By Don Lincoln, Fermilab
British scientist Thomas Young proved that light was a wave whereas Albert Einstein proved that light was a particle. However, it was Einstein who bridged the wave and the particle world, by using the frequency of photons to determine their energy. What is the connection between the two? Why do physicists believe in the wave–particle duality of light?
Akira Tonomura and Thomas Young’s Interference Observation
The question that bothered early was, how can we explain the Thomas Young interference observation if light is made of particles? In order to understand how, let’s take an example. Suppose we turned down the brightness of the light source (the laser in the modern world) so that one photon is emitted at a time. If light were a classical particle, then the photon would have traveled to the double slits, pick a slit to go through, and then appear on the distant wall. If light were a classical wave, it would pass through both slits, interfere, and then appear as a very faint interference pattern of bright and dark spots on the distant wall.
It was in 1989 that Japanese scientist Akira Tonomura, and his co-workers at Hitachi, did this experiment. They actually used electrons and not photons. They shot one particle through the double slits by simply turning down the intensity very low. It appeared on the distant wall in a single location, thus, proving that light is a particle.
Nonetheless, an interesting result occurred when they repeated the experiment time and time again and built up a pattern of where the particles appeared. After thousands and thousands of individual particles, what they found was that the pattern looked just like what Thomas Young observed in 1801. The electrons arrived and interacted with the distant wall like particles, but their motion and the places they were found acted like waves.
Louis de Broglie
Hence, this is the reason that we say that photons and electrons and indeed all atomic and subatomic particles act like waves. In fact, everything acts like a wave, even a thrown baseball. However, for big things, such as baseballs, the wavelength is so incredibly tiny, that it’s not possible to see their wave behavior.
Tonomura and his colleagues used electrons and not photons because it’s easier to do, but does that tell us something about how light works? Well, sort of, at least indirectly. It was in 1924 that French physicist, Louis de Broglie, submitted his PhD thesis. In it, he made the most amazing conjecture. He hypothesized that if photons could be both waves and particles, why couldn’t electrons have the same property? He even proposed a way to determine the wavelength of an electron. The wavelength is proportional to one over the momentum of the electron. So, a very high momentum electron has a very short wavelength and vice versa.
De Broglie turned out to be right and, a year later, Austrian physicist, Erwin Schrӧdinger, expanded on the idea to invent modern quantum mechanics. Thus, this is how we know that photons and, by extension, electrons have both a wave and particle nature.
This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.
Clinton Davisson and Lester Germer
From 1923 through 1927, American physicists, Clinton Davisson and Lester Germer, were working at Western Electric. What Davisson and Germer were doing was shooting electrons at a block of nickel.
Now, this was still early in the history of modern physics. People were casting around in the dark and progress was made in fits and starts. What Davisson and Germer expected was to see the electrons reflected in a willy-nilly manner. This was because the surface of the sample wasn’t particularly smooth. To do this properly, they needed to invent a low intensity electron source, which only had a few electrons getting shot off at once. They also needed the electron beam to be in a vacuum.
The Davisson-Germer Experiment
A familiar form of an electron source is an old-style TV. Those old ones used heaters to boil electrons off a thing, called a cathode, and shoot the electrons toward the screen using high voltages and electric fields. Davisson and Germer’s source was basically the same as an old-style TV. They moved an electron detector around it, looking where the electrons bounced. What they found was that the electrons appeared in lots of places. But then they got lucky, or unlucky, but then lucky, so here’s what happened.
The vacuum on their equipment started leaking. Air started to get into their apparatus. With the oxygen from the air interacting with the nickel, nickel oxide was formed. It ruined their measurement as they were studying nickel and not nickel oxide. To get rid of the nickel oxide, they heated their sample, expecting the nickel oxide to boil off, leaving a pure nickel sample. Unaware that the way they heated and cooled the sample changed the arrangements in the block of nickel, instead of an amorphous block of nickel, it became a block of crystalline nickel.
Electrons Are Both Particles and Waves
Crystals have the property that they have repeating and regular structures. This is akin to having the slits of the Thomas Young experiment, but instead of two slits, it’s as if there were many, many slits. So, they repeated their experiment and found something odd. They found that the electrons bouncing off the nickel would bounce in some places and not at all in others. It looked much like the pattern seen by Thomas Young, although the pattern was two dimensional.
Thus, with the Davisson-Germer experiment, de Broglie’s hypothesis was confirmed: Electrons, like photons, are both particles and waves.
Since then, many other experiments have confirmed these early conclusions. Subatomic particles are both particles and waves. That’s pretty bizarre, but even more is the fact that particles, separated by great distances, can communicate with one another in ways that both do and don’t exceed the speed of light.
Common Questions about Subatomic Particles and the Wave–Particle Duality
After thousands and thousands of individual particles appeared, Akira Tonomura and his co-workers found that the pattern looked just like what Thomas Young observed in 1801.
Louis de Broglie hypothesized that, if photons could be both waves and particles, why couldn’t electrons have the same property?
With the Davisson-Germer experiment, de Broglie’s hypothesis was confirmed: Electrons, like photons, are both particles and waves.