Mind-Blowing Lessons About 'The End of Everything'

Cosmologist Dr. Katie Mack contemplates the doom of the universe with a refreshing giddiness.

Note: Text has been edited and may not match audio exactly. 

Emily Cox: Hi, I'm Emily Cox, one of the Audible editors, and today I have the enormous pleasure of speaking with Dr. Katie Mack, a cosmologist and professor of physics at North Carolina State University, and the author of a new book, The End of Everything (Astrophysically Speaking). Welcome, Katie.

Katie Mack: Thanks, thanks for having me here.

EC: I'm so excited to be talking to you. I have to say, this is the first time I've interviewed a scientist. I tend to speak to novelists or genre fiction writers, and I think I'm feeling a little out of my depth, but I am also super jazzed to be speaking to you. And I'll admit, you're probably one of my top three favorite Twitter follows.

KM: Oh, that's very sweet, thank you.

EC: That's how I got the bug, because I've been following you on Twitter, and then when the publishers presented this book, I was like, "I need to read that." For anyone who's listening, I want to advise you to go follow @AstroKatie. You will learn a lot. So, there's a lot I want to talk to you about in your book, but first I just wanted to hear a little bit about how you got into your field of work. You don't talk a ton about your background in this work, and so I'd love to hear a bit about how you got into doing what you do.

KM: So I've wanted to be a scientist since I was very young, pretty much as long as I can remember. I was the kind of kid who would tinker with things, take things apart, put them back together, try to figure out how it all works, and that just led me into other areas of science, and then I started to read books about black holes and time travel and the Big Bang and all that kind of stuff, and I thought that that was the coolest, most mind-bending kind of thing to think about.

It's true that on a cosmic scale, entropy will always increase, but for a fixed period of time and a fixed place, we can still make things better for our own lives.

And then I read A Brief History of Time by Stephen Hawking, and I thought, "Okay, that's the work I want to do, that's what I want to be. He's a cosmologist, clearly I should be a cosmologist." And so I studied physics and went through grad school and all of that, and ended up doing cosmology. So it worked out much better than I could've guessed in terms of choosing that path and more or less staying on it. Not without challenges and detours, but I've always been fascinated by the universe, I've always been fascinated by physics in general, and the weird science that messes with your head a little bit, and these really big topics.

I know a lot of astronomers who got into the field through stargazing, and I grew up in a city where you could not see the stars, so that was not my entry. I grew up in Long Beach, California, outside of LA. So the sky was pink. You didn't get a lot of dark skies. But there was an observatory there, the Griffith Park Observatory, and I would go there for meteor showers and things like that, and eclipses, and that was always cool, but really what got me into cosmology was the physics, was trying to understand how things work at a really fundamental level.

EC: Right, so the mathematical level behind it all.

KM: Yeah.

EC: That's fascinating. In your book you propose five of the most likely endings to the universe. As a fiction editor, I'm naturally spoiler alert averse, so I don't want to go super deep on all of them. I want to save some for listeners. But I did want to ask… I have a guess at what your favorite version of the end of the world is. Would I be right saying it's vacuum decay? 

KM: It is absolutely vacuum decay, yes. Because it is just such a weird and unexpected kind of process, something that would really come out of nowhere. Most of the time when you're talking about the future evolution of the cosmos it's something about the universe is expanding, or maybe it reverses its expansion, or something goes wrong with that aspect.

But vacuum decay is very, very different because it's really physics itself breaking somewhere in the cosmos, and then that spreading through the whole thing in a way that's... it's amazing that that's even conceivable, and yet the current data and the equations all kind of point to maybe that could happen.

EC: Right. So I was actually listening to this part of the book on Alexa in our kitchen, and my husband was there, and for context he's English, and the part where it says your feet will disappear before your brain can even acknowledge that it’s happening, he just looks at me, and he goes, "Blimey. Probably the most intense thing I've heard today." But you do say that that is still probably really unlikely, or if it happens, it's really far off. But how is it both potentially somewhere immediately in our near future and also very likely far off? 

KM: It all comes down to quantum mechanics. Quantum mechanics is an idea of physics that says that really, you can't ever completely predict the outcome of an experiment. You can get probabilities for how things will turn out. So if you are shooting an electron beam at a wall, you have a vague idea of where that's going to hit, but there'll be some scatter around that, and that's inherent to how particles work in our universe. And the thing about vacuum decay that makes it so intriguing is that it's governed by a quantum mechanical process, which means that we can't predict exactly where or when it will happen. All we can say is, based on our calculations, there's a certain probability that it'll happen at a particular place at a particular time.

And so the probability of it happening anytime soon is astronomically small. In our entire volume of observable universe, the chance of it happening within the lifetime of the solar system is tiny. But because it is this unpredictable quantum process, you can't ever say for sure exactly when it will happen. And because it's something that, if it does happen, it happens... the bubble of death that gets created spreads at about the speed of light. You wouldn't see it coming. You'd have no warning, because nothing can travel faster than the speed of light to bring you that information.

So if it did appear somewhere in the cosmos and then spread out, you wouldn't know it was going to hit you before it did.

EC: Wow. But it could happen so far out in the cosmos that it couldn't get to us, right? There is a horizon beyond which it couldn't. That blew my mind, that maybe this thing is happening out there somewhere, and it's not going to get us.

KM: Yeah, so because the universe is expanding so quickly, when you look at long enough distances, there's a distance out to which, if it happened at that point, it would be moving away from us at more than the speed of light. Not that it's moving through space at more than the speed of light, because nothing can do that, but that there's more space in between us being produced so that it gets farther away at more than the speed of light, and then that light speed bubble would not get to us.

...When you do have the chance to step away from basic survival and think in these big cosmic terms, and experience that awe, and experience being part of the universe, I think that's a wonderful thing.

But the reason that I don't find that super comforting is because if we get to the point where it's likely to happen somewhere outside of our volume of space, it's also just as likely to happen inside. And so it can happen at more than one point, in principle, if you wait long enough.

EC: Right. I see.

KM: We don't even know if vacuum decay is possible or not. There's a lot of theory that we still have to work out, there's a lot of experimental evidence we still need to deal with. So it's really not something to worry about, and I'm always telling people, "Really, don't worry about this." I find it fun and cool to think about because I'm a physicist and because I love this kind of stuff, but in practical terms, you can completely forget that vacuum decay is possible if it upsets you. But if you think it's cool because it's interesting physics, then, you know.

EC: I think it's cool. I love the sort of quick and dirty nature of it versus the other four versions, which seem to be very much more like, it's going to be a while. I think that you indicated a few times throughout your book that you think that probably heat death is the most likely, or the most, sort of, approved by the community. Is that a fair way of putting it?

KM: Yeah, it's sort of the default hypothesis. It's considered to be the simplest hypothesis, where no new physics has to come in to change our picture of how the universe works. So if everything works in the simplest way we can imagine, then we're headed toward a heat death.

EC: Can you give a little top line about how heat death works? Because when I got to the chapter heading I imagined a fiery death, but it actually sounds like the opposite of that, right?

KM: Yeah, it’s kind of a misleading term, because the way the term "heat" is used in that title is the technical physics definition of heat, which just means disordered energy. Heat, like the friction heat in an engine that makes it less efficient, like the waste heat of processes in physics, that's the kind of heat we're talking about in the heat death.

What happens in the heat death is that the universe continues expanding. So right now the universe is expanding such that distant galaxies are becoming more distant from us and from each other, and if that carries on forever, and if it continues accelerating, which it's doing now, which is weird—that's a whole other topic, that's dark energy, that's a whole thing. But if the expansion continues going and continues speeding up, then every region of the cosmos, every galaxy or little group of galaxies gets more and more isolated, and so there are no more collisions of galaxies to produce new stars, and the stars within galaxies start burning out, and eventually you just kind of run out of stuff, everything starts to decay, the stars burn out and die, even black holes evaporate.

And at the ultimate end, all that's left in the cosmos is the waste heat of the universe, the little bit of radiation, tiny trace amount of radiation that's left over from all the processes of the universe winding down.

EC: So it's just sort of like the end of the Big Bang. It goes from that really intense to just...

KM: Yeah, the universe just kind of diffuses and fades away, which I find to be quite a depressing concept.

EC: Yes, it's really stark sounding. But if everything's expanding away from each other, how come we are going to run into the Andromeda Galaxy? Shouldn't we be moving away from each other?

KM: The Andromeda Galaxy is just so close to us that it's already in our gravitational field, right? So technically gravity extends infinitely, but if something's far enough away that the expansion is stronger than the gravity between the things, then they move away from each other. But if things are very close by, then they can still attract. And so we are in what we call the local group of galaxies. It's us, and Andromeda, and the Triangulum Galaxy, and a smattering of little dwarf galaxies.

And we're just in a little sort of clump in this part of space, and we're all gravitationally connected to each other. And so the Andromeda Galaxy is moving toward us at 110 kilometers per second. It's a large galaxy, a large disc-like galaxy, very similar to our galaxy but maybe about 10 times as massive. And in about four billion years, it'll run into us, and that'll mix up all of our stars and make a whole big mess. But we'll still be here, we'll just be a bigger, messier galaxy when that happens.

But anything significantly farther away is caught up in the flow of the expansion, and so the really distant things are moving away from us. Things that are already close together can still collide.

EC: I see, okay. That's very helpful. Thank you for humoring me on that. I think also, when I was listening to this part, the heat death is really a constant increase... It's an increase in entropy, right? I think at one point you actually say that a general rule of thumb is that things always get worse. Right?

KM: Yup, yup.

EC: Which sort of made me feel a little bit of despair, because it's... I don't know, I think I start to feel like if everything's getting worse, is there any hope for things like climate change and political chaos and this moment we're in? But are these just planet-level concerns that are totally, totally separate from the entropy?

KM: What the second law of thermodynamics says is that for any enclosed system, the entropy only increases over time. And so that means that if you're running a refrigerator in a closed room or something like that, it's going to heat up the room. Even though the inside of the fridge is getting colder, there's enough waste heat in the process that it's going to make the room hotter on the whole, so that's going to go badly, right?

And so, Earth is not necessarily a closed system. There's stuff that radiates into space, and there are a lot of ways you can move energy around in the planet, you can move the waste heat in different ways, and you can work on processes that are more efficient, and change how you deal with waste, and so on. So it's true that anything that we do will produce waste, anything that we do does increase entropy locally, but we can also change how we deal with that.

...The way I respond to dread is to laugh. I don't know why, but there's something about it that, because it is so distant from us, and it's so big and powerful, and yet kind of disconnected. You can look at it and you have this sort of nervous giggle, right?

It's true that on a cosmic scale, entropy will always increase, but for a fixed period of time and a fixed place, we can still make things better for our own lives. On the time scales of human lives, we can certainly improve the world, and I think that that's something that we should do while we exist. But ultimately, in the far, far future, entropy will win out.

EC: Right. I think it was in the Big Rip chapter, there was a footnote—which, by the way, in the audio edition were inserted so seamlessly, and I loved how much it livened it. I both read and listened to it, and I enjoyed listening to it more because my eyes didn't have to move around the page, it was great. But you said, "If you ask them, my colleagues will claim that their real motivation is understanding the nature of dark energy because of what it tells us about fundamental physics and our cosmological model. But I know it's really the dread."

So, are your colleagues—and is it right to call them cosmic eschatologists? You're sort of studying the end of everything?

KM: Yeah, I think there are very few of us who specialize specifically in the end of the universe. It's more like cosmologists who dabble in cosmic eschatology for the most part. Most of us also do other things that our colleagues would consider to be more sensible. But yeah, people who study dark energy are implicitly studying the future of the cosmos and the fate of the universe.

EC: Interesting. There's this real hopefulness about your book, actually. There's this levity and joy which I think comes through because of the wonder. But it's really interesting how you've paired it with the dread. And I loved how much you told us not to worry, but I don't know. I'd love to hear you talk about that a little bit, and also to hear, I think I'd be remiss not to ask you what we should be worrying about, because I think that's an important question.

KM: Yeah. I think the things that we should be worrying about in terms of, as human beings, are the things that are directly affecting the lives and the suffering of human beings. Things like climate change, things like oppression. Those are things that we need to deal with as humans, as a society, and those are things that should be our top priorities in terms of what we do here on Earth to make lives better.

But I also think that… one of the things that I love about thinking about cosmology is that it's a way of taking yourself out of the concerns of the world and getting this much wider, broader perspective on how we fit into this big picture of the whole story of the cosmos. And there's something hopeful about that, there's something that I find really comforting about that, about just having the luxury to think about that. And obviously not everybody does, and we should work on that. But when you do have the chance to step away from basic survival and think in these big cosmic terms, and experience that awe, and experience being part of the universe, I think that's a wonderful thing.

And it is scary as well. We are utterly insignificant little specks of dust in this big picture. We are totally at the mercy of the evolution of the cosmos, of big, scary processes that happen out in space that there's nothing we can do about, that we wouldn't even know were happening, necessarily. But we also have this amazing power to understand so much. We can see the Big Bang, that's incredible, right, we can see the light from the Big Bang, from when the whole universe was just a fiery, roiling plasma, because it had just been started, it just began.

EC: That first chapter. It took me a while to get through it because I had to ground myself.

KM: Yeah, so I think for me, there is this constant balance between the awe and the wonder and the dread. And for me, I think the way I respond to dread is to laugh. I don't know why, but there's something about it that, because it is so distant from us, and it's so big and powerful, and yet kind of disconnected. You can look at it and you have this sort of nervous giggle, right? And that's just how that affects me emotionally, and I think I tried to put a lot of that into the book because that's just how I feel about it.

EC: That's actually a good lead to what I wanted to talk about next. I loved your pop culture references. I feel like this is such human book on top of being such a big thinking science book. It's a little bit of everything.

KM: I consume a lot of science fiction, so it was hard to not have references to that stuff in the book.

EC: I saw, I think on Twitter, you listen to audiobooks. I'd love to hear what you listen to, I’d love to hear about some of your favorite sci-fi works and authors.

KM: Oh, gosh. I pretty much constantly listen to audiobooks, actually. I had to upgrade my Audible subscription recently because I was buying too many. I listen to a lot of science fiction, and I do relisten to things quite a lot, because a lot of times I listen as I'm going to bed and I don't want to always listen to something new when I'm falling asleep, because then I'll miss things, and I don't want to do that. So there are certain books that I'll come back to.

So right now the one I'm coming back to is Ancillary Justice by Ann Leckie, and that whole series, which is awesome. And beautifully read too. I love the narration in that series as well. I've also been listening to Mary Robinette Kowal's series, the Lady Astronaut series, which is great. And, I listened to Hank Green's two novels, [An Absolutely] Remarkable Thing and A Beautifully Foolish Endeavor. I think I listened to both of those on Audible, although I have the hard copy of one of them. Those are very good, and a neat look at how we conceptualize influence and fame, and how we see ourselves on the internet and stuff like that, that was a neat thing tied into a science fiction story. I've been listening to the Bobiverse novels recently. I just started the most recent one, which is sort of fun, light entertainment about von Neumann probes and the little society they put together. And I listened to Adrian Tchaikovsky's books recently, the Children of Time and Children of Ruin. Those were really great as well.

Mostly I listen to things involving spaceships. Both when I read and when I listen to science fiction, it's usually something that involves spaceships. And that's because when I look for entertainment in that way, I really want something escapist, and if it involves a spaceship, it's pretty clearly going to be out there. So I think that's kind of a good metric for me. Spaceships, aliens, parallel universes, whatever.

EC: Nice. It's funny, I would've guessed dystopian, but maybe you want to get away from that.

KM: I don't like things that are too bleak. When it's real bleak and gritty, I find it hard to take. Because I want my science fiction to be separate and frivolous. Not that it doesn't have important themes. I like it when there's that balance where it's not just everybody's miserable, everything's terrible, where there is some really interesting conflict and really interesting challenges, but there's basically a hopeful or at least thriving perspective. I appreciate that a lot. When I watch Star Trek, I love The Next Generation because it's that very hopeful vision. And some science fiction tries to get really dark and gritty, and it's not as much fun for me when it's like that.

EC: Yeah, that's interesting. I have a recommendation for you on that front. I listen to some sci-fi. I like it a lot, but I tend more towards the YA space. There's a great series which I just love, called the Illuminae series by Jay Kristoff and Amie Kaufman. It's about a corporate spaceship that's tracking down another spaceship and they're on the run across the galaxy, and they're trying to get some kind of portal that will take them somewhere else. I don't even know if that's something that is ever going to be scientifically possible, but I want to believe that one day it will be, and we can travel across the cosmos through some kind of portal or something.

KM: Yeah, I would love it if light-speed travel or wormholes were feasible. I think, as a physicist, I am not hopeful. But I like to think about it. I do also appreciate science fiction that doesn't make use of that. So, for example, Alastair Reynolds, I love his stuff, and he does not break the speed-of-light limit. He gets around that by having people live really long times, so you can actually go somewhere. Because that's the problem with not going faster than the speed of light, is that you don't get anywhere, right?

And so he gets around it through messing with biology, which maybe the biologists would object, but I appreciate the getting the physics right. Also The Expanse series. They bring in aliens who can do other things, but all the humans are limited. They have fun things like there's no artificial gravity on their spaceships, so they... This is The Expanse series by James S.A. Corey, which is also great, and the TV show is amazing too. So they only have gravity when they're accelerating in their spacecraft or if they have something that spins, and I think that's a nice thing because artificial gravity is always like, "Er, I don't know how that works."

EC: Yeah, so it's not something that is feeling likely.

KM: No, I mean you have similar kind of problems with trying to do artificial gravity as you do with trying to go faster than the speed of light. You're messing with relativity in ways that we probably can't do. But one is special relativity, one is general relativity, but the basic idea is the same, that you have to screw with space-time, and we don't have a good way to do that.

EC: Here's Katie Mack, crushing dreams, left, right, and center.

KM: Sorry. I still enjoy it, right, I don't mind if the rules are broken if they're done in an interesting and consistent way. I was watching a sci-fi TV series recently that did a lot of things wrong in the physics, but it was a great story, so I was like, "Oh, this is awesome." And some of my astronomer colleagues were like, "Oh, but they got the time delay between here and Mars wrong." Like, "Yeah, okay, fine, but the people are cool." It doesn't totally ruin it for me unless they're just not paying any attention to the science, and then it's like, "Well, you're not trying."

EC: Right, yeah. Do a little bit better here. I just want to say it's been such a pleasure speaking to you. Again, the title is The End of Everything (Astrophysically Speaking). And please do write some more books, because I will devour them.

KM: Okay, thanks. Give it a bit of time, but I'll get there eventually.

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