Double slit experiment (full version)

In quantum mechanics, particles are commonly described as waves, but not conventional (classical) waves. In the year of 1801, the British physicist Thomas Young performed the double-slit experiment, considered one of the most important in science. Such experiment was based on a light emitter, a plate with two slits and a "wall", where it was possible to visualize the pattern formed by the light. As light passes through both slits, it undergoes a phenomenon called diffraction, and in each slit the light begins to spread as if the slits were now new points of light emission. With the propagation of the waves, parts of these meet, amplifying and / or canceling their amplitudes, this is called wave interference. Interference occurs both in mechanical waves (which need a medium to propagate such as sound waves) and in electromagnetic waves (which can propagate in vacuum). With the interference, it is possible to observe the formation of a very characteristic pattern in the wall, with places where there is luminous intensity and places where there is absence or low light (shade or penumbra). Because of the wave-like behavior, Thomas Young thought he had taken over a debate that came from long ago.

Around the 18 century, there was debate about what nature light had, corpuscular or ondulatory behavior. Isaac Newton defended that light was made of particles and was a pioneer of what became the "corpuscular theory of light". However, with Young's experiment, the situation became perverse. In 1905 Albert Einstein published his articles about the photoelectric effect. In those, the german physicist supported the ideia that light was composed by particles-named photons- thus Newton's vision was brought back again.

More than a century later, with the insertion of the new field of science, quantum mechanics, Thomas Young's experiment was repeated, however, with a fundamental change involved. The experiment, in its recent version, used a particle emitter (electrons) rather than light (as in the original version). Think of an electron as a basketball with fresh paint, which needs to pass through one of the slits, so it can reach and mark the wall. When throwing several balls, it is reasonable to expect a portion to pass through the right and / or left slit, thus reaching the wall. With this situation, we can imagine that a pattern of two stripes (marked by the painted balls) forms on the wall, in the direction of the slits. This is the "classical" view of what you would expect to happen with electrons being fired, but the situation turned out to be totally different. On the wall, the same pattern of marks of the experiment with light was formed, which indicates that the particles were behaving as waves, passing through the interference process. How could that happen? Massive particles behaving like waves? It is factual, within our understanding of (classical) wave mechanics, that waves do not carry matter (mass), but rather energy. How could such contradictory behavior occur? This has disrupted scientists around the world. It was like the electron traversed the two slits, interfered with itself and struck the wall, forming the wave pattern mentioned. This is called the wave-particle duality.

To try to get some explanation, the physicists introduced into the double-slit experiment (the "modern" version) an apparatus capable of detecting which slit the electron actually passed through. Science surprised everyone again. With the presence of the apparatus, the electrons began to behave like "normal" particles (in the same way as the example of basketball balls), marking on the wall two bands, approximately aligned with the slits. When the apparatus was removed, the wave behavior was back again. By "forcing" the system to give us a value (formally called eigenvalue) such as the position or momenta, the electron provides it in a random way (such randomness will be explained in detail in future articles, aiming at explaining the so-called "wave function "), momentarily losing its undulatory behavior. In other words, and roughly speaking, when we treat a "quantum particle" as a particle, it will behave as such.

In order to verify if the result was the same for other types of particles, the experiment was repeted, using even entire atoms, and the result was the same. The Heisenberg uncertainty principle (which will gain a big highlight) is easily visualized in the Thomas Young experiment.

The double-slit experiment is basically the "father" of quantum mechanics and is the origin of great mysteries unresolved up to date, such as the interpretation of the wave function. Several scientists ventured into trying to find an explanation for the probabilistic behavior. The interpretations that stand out are those of Hugh Everett (with the "interpretation of many worlds" involving a kind of multiverse, that of Max Bohm (with his "pilot wave theory" and the most adopted and accepted, that of Heisenberg and Bohr (Copenhagen interpretation). However, there is no way to state which interpretation, that is, if any, is correct.

There are still other experiments related to quantum mechanics that are even more "bizarre" than "quantum rubber".

The first photo illustrates the experiment of the double slit realized with electrons, while the second photo demonstrates the phenomenon of interference / superposition of waves

Reference Material: "The Quantum Universe" (Brian Cox and Jeff Forshaw)