Copenhagen, part 2

To understand the play Copenhagen, it is necessary to have a basic understanding of the contributions that Niels Bohr and Werner Heisenberg made to quantum mechanics. But first, what is quantum mechanics?

It is the study of very small things, at atomic and subatomic scales. How do atoms behave? How do subatomic particles behave? The answer is that they behave strangely, unlike anything else we are familiar with.

The strangeness can be described by the "double slit experiment." Imagine a machine gun that sprays bullets at a thick metal plate with two parallel slits in it. Behind the metal plate is a brick wall. What will be the pattern of bullets striking the brick wall? There will be two concentrations: one behind slit #1 and one behind slit #2. Call this pattern A.

Now imagine a light source that shines onto a metal plate with two parallel slits. Behind the plate is a screen. What will we see on the screen? We will see an interference pattern of alternating light and dark bands. Call this pattern B.

So far so good. Nothing unusual yet. Light behaves like waves (pattern B) and bullets behave like particles (pattern A).

But we also know that light consists of subatomic particles called photons, like little bullets. Suppose we slow our experiment down until the light source emits just one photon at a time. Over time, we get the same interference pattern mentioned above: pattern B, the wave pattern. How does each photon "know" where to go to produce the interference pattern? Why don't the photons go straight through one slit or the other to produce pattern A? No one knows. It is as if each photon goes through both slits simultaneously, interfering with itself, to produce pattern B. This is strange behaviour for a "particle."

It gets even more strange. We know that each photon had to go through either slit #1 or slit #2. Suppose we want to know: which slit did each photon go through? As soon as we introduce any mechanism to answer that question, we destroy the interference pattern. Instead of pattern B we get pattern A. Why? No one knows. It is as if each photon goes through both slits simultaneously until it is observed, and then it goes through only one of the two slits.

The experiment can be done with electrons instead of photons. The result is the same.

The essence is that subatomic particles can behave either as waves or particles. And what the observer is doing (i.e., watching the slits or not) affects which aspect is observed. This is bizarre.

That was a dry explanation, with no pictures. For a better explanation, with pictures and cool simulations that you can play with, click here (total of 3 screens).