The Mystery which cannot Go Away
The double-slit experiment is one of the simplest scientific experiments ever devised: it requires only two things: some light and an impermeable sheet with two slits in it. What the experiment lacks in complexity it has more than compensated for by the astonishing results which it has yielded. The double-slit experiment, is in itself, a 4-dimensional experiment, that is mutable and responsive to the observer. It is an experiment which exists and is repeatable and stable in the scientific sense, but infinitely changeable and although devised with a certain hypothesis and outcome in mind, which it delivered, can also give different and contrasting results, depending on how the experiment is performed.
For instance, Thomas Young performed the experiment to analyse the nature of light which he suspected was a kind of wave. The supposed light ‘waves’ were duly produced by the ‘interference’ patterns produced when the light ‘waves’ went through the two slits, proving once and for all that light was indeed a ‘wave’ and everyone went away with happy faces. It was not until the invention of the cathode ray that single electrons could be fired through the double slits which would inexplicably lead to the same characteristic ‘wave’ diffraction patterns which Young had produced nearly a century before.
The dramatic conclusion reached was that a single particle somehow passes through both slits at once in order to create the wave interference pattern, or that light acts as both a wave and a particle at the same time, which appears to be an inherent contradiction. Furthermore, that if there were any attempt to measure or localise the light as it passed through the slits, then the ‘wave function’ collapsed and the pattern changed to show that only a single photon had passed through one of the slits and there was no wave diffraction pattern.
The particularly peculiar thing about the double-slit experiment and the collapse of the wave function is that it is not the measuring or recording device itself which collapses the wave function but it seems to be consciousness itself which does this, and many elaborations on the double-slit experiment have been devised precisely to test this strange hypothesis which has so disturbed many renowned scientists.
For instance, if an interferometer is introduced into the experiment to pinpoint the position of the 4d photon, then the wave function collapses and we see a pattern which would indicate that the photon has passed as a particle through one of the two slits. However, if there was no 'access' to the information and the observer had no idea which slit the photon had passed through, then it would behave as a wave and the wave function would be maintained and it would create a wave diffraction pattern. So, if our minds can have an effect over one photon or electron then what more control can we have over the whole light-based reality?
Quantum physics puts the human mind at
the centre of the phenomenon and, by implication the observer seems to play an
active part in ordering reality itself. But back in 1801, before any of this
was ever dreamed of, Thomas Young performed the experiment to analyse the
nature of light which he suspected was a kind of wave, and he actually used
sunlight.
Sunlight was sent through the metal sheet with two slits and an interference pattern was produced on the screen by the light ‘wave’ passing through the slits and creating the wave interference pattern. The interference pattern which is so central to quantum physics, is composed of alternating bright bands and dark bands, so light waves coming out of one slit, interacted with the light waves coming out of the other slit, and where the two waves met at peaks or troughs, they reinforced each other and created a bright band of light, while where a peak met a trough, the waves cancelled each other out and a dark band was produced on the screen. The bright and dark bands on the screen are a two-dimensional representation of the two wave fronts reinforcing (as in the bright areas) or cancelling each other out (as in the dark areas).
He proved to himself and science, that
the light ‘waves’ went through the two slits, as waves, interfered with each
other, so to speak, and then formed the interference pattern on the screen,
proving once and for all that light was indeed a ‘wave’, at least so they
thought. It was not until the invention of the cathode ray that subatomic,
single free electrons could be fired through the double slits, although now,
they can use attenuated or low power lasers to fire single photons.
Since electrons, as subatomic particles are also ‘quantum objects’ like photons, atoms and more recently, molecules up to a certain size. It might have been presumed that firing single ‘particles’ would create a different kind of pattern of light on the screen, perhaps one showing two bright bands, where the particles had passed through one slit or the other. However, instead, firing single electrons through the slits, incrementally created the same interference pattern as if light waves were passing through both slits simultaneously, and interacting with each other as before, the waves either reinforcing or cancelling each other out in order to create the interference pattern. But how can it be that single quantum electrons, passing individually through one of the two slits can create an interference pattern as if they are interacting with each other and passing through both slits at once and interfering with each other as if they are part of some kind of unified whole?
The dramatic conclusion of this experiment is that the quantum particle is both a wave and a particle at the same time. Speaking of the Double Slit experiment, our friend Richard Feynman said:
“…this one experiment.. has been designed to contain all of the mystery of quantum mechanics, to put you up against the paradoxes and mysteries and peculiarities of nature one hundred per cent. Any other situation in quantum mechanics, it turns out, can always be explained by saying, 'You remember the case of the experiment with the two holes? It's the same thing'. I am going to tell you about the experiment with the two holes. I am baring nature in her most elegant and difficult form.”[i]
