Welcome to the Multiverse Chronicles: A Journey Through Infinite Realities
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 measure it. At that point, the possibilities collapse, and one physical state becomes apparent to the observer. The “many worlds” interpretation of quantum mechanics posits that all the possible states the measurement might have shown play out in different universes, each with a different version of the observer.
Scientific American spoke to Halpern about how the multiverse concept fits with modern physics, the evidence scientists have for it so far, and what it all means for the nature of our existence.
What exactly is a multiverse, anyway?
There are different ideas about the multiverse: cultural ideas and scientific ideas. Those notions are very different. The artistic ideas apply to human life.
The idea can be split in science into the quantum multiverse and the cosmological multiverse. The quantum multiverse is a possible answer to 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 know about their doppelgängers.
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 are within the proper 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 something about our enclave of the universe, and maybe even our whole universe, is unique. Perhaps we should consider all the other possibilities and 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 are 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.
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 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 possible. We use different tools, and one of those tools is theoretical physics; another is direct observation. We hope those methods match up, but sometimes there’s a lag. Sometimes, there are experimental results that theory does not explain. Sometimes, theoretical models—such as general relativity—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 will flock to the better theory. They won’t always wait for the original theory to be confirmed.
Do you think most physicists are open to the idea of a multiverse?
When I interviewed different people, I was surprised. 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 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.
People are always people with their preferences, even in the sciences.
Even if you’re a trained physicist basing everything on laws of physics and things like general relativity and quantum physics, there’s always some room for philosophical preferences. Some physicists like to think that time is an illusion, and others want to believe that time is accurate. 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. The world is timeless.’ We can’t do that. So we can only speculate about whether time is passing physically or whether, in reality, time and space are on the same footing, and we’re just inside some imaginary realm in which time passes in our minds.
Is there an overarching idea you hope The Allure of the Multiverse conveys?
I want 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. Given all of the options, it’s a mystery why things are the way they are.