Using Me!

Quantum Mechanics, Consciousness, Reality and Computation – Part II

In Young’s Double Slit experiement a beam of light is projected towards two slits in a partition. Due to the wave-like properties of Electromagnetic radiation the light passes through both slits and disperses there just as a water wave disperses after passing through a narrow gap. These dispersed waves interfere with each other –  the peaks and troughs in the wave form interfere with each other to form bright and dark bands on the screen behind the partition.

This only makes sense in the context of wave-like light. However we can also describe other properties of light as being particle-like. For example in the way that light is absorbed and re-emitted by electrons. In fact from a given light source we can determine how many particle-like photons are being produced every second. We can engineer a source for Young’s double slit experiment of such low intensity that we can say with absolute certainty that only one photon is passing between the source and the screen at any time. Logically you would think that in such a case the photon can only possibly go through one slot or the other. So there should be no interference pattern. But if we use photographic paper to record where the photons hit the screen we still get an interference pattern, built up one tiny dot at a time. So despite there being just one photon in the experiment at a time it still behaves like a wave that passes through both slits. Incredibly if you still think of the photon as particle-like it has somehow interfered with itself. Yet since the interference pattern is built up from individual dots we can say that as far as the screen is concerned the light is particle like, not wave-like.

A momentary recap. The wave nature of light (electromagnetic radiation) has peaks and troughs just as waves on water do. When a wave at a peak combines with a wave at a trough they cancel each other out. So at each place on the screen we can work out what the magnitude of the wave is for the two paths, through the two slits. In some places the two waves will be at maximum amplitude and form a bright area. In others one will be at a positive amplitude and the other at a negative amplitude causing them to cancel each other out and we see a dark band. And at most places the two amplitudes will add to something between this maximum and minimum. And so our complete pattern on the screen shows bands of light varying from maximum to minimum brightness and back again.

How are we to make sense of this? In Quantum Mechanics we express everything as a wave function that describes the probability of finding our subject (in this case a photon) over a region of space. This is a probabilistic Wave Function completely unrelated to the wave-like way in which electromagnetic radiation propagates. This Wave Function represents the fact that we don’t really know where the photon is. Somehow the photon’s existence is smeared out such that there is a probability that it goes through one slit and a probability that it goes through the other. As confusing as this is it allows the single photon to interfere with itself.

Moreover in Quantum Mechanics we talk about ‘collapsing the wavefunction’. This is what happens when we measure our subject. We force the photon to decide where it is (not in any conscious manner on the photon’s part of course!). Our screen forces the wave function to collapse since the photon must hit it somewhere. Actually there’s always a probability that the photon will simply pass on through the screen and remain just a smeared out probability continuing along it’s way, but the ones doing that aren’t relevant to our experiment nor do they break anything discussed here.

Likewise if we place a sensor in one of the slits we force the Wave Function to collapse and the question of which slit the photon travels through becomes deterministic and no longer probabilistic. If we do this to our experiment the bright and dark bands disappear from the screen and are replaced by just two bright bands representing where the photon has travelled through each slit in a ‘particle-like’ manner. Remove the sensor and suddenly the interference pattern from the ‘wave-like’ nature of light returns!

What are we to make of this? And the obvious question… What constitutes a sensor? What collapses the wave function and what doesn’t? More to follow!