Quantum theory provides a foundation for theoretical
physics that is extremely predictive and explanatory. As per the Stanford
Encyclopedia of Philosophy, the theory is an extraordinarily well-functioning
algorithm for predicting and explaining the outcomes of observations, but there
is no agreement on what form of objective reality may underpin these findings.

One class of interpretations hypothesizes
backward-in-time causal influences, also known as retrocausality, which is the
basis for constructing a convincing foundational account of quantum theory. It
proposes that a future event could influence the past, like when someone's
stomach gets hurt today because of tomorrow's meal.

## Theoretical Support for Retrocausality

Science Alert reported in 2017 that two physicists,
one from the United States and one from Canada, examined several basic
assumptions in quantum theory and concluded that measurements made to a
particle might reverberate back in time and forward unless scientists found
that time only runs one way.

Quantum physics is known to be strange, partly due
to the fact that particles, on a fundamental level, do not behave like solid
billiard balls sliding down a table but rather like a fuzzy mist of
possibilities swirling around the room.

In 2012, physicist Huw Price suggested that if the
strange probabilities behind quantum states are genuine, and if nothing
confines time to one direction, the black ball in that cloud of maybes might
roll out of the pocket and knock the white ball.

The two physicists, Matthew S. Leifer from Chapman
University in California and Matthew F. Pusey from the Perimeter Institute for
Theoretical Physics in Ontario, wondered whether the quantum world would be
different when it comes to time. They applied their new model to Bell's theorem
to find out,

John Stewart Bell said that the universe is random
at a fundamental level. That means weird things happening in quantum mechanics
cannot be explained by actions taking place in a nearby place. But what about
taking place at a certain time? Can something far away influence that cloud
without touching it, as Albert Einstein describes it as spooky?

Measuring one of two particles linked in space at
any point instantaneously establishes the value for the other, regardless of
where it has traveled in the Universe. This 'entanglement' has been tested
several times in light of Bell's theorem, filling gaps that may indicate they
are interacting on a local level in some way, despite what appears to be a
distance.

However, if causality worked backward, a particle
might carry the action of its measurement back in time to when it was
entangled, impacting its partner. No need for faster-than-light communications.

## Implications of Retrocausality

Retrocausality could have vast implications for
understanding the foundations of quantum theory, Phys.org reported. One of
which is the implication for Bell tests that shows distant particles cannot
influence each other, but quantum theory is incomplete. If Leifer and Pusey's
theory is correct, the retrocausality may be one of the missing pieces that
complete the quantum theory.

Leifer said that the different interpretations of
quantum theory have different implications for how scientists generalize
standard quantum theory. They might need to construct the correct quantum
gravity theory or resolve some high-energy physics issues.

He added that if retrocausality does exist in the
universe, then it might be the case that there are certain eras in which there
is not a definite arrow of causality. The two physicists do not have any
experiments to test retrocausality, but perhaps the most needed is theoretical
support, as the idea is more an interpretation of observations rather than
making new observations.

Reference: Stanford Encyclopedia of Philosophy.

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