QR3.1.2 Particle or Wave?

Figure 3.1. a. Huygen’s wave front. b. Newton’s corpuscles

The question of whether light is a wave or particles has a long history. In the seventeenth century Huygens noted that light beams at right angles pass right through each other like waves while arrow-like particles should collide. He concluded that light was an expanding wave front that spreads in all directions, with each strike point the center of a new little wavelet. If the wavelets interfere as they spread, the trough of one wave will cancel the crest of another to give a forward moving envelope that at a distance from the source acts like a ray of light (Figure 3.1a). Huygen’s Principle that each wave front point is a new wavelet source spreading in all directions explained reflection, refraction and diffraction.

In contrast, Newton noted that light travels in straight lines rather than bending round corners as sound waves do when we hear someone talking in the next room, so concluded that light was particle-like corpuscles that traveled in straight lines to match the optics of the day. His particle model explained only reflection and refraction (Figure 3.1b) but for some reason carried the day.

Two hundred years later, Maxwell, building on Faraday’s idea of a field, wrote the equations of light as an electromagnetic wave based on a mechanical model of rotating vortexes. The equations worked so they were quickly accepted, and this seemed to settle the matter that light was a wave.

Maxwell’s original equations assumed that light waves travel through a “luminiferous aetherbut the Michelson-Morley experiment then dispelled the idea that light traveled in a physical medium. Then Einstein equally convincingly argued from the photo-electric effect that light comes in particle-like packets called photons. The result was two theories, both of which worked to a degree.

Over centuries, the theory of light has swung from Huygens’ waves to Newton’s corpuscles to Maxwell’s waves to Einstein’s photon packets with no clear winner, so modern physics finally gave up. It concludes that light is wave and a particle, though no-one can explain how such a wavicle is possible. Three centuries after Huygens and Newton, we still don’t know whether light is a wave or a particle and the miracle of wave-particle duality essentially enshrines our ignorance. Physical realism could have explained light as a particle or as a wave but it can’t explain how it can be both.