The beautiful experiment that showed that light was a wave is used in quantum physics, with particles on the scale of atoms.
Manuel D. Barriga-Carrasco, Universidad Castilla-La Mancha
In classical physics there are two well-differentiated worlds: waves (mechanical or electromagnetic) and particles (corpuscles), both very well defined.
Previously it was thought that there was no relationship between these two worlds, but in the late 19th century, as the small world (molecules, atoms and their components) became known, it was discovered that the smallest particles could behave like waves. If particles behaved like waves, we had to know which wave was associated with those particles: the “particle wave”.
At the same time, at that time, the opposite was revealed: a wavelike behavior similar to that of particles. Two examples are the photoelectric effect and the Compton effect.
Luis De Broglie based himself on the already existing definition of photons: they were the particles that make up light (in classical physics, a wave) that behaved like particles. Thus, it was known that the mass of photons was zero, that their speed was that of light and that they had an impulse associated with the wavelength of that light (wavelength is a characteristic of waves that tells us how far the wave is. repeats itself).
De Broglie thought that if light could behave like a particle and have a momentum associated with its wavelength, electrons could behave like waves and have a wavelength associated with their momentum.
He defined the De Broglie wavelength, “the particle wave,” as Planck’s constant (a very small number characteristic of the atomic world) divided by the particle’s momentum.
This idea was not based on any calculation or evidence. It was a hypothesis that needed to be proven.
The double slit experiment for light
The double-slit experiment is an experiment carried out in the early 19th century by the English physicist Thomas Young, with the aim of supporting the theory that light was a wave and rejecting the theory that light was composed of particles.
Young shone a beam of light through two slits and saw an interference pattern appear on a screen, a series of alternating bright and dark streaks.
Wikimedia Commons, CC BY
This result would be inexplicable if the light were composed of particles, as only two light bands should be observed in front of the slits, but it is easily interpretable assuming that the light is a wave and that it suffers interference.
Later, this experiment was considered in quantum physics to demonstrate the wave behavior of very small particles, on the scale of atoms. The experiment can be carried out with electrons, atoms or neutrons, producing interference patterns similar to those obtained when carried out with light. This, therefore, shows this wave behavior of the particles.
The double slit experiment for electrons
Let’s see what happens in the double-slit experiment if, instead of a beam of light, we have a beam of electrons.
These electrons can be launched at any speed we like, accelerating them through a difference in electrical potential. Since we can choose the speed of these electrons and the de Broglie length depends on the speed, we are actually choosing the wavelength of these electrons.
However, building a double slit in the case of electrons is far from easy. It was only many years after the idea was proposed that this experiment could be carried out.
In 1961, Claus Jönsson accelerated an electron beam by 50,000 volts and passed that beam through a double slit with very small spacing and width.
The electron beam was first passed through a single slit and counted from a distance with detectors. The detectors in front of the slit counted many more electrons.
Then, another slit was made, with which it was seen that some maxima and minima of the electron count appeared according to the position of the detectors.
That is, there were detectors at the height of the first slit that received fewer electrons when there were two slits than when there was one.
The first thing they thought was that it was because of the charge the electrons had. With a negative charge, these electrons could repel each other as they traveled together in the beam. To verify this, they threw electrons one by one with the two slits open and the same result was obtained, for which they concluded that these maxima and minima indicated that the electrons had been interfered with and therefore possessed wave properties.
The double-slit interference pattern photographed by Jönsson was similar to the double-slit patterns obtained with light sources, reinforcing the evidence in favor of the wave nature of particles.
At the same time, other experiments were done with particles that reached the same conclusion: they had wave properties. This was not explainable from the point of view of classical physics, so it would be part of a large branch of modern physics, quantum physics.
The unmeasurable experiment
Let’s estimate the De Broglie wave associated with the electron. If the electron is moving at a speed close to that of light, for example 0.6 times the speed of light, its associated wavelength is approximately 3 picometers, a very small but measurable wavelength within the spectrum of X-rays or gamma.
Now, let’s calculate the de Broglie wavelength of a car that weighs 1000 kg and is moving at a speed of 100 meters per second. The wavelength associated with this car is 6.6 x 10⁻³⁹ m, which is so small that it is impossible to measure.
Therefore, there is no experiment that can show the wave nature of macroscopic objects. Only when you penetrate inside the atom to experiment with atomic and nuclear particles is it possible to observe the de Broglie wavelength, the wavelength of particles.
Manuel D. Barriga-Carrasco, Professor of Fluid Mechanics at the Superior Technical School of Industrial Engineers, Universidad Castilla-La Mancha
This article was originally published in The conversation. read the original.