The general assumption is that quantum theory deals with phenomena of atomic size and below, but does it have effects on macroscopic systems?
The answer is: absolutely yes.
Amir Segal
Direct link to this page: https://www.hayadan.org.il/quantfuture.html
The general assumption is that quantum theory deals with phenomena of atomic size and below, but does it have effects on macroscopic systems?
The answer is: absolutely yes.
The impact on the macroscopic world comes through probability. The classical probability of a particle being at a certain energy is very different from the quantum probability. There are two main differences between the probability of a particle in classical and quantum theory:
The classical Torah allows a particle to have any energy from a sequence of numbers.
The quantum theory allows a particle to exist only at discrete energies (certain numbers from a list).
Another and very important difference for statistics is in the definition of the identical quantum particles:
First we will imagine two particles in empty space. The classic description says that the particles can be distinguished, each of them can be given a serial number. Quantum theory says something different: the particles are so identical that they cannot be distinguished at all. If the two particles are interchanged, we will not be able to tell that such an interchange has taken place. Therefore, it is not possible to give a serial number to each of them.
Now we will separate two types of quantum particles - bosons and fermions (every elementary particle belongs to one of these groups):
We will use the fact that exchanging the two particles with each other is the same as a mirroring operation in space.
Bosons - Bosons are particles whose wave function does not change under reflection in space (symmetry for reflection).
Fermions - Fermions do change under reflection in space, their wave function gets a minus sign (anti-symmetry for reflection).
Now suppose we have two fermions in the same quantum state: if we perform a reflection in space, the wave functions of the fermions will get a minus sign, but we said that the reflection operation is the same as exchanging the identical particles with each other, so there should be no change in the wave functions. The only function that is equal to the minus of itself is... zero!
Conclusion: there cannot be two (or more) fermions in the same quantum state. This is Pauli's law of exclusion.
Example:
Electrons are fermions, therefore, there cannot be two electrons in a box with the same spin and the same speed.
From these differences, it is clear that the number of possible states for a macroscopic system, and the probability of a state
A certain macroscopic, very different from classical physics. A small crystal has an order of magnitude of 10 to the 20th power of particles, so a probabilistic deviation of a single particle becomes a measurable phenomenon.
Here are some examples:
Electrical conductivity: An electric current in a normal wire is created by the movement of electrons in the wire. As I mentioned, Pauli's law of prohibition does not allow all electrons to move at the same speed, therefore, the intensity of the current that the classical theory predicts is very different from the intensity that the quantum theory predicts. In fact, the various collisions of the electrons with the material must be calculated, here too there is a difference between the classical and quantum theories. The quantum theory gives predictions that are in good agreement with experience (in contrast to the classical predictions).
Superconductivity: Many materials become superconductors at low temperatures (their electrical resistance drops to zero at a temperature of a few degrees above absolute zero). The phenomenon of superconductivity was discovered at the beginning of the twentieth century, only in the fifties was a quantum model proposed to explain the phenomenon (the BCS model of attraction between electron pairs, the model is influenced by the fact that electrons are fermions). The phenomenon has no classical explanation.
In fact, the electrons in the atom are also superconductors, but in a slightly different sense (regardless of the BCS model). According to the classical model, they should release electromagnetic radiation and fall into the nucleus of the atom within a billionth of a second. The fact that they do not fall but continue to faithfully surround the nucleus of the atom is explained in the quantum model.
Superfluidity: Similar to superconductors, there are liquids whose internal friction drops to zero at very low temperatures. Liquids like ale have strange properties, if we stir the liquid, it will continue to mix and won't stop. Here, too, the explanation is quantum. The superconducting and liquid phenomena are examples of 'Einstein boson condensation', this is a quantum effect in which many boson particles drop to a low energy state at low temperature. The idea of superconductivity is that a pair of electrons (remember, electrons are fermions) can be roughly considered a new particle behaving like a boson.
Ferromagnetic materials: There are materials (such as nickel Ni) that spread a magnetic field around them, without an external magnetic field being applied to them. Materials like iron are called ferromagnets. A magnetic field is created due to electric currents. Classical theory predicts that within a material in thermal equilibrium, the currents will cancel each other out so that no magnetic field is created. The quantum theory does not allow electrons to flow in every possible path, but only in certain paths, so the paths will not always cancel each other out. In addition, the quantum theory introduces a new property of the electron - spin. The spin of the electron (as if it is spinning around itself) can also contribute to the magnetism of the material.
Black body radiation: electromagnetic radiation trapped in a box, cannot be classically described. A classical calculation gives an infinite result for the energy of the radiation (ultraviolet catastrophe). The quantum theory gives a final result. The distribution of radiation that the quantum theory predicts corresponds perfectly to the cosmic background radiation (radiation that was dispersed about 300,000 years after the big bang). The electromagnetic radiation consists of photons, the photons are bosons.
Laser: The first laser was activated in the sixties, after quantum theory researchers suggested the possibility of producing such a light beam. The laser is a beam of photons with a very uniform wavelength and high coherence. The laser beam is created as a result of a quantum system (gas or crystal) with discrete energy levels. It is necessary to inject a lot of energy into the system, as a result, the electrons in the atoms of the system rise to a high energy level.
When they drop back down to a low level, they emit photons with a very specific wavelength. This is the laser.
There are many other macroscopic phenomena for which the explanation is quantum. We are used to thinking of quantum theory as a basic study of elementary particles (a study that I respect very much), but it is important to understand that the hand of quantum theory reaches every corner of our lives. Starting with the magnet attached to the fridge in the kitchen, through the electrical wires in our homes, to the cosmic background radiation that surrounds us.
Amir Segal
Physics expert
https://www.hayadan.org.il/BuildaGate4/general2/data_card.php?Cat=~~~915148639~~~95&SiteName=hayadan
