The Supermassive Podcast - 39: BONUS - Do black holes stop time?

Episode Date: April 7, 2023

Are gravitational waves destructive?  Will the universe ever expand faster than the speed of light? Should we only send robots to space? This month, Izzie Clarke, Dr Becky Smethurst and Dr Robert Massey take on the BIG questions in The Supermassive Mailbox. Want to support The Supermassive Podcast? Why not buy our book The Year In Space - https://geni.us/jNcrw The Supermassive Podcast from the Royal Astronomical Society is a Boffin Media Production. The producers are Izzie Clarke and Richard Hollingham. 

Hello and welcome to another bonus episode of the Supermassive podcast from the Royal Astronomical Society with me, science journalist Izzy Clark and astrophysicist Dr Becky Smethurst. Thank you to everyone who has sent lovely messages that they're enjoying the extra episodes from us. One person unfortunately didn't get the memo and left a one star review that made us sad that said the latest episode is 14 minutes. Kind of stupid. So to explain again, these are intentionally shorter bonus episodes

where we dive into the super massive mailbox to take on more of your questions that we just don't have time to cover in the main show. Yeah, but by creating these episodes, we're now getting more emails and more questions. So like the universe, our mailbox is just expanding and I don't think we'll ever get to the end of it. I think that's all that we've created here. Yeah, I love that parallel though. That's something we need to keep going, keep sending in questions. Yeah, yeah, fine. So right, on to the questions. Becky, can you help with this one from Pre

Drummond? They say, dear Supermassive team, you guys are questions. Becky, can you help with this one from Pre Drummond? They say, dear Supermassive team, you guys are brilliant. Thank you very much. Thank you for the amazing podcast. I enjoy every single episode and it's helped to understand or momentarily think I understand so much. Just one simple question. What's the deal with black holes stopping time?

Would anybody care to explain? Talk to me like I'm five years old, please. Many thanks, Pri Drammond. What a brilliant email. So good. I love that. Also, I love that just one simple question. It might have been McFresh simply, Pri,

but I don't think it has the most simple answers, but I will try. So black holes don't stop time. They only appear to do that for somebody watching from the outside. It is a little bit of an illusion that they create that's caused by the black hole's immense gravity. So Einstein's theory of general relativity, which is our best theory we have to describe gravity, says that heavy objects curve space-time. So they curve space itself, and anything moving on that curved path is essentially the effect of gravity.

But the key word there was space-time. The four dimensions, the three dimensions of space, and one dimension of time that are all tied together. So a heavy object also affects time. I'm hoping I've still got you. I'm'm following i'm following let's keep going yeah so um time slows down around heavier objects and that's true for earth too like the earth itself has an effect on time so for example the closer you are to the earth's like surface so sea level the slower time is going. So you actually experience one billionth of a second less every year at sea level than someone does if they were living at the top of Everest, for example. So it's nowhere near as extreme as for a black hole, but it's still less time. And that's been proven with atomic clocks, you know, aboard planes and

the International Space Station as well, like many times like many times now of course black holes take everything to the extreme so time starts to slow so much as you get closer and closer and closer to a black hole that at the event horizon that point of no return it appears to you as someone watching it that time has stopped but for something you know falling towards the black hole it will have just crossed that event event horizon no bother right but more time will have passed for you than they think has actually passed right okay so essentially if you're watching something fall towards a black hole you will be waiting for infinity to finally see them cross the event horizon when in fact they actually have and obviously it also comes back to this idea that obviously you can't get light from beyond the event horizon either so you're waiting around for

a signal for so long as well and it all ties together but for someone actually falling towards a black hole they experience time what they think is normally and they will just cross the event horizon fine but for you watching it it's an illusion to think that time has stopped on that event horizon. Well, Pri, I hope that answers your question. I mean, Becky's written books on this, maybe. But you've done that in about three minutes. I think that's good going. Pri can be the judge of that. Yeah.

Robert, Brian Ross has emailed with a follow-up to February's episode on gravitational waves. First off, I love your podcast and I look forward to it every month and now more often with the bonus episodes. Now for the questions. That's great news. Are gravitational waves destructive? By that, do I mean if an object was close enough to a black hole merger where the amplitude of the gravitational waves was much stronger, would that overwhelm the gravitational or chemical binding energy of a planetoid asteroid person etc in a similar way to how the gravity well near a stellar mass black hole spaghettifies matter take a breath in other words would a significantly powerful gravitational wave shred matter or would that matter simply flex and stretch with the bending of space-time itself?

Well, Brian, I think that definitely comes into the simple question category again, doesn't it? Yeah, this really got me thinking. I was thinking, you know, surely there must be an effect if they're really close. So reading around on this, as far as I can tell, if you're close enough that you could actually feel the gravitational waves because remember the detections we make a tiny fractions of the the size of an atom is the shift in the detectors really minute even though they're from incredibly powerful events that is very very far away if you were close enough for a significant shift to happen i think you'd be worrying about other things so for example if you had black holes merging uh the the catastrophic

effect of that on the earth's say would overwhelm any worry about the gravitational waves you know huge blast of gamma rays would deal with this very very quickly and richard feynman is the you know the famous physicist he he thought about this when they were thinking about the idea of detecting gravitational waves he thought well you know do they interact with this could we could would they have some kind of physical effect and he had this idea of this sticky bead system where gravitational waves would move beads on rods in one direction and not another. And if there was friction, then that would obviously generate heat. So there would be this linkage through. But for that to be really significant, you're probably looking at something going on in the solar system. And then you could get, say, a very rapid tidal change on the Earth. Now, it might only be, you know, maybe a metre or so, but if you think about that happening on a very quick timescale, that would cause quite a lot of damage to the Earth.

However, I think the overall effect of the black hole merging nearby would overwhelm that. You know, the gravitational waves would be just an incidental thing in the midst of the carnage and chaos that was going on. And then there was this quirky example of a couple of physicists, Francis Pretorius and william east who wrote a paper thinking about exotic combinations of waves curving wave in a flat plane fronted wave and they worked out in certain circumstances that could make a black hole but they did conclude it was very very unlikely so on the list of things i worry about

being ripped apart by gravitational waves is a long way down. Not even being spaghettified. Yeah, we're not adding that to our science fiction list then. Well, you could. Okay, so Becky, there's one for you now. So Yandri from Rhode Island asks, Greetings. I wanted to know, since the expansion of the universe is constantly accelerating and nothing can travel faster than the speed of light, will the universe reach a point where it expands faster than the speed of light?

If so, what would happen? Would the light from faraway galaxies never reach us? By the way, love the podcast. Oh, thanks. So, Yandri, the universe technically already is expanding faster than the speed of light. If we measure it over a huge distance, right, and we sort of take, okay, the distance to this galaxy in this direction and the distance to this galaxy in the opposite direction, and sort of do a very naive sort of speed equals distance over time calculation, then the speed you get out is greater than the speed

of light. Now, we've got to remember the speed limit of the speed of light from Einstein's special relativity theory is for objects moving through space. So no object moving through space can travel faster than the speed of light. But the thing is, the galaxies that we use to actually measure this rate of expansion, they're not the ones moving. You know, the galaxies aren't moving away from us. It's that the space between us and those galaxies is expanding, like space itself is stretching. And you can't really measure that as a speed because your unit distance is always changing so there is no upper bound on the rate expansion of the universe and then as for not sort of being able to see galaxies eventually also

that's not the case because you have to remember we're seeing the light as it was when it was emitted from those galaxies like what's a 13 years ago. And we're seeing them as they were, you know, when the universe was just 800 million years old, let's say. And so the light has been traveling since then. But now those galaxies, because of the expansion, are something like 45 billion light years away. And we'll never receive, you know, the light from those until the universe is 45 billion years old the universe has been around long enough for the light to reach us so we're always waiting for that light anyway even if they are sort of further away than they are now we'll only ever see the observable universe and even if the universe is appearing to expand faster than the speed of light it's still we're

just waiting around for the light itself okay everyone's come with the big questions this month like yeah you know well done everyone they have been paying attention okay robert david holmes emailed to ask as a child of the gemini and apollo era one who devoured every possible second of coverage it goes against the brain to suggest that the focus should be on remote exploration rather than on human perhaps the only exception could be lunar exploration but it strikes me that both the constraints of the human body in relation to long-term space travel plus certainly currently the problem of protecting humans in space from radiation and lastly and simply the impossibility of the time taken given physics for humans to get anywhere

apart from the moon or possibly mars surely means that the focus should be on remote exploration rather than waste time and money on trying to make human exploration work what do we think of that this is more of a suddenly we're now on a debating podcast uh it's definitely an opinion piece here isn't it yeah i mean look uh it's ongoing debate i mean and there was a recent book by uh martin reese and donald gosmith called the end of astronauts discussing exactly this and highlighting all those issues i think i suppose that the big change has been that over the last few decades we've seen huge improvement in in some robotic spacecraft because it used to be said it was very difficult to get robots to do what people can.

And that is getting better and better in time with AI and so on. It's getting easier to design robots that can do those things. Obviously, also, if you send a robotic probe, it's cheaper. And if it goes wrong, nobody's died. We're a bit miffed. We're going to struggle to get the budget back again to restore the mission and all of those things. But fundamentally, no one dies. So big ethical question there from the outset. You've then got things like, well, you know, I think the sort of myth that we can somehow relocate to Mars because we've messed up the Earth is just a non-starter. You know, you can't get anything like a meaningful number of people there to do it. And if you did, you would make the problems on Earth even worse along the way because of all the resources and pollution and everything you generate. So to my mind, I guess I'm not really, you know, I like human space exploration because I think it's really exciting.

You know, and let's face it, if we see astronauts on the space station, it's a lot of fun, let alone people going back to the moon as we'd like you to do in the next few years. Whether that's the best way to do the science now is a really good question. I suppose there's still the fact that human explorers on the surface of the moon, for example, will be that bit more creative and imaginative and quick to react than robotic spacecraft and probes are. But that difference has narrowed a lot since the beginning of the space age. So if I had to answer that, I'd say I think it'll be quite fun to see people back on the moon. Is that a good answer for doing it? I don't know. But I will look forward to it. When it happens, I think it'll be incredibly exciting and inspiring. Perhaps maybe the reason for doing it, if there is one, is the inspiration. Not that people imagine they can become astronauts,

but that they just think, wow, this is an incredibly exciting thing for human beings to do. If it gets quicker to get to places like Mars, like if we could go there in a couple of weeks rather than being this enormously complicated two-year round trip, I think we'd be on our way to doing it. But even that at the moment still seems quite a remote thing to me. Whatever the announcement from almost every US president says that we'll be there in 20 years i've been watching those since about 1980 and it's always 20 years away so we'll see but you know irrespective of the risks and all the rest of it i think we'll all be excited when people do go back to the moon yeah fair enough very good answer well thank you everyone and do keep the questions coming you can email podcast at ras.ac.uk tweet at royal astro sock and we're

also on instagram at supermassive pod we'll be back in a few weeks time with an episode about saturn and i'm so excited i mean you're already bouncing off the wall we haven't even got there yet i'm so excited i'm like who are the experts who are we going to talk to I'm like Becky do you want to produce this episode like you can make this one so excited but until then

everybody and make sure you tune in for that one happy stargazing