The Science of Doctor Who

WHISTLING

Ah, excellent, there you are.

# Things can only get better... #

You're a bit late. Or early.

Possibly both, hard to keep track of time, Prof.

Or shall we just go with "Bri"?

What is this place? Amazing.

Hey, come on, man, be cool. You're supposed to be a physicist.

And put a tie on.

You're not the make-up artist.

-Sharp. I can see why you got that fellowship.

-Where am I?

Bit complicated.

Sort of a spaceship/time machine/ swimming pool.

Optional hat stand. I need five minutes of your time,

and when I say five minutes, I'm lying.

-I'm just going to give a lecture.

-I know, I've just seen it.

-It's great.

-But I haven't given it yet.

Tricky to explain - seen it anyway.

You've seen it and you think it's great?

Did I say great? I meant lousy.

You need a spot of help with that. That's why I'm here.

I've bought you a gift.

Actually, I say bought. More like pinched.

But it's the thought that counts. I couldn't find any paper.

Is this what I think it is?

Yeah. Unless you think it's a hat, or a banana,

in which case Manchester Uni needs a re-boot.

-This is over 250 years old.

-About a week old, actually.

I picked it up last Saturday tea-time.

No, no, no, that is impossible

Naughty word, Brian.

2p in the swear box, please.

Space and time. Time and space.

Locked in an intricate dance across the cosmos,

and if you know the tune... anything is possible.

I was going for poetry. Forgot you were a physicist.

Right, hold on to something. Probably your sanity. Ready?

Usually it just twirls around.

-It's probably this.

-Shut up, Brian.

SONG: "Doctor Who Theme"

APPLAUSE

I have absolutely no idea what Mr Cox has in store for us tonight.

He's an enigma. It could be anything.

He might just start singing, for all I know.

It might also be something to do with science.

It might be something to do with time travel.

I've no idea.

And you're going to have a crash helmet on

to protect your beautiful shiny head.

I'm looking to Brian to prove that everything that happens

-in Doctor Who could actually happen.

-Is cast iron fact.

All right, there we are. Sorry.

Is this how you're going to collapse my mass?

LAUGHTER

Just some bloke with a really tight backpack on.

Well, knowing Brian, it will be mind-blowing.

Hopefully we'll all go home knowing how to make a TARDIS.

I'm really looking forward to it, to see what he's got in store for us.

CLOCK TICKS

If I could borrow the TARDIS just for one day,

of all the places I would travel through space and time,

I'd choose here on the 27th of December, 1860.

On that day, Michael Faraday stood on this stage

and delivered his Royal Institution Christmas Lecture

on the chemical history of the candle.

These are his original lecture notes,

singed by the burning candle he used to illuminate them

during the dark winter nights of Victorian London.

Faraday was one of the greatest scientists in the world.

He laid the foundations of our modern understanding

of electricity and magnetism, but the route he took

through time and space to change the world was unusual.

The son of a Yorkshire blacksmith,

he left school at 13 to become an apprentice bookbinder.

An ordinary young man, but someone who loved to think.

He was curious about the world.

His life changed in 1812 when he attended a series of lectures

by another of the great scientific ghosts that haunt this place

on Albemarle Street in London - Humphry Davy,

the charismatic professor of chemistry at the Royal Institution

and a passionate believer in the power and possibilities of science.

Faraday diligently transcribed the lectures and gave them to Davy,

who was so impressed that he appointed the young man

as his scientific assistant.

The rest, as they say, is history.

So this building, this lecture theatre, has a past

that is inextricably bound up with our present and our future,

not only through the great discoveries

that have shaped our scientific civilisation,

but also through the countless generations

of children and adults alike who have been inspired

by lectures given in this theatre to explore nature

and, to echo Humphry Davy, to find new worlds to conquer.

Tonight, I want to explore if, just like the Doctor,

if we could one day conquer time, allowing me to travel to that night

in this room at Christmas 1860.

Will that be forever impossible?

Are the doors to the past firmly closed?

Well, this object is known unromantically as H4.

It's a maritime timekeeper built over 250 years ago by John Harrison.

At the time of its design,

this was the most accurate portable means of telling the time

ever invented.

It was built to navigate, to map the world.

Listen, it still works.

H4 TICKS

Can you hear that?

Beautiful thing.

Time, as the Doctor knows, is the key to exploration.

If you divide our planet into strips by lines of longitude,

marching east to west, then, for every 15 degrees you travel,

noon, that's the time that the sun reaches its highest point

in the sky, shifts by one hour.

So, if you have a clock that keeps perfect time,

synchronised, let's say, with noon at Greenwich, which is here,

then you can simply read off your longitude.

For example, if I left Greenwich with my H4 and travel west

on my ship, then H4 would read 5pm Greenwich Mean Time

when the sun is directly overhead at my position.

Then I know that I'm at 15 times 5, equals 75 degrees west of Greenwich,

which is roughly on the line here that passes through New York City.

Harrison's H4 was the first watch that could keep time near-perfectly

through the rigours of an ocean voyage.

It changed the fortunes of Britain,

and it changed the fortunes of the world.

With the help of the design of this watch,

the Earth was systematically explored and mapped.

Trade and travel were no longer a lottery.

We knew, for the first time, our place on the surface of our planet.

So time can be used to determine our position in space.

But space and time still feel as if they're separate things.

Time marches inexorably on,

marked out for 250 years by the relentless ticking of H4.

This is not the world the Doctor inhabits.

He has freedom of movement through space and time,

and, surprisingly, it's not the world we inhabit either.

I'm going to show you that we too are ALMOST free

to wander through time.

During the late years of the 19th century, physicists,

and in particular Albert Einstein,

were forced to re-examine our intuitive picture of space and time,

and halfway through the 20th century, Einstein's colleague

and tutor Hermann Minkowski was compelled to write

his now-famous obituary for the simple tick-tock of the clock.

"From henceforth, space by itself and time by itself

"have vanished into the merest shadows

"and only a kind of blend of the two exists in its own right."

I'm tempted to say, as the Doctor would, "Wibbly-wobbly, timey-wimey"

But I won't.

HE CHUCKLES

I just did.

What could Minkowski have meant? Well, let me draw a map.

A map like no other.

It's a map that contains the entirety of the known universe.

Our past, our present and our future.

So, this line, here...

..represents space.

This dot represents our place in space,

here, at the Royal Institution.

Now, let me add time to the map.

So, this is our future.

This is all the time that is yet to come, if you like,

and this is our past.

Now, the dot represents what physicists call an event.

That's this lecture room, our place in space,

tonight, our place in time.

So this is a strange kind of map.

It's a map of infinite size,

and this line, space, here, represents our now.

The Doctor has complete freedom of movement around this map

in the TARDIS.

He can visit any event he likes -

any place in space, any place in time.

Now, we, of course, don't have that freedom,

although, as we'll see, we have more freedom than you might think.

Now, let me explain what I mean by that cryptic statement.

Let's travel back down the timeline.

Let's travel back to, let's say, 1830,

a point in the past.

Same place, here in Albemarle Street, different time.

Our event is Michael Faraday conducting experiments

in his laboratory, just a few feet away from us here.

And I need a volunteer to re-create one of his experiments,

and, because I can, I'm going to choose Dallas Campbell.

Where's Dallas?

-Thank you, Dallas.

-Thank you.

-I know you're a man of science...

-Well, yes...

-..and engineering.

-I try, I try.

So, if we wheel this experiment forward...

what I want you to do is re-create one of Faraday's famous experiments.

There's a bit of danger involved.

I-I thought there must be I was, expecting it.

-These will save you in the event of, er, an explosion.

-OK.

It looks quite Doctor Who.

It does look quite Doctor Who, you're right.

What it is, is a series of coils of wire.

So, I've got coil of wire, coil of wire, coil of wire.

-And on this pole there are a series of magnets.

-Yes.

-We've got magnets...

-Yes.

-..inside a coil of wire.

And to make it more televisual, an explosive, of course.

So, what I'm going to do is move over here.

What I'm going to do is ask you

to move the magnets in and out of the coils of wire.

Now, you may need to be relatively vigorous.

Stand back, stand back, here we go. Ready?

LAUGHTER AND APPLAUSE

And that's how science works.

-OK, was I too vigorous?

-Yeah.

Years of practice.

Yes, exactly.

OK, here we go. I can't do this.

You can.

You're turning the lights on.

-Hooray.

-Keep going. Come on, Dallas.

Oh!

CHEERING

A spectacular demonstration.

So, just by moving these magnets in and out of coiled wire,

Dallas created electricity, enough to light up two light bulbs

and ignite that electric match.

So, what does this mean?

Well, the answer is that electricity and magnetism

are in some way linked.

Now, Faraday and his colleagues were intrigued.

How can a moving magnet,

which seems physically unconnected to the electric wire,

cause an electric current to flow?

Well, the elegant answer was provided in 1861,

30 years after Faraday's experiments,

by the great Scottish physicist, James Clerk Maxwell.

These...

..are Maxwell's wave equations.

Now, Maxwell's genius was to discover these equations

by bringing the whole of electricity and magnetism together

into a single framework.

They describe electric and magnetic fields.

This is the electric field here

and this is the magnetic field here.

But they described fields driving themselves through space as waves.

Electromagnetic waves.

Now, in itself this is a remarkable thing, a fascinating discovery.

But even more remarkable is the prediction from Maxwell's equations

that these waves travel at a very particular speed.

Now, the speed enters as the ratio

of the strengths of the electric and magnetic forces.

It was something he'd seen before.

A number measured as far back as 1676 by an astronomer called Romer.

It was, magically, the speed of light.

This is a tremendous discovery.

Maxwell had found an explanation for the nature of light itself.

Light is a wave, electric and magnetic fields,

sloshing energy between them and propelling themselves through space

at this specific speed.

Very beautiful. But puzzling as well,

because the speed of light appears in Maxwell's equations

as a constant.

It is always, in modern units,

precisely 299,792,458 metres per second.

The crucial point is that Maxwell's equations don't say that this speed

is measured in relation to something.

They're not measured relative to anything at all.

It just states the speed of light, of electrometric waves,

is 299,792,458 metres per second.

Everybody!

LAUGHTER

Just feels like I should say that!

299,792,458 metres per second.

That is stranger than it sounds.

To understand the consequences of this,

let's return to the beautiful timepiece, the H4,

which usually resides, actually, at the Royal Observatory in Greenwich.

Is it moving?

Well, easy. No, it isn't.

Except of course, that it IS moving, after a fashion.

Remember, the Earth is spinning on its axis.

It's about 650mph at this latitude.

So, as well as being stationary, relative to this lecture theatre,

the clock is also moving at 650mph

relative to, let's say, the Doctor in his TARDIS,

looking down on the Earth from space.

So, for the watch and everything else, speed is relative.

The watch is stationary, relative to this lecture theatre,

but according to Maxwell, light doesn't play by these rules.

Instead, everyone will measure the speed of light to be the same.

This is a profoundly strange concept.

This is the way the universe is built, and it has consequences.

Now, to explore these, I need a professor of physics,

so I'm going to choose Jim Al-Khalili.

Where are you Jim?

Now, Einstein did famous experiments.

He used to do things called thought experiments,

and we're going to re-create one of those tonight.

He also had very good hair.

Not as good as yours, but he had very good hair.

Anyone with any hair has very good hair, as far as I'm concerned.

I get these jokes in before Brian does.

I wouldn't dare to comment.

You're going to get me to do something silly, aren't you?

-Please have a seat. Yes.

-Right.

-Notice that I'm in control of this.

-You're on a wheeled contraption.

-Yes, yes.

And you're going to have a crash helmet on.

Jolly good.

To protect your beautiful shiny head.

LAUGHTER

There we are.

Now Jim's going to help me demonstrate

one of Einstein's most famous thought experiments.

This will vividly illustrate

the consequences of taking Maxwell's equations,

Maxwell's constant speed of light, seriously.

I should explain what's happened. It's not just for fun, this.

So, what Jim's got on his head is a video camera.

That's why he's got this crash helmet on.

So that's going to enable us to see the world from Jim's perspective -

and we, of course, are looking at Jim,

so we can see the world from our perspective.

Now, Einstein's light clock thought experiment

is essentially a very simple idea.

Einstein just imagined a clock made of two mirrors

with a beam of light bouncing between the mirrors.

So, Jim can simulate that with this torch here, this little bulb,

by moving it up and down.

So, Einstein's clock is essentially...

LAUGHTER Shall I do that now?

It's not quite as powerful as Dallas's...

So, Einstein's light clock worked like this -

so, if you bounce that beam of light...

So, my two hands there are the mirrors, and what you can see

is that you could construct a clock out of this sort of arrangement.

Essentially, one tick, which would be, like...

tick...

..tick...

tick - so that's like the pendulum,

the beam of light bouncing between the mirrors,

and you could use that, actually, to build a very accurate clock.

Then, Einstein imagined what that clock would look like

if it were moving relative to us.

So, what I'm going to have happen is, Jim is going to be moved...

along the stage...

Keep moving the clock.

And then we can dim the light, so we can see what that looks like

from our perspective, stationary relative to Jim.

And we've also got... So there's a little box there, you can see.

That's Jim's head camera,

so Jim is seeing, of course, the clock in exactly the way

that we pictured it when it was stationary, relative to us.

The light beam is bouncing up and down between the mirrors.

But if you look, and we've got a little video effect on there

so you can see the trail,

you can see that the beam of light that we see is tracing out

a triangular pattern across the stage.

Beautiful.

Thank you.

-Can I get off now? I'm feeling a bit sick.

-You can, yes!

-Thank you, Jim.

-It was a bit fast.

-Thank you.

What a great use of that wonderful intellect.

But it was beautifully demonstrated.

What we saw there was, if I sketch it out again,

from our perspective now,

from the audience's perspective, is that here are all those mirrors,

so this is the light clock that Jim was carrying

but you saw that from your perspective, watching Jim move,

the light took a kind of triangular path

as it bounced across the stage between the mirrors.

Here is what Einstein's postulate, if you like, Einstein's suggestion

that the speed of light is constant for all observers, implies.

See, this path is obviously longer than this path.

So, if we all agree on the speed of light,

then it is obvious that it must take the light longer

to tick for the moving clock than it does for the stationary clock.

Moving clocks run slowly.

This is true.

Time really did pass at a different rate for Jim.

It passed at a different rate for him

than it did for you in the audience, watching Jim move.

There's no sleight of hand here.

Jim really is a time traveller.

Yes!

Our time is personal to us.

This is what Einstein had discovered.

There's no such thing as absolute time.

Now, why don't we notice this in everyday life?

It's because the amount by which time slowed down for Jim

was minuscule, because the speed he was travelling was so small

compared to the speed of light.

But if we'd have sent Jim off in a rocket...

Would you like that?

A rocket?

Flying out into space.

Let's say that we catapulted Jim off at 99.94% of the speed of light

for five years according to his watch.

Then we tell Jim to turn around and come back.

It takes another five years to get back to the Earth.

So, for him, the journey would take ten years.

But for us, with our watches ticking faster than Jim's,

29 years would have passed.

Jim would return in 2042 having aged only ten years.

It's a real effect, he'd be a time traveller.

Time travel into the future is possible.

In fact it's an intrinsic part of the way the universe is built.

We're all time travellers in our own small way.

APPLAUSE

What on Earth?

Oh, hello. Get your tally out.

That's a Silent. You've got to admire a monster that puts on a tie.

-It's amazing.

-Yeah. Shunned by the rest of the galaxy.

They'd be vastly more popular

if they laundered their shirts every now and then.

An intelligent bipedal life form. That's a near-impossibility.

Oh, no, don't look away.

What on Earth?

That's a Silent. Keep staring at it, would you?

I haven't got time right now to keep introducing it.

I want more aliens. Where can we go?

-Oh, you're applying for the job, then?

-Job?

-My assistant.

-What does it involve?

Oh, you know, getting captured, dying occasionally.

The benefits are obviously the travel.

I mean, Earth people need to get out more, Brian. Spread your wings,

meet the neighbours. I mean, what year is this?

From your hair I'd say the sixties. It looks like an upside down mop.

Yes, the moon is nice, but come on, my man, have a wander,

stop loitering around your own solar system like a sulky teenager.

-What on Earth?

-Shut up, Brian.

Are we alone in the universe?

Well, I'd say this is one of the most important questions

in modern science.

In Doctor Who, the answer is an emphatic no.

The universe is filled with aliens,

many with technology far more advanced than our own.

Science fiction's replete with aliens,

partly, I think, because we desperately want them to exist.

The alternative, that we're alone in a possibly infinite universe

is a frightening concept.

But what do we know about the possibility

of finding the alien life, and, in particular,

intelligent life somewhere beyond our solar system?

Well, in 1950, the great Italian physicist Enrico Fermi

took this question and rephrased it, he turned it into a paradox,

highlighting, in the process, one of the great mysteries.

Our sun and its system of eight planets

is one star out of an estimated 400 billion

that form our home galaxy, the Milky Way.

Fermi argued that with so many worlds

and such vast expanses of time

stretching back over 12 billion years

to the formation of our galaxy, there must be planets out there

with civilisations far in advance of our own.

So, our universe should be like Doctor Who.

We should expect, just on statistical grounds,

to have caught some glimpse of those spacefaring civilisations

out there amongst the stars and yet we have seen no evidence of anyone.

This is known as the Fermi paradox.

If they were out there, we should see them.

The problem, of course,

is that to send a space probe to even the nearest star

would take many thousands of years with our current technology,

so the search must proceed

without physically travelling beyond our solar system,

at least for the foreseeable future. And there is a way.

The most ancient way of observing the sky at night.

Astronomy.

By capturing light from distant star systems,

using an array of telescopes both on the ground and in orbit,

we've found 992 exoplanets,

and we can now begin to characterise those planets,

to search for signs of life

encoded in the faint light from these distant worlds.

So far, one of the best candidates for life

orbits around one of the stars in this constellation -

in the constellation of Lyra.

It's a planet called Kepler 62E,

after the recently-retired Kepler telescope that first identified it.

It seems to be just the right size and mass to make it a rocky planet

and in just the right orbit to give it a chance

of possessing liquid water on its surface.

But, remarkably, we can do better

than simply estimating what these planets are made of.

See, we're on the verge of being able to look directly

into the atmospheres of these planets

and search for the tell-tale fingerprints of life.

And I'm going to ask Charles Dance to come down

and help me show you how.

I've got a coat, have I?

-Yeah, er, I think its fireproof.

-Thank you very much, thank you.

So, what we're going to do, here...

You just want me to clean this trolley, don't you, really?

-Give you a mop!

-Yeah. Yes.

What we're going to do is,

we're going to demonstrate the technique that astronomers use

-to identify...

-Should I be standing where you are?

..the presence of chemicals in the atmosphere.

You can stand wherever you want, it's not going to help you at all.

So, er... What I'm going to ask you to do is,

we've got a selection of four chemical elements...

-Right.

-..dissolved in these solutions,

and I want you to spray them through the Bunsen flame.

Which direction would you like me to spray them?

I think, actually, probably sort of just upwards and...

-Really, are you sure?

-Er, yeah.

In any particular order?

No, no, just - let's see what happens.

So, you can see, apart from this one, they're all colourless liquids,

-but they've got chemical elements dissolved...

-OK.

-..in the solution.

-All right.

-So, let's have a go at that.

We could dim the lights a bit, actually, perhaps.

Go on. Spray that one through. Let's see what that does.

OK.

There it goes.

Shall I do that one again?

Go again. Beautiful green colour.

Bright green colour.

So, now let's try that one.

-Same thing?

-Yeah.

-AUDIENCE: Ooh!

-Oh, I like that.

A bright red.

This takes me back.

I know!

To school chemistry lessons?

No, to early psychedelic rock concerts.

APPLAUSE

I quite like that one, actually.

-Oh, where's the red?

-What if we do two together?

-Go on, let's do it, let's go for it.

-Shall we do three together?

Oh, dear.

Thank you very much.

Now, the reason that those chemical elements behaved in different ways

is down to the structure of the elements themselves.

See, what happens when you burn that element,

when you heat it up, is the electrons jump around

between different orbits around the atomic nucleus

and then fall back down again and emit light,

and so what we're seeing there

is the structure of the atoms themselves

that make up the chemical elements.

Each element will have a different signature of light that it emits

when heated, because it has a different configuration of electrons

around the nucleus.

Now, as well as emitting light when heated,

elements also absorb light of exactly the same colour

if they're present in the atmosphere of a star or a planet.

Here, for example, is a spectrum of light from the sun.

So, this is sunlight split up into all the colours of the rainbow,

by a prism, for example.

And you can see that it is covered in black lines,

all over, in every colour.

These are the fingerprints of chemical elements,

in the same way that we saw Charles show us the beautiful colours,

the fingerprint of the element in those bottles.

Now, we're on the verge of launching telescopes and detectors

so sensitive that we can analyse the light not only from stars,

like the sun, but also the light reflected and absorbed

by the atmospheres of planets around those stars.

This will allow us to look for the fingerprints of molecules

such as water, methane, and even organic molecules,

the fingerprints of life in the atmospheres of alien worlds.

These techniques might prove the first direct evidence

that we're not alone in the universe.

But they still won't allow us to resolve Fermi's paradox,

because these chemical fingerprints won't differentiate

between simple, single-celled organisms

and the complex multi-cellular life that is surely a prerequisite

for the existence of a civilisation like our own.

But there is just a possibility

that we can look for signatures of intelligent civilisations.

See, as a civilisation gets more and more advanced,

its energy consumption rises dramatically.

With every new machine we create here on Earth,

from the tiniest mobile phone to the largest power station,

we produce more heat.

I'll show you what I mean.

Here is an infrared camera.

So, this is measuring not the light from you, the audience,

but the heat from the audience,

because those colours are representing the amount of heat

that you are putting out.

Yeah, give us a wave.

I can see exactly what you're doing at the back.

That's because you are biological machines.

Every machine, no matter how sophisticated or efficient,

must do this.

It must leave a tell-tale heat signature behind

as it goes about its business.

Now, a group of researchers at Penn State University

are attempting to exploit this fundamental universal law,

using infrared cameras to search the stars

and even to search for entire galaxies

to see if they can see hot spots,

systems that are giving out more heat in the infrared spectrum

than you would expect from purely natural processes.

If they sift through all their data,

and actually find a star, a planet or even a galaxy

with this characteristic infrared signature,

then they could claim evidence, not only for complex life

but for a machine-building, star-harnessing,

transgalactic civilisation.

Doctor Who from afar.

Far-fetched? Yeah, of course it is.

But the simple act of looking, of observing nature,

is the key to science, and we shouldn't take anything for granted.

And it's worth noting, finally,

that we may already inadvertently have made contact.

The first episode of Doctor Who

was broadcast on the 23rd of November, 1963.

The programme was encoded in beams of radio waves,

as beams of light that were broadcast to the nation's TVs.

These radio waves didn't simply hang around floating above the UK,

they left our atmosphere,

expanding in spheres just like the light from Faraday's candle

and began their journey out into space.

Today, that signal will have reached 50 light years from this planet.

SONG: "Doctor Who Theme"

What would an alien civilisation think

if their first experience of our civilisation

was the adventures of the time-travelling doctor?

Oi, Cox, no.

Hands off. Complicated.

Ish.

Ish?! Hah! Don't you "ish" me.

Beyond human understanding.

Relative internal spatial co-ordinates are completely at odds

with externally observed dimensions. So, nur.

Bigger on the inside than the outside

doesn't seem too complicated to me.

Don't listen to him. Cover your ears.

Where exactly are your ears?

Listen, how do you fuel something like this?

The power requirements must be immense.

Oh, yeah? Yeah, I use a black hole.

-A black hole?

-Little bit of Time Lord engineering,

siphon off the energy. Powering this thing is like falling off a log.

A very big log, an n-dimensional log.

Read some Einstein.

The tidal forces on a black hole in there would rip it to bits.

Hah! Yeah, I know that.

Nice chap, Einstein. Bow tie wearer. Always gets my vote.

Wicked hair. But he's behind the times, Coxy.

You want to see my black hole?

I keep it down there, in the basement.

So, the Doctor's world is closer to our own

than you might have imagined.

We're all time travellers,

and we've reached out and touched alien worlds.

But I'm drawn back to these notes.

To December 1860, and Michael Faraday's Christmas Lecture

when he inspired a generation of children to become scientists,

using the simple but magical candle.

What about my dream to return to that moment in time?

So, let's take a look at our map again.

Now, we have everything in the past that has ever happened down there,

and we have everything that ever could happen in the future up here.

The Doctor has complete freedom of movement on the map.

He can go anywhere.

But what Einstein realised is that we can't have freedom of movement,

otherwise we'd run into trouble.

So, he discovered a limit.

He built it into his theory.

Something that we can all agree on.

The speed of light.

Let's think about Faraday's candle again.

If there wasn't a roof on this lecture theatre,

then this would be sending out light into the universe.

An expanding sphere of light travelling outwards

at 300,000 kilometres per second.

In one and a half seconds it would have passed by the moon.

In eight minutes it would speed past the sun,

and in around 100,000 years,

it would completely clear the Milky Way Galaxy.

Now, I can draw this onto my map.

So, this is here and now in this lecture theatre

at the Royal Institution.

So, I can draw a line on my map that represents the trajectory

of a beam of light through space-time.

Of course it expands in all directions,

so I have another one of those lines going out there.

A pair of diagonal lines.

Now, I could also draw lines on this map

which represent the paths of beams of light from the past,

if they arrived here, now, in this lecture theatre.

And here they'll be.

They'll look the same, but they'll extend out into the past.

Now, we all agree on these lines

because we all agree on the speed of light,

so they must be important in some way. And they are.

This is how Einstein protects the past from the future.

They limit how we can move around on the map,

because nothing can travel faster than the speed of light.

It is a universal speed limit.

What does that mean?

Well, imagine that there is someone sat here, let's say,

with a telescope.

If I wanted some signal,

some flash of light to get out to that event there,

which would be, let's say, an alien in some distant galaxy,

taking a telescope out and looking at us,

then it would have to travel - the influence, the light -

would have to travel faster than the speed of light.

It can't happen.

So, this line seems to restrict the movement of things.

Things that travel slower than light

are condemned to live inside this area.

This area is clearly important, and it's got a name.

It's called the future light cone.

That encompasses all of our futures.

Every event that's going to happen to any of us in this audience

or watching at home, that happens, will happen in this region

of space-time inside the future light cone.

It also applies to the past.

So, this is a special region.

It's called our past light cone.

This is the region that contains events in space and time

that could in principle have influenced us now,

at this point, here, tonight.

This is the geometry of space-time as described by Einstein

in his theory of special relativity that he published in 1905.

It allows me to trace my life through these two regions.

I can locate any event that happened in my life on this map.

So, I was born on March the 3rd 1968,

and the first picture I have of me at Christmas

was actually 1972 in Oldham.

There I am, that's that event. It's me in Oldham in Christmas 1972.

Now, there are lots of things that happened to me.

I've got a very embarrassing picture actually in 1989...

What was I thinking?

I-I...

The kind of lifestyle I had.

That was actually when I was on tour with a rock band somewhere,

I think I was somewhere in Europe. So it could have been... Actually...

Oh, where shall I put myself? Over there, that would be 1989.

That's another event, me on a tour bus,

um, drinking sensibly in Europe in 1989.

And so on.

So, my life is a series of events that I can plot on this diagram.

I'm now here, of course, the Royal Institution in 2013.

So, we could imagine plotting every event in my life on this diagram.

That would make a line, and it's a line known as a world line.

And it can wander around in space, cos I've been at different places,

and, of course, it wanders around in time

from 1968 to 2013 there.

And, of course, Faraday's Christmas Lecture on the candle,

the event I most want to visit in space-time,

is also sitting somewhere down here in my past light cone.

It's there.

Christmas 1860.

Why is it in my past light cone?

It has to be because it's influenced me.

These lecture notes were present at that event

when Faraday stood here and delivered his lecture,

and they're present in front of me now.

So, I could draw the world line at that note book on this diagram.

And they've stayed in the Royal Institution,

the same place in space, pretty much their whole life,

because they began in 1860 and they're here now with me in 2013.

But according to Einstein's Theory of Special Relativity,

I can never visit Faraday, because my future world line,

the things I can experience,

is restricted to stay inside the future light cone.

To get out, to escape into the past, what would I have to do?

Well, the first thing I'd have to do

is travel faster than the speed of light,

even before I begin to consider how I could possibly do that

and loop round to 1860, and the universe isn't built that way.

The doors to the past, unless we have a TARDIS,

appear to be firmly closed.

What if there's another way?

What if I can change the direction of my future light cone,

change the direction of my entire future,

and perhaps begin to tilt it towards the past?

Well, there are objects in our universe

that can tilt light cones, and if I could get close enough

they'd affect the direction of my future in a radical way.

There's one at the heart of the TARDIS, a black hole.

The Eye of Harmony is described in Doctor Who as a star,

frozen at the point of collapse into a black hole.

It's a poetic line, but unusually, it has to be said, for poetry,

this one turns out to be physically accurate.

Black holes form at the end of the lives

of the most massive stars in the universe.

When such stars, at least 20 times the mass of our sun,

run out of fuel in their cores,

no known force can overcome the inward pull of gravity

and prevent them from collapsing,

as far as anyone knows, to a single, infinitely dense point

known as a singularity.

I can draw one of those on a space-time diagram.

So here is space, and here is time.

And this is a diagram from the point of view of the black hole,

so that's the singularity ticking forward in time, as it were.

And these two lines, which are very important,

have the evocative names of event horizons.

These mark out a region in space and time

where the gravitational pull is so strong

that light itself cannot escape.

In the vicinity of the event horizon very strange things happen.

And I need a very strange volunteer to demonstrate that.

So, Rufus Hound, where are you?

That was perhaps a little unkind, wasn't it?

-"A very strange volunteer."

-No, it seems about right.

-Is it about right?

-Yeah.

-Um, so...

-I thought that with my fourth brain.

Did you?

What I'd like to do is to throw you into a black hole.

You wouldn't be the first.

-In the name of physics, now...

-You would be the first.

I think it's going to mean

that you're going to meet a very noble end,

a very wonderful exit from this universe.

But in order to observe you as you exit our plane of existence,

-as it were, I want to kit you out with two watches.

-OK.

This one, which I want you to put on your back,

is going to be the one that we can observe.

All right, there we are. Sorry.

Is this how you're going to collapse my mass?

Is that a bit... Is that comfortable?

You're going to do the straps up, is that how black holes work?

Just some bloke with a really tight backpack on.

There we go.

I already feel implosiony.

And I'd like to give you - well, actually, have you got a watch?

-I've got a watch.

-Oh, you've got a watch.

-And there's a second hand ticking away.

-Yep.

That's good.

Right, so, what we're going to do, is we're going to...

Right - it's low voltage, it's all right.

-I'm going to turn it...

-Where are my safety goggles, Brian?

If you just turn round...

If it'll make you feel better I can get some, but it won't help.

No. Great, fine.

If you turn around so we can see this clock,

and I'm going to turn the clock on, and there it is.

So it's whizzing forward in time.

And I want you to face the blackboard, the Eye of Harmony,

that's the black hole, there.

And what I've done is, I've speeded time up

just so we can see it ticking along.

This is the rate that time's passing for us now,

-and it would be the same on your watch here.

-Right.

And I'm going to ask you to move slowly towards the event horizon.

Very slowly.

That's it.

How do you feel?

Like this is slightly TOO slow.

It's all right.

But you see what's happening.

If you stop there, you're approaching the event horizon,

and time on the watch that we're looking at, attached to your back,

is slowing down. How's the time, though, on your watch?

Exactly the same.

It's ticking along at exactly the same rate.

Now, you might start to feel a bit uncomfortable

because for these sort of stellar mass black holes,

the gravitational force on your feet would now be significantly stronger

than the gravitation force on your head.

Now, this is called spaghettification.

Why?

-So, you're beginning to get slightly taller.

-Right.

And eventually, actually, as you approach the event horizon

I think, really, you'd get so tall

that you'd just be a long line of atoms, disassociated.

But anyway, let's ignore that for the moment. Carry on.

-So, you see...

-I don't know why I feel slightly in awe of a picture.

Right towards the black hole.

And what we see - there, stop.

That is on the event horizon

and we would see Rufus' watch, strapped to his back, freeze.

It would stop, but what does your watch look like?

Still going.

Still going at exactly the same rate.

This is precisely what Einstein tells us would happen

as Rufus fell into the black hole.

We'd see time freeze.

We would see an image of Rufus just like that, actually,

that's quite powerful.

How long can you stand on one leg, just like that?

We'd see a frozen image of Rufus on his way across the event horizon.

Time would stop, that image would still be there.

It would be a sort of immortality,

whereas from Rufus' perspective, time would pass as normal,

he would pass over the event horizon,

he would approach the singularity

and be crushed to an infinitely dense point.

Thank you.

Thanks Rufus.

Um, let me explain what happened to Rufus.

So here is my space-time diagram again.

Remember that the black hole is sat here, stationary.

There's the singularity, here are the event horizons.

And what I'm going to do is,

I'm going to superimpose Rufus' world line...

..onto this diagram.

Now, we're looking at Rufus, remember,

from the point of view of the black hole.

So it's just sat there, it's going nowhere,

and Rufus is on a journey towards the event horizon

and beyond into oblivion.

What I've also drawn are Rufus' light cones,

the various points along his world line.

These mark out Rufus' accessible future.

But look what happens to these light cones

as he approaches the event horizon.

They're tilting.

Now, this tilt, according to Albert Einstein,

is caused by the mass of the black hole itself.

It's a representation of a central idea

in Einstein's theory of gravity, general relativity.

The idea is this - mass and energy curve space and time,

the very fabric of the universe itself.

That curvature, the warping of space and time, if you like,

is what we're seeing in this diagram as the tilting of light cones,

the tilting of Rufus' future towards the event horizon.

And look what happens here on the horizon.

You see what's happened to the light cone?

It's tilted so much, space and time are curved and warped so much,

that all of Rufus' future is pointing inwards,

into the horizon, into the black hole.

His world line is heading towards the singularity.

There's no escape for Rufus

because his entire future is inside the black hole.

He'd have to travel faster than light to get out,

and that is not allowed in our universe.

This diagram is very beautiful.

It allows us to see something else,

it also allows us to see what happened to Rufus' clock

as we watched it tick slower and slower and slower

as he approached the horizon.

So, let's imagine...

let's imagine that on each tick of Rufus' clock,

the one on his back, a pulse of light was sent out

and we detected that pulse of light from our vantage point

far away from the black hole.

So, let me put them on.

There.

You see what happens.

As the light cones pulse,

then those pulses of light arrive at us at later and later times.

This is the ticking of the clock.

As far as Rufus is concerned, the clock's ticking away normally,

one second, two seconds, three seconds, four seconds.

But, as we see it, the first second is faster than the second second,

which is faster than the third second.

Tick...

tick...

..tick. And here, on the horizon,

the light pulse goes flying up the side of the light cone,

which is aligned along the event horizon itself.

This pulse never reaches us, so time stops from our perspective.

We see that frozen image of Rufus.

He never makes it across the horizon from our vantage point.

According to him everything proceeds quite normally -

although he's getting spaghettified, it has to be said -

until he gets squashed on the singularity.

This image of Rufus is frozen forever at the horizon.

But here's the wonderful thing -

the same is true for the collapsing star itself.

See, from the perspective of an outside observer,

time stops, so we'd never actually see the star collapse,

we'd see a frozen image fading away

of the dying star forever frozen in time at the moment of collapse,

that is precisely the Eye of Harmony as described in Doctor Who.

How beautiful.

But what of my ambition to get back into the past

and experience Michael Faraday deliver his lecturer?

Well, everything I've spoken about so far in this lecture

is science fact, including this description of a frozen star.

But now it's time to speculate just a little,

but still remain constrained by the known laws of physics.

Notice what the Eye of Harmony, the black hole, did.

It tilted light cones, it changed the direction

of the accessible future in space-time.

Now, could it be that we could dream up some geometry of space-time,

a distribution of matter and energy that would tilt light cones

all the way around?

What I want to do is tilt my future light cone

in such a way that it gets me back to Faraday's Christmas Lecture

in 1860.

Something like this.

So, here...

is a piece of space-time.

It's meant to map directly onto this diagram I drew here.

Here's 1860, and here's me in 2013.

Now, we've seen that a black hole can tilt light cones like that.

What if we could arrange the geometry so that the light cone

tilts around, so it bends in some way

so that...

I can reattach space-time, as it were, around into the past?

I could curve space-time in such a way that this area,

my accessible future, ends up pointing into my own past

and specifically, in this case, ends up pointing to this place,

this event I want to visit, Faraday's lecture in 1860.

Could we design some configuration of matter and energy

that would curve the light cones around

so I could get back into my own past?

The answer is...

we don't know.

But nobody has been able to prove that space-time geometries

similar to this cannot exist, at least in principle.

Although most experts believe that they must in some way be forbidden.

But there's still the faintest possibility,

given the laws of physics as we understand them today,

that someone, someday - maybe a young girl or a young boy -

will be inspired to try.

And, even if they fail, by the very act of trying

they might just go on to change the world.

APPLAUSE

Home!

Oh, I want to visit more alien worlds.

No. Greedy, Brian. Can't be greedy.

You've got a lecture to give, people to inspire, merchandise to sell.

Actually, that reminds me, could you rustle me up a lunchbox?

Maybe a T-shirt, slim-fitting.

Oh, don't forget the gift I got you, you'll need that.

So, what was this all about, then,

just taking me on a tour of the wonders of the universe?

Ah!

Well, there's someone in your audience today,

just an ordinary kid, so high, sad eyes, look out for her,

someone who loves to think about why the sky is blue

and how bees can hover like helicopters,

but after today she stops being ordinary,

she grows up to be extraordinary, a woman who changes the world.

And all she needed was a nudge from you, eh? Today, right now.

No pressure.

I do love humans.

They can be a bit defeatist. You know, "Mustn't," "Can't"...

Sometimes you just need a helping hand.

Every adventure starts with a moment, a spark. Ooh!

Whilst I'm here...

Bit of anti-shine. You'll need that.

Ah.

Don't forget to twiddle the size of the event horizon.

Shut up, Brian.

One more adventure before tea.

SONG: "Doctor Who Theme"

Subtitles by Red Bee Media Ltd