Some people talk about the Multiverse as if it’s an established fact. And some people suppose that, because it’s been connected with string theory and quantum physics, the whole thing is probably true but just over the average Joe’s head.
As both an apologist and a writer of science fiction, I’m just going to say from the outset that the Multiverse is, at present, purest speculation. In fact, by its very nature, it’s likely to remain so.
In case, I’ve jumped the gun for some of my readers, the Multiverse is the idea that our Earth is just one of an infinite number of universes, each having a slightly different set of properties. Television shows like Sliders gave us a basic idea of how the concept works, but it’s so much more. It is quite literally the Theory of Anything.
And I do mean anything. In the Multiverse, there are universes where life doesn’t even exist on Earth and where its as ubiquitous as the Star Wars universe. In the Multiverse, there are universes where you are the President, where you’re the opposite gender, and where you’re the darkest part of yourself, like that Star Trek episode where Spock had a beard. In the Multiverse, Hitler never existed, lived as a slave during the Civil War, and won and lost a thousand World War I I in a zillion different ways, including getting punched in the nose by Captain America. In the Multiverse, there is an America where the turkey became the national bird and we eat Thanksgiving fish thrice a year. I could go on. All that’s required is imagination.
One of the reasons for the Multiverse theory is to get past the Anthropic Principle, the observation that the properties of the universe we inhabit seem finely tuned for life here on Earth. It’s a big problem for folks who’ve hitched their star to pure naturalism because the Anthropic Principle implies a Creator, the ultimate negation of pure naturalism. The Multiverse converts the Anthropic Principle to a weak form; that is, the universe appears to be fine tuned for life on Earth because we just happen to exist one possible universe with these properties out of a random infinite number of possible universes. Incidentally, it’s also used to sidestep the Fermi Paradox, the observation that the Copernican Principle would predict that life would be common in the universe but, as yet, we appear to be alone. An appeal to the Multiverse washes the paradox away, in theory: we just happen to be in the universe where life should be theoretically ubiquitous but actually only exists here.
You see, it is quite literally the Theory of Anything
Scientifically speaking, the Multiverse is void for vagueness. Princeton cosmologist Paul Steinhardt used the 2014 annual Edge Foundation Question to voice his criticism:
“A pervasive idea in fundamental physics and cosmology that should be retired: the notion that we live in a multiverse in which the laws of physics and the properties of the cosmos vary randomly from one patch of space to another. According to this view, the laws and properties within our observable universe cannot be explained or predicted because they are set by chance. Different regions of space too distant to ever be observed have different laws and properties, according to this picture. Over the entire multiverse, there are infinitely many distinct patches. Among these patches, in the words of Alan Guth, “anything that can happen will happen—and it will happen infinitely many times”. Hence, I refer to this concept as a Theory of Anything.
Any observation or combination of observations is consistent with a Theory of Anything. No observation or combination of observations can disprove it. Proponents seem to revel in the fact that the Theory cannot be falsified. The rest of the scientific community should be up in arms since an unfalsifiable idea lies beyond the bounds of normal science. Yet, except for a few voices, there has been surprising complacency and, in some cases, grudging acceptance of a Theory of Anything as a logical possibility. The scientific journals are full of papers treating the Theory of Anything seriously. What is going on?” 
In other words, it’s just bad science. It can’t be falsified and makes no tangible predictions. It’s a cop-out that amounts to a shoulder-shrugging pseudoscientific, “Well, that’s just the way it is.”
When I say there’s no way to prove it, I do so with a high level of confidence. You see, our observable samples size of universes is exactly one. Even in theory, we could not observe other universes in the Multiverse. We could speculate that features of our observable universe are evidence of contact with another, but honestly how would we objectively be able to determine whether those features weren’t just properties of our lone universe and had nothing to do with the proposed Multiverse?
Now I happen to think the strong Anthropic Principle is evident because our Creator intended to give us, as Blaise Pascal put it, too little evidence to be sure but too much to ignore. I also happen to think that the answer to the Fermi Paradox is that the Copernican Principle is arrogant in its presumption of a purely material universe. Aliens that are kind of rare rather than ubiquitous would cast doubt on a purely naturalistic universe. O course, the materialist could appeal to the Multiverse and claim that aliens just happen to be rare in this universe when they could be more commonplace. Meanwhile, the creationist could observe that Creator chose to paint on more than one canvas.
One of the intriguing thing about the Multiverse for a scifi writer is that it that in a way it does exist. It simply exists in imagination and in literature. The worlds we create, especially when we write or film speculative fiction, can be just as real as the one we live and breathe in. Tolkein likened storytelling to an act of sub-creation, which he further saw as an act of worship. We are made in the image of the Creator; what greater act of worship can there be than creation? Similarly, it may also be that the Multiverse exists as all possible worlds in the mind of God, but again it seems there is no place where the Multiverse exists outside of the realm of imagination.
 Steinhardt, Paul. “Theories of Anything.” WHAT SCIENTIFIC IDEA IS READY FOR RETIREMENT? edge.org (March 9, 2014). Retrieved November 23, 2016