1 00:00:08,300 --> 00:00:11,300 WHISTLING 2 00:00:11,300 --> 00:00:13,300 Ah, excellent, there you are. 3 00:00:13,300 --> 00:00:18,300 ♪ Things can only get better... ♪ 4 00:00:18,300 --> 00:00:20,300 You're a bit late. Or early. 5 00:00:20,300 --> 00:00:22,300 Possibly both, hard to keep track of time, Prof. 6 00:00:22,300 --> 00:00:24,300 Or shall we just go with "Bri"? 7 00:00:24,300 --> 00:00:26,300 What is this place? Amazing. 8 00:00:26,300 --> 00:00:28,300 Hey, come on, man, be cool. You're supposed to be a physicist. 9 00:00:28,300 --> 00:00:30,300 And put a tie on. 10 00:00:30,300 --> 00:00:33,300 You're not the make-up artist. 11 00:00:33,300 --> 00:00:35,300 Sharp. I can see why you got that fellowship. Where am I? 12 00:00:35,300 --> 00:00:36,300 Bit complicated. 13 00:00:36,300 --> 00:00:39,300 Sort of a spaceship/time machine/ swimming pool. 14 00:00:39,300 --> 00:00:41,300 Optional hat stand. I need five minutes of your time, 15 00:00:41,300 --> 00:00:43,300 and when I say five minutes, I'm lying. 16 00:00:43,300 --> 00:00:45,300 I'm just going to give a lecture. I know, I've just seen it. 17 00:00:45,300 --> 00:00:48,300 It's great. But I haven't given it yet. 18 00:00:48,300 --> 00:00:50,300 Tricky to explain - seen it anyway. 19 00:00:50,300 --> 00:00:52,300 You've seen it and you think it's great? 20 00:00:52,300 --> 00:00:54,300 Did I say great? I meant lousy. 21 00:00:54,300 --> 00:00:56,300 You need a spot of help with that. That's why I'm here. 22 00:00:56,300 --> 00:00:57,300 I've bought you a gift. 23 00:00:57,300 --> 00:00:59,300 Actually, I say bought. More like pinched. 24 00:00:59,300 --> 00:01:02,300 But it's the thought that counts. I couldn't find any paper. 25 00:01:03,300 --> 00:01:04,300 Is this what I think it is? 26 00:01:04,300 --> 00:01:07,300 Yeah. Unless you think it's a hat, or a banana, 27 00:01:07,300 --> 00:01:09,300 in which case Manchester Uni needs a re-boot. 28 00:01:09,300 --> 00:01:12,300 This is over 250 years old. About a week old, actually. 29 00:01:12,300 --> 00:01:13,300 I picked it up last Saturday tea-time. 30 00:01:13,300 --> 00:01:16,300 No, no, no, that is impossible 31 00:01:16,300 --> 00:01:18,300 Naughty word, Brian. 32 00:01:18,300 --> 00:01:20,300 2p in the swear box, please. 33 00:01:20,300 --> 00:01:24,300 Space and time. Time and space. 34 00:01:24,300 --> 00:01:27,300 Locked in an intricate dance across the cosmos, 35 00:01:27,300 --> 00:01:31,300 and if you know the tune... anything is possible. 36 00:01:33,300 --> 00:01:35,300 I was going for poetry. Forgot you were a physicist. 37 00:01:35,300 --> 00:01:39,300 Right, hold on to something. Probably your sanity. Ready? 38 00:01:44,300 --> 00:01:46,300 Usually it just twirls around. 39 00:01:46,300 --> 00:01:48,300 It's probably this. Shut up, Brian. 40 00:01:48,300 --> 00:01:50,300 SONG: "Doctor Who Theme" 41 00:02:03,300 --> 00:02:06,300 APPLAUSE 42 00:02:10,300 --> 00:02:14,300 I have absolutely no idea what Mr Cox has in store for us tonight. 43 00:02:14,300 --> 00:02:16,300 He's an enigma. It could be anything. 44 00:02:16,300 --> 00:02:18,300 He might just start singing, for all I know. 45 00:02:18,300 --> 00:02:21,300 It might also be something to do with science. 46 00:02:21,300 --> 00:02:24,300 It might be something to do with time travel. 47 00:02:24,300 --> 00:02:25,300 I've no idea. 48 00:02:25,300 --> 00:02:27,300 And you're going to have a crash helmet on 49 00:02:27,300 --> 00:02:29,300 to protect your beautiful shiny head. 50 00:02:30,300 --> 00:02:34,300 I'm looking to Brian to prove that everything that happens 51 00:02:34,300 --> 00:02:36,300 in Doctor Who could actually happen. Is cast iron fact. 52 00:02:36,300 --> 00:02:37,300 All right, there we are. Sorry. 53 00:02:39,300 --> 00:02:41,300 Is this how you're going to collapse my mass? 54 00:02:41,300 --> 00:02:43,300 LAUGHTER 55 00:02:43,300 --> 00:02:45,300 Just some bloke with a really tight backpack on. 56 00:02:49,300 --> 00:02:52,300 Well, knowing Brian, it will be mind-blowing. 57 00:02:52,300 --> 00:02:55,300 Hopefully we'll all go home knowing how to make a TARDIS. 58 00:02:55,300 --> 00:02:57,300 I'm really looking forward to it, to see what he's got in store for us. 59 00:03:16,300 --> 00:03:19,300 CLOCK TICKS 60 00:03:26,300 --> 00:03:29,300 If I could borrow the TARDIS just for one day, 61 00:03:29,300 --> 00:03:33,300 of all the places I would travel through space and time, 62 00:03:33,300 --> 00:03:38,300 I'd choose here on the 27th of December, 1860. 63 00:03:38,300 --> 00:03:42,300 On that day, Michael Faraday stood on this stage 64 00:03:42,300 --> 00:03:46,300 and delivered his Royal Institution Christmas Lecture 65 00:03:46,300 --> 00:03:48,300 on the chemical history of the candle. 66 00:03:48,300 --> 00:03:52,300 These are his original lecture notes, 67 00:03:52,300 --> 00:03:56,300 singed by the burning candle he used to illuminate them 68 00:03:56,300 --> 00:04:00,300 during the dark winter nights of Victorian London. 69 00:04:00,300 --> 00:04:03,300 Faraday was one of the greatest scientists in the world. 70 00:04:03,300 --> 00:04:06,300 He laid the foundations of our modern understanding 71 00:04:06,300 --> 00:04:10,300 of electricity and magnetism, but the route he took 72 00:04:10,300 --> 00:04:14,300 through time and space to change the world was unusual. 73 00:04:14,300 --> 00:04:16,300 The son of a Yorkshire blacksmith, 74 00:04:16,300 --> 00:04:20,300 he left school at 13 to become an apprentice bookbinder. 75 00:04:20,300 --> 00:04:23,300 An ordinary young man, but someone who loved to think. 76 00:04:23,300 --> 00:04:25,300 He was curious about the world. 77 00:04:25,300 --> 00:04:30,300 His life changed in 1812 when he attended a series of lectures 78 00:04:30,300 --> 00:04:34,300 by another of the great scientific ghosts that haunt this place 79 00:04:34,300 --> 00:04:37,300 on Albemarle Street in London - Humphry Davy, 80 00:04:37,300 --> 00:04:41,300 the charismatic professor of chemistry at the Royal Institution 81 00:04:41,300 --> 00:04:45,300 and a passionate believer in the power and possibilities of science. 82 00:04:45,300 --> 00:04:50,300 Faraday diligently transcribed the lectures and gave them to Davy, 83 00:04:50,300 --> 00:04:52,300 who was so impressed that he appointed the young man 84 00:04:52,300 --> 00:04:54,300 as his scientific assistant. 85 00:04:54,300 --> 00:04:57,300 The rest, as they say, is history. 86 00:04:57,300 --> 00:05:01,300 So this building, this lecture theatre, has a past 87 00:05:01,300 --> 00:05:05,300 that is inextricably bound up with our present and our future, 88 00:05:05,300 --> 00:05:08,300 not only through the great discoveries 89 00:05:08,300 --> 00:05:10,300 that have shaped our scientific civilisation, 90 00:05:10,300 --> 00:05:13,300 but also through the countless generations 91 00:05:13,300 --> 00:05:17,300 of children and adults alike who have been inspired 92 00:05:17,300 --> 00:05:21,300 by lectures given in this theatre to explore nature 93 00:05:21,300 --> 00:05:25,300 and, to echo Humphry Davy, to find new worlds to conquer. 94 00:05:25,300 --> 00:05:28,300 Tonight, I want to explore if, just like the Doctor, 95 00:05:28,300 --> 00:05:33,300 if we could one day conquer time, allowing me to travel to that night 96 00:05:33,300 --> 00:05:36,300 in this room at Christmas 1860. 97 00:05:36,300 --> 00:05:40,300 Will that be forever impossible? 98 00:05:40,300 --> 00:05:43,300 Are the doors to the past firmly closed? 99 00:05:45,300 --> 00:05:50,300 Well, this object is known unromantically as H4. 100 00:05:50,300 --> 00:05:58,300 It's a maritime timekeeper built over 250 years ago by John Harrison. 101 00:05:58,300 --> 00:06:00,300 At the time of its design, 102 00:06:00,300 --> 00:06:04,300 this was the most accurate portable means of telling the time 103 00:06:04,300 --> 00:06:05,300 ever invented. 104 00:06:05,300 --> 00:06:09,300 It was built to navigate, to map the world. 105 00:06:09,300 --> 00:06:11,300 Listen, it still works. 106 00:06:11,300 --> 00:06:14,300 H4 TICKS 107 00:06:16,300 --> 00:06:18,300 Can you hear that? 108 00:06:20,300 --> 00:06:22,300 Beautiful thing. 109 00:06:22,300 --> 00:06:26,300 Time, as the Doctor knows, is the key to exploration. 110 00:06:26,300 --> 00:06:31,300 If you divide our planet into strips by lines of longitude, 111 00:06:31,300 --> 00:06:36,300 marching east to west, then, for every 15 degrees you travel, 112 00:06:36,300 --> 00:06:39,300 noon, that's the time that the sun reaches its highest point 113 00:06:39,300 --> 00:06:42,300 in the sky, shifts by one hour. 114 00:06:42,300 --> 00:06:45,300 So, if you have a clock that keeps perfect time, 115 00:06:45,300 --> 00:06:50,300 synchronised, let's say, with noon at Greenwich, which is here, 116 00:06:50,300 --> 00:06:54,300 then you can simply read off your longitude. 117 00:06:54,300 --> 00:06:58,300 For example, if I left Greenwich with my H4 and travel west 118 00:06:58,300 --> 00:07:04,300 on my ship, then H4 would read 5pm Greenwich Mean Time 119 00:07:04,300 --> 00:07:08,300 when the sun is directly overhead at my position. 120 00:07:08,300 --> 00:07:14,300 Then I know that I'm at 15 times 5, equals 75 degrees west of Greenwich, 121 00:07:14,300 --> 00:07:20,300 which is roughly on the line here that passes through New York City. 122 00:07:20,300 --> 00:07:25,300 Harrison's H4 was the first watch that could keep time near-perfectly 123 00:07:25,300 --> 00:07:27,300 through the rigours of an ocean voyage. 124 00:07:27,300 --> 00:07:29,300 It changed the fortunes of Britain, 125 00:07:29,300 --> 00:07:32,300 and it changed the fortunes of the world. 126 00:07:32,300 --> 00:07:35,300 With the help of the design of this watch, 127 00:07:35,300 --> 00:07:38,300 the Earth was systematically explored and mapped. 128 00:07:38,300 --> 00:07:40,300 Trade and travel were no longer a lottery. 129 00:07:40,300 --> 00:07:46,300 We knew, for the first time, our place on the surface of our planet. 130 00:07:46,300 --> 00:07:51,300 So time can be used to determine our position in space. 131 00:07:51,300 --> 00:07:55,300 But space and time still feel as if they're separate things. 132 00:07:55,300 --> 00:07:58,300 Time marches inexorably on, 133 00:07:58,300 --> 00:08:04,300 marked out for 250 years by the relentless ticking of H4. 134 00:08:04,300 --> 00:08:06,300 This is not the world the Doctor inhabits. 135 00:08:06,300 --> 00:08:10,300 He has freedom of movement through space and time, 136 00:08:10,300 --> 00:08:15,300 and, surprisingly, it's not the world we inhabit either. 137 00:08:15,300 --> 00:08:20,300 I'm going to show you that we too are ALMOST free 138 00:08:20,300 --> 00:08:21,300 to wander through time. 139 00:08:21,300 --> 00:08:25,300 During the late years of the 19th century, physicists, 140 00:08:25,300 --> 00:08:27,300 and in particular Albert Einstein, 141 00:08:27,300 --> 00:08:31,300 were forced to re-examine our intuitive picture of space and time, 142 00:08:31,300 --> 00:08:36,300 and halfway through the 20th century, Einstein's colleague 143 00:08:36,300 --> 00:08:39,300 and tutor Hermann Minkowski was compelled to write 144 00:08:39,300 --> 00:08:45,300 his now-famous obituary for the simple tick-tock of the clock. 145 00:08:45,300 --> 00:08:50,300 "From henceforth, space by itself and time by itself 146 00:08:50,300 --> 00:08:53,300 "have vanished into the merest shadows 147 00:08:53,300 --> 00:08:58,300 "and only a kind of blend of the two exists in its own right." 148 00:08:58,300 --> 00:09:03,300 I'm tempted to say, as the Doctor would, "Wibbly-wobbly, timey-wimey" 149 00:09:03,300 --> 00:09:05,300 But I won't. 150 00:09:05,300 --> 00:09:06,300 HE CHUCKLES 151 00:09:06,300 --> 00:09:07,300 I just did. 152 00:09:08,300 --> 00:09:12,300 What could Minkowski have meant? Well, let me draw a map. 153 00:09:12,300 --> 00:09:13,300 A map like no other. 154 00:09:13,300 --> 00:09:17,300 It's a map that contains the entirety of the known universe. 155 00:09:17,300 --> 00:09:21,300 Our past, our present and our future. 156 00:09:21,300 --> 00:09:23,300 So, this line, here... 157 00:09:25,300 --> 00:09:26,300 ..represents space. 158 00:09:29,300 --> 00:09:33,300 This dot represents our place in space, 159 00:09:33,300 --> 00:09:37,300 here, at the Royal Institution. 160 00:09:37,300 --> 00:09:39,300 Now, let me add time to the map. 161 00:09:47,300 --> 00:09:50,300 So, this is our future. 162 00:09:50,300 --> 00:09:54,300 This is all the time that is yet to come, if you like, 163 00:09:54,300 --> 00:09:58,300 and this is our past. 164 00:09:58,300 --> 00:10:02,300 Now, the dot represents what physicists call an event. 165 00:10:02,300 --> 00:10:06,300 That's this lecture room, our place in space, 166 00:10:06,300 --> 00:10:09,300 tonight, our place in time. 167 00:10:09,300 --> 00:10:11,300 So this is a strange kind of map. 168 00:10:11,300 --> 00:10:13,300 It's a map of infinite size, 169 00:10:13,300 --> 00:10:18,300 and this line, space, here, represents our now. 170 00:10:18,300 --> 00:10:21,300 The Doctor has complete freedom of movement around this map 171 00:10:21,300 --> 00:10:23,300 in the TARDIS. 172 00:10:23,300 --> 00:10:25,300 He can visit any event he likes - 173 00:10:25,300 --> 00:10:28,300 any place in space, any place in time. 174 00:10:28,300 --> 00:10:31,300 Now, we, of course, don't have that freedom, 175 00:10:31,300 --> 00:10:36,300 although, as we'll see, we have more freedom than you might think. 176 00:10:36,300 --> 00:10:39,300 Now, let me explain what I mean by that cryptic statement. 177 00:10:39,300 --> 00:10:42,300 Let's travel back down the timeline. 178 00:10:42,300 --> 00:10:47,300 Let's travel back to, let's say, 1830, 179 00:10:47,300 --> 00:10:49,300 a point in the past. 180 00:10:49,300 --> 00:10:53,300 Same place, here in Albemarle Street, different time. 181 00:10:53,300 --> 00:10:57,300 Our event is Michael Faraday conducting experiments 182 00:10:57,300 --> 00:11:00,300 in his laboratory, just a few feet away from us here. 183 00:11:00,300 --> 00:11:03,300 And I need a volunteer to re-create one of his experiments, 184 00:11:03,300 --> 00:11:06,300 and, because I can, I'm going to choose Dallas Campbell. 185 00:11:06,300 --> 00:11:07,300 Where's Dallas? 186 00:11:14,300 --> 00:11:18,300 Thank you, Dallas. Thank you. I know you're a man of science... 187 00:11:18,300 --> 00:11:20,300 Well, yes... ..and engineering. I try, I try. 188 00:11:20,300 --> 00:11:23,300 So, if we wheel this experiment forward... 189 00:11:23,300 --> 00:11:27,300 what I want you to do is re-create one of Faraday's famous experiments. 190 00:11:27,300 --> 00:11:29,300 There's a bit of danger involved. 191 00:11:29,300 --> 00:11:31,300 I-I thought there must be I was, expecting it. 192 00:11:31,300 --> 00:11:35,300 These will save you in the event of, er, an explosion. OK. 193 00:11:35,300 --> 00:11:37,300 It looks quite Doctor Who. 194 00:11:37,300 --> 00:11:39,300 It does look quite Doctor Who, you're right. 195 00:11:39,300 --> 00:11:42,300 What it is, is a series of coils of wire. 196 00:11:42,300 --> 00:11:45,300 So, I've got coil of wire, coil of wire, coil of wire. 197 00:11:45,300 --> 00:11:49,300 And on this pole there are a series of magnets. Yes. 198 00:11:49,300 --> 00:11:52,300 We've got magnets... Yes. ..inside a coil of wire. 199 00:11:52,300 --> 00:11:55,300 And to make it more televisual, an explosive, of course. 200 00:11:57,300 --> 00:12:00,300 So, what I'm going to do is move over here. 201 00:12:01,300 --> 00:12:03,300 What I'm going to do is ask you 202 00:12:03,300 --> 00:12:06,300 to move the magnets in and out of the coils of wire. 203 00:12:06,300 --> 00:12:09,300 Now, you may need to be relatively vigorous. 204 00:12:13,300 --> 00:12:16,300 Stand back, stand back, here we go. Ready? 205 00:12:16,300 --> 00:12:17,300 LAUGHTER AND APPLAUSE 206 00:12:21,300 --> 00:12:24,300 And that's how science works. 207 00:12:24,300 --> 00:12:26,300 OK, was I too vigorous? Yeah. 208 00:12:26,300 --> 00:12:27,300 Years of practice. 209 00:12:27,300 --> 00:12:29,300 Yes, exactly. 210 00:12:29,300 --> 00:12:31,300 OK, here we go. I can't do this. 211 00:12:31,300 --> 00:12:33,300 You can. 212 00:12:33,300 --> 00:12:34,300 You're turning the lights on. 213 00:12:34,300 --> 00:12:36,300 Hooray. Keep going. Come on, Dallas. 214 00:12:36,300 --> 00:12:38,300 Oh! 215 00:12:38,300 --> 00:12:41,300 CHEERING 216 00:12:41,300 --> 00:12:43,300 A spectacular demonstration. 217 00:12:51,300 --> 00:12:55,300 So, just by moving these magnets in and out of coiled wire, 218 00:12:55,300 --> 00:12:59,300 Dallas created electricity, enough to light up two light bulbs 219 00:12:59,300 --> 00:13:02,300 and ignite that electric match. 220 00:13:02,300 --> 00:13:04,300 So, what does this mean? 221 00:13:04,300 --> 00:13:08,300 Well, the answer is that electricity and magnetism 222 00:13:08,300 --> 00:13:10,300 are in some way linked. 223 00:13:10,300 --> 00:13:13,300 Now, Faraday and his colleagues were intrigued. 224 00:13:13,300 --> 00:13:15,300 How can a moving magnet, 225 00:13:15,300 --> 00:13:18,300 which seems physically unconnected to the electric wire, 226 00:13:18,300 --> 00:13:21,300 cause an electric current to flow? 227 00:13:21,300 --> 00:13:24,300 Well, the elegant answer was provided in 1861, 228 00:13:24,300 --> 00:13:27,300 30 years after Faraday's experiments, 229 00:13:27,300 --> 00:13:31,300 by the great Scottish physicist, James Clerk Maxwell. 230 00:13:34,300 --> 00:13:35,300 These... 231 00:13:37,300 --> 00:13:39,300 ..are Maxwell's wave equations. 232 00:13:39,300 --> 00:13:43,300 Now, Maxwell's genius was to discover these equations 233 00:13:43,300 --> 00:13:46,300 by bringing the whole of electricity and magnetism together 234 00:13:46,300 --> 00:13:49,300 into a single framework. 235 00:13:49,300 --> 00:13:52,300 They describe electric and magnetic fields. 236 00:13:52,300 --> 00:13:55,300 This is the electric field here 237 00:13:55,300 --> 00:13:57,300 and this is the magnetic field here. 238 00:13:57,300 --> 00:14:02,300 But they described fields driving themselves through space as waves. 239 00:14:02,300 --> 00:14:04,300 Electromagnetic waves. 240 00:14:04,300 --> 00:14:09,300 Now, in itself this is a remarkable thing, a fascinating discovery. 241 00:14:09,300 --> 00:14:13,300 But even more remarkable is the prediction from Maxwell's equations 242 00:14:13,300 --> 00:14:17,300 that these waves travel at a very particular speed. 243 00:14:17,300 --> 00:14:22,300 Now, the speed enters as the ratio 244 00:14:22,300 --> 00:14:25,300 of the strengths of the electric and magnetic forces. 245 00:14:25,300 --> 00:14:28,300 It was something he'd seen before. 246 00:14:28,300 --> 00:14:34,300 A number measured as far back as 1676 by an astronomer called Romer. 247 00:14:34,300 --> 00:14:38,300 It was, magically, the speed of light. 248 00:14:40,300 --> 00:14:41,300 This is a tremendous discovery. 249 00:14:41,300 --> 00:14:45,300 Maxwell had found an explanation for the nature of light itself. 250 00:14:45,300 --> 00:14:49,300 Light is a wave, electric and magnetic fields, 251 00:14:49,300 --> 00:14:53,300 sloshing energy between them and propelling themselves through space 252 00:14:53,300 --> 00:14:55,300 at this specific speed. 253 00:14:55,300 --> 00:14:59,300 Very beautiful. But puzzling as well, 254 00:14:59,300 --> 00:15:02,300 because the speed of light appears in Maxwell's equations 255 00:15:02,300 --> 00:15:04,300 as a constant. 256 00:15:04,300 --> 00:15:06,300 It is always, in modern units, 257 00:15:06,300 --> 00:15:13,300 precisely 299,792,458 metres per second. 258 00:15:14,300 --> 00:15:19,300 The crucial point is that Maxwell's equations don't say that this speed 259 00:15:19,300 --> 00:15:22,300 is measured in relation to something. 260 00:15:22,300 --> 00:15:24,300 They're not measured relative to anything at all. 261 00:15:24,300 --> 00:15:29,300 It just states the speed of light, of electrometric waves, 262 00:15:29,300 --> 00:15:36,300 is 299,792,458 metres per second. 263 00:15:36,300 --> 00:15:37,300 Everybody! 264 00:15:37,300 --> 00:15:39,300 LAUGHTER 265 00:15:39,300 --> 00:15:41,300 Just feels like I should say that! 266 00:15:41,300 --> 00:15:46,300 299,792,458 metres per second. 267 00:15:46,300 --> 00:15:49,300 That is stranger than it sounds. 268 00:15:49,300 --> 00:15:52,300 To understand the consequences of this, 269 00:15:52,300 --> 00:15:55,300 let's return to the beautiful timepiece, the H4, 270 00:15:55,300 --> 00:16:00,300 which usually resides, actually, at the Royal Observatory in Greenwich. 271 00:16:00,300 --> 00:16:01,300 Is it moving? 272 00:16:01,300 --> 00:16:04,300 Well, easy. No, it isn't. 273 00:16:04,300 --> 00:16:09,300 Except of course, that it IS moving, after a fashion. 274 00:16:09,300 --> 00:16:11,300 Remember, the Earth is spinning on its axis. 275 00:16:11,300 --> 00:16:15,300 It's about 650mph at this latitude. 276 00:16:15,300 --> 00:16:18,300 So, as well as being stationary, relative to this lecture theatre, 277 00:16:18,300 --> 00:16:22,300 the clock is also moving at 650mph 278 00:16:22,300 --> 00:16:26,300 relative to, let's say, the Doctor in his TARDIS, 279 00:16:26,300 --> 00:16:29,300 looking down on the Earth from space. 280 00:16:29,300 --> 00:16:35,300 So, for the watch and everything else, speed is relative. 281 00:16:35,300 --> 00:16:39,300 The watch is stationary, relative to this lecture theatre, 282 00:16:39,300 --> 00:16:43,300 but according to Maxwell, light doesn't play by these rules. 283 00:16:43,300 --> 00:16:48,300 Instead, everyone will measure the speed of light to be the same. 284 00:16:48,300 --> 00:16:51,300 This is a profoundly strange concept. 285 00:16:51,300 --> 00:16:55,300 This is the way the universe is built, and it has consequences. 286 00:16:55,300 --> 00:16:59,300 Now, to explore these, I need a professor of physics, 287 00:16:59,300 --> 00:17:02,300 so I'm going to choose Jim Al-Khalili. 288 00:17:02,300 --> 00:17:03,300 Where are you Jim? 289 00:17:08,300 --> 00:17:10,300 Now, Einstein did famous experiments. 290 00:17:10,300 --> 00:17:13,300 He used to do things called thought experiments, 291 00:17:13,300 --> 00:17:15,300 and we're going to re-create one of those tonight. 292 00:17:15,300 --> 00:17:16,300 He also had very good hair. 293 00:17:16,300 --> 00:17:19,300 Not as good as yours, but he had very good hair. 294 00:17:19,300 --> 00:17:23,300 Anyone with any hair has very good hair, as far as I'm concerned. 295 00:17:23,300 --> 00:17:24,300 I get these jokes in before Brian does. 296 00:17:24,300 --> 00:17:25,300 I wouldn't dare to comment. 297 00:17:25,300 --> 00:17:28,300 You're going to get me to do something silly, aren't you? 298 00:17:28,300 --> 00:17:31,300 Please have a seat. Yes. Right. Notice that I'm in control of this. 299 00:17:31,300 --> 00:17:33,300 You're on a wheeled contraption. Yes, yes. 300 00:17:33,300 --> 00:17:35,300 And you're going to have a crash helmet on. 301 00:17:35,300 --> 00:17:37,300 Jolly good. 302 00:17:37,300 --> 00:17:39,300 To protect your beautiful shiny head. 303 00:17:39,300 --> 00:17:41,300 LAUGHTER 304 00:17:41,300 --> 00:17:43,300 There we are. 305 00:17:43,300 --> 00:17:45,300 Now Jim's going to help me demonstrate 306 00:17:45,300 --> 00:17:48,300 one of Einstein's most famous thought experiments. 307 00:17:48,300 --> 00:17:50,300 This will vividly illustrate 308 00:17:50,300 --> 00:17:53,300 the consequences of taking Maxwell's equations, 309 00:17:53,300 --> 00:17:57,300 Maxwell's constant speed of light, seriously. 310 00:17:57,300 --> 00:18:00,300 I should explain what's happened. It's not just for fun, this. 311 00:18:00,300 --> 00:18:03,300 So, what Jim's got on his head is a video camera. 312 00:18:03,300 --> 00:18:04,300 That's why he's got this crash helmet on. 313 00:18:04,300 --> 00:18:08,300 So that's going to enable us to see the world from Jim's perspective - 314 00:18:08,300 --> 00:18:10,300 and we, of course, are looking at Jim, 315 00:18:10,300 --> 00:18:13,300 so we can see the world from our perspective. 316 00:18:13,300 --> 00:18:16,300 Now, Einstein's light clock thought experiment 317 00:18:16,300 --> 00:18:18,300 is essentially a very simple idea. 318 00:18:18,300 --> 00:18:22,300 Einstein just imagined a clock made of two mirrors 319 00:18:22,300 --> 00:18:26,300 with a beam of light bouncing between the mirrors. 320 00:18:26,300 --> 00:18:30,300 So, Jim can simulate that with this torch here, this little bulb, 321 00:18:30,300 --> 00:18:32,300 by moving it up and down. 322 00:18:32,300 --> 00:18:34,300 So, Einstein's clock is essentially... 323 00:18:34,300 --> 00:18:36,300 LAUGHTER Shall I do that now? 324 00:18:36,300 --> 00:18:39,300 It's not quite as powerful as Dallas's... 325 00:18:40,300 --> 00:18:42,300 So, Einstein's light clock worked like this - 326 00:18:42,300 --> 00:18:44,300 so, if you bounce that beam of light... 327 00:18:44,300 --> 00:18:48,300 So, my two hands there are the mirrors, and what you can see 328 00:18:48,300 --> 00:18:52,300 is that you could construct a clock out of this sort of arrangement. 329 00:18:52,300 --> 00:18:56,300 Essentially, one tick, which would be, like... 330 00:18:56,300 --> 00:18:57,300 tick... 331 00:18:58,300 --> 00:19:01,300 ..tick... 332 00:19:01,300 --> 00:19:03,300 tick - so that's like the pendulum, 333 00:19:03,300 --> 00:19:05,300 the beam of light bouncing between the mirrors, 334 00:19:05,300 --> 00:19:09,300 and you could use that, actually, to build a very accurate clock. 335 00:19:09,300 --> 00:19:13,300 Then, Einstein imagined what that clock would look like 336 00:19:13,300 --> 00:19:16,300 if it were moving relative to us. 337 00:19:16,300 --> 00:19:20,300 So, what I'm going to have happen is, Jim is going to be moved... 338 00:19:20,300 --> 00:19:22,300 along the stage... 339 00:19:22,300 --> 00:19:24,300 Keep moving the clock. 340 00:19:24,300 --> 00:19:27,300 And then we can dim the light, so we can see what that looks like 341 00:19:27,300 --> 00:19:30,300 from our perspective, stationary relative to Jim. 342 00:19:30,300 --> 00:19:34,300 And we've also got... So there's a little box there, you can see. 343 00:19:34,300 --> 00:19:35,300 That's Jim's head camera, 344 00:19:35,300 --> 00:19:39,300 so Jim is seeing, of course, the clock in exactly the way 345 00:19:39,300 --> 00:19:42,300 that we pictured it when it was stationary, relative to us. 346 00:19:42,300 --> 00:19:44,300 The light beam is bouncing up and down between the mirrors. 347 00:19:44,300 --> 00:19:47,300 But if you look, and we've got a little video effect on there 348 00:19:47,300 --> 00:19:49,300 so you can see the trail, 349 00:19:49,300 --> 00:19:53,300 you can see that the beam of light that we see is tracing out 350 00:19:53,300 --> 00:19:56,300 a triangular pattern across the stage. 351 00:19:59,300 --> 00:20:00,300 Beautiful. 352 00:20:01,300 --> 00:20:02,300 Thank you. 353 00:20:03,300 --> 00:20:07,300 Can I get off now? I'm feeling a bit sick. You can, yes! 354 00:20:07,300 --> 00:20:09,300 Thank you, Jim. It was a bit fast. Thank you. 355 00:20:18,300 --> 00:20:21,300 What a great use of that wonderful intellect. 356 00:20:22,300 --> 00:20:24,300 But it was beautifully demonstrated. 357 00:20:24,300 --> 00:20:29,300 What we saw there was, if I sketch it out again, 358 00:20:29,300 --> 00:20:31,300 from our perspective now, 359 00:20:31,300 --> 00:20:34,300 from the audience's perspective, is that here are all those mirrors, 360 00:20:34,300 --> 00:20:37,300 so this is the light clock that Jim was carrying 361 00:20:37,300 --> 00:20:42,300 but you saw that from your perspective, watching Jim move, 362 00:20:42,300 --> 00:20:45,300 the light took a kind of triangular path 363 00:20:45,300 --> 00:20:48,300 as it bounced across the stage between the mirrors. 364 00:20:51,300 --> 00:20:57,300 Here is what Einstein's postulate, if you like, Einstein's suggestion 365 00:20:57,300 --> 00:21:01,300 that the speed of light is constant for all observers, implies. 366 00:21:01,300 --> 00:21:07,300 See, this path is obviously longer than this path. 367 00:21:07,300 --> 00:21:11,300 So, if we all agree on the speed of light, 368 00:21:11,300 --> 00:21:16,300 then it is obvious that it must take the light longer 369 00:21:16,300 --> 00:21:22,300 to tick for the moving clock than it does for the stationary clock. 370 00:21:23,300 --> 00:21:28,300 Moving clocks run slowly. 371 00:21:28,300 --> 00:21:30,300 This is true. 372 00:21:30,300 --> 00:21:33,300 Time really did pass at a different rate for Jim. 373 00:21:33,300 --> 00:21:35,300 It passed at a different rate for him 374 00:21:35,300 --> 00:21:39,300 than it did for you in the audience, watching Jim move. 375 00:21:39,300 --> 00:21:41,300 There's no sleight of hand here. 376 00:21:41,300 --> 00:21:44,300 Jim really is a time traveller. 377 00:21:44,300 --> 00:21:46,300 Yes! 378 00:21:48,300 --> 00:21:51,300 Our time is personal to us. 379 00:21:51,300 --> 00:21:53,300 This is what Einstein had discovered. 380 00:21:53,300 --> 00:21:55,300 There's no such thing as absolute time. 381 00:21:55,300 --> 00:21:58,300 Now, why don't we notice this in everyday life? 382 00:21:58,300 --> 00:22:02,300 It's because the amount by which time slowed down for Jim 383 00:22:02,300 --> 00:22:08,300 was minuscule, because the speed he was travelling was so small 384 00:22:08,300 --> 00:22:10,300 compared to the speed of light. 385 00:22:10,300 --> 00:22:14,300 But if we'd have sent Jim off in a rocket... 386 00:22:14,300 --> 00:22:16,300 Would you like that? 387 00:22:16,300 --> 00:22:17,300 A rocket? 388 00:22:17,300 --> 00:22:18,300 Flying out into space. 389 00:22:18,300 --> 00:22:24,300 Let's say that we catapulted Jim off at 99.94% of the speed of light 390 00:22:24,300 --> 00:22:28,300 for five years according to his watch. 391 00:22:28,300 --> 00:22:31,300 Then we tell Jim to turn around and come back. 392 00:22:31,300 --> 00:22:34,300 It takes another five years to get back to the Earth. 393 00:22:34,300 --> 00:22:38,300 So, for him, the journey would take ten years. 394 00:22:38,300 --> 00:22:41,300 But for us, with our watches ticking faster than Jim's, 395 00:22:41,300 --> 00:22:44,300 29 years would have passed. 396 00:22:44,300 --> 00:22:51,300 Jim would return in 2042 having aged only ten years. 397 00:22:51,300 --> 00:22:54,300 It's a real effect, he'd be a time traveller. 398 00:22:54,300 --> 00:22:57,300 Time travel into the future is possible. 399 00:22:57,300 --> 00:23:02,300 In fact it's an intrinsic part of the way the universe is built. 400 00:23:02,300 --> 00:23:07,300 We're all time travellers in our own small way. 401 00:23:07,300 --> 00:23:09,300 APPLAUSE 402 00:23:14,300 --> 00:23:16,300 What on Earth? 403 00:23:23,300 --> 00:23:25,300 Oh, hello. Get your tally out. 404 00:23:25,300 --> 00:23:28,300 That's a Silent. You've got to admire a monster that puts on a tie. 405 00:23:28,300 --> 00:23:31,300 It's amazing. Yeah. Shunned by the rest of the galaxy. 406 00:23:31,300 --> 00:23:32,300 They'd be vastly more popular 407 00:23:32,300 --> 00:23:35,300 if they laundered their shirts every now and then. 408 00:23:35,300 --> 00:23:38,300 An intelligent bipedal life form. That's a near-impossibility. 409 00:23:38,300 --> 00:23:41,300 Oh, no, don't look away. 410 00:23:41,300 --> 00:23:42,300 What on Earth? 411 00:23:42,300 --> 00:23:44,300 That's a Silent. Keep staring at it, would you? 412 00:23:44,300 --> 00:23:47,300 I haven't got time right now to keep introducing it. 413 00:23:47,300 --> 00:23:50,300 I want more aliens. Where can we go? 414 00:23:50,300 --> 00:23:53,300 Oh, you're applying for the job, then? Job? 415 00:23:53,300 --> 00:23:55,300 My assistant. What does it involve? 416 00:23:55,300 --> 00:23:57,300 Oh, you know, getting captured, dying occasionally. 417 00:23:57,300 --> 00:23:59,300 The benefits are obviously the travel. 418 00:23:59,300 --> 00:24:02,300 I mean, Earth people need to get out more, Brian. Spread your wings, 419 00:24:02,300 --> 00:24:04,300 meet the neighbours. I mean, what year is this? 420 00:24:04,300 --> 00:24:06,300 From your hair I'd say the sixties. It looks like an upside down mop. 421 00:24:06,300 --> 00:24:10,300 Yes, the moon is nice, but come on, my man, have a wander, 422 00:24:10,300 --> 00:24:14,300 stop loitering around your own solar system like a sulky teenager. 423 00:24:15,300 --> 00:24:18,300 What on Earth? Shut up, Brian. 424 00:24:34,300 --> 00:24:37,300 Are we alone in the universe? 425 00:24:37,300 --> 00:24:40,300 Well, I'd say this is one of the most important questions 426 00:24:40,300 --> 00:24:41,300 in modern science. 427 00:24:41,300 --> 00:24:45,300 In Doctor Who, the answer is an emphatic no. 428 00:24:45,300 --> 00:24:47,300 The universe is filled with aliens, 429 00:24:47,300 --> 00:24:51,300 many with technology far more advanced than our own. 430 00:24:51,300 --> 00:24:54,300 Science fiction's replete with aliens, 431 00:24:54,300 --> 00:24:58,300 partly, I think, because we desperately want them to exist. 432 00:24:58,300 --> 00:25:02,300 The alternative, that we're alone in a possibly infinite universe 433 00:25:02,300 --> 00:25:04,300 is a frightening concept. 434 00:25:04,300 --> 00:25:07,300 But what do we know about the possibility 435 00:25:07,300 --> 00:25:09,300 of finding the alien life, and, in particular, 436 00:25:09,300 --> 00:25:12,300 intelligent life somewhere beyond our solar system? 437 00:25:12,300 --> 00:25:17,300 Well, in 1950, the great Italian physicist Enrico Fermi 438 00:25:17,300 --> 00:25:22,300 took this question and rephrased it, he turned it into a paradox, 439 00:25:22,300 --> 00:25:26,300 highlighting, in the process, one of the great mysteries. 440 00:25:26,300 --> 00:25:29,300 Our sun and its system of eight planets 441 00:25:29,300 --> 00:25:33,300 is one star out of an estimated 400 billion 442 00:25:33,300 --> 00:25:36,300 that form our home galaxy, the Milky Way. 443 00:25:36,300 --> 00:25:40,300 Fermi argued that with so many worlds 444 00:25:40,300 --> 00:25:42,300 and such vast expanses of time 445 00:25:42,300 --> 00:25:44,300 stretching back over 12 billion years 446 00:25:44,300 --> 00:25:48,300 to the formation of our galaxy, there must be planets out there 447 00:25:48,300 --> 00:25:52,300 with civilisations far in advance of our own. 448 00:25:52,300 --> 00:25:56,300 So, our universe should be like Doctor Who. 449 00:25:56,300 --> 00:25:59,300 We should expect, just on statistical grounds, 450 00:25:59,300 --> 00:26:04,300 to have caught some glimpse of those spacefaring civilisations 451 00:26:04,300 --> 00:26:11,300 out there amongst the stars and yet we have seen no evidence of anyone. 452 00:26:11,300 --> 00:26:13,300 This is known as the Fermi paradox. 453 00:26:13,300 --> 00:26:16,300 If they were out there, we should see them. 454 00:26:18,300 --> 00:26:19,300 The problem, of course, 455 00:26:19,300 --> 00:26:22,300 is that to send a space probe to even the nearest star 456 00:26:22,300 --> 00:26:26,300 would take many thousands of years with our current technology, 457 00:26:26,300 --> 00:26:28,300 so the search must proceed 458 00:26:28,300 --> 00:26:31,300 without physically travelling beyond our solar system, 459 00:26:31,300 --> 00:26:34,300 at least for the foreseeable future. And there is a way. 460 00:26:34,300 --> 00:26:38,300 The most ancient way of observing the sky at night. 461 00:26:38,300 --> 00:26:40,300 Astronomy. 462 00:26:40,300 --> 00:26:42,300 By capturing light from distant star systems, 463 00:26:42,300 --> 00:26:46,300 using an array of telescopes both on the ground and in orbit, 464 00:26:46,300 --> 00:26:51,300 we've found 992 exoplanets, 465 00:26:51,300 --> 00:26:54,300 and we can now begin to characterise those planets, 466 00:26:54,300 --> 00:26:56,300 to search for signs of life 467 00:26:56,300 --> 00:26:59,300 encoded in the faint light from these distant worlds. 468 00:26:59,300 --> 00:27:02,300 So far, one of the best candidates for life 469 00:27:02,300 --> 00:27:05,300 orbits around one of the stars in this constellation - 470 00:27:05,300 --> 00:27:07,300 in the constellation of Lyra. 471 00:27:07,300 --> 00:27:10,300 It's a planet called Kepler 62E, 472 00:27:10,300 --> 00:27:14,300 after the recently-retired Kepler telescope that first identified it. 473 00:27:14,300 --> 00:27:19,300 It seems to be just the right size and mass to make it a rocky planet 474 00:27:19,300 --> 00:27:21,300 and in just the right orbit to give it a chance 475 00:27:21,300 --> 00:27:24,300 of possessing liquid water on its surface. 476 00:27:24,300 --> 00:27:27,300 But, remarkably, we can do better 477 00:27:27,300 --> 00:27:30,300 than simply estimating what these planets are made of. 478 00:27:30,300 --> 00:27:33,300 See, we're on the verge of being able to look directly 479 00:27:33,300 --> 00:27:36,300 into the atmospheres of these planets 480 00:27:36,300 --> 00:27:39,300 and search for the tell-tale fingerprints of life. 481 00:27:39,300 --> 00:27:43,300 And I'm going to ask Charles Dance to come down 482 00:27:43,300 --> 00:27:44,300 and help me show you how. 483 00:27:58,300 --> 00:27:59,300 I've got a coat, have I? 484 00:27:59,300 --> 00:28:04,300 Yeah, er, I think its fireproof. Thank you very much, thank you. 485 00:28:04,300 --> 00:28:06,300 So, what we're going to do, here... 486 00:28:06,300 --> 00:28:10,300 You just want me to clean this trolley, don't you, really? 487 00:28:10,300 --> 00:28:13,300 Give you a mop! Yeah. Yes. 488 00:28:13,300 --> 00:28:15,300 What we're going to do is, 489 00:28:15,300 --> 00:28:18,300 we're going to demonstrate the technique that astronomers use 490 00:28:18,300 --> 00:28:21,300 to identify... Should I be standing where you are? 491 00:28:21,300 --> 00:28:23,300 ..the presence of chemicals in the atmosphere. 492 00:28:23,300 --> 00:28:27,300 You can stand wherever you want, it's not going to help you at all. 493 00:28:28,300 --> 00:28:31,300 So, er... What I'm going to ask you to do is, 494 00:28:31,300 --> 00:28:34,300 we've got a selection of four chemical elements... 495 00:28:34,300 --> 00:28:38,300 Right. ..dissolved in these solutions, 496 00:28:38,300 --> 00:28:40,300 and I want you to spray them through the Bunsen flame. 497 00:28:40,300 --> 00:28:42,300 Which direction would you like me to spray them? 498 00:28:42,300 --> 00:28:45,300 I think, actually, probably sort of just upwards and... 499 00:28:45,300 --> 00:28:46,300 Really, are you sure? Er, yeah. 500 00:28:46,300 --> 00:28:48,300 In any particular order? 501 00:28:48,300 --> 00:28:49,300 No, no, just - let's see what happens. 502 00:28:49,300 --> 00:28:53,300 So, you can see, apart from this one, they're all colourless liquids, 503 00:28:53,300 --> 00:28:56,300 but they've got chemical elements dissolved... OK. ..in the solution. 504 00:28:56,300 --> 00:28:57,300 All right. So, let's have a go at that. 505 00:28:57,300 --> 00:29:00,300 We could dim the lights a bit, actually, perhaps. 506 00:29:00,300 --> 00:29:03,300 Go on. Spray that one through. Let's see what that does. 507 00:29:03,300 --> 00:29:04,300 OK. 508 00:29:04,300 --> 00:29:06,300 There it goes. 509 00:29:06,300 --> 00:29:07,300 Shall I do that one again? 510 00:29:07,300 --> 00:29:09,300 Go again. Beautiful green colour. 511 00:29:09,300 --> 00:29:11,300 Bright green colour. 512 00:29:11,300 --> 00:29:13,300 So, now let's try that one. 513 00:29:13,300 --> 00:29:16,300 Same thing? Yeah. 514 00:29:16,300 --> 00:29:17,300 AUDIENCE: Ooh! Oh, I like that. 515 00:29:17,300 --> 00:29:19,300 A bright red. 516 00:29:19,300 --> 00:29:21,300 This takes me back. 517 00:29:21,300 --> 00:29:22,300 I know! 518 00:29:22,300 --> 00:29:24,300 To school chemistry lessons? 519 00:29:24,300 --> 00:29:27,300 No, to early psychedelic rock concerts. 520 00:29:27,300 --> 00:29:29,300 APPLAUSE 521 00:29:34,300 --> 00:29:36,300 I quite like that one, actually. 522 00:29:36,300 --> 00:29:38,300 Oh, where's the red? What if we do two together? 523 00:29:38,300 --> 00:29:41,300 Go on, let's do it, let's go for it. Shall we do three together? 524 00:29:46,300 --> 00:29:47,300 Oh, dear. 525 00:29:47,300 --> 00:29:49,300 Thank you very much. 526 00:29:55,300 --> 00:30:00,300 Now, the reason that those chemical elements behaved in different ways 527 00:30:00,300 --> 00:30:03,300 is down to the structure of the elements themselves. 528 00:30:03,300 --> 00:30:05,300 See, what happens when you burn that element, 529 00:30:05,300 --> 00:30:09,300 when you heat it up, is the electrons jump around 530 00:30:09,300 --> 00:30:12,300 between different orbits around the atomic nucleus 531 00:30:12,300 --> 00:30:16,300 and then fall back down again and emit light, 532 00:30:16,300 --> 00:30:17,300 and so what we're seeing there 533 00:30:17,300 --> 00:30:20,300 is the structure of the atoms themselves 534 00:30:20,300 --> 00:30:22,300 that make up the chemical elements. 535 00:30:22,300 --> 00:30:26,300 Each element will have a different signature of light that it emits 536 00:30:26,300 --> 00:30:31,300 when heated, because it has a different configuration of electrons 537 00:30:31,300 --> 00:30:33,300 around the nucleus. 538 00:30:33,300 --> 00:30:36,300 Now, as well as emitting light when heated, 539 00:30:36,300 --> 00:30:40,300 elements also absorb light of exactly the same colour 540 00:30:40,300 --> 00:30:44,300 if they're present in the atmosphere of a star or a planet. 541 00:30:44,300 --> 00:30:48,300 Here, for example, is a spectrum of light from the sun. 542 00:30:48,300 --> 00:30:52,300 So, this is sunlight split up into all the colours of the rainbow, 543 00:30:52,300 --> 00:30:54,300 by a prism, for example. 544 00:30:54,300 --> 00:30:58,300 And you can see that it is covered in black lines, 545 00:30:58,300 --> 00:31:00,300 all over, in every colour. 546 00:31:00,300 --> 00:31:03,300 These are the fingerprints of chemical elements, 547 00:31:03,300 --> 00:31:07,300 in the same way that we saw Charles show us the beautiful colours, 548 00:31:07,300 --> 00:31:10,300 the fingerprint of the element in those bottles. 549 00:31:10,300 --> 00:31:14,300 Now, we're on the verge of launching telescopes and detectors 550 00:31:14,300 --> 00:31:18,300 so sensitive that we can analyse the light not only from stars, 551 00:31:18,300 --> 00:31:23,300 like the sun, but also the light reflected and absorbed 552 00:31:23,300 --> 00:31:26,300 by the atmospheres of planets around those stars. 553 00:31:26,300 --> 00:31:29,300 This will allow us to look for the fingerprints of molecules 554 00:31:29,300 --> 00:31:33,300 such as water, methane, and even organic molecules, 555 00:31:33,300 --> 00:31:38,300 the fingerprints of life in the atmospheres of alien worlds. 556 00:31:38,300 --> 00:31:42,300 These techniques might prove the first direct evidence 557 00:31:42,300 --> 00:31:45,300 that we're not alone in the universe. 558 00:31:45,300 --> 00:31:48,300 But they still won't allow us to resolve Fermi's paradox, 559 00:31:48,300 --> 00:31:52,300 because these chemical fingerprints won't differentiate 560 00:31:52,300 --> 00:31:55,300 between simple, single-celled organisms 561 00:31:55,300 --> 00:32:00,300 and the complex multi-cellular life that is surely a prerequisite 562 00:32:00,300 --> 00:32:03,300 for the existence of a civilisation like our own. 563 00:32:03,300 --> 00:32:06,300 But there is just a possibility 564 00:32:06,300 --> 00:32:09,300 that we can look for signatures of intelligent civilisations. 565 00:32:09,300 --> 00:32:12,300 See, as a civilisation gets more and more advanced, 566 00:32:12,300 --> 00:32:15,300 its energy consumption rises dramatically. 567 00:32:15,300 --> 00:32:18,300 With every new machine we create here on Earth, 568 00:32:18,300 --> 00:32:21,300 from the tiniest mobile phone to the largest power station, 569 00:32:21,300 --> 00:32:24,300 we produce more heat. 570 00:32:24,300 --> 00:32:25,300 I'll show you what I mean. 571 00:32:25,300 --> 00:32:28,300 Here is an infrared camera. 572 00:32:28,300 --> 00:32:32,300 So, this is measuring not the light from you, the audience, 573 00:32:32,300 --> 00:32:34,300 but the heat from the audience, 574 00:32:34,300 --> 00:32:37,300 because those colours are representing the amount of heat 575 00:32:37,300 --> 00:32:39,300 that you are putting out. 576 00:32:39,300 --> 00:32:41,300 Yeah, give us a wave. 577 00:32:42,300 --> 00:32:45,300 I can see exactly what you're doing at the back. 578 00:32:47,300 --> 00:32:50,300 That's because you are biological machines. 579 00:32:50,300 --> 00:32:54,300 Every machine, no matter how sophisticated or efficient, 580 00:32:54,300 --> 00:32:55,300 must do this. 581 00:32:55,300 --> 00:32:59,300 It must leave a tell-tale heat signature behind 582 00:32:59,300 --> 00:33:01,300 as it goes about its business. 583 00:33:01,300 --> 00:33:06,300 Now, a group of researchers at Penn State University 584 00:33:06,300 --> 00:33:09,300 are attempting to exploit this fundamental universal law, 585 00:33:09,300 --> 00:33:13,300 using infrared cameras to search the stars 586 00:33:13,300 --> 00:33:16,300 and even to search for entire galaxies 587 00:33:16,300 --> 00:33:19,300 to see if they can see hot spots, 588 00:33:19,300 --> 00:33:22,300 systems that are giving out more heat in the infrared spectrum 589 00:33:22,300 --> 00:33:26,300 than you would expect from purely natural processes. 590 00:33:26,300 --> 00:33:28,300 If they sift through all their data, 591 00:33:28,300 --> 00:33:32,300 and actually find a star, a planet or even a galaxy 592 00:33:32,300 --> 00:33:35,300 with this characteristic infrared signature, 593 00:33:35,300 --> 00:33:38,300 then they could claim evidence, not only for complex life 594 00:33:38,300 --> 00:33:40,300 but for a machine-building, star-harnessing, 595 00:33:40,300 --> 00:33:44,300 transgalactic civilisation. 596 00:33:44,300 --> 00:33:46,300 Doctor Who from afar. 597 00:33:46,300 --> 00:33:49,300 Far-fetched? Yeah, of course it is. 598 00:33:49,300 --> 00:33:52,300 But the simple act of looking, of observing nature, 599 00:33:52,300 --> 00:33:56,300 is the key to science, and we shouldn't take anything for granted. 600 00:33:56,300 --> 00:33:59,300 And it's worth noting, finally, 601 00:33:59,300 --> 00:34:03,300 that we may already inadvertently have made contact. 602 00:34:03,300 --> 00:34:05,300 The first episode of Doctor Who 603 00:34:05,300 --> 00:34:09,300 was broadcast on the 23rd of November, 1963. 604 00:34:09,300 --> 00:34:13,300 The programme was encoded in beams of radio waves, 605 00:34:13,300 --> 00:34:17,300 as beams of light that were broadcast to the nation's TVs. 606 00:34:17,300 --> 00:34:22,300 These radio waves didn't simply hang around floating above the UK, 607 00:34:22,300 --> 00:34:23,300 they left our atmosphere, 608 00:34:23,300 --> 00:34:28,300 expanding in spheres just like the light from Faraday's candle 609 00:34:28,300 --> 00:34:30,300 and began their journey out into space. 610 00:34:30,300 --> 00:34:34,300 Today, that signal will have reached 50 light years from this planet. 611 00:34:34,300 --> 00:34:38,300 SONG: "Doctor Who Theme" 612 00:34:38,300 --> 00:34:40,300 What would an alien civilisation think 613 00:34:40,300 --> 00:34:44,300 if their first experience of our civilisation 614 00:34:44,300 --> 00:34:47,300 was the adventures of the time-travelling doctor? 615 00:35:01,300 --> 00:35:03,300 Oi, Cox, no. 616 00:35:03,300 --> 00:35:05,300 Hands off. Complicated. 617 00:35:05,300 --> 00:35:06,300 Ish. 618 00:35:06,300 --> 00:35:08,300 Ish?! Hah! Don't you "ish" me. 619 00:35:08,300 --> 00:35:09,300 Beyond human understanding. 620 00:35:09,300 --> 00:35:12,300 Relative internal spatial co-ordinates are completely at odds 621 00:35:12,300 --> 00:35:15,300 with externally observed dimensions. So, nur. 622 00:35:15,300 --> 00:35:17,300 Bigger on the inside than the outside 623 00:35:17,300 --> 00:35:18,300 doesn't seem too complicated to me. 624 00:35:20,300 --> 00:35:22,300 Don't listen to him. Cover your ears. 625 00:35:22,300 --> 00:35:24,300 Where exactly are your ears? 626 00:35:24,300 --> 00:35:26,300 Listen, how do you fuel something like this? 627 00:35:26,300 --> 00:35:28,300 The power requirements must be immense. 628 00:35:28,300 --> 00:35:30,300 Oh, yeah? Yeah, I use a black hole. 629 00:35:30,300 --> 00:35:32,300 A black hole? Little bit of Time Lord engineering, 630 00:35:32,300 --> 00:35:35,300 siphon off the energy. Powering this thing is like falling off a log. 631 00:35:35,300 --> 00:35:37,300 A very big log, an n-dimensional log. 632 00:35:37,300 --> 00:35:38,300 Read some Einstein. 633 00:35:38,300 --> 00:35:41,300 The tidal forces on a black hole in there would rip it to bits. 634 00:35:41,300 --> 00:35:42,300 Hah! Yeah, I know that. 635 00:35:42,300 --> 00:35:46,300 Nice chap, Einstein. Bow tie wearer. Always gets my vote. 636 00:35:46,300 --> 00:35:49,300 Wicked hair. But he's behind the times, Coxy. 637 00:35:49,300 --> 00:35:50,300 You want to see my black hole? 638 00:35:50,300 --> 00:35:52,300 I keep it down there, in the basement. 639 00:36:12,300 --> 00:36:15,300 So, the Doctor's world is closer to our own 640 00:36:15,300 --> 00:36:16,300 than you might have imagined. 641 00:36:16,300 --> 00:36:18,300 We're all time travellers, 642 00:36:18,300 --> 00:36:21,300 and we've reached out and touched alien worlds. 643 00:36:21,300 --> 00:36:25,300 But I'm drawn back to these notes. 644 00:36:25,300 --> 00:36:29,300 To December 1860, and Michael Faraday's Christmas Lecture 645 00:36:29,300 --> 00:36:33,300 when he inspired a generation of children to become scientists, 646 00:36:33,300 --> 00:36:37,300 using the simple but magical candle. 647 00:36:37,300 --> 00:36:41,300 What about my dream to return to that moment in time? 648 00:36:41,300 --> 00:36:45,300 So, let's take a look at our map again. 649 00:36:47,300 --> 00:36:52,300 Now, we have everything in the past that has ever happened down there, 650 00:36:52,300 --> 00:36:57,300 and we have everything that ever could happen in the future up here. 651 00:36:57,300 --> 00:37:00,300 The Doctor has complete freedom of movement on the map. 652 00:37:00,300 --> 00:37:02,300 He can go anywhere. 653 00:37:02,300 --> 00:37:06,300 But what Einstein realised is that we can't have freedom of movement, 654 00:37:06,300 --> 00:37:08,300 otherwise we'd run into trouble. 655 00:37:08,300 --> 00:37:11,300 So, he discovered a limit. 656 00:37:11,300 --> 00:37:13,300 He built it into his theory. 657 00:37:13,300 --> 00:37:16,300 Something that we can all agree on. 658 00:37:16,300 --> 00:37:18,300 The speed of light. 659 00:37:18,300 --> 00:37:22,300 Let's think about Faraday's candle again. 660 00:37:22,300 --> 00:37:25,300 If there wasn't a roof on this lecture theatre, 661 00:37:25,300 --> 00:37:29,300 then this would be sending out light into the universe. 662 00:37:29,300 --> 00:37:32,300 An expanding sphere of light travelling outwards 663 00:37:32,300 --> 00:37:35,300 at 300,000 kilometres per second. 664 00:37:35,300 --> 00:37:39,300 In one and a half seconds it would have passed by the moon. 665 00:37:39,300 --> 00:37:43,300 In eight minutes it would speed past the sun, 666 00:37:43,300 --> 00:37:46,300 and in around 100,000 years, 667 00:37:46,300 --> 00:37:50,300 it would completely clear the Milky Way Galaxy. 668 00:37:50,300 --> 00:37:54,300 Now, I can draw this onto my map. 669 00:37:54,300 --> 00:37:58,300 So, this is here and now in this lecture theatre 670 00:37:58,300 --> 00:37:59,300 at the Royal Institution. 671 00:37:59,300 --> 00:38:06,300 So, I can draw a line on my map that represents the trajectory 672 00:38:06,300 --> 00:38:09,300 of a beam of light through space-time. 673 00:38:09,300 --> 00:38:12,300 Of course it expands in all directions, 674 00:38:12,300 --> 00:38:15,300 so I have another one of those lines going out there. 675 00:38:15,300 --> 00:38:17,300 A pair of diagonal lines. 676 00:38:17,300 --> 00:38:20,300 Now, I could also draw lines on this map 677 00:38:20,300 --> 00:38:23,300 which represent the paths of beams of light from the past, 678 00:38:23,300 --> 00:38:28,300 if they arrived here, now, in this lecture theatre. 679 00:38:28,300 --> 00:38:29,300 And here they'll be. 680 00:38:29,300 --> 00:38:34,300 They'll look the same, but they'll extend out into the past. 681 00:38:34,300 --> 00:38:37,300 Now, we all agree on these lines 682 00:38:37,300 --> 00:38:39,300 because we all agree on the speed of light, 683 00:38:39,300 --> 00:38:45,300 so they must be important in some way. And they are. 684 00:38:45,300 --> 00:38:49,300 This is how Einstein protects the past from the future. 685 00:38:49,300 --> 00:38:52,300 They limit how we can move around on the map, 686 00:38:52,300 --> 00:38:57,300 because nothing can travel faster than the speed of light. 687 00:38:57,300 --> 00:39:00,300 It is a universal speed limit. 688 00:39:00,300 --> 00:39:02,300 What does that mean? 689 00:39:02,300 --> 00:39:07,300 Well, imagine that there is someone sat here, let's say, 690 00:39:07,300 --> 00:39:08,300 with a telescope. 691 00:39:08,300 --> 00:39:10,300 If I wanted some signal, 692 00:39:10,300 --> 00:39:13,300 some flash of light to get out to that event there, 693 00:39:13,300 --> 00:39:16,300 which would be, let's say, an alien in some distant galaxy, 694 00:39:16,300 --> 00:39:18,300 taking a telescope out and looking at us, 695 00:39:18,300 --> 00:39:21,300 then it would have to travel - the influence, the light - 696 00:39:21,300 --> 00:39:23,300 would have to travel faster than the speed of light. 697 00:39:23,300 --> 00:39:25,300 It can't happen. 698 00:39:25,300 --> 00:39:30,300 So, this line seems to restrict the movement of things. 699 00:39:30,300 --> 00:39:32,300 Things that travel slower than light 700 00:39:32,300 --> 00:39:37,300 are condemned to live inside this area. 701 00:39:37,300 --> 00:39:40,300 This area is clearly important, and it's got a name. 702 00:39:40,300 --> 00:39:42,300 It's called the future light cone. 703 00:39:42,300 --> 00:39:45,300 That encompasses all of our futures. 704 00:39:45,300 --> 00:39:48,300 Every event that's going to happen to any of us in this audience 705 00:39:48,300 --> 00:39:53,300 or watching at home, that happens, will happen in this region 706 00:39:53,300 --> 00:39:56,300 of space-time inside the future light cone. 707 00:39:56,300 --> 00:39:58,300 It also applies to the past. 708 00:39:58,300 --> 00:40:00,300 So, this is a special region. 709 00:40:00,300 --> 00:40:03,300 It's called our past light cone. 710 00:40:03,300 --> 00:40:08,300 This is the region that contains events in space and time 711 00:40:08,300 --> 00:40:12,300 that could in principle have influenced us now, 712 00:40:12,300 --> 00:40:15,300 at this point, here, tonight. 713 00:40:15,300 --> 00:40:19,300 This is the geometry of space-time as described by Einstein 714 00:40:19,300 --> 00:40:24,300 in his theory of special relativity that he published in 1905. 715 00:40:24,300 --> 00:40:28,300 It allows me to trace my life through these two regions. 716 00:40:28,300 --> 00:40:32,300 I can locate any event that happened in my life on this map. 717 00:40:32,300 --> 00:40:40,300 So, I was born on March the 3rd 1968, 718 00:40:40,300 --> 00:40:43,300 and the first picture I have of me at Christmas 719 00:40:43,300 --> 00:40:47,300 was actually 1972 in Oldham. 720 00:40:47,300 --> 00:40:53,300 There I am, that's that event. It's me in Oldham in Christmas 1972. 721 00:40:53,300 --> 00:40:56,300 Now, there are lots of things that happened to me. 722 00:40:56,300 --> 00:40:59,300 I've got a very embarrassing picture actually in 1989... 723 00:41:02,300 --> 00:41:06,300 What was I thinking? 724 00:41:06,300 --> 00:41:08,300 I-I... 725 00:41:08,300 --> 00:41:09,300 The kind of lifestyle I had. 726 00:41:09,300 --> 00:41:12,300 That was actually when I was on tour with a rock band somewhere, 727 00:41:12,300 --> 00:41:15,300 I think I was somewhere in Europe. So it could have been... Actually... 728 00:41:15,300 --> 00:41:18,300 Oh, where shall I put myself? Over there, that would be 1989. 729 00:41:18,300 --> 00:41:21,300 That's another event, me on a tour bus, 730 00:41:21,300 --> 00:41:26,300 um, drinking sensibly in Europe in 1989. 731 00:41:26,300 --> 00:41:27,300 And so on. 732 00:41:27,300 --> 00:41:33,300 So, my life is a series of events that I can plot on this diagram. 733 00:41:33,300 --> 00:41:37,300 I'm now here, of course, the Royal Institution in 2013. 734 00:41:37,300 --> 00:41:41,300 So, we could imagine plotting every event in my life on this diagram. 735 00:41:41,300 --> 00:41:47,300 That would make a line, and it's a line known as a world line. 736 00:41:47,300 --> 00:41:51,300 And it can wander around in space, cos I've been at different places, 737 00:41:51,300 --> 00:41:54,300 and, of course, it wanders around in time 738 00:41:54,300 --> 00:42:00,300 from 1968 to 2013 there. 739 00:42:00,300 --> 00:42:04,300 And, of course, Faraday's Christmas Lecture on the candle, 740 00:42:04,300 --> 00:42:07,300 the event I most want to visit in space-time, 741 00:42:07,300 --> 00:42:13,300 is also sitting somewhere down here in my past light cone. 742 00:42:13,300 --> 00:42:14,300 It's there. 743 00:42:14,300 --> 00:42:17,300 Christmas 1860. 744 00:42:17,300 --> 00:42:18,300 Why is it in my past light cone? 745 00:42:18,300 --> 00:42:21,300 It has to be because it's influenced me. 746 00:42:21,300 --> 00:42:25,300 These lecture notes were present at that event 747 00:42:25,300 --> 00:42:28,300 when Faraday stood here and delivered his lecture, 748 00:42:28,300 --> 00:42:30,300 and they're present in front of me now. 749 00:42:30,300 --> 00:42:34,300 So, I could draw the world line at that note book on this diagram. 750 00:42:34,300 --> 00:42:37,300 And they've stayed in the Royal Institution, 751 00:42:37,300 --> 00:42:40,300 the same place in space, pretty much their whole life, 752 00:42:40,300 --> 00:42:47,300 because they began in 1860 and they're here now with me in 2013. 753 00:42:47,300 --> 00:42:50,300 But according to Einstein's Theory of Special Relativity, 754 00:42:50,300 --> 00:42:55,300 I can never visit Faraday, because my future world line, 755 00:42:55,300 --> 00:42:57,300 the things I can experience, 756 00:42:57,300 --> 00:43:02,300 is restricted to stay inside the future light cone. 757 00:43:02,300 --> 00:43:07,300 To get out, to escape into the past, what would I have to do? 758 00:43:07,300 --> 00:43:10,300 Well, the first thing I'd have to do 759 00:43:10,300 --> 00:43:14,300 is travel faster than the speed of light, 760 00:43:14,300 --> 00:43:18,300 even before I begin to consider how I could possibly do that 761 00:43:18,300 --> 00:43:22,300 and loop round to 1860, and the universe isn't built that way. 762 00:43:22,300 --> 00:43:26,300 The doors to the past, unless we have a TARDIS, 763 00:43:26,300 --> 00:43:29,300 appear to be firmly closed. 764 00:43:29,300 --> 00:43:31,300 What if there's another way? 765 00:43:31,300 --> 00:43:36,300 What if I can change the direction of my future light cone, 766 00:43:36,300 --> 00:43:40,300 change the direction of my entire future, 767 00:43:40,300 --> 00:43:44,300 and perhaps begin to tilt it towards the past? 768 00:43:44,300 --> 00:43:48,300 Well, there are objects in our universe 769 00:43:48,300 --> 00:43:52,300 that can tilt light cones, and if I could get close enough 770 00:43:52,300 --> 00:43:56,300 they'd affect the direction of my future in a radical way. 771 00:43:56,300 --> 00:44:00,300 There's one at the heart of the TARDIS, a black hole. 772 00:44:00,300 --> 00:44:05,300 The Eye of Harmony is described in Doctor Who as a star, 773 00:44:05,300 --> 00:44:09,300 frozen at the point of collapse into a black hole. 774 00:44:09,300 --> 00:44:15,300 It's a poetic line, but unusually, it has to be said, for poetry, 775 00:44:15,300 --> 00:44:19,300 this one turns out to be physically accurate. 776 00:44:19,300 --> 00:44:21,300 Black holes form at the end of the lives 777 00:44:21,300 --> 00:44:24,300 of the most massive stars in the universe. 778 00:44:24,300 --> 00:44:28,300 When such stars, at least 20 times the mass of our sun, 779 00:44:28,300 --> 00:44:29,300 run out of fuel in their cores, 780 00:44:29,300 --> 00:44:33,300 no known force can overcome the inward pull of gravity 781 00:44:33,300 --> 00:44:35,300 and prevent them from collapsing, 782 00:44:35,300 --> 00:44:39,300 as far as anyone knows, to a single, infinitely dense point 783 00:44:39,300 --> 00:44:42,300 known as a singularity. 784 00:44:42,300 --> 00:44:46,300 I can draw one of those on a space-time diagram. 785 00:44:46,300 --> 00:44:50,300 So here is space, and here is time. 786 00:44:50,300 --> 00:44:54,300 And this is a diagram from the point of view of the black hole, 787 00:44:54,300 --> 00:44:59,300 so that's the singularity ticking forward in time, as it were. 788 00:44:59,300 --> 00:45:02,300 And these two lines, which are very important, 789 00:45:02,300 --> 00:45:06,300 have the evocative names of event horizons. 790 00:45:06,300 --> 00:45:10,300 These mark out a region in space and time 791 00:45:10,300 --> 00:45:13,300 where the gravitational pull is so strong 792 00:45:13,300 --> 00:45:15,300 that light itself cannot escape. 793 00:45:15,300 --> 00:45:19,300 In the vicinity of the event horizon very strange things happen. 794 00:45:19,300 --> 00:45:24,300 And I need a very strange volunteer to demonstrate that. 795 00:45:24,300 --> 00:45:27,300 So, Rufus Hound, where are you? 796 00:45:34,300 --> 00:45:36,300 That was perhaps a little unkind, wasn't it? 797 00:45:36,300 --> 00:45:38,300 "A very strange volunteer." No, it seems about right. 798 00:45:38,300 --> 00:45:39,300 Is it about right? Yeah. 799 00:45:39,300 --> 00:45:42,300 Um, so... I thought that with my fourth brain. 800 00:45:42,300 --> 00:45:44,300 Did you? 801 00:45:44,300 --> 00:45:48,300 What I'd like to do is to throw you into a black hole. 802 00:45:48,300 --> 00:45:50,300 You wouldn't be the first. 803 00:45:50,300 --> 00:45:52,300 In the name of physics, now... You would be the first. 804 00:45:52,300 --> 00:45:54,300 I think it's going to mean 805 00:45:54,300 --> 00:45:57,300 that you're going to meet a very noble end, 806 00:45:57,300 --> 00:45:59,300 a very wonderful exit from this universe. 807 00:45:59,300 --> 00:46:03,300 But in order to observe you as you exit our plane of existence, 808 00:46:03,300 --> 00:46:06,300 as it were, I want to kit you out with two watches. OK. 809 00:46:06,300 --> 00:46:09,300 This one, which I want you to put on your back, 810 00:46:09,300 --> 00:46:13,300 is going to be the one that we can observe. 811 00:46:15,300 --> 00:46:17,300 All right, there we are. Sorry. 812 00:46:17,300 --> 00:46:20,300 Is this how you're going to collapse my mass? 813 00:46:20,300 --> 00:46:22,300 Is that a bit... Is that comfortable? 814 00:46:22,300 --> 00:46:24,300 You're going to do the straps up, is that how black holes work? 815 00:46:24,300 --> 00:46:27,300 Just some bloke with a really tight backpack on. 816 00:46:27,300 --> 00:46:28,300 There we go. 817 00:46:36,300 --> 00:46:38,300 I already feel implosiony. 818 00:46:39,300 --> 00:46:42,300 And I'd like to give you - well, actually, have you got a watch? 819 00:46:42,300 --> 00:46:44,300 I've got a watch. Oh, you've got a watch. 820 00:46:44,300 --> 00:46:46,300 And there's a second hand ticking away. Yep. 821 00:46:46,300 --> 00:46:47,300 That's good. 822 00:46:47,300 --> 00:46:51,300 Right, so, what we're going to do, is we're going to... 823 00:46:51,300 --> 00:46:54,300 Right - it's low voltage, it's all right. 824 00:46:54,300 --> 00:46:57,300 I'm going to turn it... Where are my safety goggles, Brian? 825 00:46:57,300 --> 00:47:00,300 If you just turn round... 826 00:47:00,300 --> 00:47:03,300 If it'll make you feel better I can get some, but it won't help. 827 00:47:03,300 --> 00:47:04,300 No. Great, fine. 828 00:47:04,300 --> 00:47:07,300 If you turn around so we can see this clock, 829 00:47:07,300 --> 00:47:10,300 and I'm going to turn the clock on, and there it is. 830 00:47:10,300 --> 00:47:11,300 So it's whizzing forward in time. 831 00:47:11,300 --> 00:47:14,300 And I want you to face the blackboard, the Eye of Harmony, 832 00:47:14,300 --> 00:47:16,300 that's the black hole, there. 833 00:47:16,300 --> 00:47:18,300 And what I've done is, I've speeded time up 834 00:47:18,300 --> 00:47:20,300 just so we can see it ticking along. 835 00:47:20,300 --> 00:47:23,300 This is the rate that time's passing for us now, 836 00:47:23,300 --> 00:47:25,300 and it would be the same on your watch here. Right. 837 00:47:25,300 --> 00:47:29,300 And I'm going to ask you to move slowly towards the event horizon. 838 00:47:29,300 --> 00:47:31,300 Very slowly. 839 00:47:33,300 --> 00:47:35,300 That's it. 840 00:47:36,300 --> 00:47:37,300 How do you feel? 841 00:47:37,300 --> 00:47:39,300 Like this is slightly TOO slow. 842 00:47:42,300 --> 00:47:44,300 It's all right. 843 00:47:44,300 --> 00:47:46,300 But you see what's happening. 844 00:47:46,300 --> 00:47:49,300 If you stop there, you're approaching the event horizon, 845 00:47:49,300 --> 00:47:53,300 and time on the watch that we're looking at, attached to your back, 846 00:47:53,300 --> 00:47:56,300 is slowing down. How's the time, though, on your watch? 847 00:47:56,300 --> 00:47:57,300 Exactly the same. 848 00:47:57,300 --> 00:48:01,300 It's ticking along at exactly the same rate. 849 00:48:01,300 --> 00:48:04,300 Now, you might start to feel a bit uncomfortable 850 00:48:04,300 --> 00:48:08,300 because for these sort of stellar mass black holes, 851 00:48:08,300 --> 00:48:11,300 the gravitational force on your feet would now be significantly stronger 852 00:48:11,300 --> 00:48:14,300 than the gravitation force on your head. 853 00:48:14,300 --> 00:48:16,300 Now, this is called spaghettification. 854 00:48:19,300 --> 00:48:21,300 Why? 855 00:48:21,300 --> 00:48:24,300 So, you're beginning to get slightly taller. Right. 856 00:48:24,300 --> 00:48:26,300 And eventually, actually, as you approach the event horizon 857 00:48:26,300 --> 00:48:28,300 I think, really, you'd get so tall 858 00:48:28,300 --> 00:48:31,300 that you'd just be a long line of atoms, disassociated. 859 00:48:31,300 --> 00:48:34,300 But anyway, let's ignore that for the moment. Carry on. 860 00:48:36,300 --> 00:48:39,300 So, you see... I don't know why I feel slightly in awe of a picture. 861 00:48:41,300 --> 00:48:43,300 Right towards the black hole. 862 00:48:43,300 --> 00:48:46,300 And what we see - there, stop. 863 00:48:46,300 --> 00:48:48,300 That is on the event horizon 864 00:48:48,300 --> 00:48:52,300 and we would see Rufus' watch, strapped to his back, freeze. 865 00:48:52,300 --> 00:48:55,300 It would stop, but what does your watch look like? 866 00:48:55,300 --> 00:48:56,300 Still going. 867 00:48:56,300 --> 00:48:58,300 Still going at exactly the same rate. 868 00:48:58,300 --> 00:49:02,300 This is precisely what Einstein tells us would happen 869 00:49:02,300 --> 00:49:04,300 as Rufus fell into the black hole. 870 00:49:04,300 --> 00:49:07,300 We'd see time freeze. 871 00:49:07,300 --> 00:49:10,300 We would see an image of Rufus just like that, actually, 872 00:49:10,300 --> 00:49:12,300 that's quite powerful. 873 00:49:12,300 --> 00:49:15,300 How long can you stand on one leg, just like that? 874 00:49:15,300 --> 00:49:19,300 We'd see a frozen image of Rufus on his way across the event horizon. 875 00:49:19,300 --> 00:49:22,300 Time would stop, that image would still be there. 876 00:49:22,300 --> 00:49:24,300 It would be a sort of immortality, 877 00:49:24,300 --> 00:49:27,300 whereas from Rufus' perspective, time would pass as normal, 878 00:49:27,300 --> 00:49:29,300 he would pass over the event horizon, 879 00:49:29,300 --> 00:49:31,300 he would approach the singularity 880 00:49:31,300 --> 00:49:34,300 and be crushed to an infinitely dense point. 881 00:49:34,300 --> 00:49:36,300 Thank you. 882 00:49:47,300 --> 00:49:48,300 Thanks Rufus. 883 00:49:48,300 --> 00:49:51,300 Um, let me explain what happened to Rufus. 884 00:49:51,300 --> 00:49:55,300 So here is my space-time diagram again. 885 00:49:55,300 --> 00:49:58,300 Remember that the black hole is sat here, stationary. 886 00:49:58,300 --> 00:50:01,300 There's the singularity, here are the event horizons. 887 00:50:01,300 --> 00:50:04,300 And what I'm going to do is, 888 00:50:04,300 --> 00:50:09,300 I'm going to superimpose Rufus' world line... 889 00:50:11,300 --> 00:50:13,300 ..onto this diagram. 890 00:50:15,300 --> 00:50:17,300 Now, we're looking at Rufus, remember, 891 00:50:17,300 --> 00:50:19,300 from the point of view of the black hole. 892 00:50:19,300 --> 00:50:21,300 So it's just sat there, it's going nowhere, 893 00:50:21,300 --> 00:50:25,300 and Rufus is on a journey towards the event horizon 894 00:50:25,300 --> 00:50:28,300 and beyond into oblivion. 895 00:50:28,300 --> 00:50:31,300 What I've also drawn are Rufus' light cones, 896 00:50:31,300 --> 00:50:34,300 the various points along his world line. 897 00:50:34,300 --> 00:50:39,300 These mark out Rufus' accessible future. 898 00:50:39,300 --> 00:50:42,300 But look what happens to these light cones 899 00:50:42,300 --> 00:50:44,300 as he approaches the event horizon. 900 00:50:44,300 --> 00:50:46,300 They're tilting. 901 00:50:46,300 --> 00:50:50,300 Now, this tilt, according to Albert Einstein, 902 00:50:50,300 --> 00:50:53,300 is caused by the mass of the black hole itself. 903 00:50:53,300 --> 00:50:56,300 It's a representation of a central idea 904 00:50:56,300 --> 00:51:00,300 in Einstein's theory of gravity, general relativity. 905 00:51:00,300 --> 00:51:05,300 The idea is this - mass and energy curve space and time, 906 00:51:05,300 --> 00:51:08,300 the very fabric of the universe itself. 907 00:51:08,300 --> 00:51:12,300 That curvature, the warping of space and time, if you like, 908 00:51:12,300 --> 00:51:17,300 is what we're seeing in this diagram as the tilting of light cones, 909 00:51:17,300 --> 00:51:22,300 the tilting of Rufus' future towards the event horizon. 910 00:51:22,300 --> 00:51:24,300 And look what happens here on the horizon. 911 00:51:24,300 --> 00:51:26,300 You see what's happened to the light cone? 912 00:51:26,300 --> 00:51:32,300 It's tilted so much, space and time are curved and warped so much, 913 00:51:32,300 --> 00:51:37,300 that all of Rufus' future is pointing inwards, 914 00:51:37,300 --> 00:51:39,300 into the horizon, into the black hole. 915 00:51:39,300 --> 00:51:43,300 His world line is heading towards the singularity. 916 00:51:43,300 --> 00:51:45,300 There's no escape for Rufus 917 00:51:45,300 --> 00:51:50,300 because his entire future is inside the black hole. 918 00:51:50,300 --> 00:51:53,300 He'd have to travel faster than light to get out, 919 00:51:53,300 --> 00:51:57,300 and that is not allowed in our universe. 920 00:51:57,300 --> 00:51:59,300 This diagram is very beautiful. 921 00:51:59,300 --> 00:52:01,300 It allows us to see something else, 922 00:52:01,300 --> 00:52:04,300 it also allows us to see what happened to Rufus' clock 923 00:52:04,300 --> 00:52:07,300 as we watched it tick slower and slower and slower 924 00:52:07,300 --> 00:52:10,300 as he approached the horizon. 925 00:52:10,300 --> 00:52:12,300 So, let's imagine... 926 00:52:12,300 --> 00:52:15,300 let's imagine that on each tick of Rufus' clock, 927 00:52:15,300 --> 00:52:19,300 the one on his back, a pulse of light was sent out 928 00:52:19,300 --> 00:52:22,300 and we detected that pulse of light from our vantage point 929 00:52:22,300 --> 00:52:24,300 far away from the black hole. 930 00:52:26,300 --> 00:52:27,300 So, let me put them on. 931 00:52:28,300 --> 00:52:30,300 There. 932 00:52:31,300 --> 00:52:33,300 You see what happens. 933 00:52:33,300 --> 00:52:35,300 As the light cones pulse, 934 00:52:35,300 --> 00:52:41,300 then those pulses of light arrive at us at later and later times. 935 00:52:41,300 --> 00:52:43,300 This is the ticking of the clock. 936 00:52:43,300 --> 00:52:46,300 As far as Rufus is concerned, the clock's ticking away normally, 937 00:52:46,300 --> 00:52:49,300 one second, two seconds, three seconds, four seconds. 938 00:52:49,300 --> 00:52:55,300 But, as we see it, the first second is faster than the second second, 939 00:52:55,300 --> 00:52:57,300 which is faster than the third second. 940 00:52:57,300 --> 00:52:59,300 Tick... 941 00:52:59,300 --> 00:53:00,300 tick... 942 00:53:01,300 --> 00:53:05,300 ..tick. And here, on the horizon, 943 00:53:05,300 --> 00:53:09,300 the light pulse goes flying up the side of the light cone, 944 00:53:09,300 --> 00:53:12,300 which is aligned along the event horizon itself. 945 00:53:12,300 --> 00:53:18,300 This pulse never reaches us, so time stops from our perspective. 946 00:53:18,300 --> 00:53:21,300 We see that frozen image of Rufus. 947 00:53:21,300 --> 00:53:25,300 He never makes it across the horizon from our vantage point. 948 00:53:25,300 --> 00:53:28,300 According to him everything proceeds quite normally - 949 00:53:28,300 --> 00:53:31,300 although he's getting spaghettified, it has to be said - 950 00:53:31,300 --> 00:53:35,300 until he gets squashed on the singularity. 951 00:53:35,300 --> 00:53:39,300 This image of Rufus is frozen forever at the horizon. 952 00:53:39,300 --> 00:53:42,300 But here's the wonderful thing - 953 00:53:42,300 --> 00:53:46,300 the same is true for the collapsing star itself. 954 00:53:46,300 --> 00:53:50,300 See, from the perspective of an outside observer, 955 00:53:50,300 --> 00:53:55,300 time stops, so we'd never actually see the star collapse, 956 00:53:55,300 --> 00:53:59,300 we'd see a frozen image fading away 957 00:53:59,300 --> 00:54:04,300 of the dying star forever frozen in time at the moment of collapse, 958 00:54:04,300 --> 00:54:10,300 that is precisely the Eye of Harmony as described in Doctor Who. 959 00:54:10,300 --> 00:54:12,300 How beautiful. 960 00:54:12,300 --> 00:54:15,300 But what of my ambition to get back into the past 961 00:54:15,300 --> 00:54:19,300 and experience Michael Faraday deliver his lecturer? 962 00:54:19,300 --> 00:54:22,300 Well, everything I've spoken about so far in this lecture 963 00:54:22,300 --> 00:54:27,300 is science fact, including this description of a frozen star. 964 00:54:29,300 --> 00:54:32,300 But now it's time to speculate just a little, 965 00:54:32,300 --> 00:54:37,300 but still remain constrained by the known laws of physics. 966 00:54:37,300 --> 00:54:40,300 Notice what the Eye of Harmony, the black hole, did. 967 00:54:40,300 --> 00:54:44,300 It tilted light cones, it changed the direction 968 00:54:44,300 --> 00:54:47,300 of the accessible future in space-time. 969 00:54:47,300 --> 00:54:52,300 Now, could it be that we could dream up some geometry of space-time, 970 00:54:52,300 --> 00:54:56,300 a distribution of matter and energy that would tilt light cones 971 00:54:56,300 --> 00:54:58,300 all the way around? 972 00:54:59,300 --> 00:55:04,300 What I want to do is tilt my future light cone 973 00:55:04,300 --> 00:55:09,300 in such a way that it gets me back to Faraday's Christmas Lecture 974 00:55:09,300 --> 00:55:10,300 in 1860. 975 00:55:13,300 --> 00:55:14,300 Something like this. 976 00:55:16,300 --> 00:55:18,300 So, here... 977 00:55:18,300 --> 00:55:20,300 is a piece of space-time. 978 00:55:20,300 --> 00:55:23,300 It's meant to map directly onto this diagram I drew here. 979 00:55:25,300 --> 00:55:31,300 Here's 1860, and here's me in 2013. 980 00:55:31,300 --> 00:55:37,300 Now, we've seen that a black hole can tilt light cones like that. 981 00:55:37,300 --> 00:55:42,300 What if we could arrange the geometry so that the light cone 982 00:55:42,300 --> 00:55:49,300 tilts around, so it bends in some way 983 00:55:49,300 --> 00:55:51,300 so that... 984 00:55:51,300 --> 00:55:57,300 I can reattach space-time, as it were, around into the past? 985 00:55:57,300 --> 00:56:01,300 I could curve space-time in such a way that this area, 986 00:56:01,300 --> 00:56:06,300 my accessible future, ends up pointing into my own past 987 00:56:06,300 --> 00:56:11,300 and specifically, in this case, ends up pointing to this place, 988 00:56:11,300 --> 00:56:16,300 this event I want to visit, Faraday's lecture in 1860. 989 00:56:16,300 --> 00:56:20,300 Could we design some configuration of matter and energy 990 00:56:20,300 --> 00:56:22,300 that would curve the light cones around 991 00:56:22,300 --> 00:56:27,300 so I could get back into my own past? 992 00:56:27,300 --> 00:56:29,300 The answer is... 993 00:56:29,300 --> 00:56:30,300 we don't know. 994 00:56:33,300 --> 00:56:39,300 But nobody has been able to prove that space-time geometries 995 00:56:39,300 --> 00:56:44,300 similar to this cannot exist, at least in principle. 996 00:56:44,300 --> 00:56:50,300 Although most experts believe that they must in some way be forbidden. 997 00:56:50,300 --> 00:56:54,300 But there's still the faintest possibility, 998 00:56:54,300 --> 00:56:57,300 given the laws of physics as we understand them today, 999 00:56:57,300 --> 00:57:02,300 that someone, someday - maybe a young girl or a young boy - 1000 00:57:02,300 --> 00:57:04,300 will be inspired to try. 1001 00:57:04,300 --> 00:57:08,300 And, even if they fail, by the very act of trying 1002 00:57:08,300 --> 00:57:13,300 they might just go on to change the world. 1003 00:57:13,300 --> 00:57:15,300 APPLAUSE 1004 00:57:25,300 --> 00:57:27,300 Home! 1005 00:57:27,300 --> 00:57:29,300 Oh, I want to visit more alien worlds. 1006 00:57:29,300 --> 00:57:30,300 No. Greedy, Brian. Can't be greedy. 1007 00:57:30,300 --> 00:57:34,300 You've got a lecture to give, people to inspire, merchandise to sell. 1008 00:57:34,300 --> 00:57:36,300 Actually, that reminds me, could you rustle me up a lunchbox? 1009 00:57:36,300 --> 00:57:38,300 Maybe a T-shirt, slim-fitting. 1010 00:57:38,300 --> 00:57:40,300 Oh, don't forget the gift I got you, you'll need that. 1011 00:57:40,300 --> 00:57:42,300 So, what was this all about, then, 1012 00:57:42,300 --> 00:57:44,300 just taking me on a tour of the wonders of the universe? 1013 00:57:44,300 --> 00:57:46,300 Ah! 1014 00:57:46,300 --> 00:57:49,300 Well, there's someone in your audience today, 1015 00:57:49,300 --> 00:57:52,300 just an ordinary kid, so high, sad eyes, look out for her, 1016 00:57:52,300 --> 00:57:55,300 someone who loves to think about why the sky is blue 1017 00:57:55,300 --> 00:57:57,300 and how bees can hover like helicopters, 1018 00:57:57,300 --> 00:58:02,300 but after today she stops being ordinary, 1019 00:58:02,300 --> 00:58:06,300 she grows up to be extraordinary, a woman who changes the world. 1020 00:58:07,300 --> 00:58:11,300 And all she needed was a nudge from you, eh? Today, right now. 1021 00:58:11,300 --> 00:58:12,300 No pressure. 1022 00:58:12,300 --> 00:58:13,300 I do love humans. 1023 00:58:13,300 --> 00:58:17,300 They can be a bit defeatist. You know, "Mustn't," "Can't"... 1024 00:58:17,300 --> 00:58:19,300 Sometimes you just need a helping hand. 1025 00:58:19,300 --> 00:58:22,300 Every adventure starts with a moment, a spark. Ooh! 1026 00:58:22,300 --> 00:58:24,300 Whilst I'm here... 1027 00:58:24,300 --> 00:58:27,300 Bit of anti-shine. You'll need that. 1028 00:58:27,300 --> 00:58:29,300 Ah. 1029 00:58:29,300 --> 00:58:31,300 Don't forget to twiddle the size of the event horizon. 1030 00:58:31,300 --> 00:58:32,300 Shut up, Brian. 1031 00:58:39,300 --> 00:58:40,300 One more adventure before tea. 1032 00:58:47,300 --> 00:58:49,300 SONG: "Doctor Who Theme" 1033 00:58:52,300 --> 00:58:55,300 Subtitles by Red Bee Media Ltd