The Double Slit Experiment


By Steve Humphrey


The Double Slit experiment was first performed by Thomas Young, a proponent of the wave theory of light, in 1801. He lets sunlight strike a screen with a small hole in it, creating a beam of light which travels a short distance and hits another screen containing a detector. Imagine that screen covered with a photo emulsion, which is sensitive to light.

If light is corpuscular or consists of a stream of particle-like entities, called “photons,” then the detector should register the arrival of each one as a dot on the photo paper. If light is a wave, then it should be detected as a smear on the paper. Think of a water wave crashing against a sea wall or jetty. The wave hits all along the wall.

If light consists of corpuscles.

Right off the bat, we see that the paper registers a series of small dots, or points. So, it’s corpuscular, right?

Not so fast. Next Young inserted a third screen between the other two, this one containing two narrow vertical slits a short distance apart. If light consisted of a stream of photons, we would expect to see two concentrations of dots, one behind each slit. Instead, what we see is bands of dots, bright sections separated by darker sections. This is an interference phenomenon. Think of two water waves coming together. Where the crests of the two waves coincide, they are amplified and the resultant crest is taller (this is called constructive interference), and where the troughs coincide, they get lower (destructive interference). And where a crest meets a trough, the water becomes flat. In the case of light, constructive interference leads to more dots and destructive interference leads to fewer. So, even though we still have point-like dots on the detector, the distribution of the dots suggests an interference pattern. So, light is a wave phenomenon, right?

Not so fast. If we cover one of the slits, the particle distribution returns. And if we set it up so we can tell which slit the light goes through without otherwise disturbing it, we lose interference. Further, we can reduce the intensity of the light to the point where only one light ray is emitted at a time. Surely, in this case, the light must go through exactly one of the slits. But surprise, surprise, we still get interference. It is as if the light ray goes through both slits and interferes with itself on the other side of the screen. But how can a photon go through both slits?

Now, to make things even stranger, the same experiment can be, and has been, done with particles of matter, such as electrons. In fact, it has been done with even larger atoms and molecules. (This is a difficult experiment to perform but has been rated one of the five most important experiments in physics.) What do you suppose we get? An interference pattern. Even when the particles are fired one at a time. But how can particles interfere with themselves unless they go through both slits? And again, if we try to determine which slit a particle goes through, we lose interference. What’s going on?

In 1924, Louis deBroglie, a member of the French royal family, hypothesized that just as light sometimes exhibits particle behavior, matter should exhibit wave-like behavior. The double-slit experiment confirms this. But how can Nature be like that? How can something be both a particle and a wave?? Is it a “wavicle”?

If light consists of waves.

Two conclusions were ultimately drawn from the results of these experiments. First, we need a new term to describe the behavior of the photons and particles. It is said that they are in a “superposition” of states. Superposition is clearly represented in math, but has no intuitive, commonly understood meaning. If a particle is in a superposition of position states as it goes through the slits, then it doesn’t go through the left slit, it doesn’t go through the right slit, it doesn’t go through both slits and it doesn’t go through neither slit. A system in a superposition is represented by a wave function. Systems in superposition are never observed. When we observe something, the wave function “collapses” and the system takes on a determinate state. This has led to a lot of weird speculation, which I will talk about in future columns. (This is where Schrodinger’s Cat comes in.)

Second, if an electron is in a superposition of position states as it moves through the experiment, then it doesn’t follow a determinate trajectory. That is, you can’t draw a single line to represent the path of the electron. In some sense, the electron follows all paths. Imagine what this means for Rutherford’s Planetary Model of the atom, which I discussed last month.

One reason Quantum Mechanics is so difficult to grasp, even though the math is fairly straightforward, is that it challenges our ordinary conceptions of how the world is and works. Niels Bohr’s view on this, as I interpret him, and there are many who would disagree with me, is that we don’t have the conceptual apparatus necessary to visualize or understand what goes on in the microphysical realm. Quantum requires us to develop brand new concepts, like superposition and entanglement, but because they are new, we have developed no intuitions about them.

Stick around. I will be exploring further the wonderful world of Quantum Mechanics in my next column.

Steve Humphrey has a Ph.D. in the history and philosophy of science, with a specialty in the philosophy of physics. Questions? Comments? Suggestions?
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