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.
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