WHAT IS THE NATURE OF LIGHT?

So light is a particle, right? But wait! What about diffraction and interference? Didn’t Foucault show that the speed of light in air and water needed to explain diffraction disagree with experimental data if we assume that light is a stream of particles (Figure 7)? And isn’t interference supposed to be the obvious signature of a wave? So, what is the nature of light? Is light a wave or a flow of particles?

In trying to solve this quandary, it is interesting to think what would happen if we combined the double-slit experiment (Figure 4) with the single-photon experiment (Figure 33). American physicist Richard Feynman, who won the 1965 Nobel Prize in Physics, liked to introduce this hybrid experiment by thinking about what would happen if a machine gun is shot at an iron plate with two slits in it (Figure 89a). If there were a concrete wall behind the iron plate, and since bullets don’t interfere with each other like water waves, the bullets would be expected to carve the brick wall behind the two slots. A few bullets may bounce off the edges of the slots, creating just a few stray hits, but the vast majority of the damage to the wall would be expected to be in the two areas behind the slots.

Figure 89 In Feynman’s thought experiment, a machine gun shoots bullets—one at a time—against a steel plate with two slits. (a) The damage to the brick wall behind it is expected to be behind these slots. (b) When the experiment is conducted with photons shot one at a time, their cumulative hits actually form an interference pattern.

Surprisingly, when this experiment is carried out with single photons (assuming the wall would record the individual photon hits, as would happen if it were a photographic plate), the cumulative pattern is not the one predicted in the thought experiment of Figure 89a. Instead, the individual photon hits accumulate to form an interference pattern, as shown in Figure 89b. This result could be interpreted as meaning that light really is conveyed as a wave, since interference occurs, but at the same time, a single hit is detected on the screen at any given time, indicating that single photons are flying through the apparatus. Hence, the emergence of an interference pattern suggests that each photon interferes with itself, and therefore each photon must be going through both slits at once!

In 1909, about a century after Young carried out his original double-slit experiment, British physicist G. I. Taylor performed a similar experiment in a very dark room, using a photographic plate as the screen on which he showed that even the feeblest light source—equivalent to “a candle burning at a distance slightly exceeding a mile”—could lead to interference fringes.