Quantum Facts

We hear a lot these days in the popular press about quantum mechanics.  Some articles are quite thought-provoking while others, frankly, make me roll my eyes.  There are some pretty gnarly concepts at the root of quantum mechanics, but some can be grasped by the layman—which is definitely what I am—if the explanation is clear enough.

The thought problem proposed by Erwin Schrödinger in 1935, and referred to as "Schrödinger's Cat", is beyond doubt the most widely known illustration of the principles of quantum mechanics.  It involves a box containing a living cat, a flask of cyanide gas, a radioactive atom, and a Geiger counter.  If the atom decays and emits a particle, it will trigger the Geiger counter.  A mechanism attached to the Geiger counter shatters the flask, and the cat dies.  Yes, as a cat lover, this disturbs me, but it is, after all, a thought problem.

Since the box is sealed, it can't be known whether the atom has decayed—and thus whether the cat is alive or dead—until the box is opened (presumably after the cyanide has dissipated, or you would be dead).  Where the weirdness of quantum mechanics comes in is that until it is observed, the cat must be assumed to be both alive and dead.  How can this patently absurd notion be true? The key idea here is a concept called superposition. 

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We used to think that an atom looked like a tiny sun with the electrons whirling merrily around it.  This was before we had microscopes powerful enough to see atoms, something that didn't happen until 1955 when Dr. Erwin Mueller got the first glimpse through his newest “field electron microscope” at Pennsylvania State.  We now see electrons not as having orbits as such, but "shells" that are virtual spheres around the nucleus where the electron might possibly be at any given instant.  

These shells occur at various distances from the nucleus, determined by the amount of energy they possess, and the distances are called quantum levels.  This is the fact that the whole Quantum Theory rests on: an electron can only be on one or the other of its quantum levels, and no place in between.  When we speak of a "quantum leap", we are using a metaphor for what actually happens at the atomic level.  As an electron gains or loses energy, it leaps from a lower to higher to shell—or quantum level—and vice versa. 

This leap is what happens when a radioactive atom decays, as the one in Schrödinger box might or might not do.  The “decay” we speak of is the electron losing energy and dropping to the next lower level.  As you may know, there is a principle called "conservation of energy".  The energy that the electron loses doesn't just disappear.  It can't.  Instead, the lost energy become a photon, which flies away from the atom at the speed of light.

So now we come to this superposition idea, almost.  In 1801, well before the Quantum Theory was conceived, an experimenter named Thomas Young set up an apparatus to demonstrate the behavior of light.  It was believed that light consisted of either a waves or particles, and he wanted to find out which. Young's device, called a double-slit interferometer, sent a ray of sunlight through a small hole in a piece of paper, then split that ray in two by placing another piece of paper on edge directly in the middle of it.  Although subsequent experiments used an actual pair of slits, this served to do the same thing: it showed that the two beams created an interference pattern on the screen where they landed.  The same thing will happen when waves of water are split; the peaks and troughs alternately cancel and reinforce one another.  You can see what the pattern of an interferometer looks like at the top of this article.

Thus, Young had proved that light was a wave and not a particle...only he hadn't.  During the early discussions of Quantum Theory, Albert Einstein wrote:

"It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do."

How could this be?  By this time, a more sophisticated version of Young's double-slit interferometer (one with actual slits) was being used to test the properties of light.  In was now possible to aim a very narrow beam of electrons toward two small slits, and detect the pattern they made.  If electrons are particles, the pattern should be two solid bars in the shape of the slits.  If they are waves, Young’s interference pattern should emerge.  But something strange happens: if either path of the electrons is monitored, each particle appears to pass through one slit or the other, and no interference is seen.  If, on the other hand, neither is checked, the electron will appear to have passed through both slits simultaneously before interfering with itself, thus acting like a wave.  How is this possible?  The only difference is one scenario involves observation—or what physicists call a measurement—and the other does not.  This is what is referred to as the observer effect.  Yes, this sounds crazy.  The behavior of innocent, unintelligent electrons changes depending on whether they’re looked at or not.  This phenomenon is seen over and over again with increasingly sophisticated experiments.

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So, finally to superposition.  It is said that the electrons (or photons, or other particles) when unobserved, are in superposition, meaning that they can be in any possible state. In the case of the double slit experiment, this means they can go through either slit, and you don’t know which.  In this condition, they (or a significant number of them) seem to go through both slits at once.  When observed, they meekly go through one or the other.  Observation—measurement—collapses the wave function, the mathematical description of the probabilities.  There is then, no more probability, just a fact.  One slit; other slit.

As you might imagine, the implications of this phenomenon gave physicists fits, and continues to do so to this day.  This, in part, is why Einstein, who took a while to embrace some of the tenets of quantum physics, famously said, “God does not play dice”.

Most thinkers in this field are willing to concede the reality of the observer effect at the subatomic level, but draw the line at saying it has validity in the “macro” world—that is, ordinary reality.  Others are not so sure, and this is why we have respected scientists talking seriously about parallel universes that blossom from every collapsing wave function, and even the notion that we are two dimensional beings dreaming our waking lives in three dimensions.  The movie, “The Matrix” is often mentioned.

Given the above, my title, “Quantum Facts” might seem a bit tongue-in-cheek.  I had in mind to go into my suspicion that facts in today’s world seems rather arbitrary when viewed from different directions, and perhaps do a take on the observation that eye witness accounts of events can vary wildly from person to person.  But I’ve bored you long enough, and we can save that for another day.

For more intriguing information:

Through the Wormhole: The Double Slit Experiment (a video with Morgan Freeman)

Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality by Anil Ananthaswamy