How do you feel about the multiverse?” The question was not out of place in our impromptu dinner-table lecture, yet it caught me completely off-guard. It’s not that I’ve never been asked about the multiverse before, but explaining a theoretical construct is quite different to saying how you feel about it. I can put forth all the standard arguments and list the intellectual knots a multiverse would untangle; I can sail through the facts and technicalities, but I stumble over the implications.
In physics we’re not supposed to talk about how we feel. We are a hard-nosed, quantitative, and empirical science. But even the best of our dispassionate analysis begins only after we have decided which avenue to pursue. When a field is nascent, there tend to be a range of options to consider, all of which have some merit, and often we are just instinctively drawn to one. This choice is guided by an emotional reasoning that transcends logic. Which position you choose to align yourself with is, as Stanford University physicist Leonard Susskind says, “about more than scientific facts and philosophical principles. It is about what constitutes good taste in science. And like all arguments about taste, it involves people’s aesthetic sensibilities.”
The debate over the multiverse is a fight about identity and consequence.
My own research is in string theory, and one of its features is that there exist many logically consistent versions of the universe other than our own. The same process that created our universe can also bring those other possibilities to life, creating an infinity of other universes where everything that can occur, does. The chain of arguments starts from a place I’m familiar with, and I can follow the flourishes that the equations make as they dance down the page toward this particular conclusion, but, while I understand the multiverse as a mathematical construction, I cannot bring myself to believe it will leap out of the realm of theory and find a manifestation in physical reality. How do I pretend I have no problem accepting the fact that infinite copies of me might be parading around in parallel worlds making choices both identical to, and different from, mine?
I am not alone in my ambivalence. The multiverse has been hotly debated and continues to be a source of polarization among some of the most prominent scientists of the day. The debate over the multiverse is not a conversation about the particulars of a theory. It is a fight about identity and consequence, about what constitutes an explanation, what proof consists of, how we define science, and whether there is a point to it all.
Whenever I talk about the multiverse, one of the questions that inevitably comes up is one I actually have an answer to. Whether we live in a universe or multiverse, these classifications relate to scales so large they defy imagination. No matter the outcome, life around us isn’t going to change one way or another. Why does it matter?
It matters because where we are influences who we are. Different places call forth different reactions, give rise to different possibilities; the same object can look dramatically different against different backgrounds. In more ways than we are perhaps conscious of, we are molded by the spaces we inhabit. The universe is the ultimate expanse. It contains every arena, every context in which we can realize existence. It represents the sum total of possibilities, the complete set of all we can be.
A measurement makes sense only within a reference frame. Numbers are clearly abstract until paired with units, but even vague assessments such as “too far,” “too small,” and “too strange” presume a coordinate system. Too far invokes an origin; too small refers to a scale; too strange implies a context. Unlike units, which are always stated, the reference frame of assumptions is seldom specified, and yet the values we assign to things—objects, phenomena, experiences—are calibrated against these invisible axes.
If we find out that all we know, and all we can ever know, is just one pocket in the multiverse, the entire foundation upon which we have laid our coordinate grid shifts. Observations don’t change, but implications do. The presence of those other bubble universes out there might not impact the numbers we measure here on our instruments, but could radically impact the way we interpret them.
The first thing that strikes you about the multiverse is its immensity. It is larger than anything humankind has ever dealt with before—the aggrandizement is implicit in the name. It would be understandable if the passionate responses provoked by the multiverse came from feeling diminished. Yet the size of the multiverse is perhaps its least controversial feature.
At some point in our history, we appear to have made peace with the fact we are infinitesimal.
Gian Giudice, head of CERN’s theory group, speaks for most physicists when he says that one look at the sky sets us straight. We already know our scale. If the multiverse turns out to be real, he says, “the problem of me versus the vastness of the universe won’t change.” In fact, many find comfort in the cosmic perspective. Framed against the universe, all our troubles, all the drama of daily life, diminishes so dramatically that “anything that happens here is irrelevant,” says physicist and author Lawrence Krauss. “I find great solace in that.”
From the stunning photographs the Hubble Space telescope has beamed back to Octavio Paz’s poems of “the enormous night” to Monty Python’s “Galaxy Song” to be sung “whenever life gets you down,” there is Romanticism associated with our Lilliputian magnitude. At some point in our history, we appear to have made peace with the fact we are infinitesimal.
If it isn’t because we are terrified of the scale, are we resistant to the notion of the multiverse because it involves worlds that are out of sight and seem doomed to remain so? This is indeed a common complaint I hear from my colleagues. South African physicist George Ellis (who is strongly opposed to the multiverse) and British cosmologist Bernard Carr (an equally strong advocate) have discussed such issues in a series of fascinating conversations. Carr suggests their fundamental point of diversion concerns “which features of science are to be regarded as sacrosanct.” Experimentation is the traditional benchmark. Comparative observations are an acceptable substitute: Astronomers cannot manipulate galaxies, but do observe them by the millions, in various forms and stages. Neither approach fits the multiverse. Does it therefore lie outside the domain of science?
If galaxies we know of can exist in some distant region beyond sight, who’s to say other things can’t be there, too?
Susskind, one of the fathers of string theory, sounds a reassuring note. There is a third approach to empirical science: to infer unseen objects and phenomena from those things we do see. We don’t have to go as far as causally disconnected regions of spacetime to find examples. Subatomic particles will do. Quarks are permanently bound together into protons, neutrons, and other composite particles. “They are, so to speak, hidden behind a … veil,” Susskind says, “but by now, although no single quark has ever been seen in isolation, there is no one who seriously questions the correctness of the quark theory. It is part of the bedrock foundation of modern physics.”
Because the universe is now expanding at an accelerating rate, galaxies that currently lie on the horizon of our field of vision will soon be pushed over the edge. We don’t expect them to tumble into oblivion anymore than we expect a ship to disintegrate when it sails over the horizon. If galaxies we know of can exist in some distant region beyond sight, who’s to say other things can’t be there, too? Things we’ve never seen and never will? Once we admit the possibility that there are regions beyond our purview, the implications grow exponentially. The British Astronomer Royal, Martin Rees, compares this line of reasoning to aversion therapy. When you admit to there being galaxies beyond our present horizon, you “start out with a little spider a long distance away,” but, before you know it, you unleash the possibility of a multiverse—populated with infinite worlds, perhaps quite different to your own—find “a tarantula crawling all over you.”
The lack of ability to directly manipulate objects has never really figured in my personal criteria for a good physical theory, anyway. Whatever bothers me about the multiverse, I’m sure it isn’t this.
The multiverse challenges yet another of our most cherished beliefs—that of uniqueness. Could this be the root of our trouble with it? As Tufts cosmologist Alexander Vilenkin explains, no matter how large our observable region is, as long as it is finite, it can only be in a finite number of quantum states; specifying these states uniquely determines the contents of the region. If there are infinitely many such regions, the same configuration will necessarily be replicated elsewhere. Our exact world here—down to the last detail—will be replicated. Since the process continues into infinity, there will eventually be not one, but infinite copies of us.
“I did find the presence of all these copies depressing,” Vilenkin says. “Our civilization may have many drawbacks, but at least we could claim it is unique—like a piece of art. And now we can no longer say that.” I know what he means. That bothers me, too, but I’m not sure it quite gets to the root of my discontent. As Vilenkin says, somewhat wistfully: “I am not presumptuous enough to tell reality what it should be.”
The crux of the debate, at least for me, lies in a strange irony. Although the multiverse enlarges our concept of physical reality to an almost unimaginable extent, it feels claustrophobic in that it demarcates an outer limit to our knowledge and our capacity to acquire knowledge. We theorists dream of a world without arbitrariness, whose equations are entirely self-contained. Our goal is to find a theory so logically complete, so tightly constrained by self-consistency, that it can only take that one unique form. Then, at least, even if we don’t know where the theory came from or why, the structure will not seem arbitrary. All the fundamental constants of nature would emerge “out of math and π and 2’s,” as Berkeley physicist Raphael Bousso puts it.
An infinity of universes is simpler than a single universe would be—there is less to explain.
This is the lure of Einstein’s general theory of relativity—the reason physicists all over the world exclaim at its extraordinary, enduring beauty. Considerations of symmetry dictate the equations so clearly that the theory seems inevitable. That is what we have wanted to replicate in other domains of physics. And so far we have failed.
For decades, scientists have looked for a physical reason why the fundamental constants should take on the values they do, but none has thus far been found. In fact, when we use our current theories to guess at the probable value at some of these parameters, the answers are so far from what is measured that it is laughable. But then how do we explain these parameters? If there is just this one unique universe, the parameters governing its design are invested with a special significance. Either the process governing them is completely random or there must be some logic, perhaps even some design, behind the selection.
Neither option seems particularly appealing. As scientists, we spend our lives looking for laws because we believe there are reasons why things happen, even when we don’t understand them; we look for patterns because we think there is some order to the universe even if we don’t see it. Pure, random chance is not something that fits in with that worldview.
But to invoke design isn’t very popular either, because it entails an agency that supersedes natural law. That agency must exercise choice and judgment, which—in the absence of a rigid, perfectly balanced, and tightly constrained structure, like that of general relativity—is necessarily arbitrary. There is something distinctly unsatisfying about the idea of there being several logically possible universes, of which only one is realized. If that were the case, as cosmologist Dennis Sciama said, you would have to think “there’s [someone] who looks at this list and says ‘well we’re not going to have that that one, and we won’t have that one. We’ll have that one, only that one.’ ”
There may well be nothing special about our entire universe, except for the fact that it is ours.
Personally speaking, that scenario, with all its connotations of what could have been, makes me sad. Floating in my mind is a faint collage of images: forlorn children in an orphanage in some forgotten movie when one from the group is adopted; the faces of people who feverishly chased a dream, but didn’t make it; thoughts of first-trimester miscarriages. All these things that almost came to life, but didn’t, rankle. Unless there’s a theoretical constraint ruling out all possibilities but one, the choice seems harsh and unfair.
In such a carefully calibrated creation, how are we to explain needless suffering? Since such philosophical, ethical, and moral concerns are not the province of physics, most scientists avoid commenting on them, but Nobel laureate Steven Weinberg spelled it out: “Whether our lives show evidence for a benevolent designer … is a question you will all have to answer for yourselves. My life has been remarkably happy … but even so, I have seen a mother painfully die of cancer, a father’s personality destroyed by Alzheimer’s disease and scores of second and third cousins murdered in the Holocaust. Signs of a benevolent designer are pretty well hidden.”
In the face of pain, an element of randomness is far easier to accept than either the callous negligence or the deliberate malevolence of an otherwise meticulously planned universe.
The multiverse promised to extricate us from these awful thoughts, to provide a third option that overcame the dilemma of explanation.
To be sure, physicists didn’t invent it for that purpose. The multiverse emerged out of other lines of thought. The theory of cosmic inflation was intended to explain the broad-scale smoothness and flatness of the universe we see. “We were looking for a simple explanation of why the universe looks like a big balloon,” says Stanford physicist Andrei Linde. “We didn’t know we had bought something else.” This something else was the realization that our big bang was not unique, and that there should in fact be an infinite number of them, each creating a disconnected domain of spacetime.
Then string theory came along. String theory is currently the best contender we have for a unified theory of everything. It not only achieves the impossible—reconciling gravity and quantum mechanics—but insists upon it. But for a scheme which reduces the enormous variety of our universe to a minimalist set of building blocks, string theory suffers from a singularly embarrassing problem: We don’t know how to determine the precise values of the fundamental constants of nature. Current estimates say there are about 10500 potential options—a number so unfathomably large we don’t even have a name for it. String theory lists all the possible forms physical laws can take, and inflation creates a way for them to be realized. With the birth of each new universe, an imaginary deck of cards is shuffled. The hand that is dealt determines the laws that govern that universe.
The multiverse explains how the constants in our equations acquire the values they do, without invoking either randomness or conscious design. If there are vast numbers of universes, embodying all possible laws of physics, we measure the values we do because that’s where our universe lies on the landscape. There’s no deeper explanation. That’s it. That’s the answer.
But as much as the multiverse frees us from the old dichotomy, it leaves a profound unease. The questions we have spent so long pondering might have no deeper answer than just this: that it is the way it is. That might be the best we can do, but it’s not the kind of answer we’re used to. It doesn’t pull back the covers and explain how something works. What’s more, it dashes the theorists’ dream, with the claim that no unique solution will ever be found because no unique solution exists.
There are some who don’t like that answer, others who don’t think it even qualifies to be called an answer, and some who accept it.
To Nobel laureate David Gross, the multiverse “smells of angels.” Accepting the multiverse, he says, is tantamount to throwing up your hands and accepting that you’ll never really understand anything, because whatever you see can be chalked up to a “historical accident.” His fellow Nobelist Gerard ’t Hooft complains he cannot accept a scenario where you are supposed to “try all of these solutions until you find a universe that looks like the world we live in.” He says: “This is not the way physics has worked for us in the past, and it is not too late to hope that we will be able to find better arguments in the future.”
Princeton cosmologist Paul Steinhardt refers to the multiverse as the “Theory of Anything,” because it allows everything but explains nothing. “A scientific theory ought to be selective,” he says. “Its power is determined by the number of possibilities it excludes. If it includes every possibility, then it excludes nothing, then it has zero power.” Steinhardt was one of the early champions of inflation until he realized that it generically gave rise to the multiverse, carving out a space of possibilities rather than making specific predictions. He has since become one of inflation’s most vocal critics. On a recent episode of Star Talk, he introduced himself as a proponent of alternatives to the multiverse. “What did the multiverse ever do to you?” the host joked. “It destroyed one of my favorite ideas,” Steinhardt replied.
Physics was supposed to be the province of truth, of absolutes, of predictions. Things either are, or aren’t. Theories aren’t meant to be elastic or inclusive, but instead restrictive, rigid, dismissive. Given a situation, you want to be able to predict the likely—ideally, the unique and inevitable—outcome. The multiverse gives us none of that.
The debate over the multiverse sometimes gets vociferous, with skeptics accusing proponents of betraying science. But it’s important to realize that nobody chose this. We all wanted a universe that flowed organically from some beautiful deep principles. But from what we can tell so far, that’s not the universe we got. It is what it is.
Must the argument for the multiverse be negative? Must it be a distant second-best option? Many of my colleagues are trying to put the multiverse in a more hopeful light. Logically speaking, an infinity of universes is simpler than a single universe would be—there is less to explain. As Sciama said, the multiverse “in a sense satisfies Occam’s razor, because you want to minimize the arbitrary constraints you place on the universe.” Weinberg says that a theory that is free of arbitrary assumptions and hasn’t been “carefully tinkered with to make it match observations” is beautiful in its own way. It might turn out, he says, that the beauty we find here is similar to that of thermodynamics, a statistical kind of beauty, which explains the state of the macroscopic system, but not of its every individual constituent. “You search for beauty, but you can’t be too sure in advance where you’ll find it, or what kind of beauty you’ll have,” Weinberg says.
Several times, while contemplating these weighty intellectual issues, my thoughts circled back to the simple, beautiful wisdom of Antoine de Saint-Exupéry’s Little Prince who, having considered his beloved rose unique in all the worlds, finds himself in a rose garden. Bewildered by this betrayal and saddened by the loss of consequence—his rose’s and his own—he breaks down in tears. Eventually he comes to realize that his rose is “more important than all the hundreds of others” because she is his.
There may well be nothing special about our entire universe, except for the fact that it is ours. But isn’t that enough? Even if our entire lives, the sum of all we can ever know, turn out to be cosmically insignificant, they are still ours. There is something distinguished about here, now, mine. Meaning is something we confer.
Several times over these past few months, I found myself replaying my conversation with Gian Giudice. I found it reassuring how unperturbed he was by the vast range of possible universes and the seemingly arbitrary choices made by our own. Perhaps the multiverse is just telling us that we’re focusing on the wrong questions, he says. Maybe, as Kepler did with the orbits of the planets, we’re trying to read a deeper meaning into these numbers than is there.
Since the solar system was all Kepler knew, he thought the shapes of the planetary orbits and the specific values of their various distances from the sun must carry important information, but that turned out to not be the case. These quantities were not fundamental; they were merely environmental parameters. That may have seemed lamentable at the time, but looking back now from the vantage point of general relativity, we no longer feel any sense of loss. We have a beautiful description of gravity; it just happens to be one in which these values of the planetary orbits are not fundamental constants.
Perhaps, says Giudice, the multiverse implies something similar. Perhaps we need to let go of something we’re holding onto too tightly. Maybe we need to think bigger, refocus, regroup, reframe our questions to nature. The multiverse, he says, could open up “extremely satisfying, gratifying, and mind-opening possibilities.”
Of all the pro-multiverse arguments I heard, this is the one that appeals to me the most. In every scenario, for every physical system, we can pose infinitely many questions. We try to strip a problem back to the essentials and ask the most basic questions, but our intuition is built upon what came before, and it is entirely possible that we are drawing upon paradigms that are no longer relevant for the new realms we are trying to probe.
The multiverse is less like a closed door and more like a key. To me, the word is now tinged with promise and fraught with possibility. It seems no more wasteful than a bower full of roses.
Lead image: A Tate Modern employee views The Passing Winter 2005 by Japanese artist Yayoi Kusama. Credit: Daniel Leal-Olivas/AFP/Getty Images.