"I think it's safe to say that no one really understands quantum physics." Richard Feynman (1918-1988), quantum physical theorist.
Indeed quantum physics is mysterious but a century after its formulation, thanks to it, scientists fully understand the phenomena that surround us. However, the implications of quantum physics are so complex and unusual that much of the scientific community has decided to elude them. 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 the field open to all the popularizations of which we must be wary. Many articles or videos explain to 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 weirdness of matter and light has spread all over our classical world.
Effectively at the particle scale, the atom is quantum, the photon is quantum and therefore by extrapolation the whole 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 a macroscopic scale, it mainly concerns the world of the infinitely small, that of particles, atoms, molecules of a few tens of atoms.
It is only at this atomic and subatomic scale that quantum concepts of matter appear. Among these concepts which will not be explained here, there is particle wave duality, state superposition, quantum entanglement or non-locality. It is thanks to these concepts that quantum physics describes with great precision the structure of matter with its physical properties (mass, radius, nature of the chemical bond, stability, energy level, etc.).
It is likely that the future will be quantum and that more and more quantum phenomena will appear in our classical world. But today, I will simply clarify under what conditions these purely quantum properties emerge from matter and light.
Hence the underlying question: since matter on our scale is made up of quantum objects, why does it not obey the principles of superposition, duality, entanglement?
- The atom when it is isolated, it is a wave of the order of a nanometer.
- An isolated iron atom in a vacuum without light occupies an infinity of different positions at the same time.
- Two photons when produced together remain entangled regardless of the distance between them.
Why do these quantum states disappear on a macroscopic scale?
These states of matter are counter-intuitive because we do not observe them in our world of billions upon billions upon billions of particles.
At the microscopic scale, an isolated quantum object behaves more like a wave occupying all space and it is impossible to locate it precisely. This means that when an interaction acts on it, it encounters a diffuse object, rather fuzzy and not a corpuscle which would have a certain volume located in a very precise place.
It is the quantum decoherence that is widely accepted. It tells us that when the object is too big or interacts with too much matter in the environment (air, liquid, solid, light, etc.) it ceases to be quantum.
By interacting with the environment the quantum object will switch to another scale. During his wanderings he will encounter other objects in 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 theory of decoherence.
Mathematically these interactions destroy the quantum phase of the object, that is to say the manifestation of the wave. This phase shift eventually becomes zero 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 the atoms of the environment reduces the quantum object. This is called "wave packet reduction".
Since a quantum object cannot be extracted from the terrestrial environment, how do we know that its quantum states exist?
All experiments in quantum physics are carried out under extreme conditions, under ultravacuum or at very low temperatures (near absolute zero) or both. Sometimes even at very high pressures, hundreds of times that of our environment.
In any case, our particle must never encounter other particles until it is measured.
Even superconductivity (absence of electrical resistance) or superfluidity (absence of any viscosity) acting on macroscopic objects cannot occur at room temperature. They are observed when the temperature approaches absolute zero. For example, when liquid helium is brought to less than two degrees from absolute zero, the corpuscles become waves again and come together in a single giant wave, corresponding to the Bose-Einstein condensate.
As long as the extreme vacuum and temperature conditions persist, the wave will resist decoherence and persist. This is why liquid helium at two degrees from absolute zero passes through the nano holes in the wall of the glass (the wave no longer has any viscosity). Once out of the glass, the wave will interact with the material (air) and will disappear to appear as corpuscles, drops of helium will condense under the glass.
Without its 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 suddenly present or absent. We do not go from an environment that is too rich where there are no quantum effects to an environment that is very poor in information where quantum effects appear. The wave function φ(r,t) or presence probability density does not vanish instantaneously but slowly attenuates before disappearing in the classical world. All quantum objects are characterized by this wave function (psi). It describes the probability that a particle has to be located at a place in space. It is only during measurement that the particle will collapse (interact with its environment) to a probable but unpredictable place precisely.
The quantum object always has a decoherence time to appear in the classical state, it is small but not zero; this is what allows us to measure it.
In summary, the quantum object is very fragile; its fragility is due to the quality of ultrahigh vacuum or ultracold. The concepts of quantum physics in these extreme conditions are well understood and for a century, no experiment has failed its equations.
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Image: Atomic orders of magnitude.
In the world of the infinitely small, the orbitals of the electron can take different characteristic forms depending on the nature of the atom. For example the orbitals of hydrogen have a spherical shape, the orbitals of oxygen have the shape of two drops of water, the orbitals of iron have the shape of four drops of water. This shape of the atomic orbital defines the size of the atom.
The characteristic sizes of atoms or the distances between nuclei in molecules are of the order of angstroms (one tenmillionth of a millimetre) in agreement with experiment. We can say that the atoms are separated from each other by a few angstroms.
However, the electronic cloud of an atom does not have a well-defined dimension because it is a superposition of atomic orbitals of a probabilistic nature. There is therefore no very precise measurement of the size of atoms because the shape of this region of atomic space depends on the energy of the electron and its angular momentum. The characteristic energies of atoms are of the order of an electron-volt.
The sizes and energies of atoms or molecules are obviously too small to be directly observable at our scale. But their effects can be amplified and made visible thanks to the colossal number of atoms that make up the world on our scale.
1 mole of hydrogen (1H) weighs 1 g.
1 mole of iron (56Fe) weighs 56 g.
This amount of matter is made up of 6.02 x 1023 elementary entities.
Image credit: vulgarisation.fr