All language subtitles for The.Sky.at.Night.S2025E03.Greenwich.A.Journey.Through.Space.and.Time.1080p.iP.WEB-DL.AAC2.0.HFR.H.264-RAWR_track3_[eng]

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.