0:00:10 > 0:00:12WHISTLING
0:00:12 > 0:00:14Ah, excellent, there you are.
0:00:14 > 0:00:19# Things can only get better... #
0:00:19 > 0:00:21You're a bit late. Or early.
0:00:21 > 0:00:23Possibly both, hard to keep track of time, Prof.
0:00:23 > 0:00:25Or shall we just go with "Bri"?
0:00:25 > 0:00:27What is this place? Amazing.
0:00:27 > 0:00:30Hey, come on, man, be cool. You're supposed to be a physicist.
0:00:30 > 0:00:32And put a tie on.
0:00:32 > 0:00:34You're not the make-up artist.
0:00:34 > 0:00:36- Sharp. I can see why you got that fellowship.- Where am I?
0:00:36 > 0:00:37Bit complicated.
0:00:37 > 0:00:40Sort of a spaceship/time machine/ swimming pool.
0:00:40 > 0:00:42Optional hat stand. I need five minutes of your time,
0:00:42 > 0:00:44and when I say five minutes, I'm lying.
0:00:44 > 0:00:46- I'm just going to give a lecture. - I know, I've just seen it.
0:00:46 > 0:00:49- It's great. - But I haven't given it yet.
0:00:49 > 0:00:51Tricky to explain - seen it anyway.
0:00:51 > 0:00:53You've seen it and you think it's great?
0:00:53 > 0:00:55Did I say great? I meant lousy.
0:00:55 > 0:00:57You need a spot of help with that. That's why I'm here.
0:00:57 > 0:00:58I've bought you a gift.
0:00:58 > 0:01:00Actually, I say bought. More like pinched.
0:01:00 > 0:01:03But it's the thought that counts. I couldn't find any paper.
0:01:04 > 0:01:06Is this what I think it is?
0:01:06 > 0:01:08Yeah. Unless you think it's a hat, or a banana,
0:01:08 > 0:01:10in which case Manchester Uni needs a re-boot.
0:01:10 > 0:01:13- This is over 250 years old. - About a week old, actually.
0:01:13 > 0:01:15I picked it up last Saturday tea-time.
0:01:15 > 0:01:18No, no, no, that is impossible
0:01:18 > 0:01:19Naughty word, Brian.
0:01:19 > 0:01:222p in the swear box, please.
0:01:22 > 0:01:25Space and time. Time and space.
0:01:25 > 0:01:28Locked in an intricate dance across the cosmos,
0:01:28 > 0:01:32and if you know the tune... anything is possible.
0:01:34 > 0:01:37I was going for poetry. Forgot you were a physicist.
0:01:37 > 0:01:41Right, hold on to something. Probably your sanity. Ready?
0:01:45 > 0:01:47Usually it just twirls around.
0:01:47 > 0:01:49- It's probably this.- Shut up, Brian.
0:01:49 > 0:01:52SONG: "Doctor Who Theme"
0:02:04 > 0:02:07APPLAUSE
0:02:12 > 0:02:16I have absolutely no idea what Mr Cox has in store for us tonight.
0:02:16 > 0:02:17He's an enigma. It could be anything.
0:02:17 > 0:02:20He might just start singing, for all I know.
0:02:20 > 0:02:23It might also be something to do with science.
0:02:23 > 0:02:25It might be something to do with time travel.
0:02:25 > 0:02:26I've no idea.
0:02:26 > 0:02:28And you're going to have a crash helmet on
0:02:28 > 0:02:30to protect your beautiful shiny head.
0:02:31 > 0:02:35I'm looking to Brian to prove that everything that happens
0:02:35 > 0:02:37- in Doctor Who could actually happen. - Is cast iron fact.
0:02:37 > 0:02:39All right, there we are. Sorry.
0:02:40 > 0:02:42Is this how you're going to collapse my mass?
0:02:42 > 0:02:44LAUGHTER
0:02:44 > 0:02:46Just some bloke with a really tight backpack on.
0:02:50 > 0:02:53Well, knowing Brian, it will be mind-blowing.
0:02:53 > 0:02:56Hopefully we'll all go home knowing how to make a TARDIS.
0:02:56 > 0:02:59I'm really looking forward to it, to see what he's got in store for us.
0:03:17 > 0:03:20CLOCK TICKS
0:03:27 > 0:03:30If I could borrow the TARDIS just for one day,
0:03:30 > 0:03:34of all the places I would travel through space and time,
0:03:34 > 0:03:39I'd choose here on the 27th of December, 1860.
0:03:39 > 0:03:44On that day, Michael Faraday stood on this stage
0:03:44 > 0:03:47and delivered his Royal Institution Christmas Lecture
0:03:47 > 0:03:50on the chemical history of the candle.
0:03:50 > 0:03:53These are his original lecture notes,
0:03:53 > 0:03:57singed by the burning candle he used to illuminate them
0:03:57 > 0:04:01during the dark winter nights of Victorian London.
0:04:01 > 0:04:04Faraday was one of the greatest scientists in the world.
0:04:04 > 0:04:07He laid the foundations of our modern understanding
0:04:07 > 0:04:11of electricity and magnetism, but the route he took
0:04:11 > 0:04:15through time and space to change the world was unusual.
0:04:15 > 0:04:17The son of a Yorkshire blacksmith,
0:04:17 > 0:04:21he left school at 13 to become an apprentice bookbinder.
0:04:21 > 0:04:24An ordinary young man, but someone who loved to think.
0:04:24 > 0:04:27He was curious about the world.
0:04:27 > 0:04:31His life changed in 1812 when he attended a series of lectures
0:04:31 > 0:04:35by another of the great scientific ghosts that haunt this place
0:04:35 > 0:04:38on Albemarle Street in London - Humphry Davy,
0:04:38 > 0:04:42the charismatic professor of chemistry at the Royal Institution
0:04:42 > 0:04:47and a passionate believer in the power and possibilities of science.
0:04:47 > 0:04:51Faraday diligently transcribed the lectures and gave them to Davy,
0:04:51 > 0:04:54who was so impressed that he appointed the young man
0:04:54 > 0:04:55as his scientific assistant.
0:04:55 > 0:04:58The rest, as they say, is history.
0:04:58 > 0:05:02So this building, this lecture theatre, has a past
0:05:02 > 0:05:07that is inextricably bound up with our present and our future,
0:05:07 > 0:05:09not only through the great discoveries
0:05:09 > 0:05:12that have shaped our scientific civilisation,
0:05:12 > 0:05:14but also through the countless generations
0:05:14 > 0:05:18of children and adults alike who have been inspired
0:05:18 > 0:05:22by lectures given in this theatre to explore nature
0:05:22 > 0:05:27and, to echo Humphry Davy, to find new worlds to conquer.
0:05:27 > 0:05:30Tonight, I want to explore if, just like the Doctor,
0:05:30 > 0:05:35if we could one day conquer time, allowing me to travel to that night
0:05:35 > 0:05:38in this room at Christmas 1860.
0:05:38 > 0:05:41Will that be forever impossible?
0:05:41 > 0:05:44Are the doors to the past firmly closed?
0:05:46 > 0:05:52Well, this object is known unromantically as H4.
0:05:52 > 0:05:59It's a maritime timekeeper built over 250 years ago by John Harrison.
0:05:59 > 0:06:01At the time of its design,
0:06:01 > 0:06:05this was the most accurate portable means of telling the time
0:06:05 > 0:06:06ever invented.
0:06:06 > 0:06:10It was built to navigate, to map the world.
0:06:10 > 0:06:12Listen, it still works.
0:06:12 > 0:06:15H4 TICKS
0:06:18 > 0:06:19Can you hear that?
0:06:21 > 0:06:23Beautiful thing.
0:06:23 > 0:06:28Time, as the Doctor knows, is the key to exploration.
0:06:28 > 0:06:32If you divide our planet into strips by lines of longitude,
0:06:32 > 0:06:37marching east to west, then, for every 15 degrees you travel,
0:06:37 > 0:06:40noon, that's the time that the sun reaches its highest point
0:06:40 > 0:06:43in the sky, shifts by one hour.
0:06:43 > 0:06:46So, if you have a clock that keeps perfect time,
0:06:46 > 0:06:52synchronised, let's say, with noon at Greenwich, which is here,
0:06:52 > 0:06:55then you can simply read off your longitude.
0:06:55 > 0:07:00For example, if I left Greenwich with my H4 and travel west
0:07:00 > 0:07:06on my ship, then H4 would read 5pm Greenwich Mean Time
0:07:06 > 0:07:09when the sun is directly overhead at my position.
0:07:09 > 0:07:15Then I know that I'm at 15 times 5, equals 75 degrees west of Greenwich,
0:07:15 > 0:07:21which is roughly on the line here that passes through New York City.
0:07:21 > 0:07:26Harrison's H4 was the first watch that could keep time near-perfectly
0:07:26 > 0:07:28through the rigours of an ocean voyage.
0:07:28 > 0:07:31It changed the fortunes of Britain,
0:07:31 > 0:07:33and it changed the fortunes of the world.
0:07:33 > 0:07:36With the help of the design of this watch,
0:07:36 > 0:07:39the Earth was systematically explored and mapped.
0:07:39 > 0:07:42Trade and travel were no longer a lottery.
0:07:42 > 0:07:47We knew, for the first time, our place on the surface of our planet.
0:07:47 > 0:07:52So time can be used to determine our position in space.
0:07:52 > 0:07:56But space and time still feel as if they're separate things.
0:07:56 > 0:07:59Time marches inexorably on,
0:07:59 > 0:08:05marked out for 250 years by the relentless ticking of H4.
0:08:05 > 0:08:07This is not the world the Doctor inhabits.
0:08:07 > 0:08:11He has freedom of movement through space and time,
0:08:11 > 0:08:16and, surprisingly, it's not the world we inhabit either.
0:08:16 > 0:08:21I'm going to show you that we too are ALMOST free
0:08:21 > 0:08:23to wander through time.
0:08:23 > 0:08:26During the late years of the 19th century, physicists,
0:08:26 > 0:08:28and in particular Albert Einstein,
0:08:28 > 0:08:32were forced to re-examine our intuitive picture of space and time,
0:08:32 > 0:08:37and halfway through the 20th century, Einstein's colleague
0:08:37 > 0:08:41and tutor Hermann Minkowski was compelled to write
0:08:41 > 0:08:46his now-famous obituary for the simple tick-tock of the clock.
0:08:46 > 0:08:51"From henceforth, space by itself and time by itself
0:08:51 > 0:08:55"have vanished into the merest shadows
0:08:55 > 0:08:59"and only a kind of blend of the two exists in its own right."
0:08:59 > 0:09:04I'm tempted to say, as the Doctor would, "Wibbly-wobbly, timey-wimey"
0:09:04 > 0:09:06But I won't.
0:09:06 > 0:09:07HE CHUCKLES
0:09:07 > 0:09:09I just did.
0:09:10 > 0:09:13What could Minkowski have meant? Well, let me draw a map.
0:09:13 > 0:09:14A map like no other.
0:09:14 > 0:09:19It's a map that contains the entirety of the known universe.
0:09:19 > 0:09:22Our past, our present and our future.
0:09:22 > 0:09:24So, this line, here...
0:09:26 > 0:09:28..represents space.
0:09:31 > 0:09:35This dot represents our place in space,
0:09:35 > 0:09:38here, at the Royal Institution.
0:09:38 > 0:09:40Now, let me add time to the map.
0:09:49 > 0:09:51So, this is our future.
0:09:51 > 0:09:55This is all the time that is yet to come, if you like,
0:09:55 > 0:09:59and this is our past.
0:09:59 > 0:10:03Now, the dot represents what physicists call an event.
0:10:03 > 0:10:07That's this lecture room, our place in space,
0:10:07 > 0:10:10tonight, our place in time.
0:10:10 > 0:10:12So this is a strange kind of map.
0:10:12 > 0:10:14It's a map of infinite size,
0:10:14 > 0:10:19and this line, space, here, represents our now.
0:10:19 > 0:10:23The Doctor has complete freedom of movement around this map
0:10:23 > 0:10:24in the TARDIS.
0:10:24 > 0:10:26He can visit any event he likes -
0:10:26 > 0:10:29any place in space, any place in time.
0:10:29 > 0:10:33Now, we, of course, don't have that freedom,
0:10:33 > 0:10:37although, as we'll see, we have more freedom than you might think.
0:10:37 > 0:10:40Now, let me explain what I mean by that cryptic statement.
0:10:40 > 0:10:43Let's travel back down the timeline.
0:10:43 > 0:10:49Let's travel back to, let's say, 1830,
0:10:49 > 0:10:51a point in the past.
0:10:51 > 0:10:54Same place, here in Albemarle Street, different time.
0:10:54 > 0:10:58Our event is Michael Faraday conducting experiments
0:10:58 > 0:11:01in his laboratory, just a few feet away from us here.
0:11:01 > 0:11:05And I need a volunteer to re-create one of his experiments,
0:11:05 > 0:11:08and, because I can, I'm going to choose Dallas Campbell.
0:11:08 > 0:11:09Where's Dallas?
0:11:15 > 0:11:19- Thank you, Dallas.- Thank you. - I know you're a man of science...
0:11:19 > 0:11:21- Well, yes...- ..and engineering. - I try, I try.
0:11:21 > 0:11:24So, if we wheel this experiment forward...
0:11:24 > 0:11:28what I want you to do is re-create one of Faraday's famous experiments.
0:11:28 > 0:11:30There's a bit of danger involved.
0:11:30 > 0:11:32I-I thought there must be I was, expecting it.
0:11:32 > 0:11:37- These will save you in the event of, er, an explosion.- OK.
0:11:37 > 0:11:38It looks quite Doctor Who.
0:11:38 > 0:11:40It does look quite Doctor Who, you're right.
0:11:40 > 0:11:43What it is, is a series of coils of wire.
0:11:43 > 0:11:47So, I've got coil of wire, coil of wire, coil of wire.
0:11:47 > 0:11:50- And on this pole there are a series of magnets.- Yes.
0:11:50 > 0:11:53- We've got magnets...- Yes. - ..inside a coil of wire.
0:11:53 > 0:11:57And to make it more televisual, an explosive, of course.
0:11:58 > 0:12:01So, what I'm going to do is move over here.
0:12:03 > 0:12:04What I'm going to do is ask you
0:12:04 > 0:12:08to move the magnets in and out of the coils of wire.
0:12:08 > 0:12:10Now, you may need to be relatively vigorous.
0:12:14 > 0:12:17Stand back, stand back, here we go. Ready?
0:12:17 > 0:12:18LAUGHTER AND APPLAUSE
0:12:22 > 0:12:25And that's how science works.
0:12:25 > 0:12:27- OK, was I too vigorous?- Yeah.
0:12:27 > 0:12:28Years of practice.
0:12:28 > 0:12:30Yes, exactly.
0:12:30 > 0:12:32OK, here we go. I can't do this.
0:12:32 > 0:12:34You can.
0:12:34 > 0:12:35You're turning the lights on.
0:12:35 > 0:12:38- Hooray.- Keep going. Come on, Dallas.
0:12:38 > 0:12:39Oh!
0:12:39 > 0:12:42CHEERING
0:12:42 > 0:12:44A spectacular demonstration.
0:12:53 > 0:12:56So, just by moving these magnets in and out of coiled wire,
0:12:56 > 0:13:01Dallas created electricity, enough to light up two light bulbs
0:13:01 > 0:13:04and ignite that electric match.
0:13:04 > 0:13:05So, what does this mean?
0:13:05 > 0:13:09Well, the answer is that electricity and magnetism
0:13:09 > 0:13:11are in some way linked.
0:13:11 > 0:13:14Now, Faraday and his colleagues were intrigued.
0:13:14 > 0:13:16How can a moving magnet,
0:13:16 > 0:13:20which seems physically unconnected to the electric wire,
0:13:20 > 0:13:22cause an electric current to flow?
0:13:22 > 0:13:26Well, the elegant answer was provided in 1861,
0:13:26 > 0:13:2930 years after Faraday's experiments,
0:13:29 > 0:13:32by the great Scottish physicist, James Clerk Maxwell.
0:13:35 > 0:13:36These...
0:13:38 > 0:13:41..are Maxwell's wave equations.
0:13:41 > 0:13:44Now, Maxwell's genius was to discover these equations
0:13:44 > 0:13:48by bringing the whole of electricity and magnetism together
0:13:48 > 0:13:50into a single framework.
0:13:50 > 0:13:53They describe electric and magnetic fields.
0:13:53 > 0:13:56This is the electric field here
0:13:56 > 0:13:59and this is the magnetic field here.
0:13:59 > 0:14:03But they described fields driving themselves through space as waves.
0:14:03 > 0:14:05Electromagnetic waves.
0:14:05 > 0:14:10Now, in itself this is a remarkable thing, a fascinating discovery.
0:14:10 > 0:14:14But even more remarkable is the prediction from Maxwell's equations
0:14:14 > 0:14:19that these waves travel at a very particular speed.
0:14:19 > 0:14:23Now, the speed enters as the ratio
0:14:23 > 0:14:27of the strengths of the electric and magnetic forces.
0:14:27 > 0:14:29It was something he'd seen before.
0:14:29 > 0:14:35A number measured as far back as 1676 by an astronomer called Romer.
0:14:35 > 0:14:39It was, magically, the speed of light.
0:14:41 > 0:14:43This is a tremendous discovery.
0:14:43 > 0:14:47Maxwell had found an explanation for the nature of light itself.
0:14:47 > 0:14:50Light is a wave, electric and magnetic fields,
0:14:50 > 0:14:54sloshing energy between them and propelling themselves through space
0:14:54 > 0:14:57at this specific speed.
0:14:57 > 0:15:00Very beautiful. But puzzling as well,
0:15:00 > 0:15:03because the speed of light appears in Maxwell's equations
0:15:03 > 0:15:05as a constant.
0:15:05 > 0:15:07It is always, in modern units,
0:15:07 > 0:15:14precisely 299,792,458 metres per second.
0:15:15 > 0:15:20The crucial point is that Maxwell's equations don't say that this speed
0:15:20 > 0:15:23is measured in relation to something.
0:15:23 > 0:15:25They're not measured relative to anything at all.
0:15:25 > 0:15:30It just states the speed of light, of electrometric waves,
0:15:30 > 0:15:37is 299,792,458 metres per second.
0:15:37 > 0:15:38Everybody!
0:15:38 > 0:15:40LAUGHTER
0:15:40 > 0:15:42Just feels like I should say that!
0:15:42 > 0:15:47299,792,458 metres per second.
0:15:47 > 0:15:50That is stranger than it sounds.
0:15:50 > 0:15:53To understand the consequences of this,
0:15:53 > 0:15:56let's return to the beautiful timepiece, the H4,
0:15:56 > 0:16:01which usually resides, actually, at the Royal Observatory in Greenwich.
0:16:01 > 0:16:03Is it moving?
0:16:03 > 0:16:05Well, easy. No, it isn't.
0:16:05 > 0:16:10Except of course, that it IS moving, after a fashion.
0:16:10 > 0:16:13Remember, the Earth is spinning on its axis.
0:16:13 > 0:16:16It's about 650mph at this latitude.
0:16:16 > 0:16:19So, as well as being stationary, relative to this lecture theatre,
0:16:19 > 0:16:23the clock is also moving at 650mph
0:16:23 > 0:16:27relative to, let's say, the Doctor in his TARDIS,
0:16:27 > 0:16:30looking down on the Earth from space.
0:16:30 > 0:16:36So, for the watch and everything else, speed is relative.
0:16:36 > 0:16:40The watch is stationary, relative to this lecture theatre,
0:16:40 > 0:16:45but according to Maxwell, light doesn't play by these rules.
0:16:45 > 0:16:50Instead, everyone will measure the speed of light to be the same.
0:16:50 > 0:16:52This is a profoundly strange concept.
0:16:52 > 0:16:57This is the way the universe is built, and it has consequences.
0:16:57 > 0:17:00Now, to explore these, I need a professor of physics,
0:17:00 > 0:17:03so I'm going to choose Jim Al-Khalili.
0:17:03 > 0:17:04Where are you Jim?
0:17:09 > 0:17:12Now, Einstein did famous experiments.
0:17:12 > 0:17:14He used to do things called thought experiments,
0:17:14 > 0:17:16and we're going to re-create one of those tonight.
0:17:16 > 0:17:18He also had very good hair.
0:17:18 > 0:17:20Not as good as yours, but he had very good hair.
0:17:20 > 0:17:24Anyone with any hair has very good hair, as far as I'm concerned.
0:17:24 > 0:17:26I get these jokes in before Brian does.
0:17:26 > 0:17:27I wouldn't dare to comment.
0:17:27 > 0:17:29You're going to get me to do something silly, aren't you?
0:17:29 > 0:17:32- Please have a seat. Yes.- Right. - Notice that I'm in control of this.
0:17:32 > 0:17:34- You're on a wheeled contraption. - Yes, yes.
0:17:34 > 0:17:36And you're going to have a crash helmet on.
0:17:36 > 0:17:38Jolly good.
0:17:38 > 0:17:40To protect your beautiful shiny head.
0:17:40 > 0:17:42LAUGHTER
0:17:42 > 0:17:44There we are.
0:17:44 > 0:17:46Now Jim's going to help me demonstrate
0:17:46 > 0:17:49one of Einstein's most famous thought experiments.
0:17:49 > 0:17:51This will vividly illustrate
0:17:51 > 0:17:54the consequences of taking Maxwell's equations,
0:17:54 > 0:17:58Maxwell's constant speed of light, seriously.
0:17:58 > 0:18:01I should explain what's happened. It's not just for fun, this.
0:18:01 > 0:18:04So, what Jim's got on his head is a video camera.
0:18:04 > 0:18:06That's why he's got this crash helmet on.
0:18:06 > 0:18:09So that's going to enable us to see the world from Jim's perspective -
0:18:09 > 0:18:12and we, of course, are looking at Jim,
0:18:12 > 0:18:14so we can see the world from our perspective.
0:18:14 > 0:18:17Now, Einstein's light clock thought experiment
0:18:17 > 0:18:20is essentially a very simple idea.
0:18:20 > 0:18:24Einstein just imagined a clock made of two mirrors
0:18:24 > 0:18:27with a beam of light bouncing between the mirrors.
0:18:27 > 0:18:31So, Jim can simulate that with this torch here, this little bulb,
0:18:31 > 0:18:33by moving it up and down.
0:18:33 > 0:18:36So, Einstein's clock is essentially...
0:18:36 > 0:18:38LAUGHTER Shall I do that now?
0:18:38 > 0:18:40It's not quite as powerful as Dallas's...
0:18:42 > 0:18:44So, Einstein's light clock worked like this -
0:18:44 > 0:18:46so, if you bounce that beam of light...
0:18:46 > 0:18:49So, my two hands there are the mirrors, and what you can see
0:18:49 > 0:18:53is that you could construct a clock out of this sort of arrangement.
0:18:53 > 0:18:57Essentially, one tick, which would be, like...
0:18:57 > 0:18:58tick...
0:19:00 > 0:19:02..tick...
0:19:02 > 0:19:04tick - so that's like the pendulum,
0:19:04 > 0:19:06the beam of light bouncing between the mirrors,
0:19:06 > 0:19:11and you could use that, actually, to build a very accurate clock.
0:19:11 > 0:19:14Then, Einstein imagined what that clock would look like
0:19:14 > 0:19:17if it were moving relative to us.
0:19:17 > 0:19:21So, what I'm going to have happen is, Jim is going to be moved...
0:19:21 > 0:19:24along the stage...
0:19:24 > 0:19:25Keep moving the clock.
0:19:25 > 0:19:28And then we can dim the light, so we can see what that looks like
0:19:28 > 0:19:32from our perspective, stationary relative to Jim.
0:19:32 > 0:19:35And we've also got... So there's a little box there, you can see.
0:19:35 > 0:19:37That's Jim's head camera,
0:19:37 > 0:19:40so Jim is seeing, of course, the clock in exactly the way
0:19:40 > 0:19:43that we pictured it when it was stationary, relative to us.
0:19:43 > 0:19:46The light beam is bouncing up and down between the mirrors.
0:19:46 > 0:19:48But if you look, and we've got a little video effect on there
0:19:48 > 0:19:50so you can see the trail,
0:19:50 > 0:19:54you can see that the beam of light that we see is tracing out
0:19:54 > 0:19:58a triangular pattern across the stage.
0:20:00 > 0:20:01Beautiful.
0:20:02 > 0:20:03Thank you.
0:20:05 > 0:20:08- Can I get off now? I'm feeling a bit sick.- You can, yes!
0:20:08 > 0:20:11- Thank you, Jim. - It was a bit fast.- Thank you.
0:20:19 > 0:20:22What a great use of that wonderful intellect.
0:20:23 > 0:20:25But it was beautifully demonstrated.
0:20:25 > 0:20:30What we saw there was, if I sketch it out again,
0:20:30 > 0:20:32from our perspective now,
0:20:32 > 0:20:35from the audience's perspective, is that here are all those mirrors,
0:20:35 > 0:20:39so this is the light clock that Jim was carrying
0:20:39 > 0:20:43but you saw that from your perspective, watching Jim move,
0:20:43 > 0:20:46the light took a kind of triangular path
0:20:46 > 0:20:50as it bounced across the stage between the mirrors.
0:20:52 > 0:20:58Here is what Einstein's postulate, if you like, Einstein's suggestion
0:20:58 > 0:21:02that the speed of light is constant for all observers, implies.
0:21:02 > 0:21:09See, this path is obviously longer than this path.
0:21:09 > 0:21:13So, if we all agree on the speed of light,
0:21:13 > 0:21:17then it is obvious that it must take the light longer
0:21:17 > 0:21:23to tick for the moving clock than it does for the stationary clock.
0:21:25 > 0:21:29Moving clocks run slowly.
0:21:29 > 0:21:31This is true.
0:21:31 > 0:21:34Time really did pass at a different rate for Jim.
0:21:34 > 0:21:36It passed at a different rate for him
0:21:36 > 0:21:40than it did for you in the audience, watching Jim move.
0:21:40 > 0:21:42There's no sleight of hand here.
0:21:42 > 0:21:46Jim really is a time traveller.
0:21:46 > 0:21:47Yes!
0:21:50 > 0:21:52Our time is personal to us.
0:21:52 > 0:21:54This is what Einstein had discovered.
0:21:54 > 0:21:57There's no such thing as absolute time.
0:21:57 > 0:21:59Now, why don't we notice this in everyday life?
0:21:59 > 0:22:03It's because the amount by which time slowed down for Jim
0:22:03 > 0:22:09was minuscule, because the speed he was travelling was so small
0:22:09 > 0:22:11compared to the speed of light.
0:22:11 > 0:22:15But if we'd have sent Jim off in a rocket...
0:22:15 > 0:22:17Would you like that?
0:22:17 > 0:22:18A rocket?
0:22:18 > 0:22:20Flying out into space.
0:22:20 > 0:22:26Let's say that we catapulted Jim off at 99.94% of the speed of light
0:22:26 > 0:22:30for five years according to his watch.
0:22:30 > 0:22:33Then we tell Jim to turn around and come back.
0:22:33 > 0:22:35It takes another five years to get back to the Earth.
0:22:35 > 0:22:39So, for him, the journey would take ten years.
0:22:39 > 0:22:43But for us, with our watches ticking faster than Jim's,
0:22:43 > 0:22:4629 years would have passed.
0:22:46 > 0:22:52Jim would return in 2042 having aged only ten years.
0:22:52 > 0:22:55It's a real effect, he'd be a time traveller.
0:22:55 > 0:22:59Time travel into the future is possible.
0:22:59 > 0:23:03In fact it's an intrinsic part of the way the universe is built.
0:23:03 > 0:23:08We're all time travellers in our own small way.
0:23:08 > 0:23:10APPLAUSE
0:23:15 > 0:23:17What on Earth?
0:23:24 > 0:23:26Oh, hello. Get your tally out.
0:23:26 > 0:23:29That's a Silent. You've got to admire a monster that puts on a tie.
0:23:29 > 0:23:32- It's amazing.- Yeah. Shunned by the rest of the galaxy.
0:23:32 > 0:23:34They'd be vastly more popular
0:23:34 > 0:23:36if they laundered their shirts every now and then.
0:23:36 > 0:23:40An intelligent bipedal life form. That's a near-impossibility.
0:23:40 > 0:23:42Oh, no, don't look away.
0:23:42 > 0:23:43What on Earth?
0:23:43 > 0:23:45That's a Silent. Keep staring at it, would you?
0:23:45 > 0:23:48I haven't got time right now to keep introducing it.
0:23:48 > 0:23:51I want more aliens. Where can we go?
0:23:51 > 0:23:54- Oh, you're applying for the job, then?- Job?
0:23:54 > 0:23:56- My assistant.- What does it involve?
0:23:56 > 0:23:58Oh, you know, getting captured, dying occasionally.
0:23:58 > 0:24:00The benefits are obviously the travel.
0:24:00 > 0:24:03I mean, Earth people need to get out more, Brian. Spread your wings,
0:24:03 > 0:24:05meet the neighbours. I mean, what year is this?
0:24:05 > 0:24:08From your hair I'd say the sixties. It looks like an upside down mop.
0:24:08 > 0:24:11Yes, the moon is nice, but come on, my man, have a wander,
0:24:11 > 0:24:15stop loitering around your own solar system like a sulky teenager.
0:24:16 > 0:24:19- What on Earth?- Shut up, Brian.
0:24:36 > 0:24:38Are we alone in the universe?
0:24:38 > 0:24:41Well, I'd say this is one of the most important questions
0:24:41 > 0:24:42in modern science.
0:24:42 > 0:24:46In Doctor Who, the answer is an emphatic no.
0:24:46 > 0:24:48The universe is filled with aliens,
0:24:48 > 0:24:52many with technology far more advanced than our own.
0:24:52 > 0:24:55Science fiction's replete with aliens,
0:24:55 > 0:24:59partly, I think, because we desperately want them to exist.
0:24:59 > 0:25:03The alternative, that we're alone in a possibly infinite universe
0:25:03 > 0:25:05is a frightening concept.
0:25:05 > 0:25:08But what do we know about the possibility
0:25:08 > 0:25:10of finding the alien life, and, in particular,
0:25:10 > 0:25:13intelligent life somewhere beyond our solar system?
0:25:13 > 0:25:19Well, in 1950, the great Italian physicist Enrico Fermi
0:25:19 > 0:25:23took this question and rephrased it, he turned it into a paradox,
0:25:23 > 0:25:27highlighting, in the process, one of the great mysteries.
0:25:27 > 0:25:30Our sun and its system of eight planets
0:25:30 > 0:25:34is one star out of an estimated 400 billion
0:25:34 > 0:25:37that form our home galaxy, the Milky Way.
0:25:37 > 0:25:41Fermi argued that with so many worlds
0:25:41 > 0:25:43and such vast expanses of time
0:25:43 > 0:25:46stretching back over 12 billion years
0:25:46 > 0:25:49to the formation of our galaxy, there must be planets out there
0:25:49 > 0:25:53with civilisations far in advance of our own.
0:25:53 > 0:25:57So, our universe should be like Doctor Who.
0:25:57 > 0:26:01We should expect, just on statistical grounds,
0:26:01 > 0:26:05to have caught some glimpse of those spacefaring civilisations
0:26:05 > 0:26:12out there amongst the stars and yet we have seen no evidence of anyone.
0:26:12 > 0:26:14This is known as the Fermi paradox.
0:26:14 > 0:26:17If they were out there, we should see them.
0:26:19 > 0:26:20The problem, of course,
0:26:20 > 0:26:24is that to send a space probe to even the nearest star
0:26:24 > 0:26:27would take many thousands of years with our current technology,
0:26:27 > 0:26:29so the search must proceed
0:26:29 > 0:26:32without physically travelling beyond our solar system,
0:26:32 > 0:26:35at least for the foreseeable future. And there is a way.
0:26:35 > 0:26:40The most ancient way of observing the sky at night.
0:26:40 > 0:26:41Astronomy.
0:26:41 > 0:26:44By capturing light from distant star systems,
0:26:44 > 0:26:47using an array of telescopes both on the ground and in orbit,
0:26:47 > 0:26:52we've found 992 exoplanets,
0:26:52 > 0:26:55and we can now begin to characterise those planets,
0:26:55 > 0:26:57to search for signs of life
0:26:57 > 0:27:01encoded in the faint light from these distant worlds.
0:27:01 > 0:27:03So far, one of the best candidates for life
0:27:03 > 0:27:06orbits around one of the stars in this constellation -
0:27:06 > 0:27:08in the constellation of Lyra.
0:27:08 > 0:27:11It's a planet called Kepler 62E,
0:27:11 > 0:27:15after the recently-retired Kepler telescope that first identified it.
0:27:15 > 0:27:20It seems to be just the right size and mass to make it a rocky planet
0:27:20 > 0:27:22and in just the right orbit to give it a chance
0:27:22 > 0:27:25of possessing liquid water on its surface.
0:27:25 > 0:27:28But, remarkably, we can do better
0:27:28 > 0:27:31than simply estimating what these planets are made of.
0:27:31 > 0:27:34See, we're on the verge of being able to look directly
0:27:34 > 0:27:37into the atmospheres of these planets
0:27:37 > 0:27:41and search for the tell-tale fingerprints of life.
0:27:41 > 0:27:44And I'm going to ask Charles Dance to come down
0:27:44 > 0:27:45and help me show you how.
0:27:59 > 0:28:01I've got a coat, have I?
0:28:01 > 0:28:05- Yeah, er, I think its fireproof. - Thank you very much, thank you.
0:28:05 > 0:28:07So, what we're going to do, here...
0:28:07 > 0:28:11You just want me to clean this trolley, don't you, really?
0:28:11 > 0:28:14- Give you a mop!- Yeah. Yes.
0:28:14 > 0:28:16What we're going to do is,
0:28:16 > 0:28:19we're going to demonstrate the technique that astronomers use
0:28:19 > 0:28:22- to identify... - Should I be standing where you are?
0:28:22 > 0:28:25..the presence of chemicals in the atmosphere.
0:28:25 > 0:28:28You can stand wherever you want, it's not going to help you at all.
0:28:30 > 0:28:32So, er... What I'm going to ask you to do is,
0:28:32 > 0:28:36we've got a selection of four chemical elements...
0:28:36 > 0:28:39- Right. - ..dissolved in these solutions,
0:28:39 > 0:28:42and I want you to spray them through the Bunsen flame.
0:28:42 > 0:28:44Which direction would you like me to spray them?
0:28:44 > 0:28:46I think, actually, probably sort of just upwards and...
0:28:46 > 0:28:48- Really, are you sure?- Er, yeah.
0:28:48 > 0:28:49In any particular order?
0:28:49 > 0:28:51No, no, just - let's see what happens.
0:28:51 > 0:28:54So, you can see, apart from this one, they're all colourless liquids,
0:28:54 > 0:28:57- but they've got chemical elements dissolved...- OK.- ..in the solution.
0:28:57 > 0:28:59- All right. - So, let's have a go at that.
0:28:59 > 0:29:01We could dim the lights a bit, actually, perhaps.
0:29:01 > 0:29:04Go on. Spray that one through. Let's see what that does.
0:29:04 > 0:29:06OK.
0:29:06 > 0:29:07There it goes.
0:29:07 > 0:29:09Shall I do that one again?
0:29:09 > 0:29:10Go again. Beautiful green colour.
0:29:10 > 0:29:12Bright green colour.
0:29:12 > 0:29:14So, now let's try that one.
0:29:14 > 0:29:17- Same thing?- Yeah.
0:29:17 > 0:29:19- AUDIENCE: Ooh!- Oh, I like that.
0:29:19 > 0:29:20A bright red.
0:29:20 > 0:29:22This takes me back.
0:29:22 > 0:29:24I know!
0:29:24 > 0:29:25To school chemistry lessons?
0:29:25 > 0:29:29No, to early psychedelic rock concerts.
0:29:29 > 0:29:31APPLAUSE
0:29:36 > 0:29:37I quite like that one, actually.
0:29:37 > 0:29:39- Oh, where's the red? - What if we do two together?
0:29:39 > 0:29:42- Go on, let's do it, let's go for it. - Shall we do three together?
0:29:47 > 0:29:49Oh, dear.
0:29:49 > 0:29:51Thank you very much.
0:29:57 > 0:30:01Now, the reason that those chemical elements behaved in different ways
0:30:01 > 0:30:04is down to the structure of the elements themselves.
0:30:04 > 0:30:07See, what happens when you burn that element,
0:30:07 > 0:30:10when you heat it up, is the electrons jump around
0:30:10 > 0:30:14between different orbits around the atomic nucleus
0:30:14 > 0:30:17and then fall back down again and emit light,
0:30:17 > 0:30:19and so what we're seeing there
0:30:19 > 0:30:22is the structure of the atoms themselves
0:30:22 > 0:30:23that make up the chemical elements.
0:30:23 > 0:30:28Each element will have a different signature of light that it emits
0:30:28 > 0:30:32when heated, because it has a different configuration of electrons
0:30:32 > 0:30:34around the nucleus.
0:30:34 > 0:30:37Now, as well as emitting light when heated,
0:30:37 > 0:30:41elements also absorb light of exactly the same colour
0:30:41 > 0:30:46if they're present in the atmosphere of a star or a planet.
0:30:46 > 0:30:50Here, for example, is a spectrum of light from the sun.
0:30:50 > 0:30:53So, this is sunlight split up into all the colours of the rainbow,
0:30:53 > 0:30:56by a prism, for example.
0:30:56 > 0:30:59And you can see that it is covered in black lines,
0:30:59 > 0:31:01all over, in every colour.
0:31:01 > 0:31:05These are the fingerprints of chemical elements,
0:31:05 > 0:31:08in the same way that we saw Charles show us the beautiful colours,
0:31:08 > 0:31:11the fingerprint of the element in those bottles.
0:31:11 > 0:31:15Now, we're on the verge of launching telescopes and detectors
0:31:15 > 0:31:20so sensitive that we can analyse the light not only from stars,
0:31:20 > 0:31:24like the sun, but also the light reflected and absorbed
0:31:24 > 0:31:27by the atmospheres of planets around those stars.
0:31:27 > 0:31:30This will allow us to look for the fingerprints of molecules
0:31:30 > 0:31:34such as water, methane, and even organic molecules,
0:31:34 > 0:31:39the fingerprints of life in the atmospheres of alien worlds.
0:31:39 > 0:31:43These techniques might prove the first direct evidence
0:31:43 > 0:31:46that we're not alone in the universe.
0:31:46 > 0:31:49But they still won't allow us to resolve Fermi's paradox,
0:31:49 > 0:31:53because these chemical fingerprints won't differentiate
0:31:53 > 0:31:56between simple, single-celled organisms
0:31:56 > 0:32:01and the complex multi-cellular life that is surely a prerequisite
0:32:01 > 0:32:04for the existence of a civilisation like our own.
0:32:04 > 0:32:07But there is just a possibility
0:32:07 > 0:32:11that we can look for signatures of intelligent civilisations.
0:32:11 > 0:32:13See, as a civilisation gets more and more advanced,
0:32:13 > 0:32:16its energy consumption rises dramatically.
0:32:16 > 0:32:19With every new machine we create here on Earth,
0:32:19 > 0:32:23from the tiniest mobile phone to the largest power station,
0:32:23 > 0:32:25we produce more heat.
0:32:25 > 0:32:27I'll show you what I mean.
0:32:27 > 0:32:29Here is an infrared camera.
0:32:29 > 0:32:34So, this is measuring not the light from you, the audience,
0:32:34 > 0:32:35but the heat from the audience,
0:32:35 > 0:32:38because those colours are representing the amount of heat
0:32:38 > 0:32:41that you are putting out.
0:32:41 > 0:32:42Yeah, give us a wave.
0:32:43 > 0:32:47I can see exactly what you're doing at the back.
0:32:48 > 0:32:51That's because you are biological machines.
0:32:51 > 0:32:55Every machine, no matter how sophisticated or efficient,
0:32:55 > 0:32:56must do this.
0:32:56 > 0:33:00It must leave a tell-tale heat signature behind
0:33:00 > 0:33:03as it goes about its business.
0:33:03 > 0:33:07Now, a group of researchers at Penn State University
0:33:07 > 0:33:11are attempting to exploit this fundamental universal law,
0:33:11 > 0:33:14using infrared cameras to search the stars
0:33:14 > 0:33:17and even to search for entire galaxies
0:33:17 > 0:33:20to see if they can see hot spots,
0:33:20 > 0:33:24systems that are giving out more heat in the infrared spectrum
0:33:24 > 0:33:27than you would expect from purely natural processes.
0:33:27 > 0:33:29If they sift through all their data,
0:33:29 > 0:33:33and actually find a star, a planet or even a galaxy
0:33:33 > 0:33:36with this characteristic infrared signature,
0:33:36 > 0:33:39then they could claim evidence, not only for complex life
0:33:39 > 0:33:41but for a machine-building, star-harnessing,
0:33:41 > 0:33:45transgalactic civilisation.
0:33:45 > 0:33:47Doctor Who from afar.
0:33:47 > 0:33:50Far-fetched? Yeah, of course it is.
0:33:50 > 0:33:54But the simple act of looking, of observing nature,
0:33:54 > 0:33:57is the key to science, and we shouldn't take anything for granted.
0:33:57 > 0:34:00And it's worth noting, finally,
0:34:00 > 0:34:04that we may already inadvertently have made contact.
0:34:04 > 0:34:06The first episode of Doctor Who
0:34:06 > 0:34:10was broadcast on the 23rd of November, 1963.
0:34:10 > 0:34:15The programme was encoded in beams of radio waves,
0:34:15 > 0:34:18as beams of light that were broadcast to the nation's TVs.
0:34:18 > 0:34:23These radio waves didn't simply hang around floating above the UK,
0:34:23 > 0:34:25they left our atmosphere,
0:34:25 > 0:34:30expanding in spheres just like the light from Faraday's candle
0:34:30 > 0:34:32and began their journey out into space.
0:34:32 > 0:34:36Today, that signal will have reached 50 light years from this planet.
0:34:36 > 0:34:39SONG: "Doctor Who Theme"
0:34:39 > 0:34:42What would an alien civilisation think
0:34:42 > 0:34:45if their first experience of our civilisation
0:34:45 > 0:34:48was the adventures of the time-travelling doctor?
0:35:02 > 0:35:04Oi, Cox, no.
0:35:04 > 0:35:06Hands off. Complicated.
0:35:06 > 0:35:07Ish.
0:35:07 > 0:35:09Ish?! Hah! Don't you "ish" me.
0:35:09 > 0:35:11Beyond human understanding.
0:35:11 > 0:35:14Relative internal spatial co-ordinates are completely at odds
0:35:14 > 0:35:16with externally observed dimensions. So, nur.
0:35:16 > 0:35:18Bigger on the inside than the outside
0:35:18 > 0:35:20doesn't seem too complicated to me.
0:35:21 > 0:35:24Don't listen to him. Cover your ears.
0:35:24 > 0:35:25Where exactly are your ears?
0:35:25 > 0:35:27Listen, how do you fuel something like this?
0:35:27 > 0:35:29The power requirements must be immense.
0:35:29 > 0:35:31Oh, yeah? Yeah, I use a black hole.
0:35:31 > 0:35:33- A black hole? - Little bit of Time Lord engineering,
0:35:33 > 0:35:36siphon off the energy. Powering this thing is like falling off a log.
0:35:36 > 0:35:38A very big log, an n-dimensional log.
0:35:38 > 0:35:40Read some Einstein.
0:35:40 > 0:35:42The tidal forces on a black hole in there would rip it to bits.
0:35:42 > 0:35:44Hah! Yeah, I know that.
0:35:44 > 0:35:47Nice chap, Einstein. Bow tie wearer. Always gets my vote.
0:35:47 > 0:35:50Wicked hair. But he's behind the times, Coxy.
0:35:50 > 0:35:52You want to see my black hole?
0:35:52 > 0:35:53I keep it down there, in the basement.
0:36:13 > 0:36:16So, the Doctor's world is closer to our own
0:36:16 > 0:36:17than you might have imagined.
0:36:17 > 0:36:19We're all time travellers,
0:36:19 > 0:36:22and we've reached out and touched alien worlds.
0:36:22 > 0:36:27But I'm drawn back to these notes.
0:36:27 > 0:36:30To December 1860, and Michael Faraday's Christmas Lecture
0:36:30 > 0:36:34when he inspired a generation of children to become scientists,
0:36:34 > 0:36:38using the simple but magical candle.
0:36:38 > 0:36:42What about my dream to return to that moment in time?
0:36:42 > 0:36:47So, let's take a look at our map again.
0:36:49 > 0:36:53Now, we have everything in the past that has ever happened down there,
0:36:53 > 0:36:58and we have everything that ever could happen in the future up here.
0:36:58 > 0:37:01The Doctor has complete freedom of movement on the map.
0:37:01 > 0:37:03He can go anywhere.
0:37:03 > 0:37:07But what Einstein realised is that we can't have freedom of movement,
0:37:07 > 0:37:09otherwise we'd run into trouble.
0:37:09 > 0:37:12So, he discovered a limit.
0:37:12 > 0:37:15He built it into his theory.
0:37:15 > 0:37:17Something that we can all agree on.
0:37:17 > 0:37:19The speed of light.
0:37:19 > 0:37:23Let's think about Faraday's candle again.
0:37:23 > 0:37:27If there wasn't a roof on this lecture theatre,
0:37:27 > 0:37:30then this would be sending out light into the universe.
0:37:30 > 0:37:33An expanding sphere of light travelling outwards
0:37:33 > 0:37:37at 300,000 kilometres per second.
0:37:37 > 0:37:41In one and a half seconds it would have passed by the moon.
0:37:41 > 0:37:44In eight minutes it would speed past the sun,
0:37:44 > 0:37:47and in around 100,000 years,
0:37:47 > 0:37:51it would completely clear the Milky Way Galaxy.
0:37:51 > 0:37:55Now, I can draw this onto my map.
0:37:55 > 0:37:59So, this is here and now in this lecture theatre
0:37:59 > 0:38:01at the Royal Institution.
0:38:01 > 0:38:07So, I can draw a line on my map that represents the trajectory
0:38:07 > 0:38:10of a beam of light through space-time.
0:38:10 > 0:38:13Of course it expands in all directions,
0:38:13 > 0:38:16so I have another one of those lines going out there.
0:38:16 > 0:38:18A pair of diagonal lines.
0:38:18 > 0:38:21Now, I could also draw lines on this map
0:38:21 > 0:38:25which represent the paths of beams of light from the past,
0:38:25 > 0:38:29if they arrived here, now, in this lecture theatre.
0:38:29 > 0:38:31And here they'll be.
0:38:31 > 0:38:35They'll look the same, but they'll extend out into the past.
0:38:35 > 0:38:38Now, we all agree on these lines
0:38:38 > 0:38:41because we all agree on the speed of light,
0:38:41 > 0:38:46so they must be important in some way. And they are.
0:38:46 > 0:38:50This is how Einstein protects the past from the future.
0:38:50 > 0:38:54They limit how we can move around on the map,
0:38:54 > 0:38:58because nothing can travel faster than the speed of light.
0:38:58 > 0:39:01It is a universal speed limit.
0:39:01 > 0:39:03What does that mean?
0:39:03 > 0:39:08Well, imagine that there is someone sat here, let's say,
0:39:08 > 0:39:10with a telescope.
0:39:10 > 0:39:11If I wanted some signal,
0:39:11 > 0:39:14some flash of light to get out to that event there,
0:39:14 > 0:39:17which would be, let's say, an alien in some distant galaxy,
0:39:17 > 0:39:19taking a telescope out and looking at us,
0:39:19 > 0:39:22then it would have to travel - the influence, the light -
0:39:22 > 0:39:25would have to travel faster than the speed of light.
0:39:25 > 0:39:26It can't happen.
0:39:26 > 0:39:31So, this line seems to restrict the movement of things.
0:39:31 > 0:39:33Things that travel slower than light
0:39:33 > 0:39:38are condemned to live inside this area.
0:39:38 > 0:39:41This area is clearly important, and it's got a name.
0:39:41 > 0:39:43It's called the future light cone.
0:39:43 > 0:39:46That encompasses all of our futures.
0:39:46 > 0:39:50Every event that's going to happen to any of us in this audience
0:39:50 > 0:39:54or watching at home, that happens, will happen in this region
0:39:54 > 0:39:57of space-time inside the future light cone.
0:39:57 > 0:39:59It also applies to the past.
0:39:59 > 0:40:01So, this is a special region.
0:40:01 > 0:40:05It's called our past light cone.
0:40:05 > 0:40:09This is the region that contains events in space and time
0:40:09 > 0:40:13that could in principle have influenced us now,
0:40:13 > 0:40:17at this point, here, tonight.
0:40:17 > 0:40:21This is the geometry of space-time as described by Einstein
0:40:21 > 0:40:25in his theory of special relativity that he published in 1905.
0:40:25 > 0:40:30It allows me to trace my life through these two regions.
0:40:30 > 0:40:34I can locate any event that happened in my life on this map.
0:40:34 > 0:40:41So, I was born on March the 3rd 1968,
0:40:41 > 0:40:45and the first picture I have of me at Christmas
0:40:45 > 0:40:48was actually 1972 in Oldham.
0:40:48 > 0:40:54There I am, that's that event. It's me in Oldham in Christmas 1972.
0:40:54 > 0:40:57Now, there are lots of things that happened to me.
0:40:57 > 0:41:00I've got a very embarrassing picture actually in 1989...
0:41:03 > 0:41:07What was I thinking?
0:41:07 > 0:41:09I-I...
0:41:09 > 0:41:10The kind of lifestyle I had.
0:41:10 > 0:41:13That was actually when I was on tour with a rock band somewhere,
0:41:13 > 0:41:16I think I was somewhere in Europe. So it could have been... Actually...
0:41:16 > 0:41:19Oh, where shall I put myself? Over there, that would be 1989.
0:41:19 > 0:41:22That's another event, me on a tour bus,
0:41:22 > 0:41:27um, drinking sensibly in Europe in 1989.
0:41:27 > 0:41:28And so on.
0:41:28 > 0:41:34So, my life is a series of events that I can plot on this diagram.
0:41:34 > 0:41:38I'm now here, of course, the Royal Institution in 2013.
0:41:38 > 0:41:42So, we could imagine plotting every event in my life on this diagram.
0:41:42 > 0:41:48That would make a line, and it's a line known as a world line.
0:41:48 > 0:41:53And it can wander around in space, cos I've been at different places,
0:41:53 > 0:41:55and, of course, it wanders around in time
0:41:55 > 0:42:01from 1968 to 2013 there.
0:42:01 > 0:42:05And, of course, Faraday's Christmas Lecture on the candle,
0:42:05 > 0:42:08the event I most want to visit in space-time,
0:42:08 > 0:42:14is also sitting somewhere down here in my past light cone.
0:42:14 > 0:42:15It's there.
0:42:15 > 0:42:18Christmas 1860.
0:42:18 > 0:42:20Why is it in my past light cone?
0:42:20 > 0:42:23It has to be because it's influenced me.
0:42:23 > 0:42:26These lecture notes were present at that event
0:42:26 > 0:42:29when Faraday stood here and delivered his lecture,
0:42:29 > 0:42:31and they're present in front of me now.
0:42:31 > 0:42:36So, I could draw the world line at that note book on this diagram.
0:42:36 > 0:42:38And they've stayed in the Royal Institution,
0:42:38 > 0:42:41the same place in space, pretty much their whole life,
0:42:41 > 0:42:48because they began in 1860 and they're here now with me in 2013.
0:42:48 > 0:42:51But according to Einstein's Theory of Special Relativity,
0:42:51 > 0:42:56I can never visit Faraday, because my future world line,
0:42:56 > 0:42:58the things I can experience,
0:42:58 > 0:43:04is restricted to stay inside the future light cone.
0:43:04 > 0:43:09To get out, to escape into the past, what would I have to do?
0:43:09 > 0:43:11Well, the first thing I'd have to do
0:43:11 > 0:43:15is travel faster than the speed of light,
0:43:15 > 0:43:19even before I begin to consider how I could possibly do that
0:43:19 > 0:43:23and loop round to 1860, and the universe isn't built that way.
0:43:23 > 0:43:27The doors to the past, unless we have a TARDIS,
0:43:27 > 0:43:30appear to be firmly closed.
0:43:30 > 0:43:32What if there's another way?
0:43:32 > 0:43:38What if I can change the direction of my future light cone,
0:43:38 > 0:43:41change the direction of my entire future,
0:43:41 > 0:43:46and perhaps begin to tilt it towards the past?
0:43:46 > 0:43:49Well, there are objects in our universe
0:43:49 > 0:43:53that can tilt light cones, and if I could get close enough
0:43:53 > 0:43:57they'd affect the direction of my future in a radical way.
0:43:57 > 0:44:01There's one at the heart of the TARDIS, a black hole.
0:44:01 > 0:44:06The Eye of Harmony is described in Doctor Who as a star,
0:44:06 > 0:44:10frozen at the point of collapse into a black hole.
0:44:10 > 0:44:16It's a poetic line, but unusually, it has to be said, for poetry,
0:44:16 > 0:44:20this one turns out to be physically accurate.
0:44:20 > 0:44:22Black holes form at the end of the lives
0:44:22 > 0:44:25of the most massive stars in the universe.
0:44:25 > 0:44:29When such stars, at least 20 times the mass of our sun,
0:44:29 > 0:44:31run out of fuel in their cores,
0:44:31 > 0:44:34no known force can overcome the inward pull of gravity
0:44:34 > 0:44:36and prevent them from collapsing,
0:44:36 > 0:44:41as far as anyone knows, to a single, infinitely dense point
0:44:41 > 0:44:43known as a singularity.
0:44:43 > 0:44:47I can draw one of those on a space-time diagram.
0:44:47 > 0:44:51So here is space, and here is time.
0:44:51 > 0:44:55And this is a diagram from the point of view of the black hole,
0:44:55 > 0:45:00so that's the singularity ticking forward in time, as it were.
0:45:00 > 0:45:03And these two lines, which are very important,
0:45:03 > 0:45:08have the evocative names of event horizons.
0:45:08 > 0:45:11These mark out a region in space and time
0:45:11 > 0:45:14where the gravitational pull is so strong
0:45:14 > 0:45:17that light itself cannot escape.
0:45:17 > 0:45:21In the vicinity of the event horizon very strange things happen.
0:45:21 > 0:45:25And I need a very strange volunteer to demonstrate that.
0:45:25 > 0:45:28So, Rufus Hound, where are you?
0:45:35 > 0:45:37That was perhaps a little unkind, wasn't it?
0:45:37 > 0:45:39- "A very strange volunteer." - No, it seems about right.
0:45:39 > 0:45:41- Is it about right?- Yeah.
0:45:41 > 0:45:43- Um, so... - I thought that with my fourth brain.
0:45:43 > 0:45:45Did you?
0:45:45 > 0:45:49What I'd like to do is to throw you into a black hole.
0:45:49 > 0:45:51You wouldn't be the first.
0:45:51 > 0:45:54- In the name of physics, now... - You would be the first.
0:45:54 > 0:45:55I think it's going to mean
0:45:55 > 0:45:58that you're going to meet a very noble end,
0:45:58 > 0:46:01a very wonderful exit from this universe.
0:46:01 > 0:46:05But in order to observe you as you exit our plane of existence,
0:46:05 > 0:46:07- as it were, I want to kit you out with two watches.- OK.
0:46:07 > 0:46:10This one, which I want you to put on your back,
0:46:10 > 0:46:15is going to be the one that we can observe.
0:46:16 > 0:46:18All right, there we are. Sorry.
0:46:18 > 0:46:21Is this how you're going to collapse my mass?
0:46:21 > 0:46:23Is that a bit... Is that comfortable?
0:46:23 > 0:46:26You're going to do the straps up, is that how black holes work?
0:46:26 > 0:46:28Just some bloke with a really tight backpack on.
0:46:28 > 0:46:30There we go.
0:46:37 > 0:46:39I already feel implosiony.
0:46:41 > 0:46:43And I'd like to give you - well, actually, have you got a watch?
0:46:43 > 0:46:45- I've got a watch. - Oh, you've got a watch.
0:46:45 > 0:46:47- And there's a second hand ticking away.- Yep.
0:46:47 > 0:46:48That's good.
0:46:48 > 0:46:52Right, so, what we're going to do, is we're going to...
0:46:52 > 0:46:55Right - it's low voltage, it's all right.
0:46:55 > 0:46:58- I'm going to turn it... - Where are my safety goggles, Brian?
0:46:58 > 0:47:01If you just turn round...
0:47:01 > 0:47:04If it'll make you feel better I can get some, but it won't help.
0:47:04 > 0:47:05No. Great, fine.
0:47:05 > 0:47:08If you turn around so we can see this clock,
0:47:08 > 0:47:11and I'm going to turn the clock on, and there it is.
0:47:11 > 0:47:13So it's whizzing forward in time.
0:47:13 > 0:47:15And I want you to face the blackboard, the Eye of Harmony,
0:47:15 > 0:47:17that's the black hole, there.
0:47:17 > 0:47:19And what I've done is, I've speeded time up
0:47:19 > 0:47:21just so we can see it ticking along.
0:47:21 > 0:47:24This is the rate that time's passing for us now,
0:47:24 > 0:47:26- and it would be the same on your watch here.- Right.
0:47:26 > 0:47:30And I'm going to ask you to move slowly towards the event horizon.
0:47:30 > 0:47:32Very slowly.
0:47:35 > 0:47:36That's it.
0:47:37 > 0:47:39How do you feel?
0:47:39 > 0:47:41Like this is slightly TOO slow.
0:47:44 > 0:47:45It's all right.
0:47:45 > 0:47:47But you see what's happening.
0:47:47 > 0:47:51If you stop there, you're approaching the event horizon,
0:47:51 > 0:47:55and time on the watch that we're looking at, attached to your back,
0:47:55 > 0:47:57is slowing down. How's the time, though, on your watch?
0:47:57 > 0:47:58Exactly the same.
0:47:58 > 0:48:03It's ticking along at exactly the same rate.
0:48:03 > 0:48:05Now, you might start to feel a bit uncomfortable
0:48:05 > 0:48:09because for these sort of stellar mass black holes,
0:48:09 > 0:48:13the gravitational force on your feet would now be significantly stronger
0:48:13 > 0:48:15than the gravitation force on your head.
0:48:15 > 0:48:17Now, this is called spaghettification.
0:48:20 > 0:48:22Why?
0:48:22 > 0:48:25- So, you're beginning to get slightly taller.- Right.
0:48:25 > 0:48:27And eventually, actually, as you approach the event horizon
0:48:27 > 0:48:29I think, really, you'd get so tall
0:48:29 > 0:48:32that you'd just be a long line of atoms, disassociated.
0:48:32 > 0:48:35But anyway, let's ignore that for the moment. Carry on.
0:48:37 > 0:48:40- So, you see...- I don't know why I feel slightly in awe of a picture.
0:48:42 > 0:48:44Right towards the black hole.
0:48:44 > 0:48:47And what we see - there, stop.
0:48:47 > 0:48:49That is on the event horizon
0:48:49 > 0:48:54and we would see Rufus' watch, strapped to his back, freeze.
0:48:54 > 0:48:56It would stop, but what does your watch look like?
0:48:56 > 0:48:57Still going.
0:48:57 > 0:49:00Still going at exactly the same rate.
0:49:00 > 0:49:03This is precisely what Einstein tells us would happen
0:49:03 > 0:49:06as Rufus fell into the black hole.
0:49:06 > 0:49:08We'd see time freeze.
0:49:08 > 0:49:12We would see an image of Rufus just like that, actually,
0:49:12 > 0:49:14that's quite powerful.
0:49:14 > 0:49:16How long can you stand on one leg, just like that?
0:49:16 > 0:49:20We'd see a frozen image of Rufus on his way across the event horizon.
0:49:20 > 0:49:23Time would stop, that image would still be there.
0:49:23 > 0:49:25It would be a sort of immortality,
0:49:25 > 0:49:28whereas from Rufus' perspective, time would pass as normal,
0:49:28 > 0:49:30he would pass over the event horizon,
0:49:30 > 0:49:32he would approach the singularity
0:49:32 > 0:49:36and be crushed to an infinitely dense point.
0:49:36 > 0:49:37Thank you.
0:49:48 > 0:49:50Thanks Rufus.
0:49:50 > 0:49:52Um, let me explain what happened to Rufus.
0:49:52 > 0:49:56So here is my space-time diagram again.
0:49:56 > 0:49:59Remember that the black hole is sat here, stationary.
0:49:59 > 0:50:02There's the singularity, here are the event horizons.
0:50:02 > 0:50:05And what I'm going to do is,
0:50:05 > 0:50:10I'm going to superimpose Rufus' world line...
0:50:12 > 0:50:15..onto this diagram.
0:50:16 > 0:50:18Now, we're looking at Rufus, remember,
0:50:18 > 0:50:20from the point of view of the black hole.
0:50:20 > 0:50:23So it's just sat there, it's going nowhere,
0:50:23 > 0:50:26and Rufus is on a journey towards the event horizon
0:50:26 > 0:50:29and beyond into oblivion.
0:50:29 > 0:50:33What I've also drawn are Rufus' light cones,
0:50:33 > 0:50:35the various points along his world line.
0:50:35 > 0:50:40These mark out Rufus' accessible future.
0:50:40 > 0:50:43But look what happens to these light cones
0:50:43 > 0:50:46as he approaches the event horizon.
0:50:46 > 0:50:47They're tilting.
0:50:47 > 0:50:51Now, this tilt, according to Albert Einstein,
0:50:51 > 0:50:55is caused by the mass of the black hole itself.
0:50:55 > 0:50:57It's a representation of a central idea
0:50:57 > 0:51:01in Einstein's theory of gravity, general relativity.
0:51:01 > 0:51:06The idea is this - mass and energy curve space and time,
0:51:06 > 0:51:10the very fabric of the universe itself.
0:51:10 > 0:51:13That curvature, the warping of space and time, if you like,
0:51:13 > 0:51:18is what we're seeing in this diagram as the tilting of light cones,
0:51:18 > 0:51:23the tilting of Rufus' future towards the event horizon.
0:51:23 > 0:51:25And look what happens here on the horizon.
0:51:25 > 0:51:28You see what's happened to the light cone?
0:51:28 > 0:51:33It's tilted so much, space and time are curved and warped so much,
0:51:33 > 0:51:38that all of Rufus' future is pointing inwards,
0:51:38 > 0:51:41into the horizon, into the black hole.
0:51:41 > 0:51:44His world line is heading towards the singularity.
0:51:44 > 0:51:46There's no escape for Rufus
0:51:46 > 0:51:51because his entire future is inside the black hole.
0:51:51 > 0:51:55He'd have to travel faster than light to get out,
0:51:55 > 0:51:58and that is not allowed in our universe.
0:51:58 > 0:52:00This diagram is very beautiful.
0:52:00 > 0:52:02It allows us to see something else,
0:52:02 > 0:52:05it also allows us to see what happened to Rufus' clock
0:52:05 > 0:52:09as we watched it tick slower and slower and slower
0:52:09 > 0:52:11as he approached the horizon.
0:52:11 > 0:52:13So, let's imagine...
0:52:13 > 0:52:17let's imagine that on each tick of Rufus' clock,
0:52:17 > 0:52:20the one on his back, a pulse of light was sent out
0:52:20 > 0:52:24and we detected that pulse of light from our vantage point
0:52:24 > 0:52:25far away from the black hole.
0:52:27 > 0:52:28So, let me put them on.
0:52:30 > 0:52:31There.
0:52:33 > 0:52:34You see what happens.
0:52:34 > 0:52:37As the light cones pulse,
0:52:37 > 0:52:42then those pulses of light arrive at us at later and later times.
0:52:42 > 0:52:44This is the ticking of the clock.
0:52:44 > 0:52:48As far as Rufus is concerned, the clock's ticking away normally,
0:52:48 > 0:52:51one second, two seconds, three seconds, four seconds.
0:52:51 > 0:52:56But, as we see it, the first second is faster than the second second,
0:52:56 > 0:52:59which is faster than the third second.
0:52:59 > 0:53:00Tick...
0:53:00 > 0:53:02tick...
0:53:03 > 0:53:06..tick. And here, on the horizon,
0:53:06 > 0:53:10the light pulse goes flying up the side of the light cone,
0:53:10 > 0:53:13which is aligned along the event horizon itself.
0:53:13 > 0:53:19This pulse never reaches us, so time stops from our perspective.
0:53:19 > 0:53:22We see that frozen image of Rufus.
0:53:22 > 0:53:26He never makes it across the horizon from our vantage point.
0:53:26 > 0:53:30According to him everything proceeds quite normally -
0:53:30 > 0:53:33although he's getting spaghettified, it has to be said -
0:53:33 > 0:53:36until he gets squashed on the singularity.
0:53:36 > 0:53:40This image of Rufus is frozen forever at the horizon.
0:53:40 > 0:53:43But here's the wonderful thing -
0:53:43 > 0:53:48the same is true for the collapsing star itself.
0:53:48 > 0:53:51See, from the perspective of an outside observer,
0:53:51 > 0:53:56time stops, so we'd never actually see the star collapse,
0:53:56 > 0:54:00we'd see a frozen image fading away
0:54:00 > 0:54:05of the dying star forever frozen in time at the moment of collapse,
0:54:05 > 0:54:11that is precisely the Eye of Harmony as described in Doctor Who.
0:54:11 > 0:54:13How beautiful.
0:54:13 > 0:54:16But what of my ambition to get back into the past
0:54:16 > 0:54:20and experience Michael Faraday deliver his lecturer?
0:54:20 > 0:54:24Well, everything I've spoken about so far in this lecture
0:54:24 > 0:54:28is science fact, including this description of a frozen star.
0:54:30 > 0:54:34But now it's time to speculate just a little,
0:54:34 > 0:54:38but still remain constrained by the known laws of physics.
0:54:38 > 0:54:41Notice what the Eye of Harmony, the black hole, did.
0:54:41 > 0:54:45It tilted light cones, it changed the direction
0:54:45 > 0:54:48of the accessible future in space-time.
0:54:48 > 0:54:53Now, could it be that we could dream up some geometry of space-time,
0:54:53 > 0:54:57a distribution of matter and energy that would tilt light cones
0:54:57 > 0:54:59all the way around?
0:55:01 > 0:55:05What I want to do is tilt my future light cone
0:55:05 > 0:55:10in such a way that it gets me back to Faraday's Christmas Lecture
0:55:10 > 0:55:12in 1860.
0:55:14 > 0:55:16Something like this.
0:55:17 > 0:55:19So, here...
0:55:19 > 0:55:21is a piece of space-time.
0:55:21 > 0:55:25It's meant to map directly onto this diagram I drew here.
0:55:26 > 0:55:32Here's 1860, and here's me in 2013.
0:55:32 > 0:55:38Now, we've seen that a black hole can tilt light cones like that.
0:55:38 > 0:55:44What if we could arrange the geometry so that the light cone
0:55:44 > 0:55:50tilts around, so it bends in some way
0:55:50 > 0:55:53so that...
0:55:53 > 0:55:58I can reattach space-time, as it were, around into the past?
0:55:58 > 0:56:02I could curve space-time in such a way that this area,
0:56:02 > 0:56:07my accessible future, ends up pointing into my own past
0:56:07 > 0:56:12and specifically, in this case, ends up pointing to this place,
0:56:12 > 0:56:17this event I want to visit, Faraday's lecture in 1860.
0:56:17 > 0:56:21Could we design some configuration of matter and energy
0:56:21 > 0:56:24that would curve the light cones around
0:56:24 > 0:56:28so I could get back into my own past?
0:56:28 > 0:56:30The answer is...
0:56:30 > 0:56:32we don't know.
0:56:34 > 0:56:41But nobody has been able to prove that space-time geometries
0:56:41 > 0:56:45similar to this cannot exist, at least in principle.
0:56:45 > 0:56:52Although most experts believe that they must in some way be forbidden.
0:56:52 > 0:56:55But there's still the faintest possibility,
0:56:55 > 0:56:58given the laws of physics as we understand them today,
0:56:58 > 0:57:03that someone, someday - maybe a young girl or a young boy -
0:57:03 > 0:57:05will be inspired to try.
0:57:05 > 0:57:10And, even if they fail, by the very act of trying
0:57:10 > 0:57:14they might just go on to change the world.
0:57:14 > 0:57:16APPLAUSE
0:57:27 > 0:57:28Home!
0:57:28 > 0:57:30Oh, I want to visit more alien worlds.
0:57:30 > 0:57:32No. Greedy, Brian. Can't be greedy.
0:57:32 > 0:57:35You've got a lecture to give, people to inspire, merchandise to sell.
0:57:35 > 0:57:37Actually, that reminds me, could you rustle me up a lunchbox?
0:57:37 > 0:57:39Maybe a T-shirt, slim-fitting.
0:57:39 > 0:57:42Oh, don't forget the gift I got you, you'll need that.
0:57:42 > 0:57:43So, what was this all about, then,
0:57:43 > 0:57:46just taking me on a tour of the wonders of the universe?
0:57:46 > 0:57:47Ah!
0:57:47 > 0:57:51Well, there's someone in your audience today,
0:57:51 > 0:57:53just an ordinary kid, so high, sad eyes, look out for her,
0:57:53 > 0:57:56someone who loves to think about why the sky is blue
0:57:56 > 0:57:58and how bees can hover like helicopters,
0:57:58 > 0:58:03but after today she stops being ordinary,
0:58:03 > 0:58:07she grows up to be extraordinary, a woman who changes the world.
0:58:09 > 0:58:12And all she needed was a nudge from you, eh? Today, right now.
0:58:12 > 0:58:13No pressure.
0:58:13 > 0:58:15I do love humans.
0:58:15 > 0:58:18They can be a bit defeatist. You know, "Mustn't," "Can't"...
0:58:18 > 0:58:21Sometimes you just need a helping hand.
0:58:21 > 0:58:24Every adventure starts with a moment, a spark. Ooh!
0:58:24 > 0:58:26Whilst I'm here...
0:58:26 > 0:58:29Bit of anti-shine. You'll need that.
0:58:29 > 0:58:30Ah.
0:58:30 > 0:58:32Don't forget to twiddle the size of the event horizon.
0:58:32 > 0:58:34Shut up, Brian.
0:58:40 > 0:58:42One more adventure before tea.
0:58:49 > 0:58:50SONG: "Doctor Who Theme"
0:58:53 > 0:58:56Subtitles by Red Bee Media Ltd