Quantum Mechanics for Dummies: Thomas Young


Georgie McDonald returns to confuzzle our brains (or unfuzzle, if you are scientifically inclined). Be warned ― it’s not for the easily mindblown!

I previously told you about Einstein’s theory of ‘spooky action at a distance,’ which might have left your brain in a tangled mess. So, to get your brain back in gear for the next article in this series, here is a riddle:

I am the first you ever saw, what greets you every morning, and what goes out in the end. What am I?

The answer is light. Apart from when we have turned off the lights to go to bed, we probably don’t give much thought to it. But what is light? If you have done GCSE Physics you will know that what we see is visible light which is part of the electromagnetic spectrum and therefore a transverse wave. Thomas Young, however, discovered that light was a lot more complicated when you look at the quantum mechanics.

Young’s double slit experiment is very famous and shows how particles cannot only behave as conventional particles, but also waves. This experiment was first carried out using photons; however, electrons, atoms and even the Carbon 60 Bucky balls also show a wave and particle-like character.

In this experiment, light first diffracts out of one slit, allowing it to spread out and hit two more slits. The light that passes through each of these slits diffracts again, and the wave then hits a detector. If a conventional wave goes through this, we see an interference pattern on the detector screen. This is created when the waves overlap with each other to produce a pattern where they reinforce each other in some places and cancel out in others. Therefore, if two waves arrive in phase, they reinforce each other and a bright line appears on the screen; if two waves arrive in anti-phase, they cancel each other out and a dark area is seen on the screen. Conventionally, particles show no interference patterns when fired in a straight line through a slit. We would expect to see a pattern corresponding to the size and shape of the slit on the detector.

When a laboratory apparatus that could reliably fire one electron at a time through the double slit was developed, the appearance of an interference pattern suggested that even though each electron leaves the apparatus as a particle, it becomes a wave of potential and enters through both slits to interfere with itself.  The electron has therefore gone into superposition, which means that mathematically the electron takes every conceivable and inconceivable path, through both slits, through one slit and through no slits, rebounding against the sheet, around the experiment.

The double-slit experiment can be modified by adding particle detectors positioned at the slits, enabling the experimenter to find the position of a particle when it passes through each slit. This helps us work out if the electron went through only one of the slits, as would be expected of a particle, or through both, as a wave would be expected to do. When we measure what happens at one slit, the electrons go back to behaving like a particle to form two bands on the screen. The simple act of having the detectors by the slits means that the wave inference disappears! This means when you observe a quantum event, you have collapsed the wave function by observing the event.

In more basic terms, when you watch a quantum event, it becomes one or the other, and not both. This idea is the basis of Schrodinger’s cat theory, a celebrity in the physics world, and is what I will go onto explaining next time.

Check back next time for more of ‘Quantum Mechanics for Dummies’ and to read about this meowing pet cat of Schrodinger.

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