Original subtitles

I've always loved planning a party

and the anticipation while waiting for the day to arrive.

When I was a child, I always wanted to have

a birthday party on the moon.

Maybe a lunar picnic.

Or, even better, eating blue string pudding with the Clangers.

It's just a wonderful way to celebrate the passing of time -

another journey around our local star, the sun.

Hello!

Tonight, some fabulous guests are gathered to celebrate

something very special indeed.

But it's not a person, it's a place -

A place known as the home of time,

a place that has been instrumental in creating

the world around us today.

Join me in celebrating the 350th anniversary of...

..the Royal Observatory Greenwich.

Welcome to The Sky at Night!

I'm standing now on famous ground.

On either side of this line, I can go from the East

to the Western Hemisphere, and back again in an instant.

It's like the Equator,

but dividing west from east, not north from south.

It extends that way, north through the UK,

and that way, south through Spain and France, West Africa,

all the way down to Antarctica.

It's called the Prime Meridian.

But why is it here?

In 1675, King Charles II issued a royal warrant

for the building of an observatory...

..and picked a man called John Flamsteed

to be his Astronomer Royal.

The need was grave,

the difference between life and death for those at sea.

The Navy required accurate star positions to improve navigation

and reduce the number of shipwrecks.

A single reference point was needed for people to work from,

a point on the map to mark zero degrees.

And as years passed and global trade increased,

there was a need for a reference point in time as well.

And it was thanks to work done just there

that the world chose Greenwich to be that reference point.

Night by night, the Astronomer Royal would track

the Earth's rotation...

..watching stars arc overhead

as they crossed a favoured north-south reference line -

a meridian.

But it was this instrument, the Airy Transit Circle,

and the meridian which it defines

that became the one we now know as Prime.

George Biddell Airy, the seventh Astronomer Royal,

spent nights down in this pit, looking through this telescope,

recording the position of stars

as they passed through the crosshairs of his telescope,

marked with the finest spider silk.

The star positions were published in The Nautical Almanac,

a reference book carried on every ship in the Royal Navy

and countless others around the world.

Sailors could consult the almanac

and calculate their vessel's location

using the relative positions of the stars and the moon.

This was vintage GPS,

all done without a computer and far from home.

The reach and reputation of the formidable British fleet

was founded on astronomical observations made right here.

And so the world's Prime Meridian was born.

Given their unsocial hours,

it made sense for Astronomers Royal both to work and live

right here at the observatory.

That included hosting dinner parties

for the greatest thinkers of the moment to come together

and share their ideas about the universe.

Tonight, the topic of discussion is time.

- Oh, good evening, everyone. - Good evening. - Thank you.

Thank you so much for coming out here.

- Thanks for the invitation. - Yeah, delighted to be here.

The observatory is celebrating 350 years.

But was this building around at that time?

And what was it used for?

The building we know as Flamsteed House

was built at the very beginning of the observatory.

So, Christopher Wren had a great phrase when he was

describing the building.

He said, "It was built for the observer's habitation."

So, that is the place where John Flamsteed and his family

were going to live.

- But also, he said, "And a little for pomp"... - Ooh!

..which is...showing off.

And it was the King's Observatory, after all, so...

Gateway to the stars.

THEY CHUCKLE

- On the top of the hill. - Yeah. - It's elevated already.

It was an interesting time.

You know, you mentioned Christopher Wren.

There's Flamsteed, you had Newton,

and there was that group that formed the Royal Society.

We think about... The big inventions in science at that time

were the telescope and the microscope,

- but, of course, the clock as well, for accurate timekeeping... - Yes.

- You couldn't do science without it. - Yeah.

So, yeah, I mean, right up until the 19th century,

people are still using their local time -

which, for most purposes, will be the sundial.

In Oxford, there's a joke that we still use our local time.

All the students are five minutes late for their lectures!

So, we're still running on Oxford time,

which is five minutes behind Greenwich, so...

THEY LAUGH

Yeah.

Trains had replaced the horse and cart,

ushering in a standardisation of time across Britain.

Visual signals like the Greenwich Time Ball existed already,

but a new public demand for precision timekeeping was growing.

George Airy, Astronomer Royal,

was issuing signals by telegraph

for people who wanted to subscribe to a time service.

And, you know, banks, things like, you know, legal...

- The Houses of Parliament. - Yes, when contracts signed

- and things like that, yes, yes. - Absolutely, yeah.

It could be really crucial, in terms of a court case,

when did something actually happen?

But there was also an alternative,

slightly sort of cheaper service that went along.

There was one of the assistants here called John Henry Belville,

who made a little - I mean, with permission -

made a little extra money on the side by taking a chronometer,

- a very accurate clock... - Yes.

..and would then take that into London and take it around

to particularly chronometer-makers,

so people who themselves were creating precision instruments

that really needed to know what the time was.

What was interesting is, even after we had the wireless signals,

even after you start having town clocks that are more accurate,

you have... These chronometer-making businesses

- want the personal service. - Oh!

And the business, after John Henry Bell died,

was taken over by his widow. She continued the role,

and then their daughter as well, Ruth Belville,

right into the 20th century, something like 100 years

of that family carrying out this business.

I'm late for nearly everything.

If I almost had to subscribe to a time, I don't think I'd bother,

because I think it's the ideal excuse -

"Oh, I wasn't too sure what time it was."

You know, just keep it general.

- I'm the opposite. - Really?

I'm quite obsessive about knowing the time, and I think,

because I obviously observe quite a lot,

you need to have the precision of timing and whatever

to know when you're going to be in the right place at the right time

to see things like Titan crossing Saturn's disk.

But the other thing is that, once you've missed that block,

you've got to wait then about 13 years for the next lot.

And there gets a point in your life where you start thinking,

"Mmm, that's getting a little bit too far."

- "How many more of these...?" - "How many more?"

- Well, Halley's Comet is a classic, isn't it? - Yes! Yes. - Yeah.

In 2090, there is a total eclipse of the sun,

which goes over where I used to live, in Selsey.

THEY LAUGH

Wow!

- I'm quite upset about that. - Yeah.

Greenwich has been at the centre of important discoveries

about time for centuries.

I've come to meet Louise Devoy at the collection store

to discuss a test masterminded

by ninth Astronomer Royal Frank Watson Dyson

that changed our understanding of time forever.

- Hey, Louise. - Hi, welcome. - Hey.

Louise, it's lovely to be here in this fabulous place.

And I think you've got something special to show us?

I do indeed, yep.

This is a plate from the 1919 Eclipse expedition.

So, let's have a look on the light box.

So, this is a photograph that's 100 years old - more than?

Pretty much, yeah.

I'm so glad you're handling that and I'm not.

SHE CHUCKLES

- Here we go. - Oh, that's beautiful!

- It's really stunning, isn't it? - It's gorgeous.

At face value, it seems like an ordinary eclipse photo,

but this is actually quite a moment of transition

from Newtonian physics to Einsteinian physics.

In 1915, Einstein published his theory of general relativity.

He proposed that if space and time were part of the same fabric,

that fabric would be warped by any object with mass,

producing the effect we know as gravity.

This was taken in Sobral, in northern Brazil,

on the 29th of May, 1919.

And astronomers were really excited about this eclipse

because, A, it was very long - about six minutes and 50 seconds -

but also because it was going to be seen right in front of

a star cluster, the Hyades and Taurus.

So, astronomers wanted to use this event as a way

of testing general relativity.

They wanted to try and measure a very slight shift

in the position of the stars,

caused by the mass of the sun bending the starlight.

There were two teams from Britain who went out -

one from Cambridge and one from Greenwich -

and they did indeed measure a deflection of about

1.75 arc seconds.

Which, that's the value that Einstein's theory,

- not Newton's theory, predicts. - Exactly, exactly.

I mean, it was literally just fractions of a millimetre

on the plate, but really quite profound in significance.

Why was this such a big shift in thinking?

What is it about relativity that makes it this sort of moment

where it's before and after that changes everything?

So, I think there's a mindset change in thinking about

some of those fundamentals that we'd really taken for granted,

like gravity and time, that, actually, they were a lot different

to what we originally thought.

So, it must have been very unsettling.

That's right, and the sense beforehand that there are sort of

fundamental measurements that you could all agree on,

you have to throw that out for relativity.

So, suddenly, you and I can be in different places in the universe

and disagree about time. That feels crazy.

- It does indeed. - It still feels crazy.

Dyson's teams proved Einstein's theory correct.

In the 21st century, it's now atomic timekeepers

that allow us to test relativity at its limits...

..pendulums replaced by atoms...

..ticking at over nine billion oscillations per second.

Modern tests of relativity often use instruments like atomic clocks.

Yeah, sure. I think one of the most famous ones dates back from 1971,

the Hafele-Keating experiment, when caesium clocks were put

on aircraft, just on, like, a commercial airliner,

and sent eastwards and then westwards.

And they found that there was a disparity between the clocks

and a control clock down at the US Naval Observatory

that really showed that sort of time dilation through kinetic motion.

And they also looked at gravitational time dilation as well.

So if you're comparing clocks at altitude

versus clocks on the ground, you can see that variation as well.

So, it's really incredible how you can detect those tiny changes.

Even fractions of microseconds are vital in our modern world...

..for financial transactions,

for precision GPS...

..and even for spacecraft navigation.

For a further test, there's now a clock in space to do this.

That's right. Even within the last few weeks or so,

the European Space Agency has launched Aces -

- Atomic Clock Ensemble in Space. - OK, good acronym, yeah. - Yeah.

And then the idea is that that space clock can then sort of

talk to other atomic clocks on Earth as well.

So, we can use that for science, and also for applications

such as position navigation systems.

Oh, I hadn't realised it was navigation as well.

I'm interested in the science cos you can do these subtle tests

of whether Einstein was right, but this is a practical

- navigational thing as well. - Exactly, so we can use it

to map the Earth in better detail.

We can use it to potentially measure volcanoes, glaciers,

maybe even earthquakes at some point.

But it also has that application for the satellite navigation system.

The better you know your time, the better you know your location.

I love this. We're in Greenwich,

and we're talking about putting clocks in space to do navigation

and to study the Earth, as well as physics.

- That's fabulous. - Great.

Einstein tells us that the way we experience time

can be changed by our gravitational environment.

This means that we may have some natural candidates

for time machines in our universe.

So, people say to me, you know,

"Could time travel ever be possible?"

I'm like, well, technically, now,

if you were willing to get into a spacecraft that could fly,

you know, close to a black hole.

- Close? - Close!

THEY LAUGH

- Matthew McConaughey's done it! - LAUGHTER CONTINUES

- Seen it, done it! - I mean, we've all seen Interstellar, right?

If you went close to a black hole,

- you would experience time passing normally for you. - Yes.

But compared to someone you left behind on Earth,

less time would pass for you.

So, this could be a way to travel into the future.

Black holes aren't a new idea.

The thought that dark objects

could be lurking undetected in our universe

intrigued scientists for centuries.

People have been thinking about black holes

and the idea of a star so dense that it would collapse...

- Dark stars. - Dark stars, exactly, yes!

- ..way back to, I think it was the 1700s. - Right.

Mathematically, we describe them as this unknowing singularity,

where all the matter is crushed down into this infinitely dense

and infinitely small point that,

in Einstein's theory of general relativity,

that point is undefinable in either space or time.

Yes. For me, I've always thought of it as, like, you know,

punching a hole in space and time.

Because it's more than just a dark star, you know,

a lump of matter that's squashed down to some size.

It basically keeps on imploding more and more until it disappears.

As you say, it punches a hole in space.

That's a really nice...way of describing it,

cos it has zero size.

- And that's what Einstein didn't like. - Yes, exactly.

It wasn't until 1971 that the first real black hole was detected.

Just at the start of the '70s

is when Paul Murdin and Louise Webster,

who were hired by the Royal Observatory Greenwich

to do their research,

and they found a star that was moving on the sky -

so much so that there must have been something incredibly heavy

orbiting with it to get it to move and wobble around in that way.

- Yeah. - That was Cygnus X-1. - It was Cygnus X-1.

It was indeed, yes. It was the very first black hole

that was ever discovered.

What would happen to you, or me, if you fell into a black hole?

Oh, this is one of my favourite things to answer

because it involves my favourite word in the English language ever,

which is spaghettification.

Essentially, what would happen if you fell towards a black hole

is that the gravity would be so much stronger...

So, if you're falling feet first towards a black hole,

it'd be so much stronger at your feet than at your head

that you would get stretched out like spaghetti,

to the point where you'd be this long, thin chain of atoms,

- sort of on a direct trajectory. - Doesn't sound good.

THEY LAUGH

Does sound a bit painful.

But even if you do survive, weird...

- You know, time and space get twisted around. - Yeah.

I mean, is it the case that the direction you're heading towards

the singularity at the centre of the black hole is no longer then

- a direction in space? It's now a direction in time? - Yes, exactly.

Which is why you reaching this singularity

is as inevitable as you reaching tomorrow.

Yes. And you can't stop tomorrow from coming,

which is why everything ends up in the singularity,

once you cross the event horizon.

There is literally no other path that you can take through time

except to get to the singularity.

I'm trying to think what that means.

- Don't try and visualise it... - No. - THEY LAUGH

..because it's one of those things...

- More prosecco! - We haven't had enough prosecco for that!

We can talk about an infinitely small point in theory.

But in the real world, it causes havoc with space and time.

Chris is out to meet Professor Claudia de Rham

to discuss the solutions to this 100-year-old paradox.

So, Claudia, in some sense, you've spent your career exploring

the difficult bits of the universe in extreme conditions.

Tell us a bit about what you work on.

So, we understand gravity really,

really, really so well, incredibly well -

in the solar system, in the galaxy -

but I like to think of gravity in the most extreme environments.

So, that can be in very, very small regions of space and time.

So, going down to 10 to the -33cm, very, very small distances,

where maybe the notion of space and time stop making sense.

As we approach the infinitely small and infinitely dense singularity,

we get scales just 10 to the -33cm across.

That's a million, billion, billion, billionth of one centimetre.

And it's here that our current understanding of space-time fails.

As you go into the black hole, and you go deeper and deeper

and deeper into the gravitational well of the black hole,

then the curvature of the way space and time are curved

within one another is becoming so high that you can ask questions

like, what happens when I take two particles and collide them together?

And the theory tells us that the outcome, it makes no sense.

For instance, it may tell you that the probability

of a particular outcome is more than 100%.

- Right. - And that, we know, is nonsense.

And what about the ideas that people have for going beyond relativity,

- of solving this problem that the theory breaks down? - Yes, yeah.

We're reaching, really, the realm of quantum gravity.

There's different alternatives out there -

there's string theory, there's loop quantum gravity,

there's causal set -

and it is possible that something else has to come in.

So, string theory resolves that question by really saying

that the fundamental constituents of matter -

and in fact, the fundamental constituents of all particles,

including the ones that carry forces -

they're not point-like particles, they are little strings.

There's still a notion of space and time,

it's just that the particles themselves,

they are extended in nature.

And so you can realise how this helps with having quantities

which otherwise would be infinite, because it's confined in one point.

Whereas, in those strings, it gets a little bit more diluted.

It has an extent to it.

And so that is a beautiful way to get rid of these discrepancies

of what would happen at the centre of the black hole.

So, people have talked about maybe what happens if you go through

- this singularity as well. - Yeah. Yeah, yeah, yeah.

So, you can think of mathematical solutions where you can relate,

connect the centre of a black hole with a wormhole,

for instance, or other dimensions.

What... What is a wormhole?

- Let me imagine that, if we're here... - Yeah.

..and I want to go to another point -

so you are here, Chris, and someone else on the other side

of the universe, an alien, is there, wants to talk to you,

typically, it will take you a long, long time to go and...

You can only travel at the speed of light.

Unfortunately, yes, so it may take you billions of years

to get from one point to another,

and maybe you don't have that amount of time.

So, instead, what you're going to say is,

"Let me jump into a black hole."

Good idea. So it's possible,

and that's becoming more speculative,

but this is space.

This is space, and it's flat.

And it will remain flat in here, but let's just imagine that,

as you go into the black hole in here -

now you jumped into the black hole -

space is so, so curved -

and I can't draw it curved in here,

but let's imagine it's so curved that you're able to connect

at that point here with a bridge -

and let's call this bridge a wormhole -

to another point in the universe, in here.

- So it's a short cut, basically? - Exactly. Then it could be perceived

as a short cut using the curvature of space-time

to connect different points in the universe together.

And when you're at that stage,

it doesn't really need to be even in our universe.

You can go into extra dimensions if you want to.

And such things are allowed by relativity, right?

- You can solve the equations? - So... That's right.

These are mathematically correct solutions.

But physically, for this to happen,

we need some kind of matter, we need some kind of framework

which we don't think would be stable in the current state.

- So, it is very, very speculative. - Yeah.

The astronomers here 350 years ago

might be surprised by their successors

spending time on such wild and wonderful concepts of reality.

And yet, today's science builds on their efforts

to understand the cosmos.

So, that's why I'm not that bothered

that we don't have answers to the biggest questions -

what the Big Bang was, what to replace relativity with?

Those will be solved by future generations.

It's a team effort, with each successive group of astronomers

and thinkers contributing to the same cosmic story.

Thank you.

Back at the dinner table, the evening is progressing

to the most important moment of any birthday party.

Oh, wow. Look at that!

It is a proper cake.

Oh, that looks so good, yeah.

It sort of feels as if we should sing.

But it might take a while.

- THEY LAUGH - And light 350 candles. - Do a toast.

Let's raise a glass to 350 years of the Royal Observatory Greenwich.

- Happy birthday. - Amen.

THEY CHUCKLE

The cake reminds me of another aspect of time

that we haven't talked about.

You can bake a cake - well, I can't, but some people can...

- One can. - All right, one can. But you can't unbake a cake. - Mmm.

It is strange that it's so natural to us

that time should have a direction,

that it only moves in one direction, from past to the future.

And yet, in physics, all the laws of physics,

all the equations of physics,

we're told that time is symmetric.

It can move forwards and backwards and the same things happen.

I wonder if it's our perception of time running that way.

And could you leap out of that perception?

Well, that is... A lot of physicists and philosophers have argued that,

that time is really an illusion.

It's just, you know, a construct.

I don't believe that.

I think the way we perceive time,

it is, what we say, it's objective, it's not subjective.

- It's not just all in our head. - All in our head, yes.

And this is still something, you know, going all the way

back to the ancient Greeks,

this struggle to understand, does time flow?

Why does it have a direction? What is now?

- I mean, in relativity, right? - You can't define it. - Yeah. - Exactly.

- Yes. - It's one of those... - It's relative, right,

- to you, the observer. - There is no "now"! - Yes! - Exactly.

There is no moment "now" that divides the past and the future.

- Mm-mm. - Einstein says all times are equally real.

Boom.

THEY LAUGH

It's a very interesting thing to try and concentrate on this moment,

this here, which is then gone.

- Yeah. - And also, like, the current moment now is so hard to define,

because, as I look at you,

I'm seeing you as you were a few nanoseconds ago anyway.

- So, what is "now"? - Yeah. Yeah, there is no...

It's different for all of us.

It is a strange thing that we've grappled with

for thousands of years, and we haven't figured it out.

Which reminds me, when it comes to time,

it's time for me to leave, because I have a train to catch!

- THEY LAUGH - I'm going to miss out on the cake!

- No! - We shall save you a piece.

OK, see you all again soon. Bye-bye.

Sadly for Jim, we can't stop time...

..or his train from leaving the station without him.

But for the rest of us, there's one final treat.

The nature of light in the universe

allows us to see stars and galaxies as they existed in their past.

Just as astronomers have been doing here for 350 years,

it's time to look up.

This is a fantastic piece of kit.

This is the Annie Maunder Astrographic Telescope.

It is a 21st-century telescope

in a 19th-century telescope dome,

which is a bit of a quirk in itself.

So, who was Annie Maunder?

She was one of very few women who actually had paid work

at the observatory in the 19th century.

So, just in the 1890s, there was a bit of an experiment

in having women as people who were doing the calculations.

So, tonight, with a bit of clear up there,

have you got anything in mind for us?

Yes, so this evening,

- we are pointed directly towards Messier 13... - Oh, wow.

- ..which is the great Hercules Globular Cluster. - Yes.

And that is a nice, bright target that people can find,

usually even with a pair of binoculars on a clear night.

- It's a popular amateur target. - Yeah, and I think that was one

that was discovered by Edmond Halley,

- who was second Astronomer Royal. - That's right.

And there's a famous bit inside it, isn't there, called the "propeller"?

- Yes, yes. - Can you pick the propeller up?

Because I have a colleague that always denies it exists.

I don't think we can promise anything.

Let's take a look.

The light from the night sky is currently filtering through

our telescope down towards a digital camera right in the end,

instead of an eyepiece.

And then the image is being sent through this imaging software

that you can see right here.

And...you can't see a lot.

I mean, really, what you're looking at is cloud,

- though something is beginning to peek through... - Yes.

- It was there earlier. - Yes, we saw it earlier.

- Yes, me and Maggie have been keeping our eyes on this screen... - Yes.

..this live view from the telescope, and we did see something.

Yes, and it went solid, but now there's a slight...

- It's started to come back. - It's teasing us. - Yes!

Yes, and it makes us practise the most important skill

- of astronomy in Britain, which is... - Hope!

- Hope, absolutely, and patience. - THEY LAUGH

- Yes. - Yes.

But with clear skies and enough time to acquire the data

and process the image,

we get something that looks a little bit more like this.

- Oh, wow. - The one you made earlier. - Yes! - Yes.

- So, the colour in the stars... - The stars contrast beautifully.

- And can you see the propeller? - Ooh, hmm...

- No, you're going to... - It's hidden away, yeah.

It's there. It's that dark line, that one, and that one.

- I see! Yes, the three blades. - I see it now, yeah.

120 degrees apart. And that's what characterises that cluster.

It's been such a wonderful night -

good food and excellent conversation,

astronomers sharing their ideas about the universe,

just as they've done at Flamsteed House for hundreds of years.

Thank you so much for joining us.

And from here at the top of Greenwich Hill, goodnight.