Wave-Particle Duality

About 150 years ago light was thought to behave solely as a wave similar to sound and water waves. In 1905 Einstein observed that light may also act as being made out of small particles. Since then physicists found it difficult understanding the full nature of light since in some situations it acts like a particle and in others like a wave. This dual nature of light lead to the insight that all fundamental physical objects enclose a wave and a particle aspect, even electrons, protons, and students.

Interestingly, it was found that in any experiment light shows only one aspect at a time, either it behaves as a wave or as a particle. This observation was expressed in 1927 in Niels Bohr’s Principle of Complementarity. Einstein was deeply troubled by that principle, since he could not accept that any external measurement would prevent light to reveal his full dual nature. The fundamental problem, however, seemed to be that you have to destroy the light in order to measure one aspect of it. Then once destroyed there is no light left to measure the other aspect. You can’t eat the cake and have it.

In 2004, Shahriar S Afshar claimed that he had devised an experiment that challenges Bohr’s principle of complementarity. A Rowan team consisting of Eduardo Flores and Ernst Knoesel from the Rowan's Physics Department, Shahriar S. Afshar, and Keith McDonald physics major at Rowan, was formed, to verify Afshar’s claim at extremely low light intensity levels.

In this modified double-slit experiment, a laser beam hits a screen with two small pinholes. As a particle, light goes through one of the pinholes. Through a lens system the light is then imaged onto two detectors, where a certain detector measures only the photons, which went through a certain pinhole. In this way Afshar verifies the particle nature of light. As a wave, light goes through both pinholes and forms a so-called interference pattern of bright and dark fringes. Afshar’s experiment consist of the clever idea of putting small absorbing wires at the exact position of the dark interference fringes, where you expect no light. He then observes that the wires do not change the total light intensity, so there are really dark fringes at the position of the wires. That proves that light also behaves as a wave in the same experiment, in which it behaves as a particle.

The findings of the Afshar experiment were published online on 23 January 2007, in the Foundations of Physics Journal, an international journal devoted to the conceptual bases and fundamental theories of modern physics, biophysics, and cosmology. The main new contribution is that light carries the wave and particle nature at all times. It is interesting to note that even after 80 years we can still gain a better understanding about the nature of light using refined measurement techniques and creative ideas, and therefore are able add to the vast insights of former scientists.