,720 matter has to persist throughout the whole universe. 676 00:37:26,720 --> 00:37:29,091 And there's good evidence that it does. 677 00:37:29,091 --> 00:37:30,507 AUDIENCE: But how could there ever 678 00:37:30,507 --> 00:37:33,050 have been an infinite amount of matter in one point? 679 00:37:33,050 --> 00:37:33,890 NICHOLAS DIBELLA: How could there ever 680 00:37:33,890 --> 00:37:36,077 have been an infinite amount of matter at one point? 681 00:37:36,077 --> 00:37:37,701 AUDIENCE: [INAUDIBLE] had to create it. 682 00:37:37,701 --> 00:37:39,080 So it must have been all-- 683 00:37:39,080 --> 00:37:41,270 NICHOLAS DIBELLA: Well, that's a hard question. 684 00:37:41,270 --> 00:37:48,170 And actually, a process called inflation might explain that. 685 00:37:48,170 --> 00:37:51,380 And I'm going to talk about inflation in a minute. 686 00:37:51,380 --> 00:37:53,050 Where am I on time? 687 00:37:53,050 --> 00:37:56,480 Let me see. 688 00:37:56,480 --> 00:37:58,071 Yeah, we could take a break here. 689 00:37:58,071 --> 00:37:59,070 We'll take a break here. 690 00:37:59,070 --> 00:38:01,340 I'll return talking about inflation 691 00:38:01,340 --> 00:38:03,640 and the level II multi-verse. 692 00:38:11,840 --> 00:38:14,810 I mentioned that the level one multi-verse 693 00:38:14,810 --> 00:38:18,330 is the least controversial, and it's the least weird. 694 00:38:18,330 --> 00:38:21,902 So if you thought that was crazy, well, listen to this. 695 00:38:21,902 --> 00:38:28,893 Now, I'm going to discuss the level II multi-verse. 696 00:38:34,810 --> 00:38:42,550 So in the level I multi-verse, the only kind 697 00:38:42,550 --> 00:38:45,460 of weird thing that happened was that there 698 00:38:45,460 --> 00:38:48,190 were universes outside of our own 699 00:38:48,190 --> 00:38:52,300 that had different arrangements of matter. 700 00:38:52,300 --> 00:38:54,850 And there's only a finite number of arrangers of matter 701 00:38:54,850 --> 00:38:57,740 and a finite amount of space that are possible. 702 00:38:57,740 --> 00:39:00,040 So if you travel far enough, then eventually you 703 00:39:00,040 --> 00:39:02,350 reach a copy of our arrangement of matter, 704 00:39:02,350 --> 00:39:05,560 and you'd find a copy of our history and our future. 705 00:39:05,560 --> 00:39:09,091 And that's pretty much all there is to a level I multiverse. 706 00:39:09,091 --> 00:39:09,590 Question? 707 00:39:09,590 --> 00:39:10,965 AUDIENCE: There's no way anything 708 00:39:10,965 --> 00:39:13,430 could get more complicated than what we just did. 709 00:39:13,430 --> 00:39:14,380 NICHOLAS DIBELLA: There's no way that anything 710 00:39:14,380 --> 00:39:15,040 could get more complicated? 711 00:39:15,040 --> 00:39:15,340 AUDIENCE: It's not possible 712 00:39:15,340 --> 00:39:16,280 NICHOLAS DIBELLA: It's not possible. 713 00:39:16,280 --> 00:39:16,780 OK. 714 00:39:16,780 --> 00:39:20,200 Well, in a level II multiverse, not only 715 00:39:20,200 --> 00:39:26,470 are there different arrangements of matter in finite Hubble 716 00:39:26,470 --> 00:39:40,200 volumes, but we also have different dimensionalities 717 00:39:40,200 --> 00:39:41,435 of space and time. 718 00:39:58,340 --> 00:40:05,830 We also have different properties of particles. 719 00:40:08,650 --> 00:40:10,840 For instance, the mass of the electron 720 00:40:10,840 --> 00:40:20,440 is different in a parallel level II universe than it is here. 721 00:40:20,440 --> 00:40:29,800 Different properties of particles, 722 00:40:29,800 --> 00:40:32,660 and different physical constants. 723 00:40:48,310 --> 00:40:51,280 I'm going to talk more about this in the second, what 724 00:40:51,280 --> 00:40:54,770 each one of these means. 725 00:40:54,770 --> 00:41:02,140 But first, let me tell you how this could happen. 726 00:41:05,660 --> 00:41:10,404 So there's a-- hi. 727 00:41:15,400 --> 00:41:20,950 There's a popular theory that's proven enormously 728 00:41:20,950 --> 00:41:25,030 successful to explain the early universe, 729 00:41:25,030 --> 00:41:28,840 and this theory goes by the name of inflation. 730 00:41:28,840 --> 00:41:34,210 And inflation refers to an extremely, extremely rapid 731 00:41:34,210 --> 00:41:39,580 expansion of space in the early universe. 732 00:41:39,580 --> 00:41:46,420 So you can imagine, a very long time ago, 733 00:41:46,420 --> 00:41:50,290 you had the universe, for instance. 734 00:41:50,290 --> 00:41:54,235 Say this is the whole universe, and it expands very rapidly. 735 00:41:57,620 --> 00:42:02,320 It's hard to visualize that, but just try to think in your heads 736 00:42:02,320 --> 00:42:06,310 that the distance between points expands, and expands 737 00:42:06,310 --> 00:42:07,360 really rapidly. 738 00:42:07,360 --> 00:42:09,660 That's what I mean by space expanding. 739 00:42:09,660 --> 00:42:12,550 So space expands everywhere. 740 00:42:12,550 --> 00:42:14,830 Space everywhere expands. 741 00:42:14,830 --> 00:42:20,860 However, there are some regions in the universe 742 00:42:20,860 --> 00:42:23,740 where space stops expanding. 743 00:42:23,740 --> 00:42:26,470 And it stops expanding just because of what are 744 00:42:26,470 --> 00:42:28,300 called quantum fluctuations. 745 00:42:28,300 --> 00:42:31,180 There is a quantum probability that space here 746 00:42:31,180 --> 00:42:35,710 will stop expanding, and there there's a possibility here 747 00:42:35,710 --> 00:42:37,530 that space will stop expanding. 748 00:42:37,530 --> 00:42:41,770 And so, there are lots of regions 749 00:42:41,770 --> 00:42:43,900 that pop up around the universe where 750 00:42:43,900 --> 00:42:46,290 space has stopped expanding. 751 00:42:46,290 --> 00:42:48,520 And we call these regions bubbles. 752 00:42:55,460 --> 00:42:57,085 So the universe has expanded, and there 753 00:42:57,085 --> 00:43:03,892 are some bubbles where space has stopped expanding. 754 00:43:07,130 --> 00:43:10,810 And there are some other regions where space continues 755 00:43:10,810 --> 00:43:13,030 to expand really fast. 756 00:43:13,030 --> 00:43:20,080 Well, it turns out that when this happens a very mysterious 757 00:43:20,080 --> 00:43:24,490 process called symmetry breaking-- 758 00:43:24,490 --> 00:43:26,502 spontaneous symmetry breaking-- 759 00:43:38,880 --> 00:43:40,170 occurs. 760 00:43:40,170 --> 00:43:46,300 And it occurs in each one of these bubble universes. 761 00:43:46,300 --> 00:43:48,520 So this is a bubble. 762 00:43:48,520 --> 00:43:51,180 It occurs in each one of these bubbles. 763 00:43:51,180 --> 00:43:54,330 And as a result, the symmetry breaking, what happens 764 00:43:54,330 --> 00:43:59,460 is that these values become fixed-- 765 00:43:59,460 --> 00:44:02,460 the dimension of space becomes fixed, the dimension of time 766 00:44:02,460 --> 00:44:03,240 becomes fixed. 767 00:44:03,240 --> 00:44:05,670 I mean, the number of dimensions of space 768 00:44:05,670 --> 00:44:08,240 and the number of dimensions of time become fixed. 769 00:44:08,240 --> 00:44:11,160 So in our universe, for instance, 770 00:44:11,160 --> 00:44:15,660 there are probably three dimensions of space 771 00:44:15,660 --> 00:44:17,370 and just one dimension of time. 772 00:44:17,370 --> 00:44:19,920 And that's exactly what we observe. 773 00:44:19,920 --> 00:44:26,600 I can walk forward, backwards, upwards, downwards, sidewards, 774 00:44:26,600 --> 00:44:28,390 rightwards, whatever. 775 00:44:28,390 --> 00:44:31,167 So there are three different directions we can travel, 776 00:44:31,167 --> 00:44:33,000 and those are the three dimensions of space. 777 00:44:33,000 --> 00:44:36,030 And there also appears just to be one dimension of time 778 00:44:36,030 --> 00:44:38,946 that we travel, and we travel into the future. 779 00:44:38,946 --> 00:44:41,490 So there's one dimension of time. 780 00:44:41,490 --> 00:44:44,670 And that's what's symmetry breaking did for our bubble. 781 00:44:44,670 --> 00:44:52,620 It fixed the dimensionality of space and time to be 3 and 1. 782 00:44:52,620 --> 00:44:55,710 Symmetry breaking also fixed-- 783 00:44:55,710 --> 00:44:58,830 by fixed, I mean it determined-- determined 784 00:44:58,830 --> 00:45:01,380 the different properties of particles. 785 00:45:01,380 --> 00:45:07,687 For example, it fixed the value of the electron mass, which 786 00:45:07,687 --> 00:45:11,310 is, as you know, about 9.1 times 10 to the minus 31 kilograms. 787 00:45:11,310 --> 00:45:17,730 It fixed the mass of the proton, which is about 1.67 times 10 788 00:45:17,730 --> 00:45:19,860 to the minus 27 kilograms. 789 00:45:19,860 --> 00:45:21,441 It's fixed the mass of-- 790 00:45:24,600 --> 00:45:27,020 those only the two ones that are the mass I know. 791 00:45:27,020 --> 00:45:29,800 It fixed the mass of lots of other different particles, 792 00:45:29,800 --> 00:45:33,024 and it also fixed things like charges-- like, 793 00:45:33,024 --> 00:45:33,690 electric charge. 794 00:45:36,240 --> 00:45:38,134 Why? 795 00:45:38,134 --> 00:45:39,550 Well, I'll get to why in a minute. 796 00:45:39,550 --> 00:45:41,050 But it's also fixed electric charge. 797 00:45:41,050 --> 00:45:43,800 It also fixed some property called 798 00:45:43,800 --> 00:45:48,150 spin, which is a type of intrinsic angular momentum. 799 00:45:48,150 --> 00:45:50,320 Don't worry about it if you haven't heard of it. 800 00:45:50,320 --> 00:45:52,470 It also fixed-- well, it fixed a whole bunch 801 00:45:52,470 --> 00:45:54,840 of properties of particles. 802 00:45:54,840 --> 00:45:57,450 There are certain properties that particles have, 803 00:45:57,450 --> 00:46:00,330 and they have these properties because a type of symmetry 804 00:46:00,330 --> 00:46:03,390 breaking occurred in the early universe 805 00:46:03,390 --> 00:46:04,965 as a result of inflation. 806 00:46:04,965 --> 00:46:07,120 Let me write this down-- inflation. 807 00:46:13,630 --> 00:46:17,190 And so, different properties of particles were determined, 808 00:46:17,190 --> 00:46:20,442 and different dimensionalities space and time 809 00:46:20,442 --> 00:46:21,150 would determined. 810 00:46:21,150 --> 00:46:23,850 And also, different physical constants are determined. 811 00:46:23,850 --> 00:46:27,580 I mean, the speed of light is about 2.99792458 times 10 812 00:46:27,580 --> 00:46:31,710 to the 8 meters per second, for example. 813 00:46:31,710 --> 00:46:36,990 And that value was determined by symmetry breaking. 814 00:46:36,990 --> 00:46:41,850 It's also conceivable that our bubble universe 815 00:46:41,850 --> 00:46:46,650 could have been-- 816 00:46:46,650 --> 00:46:48,300 what's the word I'm looking for-- 817 00:46:48,300 --> 00:46:53,100 given, or could have been imparted with a different set 818 00:46:53,100 --> 00:46:55,540 of these numbers. 819 00:46:55,540 --> 00:47:01,500 Our universe could have had a dimensionality of space 820 00:47:01,500 --> 00:47:02,186 being four. 821 00:47:02,186 --> 00:47:03,810 There could be four dimensions of space 822 00:47:03,810 --> 00:47:06,660 and one dimension of time. 823 00:47:06,660 --> 00:47:09,690 And if that were the case, then our world 824 00:47:09,690 --> 00:47:12,370 would be quite different. 825 00:47:12,370 --> 00:47:16,860 And actually, we probably wouldn't exist in this world. 826 00:47:16,860 --> 00:47:22,410 First, it's easy to visualize what four dimensions of space 827 00:47:22,410 --> 00:47:25,265 would mean in terms of how we can move. 828 00:47:25,265 --> 00:47:27,390 We would have four different independent directions 829 00:47:27,390 --> 00:47:28,512 that we can move. 830 00:47:28,512 --> 00:47:29,970 That's what it would mean for there 831 00:47:29,970 --> 00:47:33,210 to be four dimensions of space. 832 00:47:33,210 --> 00:47:36,330 But actually, you can make some predictions 833 00:47:36,330 --> 00:47:42,840 about how atoms would behave if the laws of physics 834 00:47:42,840 --> 00:47:45,630 are the same but the dimensionalities 835 00:47:45,630 --> 00:47:48,030 of space and time are different. 836 00:47:48,030 --> 00:47:52,650 And you can make these predictions by simply fixing up 837 00:47:52,650 --> 00:47:57,150 the equations of the laws of physics 838 00:47:57,150 --> 00:48:02,260 in an appropriate manner, and then get at some results. 839 00:48:02,260 --> 00:48:05,400 By the way, all of these bubble universes 840 00:48:05,400 --> 00:48:07,230 have the same laws of physics. 841 00:48:07,230 --> 00:48:08,724 They have the same laws of physics. 842 00:48:08,724 --> 00:48:10,140 The only things that are different 843 00:48:10,140 --> 00:48:14,380 are the dimensionalities of space and time, 844 00:48:14,380 --> 00:48:18,420 the properties of matter, and the physical constants, 845 00:48:18,420 --> 00:48:20,160 like the speed of light. 846 00:48:20,160 --> 00:48:22,040 Also what could differ-- 847 00:48:22,040 --> 00:48:25,680 Well, within each one of these universes 848 00:48:25,680 --> 00:48:31,140 there could be a whole slew of type I universes. 849 00:48:31,140 --> 00:48:35,190 So type I is kind of contained in type II. 850 00:48:35,190 --> 00:48:37,040 So you have infinite number of type I, 851 00:48:37,040 --> 00:48:41,580 an infinite number of type I there, and so forth. 852 00:48:41,580 --> 00:48:44,710 But they all have the same laws of physics. 853 00:48:44,710 --> 00:48:46,980 Now, you can make certain predictions 854 00:48:46,980 --> 00:48:50,640 about what the world would be like in a universe 855 00:48:50,640 --> 00:48:54,400 with different dimensionalities from that of our own. 856 00:48:54,400 --> 00:48:58,510 For example, if the dimension of space-- 857 00:48:58,510 --> 00:49:10,320 let's say there are four space dimensions and five time 858 00:49:10,320 --> 00:49:12,300 dimensions. 859 00:49:12,300 --> 00:49:14,700 Which is conceivable. 860 00:49:14,700 --> 00:49:16,790 Its hard to imagine, but it's conceivable. 861 00:49:16,790 --> 00:49:20,010 And you can fix the equations of physics 862 00:49:20,010 --> 00:49:23,640 to include for this number of dimensions. 863 00:49:23,640 --> 00:49:26,081 Let's say there are five time dimensions. 864 00:49:28,850 --> 00:49:32,910 Well, such a world would be very different from our own. 865 00:49:32,910 --> 00:49:38,280 Because it turns out that if our universe were like this, then 866 00:49:38,280 --> 00:49:42,450 it would be absolutely impossible to make a prediction 867 00:49:42,450 --> 00:49:48,750 about any event with a non infinite uncertainty. 868 00:49:48,750 --> 00:49:50,265 So we couldn't make any prediction. 869 00:49:53,151 --> 00:49:54,900 We're able to make pretty good predictions 870 00:49:54,900 --> 00:49:56,370 in our own universe, right? 871 00:49:56,370 --> 00:49:58,890 I mean, I predict that this chalk, once I drop it, 872 00:49:58,890 --> 00:50:03,800 will fall on this paper with an uncertainty of say 1 mile. 873 00:50:03,800 --> 00:50:04,520 I don't know. 874 00:50:04,520 --> 00:50:07,410 So there's an uncertainty of one mile 875 00:50:07,410 --> 00:50:10,990 of where this chalk will fall. 876 00:50:10,990 --> 00:50:12,180 OK, It worked. 877 00:50:12,180 --> 00:50:15,189 My uncertainty was non-infinite. 878 00:50:15,189 --> 00:50:16,980 It was pretty big, but it was non infinite. 879 00:50:16,980 --> 00:50:19,740 And it turned out to correspond to what we observed. 880 00:50:19,740 --> 00:50:22,620 Well, if these were the dimensionalities 881 00:50:22,620 --> 00:50:25,770 of space and time, you wouldn't be 882 00:50:25,770 --> 00:50:29,130 able to get any better than infinite uncertainty. 883 00:50:29,130 --> 00:50:34,320 You can also can imagine there being just one space dimension, 884 00:50:34,320 --> 00:50:37,230 and there being like four time dimensions. 885 00:50:37,230 --> 00:50:42,140 So let say one space and four time. 886 00:50:45,800 --> 00:50:49,990 Well, in this universe with these dimensionalities, 887 00:50:49,990 --> 00:50:56,720 it turns out that all atoms are unstable. 888 00:50:56,720 --> 00:50:58,650 They're immediately unstable. 889 00:50:58,650 --> 00:51:01,720 If I had an atom it would just exist for a split second 890 00:51:01,720 --> 00:51:03,380 and then that would be the end it. 891 00:51:03,380 --> 00:51:07,860 It would be impossible for molecules to form. 892 00:51:07,860 --> 00:51:09,650 It would be impossible for atoms just 893 00:51:09,650 --> 00:51:15,950 to sit there, much less than stick together to other atoms. 894 00:51:15,950 --> 00:51:19,760 And it would be impossible for macromolecules to form. 895 00:51:19,760 --> 00:51:23,600 It would be impossible for simple cells to form-- 896 00:51:23,600 --> 00:51:24,915 What's that? 897 00:51:24,915 --> 00:51:26,810 AUDIENCE: No sentient life there. 898 00:51:26,810 --> 00:51:28,130 NICHOLAS DIBELLA: Yeah, no. 899 00:51:28,130 --> 00:51:29,420 No life there. 900 00:51:29,420 --> 00:51:31,910 This type of universe would be completely 901 00:51:31,910 --> 00:51:36,237 devoid of intelligent people like us. 902 00:51:36,237 --> 00:51:37,520 [LAUGHTER] 903 00:51:37,520 --> 00:51:38,217 Yeah? 904 00:51:38,217 --> 00:51:40,994 AUDIENCE: What would it be like outside of these bubbles? 905 00:51:40,994 --> 00:51:42,785 NICHOLAS DIBELLA: Outside of these bubbles? 906 00:51:42,785 --> 00:51:45,260 AUDIENCE: Would the amount of dimensions 907 00:51:45,260 --> 00:51:47,240 and all properties of physical constant 908 00:51:47,240 --> 00:51:49,524 be changing all the time? 909 00:51:49,524 --> 00:51:52,024 They would never be the same? 910 00:51:52,024 --> 00:51:53,190 NICHOLAS DIBELLA: You mean-- 911 00:51:53,190 --> 00:51:55,030 AUDIENCE: If you left the bubble, 912 00:51:55,030 --> 00:51:58,080 and you were just in the rest of space, 913 00:51:58,080 --> 00:52:00,130 I guess continuing to expand. 914 00:52:02,794 --> 00:52:06,800 NICHOLAS DIBELLA: I'm not exactly sure. 915 00:52:06,800 --> 00:52:10,460 I think-- Yeah, I'm not sure what 916 00:52:10,460 --> 00:52:13,300 it's like outside the bubbles. 917 00:52:13,300 --> 00:52:16,770 I don't completely understand this. 918 00:52:16,770 --> 00:52:18,760 I'm telling you the stuff that I do know. 919 00:52:18,760 --> 00:52:19,400 AUDIENCE: Do you know if it would 920 00:52:19,400 --> 00:52:21,800 be like just one moment they're inside the bubble 921 00:52:21,800 --> 00:52:24,040 and everything is normal, and the next minute they're 922 00:52:24,040 --> 00:52:26,450 outside the bubble and it's completely different? 923 00:52:26,450 --> 00:52:28,030 NICHOLAS DIBELLA: Oh, oh, oh, oh. 924 00:52:28,030 --> 00:52:28,820 OK. 925 00:52:28,820 --> 00:52:30,410 Well, I can kind of answer that. 926 00:52:30,410 --> 00:52:31,760 I can kind of answer that. 927 00:52:31,760 --> 00:52:34,560 These bubbles are infinite in size. 928 00:52:34,560 --> 00:52:38,415 So once you're in one of these bubbles you can't get out. 929 00:52:38,415 --> 00:52:40,040 You're not going to be able to get out. 930 00:52:40,040 --> 00:52:42,080 And you can't interact with the other bubbles unfortunately. 931 00:52:42,080 --> 00:52:43,705 AUDIENCE: How are they infinite in size 932 00:52:43,705 --> 00:52:45,605 if there's more than one of them? 933 00:52:45,605 --> 00:52:47,480 NICHOLAS DIBELLA: That's kind hard to imagine 934 00:52:47,480 --> 00:52:50,860 how they can be infinite in size and with more than one of them. 935 00:52:50,860 --> 00:52:53,420 But that's simply-- 936 00:52:53,420 --> 00:52:56,570 I know it's hard to visualize. 937 00:52:56,570 --> 00:52:59,690 But infinity is a really weird concept, 938 00:52:59,690 --> 00:53:03,915 and it does weird things to you when you think about it. 939 00:53:03,915 --> 00:53:06,650 AUDIENCE: But the entire universe in its entirety 940 00:53:06,650 --> 00:53:10,482 is infinitely larger than infinitely large things. 941 00:53:10,482 --> 00:53:11,440 NICHOLAS DIBELLA: Yeah. 942 00:53:11,440 --> 00:53:16,520 You have to be pretty careful when you use the word infinity. 943 00:53:16,520 --> 00:53:18,850 You might have heard about different types of infinity, 944 00:53:18,850 --> 00:53:21,710 like countable infinity, uncountable infinity. 945 00:53:21,710 --> 00:53:24,170 For instance, there are more irrational numbers 946 00:53:24,170 --> 00:53:26,570 than there are rational numbers. 947 00:53:26,570 --> 00:53:30,620 And I'll let you all wonder about that. 948 00:53:34,420 --> 00:53:40,220 The point is, the way you deal with infinity is kind of weird, 949 00:53:40,220 --> 00:53:43,400 and it's kind of confusing, and it does weird things to you. 950 00:53:43,400 --> 00:53:46,690 AUDIENCE: [INAUDIBLE] any amount of any number-- 951 00:53:46,690 --> 00:53:49,670 any combination of sounds has to be a number. 952 00:53:49,670 --> 00:53:51,514 NICHOLAS DIBELLA: Any combination of what? 953 00:53:51,514 --> 00:53:53,366 AUDIENCE: Any combination of sounds ever 954 00:53:53,366 --> 00:53:55,440 has to be [INAUDIBLE] of a number. 955 00:53:55,440 --> 00:53:56,880 But there are infinite numbers. 956 00:53:56,880 --> 00:53:58,944 And there are less-- 957 00:53:58,944 --> 00:54:00,455 you have to have-- 958 00:54:00,455 --> 00:54:02,830 NICHOLAS DIBELLA: I don't know what you're talking about. 959 00:54:02,830 --> 00:54:03,355 What? 960 00:54:03,355 --> 00:54:05,590 Talk to me after class. 961 00:54:05,590 --> 00:54:07,100 I don't know. 962 00:54:07,100 --> 00:54:08,960 OK. 963 00:54:08,960 --> 00:54:11,270 Well, getting back to this. 964 00:54:11,270 --> 00:54:14,556 In this universe with these dimensionalities, 965 00:54:14,556 --> 00:54:15,680 atoms would cease to exist. 966 00:54:15,680 --> 00:54:17,138 They'd just be completely unstable. 967 00:54:19,770 --> 00:54:27,740 Now, this level of multiverse actually provides 968 00:54:27,740 --> 00:54:30,870 a nice explanation for the question, 969 00:54:30,870 --> 00:54:36,920 why is there one dimension of time 970 00:54:36,920 --> 00:54:38,990 and why are there three dimensions of space? 971 00:54:38,990 --> 00:54:41,160 Why is the mass of an electron this number? 972 00:54:41,160 --> 00:54:43,070 Why is the speed of light this number? 973 00:54:43,070 --> 00:54:46,310 Why is Newton's gravitational constant this number? 974 00:54:46,310 --> 00:54:49,880 It provides a nice explanation for that. 975 00:54:49,880 --> 00:54:56,805 Because there are infinitely many bubbles. 976 00:54:56,805 --> 00:54:59,180 And there are infinitely many different types of symmetry 977 00:54:59,180 --> 00:55:00,870 breakings that can occur. 978 00:55:00,870 --> 00:55:02,720 So you'd expect to find some probability 979 00:55:02,720 --> 00:55:05,660 distribution of various dimensionalities of space 980 00:55:05,660 --> 00:55:06,852 and time. 981 00:55:06,852 --> 00:55:08,810 You'd expect to find a probability distribution 982 00:55:08,810 --> 00:55:11,510 of various properties of particles 983 00:55:11,510 --> 00:55:15,830 and various values of physical constants. 984 00:55:15,830 --> 00:55:20,070 And so, it just kind of occurred by chance 985 00:55:20,070 --> 00:55:24,875 that our universe has these certain values. 986 00:55:27,710 --> 00:55:33,650 If it were the case that these other bubbles didn't exist. 987 00:55:33,650 --> 00:55:37,910 If it were the case that they were just one universe with one 988 00:55:37,910 --> 00:55:41,450 set of numbers for physical constants, 989 00:55:41,450 --> 00:55:43,160 one set of numbers for properties 990 00:55:43,160 --> 00:55:45,845 of particles, one set of dimensionalities 991 00:55:45,845 --> 00:55:46,880 for space and time. 992 00:55:46,880 --> 00:55:50,540 If there were just one universe with those very specific sets 993 00:55:50,540 --> 00:55:58,820 of numbers, then it's very hard to explain why those numbers 994 00:55:58,820 --> 00:56:01,560 and why not other numbers. 995 00:56:01,560 --> 00:56:05,480 Well, the level II multiverse provides a very elegant 996 00:56:05,480 --> 00:56:07,820 explanation-- 997 00:56:07,820 --> 00:56:09,510 it just occurred by probability. 998 00:56:09,510 --> 00:56:12,690 It's just a result of symmetry breaking occurring 999 00:56:12,690 --> 00:56:14,780 in the early universe. 1000 00:56:14,780 --> 00:56:18,220 That's a wonderful explanation. 1001 00:56:18,220 --> 00:56:21,350 Another explanation that's been traditionally proposed 1002 00:56:21,350 --> 00:56:28,470 is that something fine-tuned those values. 1003 00:56:28,470 --> 00:56:32,060 Well, that's a kind of controversial view 1004 00:56:32,060 --> 00:56:37,391 that I think is less elegant than this multiverse theory. 1005 00:56:37,391 --> 00:56:37,890 Question? 1006 00:56:37,890 --> 00:56:39,858 AUDIENCE: Well, you were saying that all this 1007 00:56:39,858 --> 00:56:41,810 is happening from our universe. 1008 00:56:41,810 --> 00:56:44,554 So from our big bang all this happened? 1009 00:56:47,372 --> 00:56:48,330 NICHOLAS DIBELLA: Yeah. 1010 00:56:48,330 --> 00:56:50,550 From our big bang, this all happened. 1011 00:56:50,550 --> 00:56:51,050 OK. 1012 00:56:51,050 --> 00:56:55,035 So there's one huge universe, which is "the universe." 1013 00:56:55,035 --> 00:56:57,560 Which you could say is "the multiverse." 1014 00:56:57,560 --> 00:57:02,660 And inside the multiverse there are various infinite bubbles 1015 00:57:02,660 --> 00:57:05,270 that arised as a result of this process 1016 00:57:05,270 --> 00:57:06,649 called inflation occurring. 1017 00:57:06,649 --> 00:57:08,690 And a result of this symmetry breaking occurring, 1018 00:57:08,690 --> 00:57:10,100 fixed these various numbers. 1019 00:57:10,100 --> 00:57:13,400 AUDIENCE: But does that mean that all this happening now, 1020 00:57:13,400 --> 00:57:15,400 is that-- if space is infinite, could there 1021 00:57:15,400 --> 00:57:16,550 be another big bang? 1022 00:57:16,550 --> 00:57:18,800 NICHOLAS DIBELLA: Oh, could there be another big bang? 1023 00:57:18,800 --> 00:57:21,660 Yeah, there there could be. 1024 00:57:21,660 --> 00:57:22,994 There could be another big bang. 1025 00:57:26,270 --> 00:57:29,690 This whole subject of talking about the universe 1026 00:57:29,690 --> 00:57:32,330 as a whole, this is the subject of cosmology. 1027 00:57:32,330 --> 00:57:35,850 And there are a lot of really cool ideas in cosmology. 1028 00:57:38,940 --> 00:57:40,580 You can talk about multiple big bangs. 1029 00:57:40,580 --> 00:57:42,860 You could talk about the universe expanding 1030 00:57:42,860 --> 00:57:45,740 and then contracting, and then being a cyclic universe. 1031 00:57:45,740 --> 00:57:48,230 There are lots of cool ideas. 1032 00:57:48,230 --> 00:57:51,020 It's possible that there could be multiple big bangs. 1033 00:57:51,020 --> 00:57:56,960 And I think it might be possible within this theory. 1034 00:57:56,960 --> 00:57:59,524 And that might even-- 1035 00:57:59,524 --> 00:58:01,190 there are various versions of inflation, 1036 00:58:01,190 --> 00:58:03,440 and there are probably lots of versions where 1037 00:58:03,440 --> 00:58:05,330 multiple big bangs do occur. 1038 00:58:05,330 --> 00:58:07,700 But I'm not an expert on it, and so I'm just 1039 00:58:07,700 --> 00:58:10,120 feeding you what I know. 1040 00:58:10,120 --> 00:58:11,350 Was there another question? 1041 00:58:11,350 --> 00:58:11,850 Yeah. 1042 00:58:11,850 --> 00:58:13,490 AUDIENCE: How many meters is on kilometer? 1043 00:58:13,490 --> 00:58:15,615 NICHOLAS DIBELLA: How many meters is one kilometer? 1044 00:58:15,615 --> 00:58:17,562 1,000. 1045 00:58:17,562 --> 00:58:20,584 AUDIENCE: OK, that's it. 1046 00:58:20,584 --> 00:58:22,750 NICHOLAS DIBELLA: Very deep, philosophical question. 1047 00:58:25,470 --> 00:58:26,400 OK. 1048 00:58:26,400 --> 00:58:28,580 Does anybody have the time? 1049 00:58:28,580 --> 00:58:29,870 I'm not going to look at my-- 1050 00:58:29,870 --> 00:58:30,810 AUDIENCE: 2:39 1051 00:58:30,810 --> 00:58:34,560 NICHOLAS DIBELLA: 2:39 OK. 1052 00:58:34,560 --> 00:58:36,150 Let's see. 1053 00:58:36,150 --> 00:58:38,090 So this is the level II multiverse. 1054 00:58:38,090 --> 00:58:42,005 I think I'm going to skip level III and going to just go to IV. 1055 00:58:42,005 --> 00:58:43,880 I don't think we have time to do all of them. 1056 00:58:43,880 --> 00:58:48,030 But level III would actually take a long time to explain, 1057 00:58:48,030 --> 00:58:52,680 because, well, level III has to do with the many worlds 1058 00:58:52,680 --> 00:58:54,270 interpretation of quantum mechanics, 1059 00:58:54,270 --> 00:58:55,853 and I would have to explain that first 1060 00:58:55,853 --> 00:58:59,610 and then talk about crazy consequences. 1061 00:58:59,610 --> 00:59:03,150 But actually, the level III multiverse 1062 00:59:03,150 --> 00:59:07,290 doesn't add any qualitatively new features. 1063 00:59:07,290 --> 00:59:12,360 Like, you we still have these different sets of numbers. 1064 00:59:12,360 --> 00:59:14,735 You still have different arrangements of matter. 1065 00:59:14,735 --> 00:59:16,110 So I'm just going to push forward 1066 00:59:16,110 --> 00:59:20,450 to level IV, which is the highest level multiverse. 1067 00:59:20,450 --> 00:59:22,154 You guys ready? 1068 00:59:22,154 --> 00:59:22,695 AUDIENCE: No. 1069 00:59:22,695 --> 00:59:24,180 AUDIENCE: Always. 1070 00:59:24,180 --> 00:59:25,170 AUDIENCE: Never. 1071 00:59:25,170 --> 00:59:26,160 AUDIENCE: It's scary. 1072 00:59:26,160 --> 00:59:26,790 NICHOLAS DIBELLA: It's scary? 1073 00:59:26,790 --> 00:59:27,414 AUDIENCE: Yeah. 1074 00:59:27,414 --> 00:59:30,066 It's [INAUDIBLE]. 1075 00:59:30,066 --> 00:59:33,807 How did they even come to this? 1076 00:59:33,807 --> 00:59:35,640 NICHOLAS DIBELLA: How did they come to this? 1077 00:59:35,640 --> 00:59:41,965 Well, inflation was discovered by-- 1078 00:59:41,965 --> 00:59:44,006 AUDIENCE: We're still discovering things on Earth 1079 00:59:44,006 --> 00:59:45,902 that we don't know. 1080 00:59:45,902 --> 00:59:47,997 How do we figure out-- 1081 00:59:47,997 --> 00:59:50,330 NICHOLAS DIBELLA: There are a lot of really smart people 1082 00:59:50,330 --> 00:59:51,746 that have thought of these things, 1083 00:59:51,746 --> 00:59:55,400 and inflation was actually discovered by physicists 1084 00:59:55,400 --> 00:59:57,380 here at MIT, Alan Guth. 1085 00:59:57,380 --> 01:00:01,750 He discovered this inflation mechanism. 1086 01:00:01,750 --> 01:00:06,830 And it actually it explains a whole lot of phenomena 1087 01:00:06,830 --> 01:00:09,680 that are really hard to explain, and impossible to explain 1088 01:00:09,680 --> 01:00:12,131 with other mechanisms. 1089 01:00:12,131 --> 01:00:18,200 It explains why, for example, the universe 1090 01:00:18,200 --> 01:00:21,740 is roughly uniform. 1091 01:00:21,740 --> 01:00:24,412 Why the universe roughly looks the same 1092 01:00:24,412 --> 01:00:25,370 no matter where you go. 1093 01:00:25,370 --> 01:00:26,453 It looks roughly the same. 1094 01:00:26,453 --> 01:00:28,490 It explains lots of things like that. 1095 01:00:28,490 --> 01:00:28,990 Question? 1096 01:00:28,990 --> 01:00:30,220 AUDIENCE: Is he still here? 1097 01:00:30,220 --> 01:00:31,716 NICHOLAS DIBELLA: Yeah, yeah. 1098 01:00:31,716 --> 01:00:35,000 I have his book [INAUDIBLE]. 1099 01:00:35,000 --> 01:00:35,630 Oh, good. 1100 01:00:35,630 --> 01:00:36,858 Cool, cool. 1101 01:00:36,858 --> 01:00:38,767 AUDIENCE: So you must ask him-- 1102 01:00:38,767 --> 01:00:39,725 NICHOLAS DIBELLA: Yeah. 1103 01:00:39,725 --> 01:00:40,940 AUDIENCE: Is he a teacher. 1104 01:00:40,940 --> 01:00:43,587 NICHOLAS DIBELLA: Yeah, actually next semester 1105 01:00:43,587 --> 01:00:45,170 I'm taking a class that he's teaching, 1106 01:00:45,170 --> 01:00:48,030 called The Early Universe. 1107 01:00:48,030 --> 01:00:52,520 And that's really cool that we students 1108 01:00:52,520 --> 01:00:55,190 get to take classes taught by the absolute experts 1109 01:00:55,190 --> 01:00:56,295 on these fields. 1110 01:00:56,295 --> 01:00:58,478 So yeah, come to MIT. 1111 01:00:58,478 --> 01:00:59,980 [LAUGHTER] 1112 01:00:59,980 --> 01:01:00,480 OK. 1113 01:01:04,560 --> 01:01:07,630 So now level IV multi-verse. 1114 01:01:15,019 --> 01:01:16,060 I looking doing that too. 1115 01:01:19,230 --> 01:01:23,430 Now, level II multi-verse, we had different dimensionalities 1116 01:01:23,430 --> 01:01:26,020 of space and time, different properties of particles, 1117 01:01:26,020 --> 01:01:30,040 different physical constants, but the same laws of physics. 1118 01:01:30,040 --> 01:01:32,530 Well guess what's going to be different in the level IV 1119 01:01:32,530 --> 01:01:35,260 universe? 1120 01:01:35,260 --> 01:01:38,274 Different laws of physics, yeah. 1121 01:01:38,274 --> 01:01:39,256 AUDIENCE: That's weird. 1122 01:01:58,920 --> 01:02:04,930 NICHOLAS DIBELLA: It's been noticed for a long time that-- 1123 01:02:04,930 --> 01:02:09,020 well, the universe is explained very well 1124 01:02:09,020 --> 01:02:10,325 by mathematical models. 1125 01:02:13,790 --> 01:02:16,310 A lot of you probably haven't taken any mathematical classes 1126 01:02:16,310 --> 01:02:19,370 on physics, but there are lots of equations 1127 01:02:19,370 --> 01:02:22,550 in physics that give rise to these various predictions 1128 01:02:22,550 --> 01:02:25,500 that I'm talking about. 1129 01:02:25,500 --> 01:02:27,920 And I gave you some equations in the second class. 1130 01:02:27,920 --> 01:02:29,090 Maybe I shouldn't have-- 1131 01:02:29,090 --> 01:02:31,900 but somebody influenced me. 1132 01:02:31,900 --> 01:02:35,360 But there are various equations in physics. 1133 01:02:35,360 --> 01:02:36,860 And there are various equations that 1134 01:02:36,860 --> 01:02:41,510 are very specific to a particular set of laws 1135 01:02:41,510 --> 01:02:43,100 of physics. 1136 01:02:43,100 --> 01:02:46,700 And we say that these laws of physics 1137 01:02:46,700 --> 01:02:50,660 have a particular mathematical structure. 1138 01:02:50,660 --> 01:02:52,760 And it's been a view for a long time 1139 01:02:52,760 --> 01:02:56,720 that these mathematical structures merely 1140 01:02:56,720 --> 01:02:58,460 approximate the universe. 1141 01:02:58,460 --> 01:03:05,510 Maybe the universe is impossible to describe with math. 1142 01:03:05,510 --> 01:03:13,574 Our models predict very well what 1143 01:03:13,574 --> 01:03:14,990 we should observe in the universe, 1144 01:03:14,990 --> 01:03:16,260 but they're not perfect. 1145 01:03:16,260 --> 01:03:19,100 In fact, we know for a fact that we don't have the precise-- 1146 01:03:19,100 --> 01:03:25,450 we don't have the absolute correct model for our universe. 1147 01:03:25,450 --> 01:03:27,200 Well, because, actually, quantum mechanics 1148 01:03:27,200 --> 01:03:32,330 and general relativity don't work with each other. 1149 01:03:32,330 --> 01:03:33,570 They contradict each other. 1150 01:03:33,570 --> 01:03:36,590 And this is a problem in modern physics 1151 01:03:36,590 --> 01:03:38,510 that people are working on solving. 1152 01:03:38,510 --> 01:03:41,390 String theory is one hope for-- 1153 01:03:41,390 --> 01:03:41,890 what's that? 1154 01:03:41,890 --> 01:03:43,521 AUDIENCE: So only one is right? 1155 01:03:43,521 --> 01:03:44,520 NICHOLAS DIBELLA: Well-- 1156 01:03:44,520 --> 01:03:45,942 AUDIENCE: Or is it mixable? 1157 01:03:45,942 --> 01:03:46,889 [INAUDIBLE] 1158 01:03:46,889 --> 01:03:48,680 NICHOLAS DIBELLA: The question is, is there 1159 01:03:48,680 --> 01:03:53,570 only one mathematical structure that can model the universe? 1160 01:03:53,570 --> 01:04:01,370 Well, you could probably think of different sets of equations 1161 01:04:01,370 --> 01:04:02,890 to give the same results. 1162 01:04:02,890 --> 01:04:06,350 But if that's the case, then it might 1163 01:04:06,350 --> 01:04:10,070 have to be that they're all really manifestations 1164 01:04:10,070 --> 01:04:11,050 of the same thing. 1165 01:04:11,050 --> 01:04:12,200 AUDIENCE: OK. 1166 01:04:12,200 --> 01:04:14,158 NICHOLAS DIBELLA: They might all be isomorphic, 1167 01:04:14,158 --> 01:04:15,910 is the technical term-- 1168 01:04:15,910 --> 01:04:18,344 the term the mathematicians use. 1169 01:04:18,344 --> 01:04:20,804 AUDIENCE: So how would they decide which model 1170 01:04:20,804 --> 01:04:22,280 is the right or wrong one? 1171 01:04:22,280 --> 01:04:23,480 NICHOLAS DIBELLA: How would they decide which model 1172 01:04:23,480 --> 01:04:24,650 is right for our universe? 1173 01:04:24,650 --> 01:04:27,549 Well, first of all, you to have a consistent model. 1174 01:04:27,549 --> 01:04:29,090 You have to have a consistent theory. 1175 01:04:29,090 --> 01:04:30,890 It has to be consistent. 1176 01:04:30,890 --> 01:04:34,820 General relativity plus quantum mechanics is not consistent. 1177 01:04:34,820 --> 01:04:39,050 When you try to talk about the quantum mechanics 1178 01:04:39,050 --> 01:04:41,060 inside of black holes for instance, 1179 01:04:41,060 --> 01:04:43,049 you get at results like infinite probabilities. 1180 01:04:43,049 --> 01:04:44,840 And what does an infinite probability mean? 1181 01:04:44,840 --> 01:04:45,548 It's meaningless. 1182 01:04:45,548 --> 01:04:47,220 It doesn't really mean anything. 1183 01:04:47,220 --> 01:04:50,870 And so, the theory we have right now is inconsistent. 1184 01:04:50,870 --> 01:04:52,940 And people are searching for a consistent theory. 1185 01:04:52,940 --> 01:04:55,730 And so far, string theory looks like it's our best hope. 1186 01:04:55,730 --> 01:04:57,330 But that's not worked out yet. 1187 01:04:57,330 --> 01:04:59,480 But it's doing better than other theories, 1188 01:04:59,480 --> 01:05:01,310 and so, it receives a lot more attention. 1189 01:05:01,310 --> 01:05:04,490 But it's still the fundamental problem. 1190 01:05:04,490 --> 01:05:06,690 AUDIENCE: How do they determine consistency then? 1191 01:05:06,690 --> 01:05:08,815 NICHOLAS DIBELLA: How do you determine consistency? 1192 01:05:08,815 --> 01:05:11,390 Well, you check to see, here are two things, 1193 01:05:11,390 --> 01:05:13,676 do they agree with each other? 1194 01:05:13,676 --> 01:05:15,050 If this is true and this is true, 1195 01:05:15,050 --> 01:05:16,258 it implies that this is true. 1196 01:05:16,258 --> 01:05:19,060 AUDIENCE: Is it consistency with experiments 1197 01:05:19,060 --> 01:05:20,060 of what we already know? 1198 01:05:20,060 --> 01:05:20,930 NICHOLAS DIBELLA: OK. 1199 01:05:20,930 --> 01:05:22,180 Various types of consistency-- 1200 01:05:22,180 --> 01:05:23,090 OK. 1201 01:05:23,090 --> 01:05:26,264 First and foremost, it has to be consistent in itself. 1202 01:05:26,264 --> 01:05:27,680 If it's not consistent in itself-- 1203 01:05:27,680 --> 01:05:28,160 AUDIENCE: That's bad. 1204 01:05:28,160 --> 01:05:29,618 NICHOLAS DIBELLA: Then it gets bad. 1205 01:05:29,618 --> 01:05:35,060 And secondly, we hope that it models the actual universe. 1206 01:05:35,060 --> 01:05:38,780 I can think of a theory right now that's consistent, 1207 01:05:38,780 --> 01:05:40,700 but it doesn't model the universe. 1208 01:05:40,700 --> 01:05:43,820 I could propose a theory that the whole universe 1209 01:05:43,820 --> 01:05:49,400 is determined by how I move this pen. 1210 01:05:49,400 --> 01:05:52,970 I can think of various rules of how it works. 1211 01:05:52,970 --> 01:05:56,099 But it doesn't agree with experiment. 1212 01:05:56,099 --> 01:05:58,140 So we need something that agrees with experiment. 1213 01:06:06,250 --> 01:06:09,040 But it's been asked, why-- 1214 01:06:09,040 --> 01:06:12,340 well, actually, it's been remarked by Einstein, 1215 01:06:12,340 --> 01:06:15,340 that perhaps the most incomprehensible 1216 01:06:15,340 --> 01:06:19,270 thing about the universe is that it is comprehensible. 1217 01:06:19,270 --> 01:06:24,430 And it's comprehensible with the aid of mathematics. 1218 01:06:24,430 --> 01:06:28,450 And many people have wondered, why does math work so well? 1219 01:06:28,450 --> 01:06:31,610 Why does it work so well? 1220 01:06:31,610 --> 01:06:34,060 Maybe it's the case that the universe actually 1221 01:06:34,060 --> 01:06:35,440 is mathematical. 1222 01:06:35,440 --> 01:06:37,120 Math isn't merely an approximation, 1223 01:06:37,120 --> 01:06:40,750 but the universe is mathematical. 1224 01:06:40,750 --> 01:06:49,490 And that's the main idea behind this level IV multiverse. 1225 01:06:49,490 --> 01:06:55,350 So in our universe there is-- 1226 01:06:55,350 --> 01:07:00,120 well, if it's true that the universe is mathematical, 1227 01:07:00,120 --> 01:07:02,950 then in our universe there's a mathematical structure 1228 01:07:02,950 --> 01:07:05,960 that is the universe. 1229 01:07:05,960 --> 01:07:12,990 So the main assumption here is that the universe 1230 01:07:12,990 --> 01:07:14,150 is mathematical. 1231 01:07:24,650 --> 01:07:27,680 Well, what kind of mathematical models? 1232 01:07:27,680 --> 01:07:32,230 Well general relativity is, for instance, 1233 01:07:32,230 --> 01:07:36,170 some sub branch of differential geometry 1234 01:07:36,170 --> 01:07:43,400 and spacetime is a pseudo-Riemannian manifold. 1235 01:07:43,400 --> 01:07:45,260 It's very complicated math. 1236 01:07:45,260 --> 01:07:47,840 They're very complicated mathematical structures. 1237 01:07:47,840 --> 01:07:50,480 But the fact is that they are mathematical structures. 1238 01:07:50,480 --> 01:07:52,640 And the assumption behind the level IV multiverse 1239 01:07:52,640 --> 01:07:55,790 is that the universe is mathematical. 1240 01:07:55,790 --> 01:08:01,520 Now, we can ask, why this mathematical structure and not 1241 01:08:01,520 --> 01:08:03,950 other mathematical structures? 1242 01:08:03,950 --> 01:08:07,610 And a nice answer to that question, 1243 01:08:07,610 --> 01:08:10,100 similar to the answer to the question, why 1244 01:08:10,100 --> 01:08:13,580 these dimensionalities, why these concepts and not others, 1245 01:08:13,580 --> 01:08:17,990 is that there are many other universes with different laws 1246 01:08:17,990 --> 01:08:21,140 of physics, which correspond to different mathematical 1247 01:08:21,140 --> 01:08:23,359 structures. 1248 01:08:23,359 --> 01:08:25,880 So I just proposed one before with the 1249 01:08:25,880 --> 01:08:28,760 had to do with this pen. 1250 01:08:28,760 --> 01:08:34,760 If I could formulate that into a mathematical theory that's 1251 01:08:34,760 --> 01:08:39,470 consistent, then according to level for multiple IV 1252 01:08:39,470 --> 01:08:43,580 multiverse theories there exist universes 1253 01:08:43,580 --> 01:08:47,960 that obey the laws of physics as dictated by this pen. 1254 01:08:50,910 --> 01:08:52,200 In our universe-- 1255 01:08:52,200 --> 01:08:54,660 AUDIENCE: And when you say there exists, 1256 01:08:54,660 --> 01:08:56,859 would that mean that if you travelled far enough 1257 01:08:56,859 --> 01:08:59,359 or something, or if you search the whole physical universe, 1258 01:08:59,359 --> 01:09:00,279 you say, "there exists"-- 1259 01:09:00,279 --> 01:09:00,899 NICHOLAS DIBELLA: "There exists," yeah, yeah. 1260 01:09:00,899 --> 01:09:02,000 AUDIENCE: You're going to find it somewhere? 1261 01:09:02,000 --> 01:09:03,840 NICHOLAS DIBELLA: You're going to find it somewhere. 1262 01:09:03,840 --> 01:09:04,349 Yeah. 1263 01:09:04,349 --> 01:09:06,140 But you probably won't be able to get to it 1264 01:09:06,140 --> 01:09:06,830 because it's so far away. 1265 01:09:06,830 --> 01:09:07,120 AUDIENCE: Yeah. 1266 01:09:07,120 --> 01:09:08,729 NICHOLAS DIBELLA: And because it's moving away from a-- 1267 01:09:08,729 --> 01:09:09,380 AUDIENCE: A different bubble or something. 1268 01:09:09,380 --> 01:09:11,420 NICHOLAS DIBELLA: Yeah, a different bubble. 1269 01:09:11,420 --> 01:09:12,609 You probably won't be able to reach these. 1270 01:09:12,609 --> 01:09:13,984 AUDIENCE: Kind of like, existence 1271 01:09:13,984 --> 01:09:16,000 is a iffy sort of a thing. 1272 01:09:16,000 --> 01:09:18,290 NICHOLAS DIBELLA: Existence is an iffy sort of thing. 1273 01:09:18,290 --> 01:09:23,979 This is the most controversial level of multiverse. 1274 01:09:23,979 --> 01:09:26,479 The first one that I described you seemed crazy at the time, 1275 01:09:26,479 --> 01:09:27,770 but now we have different laws of physics, 1276 01:09:27,770 --> 01:09:28,603 different constants. 1277 01:09:28,603 --> 01:09:32,689 And level I we're at home with. 1278 01:09:32,689 --> 01:09:33,189 question? 1279 01:09:33,189 --> 01:09:34,890 AUDIENCE: In the multiverse, is it 1280 01:09:34,890 --> 01:09:39,140 possible for universes to overlap? 1281 01:09:39,140 --> 01:09:42,260 NICHOLAS DIBELLA: Is it possible for universes to overlap? 1282 01:09:42,260 --> 01:09:44,810 Well, it depends on how you define universe. 1283 01:09:44,810 --> 01:09:47,990 Is it possible-- are you asking for there 1284 01:09:47,990 --> 01:09:53,349 to be two different level IV universes overlapping? 1285 01:09:53,349 --> 01:09:54,890 Because if that's the case, then that 1286 01:09:54,890 --> 01:10:01,950 would require the overlapped universe to obey-- 1287 01:10:01,950 --> 01:10:04,760 well, it might require it to obey 1288 01:10:04,760 --> 01:10:07,010 two different mathematical laws at the same time, 1289 01:10:07,010 --> 01:10:09,590 or might just be an amalgam of the two. 1290 01:10:09,590 --> 01:10:10,003 AUDIENCE: Well if you have the two 1291 01:10:10,003 --> 01:10:12,002 different mathematical structures though, right, 1292 01:10:12,002 --> 01:10:13,880 sometimes mathematical structures do overlap. 1293 01:10:13,880 --> 01:10:14,838 NICHOLAS DIBELLA: Yeah. 1294 01:10:14,838 --> 01:10:17,510 Yeah-- so, it's-- yeah. 1295 01:10:17,510 --> 01:10:18,590 Yeah. 1296 01:10:18,590 --> 01:10:19,354 Question? 1297 01:10:19,354 --> 01:10:24,695 AUDIENCE: Two things-- one, how fast or far 1298 01:10:24,695 --> 01:10:26,339 can a modern rocket go? 1299 01:10:26,339 --> 01:10:27,630 NICHOLAS DIBELLA: How far what? 1300 01:10:27,630 --> 01:10:30,485 AUDIENCE: How far or fast can a modern rocket go? 1301 01:10:30,485 --> 01:10:32,110 NICHOLAS DIBELLA: I don't know how fast 1302 01:10:32,110 --> 01:10:33,214 a modern rocket can go. 1303 01:10:33,214 --> 01:10:34,380 AUDIENCE: How far can it go? 1304 01:10:34,380 --> 01:10:34,750 NICHOLAS DIBELLA: How far can it-- 1305 01:10:34,750 --> 01:10:37,380 well, if you just leave it, if you wait long enough, 1306 01:10:37,380 --> 01:10:38,610 it'll go arbitrarily far-- 1307 01:10:38,610 --> 01:10:39,497 a rocket. 1308 01:10:39,497 --> 01:10:41,485 AUDIENCE: OK. 1309 01:10:41,485 --> 01:10:43,970 AUDIENCE: It can go fast enough to cause the Doppler-- 1310 01:10:43,970 --> 01:10:45,791 I know that things [INAUDIBLE] that had 1311 01:10:45,791 --> 01:10:47,460 the Doppler effect [INAUDIBLE]. 1312 01:10:47,460 --> 01:10:48,835 NICHOLAS DIBELLA: OK, yeah, yeah. 1313 01:10:48,835 --> 01:10:50,046 Yeah. 1314 01:10:50,046 --> 01:10:51,420 Yeah, rockets can go pretty fast. 1315 01:10:51,420 --> 01:10:53,080 Let me get back to multiverses though. 1316 01:10:53,080 --> 01:10:55,230 OK. 1317 01:10:55,230 --> 01:10:57,904 What time is it? 1318 01:10:57,904 --> 01:10:59,332 AUDIENCE: 2:52 1319 01:10:59,332 --> 01:11:00,290 NICHOLAS DIBELLA: 2:52. 1320 01:11:00,290 --> 01:11:00,540 OK. 1321 01:11:00,540 --> 01:11:01,690 I have few more minutes. 1322 01:11:01,690 --> 01:11:03,870 OK. 1323 01:11:03,870 --> 01:11:08,220 So in this level IV multi-verse you 1324 01:11:08,220 --> 01:11:11,010 have different laws of physics. 1325 01:11:11,010 --> 01:11:12,660 Our universe has one specific type 1326 01:11:12,660 --> 01:11:15,600 that what we hope to one day find. 1327 01:11:15,600 --> 01:11:18,510 We might not find it, but we hope we will find it. 1328 01:11:18,510 --> 01:11:21,450 If we do find it, then that would be evidence 1329 01:11:21,450 --> 01:11:24,635 for this level IV multiverse. 1330 01:11:24,635 --> 01:11:26,364 It would be evidence for the assumption 1331 01:11:26,364 --> 01:11:27,780 that the universe is mathematical. 1332 01:11:27,780 --> 01:11:30,540 It we have evidence because we then 1333 01:11:30,540 --> 01:11:34,680 would have a mathematical structure for our universe. 1334 01:11:34,680 --> 01:11:39,780 And if other universes exist with different laws of physics, 1335 01:11:39,780 --> 01:11:42,360 then than that would be a rather nice explanation 1336 01:11:42,360 --> 01:11:46,440 for why these laws not others. 1337 01:11:46,440 --> 01:11:49,920 Well, it's because, every conceivable set 1338 01:11:49,920 --> 01:11:52,560 of physical laws would exist. 1339 01:11:52,560 --> 01:11:55,080 Every conceivable set of laws would exist would, 1340 01:11:55,080 --> 01:11:58,760 and there's some probability distribution 1341 01:11:58,760 --> 01:12:03,780 that your universe is equipped with these laws and not 1342 01:12:03,780 --> 01:12:05,054 some others. 1343 01:12:05,054 --> 01:12:06,720 AUDIENCE: How would you ever determine-- 1344 01:12:06,720 --> 01:12:07,730 NICHOLAS DIBELLA: How did you ever determine 1345 01:12:07,730 --> 01:12:08,720 those probabilities? 1346 01:12:08,720 --> 01:12:09,599 I don't know. 1347 01:12:09,599 --> 01:12:10,140 I don't know. 1348 01:12:10,140 --> 01:12:12,630 And I don't know how you would actually 1349 01:12:12,630 --> 01:12:18,630 figure out if the level IV multiverse theories are true. 1350 01:12:18,630 --> 01:12:24,750 Other than that they answered some questions rather nicely, 1351 01:12:24,750 --> 01:12:26,330 like why these laws and not others. 1352 01:12:26,330 --> 01:12:29,240 It might actually be completely impossible the answer. 1353 01:12:29,240 --> 01:12:33,160 It might be forever in the realm of metaphysics. 1354 01:12:33,160 --> 01:12:37,050 But it provides a nice sense of closure 1355 01:12:37,050 --> 01:12:40,320 to all of these universes. 1356 01:12:40,320 --> 01:12:43,410 We can explain everything so nicely 1357 01:12:43,410 --> 01:12:45,330 with the aid of parallel universes. 1358 01:12:45,330 --> 01:12:52,170 Things it seems the things that seem once to be so mysterious, 1359 01:12:52,170 --> 01:12:54,640 now they just fall under the rug or something. 1360 01:12:54,640 --> 01:12:55,365 Is that a-- 1361 01:12:55,365 --> 01:12:56,344 I don't know. 1362 01:12:56,344 --> 01:12:57,510 Am I using the phrase right? 1363 01:12:57,510 --> 01:12:58,301 Fall under the rug? 1364 01:12:58,301 --> 01:12:59,380 Swept under the rug? 1365 01:12:59,380 --> 01:13:01,267 Is swept under the rug a bad phrase? 1366 01:13:01,267 --> 01:13:02,100 Like, a mean phrase? 1367 01:13:02,100 --> 01:13:02,599 OK. 1368 01:13:02,599 --> 01:13:04,540 Well, these questions are answered so nicely 1369 01:13:04,540 --> 01:13:07,020 is what I'm trying to say with parallel universes. 1370 01:13:07,020 --> 01:13:11,060 And they seem crazy, and they are crazy. 1371 01:13:11,060 --> 01:13:14,410 But there is lots of evidence for them, 1372 01:13:14,410 --> 01:13:19,440 particularly the lower level type multiverses. 1373 01:13:19,440 --> 01:13:21,270 And so, we should take them seriously. 1374 01:13:21,270 --> 01:13:23,085 And even if you don't take them seriously, 1375 01:13:23,085 --> 01:13:24,990 you should at least appreciate them, 1376 01:13:24,990 --> 01:13:27,920 that they give us nice things to talk about, 1377 01:13:27,920 --> 01:13:29,790 nice things to think about. 1378 01:13:29,790 --> 01:13:33,160 And they give us lots of more opportunities to make jokes. 1379 01:13:33,160 --> 01:13:34,680 That's always one of my favorites. 1380 01:13:34,680 --> 01:13:36,210 AUDIENCE: Do you have any joke? 1381 01:13:36,210 --> 01:13:37,793 NICHOLAS DIBELLA: Do I have any jokes? 1382 01:13:37,793 --> 01:13:38,970 Well, sure. 1383 01:13:38,970 --> 01:13:41,900 I have lots of jokes. 1384 01:13:41,900 --> 01:13:45,060 I had one joke-- 1385 01:13:45,060 --> 01:13:47,280 I was planning on saying, one other type 1386 01:13:47,280 --> 01:13:51,670 of conceivable universe that's level IV. 1387 01:13:51,670 --> 01:13:55,050 In our universe, gravitation is not 1388 01:13:55,050 --> 01:13:57,380 responsible for people falling in love, 1389 01:13:57,380 --> 01:13:59,910 but it's conceivable that in another universe gravitation 1390 01:13:59,910 --> 01:14:01,960 is responsible for people falling in love. 1391 01:14:01,960 --> 01:14:03,810 [LAUGHTER]