Wave-Particle Duality: Nature writes: “An experiment showing that oil droplets can be propelled across a fluid bath by the waves they generate has prompted physicists to reconsider the idea that something similar allows particles to behave like waves.”
Image Credit: Dan Harris/MIT
Source: Nature

An article, by Zeeya Merali, in the journal Nature writes about the difficulty that even physicists have in explaining and testing the fundamentals of quantum physics; Merali writes:
Ever since they invented quantum theory in the early 1900s, explains Maroney, who is himself a physicist at the University of Oxford, UK, they have been talking about how strange it is — how it allows particles and atoms to move in many directions at once, for example, or to spin clockwise and anticlockwise simultaneously. But talk is not proof, says Maroney. “If we tell the public that quantum theory is weird, we better go out and test that's actually true,” he says. “Otherwise we're not doing science, we're just explaining some funny squiggles on a blackboard.”
It is this sentiment that has led Maroney and others to develop a new series of experiments to uncover the nature of the wavefunction — the mysterious entity that lies at the heart of quantum weirdness. On paper, the wavefunction is simply a mathematical object that physicists denote with the Greek letter psi (Ψ) — one of Maroney's funny squiggles — and use to describe a particle's quantum behaviour. Depending on the experiment, the wavefunction allows them to calculate the probability of observing an electron at any particular location, or the chances that its spin is oriented up or down. But the mathematics shed no light on what a wavefunction truly is. Is it a physical thing? Or just a calculating tool for handling an observer's ignorance about the world? 
Or so it remains. Much of quantum physics remains in its theoretical state, unproven beyond these mathematical symbols, these Greek letters remaining the secret, arcane language of only a few initiates. Theories, however, need testing, and one of these is the long-standing belief in wave-particle duality, which dates to the beginnings of quantum theory, generally recognized as Max Planck’s 1900 paper on blackbody radiation. It also dates to the other pioneers of quantum physics, the “they” being physicists such as Albert Einstein, Niels Bohr, Erwin SchrödingerLouis de Broglie, Max Born, Paul Dirac, Werner Heisenberg and Wolfgang Pauli. Other later pioneers are Richard Feynman and Julian Schwinger with their QED theory, or quantum electrodynamics. (You can view Feynman's lecture on the subject here.)

Quantum theory is both fascinating and frustrating. Simply explained, quantum theory says that at the molecular and atomic levels of size—very small particles—the laws that govern large objects do not apply. Quantum is Latin for “how much,” which forms the fundamental basis for quantum theory: everything, both matter and energy, can be reduced to discrete units. Equally important to this discussion is that such microscopic particles behave differently than the larger macroscopic ones that we humans can see.

Adding to the strangeness is that the observation of such particles actually influences their behaviour—called the Heisenberg Uncertainty Principle, discovered by the German theoretical physicist, Werner Heisenberg, in 1926. This effect is not common with larger particles at the macroscopic level. Einstein never fully accepted quantum mechanics and its theory of quantum entanglement (championed by Bohr), and described this phenomena, in 1927, as “spooky action at a distance.” Einstein said it contradicted his special theory of relativity.

Such disputes are expected when trying to understand something so odd, so hard to comprehend, let alone explain with a degree of clarity. This is one of the many paradoxes found in quantum physics, some of which have been resolved. But many remain, including the wave-particle duality, where light waves act like particles and particles like light waves. Some of the other interesting but mysterious phenomena include the following: “Matter can go from one spot to another without moving through the intervening space (called quantum tunnelling). Information moves instantly across vast distances. In fact, in quantum mechanics we discover that the entire universe is actually a series of probabilities.”

Is this it? This in some simple way explains the sense of quantum physics, chiefly, that much of quantum physics is a measurement of probabilities, an application of mathematical statistics to what seems unreal—as a way to make it comprehensible and real to our human senses and to our mind. Is it all by chance? Is there no deterministic force at play? Einstein, although a secular humanist and confirmed agnostic (but not a militant atheist), did not support this view, citing this as one of the faults of quantum mechanics (e.g., famously telling Bohr “God does not play dice with the universe”).

Mystery is most often linked to a religious devotion, but it can be applied to science and most notably to theoretical physics and to quantum physics. It can most assuredly be found outside the traditional view of religion, outside of a belief in a personal god; Einstein is quoted as saying in Philip Frank’s Einstein: His Life and Times (1947), chap. 12, sec. 5 - “Einstein’s Attitude Toward Religion”:
The most beautiful emotion we can experience is the mystical. It is the power of all true art and science.He to whom this emotion is a stranger, who can no longer wonder and stand rapt in awe, is as good as dead. To know that what is impenetrable to us really exists, manifesting itself as the highest wisdom and the most radiant beauty, which our dull faculties can comprehend only in their most primitive forms—this knowledge, this feeling, is at the center of true religiousness. In this sense, and in this sense only, I belong to the rank of devoutly religious men.
The mysterious might explain the strange, but beautiful, world of quantum physics. Much of it is speculation, trying to fill in the gaps of knowledge. In much of theoretical physics there is a lack of certainty, which runs contrary to our human desire for clearly defined knowledge and understanding; although this certainty might always elude physicists, it will feed their curiosity.

For more, go to [Nature].