The 2022 physics Nobel prize was awarded for experimental work demonstrating fundamental breaks in our understanding of the quantum world, leading to discussions around local realism and how it could be refuted. Many theorists believe these experiments challenge either locality (the notion that distant objects require a physical mediator to interact) or realism (the idea that theres an objective state of reality). However, a growing number of experts suggest an alternative approach, retrocausality, which posits that present actions can affect past events, thus preserving both locality and realism.

The 2022 Nobel Prize in physics highlighted the challenges quantum experiments pose to local realism. However, a growing body of experts propose retrocausality as a solution, suggesting that present actions can influence past events, thus preserving both locality and realism. This concept offers a novel approach to understanding causation and correlations in quantum mechanics, and despite some critics and confusion with superdeterminism, it is increasingly seen as a viable explanation for recent groundbreaking experiments, potentially safeguarding the core principles of special relativity.

In 2022, the physics Nobel prize was awarded for experimental work showing that the quantum world must break some of our fundamental intuitions about how the universe works.

Many look at those experiments and conclude that they challenge locality the intuition that distant objects need a physical mediator to interact. And indeed, a mysterious connection between distant particles would be one way to explain these experimental results.

Others instead think the experiments challenge realism the intuition that theres an objective state of affairs underlying our experience. After all, the experiments are only difficult to explain if our measurements are thought to correspond to something real. Either way, many physicists agree about whats been called the death by experiment of local realism.

But what if both of these intuitions can be saved, at the expense of a third? A growing group of experts think that we should abandon instead the assumption that present actions cant affect past events. Called retrocausality, this option claims to rescue both locality and realism.

What is causation anyway? Lets start with the line everyone knows: correlation is not causation. Some correlations are causation, but not all. Whats the difference?

Consider two examples. (1) Theres a correlation between a barometer needle and the weather thats why we learn about the weather by looking at the barometer. But no one thinks that the barometer needle is causing the weather. (2) Drinking strong coffee is correlated with a raised heart rate. Here it seems right to say that the first is causing the second.

The difference is that if we wiggle the barometer needle, we wont change the weather. The weather and the barometer needle are both controlled by a third thing, the atmospheric pressure thats why they are correlated. When we control the needle ourselves, we break the link to the air pressure, and the correlation goes away.

But if we intervene to change someones coffee consumption, well usually change their heart rate, too. Causal correlations are those that still hold when we wiggle one of the variables.

These days, the science of looking for these robust correlations is called causal discovery. Its a big name for a simple idea: finding out what else changes when we wiggle things around us.

In ordinary life, we usually take for granted that the effects of a wiggle are going to show up later than the wiggle itself. This is such a natural assumption that we dont notice that were making it.

But nothing in the scientific method requires this to happen, and it is easily abandoned in fantasy fiction. Similarly in some religions, we pray that our loved ones are among the survivors of yesterdays shipwreck, say. Were imagining that something we do now can affect something in the past. Thats retrocausality.

The quantum threat to locality (that distant objects need a physical mediator to interact) stems from an argument by the Northern Ireland physicist John Bell in the 1960s. Bell considered experiments in which two hypothetical physicists, Alice and Bob, each receive particles from a common source. Each chooses one of several measurement settings, and then records a measurement outcome. Repeated many times, the experiment generates a list of results.

Bell realized that quantum mechanics predicts that there will be strange correlations (now confirmed) in this data. They seemed to imply that Alices choice of setting has a subtle nonlocal influence on Bobs outcome, and vice versa even though Alice and Bob might be light years apart. Bells argument is said to pose a threat to Albert Einsteins theory of special relativity, which is an essential part of modern physics.

But thats because Bell assumed that quantum particles dont know what measurements they are going to encounter in the future. Retrocausal models propose that Alices and Bobs measurement choices affect the particles back at the source. This can explain the strange correlations, without breaking special relativity.

In recent work, weve proposed a simple mechanism for the strange correlation it involves a familiar statistical phenomenon called Berksons bias (see our popular summary here).

Theres now a thriving group of scholars who work on quantum retrocausality. But its still invisible to some experts in the wider field. It gets confused for a different view called superdeterminism.

Superdeterminism agrees with retrocausality that measurement choices and the underlying properties of the particles are somehow correlated.

But superdeterminism treats it like the correlation between the weather and the barometer needle. It assumes theres some mysterious third thing a superdeterminer that controls and correlates both our choices and the particles, the way atmospheric pressure controls both the weather and the barometer.

So superdeterminism denies that measurement choices are things we are free to wiggle at will, they are predetermined. Free wiggles would break the correlation, just as in the barometer case. Critics object that superdeterminism thus undercuts core assumptions necessary to undertake scientific experiments. They also say that it means denying free will, because something is controlling both the measurement choices and particles.

These objections dont apply to retrocausality. Retrocausalists do scientific causal discovery in the usual free, wiggly way. We say it is folk who dismiss retrocausality who are forgetting the scientific method, if they refuse to follow the evidence where it leads.

What is the evidence for retrocausality? Critics ask for experimental evidence, but thats the easy bit: the relevant experiments just won a Nobel Prize. The tricky part is showing that retrocausality gives the best explanation of these results.

Weve mentioned the potential to remove the threat to Einsteins special relativity. Thats a pretty big hint, in our view, and its surprising it has taken so long to explore it. The confusion with superdeterminism seems mainly to blame.

In addition, we and others have argued that retrocausality makes better sense of the fact that the microworld of particles doesnt care about the difference between past and future.

We dont mean that it is all plain sailing. The biggest worry about retrocausation is the possibility of sending signals to the past, opening the door to the paradoxes of time travel. But to make a paradox, the effect in the past has to be measured. If our young grandmother cant read our advice to avoid marrying grandpa, meaning we wouldnt come to exist, theres no paradox. And in the quantum case, its well known that we can never measure everything at once.

Still, theres work to do in devising concrete retrocausal models that enforce this restriction that you cant measure everything at once. So well close with a cautious conclusion. At this stage, its retrocausality that has the wind in its sails, so hull down towards the biggest prize of all: saving locality and realism from death by experiment.

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This article was first published in The Conversation.

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