Double Slit Experiment
The double-slit experiment was first performed by Thomas Young in 1803 and was initially forwarded as a demonstration of the wave nature of light in contradiction to Newton’s corpuscular or particle theory of light.
The diagram above simply tries to illustrate the essential elements of Young’s experiment in which light, as a wave, creates an interference pattern of light and dark bands on the background screen on the right. However, in the context of the timeline of developments being discussed, we have reviewed both Compton’s suggestion that light has a particle-like nature and deBroglie’s suggestion that matter particles have a wave-like nature. While aspects of the experimental evidence, which support the wave-particle nature of matter, extends beyond the ‘Pre-War’ timeline, the main arguments of the debate were laid down in this time period. So to start, we need to present what commonsense would tell us is the equivalent to Young's double-slit experiment for particles.
Of course, our previous discussions of quantum mechanics might tell us that any unusual wave-particle duality issues will only be observed, if the particles in question are much closer to the quantum scale, e.g. electrons. Given that the cathode ray tube was invented in 1897, we might reasonably update the diagram above by introducing the idea of an ‘electron gun’ to fire a stream of electrons through the double slits and onto the screen.
However, while the only real difference between the two diagrams appears to be the scale of the particles, the outcome of the pattern on the screen is very different and cannot be explained by classical mechanics; although we might wish to consider a few possibilities. For example:
Is it possible that the interference pattern is caused by electrons going through the two slits and then somehow interacting with each other?
One of the most surprising aspect of the experiment outlined above, i.e. based on the electron gun, is the idea that one electron, at a time, can be fired at the screen. Latter day experiments along these lines have been done and appear to suggest that after a large number of electrons have been fired, i.e. one at a time, the interference pattern still emerges. If so, then we appear to be led to the conclusion that no matter what is causing the interference pattern, it does not appear to involve the interaction of two or more electrons.
What implications follow from this assumption?
Within the wave model, we recognise that the interference pattern results from components of the same wave front passing through both slits, which then recombine in superposition to generate the observed interference pattern.
Might this suggest that aspects of the wave-like nature of a single electron is passing through both slits?
Subsequent experiments have apparently been carried out to try to resolve which slit the electron passes through. However, when this is done, it would appear that the electron reverts to its particle-like nature and passes through just one of the slits; but in-line with the earlier particle-like distribution pattern, the interference phenomenon disappears. Somehow, the process of measurement has an effect on the quantum system.
What explanation might quantum mechanics offer up in this situation?
Well, to some extent, we have just discussed some of the implications of Heisenberg's Uncertainty Principle, which suggests that it is impossible to interact with a quantum system, in order to make a measurement, without disturbing the original system. As described, using photons to detect the position of the electron, at either slit, will result in some form of energy exchange. As in the case of the photoelectric effect, turning down the intensity, i.e. number of photons, will simply result in some of the electrons not being detected, such that the interference pattern will start to re-emerge. We might also consider reducing the energy associated with the photons, i.e.
However, this approach only leads to an increase in the associated wavelength, such that we would lose the spatial resolution to determine which of the two slits the electron passed through. Again, the failure to interact with the quantum system, i.e. the electrons, causes the interference pattern to re-emerge. As such, the philosophical implication of quantum mechanics is that there is no experiment that can resolve the slit through which the electron passed, while maintaining the interference pattern. In fact, this conclusion led Heisenberg to state:
“The path of a particle comes into existence only when we observe it."
The implication of this statement will be discussed further under the heading of ‘Wave Function Collapse`. So, within the confines of quantum theory, the conclusion appears to be that the position of the electron is only defined in terms of a probability linked to the wave function [Ψ]. When we do not interact with the probability wave, components of this ‘conceptual’ wave pass through both slits and recombine at the screen, which results in the ‘observed’ interference pattern. In contrast, when we try to interact with the probability wave, it collapses, such that there is only a 100% chance it went through one slit and a 0% chance it went through the other. In this instance, the two components of the probability wave cease to exist and cannot therefore recombine to form an interference pattern.