When Time Flows Backwards

​New experiments conducted at Washington University St. Louis offer a further demonstration of one of the quantum world's more difficult implications: time symmetry. That is, as shown by the physicist Kater Murch and his group, the future influences the present. What might be done to a particle five minutes from now helps determine what happened to the particle five minutes ago.

We are doomed to time because of organization. We have pasts, which are just spaces of non-possibility. The past for us is a fixed guideway, a steady elimination of options culminating in this here—and this now. And this forward-universe seems like a reasonable thing, if anxiety-inducing.

In the Grand Central Terminal of everything, we've stood at the departures board and made a selection, boarded a train, and here we are on an express to Newbury (or wherever). We've left every other train and destination behind, and those trains have all themselves left, and you aren't on any of them. Their destinations are unavailable.

Why should it be so? Well, causality for one. We're strung along in a generally forward direction by things occurring that cause other things to happen or cause other things to be much more likely to happen. It would seem that the alternative is for nothing to ever happen at all, leaving us to just stand there staring at the departures board, never choosing but never really happening either.

So, that's life, but our lived reality is hardly everything. If the universe has a more fundamental perspective on time, it may be in disregarding time completely. Such is existence in the quantum realm, where the events of our classical world are traded for indeterminate states where all possibilities are allowed to coexist as though a landscape, rather than the singular frames of "present" we usually experience. This is time as space, where every direction is as valid as north, south, east, and west.

A consequence of time as space is retrocausality, in which is the future is free to influence the past. Imagine if at some future point, you were to learn something, some piece of new information. This new information would in a time-agnostic world (somehow) reach you still in Grand Central undecided, and then you would choose based on this nugget from the future. The cause has not occurred yet, but somehow it manages to have an effect in the past. This has some obvious problems.

The general idea being probed by Murch and his team is properly called post-selection. Instead of Grand Central Terminal, imagine a quantum state as a coin stuck in a state of continuous flippage, just spinning there in mid-air refusing to fall and choose between heads or tails. That's quantum reality, for the most part, which happens to be the fabric of everything. We can make this quantum reality behave "normally" by making a measurement of it, forcing it to become either heads or tails. But no matter how long we've watched the coin spin around, we can still only give even odds of either outcome. Observing the coin's past makes no difference here.

That's just what it is to be indeterminate. The coin is both heads and tails, at once 100 percent heads and 100 percent tails. A superposition. Murch's experiment replaced the coin for a qubit, which is some particle that is prepared in such a way that it exists in a superposition of two different states. The team played a guessing game of sorts in which the particle was measured, forcing it to choose one and only one state, but the results of that measurement were hidden from the experimenters.

Murch and his team then performed a series of "weak" measurements on the particle. A weak measurement is a sort of trick super-low energy measurement in which the particle and the measuring device are coupled together in such a way that some limited information can be gathered about the particle without disturbing it. The basic idea is that a quantum state can be measured very weakly but with many repetitions, allowing for a meaningful answer based on statistics through time.

And watching the experiment progress forward through time, it remains impossible to say beyond 50/50 odds what the actual final state of the particle will be in. But, if you follow the experiment backwards, following the particle's timeline from its future to its past, Murch and co. found that they could up their odds to 90 percent correctness. The implication is that everything that happened to the particle/state after the strong measurement, influences the strong measurement itself … in the past.

"We have demonstrated the use of the quantum trajectory formalism to infer the quantum state of a superconducting qubit conditioned on the outcome of continuous measurement," Murch and his group conclude. "We have also demonstrated a quantum hindsight effect, where probing of a quantum system modifies and improves the predictions about measurements already performed in the past."

"I always thought the measurement would resolve the time symmetry in quantum mechanics," Murch notes in a separate statement. "If we measure a particle in a superposition of states and it collapses into one of two states, well, that sounds like a process that goes forward in time."

Murch's results aren't some kind of one-off. A steady stream of experiments, conducted mostly in the past five or so years, ​have offered similar findings. As we probe the future of some quantum thing, it appears to effect that thing's past. Not entirely, of course, but significantly or non-trivially.

Interpret that how you will. We're still great big organizations of particles, not particles, and so are subject to entropy and all of its trappings (like time). But, at the quantum level, indeterminate-ness is being slowly chipped away, leaving something that looks a bit more like a closed loop. What is was and will be, or, as the mathematician and physicist ​Hermann Weyl put it in 1949, "The objective world simply is, it does not happen." And yet we don't quite live in that world.

"I do think much about the philosophy of this, but it takes a while and several experiments to draw conclusions," Murch told me. "The classical world is very different from the quantum, objects have definite states and local properties, two things that quantum states do not. So in the classical world it is no surprise that hindsight is 20/20, but since uncertainty and indefiniteness are intrinsic to quantum things, hindsight is not so straightforward."

In one of his famous lectures, Richard Feynman talked about this whole mess—and what messes even are and how messes drive time and fashion it into an arrow. An arrow of messes. Which is what happens when you add up a bunch of closed loops, building and building into huge things like human brains.

"As we go up in this hierarchy of complexity, we get to things like muscle twitch, or nerve impulse, which is an enormously complicated thing in the physical world, involving an organization of matter in a very elaborate complexity," he said. "Then come things like 'frog'. And then we go on, and we come to words and concepts like 'man', and 'history', or 'political expediency', and so forth, a series of concepts which we use to understand things at an ever higher level."

Feynman continued: "And going on, we come to things like evil, and beauty, and hope… Which end is nearer to God; if I may use a religious metaphor. Beauty and hope, or the fundamental laws?"

Murch's paper ​can be found in open-access form at arXiv.org.