Humans live in a universe; that is a fact. Up for debate, though, is whether that universe lives in a sea of other universes—a multiverse.
The idea of a multiverse is the subject of much science fiction—but it’s also a real possibility (or rather a set of many possibilities) that some scientists take seriously and investigate.
Multiversal concepts pop up in several branches of modern physics. In quantum mechanics, for instance, a particle exists in a superposition of all possible states at once—until, that is, someone tries to make a measurement of it. At that point, the possibilities collapse, and one physical state becomes apparent to the observer. The “many worlds” interpretation of quantum mechanics, though, posits that all the possible states the measurement might have shown play out in different universes, each with a different version of the observer.
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The many-worlds interpretation is perhaps the most famous scientific idea of a multiverse. But it’s far from the only one. In his new book, The Allure of the Multiverse: Extra Dimensions, Other Worlds, and Parallel Universes, physicist Paul Halpern of Saint Joseph’s University in Philadelphia explores these potential parallel realities, along with their histories and evolutions, their philosophies and the insights they offer into the nature of science.
Scientific American spoke to Halpern about how the concept of the multiverse fits with modern physics, the evidence scientists have for it so far, and what it all means for the nature of our existence.
An edited transcript of the interview follows.
What exactly is a multiverse, anyway?
There are different ideas about the multiverse: cultural ideas and scientific ideas. Those notions tend to be very different. The cultural ideas apply to human life. People wonder what would have happened if they had moved to a different city, taken a different job, decided to pursue different hobbies or had a different relationship. They imagine paths they didn’t take and try to picture what would happen.
In science, the idea can be split into the quantum multiverse and the cosmological multiverse. The quantum multiverse is a possible answer to the questions of what happens when measurements are taken in quantum physics and how human life is connected to the quantum world. That was addressed in 1957 by a young graduate student, Hugh Everett III, with the many-worlds interpretation. He speculated that different possibilities split into different universes—and that humans experience multiple realities but don’t really know about their doppelgängers.
And there’s the cosmological multiverse, which is the idea that a process called inflation, the rapid expansion believed to be an early stage of the universe, is relatively easy to achieve in the early universe and elsewhere and happens all the time. It results in other bubble universes that expand, and our universe has also expanded, so they’re currently beyond our scope.
What are the properties of this universe that make physicists think a multiverse might be a correct interpretation of reality?
The parameters of our universe seem to be within the right range for galaxies, stars, planets and life to form. If these constants—the strength of gravitation, the strength of the electromagnetic interactions, and so forth—were adjusted just a bit, then planets and life as we know it never would have formed. This is sometimes called the fine-tuning problem.
One reason for this fine-tuning was proposed by physicist Brandon Carter around 1970. He suggested that maybe something about our enclave of the universe, and maybe even our whole universe, is special. Maybe, then, we should think about all the other possibilities and consider why we’re in this branch of the universe rather than the others. And that might have something to do with the fact that there’s an array of possible universes, and we happen to be one of the few that could support structure formation and eventually lead to life as we know it. But in most of the other versions, we would not be here.
One problem with those other universes is that, if they exist, they’re beyond our perception. Scientists don’t currently have a way to directly test the idea of the multiverse. In your book, you lay out some ideas for indirect tests and note that in the future we might come up with clever direct experiments. But what does it mean that the multiverse is currently untestable?
There are other ideas in physics now that can never be tested directly. For example, the universe is expanding and accelerating, and light has a finite speed, so we’re never going to be able to see parts of the universe that are beyond a certain radius. Beyond roughly 46 billion light-years we can only use our imagination. But it could be that nature has surprises beyond the range of observability, and we would never know.
Quantum uncertainty also limits what we can observe at once. Because of Heisenberg’s uncertainty principle, we couldn’t, even in theory, plot out the positions and momenta of all the particles in the universe at once.
The fact is that physics has evolved to a point where there are a lot of things that are not directly measurable. If we can come up with a theory that explains everything within the observable universe, and it requires reference to a multiverse, there is a segment of scientists who would say, “Well, we need to accept that there will be things that we will never know.” There are others who stubbornly might pursue other possibilities.
I think everyone would agree that if we could come up with a theory of the universe that is self-consistent, is all-encompassing and doesn’t rely on unobservable things, then that would even be better, but it might not happen.
Some researchers say that if something is untestable in traditional ways, it’s not science—it’s pseudoscience. To you, is the goal of science to try to find the truth of the universe or simply to find things that can be proved through experimentation?
Humanity has an ambition to try to understand everything in its world, and that now has become everything in its universe. We’re a very bold group of people living on a planet that’s a relatively tiny part of everything. We use our instruments to try to understand it as much as we can. We use different tools, and one of those tools is theoretical physics; another tool is direct observation. We hope that those methods match up, but sometimes there’s a lag. Sometimes there are experimental results that theory does not explain. Sometimes there are theoretical models—such as general relativity—that seem so compelling that there’s some degree of acceptance without observation, and only later does science produce experimental results.
There are exciting ideas in theory that take a little while to test, so one has to be patient. But, of course, if a better theory comes along that matches experimental tests, then people are going to flock to the better theory. They’re not going to always wait for the original theory to be confirmed.
Do you think most physicists are open to the idea of a multiverse?
When I was interviewing different people, I had some surprises. Some people whom I thought were very observation-based, hard-headed scientists turned out to be very open to the idea of a multiverse. And then others who have their own, maybe far-reaching, ideas turned out to be drawing the line, saying, “No, we can’t have a multiverse, but we can have these other things.”
Different theorists have their own tastes. The limits for one researcher might be completely different than the limits for another researcher. There’s a certain amount of personal philosophy involved.
I guess people are always people, with their own preferences, even in the sciences.
Even if you’re a trained physicist and are basing everything on laws of physics and things like general relativity and quantum physics, there’s always some room for philosophical preferences. There are some physicists who really like to think that time is an illusion, and others like to think that time is real. That verges on philosophy because it’s hard for us, or even impossible, to step outside of time and say, ‘Hey, wait a minute, it was an illusion all along. Really, the world is timeless.’ We can’t really do that. So we can only kind of speculate about whether time is really passing physically or whether, in reality, time and space are on the same footing and we’re just inside some kind of illusory realm in which time passes in our own minds.
Is there an overarching idea that you hope The Allure of the Multiverse conveys?
I’d like people to appreciate the range of possibilities in theoretical physics—even of things that are well accepted, such as the general theory of relativity and quantum physics—and understand that it is a great mystery how all these possibilities somehow filtered down into the universe that we observe today. It’s a mystery why things are the way they are, given all of the options.