To Save Physics, Experts Suggest We Need to Assume The Future Can Affect The Past : ScienceAlert

To Save Physics, Experts Suggest We Need to Assume The Future Can Affect The Past : ScienceAlert

In 2022, the Nobel Prize in Physics was awarded for experimental work showing that the quantum world should 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 intermediary to interact. And indeed, a mysterious connection between distant particles would be one way to explain these experimental results.

Others instead think that experiments challenge “realism”—the intuition that there is an objective state of affairs underlying our experience. After all, experiments are difficult to explain if our measurements are thought to correspond to something real.

However, many physicists agree on the so-called “death by experiment” of local realism.

But what if both of these intuitions can be saved, at the expense of a third?

A growing body of experts think we should instead abandon the assumption that current actions cannot affect past events. Called “retrocausality,” this option claims to rescue both locality and realism.


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 thinks that the barometer needle is causing the weather. (2) Drinking strong coffee is associated with an increased heart rate. Here it seems fair to say that the former is causing the latter.

The difference is that if we “hurt” the barometer needle, we will not change the weather. The weather and the barometer needle are both controlled by a third thing, atmospheric pressure – that’s why they are related. When we check the needle itself, we break the connection with the air pressure and the correlation goes away.

But if we intervene to change someone’s coffee consumption, we will usually change their heart rate as well. Causal correlations are what still remain when we move one of the variables.

These days, the science of looking for these strong correlations is called “causal discovery.” It’s a big name for a simple idea: to find out what else changes when we move things around us.

In ordinary life, we usually take it for granted that the effects of a shake will appear later than the shake itself. This is such a natural assumption that we don’t even notice we’re 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 will be among the survivors of yesterday’s shipwreck, say.

We are imagining that something we do now can affect something in the past. This is retrocausality.

Quantum retrocausality

The quantum threat to locality (that distant objects need a physical intermediary to interact) stems from an argument 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 imply that Alice’s choice of environment has a subtle “nonlocal” influence on Bob’s outcome, and vice versa—even though Alice and Bob may be light years apart.

Bell’s argument is said to pose a threat to Albert Einstein’s theory of special relativity, which is an essential part of modern physics.

But this is because Bell assumed that quantum particles do not know what measurements they will encounter in the future. Retrocausal models propose that the measurement choices of Alice and Bob affect the particles back to the source. This can explain strange correlations without breaking special relativity.

In recent work, we have proposed a simple mechanism for the odd correlation – it involves a well-known statistical phenomenon called the Berkson bias (see our popular summary here).

There is now a thriving group of researchers working on quantum retrocausality. But it is still invisible to some experts in the wider field. It is confused with another view called “superdeterminism”.


Superdeterminism agrees with retrocausality that measurement choices and fundamental particle properties are somehow correlated.

But superdeterminism treats it like the correlation between the weather and the barometer needle. It posits that there is a mysterious third thing—a “superdeterminer”—that controls and connects both our choices and particles, the way atmospheric pressure controls the weather and the barometer.

So superdeterminism denies that measurement choices are things we are free to tempt at will, they are predetermined. Free movements would break the correlation, as in the case of the barometer.

Critics counter that superdeterminism thus undermines the essential assumptions needed to undertake scientific experiments. They also say that this is to deny free will because something is controlling both the measurement choices and the particles.

These objections do not apply to retrocausality. Retrocausalists make scientific causal discovery in the usual loose and sharp way. We say that it is the people who reject retrocausality who are forgetting the scientific method, if they refuse to follow the evidence where it leads.


What is the evidence for retrocausality? Critics demand experimental evidence, but that’s the easy part: the relevant experiments just won a Nobel Prize. The tricky 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 relativity. That’s a pretty big suggestion, in our opinion, and it’s surprising that it’s taken this long to explore. Confusion with superdeterminism seems to be mainly to blame.

In addition, we and others have argued that retrocausality better accounts for the fact that the microworld of particles does not care about the difference between the past and the future.

We don’t want to say that everything is sailing. The biggest concern with retrocausation is the possibility of sending signals into the past, opening the door to time travel paradoxes.

But to make a paradox, the effect in the past must be measured. If our new grandmother could not read our advice to avoid marrying grandfather, which means we would not exist, there is no paradox. And in the quantum case, it is well known that we can never measure everything at once.

However, there is work to be done in designing concrete retrocausal models that enforce this limitation that you cannot measure everything at once.

So we’ll close with a cautious conclusion. At this stage, it’s retrocausality that has the wind in its sails, so sit down to the biggest prize of all: saving 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.

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