Is Time Real?

Thank you all so much for being here at our wedding. I can't believe I get to spend the rest of my life with a woman of my dreams. Speaking of dreams, have you ever dreamed of tasting all the colours of the rainbow? Because that is exactly what you get with Skittles. Five bold fruit flavours in every pack. Lemon, orange, lime, strawberry and blackcurrant. They're chewy, they're colourful, they're perfect. Just like my wife. So thank you for coming and remember to buy Skittles. Therapy Clinic, one of Europe's leading aesthetic clinics, has arrived in Cheltenham. With over 100,000 five-star reviews and more than 10 million treatments performed, therapy are industry leaders in laser hair removal, cosmetic injections and advanced skin treatments. With over 85 clinics globally and a team of more than 200 doctors, therapy deliver safe doctor-led treatments at accessible prices. For verification head to therapyclinic.com or visit Therapy Clinic Cheltenham today, located on the High Street. When life is hectic, energy ups and downs are all you need. If you're seeking energy reassurance, Eonnext can help. From smart tech that helps you take control of your energy future to always staying below the price cap with NexPledge. We're here for whatev'lis next. Just one of the reasons why we're rated excellent on TrustPilot by our customers. Find out more at Eonnext.com. NexPledge variable rates are always below the often price cap. 25 pounds exit fee per fuel applies. Eligibility and season fees apply. TrustPilot February 2026. What is time? Is time real? How important is it to have accurate time in the modern world and space as well? So as you move around space, you are also moving in time. Hello and welcome to the supermassive podcast from the Royal Astronomical Society. With me, science journalist Izzy Clark and astrophysicist Dr Becky Smethurst. Start your stopwatches, set your timers, turn back your clocks. This is getting tiring, but you get the picture right. This month we're talking about time. What is it? How and why do we even measure it? And is time even real or is it a human construct? Yeah, it's a big show with lots of big questions and the head scratches are coming in early. So we'll be speaking with a woman who works on one of the world's most accurate clocks and editor Richard has been to the very heart of time itself in Greenwich and that's all coming up. And a quick note to say that I have sorted out the ad-free version of the podcast. It's taken me a while. Sorry everyone, thank you for your patience. But no, the supermassive club does exist. There is a link for those that don't want any adverts and would like to support the show in our episode description and we'll talk a little bit more about it later on in the show. But that is the beginning of my admin. You can breathe a sigh of relief, Izzy, now it's done. Hooray! But obviously it is not the supermassive podcast without Dr. Robert Massey from the Royal Astronomical Society. And the idea of time is quite hard to explain, but this goes back all the way to the dawn of time and humans looking at the sky, right? It does indeed, yeah. I mean, honestly, let's start with a big concept of the very outset, I think. Yeah, over to you, Robert. Explain everything for us. Yeah, exactly. Yeah, trivial question there. I mean, look, we must have, I was thinking about this, I think we must absolutely have been aware of the passage of time because, you know, how could you not be, you know, if we're in any way sentient? And I guess I'd be very interested to know, you know, would say great apes are aware of it as well as well because so much of life on Earth responds to it, you know, our whole biology is wired around day and night. And, you know, we mark that at the, well, I would say the easiest level, but a fundamental level. There's a connection with the sky because the motion of the sun across the sky rising and setting for day and night itself. The moon delimiting the months, you know, a perfect example there as it goes through the phase cycle and then, you know, repeats that roughly, but not quite exactly 12 times a year. And then the passage of the seasons where you've got the sun being higher or lower at different times of year. And, you know, early cultures marking that great big Neolithic works like Stonehenge and then moving on to Sundials and Astrolabes and still a long time ago, right? You know, Astrolabes dating back what, 1500 years or so. So it's really, really built into us. And then that kind of arrow of time, you know, it's very obvious in the passage of our lives. I mean, there's not astronomy, right? You know, we all know that we grow old and all the rest of it. And it sort of sounds a bit morbid. You think, oh, everyone dies and all that stuff. Of course we do. But at the same time, there's something quite beautiful for me about the fact that the cosmos, the stars, the universe, the planets all kind of live on regardless. And, you know, it gives us that feeling for how short our time on Earth is. But there's that brilliant beauty in the cosmos above our heads and around us. So, yeah, it's all a bit flowery this, but, you know, let's think about time. Cheers, all right. Well, we'll catch up with you later in the show for some more questions, which, as you said, I got to read some headscratchers. So, pay yourself for that. And of course, this month's Stargazing Tips as well. This is obviously a huge topic, but we'll try and cover the basics. So, what is time and why do we measure it like we do? These are both questions that I put to Ann Curtis, Principal Research Scientist at the UK's National Physical Laboratory. But first, she wanted to set the record straight for all of us and explained what is a clock? It's actually not as obvious as you might think. These, what a clock is for a person on the street might be quite different than if you needed to make a clock or make time for other people or bring time to other people. A clock has to have three parts for it to be a useful clock for science and measurement. And the first bit is something that oscillates. So, something that's periodic. And this could be how a pendulum swings in a clock. It makes that kind of ticking sound we're all used to when we think about clocks. And then we need some kind of device that can count those oscillations, those ticks, and give us information about them like a clock face does. And now you've got something that counts seconds for you, but it doesn't really tell you the time until you reference it to something that everyone can agree on and say, oh, well, that's what time it was at that time. And now we can count seconds from that point. And so for like a clock in your house, you might use a reference of the sun being up above you at high noon. So it's really the rotation of the earth is the reference for a clock. So that's kind of our basic definition of what a clock is. And the important bit is the oscillator producing a signal that you count reference to something that everyone agrees on. OK, and I suppose that is a question that I've always wondered, is there everyone agreeing on it? You know, how is the second a second? How has that been determined? I know that's only question two in, but like, can you go into that a little bit more? For a very long time, this has been an international effort of how do we collectively as the earth and all the people's on it and all the different countries and cultures decide what time is in a way that helps us interact with each other? Because as soon as people could move around, you had to start agreeing on what time was or else your trains can't run. You're trying to communicate what time to all get together and everyone gets upset because you show up at different times. I mean, these are really cultural reasons that humans have needed to agree on a reference. But essentially the earth rotating and the sun moving was a kind of key point of how you define that. And when that isn't exactly even, the rotation of the earth isn't as steady as we might think, there was an agreement that there'd be some kind of average second based on an average type of year. And that was used for a long time until even that wasn't accurate enough due to this kind of wobbling of the rotation rate of the earth. And as societies became more complex, you had to find new and innovative ways to make clocks with more and more precise timing. And then it turns out navigation becomes something that requires good clocks. And that was again a big push to fund scientists to find ways to make better clocks so they could navigate better. And so how have they done that? How can we get this accurate measurement of time that everyone can also agree on? It was agreed back in 1967 that we moved from this kind of celestial reference of the sun to one based on atoms. And the way this worked was we decided that our oscillator, our ticking bit of the clock, would be how much energy or how much frequency it takes to excite a transition between two energy states of the electrons in a cesium atom. And it turns out they have a precise number of oscillations that would occur, which is about 9.2 billion, occur in one second. And when those 9.2 with lots more digits oscillations occur, then one second has occurred by definition. Wow. Is there any reason that cesium is the go to for this? That is a really interesting question. At the time, it was one of the first clocks that was made that way. And so it became something that was a clock that many different institutes could make and could evaluate as in realizing this new definition. So you define something in terms of, oh, if you have this oscillation and it does excite this transition, then that's a second. But then you have to realize that with a real experiment. And so the first of these kind of clocks was actually made at NPL, but then a number of other institutes around the world also did this. And so they collectively decided in 1967 to go with this cesium definition. So that's the one second is the amount of time it takes 9.2 billion oscillations of energy states within a cesium atom. This is what we talk about when we talk about atomic clocks. So how important are they? And is this the main functioning of how they work? Is there more to it? That's exactly how it works. As I defined clocks before with an oscillator, a counter and a reference, we now in our atomic clocks have an oscillator. It's an electromagnetic wave that can excite this atomic transition. We have counters in terms of microwaves. You can just count them because it's literally energy oscillating. And then the reference is when you shine that radiation on your atoms. Do you get the signal that lets you know you've got the right frequency and it feeds back then to your oscillator keeping it the right way? The clock output is actually a microwave frequency and the atoms are simply the reference. Amazing. And so how important is it to have accurate time in the modern world and even in space as well? So time, a lot of people talk about it as this kind of invisible resource. It's actually underpinning almost everything we do technologically in the world right now. And that's because clocks are at the basis of communication systems in terms of synchronizing distant points. They're absolutely essential in satellite navigation. So our global navigation satellite systems like GPS, each of them has a number of atomic clocks on board. And the more precise the clocks are, the more accurately you can pinpoint your location. There's getting information for your financial transactions that all has to be synchronized. And then there's of course all sorts of really interesting science you can do. One that I think is really interesting with these kind of accurate clocks is having large arrays of radio telescopes. And the more precisely you can synchronize their signals when you're doing the analysis, the more fine detail you can see out in the universe. And even things like gravitational wave detection, both the timing of how those things come in, but more importantly the technology they use in those really large scale like kilometer long systems is the same kind of technology we've developed for clocks and those two communities really communicate together. One question that we've had sent in which is why does time seem to move forward and not backward? So it's really interesting because Rimmer I described what a clock is and I make my atomic clocks and therefore I'm realizing the SI second or I'm creating a new clock that potentially could be even better and we could redefine the second in terms of an even better clock. But when it comes to these deep philosophical questions about what is time and does time have a direction, and why does time seem to have a directionality. And so it's very interesting when you start talking to scientists across very different fields of research how time affects what they do. When you think about this arrow of time, you can see in physics equations that it shouldn't matter if you move, you know, T going in one direction versus T going in the other direction, you get the, you can't tell if you flip your T's and invert time, you get the same thing. But we know that we very much seem to be living moving forward in time. And I think the one thing people look at is in statistical mechanics or thermodynamics. If you have a pool of water and you put some dye in the middle of it, and then it disperses through the water, and it doesn't ever then come back to a point. And that's something in our reality that we see. And I think when people say the arrow of time, I think that's the best way to look at it. But it really is a statistical question. Any individual ordering of these particles is just as possible as all the rest. But there's only one ordering that has everything just how it started. And so it kind of becomes a probability question. And so people who think about thermodynamics and statistical mechanics and information theory, which again plays into black holes, they are the ones really thinking about this arrow of time. And what does it mean for us physically? Is it real? How does it affect their day to day lives? Can we can we mess around with it? Can we do an experiment that reverses the arrow of time? Interesting. So are we saying is time real? Is this where we're ending this conversation? Have humans constructed times? Do you have thoughts on that? It's funny because these are the real questions that people ask all the time. And I'm like, well, I just realized the atomic second and I try to make clocks to realize atomic seconds better and better. I think that's very separate from what is time is time real. If nothing changed this time move. And these are very much philosophical questions. But people are trying to figure out what kind of experiments could we create to create an environment that would allow us to answer these like scientists and not just philosophers. I went to a very interesting meeting. I was the experimentalist trying to keep clocks real for the group. But the task really was, but it was so interesting for me to hear how they think about time and how they think about I'm going to make a clock to test these things. And being able to bring those two communities together helps us really answer these kind of questions. Thank you to Ann Curtis at the National Physical Laboratory. So Becky, in our last episode, we talked about space time. So how is that different to time? Well, it isn't really. I guess because we think about moving through space and moving through time is two very different things in terms of how we experience it in everyday lives, right? We can control how we move through space. We go up and down and left and right and forward and background. But as humans, we can't currently anyway control how we move through time. It sort of just flows around us is how we sort of visualize time moving, right? But it was Einstein with his theory of special relativity and general relativity as well that tied these things together into space time. So four dimensions that are all intrinsically linked. And so as you move around space, you are also moving in time. So whenever we talk about time, what we're talking about is the fourth dimension of space time. You can't talk about time without talking about space either. Cool. And so when we announced this episode, we had a few people asking about time and relativity and those different time frames. So I think the best way to probably cover all of this is to talk about the twin paradox. So can you explain what that is? Yeah. So Einstein's theory of special relativity is what teaches us about something called time dilation. So it comes from this idea that as you move through space, you're also moving through time. And as you travel faster through space, time then flows around you differently. We're talking about how we spoke about it before, right? So although it feels like to you, even if you're on a space, you have to travel very, very fast, that time is passing at the same rate, less will have passed for someone who is stationary, right? Yeah. So in the twin paradox that this is explained essentially as you can imagine, if you get one twin stays on Earth, one twin hops in a spaceship, travels away from Earth at close to the speed of light for a while and then comes back again. Less time will have passed for the twin in the spaceship, although it'll feel like the same amount of time to, you know, to them. And so when they come back to Earth, they will be younger than the twin that they left on Earth when they return it. So that makes sense in terms of what we think about time dilation, but it's known as a paradox because it is quite counterintuitive from a physics perspective in terms of what we think we know about physics, at least before Einstein came along anyway. And it's not because it breaks any logic, it's just counterintuitive. So if you think about it in terms of reference frames, right? So we talk about this a lot in physics, right? All speeds have to be measured relative to something. Yeah. So for example, your car speedometer might read like 70 miles an hour on a motorway, but that's relative to like the ground, right? Yeah. And the ground on Earth is also still moving because it's spinning and the Earth is moving and whatever. So it's all relative, but also like relative to say a car you're overtaking on the motorway, you know, you're not moving seven to an hour faster than them. You might only be moving five miles an hour faster than them. Yeah. So relative to them, you're not actually moving that fast. So when it comes back to the twin paradox, like the twin in the spaceship who doesn't really feel their movement on the spaceship, right? You know, it's like if you're in a car, you're looking around, everything is stationary. It's only when you look out the window as you realize you're moving, right? When they look out of their spaceship window back at Earth, it looks like Earth is moving away from them. So it looks like the other twin who stayed on Earth is moving close to the speed of light and not them. So from their perspective in their reference frame, the other twin should be the one that's younger when they get back and experience the time dilation because they look like they're the one that's moved. This is the paradox supposedly. But that way of thinking is only true if your frame of reference is what's known as an inertial frame. So it's either not moving at all or it's moving with a constant speed. Okay. But the twin in the spacecraft will accelerate to get out to the speed of light and then decelerates again to come back to Earth. So their reference frame isn't inertial. So there is no actual paradox. Okay. Yeah. And of course, this was originally a thought experiment, but it's been proven with atomic clocks that we've like, flowed around the Earth a few times and they've come back with different, you know, readings afterwards. We need to correct for it with GPS satellites so the GPS doesn't break every single day as well. And I also love that like the twins are the twin astronauts. Mark and Scott Kelly have kind of become the face of the twin paradox now. Because you look at it and then you just get there, the sort of official photographs. You're like, yeah, they did do this. Exactly. So there's some said, oh, we've proved the twin paradox with them, but we haven't proved it right. We just, Scott went to space for a year on the International Space Station while Mark Kelly stayed behind on Earth. And so technically, if you account for the time dilation that Scott Kelly would have experienced, then he is 8.5 milliseconds younger than he would have been if he'd stayed on Earth, right? So he aged 8.5 milliseconds less than that. And you can tell. Yeah. You can really tell. You can really see the date now. Yeah. Thank you, Vicki, but we should mention that we are going to come back to this idea. More of the science fiction realm of time. Right. Because we had so many questions. Thank you, everyone. Obviously, we sort of knew it was going to happen, but we will at some point make another episode that is more focused on time travel and teleportation and that element of things. Manipulating space time to move in different ways. Got it. Yes, because we simply do not have the time today. Was it just all like, I've watched an episode of that? Yes. Brilliant. I'm so excited for that. The zesty lime and lush tropical fruits are always delicious. Try Velimaria Sauvignon Blanc, a vibrant New Zealand wine that's perfect for every occasion. Available at all good wine retailers. Quick question. When was the last time a display ad changed your mind? Now, think about the last time a friend told you about something they loved. Different feeling, right? That's how podcast advertising works. A host who's built real trust with their audience talks about your brand in their own words. In their own voice, it doesn't interrupt the experience. It's part of it. With Acast, you can access the world's largest podcast marketplace. Choose the right shows, the right audiences, the right format. Then watch the data tell you it worked. You're not buying impressions, you're buying influence. Learn more by visiting acast.com slash advertise. I don't know about you, but all of us here at the Supermassive Podcast are pretty oversheduled, our days run, or don't run on doing things on time. What is it? Yeah, I don't know. I don't know. I wonder if anyone else has experienced anything like this. But if you go back in human history, that wasn't always the case. In fact, the reason we now have accurate time is all to do with travel and technology. Editor Richard Hollingham visited the slightly windy Royal Observatory in Greenwich, London for our own brief history of time. If any place on earth represents the centre of time, it is here at the Royal Observatory Greenwich. I'm standing on possibly one of the most famous lines in the world, the Prime Meridian. And just for our Canadian listeners, I am right beside written on the ground here at Greenwich, Montreal and Ottawa. With me is Finn Burridge, a science communicator here at the Royal Observatory. Finn, what does this line represent? I mean, it's what people come from all over the world to see. They do, and it's a really historically important line. Just historically but geographically, of course, it is the gap between east and west. So this line marks the point where the eastern hemisphere meets the western hemisphere. And it has been that way since 1884. So this is the prime reading of the world, and we do get a lot of visitors here, of course. Now, in our brief history of time, we're going to actually start in the Observatory behind us in one of the larger rooms there, and the motivation for getting accurate time. Yeah, and this has been the goal of the Observatory for, well, 350 years this year, in fact. It was all about navigation, actually, finding time to navigate around the world. It was a massive problem for sailors around the 16, 17, 1800s was how to find your longitude. So latitude is how far north or south you are from the equator. Longitude is how far east to west you are from where, right? And this is the Meridian is that baseline. And before the Meridian was established, finding that position, east and west, was really tricky. OK, well, let's head up into the Observatory itself, across the cobbled courtyard, and then up the stairs. This room is just fantastic. You can hear the echo, wood-panelled room, and kind of whispering, because if I shout, the echo is just out of control. Yes, and this is our octagon room. So it is, as in the name, it is an octagon shape, and it is the oldest part of the site here at Greenwich. So if you cast your mind back 350 years, all that was here was essentially a small fort, castle, a retreat for the kings, Tudor kings and queens. And it was established to a great line of site, this hill here at Greenwich, to the south. John Flamsteed, our first royal astronomer, there was nothing here for astronomy, so he had to build his own bespoke observing suite, which was this octagon room. Installed on the right-hand side, we've got some of the first regulators. These are some fantastic clocks that he would use to time when stars crossed the Meridian to make a map. And that was his first job. He spent his entire career doing that. To solve the longitude, we needed to observe stars, and we didn't know where the stars were in the 1670s. So Flamsteed spent 40 years in this room, and just outside, wasn't all the observing was done in this room, but a lot of it was mapping the entire night sky. So you look at the stars, map the movement of stars, and use that to make time? Well, so if we get into the longitude, the problem was finding out where you were. It's easy to find out where you are north and south, especially in the northern hemisphere. You just look for the north star, or at least you look for the north celestial pole. So that's a point in the sky where the stars don't appear to move. Now, if you're at the north pole, it's directly above you. If you're at the equator, it's on your horizon. So it's a really easy way. You just measure the height of that star, you've got it. East and west is tricky. You actually need to find out a time difference between where you are and where something else is. Back in the 1600s, the only way to do that was to find your time by the stars. As they rotate in 24 hours, if you can find where one of these stars is, relative to the ones you'll get your time. But to do this, you have to know what the stars were, where they were, how they moved. And we didn't have a full understanding of that. So Flamsteed's task was to make a map by every time a star passed the meridian. He would note the time and eventually, kind of almost like a music player, as notes go past the dial, he could make a map of the stars. And came up with these clocks here, which are on the wall. The clock faces almost look like the clock faces of grandfather clock. Rather beautiful inlaid, I guess, silver there. I can't see any mechanism. What's the mechanism? They're all hidden. These are fantastic pendulum clocks. Pendulum clocks were the most accurate way of time keeping at that time in the late 1600s. Started by Huygens, some famous astronomical name, but also a clock maker. And behind this wall here, this panel is a 13-foot pendulum. The longer you make your pendulum, the better, the more accurate you can make it, which dangles through the whole building. Unfortunately, this is a listed building, and we like the octu-room looking the way it does. We can't tear the walls down to look at them. But there is some phenomenal machinery behind there, which sadly, you can't get a look at at the moment. These pendulums, another problem is you can't install a 13-foot pendulum onto your ship. It's not going to work. So that was where John Harrison comes in, who made very accurate time keepers, but also the work of our astronomers. Time keeping wasn't the only way of solving longitude, or at least having an accurate clock. You could also use the position of the stars to get your time. So our astronomers here worked on a method called the lunar distance problem. You basically look at how far is a certain star from the moon. The moon travels around the Earth every 28 days. It's a lovely time keeper, an ancient time keeper. We've been using it for thousands of years. And if you could, from wherever you are in the world, whether you're here or on the other side of the Atlantic, measure how far the moon is away from, let's say, a star like prasion. As it passes past it, you could work out the time, as long as you had a book with you, and a book that told you what time it was here at Greenwich. And that was the key. You could feel the history in this place from the wooden stairs, the portraits on the walls. We come into one of the exhibition galleries here, and at the centre, these are just beautiful mechanisms, just these intricate clocks, no pendulum here. The one that's working, it's got a fan that's going backwards and forwards, and all sorts of... I don't know how best to describe them, brass balls, really. They're in odd shape, these little golden boxes. But these were Harrison's clocks. These are Harrison's, so we've got H1 here on the right-hand side for us, all the way through to H4, which was the really revolutionary one. And they were an attempt to solve the problem we've just talked about. You can't take a pendulum on a ship. So if you needed accurate time, and this was at the core of method of finding your longitude, you needed to know what time it was where you are, and what time it was somewhere else, which was Greenwich. You had to take an accurate clock that kept Greenwich time. And if it had a pendulum, as soon as you went out onto the open sea, it would go all over the place. That pendulum starts going haywire, you're never going to get an accurate timekeeper. So how do you make a clock without a pendulum like that that's free? These strange kind of spheres that you see on top of a lot of the clocks, they are in a way pendulums, but they don't swing freely like a one-and-a-grandfather clock. They're actually kind of up against one another. They're touching, which means they can't swing freely. They rotate, oscillate, kind of like two circles that are like two wheels rolling past each other. But they just look like beautiful mechanisms with, like you say, these golden spheres there's a spring across the middle of them, there's this blade that's warring round as well. It's almost like someone's, you know, an old-fashioned pocket watch, but kind of exploded out. You can see the whole mechanism, you see all the cogs, all the intricacies of it. That's what's great actually about coming and standing here is you get to, unlike the ones upstairs in the open room, you literally can see the inner workings, the heart of these clocks, which took Harrison his whole life. This is a labour of love for Harrison from his earliest clock all the way to H4, which he made in his final decade. And it's great to see the internal mechanisms. They were actually really revolutionary at the time as well. For example, by the time you get to H3, we can't solve the pendulum problem. Easy peasy, right? You put them up against you, they're great. But what happens when you go somewhere really hot? What happens when you go somewhere really cold? Oh, so it's all going to expand or contract? Yeah, exactly. If you've got a wooden clock, it's going to expand. The length of the pendulum changes. You've not got accurate time. So he came up with what's called the bimetallic strip. You take two metals, you twine them together like a piece of thread. If one of them tries to expand because it's cold or because it's hot, the other one, which is a different metal, expands at a different rate, stops it. And vice versa, if it gets cold. So it lowers the effect of that temperature and makes them more accurate. And these clocks would begin to keep less than a second of loss a day. So we were really improving on this method until you get to H4, which is very different. Very different clock. It's a pocket watch, essentially. A pocket watch. Let's go down a look. God, I have to look at the pocket watch. It is a pocket watch. I mean, it's an enormous one. It's Alice in Wonderland style. It is. White rabbit pocket watch. But it's an enormous watch. And that is as accurate as these large clocks. It's more accurate than these clocks, in fact. This was the one that actually solved the problem. And Harrison was eventually awarded his prize from the Board of Longitude, which was set up hundreds of years beforehand from this watch. Pocket watches have been around even since the Observatory Foundation in 1670. People have had pocket watches, but they were horribly inaccurate. They would lose up to a minute a day for the earliest ones, which was fine for domestic timekeeping, but nowhere near good enough for the Navy. But what Harrison realized is if you miniaturize the device, if you contain it within its own packaging, you protect it from all that stuff, all that heat, all that cold, all that seafoam. And instead you rely, instead of a pendulum on a spring, essentially, a wound oscillator that beats very quickly. And this actually harks back to kind of the science of timekeeping. The faster you can oscillate something, the more beats per second you can make, the more accurate you can measure time because you have more to work with. And this one beats far faster than the other clocks, which made it a really accurate device. It strikes me that, you know, we're obsessed with time. So we agreed to meet here at 9 o'clock in the morning. I had to get a train, which came at 7.25 to get here. All the trains have to come through London. We'll come to trains in just a second. We're obsessed with time today, but this was just driven by a need. It was the ships needed to know where they were. It was not necessarily driven by a desire to know what time it is accurately. Most people, for thousands of years, even up until the 1700s, would just keep time by the sun. It was just when did the sunrise, when did the sunset, you know, the changing of the seasons. But you needed measurements of time to actually find longitude, and it was all a drive for the Navy. However, it had unintended consequences. As this problem began to be solved in the late 1800s, the world began to industrialise. Time became a really, really important tool for everyone, which actually elevated Greenwich's position globally. We'd already solved the longitude problem, which was very, very, very good, of course. Amoridion was voted for in 1884, and it became a centre of timekeeping for science as well, which is really important. Well, let's head downstairs to another gallery and talk about trains. Let's do it. We've come into another room of, well, lots of clocks. Lots and lots of clocks. These look a lot more modern. And I teased trains, and trains, like ships, trains are another... It's transport that drives this. The Industrial Revolution brought about ways in which people could travel quickly, and this was really key, because before then, as we were talking about before, everyone has their own time. It's the sun, my local time. Whenever the sun hits south or the highest in the sky, that's 12 noon. And it meant that actually, even in England, every town had its own time. In Bristol, for example, it was a few minutes, because it's eastern and west, of course. It was a few minutes ahead of London. And that was fine in the past, because it would take you hours, a day, to ride to Bristol on a horse. But what happens if you can get there on a train? In a few hours. You need to know when your train is coming in. So what do you keep? What do they put on the schedule board? Do they put when the train arrives London time, or do they put Bristol time? And it became a problem, because people were missing trains. People couldn't travel between locations. So the idea was set that we have a time zone, that instead of every town having its own time, all of England, all of Britain would have one central mean time. So it wouldn't be based really on the sun anymore. It would be based on one clock that would keep a 24-hour cycle for all of England, a time zone. And that's what Greenwich Mean Time is. That was voted upon. It helped trains arrive on time. And then that system was adopted globally, as we were able to travel around the world faster and faster. And eventually now, not every country, but time zones are established across the world, and most countries, all countries have adopted them. Phil should mention this clock, just because we're a podcast, but we're essentially based on broadcast. And this is the pretty famous in the UK, certainly, the pips that you get on the hour on the radio. And this is one of the original pips clocks. Yeah, it was the clock that kept the six pips. Greenwich, it's a time desk. We had a time desk here. These were all offices now in the museum. They were the ones who would keep the time, and that signal would be sent out across the world. So they would have astronomers measuring exact side real time from the stars. They would then translate that into civil time, so that we could use and update the clock, the shepherd gate clock, and keep Greenwich mean time accurate. And then that time was sent out across the world via telegrams and telegraphs. So that's the Greenwich time signal. Yes, pretty much. And the six pips were literally the beats of this clock that you would hear that would standardize the... And we still hear it on news broadcasts at the start of news broadcasts today. And this was the clock that kept that time. Well, Phil, I mean, we've whizzed through time. I guess, Phil, you should share one more story, which is this idea that you could sell time. Oh, yes, you could. And you could make money from time. You could. We didn't. So there's a clock on the gate at the front of the observatory. It's called the shepherd gate clock. It is a GMT clock, 24-hour clock. So it's strange when you first see it, because you're used to a 12-hour face. It has all 24 hours. And when this time signal was sent out to the world, that clock would also be updated for the citizens of London. Now, if you weren't nearly observatory, though, what good does that do you? Because you can't see the shepherd gate clock if you're in Peckham or if you're somewhere else. So there was this woman, Ruth Belleville. She would come to the shepherd gate clock early in the morning when the clock was first set to the time. She would record what time it was. High-tail it into central London as fast as she could with her own timekeeping device and then sell that accurate time to businessmen, to people in markets, anybody that needed the time in central London. And yeah, she made a pretty penny off it. Finn Burridge with Richard at the Royal Observatory Greenwich. And many thanks to the team there for letting us have a look around. This is the Supermassive Podcast from the Royal Astronomical Society with me, astrophysicist Dr Becky Peckist and science journalist Izzy Clark. Right. Our Supermassive Club. I would like to tell you about it. So thank you for everyone, for all of your patience while we set this up. We obviously took on adverts at the beginning of this year to support the podcast and then realised that some people might not want to hear them. So we've now... We're very vocal people. Yeah. So we've created an ad-free version. Again, you can subscribe to that. Join the Supermassive Club. And that just helps keep the show going in a different way. It's going to be £3.99 a month. So that gives you ad-free versions of the podcast. But there's also going to be a few forums on there. So places for you to share, you know, stargazing photos at home, any astrophotography that you've tried out or are learning. There's going to be the Supermassive Book Club in there as well. So you can just share your cool books that you're reading and, you know, want to share with other like-minded people. Again, there'll be another way that you can send us in some questions. But there is one rule and that is to be kind to each other. That's what we're all about on this show. So yeah, just please respect everyone that's in there. That would be lovely. Thank you very much. So there's a link in the show notes for you to subscribe. If that sounds interesting to you. It's supermassive.supportingcast.fm. But yeah, it's in the show notes. So you just click the link in the show. And a big thanks to Izzy for sorting that out as well because it was a bit of a slog. So well done, Iz. It's a bit more complicated than I expected, but it's fine. And also, I just really like the idea that what started off as just being a way of doing an ad-free version of the podcast to help support the show has actually now been a way that we can create a forum and people can share their photos. And hopefully it will be just like a nice little place to go to. But anyway, shall we get on with some questions? Let's do it. Becky, Robert, are you ready for... Just about. Go for it. I think it's a potential philosophical existential spiral that we're all about to embark on. But anyway... Take a deep breath. I think, actually, this is a fun one to start with. David Connity asks, if you had a time machine, which point in time would you go forward or backwards to? And why? Who wants to start? Good question. I was thinking about this. I reckon I'd like to go a thousand years into the future just to see how things turn out, because I know it's a time I'll never live to. Right? And just, you know, will we have managed interstellar travel with space probes or something? You know, I don't know. Those sort of things seem quite exciting. Do you think not much will have changed, but we'll live underwater? Who knows? It's either dystopia or utopia or probably something in between, I think, is probably the best guess on it all. But maybe we'll have consented some of the bigger problems. I'm just laughing at the fact that you've been influenced by the year 3000 song lovers. Yes, I know. It's like, I'm busted, we'll be there. To all the Jonas Brothers, to all the people in the US. How about you, Becky? I'd go back in time to the time of the dinosaurs. Not because dinosaurs are really cool, but they are really cool, but because I want to see Saturn without any rings. We think that's how old Saturn's rings are. Okay. Isn't that sort of slightly depressing for you, though? Yeah, a little bit. But do you think it'd be cool to look up and be like, wow, how different is that? And sort of experience that big of a change in what is in sort of universes terms, relatively short amount of time, I think would be very cool. And shorter days as well, slightly shorter. Is that now or shorter then, something like that? Yes, exactly. See the moon so much closer, and the stars would have changed. I guess for the other sort of going forward, it would be sort of, could you go forward enough so that your North Star wasn't Polaris anymore? Because of the procession of Earth axes. That would be cool too. Do you know what? The point that I think about probably too much, this is probably my Roman Empire, which is really not that exciting, is if I can tamper with the situation at the time that I travel back into, it's that split in sort of that industrial revolution time between choosing motor cars and electric cars. And I'd be like, go for the electric please. But who knows what, well, that would have ended up. But anyway, I think about that choice point far too much. So I'd like to be in the room where there would be discussion. Yes, exactly. More musical references. Great. Just a jake one of me. Robert, let's get onto some other questions. Anna Kirsten on Instagram asks, if time passes slower on the ISS, how long would you have to orbit to get a 24 hour difference to Earth time? Yeah, this is like that longevity thing where you can eek out your life a tiny, tiny bit more compared with people on Earth. So yeah, hi Anna. So for objects in Earth orbit, you've got a combination of things. And one of them is special relativity, time slowing down relative to people on Earth because you're traveling faster. And general relativity where there's the opposite effect, the azure deeper in the gravitational field. So say you're closer to a black hole, which Becky, you know, can talk about later on, then time is running slower for you there. And the two balance out roughly about 5,000 kilometers above the Earth for satellites. So for the ISS, they win. They get, well, if you see it in that way, they get slightly slower passage of time than we do on the ground. Now it's tiny, tiny, tiny, tiny, as Becky was mentioning earlier on, like 5,000ths of a second every six months, but measurable. And on that basis, the ballpark figure is, I think you'd have to wait getting on for 50,000 years for that to come to 24 hours. So sadly, unless you've got a way of isolating yourself in some coma or something or cryogenics, and you know, it's not going to be very, very helpful. But yeah, you do eek out that tiny difference. Becky also mentioned the NASA's own sort of twin paradox and the twin experiment where Scott Kelly was on the ISS and Mark was on the ground. And Mark's quite a tough guy. Actually, he's a senator now, and he's in military service and so on. He also took on Elon Musk and online and so on. But they're actually, they were doing that to test their health outcomes. And then what's surprising there wasn't a huge difference, which is quite a positive thing if you want people to stay in space for a long time. But I don't think NASA, NASA worried too much about the difference in their ages. Okay. And Becky, Ian Buchanan has emailed about photons and time. He says, I've got a question that's been bending my brain a bit. I've read that photons don't experience time because they're massless. So from their perspective, if we can call it that, they travel instantaneously. If that's the case though, how does that fit with the idea that the speed of light is the universal speed limit? If photons don't experience time, do they even have to obey the speed of light? Ian, you're asking the tough to explain in a few minutes questions here as opposed to explaining them in an entire undergraduate lecture course. It feels like one would be needed to fully explain this one. But in a nutshell, yes, they have to obey the speed of light even though they're massless. Like what you have to remember is that the speed of light is a universal speed limit because things with mass can't reach it because you need an infinite amount of energy to accelerate up to that speed. And then time dilation is so extreme that it stops. So this whole, yes, time is instantaneous for photons traveling because time has stopped when you reach the speed of light is really only coming from the perspective of something having mass. With photons being massless, they don't experience that same restriction on energy accelerating up to that limit because there is no bending of space-time for something that is massless. So yeah, they don't experience time. But that doesn't mean to say that it wouldn't be restricted to the speed of light in terms of the speed they can travel. That's just a fundamental property of light itself. If anything, the question should be, why is the speed of light that fundamental property, that fundamental speed limit for things that then have mass? It doesn't make sense in that regard, but it's all because light, space-time, et cetera, they're all intrinsically linked in the laws of physics and in the equations you get out from special relativity and from general relativity as well. As I said, that might need an entire undergraduate lecture course to go over why that is the case. Thank you, Becky. Excellent effort there. And Robert, Richard J. Barberles wants to know, how many dimensions does time have? I mean, come on. I can't have that question. Exactly. I mean, come on, Richard. Honestly, nice easy one there. I mean, look, as Becky mentioned earlier on, we mostly think of ourselves as being sort of in a 4D universe, three spatial dimensions, one in time, blended together as space-time, but time is the one, you know, we don't have that option yet, at least, and probably not ever to kind of reverse our direction in it, so it flows in one direction. And the rate of that changes depending on whether you're in a strong gravitational field, how fast you're moving, et cetera, et cetera. But there are more exotic ideas on how these additional dimensions work, and they usually tend to be wrapped up on a very, very small scale embedded in things like string theory and so on. But for time, the difficulty is that if you have multiple dimensions, you can end up with all these causal paradoxes. It's very easy to imagine that breaking causality if you have something weird going on with multiple dimensions in time. So it's challenging at best, I think, to describe it. I mean, various authors have talked about it, and I suppose one relatively easy way to think about it, you could think about things like parallel universes or, you know, that sort of sideways in time, I do. Where you're going into then things happening at different rates, but it's hard, and I think... It's philosophically and conceptually quite hard to imagine how time would work on that basis, you know, and see the universe the way it does. I don't know how you would test it, but yeah, it's not to say people haven't thought about it, and sometimes they reject the idea out of hand, and sometimes they explore it further, which is probably not the definitive answer you're looking for, I'm guessing, Richard. I always think with that, like, we wouldn't have the language to even talk about it, right? Because, okay, we can't... We can't go forward or backward through time. That language makes sense, but if I say we're moving up and down through time, that doesn't even... or left and right through time, it doesn't mean anything, right? So how would you even have... How would you even describe and put into words on paper so that we can understand this concept of multiple time dimensions, you know? It's hard enough for me to think about, you know, when people think about, say, like, four-dimensional spaces and so on, and you see these mathematical contrasts of hyper cubes and all the rest of it, look at it and you think... Mmm... At me... Exactly. It's a visual representation on paper, typically, or a screen of something, which is not even 3D, but 4D, anything. And that's just four dimensions, you know, trying to extend it into time as well. And it's pretty hard. The equations may work for it, but the concept is a real struggle. Amazing. And Becky, obviously, we needed a black hole question. Of course. And she asks, how does time behave near and inside a black hole when we say space and time are intertwined? Yeah, so... We also get time dilation, not just when we're traveling fast, but when we're near heavy objects. So time slows down... The closer you are to a heavy object. This is what Robert was talking about before, as well with astronauts on the ISS having these, like, double effects. Like, you know, when they're sort of on the ISS, they're further away from Earth, right? They get less of this time dilation. So, black holes are obviously the ultimate curves of space time, because they are so incredibly dense. And so this is where we see the biggest effects when it comes to time dilation. And so, you know, if you are thinking about what's the closest we could get to a time travel machine is, then I think scientifically speaking, a black hole would be the closest. Because you could, if you were willing to risk it, you know, skirt past a black hole, maybe loop around one side and ping out back the other side. And you would have travelled across its very curved space and you would have experienced less time than people watching you do it from a safe distance, not moving. Or not in a high gravitational field, should I say, as well. And so, because less time would have passed for you, it essentially would be a way to travel to the future, right, without waiting for as long for the future to come. So that, in some way, you know, would be a way to do that. When we talk about space and time being really intertwined, though, is when we think about what happens beyond the event horizon of a black hole. So the event horizon being that sort of, like, sphere in space where if you enter inside that sphere, you cannot escape it again. That's what we really sort of classify as the size of the black hole. And this is where things start to get weird, because, at least mathematically, on the event horizon, what we say is that all the matter is squashed down into this infinitely, small, infinitely dense point that we call this singularity. And technically, the singularity is, like, you know, radius equals zero, right? If you've got a radius of event horizon, it's right in the very center of that event horizon. It would technically be a place in space, but also if you're starting to, you know, divide by zero, oh no, the mathematician's panicking, like, basically you can't actually define where that singularity is in space, therefore, also, you can't define where it is in space, time, but it's still there. But once you cross the event horizon in terms of what you would experience in terms of space and time, the only place that you would end up after crossing the event horizon is the singularity, because you can't escape it, you can't escape that pull of gravity. So you'll think the singularity becomes a place in time as well. Essentially, all paths you can take in space would also lead to the singularity, which means that the singularity is also a place in time, because it's a place in your future as well. It'd be like trying to stop tomorrow from coming. You cannot, you know, stop the fact that your future is the singularity at that point. So yeah, so when we cross the event horizon, time and space really, really, really get sort of like all mushed together, because the singularity becomes a direction in time, not just in space. We'll... Thank you so much to everyone who sent in questions and thank you Robert and Becky for condensing, you know, what, as you say, our actual, like, year's worth of studies in the future. And for you, minutes, so good effort, team. If you do want to send in a question, please do. You can email podcast.ris.ac.uk or find us on Instagram. We'll send you a message or find us on Instagram at SupermassivePod. We'll post one in the Supermassive Club. Oh yeah, you can do that now. Fair option. Special pat on the back to whoever posts the first ever question in the forum. I'll make it a good one, no pressure, but... Yeah. So, shall we rest our minds a little bit and end with some stargazing? Robert, what can we see in the night sky this month? Yeah, this is so much easier than questions about the nature of time, frankly. I mean, you know, we're right in the middle of summer in the northern hemisphere of June. It's obviously the month of the shortest nights, the longest days, the sun at its highest in the sky, and culminating in the solstice on the 21st of June. We don't have midnight sun in the UK, but if you're up in, say, the Shetland Islands, then you've got very short nights there. The sun doesn't set until 25 to 11 and rises at 24 in the morning. And I was looking at this, the islanders call it Dasimah Dim, and I apologise for probably wrecking the pronunciation of this because it never gets properly dark. But it's also... This year is also the major lunar standstill, which we covered in a previous episode when the moon is unusually north and south in the sky. And on the 10th to 11th of June, because of that, we'll see the lowest full moon in 19 years to the left of the bright red supergiant star entire, he's in Scorpius. And it should be a great view, even if you're in London, it'll be 10 degrees above the little horizon, so pretty low throughout. If you're up in Shetland, mentioning that again, it'll be one degree above the horizon. So I was kind of thinking if you were on the sub-antip of Shetland, looking out over the sea, you've got this beautiful opportunity to capture this moon moving along, and you get this other effect called the moon illusion, which is an illusion, which is where it looks a lot bigger when it's near the horizon. So I think it's a really nice opportunity, it's a large and dramatic world, or apparently large and dramatic world down there. Because it's a short night, the summer stars tend to be a bit washed out by twilight this month, it never gets properly dark, unless you're say further south if you're going on holiday down to the Mediterranean or somewhere. But you have got Antares, as I mentioned before, you've got a bit later in the night Sagittarius, and when it gets dark again in a few weeks, when it gets properly dark, that's going to be the direction of the heart of the Milky Way, the heart of the galaxy, and so if you've got a pair of binoculars or a small telescope, it's sort of festooned with stars and clusters and nebulae and something to look forward to. A couple more things, if you're up early at dawn, as I happen to have been this morning actually as we're recording this, Venus is really spectacular in the morning sky, really stands out. Obviously sunrise is pretty early this time of day, time of year rather, so you do need to get up, you know, 4am or so, but it's really beautiful and it's pretty much as best for the year. And if you have a telescope, it'll look like a little tiny half moon, or a waxing half moon, it'll be growing getting a bit fatter over the month. And the other two things I've really got to mention, were still at solar maximum, there's lots of activity on the sun, some of those sunspots are really very dramatic, and they are leading to displays the northern lights, maybe not on the scale we saw last year, although you never quite know, but you know, very, very often it's worth keeping an eye, I'll put the word safely on the sun, get yourself a safe solar filter, look it up, just get one, get access to one, get advice, but if you do that, then they can be really dramatic, some of them are even big enough, you can see without a telescope, with a filter. And the final thing, not to lose some clouds, we're entering this season when you get this beautiful clouds, high up in the atmosphere, 80 kilometres up, ice crystals reflecting sunlight, they're so high that they catch the sun, even during the middle of the night at this time of year, so roughly June, July it's a perfect time to see those, and it's a really nice way to kind of bookend your evening, if you're out on it's a warm evening and you look out and do you're somewhere dark and you see this beautiful display, and you'll recognise them partly because they're just ethereal, they're very odd things to see, but also they're there in the middle of the night, you know, they're glowing in the middle of the night and they don't move, because unlike clouds, lower down that are being blown around, these things are so high up, they only change very, very slowly, so do look out for them and send us your photos of all of these things, or add them to the Supermassive Club. Thanks Robert, and actually I obviously had my little trip away over to the Southern Hemisphere in Bali, and I got to see like the Southern Cross for the first time, which is great, and you know, the plough upside down, which will always be fun, and just the moon looking a little bit different, so I can tick that off my bucket list. I think that's it for this, right? We will be back, also just well done to anyone who has got to the end of this episode, I think, part on the back, I mean, we threw a lot at you this episode, but we'll be back next time with an episode on everyone's favourite former planet, Pluto, and there'll be a bonus episode taking on more of your questions in a few weeks. Contact us! Get holiday ready without the faff. Search post office travel insurance. T's and C's apply. Medical Assistance Plus is only valid for trips taken outside of the UK. With LV, I can get my home insurance from just £133. They've made it easy for me to get a great price, and their 24-7 emergency helpline lets me look after what matters to me, because insurance is simple when it's me and LV. No wonder we're rated excellent on TrustPilot. Get your quote today at LV.com. 10% of new customers paid £133 or less July to December 2025. LV General Insurance is part of Allianz. If you try some astronomy at home, it's at SupermassivePod on Instagram, or you can email your questions to podcast.ris.ac.uk, and a close drop in the Supermassive Club, and we'll try and cover them in a future episode. Thanks for watching. I'll see you in the next one. But until next time, everybody, happy stargazing.