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Space Nuts: Fast-Moving Planets, Photon Pressure, and the Nature of Moons
In this engaging Q&A episode of Space Nuts, hosts Andrew Dunkley and Professor Fred Watson tackle a series of intriguing listener questions. Kicking off with Ron's query about the implications of living on a super fast-moving planet orbiting Sagittarius A*, they delve into the fascinating world of time dilation and its effects on scientific observations. Next, Jeff from Colorado poses a thought-provoking question regarding photon pressure and its potential influence on the universe's expansion, leading to a discussion on dark energy and cosmic mysteries. Brian from the Gold Coast wonders about the visibility of cosmic microwave background radiation behind black holes, while Bob rounds off the episode with a query on the characteristics that define a moon. Join Andrew and Fred Watson as they navigate these cosmic curiosities with their trademark wit and insight.
Key topics
- The effects of high-speed planetary motion near a black hole on time and observational science.
- Exploring photon pressure and its relationship to the universe's expansion and dark energy.
- The cosmic microwave background radiation and its interaction with black holes.
- Defining moons: How small and irregular can they be while still being classified as moons?
Timestamps
00:00 - Introduction and overview of listener questions
01:20 - Ron’s question about living on a fast-moving planet and time dilation effects
15:30 - Jeff’s inquiry into photon pressure and its impact on cosmic expansion
25:00 - Brian’s question regarding cosmic microwave background radiation and black holes
35:15 - Bob’s exploration of what constitutes a moon
45:00 - Closing thoughts and listener engagement
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00:00:00 --> 00:00:02 Andrew Dunkley: Hi there. Thanks for joining us again. This
00:00:02 --> 00:00:05 is Space Nuts, a Q and A edition where we,
00:00:05 --> 00:00:07 uh, get questions, we look at them and go,
00:00:08 --> 00:00:10 no clue. No, I haven't got any idea what
00:00:10 --> 00:00:13 we're doing. So we're not doing that today.
00:00:13 --> 00:00:14 Professor Fred Watson: Actually, we will.
00:00:14 --> 00:00:17 Andrew Dunkley: Uh, we've got a question from Ron, uh, asking
00:00:17 --> 00:00:20 about, uh, astronomy in regard to
00:00:20 --> 00:00:22 a super fast moving planet. How would that
00:00:22 --> 00:00:24 work if you were living on it, is what he's
00:00:24 --> 00:00:27 saying. Uh, Jeff wants to know about photon
00:00:27 --> 00:00:30 pressure. Um, Brian is asking
00:00:30 --> 00:00:33 about, um, what a black hole
00:00:33 --> 00:00:35 covers up. Interesting.
00:00:35 --> 00:00:38 And when is a moon no longer a
00:00:38 --> 00:00:41 moon? Bob asks. We'll see if we can answer
00:00:41 --> 00:00:43 all of those questions on today's episode of
00:00:43 --> 00:00:44 space nuts.
00:00:45 --> 00:00:46 Professor Fred Watson: 15 seconds.
00:00:46 --> 00:00:48 Guidance is internal. 10,
00:00:48 --> 00:00:51 9. Ignition sequence start.
00:00:51 --> 00:00:52 Professor Fred Watson: Space nuts.
00:00:52 --> 00:00:53 Professor Fred Watson: 5, 4, 3, 2.
00:00:53 --> 00:00:56 1, 2, 3, 4, 5, 5, 4, 3,
00:00:56 --> 00:00:57 2, 1.
00:00:57 --> 00:00:58 Andrew Dunkley: Space nuts.
00:00:58 --> 00:01:00 Professor Fred Watson: Astronauts report it feels good.
00:01:01 --> 00:01:03 Andrew Dunkley: He's back for more. His name is Professor
00:01:03 --> 00:01:05 Fred Watson Watson, astronomer at large.
00:01:05 --> 00:01:06 Hello, Fred Watson.
00:01:06 --> 00:01:09 Professor Fred Watson: Hello, Andrew. Uh, it's good to be back for
00:01:09 --> 00:01:09 more.
00:01:09 --> 00:01:12 Andrew Dunkley: It is. I'm trying to figure out why your
00:01:12 --> 00:01:14 parents gave you the first name Professor.
00:01:14 --> 00:01:15 That seems a little.
00:01:17 --> 00:01:20 Professor Fred Watson: I thought so too. It could have been, sir.
00:01:22 --> 00:01:24 Andrew Dunkley: I was probably not as bad as those parents
00:01:24 --> 00:01:26 in. I think it was Sweden or somewhere who
00:01:26 --> 00:01:29 named their kid Megadeth. Well,
00:01:29 --> 00:01:32 yes, and the government tried
00:01:32 --> 00:01:34 to stop them and they went to court and they
00:01:34 --> 00:01:34 won.
00:01:35 --> 00:01:37 Professor Fred Watson: Yes, I believe I remember that.
00:01:37 --> 00:01:40 Andrew Dunkley: Yeah, Quite a while back now. But m.
00:01:40 --> 00:01:43 That's beside the point. Uh, we've got, uh,
00:01:43 --> 00:01:45 answers to questions or we've got questions
00:01:45 --> 00:01:47 we're hoping we can answer.
00:01:48 --> 00:01:50 When I say we, I'm talking about you.
00:01:53 --> 00:01:54 Professor Fred Watson: No pressure.
00:01:54 --> 00:01:57 Andrew Dunkley: No, none at all. Um, this first question is
00:01:57 --> 00:01:59 brilliant, though. Uh, hi there. From upstate
00:01:59 --> 00:02:02 New York. How hope all is well,
00:02:02 --> 00:02:05 uh, as you approach spring. It's not, Ron.
00:02:06 --> 00:02:09 Hay fever sucks. I,
00:02:09 --> 00:02:11 I played 36 holes of golf this week in a, in
00:02:11 --> 00:02:13 a major local tournament. And, um,
00:02:15 --> 00:02:17 I, I paid for it dearly.
00:02:18 --> 00:02:21 Um, and I still am. But,
00:02:21 --> 00:02:23 uh, yeah, it's a tough time of year for
00:02:23 --> 00:02:26 people who suffer hay fever here. I came
00:02:26 --> 00:02:29 second, by the way. Anyway, um, sorry,
00:02:29 --> 00:02:31 this is a bit wordy. He says with the
00:02:31 --> 00:02:34 information just released on the star S300,
00:02:34 --> 00:02:36 uh, one orbiting Sagittarius A being one of
00:02:36 --> 00:02:39 the fastest stars in the Milky Way. And he
00:02:39 --> 00:02:42 sent us a link about that. Moving at 8% the
00:02:42 --> 00:02:43 speed of light. That sounds like a lot.
00:02:43 --> 00:02:46 25 kilometres per second. I was
00:02:46 --> 00:02:49 wondering how this would affect an astronomer
00:02:49 --> 00:02:52 on a hypothetical planet, um,
00:02:52 --> 00:02:55 around that region, um, relative
00:02:55 --> 00:02:57 to an Outside observer, their clocks would
00:02:57 --> 00:02:59 uh, slow dramatically due to the high
00:02:59 --> 00:03:02 velocity and uh, the depth in
00:03:02 --> 00:03:05 the black hole's gravitational well at
00:03:05 --> 00:03:08 um, I can't say that word. Peri.
00:03:08 --> 00:03:09 Professor Fred Watson: Periapsis.
00:03:09 --> 00:03:12 Andrew Dunkley: Yes. Uh, then speed up as they
00:03:12 --> 00:03:15 reach apoapsis.
00:03:15 --> 00:03:18 Yes. Uh, would that not cause their
00:03:18 --> 00:03:20 measurements of time related events like
00:03:20 --> 00:03:23 orbital periods of nearby planets or
00:03:23 --> 00:03:26 stars to vary during their orbit
00:03:26 --> 00:03:29 of the black hole? If so, it's crazy to
00:03:29 --> 00:03:31 think how this would skew their science until
00:03:31 --> 00:03:34 they learned about relativity. Thanks very
00:03:34 --> 00:03:37 much for a great podcast. Always enlightening
00:03:37 --> 00:03:38 Ron. Thank you Ron.
00:03:38 --> 00:03:41 I love this question, it's really
00:03:41 --> 00:03:41 clever.
00:03:42 --> 00:03:44 Professor Fred Watson: Well it is. Um, and um,
00:03:45 --> 00:03:47 uh, Ron's exactly right.
00:03:49 --> 00:03:51 This thing is in
00:03:51 --> 00:03:54 a very very unusual place within
00:03:56 --> 00:03:58 orbiting Sagittarius, a star which is the 4
00:03:58 --> 00:04:01 million solar mass black hole at the centre
00:04:01 --> 00:04:04 of our galaxy. So um, exactly
00:04:04 --> 00:04:06 as he says, both the relativistic
00:04:06 --> 00:04:09 speed and um, by that I mean speed close to
00:04:09 --> 00:04:12 the speed of light. 8% is close to the speed
00:04:12 --> 00:04:14 of light compared with most of the speeds
00:04:14 --> 00:04:17 that we uh, encounter uh, in astronomy.
00:04:18 --> 00:04:20 Uh, um, it is uh,
00:04:20 --> 00:04:23 that that relativistic speed changes,
00:04:23 --> 00:04:26 gives you time dilation as does the
00:04:26 --> 00:04:29 gravitational well that um, this
00:04:29 --> 00:04:31 star orbits. I
00:04:32 --> 00:04:34 didn't um, Let me just have a look at
00:04:34 --> 00:04:37 that uh, link. I have looked at it before,
00:04:38 --> 00:04:41 um, but I can't remember how elliptical its
00:04:41 --> 00:04:43 orbit is. Um, it's
00:04:44 --> 00:04:46 basically very elongated and so
00:04:46 --> 00:04:49 at what is what Ron uh,
00:04:49 --> 00:04:52 correctly calls periapsis, um
00:04:53 --> 00:04:56 the apse closest to the galactic
00:04:56 --> 00:04:57 centre.
00:04:57 --> 00:05:00 Apsidies are uh, the points in an orbit
00:05:00 --> 00:05:02 which are either ah, the nearest or the
00:05:02 --> 00:05:04 furthest periapsis nearest apoapsis
00:05:04 --> 00:05:07 furthest away. Um, we talk about perigee and
00:05:07 --> 00:05:10 apogee for satellites around the Earth.
00:05:10 --> 00:05:13 Um, that's the way the
00:05:13 --> 00:05:15 terminology works. So um, you're
00:05:15 --> 00:05:18 going to have different phenomena uh
00:05:18 --> 00:05:20 depending on where you are because the
00:05:20 --> 00:05:22 velocity will vary as you
00:05:22 --> 00:05:25 go around this elongated orbit. That's
00:05:25 --> 00:05:28 Kepler's laws. They still hold at these
00:05:28 --> 00:05:30 uh, high gravitational intensities.
00:05:30 --> 00:05:33 Um, um, so the time
00:05:33 --> 00:05:36 dilation phenomena uh, and
00:05:36 --> 00:05:38 the sorts of things that you would experience
00:05:38 --> 00:05:41 are exactly ah as Ron said there'd be another
00:05:41 --> 00:05:43 phenomenon as well. Um, I mean
00:05:45 --> 00:05:47 being near a black hole is gonna be weird uh
00:05:47 --> 00:05:49 as to what your sky looks like because the
00:05:49 --> 00:05:52 black hole is relatively nearby, there's
00:05:52 --> 00:05:54 stuff swirling around it with the accretion
00:05:54 --> 00:05:56 disc. It's kind of hard to imagine yourself
00:05:56 --> 00:05:59 in that situation. But at
00:05:59 --> 00:06:01 relativistic speeds like that, uh, it
00:06:01 --> 00:06:04 also changes the position of
00:06:04 --> 00:06:06 objects in your sky.
00:06:07 --> 00:06:09 Um, so if you imagine something hurtling
00:06:09 --> 00:06:12 along at, uh, a significant
00:06:12 --> 00:06:15 fraction of the speed of light, uh, the stars
00:06:15 --> 00:06:17 that you see tend to
00:06:18 --> 00:06:21 the positions you'll see will, um, be.
00:06:21 --> 00:06:24 They're erroneous, but the stars will look as
00:06:24 --> 00:06:26 though they're nearer to the direction in
00:06:26 --> 00:06:29 which you're heading than they really are. So
00:06:29 --> 00:06:30 there's a tendency, especially as you
00:06:30 --> 00:06:32 approach getting near the speed of light,
00:06:32 --> 00:06:34 basically all the stars that you'd see will
00:06:34 --> 00:06:37 be bunched up, uh, fairly close together in
00:06:37 --> 00:06:40 front of you, um, because that's the way
00:06:40 --> 00:06:41 their light is changing itself.
00:06:42 --> 00:06:44 Andrew Dunkley: I reckon, uh, being on a planet in a
00:06:44 --> 00:06:47 situation like that, uh, orbiting a fast
00:06:47 --> 00:06:49 moving star, your astrophotography would be
00:06:49 --> 00:06:49 awesome.
00:06:51 --> 00:06:52 Professor Fred Watson: Yeah, it would.
00:06:52 --> 00:06:55 Except, well, probably not. I mean,
00:06:56 --> 00:06:58 the star trails would all be in the wrong
00:06:58 --> 00:07:00 place. Yeah, the stars, yeah, but no, great
00:07:00 --> 00:07:03 question. And Ron, you're on the money there
00:07:03 --> 00:07:04 too.
00:07:04 --> 00:07:06 Andrew Dunkley: Yeah. And um, by comparison, I suppose we can
00:07:06 --> 00:07:08 draw on a science fiction movie,
00:07:08 --> 00:07:11 Interstellar, where they visited
00:07:11 --> 00:07:14 Miller's planet, which was near Gargantua,
00:07:14 --> 00:07:17 uh, which is, um, a black
00:07:17 --> 00:07:19 hole in the film. And for every
00:07:19 --> 00:07:22 hour they were on the surface of
00:07:22 --> 00:07:25 the planet, seven years passed on Earth. So,
00:07:25 --> 00:07:27 uh, that's the sort of gravitational effect
00:07:27 --> 00:07:29 we're talking about. But, um, in terms
00:07:29 --> 00:07:32 of the speed of a planet,
00:07:32 --> 00:07:35 um, in accordance with the fast moving star,
00:07:36 --> 00:07:38 it's a different kettle of fish, isn't it?
00:07:38 --> 00:07:41 Um, you're observing everything out of sync,
00:07:41 --> 00:07:42 I suppose would be the best way to describe
00:07:42 --> 00:07:43 it.
00:07:43 --> 00:07:45 Professor Fred Watson: Yes, that's right, exactly.
00:07:45 --> 00:07:48 Andrew Dunkley: So quite interesting. Thanks for the
00:07:48 --> 00:07:50 question, Ron, and great to hear from you.
00:07:54 --> 00:07:56 Professor Fred Watson: Also space nuts.
00:07:56 --> 00:07:58 Andrew Dunkley: Uh, our next question, Fred Watson, comes
00:07:58 --> 00:08:00 from Jeff, uh, also in the United States.
00:08:00 --> 00:08:02 He's in Colorado. Uh, hello
00:08:02 --> 00:08:04 Andrew and Fred Watson. Thanks for the
00:08:04 --> 00:08:06 amazing shows. Oh, have we done one? Have we.
00:08:08 --> 00:08:11 Uh, the sun emits millions of billions of
00:08:11 --> 00:08:14 photons every second and it's pretty
00:08:14 --> 00:08:16 much average as a star.
00:08:17 --> 00:08:19 Uh, could the radiation pressure from all the
00:08:19 --> 00:08:22 universe's photons exert enough force to
00:08:22 --> 00:08:25 affect the expansion of the universe? Now
00:08:25 --> 00:08:26 there's an interesting theory, Fred Watson.
00:08:27 --> 00:08:30 Professor Fred Watson: Uh, there is, yes. Uh, it's
00:08:30 --> 00:08:33 um, so 10 to the 45. 10 to
00:08:33 --> 00:08:35 the power. 45 photons per second coming from
00:08:35 --> 00:08:38 the sun. Um, I like that number. It's quite a
00:08:38 --> 00:08:39 lot. It is
00:08:41 --> 00:08:43 so that, uh,
00:08:45 --> 00:08:48 that basically that, um,
00:08:48 --> 00:08:50 flux of photons, if I can put it that way,
00:08:51 --> 00:08:53 is for every star is an
00:08:53 --> 00:08:56 outward flux, um, for the universe
00:08:56 --> 00:08:59 as a whole, it's kind of going to cancel out.
00:08:59 --> 00:09:01 Um, but it is something
00:09:02 --> 00:09:05 that is taken into account,
00:09:06 --> 00:09:09 um, in the study of
00:09:09 --> 00:09:11 cosmology, the behaviour of the universe as a
00:09:11 --> 00:09:14 whole. And so in particular, you know,
00:09:14 --> 00:09:16 the potentially nice idea that this could
00:09:16 --> 00:09:19 account for dark energy, uh, which is the
00:09:19 --> 00:09:22 energy of the universe itself. That
00:09:22 --> 00:09:25 is, um, two quite different
00:09:25 --> 00:09:28 things. Uh, dark energy is
00:09:28 --> 00:09:30 independent of any kind of photon pressure.
00:09:31 --> 00:09:34 Uh, and so, um, it's basically the
00:09:34 --> 00:09:36 idea of all the photons floating around the
00:09:36 --> 00:09:39 universe. Uh, I think they are the most
00:09:39 --> 00:09:42 common subatomic particle in the universe, if
00:09:42 --> 00:09:45 I remember rightly. And neutrinos are the
00:09:45 --> 00:09:47 second. I think that was one we covered a
00:09:47 --> 00:09:48 couple of weeks ago.
00:09:48 --> 00:09:48 Andrew Dunkley: Yep.
00:09:49 --> 00:09:51 Professor Fred Watson: Uh, so, um, they're accounted for
00:09:52 --> 00:09:54 in our understanding of the expansion of the
00:09:54 --> 00:09:57 universe. And it doesn't explain dark energy.
00:09:57 --> 00:10:00 Dark energy is still an issue. Uh, it's,
00:10:00 --> 00:10:02 uh, something just connected with space
00:10:02 --> 00:10:04 itself. It will be there. Even if the
00:10:04 --> 00:10:07 universe was completely dark, you'd still
00:10:07 --> 00:10:10 have dark energy, we believe. And
00:10:10 --> 00:10:11 we don't know what it is.
00:10:12 --> 00:10:14 Andrew Dunkley: No, we don't. So probably
00:10:15 --> 00:10:17 not contributing to the expansion of the
00:10:17 --> 00:10:17 universe.
00:10:18 --> 00:10:19 Professor Fred Watson: Yes, that's correct.
00:10:20 --> 00:10:21 Andrew Dunkley: And we don't really know what is
00:10:22 --> 00:10:22 contributing.
00:10:23 --> 00:10:25 Professor Fred Watson: Well, the main contribution is the Big Bang
00:10:25 --> 00:10:26 itself. Yeah, uh,
00:10:28 --> 00:10:31 Andrew Dunkley: it's just a continuing explosion of some
00:10:31 --> 00:10:31 kind.
00:10:32 --> 00:10:35 Professor Fred Watson: Um, so the
00:10:35 --> 00:10:37 Big Bang was what gave the
00:10:37 --> 00:10:40 universe the impetus to start expanding.
00:10:40 --> 00:10:43 Uh, and, uh, for the first.
00:10:43 --> 00:10:46 Well, uh, it actually, the expansion
00:10:46 --> 00:10:49 rate was very, very rapid at the beginning of
00:10:49 --> 00:10:51 this period that we call inflation, which
00:10:51 --> 00:10:53 lasted for a gazillionth of a second.
00:10:54 --> 00:10:56 Uh, and then it sort of settled down to
00:10:56 --> 00:10:57 something a bit more reasonable.
00:10:58 --> 00:10:58 Professor Fred Watson: Uh,
00:11:00 --> 00:11:02 Professor Fred Watson: but about 5 billion years ago,
00:11:03 --> 00:11:05 that's when the expansion started
00:11:05 --> 00:11:08 accelerating. Uh, and that's where we
00:11:08 --> 00:11:11 think the effect of dark energy really
00:11:11 --> 00:11:13 kicked in. Because dark energy was,
00:11:14 --> 00:11:16 uh, strong enough to overcome the
00:11:16 --> 00:11:17 gravitational pull of everything in the
00:11:17 --> 00:11:19 universe, which is trying to slow the
00:11:19 --> 00:11:22 expansion down gravitationally. And that's
00:11:22 --> 00:11:23 what we used to think would happen. And you
00:11:23 --> 00:11:26 get the big crunch. Uh, but yeah, when
00:11:26 --> 00:11:29 dark energy was. Oh, when the accelerated
00:11:29 --> 00:11:30 expansion was discovered back in
00:11:30 --> 00:11:33 1998,
00:11:33 --> 00:11:36 uh, that's when we realised there was
00:11:36 --> 00:11:39 something, um, lacking in our understanding
00:11:39 --> 00:11:41 of the universe. Something that is there,
00:11:41 --> 00:11:43 this energy that seems to
00:11:44 --> 00:11:46 impel the universe to be continuously
00:11:46 --> 00:11:49 accelerating. Uh, um, accelerating
00:11:49 --> 00:11:51 faster than it was before. And that's what's
00:11:51 --> 00:11:53 happening now? M. Yeah, I think
00:11:54 --> 00:11:56 just accelerating. It's the acceleration of
00:11:56 --> 00:11:58 the universe. The acceleration itself may be
00:11:58 --> 00:12:01 slowing down, but it is
00:12:01 --> 00:12:02 still going, I'm getting myself.
00:12:02 --> 00:12:04 Andrew Dunkley: Yeah, it's very confusing. It expanded
00:12:04 --> 00:12:06 initially. When the Big Bang happened, it
00:12:06 --> 00:12:08 expanded. Uh, am I right in saying fast?
00:12:08 --> 00:12:11 Faster than the speed of light for a period
00:12:11 --> 00:12:11 of time.
00:12:12 --> 00:12:15 Professor Fred Watson: Yes. I mean, space, um, can do
00:12:15 --> 00:12:16 whatever it likes.
00:12:16 --> 00:12:17 It's not limited to the speed of light.
00:12:17 --> 00:12:20 Andrew Dunkley: Then it's down now, started
00:12:20 --> 00:12:23 accelerating again. But now we think that
00:12:23 --> 00:12:26 acceleration is slowing down
00:12:26 --> 00:12:27 possibly.
00:12:28 --> 00:12:30 Professor Fred Watson: Ah, that's still an observation that's yet to
00:12:30 --> 00:12:32 be confirmed. That the acceleration is
00:12:32 --> 00:12:34 decreasing, but it's still accelerating.
00:12:34 --> 00:12:36 That's the thing. It's not, you know, the
00:12:36 --> 00:12:38 expansion of the universe is not slowing
00:12:38 --> 00:12:41 down. Uh, still
00:12:41 --> 00:12:43 accelerating, but the acceleration might be
00:12:43 --> 00:12:45 slowing down. It's.
00:12:46 --> 00:12:48 Yeah, it's a double derivative for those.
00:12:48 --> 00:12:49 Andrew Dunkley: It is, isn't it?
00:12:49 --> 00:12:50 Professor Fred Watson: Yeah, exactly.
00:12:50 --> 00:12:52 Andrew Dunkley: Yeah. Uh, one of our live viewers,
00:12:53 --> 00:12:56 uh, has suggested that the universe could
00:12:56 --> 00:12:58 be a doughnut shape. He doesn't have anything
00:12:58 --> 00:13:01 to prove that he's. It's just a hunch. I,
00:13:02 --> 00:13:04 uh, we. We can prove certain things, can't
00:13:04 --> 00:13:04 we?
00:13:05 --> 00:13:07 Professor Fred Watson: Yeah, Um, I mean, it's.
00:13:09 --> 00:13:11 You can have a doughnut. You can have, uh,
00:13:11 --> 00:13:14 what you might call, uh, the geometry of a
00:13:14 --> 00:13:16 doughnut, uh, without the thing actually
00:13:16 --> 00:13:19 looking like a doughnut. If I put it that
00:13:19 --> 00:13:22 way. Yeah. Um, uh, this is all
00:13:22 --> 00:13:24 to do with, for example, the geometry that we
00:13:24 --> 00:13:27 all deal with in everyday life is Euclidean
00:13:27 --> 00:13:29 geometry. Where parallel lines don't meet.
00:13:29 --> 00:13:31 But there are other geometries, uh, which
00:13:31 --> 00:13:33 could affect the shape of the universe. And
00:13:33 --> 00:13:36 doughnuts, I think, do come into
00:13:36 --> 00:13:36 consideration.
00:13:36 --> 00:13:38 Andrew Dunkley: Well, there you go, Moose. Well done.
00:13:40 --> 00:13:41 See, people just.
00:13:41 --> 00:13:43 They just think and ideas
00:13:44 --> 00:13:46 come forward. And sometimes they're worth
00:13:46 --> 00:13:48 investigating, indeed.
00:13:49 --> 00:13:52 Um, thank you for the question. I think it
00:13:52 --> 00:13:53 was from Jeff in Colorado.
00:13:53 --> 00:13:54 Professor Fred Watson: It was.
00:13:54 --> 00:13:57 Andrew Dunkley: This is Space Nuts, a Q A edition with Andrew
00:13:57 --> 00:13:59 Dunkley and Professor Fred Watson Watson.
00:14:00 --> 00:14:03 Professor Fred Watson: Hey, that's one of the better sims, believe
00:14:03 --> 00:14:05 me. We've had a couple of cool cardiac arrest
00:14:05 --> 00:14:08 down here too. Pete. Up
00:14:08 --> 00:14:08 here.
00:14:08 --> 00:14:09 Professor Fred Watson: Space Nuts.
00:14:11 --> 00:14:13 Andrew Dunkley: Uh, our next question comes from the Gold
00:14:13 --> 00:14:15 Coast. Not the one in, uh, Africa, the one in
00:14:15 --> 00:14:18 Australia. Uh, from Brian. He says.
00:14:18 --> 00:14:21 Hi. Long time listener, first question. Thank
00:14:21 --> 00:14:22 you, Brian. Good to hear from you. Sorry.
00:14:22 --> 00:14:23 About black holes.
00:14:23 --> 00:14:23 Professor Fred Watson: Ah.
00:14:23 --> 00:14:26 Andrew Dunkley: Uh, uh, since we cannot see behind a black
00:14:26 --> 00:14:29 hole, does it cover up the cosmic microwave
00:14:29 --> 00:14:32 background radiation. Or does that light get
00:14:32 --> 00:14:34 lensed around the black hole?
00:14:35 --> 00:14:38 Professor Fred Watson: Um. Uh, yes,
00:14:38 --> 00:14:41 it does. Uh, to an
00:14:41 --> 00:14:41 extent.
00:14:42 --> 00:14:42 Professor Fred Watson: Um,
00:14:45 --> 00:14:47 Professor Fred Watson: there are funny effects
00:14:47 --> 00:14:50 that come into play on the
00:14:50 --> 00:14:53 background radiation. And
00:14:54 --> 00:14:56 basically this sent my thinking,
00:14:57 --> 00:14:59 uh, to something I really read about for a
00:14:59 --> 00:15:02 long time. And, um,
00:15:02 --> 00:15:05 I've now lost it. Yes, here it is. Something
00:15:05 --> 00:15:06 called the Sunyaev
00:15:07 --> 00:15:09 Zeldovich effect, which
00:15:10 --> 00:15:13 is the way, um,
00:15:13 --> 00:15:15 the spectrum of the cosmic
00:15:15 --> 00:15:18 microwave background is
00:15:18 --> 00:15:21 modified as the Light
00:15:21 --> 00:15:23 passes through galaxy clusters.
00:15:23 --> 00:15:25 It comes about through a phenomenon called
00:15:25 --> 00:15:28 inverse Compton scattering. Uh,
00:15:29 --> 00:15:31 so that the radiation of the background
00:15:33 --> 00:15:36 is uh, modified. If that light passes through
00:15:36 --> 00:15:39 a galaxy cluster. I
00:15:39 --> 00:15:42 can't remember which way it works, whether
00:15:42 --> 00:15:44 it's. I, uh, think it is
00:15:45 --> 00:15:47 a decrease in the intensity
00:15:48 --> 00:15:51 of the background. It might be an
00:15:51 --> 00:15:53 increase. I might have that. It's a long time
00:15:53 --> 00:15:55 since I've thought about this. But that's the
00:15:55 --> 00:15:58 sort of thing that. That is taken into
00:15:58 --> 00:16:00 account when people consider
00:16:01 --> 00:16:04 uh, um. Basically
00:16:04 --> 00:16:07 the cosmic microwave background as we see it
00:16:07 --> 00:16:10 from planet Earth. Uh, and
00:16:10 --> 00:16:13 so, um, phenomena like black holes.
00:16:14 --> 00:16:17 Uh, Brian's question is
00:16:17 --> 00:16:19 a good one because there would be a
00:16:19 --> 00:16:22 modification of the radiation, um, and
00:16:23 --> 00:16:25 lensing would be part of that. You've got a
00:16:25 --> 00:16:28 black hole that uh, intensely
00:16:28 --> 00:16:31 modifies the gravitational field around it
00:16:31 --> 00:16:34 that makes it act like a lens. So that's
00:16:34 --> 00:16:37 going to happen with not just uh, the light
00:16:37 --> 00:16:39 of any stars behind it, but the
00:16:39 --> 00:16:41 gravitational, uh, sorry, the background
00:16:41 --> 00:16:43 radiation itself, the microwave background.
00:16:44 --> 00:16:47 Um, it's an interesting, um,
00:16:47 --> 00:16:49 question actually. I'd like to follow up a
00:16:49 --> 00:16:51 little bit more on that one, Brian, and maybe
00:16:51 --> 00:16:53 we'll get back to you. Uh, again, just to the
00:16:53 --> 00:16:56 extent, you know, is that a way of
00:16:56 --> 00:16:58 determining how many black holes there are in
00:16:58 --> 00:17:00 space? Uh, because we don't really have.
00:17:00 --> 00:17:03 We've got a few estimates based on
00:17:03 --> 00:17:06 um, our uh, knowledge of stellar
00:17:06 --> 00:17:09 evolution. Uh, we've got a few estimates
00:17:09 --> 00:17:12 based on galaxies themselves. Um,
00:17:12 --> 00:17:14 I guess, um,
00:17:15 --> 00:17:17 thinking aloud a bit and maybe this is the
00:17:17 --> 00:17:20 answer to the question, uh,
00:17:20 --> 00:17:23 the black hole itself.
00:17:23 --> 00:17:25 Let's just think about the black holes that
00:17:25 --> 00:17:28 uh, are uh, in the centres of
00:17:28 --> 00:17:30 galaxies, the supermassive black holes.
00:17:31 --> 00:17:34 So that uh, you might have a supermassive
00:17:34 --> 00:17:36 black hole that's a few billion times the
00:17:36 --> 00:17:38 solar mass, but that's still
00:17:39 --> 00:17:41 probably small compared with the mass of the
00:17:41 --> 00:17:44 galaxy itself. So the lensing effect
00:17:45 --> 00:17:48 is going to be what you would see from the
00:17:48 --> 00:17:50 galaxy and the black hole at the middle.
00:17:51 --> 00:17:53 Uh, and I think that's probably the likely
00:17:53 --> 00:17:56 answer to the question. Um, so I might
00:17:56 --> 00:17:58 not need to follow up on that,
00:17:59 --> 00:18:01 having kind of worked it out. Uh,
00:18:02 --> 00:18:05 and that lensing effect of galaxies is indeed
00:18:05 --> 00:18:07 taken into account when we look at
00:18:08 --> 00:18:10 the cosmic um, microwave background
00:18:10 --> 00:18:12 radiation. So yes is the answer.
00:18:13 --> 00:18:14 Andrew Dunkley: Okay, Easy peasy.
00:18:15 --> 00:18:18 Uh, and uh, thank you for the question, um,
00:18:18 --> 00:18:21 Brian. And uh, yeah, hope all is well in that
00:18:21 --> 00:18:22 horrible, horrible part of the world
00:18:24 --> 00:18:27 known as the Gold Coast. It's well named
00:18:27 --> 00:18:29 because it's got beautiful beaches,
00:18:29 --> 00:18:32 beautiful, uh, climate. Don't um, know
00:18:32 --> 00:18:34 about the people. No, I'm kidding. Queensland
00:18:34 --> 00:18:37 is a lovely. Wow.
00:18:38 --> 00:18:41 Like I mentioned last episode, my best friend
00:18:41 --> 00:18:42 Dave, he's a Queenslander. So there you are.
00:18:43 --> 00:18:45 Professor Fred Watson: Don't worry, Brian. He doesn't like
00:18:45 --> 00:18:46 Yorkshiremen either.
00:18:47 --> 00:18:50 Andrew Dunkley: Uh, I like everybody, I do.
00:18:50 --> 00:18:53 Ah, our final question, Fred Watson, comes
00:18:53 --> 00:18:55 from Bob. I don't know where Bob's from, but
00:18:55 --> 00:18:58 hello, Bob, wherever you are. Uh, how small
00:18:58 --> 00:19:01 and misshapen can a moon be and
00:19:01 --> 00:19:03 still with a straight face
00:19:04 --> 00:19:05 be called a moon?
00:19:05 --> 00:19:07 That's an interesting question. Brings Mars
00:19:07 --> 00:19:09 into the equation here because its moons are,
00:19:09 --> 00:19:12 uh, certainly not what you would consider
00:19:12 --> 00:19:15 moon shaped. We think of our moon as a
00:19:15 --> 00:19:18 sphere, and that's generally how it goes. And
00:19:18 --> 00:19:20 a lot of the other ones in the solar system
00:19:20 --> 00:19:22 are spherical. Uh, Mars is an
00:19:22 --> 00:19:24 exception. Um, we think they're captured
00:19:24 --> 00:19:26 asteroids, don't we? But, uh, yeah, when is a
00:19:26 --> 00:19:28 moon not a moon? I think is the precid
00:19:28 --> 00:19:29 version of the question.
00:19:31 --> 00:19:34 Professor Fred Watson: Yeah. Um, so this
00:19:34 --> 00:19:36 could be my old mate Bob Argyle, uh,
00:19:36 --> 00:19:39 who's in Cambridge. Ah, but
00:19:39 --> 00:19:42 I don't know. So, Bob, if this is you, drop
00:19:42 --> 00:19:44 us a line. Um,
00:19:45 --> 00:19:48 yeah, there's a very good friend, uh, going
00:19:48 --> 00:19:50 back a long way. We used to work together at
00:19:50 --> 00:19:52 the Royal Greenwich Observatory. Bob
00:19:52 --> 00:19:55 originated, if it's the same Bob, in, uh,
00:19:55 --> 00:19:57 Newcastle on Tyne, up there in the north of
00:19:57 --> 00:20:00 England. And I used to live there too. So,
00:20:00 --> 00:20:03 yeah, still got some records that he sold me.
00:20:05 --> 00:20:07 Um, okay. We've done all that before because
00:20:07 --> 00:20:10 I think Bob's asked us questions before and,
00:20:10 --> 00:20:13 um, it probably isn't the same, Bob,
00:20:13 --> 00:20:15 but it's a great question. Um,
00:20:16 --> 00:20:19 how small and misshapen can a moon be and
00:20:19 --> 00:20:21 still with a straight face be called a moon?
00:20:21 --> 00:20:23 Um, pretty small, actually. Uh,
00:20:24 --> 00:20:26 certainly. Um, some of
00:20:26 --> 00:20:29 the moons of Saturn, uh,
00:20:30 --> 00:20:33 their diameters are measured, you know,
00:20:33 --> 00:20:36 in single figure kilometres.
00:20:36 --> 00:20:39 Um, uh, we're kind of getting
00:20:39 --> 00:20:41 on that way. You mentioned the moons of Mars,
00:20:41 --> 00:20:42 Andrew.
00:20:42 --> 00:20:43 Professor Fred Watson: Yes.
00:20:43 --> 00:20:45 Professor Fred Watson: Phobos, 23 kilometres across or
00:20:45 --> 00:20:47 thereabouts. Deimos, the smaller one. I think
00:20:47 --> 00:20:50 it's about 15 kilometres and as you said,
00:20:50 --> 00:20:53 probably captured asteroids. But the fact
00:20:53 --> 00:20:55 that it's in orbit around, that these are in
00:20:55 --> 00:20:57 orbit around Mars makes them moons. They are
00:20:57 --> 00:21:00 satellites of Mars. Uh, and I think they get
00:21:00 --> 00:21:02 smaller when you look at Saturn. Some of the
00:21:02 --> 00:21:05 moons sort of wed into the
00:21:05 --> 00:21:08 ring system. I think, as I said,
00:21:08 --> 00:21:11 the diameters measured in single kilometres,
00:21:11 --> 00:21:13 but we still call them moons. They add to the
00:21:13 --> 00:21:16 total of. I think it's 297 at the moment. The
00:21:16 --> 00:21:18 total number of moons known around Saturn.
00:21:19 --> 00:21:22 Uh, so, um, yeah. How small can a moon be
00:21:22 --> 00:21:24 and still with a straight face be called a
00:21:24 --> 00:21:26 moon? Pretty damn small. A few kilometres.
00:21:26 --> 00:21:28 Yeah, maybe even a few metres.
00:21:29 --> 00:21:32 If you're talking about, um, you know,
00:21:32 --> 00:21:34 debris around a planet, there might be
00:21:34 --> 00:21:36 objects that size in orbit around Earth.
00:21:36 --> 00:21:39 Although we do tend to know now about near
00:21:39 --> 00:21:41 Earth objects, whether they're um, asteroids
00:21:41 --> 00:21:44 or moons. We think that
00:21:45 --> 00:21:47 our um, big moon is the only natural
00:21:47 --> 00:21:50 satellite of Earth. We've discussed some of
00:21:50 --> 00:21:53 that stuff as well. We do, sorry. There are
00:21:53 --> 00:21:56 objects that accompany Earth, uh, temporarily
00:21:56 --> 00:21:56 in its orbit.
00:21:56 --> 00:21:59 Andrew Dunkley: Well, I was about to mention 1, uh, 2025
00:21:59 --> 00:22:01 PN7 which was discovered in
00:22:02 --> 00:22:05 Aug. 2025 by the Pan Starrs
00:22:05 --> 00:22:08 Observatory. It's referred to as a
00:22:08 --> 00:22:10 quasi moon. Uh, and
00:22:10 --> 00:22:12 it's like 19 metres across,
00:22:13 --> 00:22:14 62ft.
00:22:14 --> 00:22:15 Professor Fred Watson: Yeah, yeah.
00:22:15 --> 00:22:17 Andrew Dunkley: So it's, it's, you know, it's only
00:22:18 --> 00:22:19 about the size of a house.
00:22:20 --> 00:22:21 Professor Fred Watson: Yeah, that's right.
00:22:22 --> 00:22:24 Andrew Dunkley: So, um, but, but it won't stay.
00:22:26 --> 00:22:29 Professor Fred Watson: That's correct. Um, and it's only
00:22:29 --> 00:22:31 a quasi moon. It's actually in orbit around
00:22:31 --> 00:22:34 the sun. Uh, a number of objects like that,
00:22:34 --> 00:22:35 they'.
00:22:35 --> 00:22:37 Andrew Dunkley: It's caught up with us at the moment, isn't
00:22:37 --> 00:22:37 it?
00:22:37 --> 00:22:40 Professor Fred Watson: That's right. When you look at its orbit, you
00:22:40 --> 00:22:42 realise that uh, uh,
00:22:43 --> 00:22:45 uh, it's in orbit around the sun, but it
00:22:45 --> 00:22:47 always hangs around the Earth as well.
00:22:47 --> 00:22:50 But it's not in orbit around the Earth. Uh, I
00:22:50 --> 00:22:52 think that's what defines a quasi moon.
00:22:53 --> 00:22:55 Andrew Dunkley: Here's a question. Um, with
00:22:56 --> 00:22:59 Mars probably capturing its moons
00:23:00 --> 00:23:03 because of their odd potato shapes, is it
00:23:03 --> 00:23:05 possible sometime that Earth could capture,
00:23:05 --> 00:23:08 uh, an asteroid and claim a second moon?
00:23:09 --> 00:23:11 Professor Fred Watson: Uh, yes. Yeah, that's right, it is.
00:23:11 --> 00:23:12 Andrew Dunkley: Wow.
00:23:12 --> 00:23:14 Professor Fred Watson: And it may have happened in the past as well.
00:23:14 --> 00:23:16 Andrew Dunkley: Yeah. Okay. Very
00:23:16 --> 00:23:17 interesting.
00:23:17 --> 00:23:19 Professor Fred Watson: All right. The um, thing about the Earth
00:23:19 --> 00:23:22 compared with Mars, Andrew though, is that
00:23:22 --> 00:23:25 what might inhibit that capture on
00:23:25 --> 00:23:27 Earth is our uh, main moon because
00:23:28 --> 00:23:31 it's compared with our uh, planet,
00:23:31 --> 00:23:33 it's a massive object. Uh,
00:23:33 --> 00:23:36 it's 1-80th of the mass of Earth.
00:23:36 --> 00:23:39 Uh, the mass of Phobos and Deimos together
00:23:39 --> 00:23:42 is, you know, basically
00:23:42 --> 00:23:44 infinitesimal compared with the mass of Mars.
00:23:45 --> 00:23:48 So the moon itself, its gravity is
00:23:48 --> 00:23:50 going to be something that, um, any
00:23:50 --> 00:23:52 wandering asteroid will feel, uh,
00:23:53 --> 00:23:55 alongside the gravity of Earth and that might
00:23:55 --> 00:23:58 tend to kick it out relatively quickly from
00:23:58 --> 00:24:00 being in orbit around the Earth.
00:24:00 --> 00:24:01 Andrew Dunkley: Okay, very cool.
00:24:02 --> 00:24:04 Um, Gosh, we went through those fast.
00:24:06 --> 00:24:08 Professor Fred Watson: We did. Great questions too.
00:24:08 --> 00:24:11 Andrew Dunkley: They were indeed, yes. If, uh, you have
00:24:11 --> 00:24:13 questions for us, um, please send them in.
00:24:13 --> 00:24:15 We'd love to hear from you. You, uh, can jump
00:24:15 --> 00:24:17 on our website to get more information about
00:24:17 --> 00:24:19 how to do that. Well, I'll tell you. Click
00:24:19 --> 00:24:22 the AMA button at the top of the
00:24:22 --> 00:24:25 screen and you can send text and audio
00:24:25 --> 00:24:27 questions. Don't forget to tell us who you
00:24:27 --> 00:24:29 are and maybe where you're from. That'd be
00:24:29 --> 00:24:32 nice. We love to know where our audience is.
00:24:32 --> 00:24:33 We had someone from Yemen the other day,
00:24:33 --> 00:24:36 which was terrific. Uh, but, uh,
00:24:36 --> 00:24:39 nice to have an international flavour in our,
00:24:39 --> 00:24:41 in our shows. And thanks, uh, to our live
00:24:41 --> 00:24:44 viewers as well, who pop questions in once
00:24:44 --> 00:24:47 in a while, which is terrific. And thank you,
00:24:47 --> 00:24:48 Fred Watson. Uh, we'll catch you on the next
00:24:48 --> 00:24:50 episode of Space Nuts.
00:24:50 --> 00:24:53 Professor Fred Watson: Great pleasure, Andrew, and thanks again to
00:24:53 --> 00:24:54 all the listeners sending in those fabulous
00:24:54 --> 00:24:55 questions.
00:24:55 --> 00:24:58 Andrew Dunkley: Indeed. And thanks to Huw in the studio.
00:24:58 --> 00:25:00 Couldn't be with us today because he's a
00:25:00 --> 00:25:03 quasi Huw. And from me, Andrew
00:25:03 --> 00:25:05 Dunkley, thanks for your company. We'll see
00:25:05 --> 00:25:07 you on the next episode of Space Nuts.
00:25:07 --> 00:25:08 Professor Fred Watson: Bye. Bye.
00:25:08 --> 00:25:11 Andrew Dunkley: Uh, you've been listening to the Space Nuts
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