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| A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (NASA / WMAP Science Team |
That expansion stopped after just a fraction of a second. But according to an idea called the “inflationary multiverse,” it continues—just not in our universe where we could see it. And as it does, it spawns other universes. And even when it stops in those spaces, it continues in still others. This “eternal inflation” would have created an infinite number of other universes.
Together, these cosmic islands form what scientists call a “multiverse.” On each of these islands, the physical fundamentals of that universe—like the charges and masses of electrons and protons and the way space expands—could be different.
Cosmologists mostly study this inflationary version of the multiverse, but the strange scenario can takes other forms, as well. Imagine, for example, that the cosmos is infinite. Then the part of it that we can see—the visible universe—is just one of an uncountable number of other, same-sized universes that add together to make a multiverse. Another version, called the “Many Worlds Interpretation,” comes from quantum mechanics. Here, every time a physical particle, such as an electron, has multiple options, it takes all of them—each in a different, newly spawned universe.
But all of those other universes might be beyond our scientific reach. A universe contains, by definition, all of the stuff anyone inside can see, detect or probe. And because the multiverse is unreachable, physically and philosophically, astronomers may not be able to find out—for sure—if it exists at all.
Determining whether or not we live on one of many islands, though, isn’t just a quest for pure knowledge about the nature of the cosmos. If the multiverse exists, the life-hosting capability of our particular universe isn’t such a mystery: An infinite number of less hospitable universes also exist. The composition of ours, then, would just be a happy coincidence. But we won’t know that until scientists can validate the multiverse. And how they will do that, and if it even possible to do that, remains an open question.
Null results
This uncertainty presents a problem. In science, researchers try to explain how nature works using predictions that they formally call hypotheses. Colloquially, both they and the public sometimes call these ideas “theories.” Scientists especially gravitate toward this usage when their idea deals with a wide-ranging set of circumstances or explains something fundamental to how physics operates. And what could be more wide-ranging and fundamental than the multiverse?
For an idea to technically move from hypothesis to theory, though, scientists have to test their predictions and then analyze the results to see whether their initial guess is supported or disproved by the data. If the idea gains enough consistent support and describes nature accurately and reliably, it gets promoted to an official theory.
As physicists spelunk deeper into the heart of reality, their hypotheses—like the multiverse—become harder and harder, and maybe even impossible, to test. Without the ability to prove or disprove their ideas, there’s no way for scientists to know how well a theory actually represents reality. It’s like meeting a potential date on the internet: While they may look good on digital paper, you can’t know if their profile represents their actual self until you meet in person. And if you never meet in person, they could be catfishing you. And so could the multiverse.
Physicists are now debating whether that problem moves ideas like the multiverse from physics to metaphysics, from the world of science to that of philosophy.
What the heck is a Multiverse?
Show-me state
Some theoretical physicists say their field needs more cold, hard evidence and worry about where the lack of proof leads. “It is easy to write theories,” says Carlo Rovelli of the Center for Theoretical Physics in Luminy, France. Here, Rovelli is using the word colloquially, to talk about hypothetical explanations of how the universe, fundamentally, works. “It is hard to write theories that survive the proof of reality,” he continues. “Few survive. By means of this filter, we have been able to develop modern science, a technological society, to cure illness, to feed billions. All this works thanks to a simple idea: Do not trust your fancies. Keep only the ideas that can be tested. If we stop doing so, we go back to the style of thinking of the Middle Ages.”
He and cosmologists George Ellis of the University of Cape Town and Joseph Silk of Johns Hopkins University in Baltimore worry that because no one can currently prove ideas like the multiverse right or wrong, scientists can simply continue along their intellectual paths without knowing whether their walks are anything but random. “Theoretical physics risks becoming a no-man's-land between mathematics, physics and philosophy that does not truly meet the requirements of any,” Ellis and Silk noted in a Nature editorial in December 2014.
It’s not that physicists don’t want to test their wildest ideas. Rovelli says that many of his colleagues thought that with the exponential advance of technology—and a lot of time sitting in rooms thinking—they would be able to validate them by now. “I think that many physicists have not found a way of proving their theories, as they had hoped, and therefore they are gasping,” says Rovelli.
“Physics advances in two manners,” he says. Either physicists see something they don’t understand and develop a new hypothesis to explain it, or they expand on existing hypotheses that are in good working order. “Today many physicists are wasting time following a third way: trying to guess arbitrarily,” says Rovelli. “This has never worked in the past and is not working now.”
The multiverse might be one of those arbitrary guesses. Rovelli is not opposed to the idea itself but to its purely drawing-board existence. “I see no reason for rejecting a priori the idea that there is more in nature than the portion of spacetime we see,” says Rovelli. “But I haven't seen any convincing evidence so far.”
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Source: Smithsonian and Fermilab Channel (YouTube)
