Cosmic Queries: Gravitational Waves & the Great Dust Debate

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

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is Space Nuts, a Q and A edition. My name is

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Andrew Dunkley, your host. Uh, terrific to

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have your company. Questions that we will be

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answering on today's program include the Big

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Crunch, gravitational waves,

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shifting magnetic poles, uh,

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the use of the term dust. Somebody's got

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maybe an issue with that. And questions

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about gas and ice giants and why do we

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think they've got rocky cores. That's all

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coming up on this episode of space nuts.

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Voice Over Guy: 15 seconds. Guidance is internal.

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10, 9. Ignition

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sequence time. Space nuts. 5, 4, 3,

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2. 1, 2, 3, 4, 5, 5, 4,

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3, 2, 1. Space nuts. Astronauts It

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feels good.

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Andrew Dunkley: And joining us for what will be the last time

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in a little while, because Fred's coming back

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next week, Jonti Horner, professor of

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astrophysics at the University of Southern

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Queensland. Hi, Jonti.

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Jonti Horner: Good afternoon. How are you going?

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Andrew Dunkley: Ah, uh, pretty good. And you?

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Jonti Horner: Uh, not too bad, you know, dealing with the

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usual kind of too much work, not enough fun.

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Looking forward to a trip to a conference

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next week. I'm down to the Australian Space

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Research Conference, which is always my

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favorite meeting of the year. So it's perfect

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timing for Fred to return because I wouldn't

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have been easily available next week anyway.

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And, um, time to hand over. And everybody

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listening can breathe a huge sigh of relief

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because normality has been restored.

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Andrew Dunkley: Ah, no, it's not like that.

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Uh, in fact, um, in fact, that's where we can

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start because we, uh, do have some

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comments from the audience. Uh, this came

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from Sam in British Columbia. He says, I just

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wanted to say how helpful I

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found the answer to the Lagrange points in

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Mass question

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and how much I enjoy Johnny Horner's

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explanations, musings and answers. I know

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sometimes they seem a little more detailed

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than chatty, but I really enjoy that

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extra detail and context. I found the spatial

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contours explanation extremely useful. Thank

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you. So, um, you got a bit of a fan there.

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And another comment that I came across

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on our, um, podcast

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group Facebook page. I appreciated all the

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attention Andrew and Jonti devoted to the

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government shutdown. My family suffered

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personally. That came from Martin. Although,

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uh, there was someone else who didn't

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appreciate us going down the political line.

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But because of the impact that had on NASA

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particularly, uh, it was probably something,

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uh, that was worth discussing.

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Jonti Horner: Yeah, I think it is important. I understand

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that people don't like it when you get into

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politics too much and to your political

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views. But I think in this case it's

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something where colleagues of mine were being

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directly affected. I know people

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who had more than four weeks without pay. And

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we're here to talk about what's happening

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with space and um, exploration and

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research. And when there's something that's

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impeding that, it's important to discuss it.

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And it's doubly important I think when people

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are trying to use it for political capital to

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perpetuate lies about alien

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spacecraft, you know, um, you need to

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set the record straight to correct other

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people training into politics when they

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shouldn't do so. You know, I appreciate the

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comments. I love the positive feedback. I ah,

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try and not get too political in terms of my

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own views on stuff, but there are some topics

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which we do need to cross. And you know, my

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heart does go out to those who were directly

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impacted by the shutdown for whatever the

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reasons the shutdown was happening. It's not

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good when you have to go m more than a month

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without food, particularly for those families

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who have two people who are both government

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employees and have children with mouths to

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feed.

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Andrew Dunkley: Yeah, and we were talking, we're talking

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thousands upon thousands of people. So it

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wasn't just a handful.

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Uh, let's move on to our first set of

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questions. Beau in Melbourne has sent us two

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questions, uh, via our audio stream.

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Uh, let's see what he wants to find out.

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Beau: Hello, Andrew and Professor, uh, Jonti

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Horner. Is Beau here? Yes. Your second

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favorite B.O. from Melbourne, Australia.

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I have a question for you, but first I would

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like to do a fact check please. Um,

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a couple of episodes ago, um, Professor

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Watson, uh, talked about the Gnab Gib or

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the Big Crunch. And basically he said,

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ah, at the end of the Gnab Gib, um, matter

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will come closer to one another, uh, as the

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effect of gravity takes over and we uh, will

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end up in one giant singularity and

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collapse. Uh, what he didn't say

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was the uh, effect of that on

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light. Now my understanding is that

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um, obviously as stars and galaxies come

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closer together, the sky will get brighter

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and brighter and uh, as matter starts to

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fuse, uh, will give out more heat and more

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uh, light as well. So essentially

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will end up in a reverse Big Bang, uh, and

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then we will all come to a big blinding,

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uh, end, um, both matter and uh,

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light coming together in a reverse Big Bang.

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So I just wanted to see if that is correct,

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uh, regarding light. I'd love to hear

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Jonty's view on that.

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Um, now my question is related to

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gravitational waves. Uh, we

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know that gravitational waves, ah,

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distort the fabric of space time.

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Um, In a wave pattern. We also know

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that multiple gravitational wave exist,

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um, because there are, you know, black hole

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collisions and black hole neutron star

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collisions happening, um, throughout the

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universe. Now what happens when

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those two gravitational waves meet

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each other? Um, particularly what would

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happen to, um, I guess the interference

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patterns as the waves, uh, starts overlapping

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each other at the peaks and the troughs

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during, do we see any

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changes to space time itself?

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Do we see, for example, time speed up,

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slow down or stop? Do we see gravity,

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um, cease or increase

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or decrease? Um, um,

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just wanted to know what would happen to

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space time and that interference patterns,

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the peaks to troughs. Um, love to hear

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Professor John de Horner's view on that. Um,

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thank you very much and please.

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Jonti Horner: Keep up your good work.

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Andrew Dunkley: Thank you, Beau. Uh, great questions.

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Uh, we'll probably start with the big crunch

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and the effect on light. Now,

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um, I suppose we have to consider

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the timing of events because the universe

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is still expanding, Although now they're

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starting to think that acceleration is no

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longer speeding up, it's slowing down

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or that the expansion, um, but

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it's still expanding. Far as we're aware at

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this point in time. Uh, Fred has told us in

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the past that it will expand to the point

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where everything will move so far apart that

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we will just be by ourselves in the universe,

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in blackness. Um, so

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the question is, is that still going to

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happen? And even if it

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does, and there is a big crunch,

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what's going to happen to all the light

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anyway? So it's a really

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fascinating area.

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Jonti Horner: It is, and it's really complicated. It's

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dealing with things that are incredibly far

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in the distant future.

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Andrew Dunkley: Um, it is week or the week after, I think.

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Jonti Horner: Absolutely. Um, well, with the way that time

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seems to pass quicker and quicker as I get

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older, it does probably mean that it will be

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next week, but it's a difficult one.

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So there is still some debate over whether

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the universe will continue to expand forever

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or whether it will turn around and begin to

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collapse. And reminds me of the Arthur C.

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Clarke quote about life elsewhere, which I'm

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going to butcher and paraphrase in this case,

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which is that two possibilities exist and

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both are equally terrifying. You know,

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either, you know, we expand forever or we

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don't. And they're equally scary in many

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ways. But assuming that we did collapse back

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down to a point. Now that will likely happen

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at the point when all the stars have died,

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um, when everything has come to an end. And

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so you'll probably have a universe full of

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non luminous stuff and black holes. And

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that's about it maybe so far away in the

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future that even the biggest black holes have

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evaporated from Hawking radiation. But

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whatever will happen, whatever is left will

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be squashed into an ever smaller place that

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will include all of the radiation that's

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going through the universe. Now we see the

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cosmic microwave background, and we see it

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at, uh, um, very long wavelengths, at

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microwave wavelengths, with an approximate

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temperature of like 2.9 Kelvin or something

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like that. I can't remember the exact number.

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That's because that light is redshifted,

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because the universe has expanded and

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stretched that energy out. If the universe

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collapsed back in, you'd be going the

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opposite. You'd be blue, shifting all the

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radiation. So as you squash the universe into

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an ever smaller space because of the quirk of

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the fact that there is nothing outside the

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universe, the universe is both infinite and

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finite at the same time. So you can't be

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outside the universe, because that's

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meaningless. All of the light and all of the

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energy in the universe will remain in the

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universe as the universe gets smaller. So my

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understanding is that as you get towards a

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high, hypothetical Big Crunch, the

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temperature, the pressure and the density

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will just increase and increase and increase.

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And, um, the universe will end in a very,

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very hot mess, effectively. So it will be

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like running the Big Bang backwards. There'll

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be differences. We don't fully understand

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what will happen and how it will all go. We

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don't know whether that would trigger another

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Big Bang, because, to be honest, we don't

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know enough about that time of the universe.

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And certainly I'm nowhere near, uh, the

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forefront of researching that to give a more

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educated opinion. But I know that the closer

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you get to the Big Bang looking back, the

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harder it is to be exactly sure what

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happened. Because the less information we

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have and the harder you're having to push our

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understanding of physics to the point it

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breaks down. And the same will be true going

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the other way. You're just reaching

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temperatures and pressures that make no

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sense. You have periods when different

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forces were combined into a single force. I

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do not know with my level of knowledge

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whether the expectation is that those

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transitions would happen at the same point

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going back as they did coming forward.

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So I think that the exact details of

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how the Big Crunch would go, uh, are still

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very much up for debate if it were to happen.

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But I think it's very fair to say that it

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will be very bright, very hot, very

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unpleasant, and we wouldn't be around to

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enjoy it.

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Andrew Dunkley: No, definitely. Well, yeah. It's like the

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restaurant at the end of the universe in

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hitchhikers. You know, if we're not going

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to sit there and enjoy a wonderful dinner

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while it all happens around us, it's um,

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yeah, I'd say humanity be long gone by then

266
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or transition into something else, I don't

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know. But I certainly don't think it would be

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00:10:34.530 --> 00:10:36.730
like rewinding a film and watching it all

269
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just happen in reverse. There'll be some

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00:10:39.650 --> 00:10:41.370
cataclysmic effect for sure.

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Uh, the main question Beau wanted answered

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00:10:44.850 --> 00:10:46.850
was about gravitational waves. And they're

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out there, they're happening, we're detecting

274
00:10:48.490 --> 00:10:50.920
them all the time. Um,

275
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but what happens when they cross each other?

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What's the effect? I would equate it to

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throwing two pebbles in a pond and the waves

278
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just cross over and that'd be it.

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00:11:00.460 --> 00:11:02.100
Jonti Horner: Yeah, I've done a bit of reading around on

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00:11:02.100 --> 00:11:04.380
this one because honestly, I haven't got the

281
00:11:04.380 --> 00:11:07.140
foggies coming into this. So my

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00:11:07.140 --> 00:11:10.100
default assumption is that, ah, the waves

283
00:11:10.100 --> 00:11:11.660
would interfere in the same way that

284
00:11:11.660 --> 00:11:13.420
electromagnetic waves interfere in that

285
00:11:13.420 --> 00:11:15.620
they'd add, um, together. So you'd get a peak

286
00:11:15.620 --> 00:11:17.340
and a trough would cancel out a peak and a

287
00:11:17.340 --> 00:11:20.210
peak would lead to constructive interference.

288
00:11:20.210 --> 00:11:22.450
So you'd get bigger and smaller

289
00:11:23.010 --> 00:11:25.410
instantaneous amplitudes.

290
00:11:26.130 --> 00:11:28.490
You'd get an interference pattern reading

291
00:11:28.490 --> 00:11:31.490
around online. Um, it seems that that is

292
00:11:31.650 --> 00:11:34.490
broadly the consensus, so long as you

293
00:11:34.490 --> 00:11:36.410
are a long way away from a strong

294
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gravitational field, so you're a long way

295
00:11:38.410 --> 00:11:40.730
away from the source of these things, or

296
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you're a long way away from something like a

297
00:11:42.410 --> 00:11:45.380
black hole. And apparently the physics of

298
00:11:45.380 --> 00:11:48.340
the general relativistic treatment of

299
00:11:48.340 --> 00:11:51.340
this gets incredibly gnarly. When you get

300
00:11:51.340 --> 00:11:53.300
to those kind of situations and nobody's

301
00:11:53.300 --> 00:11:55.740
really sure what happened, the maths gets

302
00:11:55.740 --> 00:11:57.300
difficult. And the point is you're pushing

303
00:11:57.300 --> 00:11:58.780
the boundaries of what we know and what we

304
00:11:58.780 --> 00:12:01.260
can observe into the unknown. So what you

305
00:12:01.260 --> 00:12:03.900
have to do is you have to develop possible

306
00:12:04.620 --> 00:12:07.460
answers and um, test them, build

307
00:12:07.460 --> 00:12:09.660
theories, make predictions, see what happens.

308
00:12:09.900 --> 00:12:12.850
But I think in general, if, for example,

309
00:12:12.850 --> 00:12:14.930
one of our big gravitational wave detectors,

310
00:12:14.930 --> 00:12:17.490
two waves came in at once, you would

311
00:12:17.490 --> 00:12:20.210
probably, at that instantaneous location, you

312
00:12:20.210 --> 00:12:21.970
get an extra large peak or an extra large

313
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trough, or they'd cancel out. But because you

314
00:12:24.450 --> 00:12:26.170
might have more than one detector around the

315
00:12:26.170 --> 00:12:28.690
earth, thanks to the directions of motion,

316
00:12:28.850 --> 00:12:30.810
you'd only have that specific type of

317
00:12:30.810 --> 00:12:33.530
interference at that specific detector. So it

318
00:12:33.530 --> 00:12:35.130
will probably give us a signal that, if

319
00:12:35.130 --> 00:12:36.730
you've got multiple gravitational wave

320
00:12:36.730 --> 00:12:39.190
detectors around the globe, would be distinct

321
00:12:39.190 --> 00:12:41.110
and identifiable and would allow you to test

322
00:12:41.110 --> 00:12:44.030
that interference, if that makes sense.

323
00:12:44.110 --> 00:12:46.510
Now my understanding of the typical

324
00:12:46.510 --> 00:12:48.750
gravitational wave events that we see is that

325
00:12:48.910 --> 00:12:50.390
you've, ah, got these waves that are building

326
00:12:50.390 --> 00:12:53.310
up from inspiraling neutron stars or black

327
00:12:53.310 --> 00:12:54.990
holes, or a neutron star and a black hole

328
00:12:54.990 --> 00:12:56.990
about to collide, where you get

329
00:12:57.710 --> 00:13:00.350
very low frequency, very low amplitude waves

330
00:13:00.350 --> 00:13:02.470
that build to a sharp crescendo, which is why

331
00:13:02.470 --> 00:13:05.230
you get these attempts to sonify the data,

332
00:13:05.230 --> 00:13:07.990
where you get this rising whistle, rising in

333
00:13:07.990 --> 00:13:10.430
pitch and rising in volume. So the idea is

334
00:13:10.430 --> 00:13:12.110
that you get a lot of small waves first and

335
00:13:12.110 --> 00:13:13.630
then you get a really big build to a

336
00:13:13.630 --> 00:13:16.230
crescendo and then fall off. So typically you

337
00:13:16.230 --> 00:13:17.910
probably wouldn't observe this happening with

338
00:13:17.910 --> 00:13:20.030
our current technology unless you have the

339
00:13:20.030 --> 00:13:22.750
incredible good fortune to have two events

340
00:13:23.070 --> 00:13:25.350
where you get the peak arriving at the same

341
00:13:25.350 --> 00:13:27.550
time. And that'll be the interesting test.

342
00:13:28.110 --> 00:13:30.470
So, yeah, to summarize, I don't think anybody

343
00:13:30.470 --> 00:13:32.510
fully knows, but because you're pushing the

344
00:13:32.510 --> 00:13:34.550
bounds of what is known. But it seems to be

345
00:13:34.550 --> 00:13:36.670
that the consensus is that in open space,

346
00:13:36.990 --> 00:13:39.670
away from really significant masses or away

347
00:13:39.670 --> 00:13:42.070
from the sources of the waves, they would

348
00:13:42.070 --> 00:13:43.950
just have normal kind of constructive and

349
00:13:43.950 --> 00:13:46.110
destructive interference as the peaks and

350
00:13:46.110 --> 00:13:47.550
troughs go across each other.

351
00:13:48.590 --> 00:13:51.310
Andrew Dunkley: Okie dokie. There you are. Uh, thank you, Bo.

352
00:13:51.390 --> 00:13:52.110
Great question.

353
00:13:54.990 --> 00:13:57.950
Jonti Horner: 0G and I feel fine. Space nuts.

354
00:13:58.750 --> 00:14:00.940
Andrew Dunkley: Uh, our next question comes from Paddy,

355
00:14:01.710 --> 00:14:04.300
uh, reflecting on the discussion around the

356
00:14:04.300 --> 00:14:06.620
shifting of the magnetic poles. If they were

357
00:14:06.620 --> 00:14:09.420
to flip, how would the field

358
00:14:09.420 --> 00:14:12.380
behave as it transitioned? Uh,

359
00:14:12.380 --> 00:14:15.020
the equator, uh, would it

360
00:14:15.020 --> 00:14:17.820
spin with the Earth's, uh, rotation? Would it

361
00:14:17.820 --> 00:14:20.620
let in more debris, solar radiation and

362
00:14:20.620 --> 00:14:23.180
or, uh, uh, cosmic particles?

363
00:14:23.820 --> 00:14:26.380
And to go full Hollywood disaster

364
00:14:26.380 --> 00:14:29.180
movie, given the, uh, visual representation

365
00:14:29.180 --> 00:14:31.920
of the mega magnetic field suggests an apple

366
00:14:31.920 --> 00:14:34.560
shape. Uh, could the funnel of the

367
00:14:34.560 --> 00:14:37.320
magnetic field become like a magnifying glass

368
00:14:37.320 --> 00:14:39.120
scorching the earth as it crosses the

369
00:14:39.120 --> 00:14:39.560
equator?

370
00:14:40.280 --> 00:14:40.960
Jonti Horner: Love, uh, the show.

371
00:14:40.960 --> 00:14:42.840
Andrew Dunkley: Keep up the great work. That's from Paddy.

372
00:14:42.920 --> 00:14:44.800
He's put a bit of thought into this and I

373
00:14:44.800 --> 00:14:47.540
love the sci fi component. But, um,

374
00:14:47.880 --> 00:14:50.480
yeah, is this in your

375
00:14:50.480 --> 00:14:51.960
ballpark, this kind of thing?

376
00:14:52.250 --> 00:14:55.080
Jonti Horner: Uh, as an astrophysicist, it's

377
00:14:55.080 --> 00:14:57.560
closer to my ballpark than the gnabs and the

378
00:14:57.560 --> 00:15:00.000
dark energy stuff. I mean, I'm still not, I

379
00:15:00.000 --> 00:15:02.260
would argue, an expert, but I'm close to it

380
00:15:02.260 --> 00:15:03.900
and I have done a bit of reading. Now what I

381
00:15:03.900 --> 00:15:06.580
would say here is actually, um, the

382
00:15:06.580 --> 00:15:09.300
Wikipedia page for geomagnetic reversal

383
00:15:09.300 --> 00:15:11.740
is a really interesting read. It's very in

384
00:15:11.740 --> 00:15:13.700
depth and contains a lot of good historical

385
00:15:13.700 --> 00:15:16.020
information. So while I acknowledge Wikipedia

386
00:15:16.020 --> 00:15:17.780
is very much secondary rather than primary

387
00:15:17.780 --> 00:15:20.260
Resource, I think for topics like this and

388
00:15:20.260 --> 00:15:22.060
topics in astronomy, the articles tend to

389
00:15:22.060 --> 00:15:24.740
stay fairly on task and fairly accurate

390
00:15:24.740 --> 00:15:26.420
because people will fix them if they break

391
00:15:26.420 --> 00:15:28.750
very quickly. Um, and that

392
00:15:29.230 --> 00:15:30.870
reading that should, to some degree,

393
00:15:30.870 --> 00:15:32.950
immediately put Paddy's mind at rest in terms

394
00:15:32.950 --> 00:15:34.870
of the Earth getting baked or scorched or

395
00:15:34.870 --> 00:15:36.510
Hollywood disaster movie type things

396
00:15:36.510 --> 00:15:38.790
happening at the time of a field reversal.

397
00:15:38.790 --> 00:15:40.510
Because we've had at least

398
00:15:40.510 --> 00:15:43.310
183reversals in the last 83 million

399
00:15:43.310 --> 00:15:45.590
years, which means that these things have

400
00:15:45.590 --> 00:15:48.430
happened regularly through the period of

401
00:15:48.510 --> 00:15:51.350
the Earth being inhabited and have not caused

402
00:15:51.350 --> 00:15:53.870
any mass extinctions. There have been some

403
00:15:53.950 --> 00:15:56.800
suggestions that periods where

404
00:15:56.800 --> 00:15:58.640
you get magnetic field locked in one

405
00:15:58.640 --> 00:16:01.160
direction for very long periods of time,

406
00:16:01.240 --> 00:16:03.680
which last happened during the Cretaceous

407
00:16:03.680 --> 00:16:06.320
period, where you had something like a 50

408
00:16:06.320 --> 00:16:08.320
million year period where the magnetic field

409
00:16:08.320 --> 00:16:10.800
didn't flip. There have been some suggestions

410
00:16:10.800 --> 00:16:13.240
that when those very long periods of time

411
00:16:13.720 --> 00:16:16.000
come to an end, that it could trigger a

412
00:16:16.000 --> 00:16:18.800
certain amount of added volcanic

413
00:16:18.800 --> 00:16:21.240
activity and stuff like this. And that may

414
00:16:21.240 --> 00:16:23.520
lead to some traumas for life, but never

415
00:16:23.520 --> 00:16:26.280
quite at the level of a mass extinction. And

416
00:16:26.280 --> 00:16:29.160
there's a couple of beautiful, um, figures

417
00:16:29.160 --> 00:16:30.560
plotting out the

418
00:16:31.680 --> 00:16:34.440
flips that have happened going back about

419
00:16:34.440 --> 00:16:36.000
180 million years,

420
00:16:37.360 --> 00:16:39.160
talking about these periods where the

421
00:16:39.160 --> 00:16:41.000
magnetic field gets locked into a single

422
00:16:41.000 --> 00:16:43.600
orientation. Nothing much happens for a long

423
00:16:43.600 --> 00:16:46.200
time, which is known as a superchron. And

424
00:16:46.200 --> 00:16:48.760
then you get other times when you get more

425
00:16:48.760 --> 00:16:51.150
flips in a short period than typical. There's

426
00:16:51.150 --> 00:16:53.910
one here, 51 reversals occurred during a 12

427
00:16:53.910 --> 00:16:56.830
million period centered on, I think it's 15

428
00:16:56.830 --> 00:16:59.190
million years ago. So you get periods where

429
00:16:59.190 --> 00:17:01.110
there's a lot more of them happening. You

430
00:17:01.110 --> 00:17:03.030
also get periods where it tries to flip and

431
00:17:03.030 --> 00:17:05.910
then goes back to how it was. Uh, so the idea

432
00:17:05.910 --> 00:17:07.470
that you had from school that the Earth's

433
00:17:07.470 --> 00:17:09.070
magnetic field is essentially, we have a

434
00:17:09.070 --> 00:17:10.670
giant bar magnet in the middle of the Earth,

435
00:17:10.670 --> 00:17:12.710
and it's very controlled and static. As we

436
00:17:12.710 --> 00:17:14.470
said on the podcast a few weeks ago, that has

437
00:17:14.470 --> 00:17:17.450
fallen by the wayside. Now, the magnetic

438
00:17:17.450 --> 00:17:20.330
field being generated by wibbly wobbliness

439
00:17:20.330 --> 00:17:22.370
and convection currents and all sorts in the

440
00:17:22.370 --> 00:17:24.810
Earth's outer core through a dynamo effect is

441
00:17:24.810 --> 00:17:27.290
fairly well understood. And, um, these field

442
00:17:27.290 --> 00:17:30.130
reversals are something that falls out

443
00:17:30.130 --> 00:17:32.410
naturally in modeling. So people have not had

444
00:17:32.410 --> 00:17:34.970
to hugely increase the capacity of their

445
00:17:34.970 --> 00:17:36.770
modeling ability when modeling the behavior

446
00:17:36.770 --> 00:17:39.250
of the outer core to make them happen. They

447
00:17:39.330 --> 00:17:41.250
happen naturally from the way the models are

448
00:17:41.250 --> 00:17:43.650
set up, which is really interesting. What

449
00:17:43.650 --> 00:17:46.490
seems to happen is that, uh, unlike the sun,

450
00:17:46.490 --> 00:17:48.450
where you get the magnetic field reversals at

451
00:17:48.450 --> 00:17:50.170
about the time when the Sun's magnetic field

452
00:17:50.170 --> 00:17:52.570
gets the strongest. And that's all down to

453
00:17:52.570 --> 00:17:55.130
the tangling up of the magnetic field lines

454
00:17:55.130 --> 00:17:57.210
as the sun rotates as a fluid body, not a

455
00:17:57.210 --> 00:18:00.010
solid body. On the Earth, the magnetic

456
00:18:00.010 --> 00:18:02.530
field reversals tend to occur at times of low

457
00:18:02.530 --> 00:18:05.010
magnetic field. So what tends to happen is

458
00:18:05.010 --> 00:18:06.970
that the dynamo becomes less effective.

459
00:18:07.370 --> 00:18:09.920
Things become confused in the inner core. You

460
00:18:09.920 --> 00:18:11.720
can even get periods where you get multiple

461
00:18:11.720 --> 00:18:14.240
north and south poles while the magnetic

462
00:18:14.240 --> 00:18:16.360
field in the dynamo breaks down and reasserts

463
00:18:16.360 --> 00:18:18.840
itself, and then it flips over. There is some

464
00:18:18.840 --> 00:18:20.760
discussion over how quick this can happen

465
00:18:20.760 --> 00:18:23.160
with most studies seem to suggest it can take

466
00:18:23.160 --> 00:18:25.560
anything from 2,000 to 12,000 years.

467
00:18:26.040 --> 00:18:28.480
But sometimes it could be quicker, sometimes

468
00:18:28.480 --> 00:18:30.960
it could be slower. It's all complex, and

469
00:18:30.960 --> 00:18:33.040
it's because it's all tied to this turbulent

470
00:18:33.040 --> 00:18:35.520
roiling of the liquid metal in the outer

471
00:18:35.520 --> 00:18:38.370
core. What this means is that,

472
00:18:38.370 --> 00:18:40.820
uh, firstly, if you shift where the north and

473
00:18:40.820 --> 00:18:42.900
south magnetic poles of the Earth are, they

474
00:18:42.900 --> 00:18:44.740
will rotate with the Earth. Uh, that's in

475
00:18:44.740 --> 00:18:47.540
fact what we see with pulsars. Why we get the

476
00:18:47.540 --> 00:18:49.500
pulsars is that the magnetic fields and the

477
00:18:49.500 --> 00:18:52.420
rotation axis are not lined up. So you get a

478
00:18:52.420 --> 00:18:54.180
magnetic hotspot on the surface of the

479
00:18:54.180 --> 00:18:56.580
pulsar, uh, where you get the magnetic polis,

480
00:18:56.580 --> 00:18:58.580
where any material around will be funneled

481
00:18:58.580 --> 00:19:00.260
down the magnetic field to hit there. You get

482
00:19:00.260 --> 00:19:02.420
this hot spot. You get lots of radiation

483
00:19:02.420 --> 00:19:05.360
emitted from the poles. And as, uh,

484
00:19:05.360 --> 00:19:08.190
the pulsar rotates, those poles sweep

485
00:19:08.190 --> 00:19:10.350
like lighthouse beams, and we get pulses of

486
00:19:10.350 --> 00:19:13.030
radio waves when that beam sweeps across us.

487
00:19:13.590 --> 00:19:15.150
So it's fairly well understood that the

488
00:19:15.150 --> 00:19:17.830
magnetic field rotates with the Earth. And

489
00:19:17.830 --> 00:19:20.310
therefore, if the

490
00:19:20.470 --> 00:19:23.310
magnetic pole was in Kenya or somewhere like

491
00:19:23.310 --> 00:19:25.310
that, it was somewhere near the equator, it

492
00:19:25.310 --> 00:19:27.230
will be rotating with the Earth. That's kind

493
00:19:27.230 --> 00:19:28.910
of how it would work. And, um, that will

494
00:19:28.910 --> 00:19:31.460
probably happen if the flip was the north

495
00:19:31.460 --> 00:19:34.100
pole wandering to the Earth's south pole. In

496
00:19:34.100 --> 00:19:35.620
reality, though, it seems that these

497
00:19:35.700 --> 00:19:38.540
reversals are more almost like the Earth's

498
00:19:38.540 --> 00:19:40.700
magnetic fields weaken. They become

499
00:19:40.700 --> 00:19:43.260
disestablished, you get all this confusion,

500
00:19:43.260 --> 00:19:45.220
and then a new field establishes itself,

501
00:19:45.620 --> 00:19:48.460
which I think is probably part of the

502
00:19:48.460 --> 00:19:51.260
reason that the flips are even less periodic

503
00:19:51.260 --> 00:19:53.100
than you think. They're talked about as being

504
00:19:53.100 --> 00:19:55.740
totally random. But I suspect that's added to

505
00:19:55.740 --> 00:19:57.990
by the fact that, that if you wipe out the

506
00:19:57.990 --> 00:19:59.710
Earth's magnetic field and turn it on again,

507
00:19:59.950 --> 00:20:02.190
if you imagine you had a 50, 50 chance of it

508
00:20:02.190 --> 00:20:04.110
being north south, and a 50, 50 transmit

509
00:20:04.110 --> 00:20:06.790
being south north, then only half of the Time

510
00:20:06.790 --> 00:20:08.510
it weakened, would you get it flipped to the

511
00:20:08.510 --> 00:20:10.910
other polarity. And so that might be part of

512
00:20:10.910 --> 00:20:13.870
what's going on there. So it's all really,

513
00:20:13.870 --> 00:20:16.070
really complex. What would happen is that we

514
00:20:16.070 --> 00:20:18.830
would get to some degree a greater flux of

515
00:20:18.830 --> 00:20:20.510
radiation hitting the top of the Earth's

516
00:20:20.510 --> 00:20:22.630
atmosphere. The charged particles that get

517
00:20:22.630 --> 00:20:24.550
diverted around us by the magnetic field, it

518
00:20:24.550 --> 00:20:27.200
will get less effective. But it's worth

519
00:20:27.200 --> 00:20:29.600
noting that our atmosphere is incredibly

520
00:20:29.600 --> 00:20:32.400
effective protection for us anyway. I saw one

521
00:20:32.400 --> 00:20:34.960
article saying our atmosphere is as effective

522
00:20:34.960 --> 00:20:36.560
at protecting against the solar wind and

523
00:20:36.560 --> 00:20:38.560
charged particles as a 3 meter layer of

524
00:20:38.560 --> 00:20:40.920
concrete would be. So the atmosphere does a

525
00:20:40.920 --> 00:20:43.560
very, very good job. Uh, which is why it

526
00:20:43.560 --> 00:20:46.120
seems that these magnetic field weakenings

527
00:20:46.520 --> 00:20:48.440
don't lead to m mass extinctions and things.

528
00:20:48.440 --> 00:20:50.080
They will have a bit of an effect on the

529
00:20:50.080 --> 00:20:53.080
upper atmosphere stuff will happen. There

530
00:20:53.080 --> 00:20:55.000
are suggestions that maybe you could get a

531
00:20:55.000 --> 00:20:56.400
little bit of additional atmospheric

532
00:20:56.400 --> 00:20:58.800
stripping happening during these times from

533
00:20:58.800 --> 00:21:01.360
solar radiation, but effectively the impact

534
00:21:01.360 --> 00:21:03.400
would not be that great on the surface of the

535
00:21:03.400 --> 00:21:06.040
Earth. It would probably play merry havoc

536
00:21:06.040 --> 00:21:08.720
with scouts who are doing orienteering and

537
00:21:08.720 --> 00:21:10.400
people doing the Duke of Edinburgh Reward and

538
00:21:10.400 --> 00:21:12.240
things like this where you follow a map and

539
00:21:12.240 --> 00:21:13.600
you've got to use a map and a compass.

540
00:21:13.600 --> 00:21:15.720
Because if the North Pole is in a different

541
00:21:15.720 --> 00:21:17.960
place every year and weaker, uh, that's going

542
00:21:17.960 --> 00:21:20.040
to be a pain for navigation. There would

543
00:21:20.040 --> 00:21:21.760
doubtless be significant effects on

544
00:21:21.760 --> 00:21:24.720
technology, obviously, and we

545
00:21:24.720 --> 00:21:26.520
saw last week with a really good solar storm

546
00:21:26.520 --> 00:21:28.640
and Aurora again, that we are to some degree

547
00:21:28.640 --> 00:21:31.080
at the mercy of big solar storms. We

548
00:21:31.080 --> 00:21:32.600
discussed in the past the likelihood of

549
00:21:32.600 --> 00:21:34.280
events like the Carrington event being a

550
00:21:34.280 --> 00:21:36.480
problem for satellites and for unshielded

551
00:21:36.480 --> 00:21:38.560
electronics on the surface of the Earth. And

552
00:21:38.560 --> 00:21:40.800
if the Earth's magnetic field were weaker or

553
00:21:40.800 --> 00:21:43.040
were in the process of reversing, then an

554
00:21:43.040 --> 00:21:44.960
equal strength solar storm would do more

555
00:21:44.960 --> 00:21:47.560
damage because less of it would be deflected.

556
00:21:48.010 --> 00:21:49.920
Um, but you wouldn't end up with the kind of

557
00:21:49.920 --> 00:21:52.420
giant lens baking strip along the Earth.

558
00:21:52.790 --> 00:21:54.620
Um, fortunately or unfortunately, depending

559
00:21:54.620 --> 00:21:55.980
on your point of view and your love of

560
00:21:55.980 --> 00:21:58.740
Hollywood dramatics, that should be fine.

561
00:21:59.460 --> 00:22:00.740
It would be an interesting event.

562
00:22:00.820 --> 00:22:03.420
There are people who keep suggesting that

563
00:22:03.420 --> 00:22:05.820
this kind of thing is imminent. The problem

564
00:22:05.820 --> 00:22:08.700
there is imminent in geological timescales

565
00:22:08.700 --> 00:22:10.660
doesn't mean imminent on a human timescale.

566
00:22:10.660 --> 00:22:12.420
So the last reversal, I believe, was about

567
00:22:12.580 --> 00:22:15.340
780,000 years ago. The

568
00:22:15.340 --> 00:22:17.420
average timing of them seems to be out every

569
00:22:17.420 --> 00:22:19.220
half a million years. So people say we're

570
00:22:19.220 --> 00:22:22.050
overdue. That skips the fact

571
00:22:22.050 --> 00:22:23.730
that actually the timings are very random.

572
00:22:23.730 --> 00:22:25.370
It's A bit like waiting for a bus. I use this

573
00:22:25.370 --> 00:22:26.890
analogy all the time. You know, if you've got

574
00:22:26.890 --> 00:22:28.690
a bus due every five minutes, you may wait

575
00:22:28.690 --> 00:22:31.010
half an hour and five come along at once. You

576
00:22:31.010 --> 00:22:33.370
did? No. And, um, with these kind of

577
00:22:33.370 --> 00:22:35.530
reversals, that's exacerbated by the fact

578
00:22:35.530 --> 00:22:37.450
that we tend to get long blocks and short

579
00:22:37.450 --> 00:22:39.330
blocks. So I'm looking just at the last 5

580
00:22:39.330 --> 00:22:41.770
million years. And if you go from 5 million

581
00:22:41.770 --> 00:22:44.530
years ago, um, black on this

582
00:22:44.530 --> 00:22:46.730
plot is the polarity we have now on white is

583
00:22:46.730 --> 00:22:49.480
the other one. 5.01 million years ago, it

584
00:22:49.480 --> 00:22:51.880
flipped so that south was at the top. Then

585
00:22:51.880 --> 00:22:54.760
4.89 million years ago, we had, what

586
00:22:54.760 --> 00:22:57.120
is it, 80,000 years of our current polarity.

587
00:22:57.120 --> 00:22:59.680
Then it flipped back and we had 17,000 years.

588
00:22:59.920 --> 00:23:02.800
Then it flipped back for 17,000 years, back

589
00:23:02.800 --> 00:23:05.440
for 18,000 years, back for 8,000

590
00:23:05.440 --> 00:23:07.876
years, and then there was a 60,000

591
00:23:08.044 --> 00:23:09.840
600,000 year gap.

592
00:23:11.200 --> 00:23:13.720
And so it's very, very spotty. The last flip

593
00:23:13.720 --> 00:23:16.530
was 780,000 years ago. Before

594
00:23:16.530 --> 00:23:18.690
that it was only a 12,000 year gap.

595
00:23:19.300 --> 00:23:21.050
Um, and then there was a very long period

596
00:23:21.050 --> 00:23:23.850
between 1.0, uh, 6 and 1.78 million years

597
00:23:23.850 --> 00:23:26.210
ago, when it was the opposite polarity,

598
00:23:26.530 --> 00:23:29.130
except for a single measurement at 1.19

599
00:23:29.130 --> 00:23:30.810
million years ago, when it was the other way

600
00:23:30.810 --> 00:23:33.650
around. So that was a very short flip. So, in

601
00:23:33.650 --> 00:23:35.690
all honesty, saying that we're overdue for it

602
00:23:35.690 --> 00:23:37.410
is a bit like bumping into somebody grumpy at

603
00:23:37.410 --> 00:23:39.290
the bus stop because the bus is 30 seconds

604
00:23:39.290 --> 00:23:41.610
late. In all honesty, you've got no clue when

605
00:23:41.610 --> 00:23:43.970
that bus is going to arrive. And looking at

606
00:23:43.970 --> 00:23:46.190
the time periods in the Cretaceous, there's

607
00:23:46.190 --> 00:23:49.030
two or three of these megalong breaks, these

608
00:23:49.030 --> 00:23:51.110
superchrons that have been identified. Two

609
00:23:51.110 --> 00:23:52.510
are very confident ones, a bit more

610
00:23:52.510 --> 00:23:54.710
controversial, but the most recent one in the

611
00:23:54.710 --> 00:23:57.630
Cretaceous was more than 50 million years

612
00:23:57.630 --> 00:23:59.750
with a single polarity. And that's the

613
00:23:59.750 --> 00:24:01.269
equivalent of being at the bus stop. But the

614
00:24:01.269 --> 00:24:02.150
buses are on strike.

615
00:24:02.710 --> 00:24:05.310
Andrew Dunkley: Yes, yes. Wouldn't be a problem in

616
00:24:05.310 --> 00:24:07.190
Japan. They are very strict about their

617
00:24:07.190 --> 00:24:09.670
timing. In fact, I remember a story a couple

618
00:24:09.670 --> 00:24:11.510
of years ago about a train driver who lost

619
00:24:11.510 --> 00:24:13.110
his job for being two minutes late.

620
00:24:13.340 --> 00:24:15.540
Jonti Horner: Yeah. So I remember that in Switzerland. One

621
00:24:15.540 --> 00:24:17.820
of the bizarre experiences when I first moved

622
00:24:17.820 --> 00:24:19.900
to Switzerland for my first postdoc, kind of,

623
00:24:20.170 --> 00:24:22.700
um, 20 years ago, 22 years ago, was being on

624
00:24:22.700 --> 00:24:24.500
the train platform and the train was slightly

625
00:24:24.500 --> 00:24:26.340
late and, um, people were checking their

626
00:24:26.340 --> 00:24:28.420
watches and correcting their watches because

627
00:24:28.420 --> 00:24:30.100
they thought that their watch was wrong

628
00:24:30.100 --> 00:24:31.340
rather than the train being late.

629
00:24:31.660 --> 00:24:32.060
Andrew Dunkley: Wow.

630
00:24:32.060 --> 00:24:34.580
Jonti Horner: And it's like, I'm used to British trends,

631
00:24:34.580 --> 00:24:36.100
where if they come on the correct week,

632
00:24:36.100 --> 00:24:38.780
you're lucky, you know? Yes.

633
00:24:39.100 --> 00:24:40.900
Andrew Dunkley: The. Australia's a bit like that. Although

634
00:24:40.900 --> 00:24:42.660
they're pretty good most of the time. You

635
00:24:42.660 --> 00:24:44.460
only ever hear about them when the press has

636
00:24:44.460 --> 00:24:45.960
decided to stick the knife in.

637
00:24:45.960 --> 00:24:47.230
Jonti Horner: Absolutely. Yeah.

638
00:24:47.230 --> 00:24:49.000
Andrew Dunkley: Ah, nine times out of ten that'll be okay.

639
00:24:49.000 --> 00:24:50.640
Jonti Horner: At least most places have trains. I don't

640
00:24:50.640 --> 00:24:52.080
know if I've told this story before, but my

641
00:24:52.080 --> 00:24:53.800
understanding of the reason that we don't

642
00:24:53.800 --> 00:24:55.880
have a fast train from Toowoomba to Brisbane

643
00:24:56.120 --> 00:24:57.920
is that there used to be a train service. And

644
00:24:57.920 --> 00:25:00.520
in the 1950s, the family that ran the coach

645
00:25:00.600 --> 00:25:03.080
service on the roads from Toowoomba to

646
00:25:03.080 --> 00:25:04.920
Brisbane got elected to the Toowoomba Council

647
00:25:05.080 --> 00:25:06.840
and shut down the railway, because it was.

648
00:25:08.360 --> 00:25:10.640
And so 70 years later, we still have no fast

649
00:25:10.640 --> 00:25:12.240
rail to Brisbane. And it comes up every few

650
00:25:12.240 --> 00:25:13.800
years that we should have it. And it just

651
00:25:13.800 --> 00:25:14.680
never got going again.

652
00:25:16.320 --> 00:25:18.160
Andrew Dunkley: Yeah, I'm sure there's a lot of that going

653
00:25:18.160 --> 00:25:21.120
on. Um, but. Great question, Patty. And

654
00:25:21.200 --> 00:25:23.370
it sort of throws a curveball, um,

655
00:25:24.000 --> 00:25:26.800
into, um, you know, if it happens, if

656
00:25:26.800 --> 00:25:29.440
there is a magnetic pole flip,

657
00:25:30.050 --> 00:25:32.440
um, does that mean we are no longer down

658
00:25:32.440 --> 00:25:33.520
under, but up over?

659
00:25:34.160 --> 00:25:34.880
Jonti Horner: Absolutely.

660
00:25:36.880 --> 00:25:39.360
Andrew Dunkley: Yes, that could be the case.

661
00:25:39.920 --> 00:25:41.720
Oh, uh, gosh, no. We don't want to cause any

662
00:25:41.720 --> 00:25:43.550
trouble. Let's just leave things as they, uh,

663
00:25:43.550 --> 00:25:45.720
are. Thanks, Paddy. This is Space Nuts with

664
00:25:45.720 --> 00:25:47.980
Andrew Dunkley and John Dee Horner.

665
00:25:50.300 --> 00:25:52.420
Okay, we checked all four systems, and.

666
00:25:52.420 --> 00:25:55.260
Jonti Horner: Being with a go, Space Nuts, our.

667
00:25:55.260 --> 00:25:57.620
Andrew Dunkley: Next question comes from Howard Bennett.

668
00:25:57.620 --> 00:26:00.220
Howard is in Penang in Malaysia.

669
00:26:00.730 --> 00:26:03.020
Uh, I have a question about the term

670
00:26:03.260 --> 00:26:03.820
dust.

671
00:26:04.140 --> 00:26:04.780
Jonti Horner: Dust.

672
00:26:05.180 --> 00:26:07.980
Andrew Dunkley: Dust. The word is used indiscriminately

673
00:26:08.300 --> 00:26:10.940
throughout astrophysics with no real

674
00:26:10.940 --> 00:26:13.320
definition. I don't know. Um,

675
00:26:14.430 --> 00:26:17.330
uh, I know it's not the same as the dust

676
00:26:17.330 --> 00:26:20.210
bunnies under my bed, but what exactly is the

677
00:26:20.210 --> 00:26:22.810
space dust that obscures our heart

678
00:26:23.210 --> 00:26:26.010
of, uh, galaxies and inhabits the empty space

679
00:26:26.010 --> 00:26:28.730
between galaxies, not to mention moon dust

680
00:26:28.730 --> 00:26:31.650
and deadly dust storms on Mars? Most

681
00:26:31.650 --> 00:26:34.490
confusing. Uh, maybe we need a new word.

682
00:26:35.050 --> 00:26:37.450
So when we refer to dust in space,

683
00:26:38.410 --> 00:26:40.530
what are we talking about? And is it all the

684
00:26:40.530 --> 00:26:41.210
same stuff?

685
00:26:41.880 --> 00:26:44.840
Jonti Horner: It's all sorts of stuff, basically, but the

686
00:26:44.840 --> 00:26:47.600
commonality is that it's small pieces of

687
00:26:47.600 --> 00:26:49.720
solid material. So that's effectively what

688
00:26:49.720 --> 00:26:52.600
you're talking about. It becomes

689
00:26:53.000 --> 00:26:54.920
confusing occasionally in the solar system,

690
00:26:54.920 --> 00:26:56.640
for example, when we draw the line between

691
00:26:56.640 --> 00:26:59.400
meteoroids, which are, uh, particles of,

692
00:26:59.400 --> 00:27:00.960
effectively, dust going around the sun, and

693
00:27:00.960 --> 00:27:02.600
asteroids, which are bigger things going

694
00:27:02.600 --> 00:27:04.960
around the sun. And typically, people place A

695
00:27:04.960 --> 00:27:07.820
division there at about a meter diameter. So

696
00:27:07.820 --> 00:27:09.980
the same object that's 1.1 meters across,

697
00:27:09.980 --> 00:27:12.940
you'd call a small asteroid at uh, 0.9 meters

698
00:27:12.940 --> 00:27:14.660
would be a meteoroid. And that's just because

699
00:27:14.660 --> 00:27:17.360
we have to have a boundary somewhere. Um,

700
00:27:17.380 --> 00:27:19.900
and materials that are considered dust in

701
00:27:19.900 --> 00:27:22.540
space will include things that at home you'd

702
00:27:22.540 --> 00:27:25.420
consider ice. If it's solid, it's

703
00:27:25.420 --> 00:27:28.340
dust. And um, the hodred is the lessings can

704
00:27:28.340 --> 00:27:30.460
be solid. When it comes to the stuff on the

705
00:27:30.460 --> 00:27:32.820
moon then you're talking about the dust being

706
00:27:33.530 --> 00:27:35.570
surface rocks that have been pulverized by

707
00:27:35.570 --> 00:27:38.090
impacts. So you have these tiny

708
00:27:38.490 --> 00:27:41.490
particles of martian, of lunar regoliths,

709
00:27:41.490 --> 00:27:43.490
sorry, which are uh, small pieces of dust

710
00:27:43.490 --> 00:27:45.290
because they're small pieces of solid

711
00:27:45.290 --> 00:27:48.090
material. Lunar dust is pretty brutal

712
00:27:48.090 --> 00:27:49.930
because there's no moisture and no weathering

713
00:27:49.930 --> 00:27:52.250
there. So it's incredibly sharp edged and

714
00:27:52.250 --> 00:27:54.570
abrasive. And that's why it's such a problem

715
00:27:54.650 --> 00:27:57.210
for future astronauts. It's why it's a

716
00:27:57.210 --> 00:27:58.690
problem technologically. It's why when they

717
00:27:58.690 --> 00:28:01.240
came back they had to clean the astronauts

718
00:28:01.480 --> 00:28:02.240
vacuum them.

719
00:28:02.240 --> 00:28:05.040
Andrew Dunkley: I think they did. Ah, I remember Buzz Aldrin

720
00:28:05.040 --> 00:28:07.160
described walking on the moon as walking on

721
00:28:07.160 --> 00:28:08.200
talcum powder.

722
00:28:08.200 --> 00:28:10.920
Jonti Horner: Yeah, very, very slippery, lots of very fine

723
00:28:10.920 --> 00:28:12.440
dust particles. With the exception that

724
00:28:12.440 --> 00:28:15.330
talcum powder is a lot less abrasive. Um,

725
00:28:15.639 --> 00:28:17.440
I think a better analogy, although it's not

726
00:28:17.440 --> 00:28:20.000
perfect, to the kind of things you get that

727
00:28:20.000 --> 00:28:21.680
cause miner's lung and things like that,

728
00:28:21.680 --> 00:28:23.880
where you've got particles of dust being

729
00:28:23.880 --> 00:28:26.200
created by explosions or digging underground

730
00:28:26.790 --> 00:28:28.950
that haven't had time to be rounded off by

731
00:28:29.670 --> 00:28:31.750
moisture and weathering yet. And they cause

732
00:28:31.750 --> 00:28:34.390
huge problems for people who inhale them. I

733
00:28:34.390 --> 00:28:35.710
believe that was a part of the problem with

734
00:28:35.710 --> 00:28:37.430
asbestos when you inhale it actually it's to

735
00:28:37.430 --> 00:28:38.830
do with the sharpness of the particles and

736
00:28:38.830 --> 00:28:41.350
the damage that they do. So that's the kind

737
00:28:41.350 --> 00:28:43.670
of mundus stuff. Similarly when we talk about

738
00:28:43.670 --> 00:28:46.510
dust zones on Mars, the dust there are those

739
00:28:46.510 --> 00:28:49.190
particles of solid material that are small

740
00:28:49.190 --> 00:28:50.870
enough that they can be lofted into the

741
00:28:50.870 --> 00:28:52.830
atmosphere through a variety of processes.

742
00:28:52.830 --> 00:28:55.440
Not just the wind, but there are solar, ah,

743
00:28:55.800 --> 00:28:58.320
radiation processes that can levitate dust

744
00:28:58.400 --> 00:29:01.360
off the surface of Mars, um, including

745
00:29:02.160 --> 00:29:04.240
um, one that is really fascinating that I did

746
00:29:04.240 --> 00:29:06.760
some research on with colleagues again 20

747
00:29:06.760 --> 00:29:09.440
years ago now, which is this weird

748
00:29:09.760 --> 00:29:12.520
photo, um, with light

749
00:29:12.520 --> 00:29:14.640
based effect. We're familiar with kind of

750
00:29:15.600 --> 00:29:17.760
radiation pressure and the Ponting Robertson

751
00:29:17.760 --> 00:29:19.200
effect. These are things we talk about a lot.

752
00:29:19.200 --> 00:29:20.320
But there's also something called

753
00:29:20.320 --> 00:29:23.240
photophoresis which is

754
00:29:23.320 --> 00:29:25.640
to do with the Absorption and re emission

755
00:29:26.200 --> 00:29:28.760
of light from very small dust grains

756
00:29:29.160 --> 00:29:32.000
that when you're at a, ah, very specific size

757
00:29:32.000 --> 00:29:34.760
range, can exert a really intense force.

758
00:29:35.240 --> 00:29:37.839
So what happens is, uh, when your dust

759
00:29:37.839 --> 00:29:40.440
grain absorbs some light, it

760
00:29:40.440 --> 00:29:42.600
temporarily has a temperature gradient on it.

761
00:29:43.000 --> 00:29:45.200
That temperature gradient depends on how big

762
00:29:45.200 --> 00:29:47.000
the dust grain is. Whether the near side or

763
00:29:47.000 --> 00:29:48.640
the far side of the dust grain gets hot.

764
00:29:48.640 --> 00:29:50.320
Because if the light penetrates most of the

765
00:29:50.320 --> 00:29:52.320
way through, the far side is a bit that

766
00:29:52.320 --> 00:29:54.320
absorbs it and gets hot. So you get a dust

767
00:29:54.320 --> 00:29:55.960
grain that's hotter on one side than another.

768
00:29:56.520 --> 00:29:58.800
If that dust is in an atmosphere that is not

769
00:29:58.800 --> 00:30:00.760
too dense and not too low density,

770
00:30:01.560 --> 00:30:04.480
the gas particles from the point of

771
00:30:04.480 --> 00:30:06.440
view of the dust grain will be perceived as

772
00:30:06.440 --> 00:30:09.280
single impactors, single billiard

773
00:30:09.280 --> 00:30:11.880
balls. And when they hit the dust

774
00:30:11.880 --> 00:30:13.920
grain, they stick briefly and then leave

775
00:30:13.920 --> 00:30:15.880
again. And if they hit the hot side, they'll

776
00:30:15.880 --> 00:30:17.360
leave with more energy than when they leave

777
00:30:17.360 --> 00:30:20.000
the cool side. So you get a

778
00:30:20.000 --> 00:30:22.680
force. Now, this is a really

779
00:30:22.680 --> 00:30:24.680
quirky force I'd never come across until I

780
00:30:24.680 --> 00:30:26.720
saw talk from a couple of physicists who were

781
00:30:26.720 --> 00:30:29.320
talking about dust grains on Mars. Um, we

782
00:30:29.320 --> 00:30:31.160
looked into it in the form of what this would

783
00:30:31.160 --> 00:30:33.200
have as an effect for planet formation disks

784
00:30:33.200 --> 00:30:36.040
and stuff like this. But what's really quirky

785
00:30:36.040 --> 00:30:38.080
is that this is only effective over a

786
00:30:38.080 --> 00:30:40.640
relatively small range of gas pressures.

787
00:30:40.960 --> 00:30:43.280
If the gas is too thin, doesn't happen. M if

788
00:30:43.280 --> 00:30:46.140
the gas is too dense or that individual

789
00:30:46.140 --> 00:30:48.900
probabilistic single gas molecules adhering

790
00:30:48.900 --> 00:30:51.220
and leaving doesn't happen. But in those

791
00:30:51.220 --> 00:30:54.140
range of pressures, it can be up to 10 or 100

792
00:30:54.140 --> 00:30:55.580
times stronger than all the other forces.

793
00:30:56.060 --> 00:30:58.460
And, um, can Mars atmosphere, particularly in

794
00:30:58.460 --> 00:31:00.220
the highlands, is the right pressure that

795
00:31:00.220 --> 00:31:02.100
this can actually levitate dust grains off

796
00:31:02.100 --> 00:31:04.100
the surface of Mars and is viewed as

797
00:31:04.100 --> 00:31:05.900
potentially been helping to trigger those

798
00:31:05.900 --> 00:31:07.900
dust zones to start the dust getting kicked

799
00:31:07.900 --> 00:31:10.300
up into the atmosphere. So all sorts of

800
00:31:10.830 --> 00:31:12.230
really cool stuff there. The other thing that

801
00:31:12.230 --> 00:31:14.110
I found out from those physicists that we

802
00:31:14.110 --> 00:31:17.110
worked with, um, way back then is that

803
00:31:17.110 --> 00:31:19.110
when you buy the little light windmills that

804
00:31:19.110 --> 00:31:21.470
you can get in an evacuated shell that are

805
00:31:21.470 --> 00:31:23.670
meant to show radiation pressure, they're

806
00:31:23.670 --> 00:31:25.590
actually not there using photophoresis

807
00:31:25.590 --> 00:31:27.110
because there is some atmosphere in that

808
00:31:27.110 --> 00:31:29.430
bubble still. And, um, the Havel's one is

809
00:31:29.430 --> 00:31:31.750
colored white, one is colored black, the

810
00:31:31.750 --> 00:31:33.710
black side gets hotter and you get this

811
00:31:33.710 --> 00:31:36.510
photophoresis force happening rather than

812
00:31:36.510 --> 00:31:39.490
radiation pressure, which is interesting and

813
00:31:39.490 --> 00:31:42.490
quirky. Coming back to the question, all the

814
00:31:42.490 --> 00:31:44.530
way to the question is whenever astronomers

815
00:31:44.530 --> 00:31:47.050
use the term dust, then what they're meaning

816
00:31:47.050 --> 00:31:49.210
is particles of solid material

817
00:31:49.850 --> 00:31:52.090
that are too small to be considered asteroids

818
00:31:52.090 --> 00:31:55.090
or planets or things like this. That gets

819
00:31:55.090 --> 00:31:57.850
a catch all of dust. And it behaves very much

820
00:31:57.850 --> 00:32:00.090
like dust in the Earth's atmosphere. Red

821
00:32:00.090 --> 00:32:01.970
light penetrates it more easily than yellow

822
00:32:01.970 --> 00:32:03.730
light, which penetrates more easily than blue

823
00:32:03.730 --> 00:32:05.250
light. Because the longer the wavelength, the

824
00:32:05.250 --> 00:32:07.700
better you can pass through. Which is why if

825
00:32:07.700 --> 00:32:10.180
you look at photographs of some of the

826
00:32:10.180 --> 00:32:12.740
wonderful dark nebulae in the night sky, like

827
00:32:12.740 --> 00:32:14.780
the Coalsack Nebula, which is ahead of the

828
00:32:14.780 --> 00:32:16.620
EMU in the sky to the traditional owners of

829
00:32:16.620 --> 00:32:18.700
the M land here in Australia. Like many of

830
00:32:18.700 --> 00:32:20.860
the Barnard Nebulas, Barnard did a big study

831
00:32:20.860 --> 00:32:23.020
of finding dark nebulae all across the sky.

832
00:32:23.260 --> 00:32:25.500
If you look at photographs of those that have

833
00:32:25.500 --> 00:32:27.780
been taken in full color and you zoom in

834
00:32:27.780 --> 00:32:29.540
around the peripheries of those clouds,

835
00:32:29.540 --> 00:32:31.140
you'll see that the stars right at the edge

836
00:32:31.140 --> 00:32:33.430
look red. And that's because he's seeing them

837
00:32:33.430 --> 00:32:35.550
through the outer edge of the dust cloud. And

838
00:32:35.550 --> 00:32:37.150
the blue and the yellow light is scattered

839
00:32:37.150 --> 00:32:39.710
away. The red light penetrates through. And

840
00:32:39.710 --> 00:32:41.230
you can see this very well. If you look at

841
00:32:41.230 --> 00:32:42.950
some of the famous photos of the Coalsack

842
00:32:42.950 --> 00:32:45.030
Nebula, it's really, really distinct and

843
00:32:45.030 --> 00:32:47.990
noticeable. And it's because dust is dust is

844
00:32:47.990 --> 00:32:50.150
dust. I appreciate it gets confusing because

845
00:32:50.150 --> 00:32:53.110
we use the term in so very many contexts

846
00:32:53.110 --> 00:32:55.510
as a throwaway thing and

847
00:32:55.990 --> 00:32:58.470
to our experience on Earth because it's warm

848
00:32:58.470 --> 00:33:01.230
here. You don't consider flakes of ice and

849
00:33:01.230 --> 00:33:03.600
snowflakes as dust, but if you were.

850
00:33:03.760 --> 00:33:05.800
Andrew Dunkley: Or smoke, you don't think about smoke as

851
00:33:05.800 --> 00:33:08.000
dust, but that's exactly what it is. Can you

852
00:33:08.000 --> 00:33:10.640
see that photo I took during the bushfires a

853
00:33:10.640 --> 00:33:11.120
few years ago?

854
00:33:11.120 --> 00:33:14.040
Jonti Horner: Yeah. What's spooky about that is that I've

855
00:33:14.040 --> 00:33:17.040
seen the sky diminished and denuded

856
00:33:17.040 --> 00:33:19.600
by bushfire smoke. And I've

857
00:33:19.920 --> 00:33:22.520
also seen it from, um,

858
00:33:23.040 --> 00:33:25.080
dust storms. Dust that's been kicked up and

859
00:33:25.080 --> 00:33:26.880
alligated off the surface of the Earth. And I

860
00:33:26.880 --> 00:33:29.280
would have never expected this. But when it's

861
00:33:29.280 --> 00:33:32.000
really, really bad, they both lead to a very

862
00:33:32.000 --> 00:33:34.640
red sky. But when it's not that

863
00:33:34.640 --> 00:33:35.960
intense, you can actually tell the

864
00:33:35.960 --> 00:33:37.680
difference. Because the sky looks different

865
00:33:37.760 --> 00:33:40.480
between lofted dust and smoke, you actually

866
00:33:40.480 --> 00:33:43.000
get a very different kind of reddening that

867
00:33:43.000 --> 00:33:45.320
makes the particles of different sizes. But

868
00:33:45.320 --> 00:33:47.640
if I took a bucket of smoke or a bucket of

869
00:33:47.640 --> 00:33:50.160
snowflakes into space and scattered them into

870
00:33:50.160 --> 00:33:52.200
the solar system, they'd just be considered

871
00:33:52.200 --> 00:33:54.840
dust. Yeah. Small pieces of solid

872
00:33:54.840 --> 00:33:55.230
material.

873
00:33:55.780 --> 00:33:58.700
Andrew Dunkley: There you go. Um, Howard, if you can think

874
00:33:58.700 --> 00:34:01.420
of a set of names to cover

875
00:34:01.420 --> 00:34:04.380
the Various categories let, uh, us know.

876
00:34:04.380 --> 00:34:07.380
But, um, I think just using the term dust

877
00:34:07.380 --> 00:34:10.380
is probably the easiest way to deal with

878
00:34:10.380 --> 00:34:12.300
it, by the sound of things. Thanks for your

879
00:34:12.300 --> 00:34:14.340
question. Hope all is well in Malaysia.

880
00:34:17.140 --> 00:34:19.300
Jonti Horner: Three, two, one.

881
00:34:20.020 --> 00:34:20.760
Space. No.

882
00:34:21.750 --> 00:34:23.670
Andrew Dunkley: Uh, our final question today comes from

883
00:34:23.670 --> 00:34:24.350
Martin.

884
00:34:25.390 --> 00:34:27.710
Berman Gorvine: Hello, space nuts.

885
00:34:28.190 --> 00:34:30.350
Martin Berman Gorvine here,

886
00:34:30.750 --> 00:34:33.310
writer extraordinaire, uh, in many

887
00:34:33.470 --> 00:34:35.950
genres, with yet another question.

888
00:34:36.750 --> 00:34:39.310
How do we determine whether the gas

889
00:34:39.310 --> 00:34:42.190
giants and. Or the ice

890
00:34:42.190 --> 00:34:44.790
giants have rocky

891
00:34:44.790 --> 00:34:47.710
cores? And if they do not

892
00:34:47.710 --> 00:34:50.420
have rocky cores, what might they

893
00:34:50.420 --> 00:34:51.580
have inside?

894
00:34:52.940 --> 00:34:55.820
Possibly of tangential relevance.

895
00:34:56.620 --> 00:34:59.340
I saw an article that

896
00:34:59.340 --> 00:35:01.620
appeared earlier this year in

897
00:35:01.620 --> 00:35:04.140
scitech Daily saying

898
00:35:04.540 --> 00:35:04.940
that

899
00:35:07.100 --> 00:35:09.860
analysis of Hubble data shows that

900
00:35:09.860 --> 00:35:12.460
methane has been depleted at

901
00:35:12.460 --> 00:35:15.340
Uranus poles in recent

902
00:35:15.500 --> 00:35:18.190
decades, which begs the question,

903
00:35:19.230 --> 00:35:22.190
is Uranus outgassing methane?

904
00:35:22.430 --> 00:35:25.270
Oh, sorry, sorry. I shouldn't have said that.

905
00:35:25.270 --> 00:35:27.820
I don't know what came over me. Uh,

906
00:35:27.950 --> 00:35:30.750
I will, uh, do penance immediately.

907
00:35:31.230 --> 00:35:33.710
Berman Gorvine, over and

908
00:35:34.190 --> 00:35:34.590
out.

909
00:35:35.470 --> 00:35:38.390
Andrew Dunkley: Thanks, Martin. I did wonder where

910
00:35:38.390 --> 00:35:40.030
he was going with that. I shouldn't have been

911
00:35:40.030 --> 00:35:42.510
surprised. Um, so to

912
00:35:42.750 --> 00:35:45.470
gas and ice giants, um,

913
00:35:46.990 --> 00:35:48.870
if they don't have rocky cores, what do they

914
00:35:48.870 --> 00:35:51.310
have? I mean, there's been some suggestions

915
00:35:51.310 --> 00:35:53.990
that some of them just have a liquid center,

916
00:35:53.990 --> 00:35:56.310
like a nice, you know, chocolate you get at

917
00:35:56.310 --> 00:35:59.070
Christmas. Um, could they all

918
00:35:59.070 --> 00:36:01.350
be different? I mean, did they all have to

919
00:36:01.350 --> 00:36:03.750
have the same kind of thing? It's not, you

920
00:36:03.750 --> 00:36:05.150
know, we're not talking about dust here.

921
00:36:05.470 --> 00:36:06.990
Jonti Horner: Well, there's a couple of different things

922
00:36:06.990 --> 00:36:09.790
that lead into this and should make the

923
00:36:09.790 --> 00:36:11.350
distinction between the planets we have in

924
00:36:11.350 --> 00:36:13.860
the solar system and objects elsewhere. Um,

925
00:36:14.030 --> 00:36:15.590
because the only planets that we can get up

926
00:36:15.590 --> 00:36:17.170
close and personal to are the ones here at,

927
00:36:17.170 --> 00:36:17.470
um, Home.

928
00:36:17.870 --> 00:36:18.390
Andrew Dunkley: Yeah.

929
00:36:18.390 --> 00:36:20.750
Jonti Horner: The background here is that traditional views

930
00:36:20.750 --> 00:36:23.110
of planet formation involve a process called

931
00:36:23.110 --> 00:36:26.070
core accretion. So this is where you take the

932
00:36:26.070 --> 00:36:27.910
solid material, the dust from the

933
00:36:27.910 --> 00:36:30.270
protoplanetary disk. And if you're out beyond

934
00:36:30.270 --> 00:36:32.310
the ice line, that dust includes a lot of icy

935
00:36:32.310 --> 00:36:34.910
material, solid material, agglomerates,

936
00:36:34.910 --> 00:36:36.470
forming bigger and bigger objects until

937
00:36:36.470 --> 00:36:38.230
eventually get something massive enough to

938
00:36:38.230 --> 00:36:40.260
start gathering the gases and hold onto them.

939
00:36:40.890 --> 00:36:41.970
Because whether you keep hold of an

940
00:36:41.970 --> 00:36:43.610
atmosphere or not depends on your mass and

941
00:36:43.610 --> 00:36:45.410
the strength of your gravity. The more

942
00:36:45.410 --> 00:36:47.090
massive you are, the more gas you can hold

943
00:36:47.090 --> 00:36:49.890
onto, but also the more capable you'll be of

944
00:36:49.890 --> 00:36:51.970
capturing hydrogen and helium, which are the

945
00:36:51.970 --> 00:36:53.930
main gases in the universe.

946
00:36:55.130 --> 00:36:58.050
So the idea is that Jupiter and Saturn got

947
00:36:58.050 --> 00:37:00.930
to 10 or 12 earth masses, which is kind of

948
00:37:00.930 --> 00:37:02.530
viewed as being the threshold for gathering

949
00:37:02.530 --> 00:37:04.730
up the hydrogen and helium gas

950
00:37:05.130 --> 00:37:07.790
fairly quickly. Hoovered up a lot of hydrogen

951
00:37:07.790 --> 00:37:09.550
and helium. And they became the gas giants.

952
00:37:09.550 --> 00:37:11.230
And that's why the name gas giants has been

953
00:37:11.230 --> 00:37:14.110
used for Uranus and Neptune. They

954
00:37:14.110 --> 00:37:15.550
never really got big enough to gather

955
00:37:15.550 --> 00:37:17.430
hydrogen and helium before the hydrogen and

956
00:37:17.430 --> 00:37:19.830
helium had been blown away. But they gathered

957
00:37:19.830 --> 00:37:22.590
huge mantles of methane,

958
00:37:22.590 --> 00:37:25.510
ethane, ammonia, things that are

959
00:37:25.510 --> 00:37:27.630
typically ice at that kind of distance

960
00:37:28.190 --> 00:37:30.550
under gas phase, depending exactly how far

961
00:37:30.550 --> 00:37:32.910
away you are. And so you've got these objects

962
00:37:32.990 --> 00:37:35.870
that are uh, significantly composed of

963
00:37:35.870 --> 00:37:38.830
material that could be considered ices or

964
00:37:39.710 --> 00:37:41.990
gases that are more massive, so therefore

965
00:37:41.990 --> 00:37:44.750
have a lower, a higher escape velocity

966
00:37:44.910 --> 00:37:47.070
and therefore are easier to hold onto with a

967
00:37:47.070 --> 00:37:49.750
lower mass. So the distinction between the

968
00:37:49.750 --> 00:37:51.670
ice giants and the gas giants is a

969
00:37:51.670 --> 00:37:53.990
compositional one. And it's to do with how

970
00:37:53.990 --> 00:37:56.230
they formed. They always used to just all be

971
00:37:56.230 --> 00:37:59.230
called gas giants. The ice giants idea came

972
00:37:59.230 --> 00:38:01.630
in with different models of planet formation.

973
00:38:01.630 --> 00:38:04.590
Because what we tend to do with planet name

974
00:38:04.590 --> 00:38:07.150
classification with things like

975
00:38:07.630 --> 00:38:09.590
whether Ceres is an asteroid or a dwarf

976
00:38:09.590 --> 00:38:11.430
planet, or both, whether Pluto's a planet or

977
00:38:11.430 --> 00:38:13.910
a dwarf planet. What we tend to do is we tend

978
00:38:13.910 --> 00:38:16.670
to place boundaries as humans to allow us to

979
00:38:16.670 --> 00:38:19.030
group like with like and separate things that

980
00:38:19.030 --> 00:38:21.670
are functionally different in origin or have

981
00:38:21.670 --> 00:38:23.670
a different history. And we do this in our

982
00:38:23.670 --> 00:38:25.270
day to day lives. We have children and

983
00:38:25.270 --> 00:38:28.060
pensioners, we have adults, we have

984
00:38:28.300 --> 00:38:30.260
people who suddenly wake up one morning and

985
00:38:30.260 --> 00:38:32.260
they can drive a car the day before. They

986
00:38:32.260 --> 00:38:33.700
were legally not allowed to do so because

987
00:38:33.700 --> 00:38:35.700
they've crossed this magic threshold. It's a

988
00:38:35.700 --> 00:38:38.700
very human thing. The nature of them

989
00:38:38.700 --> 00:38:40.780
in terms of having cores is therefore

990
00:38:41.180 --> 00:38:43.060
initially an outcome of our best

991
00:38:43.060 --> 00:38:44.860
understanding of how these things could form.

992
00:38:45.100 --> 00:38:47.220
The idea is that you need to form a kernel of

993
00:38:47.220 --> 00:38:49.660
solid material to get enough mass

994
00:38:49.980 --> 00:38:52.860
in order to accrete the gas. Now there's an

995
00:38:52.860 --> 00:38:55.220
alternate model which probably ties into the

996
00:38:55.220 --> 00:38:57.700
formation of objects in binary star systems,

997
00:38:57.860 --> 00:38:59.820
where when you've got a much more massive

998
00:38:59.820 --> 00:39:02.420
disk of material around a star, you can get

999
00:39:02.420 --> 00:39:04.500
an instantaneous gravitational instability

1000
00:39:05.060 --> 00:39:07.860
where you get a, ah, very gas heavy object

1001
00:39:08.100 --> 00:39:10.860
formed very, very quickly that

1002
00:39:10.860 --> 00:39:12.980
wouldn't need a core as a nucleus around

1003
00:39:12.980 --> 00:39:15.500
which it forms. It would have solid material

1004
00:39:15.500 --> 00:39:17.460
in it. But that solid material would just be

1005
00:39:17.920 --> 00:39:20.080
at the level that the background material

1006
00:39:20.080 --> 00:39:22.400
would have. So the composition of an object

1007
00:39:22.400 --> 00:39:24.240
formed in this way from this gravitational

1008
00:39:24.240 --> 00:39:27.240
instability would be the same as a bulk

1009
00:39:27.240 --> 00:39:29.480
composition of the disk. The composition of

1010
00:39:29.480 --> 00:39:31.200
an object that forms from core accretion

1011
00:39:31.200 --> 00:39:33.240
would be richer in the solid material because

1012
00:39:33.240 --> 00:39:35.280
they form a big amount of solids before they

1013
00:39:35.280 --> 00:39:37.560
gather any gas. Once they're at the point of

1014
00:39:37.560 --> 00:39:40.040
gathering the gas, they gather everything in

1015
00:39:40.040 --> 00:39:42.080
the same amounts as they're in the disk. So

1016
00:39:42.080 --> 00:39:44.470
you end up with something that has a large

1017
00:39:44.470 --> 00:39:47.390
amount of disk like composition, plus

1018
00:39:47.390 --> 00:39:50.270
a chunk of solids added in. But the thing is,

1019
00:39:50.270 --> 00:39:51.670
the bulk of those solids are down at the

1020
00:39:51.670 --> 00:39:53.270
bottom, so you can't really measure that

1021
00:39:53.270 --> 00:39:55.790
remotely. So how do you tell them apart?

1022
00:39:55.790 --> 00:39:58.470
Well, to be honest, for things around other

1023
00:39:58.470 --> 00:40:01.150
stars, we can't yet. So what we need to do

1024
00:40:01.150 --> 00:40:03.790
instead is look at their orbits and the

1025
00:40:03.790 --> 00:40:06.230
structures of the system and um, see how they

1026
00:40:06.230 --> 00:40:09.030
fit with these different formation

1027
00:40:09.030 --> 00:40:12.010
models. Um, and this is, I think going to,

1028
00:40:12.010 --> 00:40:13.730
in the next few years lead to a shift in how

1029
00:40:13.730 --> 00:40:16.410
we define what a brown dwarf is. Where

1030
00:40:16.410 --> 00:40:18.170
historically a brown dwarf was something

1031
00:40:18.170 --> 00:40:20.490
between 13 Jupiter masses and about 80

1032
00:40:20.490 --> 00:40:23.130
Jupiter masses, was something that was a

1033
00:40:23.130 --> 00:40:25.210
failed star rather than a giant planet. But

1034
00:40:25.210 --> 00:40:27.170
we're finding objects that blur that boundary

1035
00:40:27.170 --> 00:40:29.090
more and more. And I think we'll probably

1036
00:40:29.090 --> 00:40:31.050
shift to a different definition which looks

1037
00:40:31.050 --> 00:40:33.210
at, uh, the formation mechanism and the

1038
00:40:33.210 --> 00:40:35.050
presence of a core. So if you've got

1039
00:40:35.050 --> 00:40:36.810
something twice the mass of Jupiter bit

1040
00:40:36.810 --> 00:40:38.690
formed through this gravitational instability

1041
00:40:38.690 --> 00:40:41.150
method, that will be a very low mass brown

1042
00:40:41.150 --> 00:40:43.430
dwarf. Whereas if you've got something 20

1043
00:40:43.430 --> 00:40:45.790
Jupiter masses, that has a solid core, that

1044
00:40:45.790 --> 00:40:48.310
will be a very massive planet because it

1045
00:40:48.310 --> 00:40:49.990
formed through core accretion. I think that's

1046
00:40:49.990 --> 00:40:52.790
probably where we're going. That means

1047
00:40:52.790 --> 00:40:54.870
then that you can draw inferences on this

1048
00:40:54.870 --> 00:40:56.670
based on the structure of the planetary

1049
00:40:56.670 --> 00:40:58.670
system you've got, based on the orbits of the

1050
00:40:58.670 --> 00:41:00.230
objects, because these different formation

1051
00:41:00.230 --> 00:41:02.430
mechanisms would form very different systems.

1052
00:41:02.830 --> 00:41:05.720
But here in the solar system, we actually

1053
00:41:05.880 --> 00:41:08.360
can eventually figure out whether

1054
00:41:08.600 --> 00:41:10.600
giant planets have got a solid core or not.

1055
00:41:10.760 --> 00:41:13.160
In order to do that, we need spacecraft to be

1056
00:41:13.160 --> 00:41:15.600
orbiting those planets for a lengthy period

1057
00:41:15.600 --> 00:41:17.640
of time, preferably on highly elongated

1058
00:41:17.640 --> 00:41:19.400
orbits like Juno. This was one of the key

1059
00:41:19.560 --> 00:41:22.080
points of the Juno mission, where you've got

1060
00:41:22.080 --> 00:41:24.560
a spacecraft going round on highly

1061
00:41:24.560 --> 00:41:26.920
elongated orbit which is

1062
00:41:27.000 --> 00:41:29.200
experiencing the gravitational pull from the

1063
00:41:29.200 --> 00:41:31.400
planet. And when you're very close to the

1064
00:41:31.400 --> 00:41:34.360
planet, your orbit is not just sensitive

1065
00:41:34.360 --> 00:41:36.000
to the mass of the planet, as if all of the

1066
00:41:36.000 --> 00:41:37.720
mass was at a single point in the middle of

1067
00:41:37.720 --> 00:41:40.000
the planet, you actually become sensitive to

1068
00:41:40.000 --> 00:41:42.680
the distribution of mass within the planet.

1069
00:41:43.160 --> 00:41:45.400
Fundamentally, a planet that has a lot of gas

1070
00:41:45.400 --> 00:41:47.720
on top and a small dense core that has a

1071
00:41:47.720 --> 00:41:50.440
varying density throughout will affect the

1072
00:41:50.440 --> 00:41:53.040
spacecraft differently to how a planet that

1073
00:41:53.040 --> 00:41:55.320
was uniform in density throughout would do.

1074
00:41:55.720 --> 00:41:57.400
Now, to some degree, we do this on Earth,

1075
00:41:57.400 --> 00:42:00.280
where people map the density variations at a

1076
00:42:00.280 --> 00:42:03.080
very local scale for, um, GPS

1077
00:42:03.080 --> 00:42:04.800
satellites and things like that. And you've

1078
00:42:04.800 --> 00:42:06.960
seen beautiful gravitational maps of the

1079
00:42:06.960 --> 00:42:09.000
Earth where it looks like a deformed potato

1080
00:42:09.000 --> 00:42:11.880
effect. Yes. So same kind of idea with

1081
00:42:11.880 --> 00:42:14.360
Jupiter and Saturn. By using the data from

1082
00:42:14.360 --> 00:42:17.000
Juno, by using Cassini data from around

1083
00:42:17.000 --> 00:42:19.600
Saturn, we have a fairly good idea that those

1084
00:42:19.600 --> 00:42:21.480
planets do actually have

1085
00:42:22.360 --> 00:42:25.360
cores of solid and liquid material deep

1086
00:42:25.360 --> 00:42:27.120
within them that would have formed through

1087
00:42:27.120 --> 00:42:29.200
this core accretion process. So that's why we

1088
00:42:29.200 --> 00:42:31.810
can be fairly confident that they have rocky

1089
00:42:31.810 --> 00:42:34.010
cores here, where rocky is basically meaning

1090
00:42:34.090 --> 00:42:36.650
anything solid. There'll be iron and nickel,

1091
00:42:36.730 --> 00:42:38.690
there'll be water ice, and there'll also be

1092
00:42:38.690 --> 00:42:40.530
liquid metallic hydrogen and things like

1093
00:42:40.530 --> 00:42:42.930
this. But there will be a solid kernel at the

1094
00:42:42.930 --> 00:42:45.370
core from which those planets form. There is

1095
00:42:46.090 --> 00:42:48.170
some interest that comes from this because I

1096
00:42:48.170 --> 00:42:50.610
think Jupiter's core, I think it was

1097
00:42:50.610 --> 00:42:53.370
Jupiter's rather than Saturn's, the data has

1098
00:42:53.370 --> 00:42:55.090
revealed is there, uh, it's a bit more

1099
00:42:55.090 --> 00:42:57.930
massive than expected, but also more spread

1100
00:42:57.930 --> 00:43:00.770
out and slushy. And that is thought to be

1101
00:43:00.770 --> 00:43:02.690
potentially evidence of a late giant impact

1102
00:43:02.690 --> 00:43:04.970
on Jupiter, where there was a late addition

1103
00:43:04.970 --> 00:43:07.970
of a big chunk of solid material in much same

1104
00:43:07.970 --> 00:43:10.610
way that there was a giant impact that formed

1105
00:43:10.610 --> 00:43:12.130
the Earth and the moon, A giant impact that

1106
00:43:12.130 --> 00:43:14.010
stripped the surface of Mercury away, leaving

1107
00:43:14.010 --> 00:43:16.690
Mercury denuded. Giant impacts were a huge

1108
00:43:16.690 --> 00:43:19.610
part of planet formation. But in order to be

1109
00:43:19.610 --> 00:43:22.290
absolutely definitively sure that you have a

1110
00:43:22.290 --> 00:43:24.470
solid core, you need those close up

1111
00:43:24.470 --> 00:43:26.110
spacecraft measurements to be able to

1112
00:43:26.110 --> 00:43:27.110
distinguish the

1113
00:43:28.630 --> 00:43:30.830
subtleties in the gravitational field that

1114
00:43:30.830 --> 00:43:32.830
result from something that is not uniformly

1115
00:43:32.830 --> 00:43:35.590
dense but has a varying density and has a,

1116
00:43:35.590 --> 00:43:38.270
I guess, significant internal structure. We

1117
00:43:38.270 --> 00:43:40.910
can do that in the solar system. We haven't

1118
00:43:40.910 --> 00:43:42.590
yet done that for Uranus and Neptune because

1119
00:43:42.590 --> 00:43:44.470
we've never had orbiters go to those planets.

1120
00:43:44.470 --> 00:43:46.430
And I look forward to the day that we manage

1121
00:43:46.430 --> 00:43:48.150
that. But even if those missions start being

1122
00:43:48.150 --> 00:43:49.670
planned now, they probably won't launch till

1123
00:43:49.670 --> 00:43:52.460
the 2000-40s. I will be retired by the time

1124
00:43:52.460 --> 00:43:53.940
they get there, but I'll still be watching on

1125
00:43:53.940 --> 00:43:56.420
eagerly for the planets round of the stars.

1126
00:43:56.420 --> 00:43:59.340
We have to draw on the nature of the

1127
00:43:59.340 --> 00:44:01.020
planetary system. They're moving the orbits

1128
00:44:01.020 --> 00:44:02.940
and draw inferences then on which of the

1129
00:44:02.940 --> 00:44:05.660
formation mechanisms that they had. And

1130
00:44:05.660 --> 00:44:07.820
that's where the complexity about brown dwarf

1131
00:44:07.820 --> 00:44:10.180
versus giant planet comes from as well.

1132
00:44:10.580 --> 00:44:12.420
So it's a wonderfully deep and complex

1133
00:44:12.420 --> 00:44:14.940
question. In terms of the methane on

1134
00:44:14.940 --> 00:44:17.920
Uranus, I think that is not that Uranus

1135
00:44:17.920 --> 00:44:19.520
is outgassing the methane, it's keeping the

1136
00:44:19.520 --> 00:44:21.640
methane to itself. A bit like when I put the

1137
00:44:21.640 --> 00:44:23.720
dogs in a locked room um, they keep their

1138
00:44:23.720 --> 00:44:25.360
methane to themselves, and it's sometimes not

1139
00:44:25.360 --> 00:44:27.710
that pleasant when I go back in there. Um,

1140
00:44:27.710 --> 00:44:29.640
but rather the methane levels varying because

1141
00:44:29.640 --> 00:44:32.000
of the time of year and the seasonality of

1142
00:44:32.000 --> 00:44:34.240
weather on Uranus. I think that's probably

1143
00:44:34.240 --> 00:44:35.080
what's happening there.

1144
00:44:36.040 --> 00:44:38.720
Andrew Dunkley: Okay. Uh, it's a great question. Uh, Martin

1145
00:44:38.720 --> 00:44:41.080
always comes up with a ripper or two from

1146
00:44:41.080 --> 00:44:42.800
time to time. And some good questions as

1147
00:44:42.800 --> 00:44:44.840
well. And, uh, yeah, that was.

1148
00:44:45.940 --> 00:44:47.780
That was a good one. Thank you, Martin. And

1149
00:44:48.100 --> 00:44:50.660
thanks. Thanks for the joke. Loved it.

1150
00:44:51.200 --> 00:44:53.940
Um, and that's where we are going to

1151
00:44:54.100 --> 00:44:56.900
finish up. And, Jonti, thank you for filling

1152
00:44:56.900 --> 00:44:59.660
in for the last seven weeks or so while

1153
00:44:59.660 --> 00:45:02.580
Fred took a vacay. Uh, we really

1154
00:45:02.580 --> 00:45:04.660
do appreciate it, and, uh, we'll certainly

1155
00:45:04.660 --> 00:45:06.340
have you back down the track. Thank you.

1156
00:45:06.340 --> 00:45:07.740
Jonti Horner: It's always a pleasure. And in the meantime,

1157
00:45:07.740 --> 00:45:09.620
I'll keep my eye on the Facebook group and

1158
00:45:09.700 --> 00:45:12.160
cheer on people sharing Nightwish videos. Uh,

1159
00:45:12.160 --> 00:45:13.460
I saw that. That made me happy.

1160
00:45:13.460 --> 00:45:13.820
Berman Gorvine: Yeah.

1161
00:45:13.820 --> 00:45:15.220
Andrew Dunkley: Yeah, I knew someone would.

1162
00:45:15.300 --> 00:45:15.680
Jonti Horner: Yeah.

1163
00:45:15.680 --> 00:45:18.040
Andrew Dunkley: Uh, fantastic than Jonti. Thank you very

1164
00:45:18.040 --> 00:45:18.320
much.

1165
00:45:18.320 --> 00:45:20.000
Jonti Horner: That's a pleasure. I'll catch you next time.

1166
00:45:20.240 --> 00:45:22.800
Andrew Dunkley: Okay, Bye. Bye. Uh, Jonti Horner, professor

1167
00:45:22.800 --> 00:45:24.960
of astrophysics at the university University

1168
00:45:25.040 --> 00:45:27.160
of Southern Queensland, uh, filling in for

1169
00:45:27.160 --> 00:45:30.160
Fred for the last several weeks. And we will,

1170
00:45:30.200 --> 00:45:32.520
uh, get him back on in the not too distant

1171
00:45:32.520 --> 00:45:35.200
future. And thanks to Huw in the studio. Huw

1172
00:45:35.200 --> 00:45:38.000
couldn't be with us today because, um, he's

1173
00:45:38.000 --> 00:45:40.520
been having trouble sitting. Uh, and he went

1174
00:45:40.520 --> 00:45:42.160
to the doctor, and the doctor said, you've

1175
00:45:42.160 --> 00:45:44.160
got a ring around your anus. Oh, I couldn't

1176
00:45:44.160 --> 00:45:45.400
help it. Thanks, Martin.

1177
00:45:45.400 --> 00:45:48.220
Jonti Horner: You inspired me. I'm done being

1178
00:45:48.220 --> 00:45:49.540
locked in a room with my dog.

1179
00:45:51.140 --> 00:45:53.060
Yes. Yes, indeed.

1180
00:45:53.380 --> 00:45:55.180
Andrew Dunkley: All right, we're done. Thanks for your

1181
00:45:55.180 --> 00:45:56.780
company. We'll catch you on the next episode

1182
00:45:56.780 --> 00:45:58.340
of Space Nuts. Bye. Bye.

1183
00:45:59.620 --> 00:46:01.820
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1184
00:46:01.820 --> 00:46:04.780
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1185
00:46:04.780 --> 00:46:06.740
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1186
00:46:06.980 --> 00:46:09.700
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1187
00:46:09.700 --> 00:46:12.150
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1188
00:46:12.150 --> 00:46:14.810
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1189
00:46:14.810 --> 00:46:17.050
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