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As memes go, it wasn’t particularly viral. But for a couple of hours on the morning of November 6, the term “darkest timeline” trended in Google searches, and several physicists posted musings on social media about whether we were actually in it. All the probabilities expressed in opinion polls and prediction markets had collapsed into a single definite outcome, and history went from “what might be” to “that just happened.” The two sides in this hyperpolarized U.S. presidential election had agreed on practically nothing—save for their shared belief that its outcome would be a fateful choice between two diverging trajectories for our world. That raises rather obvious (but perhaps pointless) questions: Could a “darkest

timeline” (or any other “timeline,” for that matter) be real? Somewhere out there in the great beyond, might there be a parallel world in which Kamala Harris electorally triumphed instead?

It turns out that, outside of fostering escapist sociopolitical fantasies and putting a scientific gloss on the genre of counterfactual history, the notion of alternate timelines is in fact something physicists take very seriously. The concept most famously appears in quantum mechanics, which predicts a multiplicity of outcomes—cats that are both alive and dead and all that. If a particle of light—a photon—strikes a mirror that is only partially silvered, the particle can, in a sense, both pass through and reflect off that surface—two mutually exclusive outcomes, known in physics parlance as a superposition. Only one of those possibilities will manifest itself when an observation is made, but until then, the particle juggles both possibilities simultaneously. That’s what the mathematics says—and what experiments confirm. For instance, you can create a superposition and then uncreate it by directing the light onto a second partially silvered mirror. That wouldn’t be possible unless both possibilities remained in play. Although this feature is usually framed in terms of subatomic particles, it is thought to be ubiquitous across all scales in the universe.

What supports the idea that these timelines are real, and not just imaginative fictions, is that they can “interfere” with one another, either enhancing or diminishing the probability of their occurrence. That is, something that might have happened but doesn’t has a measurable effect on what does, as if the former reaches from the shadowy realm of the possible into the world of the actual.

Consider the bomb detector that physicists Avshalom Elitzur and Lev Vaidman proposed in 1993 and that has since been demonstrated (fortunately not with real bombs): Perform the experiment with the partially silvered mirror but place a light-sensitive bomb along one of the two paths the photon can take. This blockage prevents you from uncreating the superposition to restore the traveling photon to its original state. It does so even if the bomb never goes off, indicating that the photon never touched it. The mere possibility that the photon could strike the bomb affects what happens. In theory, you could use this principle—known as counterfactual definiteness—to take x-ray images of cells without subjecting them to damaging radiation. In an emerging subject known as counterfactual quantum computing, a computer outputs a value even if you never press the “run” button.

One way to think about counterfactual definiteness is known as the many-worlds interpretation. A photon striking a mirror causes the cosmic timeline to branch, creating one world in which the particle passes through the mirror and one in which it reflects off that surface. Each of us is stuck inside our world and therefore sees only one outcome at a time, but the other is still there, visible to an inhabitant of the alternate world. All such worlds, taken together, constitute a “multiverse.”

Whether they agree with the many-worlds interpretation or not, physicists and philosophers certainly love to argue about it. Some admire its elegance; others grouse about conceptual difficulties, such as the slippery matter of what exactly constitutes a “world.” Quantum theory not only allows multiple worlds but also offers an infinity of ways to define them.

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