Can Retrocausality Help Save the Death of Local Realism in Quantum Mechanics?

Tuesday - May 9 2023, 02:21 UTC - 1 year ago

The concept of retrocausality has been put forward as a novel explanation for the death of local realism in quantum mechanics. It states that the present can influence the past, and provides a way of understanding correlations and causation in quantum mechanics. It is non-local and non-linear and requires the use of entangled photons in experiments, which could provide a testable explanation for recent groundbreaking experiments and potentially safeguard the core principles of special relativity.

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 there’s 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 what’s 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 can’t 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 causation, but not all. What’s the difference? .

Consider two examples. (1) There’s a correlation between a barometer needle and the weather – that’s 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 won’t change the weather. The weather and the barometer needle are both controlled by a third thing, the atmospheric pressure – that’s 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 someone’s coffee consumption, we’ll 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." It’s 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 don’t notice that 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 are among the chosen ones who will be "saved" from the destruction to come.

Retrocausality steps in to fill the gap and offers a novel approach to understanding causation and correlations in quantum mechanics. It is non-local and non-linear, meaning that it does not rely on anything moving from one place to another, and occurs with spontaneity. It requires the use of entangled photons in experiments, wherein one photon can be used to affect the state of the other, even at a large distance. When this phenomenon is combined with an intuitive explanation of retrocausality, it could provide a testable explanation for recent groundbreaking experiments and potentially safeguard the core principles of special relativity at the same time.