In The Number of the Heavens, Tom Siegfried, the award-winning former editor of Science News, shows that one of the most fascinating and controversial ideas in contemporary cosmology—the existence of multiple parallel universes—has a long and divisive history that continues to this day. We spoke to him about the possible existence of a multiverse and the controversy surrounding it.
Trying to conceptualize something like the universe, let alone a multiverse, feels like a big undertaking. Where should people start in trying to comprehend the existence of something so vast?
You have to start small, say with the Earth. When H. G. Wells wrote his history of the world, he imagined shrinking the Earth to the size of a ball one inch across. The sun would then be a globe of fire, 9 feet across, a little more than 300 yards away. The planet Neptune, on this reduced scale, would be more than five miles away. The nearest star, the Alpha Centauri system, would be about 40,000 miles away. And the farthest objects observable in space would be something like 100 trillion miles away, even on the scale with the Earth smaller than a golf ball. Put another way, it takes light eight minutes to reach the Earth from the sun, four years to travel from the sun to the nearest star, and billions of years to travel to the farthest visible objects. That’s all within the universe we can see. If there’s a multiverse, countless other such spaces, of similar extent, exist beyond what we can see.
This book breaks down an eternal debate about whether there is one universe or many. You describe it like a long-running TV sitcom, where characters keep cycling out but common themes continue to reemerge. What are some of those common themes?
One common theme is that logic rules out more than one universe, because the universe contains everything so there can be only one. But the definition of “universe” at any point in time may turn out to be incorrect—it has often been the case that what people believed to be the universe was not, in fact, everything that existed. So the definition of universe needed to change.
Another common theme is that universes beyond our sight are not a proper subject for science—if we can’t see them, it is meaningless to believe that they exist. But advances in technology (such as better telescopes) can bring previously unseen objects into view. And in some cases the explanation for things that we can see unavoidably entail the existence of other things we cannot. Similar arguments were made against the existence of atoms—they could not be seen, so they could not be real. But plenty of evidence indirectly indicated their existence, and modern microscopes can image them. It’s not a coincidence, I think, that the ancient Greek philosophers who proposed the existence of atoms also believed in the existence of multiple universes.
What is your favorite figure from this historic debate about the multiverse? Whose work have you found particularly compelling?
Many to choose from. I guess I like Bernard le Bovier de Fontenelle the best, because he was a popularizer of science like me. He articulated the 17th-century case for a plurality of worlds better than anyone. But I should also mention Ormsby MacKnight Mitchel. He single-handedly made Cincinnati the leading center for astronomy in America in the 1840s, and was a most enthusiastic advocate for the view that the distant fuzzy nebulae in space were “island universes” like the Milky Way.
You mentioned that many serious scientists today consider the theory of the multiverse to be unscientific. Please explain.
There is a widespread view among scientists that the multiverse is an idea that cannot be tested, or “falsified,” and therefore it is not scientific. Many other scientists challenge that view, however. For one thing, the requirement of falsifiability is no longer widely accepted by many scientists and philosophers. For another, the notion of a multiverse is not itself a theory, but is rather a prediction of other theories that can, in fact, be tested in various ways.
Books and movie often portray the idea of a parallel universe as this alternate reality, where another version of you is living a totally different life. Does this idea of alternate selves hold up with scientists, or is it wishful thinking?
The idea of alternate selves is most closely connected with the Many Worlds Interpretation of quantum mechanics. In that interpretation, each of us does split into additional versions of ourselves as quantum measurements are made. Some physicists advocate strongly for this view, while others reject it. It’s a debate that will not be resolved anytime soon.
It’s also possible that the universe is infinite, and in a multiverse there could be an infinite number of universes. If so, and if the number is truly infinite and not just really big, all possible combinations of atoms will occur over and over again in different places. And so there would be other combinations of atoms identical to those that make up you (and me, and everybody else). Whether that would count as another version of you or just somebody exactly like you is a good question.
You write, “The possibility of a multiverse implies larger lessons about how science should be done” and “The rules of science that work well for explaining nature in a lone universe may not apply in a multiverse.” Could you speak more to this? How would science change by taking the rules of a multiverse into account?
The traditional goal of science (or at least of physicists) is to explain the world by deduction from a small number of fundamental laws and principles. Ideally, in this view, one deep theory would enable calculations of all the physical features of the universe. This view implicitly assumes that the universe is governed everywhere (and at all times) by the same laws. If so, and there is only one universe, then this is a sensible approach to doing science. But rigorously applying the rules that stem from this approach deny the possibility of other universes from the beginning. If, in fact, other universes exist, the premises upon which the standard approach rests no longer apply. In that case the explanation for phenomena we observe may be that multiple possibilities are realized in different universes, and ours is the way it is because its peculiar features permit us to live here. It’s like the way no fundamental law determines the Earth’s temperature. It’s not the only planet—it’s one of many, and its temperature is what it is because that’s the temperature suitable for habitation.
