If you can’t see it, it doesn’t exist. That’s an old philosophy, one that many scientists swallowed whole. But as Ziva David of NCIS would say, it’s total salami. After all, you can’t see bacteria and viruses, but they can still kill you.
Yet some scientists still invoke that philosophy to deny the scientific status of all sorts of interesting things. Like the theoretical supertiny loops of energy known as superstrings. Or the superhuge collection of parallel universes known as the multiverse.It’s the same attitude that led some 19th century scientists and philosophers to deny the existence of atoms.
Ernst Mach, leader of the anti-atomists, said atoms were a “mental artifice,” existing only in thought. “Atoms cannot be perceived by the senses,” he said.
To be fair, Mach was an accomplished physicist and a deep thinker. He wouldn’t have objected to believing in bacteria once you looked at them through a microscope. But Mach thought atoms were unobservable in principle. “From their very nature,” he wrote, they “can never be made the objects of sensuous contemplation.” He turned out to be wrong—today super-duper microscopes can not only show you images of atoms but can also drag them around to spell out IBM on magazine covers.
Even if atoms had been forever unseeable, their existence could still be established indirectly. That’s what Einstein did in 1905 when he analyzed Brownian motion, the random fluttering of tiny particles suspended in a liquid. Einstein showed mathematically that Brownian motion was a consequence of atoms and molecules bouncing off the suspended particles. So atoms do exist, even if they can’t be observed, as their existence implies phenomena that can be observed.
Similar reasoning can be applied to parallel universes. If other universes exist, they may well be forever beyond the power of humankind’s observational instruments. But perhaps the laws explaining observable things also require unobservable universes.
“It might be that the laws we use successfully to describe the observable universe are most naturally formulated in a larger framework, that includes unobservable parts,” writes Nobel laureate physicist Frank Wilczek of MIT.
In a new paper, Wilczek ruminates on the various ways that a multiverse could reveal its reality. By multiverse, he means realms where different laws of physics can apply at different places and times — a property he calls “multiversality.” It makes sense to consider the multiverse real, Wilczek contends, if aspects of observable reality can be explained only by recourse to the principle of multiversality.
One such aspect of observable reality, he points out, is the observable universe itself, and the fact that it contains observers. It turns out that a very limited range of physical conditions allow the ingredients needed for observers to exist (stars, planets, atoms). When you investigate what would happen if you changed some basic quantities of physics (say the speed of light or the charge on an electron), you find that “the emergence of complex structures capable of supporting intelligent observation appears quite fragile,” Wilczek writes.
One such feature of the universe is the amount of energy residing throughout the vacuum of space. This “dark energy” exerts a repulsive force; in large amounts it would dilute matter in the universe too much for stars and galaxies to form. Fortunately the actual observed amount of dark energy is very small, allowing the universe to build the infrastructure needed for life.
Attempts to calculate why the dark energy is so weak have failed, and “lucky coincidence” does not really count as a scientific explanation. But if the multiverse exists, with a multiplicity of dark energy densities, then the mystery is solved: Humans live in the part of the multiverse where dark energy is meager. In regions with a lot of dark energy, nobody is around to wonder about it.
Wilczek examines other examples of multiversality, including the role that superstring theory plays in explaining the dark energy mystery. In several cases he finds that multiversality can explain the otherwise inexplicable.
He warns that some phenomena may be unexplained simply because physicists haven’t yet been sufficiently clever to find the true explanation. But his deeper point remains: the need to recognize that science should not limit its vision.
“In the past scientists have repeatedly reached ‘intellectual closure’ on inadequate pictures of the universe, and underestimated its scale,” he notes. To Copernicus, the universe was little more than the solar system. A century ago, it was not yet clear that the universe was anything more than the Milky Way galaxy. Now telescopes reveal a universe with billions of billions of galaxies.
It shouldn’t be considered silly to propose that existence will once again turn out to be bigger than humans have previously supposed. It’s just that this time seeing farther will require abandoning the link between seeing and existing. As Wilczek notes, the dictum that “nothing that is not observed exists” has “no empirical content.”
“It is more in the nature of a moral exhortation,” he writes, “whose application, though usually appropriate, might be outweighed by other scientific considerations.”