- 6/10/2025
The Sky at Night - Greenwich: A Journey Through Space and Time
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00:00I've always loved planning a party and the anticipation while waiting for the
00:07day to arrive. When I was a child I always wanted to have a birthday party
00:14on the moon, maybe a lunar picnic or even better eating blue string pudding with
00:21the Clangers. It's just a wonderful way to celebrate the passing of time, another
00:26journey around our local star the Sun. Tonight some fabulous guests are gathered
00:35to celebrate something very special indeed. It is not a person, it's a place. A place
00:43known as the home of time. A place that has been instrumental in creating the world
00:49around us today. Join me in celebrating the 350th anniversary of the Royal
00:55Observatory Greenwich. Welcome to the Versailles Knight.
01:25I'm standing now on famous ground. On either side of this line I can go from the east to
01:38the western hemisphere and back again in an instant. It's like the equator but dividing
01:43west from east not north from south. It extends that way north through the UK and that way
01:48south through Spain and France, West Africa, all the way down to Antarctica. It's called the
01:54Prime Meridian. But why is it here?
01:57In 1675 King Charles II issued a royal warrant for the building of an observatory and picked
02:07a man called John Flamstein to be his astronomer royal.
02:11The need was grave. The difference between life and death for those
02:18at sea. The Navy required accurate star positions
02:23to improve navigation and reduce the number of shipwrecks.
02:27A single reference point was needed for people to work from, a point on the map to mark zero degrees.
02:35And as years passed and global trade increased, there was a need for a reference point in time as well.
02:42And it was thanks to work done just there that the world chose Greenwich to be that reference point.
02:50Night by night the astronomer royal would track the Earth's rotation.
02:56Watching stars arc overhead as they crossed a favoured north-south reference line, a meridian.
03:03But it was this instrument, the Airy Transit Circle, and the meridian which it defines, that became the one we now know as Prime.
03:12George Biddell Airy, the seventh astronomer royal, spent nights down in this pit, looking through this telescope,
03:18recording the position of stars as they passed through the cross-heads of his telescope, marked with the finest spider silk.
03:26The 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.
03:39Sailors could consult the almanac and calculate their vessel's location using the relative positions of the stars and the moon.
03:46This was vintage GPS, all done without a computer and far from home.
03:54The reach and reputation of the formidable British fleet was founded on astronomical observations made right here.
04:01And so the world's prime meridian was born.
04:09Given their unsocial hours, it made sense for astronomers royal both to work and live right here at the observatory.
04:16That included hosting dinner parties for the greatest thinkers of the moment to come together and share their ideas about the universe.
04:29Tonight, the topic of discussion is time.
04:33Good evening, everyone.
04:34Good evening.
04:35Thank you so much for coming out here.
04:37Thanks for the invitation.
04:38Delighted to be here.
04:39The observatory is celebrating 350 years, but was this building around at that time and what was it useful?
04:45The building we know as Flamsteed House was built at the very beginning of the observatory.
04:50So Christopher Wren had a great phrase when he was describing the building.
04:54He said it was built for the observer's habitation.
04:56So that is the place where John Flamsteed and his family are going to live.
05:01But also, he said, and a little for pomp, which is showing off.
05:05And this was the king's observatory after all.
05:07So gateway to the stars.
05:09On top of the hill.
05:10It's elevated already.
05:12It was an interesting time.
05:13You know, you mentioned Christopher Wren.
05:15There's Flamsteed.
05:16You had Newton.
05:17There was that group that formed the Royal Society.
05:19Yes.
05:20We think about the big inventions in science at that time with the telescope and the microscope.
05:24But, of course, the clock as well.
05:26You know, for accurate timekeeping, you couldn't do science without it.
05:30Yeah.
05:31So, yeah, I mean, right up until the 19th century, people are still using their local time, which for most purposes will be the sundial.
05:36In Oxford, there's a joke that we still use our local time.
05:39All the students are five minutes late for their lectures.
05:41So it's still running on Oxford time, which is five minutes behind Greenwich.
05:45So.
05:46Yeah.
05:47Yeah.
05:48Yeah.
05:51Trains had replaced the horse and cart, ushering in a standardisation of time across Britain.
05:59Visual signals like the Greenwich Time Ball existed already.
06:03But a new public demand for precision timekeeping was growing.
06:08George Eyrie as astronomer royal was issuing signals by telegraph for people who wanted to subscribe to a time service.
06:17And, you know, banks, things like, you know, legal, hazards of parliament.
06:22When the contracts are signed and things like that.
06:23Yeah.
06:24Absolutely.
06:25Yeah.
06:26It could be really crucial in terms of the court case.
06:27When did something actually happen?
06:28But there was also an alternative, slightly sort of cheaper service that went along.
06:32There was one of the assistants here called John Henry Belleville, who made a little, I mean, with permission,
06:37made a little extra money on the side by taking a chronometer, a very accurate clock,
06:42and would then take that into London and take it round to particularly chronometer makers.
06:47So people who themselves were creating precision instruments who really needed to know what the time was.
06:52What was interesting is it, even after we had the wireless signals, even after you start having tan clocks that are more accurate,
06:58you have these chronometer making businesses want the personal service.
07:03And the business, after John Henry Belleville died, was taken over by his widow.
07:08She continued the role.
07:09And then their daughter as well, Ruth Belleville, right into the 20th century.
07:13Something like a hundred years of that family carrying out this business.
07:17I'm late for nearly everything.
07:19If I almost had to subscribe to a time, I don't think I'd bother.
07:22Because I think it's the ideal excuse.
07:24I wasn't too sure what time was it.
07:25Yeah.
07:26Just keep it general.
07:27I'm the opposite.
07:28Really?
07:29I'm quite obsessive about knowing the time.
07:30And I think, because I obviously observe quite a lot, you need to have the precision of timing and whatever
07:38to know when you're going to be in the right place at the right time to see things like Titan crossing Saturn's disc.
07:43But the other thing is that once you've missed that block, you've got to wait then about 13 years for the next slot.
07:52And there gets a point in your life where you start thinking, hmm, that's getting a little bit too far.
07:59How many more of these do I have?
08:01Halley's Comet is a classic, isn't it?
08:03Yes.
08:04In 2090, there is a total eclipse of the sun which goes over where I used to live in Selsie.
08:10Wow.
08:11I'm quite upset about that.
08:12Yeah.
08:16Greenwich has been at the centre of important discoveries about time for centuries.
08:22I've come to meet Louise Devoy at the collection store to discuss a test masterminded by 9th Astronomer Royal Frank Watson Dyson
08:32that changed our understanding of time forever.
08:36Hi, welcome.
08:37Louise, it's lovely to be here in this fabulous place.
08:40And I think you've got something special to show us.
08:42I do indeed, yeah.
08:43This is a plate from the 1919 Eclipse Expedition.
08:47So let's have a look on the light box.
08:50So this is a photograph that's 100 years old, more than.
08:53Pretty much.
08:54Yeah.
08:55I'm so glad you're handling that when I'm not.
08:58Here we go.
08:59Oh, that's beautiful.
09:01It's really stunning, isn't it?
09:03It's gorgeous.
09:04At face value, it seems like an ordinary eclipse photo.
09:07But this is actually quite a moment of transition from Newtonian physics to Einsteinian physics.
09:15In 1915, Einstein published his theory of general relativity.
09:21He 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.
09:33This was taken in Sobral in northern Brazil on the 29th of May 1919.
09:39And astronomers were really excited about this eclipse because, A, it was very long, about 6 minutes and 50 seconds.
09:45But also because it was going to be seen right in front of a star cluster, the Hyades and Taurus.
09:50So astronomers wanted to use this event as a way of testing general relativity.
09:54They 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.
10:01There were two teams from Britain who went out, one from Cambridge and one from Greenwich.
10:05And they did indeed measure a deflection of about 1.75 arc seconds.
10:10Which gives you that's the value that Einstein's theory, not Newton's theory, predicts.
10:14Exactly, exactly.
10:15I mean, it was literally just fractions of a millimetre on the plate, but really quite profound and significant.
10:20Why was this such a big shift in thinking?
10:23What is it about relativity that makes it this sort of moment where it's before and after that changes everything?
10:29So I think there's a mindset change in thinking about some of those fundamentals that we'd really taken for granted,
10:34like gravity and time, that actually they were a lot different to what we originally thought.
10:38So it must have been very unsettling.
10:40That's right.
10:41In this sense beforehand that there are sort of fundamental measurements that you could all agree on,
10:45you have to throw that out for relativity.
10:47So suddenly you and I can be in different places in the universe and disagree about time.
10:51That feels crazy.
10:52It does indeed.
10:53It still feels crazy.
10:54Dyson's teams proved Einstein's theory correct.
10:59In the 21st century, it's now atomic timekeepers that allow us to test relativity at its limits.
11:11Pendulums replaced by atoms, ticking at over 9 billion oscillations per second.
11:18Modern tests of relativity often use instruments like atomic clocks.
11:23Yeah, sure.
11:24I think one of the most famous ones dates back from 1971, the Hafila Keating experiment,
11:29when sodium clocks were put on aircraft, just on like a commercial airliner,
11:34and sent eastwards and then westwards.
11:36And they found that there was a disparity between the clocks and a control clock down at the US Naval Observatory
11:42that really showed that sort of time dilation through kinetic motion.
11:47They also looked at gravitational time dilation as well,
11:50so if you're comparing clocks at altitude versus clocks on the ground,
11:54you can see that variation as well.
11:56So it's really incredible how you can detect those tiny, tiny changes.
12:02Even fractions of microseconds are vital in our modern world.
12:06For financial transactions, for precision GPS, and even for spacecraft navigation.
12:16For a further test, there's now a clock in space to do this.
12:20That's right. Even within the last few weeks or so,
12:22the European Space Agency has launched ACES, Atomic Clock Ensemble in Space.
12:27OK, good acronym. Yep.
12:29And then the idea is that that space clock can then sort of talk to other atomic clocks on Earth as well.
12:35So we can use that for science and also for applications such as position navigation systems.
12:40Oh, I hadn't realised it was navigational as well.
12:42I'm interested in the science because you can do these subtle tests of whether Einstein was right,
12:45but this is a practical navigational thing as well.
12:48Exactly. So we can use it to map the Earth in better detail.
12:51We can use it to potentially measure volcanoes, glasses, maybe even earthquakes at some point.
12:56But it also has that application for the satellite navigation system.
13:00The better you know your time, the better you know your location.
13:03I love this. We're in Greenwich and we're talking about putting clocks in space to do navigation
13:07and to study the Earth as well as physics. That's fabulous.
13:09Great.
13:13Einstein tells us that the way we experience time can be changed by our gravitational environment.
13:20This means that we may have some natural candidates for time machines in our universe.
13:26So people say to me, you know, could time travel ever be possible?
13:29I'm like, well, technically, now, if you were willing to get into a spacecraft that could fly, you know, close to a black hole.
13:36Close.
13:37Close.
13:38Matthew McConaughey's done it.
13:41I mean, we've all seen Interstellar, right?
13:44If you went close to a black hole, you would experience time passing normally for you.
13:50But compared to someone you left behind on Earth, less time would pass for you.
13:54So this could be a way to travel into the future.
13:57Black holes aren't a new idea. The thought that dark objects could be lurking undetected in our universe intrigues scientists for centuries.
14:12People have been thinking about black holes and the idea of a star so dense that it would collapse.
14:18Dark stars.
14:19Exactly, yes.
14:20Way back, I think it was the 1700s.
14:22Right.
14:23Mathematically, we describe them as this unknowing singularity, you know, where all the matter is crushed down into this infinitely dense and infinitely small point.
14:31Yes.
14:32That in Einstein's theory of general relativity, that point is undefinable in either space or time.
14:38Yes.
14:39For me, I've always thought of it as, you know, punching a hole in space and time.
14:43Because it's more than just a dark star, you know, a lump of matter that's squashed down to some size.
14:50It basically keeps on imploding more and more until it disappears.
14:54Yes.
14:55As you say, it punches a hole in space.
14:56That's a really nice way of describing it because it has zero size and that's what Einstein didn't like.
15:02Yes, exactly.
15:06It wasn't until 1971 that the first real black hole was detected.
15:13Just 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,
15:22and they found a star that was moving on the sky.
15:26So much so that it must have been something incredibly heavy orbiting with it to get it to move and wobble around in that way.
15:33Yeah.
15:34That was Cygnus X-1.
15:35It was Cygnus X-1, it was indeed, yes.
15:36It was the very first black hole that has ever been discovered.
15:38What would happen to you or me if you fell into a black hole?
15:42Oh, this is one of my favourite things to answer because it involves my favourite word in the English language ever,
15:47which is spaghettification.
15:49Essentially what would happen if you fell towards a black hole is that the gravity would be so much stronger,
15:55if you were falling feet first towards a black hole, it would be so much stronger at your feet than at your head,
15:59that you would get stretched out like spaghetti to the point where you'd be this long, thin chain of atoms,
16:06sort of on a direct trajectory.
16:08Doesn't sound good.
16:09Doesn't sound a bit painful.
16:12But even if you do survive, weird, you know, time and space get twisted around.
16:16Yeah.
16:17It's weird.
16:18I mean, is it the case that the direction you're heading towards the singularity at the centre of the black hole
16:24is no longer than a direction in space, it's now a direction in time.
16:27Yes, exactly.
16:28Which is why you reaching the singularity is as inevitable as you reaching tomorrow.
16:33Yes.
16:34And you can't stop tomorrow from coming, which is why everything ends up in the singularity
16:37once you cross that event horizon.
16:39There is literally no other path that you can take through time except to get to the singularity.
16:44I'm trying to think what that means.
16:46Don't try and be like that because it's one of those things that...
16:49I don't want for a second.
16:50We've had enough for a second for that.
16:57We can talk about an infinitely small point in theory.
17:00But in the real world, it causes havoc with space and time.
17:06Chris is out to meet Professor Claudia Duran to discuss the solutions to this hundred-year-old paradox.
17:15So, Claudia, in some sense, you've spent your career exploring the difficult bits of the universe in extreme conditions.
17:21Tell us a bit about what you work on.
17:23So, we understand gravity really, really, really so well, incredibly well, in the solar system, in the galaxy.
17:30But I like to think of gravity in the most extreme environments,
17:34so that can be in very, very small region of space and time.
17:38So, going down to 10 to the minus 33 centimetres, very, very small distances,
17:43where maybe the notion of space and time start making sense.
17:47As we approach the infinitely small and infinitely dense singularity,
17:56we get scales just 10 to the minus 33 centimetres across.
18:01That's a million, billion, billion, billionth of one centimetre.
18:07And it's here that our current understanding of space-time fails.
18:16As you go into the black hole, and you go deeper and deeper and deeper into the gravitational well of the black hole,
18:22then the curvature of the way space and time are curled within one another is becoming so high,
18:30so you can ask questions like, what happens when I take two particles and I collide them together?
18:36And the theory tells us that the outcome, it makes no sense.
18:40For instance, it may tell you that the probability of a particular outcome is more than 100%.
18:46Right.
18:47And then we know it's nonsense.
18:49And what about the ideas that people have for going beyond relativity,
18:53of solving this problem that the theory breaks down?
18:55Yes, yes, yes.
18:56We're reaching, really, the realm of quantum gravity.
18:58There's different alternatives out there.
19:00There's string theory, there's loop quantum gravity, there's causal set,
19:05and it is possible that something else has to come in.
19:09So string theory resolves that question by really saying that the fundamental constituents of matter,
19:15and in fact the fundamental constituents of all particles, including the ones that carry forces,
19:21they're not point-like particles, they're little strings.
19:24There's still a notion of space and time.
19:26It's just that the particles themselves, they are extended in nature.
19:32And so you can realise how this helps with having quantities which otherwise would be infinite,
19:37because it's confined in one point, whereas in those strings it gets a little bit more diluted.
19:42It has an extent to it.
19:44And so that is a beautiful way to get rid of these discrepancies of what would happen at the centre of the black hole.
19:52So people have talked about maybe what happens if you go through this singularity as well.
19:56Yeah, yeah, yeah.
19:57So 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.
20:08What is a wormhole?
20:10Let me imagine that if we're here, and I want to go to another point, so you are here, Chris, and someone else on the other side of the universe,
20:19an alien is there and wants to talk to you, and typically it will take you a long, long time to go.
20:25And you can only travel at the speed of light?
20:27Unfortunately, yes.
20:28So it may take you billions of years to get from one point to another, and maybe you don't have that amount of time.
20:33So in such, what you're going to say is, let me jump into a black hole.
20:37Good idea.
20:38So it's possible, and that's becoming more speculative, but this is space.
20:43This is space, and it's flat.
20:45And it will remain flat in here, but let's just imagine that as you go into the black hole in here,
20:51now you jumped into the black hole, space is so, so curved, and I can't draw it curved in here,
20:57but let's imagine it's so curved that you're enabled to connect that point here with a bridge,
21:04and let's call this bridge a wormhole, to another point in the universe in here.
21:09So it's a shortcut, basically.
21:10Exactly.
21:11Then it could be perceived as a shortcut using the curvature of space-time to connect different points in the universe together.
21:20And when you're at that stage, it doesn't really need to be even in our universe.
21:24You can go into extra dimensions if you want to.
21:26And such things are allowed by relativity, right?
21:29So...
21:30You can solve the equation.
21:31That's right.
21:32These are mathematically correct solutions, but physically, for this to happen, we need some kind of matter,
21:39we need some kind of framework which we don't think would be stable in the current stage.
21:45So it is very, very speculative.
21:47Yeah.
21:48The astronomers here 350 years ago might be surprised by their successors spending time on such wild and wonderful concepts of reality.
22:01And yet today's science builds on their efforts to understand the cosmos.
22:06So that's why I'm not that bothered that we don't have answers to the biggest questions,
22:11what the Big Bang was, what to replace relativity with.
22:15Those will be solved by future generations.
22:17It's a team effort with each successive group of astronomers and thinkers contributing to the same cosmic story.
22:24Back at the dinner table, the evening is progressing to the most important moment of any birthday party.
22:36Oh, wow.
22:37Ooh.
22:38Look at that.
22:39It is a proper cake.
22:40Oh, that's so good.
22:41Yeah.
22:42It sort of feels as if we should sing, but it might take a while.
22:46And like 350 pounds of cake to attest.
22:49Let's raise a glass for 350 years of the Royal Observatory of Greenwich.
22:54Happy birthday, Greenwich.
22:55Happy birthday, Greenwich.
22:56Happy birthday, Greenwich.
22:57Yes.
22:58The cake reminds me of another aspect of time that we haven't talked about.
23:05You can bake a cake.
23:06Well, I can't.
23:07Some people can.
23:08One can.
23:09One can.
23:10But you can't unbake a cake.
23:13It is strange that it's so natural to us that time should have a direction, that it only moves in one direction from the past to the future.
23:22And yet in physics, all the laws of physics, all the equations of physics, we're told that time is symmetric.
23:28It can move forwards and backwards and the same things happen.
23:31I wonder if it's our perception of time running that way.
23:34And could you leap out of that perception?
23:37Well, that is a lot of physicists and philosophers have argued that, that it's that time is really an illusion.
23:43It's just, you know, a construct.
23:46I don't believe that.
23:47I think the way we perceive time, it is what we say. It's objective.
23:52It's not subjective.
23:53It's not all in our head.
23:54It's not just in our mind.
23:55All in our head, yes.
23:56And this is still something after thousands, going all the way back to the ancient Greeks.
24:00This struggle to understand, does time flow?
24:04Why does it have a direction?
24:06What is now?
24:07I mean, that's the other, in relativity, right?
24:08You want to define it.
24:09Exactly.
24:10Yeah.
24:11It's relative, right?
24:12To you, the observer.
24:13Yeah, there is no now.
24:14Yes, exactly.
24:15Yeah, there is no moment now that divides the past and the future.
24:18Einstein says, all times are equally real.
24:21It's a very interesting thing to try and concentrate on this moment, this one here, which is then gone.
24:30Yeah.
24:31And also, like, the current moment now is so hard to define, because as I look at you,
24:35I'm seeing you as you were a few nanoseconds ago anyway.
24:38Right.
24:39So what is now?
24:40Yeah, yeah, there is no.
24:41It's different for all of us.
24:42It is a strange thing that we've grappled with for thousands of years and we haven't figured it out.
24:46Which reminds me, when it comes to time, it's time for me to leave because I have a train to catch.
24:54I'm going to miss out on the cake.
24:56We shall sail to a beach.
24:57See you all again soon.
24:58Bye-bye.
24:59Sadly for Jim, we can't stop time, or his train from leaving the station without him.
25:08Oh, wow.
25:10But for the rest of us, there's one final treat.
25:15The nature of light in the universe allows us to see stars and galaxies as they existed in their past.
25:23Just as astronomers have been doing here for 350 years, it's time to look up.
25:32This is a fantastic piece of kit.
25:35This is the Annie Maunder Astrographic Telescope.
25:38It is a 21st century telescope in a 19th century telescope dome, which is a bit of a quirk in itself.
25:44So who is Annie Maunder?
25:46She was one of very few women who actually had paid work at the observatory in the 19th century.
25:52So just in the 1890s, there was a bit of an experiment in having women as people who were doing the calculations.
25:59So tonight, with a bit of clear up there, have you got anything in mind for us?
26:07Yes, so this evening we are pointed directly towards Messier 13, which is the great Hercules globular cluster.
26:14Yes.
26:15And that is a nice bright target that people can find usually even with a pair of binoculars on a clear night.
26:20It's a popular amateur target.
26:22Yeah, and I think that was one that was discovered by Edmund Halley, who was second astronomer.
26:27That's right.
26:28And there's a famous bit inside it, isn't there, called the propeller?
26:31Yes, yes.
26:32If you pick the propeller up, because I have a colleague that always denies it exists.
26:37I don't think we can promise anything.
26:39Let's take a look.
26:40The light from the night sky is currently filtering through our telescope down towards a digital camera right in the end instead of an eyepiece.
26:47And then the image is being sent through this imaging software that you can see right here.
26:52And you can't see a lot.
26:55I mean, really what you're looking at is cloud, though something is beginning to peek through.
27:01What's there earlier?
27:02Yes, we saw it earlier.
27:03Yes.
27:04Me and Maggie have been keeping our eyes on this screen, this live view from the telescope.
27:07And we did see something.
27:08Yes, and it went solid, but now there's a slight thing.
27:11It started.
27:12Yes.
27:13It's teasing us.
27:14Yes.
27:15Yes, and it makes us practise the most important skill of astronomy in Britain, which is hope.
27:20Hope.
27:21Absolutely.
27:22And patience.
27:23Yes.
27:24Yes.
27:25But 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.
27:34Oh, wow.
27:35One you make early.
27:36Yes.
27:37Yes.
27:38Yes.
27:39Absolutely.
27:40The colour in the stars.
27:41The stars contrast beautifully.
27:43And can you see the propeller?
27:44Ooh.
27:45Ooh.
27:46I know.
27:47Straight in the way.
27:48Yeah.
27:49There.
27:50It's that dark line, that one, and that one.
27:52I see.
27:53Yes.
27:54I see it now.
27:55120 degrees apart.
27:56And that's what characterises that cluster.
27:59Mm-hmm.
28:03It's been such a wonderful night.
28:05Good food and excellent conversation.
28:08Astronomers sharing their ideas about the universe, just as they've done at Flamseed House for hundreds of years.
28:14Thank you so much for joining us.
28:16And from here, at the top of Greenwich Hill, goodnight.
28:44Florian!
28:45Stadium!
28:46Thunderbird!
28:47Flamseed House.
Recommended
28:55
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