Wednesday, 1 October 2014

Quantum Mechanics

The Dreams Stuff is Made Of

Someone recently said of quantum mechanics that we don't get it because the way we think about it is all wrong. That we should think of reality as a dream. I probably agree.

I mentioned, at our last meeting, that I would post an article I wrote some time ago. It is rather long so I'll split it over a few posts. Needless to say that I am no physicist - my 1968 O Level didn't even mention QM. But, like so many others, I find the subject fascinating because it is the one area of science that hints at a greater reality that is beyond the lab, beyond the arrogant certainty of materialists and it may hold the key to a whole new world.

This could well be my last contribution to the group, so I hope you enjoy it.

Part One: From the Strange to the Bizarre

The rock solid foundations of classical physics were shaken by Einstein and relativity and the door was now open for even more weirdness. The early 20th century saw the development of a new branch of theoretical physics which, at times, more closely resembled philosophy than traditional science. Here the work was done not so much in the empirical world of the lab but increasingly in the abstract, using mathematical models and in “thought experiments”. One of the originators of this new science was Niels Bohr, a Danish physicist now regarded as the father of Quantum Mechanics.

Neils Bohr

But let’s go back a little. In the late 1890′s, Max Planck had applied his imagination to explain another of these stubborn late 19th century scientific anomalies: so called “black body radiation”. Put simply, this is to do with why materials glow brighter the hotter they get. Planck didn’t much like what he found: that the energy emitted by these black bodies behaved, not as waves, but as discrete packets called “quanta”. In 1905, Albert Einstein published a paper on the quantum nature of light (the photoelectric effect): a paper which was to win him the Nobel Prize in 1921. Einstein proposed that light can be thought of as a constant stream of particles (think of night-time tracer bullets in a war movie). Each of these particles, or photons, contained an amount of energy proportional to the frequency of the radiation. Thus, photons of red light would contain less energy than those of blue light because blue has a higher frequency than red.

Now, all this led to a degree of discomfort among the physicists of the time including Planck and Einstein themselves. Almost a hundred years earlier, an Englishman named Thomas Young invented his famous two-slit experiment to demonstrate the wave-like properties of light (see the video below). On the other hand, Planck and Einstein had now shown a distinct particle-like behaviour. It appeared that both positions were correct though they should have been mutually exclusive.

Back to Niels Bohr.

Perhaps the most significant contribution of Bohr’s long and productive career was his principle of complementarity. The two-slit experiment mentioned above shows the wave nature of light because, if light is projected through two slits on to a screen behind, wave interference patterns can be seen on the screen. Think of two pebbles dropped into a pond: the ripples of one will interfere with the ripples of the other. This can only happen with waves. However, what if we had a projector that could send discrete particles of light (photons) through the slits? Individual particles, one at a time, cannot possibly interfere with each other because only one particle is going through either of the slits at any moment. Thus, common sense would insist, particles cannot produce interference patterns. The problem for the common sense view is that they do! How? To this day nobody really knows although there are several competing interpretations. Nevertheless, this is not just theoretical musing on the part of quantum physicists: the particle gun two-slit experiment has been performed.

If it has not become clear yet, we are now into an area of physics where the nature of reality itself is in question. How can something like light be two things at once, each valid, each dependent upon how we observe it. If we design an instrument to observe the wave properties of light, then light is a wave. If we design experiments to show the particle nature of light, then light is made up of particles. Common sense says it can’t be both. Niels Bohr says “Oh yes it can!”. Bohr tells us that we cannot think in classical “either-or” terms when considering quantum effects. In the two-slit experiment, the nature of light is indeterminate until we make a measurement: the act of measurement determines its “reality”. This is complementarity and it is the basis of the so-called “Copenhagen Interpretation” of Quantum Mechanics (Bohr was a professor at Copenhagen). [This link to Robert M. Pirsig's essay is well worth a close look.]

Werner Heisenberg

The debate over wave-particle duality rages on to this day. Another aspect of Quantum Mechanics that has produced even more controversy is the "Uncertainty Principle". This was the work of German physicist, Werner Heisenberg and it became the other main ingredient of the Copenhagen Interpretation. Like complementarity, Heisenberg’s uncertainty maintained the position that – at least at the sub-atomic level – reality is nebulous.

Particles such as electrons have properties such as position and momentum but the Uncertainty Principle states that if we attempt to measure one of these values, it is then impossible to know the precise value of the other. In the big world of planes, trains and automobiles, this would be like a driver saying: “my speedometer tells me that I’m doing 40 mph but, because I’ve determined that, I can’t say where I am”. Of course, the quantum effects are not really noticeable in the big world. So, again, uncertainty says that the more accurately you measure the position of a particle, the less sure you are of its momentum (and vice-versa).

The logical conclusion of all this is that, if we cannot say anything precise about the physical nature of a particle until we interact with it (observe or measure it), then it does not have a precise reality until that interaction takes place. Some interpret this by saying that I (the observer) am required to bring into physical reality those things which I observe. Others maintain that an observer is not required, only some form of interaction. But as far as I can tell, few really dispute the uncertainty principle.

Erwin Schrödinger

Erwin Schrödinger, an Austrian physicist and contemporary of Heisenberg, devised a now famous “thought experiment” to illustrate quantum uncertainty. This has become known, simply, as “Schrödinger’s Cat”. To paraphrase this oft-repeated story: a cat is shut in a box with a sealed bottle of poison gas and a triggering device. This device is actuated (or not) by a random quantum event (a radioactive particle decay) with a 50% probability of happening within a certain time. If the event does take place, the device triggers a hammer which breaks the glass and releases the poison. When the time is up, an observer opens the box and the cat is either alive or dead but the question is: in what state was the cat before the observation? Uncertainty would have it that it was both alive and dead!

Schrödinger’s important legacy to Quantum Mechanics is, however, his wave equation. Another physicist, Louis de Broglie, theorised that if electromagnetic energy can behave as particles then perhaps particles such as electrons also behave as waves. Schrödinger agreed and formalised the wave theory of matter in his equations. So now, instead of imagining electrons as little balls of matter in orbit around a much bigger ball called the nucleus, we have a “standing” wave surrounding the nucleus. In this picture, the electron is not a particle at a specific orbital position unless and until we measure it and “collapse the wave”. Later, Max Born – another German physicist and good friend of Albert Einstein – discovered a statistical property of the wave equation: if it was multiplied by itself (squared), it would predict the probability of finding the position of a particle. He concluded that the wave function was a mere mathematical abstraction and that the particles were – always – physically discrete, classical points of matter. Einstein agreed. He was one of the opponents of the nebulous view of the Copenhagen Interpretation, arguing famously that “God doesn’t play dice”.

No comments:

Post a Comment

If you are signed-in or have a User ID, then comment as that user.
Otherwise select Name/URL from the drop-down and just enter your name.