Have you ever found yourself in a self-imposed jam and thought, “Well, if it isn’t the consequences of my own actions”? It’s a common refrain that exposes a deeper truth about the way we humans understand time and causality. Our actions in the past are correlated to our experience of the future, whether that’s a good outcome, like acing a test because you prepared, or a bad one, like waking up with a killer hangover.
But what if this forward causality could somehow be reversed in time, allowing actions in the future to influence outcomes in the past? This mind-bending idea, known as retrocausality, may seem like science fiction grist at first glance, but it is starting to gain real traction among physicists and philosophers, among other researchers, as a possible solution to some of the most intractable riddles underlying our reality.
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In other words, people are becoming increasingly “retro-curious,” said Kenneth Wharton, a professor of physics at San Jose State University who has published research about retrocausality, in a call with Motherboard. Even though it may feel verboten to consider a future that affects the past, Wharton and others think it could account for some of the strange phenomena observed in quantum physics, which exists on the tiny scale of atoms.
“We have instincts about all sorts of things, and some are stronger than others,” said Wharton, who recently co-authored an article about retrocausality with Huw Price, a distinguished professor emeritus at the University of Bonn and an emeritus fellow of Trinity College, Cambridge.
“I’ve found our instincts of time and causation are our deepest, strongest instincts that physicists and philosophers—and humans—are loath to give up,” he added.
Scientists, including Price, have speculated about the possibility that the future might influence the past for decades, but the renewed curiosity about retrocausality is driven by more recent findings about quantum mechanics.
Unlike the familiar macroscopic world that we inhabit, which is governed by classical physics, the quantum realm allows for inexplicably trippy phenomena. Particles at these scales can breeze right through seemingly impassable barriers, a trick called quantum tunneling, or they can occupy many different states simultaneously, known as superposition.
The properties of quantum objects can also somehow become synced up together even if they are located light years apart. This so-called “quantum entanglement” was famously described by Albert Einstein as “spooky action at a distance,” and experimental research into it just earned the 2022 Nobel Prize in Physics.
Quantum entanglement flouts a lot of our assumptions about the universe, prompting scientists to wonder which of our treasured darlings in physics must be killed to account for it. For some, it’s the idea of “locality,” which essentially means that objects should not be able to interact at great distances without some kind of physical mediator. Other researchers think that “realism”—the idea that there is some kind of objective bedrock to our existence—should be sacrificed at the altar of entanglement.
Wharton and Price, among many others, are embracing a third option: Retrocausality. In addition to potentially rescuing concepts like locality and realism, retrocausal models also open avenues of exploring a “time-symmetric” view of our universe, in which the laws of physics are the same regardless of whether time runs forward or backward.
“In any model where you had an event in the past correlated with your future choice of setting, that would be retrocausal”
“If you think things should be time-symmetric, there’s an argument to be made that you need some retrocausality to make sense of quantum mechanics in a time-symmetric way,” said Emily Adlam, a postdoctoral associate at Western University’s Rotman Institute of Philosophy who studies retrocausality, in a call with Motherboard. “There’s a bunch of different reasons that have come together to make people interested in this possibility.”
To better understand retrocausality, it’s worth revisiting a common thought experiment featuring characters called Alice and Bob, who each receive a particle from the same source, even though they may be light years apart. After conducting measurements on their particles, Alice and Bob discover that these objects are oddly correlated despite the vast distance between them.
Traditionally, this story—which stems from famous experiments made by physicist John Bell—is interpreted to mean that there are non-local quantum effects that cause the particles to be linked across great distances. However, proponents of retrocausality suggest that the particles display correlations that emerge from their past. In other words, the measurements that Alice and Bob conduct on their particles affect the properties of those particles in the past.
“Instead of having magic non-local connections between these two points, maybe the connection is through the past, and that’s what more of us are interested in these days,” Wharton said.
“In any model where you had an event in the past correlated with your future choice of setting, that would be retrocausal,” he added.
This idea seems so unintuitive because we imagine time as a river, an arrow, or an arrangement of sequential boxes on a calendar. At their core, these paradigms envision cause in the past and effect in the future as a forward flow, but retrocausality raises the prospect that these elements could be reversed. It may seem eerie to our brains, which process events sequentially, but the history of science is also littered with examples of human biases leading to bad conclusions, such as the Earth-centric model of the solar system.
“Obviously, as scientists, one thing that it is very useful to do is write down a law which says, ‘given the situation now, what is the situation going to be next? How will things evolve?’” Adlam said. “From a practical point of view, it makes a lot of sense for scientists to write down time evolution laws, because most of the time what we’re interested in doing with the laws is predicting the future.”
“But that’s a pragmatic consideration,” she continued. “That doesn’t mean that the laws of nature must really work that way. There’s no particular reason why they should be aligned with our practical interests in that sense. So, I think it is important to be cautious to distinguish the form of the laws that scientists like to write down for practical reasons from whatever nature is really doing.”
It’s important to emphasize at this point in time, whatever that means, that retrocausality is not the same as time travel. These models don’t predict that signals or objects—including human beings—could be dispatched to the past, in part because there is no evidence that we are currently being deluged with any such future messages, or messengers.
“You have to be very careful in a retrocausal model because the fact of the matter is, we can’t send signals back in time,” Adlam explained. “It’s important that we can’t, because if we could, then we could produce all sorts of vehicles or paradoxes. You have to make sure your model doesn’t allow that.”
Instead, retrocausal models suggest that there is a mechanism that allows circumstances in the future to correlate with past states. This scenario could remove the threats to locality and realism, according to Wharton and Price, though there’s disagreement among experts about the implications of these models. (For instance, Adlam has published work suggesting that retrocausality doesn’t save locality.)
“I’m heartened that more and more physicists are taking this seriously as an unexplored option”
While there are a range of views about the mechanics and consequences of retrocausal theories, a growing community of researchers think this concept has the potential to answer fundamental questions about the universe.
“Many people in the ‘foundations of physics’ community, both physicists and philosophers, have been interested in the question ‘Why the quantum?’ or ‘Why is the world like quantum mechanics says it is?’” Price said in an email to Motherboard. “That is, they’re trying to understand how quantum mechanics is a natural or inevitable result of simple and plausible principles.”
“I think that if our proposed explanation of entanglement works, then it would be a significant new part of the answer,” he continued. “It would show how the correlations we call ‘entanglement’ arise naturally from a combination of ingredients which are all really more basic than quantum mechanics.”
To that point, perhaps the most monumental quest in physics is the search for the “theory of everything” that would at last explain how the quantum and classical realms manage to coexist despite having completely contradictory laws. A huge number of scientists believe that the key to this endeavor is figuring out how gravity works on a quantum level, but retrocausality could also be part of the explanation, according to researchers who study it.
“The problem facing physics right now is that our two pillars of successful theories don’t talk to each other,” Wharton explained. “One is based in space and time, and one has left space and time aside for this giant quantum wave function.”
“The solution to this, as everyone seems to have agreed without discussing it, is that we’ve got to quantize gravity,” he continued. “That’s the goal. Hardly anyone has said, ‘what if things really are in space and time, and we just have to make sense of quantum theory in space and time’? That will be a whole new way to unify everything that people are not looking into.”
Price agreed that this retrocausality could provide a new means to finally solve “eliminate the tension” between quantum mechanics and classical physics (including special relativity).
“That’s such a huge payoff that I’m always puzzled that retrocausality wasn’t taken more seriously decades ago,” Price said, adding that part of the answer may be that retrocausality has frequently been conflated with another far-out concept called superdeterminism.
“Another possible big payoff is that retrocausality supports the so-called ‘epistemic’ view of the wave function in the usual quantum mechanics description—the idea that it is just an encoding of our incomplete knowledge of the system,” he continued. “That makes it much easier to understand the so-called collapse of the wave function, as a change in information, as folk such as Einstein and Schoedinger thought, in the early days. In this respect, I think it gets rid of some more of the (apparently) non-classical features of quantum mechanics, by saying that they don’t amount to anything physically real.”
To that end, scientists who work on retrocausality will continue to develop new theoretical models that attempt to account for more and more experimental phenomena. Eventually, these concepts could inspire experimental techniques that might provide evidence either for, or against, a future that can influence the past.
“The goal is to come up with a more general model,” Wharton concluded. “Whether or not me, or anyone else, will be successful remains to be seen, but I’m heartened that more and more physicists are taking this seriously as an unexplored option. Maybe we should explore it.”
