Quantum Mechanics:
From the Strange to the Bizarre
Now that the rock solid foundations of classical physics had been shaken, the
door had opened 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.
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 (the
link above is well worth a
visit). 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, again, well worth a
close look.]
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 [1]". 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, 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".
