Figure 2: Interference pattern created in a double slit experiment.

Just how troubling quantum mechanics can be, is illustrated by the famous double-slit experiment, originally performed by Thomas Young in 1801 (well before the birth of quantum mechanics). Richard Feynman had this to say about the phenomenon uncovered by the double-slit experiment: “a phenomenon which is impossible (…) to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery (of quantum mechanics)” (Feynman, Leighton, & Sands, 1969, pp. 1-1)6. It makes one wonder whether reality exists at all.

In the basic set up, a coherent light source illuminates a plate containing two parallel slits, and the light passing through the slits is observed on a screen behind the plate. A pattern of light band and the dark bands is observed on the screen that is analogous to what waves will do in a similar situation. Young reasoned if light were a wave, it would pass through both slits, separating into secondary waves that would then recombine on the other side—i.e., they interfere with each other. Thus the interference pattern is a proof of the wave nature of light because if light were made of particles, or corpuscles as Isaac Newton had claimed, the screen would have shown two bright parallel lines where the light particles had passed through one slit or the other instead of the interference pattern.

Young’s conclusions about the wave nature of light were initially rebuffed by scientists, who favored Newton’s corpuscular theory until the French physicist Augustin Fresnel conducted a series of more comprehensive improvements of Young’s basic experimental setup. He succeeded where Young had failed and finally convinced the world that light really was a series of waves, rather than streams of tiny particles.

But the matter was not resolved until well into the 20th century, when it became clear from both experiment and the theory of quantum mechanics that light is both waves and particles —moreover, particles, including electrons also have a wave nature. Max Planck, Albert Einstein, and Arthur Compton were among the architects whose work led to the realization that light was both particle and wave: specifically, light is made of photons that collectively behave as a wave.

Figure 3: The double-slit experiment performed with single photons (A) the usual interference pattern appears with no detector is placed behind a slit to register individual photons (B) no interference pattern is produced when detectors are placed next to each slit.

Advances in technology and scientific instrumentation have now reached a stage where physicists can produce and detect single photons. What happens if the double-slit experiment is repeated with individual photons?

The modern version of the double-slit experiment consists of a laser assembly, the light from which is attenuated so that only one photon emerges at a time. The photon passes through a two slit assembly that has a camera behind it. The camera registers the point where the photon strikes the screen (the sensor of the camera) behind the slits. Because the photons arrive one at a time and can pass through either one slit or the other, one would expect them to leave a pattern of two stripes on the screen. However, the distribution is rather random (grainy) at the beginning, but eventually, a band of stripes emerges. But this is what one would expect to see if a coherent beam of light was shined through the two slits—photons collectively would behave like a wave creating an interference pattern. How can single photons create an interference pattern as well? It could only happen if individual photons passed through either slits at the same time. The photon is either in two places at once, or in other words, a single photon behaves like a wave. But strangest of all is what happens when detectors are placed next to each slit. Surprisingly, when the photons are being watched, the wave pattern disappears. The interference pattern comes back as soon as the detectors are taken away!

In summary, individual photons behave like waves and an interference pattern emerges if the observer-participator’s lets them travel from the light source to the screen unimpeded. But if the observer-participator observes them en route, he or she knows which path the photons took; this knowledge forces the photons to behave like particles, passing through one slit or the other. Thus, an experiment can be constructed either to produce an interference pattern, or to determine which way the single photons went. But both scenarios cannot be attained at the same time.

As if we can change reality just by looking at it!

The double-slit results can be explained in light of the Heisenberg uncertainty principle which tells us the smaller the uncertainty in position, the larger the uncertainty in momentum, and vice versa. The interference pattern obtained in the double-slit experiment is actually the result of a distribution of momenta of the photons since the wavelength of a photon is intimately related to its momentum. But the act of determining which slit a photon chooses to pass through entails in pinning down its position, thereby making its momentum measurement increasingly uncertain. Although the graininess of the interference pattern indicates where an individual photon lands on the screen, it is not possible to determine what path it actually took to get to that spot.

6. Feynman, R., Leighton, R., and Sands, M. in The Feynman Lectures on Physics, Vol. III, page 1-1.

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