The Supermassive Podcast - What is Gravity?

Episode Date: April 24, 2025

This episode is a weighty one. Some might say massive. It’s all about gravity. What is it? Why does it matter? Izzie and Dr Becky explore Einstein and Newton’s different ideas on the subject, plus Izzie visits the Royal Society to see Isaac Newton's original manuscript for his works on gravity and the laws of motion. Special thank you to listener Hanna_m_e for suggesting this episode topic and to Professor Tessa Baker, at the Institute of Cosmology and Gravitation at the University of Portsmouth, and to Keith Moore, head of collections at the Royal Society for appearing on the show. If you have a question for the team, or an episode suggestion, send them to [e-mail] or find us on Instagram, @SupermassivePod.The Supermassive Podcast is a Boffin Media production in partnership with the Royal Astronomical Society. The producers are Izzie Clarke and Richard Hollingham. Hosted on Acast. See acast.com/privacy for more information.

This is a big topic. Essentially you're stealing energy from planets for our own personal gain. What if there is no dark man? What if there is no dark energy? What if we're just applying the wrong laws of gravity? Hello and welcome to the Supermassive podcast from the Royal Astronomical Society with me, science journalist Izzy Clark and astrophysicist Dr Becky Smethurst. You could say Izzy that this episode is going to be a weighty one. Some might say it's massive.

Because it's all about gravity. What is gravity? Why does it matter? And we'll explore Einstein and Newton's different laws on the subject as well. A special thank you to listener hannah underscore m underscore e for suggesting this. Yes, it's a good one. I can't believe we haven't done it yet. I feel like we say that about so many of like the fundamentals like why haven't we done that yet? Yeah, because also without Hannah's recommendation, I would not have gone to the Royal Society for our second interview and I would not have seen so much of Isaac Newton's work. Oh my god, I had this amazing afternoon so you will hear that and my over excitement later in the show. You see this is when I'm like why am I an astrophysicist

and a science journalist? I'm just being like professionally nosy. Hello, can I just come and see all the fabulous things in your archive? Thank you. So I'm going to say it now. This is a big topic. So Robert, Becky, ready yourselves while we try and tackle this in under an hour. And obviously Dr. Robert Massey is here, the deputy director of the Royal Astronomical Society. So Robert, perhaps we'll start with an easier question from Matt P on Instagram, who asks,

what is the etymology of the word gravity and who first used that word? Yeah, Matt P, no such thing as an easy question on this podcast, I think on supermassive or supermassivity. Yeah, I want to think about today's episode. But I looked into this and the word appears to originate from the old French gravite, with an accented E meaning seriousness or thoughtfulness, and the Latin word gravitate meaning weight, heaviness or pressure. You find similar words in other languages like Slavic, Urdu and Sanskrit, and surprisingly. But it was Newton who shifted it to something more akin to the modern sense, which isn't surprising as it was Newton who shifted it to something more akin to the modern sense, which isn't surprising as it was his basic understanding that took us there. A force rather

than a kind of quality in his book Principia in 1687. And that was this transformation to the idea that two bodies attracted each other through gravity rather than just bodies having a tendency to fall towards the centre of the earth. And as it happens, as well as the Royal Society, we've got a later copy of that amazing book Principia incip, here in our library at the RAS, but that's for another time. Maybe I'll get to see that one. You can get to see that, I promise you. We put that on display quite a lot. So yeah, come and have a look. And obviously we should also put a nod to the first person to use the word Mavity for all the Doctor Who fans out there as well. But cheers, Robert, we'll catch up with you later in

the show for some more questions. And course this month stargazing tips. Right, let's get into this properly. What the heck is gravity? Right, it's over to Tessa Baker, Professor of Cosmology at the Institute of Cosmology and Gravitation at the University of Portsmouth. Gravity is one of the four fundamental forces that are sort of known about in the standard paradigm of physics. So the other three being electromagnetism and the strong and weak nuclear forces. Strong and weak nuclear forces you probably don't experience very much in your daily life, but they control what goes on inside atoms. But gravity and electromagnetism, you do experience a lot,

particularly gravity, if you jump off and down. So the big question is then, what is gravity? What causes that fundamental force? And our modern understanding of that comes from Albert Einstein and his general theory of relativity. And his big kind of leap was to understand that gravity is really the curvature of the fabric of space and time itself. Just like that little nugget. A load of mind blowing concepts in quick succession. Yeah. Right. So what Einstein realized is what you think of as a patch of empty space, like

go deep space, far away from any planet, stars, anything, get a box of the universe. You think that's empty, there's nothing there. But that nothingness itself kind of has dynamics. So it's not just a sort of empty stage on which the rest of the universe happens that the fabric of space and time itself is a malleable thing, kind of like a fabric or a fluid. And so rather than just sort of being constant and sort of steady everywhere, that fabric of space and time can be bent and stretched and warped. Yep. And what causes that bending is the presence of massive objects.

And the more massive you are, the more you bend the fabric of space and time. So things like stars, really big objects cause huge dents in the fabric of space and time. And what we feel as the force of gravity is us really moving around on that distorted bent space-time surface. Can everything be explained through that lens? Can all of gravity always just been explained as a fabric and all of those distortions? Sure. So I guess one thing to comment on here, which we've sort of implicitly been saying it already, but we haven't addressed it, is that this fabric isn't just made up of space, the three dimensions of space that you walk around in. We did say the word space-time.

And so actually this fabric, what it is, is four-dimensional, and it kind of has a dimension of time folded into it. So with that picture, yes, in principle, that is the fundamental underlying picture with which you can always view gravity. It's just that in kind of mild fields, you almost don't need to look at that layer of complexity. You can take your Newtonian picture and it will work. Now, there are things that we don't understand in the universe related to gravity. So you've probably talked about the existence of dark matter on the podcast before. And that's a little bit of a puzzle. It's not saying that Einstein's theory is wrong, but that we do need more stuff than we can see in the universe, more stuff sourcing gravity. There's so much to unpack there. So that is one idea and that's how we talk about the

universe and we use Einstein's ideas when we talk about gravity. And so how does that then differ from what Newton thought about? Because Newton had his theories of gravity, which were before Einstein's. So how are they different? So yeah, Newton predated Einstein by about 300 years. Probably up until that point, no one really questioned what gravity was, right? You just knew from the day you were born, you just were stuck to the surface of the earth and that was it. And Newton was the first person to really formalise that mathematically. So what he was able to do is write down mathematical expressions that tell you how big is that gravitational force that sticks you to the earth. But Newton

didn't really fundamentally understand where gravity came from, he developed a kind of a cookbook, if you like, a recipe for computing the size of it. Now, actually, what was realized later on is Newton's laws of gravity drop out of Einstein's big picture of gravity. And in particular, you get Newton's laws when you're dealing with things that are moving much slower than the speed of light and are not anywhere near a black hole. So when you're in very nice, gentle, mild gravitational fields, like Newton would have experienced, you get his laws. Okay. And does everything experience gravity in the same way or how does that differ?

So yes, everything experiences gravity. We do not yet know of a particle or substance that doesn't. Interestingly, what Newton's laws of gravity tell us is that actually everything in the universe is exerting a gravitational force on everything else. So it's not just that the Earth exerts a gravitational force on you, you actually also exert a force on the Earth. It's just that unfortunately, given your relative difference in sizes, the Earth is going to move you, you're not going to move the egg, grow up to north. So everything and everybody experiences gravity. However, it can differ depending on where

you are in the universe. You have a constant property, it's the same everywhere, which is your mass, which is measured in kilograms. But your weight, the force that you feel pulling you down can differ depending on where you are in the universe. So you would weigh a different amount on different planets. If you can go up to a very high altitude place, technically you weigh slightly less than you do in a very low altitude place on the earth. It's just that that difference is very small,

it's less than a percent. If you go up as far as the space station, it's still not huge. There it's about 90%. So your weight there is about 90% of what it would be if you're on the surface of the Earth. So it's falling off quite slowly. Now you might think that's a bit strange because when you see people on the space station, they're floating around, they don't experience any weight at all. But that's because they're in free fall around the earth. It's not that they're weightless, they haven't gone so far, they've escaped the earth's gravitational field. Quite the opposite. It's just that the spacecraft

they're in and themselves are all kind of falling, orbiting, which is a kind of fall. A continuous fall, isn't it really? Yeah, exactly. I guess if we think about what's going on in the field, and you work on a newer idea, I suppose, in the long conversation of gravity, which is something called modified gravity. So what is that? Is the idea that there might be corrections to Einstein's theory of gravity. In the same way that we now know that Newton's ideas about gravity are really a sort of limit of Einstein's bigger picture, that might not be the final story. It's because there are some might not be the final story. It's because there are some strange results in cosmology. When we apply Einstein's theory of gravity to the universe as a whole, it seems to give

us things like dark matter and dark energy, or rather I should say we need to invoke the existence of those things to make all the results make sense. There is an alternative idea of this idea of modified gravity that says, what if there is no dark matter? What if there is no dark energy? Or maybe there is dark matter, but not dark energy. What if we're just applying the wrong laws of gravity? And so I work on trying to understand what those bigger ideas about gravity could be. And if you did have one of those ideas, what would it do to the universe? How could you kind of prove that idea true or false?

Thank you to Dr. Tessa Baker from the University of Portsmouth. And on that bombshell, that's so much for me to wrap my head around. I think this is why I love this topic because I think I'm quite a visual learner and you're just trying to like contemplate it and try and see it and it's like no that doesn't work and I kind of enjoy that. But we've had a follow-up question from Hannah who suggested this episode and I think it's a good one. So she says that we've all seen the bowling ball on the trampoline model and it's beautifully simple of showing how matter bends space-time, why the moon orbits earth and earth orbits the sun but and this is the main thing of this a trampoline is 2d and the universe is 3d and it's more like a body of water so would it be correct to think of gravity as pulling space time towards it, making it more dense?

This is a great question, Hannah, because sometimes I do wonder if that analogy of the trampoline does more harm than good, especially for all the visual learners out there. Because if you think about that analogy when we say put a bowling ball on a trampoline, you need gravity of Earth for the object on the trampoline to curve the surface.

And I think that's where a lot of the confusion comes from, right? So when we think about this, like all of this comes from the maths of Einstein's theory of general relativity, right? The equations in general relativity describe like the motion of objects on space-time itself as moving over a curved space-time.

So all the equations are doing is like, you've curved that because there's a massive object there. That's not to say necessarily this is how it works physically, but that's what the equations are describing. And it's very hard to picture this in three dimensions and also to make a visualization of this in three dimensions as well that people can be like, oh, I kind of can see what's going on.

And then there's the fourth dimension of time, but we'll just skip over that part. Yeah. And so I don't think the right word is pulling, right? It's not pulling on anything, nothing in the maths and the equations general relativity says pulling, it's curved space and trying to imagine curved space in three dimensions, it's very difficult.

What I like to picture in my head is like, if you think of a perfect three dimensional grid, right? Kind of like a, you know, a squared, you know, graph paper. You might get in a lab book, right? But in three dimensions where you've got everything and all these lines parallel to each other and then all these lines perpendicular to it in three dimensions, right?

If you put a massive object in the middle of that grid, right, you'd mess up that neat grid a little bit. What would be happening would be those grid lines would start curving towards the object, right, curved space. Again, that is very difficult to picture what that would actually look like though. So what really helped me actually was this incredible YouTube video from a carpenter

called Oliver Gomez. I think I'm pronouncing that correctly. Apologies if not. And they did exactly this, but with wood. Sounds strange, right? But he made a load of small cubes of wood, two different types of wood. And then he glued them all together like a Battenberg, just like alternating colors of the different wood. So you could see the difference, right? And the glue also he made really thick. So you could see the white lines, which was like the grid lines I was just talking about, right? And then he started to turn the wood, you know,

you put it on a spool and then you turn it and you carve into it in the round. And he essentially made what he called his wormhole coffee table. And if you haven't watched it, it's incredible. I'll put it in the show notes down for you. It's an incredible piece of craftsmanship first of all, but it's a great 3D visualization

of space curvature in three dimensions. So what it is is essentially like the base of the coffee table is this flat sort of like glued together pieces of wood, and then it curves round and up to the very top of the coffee table. But at the ends, you get this wormhole like thing that connects the two surfaces.

And in that bit of the sort of like wormhole connection, you can really see like, you know, the individual squares and the grid lines and the glue and that curvature and almost like that warping due to the fact that you've turned and carved into the wood that's, you that's slowly been created from this perfect grid that he started with. He didn't actually bend the wood, he didn't curve the wood.

He started with the perfect grid. He sort of carved into it and you can see then what this actually looks like in terms of 3D curvature of space. And it's honestly mind blowing. Not just watching it come together because the craftsmanship is incredible but then seeing the final product and going, oh, yeah, that's what that looks like in three

dimensions. I need to see this, but my dad is also a carpenter, so I just need to be like, and now can you make me one for Christmas? It looked like it took him years. Okay. So good luck to your dad. Okay, great. Yeah, I really, really want one. I think're absolutely just perfect. You don't even have a coffee table book on that coffee table. Look at it. Yeah amazing. And obviously gravity is a fundamental force and it can be applied to so many different things but it's really helpful in space exploration as well. Helpful? Hindering for quite a while. Well, yeah. In some circumstances, it can be helpful. So for example, if we look at gravity slingshots, like how does that work?

How is that a helpful rather than a hindrance? Yeah, the hindrance would be keeping us here on Earth until the 60s. But yes, slingshots, essentially you're stealing energy from planets for our own personal gain. So a spacecraft, you know, heading towards a planet will get accelerated by the fact that it's traveling on the curved space and it's falling towards that planet's gravity. But if you get it on a trajectory so that it just swings around the other side of the planet, you can sort of steal that acceleration so that it was going slow originally and then as it accelerates towards and around

and away from it, it will keep it right because there's no friction in space. So once it's going that fast, it will just keep going that speed and it will then accelerate it up. And so what happens is any gain in kinetic energy and movement energy the spacecraft has, the planet loses in kinetic energy. Which sometimes people are like, do we want to do that too? If into other planets, we lose energy. But just given the sheer size of the planet and how minuscule the spacecraft is in comparison,

that energy loss of the planet is basically negligible, right, in the grand scheme of things. So yes, slingshots, gravitational slingshots are very handy for space exploration because it takes a lot of fuel to get a spacecraft off the ground just because of how heavy the thing is that you're lifting it.

But then even more, if you wanna send it out to say the edge of the solar system or the outer parts of the solar system. So instead, if you use another planet like Earth or Mars or Venus to give you a boost, right? You don't need as much of that expensive fuel. Your launch is then lighter in the first place, which makes your launch less expensive

because you need less fuel. So it's just generally cheaper. It's why you sort of hear like some of some missions and some people report on the mission, like it's just launched, it won't get there for 10 years. You're like 10 years, why is it gonna take that long? But it's just because they're using slingshots, which takes a lot of time for everything to line up.

So we launch from Earth, and these spacecraft sort of trail us in their orbit for a while, and then they just wait for Earth to catch up again, and then they do the slingshot. All right, so it takes a while for all of that to happen. So for example, like ESA's JUICE mission, which launched in 2023, pretty sure we covered on the podcast at the time,

that won't get to Jupiter until 2031. Whereas NASA's Europa Clipper mission, which is going to the same place, it's just going to Jupiter's moon Europa, that launched last year in 2024, and that gets Jupiter quicker in 2030. So much quicker time than JUICE, but it's because juice was cheaper.

It's doing three flybys of two of three flybys of earth and one of Venus. So four flybys in total to make out Jupiter. Whereas Europa Clipper more expensive in terms of fuel because it only needs one flyby of earth to accelerate it out to Jupiter. Okay. That makes sense. And obviously as our black hole expert, we cannot, we have to talk about black holes in the context of gravity.

Yeah, you kind of do one without the other guy. Yeah, exactly. So what can you tell us about that there? And is there, I suppose, like a minimum amount of gravity needed to form a black hole? Yeah, yeah, exactly that is. Yeah, there's like a minimum amount that you essentially need to overcome the forces between the particles that make up atoms to crush them together into who knows what, right? Because you end up forming a black hole with an event horizon that we don't receive any information from. So you crush them together into something, whether that's the singularity, whether, you know,

the matter's crushed down into an infinitely small, infinitely dense point, or whether it's some other exotic form of matter that we've never been able to observe because event horizons are annoying. We don't know. But essentially, if you don't have enough gravity, if you have less than that minimum gravity, then you have a neutron star.

So that's just a star made purely of neutrons, which is the neutral particle that makes up atoms, and they're just sort of tightly packed as they can go, and that's what's resisting the crush of gravity down. It's something known as neutron degeneracy pressure. And people have been trying very hard to both theoretically and observationally determine

that exact limit, that exact minimum amount of gravity, you know, minimum amount of mass. It's actually known as the Tolman-Oppenheimer-Volkoff limit. And that is Oppenheimer is in. Oppenheimer is in the Manhattan Project. And the film that came out recently as well, I liked was they actually showed Oppenheimer and Tolman actually, you know, sort of at the beginning of the movie

working on that. Yeah, he's sort of like in the almost like a tutorials type with, you know, his PhD students and Tolman's one of them. And it's a really nice scene as well for people who are like super nerdy like me. And I'm like, oh, they're talking about the Tom and I'm like, fuck a minute. Anyway.

People in the cinema are like, shush, we're trying to watch the film. I was like, this was the most exciting part of the film. Anyway, that limit does depend on a few things. So it's not just the mass of the neutron star. It's also the spin as well. Higher spinning things can resist things for longer. But it's thought to be that limit around about, or it was thought to be at one point when we were sort of going through the theory,

as high as three times the mass of the sun. But since then, in terms of observational studies, we've refined that a bit more. So for example, do you remember there was a neutron star neutron star merger a few years back? We saw a big bright flash of light from that called a kilonova and we detected gravitational waves. So like ripples through space because you've changed the curvature so much where these two very dense objects are orbiting each other. We detected those and that actually managed to put a limit on it that made it that made that lower. So it actually brought it down to somewhere between 2.01 and 2.17 times the mass of the sun. So that was very helpful for that.

And we're sort of always searching for the smallest, like the least massive black holes we can find and the most massive neutron stars that are always sort of approaching those limits from both sides as well. And really the aim would be to try and break those numbers because if you could break them, you learn something more. But basically we're just trying to get to see from from both sides as well and really the aim would be to try and break those numbers because if you can break them you learn something more. But basically we're just trying to get to see if

we can actually sort of sample the entire population of things around those numbers of those limits. Okay thanks Becky and coming up after this is my very over excited trip to the Royal Society. All right so Tessa touched on the differences between Newton and Einstein's interpretation of gravity. And while we use Einstein's theory of general activity to attempt to understand heavier objects in space, Newton's laws of gravity are still a good approximation of Einstein's equations for lower mass objects. And so they're really key to how we describe the motion of things and gravity here on Earth. And how did he attempt to calculate how the world works? I visited the Royal Society in London and they published Newton's Principia which laid out his theories. Keith Moore,

their head of collections, showed me around and, disclaimer, there was so much to see and talk about that this is a slightly longer than usual edit. Sure, no, I and talk about that this is a slightly longer than usual edit. Sure, no one's sorry about that. I'm not sorry about that. And we started up by staring at a massive and somewhat intimidating portrait of the man, which overlooks one of the grand rooms at the Royal Society. Well he's born in Lincolnshire at Walsall Manor and he's born into a kind of yeoman class of people so it's farming country up there. Newton wasn't great at farming there are lots of stories about him as a young man looking after cattle and not being very good at it. He had his

mind on other things so he goes to Trinity College Cambridge to take a standard course there but very early begins his own reading programme, so he's interested in lots of things, but particularly mathematics at that time. And he pretty quickly starts outstripping his tutors and he begins to think about particularly gravity and the way that, well why is the moon staying up there? What's happening with that? Why isn't it flying off somewhere? So he begins the process which is going to end with a Principia Mathematica thinking about what we would call now orbital dynamics.

There's lots of interesting things to see in the Royal Society, so shall we go for a little walk to see some of the other things that belong to Newton that are here? We've got lots of Newton material, so yeah, you could be here for the afternoon. So we're now in the foyer of the Royal Society and there's a display case in front of us with another portrait of Newton and I love this, a beer flagon. That's absolutely right. The flagon is on loan to the Royal Society and this is, it's a connection with Trinity College Cambridge when Newton was a student there. When he left, he gave some of his furniture and effects to his roommate, John Wickens. And it survived in the family, apparently.

So you can see here an illustration of this wooden beer mug in the Gentleman's Magazine in the early 19th century. You don't think of Newton as being a party animal, really, do you? But here we have his beer mug. So I guess he must have used it a little bit. Yes, to sort of help the writing process, I suppose. Yeah, and in fact, the scholars who wrote this up thought just that. Part of Newton's formula for making ink included beer wart.

So the Principia may have been written in beer. And also, I mean, we'll get towards the end of his life later, but we actually have his death mask in front of us as well. So what is this made of and how would that have been made? So this is made from plaster and wax. It is as close as you will get to the great man this is exactly his likeness on his deathbed the process is quite an interesting one you might just be able to see if you look along the center line of the mask here a thread marks what they would do would be put us to put a thread from the forehead down the nose across the lips and chin then place wax over the face.

And if you pull the thread out before it's set, it broke into two halves, so you could then rejoin them and take casts from that. So that's what's going on here. And this is very useful for artists of the period. If you wanted to make a marble bust of Newton, and we have some of those, or you wanted to make a monument for Westminster Abbey,

having a record of his likeness was a very useful thing. And so we've got quite a few items in front of us here and they give us a bit of an idea of what Newton was like but what do we know about his character? Yes these show his likeness but not his personality and we know he could be quite argumentative. He got into disputes with Flamsteed, with Robert Hooke, with Leibniz. He was very good with young acolytes who worshipped and agreed with him. If people argued with him he was less impressed. But that was good for him, You know, it stimulated his thinking, it pushed him into publishing. And, you know, science has a lot to thank him for

and for his rather fractious temper. And so I think we've got one more thing to see and it's the biggie. So shall we go down to the library? Let's go down. So we've just walked into the library. So we've just walked into the library there is a very epic leather bound brown book in front of us and as a physics, as a former physicist I suppose I'm so excited to see this because this is the Principia, this is Newton's sort of lifetime's work. It is yes so this is the manuscript version of Principia

Mathematica. We also have a first edition here which belonged to John Flamsteed, the first astronomer royal. So we have a cornucopia of Newton material here for you. Shall we look at the Principia first? Yeah, my palms are actually sweaty, like I can't touch anything. I'm not going to touch anything. Oh my gosh. I'm actually nervous. Here we have it. So, Philosophia Naturalis Principia Mathematica beginning with what was originally a separate work, De mortu corporum. So this is where Newton has been persuaded to write this by Edmund Halley primarily.

The manuscript here is actually not nice at Newton's hand. This is in the hand of his amanuensis, Humphrey Newton, no relation as far as we know, but this is what went to the printers to produce the first edition. You don't just have the text of the Principia here, but you have all the bits that were taken out and all bits where it tells you where to put the figures which are the little woodcut illustrations which you can see in the printed volume. So if I just turn a few pages here you can begin to see some of this going on. What's that? A4 size and it's this cream parchment paper with brown ink and there is, you know, it's covered in writing but in sections you

see the word out circled and so these are the little sections that need to be removed. That's right so the sheets would be sent to the compositors, they'd set the type and right to the end there could seem to be correcting and you can still see in some places the kind of inky thumb prints of the printers in the margins there where they've been rolling the ink onto the tides. When we talk about gravity, a story that always comes up is this idea of the apple tree and Newton and his apple tree. So what is the story there and is it true? Well I can read you the story because we have that manuscript here as well. You've got everything here, of course you do.

This is a manuscript by William Stukeley who was a physician and an antiquary. He also went to Lincolnshire. This is a much smaller book now. This is sort of, you know, your standard reading-sized book again. We've got these beautiful cream pages with lovely, lovely handwriting from here, from Stukely. So what does this tell us? Stukely was slightly incensed that by the 1740s and 1750s there hadn't been a proper English biography of this great man. So he gathered stories from London from his own experience of knowing Newton as a young man. And he gathered stories from people who'd known Newton in Lincolnshire and he put them into this manuscript. So here we have William Stokely in Kensington talking to the elderly Newton and he says

after dinner, the weather being warm, we went into the garden and drank tea under the shade of some apple trees. Only he and myself. So this is Newton and Stukely. Amidst other discourse, he told me he was just in the same situation as when formally the notion of gravitation came into his mind. Why should that apple always descend perpendicularly to the ground, thought he to himself, occasioned by the fall of an apple, as he sat in contemplative mood? Why should it not go sideways or upwards, could constantly to the earth's centre? It doesn't say though that it fell on his head, so did that happen? No it didn't happen. So that's a Victorian invention by one of the

Disraelis writing about Isaac Newton. You can understand why Newton told this story. So the Principia Mathematica is densely mathematical. It's in Latin. It's not the sort of thing that you would take as holiday reading, unless you're a physicist such as yourself, you'd probably take it to the beach. However, the apple tree story is nicely tailored to a general audience. So this great idea can be explained by a very simple story

where an Apple is quite planet shaped. You sort of get the idea of that. Newton's very religious. He knows about the tree of knowledge and the Bible. So these are the things he's drawing into the story. And it's just a really good piece of public engagement and science. Thank you to Keith Moore.

I honestly could have spent all day there. You found your new home. You should have just camped out. And it was really weird. So Richard, you know, editor Richard and I went into the Royal Society and I don't really know what I was expecting to do that day. I wasn't expecting to see the Principia. And so when we walked into the library, I was just like, oh, my hands are clammy. Like, this is what no one briefed me. Like I wasn't mentally prepared for this. So it was just super exciting. And then I went and carried on editing after we'd done that interview and just was kind of in the basement of the Royal Society. And then pinching yourself. Yeah. What is going on? And then just in the cabinet next to me was one of his early telescopes

I think it was like the second one that he had made and that's just in the case in the Royal Society So it was just a really lovely day. So thank you to everyone at the Royal Society that made that happen This is the super massive podcast from the Royal Astronomical Society with me, astrophysicist Dr Becky Smethurst and science journalist Izzy Clark. I have a quick update about the ads in the podcast. We were hoping to have the subscription service in place by now to give you or to give those that want it an ad free version of the podcast. But for various reasons, I'm not going to go there.

It's proved a lot more challenging than we thought, but it will happen. Just please know I'm working on it in the background and it will happen, I believe. So for everyone who got in touch, it is coming. We are working on it. We haven't forgotten about it. Okay. Let's get on to some listener questions. Robert, Becky, are you ready?

Born ready? Yeah, just about. questions. Robert, Becky, are you ready? Born ready? Yeah, just about. So, Robert, Reggie D asks, where is that cheeky Graviton hiding? And Ginger Holt similarly asks, Graviton, what are they? So, Robert, thanks for you. Yeah, great questions. Obviously nothing like as easy as you'd like. By the way, Becky, I was looking up Oliver Gomez's coffee table and I'm not suggesting one of your wedding guests might want this as a honeymoon gift, but it's a mere £7,000 with a three to four

month lead in time. A mere joy, yeah. And it does look absolutely beautiful, I have to say. Right, that was a way of avoiding the question. So anyway, taking the second one first, so yeah, Ginger Hulk and Reggie Deet, these are good questions. Second one first, so yeah, Ginger Hulk and Reggie D, these are good questions. Second one first, the idea is that the force of gravity is mediated by a, well, you've still got to call it a hypothetical particle, because we haven't actually detected them called a graviton, even though most people think it exists, or most physicists.

And we see similar particles described as bosons, and they carry things like the electromagnetic force. So think, you know, if you want light and so on, but you've got to think about things like electromagnetism basically as an example of that. But in the strong force, the subatomic level between quarks are the fundamental particles that make up neutrons and protons in the nuclei of atoms called gluons. And then you've got these W and Z bosons and they carry this weak force that leads to radioactive decay. And these are these are all basically well established. I wouldn't say we understand them perfectly but we've detected them, we can see them in

operation. But we have yet to find gravitons and part of this is because gravity is incredibly weak in comparison and a great point on this is that we are all capable of pushing ourselves against the gravity of the whole earth just by jumping up and down. Our ability to manipulate the electromagnetic forces in our body leads us to be able to do that fairly trivially. We might not stay up, we fall down again, but we can get off the ground. Well, like I always say, think of a tiny magnet, like holding a nail against the entire Earth. Or even static electricity and a bit of stuff stuck to something too static. All of these

things we can resist the whole gravitational force of the Earth. We can move along, we can jump and all those kinds of things. In 2024, a Swedish research group led by Igor Pikovsky at Stockholm University suggested it might just be possible to find gravitons using a 15 kilogram bar of beryllium metal and looking for a single resonance, so sort of quantized effect as a graviton went through it, coming from an event like the merger of neutron stars that leads to a big pulse of gravitational waves. But, you know, that said, even if this idea works, and I am not qualified to comment on how well it might work, I'm just thinking of how hard it was to find

gravitational waves. It took decades of work from the first ideas of doing this. So maybe this is a similar kind of challenge. Some sources will say it's impossible. It's just that they're so weak. We really struggled to do it. But there are these tentative ideas around it. It's really intriguing to imagine that might just be possible. Yeah, that's so true. Okay. And Becky, Big Benfulam asks, is it possible to create artificial gravity in space? Has it ever been done? Oh, good question, Ben. I knew this was coming for this episode. It's maybe why I chose it. I mean, this is something people have been thinking about since the early days of the

space race. I mean, like seriously, scientifically considering it, right, because people actually thought that we would need artificial gravity to survive in space for any human on any length of mission to survive in space. But obviously developing this kind of tech was so expensive. It was put on hold during the Mercury and Apollo eras, essentially because people realized, people seem to be fine with some short exposure to zero G.

Let's just put it on hold until long missions would require it. And you could argue with the International Space Station now with people doing six months in space, maybe even a year in space, we're at that point where it would necessarily require some sort of artificial gravity because we've seen, thanks to studies done

on the International Space Station astronauts that long exposure to zero G does have effects on the human body, right? So bone density loss, muscle atrophy, weakening of the heart, vision lost, right? The list goes on and on, right? So artificial gravity would be great for astronauts and I'm sure they would like it themselves.

And our best ideas for how to do this have really, really been explored so well by sci-fi already. So I think that's probably what people are going to resonate with here. But for example, like if you had a static space station, rotation can mimic gravity, right? So that feeling that you get if you're on a merry-go-round and you're spinning around, it feels like you're being pushed backwards. Or if you're holding your friend's hand like out in front of you and you're spinning around, you feels like you're being pushed backwards. Or if you're holding your friend's hand out in front of you and you're spinning around, you feel like you're going to fly off backwards. So that feeling, if you can spin something at the right speed so that you can create that feeling, that force of the same strength as gravity on earth, then great. You've

got artificial gravity. That's what the spacecraft in Andy Weir's The Martian did, right, the Hermes is it spun. Or if you've got a traveling spacecraft, right, something that's not static in space necessarily, then you can actually do this with acceleration, right, can do the same job. Think about when someone puts their foot down in a car when you're driving and you again, you feel that force like pushing you backwards into the seat. If you can accelerate at one G, the same force that we feel on Earth, right, from gravity, then you've created artificial gravity through acceleration. That's what the spacecrafts do in James Corey's The Expanse as well, for those who are familiar. So technically, if you've ever been on a merry-go-round or accelerated in a car, then you have created

what you could dub artificial gravity, right? And technically, to come back to Ben's question about has it ever been done, the Gemini 11 mission in 1966 actually did manage to test this. So they had a 36 meter long tether, that tethered them to a static sort of spacecraft, and then they fired their thrusters. So they went in a circle and recorded creating a force of 0.0000015g. So they're very small in terms of the Earth's gravity. Neither of the astronauts recorded saying that they actually felt anything at all, but they did notice smaller objects getting affected and like rolling towards like what would have been the floor, you know, in sort of the artificial gravity sense. The problem with testing this

in space is that anything that moves or has moving parts in space is just an expensive failure waiting to happen, right? I really do think we'd need like an extended zero-g mission to justify developing the tech. I don't know how long that would be, five years, 10 years, or what point do you need it in terms of like the health of your astronauts to be able to fulfill the end goals of the mission. In terms of like, what people are probably listening and going, but what about like real artificial gravity? Yeah, exactly. You know, like not something that's just like a fictitious force that's created from rotation or from acceleration, right? And the problem is this is tied to energy and mass, right? If

you want to artificially create curvature of space, right? Think about how much mass you need. You need the entire earth to create one G. Like it's huge, huge amounts. And we don't know of a way to generate like a fake gravitational field that's not from having that much mass and energy there. Like say we do with an electric field or a magnetic field,

which are obviously so much stronger to work with as well. Right, so you need less energy for the strength of the field. So, honestly, I think artificial gravity in that sense is, I mean, I don't want to say never say never is my motto, but at the same time, it seems so way, way out of what we could actually achieve compared to that sort of like fictitious force, artificial gravity through rotation or acceleration.

Thank you, Becky. And Robert has cats asks how much mass would an object need to have a gravitational pull? And I suppose adding to that, what is the pull needed for an object to remain in orbit? Yeah. So, uh, okay. By the way, I was going to add one thing to what Becky was saying, which was the Apollo 8, as to Gemini 8 rather, where there was an uncontrolled rotation that nearly killed Neil Armstrong and David Scott. So they got out of it, a very high speed rotation. I don't know about the gravitational forces, but they almost blacked out. I think they probably deserve the moon landing as they got to do later as a result. But yeah, anyway, to answer the question. So the first question on how much does it need to have a gravitational as far as we know,

if something has mass, it will be subject to an exert gravitational pull, you know, we'll have that mutual attraction with other masses. So even the lightest particle has a pull. Now the caveat is it's virtually impossible to measure those because the strength of the gravitational pull as we've been saying before it's 10 million billion billion billion times weaker than the electromagnetic force so it's just incredibly hard to measure and we've just not been able to do it. And there's an interesting parallel actually which is that we don't we also can't measure gravity at very short distances too. I was hearing this in a great talk by theoretical physicists Nick Evans and Southampton last month and he was saying you know we can't measure gravity at a distance of less

than a millimetre so we don't even entirely know how it behaves at very short distances. There are some aspects of this you know that's still really intriguing and we take these things for granted but we can't quite verify them. Now as for the question of the pool needed to have an object remain in orbit well in theory you know very small things can orbit very other small things it's just they do so at a lower rate and they you know they're easier to disrupt so something staying in orbit around the earth if it's already there it will stay in orbit around the earth unless something acts to change that and that might be if it's low down that it enters the Earth's or the upper reaches the Earth's atmosphere slow it down because of air resistance it loses

energy and then drops down or if you fire a thruster and you increase the speed then essentially you're going to rise away from the Earth increase your energy and at some point if you keep doing that you reach escape velocity and you drift off so it's not so much about you know the pull needed it's basically that that equilibrium isn't disturbed that you end up you know either you if you go faster you will leave Earth's gravity entirely if you slow down you fall back to Earth and that's really how this works. Thanks Robert and Becky Jim Henrichs has emailed to say, Hi all, I love the podcast and hope you can explain the differences between the force of gravity that keeps us grounded here on Earth and the gravity, or is it gravitational waves,

that are generated by colliding black holes or whatever. This is a great question Jim and weirdly there's no difference. And again this is just something for our brains that struggle to conceptualize. Yeah, both are just curvature of space, right? So as the earth orbits around the sun, it moves through space. So as it moves into a new patch of space, it curves that patch.

And then as it leaves that bit behind, it uncurves again. And that constant curving and uncurving of space does send out ripples into space, gravitational waves. They're very, very small. However, as far as the gravitational waves made from black holes, like they actually, the energy required to actually produce these gravitational waves as well,

it takes energy to generate them and that comes from Earth's mass. Now it's a very small amount because Earth's not that big. It's around 200 watts of gravitational wave energy that sent rippling out into space because of Earth's orbit. Compare that is to the 100 million gigawatts of infrared energy we radiate out into space as just heat, as just infrared light, right?

So gravitational waves versus the rest of the stuff we're ready to get out into space, right? It's just absolutely miniscule. Now, it's only when you have two incredibly heavy and dense objects like black holes or neutron stars, right? That their movement through space as say they orbit each other, for example, and eventually merge, which is what we detects gravitational waves. They create ripples with enough energy that our detectors here on Earth have a hope of detecting them, even billions of light years away from where the merger happened and where those ripples have been sent out from. So the energies that we're talking about there

in terms of gravitational waves is 10 to the power of 50 watts. So remember before it was 200 watts from Earth and this is 10,000 giga giga giga giga gigawatts, five gigas. So Jim, there is no difference between them. It is just a difference in energy. Yeah. And if you want more on gravitational waves, we have done an episode on this back in February, 2023. It was called Getting Gravitational Wavy. So that's the one that you need if you want more on gravitational waves. But we again, along with the YouTube video, we'll link that episode here as well. I love how we do gravitational waves before gravity. I know. I was like, what was the choice there? Let's not question that. Anyway, carry on.

If we want to send in any questions, please do. We love reading them. You can email them to us at podcast at ras.ac.uk or find us on Instagram at supermassivepod. So let's finish as usual with some stargazing. Robert, what can we see in the night sky this month? Yeah, okay. Well, the obvious change is that in the Northern Hemisphere, at least you're getting a rather shorter length night sky right now as we head towards the Junus Solstice, although it's obviously going the other way in the Southern Hemisphere. But if you stay up late, there's still a lot to see. As I mentioned last month, I think the zodiac constellation of Virgo is really obvious

in the South now and not for planets, which is sometimes going through there, sometimes going through there, but it's actually a home of where there's thousands of galaxies in one of the nearest galaxy clusters, the Earth earth so i was thinking about this and thinking i know other telescopes are available but those sea star owners this is definitely the kind of target so do tag us on social media if you take pictures there um vergo is also though the home or the the location in the sky you'll need charts or an app for this, not of a planet but of the one of the brightest asteroids Vesta right now. So it's not particularly big, it's not the largest asteroid, I think it's a few hundred kilometers across, but it looks like a very faint naked eye star, just about visible to

the naked eye. But if you want to confirm it, if you look at that part of the sky, then what you can do is repeat the kind of exercise the 19th century astronomers did. Maybe if you want to really go old school, you can sit there and try to draw the star field and mark the dots. Or you can take successive pictures and what you should see is the dot moving from one night to another. I remember doing this about 30 years ago. It's not that hard to do. So if you want to see an asteroid, that's one guaranteed way to do it. Now, if you go further to the east, you've got Libra and Scorpius which are also in the zodiac they're never very high from here but Scorpius has this beautiful red supergiant star Antares and that marks the direction of the heart

of our galaxy the Milky Way so later on in the year in late August when it's a bit darker again in summer that you see the Milky Way stretching up from that region of sky absolutely beautiful and even more so if you're further south. But right now you can kind of have a preview of it. Again, if you stay up reasonably late, that's a time to see it just before we go towards the solstice. And it's really packed with these clusters

of stars and nebulae. So pick a pair of binoculars and look around there if you've got that beautiful clear southern horizon. And then further around, we've got Lyra and the bright star Vega, which is part of the summer triangle, and that's starting to become more prominent as well. And if you look near Vega, you can see things like this double, double star, it needs a small telescope for that, but it's like two binary pairs going around each other, which is always a nice sight.

For planets, we're not doing quite so well. Jupiter and Mars are hanging on a bit longer in the western sky, although Jupiter is getting really difficult to see now. Lower down Mars is getting further away, so it's really hard to see much detail. But in the morning sky, if you get up before dawn, then Venus is back and so is Saturn. And Saturn will definitely be a lot better in the autumn. I know, Becca, I can hear Becky cheering. The almost edge on rings Saturn is back and those rings are starting to open up again now. So in a few years time, it'll look as classically beautiful as ever. And then finally, remember last May in 2024, we had the first of the two amazing displays

of the Aurora. So do keep that in mind as well. You can look at apps like Aurora Watch to get alerts and to get a bit of warnings. Spaceweather.com will tell you when you've got these coronal mass ejections, these big ejections of charged particles from the sun towards the earth to give you warnings, say a couple of days ahead as well. And there are still a lot of sunspots right now. We're still very much in solar maximum. So my guess is, you know, there's every chance we'll see at least some kind of display in the months ahead. So do keep an eye on that. Woohoo! Let's keep our fingers crossed. Yes Just please. I'm excited because in a few weeks

time, for the first time ever, I'm going to see the southern hemisphere night sky. Oh wow. That's exciting. That's fantastic. So I now appreciate the frustration of all of our southern hemisphere listeners who are like, what are you going to see in the sun? Yeah. Well, you can do it next week then. Yeah, next episode you can be like, well. Thank you for having been on to this, you'll love it. Look at the night, absolutely stunning.

What's the game plan? Are you doing your homework? Are you revising the southern sky? Or are you going just to be like, I wanna be wowed, I wanna recognise nothing. I wanna recognise nothing and then when I'm there, I'll get nerdy and like, we'll be spending a long time like, what is this? What is this? What is this? And probably really infuriate my partner.

I mean, have you remembered about the moon? What? Oh, I don't know what phase it's in. No, no, the moon will be upside down for you. And that was my favourite thing. I think it's just because it's so impactful to see it and be like, no, no, no, no, no. I haven't thought about that, of course it is. Yeah, yeah, yeah. Moons upside down. Constellations are upside down.

Yeah, Orion upside down was, yeah, that was just like crazy to see. It's that that I'm excited for as well. So yeah, I'll come back with updates when we're next. When we're next live. Please do. But that's it for this month, but we thought we'd continue with the big episodes.

So next time we're going to wrap our heads around the topic of time. Next time. Next time. Which time? What time? Next time. Plus there'll be a bonus episode in a few weeks with all of your wonderful questions. So yeah, just keep adding to the Supermassive mailbox.

Yes, please. And contact us if you try some astronomy at home as well. It's at SupermassivePod on Instagram, or you can email your questions, your images to podcast.ras.ac.uk. And we'll try and cover them in a future episode. But until next time, everybody, happy stargazing.