The world of the extremely small (world of particles like the electron, the photon, the proton, the atom, etc.) is not accessible by our organs of perception, including the brain. No image, no interpretation can represent the real of the quantum world, even the words of our language are aproximatives to describe the quantum phenomena. All that one can say and show of this reality is wrong but I will still try to give you an idea of this fundamental concept of quantum physics that is wave particle duality (this term is now obsolete because it would be necessary to speak of fields). In quantum mechanics, it seems that a particle is both a corpuscle and a wave, it is not the only quirk but the others (quantum superposition, quantum entanglement or non-locality) derive from this one. What this assertion tells us is that any elementary particle can be seen as a concrete solid corpuscle but also as a wave that is an abstract concept, there is a paradox. The state of a particle describes all the aspects of this particle, ie all the knowledge (speed, kinetic momentum, position, energy, etc.) that we can obtain on the particle if we make experimental measurements on it. So let's look at what the famous experiment called double-slit Young's experiment tells us. The video opposite describes this experience in a modern way.

1 - When sending corpuscles (solids) on a wall with two slits, each corpuscle passes through either slit, bouncing in all directions and points of impact mark the screen a little anywhere, behind the slits. 2 - When we send a wave on the same wall, the wave passes through the two slits and the passage through the slits creates two small waves that will overlap, in some places they add up and others they cancel each other out, interference fringes appear on the screen. 3 - When you send a quantum object, it goes through the two slits, it interferes like a wave but when it touches the screen, it suddenly reduces to a point, rather where the two small waves add up. After a large number of tests, it appears both impacts as with the corpuscles and fringes of interference as with waves. 4 - But if we add an observer to know by which slot the particle passes, the wave is now reduced to a corpuscle at the level of the slots and passes only one slot at a time. We then measure on the screen points of impact and not interference. The observer modified the experience by his presence! If we want to determine the state of a quantum system, we must observe it but this observation has the effect of destroying the state in question.

This popularization video of Tout est quantique is a modern interpretation of wave-particle duality.

How to interpret this experience?

The scale of quantum physics is so small that you can not see a quantum object like you see a wave or a ball on a beach. For example, the size of a hydrogen atom is 53 μm (53 x 10^{-12} meter), one can align 10 million atoms to one millimeter. So to see for a physicist is not to see but to measure or detect something by relying on the tools he has built. Young's experiment shows us that when measuring a quantum object it changes its nature. Sometimes it is a corpuscle sometimes it is a wave and in addition, it depends on the measuring device or the observer. What the experience of Young's also tells us is that when the quantum object is free of any environment, it is a wave. But if the environment (screen, wall, observer or even molecules of air) forces it to interact, the object or rather its energy is suddenly reduced to a point and takes on the appearance of a corpuscle. It can be seen on the screen that the wavefront is not reduced anywhere, it is reduced where the wave is intense, ie on peaks or valleys. In other words, the probability of reduction is greater at the top and bottom of the wave than on the slopes. It is even nil where the waves are out of phase. The most surprising is that on a large number of measurements, if we send the particles one by one, in the end, despite the reduction of the wave packet, we obtain interference fringes.

An explanation was proposed in 1927 by Max Born (1882 - 1970). The particle is a wave of probability. This terrible definition shows the difficulty we encounter when we want to talk about quantum objects. In simpler terms, it is the amplitude of a wave at a given position that predicts the probability that the particle is at that position. A high amplitude does not mean that it is where the particle is, but it is there that we have the best chance of finding it (after the reduction of the wave packet). In summary: In quantum mechanics, we can not know if the particle is at a specific place in space, but what is the probability that it is there. It will have a position that if it has to interact with the environment, before it has no position, it is everywhere and her nature is undulatory. Exactly like a photon emitted by a star. While it has traveled freely for millions of years, as a wave, it will die by reaching your retina with which it will interact. The equations of quantum mechanics are surprisingly precise when we accept that it is about probability. All the matter that constitutes the universe (stars, planets, you, me) is made of atoms and subatomic particles governed by probability and not by certainty.

The double-slit experiment of Thomas Young (1773-1829), dates from 1821.
It is a physics experiment that involves interfering two beams of light from the same source.
This experiment made with photons has since been carried out with all the particles.
With electrons in the 1920s, with neutrons in the 1950s, with atoms in the 1980s and with molecules in the 1990s.
All microscopic particles show quantum aspects and are likely to interfere as in the Young experiment.

Quantum mechanics describes the fundamental physical phenomena at work at the atomic and subatomic scale. It was developed in the early twentieth century by a dozen physicists whose Planck, Einstein,
Heisenberg, Bohr, de Broglie, Schrödinger, Feynman to solve various problems such as black body radiation, the photoelectric effect, or existence of spectral lines. Quantum mechanics proved so fruitful
that it solved the mystery of the structure of the atom. It also describes the behavior of elementary particles and is the foundation of modern physics.

An interference is a superposition of two vibratory phenomena of neighboring frequencies.
Interference is the combination of two interacting waves (light waves, electromagnetic waves, sound waves, etc.).
Like the waves, the waves split and combine.
When a wave passes through two slits, it splits first in two and then the two resulting waves meet and the ridges and hollows combine.
Two ridges that meet form a larger ridge, two hollows that meet form a larger hollow and when a ridge and a hollow meet, they cancel each other out.
The ridges and troughs create a succession of bands, what is called an interference pattern.