Why the Theory of the Multiverse is Unscientific

Note: I wrote this in another place in response to someone's question. We were discussing this video about the theory of multiverses that was posted to YouTube a few days ago.

Sean Carroll explains that there are two possibilities, either the branching is infinite or finite (I think that covers just about everything).

With infinite branching there would have to be an infinite amount of energy and time, because with an infinite amount of branching drawing from a finite pool of energy, at some point the energy would be exhausted. So there would have to be an infinite amount of energy. I heard Sean Carroll make exactly this argument at a talk a few years ago.

By his own admission, if there were an infinite amount of energy and time then ALL possible universes will be seen. This includes our current universe, complete with 13+ billion years of existence, AND it would mean that an exact copy of this universe exists except in that universe Harry Potter, Hogwarts, and magic are all real and part of the universe. There also exists a universe that consists of ONLY the room you are currently in, complete with computer/phone/tablet that give the appearance of you interacting with the outside world, but the outside world doesn't exist. And whenever you leave the room that universe will cease to exist.

Again these are all arguments that Sean Carroll makes himself.

But this means that there exists a universe like our own where there is no such thing as the multiverse and it behaves as if there were only one timeline and absolutely no branching. There would also exist a universe that looks exactly like ours but the multiverse exists just as he describes it.

But how do we know which one we are in? Because if we go looking for evidence and don't find it then we don't know if we are in the universe with no multiverses, or if we could be in a universe where it is impossible to detect the multiverse. Either way we can never know until we find evidence to conclusively show it one way or the other. But that evidence does not exist. So by his own logic, if the branching is infinite, and there is an infinite amount of energy, then we can never know if multiverses don't exist, or if we just haven't seen them yet. Either way the universe remains the same and the concept of multiverses means nothing.

Next, the second possibility he brings up is a finite amount of branching. This solves the infinite energy problem, but without an infinite amount of energy there is only one universe, even if locally it functions like a multiverse. (By local, that can mean just on earth, or within the visible universe 13+ billion lightyears away. On these scales the room around you and all the galaxies 13+ billion lightyears away are all considered local.) A locally branching multiverse would go against our current understanding of physics, but it would still be possible if and only if the things making it possible are beyond our current ability to understand, calculate, or observe.

In the video he admits this (starting at time 14:40) where he says "but the details hinge on quantum gravity, cosmology, the theory of everything, and all that stuff." He is essentially saying that there exists something that we don't know about right now that makes the multiverse work. This is essentially a scientific variation of the God of the Gaps argument. It comes down to "there is no other way for this to work, so there is something, we don't know what it is, that makes it work." You can call it quantum gravity, cosmology, the theory of everything, God, Bob your neighbor, magic, a lazy dog, or anything you want it doesn't matter. It simply is a "thing" that makes it possible for his idea to be correct.

But again, we have nothing that specifically points to a multiverse, so it doesn't matter what you call the thing that makes it possible, because in the end it is something undefined to support something unproven. You could just as easily say, "The magic of Harry Potter is real but the details hinge on quantum gravity, cosmology, the theory of everything, and all that stuff" and be just a scientifically valid. Which means not at all.

"But! There is MATH behind it!"
That's nice. You can put math behind any idea. It doesn't make it real.

There is no evidence that points us specifically towards a multiverse. There is no physical motivation other than to resolve a paradox that we made for ourselves. The paradox does not come from the universe. It comes from how we think about the universe. We do not resolve a paradox that we made ourselves by insisting that the universe change to fit our ideas. Our ideas must change to fit reality.