1 00:00:00,000 --> 00:00:02,890 MAN: 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 00:00:21,245 --> 00:00:22,120 NICHOLAS DIBELLA: Hi. 9 00:00:22,120 --> 00:00:25,570 Welcome to Excitatory Topics in Physics. 10 00:00:25,570 --> 00:00:27,650 Has everybody gotten a syllabus? 11 00:00:27,650 --> 00:00:28,660 OK, good. 12 00:00:28,660 --> 00:00:30,118 Well, if you haven't gotten it, you 13 00:00:30,118 --> 00:00:31,910 can just get it after class. 14 00:00:31,910 --> 00:00:36,730 My name is Nick DiBella, and I'm a junior physics major here 15 00:00:36,730 --> 00:00:37,270 at MIT. 16 00:00:39,910 --> 00:00:42,860 I'm from Florida, originally from New York. 17 00:00:42,860 --> 00:00:45,700 And I love physics and pizza. 18 00:00:45,700 --> 00:00:49,340 And I think that's all you need to know. 19 00:00:49,340 --> 00:00:53,230 Well, so excitatory topics in physics, 20 00:00:53,230 --> 00:00:55,170 what is this class all about? 21 00:00:55,170 --> 00:00:56,370 What does excitatory mean? 22 00:00:56,370 --> 00:00:58,850 Have any of you even heard that word? 23 00:00:58,850 --> 00:01:02,035 Well, excitatory basically means exciting. 24 00:01:02,035 --> 00:01:03,910 I could have easily called the class Exciting 25 00:01:03,910 --> 00:01:07,380 Topics in Physics, but I think the word "excitatory" 26 00:01:07,380 --> 00:01:09,800 adds a little bit of mystique to the class 27 00:01:09,800 --> 00:01:12,816 and makes it a little more interesting, maybe. 28 00:01:12,816 --> 00:01:15,880 Well, anyway, you could look up the word "excitatory." 29 00:01:15,880 --> 00:01:18,610 It's actually a real word. 30 00:01:18,610 --> 00:01:24,700 So the goal of this class is basically to entertain you all. 31 00:01:24,700 --> 00:01:27,760 And I hope to do that by telling you 32 00:01:27,760 --> 00:01:30,160 about some of the most interesting things 33 00:01:30,160 --> 00:01:33,770 that I've ever heard in physics-- 34 00:01:33,770 --> 00:01:36,220 well, the most interesting things that I've heard 35 00:01:36,220 --> 00:01:37,137 that I can understand. 36 00:01:37,137 --> 00:01:39,053 Because I've heard a lot of interesting things 37 00:01:39,053 --> 00:01:41,710 that I don't understand, and I can't tell you about that. 38 00:01:41,710 --> 00:01:43,461 I'm sorry. 39 00:01:43,461 --> 00:01:43,960 Hi. 40 00:01:48,430 --> 00:01:52,170 As you've probably read from the course description, 41 00:01:52,170 --> 00:01:56,050 I plan for this class to be as conceptual as possible. 42 00:01:56,050 --> 00:01:59,650 And the main reason I've decided to do that 43 00:01:59,650 --> 00:02:03,580 is because it's really the ideas of physics that are exciting. 44 00:02:06,430 --> 00:02:12,950 Of course, to really write down the laws-- 45 00:02:12,950 --> 00:02:14,042 AUDIENCE: [LAUGHTER] 46 00:02:14,042 --> 00:02:14,600 NICHOLAS DIBELLA: Don't worry. 47 00:02:14,600 --> 00:02:15,100 That's OK. 48 00:02:19,022 --> 00:02:23,680 To really describe the universe, you have to use math. 49 00:02:23,680 --> 00:02:26,120 There's simply no other way to do it. 50 00:02:26,120 --> 00:02:28,220 You can't be precise. 51 00:02:28,220 --> 00:02:33,080 You can't really describe the universe without using math. 52 00:02:33,080 --> 00:02:37,000 But equations themselves don't really mean anything. 53 00:02:37,000 --> 00:02:39,730 It's the interpretations that we give to the equations that 54 00:02:39,730 --> 00:02:40,630 are meaningful. 55 00:02:40,630 --> 00:02:43,136 And it's those interpretations that are interesting. 56 00:02:43,136 --> 00:02:44,760 And it's those interpretations that I'd 57 00:02:44,760 --> 00:02:48,610 like to talk to you all about for these coming weeks. 58 00:02:48,610 --> 00:02:52,330 Now, I will, a few times in the class, 59 00:02:52,330 --> 00:02:57,890 pull out an equation just to impress you all, but no-- 60 00:02:57,890 --> 00:03:00,100 pull out an equation that you can appreciate 61 00:03:00,100 --> 00:03:01,570 and that you can actually use. 62 00:03:01,570 --> 00:03:06,370 But they won't be complicated. 63 00:03:06,370 --> 00:03:08,530 Probably if you just know algebra, that's enough. 64 00:03:08,530 --> 00:03:09,946 But there will be a few times when 65 00:03:09,946 --> 00:03:13,240 you'll be able to enjoy these equations that I'll give you. 66 00:03:13,240 --> 00:03:16,090 You'll be able to enjoy them. 67 00:03:16,090 --> 00:03:18,460 Oh, another reason, by the way, that I 68 00:03:18,460 --> 00:03:21,190 wanted to make this a qualitative class was 69 00:03:21,190 --> 00:03:23,780 so that I can get as many of you here as possible. 70 00:03:23,780 --> 00:03:28,810 And, well, it's good to see so many of you here. 71 00:03:28,810 --> 00:03:30,910 Now, the way this class will work 72 00:03:30,910 --> 00:03:35,810 is we'll meet here Sundays at 1:30. 73 00:03:35,810 --> 00:03:38,050 Today's the only day we're meeting at 2:30. 74 00:03:38,050 --> 00:03:40,600 We'll meet here Sundays at 1:30 and go 75 00:03:40,600 --> 00:03:46,030 until-- well, 1:30 PM, 1:30 PM. 76 00:03:46,030 --> 00:03:49,570 And we'll go until 3:00 PM. 77 00:03:49,570 --> 00:03:53,890 Technically, we'll go from 1:35 PM to 2:55 PM 78 00:03:53,890 --> 00:03:57,160 because that's the way MIT always does these classes. 79 00:03:57,160 --> 00:04:00,190 And we'll meet in this room, as usual. 80 00:04:00,190 --> 00:04:05,350 And we'll have a short break about an hour 81 00:04:05,350 --> 00:04:09,670 into the class, maybe 4 and 1/2 minutes 82 00:04:09,670 --> 00:04:11,640 or 5 minutes, something like that. 83 00:04:11,640 --> 00:04:12,700 And then we'll continue. 84 00:04:12,700 --> 00:04:17,410 And then there's another class meeting right afterwards, 85 00:04:17,410 --> 00:04:20,209 so we have to get out of here when I finish. 86 00:04:20,209 --> 00:04:26,521 But if any of you guys have questions, 87 00:04:26,521 --> 00:04:28,270 I'm happy to talk to you guys after class. 88 00:04:28,270 --> 00:04:29,728 I mean, you could follow me around, 89 00:04:29,728 --> 00:04:33,030 or I could follow you around, or wherever. 90 00:04:33,030 --> 00:04:34,310 AUDIENCE: [LAUGHTER] 91 00:04:34,310 --> 00:04:35,530 NICHOLAS DIBELLA: No, not going to follow you around. 92 00:04:35,530 --> 00:04:35,980 Sorry. 93 00:04:35,980 --> 00:04:37,270 I'm not going to follow you around. 94 00:04:37,270 --> 00:04:38,311 You can follow me around. 95 00:04:38,311 --> 00:04:41,530 You can follow me around and just ask 96 00:04:41,530 --> 00:04:43,310 any questions you have. 97 00:04:43,310 --> 00:04:45,420 I'm very happy to talk to you all. 98 00:04:49,290 --> 00:04:53,300 As you can see in the syllabus-- 99 00:04:53,300 --> 00:04:56,590 so you can see in the syllabus my name is Nicholas DiBella. 100 00:04:56,590 --> 00:04:59,170 You can call me Nick. 101 00:04:59,170 --> 00:05:01,510 You can email me with the email address 102 00:05:01,510 --> 00:05:03,010 that's listed right there. 103 00:05:03,010 --> 00:05:06,670 And you can feel free to call me as well if you have questions. 104 00:05:06,670 --> 00:05:08,375 AUDIENCE: [LAUGHTER] 105 00:05:08,375 --> 00:05:09,250 NICHOLAS DIBELLA: No. 106 00:05:09,250 --> 00:05:10,840 I really-- oh, sure. 107 00:05:10,840 --> 00:05:13,690 Sure, yeah. 108 00:05:13,690 --> 00:05:15,137 The rest-- here. 109 00:05:15,137 --> 00:05:17,470 You can get it after class if you don't already have it. 110 00:05:17,470 --> 00:05:18,970 You can get the syllabus after class 111 00:05:18,970 --> 00:05:21,760 if you don't already have it. 112 00:05:21,760 --> 00:05:29,030 But I love questions, and I love listening to questions. 113 00:05:29,030 --> 00:05:32,670 And I hope you guys feel the same. 114 00:05:32,670 --> 00:05:35,590 Oh, by the way, during lecture, you 115 00:05:35,590 --> 00:05:39,670 can feel free to ask questions if I said something confusing 116 00:05:39,670 --> 00:05:42,910 or if I'm going too fast, or if I'm mumbling. 117 00:05:45,430 --> 00:05:47,530 But try not to ask a question once every minute. 118 00:05:47,530 --> 00:05:50,590 Because then, we kind of lose the flow to the class. 119 00:05:55,080 --> 00:05:57,460 Are there any questions about how the class will 120 00:05:57,460 --> 00:06:00,250 work or anything like that? 121 00:06:00,250 --> 00:06:02,240 OK, good. 122 00:06:02,240 --> 00:06:03,190 So now that we've-- 123 00:06:03,190 --> 00:06:03,730 OK, good. 124 00:06:03,730 --> 00:06:06,940 Now we can get to some physics. 125 00:06:06,940 --> 00:06:09,257 OK, so let's get to the physics now. 126 00:06:09,257 --> 00:06:10,840 Well, I just want to make sure there's 127 00:06:10,840 --> 00:06:13,780 no doubt in your mind as to what physics actually is. 128 00:06:13,780 --> 00:06:17,950 Physics is the branch of science that attempts 129 00:06:17,950 --> 00:06:20,900 to explain everything-- 130 00:06:20,900 --> 00:06:27,040 every atom, every molecule, every insect, every dog, 131 00:06:27,040 --> 00:06:30,430 every cat, every person, every planet, every galaxy, 132 00:06:30,430 --> 00:06:36,100 every bit of energy, everything that's 133 00:06:36,100 --> 00:06:39,670 out there, everything that's out there that's observable. 134 00:06:39,670 --> 00:06:44,540 Well, I say observable because things that aren't observable 135 00:06:44,540 --> 00:06:46,250 aren't part of science. 136 00:06:46,250 --> 00:06:49,870 And that's just by definition. 137 00:06:49,870 --> 00:06:58,450 Now, physics-- well, physics which is the body of laws that 138 00:06:58,450 --> 00:07:00,385 describe everything-- 139 00:07:03,540 --> 00:07:07,120 well, physics is this collection of laws 140 00:07:07,120 --> 00:07:12,040 that describes what can happen and what can't happen. 141 00:07:12,040 --> 00:07:14,650 Just to give you an idea of a law of physics, 142 00:07:14,650 --> 00:07:17,560 there's a law of physics that says you can't travel faster 143 00:07:17,560 --> 00:07:19,060 than light. 144 00:07:19,060 --> 00:07:20,950 Light's the ultimate speed limit. 145 00:07:20,950 --> 00:07:25,460 Now, I can think, in my mind, of theoretically traveling faster 146 00:07:25,460 --> 00:07:25,960 than light. 147 00:07:25,960 --> 00:07:28,800 I mean, going on my bicycle and just going-- 148 00:07:28,800 --> 00:07:29,950 well, maybe a rocket ship. 149 00:07:29,950 --> 00:07:32,489 Going on a rocket ship and going really, really fast, 150 00:07:32,489 --> 00:07:34,030 looking at the speedometer and seeing 151 00:07:34,030 --> 00:07:36,229 it pass the speed of light, I can imagine that. 152 00:07:36,229 --> 00:07:38,020 But just because I can imagine that doesn't 153 00:07:38,020 --> 00:07:42,040 mean it can happen because it violates a law of physics-- 154 00:07:42,040 --> 00:07:45,430 namely, the law that says you can't go faster than light. 155 00:07:45,430 --> 00:07:49,600 And so we're trapped in this world where some things can 156 00:07:49,600 --> 00:07:52,600 happen, and some things can't. 157 00:07:52,600 --> 00:07:56,800 Now, humans have been around for kind of a long time, 158 00:07:56,800 --> 00:08:00,910 but it wasn't really until the 1600s or so 159 00:08:00,910 --> 00:08:04,270 that we began to make much progress in uncovering 160 00:08:04,270 --> 00:08:06,620 these laws of physics. 161 00:08:06,620 --> 00:08:10,850 And there's a big name that you probably all know. 162 00:08:10,850 --> 00:08:16,390 And it was Sir Isaac Newton that really started this revolution 163 00:08:16,390 --> 00:08:18,430 in our understanding of the world. 164 00:08:18,430 --> 00:08:20,410 He proposed three laws, which you all 165 00:08:20,410 --> 00:08:22,300 know as the laws of motion. 166 00:08:22,300 --> 00:08:25,710 He discovered a law for gravity, and he 167 00:08:25,710 --> 00:08:27,430 did a lot of other things too. 168 00:08:27,430 --> 00:08:31,780 But he really set the foundation to what we now refer 169 00:08:31,780 --> 00:08:34,210 to as classical mechanics. 170 00:08:37,377 --> 00:08:39,460 Oh, you guys don't have to take notes, by the way. 171 00:08:39,460 --> 00:08:43,120 You could if you want, but you don't have to-- 172 00:08:43,120 --> 00:08:44,532 classical mechanics. 173 00:08:47,624 --> 00:08:49,290 And by the way, I wasn't encouraging you 174 00:08:49,290 --> 00:08:50,680 to take notes by that comment. 175 00:08:50,680 --> 00:08:56,050 I was just trying to clarify something. 176 00:08:56,050 --> 00:08:59,440 Classical mechanics, this was Newton. 177 00:09:02,362 --> 00:09:07,860 And that was in the 1600s or something like that. 178 00:09:07,860 --> 00:09:08,769 It doesn't matter. 179 00:09:08,769 --> 00:09:10,810 Well, it does matter that it was a long time ago, 180 00:09:10,810 --> 00:09:17,860 but it doesn't matter so much about the precise century, 181 00:09:17,860 --> 00:09:21,100 17th century. 182 00:09:21,100 --> 00:09:25,300 Now, we call it classical just because it 183 00:09:25,300 --> 00:09:26,310 was from way back when. 184 00:09:26,310 --> 00:09:31,210 Like, we refer to classical music, a long time ago. 185 00:09:31,210 --> 00:09:36,130 Classical mechanics describes how everyday things work. 186 00:09:40,376 --> 00:09:46,280 It describes how this table will move if I start to push it. 187 00:09:46,280 --> 00:09:49,300 It can describe how friction works, 188 00:09:49,300 --> 00:09:53,790 friction between the ground and the leg of the table. 189 00:09:53,790 --> 00:09:57,580 It describes how an object will move if I throw it. 190 00:09:57,580 --> 00:10:01,590 Like, if I threw my cell phone, you would know. 191 00:10:01,590 --> 00:10:05,380 Classical mechanics predicts, with its equations, 192 00:10:05,380 --> 00:10:08,470 which I won't talk about, but it predicts, with its equations, 193 00:10:08,470 --> 00:10:12,370 for, instance that the motion of my cell phone 194 00:10:12,370 --> 00:10:15,250 is something like this. 195 00:10:15,250 --> 00:10:22,020 If this is height, and this is time, 196 00:10:22,020 --> 00:10:23,720 it moves something like that. 197 00:10:23,720 --> 00:10:27,790 It makes this definite prediction about the trajectory 198 00:10:27,790 --> 00:10:29,170 of my cell phone. 199 00:10:29,170 --> 00:10:32,890 And it's exactly what you see. 200 00:10:32,890 --> 00:10:37,060 And classical mechanics is enormously 201 00:10:37,060 --> 00:10:41,022 successful in predicting the motion of everyday things. 202 00:10:41,022 --> 00:10:43,480 And not only does it predict the motion of everyday things, 203 00:10:43,480 --> 00:10:45,790 but it also predicts the motion of, I guess, 204 00:10:45,790 --> 00:10:51,520 every-night things, like the planets, the planets moving 205 00:10:51,520 --> 00:10:53,550 around the sun. 206 00:10:53,550 --> 00:10:56,410 It makes these very definite predictions, 207 00:10:56,410 --> 00:10:59,140 and they're confirmed extremely well 208 00:10:59,140 --> 00:11:03,430 by what we observe when we look through a telescope in the sky. 209 00:11:03,430 --> 00:11:05,710 Or if you look in the sky with your eyes-- 210 00:11:05,710 --> 00:11:08,237 no, I don't know how good our eyes are. 211 00:11:08,237 --> 00:11:10,070 But if you look at the sky with a telescope, 212 00:11:10,070 --> 00:11:13,600 then you can see the motions of planets and stars. 213 00:11:13,600 --> 00:11:17,530 And classical mechanics is amazing at predicting 214 00:11:17,530 --> 00:11:19,480 how these things move. 215 00:11:19,480 --> 00:11:23,860 Now, in the 1600s and 1700s, we had classical mechanics. 216 00:11:23,860 --> 00:11:26,710 And so we could describe everyday things. 217 00:11:26,710 --> 00:11:29,627 But, well, what about some kind of everyday things 218 00:11:29,627 --> 00:11:31,210 that aren't extremely everyday things, 219 00:11:31,210 --> 00:11:35,230 like electricity and magnetism, lightning, rainbows? 220 00:11:35,230 --> 00:11:38,950 Well, it was really the 1800s that we 221 00:11:38,950 --> 00:11:42,520 started to uncover these laws of electromagnetism, electricity 222 00:11:42,520 --> 00:11:43,990 and magnetism. 223 00:11:43,990 --> 00:11:49,660 And people did tons of experiments 224 00:11:49,660 --> 00:11:53,830 to figure out how these forces worked. 225 00:11:53,830 --> 00:11:55,360 Newton really figured out gravity. 226 00:11:55,360 --> 00:11:57,820 And electricity-- well, people in the 1800s 227 00:11:57,820 --> 00:11:59,800 were working on electromagnetism. 228 00:11:59,800 --> 00:12:01,270 And the main name that's associated 229 00:12:01,270 --> 00:12:08,990 with electromagnetism is James Clerk Maxwell, 230 00:12:08,990 --> 00:12:14,680 who figured out a puzzle in electromagnetism that 231 00:12:14,680 --> 00:12:17,400 was kind of troubling. 232 00:12:17,400 --> 00:12:20,260 He figured out this key component 233 00:12:20,260 --> 00:12:23,470 that you really needed to really understand electromagnetism. 234 00:12:23,470 --> 00:12:26,800 And then he really unified the whole theory. 235 00:12:26,800 --> 00:12:29,830 And so today, we refer to Maxwell's electromagnetism 236 00:12:29,830 --> 00:12:32,755 as classical electromagnetism. 237 00:12:44,140 --> 00:12:45,680 And that was Maxwell. 238 00:12:45,680 --> 00:12:50,730 This was the 1800s. 239 00:12:50,730 --> 00:12:52,026 Oh, whoops. 240 00:12:52,026 --> 00:12:54,750 I just realized I'm not supposed to write on this board. 241 00:12:54,750 --> 00:12:57,730 Oh, well. 242 00:12:57,730 --> 00:13:04,970 Now, electromagnetism as Maxwell formulated it, 243 00:13:04,970 --> 00:13:07,520 these people figured out, also works 244 00:13:07,520 --> 00:13:09,900 extremely, extremely well. 245 00:13:09,900 --> 00:13:14,300 I mean, it's just as good at describing the things 246 00:13:14,300 --> 00:13:16,430 that it describes as classical mechanics is 247 00:13:16,430 --> 00:13:19,651 at describing the things that it describes, 248 00:13:19,651 --> 00:13:20,870 if that makes any sense. 249 00:13:24,580 --> 00:13:28,730 And so by the end of the 1800s, people 250 00:13:28,730 --> 00:13:30,860 were so confident in these theories 251 00:13:30,860 --> 00:13:34,829 because they described pretty much everything 252 00:13:34,829 --> 00:13:35,870 that they could think of. 253 00:13:35,870 --> 00:13:37,661 And they were able to describe these things 254 00:13:37,661 --> 00:13:39,780 with such amazing accuracy. 255 00:13:39,780 --> 00:13:41,705 And there was a feeling, at the time, 256 00:13:41,705 --> 00:13:43,250 a growing feeling among physicists, 257 00:13:43,250 --> 00:13:45,890 that we had solved physics. 258 00:13:45,890 --> 00:13:49,620 We had figured it all out. 259 00:13:49,620 --> 00:13:52,260 But there were a couple problems. 260 00:13:52,260 --> 00:13:54,380 There were a couple of problems with the theories 261 00:13:54,380 --> 00:13:57,360 that had to be fixed. 262 00:13:57,360 --> 00:14:01,389 There were problems between conflicts of these two, 263 00:14:01,389 --> 00:14:02,680 mechanics and electromagnetism. 264 00:14:02,680 --> 00:14:06,640 There were conflicts in things they predicted. 265 00:14:06,640 --> 00:14:08,200 Well, one of these-- 266 00:14:08,200 --> 00:14:10,550 well, it wasn't a con-- well, I guess, 267 00:14:10,550 --> 00:14:15,134 one of these problems that had to be fixed was called-- 268 00:14:15,134 --> 00:14:16,800 well, one of these problems we now refer 269 00:14:16,800 --> 00:14:19,504 to as the ultraviolet catastrophe. 270 00:14:19,504 --> 00:14:20,670 I'll explain it in a second. 271 00:14:20,670 --> 00:14:22,316 Let me just write it down. 272 00:14:22,316 --> 00:14:26,600 The ultraviolet-- doesn't that sound really bad, 273 00:14:26,600 --> 00:14:28,254 ultraviolet catastrophe. 274 00:14:38,910 --> 00:14:43,000 We call it catastrophe because it was so bad. 275 00:14:43,000 --> 00:14:46,060 And ultraviolet because-- well, let 276 00:14:46,060 --> 00:14:48,820 me explain first before I tell you why ultraviolet. 277 00:14:48,820 --> 00:14:52,782 So according to the physics of the time, which was-- 278 00:14:52,782 --> 00:14:55,240 well, I don't think they called it classical mechanics back 279 00:14:55,240 --> 00:14:55,340 then. 280 00:14:55,340 --> 00:14:57,080 But according to Newtonian mechanics 281 00:14:57,080 --> 00:15:01,140 and according to Maxwellian electromagnetism, 282 00:15:01,140 --> 00:15:05,140 the theories predicted that if you turn on a microwave, 283 00:15:05,140 --> 00:15:06,252 then there would-- 284 00:15:06,252 --> 00:15:07,960 well, one of the things that they predict 285 00:15:07,960 --> 00:15:09,250 is that if you turn on a microwave, then 286 00:15:09,250 --> 00:15:11,000 there would suddenly be an infinite amount 287 00:15:11,000 --> 00:15:13,210 of energy inside of it. 288 00:15:13,210 --> 00:15:16,210 And that seems odd, right-- 289 00:15:16,210 --> 00:15:19,525 infinite amount of energy inside of a finite box? 290 00:15:22,310 --> 00:15:27,400 Well, this type of energy, which, when you calculate it, 291 00:15:27,400 --> 00:15:29,930 it ends up being infinite, is electromagnetic energy. 292 00:15:29,930 --> 00:15:36,850 And one of the more energetic forms of electromagnetic energy 293 00:15:36,850 --> 00:15:40,780 is ultraviolet energy, part of the ultraviolet part 294 00:15:40,780 --> 00:15:43,620 of the electromagnetic spectrum, if you've heard of that. 295 00:15:43,620 --> 00:15:45,699 And so we call it the ultraviolet catastrophe. 296 00:15:45,699 --> 00:15:47,240 I guess you could also call it, like, 297 00:15:47,240 --> 00:15:49,840 the X-ray catastrophe, or the gamma ray catastrophe, 298 00:15:49,840 --> 00:15:53,260 or anything else, because those are actually more 299 00:15:53,260 --> 00:15:54,385 energetic than ultraviolet. 300 00:15:54,385 --> 00:15:56,301 But if those words don't mean anything to you, 301 00:15:56,301 --> 00:15:58,040 then don't worry about that. 302 00:15:58,040 --> 00:16:03,970 So, well, I haven't really defined energy, have I? 303 00:16:03,970 --> 00:16:06,649 Now, I haven't define energy for you guys yet. 304 00:16:06,649 --> 00:16:08,440 Well, I'm not going to define it precisely, 305 00:16:08,440 --> 00:16:11,830 but you all have some kind of idea of what energy is. 306 00:16:11,830 --> 00:16:14,705 Probably the most familiar type is kinetic energy, 307 00:16:14,705 --> 00:16:16,440 the energy of motion. 308 00:16:16,440 --> 00:16:19,240 If I run fast, then I have a lot of kinetic energy. 309 00:16:21,980 --> 00:16:24,400 You've probably also heard of potential energy-- 310 00:16:24,400 --> 00:16:26,780 specifically, gravitational potential energy, 311 00:16:26,780 --> 00:16:29,110 which is the energy that you have 312 00:16:29,110 --> 00:16:31,330 when you're in a gravitational-- well, when you're 313 00:16:31,330 --> 00:16:32,950 around something that has mass. 314 00:16:32,950 --> 00:16:36,940 So the higher up you go, the more gravitational 315 00:16:36,940 --> 00:16:38,380 potential energy you have. 316 00:16:38,380 --> 00:16:40,870 The lower you are, the less gravitational potential 317 00:16:40,870 --> 00:16:42,460 you have. 318 00:16:42,460 --> 00:16:48,160 Now, one of the things that physics allows 319 00:16:48,160 --> 00:16:52,150 is energy conversions from one type to another. 320 00:16:58,270 --> 00:16:59,710 So for instance, potential energy 321 00:16:59,710 --> 00:17:02,050 can be converted to kinetic energy. 322 00:17:02,050 --> 00:17:07,200 And you all know how this happens. 323 00:17:07,200 --> 00:17:08,470 Say you go on a plane. 324 00:17:08,470 --> 00:17:09,845 It starts flying up, and then you 325 00:17:09,845 --> 00:17:11,859 jump out of the plane with a parachute, 326 00:17:11,859 --> 00:17:15,040 jump out of the plane with a parachute. 327 00:17:15,040 --> 00:17:16,810 At first, you're very high up, and you 328 00:17:16,810 --> 00:17:19,900 have a lot of gravitational potential energy. 329 00:17:19,900 --> 00:17:22,060 But you're not moving very fast. 330 00:17:22,060 --> 00:17:25,150 And when you jump out, you start moving faster. 331 00:17:25,150 --> 00:17:28,040 And then you eventually get faster and faster and faster. 332 00:17:28,040 --> 00:17:31,570 And so eventually, a lot of gravitational potential energy 333 00:17:31,570 --> 00:17:35,440 gets converted to kinetic energy. 334 00:17:35,440 --> 00:17:41,110 And so that's a very simple type of energy conversion. 335 00:17:41,110 --> 00:17:45,850 Now, I want to tell you why the ultraviolet catastrophe's bad. 336 00:17:45,850 --> 00:17:49,500 I mean, just because there's an infinite amount of something 337 00:17:49,500 --> 00:17:51,250 doesn't necessarily mean it's a bad thing. 338 00:17:54,151 --> 00:17:55,900 I'll just give you an example why infinite 339 00:17:55,900 --> 00:17:58,530 doesn't necessarily mean that. 340 00:17:58,530 --> 00:18:00,700 A priori, it doesn't necessarily mean bad. 341 00:18:00,700 --> 00:18:03,970 Because, I mean, this room is finite, right? 342 00:18:03,970 --> 00:18:05,722 There's a finite amount of space in here. 343 00:18:05,722 --> 00:18:08,830 But there's an infinite number of points in it, 344 00:18:08,830 --> 00:18:10,300 infinite number of points of space. 345 00:18:10,300 --> 00:18:12,550 So there's an infinite of something in a finite amount 346 00:18:12,550 --> 00:18:14,990 of something. 347 00:18:14,990 --> 00:18:17,660 So why's it bad with energy? 348 00:18:17,660 --> 00:18:19,521 Well, it's actually very-- 349 00:18:19,521 --> 00:18:21,770 well, there are a lot of obvious reasons why it's bad. 350 00:18:21,770 --> 00:18:24,940 But actually, I was thinking about why it's bad when 351 00:18:24,940 --> 00:18:26,350 I was preparing this lecture. 352 00:18:26,350 --> 00:18:29,410 And I realized that the books that talk about this 353 00:18:29,410 --> 00:18:31,270 don't really talk a whole lot about why 354 00:18:31,270 --> 00:18:32,900 infinite energy in a box is bad. 355 00:18:32,900 --> 00:18:35,450 They just say, well, of course it's-- 356 00:18:35,450 --> 00:18:39,195 so when you calculate this, when you perform this integral, 357 00:18:39,195 --> 00:18:41,320 when you calculate this, you get an infinite amount 358 00:18:41,320 --> 00:18:45,460 of energy inside the container. 359 00:18:45,460 --> 00:18:46,460 And that's all they say. 360 00:18:46,460 --> 00:18:49,960 Or they'll say, which clearly violates 361 00:18:49,960 --> 00:18:52,390 experimental observation. 362 00:18:52,390 --> 00:18:53,510 OK, well, yeah, sure. 363 00:18:53,510 --> 00:18:59,740 But what about those of us for whom it's not completely clear? 364 00:18:59,740 --> 00:19:04,330 I mean, we should be able to understand this very simply. 365 00:19:07,240 --> 00:19:10,310 I mean, OK, you could understand it by saying, 366 00:19:10,310 --> 00:19:13,709 OK, well, it's clearly contrary to what we see. 367 00:19:13,709 --> 00:19:14,500 You could say that. 368 00:19:14,500 --> 00:19:17,170 But from a physics point of view, why is it bad? 369 00:19:17,170 --> 00:19:20,930 Well, one reason why it's bad is that, well, 370 00:19:20,930 --> 00:19:22,930 all those electromagnetic energy stored 371 00:19:22,930 --> 00:19:25,780 inside of the microwave or inside of this box 372 00:19:25,780 --> 00:19:30,820 gets converted into kinetic energy of the atoms moving 373 00:19:30,820 --> 00:19:31,990 around. 374 00:19:31,990 --> 00:19:34,420 And so these atoms pick up this energy, 375 00:19:34,420 --> 00:19:36,790 and start moving faster and faster and faster. 376 00:19:36,790 --> 00:19:40,240 And eventually, they have infinite kinetic energy, 377 00:19:40,240 --> 00:19:42,460 and they'll break the container. 378 00:19:42,460 --> 00:19:45,590 And you don't-- 379 00:19:45,590 --> 00:19:47,890 I mean, you see microwaves. 380 00:19:47,890 --> 00:19:48,610 Microwaves exist. 381 00:19:53,650 --> 00:19:58,510 Microwaves exist, although I guess I am kind of using now, 382 00:19:58,510 --> 00:20:05,350 well, you don't see the fact that microwaves explode. 383 00:20:05,350 --> 00:20:07,870 So I am kind of using the "it goes 384 00:20:07,870 --> 00:20:09,820 against observation" argument. 385 00:20:09,820 --> 00:20:15,750 But, well, you could interpret what I just said as a proof 386 00:20:15,750 --> 00:20:19,270 that physics, at the time, proves that microwaves today 387 00:20:19,270 --> 00:20:23,200 cannot exist, OK? 388 00:20:23,200 --> 00:20:26,920 Now, this was a problem. 389 00:20:26,920 --> 00:20:30,490 This ultraviolet catastrophe was a problem for a while 390 00:20:30,490 --> 00:20:32,910 before someone figured out how to fix it. 391 00:20:32,910 --> 00:20:35,685 And it was Max Plank that figured it out. 392 00:20:35,685 --> 00:20:37,300 Or you might have heard of Max Planck. 393 00:20:37,300 --> 00:20:37,841 I don't know. 394 00:20:37,841 --> 00:20:39,320 I hear both pronunciations. 395 00:20:39,320 --> 00:20:40,850 It's just a name. 396 00:20:40,850 --> 00:20:47,910 Max Planck is the one that fixed it. 397 00:20:47,910 --> 00:20:52,840 He came up with this awesome idea of how to fix it, 398 00:20:52,840 --> 00:20:54,100 and it worked. 399 00:20:54,100 --> 00:21:01,937 And it actually-- it's a K. Max Planck fixed it. 400 00:21:05,760 --> 00:21:06,660 Sorry. 401 00:21:06,660 --> 00:21:09,210 But that's-- I was trying to draw a smiley. 402 00:21:09,210 --> 00:21:12,672 Remind me never to draw a smiley ever again on a chalkboard. 403 00:21:16,050 --> 00:21:19,410 OK, how's that-- fixed it. 404 00:21:19,410 --> 00:21:25,380 And this eventually led to the development of what's 405 00:21:25,380 --> 00:21:28,362 called quantum mechanics. 406 00:21:31,810 --> 00:21:33,490 It's just one of the steps leading up 407 00:21:33,490 --> 00:21:37,800 to the development of quantum mechanics. 408 00:21:37,800 --> 00:21:42,270 Now, I don't plan to talk more about quantum mechanics 409 00:21:42,270 --> 00:21:43,620 until a couple of lectures. 410 00:21:43,620 --> 00:21:46,161 Well, I don't plan to talk more today about quantum mechanics 411 00:21:46,161 --> 00:21:48,810 because I'm going to do a whole lecture on it in a few weeks, 412 00:21:48,810 --> 00:21:50,140 or something like that. 413 00:21:50,140 --> 00:21:56,230 So I'll just let you ponder what it could possibly be. 414 00:21:56,230 --> 00:21:59,750 Quantum mechanics, what could it be? 415 00:21:59,750 --> 00:22:02,500 I'll let you think about that. 416 00:22:02,500 --> 00:22:07,120 Anyway, quantum mechanics, the development 417 00:22:07,120 --> 00:22:11,570 of quantum mechanics was one of the two major developments-- 418 00:22:11,570 --> 00:22:12,806 yes? 419 00:22:12,806 --> 00:22:16,980 AUDIENCE: So what did Max Planck fix again? 420 00:22:16,980 --> 00:22:19,360 NICHOLAS DIBELLA: He fixed-- 421 00:22:19,360 --> 00:22:22,660 he inserted something into the theory 422 00:22:22,660 --> 00:22:25,630 that made it so that the ultraviolet catastrophe 423 00:22:25,630 --> 00:22:26,920 wouldn't happen. 424 00:22:26,920 --> 00:22:30,336 He allowed microwaves to exist. 425 00:22:30,336 --> 00:22:31,440 Well, what he actually-- 426 00:22:31,440 --> 00:22:33,810 AUDIENCE: [LAUGHTER] 427 00:22:34,702 --> 00:22:36,160 NICHOLAS DIBELLA: Well, what he did 428 00:22:36,160 --> 00:22:40,840 was he introduced the idea of the quantum, 429 00:22:40,840 --> 00:22:46,960 that light comes in individual chunks called quanta. 430 00:22:46,960 --> 00:22:50,980 And when you add this into the theory, 431 00:22:50,980 --> 00:22:54,100 it turns out that everything is fixed. 432 00:22:54,100 --> 00:22:57,250 And Planck actually didn't really 433 00:22:57,250 --> 00:22:58,780 have a physical motivation for that. 434 00:22:58,780 --> 00:23:01,330 It was more of a mathematical trick 435 00:23:01,330 --> 00:23:03,220 that he was able to fix the theory. 436 00:23:03,220 --> 00:23:06,360 But it was Einstein afterwards that 437 00:23:06,360 --> 00:23:09,160 recognized there was something deeply physical about this 438 00:23:09,160 --> 00:23:10,330 and deeply important. 439 00:23:10,330 --> 00:23:12,700 But I'll get to that in a few weeks. 440 00:23:12,700 --> 00:23:15,226 So I'll let you guys have dreams or nightmares 441 00:23:15,226 --> 00:23:16,600 about the ultraviolet catastrophe 442 00:23:16,600 --> 00:23:18,350 and how it's fixed for the next few weeks. 443 00:23:20,840 --> 00:23:26,500 Now, quantum mechanics, as I said, 444 00:23:26,500 --> 00:23:28,710 was one of the two major developments 445 00:23:28,710 --> 00:23:43,610 in modern physics, which is the physics of the 20th century, 446 00:23:43,610 --> 00:23:44,270 20th century. 447 00:23:48,850 --> 00:23:51,877 Now, I realize we're in the 21st century. 448 00:23:51,877 --> 00:23:54,210 So maybe what we do today is also called modern physics. 449 00:23:54,210 --> 00:23:56,260 Maybe postmodern would be a better word. 450 00:23:56,260 --> 00:24:00,800 Like, you have postmodern arts, postmodern architecture, 451 00:24:00,800 --> 00:24:03,070 and things like that. 452 00:24:03,070 --> 00:24:05,253 I don't know-- modern physics. 453 00:24:05,253 --> 00:24:08,880 Now, quantum mechanics was one of the two, one of the two 454 00:24:08,880 --> 00:24:10,380 major developments in modern-- 455 00:24:10,380 --> 00:24:12,210 quantum mechanics was one part. 456 00:24:21,720 --> 00:24:25,555 The other part was relativity. 457 00:24:34,770 --> 00:24:37,340 And relativity was Einstein's baby. 458 00:24:44,950 --> 00:24:50,860 Einstein was pretty much the one who figured it all out. 459 00:24:50,860 --> 00:24:52,860 Now, how much time do we have? 460 00:24:52,860 --> 00:24:53,440 OK. 461 00:24:53,440 --> 00:25:03,520 Now, relativity was necessary because these two theories, 462 00:25:03,520 --> 00:25:05,800 Newtonian mechanics and Maxwell's theory 463 00:25:05,800 --> 00:25:09,260 of electromagnetism, these two theories 464 00:25:09,260 --> 00:25:11,860 were ultimately at odds with each other. 465 00:25:11,860 --> 00:25:13,870 They couldn't both be true. 466 00:25:13,870 --> 00:25:17,530 Only one of them could be true, or none of them. 467 00:25:17,530 --> 00:25:20,260 But at most, one of them could be true. 468 00:25:20,260 --> 00:25:23,740 And how many do you think Newton was right? 469 00:25:26,534 --> 00:25:28,200 How many of you think Maxwell was right? 470 00:25:31,470 --> 00:25:32,120 You can vote. 471 00:25:32,120 --> 00:25:32,620 I mean-- 472 00:25:32,620 --> 00:25:34,580 AUDIENCE: [LAUGHTER] 473 00:25:34,580 --> 00:25:38,250 NICHOLAS DIBELLA: Well, it turned out that it was actually 474 00:25:38,250 --> 00:25:40,080 Maxwell that was right. 475 00:25:40,080 --> 00:25:46,320 And so Newton's mechanics had to be fixed. 476 00:25:46,320 --> 00:25:50,340 And how to fix it? 477 00:25:50,340 --> 00:25:51,910 How do you fix it? 478 00:25:51,910 --> 00:25:55,530 Well, this is what Einstein figured out in the early 1900s. 479 00:25:55,530 --> 00:26:00,360 And he wrote a paper in 1905 called 480 00:26:00,360 --> 00:26:03,510 "On the Electrodynamics of Moving Bodies," 481 00:26:03,510 --> 00:26:07,800 where he, in one paper, completely set the groundwork 482 00:26:07,800 --> 00:26:10,340 for special relativity. 483 00:26:10,340 --> 00:26:15,390 Now, special relativity is one of the two main parts 484 00:26:15,390 --> 00:26:23,530 of relativity theory, which I'll write over here. 485 00:26:35,060 --> 00:26:38,850 Relativity has a special part. 486 00:26:38,850 --> 00:26:42,570 Well, I guess that "part" might not be the best word to use, 487 00:26:42,570 --> 00:26:45,550 but I'll just say part. 488 00:26:45,550 --> 00:26:47,590 There's special theory of relativity. 489 00:26:47,590 --> 00:26:51,550 It's one branch of the whole theory, special relativity. 490 00:26:51,550 --> 00:26:55,175 And there's the general theory. 491 00:26:55,175 --> 00:26:58,740 Now, special relativity Einstein figured out in 1905, 492 00:26:58,740 --> 00:27:03,710 but it took him another 10 years or so 493 00:27:03,710 --> 00:27:05,350 to figure out general relativity. 494 00:27:05,350 --> 00:27:10,070 Now, general relativity is a theory 495 00:27:10,070 --> 00:27:14,860 that modifies Newtonian mechanics by-- 496 00:27:14,860 --> 00:27:19,400 well, both of these theories fixed Newtonian mechanics. 497 00:27:19,400 --> 00:27:20,960 They both fix it in a certain way. 498 00:27:24,050 --> 00:27:27,464 Special relativity fixes it if you're living in a universe 499 00:27:27,464 --> 00:27:28,505 where there's no gravity. 500 00:27:35,130 --> 00:27:37,890 General relativity fixes it if you're living in a universe 501 00:27:37,890 --> 00:27:39,039 where there is gravity. 502 00:27:39,039 --> 00:27:41,580 And so clearly, we need general relativity in the real world. 503 00:27:47,630 --> 00:27:52,880 Now, it turns out that general relativity 504 00:27:52,880 --> 00:27:55,865 is a lot harder to understand than special relativity. 505 00:27:55,865 --> 00:28:00,200 At least mathematically, it's a lot harder to understand, 506 00:28:00,200 --> 00:28:03,740 which was one of the reasons it took Einstein a long time 507 00:28:03,740 --> 00:28:05,820 to figure out how to do it. 508 00:28:05,820 --> 00:28:09,830 So I'm going to start with special relativity today. 509 00:28:09,830 --> 00:28:21,290 And special relativity really has only two laws to it, 510 00:28:21,290 --> 00:28:22,490 two postulates. 511 00:28:22,490 --> 00:28:26,180 There are two postulates of special relativity, which, 512 00:28:26,180 --> 00:28:29,290 when you use them, you get everything. 513 00:28:29,290 --> 00:28:31,040 And there are a lot of things that you can 514 00:28:31,040 --> 00:28:34,280 get from special relativity. 515 00:28:34,280 --> 00:28:38,235 Let me just write down these two postulates. 516 00:28:38,235 --> 00:28:39,610 Well, actually, there's something 517 00:28:39,610 --> 00:28:41,650 I wanted to say before. 518 00:28:41,650 --> 00:28:47,240 Going back to modern physics, just going back 519 00:28:47,240 --> 00:28:50,090 to the whole thing that was figured out 520 00:28:50,090 --> 00:28:54,500 in the 20th century, it's important to understand 521 00:28:54,500 --> 00:28:59,510 that these new laws of physics, these new theories, 522 00:28:59,510 --> 00:29:03,470 were completely crazy, really. 523 00:29:03,470 --> 00:29:04,250 They're crazy. 524 00:29:04,250 --> 00:29:09,320 And there's a notion among the unscientific people 525 00:29:09,320 --> 00:29:12,320 of the world that relativity and quantum 526 00:29:12,320 --> 00:29:16,770 mechanics are confusing and crazy and mysterious. 527 00:29:16,770 --> 00:29:20,820 And that's because they are. 528 00:29:20,820 --> 00:29:23,060 But it is possible to understand them. 529 00:29:23,060 --> 00:29:25,480 There's a notion that's impossible to understand, 530 00:29:25,480 --> 00:29:27,879 but you can understand them. 531 00:29:27,879 --> 00:29:28,670 But they are crazy. 532 00:29:31,220 --> 00:29:36,011 And just to give you an idea of how crazy they are, 533 00:29:36,011 --> 00:29:37,385 I want to show you an experiment. 534 00:29:37,385 --> 00:29:39,960 I want to do a little experiment for you. 535 00:29:39,960 --> 00:29:42,950 One of the things that quantum mechanics predicts 536 00:29:42,950 --> 00:29:49,950 is that there is a chance that I can walk through the wall. 537 00:29:49,950 --> 00:29:53,530 So if I walk through the wall right now, 538 00:29:53,530 --> 00:29:58,460 then that would be evidence that quantum mechanics is right. 539 00:29:58,460 --> 00:30:01,240 So let me try to walk through the wall, OK? 540 00:30:01,240 --> 00:30:02,179 AUDIENCE: [LAUGHTER] 541 00:30:02,179 --> 00:30:03,470 NICHOLAS DIBELLA: So let's see. 542 00:30:08,939 --> 00:30:10,730 Shall I try this one, or shall I try green? 543 00:30:10,730 --> 00:30:11,100 I'll try-- 544 00:30:11,100 --> 00:30:11,840 AUDIENCE: Green. 545 00:30:11,840 --> 00:30:12,170 NICHOLAS DIBELLA: Green? 546 00:30:12,170 --> 00:30:13,850 AUDIENCE: Green is more [INAUDIBLE].. 547 00:30:13,850 --> 00:30:15,183 NICHOLAS DIBELLA: Green is what? 548 00:30:15,183 --> 00:30:16,450 AUDIENCE: More [INAUDIBLE]. 549 00:30:16,450 --> 00:30:16,810 NICHOLAS DIBELLA: What's that? 550 00:30:16,810 --> 00:30:17,218 AUDIENCE: [INAUDIBLE] 551 00:30:17,218 --> 00:30:17,859 AUDIENCE: Run. 552 00:30:17,859 --> 00:30:18,442 AUDIENCE: Run. 553 00:30:18,442 --> 00:30:20,692 NICHOLAS DIBELLA: Oh, I'm going to walk into the wall. 554 00:30:24,194 --> 00:30:25,146 AUDIENCE: Try again. 555 00:30:27,536 --> 00:30:29,660 NICHOLAS DIBELLA: Well, OK, I'll try one more time. 556 00:30:32,450 --> 00:30:34,260 OK, well, I just tried it three times. 557 00:30:34,260 --> 00:30:35,990 I just tried it three times, and I didn't walk through it. 558 00:30:35,990 --> 00:30:37,580 Oh, did anybody see me walk through it, though? 559 00:30:37,580 --> 00:30:39,080 I'm not sure if I walked through it. 560 00:30:39,080 --> 00:30:40,298 Did I walk through it? 561 00:30:40,298 --> 00:30:40,798 No? 562 00:30:40,798 --> 00:30:41,339 AUDIENCE: No. 563 00:30:41,339 --> 00:30:42,429 NICHOLAS DIBELLA: OK, OK. 564 00:30:42,429 --> 00:30:44,720 Well, according to quantum mechanics, there's a chance. 565 00:30:44,720 --> 00:30:47,360 There's a non-zero probability that I can walk through it. 566 00:30:47,360 --> 00:30:49,940 But it turns out that that probability's actually 567 00:30:49,940 --> 00:30:58,560 something like 10 to the minus 50th. 568 00:30:58,560 --> 00:31:06,360 So that's a 0.0, 49 zeros, and then a 1. 569 00:31:06,360 --> 00:31:12,110 So I have to attempt to walk through the wall a billion 570 00:31:12,110 --> 00:31:18,896 billion billion billion billion million times before I finally 571 00:31:18,896 --> 00:31:20,020 will probably walk through. 572 00:31:20,020 --> 00:31:22,004 AUDIENCE: [LAUGHTER] 573 00:31:24,779 --> 00:31:26,820 NICHOLAS DIBELLA: And so that's actually evidence 574 00:31:26,820 --> 00:31:28,774 that quantum mechanics is right, because I 575 00:31:28,774 --> 00:31:30,690 didn't walk through the wall with three tries. 576 00:31:30,690 --> 00:31:31,980 You wouldn't expect it to take three tries. 577 00:31:31,980 --> 00:31:34,740 You would expect it to take a billion billion billion billion 578 00:31:34,740 --> 00:31:36,630 million times, or something like that. 579 00:31:39,360 --> 00:31:42,540 Oh, by the way, that probability is actually 580 00:31:42,540 --> 00:31:47,830 increased when I increase my kinetic energy. 581 00:31:47,830 --> 00:31:50,084 So if I try running into the wall, 582 00:31:50,084 --> 00:31:52,590 then there's a greater chance that I'll actually 583 00:31:52,590 --> 00:31:54,700 go through it. 584 00:31:54,700 --> 00:31:56,306 Should I try that? 585 00:31:56,306 --> 00:31:57,152 AUDIENCE: Yeah. 586 00:31:57,152 --> 00:31:59,360 NICHOLAS DIBELLA: OK, OK. 587 00:31:59,360 --> 00:32:00,264 Let's see. 588 00:32:02,790 --> 00:32:04,790 AUDIENCE: [LAUGHTER] 589 00:32:05,790 --> 00:32:06,750 NICHOLAS DIBELLA: Oh. 590 00:32:06,750 --> 00:32:07,710 I hope I didn't-- 591 00:32:07,710 --> 00:32:08,370 OK. 592 00:32:08,370 --> 00:32:13,410 I hope I didn't break my microphone or these equipment. 593 00:32:13,410 --> 00:32:14,360 But yeah. 594 00:32:14,360 --> 00:32:16,530 But if I did go through it, wouldn't that 595 00:32:16,530 --> 00:32:19,557 be amazing if I actually did go through it? 596 00:32:19,557 --> 00:32:21,390 I've probably tried walking through the wall 597 00:32:21,390 --> 00:32:22,764 hundreds of times, and I've never 598 00:32:22,764 --> 00:32:24,030 been able to go through it. 599 00:32:24,030 --> 00:32:27,150 And it would have been amazing if we could capture this moment 600 00:32:27,150 --> 00:32:31,230 on video because I claim to people 601 00:32:31,230 --> 00:32:33,990 many times that I can do it, and they 602 00:32:33,990 --> 00:32:35,250 don't have confidence in me. 603 00:32:35,250 --> 00:32:36,800 But I think I can do it. 604 00:32:36,800 --> 00:32:40,470 And it would have been cool if I could have done it. 605 00:32:40,470 --> 00:32:42,220 I'll try it again some other time, maybe. 606 00:32:42,220 --> 00:32:42,720 Yes? 607 00:32:42,720 --> 00:32:44,428 AUDIENCE: How did they come up with that? 608 00:32:44,428 --> 00:32:46,265 Is that really the chance of it happening? 609 00:32:46,265 --> 00:32:48,390 NICHOLAS DIBELLA: Oh, how did they come up with it? 610 00:32:48,390 --> 00:32:50,140 I haven't done the calculation in a while, 611 00:32:50,140 --> 00:32:53,460 but you can do a simple one-dimensional 612 00:32:53,460 --> 00:32:58,010 quantum-tunneling calculation to get it. 613 00:32:58,010 --> 00:32:59,970 If you know some quantum mechanics, 614 00:32:59,970 --> 00:33:02,120 I can show you after class how to do it. 615 00:33:02,120 --> 00:33:02,620 OK. 616 00:33:02,620 --> 00:33:05,870 AUDIENCE: What if you only went halfway through the wall? 617 00:33:05,870 --> 00:33:09,065 NICHOLAS DIBELLA: Then I'd have a lonely life afterwards. 618 00:33:13,025 --> 00:33:18,480 Yeah, it's one of the mysteries of quantum mechanics. 619 00:33:18,480 --> 00:33:21,470 Well, one of the really cool things 620 00:33:21,470 --> 00:33:23,900 is this phenomenon called quantum tunnelling, 621 00:33:23,900 --> 00:33:27,770 and I just love it because it makes so many things possible. 622 00:33:27,770 --> 00:33:30,650 It makes it possible, for instance, 623 00:33:30,650 --> 00:33:32,485 for me to fly across the room. 624 00:33:32,485 --> 00:33:33,776 There's a chance I can do that. 625 00:33:33,776 --> 00:33:36,910 I haven't done it yet, but there's a chance I can do it. 626 00:33:36,910 --> 00:33:37,569 Yeah? 627 00:33:37,569 --> 00:33:41,561 AUDIENCE: If it's possible to run through the wall, 628 00:33:41,561 --> 00:33:44,794 then isn't it possible for you to fall through the floor? 629 00:33:44,794 --> 00:33:45,710 NICHOLAS DIBELLA: Yes. 630 00:33:45,710 --> 00:33:49,063 AUDIENCE: And there are 6 billion people in this world. 631 00:33:49,063 --> 00:33:51,937 And they're supposedly trying every second 632 00:33:51,937 --> 00:33:54,100 to fall through the floor. 633 00:33:54,100 --> 00:33:59,038 So that would break that absolutely. 634 00:33:59,038 --> 00:34:02,400 You'd have to try so many times to fall through the floor. 635 00:34:02,400 --> 00:34:07,730 NICHOLAS DIBELLA: Actually-- well, 636 00:34:07,730 --> 00:34:11,210 I haven't really told you, though, the full story. 637 00:34:11,210 --> 00:34:13,370 Real materials are actually a little bit more 638 00:34:13,370 --> 00:34:20,179 complicated than how you might calculate this probability. 639 00:34:20,179 --> 00:34:22,190 It's actually the Pauli exclusion principle 640 00:34:22,190 --> 00:34:25,050 that kind of saves us. 641 00:34:25,050 --> 00:34:27,710 It kind of saves us from falling through the floor. 642 00:34:27,710 --> 00:34:30,889 But because-- well, real materials 643 00:34:30,889 --> 00:34:33,260 are made up of electrons and protons 644 00:34:33,260 --> 00:34:36,719 and other kinds of particles that are called fermions. 645 00:34:36,719 --> 00:34:38,719 And there's this rule called the Pauli exclusion 646 00:34:38,719 --> 00:34:40,670 principle that applies to them. 647 00:34:40,670 --> 00:34:44,239 And it says that you can never be in the same quantum 648 00:34:44,239 --> 00:34:48,199 state as these particles, and that 649 00:34:48,199 --> 00:34:53,179 actually makes this probability a lot lower 650 00:34:53,179 --> 00:34:55,000 and makes me sad because-- 651 00:34:55,000 --> 00:34:56,500 AUDIENCE: [LAUGHTER]. 652 00:34:56,500 --> 00:35:00,666 NICHOLAS DIBELLA: But yeah, the probability 653 00:35:00,666 --> 00:35:03,290 is actually much lower than that because of the Pauli exclusion 654 00:35:03,290 --> 00:35:03,850 principle. 655 00:35:03,850 --> 00:35:06,830 But I might talk more about the Pauli exclusion 656 00:35:06,830 --> 00:35:09,410 principle in a few lectures, but I might not. 657 00:35:09,410 --> 00:35:12,080 But if you're interested about it, then talk to me. 658 00:35:15,430 --> 00:35:17,020 So quantum mechanics is really weird. 659 00:35:17,020 --> 00:35:19,436 That's one of the weirdest things about quantum mechanics, 660 00:35:19,436 --> 00:35:23,150 that it allows so many things to happen. 661 00:35:23,150 --> 00:35:26,270 Relativity is also really weird. 662 00:35:26,270 --> 00:35:29,840 First, according to relativity, I'm 663 00:35:29,840 --> 00:35:35,390 aging at a slower rate than the people in the back of the room. 664 00:35:35,390 --> 00:35:37,765 AUDIENCE: What? 665 00:35:37,765 --> 00:35:39,140 NICHOLAS DIBELLA: Yeah, I'm aging 666 00:35:39,140 --> 00:35:42,440 at a slower rate than the people in the back of the room. 667 00:35:42,440 --> 00:35:43,690 AUDIENCE: Why? 668 00:35:43,690 --> 00:35:43,890 NICHOLAS DIBELLA: Why? 669 00:35:43,890 --> 00:35:45,973 Because of gravitational time dilation, which I'll 670 00:35:45,973 --> 00:35:47,240 tell you about next lecture. 671 00:35:47,240 --> 00:35:49,640 AUDIENCE: [LAUGHTER] 672 00:35:55,012 --> 00:35:57,470 NICHOLAS DIBELLA: Well, I could tell you more crazy things, 673 00:35:57,470 --> 00:36:01,040 but I should probably get to telling you 674 00:36:01,040 --> 00:36:03,530 about special relativity. 675 00:36:03,530 --> 00:36:06,230 By the way, that effect that I just told you about 676 00:36:06,230 --> 00:36:07,670 comes from general relativity. 677 00:36:07,670 --> 00:36:09,290 You also get lots of cool things from special relativity. 678 00:36:09,290 --> 00:36:11,123 But you I'll tell you about them in due time 679 00:36:11,123 --> 00:36:13,810 because first, I want to talk about special relativity, 680 00:36:13,810 --> 00:36:15,950 and then I'll talk about general relativity. 681 00:36:15,950 --> 00:36:19,071 Then I'll talk about quantum mechanics. 682 00:36:19,071 --> 00:36:20,570 Then I'll tell you about everything. 683 00:36:20,570 --> 00:36:22,700 I'll give you the theory of everything, maybe. 684 00:36:22,700 --> 00:36:25,280 And I'll tell you things that people are working on, 685 00:36:25,280 --> 00:36:27,020 and some really, really cool ideas. 686 00:36:29,720 --> 00:36:33,620 So I just wanted to tell you how crazy modern physics is. 687 00:36:33,620 --> 00:36:39,520 It's totally crazing, mind-blowing. 688 00:36:39,520 --> 00:36:41,520 You can't sleep. 689 00:36:41,520 --> 00:36:44,480 Once you learn this stuff, you'll never sleep again, ever, 690 00:36:44,480 --> 00:36:45,320 ever, really. 691 00:36:50,000 --> 00:36:53,540 So special relativity-- 692 00:36:53,540 --> 00:36:56,300 I want to tell you about special relativity. 693 00:36:56,300 --> 00:37:02,045 Special relativity has only two postulates. 694 00:37:02,045 --> 00:37:03,920 You might be familiar with the term postulate 695 00:37:03,920 --> 00:37:05,750 from geometry class or something like that. 696 00:37:05,750 --> 00:37:06,420 AUDIENCE: Oh. 697 00:37:06,420 --> 00:37:08,110 NICHOLAS DIBELLA: There are-- 698 00:37:08,110 --> 00:37:10,074 OK. 699 00:37:10,074 --> 00:37:11,740 You don't have to do any geometry for me 700 00:37:11,740 --> 00:37:12,844 if you don't want to. 701 00:37:12,844 --> 00:37:14,760 I'm not going to force you guys to do geometry 702 00:37:14,760 --> 00:37:19,610 if you guys have bad memories, nightmares from classes. 703 00:37:19,610 --> 00:37:22,370 But there are two postulates of special relativity, postulates 704 00:37:22,370 --> 00:37:23,710 that you can't prove. 705 00:37:23,710 --> 00:37:27,985 They're simply fundamental to the theory. 706 00:37:27,985 --> 00:37:28,860 You can't prove them. 707 00:37:28,860 --> 00:37:30,080 You simply can't prove them. 708 00:37:30,080 --> 00:37:38,590 And I'll tell you what they are, the two postulates 709 00:37:38,590 --> 00:37:42,019 of special relativity. 710 00:37:54,450 --> 00:37:57,800 The first is that the laws of physics 711 00:37:57,800 --> 00:38:00,980 are the same for everybody. 712 00:38:00,980 --> 00:38:03,020 It's a fair world. 713 00:38:03,020 --> 00:38:04,610 Nature is kind to us. 714 00:38:07,730 --> 00:38:22,672 The laws of physics are the same for everybody. 715 00:38:26,370 --> 00:38:28,140 Now, I put "everybody" in quotation marks 716 00:38:28,140 --> 00:38:30,510 because I'll have to be a little bit more precise what 717 00:38:30,510 --> 00:38:31,671 I mean by everybody. 718 00:38:34,500 --> 00:38:39,490 The second postulate is that the speed of light is-- 719 00:38:39,490 --> 00:38:39,991 yes? 720 00:38:39,991 --> 00:38:41,656 AUDIENCE: What do you mean by everybody? 721 00:38:41,656 --> 00:38:43,950 NICHOLAS DIBELLA: Yeah, I'll tell you in one minute. 722 00:38:43,950 --> 00:38:48,297 AUDIENCE: Because what happens if you're fatter? 723 00:38:48,297 --> 00:38:49,900 NICHOLAS DIBELLA: OK. 724 00:38:49,900 --> 00:38:51,890 Well, it's been said that-- 725 00:38:51,890 --> 00:38:53,580 AUDIENCE: Will it change? 726 00:38:53,580 --> 00:38:58,300 NICHOLAS DIBELLA: Well, it's been said that we're all equal, 727 00:38:58,300 --> 00:39:00,286 but some of us are more equal than others. 728 00:39:00,286 --> 00:39:02,770 AUDIENCE: [LAUGHTER] 729 00:39:04,725 --> 00:39:07,305 NICHOLAS DIBELLA: I'll be more precise in one minute. 730 00:39:07,305 --> 00:39:08,190 Just wait a second. 731 00:39:17,010 --> 00:39:18,710 The speed of light-- 732 00:39:18,710 --> 00:39:19,591 whoa. 733 00:39:19,591 --> 00:39:20,090 Oops. 734 00:39:24,870 --> 00:39:27,450 The speed of light-- 735 00:39:27,450 --> 00:39:29,010 whatever. 736 00:39:29,010 --> 00:39:33,510 The speed of light is the same for everybody. 737 00:39:33,510 --> 00:39:34,480 Clumsiness isn't. 738 00:39:37,020 --> 00:39:38,864 Now, here's what I mean by everybody. 739 00:39:47,580 --> 00:39:54,770 By everybody, I mean all inertial observers. 740 00:39:54,770 --> 00:39:55,910 What does inertial mean? 741 00:39:55,910 --> 00:40:00,129 Inertial means moving at a constant speed. 742 00:40:00,129 --> 00:40:00,920 I'll write it down. 743 00:40:00,920 --> 00:40:01,711 I'll write it down. 744 00:40:19,428 --> 00:40:21,420 Moving at a constant-- 745 00:40:24,410 --> 00:40:26,930 sorry for my handwriting, by the way, if it's awful. 746 00:40:26,930 --> 00:40:29,910 I think it's awful. 747 00:40:29,910 --> 00:40:32,130 Inertial means moving at a constant speed. 748 00:40:32,130 --> 00:40:37,220 So if I walk across the room at a constant speed 749 00:40:37,220 --> 00:40:40,880 and walk over all the chalk that I knocked over, 750 00:40:40,880 --> 00:40:43,330 then I'm an inertial observer. 751 00:40:43,330 --> 00:40:45,290 An observer is simply somebody that 752 00:40:45,290 --> 00:40:49,190 makes measurements or a device that makes measurements. 753 00:40:49,190 --> 00:40:50,030 So yes? 754 00:40:50,030 --> 00:40:52,912 AUDIENCE: Do you mean speed or velocity? 755 00:40:52,912 --> 00:40:54,037 NICHOLAS DIBELLA: Velocity. 756 00:40:56,777 --> 00:40:58,610 But if you're moving at a constant velocity, 757 00:40:58,610 --> 00:41:00,110 then you're necessarily moving at a constant speed. 758 00:41:00,110 --> 00:41:00,380 AUDIENCE: Right. 759 00:41:00,380 --> 00:41:01,460 But if you're moving at a constant speed, 760 00:41:01,460 --> 00:41:02,300 you're not accelerating. 761 00:41:02,300 --> 00:41:03,550 NICHOLAS DIBELLA: That's true. 762 00:41:07,270 --> 00:41:10,910 OK, so if I started to accelerate, 763 00:41:10,910 --> 00:41:13,040 then I wouldn't be an inertial observer. 764 00:41:13,040 --> 00:41:16,160 And in that case, special relativity just wouldn't apply. 765 00:41:19,150 --> 00:41:22,760 It turns out that observers that are accelerating 766 00:41:22,760 --> 00:41:25,820 are complicated to deal with. 767 00:41:25,820 --> 00:41:33,066 And you actually have to use general relativity 768 00:41:33,066 --> 00:41:36,920 if you want to make measurements while accelerating. 769 00:41:36,920 --> 00:41:39,835 Because, well-- and this is actually really big, 770 00:41:39,835 --> 00:41:40,960 what I'm about to tell you. 771 00:41:40,960 --> 00:41:43,880 It turns out that there's an intimate connection 772 00:41:43,880 --> 00:41:48,020 between acceleration and gravity. 773 00:41:48,020 --> 00:41:52,160 And Einstein called it the happiest moment of his life 774 00:41:52,160 --> 00:41:55,790 when he figured out that that was true. 775 00:41:55,790 --> 00:42:00,145 I'll leave that for you to think about for a week. 776 00:42:00,145 --> 00:42:02,270 I'll tell you about it next week or the week after, 777 00:42:02,270 --> 00:42:04,430 something like that. 778 00:42:04,430 --> 00:42:08,640 Now, the first postulate is easy to accept. 779 00:42:11,960 --> 00:42:13,460 We all get the same laws of physics. 780 00:42:13,460 --> 00:42:14,660 That's cool. 781 00:42:14,660 --> 00:42:18,890 The second postulate, though, is crazy. 782 00:42:18,890 --> 00:42:20,090 It's crazy. 783 00:42:20,090 --> 00:42:24,322 And to show you just how crazy it is, I need a volunteer. 784 00:42:29,150 --> 00:42:31,146 Sir, what's your name? 785 00:42:31,146 --> 00:42:31,900 AUDIENCE: Lyf. 786 00:42:31,900 --> 00:42:32,470 NICHOLAS DIBELLA: Life? 787 00:42:32,470 --> 00:42:33,010 AUDIENCE: Yes. 788 00:42:33,010 --> 00:42:34,468 NICHOLAS DIBELLA: What a cool name. 789 00:42:34,468 --> 00:42:35,450 AUDIENCE: Thank you. 790 00:42:35,450 --> 00:42:37,520 NICHOLAS DIBELLA: OK. 791 00:42:37,520 --> 00:42:39,600 OK, can you walk for me just a little bit? 792 00:42:43,044 --> 00:42:44,960 I just wanted to make sure you're a suitable-- 793 00:42:44,960 --> 00:42:47,420 well, that you're working experimental apparatus 794 00:42:47,420 --> 00:42:49,836 because I'm going to use you as an experimental apparatus. 795 00:42:49,836 --> 00:42:50,900 AUDIENCE: [LAUGHTER] 796 00:42:51,400 --> 00:42:52,280 NICHOLAS DIBELLA: I'm also going to use myself 797 00:42:52,280 --> 00:42:53,660 as an experimental apparatus. 798 00:42:53,660 --> 00:42:55,560 We're just going to walk across the room. 799 00:42:55,560 --> 00:42:56,570 Here, come with me. 800 00:43:03,770 --> 00:43:05,960 We're going to walk from here until the end. 801 00:43:05,960 --> 00:43:10,279 And I want you to walk at about half my speed, OK? 802 00:43:10,279 --> 00:43:10,820 AUDIENCE: OK. 803 00:43:10,820 --> 00:43:11,695 NICHOLAS DIBELLA: OK. 804 00:43:11,695 --> 00:43:12,634 So let's see. 805 00:43:17,026 --> 00:43:18,002 So slow. 806 00:43:22,410 --> 00:43:23,280 Finally. 807 00:43:23,280 --> 00:43:23,780 OK. 808 00:43:26,320 --> 00:43:31,560 Oh, give me a few more minutes, a few more minutes. 809 00:43:31,560 --> 00:43:33,182 OK, that was fine. 810 00:43:33,182 --> 00:43:34,640 Let's just do it one more time, OK? 811 00:43:34,640 --> 00:43:35,723 Let's do it one more time. 812 00:43:45,830 --> 00:43:47,990 Good job. 813 00:43:47,990 --> 00:43:53,500 Now, let's see. 814 00:43:53,500 --> 00:43:54,360 Oh, come back up. 815 00:43:54,360 --> 00:43:55,605 Come back up. 816 00:43:55,605 --> 00:43:57,960 Oh, I wanted to write on there, but I guess 817 00:43:57,960 --> 00:44:01,190 I'll have to erase something. 818 00:44:01,190 --> 00:44:04,077 Now, your name is Life? 819 00:44:04,077 --> 00:44:04,660 AUDIENCE: Yes. 820 00:44:04,660 --> 00:44:05,530 NICHOLAS DIBELLA: L-I-F-E? 821 00:44:05,530 --> 00:44:06,204 AUDIENCE: L-Y-F. 822 00:44:06,204 --> 00:44:08,120 NICHOLAS DIBELLA: L-Y-- oh, that's still cool. 823 00:44:08,120 --> 00:44:09,056 AUDIENCE: [LAUGHTER]. 824 00:44:09,835 --> 00:44:10,710 NICHOLAS DIBELLA: OK. 825 00:44:13,810 --> 00:44:18,140 Now, class, how fast would you say I was walking? 826 00:44:18,140 --> 00:44:19,400 AUDIENCE: 3 miles per hour. 827 00:44:19,400 --> 00:44:20,120 NICHOLAS DIBELLA: 3 miles per hour? 828 00:44:20,120 --> 00:44:20,780 AUDIENCE: Yeah. 829 00:44:20,780 --> 00:44:22,821 NICHOLAS DIBELLA: OK, let's say 3 miles per hour. 830 00:44:22,821 --> 00:44:26,470 So the class measured-- 831 00:44:26,470 --> 00:44:28,192 AUDIENCE: [LAUGHTER] 832 00:44:28,192 --> 00:44:29,150 NICHOLAS DIBELLA: What? 833 00:44:33,550 --> 00:44:35,510 Oh. 834 00:44:35,510 --> 00:44:36,010 OK. 835 00:44:39,560 --> 00:44:43,000 I failed spelling. 836 00:44:43,000 --> 00:44:50,156 Class measures three miles per hour for Nick. 837 00:44:50,156 --> 00:44:51,280 My name's Nick, by the way. 838 00:44:51,280 --> 00:44:54,720 I think I told you-- for Nick. 839 00:44:54,720 --> 00:44:57,919 And how fast would you say Lyf was walking? 840 00:44:57,919 --> 00:44:58,502 AUDIENCE: 1.5. 841 00:44:58,502 --> 00:45:00,174 AUDIENCE: 1.5. 842 00:45:00,174 --> 00:45:01,840 NICHOLAS DIBELLA: I don't think it was-- 843 00:45:01,840 --> 00:45:03,350 I think he was actually going a 1. 844 00:45:03,350 --> 00:45:06,030 No, maybe 1-- actually, I think I was going 3.5. 845 00:45:06,030 --> 00:45:08,680 But OK, let's say he was going 1. 846 00:45:08,680 --> 00:45:12,277 Well, let's say he was going at one mile per hour. 847 00:45:12,277 --> 00:45:13,110 Let's just say that. 848 00:45:17,290 --> 00:45:22,510 Now-- for Lyf. 849 00:45:22,510 --> 00:45:30,370 Now, Lyf, how fast would you say I was walking relative to you? 850 00:45:30,370 --> 00:45:33,232 AUDIENCE: Five miles per hour. 851 00:45:33,232 --> 00:45:35,440 NICHOLAS DIBELLA: Well, I was probably walking slower 852 00:45:35,440 --> 00:45:36,502 than that, right? 853 00:45:36,502 --> 00:45:37,960 I was walking slower than what they 854 00:45:37,960 --> 00:45:42,292 would see because you're already going at 1 miles 855 00:45:42,292 --> 00:45:43,000 relative to them. 856 00:45:43,000 --> 00:45:45,190 I'm going at three, so probably something 857 00:45:45,190 --> 00:45:48,010 like two because I'm going a little bit slower, right, 858 00:45:48,010 --> 00:45:49,050 relative to you? 859 00:45:49,050 --> 00:45:49,930 AUDIENCE: Relative? 860 00:45:49,930 --> 00:45:50,700 NICHOLAS DIBELLA: Yeah, relative-- 861 00:45:50,700 --> 00:45:52,116 AUDIENCE: Relative to me, yes, you 862 00:45:52,116 --> 00:45:53,474 would be two miles per hour. 863 00:45:53,474 --> 00:45:54,390 NICHOLAS DIBELLA: Yes. 864 00:45:54,390 --> 00:46:03,974 Lyf measures 2 miles per hour for Nick. 865 00:46:03,974 --> 00:46:06,140 And how fast would you say you were walking relative 866 00:46:06,140 --> 00:46:06,972 to yourself? 867 00:46:06,972 --> 00:46:08,860 AUDIENCE: 0. 868 00:46:08,860 --> 00:46:12,164 NICHOLAS DIBELLA: 0, right? 869 00:46:12,164 --> 00:46:13,040 AUDIENCE: Yeah. 870 00:46:13,040 --> 00:46:14,750 NICHOLAS DIBELLA: Were you actually doing 871 00:46:14,750 --> 00:46:15,583 something like this? 872 00:46:15,583 --> 00:46:16,160 AUDIENCE: No. 873 00:46:16,160 --> 00:46:17,826 NICHOLAS DIBELLA: I wasn't watching you. 874 00:46:17,826 --> 00:46:19,040 AUDIENCE: You weren't? 875 00:46:19,040 --> 00:46:21,830 NICHOLAS DIBELLA: Well, I was walking with him. 876 00:46:21,830 --> 00:46:25,790 OK, so you were going at 0 miles per hour relative to yourself, 877 00:46:25,790 --> 00:46:27,530 because you're just walking. 878 00:46:27,530 --> 00:46:29,540 You're not moving past yourself. 879 00:46:29,540 --> 00:46:34,370 You couldn't unless you were special in some way, 880 00:46:34,370 --> 00:46:36,910 more special than special relativity. 881 00:46:36,910 --> 00:46:41,360 0 miles per hour for Lyf-- 882 00:46:44,930 --> 00:46:46,230 like forever, for Lyf. 883 00:46:46,230 --> 00:46:47,567 That's cool. 884 00:46:47,567 --> 00:46:48,650 That's a really cool name. 885 00:46:48,650 --> 00:46:51,920 OK, so you measured 0 miles per hour for yourself, 886 00:46:51,920 --> 00:46:55,340 and 2 miles per hour for me. 887 00:46:55,340 --> 00:47:01,140 This, my friends, is relativity at its simplest. 888 00:47:01,140 --> 00:47:04,460 It's simply the relativity of speed. 889 00:47:04,460 --> 00:47:08,870 Relativity simply means that some people measure one thing, 890 00:47:08,870 --> 00:47:11,150 and some people measure something else. 891 00:47:11,150 --> 00:47:13,310 You, sitting down in these seats, 892 00:47:13,310 --> 00:47:16,760 measured certain speeds for me and Lyf. 893 00:47:16,760 --> 00:47:19,790 And Lyf, as an inertial observer-- 894 00:47:19,790 --> 00:47:21,530 we're both walking at a constant speed-- 895 00:47:21,530 --> 00:47:22,904 as an inertial observer, measured 896 00:47:22,904 --> 00:47:25,760 2 miles per hour for me, and 0 miles per hour for himself. 897 00:47:25,760 --> 00:47:27,650 That's relativity at its most basic. 898 00:47:27,650 --> 00:47:38,660 It turns out that these rules that relate 899 00:47:38,660 --> 00:47:44,200 velocity addition, velocity addition and subtraction. 900 00:47:44,200 --> 00:47:47,551 You could can get this by subtracting 3 from 2 to get 2. 901 00:47:47,551 --> 00:47:49,550 And you can simply get this one by subtracting 1 902 00:47:49,550 --> 00:47:50,960 from 1 to get 0. 903 00:47:50,960 --> 00:47:56,210 These rules actually don't apply for light. 904 00:47:56,210 --> 00:47:58,700 They don't apply for light. 905 00:47:58,700 --> 00:48:04,270 And well, I don't have that much more time. 906 00:48:04,270 --> 00:48:07,021 But you can sit down, by the way, if you'd like. 907 00:48:07,021 --> 00:48:07,520 Thanks. 908 00:48:11,740 --> 00:48:13,220 These rules don't apply for light. 909 00:48:13,220 --> 00:48:14,920 The second postulate says that the speed of light 910 00:48:14,920 --> 00:48:16,240 is the same for everybody. 911 00:48:16,240 --> 00:48:19,350 So if I started walking-- 912 00:48:19,350 --> 00:48:20,502 let's see. 913 00:48:20,502 --> 00:48:22,270 Do I have a flashlight? 914 00:48:22,270 --> 00:48:24,370 I don't have a flashlight, but I have 915 00:48:24,370 --> 00:48:26,400 a thermometer that makes light. 916 00:48:26,400 --> 00:48:27,296 AUDIENCE: [LAUGHTER] 917 00:48:29,670 --> 00:48:31,720 NICHOLAS DIBELLA: If I walked across the room, 918 00:48:31,720 --> 00:48:34,480 turn this on, then you would think 919 00:48:34,480 --> 00:48:37,320 I would measure a certain speed for the speed of light. 920 00:48:37,320 --> 00:48:41,020 I would measure 186,282.4 miles per second for light. 921 00:48:41,020 --> 00:48:43,930 That's good to know for parties, by the way. 922 00:48:43,930 --> 00:48:45,360 I would measure that speed. 923 00:48:45,360 --> 00:48:51,910 And you would guess that you would measure 3 miles per hour 924 00:48:51,910 --> 00:48:53,650 plus that speed I just gave you. 925 00:48:53,650 --> 00:48:57,190 But it turns out that that calculation's wrong. 926 00:48:57,190 --> 00:48:58,399 You get the same exact speed. 927 00:48:58,399 --> 00:49:00,398 You get the same speed that I measure for light. 928 00:49:00,398 --> 00:49:01,660 Everybody gets the same speed. 929 00:49:01,660 --> 00:49:03,810 Speed of light is--