Quantum physics has implications for a lot of deeper issues in physics. For one thing, it suggests that within the quantum world time runs each backward and forward whereas within the classical world it solely runs forward. Until recently physicists may explore the quantum mechanical properties of single particles solely through thought experiments, because any plan to observe them directly caused them to shed their mysterious quantum properties. however in the 1980s and 1990s physicists made-up devices that allowed them to measure these fragile quantum systems so gently that they do not immediately collapse to a certain state.
We all recognize quantum physics is weird. And a part of that strangeness comes right down to the very fact that at a elementary level, particles do not act like solid billiard balls rolling down a table, But rather sort of a blurry cloud of possibilities shifting round the room. This fuzzy cloud comes into sharp focus after we attempt to measure particles, that means we are able to solely ever see a white ball hit a black one into the corner pocket, and ne’er countless white balls hit black balls into each pocket.There is an argument among physicists over whether or not that cloud of maybes represents something real, or if it’s simply a convenient representation.
A pair of physicists from the US and Canada took a closer look at some basic assumptions in quantum theory and decided unless we discovered time necessarily ran one way, measurements made to a particle could echo back in time as well as forward. Physicist Huw Price claimed back in 2012 that if the strange probabilities behind quantum states reflect something real, and if nothing restricts time to one direction, the black ball in that cloud of maybes could theoretically roll out of the pocket and knock the white ball.
There are many counterintuitive ideas in quantum theory, the idea that influences can travel backwards in time (from the future to the past) is generally not one of them. However, recently some physicists have been looking into this idea, called “retrocausality,” because it can potentially resolve some long-standing puzzles in quantum physics. if retrocausality is allowed, then the famous Bell tests can be interpreted as evidence for retrocausality and not for action-at-a-distance—a result that Einstein and others skeptical of that “spooky” property may have appreciated.
In a new paper published in Proceedings of The Royal Society A, physicists Matthew S. Leifer at Chapman University and Matthew F. Pusey at the Perimeter Institute for Theoretical Physics have lent new theoretical support for the argument that, if certain reasonable-sounding assumptions are made, then quantum theory must be retrocausal.
Retrocausality means that, when an experimenter chooses the measurement setting with which to measure a particle, that decision can influence the properties of that particle (or another particle) in the past, even before the experimenter made their choice. In other words, a decision made in the present can influence something in the past.
In the original Bell tests, physicists assumed that retrocausal influences could not happen. Consequently, in order to explain their observations that distant particles seem to immediately know what measurement is being made on the other, the only viable explanation was action-at-a-distance. That is, the particles are somehow influencing each other even when separated by large distances, in ways that cannot be explained by any known mechanism. But by allowing for the possibility that the measurement setting for one particle can retrocausally influence the behavior of the other particle, there is no need for action-at-a-distance—only retrocausal influence.
One of the main proponents of retrocausality in quantum theory is Huw Price, a philosophy professor at the University of Cambridge. In 2012, Price laid out an argument suggesting that any quantum theory that assumes that 1) the quantum state is real, and 2) the quantum world is time-symmetric (that physical processes can run forwards and backwards while being described by the same physical laws) must allow for retrocausal influences. Understandably, however, the idea of retrocausality has not caught on with physicists in general.
It is more of an idea for an interpretation at the moment that other physicists are rightly skeptical, and the onus is on proponents of this theory to flesh out the idea.
Leifer and Pusey’s arguments holds whether the quantum state is real or not—a matter that is still of some debate. A quantum state that is not real would describe physicists’ knowledge of a quantum system rather than being a true physical property of the system. Although most research suggests that the quantum state is real, it is difficult to confirm one way or the other, and allowing for retrocausality may provide insight into this question. Allowing for this openness regarding the reality of the quantum state is one of the main motivations for investigating retrocausality in general, Leifer explained.
“The reason I think that retrocausality is worth investigating is that we now have a slew of no-go results about realist interpretations of quantum theory, including Bell’s theorem, Kochen-Specker, and recent proofs of the reality of the quantum state,” he said. “These say that any interpretation that fits into the standard framework for realist interpretations must have features that I would regard as undesirable. Therefore, the only options seem to be to abandon realism or to break out of the standard realist framework.
“Abandoning realism is quite popular, but I think that this robs science of much of its explanatory power and so it is better to find realist accounts where possible. The other option is to investigate more exotic realist possibilities, which include retrocausality, relationalism, and many-worlds. Aside from many-worlds, these have not been investigated much, so I think it is worth pursuing all of them in more detail. I am not personally committed to the retrocausal solution over and above the others, but it does seem possible to formulate it rigorously and investigate it, and I think that should be done for several of the more exotic possibilities.”
“The case for embracing retrocausality seems stronger to me for the following reasons,” Leifer said. “First, having retrocausality potentially allows us to resolve the issues raised by other no-go theorems, i.e., it enables us to have Bell correlations without action-at-a-distance. So, although we still have to explain why there is no signaling into the past, it seems that we can collapse several puzzles into just one. That would not be the case if we abandon time symmetry instead.
“Second, we know that the existence of an arrow of time already has to be accounted for by thermodynamic arguments, i.e., it is a feature of the special boundary conditions of the universe and not itself a law of physics. Since the ability to send signals only into the future and not into the past is part of the definition of the arrow of time, it seems likely to me that the inability to signal into the past in a retrocausal universe could also come about from special boundary conditions, and does not need to be a law of physics. Time symmetry seems less likely to emerge in this way (in fact, we usually use thermodynamics to explain how the apparent time asymmetry that we observe in nature arises from time-symmetric laws, rather than the other way round).”
As the physicists explain further, the whole idea of retrocausality is so difficult to accept because we don’t ever see it anywhere else. The same is true of action-at-a-distance. But that doesn’t mean that we can assume that no-retrocausality and no-action-at-a-distance are true of reality in general. In either case, physicists want to explain why one of these properties emerges only in certain situations that are far removed from our everyday observations.