Figure 1

In the early 1800šs there was a debate over whether or not light was a wave or a particle. Isaac Newton believed that it was a particle. Then an English physicist, Thomas Young, conducted the double slit experiment, and Newton was proved wrong. The double slit experiment works like this. When light is shined toward a partition with a slit in it, a special screen behind the partition records the arrival of the light in white. When there are two holes open, the pattern formed on the screen is striped (see figure 1). This is because light travels as a wave, which can go through both holes at once and interfere with itself. When the waves add together, it is called constructive interference. If they cancel each other out (when the crest of one meets the trough of the other), it is called destructive interference. Because of this interference, the striped pattern is formed.

Figure 2	Figure 3

You may surmise that light particles simply travel in waves, analogous to water molecules that can travel in waves. As Einstein discovered, this is not the case. Consider the double-slit experiment again. When photons (light particles) are emitted one by one directed toward a single slit, the results look like figure 2. When there are two holes however, things start to get interesting. What is actually recorded looks very different from what we would expect. Because each particle would have to go through one hole or the other, the pattern formed should look the same as when there is only one hole open at a time, as in figure 3. But it looks drastically different. It looks like figure 1 because every photon interferes with itself - it is both a wave and a particle. This odd property is known as the wave-particle duality.

The question of wave particles was asked again in 1923 by a French nobleman, Prince Louis de Brolie. This time, however, it was about matter, not light. He used a logical connection reguarding E=mc². Energy (photon) particles are waves. Stemming from this, he hypothesized, electrons (matter) must be waves also. His idea was accepted by Einstein.

It was proven in the mid-1920šs when Clinton Davison and Lester Germer, who worked for Bell Telephone Company, studied the way a beam of light hits a chunk of nickel. This experiment worked the same way the double-slit experiment worked. They recorded where the electrons went on a phosphorescent screen. An interference pattern showed up, proving that de Brogliešs theory was right and electrons are waves too.

Similar experiments showed that all matter particles are waves. We don't encounter this waviness in daily life because de Brogliešs formula for the wavelength of matter is h-bar divided by the material bodyšs momentum. Because h-bar is very tiny, the wavelengths they make are very tiny and matter we see appears solid.