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Black Holes, Navigation, The Big Crunch & Re-Entry Speeds: Your Cosmic Questions Answered · Space Nuts: Astronomy Insights & Cosmic Discoveries

0:00
Andrew Dunkley: Hi there. Thanks for joining us. Once again,
0:02
this is a Q and A edition of Space Nuts. My
0:05
name is Andrew Dunkley. Great to have your
0:07
company. Uh, we will be answering audience
0:09
questions exclusively on this episode. We'll
0:12
never do it again. Yes, we will. Uh, we're
0:15
going to be, um, looking at
0:18
a, uh, concept that Ross has put up about,
0:20
uh, black holes equaling dark matter. Uh,
0:23
we'll explain that. Or he will and we'll try
0:25
and tear it apart. Uh, Sandy is
0:28
asking about navigation in space.
0:32
Uh, John is talking relativity, time,
0:35
black holes and the big crunch. I
0:38
knew we'd get a question about the big crunch
0:40
because we talked about it so recently. And
0:43
the speed of re entry is a question from
0:45
Andy. We'll deal with all of that on this
0:48
episode of space nuts.
0:50
Voice Over Guy: 15 seconds. Guidance is internal.
0:53
10, 9. Uh, ignition
0:55
sequence start. Uh, space nuts. 5, 4, 3,
0:58
2. 1, 2, 3, 4, 5, 5, 4,
1:01
3, 2, 1. Space nut astronauts
1:04
report it feels good.
1:06
Andrew Dunkley: And with us once again is Professor Fred
1:08
Watson, astronomer at large. Hello, Fred.
1:11
Professor Fred Watson: Hi, Andrew. Good to talk again. Uh, it
1:14
seems like only a few minutes ago that we
1:16
were talking. It does, doesn't it?
1:17
Andrew Dunkley: Yes, that's called relativity, I think.
1:20
Professor Fred Watson: Time dilation. Yeah.
1:21
Andrew Dunkley: Um, I'll tell you something funny. We had our
1:24
granddaughters around last night. They were
1:26
supposed to stay the night, but they both
1:27
chickened out so dad had to come and pick
1:29
them up at 9 o'.
1:30
Speaker C: Clock.
1:30
Andrew Dunkley: But um, um, they were, um,
1:33
having a bit of fun and um, they liked doing
1:35
craft. And one of them built a telescope with
1:38
a piece of paper and was looking through it.
1:41
And Judy said, what have you made? And it was
1:43
the four year old she said, I made
1:47
a looking through thing.
1:50
That's what a telescope. It's a looking
1:52
through thing.
1:53
Professor Fred Watson: Um, which is nearly what they were
1:56
originally called before the telescope was
1:58
intended of a time
2:01
device. Device for seeing afar. Uh,
2:05
yeah, it's.
2:06
Andrew Dunkley: Well, m. Looking through things.
2:07
Professor Fred Watson: Looking through thing. Yeah.
2:08
Andrew Dunkley: Very cute.
2:09
Professor Fred Watson: I love that. Yeah. Um,
2:12
yeah, you know, you know, you don't know what
2:14
you might have, you know, released in that
2:16
child's brain. She might become the
2:19
next great astronomer. Uh, uh, using a
2:22
looking through thing to make discoveries
2:24
about the universe.
2:25
Andrew Dunkley: Yeah. Yeah. Well, that's why we called it
2:27
Vera. No, her name's. Her name's
2:30
Felicity.
2:32
Professor Fred Watson: That's a good name as well. I like it.
2:34
Andrew Dunkley: It is nice. Shall we get to
2:37
our first question?
2:38
Professor Fred Watson: Oh, all right.
2:40
Andrew Dunkley: All right. Uh, if black holes are, uh, the
2:43
center of most galaxies, uh, and have
2:46
been eating up matter almost from the
2:48
Beginning of the universe. Can this be a
2:50
possible explanation of dark matter? The
2:53
black holes have eaten it.
2:56
Now this, uh, comes from Ross Simon. I
2:59
uh, had to smile when I read his name because
3:01
Ross Simon used to be a famous newsreader on
3:03
the Australian Broadcasting Corporation's TV
3:06
news service. I remember Ross. He was, he was
3:09
brilliant. Might be the same one. You
3:12
never know.
3:12
Professor Fred Watson: I was gonna say it's not the same Ross, is
3:14
it? I hope so.
3:16
Andrew Dunkley: It would be lovely. But, uh, it's not.
3:19
Professor Fred Watson: Yeah. So, um, whether or not you are the
3:21
famous Ross Simon. Ross, lovely to hear
3:24
from you. Uh, and um, I mean
3:27
it's, it's, it is tempting,
3:30
uh, to lump black holes and dark
3:32
matter together. Indeed. Um, that was
3:36
looked at as being one of the first
3:38
explanations of dark matter. Uh,
3:40
that we've got space full of black holes
3:43
that we don't see because they're black
3:45
holes, um, and that they might account
3:48
for the dark matter. This was the so called
3:50
macho theory. Massive compact halo
3:53
objects, uh, which was
3:56
popular in the 80s,
3:59
um, because it was only in the late
4:02
70s that people started taking the idea of
4:04
dark matter seriously when we realized
4:07
that um, something like
4:10
80% of the matter in the
4:13
universe is invisible to us.
4:16
Uh, now that's perhaps
4:19
slightly different from what Ross is asking
4:21
about because he's talking about material
4:23
being sucked into black holes. Uh, and
4:25
um, that is certainly
4:28
something that happens. But that's not matter
4:31
that's missing. That's just gone. Uh,
4:34
the bottom line is that the universe as we
4:37
see it today has this mystery in
4:39
that we know that there is stuff there that
4:42
has a gravitational effect. It holds galaxies
4:44
together, it holds galaxy clusters together,
4:47
causes uh, gravitational lensing
4:50
all over the place. Um, but
4:52
we have no way of detecting what it is other
4:55
than through its gravity. So it's, it's. Some
4:58
people used to call it missing matter. It's
5:00
not missing. It's definitely there. Uh, this
5:03
dark matter is around and it's probably in
5:06
the rooms that you and I are sitting in, uh,
5:08
at the moment. Uh, because it, it tends to be
5:11
where normal matter is and we can.
5:13
Andrew Dunkley: Judy and I were actually talking about a dark
5:15
matter the other day, but you know, I won't
5:17
elaborate.
5:20
Professor Fred Watson: Uh, well, what you do in your spare time,
5:22
Andrew, is entirely up m to you,
5:25
especially with your wife. Um,
5:27
um, so, yeah, so, so, but, um, but the,
5:30
the, the black hole thing did come
5:33
in because of this theory back in the 80s
5:36
that Machos massive compact halo objects,
5:38
objects that um, kind of dead
5:41
stars or orphan planets or more Especially
5:44
black holes might be, uh, the
5:47
source of dark matter. The source of this GR
5:50
we see present in large, on large scales
5:53
like galaxy clusters and galaxies. What ruled
5:56
that out was, uh, work carried out at a
5:58
number of observatories, including here, uh,
6:00
in Australia, uh, in fact, in a survey
6:03
which was called macho, uh, looking for
6:06
these things. Uh, and it was, um,
6:09
that if you, if you had a universe full of
6:11
black holes that you can't see, you would
6:14
still be able to detect them by what's called
6:16
gravitational microlensing. Because
6:18
occasionally one of these black holes would
6:20
pass in front of a distant star. Uh, and
6:22
because black holes distort the space around
6:25
them that behaves like a lens. And
6:27
you magnify the light of the distant star.
6:30
So you get a microlensing event has a
6:33
very characteristic shape. It's a star
6:35
getting brighter, uh, to a
6:37
sharp cusp and then fading away again quite
6:40
symmetrically. Uh, and we do
6:43
see them. They're caused by normal stars and
6:46
their planets. But in the numbers that you
6:48
would have to have for black holes to
6:51
be dark matter, they are not
6:54
there. There weren't enough. The numbers were
6:56
far too low. And that's when the emphasis
6:59
shifted to WIMPs, the weakly interacting
7:01
massive particles, which is just one class
7:03
of, uh, subatomic particles that we think
7:06
dark matter might be. So that's where the
7:08
theory stands at the moment. So black holes,
7:11
uh, you know, and I think Ross is
7:14
talking about supermassive black holes at the
7:16
centers of galaxies. Yes, They've been
7:17
swallowing stuff up for 13.8 billion years,
7:20
as far as we can tell. Um, but they don't
7:23
explain why today in
7:26
universe, um, something like 4/5 of the
7:29
matter in the universe is invisible to us.
7:32
Andrew Dunkley: Yeah, um, well, there's so many things we
7:35
don't understand. And as yet, uh,
7:37
which was brought up in a question recently,
7:40
we have not been able to capture
7:43
or identify, uh,
7:46
a dark matter particle. So, um,
7:49
until we can find some absolute proof
7:52
and study it, we're probably going
7:55
to just keep working with theory, I would
7:58
imagine.
7:58
Professor Fred Watson: Yeah, there are, um,
8:01
techniques that can be brought to bear. Um,
8:04
one of the theories about
8:06
dark matter is that if
8:09
that while dark matter particles don't
8:12
interact with normal matter particles, they
8:14
may interact with each other. In other words,
8:16
if you bring two dark matter particles
8:18
together, it's thought they might annihilate
8:20
and produce a signal in gamma radiation.
8:23
So you get this flash of gamma rays which
8:25
might have a characteristic spectrum. And
8:28
people are looking for that
8:30
phenomenon in the centers of
8:33
galaxies because that's where you would
8:35
expect the dark matter to be at its densest.
8:38
So it's where you would expect the dark
8:39
matter particles to interact with each other.
8:42
Um, so far the results have been a bit mixed
8:45
on that. But it's one possible way that we
8:47
might eventually discover, uh, what
8:50
dark matter is.
8:52
Andrew Dunkley: Maybe dark matter is like a, uh, negative
8:55
photograph. Remember in the days of manual,
8:57
um, photography, you'd take the film and
9:00
it would be negative, and then you turn it
9:02
into the photograph. Maybe dark matter
9:05
is the negative of the universe.
9:07
Professor Fred Watson: Well, yeah, I mean,
9:11
there might well be, uh, a way that
9:14
there is a sort of dark. What can I
9:17
call it? A dark particle physics.
9:20
A whole, uh, sweep of
9:23
subatomic particles which
9:26
fall under what we lump together as dark
9:29
matter. But it's not just a single particle,
9:31
it's many different ones. Just like the
9:32
particles of normal matter. The 16 normal,
9:35
uh, matter. Sorry, 16 subatomic
9:38
particles. They include forces as well as
9:40
matter when you count the 16. But you know
9:42
what I mean, You've got this suite of
9:44
different particles that make up normal
9:46
matter. Maybe there's a suite of different
9:47
particles that in some ways are a negative,
9:50
uh, um, that make up dark matter.
9:54
So I think that's not a bad one.
9:56
Andrew Dunkley: We've been assuming it's just the one thing.
9:57
It could be all sorts of things.
10:01
Professor Fred Watson: There could be atoms and molecules made out
10:03
of dark matter because
10:06
they interact with each. I don't know. Look,
10:09
I'm not a particle physicist, but, um, the
10:12
possibilities seem, not exactly endless
10:14
because particle physics has certain rules
10:16
that you've got to follow. Uh, but I'm
10:19
still pretty optimistic that we're going to
10:21
get to the bottom of dark matter, uh,
10:23
hopefully while I'm still al. Because I want
10:26
to know.
10:27
Andrew Dunkley: Yeah, we all do. We all do. Uh, thanks for
10:30
the question, Ross. Uh, really good
10:31
discussion point.
10:32
We get a lot of questions about dark matter
10:34
and black holes. Uh, while we're on the
10:36
subject of black holes, there was, uh, an
10:38
article released on the BBC, uh,
10:41
early this year. Uh, I know it's still early
10:44
this year, but.
10:44
Professor Fred Watson: Right.
10:44
Andrew Dunkley: You know, I'm talking the 3rd of January,
10:46
where scientists captured the first ever
10:48
visual proof of two supermassive black holes
10:51
in a death spiral. So we're really starting
10:54
to be able to find out more and more,
10:57
uh, through our increased technology and the
11:00
capacity to observe and create images
11:03
of these things. So, uh, that was pretty
11:05
exciting story. I read that one the other
11:07
day. I thought I'd, um, give it a mention.
11:09
But, um. Yeah, they,
11:11
uh, of Course, uh, the popular
11:14
press, uh, created their own photo which
11:17
has absolutely got nothing to do with it.
11:20
But, uh, yeah, it sells the story,
11:23
doesn't it? Um, but uh, they've got the image
11:26
of these two black holes, um,
11:29
basically getting ready to devour each other.
11:31
I think the big one will win.
11:35
Professor Fred Watson: Yes, probably. Well, you.
11:37
Andrew Dunkley: Thanks, Ross.
11:38
Professor Fred Watson: Yeah, probably.
11:39
Andrew Dunkley: Yeah, thanks, Ross. Good to hear from you.
11:44
Professor Fred Watson: Okay, we checked all four systems and.
11:46
Speaker C: Being with a go space nats, our.
11:48
Andrew Dunkley: Uh, next question comes from Sandy.
11:53
Speaker C: G', day, Fred and Andrew. It's Sandy here
11:55
from Melbourne again. Thanks for a cracking,
11:57
ah, show as usual. Um, my question today
12:00
is about navigation in the solar system for
12:03
the various spacecraft that we've sent into
12:05
deep space. Being a sci fi
12:08
nerd, my mind naturally goes to fancy
12:10
graphics of star charts and orbit parts on a
12:13
giant screen. However, wanted to
12:16
ask how mission planners at various space
12:18
agencies plot orbits. Do they take
12:21
into account objects like asteroids for any
12:23
close calls or is the space so vast
12:26
it's not really necessary? Thanks heaps,
12:29
Sandy. Cheers.
12:31
Andrew Dunkley: Thank you, Sandy. Good, uh, to hear from you.
12:33
I don't think we've heard from Sandy in a
12:34
little while, but, um. Yeah, um,
12:38
he does a lot of great
12:41
astrophotography and he's got a pretty
12:44
amazing setup that he's shown me in the past
12:46
about, uh, how he does it. Computers all
12:48
plugged into telescopes and yeah, all this
12:51
great software. It's a bit out of my league.
12:54
Um, I might get there one day. Uh,
12:56
navigation in space, plotting orbits, all
12:59
that kind of jazz. Um, I
13:02
must confess I struggle to get my head around
13:05
it. Uh, it's not like driving a car. You've
13:07
got to do, um, you know, when it comes to
13:10
space, you've got, um, much less
13:12
resistance, much, uh, more, much
13:15
more reaction to minute, um,
13:19
thrust and micro thrust and all
13:22
sorts of other things. But you've got to be
13:23
looking in all directions, not just, you
13:25
know, on the plane of the planet. When
13:28
you're driving a car type of thing. I don't
13:31
know what I'm trying to say, but, um. Yeah.
13:33
How does it all work, Fred?
13:35
Professor Fred Watson: Uh, it's. It's sort of
13:38
the equivalent of plotting things on a
13:40
screen, but not quite the same. Um,
13:43
but yeah, you know, the,
13:46
uh, idea that, um. Excuse me
13:49
a minute. Um, I'm sorry, I've got this cough.
13:52
Sandy's, uh, idea that we
13:55
need to take into account. I'm all right. I'm
13:58
all right. Yeah,
14:01
we need to take into account the positions of
14:03
asteroids and things of that sort. That's
14:04
exactly right. Um, so when you,
14:07
um, chart, uh, the
14:10
pathway through space, which
14:13
is all done numerically, you know, it
14:15
doesn't. We can make displays of them, and
14:18
I think people do as well. But the
14:21
reality is that the real hard core is locked
14:24
up in the numbers and the equations. Um, what
14:26
you have to do is to, uh, at any
14:29
instant along the orbit, uh, of the
14:32
spacecraft. Because it is always an orbit.
14:35
Usually, uh, for something, you know, going
14:37
between the planets, it will be in orbit
14:39
around the sun. Uh, that's the way
14:41
orbital mechanics work. As, uh, soon as you
14:44
switch on your thrusters, then you change
14:47
that orbit. Uh, but when all the
14:50
thrusters are off and your main engines are
14:52
off, you are following a trajectory which is
14:54
essentially an orbit. Um,
14:57
not always a closed one. It could be an open
15:00
orbit, uh, which is what's happening to the
15:02
five spacecraft that are leaving the solar
15:04
system. Uh, but that
15:06
orbit, uh, the future position
15:09
of your spacecraft is dictated by the
15:12
gravitational influence. Not just of the sun
15:15
and the Earth and probably the Moon, but all
15:17
the planets. All of them exert a
15:20
gravitational pull. Uh, and, um,
15:23
um, that goes down to the asteroids as well.
15:26
If you're passing through the asteroid belt,
15:28
you need to know where they all are, all the
15:30
ones which are known. And there's more than a
15:32
million known asteroids now.
15:35
You would have them kind of built into your
15:38
software that's looking, uh, at
15:41
the direction that your spacecraft is going
15:42
in. If there was any risk of a collision, it
15:44
would flag that. And, um, it would also
15:47
take into account the gravitational influence
15:50
of any close encounters of asteroids. So
15:52
it's a very precise science, um,
15:56
as you know, because we know
15:59
when, for example, the New Horizons, uh,
16:01
flyby of Pluto a decade ago,
16:04
uh, in 2015, um, the precision
16:07
with which that was executed was
16:09
unbelievable. And it's because of orbital
16:11
mechanics and how well we understand these
16:13
gravitational influences, uh, that let you do
16:16
that. Um, so, uh, yes, space navigation,
16:19
in some ways it's easier, uh, than navigating
16:23
on, uh, than driving a car. Because
16:25
with driving a car, you've always got the
16:28
unpredictability of the other road users.
16:31
The great thing about orbital mechanics is,
16:34
you know, what the other planets, the other
16:36
asteroids and all the rest of it are going to
16:37
do. And just one other
16:40
adjunct to this, if I may. Um,
16:43
uh, some years ago, there was, um, several
16:46
papers which talked about the interplanetary
16:48
superhighway. Uh, and these are
16:51
effectively low energy trajectories between
16:53
the planets. And it's based on exactly what
16:56
I've just been Saying you can map where, um,
16:59
the gravitational pull of all the objects
17:02
will take you. And it turned out that
17:05
if you can put um, a spacecraft at one of
17:07
your Lagrange points, these gravitationally
17:10
stable points, then leading from that
17:13
are these various low energy pathways that
17:15
take you to the Lagrange points of other
17:17
planets. Uh, and that's the interplanetary
17:20
superhighway. It might take you decades to
17:22
get, uh, um, from
17:26
the Earth Lagrange points to something like
17:28
Mars or Jupiter's Lagrange points. It's a
17:31
very slow process, but it does exist.
17:35
Almost like an imaginary highway which is
17:38
changing all the time as the planets go
17:39
around in their orbits. Uh, just an
17:41
interesting aspect of the navigation in
17:44
space.
17:45
Andrew Dunkley: Yeah, I would imagine that a lot of
17:47
this, uh, would be pre
17:50
programmed into the uh, computers of
17:53
these vessels. Um, they do
17:56
everything ahead of time because these things
17:58
are on autopilot, the long haul spacecraft
18:01
that are going out to do these missions.
18:04
Uh, so it would. And I've been
18:07
in the cockpit of a commercial uh,
18:10
airliner, um, long before you can't
18:13
do that anymore. Long before we had any
18:15
issues like that. And
18:17
watching the process, the plane flies itself
18:21
and the pilots sit back and tell dad jokes to
18:24
the tower. Um, that's what happened.
18:28
But I would imagine it's the same in space.
18:30
All these things are pre programmed, pre
18:33
calculated, uh, and then
18:35
contingencies built in just in case something
18:38
gets in the way that you didn't anticipate.
18:40
Um, they modify the
18:43
spacecraft to sense a problem and go around
18:46
it, I would imagine.
18:49
Professor Fred Watson: Yeah. In fact
18:52
the likelihood of something, it's so
18:55
predictable. And our uh, knowledge
18:58
of the sort of
19:01
congestion in space, if I can put it that
19:03
way, is so deep that um,
19:06
it's unlikely that something's going to come
19:08
along to surprise you. You suddenly see
19:10
something ahead that you've got to avoid, uh,
19:13
because that avoidance might actually be very
19:15
difficult. Um, you can do things. So
19:17
apparently, perhaps the best example
19:20
I can give you again, it goes back to New
19:22
Horizons and that is that once the
19:25
Jupiter encounter, sorry, the Pluto encounter
19:27
had happened, uh, back in July
19:30
2015, um,
19:33
they looked for other potential targets
19:36
and eventually found the object
19:39
Arrokoth that was discovered as part of
19:41
surveys looking for future targets. And they
19:44
worked out at what point they had to
19:47
apply a thrust to the spacecraft to change
19:50
its trajectory so that it would intersect
19:52
with Arrokoth. And it all happened,
19:55
you know, perfectly smoothly. Um,
19:58
I think it was a couple of years later when
20:00
the Arrokoth, uh, flyby took place. I Can't
20:03
remember when it was. Maybe even a bit later
20:05
than that. Maybe five years later.
20:09
But yeah, that all happened. That was the
20:12
nearest thing to, oh, there's something
20:14
ahead, we need to change course to either
20:17
interact with it or avoid it. Um, and it was
20:19
a very leisurely process.
20:22
Andrew Dunkley: And you're right about, uh, navigation on the
20:24
planet on roads being much more dangerous.
20:27
We were walking along the street the other
20:29
day and somebody turned right off the main
20:31
road into our, uh, part of town and,
20:34
uh, went to the right hand side of the
20:37
traffic island instead of the left.
20:38
Professor Fred Watson: Right where we were walking.
20:42
Yeah.
20:42
Andrew Dunkley: Ah, I don't think she noticed, to be honest.
20:44
Honest, she just went up the wrong.
20:46
Professor Fred Watson: Side of the road. Hm.
20:48
Andrew Dunkley: Anyway, it happens, but we, we always keep an
20:51
eye out for that kind of thing. Um, there you
20:53
go, Sandy. Thanks for the question. The
20:54
answer is easy peasy, really.
20:58
Professor Fred Watson: With modern computers, it's a lot harder if
21:00
you're doing it by hand. Yeah.
21:03
Andrew Dunkley: All right. This is a Q and A edition of Space
21:05
Nuts with Andrew Dunkley and Professor Fred
21:07
Watson.
21:11
Space Nuts. Okay. Uh, our next question
21:14
comes from John in 27
21:17
parts. Hey, guys. Love the show. Every time I
21:19
listen to a new episode, my mind goes crazy
21:22
thinking about new possibilities and
21:23
questions. I have two questions about
21:26
time dilation. Uh, in general
21:29
relativity, uh, if general
21:31
relativity causes time to be observed at
21:34
different rates, would that mean
21:36
someone orbiting very close to one of the
21:39
first black holes in existence would
21:41
experience a universe that has existed for a
21:44
much shorter period of time?
21:47
Uh, as a follow up to this, if the
21:50
new theory about the Big Crunch turns out to
21:52
be true, would the finite
21:55
time of the universe form the Big Bang
21:58
to, uh, uh, from the Big Bang to the Big
22:01
Crunch be considerably shorter if again,
22:04
you were orbiting close to a black hole,
22:07
uh, we see the universe as being
22:09
13.79 billion years old.
22:12
And uh, new estimates put the big crunch
22:15
at 20 billion years into the
22:18
future. My brain hurts thinking that these
22:20
timescales could be considerably different
22:22
due to time dilation. All the best, John. Uh,
22:25
from Suffolk in the uk.
22:29
There's a lot in there.
22:31
Professor Fred Watson: Um, yeah. And so, so
22:34
it's quite complicated because time
22:37
dilation depends, uh, on your
22:39
vantage point. So yeah, if you're in
22:42
orbit around a black hole, uh, you're in an
22:45
intense gravitational field.
22:47
Andrew Dunkley: You're also stuffed. But we'll just deal with
22:50
that another time.
22:52
Professor Fred Watson: Um, yeah,
22:55
you experience time just at the normal rate.
22:59
Uh,
23:02
what, um, an outside observer looking at you
23:05
would see would be your time going very
23:08
slowly. The time would be Dilated. So,
23:11
um, I suppose what we're talking about
23:14
here is that in terms of
23:17
what you might call the frame of rest of the
23:19
universe itself, uh,
23:22
that's what we
23:25
see when we look at the universe in general.
23:27
And that's what gives us the 13 point, 13.79
23:30
or 13.8 billion year age of the
23:33
universe. Um, your perception of that,
23:36
so that time effectively wouldn't change,
23:39
uh, but your perception of it if you were in
23:41
orbit around the black hole would. It would
23:43
probably appear to look as though it was
23:45
going very quickly. Uh, but that's because
23:48
your time's slower. And uh,
23:51
likewise, um, uh,
23:54
with the density of the universe being
23:56
higher, uh, at earlier
23:59
stages, yes, we do know time dilation takes
24:02
place. You can actually see that, uh,
24:04
because, um, when scientists, uh,
24:08
look at, uh, the light curves of
24:10
supernovae, exploding stars, they have a
24:13
light curve. Their light increases and then
24:15
decreases, uh, in a more gradual
24:18
way with a very characteristic shape. Uh,
24:21
those light curves, uh, are dilated, they're
24:24
stretched when we look at ones in the early
24:26
universe. So the phenomenon does happen,
24:29
but, but, um, it doesn't happen at a
24:32
level that's going to significantly shorten
24:35
our, um, perception. You know, the
24:37
universe's perception of its own history. If
24:40
I put it that way. I think I'm talking a
24:42
little bit in riddles here, but I hope John
24:45
follows me, that it really is all about your,
24:48
um, frame of rest, as we call it, your
24:50
vantage point, uh, on the universe. Because
24:53
that's what time dilation is all about. It's
24:55
about people seeing time going differently,
24:57
depending on their viewpoint. Our viewpoint
25:00
here, uh, from Earth is probably
25:03
that of the universe as a whole, effectively,
25:06
uh, because we are not in an intense
25:09
gravitational field. The gravitational
25:12
field of the sun is the strongest thing we
25:14
feel that keeps the Earth in orbit. But it's
25:17
nothing like what you would find around a
25:18
black hole. And so we've got probably a
25:21
fairly unbiased view of the
25:24
universe and its history.
25:26
Andrew Dunkley: Now, uh, are we in a gravity well?
25:30
Professor Fred Watson: Yeah, we are. I mean, the, the Earth itself
25:32
creates a gravity well and that's what keeps
25:34
us stuck to the Earth. Because the
25:36
gravitational potential at your head
25:39
is a little bit different from what it is at
25:41
your feet. And that's what's pulling you
25:43
down. The change in shape of space. There you
25:46
go.
25:47
Andrew Dunkley: All right.
25:48
Professor Fred Watson: I'm not spaghettifying you.
25:50
Andrew Dunkley: No, no. So, um,
25:53
did we unpack everything there?
25:58
Professor Fred Watson: Yeah, I think so. I think we. I think we've
25:59
covered most of it. Okay, look,
26:02
I know. Sorry. Go ahead.
26:05
Andrew Dunkley: No, I was going to say that, uh, he also
26:07
asked if the Big Crunch would happen
26:10
faster than the expansion.
26:12
Professor Fred Watson: So. Yes. So that I kind of, you know, lumped
26:15
that into the. The fact that the times that
26:17
we observe from, uh, our location in the
26:20
universe, probably. Yes. 20 billion years
26:22
down the track seems about right from the Big
26:24
Crunch. If the recent desi results.
26:27
Yeah, it's what happens. Um, um.
26:30
Uh, I was going to say that,
26:33
like you, John, uh, these things make my
26:35
brain hurt. So don't think it's
26:38
peculiar to, uh, um, a
26:41
few people. Uh, I think most physicists,
26:45
you know, they get their. They really have to
26:47
get their heads around the things, like
26:51
seeing things from different vantage points.
26:52
It's not entire. Uh, it's not intuitive at
26:55
all.
26:56
Andrew Dunkley: No, no. All right, John, thank you. And I
26:59
hope all is well in Suffolk. That's, uh,
27:01
that's basically like one of.
27:04
One of the easternmost points of England,
27:05
isn't it?
27:06
Professor Fred Watson: I used to live in Suffolk. Oh, there you are.
27:08
Uh, yeah, not. Not far from Cambridge, which
27:11
is in Cambridgeshire, but Suffolk. I was over
27:14
the border in Suffolk. Uh, you're right,
27:16
Suffolk. Uh, Norfolk and Suffolk are the two
27:18
counties in East Anglia. That sort of
27:21
semicircular bit that sticks out not very,
27:23
very much north of the Thames Estuary.
27:25
Andrew Dunkley: There you go. All right.
27:26
Professor Fred Watson: Very pretty.
27:27
Andrew Dunkley: Thanks, John.
27:28
Professor Fred Watson: Yeah.
27:28
Andrew Dunkley: Yes.
27:30
Good to hear from you. Our final question
27:32
today comes from Andy.
27:35
Speaker D: Hi, guys, this is Andy from London. I'm a new
27:38
listener to your podcast and quite new to
27:40
science, so forgive me if this is a stupid
27:43
question, but I was just wondering, um, when
27:46
craft re enter atmosphere and they have to
27:49
come in at a certain angle to stop from
27:51
burning up, um,
27:54
why do they not just come through slowly to.
27:57
To get away with the friction effect
28:00
which causes, uh, the heat. Thanks.
28:04
Andrew Dunkley: Thank you, Andy. Um, and thanks for finding
28:07
space Nuts and being a new listener. Uh,
28:09
you've only got 590 odd
28:11
episodes to catch up now, so. Yeah,
28:14
have fun with that. We've certainly had fun
28:17
with it. Um, I think we've had this question
28:20
before, maybe asked a different way. Um,
28:27
And when it comes to a spacecraft, we're
28:29
orbiting the planet, which is essentially,
28:31
it's just constantly falling. You're just
28:33
maintaining a velocity that stops you falling
28:36
in. Um, you do have to
28:39
slow down to re enter so that that
28:42
arc is,
28:45
you know, reduced or increased, can't
28:47
remember which. Uh, and then you fall through
28:49
the atmosphere. Um, you can't stop
28:52
and just sort of ease your way back in
28:55
as against a space elevator, which would be
28:58
able to do that if we ever build
29:00
one. Uh, but that's a different set. But a
29:03
space elevator is essentially not orbiting.
29:06
It is stationary to a point on
29:09
the planet, which means it goes up and down.
29:11
Is that making sense?
29:13
Professor Fred Watson: Yeah, um, yes, it is.
29:16
Andrew Dunkley: That's good.
29:17
Professor Fred Watson: That's the first time ever everything you've
29:19
said is correct. Um, space elevators are, uh,
29:22
hypothesized. Buzz, uh, Aldrin told me
29:25
that night I had dinner with him, it's never
29:27
going to happen. And he was quite right
29:29
because, um, the space elevator has to sit on
29:32
the equator. Uh, uh, every spacecraft in the
29:34
sky crosses the equator. So you're always
29:36
going to get things banging into, uh, will
29:39
be very difficult to build one, uh, you know,
29:42
apart from the structural thing, uh, so
29:44
neglecting the space elevator for a minute,
29:46
what you said is absolutely right. In order
29:48
to stay in orbit, you have to achieve
29:51
basically a horizontal velocity of about
29:54
8 kilometers per second. Uh,
29:57
and that's, uh, otherwise you just fall back
29:59
to Earth. So that's what all
30:02
the, you know, the, the huge amount of
30:05
fuel that is carried by a rocket being
30:07
launched. That's what it's all about. It's
30:10
about getting up to a height of 2 or
30:12
300 km and getting that
30:15
orbital velocity, getting that horizontal
30:17
velocity of 8 kilometers per second. So,
30:21
um, what you could do,
30:24
uh, is, and, you know, going, this
30:27
is hopefully helping Andy,
30:29
uh, if you, you had
30:32
unlimited amounts of fuel, you could
30:35
turn the rocket round, uh, from its orbital
30:38
position and fire, uh,
30:40
your rockets to act as a braking system
30:43
to slow the thing down. And then you could
30:45
gently tiptoe down through the atmosphere,
30:48
constantly firing your rockets. It's actually
30:50
what, um, Musk does with
30:53
his, uh, Falcon rockets.
30:57
He's got enough fuel left that he can bring
30:59
the empty spacecraft back down intact,
31:02
uh, and use it again, uh, without
31:05
needing a heat shield. Um,
31:08
so you could do that. Uh, and he's
31:10
demonstrated that we can. Uh, but it
31:13
turns out that, uh, it's much more
31:16
effective to use this process called
31:19
aerobraking, where you actually use the
31:22
atmosphere itself to slow the
31:25
spacecraft down, uh, because you don't need
31:27
any fuel for that. You just need something
31:29
that's going to stop it burning up. Uh, so
31:31
getting from this 8km per second down
31:34
to a few, you know, a meter or two per
31:37
second, uh, for a splashdown or a touchdown,
31:40
uh, is the tricky bit. And you've got to,
31:44
uh, you know, you've got to use whatever
31:46
means are at your disposal. And the easiest
31:49
one is aerobraking using the atmosphere
31:51
itself to slow you down. I should point out
31:53
that, um, I think the first stage Falcon
31:56
rockets, they're not at orbital velocity. Uh,
31:59
when they turn around and come back, they
32:00
haven't got up to that 8km per second because
32:03
there's a second stage that lets them do that
32:05
and they still burn up basically coming back
32:08
into the atmosphere. The second stages.
32:11
Um, but yeah, good question. Not a stupid
32:14
question at all.
32:15
Andrew Dunkley: No, no. And, and you know,
32:19
it's rocket science. I mean we
32:21
often say when something's not difficult,
32:24
it's not rocket science. This is rocket
32:25
science. Um, orbital speeds
32:28
are, ah, significant. They're to stay
32:31
out there, they've got to, um, do
32:34
17,000 miles an hour.
32:35
Professor Fred Watson: Hour, uh, which is eight kilometers per
32:37
second. Yeah.
32:38
Andrew Dunkley: Mark 25. Um, to slow
32:41
down so that you can return to Earth safely,
32:43
you've got to go from that speed to
32:46
subsonic speed. Uh, and
32:49
using fuel to do that would be exhaustive.
32:52
Professor Fred Watson: Yeah, that's right. That's exactly right. You
32:54
know, basically you're looking at the
32:57
same amount of hardware
33:00
in terms of a rocket and its fuel to
33:03
get you up there. You'd need the same amount
33:05
to bring you back just to do a gentle
33:07
touchdown on the Earth. M.
33:10
Um, notwithstanding the first stage
33:13
recovery that we're starting to see with.
33:15
Well, not starting. They've been doing it for
33:17
10 years with the Falcon 9 rockets.
33:20
Andrew Dunkley: M. Yeah, I'm sure the technology will improve
33:23
and we'll find better ways, but at the
33:25
moment, using the atmosphere as a free
33:27
braking system. Yeah, yeah, works really
33:30
well. Except when it doesn't. But that's only
33:33
happened a couple of times. Yeah, um,
33:35
yeah, thank you Andy. Great question though.
33:38
Uh, really appreciate it. And don't forget,
33:40
if you've got questions for us, you can send
33:42
them through via our website,
33:44
spacenutspodcast.com spacenuts
33:47
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33:52
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33:55
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34:11
you might want to hit the supporter button
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34:47
Uh, I think we're all done. Fred. Thank you
34:49
so much.
34:51
Professor Fred Watson: Uh, a pleasure, Andrew. Great to hear from
34:53
the listeners, as always. Great questions,
34:56
uh, get my mind, mind thinking again.
34:59
Uh, so thank, uh, you everybody and thanks,
35:02
uh, to you, Andrew too, for keeping on the
35:04
rails.
35:06
Andrew Dunkley: Uh, I do my best and sometimes I don't,
35:09
but yes, thank you, Fred. We'll see you next
35:11
time. Professor Fred Watson, astronomer, uh,
35:14
at large. And thanks to Huw in the studio who
35:16
definitely does keep it all together.
35:17
Although he couldn't be with us today
35:19
because, um, he re entered his garage at
35:21
excess velocity, couldn't get down to some
35:25
sonic speed. And, uh, well, he'll be in
35:27
traction for, for a couple of weeks. Uh, from
35:29
me, Andrew Dunkley. Thanks for your company.
35:31
You'll see on the next episode of Space Nuts.
35:33
Bye Bye.
35:35
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