Double-slit experiment

The double-slit experiment (also referred to as the dual-slit or two-slit experiment) is a classic demonstration of the wave properties of light. The experiment was first performed by Thomas Young (1773-1829) in the early years of the 19th century in a demonstration of Christiaan Huygen’s Principle.

The experiment has been extended using individual atomic particles to demonstrate quantum mechanics particle/wave duality. The resultant phenomenon has presented an extraordinary mystery that has not been successfully explained by quantum mechanics or any other approach.

The experiment

If a beam of light (photons) is shone through a slit in a screen onto a second screen behind the first, as one may expect, it will create a single band of light on the second screen and the rest of the screen will be dark. There will be a small amount of faint light on either side of the band of light that is due to light diffraction, when light bends passing through the slit and emerging at a slightly different angle.

As an illustrative comparison, if the same experiment were to be conducted with fine grains of sand, pouring sand onto a screen with one slit, the sand will simply fall through the slit and create a single mound on the lower surface leaving the rest of the surface bare with the odd grain of sand landing a short distance away from the mound.

In the dual-slit experiment, the same demonstration takes place but with two slits parallel to each other. A beam of light is shone on the front screen, which now has two narrow slits that allow light to pass through to the second screen behind the first. However, instead of making two bands of light on the second screen, a series of dark and illuminated bands appear which look like stationary ripples, ripples that do not move as one would expect ripples on the surface of water. In between the light bands and the dark bands there is some amount of light due, again, to diffraction.

If, by comparison, the same experiment were to be repeated using fine grain sand pouring sand onto a screen with two slits, the sand will simply fall through the slits and create just two mounds directly beneath the slits that would correspond to the light shone through a slit. The reason for this is that sand grains will pass through one of the two slits and will not interfere with other grains, so they do not cause a series of bands of sand grains. To that extent, it may be obvious that solid matter does not act like light.

The series of light bands created by photons in the dual-slit experiment is explained by the wave nature of light. The light and dark bands are caused by interference, the way light waves interact or interfere with each other as they pass through the slits. As the light waves pass through each slit they emerge at different angles due to diffraction and they then encounter light waves emerging from the other slit. The resultant light bands are areas where light waves amplify each other—constructive interference: wave crests or troughs have combined synchronistically to create a higher wave crest or lower trough. The dark bands with little or no light are those that correspond to the way light waves cancel each other—destructive interference: waves are out of synchronisation and a crest interacts with a trough resultings in a band of no light.

Quantum mechanics and wave-particle duality

The research into the nature of light beginning in 1900 has led to the development of quantum physics and Quantum ElectroDynamic (QED) theory.

A primary difference between the concept of particles and waves is this, a particle is localised, that is to say, in a single place at a specific time. A wave, on the other hand, is an extended, non-localised phenomenon.

The two-slit experiment has been performed with solid matter particles, such as a beam of individual electrons, atoms or molecules, which, one would expect, would show localised characteristics. (Arthur Holly Compton's work on photon-electron scattering in 1922 established the particle nature of photons, the Compton effect, showing that light itself, photons, has both particle and wave nature.)

In this version of the dual-slit experiment, particles are beamed onto a second screen that has been coated with a material to show a bright spot when the electrons strike the second screen. If the particles are beamed at the target screen without an intervening screen and slit, they will form a single band, like the light or sand demonstration. With a single slit, they will form a broader band, displaying the phenomenon of diffraction, just as would be expected with light. This can also be explained by particle deflection at the edges of the slit which causes a larger scattering area.

However, when the dual slit experiment is performed firing a beam of particles at a screen with two slits, this results in a series of bands just as would be expected with the light, not two single groupings as happens with the sand. Furthermore, there is also an impact band that corresponds with the area between the two slits, an area which is not directly in line with the particle beam. So, in this case, solid matter is behaving as if it is a wave, and the particles are grouping in areas that are not directly in line with the two slits.

A possible interpretation of the way these particles and photons interact was suggested by the fact that in between the light bands and the dark bands there is some amount of light or evidence of individual particles striking away from the main groupings just as in between the mounds of grains of sand there are some scattered grains. In all three cases a possible explanation was that the light waves, the grains of sand and the individual particles of matter have bounced off the edges of the slits and been scattered.

Another possible explanation was that the individual particles were colliding with each other and creating the scattering effect. To test this, individual particles have been fired one at time at the dual-slit screen and as happened with the steady stream of particles, rather than just two bands there are a series of bands that showed empty areas of interferences as well as areas with numerous strikes, the same interference pattern observed with light. No matter how widely timed the individual particles, they will make a series of bands. In other words, the individual particles behaved the same way that light waves behave and are not interfering with each other causing a small scattering effect. In quantum mechanics this dual character is called the particle/wave duality.

The series of light bands created by photons and grouped particles in the dual-slit experiment has been explained by the wave nature of light and particles. In the dual-slit experiment with light, the light and dark bands are caused by the way light waves interact as they pass through the slits. The illuminated bands are areas where light waves amplify each other: wave crests or troughs have combined to create a higher wave crest or lower trough. The dark bands with little or no light are those that correspond to the way light waves cancel each other, a crest interacts with a trough and this results in a band of no light. This is what is also believed to be happening with the individual particles.

The particles are localised when they arrive at the front screen and did not ‘bump’ into each other since they were fired one at a time and for that reason the interference bands can not be explained as collective behaviour, in other words, the particles are not interacting with each other. When the particles arrive at the back screen, they are again acting as localised particles. However, when the particles arrive at the front screen with the two slits, they seem to change their character, emerging as waves which interfere with each other.

Another unexpected phenomenon that has not been explained is that with the single slit open, the particles struck some areas of the back screen which were untouched when two slits were open. In other words, with more particles streaming through the front screen it was expected that there would be more striking the previously observed areas, however, the particles did not behave as expected.

Observation paradox

There are several things that might happen to the particles. They could be expected to emerge through either slit and produce two bands on the back screen as would be expected with particle behaviour, or they can bounce off the surface of the plate with the two slits and not pass through the front screen. However, they apparently reach the dual-slit as a single particle, then become a wave and emerge though both slits exhibiting interference patterns.

To ascertain what is happening, apparatus to observe a single slit has been used to delineate the activities of single particles. This however produced a startling result. Under observation, the particles exhibit exclusively particle behaviour, forming two bands rather than an interference pattern. The act of observing affected the behaviour of the particles.

Under observation, it has been determined that parts of particles do not emerge from the slits, they emerge as whole particles and exhibit particle behaviour forming single bands on the back screen. Currently, this behaviour is believed to be due to the intrusiveness of the act of detecting the particles as they emerge, and that intrusiveness interferes with the particle’s wave behaviour causing it to behave exclusively as a particle. The question now becomes, how does a photon (light) or an particle “know” about the two slits? At this point, the idea of location has become a hindrance to explaining this behaviour. Where a photon or an electron is located is only ascertained when it is actually observed.

Niels Bohr's explanation for this phenomenon, an extension of what is referred to as the Copenhagen Interpretation of Quantum Mechanics, was presented in 1927, in his Principle of Complementarity. Bohr posited that in any experiment light shows only one aspect at a time, either it behaves as a wave or as a particle.