The notion that there are universes beyond our own — the idea that we are but one member of a vast collection of universes called the multiverse — is highly speculative, but both exciting and humbling. It's also an idea that suggests a radically new, but inherently risky approach to certain scientific problems.
An essential working assumption in the sciences is that with adequate ingenuity, technical facility, and hard work, we can explain what we observe. The impressive progress made over the past few hundred years is testament to the apparent validity of this assumption. But if we are part of a multiverse, then our universe may have properties that are beyond traditional scientific explanation. Here's why:
Theoretical studies of the multiverse (within inflationary cosmology and string theory, for example) suggest that the detailed properties of the other universes may be significantly different from our own. In some, the particles making up matter may have different masses or electric charges; in others, the fundamental forces may differ in strength and even number from those we experience; in others still, the very structure of space and time may be unlike anything we've ever seen.
In this context, the quest for fundamental explanations of particular properties of our universe — for example, the observed strengths of the nuclear and electromagnetic forces — takes on a very different character. The strengths of these forces may vary from universe to universe and thus it may simply be a matter of chance that, in our universe, these forces have the particular strengths with which we're familiar. More intriguingly, we can even imagine that in the other universes where their strengths are different, conditions are not hospitable to our form of life. (With different force strengths, the processes giving rise to long-lived stars and stable planetary systems — on which life can form and evolve — can easily be disrupted.) In this setting, there would be no deep explanation for the observed force strengths. Instead, we would find ourselves living in a universe in which the forces have their familiar strengths simply because we couldn't survive in any of the others where the strengths were different.
If true, the idea of a multiverse would be a Copernican revolution realized on a cosmic scale. It would be a rich and astounding upheaval, but one with potentially hazardous consequences. Beyond the inherent difficulty in assessing its validity, when should we allow the multiverse framework to be invoked in lieu of a more traditional scientific explanation? Had this idea surfaced a hundred years ago, might researchers have chalked up various mysteries to how things just happen to be in our corner of the multiverse, and not pressed on to discover all the wondrous science of the last century?
Thankfully that's not how the history of science played itself out, at least not in our universe. But the point is manifest. While some mysteries may indeed reflect nothing more than the particular universe, within the multiverse, we find ourselves inhabiting, other mysteries are worth struggling with because they are the result of deep, underlying physical laws. The danger, if the multiverse idea takes root, is that researchers may too quickly give up the search for such underlying explanations. When faced with seemingly inexplicable observations, researchers may invoke the framework of the multiverse prematurely — proclaiming some or other phenomenon to merely reflect conditions in our bubble universe — thereby failing to discover the deeper understanding that awaits us.