by Katie Mack
To what extent there even exists an explanation for the nature of our universe is an open question too. In the last decade or so, physicists have been grappling with the concept of the landscape—a theoretical multiverse of different possible spaces which could have drastically different conditions from our own. If such a landscape really exists, it could mean that the properties of the space we live in are merely environmental, rather than being set by some deep principle we haven’t been clever enough yet to find. This kind of multiverse can arise out of certain versions of inflation where new bubble universes inflate out of some eternal pre-existing space forever. “The idea that we’re the unique solution of the world does not seem right to me,” says Arkani-Hamed. “But on the other hand, when you try and make sense of the landscape and eternal inflation and all that stuff, it’s such a morass that I think that the whole conception of the problem is wrong to begin with.” Even with a landscape of possible universes, the basic problem still remains. “These questions about how to apply quantum mechanics to cosmology have been there almost from day zero. They’re not new. They were very difficult fifty years ago; they’re very difficult now.”
“I very firmly believe what we should be doing is actually just retracing our steps,” says Neil Turok, a cosmology theorist who has been looking at alternatives to cosmic inflation and who spent many years as the director of the Perimeter Institute for Theoretical Physics in Canada. “Go back, rewind fifty years and say, ‘Guys, we’re building on sand.’ ”
THE LONG VIEW
There’s a famous equation in astrobiology called the Drake Equation. In theory, it’s a way to calculate the number of civilizations in our galaxy with whom we might be able to communicate. All you have to do is input the number of stars, the fraction of those with planets, the fraction of those with life, the fraction with intelligent life, and so on, and in the end you get the number of messages you should expect on your interstellar voicemail. Of course, many of these input numbers are, at least with current data, completely impossible to determine, which means that the final answer isn’t meaningful. The thing that’s useful about the Drake Equation is that it makes us think about our assumptions about extraterrestrial life, and to figure out what we do and don’t know about this whole question.
Talking with Hiranya Peiris, it occurs to me that contemplating our ultimate cosmic destruction might be much the same. I suggest to her that perhaps we’re doing a calculation where the final number doesn’t matter, but the calculation does. “The number doesn’t matter,” she agrees, “but the exercise of thinking through the different options on the table, I think, is good.” And the implications of this thought experiment might ultimately pay off. “It could lead to some cool way to test between the hypotheses that doesn’t wait for seven billion years.”
How long do we have to wait for a breakthrough? We don’t (and can’t) know. We’re exploring off the edge of the map now. Clifford Johnson is very optimistic that we’re heading toward a better, deeper understanding of physics, but he acknowledges the caveat. “It might be that we go for a couple hundred years gathering all of this data before we see the signal and then we go back and realize that, oh, it was there staring us in the face all along. That’s an annoying possibility. But for questions as big as the ones we’re trying to answer, I feel that that’s okay. Why need it be of the length scale of a human lifetime?”
In the meantime, we’ll continue on, making new paths through the woods to see what we might find hiding there. Someday, deep in the unknown wilderness of the distant future, the Sun will expand, the Earth will die, and the cosmos itself will come to an end. In the meantime, we have the entire universe to explore, pushing our creativity to its limits to find new ways of knowing our cosmic home. We can learn and create extraordinary things, and we can share them with each other. And as long as we are thinking creatures, we will never stop asking: “What comes next?”
I. Unfortunately, the only kind of open universe model that will allow this is one without a cosmological constant, so even this tiny spark of hope seems to have been snuffed out by the current data.
II. Amazingly, Dyson himself never submitted his own paper. It was submitted to Reviews of Modern Physics on his behalf by a friend, who didn’t ask permission. Dyson told me recently, “I didn’t think it was worth publishing,” considering it not to be appropriate for the journal. “It’s always a matter of opinion,” he added.
III. “See that star there? That star is going to EXPLODE in one year. Plus or minus four months. Just watch, you’ll see.” (Adapted liberally from Treu et al. 2016, The Astrophysical Journal.)
IV. Thanks, Madrid-based theorist and CERN Scientific Associate José Ramón Espinosa. Very helpful.
V. Probably closer to 1015, but I have a moral objection to the word “quadrillion.”
VI. This method is especially undesirable, Espinosa points out, since it “will not teach us anything, because we won’t even see it coming.”
VII. Another thing Sean Carroll pointed out to me is that if his interpretation of quantum mechanics is correct, there are countless copies of ourselves in parallel universes that are at this very moment succumbing to vacuum decay. So he was probably never really going to be the best place to turn for relief from existential crisis.
CHAPTER 9: Epilogue
“But if nothing we do here has any guarantee of lasting, if even the best gestures have only a slim chance of outliving us, is there any reason not to just give up?”
“Every reason in the world,” Rudd said. “We’re here, and we’re alive. It’s a beautiful evening, on the last perfect day of summer.”
Alastair Reynolds, Pushing Ice
Martin Rees isn’t building any cathedrals.
We are sitting in his office at Cambridge University’s Institute of Astronomy on a sunny June morning, and he is telling me that humanity as we know it will be forgotten. “In the Middle Ages, the cathedral builders were happy to build a cathedral that would be there for more than their lifetime, because they thought that their grandchildren would appreciate it and would live lives like them. Whereas I don’t think we have that.” Rees is no stranger to far-future speculation, having written books about the future of humanity and all the different ways we might accidentally doom ourselves. According to him, evolution, in the cultural and technological sense, is accelerating so fast that whatever the dominant intelligence is in the next few hundred or thousand years, we can’t predict what it will be like. But we can be sure that it won’t care about us. “I think to leave a legacy for a hundred years is a bolder ambition now than it would have been for our ancestors,” he says.
“Does that upset you?” I ask him.
“It upsets me very much. But why should the world be made the way we like it?”
It’s impossible to seriously contemplate the end of the universe without ultimately coming to terms with what it means for humanity. Even if you take the position that Rees’s view is overly pessimistic, there has to come a point in any timeline with a finite extent where our legacy as a species just… stops. Whatever legacy-based rationalization we use to make peace with our own personal deaths (perhaps we leave behind children, or great works, or somehow make the world a better place), none of that can survive the ultimate destruction of all things. At some point, in a cosmic sense, it will not have mattered that we ever lived. The universe will, more likely than not, fade into a cold, dark, empty cosmos, and all that we’ve done will be utterly forgotten. Where does that leave us now?
Hiranya Peiris sums it up in one word: “sad.”
“It’s very depressing,” she says. “I don’t know what else to say about it. I give talks where I mention that this is probably the fate of the universe, and people have cried.”
It does provoke some perspective-taking. “It’s very intriguing to me that the universe has produced a very interesting period where a lot happens,” she says. “And yet we seem to face a much longer period in utter darkness, cold. It’s h
orrible. I feel, actually, from that point of view, very lucky, to be in the few years of cosmology where we are learning all this stuff for the first time.”
“It momentarily makes me feel sad,” agrees Andrew Pontzen. “Then I very quickly start worrying about our problems here on Earth right now and think, ‘Come on.’ We’re in so much deeper trouble than the Heat Death of the universe. So I suppose it makes me start thinking about the problems that we face as a civilization on much shorter timescales. If I’m going to worry about anything, it’s gonna be those, not the Heat Death.”
“I just don’t really have an emotional connection, I suppose, to the death of the universe,” Pontzen continues, “but I do to the death of the Earth. I don’t mind the fact that I’m going to die in fifty years or whatever, but I don’t want the Earth to die in fifty years.”
I have a lot of sympathy for this view. In terms of things we should actually worry about, the Heat Death, or vacuum decay, or the Big Rip, or whatever, cannot be at the top of that list (even setting aside the fact that we are utterly powerless to do anything about them). As living beings, we naturally care most about our own lives, and the lives of those close to us in space and time, and for the most part we leave the unimaginably distant cosmic future to its own devices.
But personally, I still feel there’s a big difference, in some emotional sense, between “we go on forever” and “we don’t.” Nima Arkani-Hamed feels the same way. “At the absolute, absolute deepest level… whether or not people explicitly admit to thinking about it or not (and if they don’t they’re all the poorer for it)… If you think there is a purpose to life, then I at least don’t know how to find one that doesn’t connect to something that transcends our little mortality,” he tells me. “I think a lot of people at some level—again, either explicitly or implicitly—will do science or art or something because of the sense that you do get to transcend something. You touch something eternal. That word, eternal: very important. It’s very, very, very important.”
Freeman Dyson had hoped to find a way to preserve intelligent life for all time. His 1979 paper proposed a way to propagate some kind of intelligent machine into an infinite future, through a scheme involving the constant slowing of processing and intermittent hibernation. Unfortunately, those calculations were done under the assumption that the universe’s expansion doesn’t accelerate, and now, it appears that it does. And if the acceleration continues, Dyson’s plan won’t work. “It would be disappointing,” he admits. “I mean, you have to accept what nature provides. It’s like the fact that we have finite lifetimes. It’s not so tragic. In many, many ways it makes the universe more interesting. It’s always evolving to something different. But having a finite lifetime for the whole thing… maybe that’s our fate. But certainly, I would prefer to have evolution going on forever.”
And who knows? Maybe there’s a sense in which it does. Roger Penrose thinks there’s a better way. He’s spent the last decade or so developing his Conformal Cyclic Cosmology, which postulates a universe cycling from Big Bang to Heat Death, over and over again, forever, with the tantalizing possibility that something—some imprint from a previous cycle—might make it through the transition. The notion that what passes through could contain meaningful information about any conscious inhabitants is just idle speculation at the moment, he says, but the implications of that possibility could be profound. “I’m not certainly saying that I think this, but in some ways I find it less depressing… that maybe after one’s death, it’s conceivable there could be some legacy.”
Or maybe the possibility of a multiverse landscape can comfort us. Jonathan Pritchard, a cosmologist at Imperial College London whose work has run the gamut from cosmic inflation to the evolution of galaxies, finds hope in the idea that in some other distant, unconnected region, something might exist long after we’re nothing but waste heat. “Somewhere out there, there is a multiverse where stuff is always going on,” he says. “Emotionally, I like the idea of that.”
But we still die, I say.
He’s unfazed. “It’s not all about us, you know.”
If we don’t get to join the eternal multiverse party ourselves, at least our looming death can be good for physics. Neil Turok points out that the prospect of an end of time in the future, combined with the existence of our cosmic horizon, places hard boundaries on the universe, and thus helpful limits to the problem of understanding it all. A light wave traveling across a limited, expanding, accelerating universe can undergo only so many oscillations, even into the infinite future. “We live, effectively, in a box, okay? Which is finite. And if that’s true, I think it’s to be welcomed because we could understand it. The problem of understanding the universe just got a whole lot easier because it’s finite,” he says. “Finite to the past, finite in space because of the horizon, finite to the future because everything will only oscillate a finite number of times. Wow! I mean, that’s understandable. I’m an optimist by nature, but I think the world is our oyster.”
If the universe is going to end, one way or another, I concede that we may as well make our peace with it. Pedro Ferreira is way ahead of me on that one. “I think it’s great,” he says. “It’s so simple and so clean.
“I’ve never understood why people get so depressed about the end, the death of the Sun and all,” he continues. “I just like the serenity of it.”
“So it doesn’t bother you that we ultimately have no legacy in the universe?” I ask him.
“No, not at all,” he says. “I very much like our blip-ness… It’s always appealed to me,” he continues. “It’s the transience of these things. It’s the doing. It’s the process. It’s the journey. Who cares where you get to, right?”
I admit it, I still care. I’m trying not to get hung up on it, on the ending, the last page, the end of this great experiment of existence. It’s the journey, I repeat to myself. It’s the journey.
There is perhaps some solace in the fact that whatever happens, it’s not our fault. Renée Hložek considers this a definite plus.
“I love the fact that my work, even if I do it 100 percent perfectly and I’m an incredible scientist, it changes nothing about the fate of the universe,” she says. “All we are trying to do is understand it. And even if you do understand it, we can do nothing to change it. I think that’s freeing rather than scary.”
To Hložek, the Heat Death isn’t depressing, or boring. She calls it “cold and beautiful.” “It’s like the universe just sorts itself out,” she says.
“What I hope that people get out of your book is that it’s possible for the human mind to use observations of light—and/or gravitational waves, but let’s stick with light for now—and make incredible inferences with relatively simple mathematics about the picture of the universe,” Hložek says. “And even if we can do nothing to change it, that knowledge… even if that knowledge goes away, if all humans die, that knowledge right now is incredible. That’s basically why I do what I do.”
I think I see what she’s saying. Would I want to uncover the secrets of the universe, even if I didn’t get to share that knowledge, or keep it? I would. This seems important. “There’s some purpose to doing it, even if it is lost.”
“Because it changes who you are now, right?” she agrees. “I am delighted that we get to live at a time in the universe when we can see dark energy and not be ripped apart by it. But that means the whole point is that you understand it, and then you enjoy it, and then… ‘so long and thanks for all the fish.’ Cool.”
Cool.
Acknowledgments
I never really imagined I would be an author, and I never would have managed it, if it were not for the help of far more people than I can name. I’m going to make an attempt to list just a small subset of those people here, but over the last few years I have taken much more support and advice than I could ever give back from countless friends and colleagues. If you are one of those people, whether your name appears here or not, please accept my thanks f
or all you have done and know that this book is partly yours as well. (I hope you like it!)
When I first set out to write this book, I had only a vague idea that I could put some words on paper and someone would, hopefully, eventually read them. Fortunately, I’ve been deftly guided through the whole process by my wonderfully patient, professional, and encouraging literary agent, Mollie Glick, and a whole team of enthusiastic book-crafters at Scribner. I’m especially grateful to Daniel Loedel for the feedback and edits that substantially honed and shaped this manuscript, and to Nan Graham for believing in my ability to write it in the first place. Thanks also to Sarah Goldberg, Rosaleen Mahorter, Abigail Novak, and Zoey Cole at Scribner, and Casiana Ionita, Etty Eastwood, and Dahmicca Wright at Penguin UK, who have all been working tirelessly for the last few months to get this book out into the world. I’m grateful to Nick James for the wonderful illustrations that appear in these pages and to Laurel Tilton and Ana Gabela for organizational support.
One of the biggest joys of this whole process has been having an excuse to connect and talk science with a huge number of amazing physicists and astronomers who have influenced the way I think about the cosmos. For indulging me and my many, many questions, I am grateful to Andy Albrecht, Nima Arkani-Hamed, Freya Blekman, Sean Carroll, André David, Freeman Dyson, Richard Easther, José Ramón Espinosa, Pedro Ferreira, Steven Gratton, Renée Hložek, Andrew Jaffe, Clifford V. Johnson, Hiranya Peiris, Sterl Phinney, Roger Penrose, Andrew Pontzen, Jonathan Pritchard, Meredith Rawls, Martin Rees, Blake Sherwin, Paul Steinhardt, Andrea Thamm, and Neil Turok. For volunteering to look over various chapters and give me extremely helpful feedback, I’m additionally indebted to several of the above and to Adam Becker, Latham Boyle, Sébastien Carrassou, Brand Fortner, Hannalore Gerling-Dunsmore, Sarah Kendrew, Tod Lauer, Weikang Lin, Robert McNees, Toby Opferkuch, and Raquel Riberio. Whatever errors are still in the manuscript (and I’m sure there are many) originate in my own failure to reliably commit the considerable collective wisdom of all of the above to the page.
