Predictions made by quantum mechanics appear to conflict with the classical understanding of causation. In this view of causation it is generally accepted that separate events can only affect one another sequentially in forward motion through time. Quantum mechanics rather necessitates either a nonstandard view of causal relationships, or instead several clear violations of the principle of locality, which states that an object may only be influenced by its relatively constrained environment. This has been illustrated with evidence by thought experiments like the Einstein, Podolsky, and Rosen (EPR) argument and Bell’s theory of inequality. Various interpretations of quantum mechanics usually try to deal with this dilemma by adjusting the formalism to preserve either classical causation or the principle of locality. Although both certainly may eventually have to be discarded due to the findings of quantum mechanics, I will argue that instead it suggests a form of advanced causation beyond that of our classical conception in order to maintain the standard principle of locality. This is also in fact compatible with multiple interpretations of quantum mechanics, beginning with the very division between instrumentalist and realist theories.
Instrumentalist interpretations of quantum mechanics are only concerned with matching the formalism to the outcome of experiments. Just as the Earth is not exactly the rigid sphere it is often generalized as in cosmology, electrons are not necessarily waves or particles as we often conceive of such things. Historically, the most prominent interpretation of quantum mechanics has been that formed by Bohr’s Copenhagen school, based on the Complementarity rule of wave-particle duality, because it allows physicists to focus on experiment and develop the mathematical formalism without worrying too much about deeper philosophical consequences. To this end, the process of measurement of a quantum system is thought to inexplicably result in the collapse of the measured system’s wave function. That is to say, there is no underlying reason, or causation, given for why measurement results in collapse, just that it does occur. Moreover, there is no description provided at all for what exactly entails measurement of a quantum system. It’s suggested that such quantum measurements always involve interaction with a large clunky macroscopic apparatus that leads to an inherent impact on the state of the quantum system. In this way, measuring the position of a particle with great precision involves directly interfering with it in such a way so as to change its state entirely.
However, the greatest fault in this interpretation is the lack of a clear distinction between such proposed quantum indeterminacy and the entirely determinate Schrodinger equation. How can such a non-probabilistic description of the unitary evolution of quantum systems result in series of probability outcomes? And should not measurement in quantum mechanics be described on its own rather than in the terms of classical physics? The answers offered to these questions by the Copenhagen interpretation are left seemingly incomplete, largely due to the nature of complementarity, in that our language based on ordinary everyday macroscopic human experience is generally insufficient to describe the dualistic wave-particle phenomena exhibited by quantum systems. Rather, what is important about this interpretation is exactly that realism of the quantum world is rejected. Instead, the wave function is used as nothing more than a mathematical description for experimental probability outcomes, and so the nonlocal implications of the EPR thought experiment to be discussed can be brushed aside completely. Of course, it still leaves large open questions about the quantum nature of causation, especially in regard to measurement.
As quantum mechanics was being developed, so much of classical thought was already being thrown out that it made sense as a sort of grounding to describe the microscopic phenomena being characterized in terms of its macroscopic experimental outcomes, and so the Copenhagen interpretation became exceedingly popular. Conversely, realist interpretations are ultimately concerned with the idea of fundamental causation and providing an actual description of the physical world. For example, the many-worlds interpretation is one that preserves both realism of the wave function and the principle of locality. It suggests that all outcomes predicted by a quantum experiment are actually realized, each in their own separate world, despite the experience of our own world where only one outcome is obtained, and therefore rejects wave collapse. Of course this creates a rather messy ontology of seemingly infinite simultaneously existing worlds. The many worlds interpretation, like the Copenhagen and most others, actually only differs in its philosophical implications, so there is no experiment that could be performed to verify its reality over anothers. And although it at first appears to preserve locality, it does not seem to solve incompatibility with relativity at all. How could a quantum measurement immediately result in the propagation of many diverging worlds at much faster than light speed? The proponent of many-worlds would suggest that this divergence is in fact local and occurs exactly at the speed of light. Further attempts are still being made to link the mechanics of the atomic world with a complete description of reality.
The EPR thought experiment raises the question of whether the wave function as described by quantum mechanics provides this complete description of reality. It regards quantum entanglement, in which two particles have their position and momentum intertwined as a single system, and thus determining position and/or momentum for either one will immediately bind the properties of the other in turn. This phenomenon is assumed to occur despite any vast distance left between the two particles by the time of measurement. However, this is a clear violation of the principle of locality, in that it provides for the possibility of the travel of information faster than the speed of light between the two particles. The experiment suggests that either the ordinary description provided by quantum mechanics is incomplete, or that if it is complete it necessitates nonlocal interaction. In the end, it seems easier to reject the collapse of the wave function, thereby saving both determinism and locality in favor of classical causation.
The collapse of the wave function in quantum mechanics is generally considered to be a non-deterministic event. Of course this is not to say that the theory inevitably gives us an incomplete description of the world, only that the microscopic world is necessarily one statistical in nature. For example, rather than the formalism telling us what value a given measurement will result in, we may only predict the probabilities of its various outcomes. How could such a world hope to maintain solely local interaction? These probability outcomes are connected with experiment by comparing them to the relative frequency of observed measurements. The statistical nature of quantum systems therefore can be observed despite otherwise seemingly complete independence of trajectory between separate trials.
Einstein and other realists originally thought this was actually a problem with the theory and that it could be solved by providing a description for some then hidden variable inherent in each particle that would work to restore determinism. However, the experiments done on Bell’s inequality, which is based on the same thought experiment as the EPR, proved that no hidden variable theory is compatible with locality in quantum mechanics. However, the results of the Bell experiments can be explained by a theory of advanced causation. If it is assumed that particles can affect one another backwards in time, it is highly plausible that entanglement could be explained without loss of locality. Interestingly enough, both relativity and classical physics already provide the same sort of explanation for time, as does Schrodinger’s unitary evolution equation. In fact, it is only the highly disputed description of wave collapse that violates time symmetry in quantum mechanics.
Humans have great interest in their surroundings and the idea of something happening without reason tends to put us off. To this end we often try to subscribe series of causation to past events. The way we think of causation classically is that it can only occur sequentially forward through time. However, the classical laws of physics can be applied regardless of the specified direction of time. In other words, an event that happens forward through time can just as easily be described as happening in reverse backwards through time. Likewise, every particle travelling forward through time can alternatively be seen as its counterpart antiparticle travelling reversely backwards through time. If time is honestly seen to be symmetric as so, then why does causation not act in the same way? If retro-causation is thus accepted, entangled particles can just as easily be seen as travelling backwards through time until their inevitable convergence, rather than the originally conflicting view of their identical nature in light of even incredible lengths of divergence.