Skiing through the mountainside - the strange world of quantum tunnelling

3 Dec 2013

Adam Reid of the University of Cambridge gave the Thames and Kennet Regional Group a fascinating talk on 28 November 2013, entitled, Skiing through the mountainside: the strange world of quantum tunnelling.

Mr Reid began his lecture by giving a brief comparison between classical physics and quantum physics. He described classical physics as having a 'well-defined quality', works on the macroscopic world which makes the results of it intuitive. At the start of the 20th century, another type of physics came into use: quantum physics. Unlike classical physics, quantum physics has a 'fuzzy' quality (you are not able to know everything about a particle at a particular time), applies to microscopic objects and, for a majority of cases, is counterintuitive.

Mr Reid then used an example of proton transfer in water to show that classical physics and quantum physics give different answers to explain the same event. Hydrogen ions are able to travel between water molecules by forming hydroxonium ions with them. A graph can be plotted of distance of hydrogen ion when between two water molecules vs. energy. This is a 'W' shaped graph.

If the PE of the hydrogen ion is greater than the electrostatic force of the water holding it in place, the hydrogen ion is able to move between the water molecules as it is able to traverse the central region. If it is below the electrostatic force, classical physics dictates the hydrogen ion is unable to move between the water molecules as it does not have the required energy. On the other hand, quantum mechanics says the particle is able to tunnel to the neighbouring water molecule, even if the hydrogen ion does not have the required energy.

In classical physics, a substance behaves as a wave or a particle. An experiment can be set up to show this is not applicable to all objects. The experiment carried out was firing electrons at a detector with a barrier (with two apertures that can be opened and closed) between the two. When fired through a with just one aperture open, the electron does not experience any interference.

Opening the second aperture, the electron appears to behave like a particle as it forms a random, uniform distribution on the detector. After a while, interference patterns begin to show which implies it also acts as a wave. From these experiments, the wave function can be obtained. Squaring the wave function gives the probability of finding the object at specific locations.

Returning to the example of proton transfer in water and now looking at its wave function, the wave function shows there is a greater chance of finding a hydrogen ion with a water molecule rather than between the molecules. If the wave function of the particle whose total energy is lower electrostatic force experienced by water is squared, there is a probability of finding this particle in the region where it is not bonded to any waters. This implies the hydrogen ion has experienced quantum tunnelling.

Mr Reid concluded the lecture by explaining why the phenomenon of quantum tunnelling cannot happen to large objects. This is because all of the particles that comprise the object each have their own wave function which will interfere and cancel each other out. This is called quantum decoherence. Unfortunately this conclusion left a few of the audience disappointed in their ambition to walk through walls.

James Nugent

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