Retrocausality and Quantum Mechanics

The exact empirical evidence for retrocausality does not exist yet, but the existing empirical data as those from Bell tests may be interpreted in a way to support the retrocausal framework.

^{By Amal Pushp, Affiliate Physicist at the International Space Federation}

Have you ever thought that future states could affect the events that have occurred in the past? Although this idea sounds quite bizarre, it is indeed possible according to a quantum mechanical effect called retrocausality. According to the concept, causality and time do not work in the conventional sense and remarkably, an effect can predate its cause, thus reversing the directionality of time as well.  

Usually, in the classical world, this is not what we actually experience. For every cause, there is a corresponding effect, but they work sequentially rather than in the reverse way. Conventional thought process suggests that once a particular event has occurred, there’s almost zero probability that it can be reversed. The physical reason is simple, and it has to do with the arrow of time. In general, the arrow of time points in a single forward direction and this is one of the major unsolved challenges of the foundations of physics because physicists are uncertain of why the nature of time is such.  

The idea of retrocausality stems from known interpretations of quantum mechanics. For instance, the possibility of retrocausal influences has been well explored by the transactional interpretation of quantum mechanics proposed by physicist John G Cramer. In this worldview, quantum mechanical wavefunctions can interact in both the forward and backward directions of time thus opening the path to the realization of retrocausality.  

However, certain concepts about retrocausality must be made explicit before it confuses the reader. Retrocausality does not indicate that actual signals can flow from the future to the past but rather any decision that an experimenter makes on the physical system changes some variables of the system in the past and this essentially occurs preliminary to the experimenter making the choice.  

Physical phenomena always have an associated peculiarity to it. A certain experimental test can be interpreted in numerous possible ways and each interpretation may seem to be equally accurate in the eyes of the proponents. However, repeated empirical tests and modelling has the potential to reveal the true nature of the workings of physical phenomena. The reason we are referring to this is because the infamous Bell experiments which led up to the realization of non-local correlations and support for the action-at-a-distance phenomenon can also be interpreted distinctly. Originally, physicists had ignored the possibility of retrocausal influences in the Bell test experiments. But bringing retrocausality into the picture could also explain the results with the caveat that action-at-a-distance would have to be discarded. So, it basically is an alternative.  

^{Figure: A general setup of EPR-Bell experiment} 

Numerous experts have a different take on retrocausality. For instance, in a paper published in the Proceedings of The Royal Society A, physicists Matthew Leifer and Matthew Pusey explore whether retrocausality could be successfully incorporated into the theory of quantum mechanics [1]. They have discovered that quantum theory is only true if it allows for either time symmetry or no retrocausality. Leifer and Pusey are betting on retrocausality as a viable mechanism to resolve the issues encircling within the quantum world. The reasons stated by them are essentially twofold: 

• It allows incorporating Bell correlations without the need for action-at-a-distance 
• The arrow of time is not a law of physics and follows from the special boundary conditions of the universe. Similarly, in a universe wherein retrocausal influences play a major role, the inability to send signals into the past could also emerge from the special boundary conditions of the universe rather than the requirement of being a law of physics. Time symmetry on the other hand, which allows for the validity of the laws of physics irrespective of the directionality of time, is less likely to occur in that manner.  

Another set of scholars who are actively working to bring justice to the idea of retrocausality are Ken Wharton from San Jose State University and Huw Price based at Trinity College, Cambridge. In 2012, the latter published a paper arguing that a quantum theory that essentially assumes the quantum state to be ontic and the quantum world to be time-symmetric must account for retrocausality [2]. Now, recently in collaboration with Wharton, he has published a work proposing a mechanism based on retrocausal models to explain the non-local correlations observed in Bell tests [3].  

As Wharton and Price write in their article, there is a small but flourishing group of experts working on the idea of retrocausality  although it is yet to be accepted widely in the scientific community. They also reveal that the idea is mistaken for another worldview called “Superdeterminism”. The concept aligns with the retrocausal view that measurement choices and physical observables of particles are correlated but that is essentially at the expense of a “superdeterminer” who seems to control both the things. However, this would mean denying free will, which is not true in the case of retrocausality.  

The exact empirical evidence for retrocausality does not exist yet. But as said earlier, the existing empirical data such as those from Bell tests may be interpreted in a way to support the retrocausal framework. However, this would require a lot of work because the fundamentals would have to be tinkered so that it yields precise results. It would be interesting to see how this idea works out in the future as it could lead to the resolution of hot topics such as time travel and its associated paradoxes as well as development of promising technology.  

Unified Science- In Perspective

The concept of retrocausality poses many intriguing questions for physics and its shortcomings that we are aware of currently. The scientific models developed by researchers at RSF go to the heart of such puzzles. For instance, the idea of spacememory network involves trans-temporal communication which is central to the retrocausal framework wherein future states are directly influencing past states. Furthermore, the spacememory network in combination with the retrocausal framework remarkably accounts for the arrow of time paradox. As a result, time could no longer be regarded as unidirectional at least in the global perspective. Locally however, time may seem to follow a linear path when dealing with a closed physical system with certain boundary conditions. Thus, a theory of reality based on the unified spacememory network resolves some of the major longstanding problems of modern physics and in this theory, retrocausal signalling is a natural consequence so it may seem fructuous to adopt this novel framework for further developments.  

References

[1] Matthew S. Leifer and Matthew F. Pusey. "Is a time symmetric interpretation of quantum theory possible without retrocausality?" Proceedings of The Royal Society A. DOI: 10.1098/rspa.2016.0607  

[2] Huw Price, “Does time-symmetry imply retrocausality? How the quantum world says “Maybe”?, Studies in History and Philosophy of Science B: Studies in History and Philosophy of Modern Physics. DOI: 10.1016/j.shpsb.2011.12.003  

[3] Huw Price and Ken Wharton. “Entanglement Swapping and Action at a Distance” Foundations of Physics. DOI: 10.1007/s10701-021-00511-3