Many look at these 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 believe instead that the experiments challenge “realism” – the intuition that there is an objective state behind our experience. After all, the experiments are only difficult to explain if our measurements are believed to correspond to something real.
Regardless, many physicists agree on what has been called the “experimental death” of local realism.
But what if both of these intuitions can be saved at the expense of a third?
A growing body of experts believes that we should instead abandon the assumption that current actions cannot affect past events. Called “retrocausality,” this option claims to save both locality and realism.
Causation
What is causation anyway? Let’s start with the line everyone knows: correlation is not causation. Some correlations are causal, but not all. What is the difference?
Consider two examples. (1) There is a connection between a barometer needle and the weather – this is why we learn about the weather by looking at the barometer. But no one believes that the barometer needle is the cause of the weather. (2) Drinking strong coffee is correlated with an elevated heart rate. Here it seems right to say that the first causes the second.
The difference is that if we “wiggle” the barometer needle, we don’t change the weather. The weather and the barometer needle are both controlled by a third thing, atmospheric pressure – that’s why they are correlated. When we control the needle ourselves, we break the connection with the air pressure and the connection disappears.
But if we intervene to change someone’s coffee consumption, we usually also change their heart rate. Causal relationships are those that still hold when we tweak one of the variables.
These days, the science of looking for these robust connections is called “causal discovery.” It’s a big name for a simple idea: figuring out what else changes when we wiggle the 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 don’t notice that we are doing 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 yesterday’s shipwreck, e.g.
We imagine that something we do now can affect something in the past. It is retrocausality.
Quantum retrocausality
The quantum threat to locality (that distant objects need a physical mediator to interact) originates from an argument made by Northern Irish 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 selects one of several measurement settings and then records a measurement result. 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 suggest that Alice’s choice of setting has a subtle “non-local” influence on Bob’s outcome, and vice versa—even though Alice and Bob might be light years apart.
Bell’s argument is said to pose a threat to Albert Einstein’s special theory of relativity, which is an essential part of modern physics.
But that’s because Bell assumed that quantum particles don’t know what measurements they’re going to encounter in the future. Retrocausal models suggest that Alice’s and Bob’s measurement choices affect the particles back at the source. This can explain the strange correlations without breaking the special theory of relativity.
In recent work, we have proposed a simple mechanism for the strange correlation – it involves a well-known statistical phenomenon called Berkson’s bias (see our popular summary here).
There is now a thriving group of researchers working on quantum retrocausality. But it remains invisible to some experts in the wider field. It gets confused for another view called “superdeterminism”.
Superdeterminism
Superdeterminism agrees with retrocausality, that measurement choices and the underlying properties of the particles are somehow correlated.
But superdeterminism treats it like the connection between the weather and the barometer needle. It assumes that there is 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 can freely wiggle at will, they are predetermined. Free wiggles would break the connection, just like in the barometer case.
Critics object that superdeterminism thus undermines core assumptions necessary to conduct scientific experiments. They also say that it means denying free will, because something controls both measurement choices and particles.
These objections do not apply to retrocausality. Retrocausalists do scientific causal discovery in the usual freewheeling way. We say that it is people who reject retrocausality who forget the scientific method if they refuse to follow the evidence where it leads.
Proof
What is the evidence for retrocausality? Critics ask for experimental evidence, but that’s the easiest: the relevant experiments just won a Nobel Prize. The difficult part is to show that retrocausality provides the best explanation for these results.
We have mentioned the potential to remove the threat to Einstein’s special theory of relativity. That’s a pretty big tip in our opinion, and it’s surprising it’s taken this long to explore. The confusion with superdeterminism seems mainly to blame.
In addition, we and others have argued that retrocausality makes better sense in the fact that the microworld of particles does not care about the difference between past and future.
We don’t think it’s all ordinary. The main concern about retrocausation is the possibility of sending signals to the past and opening the door to the paradoxes of time travel.
But to make a paradox, the effect in the past must be measured. If our young grandmother can’t read our advice to avoid marrying grandfather, which means we wouldn’t exist, there is no paradox. And in the quantum case, it is well known that we can never measure everything at once.
Still, there is work to be done in devising concrete retrocausal models that enforce this limitation that you cannot measure everything at once.
So we end with a cautious conclusion. At this stage, retrocausality has the wind in its sails, so hull down towards the greatest prize of all: rescuing locality and realism from “death by experiment”.
Huw Price, Emeritus Fellow, Trinity College, University of Cambridge and Ken Wharton, Professor of Physics and Astronomy, San José State University
This article is republished from The Conversation under a Creative Commons license. Read the original article.