Time Dilation & Cosmic Mysteries: Unpacking Your Space Queries

0:00Andrew Dunkley Hi there. Thanks for joining us again. This

0:02 is Space Nuts, a Q and A edition where we, uh, get questions, we look at them and go, no clue. No, I haven't got any idea what we're doing. So we're not doing that today.

0:13Professor Fred Watson Actually, we will.

0:14Andrew Dunkley Uh, we've got a question from Ron, uh, asking

0:17 about, uh, astronomy in regard to a super fast moving planet. How would that work if you were living on it, is what he's saying. Uh, Jeff wants to know about photon pressure. Um, Brian is asking about, um, what a black hole covers up. Interesting. And when is a moon no longer a moon? Bob asks. We'll see if we can answer all of those questions on today's episode of space nuts.

0:45Professor Fred Watson 15 seconds.

0:46 Guidance is internal. 10, 9. Ignition sequence start.

0:51Professor Fred Watson Space nuts. 5, 4, 3, 2.

0:53 1, 2, 3, 4, 5, 5, 4, 3, 2, 1.

0:57Andrew Dunkley Space nuts.

0:58Professor Fred Watson Astronauts report it feels good.

1:01Andrew Dunkley He's back for more. His name is Professor

1:03 Fred Watson Watson, astronomer at large. Hello, Fred Watson.

1:06Professor Fred Watson Hello, Andrew. Uh, it's good to be back for

1:09 more.

1:09Andrew Dunkley It is. I'm trying to figure out why your

1:12 parents gave you the first name Professor. That seems a little.

1:17Professor Fred Watson I thought so too. It could have been, sir.

1:22Andrew Dunkley I was probably not as bad as those parents

1:24 in. I think it was Sweden or somewhere who named their kid Megadeth. Well, yes, and the government tried to stop them and they went to court and they won.

1:35Professor Fred Watson Yes, I believe I remember that.

1:37Andrew Dunkley Yeah, Quite a while back now. But m.

1:40 That's beside the point. Uh, we've got, uh, answers to questions or we've got questions we're hoping we can answer. When I say we, I'm talking about you.

1:53Professor Fred Watson No pressure.

1:54Andrew Dunkley No, none at all. Um, this first question is

1:57 brilliant, though. Uh, hi there. From upstate New York. How hope all is well, uh, as you approach spring. It's not, Ron. Hay fever sucks. I, I played 36 holes of golf this week in a, in a major local tournament. And, um, I, I paid for it dearly. Um, and I still am. But, uh, yeah, it's a tough time of year for people who suffer hay fever here. I came second, by the way. Anyway, um, sorry, this is a bit wordy. He says with the information just released on the star S300, uh, one orbiting Sagittarius A being one of the fastest stars in the Milky Way. And he sent us a link about that. Moving at 8% the speed of light. That sounds like a lot.

2:43 25,000 kilometres per second. I was wondering how this would affect an astronomer on a hypothetical planet, um, around that region, um, relative to an Outside observer, their clocks would uh, slow dramatically due to the high velocity and uh, the depth in the black hole's gravitational well at um, I can't say that word. Peri.

3:08Professor Fred Watson Periapsis.

3:09Andrew Dunkley Yes. Uh, then speed up as they

3:12 reach apoapsis. Yes. Uh, would that not cause their measurements of time related events like orbital periods of nearby planets or stars to vary during their orbit of the black hole? If so, it's crazy to think how this would skew their science until they learned about relativity. Thanks very much for a great podcast. Always enlightening Ron. Thank you Ron.

3:38 I love this question, it's really clever.

3:42Professor Fred Watson Well it is. Um, and um,

3:45 uh, Ron's exactly right. This thing is in a very very unusual place within orbiting Sagittarius, a star which is the 4 million solar mass black hole at the centre of our galaxy. So um, exactly as he says, both the relativistic speed and um, by that I mean speed close to the speed of light. 8% is close to the speed of light compared with most of the speeds that we uh, encounter uh, in astronomy.

4:18 Uh, um, it is uh, that that relativistic speed changes, gives you time dilation as does the gravitational well that um, this star orbits. I didn't um, Let me just have a look at that uh, link. I have looked at it before, um, but I can't remember how elliptical its orbit is. Um, it's basically very elongated and so at what is what Ron uh, correctly calls periapsis, um the apse closest to the galactic centre.

4:57 Apsidies are uh, the points in an orbit which are either ah, the nearest or the furthest periapsis nearest apoapsis furthest away. Um, we talk about perigee and apogee for satellites around the Earth. Um, that's the way the terminology works. So um, you're going to have different phenomena uh depending on where you are because the velocity will vary as you go around this elongated orbit. That's Kepler's laws. They still hold at these uh, high gravitational intensities.

5:30 Um, um, so the time dilation phenomena uh, and the sorts of things that you would experience are exactly ah as Ron said there'd be another phenomenon as well. Um, I mean being near a black hole is gonna be weird uh as to what your sky looks like because the black hole is relatively nearby, there's stuff swirling around it with the accretion disc. It's kind of hard to imagine yourself in that situation. But at relativistic speeds like that, uh, it also changes the position of objects in your sky.

6:07 Um, so if you imagine something hurtling along at, uh, a significant fraction of the speed of light, uh, the stars that you see tend to the positions you'll see will, um, be. They're erroneous, but the stars will look as though they're nearer to the direction in which you're heading than they really are. So there's a tendency, especially as you approach getting near the speed of light, basically all the stars that you'd see will be bunched up, uh, fairly close together in front of you, um, because that's the way their light is changing itself.

6:42Andrew Dunkley I reckon, uh, being on a planet in a

6:44 situation like that, uh, orbiting a fast moving star, your astrophotography would be awesome.

6:51Professor Fred Watson Yeah, it would.

6:52 Except, well, probably not. I mean, the star trails would all be in the wrong place. Yeah, the stars, yeah, but no, great question. And Ron, you're on the money there too.

7:04Andrew Dunkley Yeah. And um, by comparison, I suppose we can

7:06 draw on a science fiction movie, Interstellar, where they visited Miller's planet, which was near Gargantua, uh, which is, um, a black hole in the film. And for every hour they were on the surface of the planet, seven years passed on Earth. So, uh, that's the sort of gravitational effect we're talking about. But, um, in terms of the speed of a planet, um, in accordance with the fast moving star, it's a different kettle of fish, isn't it?

7:38 Um, you're observing everything out of sync, I suppose would be the best way to describe it.

7:43Professor Fred Watson Yes, that's right, exactly.

7:45Andrew Dunkley So quite interesting. Thanks for the

7:48 question, Ron, and great to hear from you.

7:54Professor Fred Watson Also space nuts.

7:56Andrew Dunkley Uh, our next question, Fred Watson, comes

7:58 from Jeff, uh, also in the United States. He's in Colorado. Uh, hello Andrew and Fred Watson. Thanks for the amazing shows. Oh, have we done one? Have we. Uh, the sun emits millions of billions of photons every second and it's pretty much average as a star. Uh, could the radiation pressure from all the universe's photons exert enough force to affect the expansion of the universe? Now there's an interesting theory, Fred Watson.

8:27Professor Fred Watson Uh, there is, yes. Uh, it's

8:30 um, so 10 to the 45. 10 to the power. 45 photons per second coming from the sun. Um, I like that number. It's quite a lot. It is so that, uh, that basically that, um, flux of photons, if I can put it that way, is for every star is an outward flux, um, for the universe as a whole, it's kind of going to cancel out.

8:59 Um, but it is something that is taken into account, um, in the study of cosmology, the behaviour of the universe as a whole. And so in particular, you know, the potentially nice idea that this could account for dark energy, uh, which is the energy of the universe itself. That is, um, two quite different things. Uh, dark energy is independent of any kind of photon pressure.

9:31 Uh, and so, um, it's basically the idea of all the photons floating around the universe. Uh, I think they are the most common subatomic particle in the universe, if I remember rightly. And neutrinos are the second. I think that was one we covered a couple of weeks ago.

9:48Andrew Dunkley Yep.

9:49Professor Fred Watson Uh, so, um, they're accounted for

9:52 in our understanding of the expansion of the universe. And it doesn't explain dark energy. Dark energy is still an issue. Uh, it's, uh, something just connected with space itself. It will be there. Even if the universe was completely dark, you'd still have dark energy, we believe. And we don't know what it is.

10:12Andrew Dunkley No, we don't. So probably

10:15 not contributing to the expansion of the universe.

10:18Professor Fred Watson Yes, that's correct.

10:20Andrew Dunkley And we don't really know what is

10:22 contributing.

10:23Professor Fred Watson Well, the main contribution is the Big Bang

10:25 itself. Yeah, uh,

10:28Andrew Dunkley it's just a continuing explosion of some

10:31 kind.

10:32Professor Fred Watson Um, so the

10:35 Big Bang was what gave the universe the impetus to start expanding. Uh, and, uh, for the first. Well, uh, it actually, the expansion rate was very, very rapid at the beginning of this period that we call inflation, which lasted for a gazillionth of a second. Uh, and then it sort of settled down to something a bit more reasonable.

10:58Professor Fred Watson Uh, but about 5 billion years ago,

11:03 that's when the expansion started accelerating. Uh, and that's where we think the effect of dark energy really kicked in. Because dark energy was, uh, strong enough to overcome the gravitational pull of everything in the universe, which is trying to slow the expansion down gravitationally. And that's what we used to think would happen. And you get the big crunch. Uh, but yeah, when dark energy was. Oh, when the accelerated expansion was discovered back in 1998, uh, that's when we realised there was something, um, lacking in our understanding of the universe. Something that is there, this energy that seems to impel the universe to be continuously accelerating. Uh, um, accelerating faster than it was before. And that's what's

11:51 happening now? M. Yeah, I think just accelerating. It's the acceleration of the universe. The acceleration itself may be slowing down, but it is still going, I'm getting myself.

12:02Andrew Dunkley Yeah, it's very confusing. It expanded

12:04 initially. When the Big Bang happened, it expanded. Uh, am I right in saying fast? Faster than the speed of light for a period of time.

12:12Professor Fred Watson Yes. I mean, space, um, can do

12:15 whatever it likes. It's not limited to the speed of light.

12:17Andrew Dunkley Then it's down now, started

12:20 accelerating again. But now we think that acceleration is slowing down possibly.

12:28Professor Fred Watson Ah, that's still an observation that's yet to

12:30 be confirmed. That the acceleration is decreasing, but it's still accelerating. That's the thing. It's not, you know, the expansion of the universe is not slowing down. Uh, still accelerating, but the acceleration might be slowing down. It's. Yeah, it's a double derivative for those.

12:48Andrew Dunkley It is, isn't it?

12:49Professor Fred Watson Yeah, exactly.

12:50Andrew Dunkley Yeah. Uh, one of our live viewers,

12:53 uh, has suggested that the universe could be a doughnut shape. He doesn't have anything to prove that he's. It's just a hunch. I, uh, we. We can prove certain things, can't we?

13:05Professor Fred Watson Yeah, Um, I mean, it's.

13:09 You can have a doughnut. You can have, uh, what you might call, uh, the geometry of a doughnut, uh, without the thing actually looking like a doughnut. If I put it that way. Yeah. Um, uh, this is all to do with, for example, the geometry that we all deal with in everyday life is Euclidean geometry. Where parallel lines don't meet.

13:29 But there are other geometries, uh, which could affect the shape of the universe. And doughnuts, I think, do come into consideration.

13:36Andrew Dunkley Well, there you go, Moose. Well done.

13:40 See, people just. They just think and ideas come forward. And sometimes they're worth investigating, indeed. Um, thank you for the question. I think it was from Jeff in Colorado.

13:53Professor Fred Watson It was.

13:54Andrew Dunkley This is Space Nuts, a Q A edition with Andrew

13:57 Dunkley and Professor Fred Watson Watson.

14:00Professor Fred Watson Hey, that's one of the better sims, believe

14:03 me. We've had a couple of cool cardiac arrest down here too. Pete. Up here.

14:08Professor Fred Watson Space Nuts.

14:11Andrew Dunkley Uh, our next question comes from the Gold

14:13 Coast. Not the one in, uh, Africa, the one in Australia. Uh, from Brian. He says. Hi. Long time listener, first question. Thank you, Brian. Good to hear from you. Sorry. About black holes.

14:23Professor Fred Watson Ah.

14:23Andrew Dunkley Uh, uh, since we cannot see behind a black

14:26 hole, does it cover up the cosmic microwave background radiation. Or does that light get lensed around the black hole?

14:35Professor Fred Watson Um. Uh, yes,

14:38 it does. Uh, to an extent.

14:42Professor Fred Watson Um, there are funny effects

14:47 that come into play on the background radiation. And basically this sent my thinking, uh, to something I really read about for a long time. And, um, I've now lost it. Yes, here it is. Something called the Sunyaev Zeldovich effect, which is the way, um, the spectrum of the cosmic microwave background is modified as the Light passes through galaxy clusters.

15:23 It comes about through a phenomenon called inverse Compton scattering. Uh, so that the radiation of the background is uh, modified. If that light passes through a galaxy cluster. I can't remember which way it works, whether it's. I, uh, think it is a decrease in the intensity of the background. It might be an increase. I might have that. It's a long time since I've thought about this. But that's the sort of thing that. That is taken into account when people consider uh, um. Basically the cosmic microwave background as we see it from planet Earth. Uh, and so, um, phenomena like black holes.

16:14 Uh, Brian's question is a good one because there would be a modification of the radiation, um, and lensing would be part of that. You've got a black hole that uh, intensely modifies the gravitational field around it that makes it act like a lens. So that's going to happen with not just uh, the light of any stars behind it, but the gravitational, uh, sorry, the background radiation itself, the microwave background.

16:44 Um, it's an interesting, um, question actually. I'd like to follow up a little bit more on that one, Brian, and maybe we'll get back to you. Uh, again, just to the extent, you know, is that a way of determining how many black holes there are in space? Uh, because we don't really have. We've got a few estimates based on um, our uh, knowledge of stellar evolution. Uh, we've got a few estimates based on galaxies themselves. Um, I guess, um, thinking aloud a bit and maybe this is the answer to the question, uh, the black hole itself.

17:23 Let's just think about the black holes that uh, are uh, in the centres of galaxies, the supermassive black holes. So that uh, you might have a supermassive black hole that's a few billion times the solar mass, but that's still probably small compared with the mass of the galaxy itself. So the lensing effect is going to be what you would see from the galaxy and the black hole at the middle.

17:51 Uh, and I think that's probably the likely answer to the question. Um, so I might not need to follow up on that, having kind of worked it out. Uh, and that lensing effect of galaxies is indeed taken into account when we look at the cosmic um, microwave background radiation. So yes is the answer.

18:13Andrew Dunkley Okay, Easy peasy.

18:15 Uh, and uh, thank you for the question, um, Brian. And uh, yeah, hope all is well in that horrible, horrible part of the world known as the Gold Coast. It's well named because it's got beautiful beaches, beautiful, uh, climate. Don't um, know about the people. No, I'm kidding. Queensland is a lovely. Wow.

18:38 Like I mentioned last episode, my best friend Dave, he's a Queenslander. So there you are.

18:43Professor Fred Watson Don't worry, Brian. He doesn't like

18:45 Yorkshiremen either.

18:47Andrew Dunkley Uh, I like everybody, I do.

18:50 Ah, our final question, Fred Watson, comes from Bob. I don't know where Bob's from, but hello, Bob, wherever you are. Uh, how small and misshapen can a moon be and still with a straight face be called a moon? That's an interesting question. Brings Mars into the equation here because its moons are, uh, certainly not what you would consider moon shaped. We think of our moon as a sphere, and that's generally how it goes. And a lot of the other ones in the solar system are spherical. Uh, Mars is an exception. Um, we think they're captured asteroids, don't we? But, uh, yeah, when is a moon not a moon? I think is the precid version of the question.

19:31Professor Fred Watson Yeah. Um, so this

19:34 could be my old mate Bob Argyle, uh, who's in Cambridge. Ah, but I don't know. So, Bob, if this is you, drop us a line. Um, yeah, there's a very good friend, uh, going back a long way. We used to work together at the Royal Greenwich Observatory. Bob originated, if it's the same Bob, in, uh, Newcastle on Tyne, up there in the north of England. And I used to live there too. So, yeah, still got some records that he sold me.

20:05 Um, okay. We've done all that before because I think Bob's asked us questions before and, um, it probably isn't the same, Bob, but it's a great question. Um, how small and misshapen can a moon be and still with a straight face be called a moon? Um, pretty small, actually. Uh, certainly. Um, some of the moons of Saturn, uh, their diameters are measured, you know, in single figure kilometres.

20:36 Um, uh, we're kind of getting on that way. You mentioned the moons of Mars, Andrew.

20:42Professor Fred Watson Yes. Phobos, 23 kilometres across or

20:45 thereabouts. Deimos, the smaller one. I think it's about 15 kilometres and as you said, probably captured asteroids. But the fact that it's in orbit around, that these are in orbit around Mars makes them moons. They are satellites of Mars. Uh, and I think they get smaller when you look at Saturn. Some of the moons sort of wed into the ring system. I think, as I said, the diameters measured in single kilometres, but we still call them moons. They add to the total of. I think it's 297 at the moment. The total number of moons known around Saturn.

21:19 Uh, so, um, yeah. How small can a moon be and still with a straight face be called a moon? Pretty damn small. A few kilometres. Yeah, maybe even a few metres. If you're talking about, um, you know, debris around a planet, there might be objects that size in orbit around Earth. Although we do tend to know now about near Earth objects, whether they're um, asteroids or moons. We think that our um, big moon is the only natural satellite of Earth. We've discussed some of that stuff as well. We do, sorry. There are objects that accompany Earth, uh, temporarily in its orbit.

21:56Andrew Dunkley Well, I was about to mention 1, uh, 2025

21:59 PN7 which was discovered in Aug. 2025 by the Pan Starrs Observatory. It's referred to as a quasi moon. Uh, and it's like 19 metres across, 62ft.

22:14Professor Fred Watson Yeah, yeah.

22:15Andrew Dunkley So it's, it's, you know, it's only

22:18 about the size of a house.

22:20Professor Fred Watson Yeah, that's right.

22:22Andrew Dunkley So, um, but, but it won't stay.

22:26Professor Fred Watson That's correct. Um, and it's only

22:29 a quasi moon. It's actually in orbit around the sun. Uh, a number of objects like that, they'.

22:35Andrew Dunkley It's caught up with us at the moment, isn't

22:37 it?

22:37Professor Fred Watson That's right. When you look at its orbit, you

22:40 realise that uh, uh, uh, it's in orbit around the sun, but it always hangs around the Earth as well. But it's not in orbit around the Earth. Uh, I think that's what defines a quasi moon.

22:53Andrew Dunkley Here's a question. Um, with

22:56 Mars probably capturing its moons because of their odd potato shapes, is it possible sometime that Earth could capture, uh, an asteroid and claim a second moon?

23:09Professor Fred Watson Uh, yes. Yeah, that's right, it is.

23:11Andrew Dunkley Wow.

23:12Professor Fred Watson And it may have happened in the past as well.

23:14Andrew Dunkley Yeah. Okay. Very

23:16 interesting.

23:17Professor Fred Watson All right. The um, thing about the Earth

23:19 compared with Mars, Andrew though, is that what might inhibit that capture on Earth is our uh, main moon because it's compared with our uh, planet, it's a massive object. Uh, it's 1-80th of the mass of Earth. Uh, the mass of Phobos and Deimos together is, you know, basically infinitesimal compared with the mass of Mars.

23:45 So the moon itself, its gravity is going to be something that, um, any wandering asteroid will feel, uh, alongside the gravity of Earth and that might tend to kick it out relatively quickly from being in orbit around the Earth.

24:00Andrew Dunkley Okay, very cool.

24:02 Um, Gosh, we went through those fast.

24:06Professor Fred Watson We did. Great questions too.

24:08Andrew Dunkley They were indeed, yes. If, uh, you have

24:11 questions for us, um, please send them in. We'd love to hear from you. You, uh, can jump on our website to get more information about how to do that. Well, I'll tell you. Click the AMA button at the top of the screen and you can send text and audio questions. Don't forget to tell us who you are and maybe where you're from. That'd be nice. We love to know where our audience is.

24:32 We had someone from Yemen the other day, which was terrific. Uh, but, uh, nice to have an international flavour in our, in our shows. And thanks, uh, to our live viewers as well, who pop questions in once in a while, which is terrific. And thank you, Fred Watson. Uh, we'll catch you on the next episode of Space Nuts.

24:50Professor Fred Watson Great pleasure, Andrew, and thanks again to

24:53 all the listeners sending in those fabulous questions.

24:55Andrew Dunkley Indeed. And thanks to Huw in the studio.

24:58 Couldn't be with us today because he's a quasi Huw. And from me, Andrew Dunkley, thanks for your company. We'll see you on the next episode of Space Nuts.

25:07Professor Fred Watson Bye. Bye.

25:08Andrew Dunkley Uh, you've been listening to the Space Nuts

25:11 podcast, available at Apple Podcasts, Spotify, iHeartRadio or your favourite podcast player. You can also stream on demand@bytes.com. this has been another quality podcast production from bytes.com.