In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles,
composite particles (hadrons), and atomic nuclei.
All elementary particles of a given kind have the same spin quantum number, an important part of the (quantum) state of a particle.
The intrinsic property of subatomic particles called spin and discussed in this page,
is related in some small ways, but is very different from the everyday concept of spin, for example, as used
when describing a spinning ball.
Spin, as used by particle physicists in the quantum world, is a property of subatomic particles, which has certain qualities
and obeys certain rules.
When combined with the spin-statistics theorem, the spin of electrons results in the Pauli exclusion principle,
which in turn underlies the periodic table of chemical elements.
The spin direction (also called spin for short) of a particle is an important intrinsic degree of freedom.
Wolfgang Pauli was the first to propose the concept of spin, but he did not name it.
In 1925, Ralph Kronig, George Uhlenbeck, and Samuel Goudsmit suggested a physical interpretation of particles spinning around
their own axis.
The mathematical theory was worked out in depth by Pauli in 1927.
When Paul Dirac derived his relativistic quantum mechanics in 1928, electron spin was an essential part of it.
There are two types of angular momentum in quantum mechanics: Orbital angular momentum, which is a generalization
of angular momentum in classical mechanics (L = r x p), and spin, which has no analogue in classical mechanics.
Since spin is a type of angular momentum, it has the same dimensions: J.s in International System Units.
In practice, however, International System Units are never used to describe spin.
Instead, it is written as a multiple of the reduced Planck constant ħ.
In natural units, the ħ is omitted, so spin is written as a unitless number.
What is spin?
No one can explain the spin mechanism.
Since spin is not proportional to the charge or to mass, it could be related to another quantity.
One of the most adapted quantity is the shape of the wave when the particle is moving (see Part 2: Wave-Particle Duality).
Here we explain the idea.
In the following figure, the two particles, an electron and a neutrino, go towards the reader.
On the left side (A), the two particles are motionless, i.e. in their corpuscular form.
The spin theoretically does not exist (this assumption must be verified).
On the right side (B), these two particles are moving as waves, as explained in Part 2.
So, the immediate idea is that spin could be a simple ratio (relative to the reduced Planck constant ħ),
like |H/L|, the height (H) and length (L) of the wave.
If this point of view is correct, the spin would not be a value attached to any particle but rather a value attached
to the movement or to the mode of propagation of the waves.
More precisely, the spin would be a direction of polarization of sCells.
In that way, we could expect that the projection of spin over a direction gives two values, +H/L and -H/L.
This point of view also means that any motionless particle can't have a spin.
In other words, the theory described in this document predicts that the spin exists only when the particle is moving,
i.e. only when the particle is in its waveform.
This idea seems to be confirmed by experimentations (superfluid atoms).
This approach may be illustrated by the following examples.
Three cases must be considered:
A/ Basic particles
The spin is 1/2 in absolute value.
The projection of the spin over an axis depends of the polarization of sCells.
B/ EM waves (bipolar polarization)
The spin of a half-period cancels each other.
C/ Associations of particles
For example, a nucleus is moving as a complex set of waves (quarks, electrons...) and their individual spins can be added or cancelled.
This also depends on the overlap of the individual waves.
A first confirmation
The point of view here described seems already be confirmed by at least one experiment: the Bose-Einstein Condensat.
For example, Helium atoms become superfluid at -273°K.
At this temperature, Physicists suppose that a superposition of states occurs, but this is only speculation.
Our explanation is also speculative, but it makes sense and is more logical and consistent.
Moreover, it solves the "spin crisis of proton" (see below).
Since we suppose that spin exists only when the particle is moving, i.e. when the particle is in its wave form,
spin must theoretically disappear at -273°K.
It means that the Pauli Exclusion does not exist anymore at this temperature.
To date, no experimentation has been done in a really absolute 0° temperature but it is probable that orbitals are
completely disorganized at such a temperature.
For example, instead having 2 and 8 electrons on the two first layers, it is possible to have an unique layer
with 10 electrons since spin disappears.
It would be interesting to conduct experimentations in that way.
Nevertheless, to date our knowledge on superfluid atoms seems to agree our theory:
"Spin does not exist if the particle is motionless".
Rule of addition of spins
The rule of addition of spins has been devised taking into account the experimentations conducted since 1925.
This theory is applied with success in a large majority of cases, but extrapolation of this
rule to all the components of physics without any reservation is highly debatable for the following reasons:
Extrapolation of spin
The rule of addition of spins applies in most cases, but this does not mean that it applies in all cases.
Any extrapolation toward the quarks or other particles is very debatable because our knowledge of the nature
of these components or spin is very poor.
To date, no one knows the spin mechanism, or the constitution of quarks, or why their charge is a multiple of 1/3...
(see the footnote 1 about the proton spin crisis).
In the same way, no one can explain the mechanism of charge, mass, electromagnetism, gravitation...
This is why before extrapolating the rule of addition of spin, it would seem logical to try to fully understand
the spin mechanism in order to see if an extrapolation towards quarks is possible.
Indeed, the real question is not "Why the rule of addition does not work?", but more simply "What is spin?".
Overlap of waves
Moreover, the spin seems to be a function of the overlap of waves.
As we know, molecules and atoms are much larger than protons or quarks.
When these elements are moving, the overlap of their waves is different.
Therefore, we get erroneous results if we extrapolate the rule of addition of spin toward elementary particles
since we do not know exactly the wave shape of each particle.
To summarize, to date:
The real nature of the spin is unknown;
the definition of "quantum value" does not explain the mechanism of spin,
The mechanism of addition of spins is unknown,
The overlap of waves is unknown,
The wave of a nucleus or atom is much larger than the wave of a quark,
The nature of spacetime is unknown,
The mechanism of electromagnetism is unknown,
The nature of electrons, quarks and other particles is unknown...
Origin of charge is unknown,
No one knows why quarks have a fractional charge 1/3 - 2/3,
Origin of mass is unknown,
Origin of gravity is unknown, even this quantity is not related to spin,
Origin of electrostatic field is unknown,
The mass of quarks is measured with poor precision, even if mass and spin are two different quantities,
Under these conditions, is it reasonable to assume that the rule of addition of spins, which can be applied in 99% of cases,
can be extrapolated to all particles without reservation? (footnote 2). Of course, not.
For all these reasons:
Our knowledge of spin is too poor to
make extrapolations toward quarks and
all particles. Therefore, the violation of
the rule of spin addition cannot be retained
as a valid objection to a new theory.
The key question is how the nucleon's spin, whose magnitude is 1/2ħ, is carried by its constituent partons (quarks and gluons).
It was originally expected before the 1980s that quarks carry all of the nucleon spin, but later experiments
contradict this expectation.
In the late 1980s, the European Muon Collaboration (EMC) conducted experiments that suggested the spin carried by quarks
is not sufficient to account for the total spin of the nucleons.
This finding astonished particle physicists at that time, and the problem of where the missing spin lies is sometimes referred
to as the "proton spin crisis".
Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) and the COMPASS experiment
at CERN, experiments E154 and E155 at SLAC, HERMES at DESY, experiments at JLab and RHIC, and others.
Global analysis of data from all major experiments confirmed the original EMC discovery and showed that the quark spin
did contribute about 30% to the total spin of the nucleon.
A major topic of modern particle physics is to find the missing angular momentum, which is believed to be carried either
by gluon spin, or by gluon and quark orbital angular momentum.
The gluon spin components are being measured by many experiments.
Quark and gluon angular momenta will be studied by measuring so-called generalized parton distributions (GPD) through
deeply virtual compton scattering (DVCS) experiments, conducted mainly at JLab.
As a result, extrapolating the spin to the protons and quarks is highly speculative.
About spin, the definition "a quantum value" is not sufficient to explain spin.
Here, we do not claim to solve the spin enigma but we try to have a more rational explanation
than "a quantum value", which is a definition without any meaning.
Of course, we could argue: "Quantum Mechanics is different from classical mechanics", but we also could say "Quantum mechanics
is identical to classical mechanics, but our brain is not clever enough to fully understand quantum mechanics".
The second definition is probably closer to reality than the first one.
Indeed, concerning spin, we are faced with a major question: "What is spin?".
In such a case, we must try to solve this problem instead to take the easy way out saying "Spin is a quantum value".
As in everyday life, we should always bear in mind this remarkable Mark Twain's citation which can be applied to spin:
"What gets us into trouble is not what we do not know. It's what we know for sure that just ain't so".
The explanation of spin presented in this webpage needs a validation by experimentation.
If this theory is right, it modifies Quantum Mechanics as follows:
- Spin disappears if the particle or the
association of particles is motionless,
- Neutrinos would have a very weak charge.
Please note that Parts 2 and 3 of the Spacetime Model also cover this subject.