Solar Eclipses, Speeding Comets & the Enigma of Cosmic Event Horizons

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Andrew Dunkley: Hi there. Thanks for joining us on a Q and A

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edition of Space Nuts. My name is Andrew

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Dunkley. Thanks for your company. on today's

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episode, questions from the audience. Rusty

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is asking about the similarities between the

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sun and the moon. And there are several, and,

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most are, coincidental, as it turns out.

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Trevor wants to talk about speeding through

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space. Slap a couple of pay plates on and

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you're on your way. Austin, has asked,

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us about Hoag's object. This is really

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interesting. And Dan wants us to explain

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the cosmic event horizon. We'll do all of

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that shortly on this edition of

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space nuts. 15 seconds. Guidance is

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internal. 10, 9.

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Ignition sequence start.

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Jonti Horner: space nuts.

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Andrew Dunkley: 5, 4, 3, 2. 1, 2, 3, 4,

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

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Jonti Horner: Space nuts.

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Andrew Dunkley: Astronauts report it.

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Jonti Horner: Neil's good.

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Andrew Dunkley: And to help us along with all of those

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questions is Jonti Horner, professor of

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astrophysics at the University of Southern Qu

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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: I'm well. Good to see you again. Nice T

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shirt, by the way. Is that a comet that's

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on a dinosaur? Yep.

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Jonti Horner: Fighting off the oncoming comment. These T

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shirts are great, actually. I use these for,

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when I'm doing outreach talks. And they're

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particularly good with kids. And I didn't

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realize when I got them, I just thought,

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these are fabulous. So these kind of printed

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T shirts with dinosaurs and rocks from space

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falling down and they're kind of funny and

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they're kind of cute. I didn't realize when I

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got them. I was giving all these talks and

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someone came up to me and said, oh, is that

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from Happy Little Dinosaurs? What's Happy

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Little Dinosaurs? Turns out that there is a

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card game. that's one of the many, many

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kind of board game card games that are out

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there. Fabulous. If I've got loads of games

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on the shelves behind me. Board gaming is

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great. game out there, which is basically

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about the end of the times for the dinosaurs.

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And they're not happy at all in actuality,

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but it's apparently really good fun and it's

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kid friendly. And apparently all these T

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shirts I've got are free advertising for

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game.

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Andrew Dunkley: Oh, there you go.

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Jonti Horner: Yeah, there you go. I'm a walking

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billboard and didn't even realize it.

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Andrew Dunkley: Yes, indeed. that's not a fat joke either.

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All right, let's answer, some questions. Now.

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I'm going to have to put an apology forward.

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Straight up. I've, been to my optometrist

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this week. I do have a Little bit of an eye

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issue, so everything's super blurry. And

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they're all text questions, as it turns out.

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But, we'll, we'll do our best. Rusty from

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Donnybrook is first up. I'm wondering just

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how many coincidences there are between the

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sun and the Moon. First, we have the almost

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exact apparent size coincidence, which has

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enabled us to, learn

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an amazing amount about the sun during solar

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eclipses. Yes, Fred and I have talked about

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that many times. Second, is the rotation

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rate, which is the same for the sun and the

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Moon at about the solar latitude where

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most sunspots are seen. What will

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this coincidence enable as we begin to

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inhabit the Moon permanently? Will we be able

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to avoid most solar radiation storms by

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moving to the night side when large groups of

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sunspots, are visible? After all, we

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don't want to be stuck dug in at, the

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poles forever. could we do this with

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spacecraft in large orbits around the Moon?

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Does the coincidence mean that large deposits

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of helium 3 may be found at certain lunar

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longitudes? are there any more Sun, Moon

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coincidence that, coincidences that you know

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of? Thanks for the show. moving on up, guys.

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you'll be number one soon. Well, we'll see

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about that. But we're happy where we are.

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Rusty, from Donnybrook. that was more than

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one question, basically, but, all centered

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around these, these

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coincidences that exist between the sun and

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the Moon.

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Jonti Horner: Absolutely. And the coincidences are really

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astonishing. And it's just our very good

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fortune to live at exactly the right time in

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the Earth's history where things line up like

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this. Because when the Moon formed air, the

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Earth was spinning much, much quicker, the

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Moon formed much closer to us. And at that

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point when the Moon moved in front of the

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sun, it would block the sun out totally by a

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long, long way. And, you'd have a fairly

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underwhelming eclipse of the sun, to be

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honest. And, as time has gone on, the Moon

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has moved further and further away from the

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Earth. The further it moves away, the slower

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the Earth, spins, the longer the Moon takes

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to orbit the, Earth and the slower it

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recedes. And we're now in this very

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privileged window where most

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of the time the Moon and the sun are

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so similar in size that if they line up

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perfectly, the sun will be blocked out by the

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disk of the Moon. And we can see the Sun's

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our atmosphere. And we get the wonders of a

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total solar eclipse. Now. Doesn't happen

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all the Time, because when the Moon is near

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apogee, when it's furthest from the Earth,

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it is small enough that you instead get the

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ring of fire type eclipse. You get an annular

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eclipse, where you get an annulus or a ring

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of the Sun's disk around the Moon. And

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that is presaging what is to come in millions

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of years into the future. So as time goes on

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and the Moon keeps edging away from us, and

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as the sun continues to get very, very, very

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slightly bigger as well, because these things

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also happen, what that means is that as time

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goes on, we will get fewer and fewer total

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eclipses and more and more annular eclipses.

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And eventually there will come a day in

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millions of years in the future where we no

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longer get total eclipses at all. And we will

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see the final ever total eclipse from the

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Earth, which will probably be sad, but

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it's so far away in the future that we don't

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really have to worry about it right now. So

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that's coincidence number one. And it's

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effectively that the Moon is about, I think

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it's 1 400th of the distance to the sudden

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1 400th of the size of the Sun. So,

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you know, similar triangles that we learned

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at school and thought we'd never use mean

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that they're about the same angle in the sky

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and they block each other out. That's all

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great. The second one, which

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Rusty, has pointed out, I'd actually never

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really given any thought to, to be honest.

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But it is true that the length of the

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lunar month, the length of the orbit of

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the Moon around the, Earth, is about the same

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as the period of rotation of the sun with an

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asterisk. And, the rotation period of the

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Moon when compared to the

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background stars is the same

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as its orbital period around the Earth

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compared to the background stars. It's

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tidally locked. It's trapped in this orbit.

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So it always keeps effectively one face

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towards the Earth, one face away from the

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Earth. And, that's a natural result of the

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tidal interaction between the two objects.

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And it's tied to these effects that are

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causing the Moon to gradually drift away.

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Now, the sun is a fluid object. It

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doesn't have the same rotation period at all,

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locations. It rotates quicker at the equator

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and slower at the poles. And that's thought

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to be part of the reason we get the sunspot

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cycles. What happens with that, is that the

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equator rotates around once every 24

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Earth days. The poles rotate about once every

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32, 34 Earth days and locations

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between have different rotation periods. So

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there is a latitude on the Sun's disk that

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rotates with a period of exactly one lunar

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month. And so what Rusty's talking about here

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is the idea that when sunspots are at that

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latitude, if you've got one dominant group of

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sunspots, then one side of the Moon will be

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getting hammered when those sunspots are

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creating activity. And the other side of the

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Moon will see the sun when the sunspots are

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on the other side and they won't see them.

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The reason that

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that doesn't mean you get one particular side

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of the moon getting all the solar radiation

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on the other side being protected, it is the

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fact that sunspots form at different

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latitudes on the sun and so go around at

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slightly different speeds. Early in a solar

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cycle, they're at higher latitude. Later in

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the solar cycle, they're at lower latitudes,

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so the rotation period changes a bit. Also,

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you get multiple groups of sunspots at a

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given time. And, the activity from a given

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sunspot group can hit the Earth and influence

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the Earth when it's at different locations on

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the sun, depending exactly what the magnetic

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field between the two is in the solar window

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doing. So. I've seen cases in the past where

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we've had a flare and they've been aurora.

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And you look at it, and the sunspot that had

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the flare wasn't dead center of the Sun's

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disk. Your intuitive thing is that when the

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sunspot is back in the middle of the Sun's

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disk, anything it ejects will come directly

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towards the Earth. But in actuality, we've

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had a lot of aurora when the sunspot is

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offset because things follow a curved path,

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all this kind of stuff. So in that sense,

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I don't think there is any particular

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longitude on the moon that would be favored

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over the others because, on longer

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timescales, everything would totally smooth

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out, totally average out. On a

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given sunspot group on a given event,

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you'll obviously have one half of the moon

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exposed and the other half protected by

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looking the other way. Effectively, I think

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in terms of people wandering around on the

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Moon and our, spacecraft on the Moon, it

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would be very inefficient to have to move

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halfway around the Moon to get into shelter.

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Yeah, I think it makes more sense to drill

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down and hide a few meters below the surface

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where the rocks shield you. Yeah. And so

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while I love the idea of us having the

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ability to duck around the corner, you'd

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actually have to move quite a long way. And

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that would be inefficient and time, time

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intensive and all the rest of it. When what

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you can do is just say, well, we're just

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going to nip indoors and watch movies for a

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couple of days. You know, we'll be under the

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shielding of all the rock and the rubble

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above us. That makes more sense.

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And, in terms of other

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coincidences, the one that I think of that

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isn't really a coincidence at all is if you

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look at the bulk composition of

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the solid material in the solar system

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is to first order, the same as the bulk

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composition of the sun minus the gases.

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And that's just because everything formed

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from the same stuff. What's interesting is

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that the Moon is depleted in the heavier

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elements and enriched in the lighter

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elements. Compared to that.

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And in the flip side, the Earth is actually a

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bit enriched in the heavier elements and

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depleted in the lighter elements. They formed

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at the same place in the solar system at the

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same time. So you'd expect them to form from

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the same stuff. And so the difference in

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their compositions actually telling us about

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the story of their formation and the fact

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that the Earth formed as a single object and

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then was smashed and torn asunder and formed

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the Earth and Moon. And the Moon was formed

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from the lightest stuff that had floated to

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the top, so primarily from the crust and the

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mantle. And therefore you get this different

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in composition where the overall

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bulk composition will be the same as the bulk

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composition of those materials in the, sun.

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But the differences are what tell us the

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story. So it's not really a coincidence, but

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it is an interesting link between a lot of

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them because we all formed at the same place

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at the same time from the same stuff. But the

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formation then shaped us.

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Andrew Dunkley: M Fascinating. There you go, Rusty. Hopefully

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that covered all your bases. Thanks for the

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question.

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0G and I feel fine. Space nuts.

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And our next question comes from Trevor in

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Port Macquarie. with Comet 3I

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Atlas heading through our solar system at a

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record speed of around 60 kilometers per

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second, it's got me thinking about how fast

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an object like a comet or indeed a

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spacecraft could reach. I'm assuming

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Comet 3i Atlas has acquired some of this

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speed with maybe a slingshot around

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another star or two in the past.

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And as it passes, the sun,

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it will probably pick up additional speed.

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Is there any limit to how fast a

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comet like this could reach if it continues

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to receive gravitational assistance from

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stars in the future? I know with a spacecraft

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we use gravitational assistance by going

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around, say, Venus and back to earth to pick

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up speed. But what would happen if

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we did, say, 50 times,

301
00:11:40.471 --> 00:11:43.391
before heading off into the direction we want

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to go? So 50 passes, he's saying, ah, could

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he reach speeds well in excess of, what deep

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space probes currently travel at? Or is

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there a level of diminishing return to this

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approach? Looking forward to your answer.

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Jonti Horner: Trevor.

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Andrew Dunkley: Love this question.

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Jonti Horner: Yes, this is really, really good fun. So,

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00:12:01.221 --> 00:12:04.001
it's a bit of both, to be honest. The long

311
00:12:04.001 --> 00:12:06.761
and short of it is that if you could set up

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the perfect chain of

313
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slingshot assists with

314
00:12:12.001 --> 00:12:13.801
ever more massive objects moving at ever

315
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greater speeds around, there is no real limit

316
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up until the point you get very close to the

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speed of light, because you can't get faster

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than the speed of light. But the

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challenge inherent in that is that you're

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transferring a bit of the kinetic energy from

321
00:12:27.001 --> 00:12:28.761
one object to another. You're sealing

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momentum through a gravitational slingshot.

323
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So if you fly by Jupiter and Jupiter gives

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you a kick to speed you up, Jupiter actually

325
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slows down a little bit. Because Jupiter is

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much, much, much, much, much, much, much more

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massive than you are. The change in its speed

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is much, much, much smaller. Momentum is

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speed times mass, effectively. But

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you're taking a bit of that kinetic energy.

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And because of that, there's no real limit to

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how much you can boost. But any given

333
00:12:53.921 --> 00:12:56.371
encounter will only give you so much.

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00:12:56.931 --> 00:12:59.331
The closer you get to the mass, the more of a

335
00:12:59.331 --> 00:13:01.251
kick you can get. If the orientation is

336
00:13:01.251 --> 00:13:04.011
right, the faster the mass is moving compared

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00:13:04.011 --> 00:13:05.771
to you, the more of a kick you can get as

338
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well. So you can imagine setting up a

339
00:13:08.171 --> 00:13:10.211
situation where you have a few flybys and you

340
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gradually get more and more boosts. And

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that's how people have taken the cheapskate

342
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route to the outer solar system. You get

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these missions that have a flyby of the Earth

344
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and a flyby of Venus and a flyby of the Earth

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and a flyby of Venus and a flyby of the Earth

346
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and a flower flyby of Mars and so on. And

347
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they get gradual, little incremental kicks to

348
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get out to Jupiter. It's also

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how we saw the Voyager spacecraft get a kick

350
00:13:31.291 --> 00:13:33.211
from Jupiter to Saturn, then Voyager 2 got a

351
00:13:33.211 --> 00:13:35.291
kick from Saturn to Uranus, and Uranus kicked

352
00:13:35.291 --> 00:13:37.331
it onto Neptune, and you got to speed up

353
00:13:37.331 --> 00:13:39.931
every time. The problem becomes

354
00:13:40.491 --> 00:13:42.371
you eventually hit the escape velocity for

355
00:13:42.371 --> 00:13:43.891
the solar system, and Jupiter is really good

356
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at doing that. Once you're at the escape

357
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velocity, you're not coming back for a second

358
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pass by Jupiter. So what are you going to get

359
00:13:48.891 --> 00:13:51.851
your next slingshot by? Well, if everything's

360
00:13:51.851 --> 00:13:53.371
lined up just right, you can do like the

361
00:13:53.371 --> 00:13:55.291
Voyager spacecraft did and do the grand tour

362
00:13:55.291 --> 00:13:56.971
where you get one to the next to the next.

363
00:13:57.051 --> 00:13:58.931
Yeah, but that's got a bit of diminishing

364
00:13:58.931 --> 00:14:01.091
returns because they're only so massive. So

365
00:14:01.091 --> 00:14:03.131
you can only get so much out of a given kick.

366
00:14:03.771 --> 00:14:06.290
You could possibly in theory aim out of our

367
00:14:06.290 --> 00:14:09.051
solar system to fly past Alpha

368
00:14:09.051 --> 00:14:10.931
Centauri and get a kick from that, if you

369
00:14:10.931 --> 00:14:13.251
oriented right and try and aim your

370
00:14:13.251 --> 00:14:15.051
spacecraft to get a kick from Alpha Centauri

371
00:14:15.051 --> 00:14:17.091
to kick you onto, I don't know, Sirius and

372
00:14:17.091 --> 00:14:18.491
get a kick there and bounce around, and

373
00:14:18.491 --> 00:14:20.571
bounce around and gradually accumulate speed.

374
00:14:21.161 --> 00:14:22.881
But eventually you'd again reach the escape

375
00:14:22.881 --> 00:14:24.441
velocity of the galaxy and eventually you'd

376
00:14:24.441 --> 00:14:27.321
be on a one way trip out of our galaxy. So it

377
00:14:27.321 --> 00:14:29.601
would be very hard to set you up to get to a

378
00:14:29.601 --> 00:14:32.371
speed that is ludicrous speed, you know, a

379
00:14:32.371 --> 00:14:34.251
speed that is approaching the speed of light.

380
00:14:34.571 --> 00:14:36.371
You'd probably need to come incredibly close

381
00:14:36.371 --> 00:14:38.771
to a black hole to do that. And you need to

382
00:14:38.771 --> 00:14:41.171
engineer things such that you didn't get

383
00:14:41.171 --> 00:14:42.691
destroyed by the radiation and everything

384
00:14:42.691 --> 00:14:45.051
from the accretion disk around it and you had

385
00:14:45.051 --> 00:14:46.811
your angle right so that you gained enough

386
00:14:46.811 --> 00:14:49.201
momentum to get much quicker. So you could

387
00:14:49.201 --> 00:14:51.681
probably play games like that. It's certainly

388
00:14:51.681 --> 00:14:53.921
really useful in the solar system. It's very

389
00:14:53.921 --> 00:14:55.881
effective at getting things up to high speed

390
00:14:55.881 --> 00:14:58.641
without requiring huge amounts of fuel. You

391
00:14:58.641 --> 00:15:01.561
can supplement it by timing the burn of your

392
00:15:01.561 --> 00:15:03.561
rockets and using your limited propellant to

393
00:15:03.561 --> 00:15:06.001
get the maximum kick possible. And I've read

394
00:15:06.001 --> 00:15:08.271
about, special science fiction style

395
00:15:08.271 --> 00:15:11.151
maneuvers where you whiz around and

396
00:15:11.151 --> 00:15:13.311
you do your burn at the very closest approach

397
00:15:13.311 --> 00:15:15.271
to an object to get the maximum change in

398
00:15:15.271 --> 00:15:17.311
velocity and the maximum energy change for

399
00:15:17.311 --> 00:15:20.271
that given packet of fuel. I

400
00:15:20.271 --> 00:15:22.071
think it's called the Oberth manoeuvre or

401
00:15:22.071 --> 00:15:23.591
something like that. And it's really

402
00:15:23.591 --> 00:15:25.991
interesting because it, at, ah, first thought

403
00:15:25.991 --> 00:15:28.711
it defeats common sense.

404
00:15:28.951 --> 00:15:30.831
Do you think if you're speeding up by 1 meter

405
00:15:30.831 --> 00:15:33.071
per second, doing it anywhere in the orbit

406
00:15:33.071 --> 00:15:34.391
would have the same effect, but in fact

407
00:15:34.391 --> 00:15:36.311
speeding up by 1 meter per second when you're

408
00:15:36.311 --> 00:15:38.831
traveling really quickly creates a bigger

409
00:15:38.831 --> 00:15:41.551
change in your final speed once gravity and

410
00:15:41.551 --> 00:15:43.031
everything works out, than doing it when

411
00:15:43.031 --> 00:15:44.351
you're moving really slowly? There's all

412
00:15:44.351 --> 00:15:46.841
sorts of odd things like that going on. So

413
00:15:46.841 --> 00:15:48.721
there are games you can play with it and you

414
00:15:48.721 --> 00:15:51.401
could set up a theoretical path that could

415
00:15:51.401 --> 00:15:53.281
take you millions of years to complete, where

416
00:15:53.281 --> 00:15:56.201
you Chain path objects. You have a

417
00:15:56.201 --> 00:15:57.681
little bit of control in your spacecraft to

418
00:15:57.681 --> 00:16:00.521
refine your orbit, refine your path,

419
00:16:01.001 --> 00:16:03.241
so that you aim perfectly to the next target,

420
00:16:03.891 --> 00:16:05.901
and effectively get a slingshot from each.

421
00:16:06.301 --> 00:16:08.501
But the practical limits are that unless you

422
00:16:08.501 --> 00:16:11.261
could get very close to a black hole, you're

423
00:16:11.261 --> 00:16:13.061
going to be limited to some degree. And it

424
00:16:13.061 --> 00:16:15.731
becomes, like you said, diminishing returns.

425
00:16:16.131 --> 00:16:19.131
Not because the physics of it prevent you

426
00:16:19.131 --> 00:16:21.091
from getting any arbitrary speed you want,

427
00:16:21.571 --> 00:16:23.571
but because the faster you're moving, the

428
00:16:23.571 --> 00:16:25.331
less targets there are to hit and the longer

429
00:16:25.331 --> 00:16:27.011
you've got to wait to get there.

430
00:16:28.051 --> 00:16:30.331
Andrew Dunkley: Do you think the day will come, though, where

431
00:16:30.331 --> 00:16:32.931
we develop technology that stops us

432
00:16:32.931 --> 00:16:34.611
needing gravity assist,

433
00:16:35.631 --> 00:16:38.551
maybe ion engines or scramjets

434
00:16:38.551 --> 00:16:40.591
or whatever the things they're developing at

435
00:16:40.591 --> 00:16:40.991
the moment?

436
00:16:42.991 --> 00:16:45.101
Jonti Horner: We're always developing new things. And, as

437
00:16:45.101 --> 00:16:46.781
I've said before, you know, the one

438
00:16:46.781 --> 00:16:47.981
prediction you can make is that all

439
00:16:47.981 --> 00:16:49.941
predictions are wrong. Science fiction is

440
00:16:50.501 --> 00:16:52.901
amazing at coming up with things that have no

441
00:16:52.901 --> 00:16:55.381
grounding in current physics, but that

442
00:16:55.381 --> 00:16:57.541
suggest possible routes to do things that

443
00:16:57.541 --> 00:16:59.741
break the current laws of physics. And to

444
00:16:59.741 --> 00:17:01.661
some degree, everybody writes them off as

445
00:17:01.661 --> 00:17:04.141
purely fiction because they can't work in our

446
00:17:04.141 --> 00:17:06.061
current understanding of physics. But I

447
00:17:06.061 --> 00:17:07.950
remember that a long time ago, a lot of

448
00:17:07.950 --> 00:17:09.590
people were arguing that it was utterly

449
00:17:09.590 --> 00:17:11.350
impossible and physically impossible to have

450
00:17:11.350 --> 00:17:13.390
heavier than air travel. And our

451
00:17:13.390 --> 00:17:15.669
understanding of, physics and the cosmos is

452
00:17:15.669 --> 00:17:18.429
limited by the quality of observations that

453
00:17:18.429 --> 00:17:21.029
we have. And our theories are

454
00:17:21.029 --> 00:17:22.989
exceptionally good at explaining things to

455
00:17:22.989 --> 00:17:24.909
the level that our observations can currently

456
00:17:24.909 --> 00:17:27.789
be carried out and a bit more. But it's

457
00:17:27.789 --> 00:17:29.709
entirely feasible that as we get better at

458
00:17:29.709 --> 00:17:31.589
making observations, we reach the limits of

459
00:17:31.589 --> 00:17:34.450
where our current models, are accurate.

460
00:17:34.450 --> 00:17:36.570
And we need new and better physics, and new

461
00:17:36.570 --> 00:17:39.050
things will come out of that. We'll also get

462
00:17:39.050 --> 00:17:41.040
more efficient at, using the tools we know do

463
00:17:41.040 --> 00:17:43.600
work. And things like the ion engines which

464
00:17:43.600 --> 00:17:45.720
they have tested with spacecraft are

465
00:17:45.720 --> 00:17:48.240
interesting. Our common conventional kind of

466
00:17:48.240 --> 00:17:51.160
chemical rockets apply a very large amount of

467
00:17:51.160 --> 00:17:53.200
thrust for a very short period of time and

468
00:17:53.200 --> 00:17:55.760
burn fuel very intensively. And so you get a

469
00:17:55.760 --> 00:17:58.320
high acceleration for a short time. Ion

470
00:17:58.320 --> 00:18:00.120
engines are almost exactly the opposite in

471
00:18:00.120 --> 00:18:01.800
that they give you a very small acceleration,

472
00:18:01.800 --> 00:18:04.160
but that can be applied continuously for very

473
00:18:04.160 --> 00:18:06.420
long periods of time. And so they're much

474
00:18:06.420 --> 00:18:08.980
more efficient. And you can imagine stuff

475
00:18:08.980 --> 00:18:11.620
like that being scaled up. But the science

476
00:18:11.620 --> 00:18:12.780
fiction that

477
00:18:14.380 --> 00:18:16.380
leads to people moving at significant

478
00:18:16.460 --> 00:18:18.220
fractions of the speed of light nearly

479
00:18:18.220 --> 00:18:20.900
universally uses technologies that are beyond

480
00:18:20.900 --> 00:18:23.730
what our physics currently allows. And, it's

481
00:18:23.730 --> 00:18:25.570
where you get the blurring of hard sci Fi and

482
00:18:25.570 --> 00:18:28.090
soft sci fi of the degree to which you have

483
00:18:28.410 --> 00:18:30.370
things grounded in current science and things

484
00:18:30.370 --> 00:18:33.120
grounded in fantasy almost in

485
00:18:33.280 --> 00:18:35.160
science that is so advanced that it's magic

486
00:18:35.160 --> 00:18:36.840
to us because we can't explain how it works

487
00:18:36.840 --> 00:18:39.560
or know that it will. And so trying to say

488
00:18:39.560 --> 00:18:42.560
whether we'll ever get to a stage where

489
00:18:42.560 --> 00:18:44.960
our technology allows us to do these things

490
00:18:46.560 --> 00:18:48.920
quickly, to get to Mars in a week, to get to

491
00:18:48.920 --> 00:18:51.520
Pluto in a month or a week,

492
00:18:52.480 --> 00:18:54.720
you can imagine it is possible if you do the

493
00:18:54.720 --> 00:18:57.360
maths. If you accelerate at 1g, the

494
00:18:57.360 --> 00:18:59.120
acceleration due to gravity and you can

495
00:18:59.120 --> 00:19:01.440
maintain the acceleration for a while, you

496
00:19:01.440 --> 00:19:04.120
get to very high speeds very quickly. So if

497
00:19:04.120 --> 00:19:06.280
you accelerate at 10 meters per second per

498
00:19:06.280 --> 00:19:09.080
second, after 100 seconds you're going at

499
00:19:09.080 --> 00:19:11.720
1 kilometer a second, after 200 seconds,

500
00:19:11.720 --> 00:19:13.600
you're going at 2 kilometers a second. So you

501
00:19:13.600 --> 00:19:16.240
gain speed very, very, very quickly. But

502
00:19:16.240 --> 00:19:18.600
you've got to use fuel very efficiently to do

503
00:19:18.600 --> 00:19:21.480
that. And a sample of a lot of hard

504
00:19:21.480 --> 00:19:23.200
science fiction that's kind of near future

505
00:19:23.200 --> 00:19:25.850
sci fi relies on the idea

506
00:19:25.850 --> 00:19:28.810
of some form of drive system that allows

507
00:19:28.810 --> 00:19:30.810
you to maintain that level of acceleration

508
00:19:31.050 --> 00:19:33.370
for periods of hours or weeks or months.

509
00:19:34.010 --> 00:19:36.490
And if you could do that, the travel time to

510
00:19:36.490 --> 00:19:39.170
get to places like Saturn becomes

511
00:19:39.170 --> 00:19:41.290
manageable. On a holiday you could do what

512
00:19:41.290 --> 00:19:42.770
Fred's doing, but instead of going to

513
00:19:42.770 --> 00:19:45.290
Scandinavia, you could go and visit Titan.

514
00:19:45.930 --> 00:19:47.730
If you've got that level of technology and

515
00:19:47.730 --> 00:19:49.210
you do it in a certain degree of comfort

516
00:19:49.210 --> 00:19:50.810
because what you do is your Spacecraft's

517
00:19:50.810 --> 00:19:53.150
accelerating at 1G, so you feel an

518
00:19:53.150 --> 00:19:54.830
acceleration of 1G and it's like you're on

519
00:19:54.830 --> 00:19:57.310
Earth. You don't need artificial gravity. You

520
00:19:57.310 --> 00:19:59.070
just have that short period halfway through

521
00:19:59.070 --> 00:20:00.470
the flight where you're weightless while the

522
00:20:00.470 --> 00:20:02.270
spacecraft turns dark, turns around and

523
00:20:02.270 --> 00:20:03.910
starts accelerating at 1G in the other

524
00:20:03.910 --> 00:20:06.070
direction to slow you down for approach.

525
00:20:06.870 --> 00:20:08.870
And that's a step level mode of the more

526
00:20:08.870 --> 00:20:11.190
realistic sci fi. But it still

527
00:20:11.190 --> 00:20:13.470
requires technology that we don't yet have

528
00:20:13.470 --> 00:20:15.910
and we may never develop or we might. And you

529
00:20:15.910 --> 00:20:17.630
know, I'd love it if we did, but we'll just

530
00:20:17.630 --> 00:20:18.290
have to see.

531
00:20:18.610 --> 00:20:21.330
Andrew Dunkley: Time will tell. but you know, Flash Gordon

532
00:20:21.330 --> 00:20:23.570
was the first to put rockets,

533
00:20:24.130 --> 00:20:26.290
you know, that could reland themselves. And

534
00:20:26.850 --> 00:20:29.830
now, now we can do that. so yeah, there's

535
00:20:29.830 --> 00:20:31.910
all sorts of weird and wonderful stuff in sci

536
00:20:31.910 --> 00:20:34.720
fi that has become reality. Thanks,

537
00:20:35.060 --> 00:20:37.180
so much for the question, Trevor. This is

538
00:20:37.180 --> 00:20:39.340
Space Nuts with Andrew Dunkley and John de

539
00:20:39.340 --> 00:20:39.860
Ora.

540
00:20:42.260 --> 00:20:44.660
0G and I feel fine. Space

541
00:20:44.660 --> 00:20:47.220
Nuts. Our next Question comes from

542
00:20:47.380 --> 00:20:50.360
Austin. He said, I've, just been watching a

543
00:20:50.360 --> 00:20:52.640
long YouTube Music presentation on Hoag's

544
00:20:52.640 --> 00:20:55.520
object. A lonesome galaxy with

545
00:20:55.520 --> 00:20:57.840
lots of features that don't seem to fit with

546
00:20:57.840 --> 00:21:00.720
present knowledge. Do objects like that

547
00:21:00.720 --> 00:21:03.360
require a complete rethink of the standard

548
00:21:03.360 --> 00:21:05.570
model or are, there ready

549
00:21:05.570 --> 00:21:08.290
explanations of its features now?

550
00:21:08.290 --> 00:21:09.850
Hoag's object is

551
00:21:10.660 --> 00:21:13.260
astonishing, to say the very least. It's a

552
00:21:13.260 --> 00:21:15.220
very unusual ring galaxy.

553
00:21:16.520 --> 00:21:18.160
I know you've been doing some research on

554
00:21:18.160 --> 00:21:20.620
this, so we can sort this one out for Austin.

555
00:21:21.660 --> 00:21:23.700
Jonti Horner: I have. It's beautiful. The photos are

556
00:21:23.700 --> 00:21:25.380
amazing. I do encourage people to look at

557
00:21:25.380 --> 00:21:28.340
this. It is one of a

558
00:21:28.340 --> 00:21:30.020
number of galaxies that we've seen that have

559
00:21:30.020 --> 00:21:32.180
this kind of ring like structure. And the

560
00:21:32.180 --> 00:21:35.100
typical explanation is that you've had some

561
00:21:35.100 --> 00:21:37.680
kind of violent encounter between two

562
00:21:37.680 --> 00:21:40.680
galaxies where one has pushed through the

563
00:21:40.680 --> 00:21:43.160
other really quickly and set up a shockwave.

564
00:21:43.160 --> 00:21:45.160
And that shockwave has propagated outwards.

565
00:21:45.480 --> 00:21:47.510
And, the shockwave causes gas and dust pile

566
00:21:47.510 --> 00:21:49.550
up and drive the bursts of star formation.

567
00:21:50.030 --> 00:21:52.910
And as we know, the bulk of the

568
00:21:52.910 --> 00:21:55.190
most luminous stars in a galaxy are the

569
00:21:55.190 --> 00:21:57.670
hottest, brightest, shortest lived ones.

570
00:21:57.670 --> 00:21:58.190
Andrew Dunkley: Yes.

571
00:21:58.270 --> 00:22:00.910
Jonti Horner: And so it's natural that you'd get a ring

572
00:22:01.230 --> 00:22:03.230
where the shock wave has recently passed,

573
00:22:03.710 --> 00:22:05.190
where you've had a lot of massive stars

574
00:22:05.190 --> 00:22:07.750
formed that have not yet died. And you'd have

575
00:22:07.750 --> 00:22:09.270
a few reddish stars dotted through there,

576
00:22:09.270 --> 00:22:10.990
which are the stars that are about to die.

577
00:22:11.070 --> 00:22:12.750
And that the middle of the galaxy would look

578
00:22:12.750 --> 00:22:14.430
fairly yellowish and dull because there's no

579
00:22:14.430 --> 00:22:15.830
young stars there because all the gas and

580
00:22:15.830 --> 00:22:18.200
dust have been swept out. And, we do see a

581
00:22:18.200 --> 00:22:21.040
lot of these kind of galaxies, but I've never

582
00:22:21.040 --> 00:22:23.560
seen any of them that is as

583
00:22:23.560 --> 00:22:26.200
symmetrical and beautiful as Hoag's object.

584
00:22:26.200 --> 00:22:29.040
Hoag's object looks like if you

585
00:22:29.040 --> 00:22:31.520
got somebody to make a perfect fried egg

586
00:22:32.160 --> 00:22:34.760
and then they got two of those weird little

587
00:22:34.760 --> 00:22:36.760
poached egg rings and they put one around the

588
00:22:36.760 --> 00:22:39.060
yolk, and then they put a much wider one

589
00:22:39.060 --> 00:22:40.780
around near the outside of the white.

590
00:22:40.860 --> 00:22:43.220
Everything between those two rings was lifted

591
00:22:43.220 --> 00:22:44.700
out and thrown away. And then the rings were

592
00:22:44.700 --> 00:22:46.220
taken away and you were left with a yolk in

593
00:22:46.220 --> 00:22:48.620
the middle and then an empty gap and then a

594
00:22:48.620 --> 00:22:50.300
ring of white around the outside. Although in

595
00:22:50.300 --> 00:22:53.020
this case a ring of blue. It is beautifully

596
00:22:53.020 --> 00:22:55.220
symmetric. There's a little bit of a hint of

597
00:22:55.220 --> 00:22:56.820
rotation there. It almost looks like a

598
00:22:56.820 --> 00:22:58.540
turning salt to its blade when you look at

599
00:22:58.540 --> 00:22:58.780
it.

600
00:22:58.970 --> 00:22:59.100
Andrew Dunkley: It does.

601
00:22:59.100 --> 00:23:02.060
Jonti Horner: which is leftover of the original rotation

602
00:23:02.060 --> 00:23:03.780
of the galaxy itself.

603
00:23:04.900 --> 00:23:07.660
The level of symmetry and stuff is

604
00:23:07.660 --> 00:23:10.340
really Unusual. Now, what it

605
00:23:10.340 --> 00:23:12.810
suggests is that, and it should be said

606
00:23:12.810 --> 00:23:15.090
nobody's absolutely certain how this object

607
00:23:15.090 --> 00:23:17.690
formed, but there are ready explanations

608
00:23:18.170 --> 00:23:20.890
that people have put forward. One which seems

609
00:23:20.890 --> 00:23:23.450
to have been shot down is that this is an

610
00:23:23.450 --> 00:23:26.440
extreme example of what's described as a

611
00:23:26.440 --> 00:23:29.010
bar, instability. So you get these barbed

612
00:23:29.010 --> 00:23:30.690
spiral galaxies where you get a big, long,

613
00:23:30.690 --> 00:23:32.570
straight central bar and then beautiful

614
00:23:32.570 --> 00:23:34.530
curved spiral arms coming off the end of the

615
00:23:34.530 --> 00:23:37.450
bar. You can almost imagine the curved arms

616
00:23:37.450 --> 00:23:39.130
joining up and then the bar disappearing. For

617
00:23:39.130 --> 00:23:40.250
some reason, you'd be left with something

618
00:23:40.250 --> 00:23:43.170
that almost looks like this. That seems to

619
00:23:43.170 --> 00:23:45.370
be very disfavored for this object

620
00:23:46.410 --> 00:23:49.170
because of the shape of the central blob. The

621
00:23:49.170 --> 00:23:51.690
central blob is fairly circular rather than

622
00:23:51.690 --> 00:23:54.690
an elongated. And bad spirals tend to have an

623
00:23:54.690 --> 00:23:56.850
elongated central blob. So that seems to rule

624
00:23:56.850 --> 00:23:59.050
that out. Although there is still debate,

625
00:23:59.530 --> 00:24:02.010
the explanation that would make most sense

626
00:24:02.330 --> 00:24:04.360
is that, this was the result of a collision

627
00:24:04.360 --> 00:24:06.320
between two galaxies at high speed, where

628
00:24:06.320 --> 00:24:08.800
you've almost got one galaxy punching through

629
00:24:08.800 --> 00:24:11.770
very near to the center of Hurd's object at,

630
00:24:11.720 --> 00:24:14.640
high speed, triggering this shockwave and

631
00:24:14.640 --> 00:24:17.440
then running off and vanishing. Now, given

632
00:24:17.440 --> 00:24:19.760
the scale of that ring, the suggestion will

633
00:24:19.760 --> 00:24:21.480
be that the collision happened about 3

634
00:24:22.000 --> 00:24:24.000
billion years ago. Billion with a B, not

635
00:24:24.000 --> 00:24:26.960
million with an M. The problem with this is

636
00:24:26.960 --> 00:24:29.080
that nobody can see any object. That would be

637
00:24:29.080 --> 00:24:31.960
the bullet seems m. That would

638
00:24:31.960 --> 00:24:34.720
have disappeared. However, 3 billion

639
00:24:34.720 --> 00:24:36.640
years is a lot of time for things to happen.

640
00:24:36.640 --> 00:24:38.480
So I think the suggestion that most people

641
00:24:38.480 --> 00:24:40.510
have is that, the bullet,

642
00:24:41.470 --> 00:24:43.590
has been lost in 3 billion years. There's

643
00:24:43.590 --> 00:24:45.480
also the fact that this is so, perfectly

644
00:24:45.480 --> 00:24:48.210
symmetrical, so perfectly face on,

645
00:24:48.530 --> 00:24:50.370
that you could possibly wonder whether the

646
00:24:50.370 --> 00:24:52.210
bullet is actually hidden behind that central

647
00:24:52.210 --> 00:24:54.450
blob. In other words, the bullet has moved

648
00:24:54.450 --> 00:24:56.010
perfectly along our line of sight and we

649
00:24:56.010 --> 00:24:57.810
don't see it because the galaxy's in the way.

650
00:24:57.890 --> 00:25:00.610
Yeah, that would make sense to me. And that

651
00:25:00.610 --> 00:25:02.689
sounds like it's vanishingly unlikely. But to

652
00:25:02.689 --> 00:25:04.650
have this thing be so symmetrical means it

653
00:25:04.650 --> 00:25:07.610
must be almost perfectly face on, which means

654
00:25:07.610 --> 00:25:09.370
that the impact that created the shockwave

655
00:25:09.370 --> 00:25:11.090
must have been quite close to our line of

656
00:25:11.090 --> 00:25:13.740
sight. And so I can see some logical

657
00:25:14.220 --> 00:25:17.020
consistency there. But this

658
00:25:17.020 --> 00:25:19.180
is, and I know I say this all the time, but

659
00:25:19.180 --> 00:25:21.420
this is a really good example of that

660
00:25:21.420 --> 00:25:23.340
interplay between theory and observation,

661
00:25:23.660 --> 00:25:25.380
where whenever we see something new, we

662
00:25:25.380 --> 00:25:27.020
struggle to explain it. We get a better

663
00:25:27.020 --> 00:25:28.820
understanding of how the universe works and

664
00:25:28.820 --> 00:25:30.660
what all the models should say and what they

665
00:25:30.660 --> 00:25:32.820
should tell us, and that improves our

666
00:25:32.820 --> 00:25:34.340
understanding of other objects, and then we

667
00:25:34.340 --> 00:25:35.740
find something else that pushes the

668
00:25:35.740 --> 00:25:38.100
boundaries of that knowledge. It is clear

669
00:25:38.100 --> 00:25:40.640
that we do not have a defined, definitive

670
00:25:40.720 --> 00:25:43.320
answer for this yet. And that's true for many

671
00:25:43.320 --> 00:25:45.880
of these objects in space. I think the

672
00:25:45.880 --> 00:25:48.440
argument of it being formed through a

673
00:25:48.440 --> 00:25:50.000
collision is

674
00:25:51.200 --> 00:25:54.080
fairly compelling, even though, you know,

675
00:25:54.160 --> 00:25:56.159
there are a lot of reasons why a typical

676
00:25:56.159 --> 00:25:57.840
formation will be unlikely. I'm just looking

677
00:25:57.840 --> 00:26:00.570
here at, the summary of this on Wikipedia, to

678
00:26:00.570 --> 00:26:02.210
be honest, and that's got links to a few

679
00:26:02.210 --> 00:26:05.130
papers that have discussed this. now I'm

680
00:26:05.130 --> 00:26:06.730
sure some of the listeners are probably

681
00:26:06.730 --> 00:26:08.290
recalling in shock at me looking on

682
00:26:08.290 --> 00:26:09.010
Wikipedia.

683
00:26:09.350 --> 00:26:11.950
Andrew Dunkley: No, no, we actually do have it quite a lot

684
00:26:11.950 --> 00:26:14.750
because there's m. Some very valid stuff in

685
00:26:14.750 --> 00:26:16.430
there. You just got to work your way through

686
00:26:16.430 --> 00:26:17.190
the garbage.

687
00:26:17.590 --> 00:26:19.900
Jonti Horner: Stress this a lot to my students that, you've

688
00:26:19.900 --> 00:26:21.380
been brought up through high school that

689
00:26:21.380 --> 00:26:23.660
Wikipedia is unreliable because it's an

690
00:26:23.660 --> 00:26:25.580
alterable resource and people can change it.

691
00:26:25.580 --> 00:26:27.740
And I've known friends who are teachers who

692
00:26:27.740 --> 00:26:30.700
set their class some really obscure

693
00:26:30.700 --> 00:26:32.740
topic to research and then deliberately go in

694
00:26:32.740 --> 00:26:34.780
and edit the Wikipedia page to say something

695
00:26:34.780 --> 00:26:36.830
wrong and then change it back after their

696
00:26:36.830 --> 00:26:38.550
class have done the work to demonstrate.

697
00:26:38.550 --> 00:26:39.350
Andrew Dunkley: You're kidding.

698
00:26:39.830 --> 00:26:41.750
Jonti Horner: Cruel, but entertaining.

699
00:26:43.070 --> 00:26:45.030
what I stress to my undergrad students is

700
00:26:45.030 --> 00:26:47.630
that for astronomy, Wikipedia is a good first

701
00:26:48.190 --> 00:26:50.590
look quite often. And that's because

702
00:26:51.069 --> 00:26:53.990
we have people around the world who

703
00:26:53.990 --> 00:26:56.230
are into astronomy, have a lot of knowledge,

704
00:26:56.230 --> 00:26:58.700
and tend to be very obsessive. And I mean

705
00:26:58.700 --> 00:27:00.780
that in a really positive light and have

706
00:27:00.780 --> 00:27:03.300
their favorite objects. And if something is

707
00:27:03.300 --> 00:27:05.060
wrong, they're not backward in coming forward

708
00:27:05.060 --> 00:27:07.740
at fixing it. Added to which, a lot of the

709
00:27:07.740 --> 00:27:09.660
astronomical sites are not controversial, and

710
00:27:09.660 --> 00:27:11.140
they're not the kind of places that

711
00:27:11.460 --> 00:27:13.620
youngsters who want to be a bit rebellious

712
00:27:13.620 --> 00:27:16.020
will go to to put something funny in. They're

713
00:27:16.020 --> 00:27:17.860
not going to be your prime targets for

714
00:27:18.180 --> 00:27:21.030
malfeasance, let's say. And, so what that

715
00:27:21.030 --> 00:27:22.990
combined means is that a lot of Wikipedia

716
00:27:22.990 --> 00:27:25.670
articles out there are, a relatively good

717
00:27:25.670 --> 00:27:27.710
first stab for astronomy topics. Now, they're

718
00:27:27.710 --> 00:27:30.500
not often spot on. You know, I found things

719
00:27:30.500 --> 00:27:33.380
that have not included my own research

720
00:27:33.700 --> 00:27:35.220
when they've been talking about a subject.

721
00:27:35.220 --> 00:27:37.210
And that makes me sad. and sometimes

722
00:27:37.210 --> 00:27:38.770
therefore give a different opinion to what

723
00:27:38.770 --> 00:27:41.010
I'd have. And so it's always a case of use it

724
00:27:41.010 --> 00:27:43.410
with caution, go to the primary

725
00:27:43.729 --> 00:27:46.690
resources. But Wikipedia is really good

726
00:27:46.690 --> 00:27:48.290
at pointing you to some of the primary

727
00:27:48.290 --> 00:27:51.210
resources to get a good feel for it. And so I

728
00:27:51.210 --> 00:27:53.330
actually find using Wikipedia is often for

729
00:27:53.330 --> 00:27:56.030
astronomy things fairly Reliable and

730
00:27:56.030 --> 00:27:58.030
fairly accurate. Because when an error creeps

731
00:27:58.030 --> 00:27:59.870
in, it is fixed very, very quickly.

732
00:27:59.950 --> 00:28:02.580
Andrew Dunkley: Yes, that's absolutely true. I actually have

733
00:28:02.580 --> 00:28:04.880
found it very helpful in the past. when I was

734
00:28:04.880 --> 00:28:07.340
writing, the book about my grandfather in

735
00:28:07.340 --> 00:28:10.220
World War I, researching the minute by

736
00:28:10.220 --> 00:28:13.180
minute processes of the particular battles

737
00:28:13.180 --> 00:28:13.940
that he was in,

738
00:28:16.280 --> 00:28:18.920
I found a lot of good data on Wikipedia.

739
00:28:18.920 --> 00:28:21.160
So, yeah, don't write it off unless you're

740
00:28:21.160 --> 00:28:21.520
doing it.

741
00:28:21.520 --> 00:28:24.390
Jonti Horner: It wouldn't surprise me. Yeah, the kind of

742
00:28:24.390 --> 00:28:26.750
community that would look into that kind of

743
00:28:26.750 --> 00:28:27.910
thing probably has a lot in common with

744
00:28:27.910 --> 00:28:29.470
astronomy and that they'd be very detail

745
00:28:29.470 --> 00:28:32.350
oriented, are very precise and

746
00:28:32.350 --> 00:28:34.110
very knowledgeable about their particular

747
00:28:34.190 --> 00:28:36.670
topic. But again, I'd have thought that it's

748
00:28:36.670 --> 00:28:38.220
unlikely that someone who wanted, to do

749
00:28:38.220 --> 00:28:40.620
something funny would go to the report of a

750
00:28:40.620 --> 00:28:42.820
particular world, one battle and edit it.

751
00:28:42.900 --> 00:28:44.660
They'd probably go to Taylor Swift's

752
00:28:44.660 --> 00:28:46.500
Wikipedia page. I suspect the editors for

753
00:28:46.500 --> 00:28:48.180
Taylor Swift's Wikipedia page have been

754
00:28:48.180 --> 00:28:50.980
working very hard. I'm sure they are.

755
00:28:52.010 --> 00:28:52.330
Andrew Dunkley: Yeah.

756
00:28:53.210 --> 00:28:55.050
Jonti Horner: So, yeah, it does not surprise me, is a

757
00:28:55.050 --> 00:28:56.090
fabulous resource.

758
00:28:56.250 --> 00:28:58.620
Andrew Dunkley: Yes, indeed. yeah, I think most of the people

759
00:28:58.620 --> 00:29:00.600
who want to stir things up go to, Facebook,

760
00:29:00.840 --> 00:29:03.600
Instagram or TikTok. That's, that's generally

761
00:29:03.600 --> 00:29:05.270
where it all ends up. thank you, Austin.

762
00:29:05.270 --> 00:29:07.320
Great question. And I, would encourage people

763
00:29:07.320 --> 00:29:09.760
to have a look at Hoag's object online

764
00:29:09.840 --> 00:29:11.920
because it is quite a spectacular

765
00:29:12.320 --> 00:29:15.080
galaxy. Well worth a look. Our final

766
00:29:15.080 --> 00:29:17.760
question today, Jonti, comes from Dan in

767
00:29:17.760 --> 00:29:20.550
California. he said, a

768
00:29:20.550 --> 00:29:22.750
new term, I heard the other day was the

769
00:29:22.750 --> 00:29:25.600
cosmic event horizon. could

770
00:29:25.600 --> 00:29:28.440
you talk about this a bit? Thanks, Dan. Yes,

771
00:29:28.440 --> 00:29:31.440
we, we. This is another one that's a bit of a

772
00:29:31.440 --> 00:29:33.720
mystery. I mean, it's a thing,

773
00:29:35.430 --> 00:29:37.670
it's well known. It's. It's not something

774
00:29:37.670 --> 00:29:40.310
that someone's sort of suggested may exist.

775
00:29:40.890 --> 00:29:43.620
the question is, you know, where, where

776
00:29:43.620 --> 00:29:46.440
is the, the limit of

777
00:29:46.440 --> 00:29:49.070
this object? it's not really an object. It's

778
00:29:49.070 --> 00:29:50.430
a status, I suppose.

779
00:29:50.510 --> 00:29:53.390
Jonti Horner: Yes, yes, it's an interesting one.

780
00:29:53.470 --> 00:29:56.110
And more generally,

781
00:29:56.270 --> 00:29:57.790
an event horizon

782
00:29:59.070 --> 00:30:01.710
is a line that marks the

783
00:30:01.710 --> 00:30:03.710
boundary between things we can observe and

784
00:30:03.710 --> 00:30:06.070
things we cannot observe in the simplest

785
00:30:06.070 --> 00:30:08.830
possible terms. So the event horizon of a

786
00:30:08.830 --> 00:30:11.030
black hole. Anything outside the event

787
00:30:11.030 --> 00:30:13.850
horizon we can see happening. Anything

788
00:30:14.410 --> 00:30:16.730
interior to the event horizon we can't see

789
00:30:16.730 --> 00:30:19.010
because light can't escape. So the event

790
00:30:19.010 --> 00:30:21.010
horizon in that sense marks a boundary

791
00:30:21.010 --> 00:30:22.890
between what is observable and what is not.

792
00:30:23.290 --> 00:30:25.650
And that has led people to this concept of

793
00:30:25.650 --> 00:30:27.970
the cosmic event horizon or cosmological

794
00:30:27.970 --> 00:30:30.450
Event horizon. And when you look into it,

795
00:30:30.450 --> 00:30:32.090
there's actually two different definitions,

796
00:30:33.050 --> 00:30:35.770
two different types. One is

797
00:30:36.090 --> 00:30:38.490
effectively the maximum distance,

798
00:30:39.050 --> 00:30:41.570
at which a source can be and have

799
00:30:41.570 --> 00:30:43.810
emitted light in the past that we can see

800
00:30:43.810 --> 00:30:46.730
today. and that distance is

801
00:30:46.730 --> 00:30:49.610
smaller than the real extent of the universe.

802
00:30:49.610 --> 00:30:51.650
It's kind of set by the point of the

803
00:30:51.650 --> 00:30:53.290
cosmological microwave background, in

804
00:30:53.290 --> 00:30:55.050
actuality, because before that, the universe

805
00:30:55.050 --> 00:30:57.330
was foggy and the light couldn't escape.

806
00:30:57.970 --> 00:31:00.930
Now the universe is very roughly 14 billion

807
00:31:00.930 --> 00:31:02.490
years old. I know the numbers are more

808
00:31:02.490 --> 00:31:04.170
accurately than that, but we'll call it 14

809
00:31:04.170 --> 00:31:07.110
billion billion years. So you're. And

810
00:31:07.110 --> 00:31:09.350
certainly my naive expectation would be that

811
00:31:09.350 --> 00:31:12.110
the most distant thing we can see is 14

812
00:31:12.110 --> 00:31:14.910
billion light years away. And that

813
00:31:14.990 --> 00:31:17.550
is true for a given version of true.

814
00:31:18.110 --> 00:31:20.920
What it actually is, the case is that, the

815
00:31:20.920 --> 00:31:23.880
things we see that are 14 billion

816
00:31:23.880 --> 00:31:26.520
light years away, we're seeing,

817
00:31:26.920 --> 00:31:28.920
and I've got to word this very carefully,

818
00:31:29.240 --> 00:31:31.940
we're seeing where they were 14

819
00:31:32.100 --> 00:31:34.860
billion years ago, and the light

820
00:31:34.860 --> 00:31:37.100
has traveled from that point for 14 billion

821
00:31:37.100 --> 00:31:40.020
years to reach us today. So we see them

822
00:31:40.340 --> 00:31:43.180
14 billion years ago at the distance

823
00:31:43.180 --> 00:31:46.020
of 14 billion light years, but they are

824
00:31:46.020 --> 00:31:48.060
heavily redshifted. They're moving away from

825
00:31:48.060 --> 00:31:50.900
us. So the objects that emitted that light 14

826
00:31:50.900 --> 00:31:53.780
billion light years 14 billion years ago

827
00:31:54.180 --> 00:31:56.270
at, a distance of 14 billion light years,

828
00:31:56.840 --> 00:31:59.640
would now be 47 billion light years

829
00:31:59.640 --> 00:32:02.280
away. And light they emit now would never

830
00:32:02.280 --> 00:32:03.840
reach us because of the expansion of the

831
00:32:03.840 --> 00:32:04.240
universe.

832
00:32:04.240 --> 00:32:04.760
Andrew Dunkley: Yep.

833
00:32:05.000 --> 00:32:07.640
Jonti Horner: So what that means is that one sense of the

834
00:32:07.640 --> 00:32:10.520
cosmic event horizon is

835
00:32:10.520 --> 00:32:13.040
that we can see objects that can be as

836
00:32:13.040 --> 00:32:15.640
distant from us today as 47 billion

837
00:32:16.120 --> 00:32:18.960
light years in any direction. But

838
00:32:18.960 --> 00:32:21.800
we see them as they were 14 billion years

839
00:32:21.800 --> 00:32:24.770
in the past, when they were only 14 billion

840
00:32:24.770 --> 00:32:27.290
light years away from us, and they've moved

841
00:32:27.290 --> 00:32:30.290
away since. So that is the event horizon in

842
00:32:30.290 --> 00:32:32.930
terms of events that we can see today that

843
00:32:32.930 --> 00:32:35.850
happened in the past. The other

844
00:32:35.850 --> 00:32:38.130
version, which makes my head hurt slightly

845
00:32:38.130 --> 00:32:40.970
more, to be honest, is the concept in

846
00:32:40.970 --> 00:32:43.370
cosmology that there is an event

847
00:32:43.450 --> 00:32:46.170
horizon which is the

848
00:32:46.170 --> 00:32:48.770
most distant objects that if they

849
00:32:48.770 --> 00:32:51.750
emitted a photon of light today, could ever

850
00:32:51.750 --> 00:32:53.550
be seen from the Earth in the future.

851
00:32:54.910 --> 00:32:57.710
Now they're moving away as well. Now, this

852
00:32:57.710 --> 00:32:59.510
seems to be well defined, and there's maths

853
00:32:59.510 --> 00:33:01.390
around it, and people have discussed it, that

854
00:33:01.390 --> 00:33:03.710
because of the event horizon. Sorry, because

855
00:33:03.710 --> 00:33:06.150
of the expansion of the universe, you can't

856
00:33:06.150 --> 00:33:09.070
have an object that's arbitrarily far away

857
00:33:09.230 --> 00:33:11.350
emit a photon of light and expect that it

858
00:33:11.350 --> 00:33:13.710
would ever reach us, because the expansion of

859
00:33:13.710 --> 00:33:15.790
the universe is such that before that photon

860
00:33:15.790 --> 00:33:18.260
of light reaches us, we will be moving away

861
00:33:18.260 --> 00:33:20.540
from it at a speed faster than the speed of

862
00:33:20.540 --> 00:33:22.780
light. And so it can never catch us up. It's

863
00:33:22.780 --> 00:33:25.420
a bit like the hare and the tortoise. And,

864
00:33:25.660 --> 00:33:28.340
you know, at some point the hair is going so

865
00:33:28.340 --> 00:33:30.060
quickly that if you throw ping pong balls

866
00:33:30.060 --> 00:33:31.900
after it, they won't catch it up because it's

867
00:33:31.900 --> 00:33:33.260
going quicker than the speed of a ping pong

868
00:33:33.260 --> 00:33:35.460
ball. Yeah. Hugely mixed metaphor, but you

869
00:33:35.460 --> 00:33:38.300
can kind of see what I mean there. Now, this

870
00:33:38.300 --> 00:33:40.660
is fairly well defined as a concept. It's

871
00:33:40.660 --> 00:33:43.519
this idea that objects far

872
00:33:43.519 --> 00:33:45.639
away from us emit light towards us, but

873
00:33:45.639 --> 00:33:47.239
they're receding and they're receding from us

874
00:33:47.239 --> 00:33:49.370
at an ever increasing speed. And, we're

875
00:33:49.370 --> 00:33:51.490
receding from them at an ever increasing

876
00:33:51.490 --> 00:33:53.970
speed because the expansion of the universe

877
00:33:53.970 --> 00:33:55.330
doesn't matter where you are, it's all

878
00:33:55.330 --> 00:33:57.450
expanding. Yeah. And so there must be a

879
00:33:57.450 --> 00:34:00.050
horizon at some point, at some distance from

880
00:34:00.050 --> 00:34:02.850
us, where an object at that distance today

881
00:34:03.410 --> 00:34:05.890
would emit a photon of light and that light

882
00:34:05.890 --> 00:34:08.010
would never reach us because of the

883
00:34:08.010 --> 00:34:10.880
expansion. I cannot find

884
00:34:10.960 --> 00:34:13.720
a definitive number for that size

885
00:34:13.720 --> 00:34:15.600
anywhere. I've looked around.

886
00:34:16.560 --> 00:34:18.440
There's a lot of mathematical equations that

887
00:34:18.440 --> 00:34:20.040
people use to quantify it. But the reason

888
00:34:20.040 --> 00:34:21.920
that we don't have a definitive number for

889
00:34:21.920 --> 00:34:24.839
that is that there are many models that look

890
00:34:24.839 --> 00:34:26.440
at the expansion of the universe into the

891
00:34:26.440 --> 00:34:28.960
future that all have very slightly different

892
00:34:29.040 --> 00:34:31.720
expansion rates going into the future. And

893
00:34:31.720 --> 00:34:33.560
different expansion rates would move this

894
00:34:33.560 --> 00:34:35.440
event horizon to different distances.

895
00:34:36.751 --> 00:34:39.471
If the universe is expanding quicker, then

896
00:34:39.471 --> 00:34:41.431
that event horizon will get nearer to us

897
00:34:41.431 --> 00:34:43.351
because you'd have to be closer to us to

898
00:34:43.351 --> 00:34:45.831
overcome this barrier of the expansion. If

899
00:34:45.831 --> 00:34:47.951
the expansion is a bit slower in the future,

900
00:34:47.951 --> 00:34:50.591
the event horizon will be further away. If

901
00:34:50.591 --> 00:34:53.591
you go again, big up to the Wikipedia page.

902
00:34:53.591 --> 00:34:55.391
If you go to the Wikipedia page on Event

903
00:34:55.391 --> 00:34:57.791
Horizon and go down to the article

904
00:34:58.271 --> 00:35:00.991
heading Cosmic Event Horizon, there is a

905
00:35:00.991 --> 00:35:02.871
figure on the right that shows the reachable

906
00:35:02.871 --> 00:35:04.991
universe as a function of time and distance

907
00:35:05.541 --> 00:35:08.501
in the context of the expanding universe that

908
00:35:08.581 --> 00:35:11.501
has lots of different things on it. And it

909
00:35:11.501 --> 00:35:13.581
seems to suggest that if light were emitted

910
00:35:13.581 --> 00:35:16.461
from the Milky Way galaxy now, right now, if

911
00:35:16.461 --> 00:35:19.341
we shone a laser up into the sky, the most

912
00:35:19.341 --> 00:35:22.221
distant object from us that could

913
00:35:22.221 --> 00:35:24.981
ever reach is at 26

914
00:35:25.301 --> 00:35:28.301
billion light years from the edge of the

915
00:35:28.301 --> 00:35:31.271
Big Bang. that's not the same

916
00:35:31.271 --> 00:35:33.431
as saying 25 billion light years from us.

917
00:35:33.591 --> 00:35:35.831
This is where it all gets really, really,

918
00:35:36.391 --> 00:35:38.911
really confusing. They've got these light,

919
00:35:38.911 --> 00:35:41.671
light curves and things like this light ray

920
00:35:41.671 --> 00:35:43.911
emitted at 13 gig years from now

921
00:35:44.391 --> 00:35:46.510
would reach further out. So that figure

922
00:35:46.510 --> 00:35:49.331
suggests that, event

923
00:35:49.331 --> 00:35:51.651
horizon is something like 13 or 14 billion

924
00:35:51.651 --> 00:35:54.411
light years from us right now. If we

925
00:35:54.411 --> 00:35:56.781
emitted that light now, something further

926
00:35:56.781 --> 00:35:59.581
away from that than that could never see us.

927
00:35:59.581 --> 00:36:01.901
Yeah, but it's all up in the air. I can't,

928
00:36:01.901 --> 00:36:03.901
like I said, find an exact answer. And I

929
00:36:03.901 --> 00:36:05.421
think the reason I can't find an exact

930
00:36:05.421 --> 00:36:08.341
calculated distance is if you even

931
00:36:08.341 --> 00:36:10.581
vary the expansion rate of the universe by a

932
00:36:10.581 --> 00:36:13.101
very small amount, you change the location of

933
00:36:13.101 --> 00:36:15.181
that event horizon by a very large distance.

934
00:36:15.501 --> 00:36:17.181
So it's just hugely uncertain. So at the

935
00:36:17.181 --> 00:36:19.461
minute it remains a kind of theoretical

936
00:36:19.461 --> 00:36:21.951
conceit, a philosophical concept, but one

937
00:36:21.951 --> 00:36:24.911
that is important in us actualizing,

938
00:36:24.911 --> 00:36:27.591
in terms of. In us conceptualizing, I

939
00:36:27.591 --> 00:36:30.591
guess, the idea that no matter how far in

940
00:36:30.591 --> 00:36:32.911
the future or the past you go, there will

941
00:36:32.911 --> 00:36:34.711
never have been a time when the entire

942
00:36:34.711 --> 00:36:37.591
universe is visible from here. So it's a bit

943
00:36:37.591 --> 00:36:39.501
like walking around on the surface of the,

944
00:36:39.451 --> 00:36:40.861
Earth. No matter where you are on the, Earth,

945
00:36:40.861 --> 00:36:42.581
you cannot see the whole of our planet unless

946
00:36:42.581 --> 00:36:44.741
you look at photos. Because from your

947
00:36:44.741 --> 00:36:47.581
location, no matter how high above sea level

948
00:36:47.581 --> 00:36:49.451
you are, there is always some of the Earth

949
00:36:49.451 --> 00:36:52.331
heat you cannot see even when you're in

950
00:36:52.331 --> 00:36:54.211
space. It's absolutely.

951
00:36:54.211 --> 00:36:56.211
Andrew Dunkley: There's always another side to it. That's.

952
00:36:56.771 --> 00:36:59.411
Jonti Horner: So when we get. It makes people's head hurt

953
00:36:59.411 --> 00:37:02.331
hugely, the idea that the universe can be

954
00:37:02.331 --> 00:37:04.490
infinite and finite at the same time, that we

955
00:37:04.490 --> 00:37:06.251
only see a small fraction of the universe

956
00:37:06.251 --> 00:37:09.091
that's out there. But to me, it does

957
00:37:09.091 --> 00:37:10.571
make sense when you think about it in the

958
00:37:10.571 --> 00:37:12.291
context of the Earth. I look out of my window

959
00:37:12.291 --> 00:37:13.771
here and I can see the beautiful bunny

960
00:37:13.771 --> 00:37:16.001
mountains in the distance. They're about 60

961
00:37:16.001 --> 00:37:17.641
kilometers away, something like that.

962
00:37:18.841 --> 00:37:20.521
That's a long way away. I can see a large

963
00:37:20.521 --> 00:37:22.121
part of the Earth's surface, but that's a

964
00:37:22.121 --> 00:37:24.881
trivially small amount of the Earth. I'm

965
00:37:24.881 --> 00:37:26.951
aware at this instant of a pool of, Earth

966
00:37:26.951 --> 00:37:29.471
around me. And in all honesty, the rest of

967
00:37:29.471 --> 00:37:30.710
the Earth could have disappeared and I

968
00:37:30.710 --> 00:37:32.071
wouldn't know now. I mean, obviously we're

969
00:37:32.071 --> 00:37:34.311
still on our connection, so that hasn't

970
00:37:34.311 --> 00:37:37.231
happened. But we can visualize in

971
00:37:37.231 --> 00:37:39.751
that sense that the whole of something can be

972
00:37:39.751 --> 00:37:41.931
bigger than the fraction that we see. The

973
00:37:41.931 --> 00:37:43.171
whole of the Earth is bigger than the

974
00:37:43.171 --> 00:37:44.411
fraction of the Earth that you can see from

975
00:37:44.411 --> 00:37:47.291
any one location. The universe is a bit like

976
00:37:47.291 --> 00:37:48.931
that. And this is another way of discussing

977
00:37:48.931 --> 00:37:51.011
that, where it's discussing the maximum

978
00:37:51.011 --> 00:37:52.491
extent that you could see in the past, the

979
00:37:52.491 --> 00:37:54.011
maximum extent that you could see in the

980
00:37:54.011 --> 00:37:56.851
future. Yeah, so hopefully that makes a

981
00:37:56.851 --> 00:37:59.831
bit of sense, and gives you some direction.

982
00:37:59.831 --> 00:38:01.351
If you want to read more about it, I guess.

983
00:38:01.351 --> 00:38:03.301
Andrew Dunkley: Yes, yes, it's, it's when you do

984
00:38:04.021 --> 00:38:06.451
research on it, it yeah, you come up with all

985
00:38:06.451 --> 00:38:09.061
sorts of theories about the universe. Is it

986
00:38:09.061 --> 00:38:12.011
infinite, is it finite? But you know, beyond

987
00:38:12.011 --> 00:38:14.651
our capability to see because of its

988
00:38:14.811 --> 00:38:17.331
expanding rate and you know, the position of

989
00:38:17.331 --> 00:38:19.611
objects emitting light that could not reach

990
00:38:19.611 --> 00:38:22.291
us because of this, that, it just goes on.

991
00:38:22.291 --> 00:38:22.971
Jonti Horner: And on and on.

992
00:38:22.971 --> 00:38:25.731
Andrew Dunkley: It's, it is really interesting and it, it

993
00:38:25.731 --> 00:38:28.481
does get your, your mind swimming. but the

994
00:38:28.481 --> 00:38:31.311
bottom line is that the objects

995
00:38:31.311 --> 00:38:33.791
behind, beyond the cosmic event horizon,

996
00:38:34.721 --> 00:38:36.881
they just, there's just not enough time for

997
00:38:36.881 --> 00:38:39.261
that light to ever reach Earth. that's the

998
00:38:39.261 --> 00:38:42.061
bottom line, isn't it? So hopefully that

999
00:38:42.061 --> 00:38:43.741
explains it for you. Dan, thanks for the

1000
00:38:43.741 --> 00:38:46.701
question. It's been great. A couple of real

1001
00:38:46.701 --> 00:38:49.171
thought provoking questions today. Much

1002
00:38:49.171 --> 00:38:51.451
appreciated. And if you do have a question

1003
00:38:51.451 --> 00:38:53.931
for us, please send it through via our

1004
00:38:53.931 --> 00:38:56.341
website. we've got a new batch of audio

1005
00:38:56.341 --> 00:38:58.571
questions, two thirds of which come from one

1006
00:38:58.571 --> 00:39:00.971
person. But that's okay.

1007
00:39:01.061 --> 00:39:03.401
we'll get through those, but we, we do need

1008
00:39:03.401 --> 00:39:05.711
more. if you've ever considered sending a

1009
00:39:05.711 --> 00:39:07.271
question, just never got around to it, jump

1010
00:39:07.271 --> 00:39:09.649
on our website, Space Nuts SpaceNutspodcast

1011
00:39:09.813 --> 00:39:11.991
uh.com spacenuts IO

1012
00:39:12.551 --> 00:39:14.991
and click on the AMA link at the top and send

1013
00:39:14.991 --> 00:39:17.511
us text or audio questions. That away

1014
00:39:17.671 --> 00:39:19.831
and we look forward to hearing from you.

1015
00:39:19.831 --> 00:39:21.991
Don't forget to tell us who you are and where

1016
00:39:21.991 --> 00:39:24.961
you are from. And thanks to all our

1017
00:39:24.961 --> 00:39:26.871
patrons too. I don't thank you enough. these

1018
00:39:26.871 --> 00:39:29.311
are the people who enjoy the program and

1019
00:39:29.311 --> 00:39:31.771
pitch in with a couple of dollars here or

1020
00:39:31.771 --> 00:39:34.661
there to keep us afloat. you are amazing

1021
00:39:34.661 --> 00:39:36.571
people. We never asked for that. But we

1022
00:39:36.571 --> 00:39:38.331
certainly appreciate your support and if

1023
00:39:38.331 --> 00:39:40.531
you'd like to become a patron, you can find

1024
00:39:40.531 --> 00:39:43.211
out more on our website. Not mandatory,

1025
00:39:43.211 --> 00:39:45.981
but certainly appreciate it. Jonti,

1026
00:39:45.981 --> 00:39:47.821
you're appreciated too. Thank you very much.

1027
00:39:48.461 --> 00:39:49.901
Jonti Horner: That's absolute pleasure. Thank you for

1028
00:39:49.901 --> 00:39:51.461
having me. And yeah, the more questions the

1029
00:39:51.461 --> 00:39:51.741
better.

1030
00:39:51.741 --> 00:39:54.021
Andrew Dunkley: Yes, they're good fun. It's a great segment.

1031
00:39:54.021 --> 00:39:55.781
I'm glad it's developed into that. It used to

1032
00:39:55.781 --> 00:39:57.771
just be something that we tacked onto the end

1033
00:39:57.771 --> 00:39:59.691
of one episode, but we've made it its own

1034
00:39:59.691 --> 00:40:01.691
show. It's become bigger than Ben Hur,

1035
00:40:01.691 --> 00:40:04.411
really. and I'm wearing a wristwatch in the

1036
00:40:04.411 --> 00:40:06.491
scene as well. Some people will understand

1037
00:40:06.571 --> 00:40:08.861
that. and, yeah, thanks, Jonti. We'll catch

1038
00:40:08.861 --> 00:40:10.741
you soon. Jonti Horner, professor of

1039
00:40:10.741 --> 00:40:12.461
Astrophysics at the University of Southern

1040
00:40:12.461 --> 00:40:14.021
Queensland. Also, thanks to Huw in the

1041
00:40:14.021 --> 00:40:16.141
studio, who couldn't be with us, he's taken a

1042
00:40:16.141 --> 00:40:18.781
holiday beyond the cosmic event horizon.

1043
00:40:19.021 --> 00:40:21.111
So, we can't see his light, but I'm sure

1044
00:40:21.111 --> 00:40:23.051
it'll return and he can tell us how he got

1045
00:40:23.051 --> 00:40:25.931
there when he gets back in about 47 billion

1046
00:40:25.931 --> 00:40:28.051
years. And from me, Andrew Dunkley, thanks

1047
00:40:28.051 --> 00:40:29.291
for your company. Catch you on the next

1048
00:40:29.291 --> 00:40:31.651
episode of Space Nuts. Bye. Bye.

1049
00:40:32.851 --> 00:40:35.051
Jonti Horner: You'll be listening to the Space Nuts

1050
00:40:35.051 --> 00:40:37.651
podcast, available

1051
00:40:37.811 --> 00:40:40.051
at Apple Podcasts, Spotify,

1052
00:40:40.211 --> 00:40:42.971
iHeartRadio, or your favorite podcast

1053
00:40:42.971 --> 00:40:44.731
player. You can also stream on

1054
00:40:44.731 --> 00:40:46.481
demand@bytes.com M.

1055
00:40:46.691 --> 00:40:48.771
Andrew Dunkley: This has been another quality podcast

1056
00:40:48.771 --> 00:40:50.581
production from bytes.com.