I have been in some way, quite amused, as to how many consider this particle or wave debate in quantum physics as something new as in that quantum realities such as non-locality as that being a recent concern. That is, if you count recent as being within the last hundred years or so. Truly this debate has persisted for millenniums, more so then just in the last century. It actually all began with the concept of Atomic Theory first proposed in India in the 6th Century B.C.E. and then again about a hundred years later in ancient Greece with the first notions of the concept. Most notably within what is considered the era of modern science, the debate has continued as to what constitutes action and physicality in nature.
This is most poignantly demonstrated in terms of the nature of light. As we know, Newton considered light to be a particle phenomena and published a theory in support of it. Then enters Thomas Young who in the early 1800’s, with his famous two slit experiment demonstrates the wave nature of light. During the same period Augustin Fresnel, who as I have often found to ironically have being born in Broglie France, came up with a complete wave theory of light. This theory was challenged by Simeon Poisson in a proposed experiment suggested by results he calculated from the formalism of Fresnel’s theory. What the theory indicated, by Poisson’s calculation, is that if a narrow beam of light was cast onto a small round object, the effects of wave interference produced would have light bend around the disk to form a bright spot within the middle of the shadow cast. This Poisson (a Newton supporter) claimed to be absurd and set up a experiment to show Fresnel the error of his ways. Unfortunately for Poisson, but fortunate for Fresnel, the bright spot in the centre of the shadow appeared exactly as the theory had predicated. Thus Poisson was embarrassed and Fresnel won a prize in relation to this along with being elected to the French academy of science and then after made a fellow of the Royal Society. Ironically this phenomena ever since has been referred to as Poisson’s spot as apposed to Fresnel’s.
Then in 1905, Albert Einstein was to reverse this all again, with his paper on the photoelectric effect, where only a particle explanation would satisfy the results of then recent experiments. This resolved what was called the ultra-violet catastrophe, yet in turn only served to broadened the mystery, for now it appears as nature acts sometimes as a particle and in other situations as a wave. Then in the early 1920's this was to be all reversed once more, with the birth of Quantum Mechanics, first with what came to be known as the Copenhagen Interpretation. This was the result of the combined contributions of Neils Bohr, Werner Heisenberg, Wolfgang Pauli and Erwin Schrodinger. In this theory all again was attributed to a wave or more appropriately the function or action of a wave, where particle phenomena is said to be the collapse of such only upon observation. This as to say that particles are not so much, more or less, figments of our imaginations, yet rather figments of our perception.
Now as to the non-local aspect of the issue. This also is often taken as something relatively new, beginning with Einstein’s objection and culminating with the experimentally demonstrated results of John Bell's theory. However, where I would say the whole thing began was with Pierre de Fermat when in the mid 1600’s he proposed “the actual path between two points taken by a beam of light is the one which is traversed in the least time”. This became known as Fermat’s Least Action Principle and was criticized not for its inability to predict outcome, yet rather for its inference as to how nature behaves. In truth, this action as it relates to the economy of nature, is still used and considered in physics and forms the base of Lagrangian mechanics, developed by Joseph Louis Lagrange. However, at the time it was largely scoffed at for what it appeared to infer. In fact it was first immediately rebuffed by Claude Closely (a optics expert at the time) as he said, “Fermat's principle can not be the cause, for otherwise we would be attributing knowledge to nature: and here, by nature, we understand only that order and lawfulness in the world, such as it is, which acts without foreknowledge, without choice, but by a necessary determination.”
In the the contemporary context what we are referring to with this aspect of the debate is if nature behaves at some level as non-local? Thst as in instantaneous or faster then light, at the quantum level?. At this juncture, I recall reading that when Richard Feynman was first presented with this contention of Fermat’s, by his high school teacher, he thought it equally absurd. Later he would go on to develop his “Sum Over Histories” approach to quantum mechanics. I have often wondered how this served as a resolution to Fermat’s conjecture? That is I never was able to fathom where all these histories were to happen and then be free to cancel themselves to the allowed paths. That is unless one subscribes to Many Worlds or John Crammer’s ideas.
Alas in the end, one should not dispair, as there has been, despite the perceived paradox existed a fully consistent explanation of this for all to see for quite some time. And that is Bohmian Mechanics, better described as the deBroglie-Bohm pilot wave theory. It’s only that many have chosen not to look. With this I am reminded as in Plato’s Allegory of the Cave that still many refuse to look at the light to see what it may truly be. That is except for Bohm and even now his mostly ridiculed disciples, who have come to find, as Bohm discovered and Bell reminded, that it is not “particle or wave”, but rather “particle and wave”. This all has taken a little less then three thousand years. Let us hope the rest will not take as long.