The mysteries of quantum mechanics are usually portrayed as mysteries of space: how strange that the fates of particles on opposite sides of the universe could be joined. But they are also mysteries of time. A particle you create today could be connected in some unaccountable way to one that does not exist yet. Einstein fretted about spooky action at a distance. He should have been equally exercised by spooky action at a delay.
In my favorite analogy, which is a direct translation of experiments with photons or other particles, you have a pair of magic coins. You toss one, your friend the other. Each lands on heads or tails at random, yet the two of them reliably land on the same side. Two events that should be independent are weirdly linked. And those events could be separated in space or time or both. You can toss one in Siena and the other in Savannah, and the outcomes always match. You can toss one today and the other tomorrow, and they match. You can even toss the same coin at two different times, and again the outcomes match.
The mystery isn’t that they match. Quantum theory demands as much. The mystery is how they match. No force passes through the space between the particles; no record is carried forward through time. It is weird enough for a coin toss today to decide the outcome of a toss tomorrow, weirder still that its influence cannot be traced through the intervening moments. It is as if knocking down the first domino in a line brought down the 483rd domino while leaving the ones in between standing. This lack of intermediation, or nonlocality, was Einstein’s top complaint about quantum theory.
As Adlam explains, spatial nonlocality automatically implies the temporal sort. According to relativity theory, if one observer sees you and your friend toss your coins at the same time, another might see you toss them at different times.
She lists three forms of temporal nonlocality. The mildest is retrocausality: what happens now depends not just on what happened in the past, but also on what will happen in the future. The outcome of that coin toss might hinge on a decision you make tomorrow. I say “mildest” because the putative influences from the future do not jump across time; they must still wend their way through each intervening moment. The domino effect of cause and effect still operates—it’s just that the dominos fall backwards. Mild or not, retrocausality would mean that the world is affected by events that it shouldn’t be.
In a second type of temporal nonlocality, known as non-Markovian dynamics, influences do vault across temporal gaps. A non-Markovian system remembers without remembering. What it does now depends not just on its present state, including any records it has kept of the past, but also on its history, including events it didn’t bother to log. Non-Markovian systems are common in physics, but in all known cases, these systems have simply externalized their memory. What happens to them is recorded in, for example, molecular motions that a high-level description omits. Deep down, the universe lives purely in the moment, or so it is usually thought. Adlam speculates it might have direct access to its past.
The third option she puts forward is the universe stands outside time. Past, present, and future are equally real and cannot be cleanly separated. What happens at one moment depends on what happens at all moments. This holistic view is plausible because the known laws of physics can be formulated atemporally. Instead of describing how a process unfolds step by step, physicists can look at the whole thing, start to finish, in one go. Physicists usually regard this as a calculational trick, but Adlam and others see it as a deep truth.
In her paper, Adlam presses the case for the atemporal view. For her, retrocausality is a rickety halfway house. Any influences moving backward in time must mesh with those moving forward, and that would take coordination imposed from the outside. By treating the universe in its entirety, the atemporal view is also aspatial. As I argue in Chapter 4 of my book, many physicists and philosophers advocate retrocausality to avoid spatial nonlocality, yet end up endorsing a thoroughgoing nonlocality.
Adlam’s paper also gets into the difficulty of proving temporal nonlocality experimentally. Two simultaneous coin tosses in different cities are clearly independent events; two successive tosses in the same city, not so clearly. To confirm that the later toss doesn’t retain some memory of the earlier, you need to conduct an experiment so elaborate that no plausible memory could keep up, as I discussed in a Quanta magazine article.
Adlam’s goal is to inject some fresh ideas into the search for an interpretation of quantum mechanics. Even proposals that style themselves radical still assume that natural processes unfold over time, and is that really justified? For instance, the atemporal view could explain away the notorious indeterminism of quantum physics. Quantum events may not be inherently random, but merely seem to be, because their causes lie in a past or future we cannot see. Temporal nonlocality seems at first to worsen the puzzles of quantum physics, but maybe it is the missing piece to solve them.