A growing number of scientists believe that the future can influence the past
Have you ever found yourself in a difficult situation and thought: “This is all the result of my own actions.” This common phrase reveals how we humans actually understand time and cause and effect. Our actions in the past are correlated with our perception of the future—whether it’s a good outcome, such as passing an exam after studying, or a bad one, such as when we wake up with a killer hangover.
But what if this direct causation could somehow be reversed in time, allowing future actions to influence past outcomes? This amazing idea, known as retrocausalitymay seem like science fiction at first glance, but it is beginning to gain real popularity among physicists and philosophers, as well as other researchers, as a possible solution to some of the most intractable mysteries underlying our reality.
In other words, people are becoming more “retro-curious,” Kenneth Wharton, a physics professor at San Jose State University who published the study on retrocausality, told Motherboard. Although it may seem strange to talk about the future influencing the past, Wharton and others believe that this can explain some of the strange phenomena observed in quantum physics that exist on a tiny atomic scale.
“We have an instinctive perception of a wide variety of things, and for some of them it is more pronounced,” said Wharton, who recently wrote article on retrocausality co-authored with Hugh Price, Distinguished Professor at the University of Bonn and Honorary Fellow of Trinity College, Cambridge.
“I have found that our instincts about time and causality are our deepest and strongest instincts, which physicists and philosophers – and the rest of the people – do not want to give up,” he added.
Some scientists, including Price, have been argue that the future can influence the past, but the renewed curiosity about retrocausality is due to recent discoveries in the field of quantum mechanics.
Unlike the familiar macroscopic world in which we live and which is governed by classical physics, the quantum realm allows inexplicably strange phenomena. Particles on these scales can go straight through seemingly impenetrable barriers (this is called tunnel effect), or be in many different states at the same time (this is called superposition).
The properties of quantum objects can also somehow become synchronized, even if they are many light-years apart. This so-calledquantum entanglement” Albert Einstein described as “frightening long-range action”, and experimental research in this area most recently brought the Nobel Prize in Physics in 2022.
Quantum entanglement goes against many of our understandings of the universe, prompting scientists to wonder what our favorite values in physics we must give up in order to explain it. Some are willing to sacrifice the idea of ”locality”, which essentially means that objects should not interact over long distances without some kind of physical intermediary. Other researchers are ready to abandon “realism” – the idea that there is some objective basis for our reality.
Wharton and Price, like many others, favor a third option: retrocausality. In addition to potentially rescuing concepts such as locality and realism, retrocausal models also open up possibilities for exploring a “time-symmetric” view of our universe, in which the laws of physics are the same whether time goes forward or backward.
“If you think that all processes must be time-symmetric, then there is a need for a retrocausality that can explain quantum mechanics in a time-symmetric way,” said Emily Adlam, a postdoctoral fellow at the Rothman Institute of Philosophy at Western University who studies retrocausality, in talking to Motherboard. “People are interested in this issue for many different reasons, converged on one topic.”
To better understand retrocausality, one can use a popular thought experiment involving characters named Alice and Bob receiving a particle from the same source, although some distance separates them. After measuring the quantum properties of each of the particles, Alice and Bob discover that these objects are strangely related to each other, despite a certain distance between them.
Traditionally, this story, which began with the famous experiments of the physicist John Bell, is interpreted as non-local quantum effects that instantly create a connection between particles located at some distance from each other at the moment of measurement. However, proponents of retrocausality suggest that the apparent correlations in particle properties stem from their past. In other words, the measurements that Alice and Bob make on their particles affect the properties of those particles in the past.
“Instead of looking at some magical non-local connections between these two points in space, you can imagine that the connection between the particles goes through the past – this is the hypothesis that we are interested in today,” Wharton said. “Any model in which an event in the past is correlated with a future choice is retrocausal,” he added.
This idea seems so counter-intuitive because we think of time as a river, an arrow, or successive squares on a calendar. At their core, these paradigms present cause in the past and its effect in the future as a direct flow. But retrocausality suggests considering the possibility that these elements can work in the opposite direction. This may seem terribly strange to our brains, which process events sequentially, but the history of science is also littered with examples of human bias leading to incorrect conclusions, such as the geocentric model of the solar system.
“Obviously, it’s easier for a scientist to write down some law of nature that takes into account the current state of things and draws conclusions about their future based on the present,” Adlam said. “From a practical point of view, it makes sense for scientists to write down laws that evolve over time, because in most cases we are interested in using these laws to predict the future.”
“But this is purely a pragmatic consideration,” she continued. “That doesn’t mean that the laws of nature really have to work that way. There is no particular reason why they should be in our practical interest in this sense. So I think it’s important to be able to distinguish between the form of the laws that scientists like to write down for practical reasons and what actually happens in nature.”
It is important to emphasize now that retrocausality, whatever that means, is not the same as time travel. These models do not say that signals or objects – people, for example – can be sent into the past. Partly because there is no evidence that we are currently receiving any messages or visitors from the future.
“You have to be very careful when working with the retrocausal model, and keep in mind that it does not allow us to send signals to the past,” Adlam explained. – It is important that we cannot do this – otherwise we could create time machines or time paradoxes. You have to make sure your retrocausality model doesn’t allow for that.”
Instead, retrocausal models suggest that there is a mechanism that allows circumstances in the future to correlate with past states. Such an approach could eliminate the threats to locality and realism, Wharton and Price argue, although there is disagreement among experts about the implications of these models. (For example, Adlam published work that retrocausality may not save locality).
While there are a number of opinions about how retrocausal theories work and the consequences, a growing number of researchers believe that this concept has the potential to answer fundamental questions about the universe.
“Many scientists interested in the foundations of physics, both physicists and philosophers, were interested in the question “Why quanta?” or “Why is the world the way quantum mechanics thinks it is?” Price said. “That is, they are trying to understand whether quantum mechanics is a natural or inevitable result of the simple principles of the organization of the universe.”
“I think that if our proposed explanation of the nature of quantum entanglement works, then it will help answer these questions as well,” he continued. “It will demonstrate how this very ‘entanglement’ naturally arises from a combination of ingredients that are actually more fundamental than quantum mechanics itself.”
To date, perhaps the most ambitious project in physics is the search for “Theories of everything”, which will finally explain how the quantum and classical spheres manage to coexist, despite completely incompatible laws. A huge number of scientists believe that the key to this is figuring out how gravity works at the quantum level, but retrocausality could also be part of the explanation, according to the researchers who study it.
“The problem facing physics now is that our two most successful theories do not fit together,” Wharton explained. “One talks about continuous space-time, and the other abandoned this concept in favor of a giant quantum wave function.”
“The solution to this problem, as everyone seemed to silently agree, is that we have to quantize gravity,” he continued. “That is the goal. Almost no one said: “What if objects really are in space and time, and we just need to understand the meaning of quantum theory in space and time”? It’s going to be a whole new way to bring everything together – a way that people don’t pay attention to.”
Price agreed that this retro-causality could provide a new means to finally resolve the contradictions between quantum mechanics and classical physics (including Special Relativity).
“This is such a huge gain that I always wonder why retrocausality wasn’t taken more seriously decades ago,” Price said, adding that part of the answer may be that retrocausality has often been confused with another distant concept called superdeterminism.
“Another possible benefit is that retrocausality supports the so-called ‘epistemic’ view of the wave function in quantum mechanics — the idea that it is just an encoding of our incomplete knowledge of the system,” he continued. “This greatly facilitates the understanding of the so-called “wave function collapse.” This collapse can be described as a process of changing information – as some scientists imagined it, for example, Einstein and Schedinger. In this regard, I think this concept allows you to get rid of some more non-classical features of quantum mechanics – simply because they do not describe anything physically real.
To this end, scientists working on retrocausality will continue to develop new theoretical models that will attempt to explain more and more experimental phenomena. Ultimately, these concepts may inspire experimental methods that can provide evidence both for and against the concept that the future influences the past.
“Our goal is to come up with a more general model,” Wharton concluded. “Whether I or anyone else will succeed remains to be seen, but I am pleased that more and more physicists are taking this task seriously.”
