Is matter a particle or a wave?

In this experiment, monochromatic light from two sources underwent constructive and destructive interference, causing alternating bright and dark “bands", or fringes, to form. This was explained with Huygens’ Principle together with diffraction, where cylindrical wavefronts spread out from the two slits. Constructive interference, or “reinforcement" of the two waves, and destructive interference, or “cancellation" of the two waves, occurred, hence the bright and dark fringes formed.

During Young's time, many thought that light was composed of tiny particles known as corpuscles. It was through this experiment Young showed that light was not made out of particles, but that it was actually a wave! This proved the wave nature of light.

Born in 1892 to a noble family in France, Louis de Broglie hypothesised the wave-particle duality of matter. Although he planned for a career in the humanities, graduating with a degree in history, he later obtained a degree in science, followed by a PhD in Physics.

During the time of de Broglie, the wave-particle duality of light, which is the theory that light behaves both as a wave and as a particle, was quite established. As such, for his PhD thesis, de Broglie proposed that just like light having the properties of both a wave and a particle, electrons too, should have the properties of both a particle and a wave.

Based on the already established relation E = hf, where E is the energy of a photon (a discrete packet of light), h is the Planck constant (6.63 × 1034 J s) and f is the frequency of the light, de Broglie further hypothesised the relation p = h / λ, where p is the momentum of an electron and λ is a quantity known as the de Broglie wavelength. This equation actually applies to all matter, and certain particles like photons.

Despite de Broglie's equation being purely a hypothesis then, this hypothesis prompted many other physicists to investigate the possible wave-particle duality of matter. Such an example would be physicists Clinton Davisson and Lester Germer, who performed an experiment that confirmed de Broglie’s hypothesis.

The Davisson-Germer Experiment

At that time, diffraction was a property thought to be shown by only waves. As such, since the electrons formed the interference pattern, they did undergo diffraction. This meant that since matter (in this case, electrons) demonstrated the diffraction property of waves, matter was indeed wave-like. This also confirmed the de Broglie’s hypothesis for electrons, a pivotal step in the discovery of matter waves.

So ... are electrons particles or waves?

A simple answer would be both.

In circumstances such as firing electrons at each other to observe their repulsion, they behave as particles.

However, in circumstances such as firing them through a double slit, they behave like a wave.

Indeed, quantum physics is rather mind-boggling, but weirdly satisfying.