Description of the image: In the world of the infinitely small, electron orbitals can take on different characteristic shapes depending on the nature of the atom. Hydrogen orbitals have a spherical shape, oxygen orbitals are shaped like two water droplets, and iron orbitals are shaped like four water droplets. This shape of the atomic orbital defines the "size of the atom" (≈ 10-10) although the size of an atom does not have a precise measure because the shape of this region of atomic space depends on the energy of the electron and its angular momentum. Image credit: vulgarisation.fr
"I think I can safely say that nobody understands quantum mechanics." Richard Feynman (1918-1988), quantum physics theorist.
The implications of quantum mechanics are so complex and unusual that much of the scientific community has decided to avoid them. However, physicists agree on how to perform calculations to account for quantum phenomena, but there is no consensus on a single way to explain them. This leaves room for all kinds of popularizations, which should be approached with caution. Many articles or videos tell us that everything is quantum.
The term quantum is often used indiscriminately in many areas of daily life (nuclear physics, chemistry, solid-state physics, optics, cosmology, electronics, medicine, biology, etc.). This quantum strangeness of matter and light has spread throughout our classical world.
At the particle scale, the atom is quantum, the photon is quantum, and thus by extrapolation, the entire universe (matter and energy) is quantum. Thus, it is easy to generalize the term quantum to everything that exists. But although quantum physics has repercussions on the macroscopic scale, it primarily concerns the world of the infinitely small, that of particles, atoms, molecules of a few dozen atoms. It is only at this atomic and subatomic scale that the quantum concepts of matter appear. Among these concepts, which will not be explained here, are wave-particle duality, state superposition, quantum entanglement, and non-locality. It is thanks to these concepts that quantum physics describes the structure of matter with great precision, along with its physical properties (mass, radius, nature of the chemical bond, stability, energy level, etc.).
- The atom, when isolated, is a wave of the order of a nanometer.
- An isolated iron atom in a vacuum occupies an infinite number of different positions simultaneously.
- Two photons, when produced together, remain entangled regardless of the distance that separates them later.
These states of matter are counterintuitive because we do not observe them in our world, which consists of billions upon billions of particles. At the microscopic scale, an isolated quantum object behaves more like a wave occupying all space, making it impossible to localize precisely. This means that when an interaction acts on it, it encounters a diffuse, rather blurry object, not a particle that has a certain volume located at a specific place. The theory of decoherence, which is widely accepted, tells us that once the object is too large or interacts with too much matter from the environment (air, liquid, solid, light, etc.), it ceases to be quantum.
By interacting with the environment, the quantum object will transition to another scale. During its wanderings, it will encounter other objects from the environment (matter and light) and interact with them.
The complexity of these interactions is such that it will have to take a position because all its quantum states quickly become incoherent, hence the name of the decoherence theory. Mathematically, these interactions destroy the quantum phase of the object, i.e., the manifestation of the wave. This phase shift eventually becomes null, and the object will appear in our macroscopic world in one of the physical states of the system, the most probable one. In other words, any collision with atoms of the environment reduces the quantum object. This is called "wave function collapse."
All experiments in quantum physics are conducted under extreme conditions, in ultra-high vacuum or at very low temperatures (close to absolute zero) or both. Sometimes even, at very high pressures, hundreds of times that of our environment.
In all cases, our particle must never encounter other particles until it is measured. Even superconductivity (absence of electrical resistance) or superfluidity (absence of any viscosity) occurring on macroscopic objects cannot manifest at room temperature. They are observed when the temperature approaches absolute zero. For example, when liquid helium is brought to less than two degrees above absolute zero, the corpuscles revert to waves and join together in a single giant wave, corresponding to the Bose-Einstein condensate.
As long as the extreme conditions of vacuum and temperature persist, the wave will resist decoherence and persist. That is why liquid helium at two degrees above absolute zero passes through the nano-holes of the glass wall (the wave no longer has any viscosity). Once outside the glass, the wave interacts with matter (air) and disappears to manifest as corpuscles, with helium droplets condensing under the glass.
Without these extreme conditions, in our daily life, quantum effects do not exist, our environment is too rich, too chaotic, too agitated, too disordered. However, quantum effects are not present or absent all at once. We do not go from a rich environment where there are no quantum effects to an environment very poor in information where quantum effects appear. The wave function φ(r,t) or the probability density of presence does not nullify instantly but attenuates slowly before disappearing in the classical world. All quantum objects are characterized by this wave function (psi). It describes the probability of a particle being located at a point in space. It is only upon measurement that the particle will reduce (interact with its environment) to a probable but unpredictable location precisely.
The quantum object always has a decoherence time to appear in the classical state, it is small but not zero; this allows us to measure it.
The quantum object is very fragile; its fragility is due to the quality of the ultra-high vacuum or ultra-low temperature. The concepts of quantum physics under these extreme conditions are well understood. For a century, no experiment has invalidated its equations.
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