Quantum mechanics operates on a spectrum of possibility or probability, while classical physics operates on a binary understanding of reality where things either happen or they don’t. Some physicists, relying on the groundbreaking work of Hugh Everett III (1930-1982), have taken to arguing that there’s an easy way to resolve this problem: if we use an extreme version of the many-worlds interpretation (MWI), which suggests that all possibilities are actualized in different universes, we can respect both the empirical evidence of certainty suggested by classical physics and the empirical evidence of ambiguity suggested by quantum mechanics. Every hypothetical, non-actualized uncertainty becomes a range of real, actualized certainties, each existing as a different universe.
The trouble is that if we accept the hypothesis that all possibilities are actualized, that means an awful lot of universes—some of them really, really freaky. Star Trek explored this idea in its episode “Mirror, Mirror” (1967), where an evil “mirror" version of the Enterprise crew (including a famously goateed Mr. Spock) spread fascism and terror throughout the universe, but it gets worse. Much, much worse.
We can begin with the obvious large-scale historical examples: in some universes, the Nazis win World War II. The Confederacy wins the American Civil War. The United States and the Soviet Union bomb each other, and the rest of us, into oblivion. Penicillin is never invented. Polio is never cured. We could go on.
Then we can look at more remote, but still possible (and therefore actualized), universes: A world where no serial killer is ever caught. A world where every police department has always been corrupt. A world where every military force kills civilians. A world where every deadly disease adapts to become airborne. Unlikely though all of these scenarios are, they’re possible—so in some universes, they’re certain.
Imagine how this changes our understanding of heroism. If you beat the odds by accomplishing something heroic, all that means is that you failed in most universes and just happened to slip into one of the few universes where you succeeded. From a many-worlds point of view, the best decisions are always the safe, moderate ones—because risking possible disaster, even if it can be averted, creates universes where that disaster is actualized. Better to do nothing dangerous, say nothing dangerous, and remain—above all—maximally safe and harmless. No angry thoughts. No bold ideas. Just maximum personal, medical, and cultural homeostasis.
But even that is not enough. If your mind is itself subject to the same probabilities as the rest of the universe, there are as many MWI versions of you as there are of any other phenomenon. So there are universes where you assault, murder, and steal; universes where you have betrayed everybody and everything you care about in every potential way; universes where you have killed and/or married and/or mugged every human being you’ve ever met.
There are, of course, milder versions of the MWI, and there are many scientists who do not accept any version of the MWI. But if we’re going to talk about this idea in a philosophical sense, we need to acknowledge that it would—if it is ever confirmed—completely change the way we look at morality. An ethic that fully incorporates the more radical forms of MWI would tell us it isn’t enough to simply be good; if we are to be responsible for our actions and all of their consequences, in ever universe, we have to limit the possibility that bad things will happen whether they ever actually do or not. And we have to live with the knowledge that, no matter what we do or don't do, improbably horrible things will come of it somewhere down the line.
There are, of course, many religious systems of morality that already say what we are is more important than what we do, that nobody is fundamentally good, and that other realities exist separate from our own. And if radical MWI is proven to be accurate, we may find ourselves returning to some of these very old ideas in an effort to grapple with it.