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.
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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.
In all the debate over many worlds, though, the key insight of the idea’s originator, physicist Hugh Everett, is often forgotten. Everett developed his view in reaction to assumptions by other physicists that, because we can see only one of the possibilities of a superposition if a particle enters into that state, something must cause all the other possibilities to be discarded. In other words, some mechanism must collapse the superposition—perhaps the act of observation itself or some sporadic randomness inherent to the fabric of reality. Everett noticed a fallacy in this reasoning: it will always look as though the superposition has collapsed, even if it remains intact. The reason is that, in making our observation, we interact with the particle, and together we and it become a single combined system. Because the particle is in superposition, so are we. But we can’t tell. Everett’s fundamental point is this: We are part of the reality we seek to observe, yet no part can fully apprehend the whole, and thus our view is limited. Multiple timelines arise in the hidden recesses imposed by our very embedding within the universe.
Other branches of physics also conceive of existence as comprising forking timelines. Physicists consider counterfactuals when calculating the path of a particle; according to what they call the principle of least action, even a classical particle that exhibits no distinctively quantum effects susses out all the possibilities. In statistical physics, researchers study particles by the septillion by thinking in terms of “ensembles,” which are another kind of multiverse, spanning all the possible ways the particles can be arranged and evolve. Over time, the particles explore all possibilities open to them. We sense their machinations indirectly as the flow of heat and establishment of thermodynamic equilibrium. Going outside physics, evolutionary biologists also routinely talk about multiple timelines: If you reran the evolution of species, would things turn out the same?
All these scientific issues are rooted in a fundamental puzzle: What does it mean to be possible but not actual? Why is there something rather than something else? The physicist Paul Davies has called this the “puzzle of what exists.” It touches not just on esoteric ideas about branching timelines but also on aspects of everyday life such as causation. To say that something causes something else, there must be the possibility that the “something else” would never have happened in the first place. In astrobiologist Sara Imari Walker’s recent book on the physics of life, Life As No One Knows It, she noted that the entire observable universe doesn’t contain enough material to create every single possible small organic molecule, let alone big ones such as the DNA strands we know and love. For her, living things distinguish themselves by making molecules and other structures that are otherwise vanishingly unlikely to exist. Life blazes a path through the void of possibility space.
Perhaps some deep rule selects the actual reality from among the possible realities, but efforts to identify that principle have been serially dashed. It is hard to argue that ours is the best of all possible worlds. Nor, despite what the 19th-century philosopher Arthur Schopenhauer proclaimed, does it seem to be the worst—things could always get worse, Google searches for the “darkest timeline” notwithstanding. For many, such as philosopher David Lewis and cosmologist Max Tegmark, the most straightforward conclusion is that all possible realities exist.
The real question, then, is not whether there are other timelines; there certainly are. Rather it is why we see only one. Perhaps life or intelligence would not be possible if the branching were too evident to us. Physics is replete with such preconditions for our existence. For instance, if temporal flow did not have a directionality—an arrow of time—there could be no lasting change, no memories, no intelligence, no agency. Keeping other timelines hidden might be of similar importance. Quantum superposition may serve some specialized functions in our bodies, but otherwise it—along with any traces of alternate timelines—is dissipated in biology’s vigorous exchange of material and energy with the environment. The very nature of intelligence is to be selective; we would be paralyzed if we had to assay boundless infinitudes. Rather than holding open all possibilities, a mind must settle—at least tentatively—on one. The effort required to make that choice—and, from there, to act upon it—may be key to giving us at least the subjective feeling of free will.
So be careful what you wish for. In dark hours we may imagine alternate timelines and long for escape to another, but we seem to be inseparable from our own. Were it easier to flit between them, we might arrive only at oblivion. Like it or not, we’re stuck in this one—if we want to change it, we’ll have to do that the old-fashioned way.
