Latest / Space Nuts: Astronomy Insights & Cosmic Discoveries / Time Dilation, Black Hole Mysteries & Cosmic Curiosities

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Time Dilation, Black Hole Mysteries & Cosmic Curiosities · Space Nuts: Astronomy Insights & Cosmic Discoveries

• 0:00
  Andrew Dunkley: Hello again. Thanks for joining us. This is
• 0:01
  Space Nuts Q and A edition. My name is Andrew
• 0:04
  Dunkley. This will be our last official show
• 0:06
  for 2025. We'll go into a short
• 0:09
  recess and be back with you early in the
• 0:12
  new year. but we've got some questions
• 0:15
  to nail down before any of that happens.
• 0:17
  And we've got a whole bunch of topics that
• 0:20
  seem to have stirred the imaginations of
• 0:23
  our audience. Andrew wants to know about time
• 0:25
  dilation of stuff. Stars.
• 0:28
  Adriano is talking black holes becoming
• 0:31
  stars. Ishtok is wanting
• 0:34
  to ask about free space.
• 0:38
  I always thought it was expensive, especially
• 0:41
  all the space around where we live. Yo.
• 0:43
  And Gergo, Redshift and Gravastars.
• 0:46
  We will tackle all of that in this edition of
• 0:49
  space nuts. 15 seconds. Guidance is
• 0:52
  internal.
• 0:53
  Professor Fred Watson: 10, 9. Ignition
• 0:56
  sequence. Star space nuts.
• 0:57
  Andrew Dunkley: 5, 4, 3, 2.
• 0:59
  Professor Fred Watson: 1. 2, 3, 4, 5, 5, 4, 3,
• 1:02
  2, 1.
• 1:03
  Andrew Dunkley: Space nuts. Astronauts report it feels good.
• 1:07
  And here he is again. It's professor Fred
• 1:09
  Watson, astronomer at large. Hello Fred.
• 1:12
  Professor Fred Watson: Hello Andrew.
• 1:14
  Andrew Dunkley: Good to see you again.
• 1:15
  Professor Fred Watson: Fancy seeing you here.
• 1:16
  Andrew Dunkley: How odd. How strange.
• 1:18
  Professor Fred Watson: How strange.
• 1:20
  Andrew Dunkley: getting ready for your Christmas break. I
• 1:22
  mean you've just come back from a break so
• 1:24
  you'd be, you know, probably
• 1:28
  feeling rather relaxed.
• 1:30
  Professor Fred Watson: Well, no, because only the last six days
• 1:32
  weren't work.
• 1:35
  Yeah. So, no, it's not quite
• 1:38
  true because we did have some time off with
• 1:39
  my family in Scotland. but we did have a
• 1:42
  proper holiday at the end of our trip. But
• 1:44
  yes, we did two months of pretty hard work
• 1:46
  actually. We had a tour in Japan and then a
• 1:48
  conference in Ireland and a few other things
• 1:50
  like that that kept us busy. So
• 1:53
  if you, if you want.
• 1:54
  Andrew Dunkley: To call international travel a job, that's,
• 1:57
  you know, that's fine.
• 2:00
  Professor Fred Watson: want to call us work. if you. Yeah,
• 2:03
  but when you've got a tour group, when you
• 2:04
  got 20 people who you entertained
• 2:07
  for a month, it's actually three and a half
• 2:10
  weeks. it is work.
• 2:13
  Andrew Dunkley: Yeah. Yeah. Well we've got a similar problem
• 2:15
  in the coming week or two with four
• 2:17
  grandchildren that we're.
• 2:19
  Professor Fred Watson: To the cave.
• 2:20
  Speaker C: Yeah.
• 2:20
  Professor Fred Watson: To be honest, I'd rather have 20 tourists
• 2:22
  than four grandchildren. Although my
• 2:24
  grandchildren are now totally self propelled.
• 2:26
  But for your Yankee swan, Aggie, she's only,
• 2:29
  she's, she's nine months yesterday actually.
• 2:32
  Anyway, anyway, yes, it's
• 2:35
  a matter of. But we. Yeah. So the bottom
• 2:38
  line is we will have a relaxing end of year
• 2:41
  break.
• 2:41
  Andrew Dunkley: I hope very good you mentioned Edinburgh.
• 2:43
  Well, our first question comes from Andrew
• 2:46
  in Edinburgh. he says, I have a two part
• 2:49
  question about the gravity and subsequent
• 2:51
  time dilation that occurs in and around
• 2:54
  supergiant stars. If the supergiants
• 2:56
  can collapse into black holes, then they must
• 2:59
  have as much or even more mass than the
• 3:02
  resulting black hole. Just spread over a much
• 3:04
  large, larger area. I guess my question is,
• 3:07
  is there significant time dilation near these
• 3:10
  stars or are they simply not dense
• 3:13
  enough to have meaningful amounts of time
• 3:16
  dilation? If they do, it's weird that
• 3:19
  comes up and a slight follow up would
• 3:22
  be what about time dilation within
• 3:25
  the star itself? Presumably near the core of
• 3:28
  these stars, the density ramps right up.
• 3:31
  Does a large difference in time dilation
• 3:33
  within a star have any impact on how
• 3:36
  it behaves? Hope that all makes sense.
• 3:38
  Thanks. Love the show. That's Andrew in
• 3:40
  Edinburgh.
• 3:42
  Professor Fred Watson: They're great questions.
• 3:46
  I'm just not sure about
• 3:50
  the first sentence. If the
• 3:52
  supergiants can collapse into black
• 3:55
  holes, then they must have as much or
• 3:58
  even more mass than the resulting black hole.
• 4:00
  Yeah, okay, I've read that properly now. just
• 4:02
  spread it maybe.
• 4:02
  Andrew Dunkley: I didn't read it properly.
• 4:04
  Professor Fred Watson: No, it's all right. No, it's fine.
• 4:06
  so yeah, Andrew's first question. Is there
• 4:08
  significant time dilation near these stars?
• 4:10
  And the answer is yes.
• 4:13
  there would be. it's, I mean the time
• 4:16
  dilation in a black hole is
• 4:19
  so great that to an outside observer,
• 4:22
  time stops on the event horizon.
• 4:25
  for a star because it's spread over
• 4:28
  a larger volume of space, the time dilation
• 4:30
  is nowhere near as great. but time dilation
• 4:33
  will be something that you would have to take
• 4:35
  into account if you had, a
• 4:37
  spacecraft orbiting near a giant star.
• 4:40
  the bottom line is with, and time dilation,
• 4:44
  it's a little bit spooky in the sense that
• 4:50
  to the star itself and to something,
• 4:53
  say you've got something in orbit around this
• 4:55
  star, their time's ticking away at the normal
• 4:57
  rate. The time dilation is only what you see
• 4:59
  from the outside.
• 5:01
  Andrew Dunkley: So this is basically the same as we were
• 5:03
  talking about in the last episode regarding
• 5:05
  Mars. Same problem.
• 5:07
  Professor Fred Watson: Yes, that's right. It is the same thing.
• 5:09
  Yeah. So time ticks away
• 5:12
  normally for the star, but to watch
• 5:14
  it from the outside, you basically see time
• 5:17
  ticking away a little bit more slowly.
• 5:21
  so they would have time dilation. M. Andrew's
• 5:24
  asking whether they're simply not dense
• 5:27
  enough to have a meaningful amount of time
• 5:29
  dilation. And I don't think that's true. I
• 5:31
  think this time dilation is significant,
• 5:33
  especially if you're looking at microseconds,
• 5:35
  as we were in the last episode. yeah,
• 5:38
  so they do. And look,
• 5:40
  you're not talking about time dilation of the
• 5:43
  kind that was depicted in interstellar
• 5:46
  where time kind of grinds to a halt
• 5:49
  almost. it's a more modest amount of
• 5:52
  time dilation, but it would actually happen.
• 5:55
  And Andrew's follow up question. What about
• 5:57
  time dilation within the star itself?
• 5:59
  Presumably near the core of these stars, the
• 6:02
  density ramps right up. There's a large
• 6:04
  difference in.
• 6:05
  Andrew Dunkley: I'm not going in there to find.
• 6:06
  Professor Fred Watson: Out there's a difference
• 6:09
  in time dilation within a star have any
• 6:11
  impact on how it behaves. and
• 6:14
  there's a curious thing there because as you,
• 6:17
  get near the core of an object, with
• 6:20
  spherical symmetry, your
• 6:22
  gravitational field gets less and less.
• 6:25
  and in fact at the center you wouldn't feel
• 6:27
  any gravity. And that's because everything's
• 6:28
  pulling you in the same direction all around.
• 6:31
  And so I believe that time dilation will
• 6:33
  probably stop in the middle of a star. That
• 6:35
  might be something I've never thought about
• 6:37
  before. maybe that's not true because
• 6:40
  you're still in a gravitational field. The
• 6:42
  fact that it cancels out everywhere. I'll
• 6:45
  check that one out actually and try and
• 6:47
  remember for our, first show next year,
• 6:49
  because that's a really interesting question.
• 6:51
  Time dilation in the center of a star, how
• 6:53
  does it behave?
• 6:55
  Andrew Dunkley: very interesting.
• 6:56
  Professor Fred Watson: But there wouldn't be a. I think the bottom
• 6:58
  line is there wouldn't be a, a big difference
• 7:00
  in time dilation from one part of a star to
• 7:02
  another. That's, that's what I'm trying to
• 7:04
  say.
• 7:04
  Andrew Dunkley: But he brings up another interesting point.
• 7:06
  You've got time dilation around a massive
• 7:08
  star.
• 7:09
  Professor Fred Watson: Yep.
• 7:10
  Andrew Dunkley: Then it goes, you know, whatever black hole,
• 7:14
  the time dilation changes.
• 7:16
  Professor Fred Watson: Yes, it does because, as it collapses, the
• 7:19
  gravitational field increases.
• 7:22
  it increases in sort of
• 7:25
  angle in the sense that, you know, it's a
• 7:27
  steeper gravitational field as you
• 7:29
  get as the black hole collapses. And by that
• 7:32
  I'm thinking of the gravitational well, you
• 7:34
  know, this dip in the trampoline sheet.
• 7:37
  That's the gravitational well of an object
• 7:39
  which turns into something like a plug hole
• 7:42
  with water going around it as a vortex for
• 7:45
  a black hole. so that's what I mean by the
• 7:47
  steepness of the gravitational field. and
• 7:50
  yes, it is so steep that the Event horizon
• 7:52
  delineates where the time dilation becomes,
• 7:55
  such that time appears to stop on the surface
• 7:57
  of the event horizon.
• 7:59
  Andrew Dunkley: Yeah, I've seen that demonstration done with
• 8:01
  like a big rubber sheet and they say
• 8:04
  that that's the time, space time
• 8:07
  continuum and they put a bowling ball in it
• 8:09
  and they say. And that's gravity.
• 8:11
  Professor Fred Watson: That's right, yep. I know,
• 8:14
  yeah.
• 8:17
  Andrew Dunkley: It'S a simple way of explaining it, but
• 8:19
  that's what it is. I suppose. Great.
• 8:22
  questions, Andrew. I hope all is well in
• 8:24
  Edinburgh. Fred's home
• 8:27
  stomping ground. yep, yep. I'll give you his
• 8:30
  address and you can go and rock his roof.
• 8:32
  This is Space Nuts with Andrew Dunkley and
• 8:34
  Professor Fred Watson.
• 8:35
  Ah, we have got an audio question. Fred,
• 8:38
  this is from Adriano.
• 8:41
  Speaker C: Hi guys. Adriano from Florence in Italy. I
• 8:44
  have my first question about black holes. So
• 8:46
  if I understood correctly, a star continued
• 8:48
  to burn his fuel like hydrogen and helium.
• 8:52
  And there are nuclear fusions and there is
• 8:54
  enough energy for the star to fight against
• 8:56
  its own gravitational pull. But, at
• 8:59
  some point there is not enough, fuel and the
• 9:01
  star collapses into a black hole. After this,
• 9:04
  the black hole will start to absorb material
• 9:07
  like hydrogen and then it should have enough
• 9:10
  energy, enough fuel to have, nuclear
• 9:13
  fusions and to fight against the
• 9:16
  gravitational pull. But, so why a
• 9:19
  black hole cannot, turn back into a star?
• 9:22
  I'm sure this is not possible, but I cannot
• 9:24
  understand why. And also guys, we had a lot
• 9:27
  of beautiful updates from the princess. Can
• 9:29
  we also have some updates from Fred? Thank
• 9:32
  you guys.
• 9:32
  Andrew Dunkley: Bye bye, Adriano, thank you very much. Fred
• 9:35
  gave us his update when he got back,
• 9:39
  but. Yeah, your point is well made. Florence,
• 9:41
  what a beautiful, beautiful city.
• 9:43
  Speaker C: Yeah.
• 9:43
  Professor Fred Watson: isn't it just?
• 9:44
  Andrew Dunkley: we, we visited Florence a few years ago and
• 9:47
  it was, it was amazing.
• 9:50
  But it was also terrible timing because it
• 9:52
  was All Saints weekend, which is a four day
• 9:54
  long weekend and there were like tens of
• 9:56
  thousands of people there. You couldn't move.
• 9:59
  You absolutely couldn't move. So,
• 10:02
  we went to, what was it called, the Ponte
• 10:04
  Vecchia, and we couldn't get near it.
• 10:07
  You just couldn't. It was it was insane.
• 10:10
  Yeah, we didn't know until we got there
• 10:12
  that's what was happening. But yeah, we still
• 10:14
  got to see it. It was a beautiful place and
• 10:17
  all those amazing statues and Galileo got,
• 10:19
  got, got up close with Galileo.
• 10:22
  Professor Fred Watson: Very good.
• 10:22
  Andrew Dunkley: Yeah, yeah.
• 10:24
  Professor Fred Watson: Did you see his? I think it's his. It's one
• 10:27
  of his fingers or his thumb, I can't remember
• 10:28
  which is on display in the science museum
• 10:30
  there.
• 10:31
  Andrew Dunkley: Oh, no, no, couldn't get near that. yeah,
• 10:33
  honestly, it was just mayhem. But, yeah,
• 10:36
  understandable though.
• 10:37
  all right, so the bottom line with Adriano's
• 10:40
  question is, why can't a black hole
• 10:43
  turn back into a star? yeah, I
• 10:46
  would think there'd be all sorts of reasons
• 10:48
  why not.
• 10:50
  Professor Fred Watson: Well, that's right. I think once you've
• 10:52
  turned into a singularity, as the, you can't
• 10:54
  double down.
• 10:58
  Andrew Dunkley: Sorry that,
• 10:59
  Professor Fred Watson: You took the words out of my mouth. No, you
• 11:02
  didn't. I mean, all bets are off basically
• 11:05
  once you, Once you've gone into a
• 11:07
  singularity. and so I,
• 11:09
  think, you know, it's a great thought that,
• 11:12
  Adriano's had. I. And it's never occurred
• 11:15
  to me before, but, but you know, you're
• 11:18
  talking about hydrogen, which certainly would
• 11:20
  get sucked into a black hole because a lot of
• 11:23
  the gas clouds that, the black
• 11:26
  hole, accretion disk would draw
• 11:28
  in and suck into the center,
• 11:33
  that's hydrogen. and hydrogen is the raw
• 11:35
  material of stars. Why can't nuclear fusion
• 11:38
  kick in again and drive the star back into
• 11:41
  being a star rather than a black hole? And I
• 11:43
  think the answer is in structure. so
• 11:46
  stars have quite a complex
• 11:48
  structure, to make them work.
• 11:51
  with the, core, with all the nuclear burning
• 11:54
  taking place, then there's a convection zone
• 11:56
  and then there's a sort of outer layer before
• 11:59
  you get to the photosphere, the layer that
• 12:00
  you can see. when you've put something into a
• 12:03
  singularity, all structure disappears.
• 12:06
  And it
• 12:09
  almost relates to, an issue
• 12:12
  that occupied the m. Mind of Stephen Hawking
• 12:14
  for a while, which is that does information
• 12:17
  get lost when it goes into a black
• 12:20
  hole? And I think there was some
• 12:22
  argument with another well known physicist.
• 12:24
  In fact, I think they had a bet, which
• 12:26
  Hawking lost. because,
• 12:31
  I think the bottom line was Hawking back that
• 12:33
  information couldn't come out of a black
• 12:35
  hole. But somebody proved a theory that
• 12:37
  information could come out of a black hole. I
• 12:39
  think I've got the right way.
• 12:42
  Basically, it's all completely mangled in
• 12:44
  terms of, we don't understand the
• 12:47
  physics of what would happen inside a
• 12:49
  singularity. We just have no idea what the
• 12:51
  physical processes would be. And they almost
• 12:54
  certainly would rule out hydrogen
• 12:56
  atoms getting together, and
• 12:59
  with enough temperature to produce the
• 13:01
  nuclear fusion that we see in a normal star.
• 13:04
  a black hole is a very abnormal object.
• 13:08
  Nothing relates to normal in a black
• 13:11
  hole. And so I think that is the answer to
• 13:13
  Adriana's question. physics
• 13:16
  doesn't work the way it works on the outside
• 13:18
  of a black hole, and I think that's why we
• 13:20
  don't see black holes turning into stars.
• 13:22
  Andrew Dunkley: Yeah, well, there's also the fuel issue,
• 13:24
  like, you know, the star has collapsed
• 13:27
  because of fuel depletion, has it
• 13:30
  not?
• 13:31
  Professor Fred Watson: Yes, that's right. But what we're saying and
• 13:32
  what Adriano is saying is that, among the
• 13:34
  stuff that is accreted by the black hole,
• 13:37
  when it's sitting there gobbling stuff up, a
• 13:40
  lot of that is hydrogen, which is the fuel.
• 13:43
  So they're getting more fuel, but they don't
• 13:45
  any longer have the process to make it turn
• 13:48
  into something that will deliver energy. I
• 13:50
  think that's the bottom line.
• 13:51
  Andrew Dunkley: I get it, I get it. Okay.
• 13:54
  great question, though, because,
• 13:59
  we've been talking black holes
• 14:01
  for I don't know how long, probably since the
• 14:04
  very beginning of the time that this
• 14:07
  podcast began, and I don't think we've ever
• 14:09
  been asked that question before.
• 14:10
  Professor Fred Watson: No, I think that's right.
• 14:12
  Andrew Dunkley: Yeah.
• 14:13
  Professor Fred Watson: So it's a lot for our. For our, listeners,
• 14:16
  doesn't it, that they can produce questions
• 14:18
  that we've never had before after however
• 14:21
  many episodes. It's getting on for 500 now.
• 14:23
  Andrew Dunkley: This is 582.
• 14:25
  Professor Fred Watson: Oh, 582. Okay.
• 14:27
  Andrew Dunkley: 582.
• 14:28
  Speaker C: Yeah.
• 14:28
  Professor Fred Watson: Right. There you go. Getting on for 600.
• 14:30
  Andrew Dunkley: Oh, no, it's not. I mean, it's happening
• 14:33
  faster because of time dilation and the fact
• 14:35
  that we decided to do two episodes
• 14:38
  a week instead of one. But.
• 14:41
  Professor Fred Watson: But I think it's nuts by definition, isn't
• 14:43
  it? I've got a feeling.
• 14:46
  Yeah.
• 14:47
  Andrew Dunkley: anyway, thank you, Adriano, and hope all is
• 14:50
  well in the beautiful Florence. This is Space
• 14:53
  Nuts with Andrew Dunkley and Professor Fred
• 14:55
  Watson.
• 14:58
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  Space nuts.
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  Our, next question doesn't come from Italy,
• 16:40
  it comes from Slovenia. Russian store.
• 16:43
  Yeah. I am listening to your podcast while
• 16:45
  driving to and from work. Great show. I hope
• 16:48
  you managed to keep control because, you
• 16:50
  know, this gets a bit crazy sometimes. I'm
• 16:53
  curious about. Wait for it, Fred. Black
• 16:55
  holes. we know that an atom is actually a
• 16:57
  lot of free space where electrons fly around.
• 17:01
  eliminating that, we probably, probably
• 17:04
  get a neutron star. Ah, with high density.
• 17:06
  But what about a black hole? How does this
• 17:09
  work? Where is the free space, that can
• 17:11
  be squeezed even further to get a black hole?
• 17:14
  get black hole material and density and to
• 17:16
  calculate the density of the black hole.
• 17:18
  Would it be a correct assumption to take the
• 17:21
  event horizon as the boundary and,
• 17:24
  based on that, calculate the volume? Or is
• 17:27
  it something else? Thank you. Best regards.
• 17:30
  Ishtok. another black hole question. Not
• 17:33
  surprising. We get a lot of them.
• 17:35
  Professor Fred Watson: We do, yeah. So,
• 17:38
  it's a great question and, that's absolutely
• 17:41
  right. An atom is a lot of free space, empty
• 17:43
  space, with a, cloud of electrons doing
• 17:46
  their quantum thing. if, you collapse
• 17:49
  the space down so that only the electrons are
• 17:51
  pushing the, atoms apart, you've got a white
• 17:54
  dwarf star, which is called electron
• 17:56
  degenerate. and if you get rid of the
• 17:58
  electrons, then you get a neutron star
• 18:00
  exactly as Ishtok, says, with
• 18:03
  very, high density, where only the
• 18:05
  neutrons keep the thing from collapsing into
• 18:08
  a black hole. But with a black hole,
• 18:12
  well, the free space is basically
• 18:14
  disappeared down the black hole.
• 18:18
  and in terms of its density,
• 18:21
  you have a definition of a black
• 18:24
  hole. one of the definitions is A point in
• 18:27
  space with infinite density. So the
• 18:29
  volume is zero. See, Jordy thinks that ah, as
• 18:32
  well. He does. Gosh, I don't know what's
• 18:35
  happening out there, but yeah, I love it.
• 18:40
  Andrew Dunkley: We had to hear him from.
• 18:41
  Professor Fred Watson: Yeah.
• 18:41
  Andrew Dunkley: For the last show of the year.
• 18:44
  Professor Fred Watson: That's right. In full
• 18:47
  flight. so. Yes, so it's a point of
• 18:50
  infinite density. So it talks a comment about
• 18:52
  calculating the density of the black hole.
• 18:54
  Would it be a correct assumption to take the
• 18:56
  event horizon as the boundary? no, it
• 18:58
  wouldn't. The event horizon's just that
• 19:00
  imaginary point where of no return.
• 19:03
  and the volume is zero. the volume of the
• 19:06
  black hole is zero, which is how the, the
• 19:08
  density gets infinite because,
• 19:11
  mass over density. Sorry, Mass over volume is
• 19:14
  density. The mass is a, ah, is a parameter,
• 19:17
  but the volume is zero. I've no idea what's
• 19:20
  happening out there, Andrew, with Jordy, but
• 19:22
  he obviously likes this conversation.
• 19:24
  Andrew Dunkley: Yes, yes, he does. He wants in.
• 19:29
  dear. yeah, look, I still
• 19:32
  don't get black holes
• 19:35
  receding into the distance. Yeah,
• 19:37
  probably chasing a snake. yeah,
• 19:41
  go ahead, Andrew.
• 19:42
  Professor Fred Watson: No, it's.
• 19:43
  Andrew Dunkley: It's hard to get your head around
• 19:46
  something like a black hole
• 19:49
  having.
• 19:52
  No, no density.
• 19:54
  Professor Fred Watson: M. No.
• 19:55
  Andrew Dunkley: In my brain.
• 19:56
  Professor Fred Watson: No volume.
• 19:57
  Andrew Dunkley: No volume.
• 19:57
  Professor Fred Watson: It's got no size. It's got zero
• 20:00
  dimensions.
• 20:01
  Andrew Dunkley: I mean, we, we give them names
• 20:04
  based on size and yet it has no size.
• 20:07
  Professor Fred Watson: Superlastic. Yeah, well, but it's the mass
• 20:09
  that's the thing. So the mass is defined for
• 20:11
  a black hole. It's one of the properties that
• 20:13
  they have. There's this thing called the no
• 20:16
  hair theorem, which I like very much.
• 20:19
  Yeah. And it's about, you know, wouldn't.
• 20:22
  Andrew Dunkley: They wouldn't.
• 20:23
  Professor Fred Watson: Yeah, that's right. Which is.
• 20:26
  It's about the very few parameters that you
• 20:28
  can get from a black hole. I think it's mass,
• 20:31
  charge and spin. I think that's all you know
• 20:33
  about a black hole. because the volume zero.
• 20:36
  And that's why the density zero. Density is
• 20:39
  mass over volume, volume zero. So the density
• 20:41
  goes to infinite infinity, but you can vary
• 20:44
  the mass.
• 20:45
  And that's why we talk about supermassive
• 20:47
  black holes and intermediate mass black holes
• 20:49
  and things of that sort.
• 20:50
  Andrew Dunkley: Okay, so what was the answer to the
• 20:53
  question?
• 20:55
  Professor Fred Watson: no.
• 20:57
  Andrew Dunkley: Righto.
• 20:58
  Professor Fred Watson: What was the question again? Hang on.
• 21:01
  yeah. Would it be. Yes. Would it be correct
• 21:03
  assumption to take the event horizon as a
• 21:04
  boundary and use that to calculate the
• 21:06
  volume? No. The event horizon is an imaginary
• 21:09
  sphere that is where the thing
• 21:12
  turns black Basically because no light can
• 21:14
  escape.
• 21:15
  Andrew Dunkley: Precisely. hope that helped Ishok.
• 21:18
  Professor Fred Watson: it's a great question.
• 21:19
  Andrew Dunkley: It is terrific question. just a very
• 21:22
  difficult subject because we just don't
• 21:25
  know a hell of a lot about black
• 21:27
  holes. They're just such a mysterious and
• 21:31
  weird object. And,
• 21:34
  we're still trying to gather information
• 21:36
  about them and they just keep throwing up
• 21:38
  these curveballs at us and not letting us in.
• 21:41
  Not that you want to go in, but you know what
• 21:43
  I mean.
• 21:44
  Professor Fred Watson: Yes, that's right.
• 21:46
  Andrew Dunkley: Yeah.
• 21:46
  Professor Fred Watson: Quite soon.
• 21:47
  Andrew Dunkley: All right, thanks Ishtok.
• 21:50
  Time to take a break from the show to tell
• 21:52
  you about our, sponsor, NordVPN. If you're
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• 23:18
  Professor Fred Watson: Space Nuts.
• 23:20
  Andrew Dunkley: We have one more question to finish things
• 23:22
  off for 2025. And
• 23:25
  it's a real European flavor for this episode.
• 23:28
  This is Gergo. Greetings gentlemen.
• 23:31
  Gergo from Slovakia here. I have a question
• 23:34
  about redshifting. Does it have a limit?
• 23:37
  Is there a point beyond which light cannot be
• 23:39
  stretched any further? If so, what happens if
• 23:42
  the light continues to travel through
• 23:44
  expanding space? And the second question.
• 23:47
  Could you talk a bit about Gravastars? Do you
• 23:49
  think they might be real? thank you for your
• 23:52
  time and for The Great show.
• 23:54
  Speaker C: Bye.
• 23:55
  Andrew Dunkley: Thanks, Georgia. yeah, it's
• 23:58
  an eclectic mix of nationalities. This
• 24:01
  Fabius was. Yeah, it's terrific.
• 24:03
  Professor Fred Watson: Yeah, it's great.
• 24:04
  Andrew Dunkley: so two questions he's, thrown into the mix.
• 24:07
  yeah. Is there a limit on redshift?
• 24:11
  yeah. Good one.
• 24:13
  Professor Fred Watson: So, yeah,
• 24:16
  it is a good question. I mean, so redshift,
• 24:19
  as a term we
• 24:22
  define as being due to the expansion of the
• 24:25
  universe. and it's slightly
• 24:28
  different from the Doppler shift. Doppler,
• 24:31
  shift is something we understand. Well, it's
• 24:33
  the way the light changes wavelength from a
• 24:35
  moving object. But with redshift, we're
• 24:38
  talking about space itself. Rather
• 24:41
  than objects moving through space. We're
• 24:43
  talking about the way space behaves. and so
• 24:45
  it's a much more fundamental thing than the
• 24:48
  Doppler shift. So in a sense,
• 24:51
  there's already a limit to redshift.
• 24:54
  but it's one that is
• 24:58
  exactly related to the age of the universe.
• 25:03
  so what I'm thinking of here is
• 25:06
  the, cosmic microwave background
• 25:09
  radiation. That's the wall of radiation
• 25:12
  which corresponds to the brightness of
• 25:15
  the Big Bang fireball. Which
• 25:18
  we're still seeing. Because as we look
• 25:20
  further into space, we look back in time. So
• 25:23
  everywhere in space we see this wall of
• 25:25
  radiation. Which is now in the microwave
• 25:28
  region of the spectrum. Which is why we call
• 25:29
  it the cosmic m. Microwave background
• 25:31
  radiation. Yeah. And so, if I
• 25:34
  remember rightly, that
• 25:36
  is basically the visible flash
• 25:40
  of the Big Bang. Because
• 25:43
  it was, basically
• 25:46
  a visible light flash. It's the visible
• 25:49
  flash redshifted by, I think,
• 25:52
  about 1300 times.
• 25:55
  So everything in the universe must have a.
• 25:57
  That we can observe. Must have a redshift
• 26:00
  less than that. I think 1300
• 26:03
  is the number that comes into my mind. I've
• 26:06
  looked at this for a long time, but it's
• 26:08
  visible light, Whose
• 26:11
  waves have been stretched by that amount to
• 26:14
  give us microwaves. So stretched about
• 26:17
  1300 times thereabouts. Now,
• 26:22
  as the universe expands and time goes
• 26:24
  on, that number will increase not
• 26:27
  by much. Might become 1301 or
• 26:30
  1305. But as time goes on, that number's
• 26:33
  increasing. So in a sense, that's a limit
• 26:36
  to redshift. physically though,
• 26:39
  I don't think there is a limit. You could,
• 26:41
  you know, if you expand the universe. If
• 26:43
  you're talking about 40 billion years into
• 26:45
  the future. And the universe is expanding
• 26:47
  more. Yes. The cosmic microwave background.
• 26:49
  Is going to be the cosmic long wavelength
• 26:51
  radio background. and so,
• 26:55
  the wavelength Will have stretched more.
• 26:58
  So there isn't a physical limit, but
• 27:01
  there is a, ah, a limit in the real
• 27:04
  universe, simply because of the age of the
• 27:06
  universe. The universe hasn't expanded
• 27:09
  for long enough for the redshift to be more
• 27:11
  than about 1300. Right,
• 27:14
  okay.
• 27:15
  Andrew Dunkley: Yeah, got it.
• 27:16
  Professor Fred Watson: Good.
• 27:18
  What was the other thing? Oh, Gravisars.
• 27:19
  Andrew Dunkley: Oh, Gravastars. Yeah, we've had, we've had
• 27:21
  questions about Gravastars before, more than
• 27:24
  once. it seems to be something that sort of
• 27:26
  captured the imagination of people
• 27:29
  that are so interested in astronomy and space
• 27:31
  science. So I suppose we should
• 27:34
  start by reminding people what a gravastar
• 27:37
  is supposed to be, because I don't think
• 27:39
  we've ever found one.
• 27:41
  Professor Fred Watson: No, that's correct. I'm going to read
• 27:43
  from that font of all knowledge,
• 27:46
  Wikipedia, who I do subscribe to, despite
• 27:49
  the fact that they keep asking me for another
• 27:51
  subscription. Anyway, that's probably because
• 27:53
  I've got more than one username. Never mind,
• 27:55
  let me read from Wikipedia. In astrophysics,
• 27:59
  a Gravis star, which is a blend word
• 28:01
  of gravitational vacuum star,
• 28:04
  is an object Hypothesized in a
• 28:06
  2001 paper by Pavel O.
• 28:09
  Mazur and Emil Motola
• 28:12
  as an alternative to the black hole theory.
• 28:15
  It has the usual black hole metric
• 28:18
  outside of the horizon. And the metric is
• 28:20
  just a way of describing space, but de
• 28:23
  sittomatric inside. And that's a different
• 28:26
  one, don't worry about that. A typical
• 28:29
  gravastar is as big as London,
• 28:32
  but weighs 10 solar masses.
• 28:35
  Yeah. So a neutron star would be about the
• 28:38
  size of London, but weigh one solar mass,
• 28:41
  basically.
• 28:42
  Andrew Dunkley: Didn't they find one in a sewer? they called
• 28:44
  it a fratberg or something.
• 28:45
  Professor Fred Watson: Fatberg. That's right, yeah. Which was
• 28:48
  just about to turn into a gravastar.
• 28:49
  Andrew Dunkley: Yes.
• 28:51
  Professor Fred Watson: on the horizon there is an ultra thin,
• 28:54
  incredibly tight shell of entirely
• 28:56
  new unique exotic matter
• 29:00
  named galactic Flubber.
• 29:02
  Andrew Dunkley: That was close.
• 29:04
  Professor Fred Watson: You weren't far off. That's right. Which is
• 29:06
  the next thing to a fatberg. Yeah. Anyway,
• 29:09
  continuing to read this solution to the
• 29:11
  Einstein equations is stable and has
• 29:13
  no singularities, which we've just been
• 29:16
  talking about singularities, points of zero
• 29:19
  volume. Instead, Gravastar is filled with
• 29:22
  either dark energy or with vacuum energy,
• 29:24
  but also vacuum. only
• 29:27
  the inside one, 10 to the 44 times denser
• 29:30
  than the outside. I'm not sure how you can
• 29:33
  have a vacuum that's 10 to the 44 times
• 29:36
  denser than another one, but I'll just let
• 29:38
  that pass. Yes.
• 29:41
  as a bonus, further theoretical
• 29:43
  considerations of gravastars include the
• 29:45
  notion of a nestar. A second gravastar
• 29:48
  nested within the first one. So that's
• 29:50
  the technical definition. I bet you're no
• 29:53
  wiser than I am. but the bottom
• 29:55
  line is that,
• 29:58
  And I'll read again. Mazur and Mottola
• 30:01
  suggest that the violent creation of a
• 30:03
  gravastar might be an explanation for the
• 30:06
  origin of our universe and, many other
• 30:09
  universes, because all the matter from a
• 30:11
  collapsing star would implode through the
• 30:14
  central hole and explode into a new dimension
• 30:16
  and expand forever, which would be consistent
• 30:19
  with the current theories regarding the Big
• 30:22
  Bang.
• 30:23
  Andrew Dunkley: Okay, so now that we know what
• 30:26
  it is, do you think they exist and
• 30:29
  will we ever find one?
• 30:31
  Professor Fred Watson: no and no. Basically, it's,
• 30:35
  an alternative theory for the Big Bang,
• 30:39
  and it's certainly interesting. And,
• 30:41
  I, you know, I, I think m.
• 30:44
  Gago's asked us to talk about it, and now we
• 30:46
  have. So, so,
• 30:49
  that's perhaps doing the best we can.
• 30:52
  Interesting. There's. There's just one other
• 30:54
  sentence I might like to read.
• 30:58
  if I can find
• 31:01
  it, I've lost it now. Oh, yeah. The
• 31:04
  new dimension that will be created in this
• 31:07
  implosion. The new dimension
• 31:09
  exerts an outward pressure on the Bose
• 31:12
  Einstein condensate layer and
• 31:15
  prevents it from collapsing further. So
• 31:18
  the Bose Einstein condensate. It sounds as
• 31:21
  though that's this thinned crust that it's
• 31:23
  got rather than an event horizon. And the
• 31:26
  Bose Einstein condensate is really
• 31:29
  interesting. I think we've just celebrated.
• 31:32
  Is it the 30th anniversary
• 31:35
  of the first example of a Bose
• 31:38
  Einstein condenser being produced? I
• 31:41
  think that's right. I think it's 30 years. I
• 31:43
  think it's 1995. what is it?
• 31:46
  it's a condensation of atoms at very low
• 31:49
  temperature that behave like one quantum
• 31:52
  object. that's the crucial things. So
• 31:55
  it's almost like entanglement, Andrew, where
• 31:58
  you've got quantum particles being entangled.
• 32:00
  This is a whole bunch of stuff that is so
• 32:02
  entangled it just looks like one quantum
• 32:04
  object and we can now create them. so
• 32:07
  that's what they're saying, that maybe this
• 32:09
  thing is made of a Bose Einstein condenser. I
• 32:11
  think this is a really good way to end, the
• 32:14
  year's, Space Nuts episode because it is
• 32:17
  completely off the wall and talking about
• 32:20
  stuff that is right at the cutting edge of
• 32:22
  physics, which I love.
• 32:23
  Andrew Dunkley: Indeed. Thank you for your questions. Gergo
• 32:26
  and Hope, you're well. Good, to hear from
• 32:29
  you. He's sending questions before so it's
• 32:31
  nice to catch up. in fact, I think, I think
• 32:33
  Ishtok, has sent questions in before as
• 32:36
  well. But, yeah, thank you for your questions
• 32:38
  everybody, for contributing to this, the
• 32:40
  final episode of 2025. Keep the questions
• 32:42
  coming in because we're coming back next
• 32:45
  year and we'll need some fresh stuff because
• 32:48
  we're down to the last one or two,
• 32:50
  which I didn't use because they all came from
• 32:52
  the same source and I like to spread the love
• 32:54
  a bit. So, we'll get into those next year.
• 32:56
  But, go to our website if you'd like to send
• 32:59
  a question in. Click on the AMA link at the
• 33:01
  top and you can send text and audio questions
• 33:04
  there. As always, please remember to tell us
• 33:07
  who you are and where you're from while
• 33:09
  you're at, on the website. check out how you
• 33:11
  might be able to support us, through various
• 33:14
  channels. whatever you choose or don't choose
• 33:16
  to, we're not going to make you do it. you
• 33:18
  can check out the shop as well. That's
• 33:20
  another way of supporting us and so on and
• 33:23
  so forth. while I think.
• 33:25
  Professor Fred Watson: Andrew, while you're talking about the
• 33:26
  questions, I think we've got a pending one
• 33:28
  still from Rusty, which we just wish to.
• 33:32
  We'll take it next year.
• 33:33
  Andrew Dunkley: Yes, yes, I recall that. But,
• 33:36
  we. I thought we'd sit on it till the new
• 33:38
  year because reading the question will
• 33:40
  actually take the pulp of the episode.
• 33:46
  Professor Fred Watson: Thank you, Andrew. Sorry to interrupt you.
• 33:47
  Andrew Dunkley: That's okay. No, that's okay. I just want to
• 33:50
  say thank you to you, Fred.
• 33:53
  and, and I should also, thank
• 33:55
  Jonti because he, he did a, fair chunk of the
• 33:58
  show and we also
• 34:01
  had our guest presenter, Heidi while I was
• 34:03
  away. So thank you to Heidi for her amazing,
• 34:06
  contribution because, it really saved my,
• 34:09
  my back because, there's probably no way in
• 34:12
  the world I could have recorded from a cruise
• 34:14
  ship and got away with it. But, yeah,
• 34:16
  fantastic. we've got a great team.
• 34:19
  and, and you know, bring on the next, the
• 34:22
  next year of Space Nuts.
• 34:23
  Speaker C: And I.
• 34:24
  Andrew Dunkley: Look, I give him a hard time every week, I
• 34:26
  do. But I've got to say thanks to Huw in the
• 34:28
  studio for his, amazing work.
• 34:31
  It's not just our podcast that he looks
• 34:33
  after. He's got a whole stable of them and
• 34:35
  it's it's basically a full time job trying to
• 34:38
  run all and you know there's not much
• 34:41
  money in it but there's certainly joy in
• 34:43
  putting our skills into something
• 34:46
  in our semi retirement from, from radio.
• 34:49
  So. Yeah. But also without the
• 34:52
  audience we would be nothing. So we
• 34:54
  send out our our
• 34:57
  thanks. We are so grateful to have you behind
• 34:59
  us and I do keep an eye on the
• 35:02
  audience through the Space Nuts podcast group
• 35:05
  on Facebook because they they spend a lot of
• 35:07
  time there talking to each other, sharing
• 35:09
  pictures, and posing unusual
• 35:12
  questions which occasionally we will bring up
• 35:14
  on the show. And special thanks to our
• 35:16
  sponsors. We've had a few sponsors who've
• 35:18
  been with us for quite some time now and, and
• 35:21
  you know, obviously we're doing something
• 35:23
  right if they're willing to stick with us. So
• 35:25
  very much appreciated. thank you
• 35:28
  Fred. thank you Jordi and,
• 35:31
  and we'll talk to you in the new year.
• 35:35
  Professor Fred Watson: Sounds great. Look forward to it Andrew. And
• 35:37
  all the very best for the festive season to
• 35:38
  you.
• 35:39
  Andrew Dunkley: And to you Imani. thank you very much,
• 35:41
  Professor Fred Watson, Astronomer at large,
• 35:43
  and from me, Andrew Dunkley. Have a great
• 35:45
  Christmas. A happy new year. We'll see you in
• 35:47
  2026. Until then, bye
• 35:50
  bye. You'll be listening to the
• 35:53
  Space Nuts podcast
• 35:56
  available at Apple Podcasts, Spotify,
• 35:59
  iHeartRadio or your favorite podcast
• 36:02
  player. You can also stream on
• 36:03
  demand@bytes.comm this.
• 36:06
  Professor Fred Watson: Has been another quality podcast production
• 36:08
  from bytes.com