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Sunday, 5 February 2012

Wave-particle duality of nature challenged at Kelaniya University

In 1807, Thomas Young demonstrated that light had wave properties by using a simple experiment now famously known as the Young’s double slit experiment. Young showed that the pattern of bright and dark fringes that forms on a screen when light passes through two narrow slits can be explained using the wave theory. Since Young’s revelations, the wave theory of light (of electromagnetic radiation in general) took firm root and dominated the physics world for nearly a century. Until Max Plank, in 1901, showed that the energy of the radiation emitted by heated objects was discretized as if light was actually composed of particles! Since the wave nature of light as demonstrated in the Young’s experiment could not be discarded, scientists reconciled that light had both wave-like and particle-like properties. As it turned out just as light possess particle like properties, particles such as electrons, neutrons and even large molecules like fullerenes possess wave like properties. In 1989 experiments conducted by Tonomura demonstrated that a beam of electrons passing through a double slit, as in Young’s experiment, actually forms a dark and bright fringe pattern on a screen next to it. It is believed that these interference patterns, as they are called, arise because of the wave nature of electrons.
Young’s double slit experiment is still an active research topic among physicists as they have not fully embraced the strange behavior of nature implied by it. Physicists have been particularly bothered by the fact that when a single electron, instead of a beam of electron is sent through the double slit what is observed on the screen is not a faint interference pattern but a single dot on the screen depicting an electron striking the screen. If an electron represented a physical wave of some sort then one would expect a series of bright and dark fringes, however faint it might be on the screen but not a single dot. This unexplained phenomenon is the focus of recent theoretical and experimental work conducted by Dr. Nalin de Silva and his student Suraj Chandana at University of Kelaniya.
Dr. de Silva rejects the wave nature of particles. According to him what gives rise to a bright and dark fringe pattern in the Young’s double slit experiment is not the so called wave nature but the particle nature itself though not necessarily those of ordinary classical particles. Dr. de Silva theorizes that each particle that hit the screen on a double slit experiment appears at both the slits simultaneously prior to reaching the screen. This view contrasts with what physicists believe to date. They believe that particles come thorough one of the slits in the Young’s experiment but not both. But particles coming through individual slits cannot explain the bright and dark fringe pattern forming on the screen. Therefore to account for the fringe pattern they theorize that particles behave like waves. However this so called wave property cannot be inferred when only a single particle is fired through the double slit, which results in a single dot on the screen and not a fringe pattern.
In the case of the double-slit experiment, quantum physics dictates that at the level of the slits, a particle is in a superposition of states corresponding to the position of the two slits. The particle is in this superposition as long as it is not disturbed. Placing a screen causes to collapse the superposition into one single state (position). Dr. de Silva interprets the superposition of positional states to mean that the particle actually exists at multiple positions prior to striking the screen. Using the super-positional states quantum physics can predict the probability of finding the particle at the screen or anywhere else between the screen and the double slits. The fringe pattern merely corresponds to the probability of finding the particle at various points of the screen. Dark fringes are where there is a zero probability of finding the particle and bright fringes are where there is higher probability of finding the particle. According to this interpretation, there is no interference of sort taking place as there are no waves to be considered. Why there is only a dot on the screen when a single electron is fired through the double slit is simply because the superposition of positional states collapses into one single position at the screen.
To prove that quantum particles do not have wave properties, Dr. de Silva designed an ingenious yet a simple experiment using a laser beam and double slit apparatus. Here, a series of aluminum sheets are placed along the zero probability positions as predicted by quantum physics, between the slits and the screen. Since light cannot penetrate aluminum sheets, no interference pattern should form on the screen if light behaved like a wave. On the other hand if light behaved as quantum particles, placing aluminum sheets at zero probability positions should not affect the fringe pattern since there is no particle present at those positions to interact with the sheet. The experiment was carefully set up by his student, Suraj Chandana. When the laser was turned on, the fringe pattern appeared on the screen as they had expected confirming the hypothesis that light consists of particles that did not have wave properties.
Physicists had to artificially attach a wave property to quantum particles to explain the fringe pattern in the Young’s experiment because they did not think that the superposition of positional states meant particles were at both the slits at the same time. According to the Copenhagen interpretation of quantum mechanics formulated by Niels Bohr and others, nothing can be said about the superposition of positional states before a measurement is made; it is the act of measurement that creates the knowledge of the position of the particle. According to Dr. de Silva’s interpretation or Vidyalankara interpretation as he prefers to call it, a particle actually exist at number of positions in superposition until the superposition is disturbed by an observer. Western physicists could not have considered this possibility since according to the Aristotelian logic a particle is either here or not here. But for people of many eastern cultures it is not a problem to believe that a particle can be at multiple locations at the same time, as they can relate to certain deities and supernatural beings that can manifest at different places at the same time. Today, the existence of such superposition of states is not just a legend but is supported by experiments. In 1996 Monroe and his colleagues had demonstrated the simultaneous existence of two spin states of Beryllium cation. Since then similar experiments have been carried out by others to prove the existence of superposition of states.
The Vidyalankara interpretation also resolves other paradoxes in quantum physics such as the Schrödinger's cat, the results of “which way” experiment and the Wheeler’s delayed choice experiment. For long physicists have been forced to understand quantum physics using particle and wave properties which are essentially classical ideas. Dr. de Silva expels these and introduces a nature that is more akin to quantum physics. He has created a paradigm shift in the knowledge of quantum nature.

Janaka Wansapura, Ph.D.