# Why does an electron have spin? [duplicate]

An electron is assumed to be a point particle that does not have structure and volume. Why does it have spin? Does this imply that the electron has volume? It is hard to imagine that a point (without volume) can spin.

• Possible duplicate of Why does an electron have left and right components? Jul 22 '18 at 7:31
• An electron is not assumed to be a point particle at the nano scale. Rather think of it as a probability "cloud". Jul 22 '18 at 7:40
• @Steeven the electron is presently assumed to be a point particle at femtoscale. No structure has been observed so far, except spin. Jul 22 '18 at 9:16
• The same user seems to have asked two extremely similar questions over the span of 10 minutes: physics.stackexchange.com/q/418864 (Suggested duplicate by @Noiralef)
– user191954
Jul 22 '18 at 10:42
• Related: physics.stackexchange.com/q/1/2451 , physics.stackexchange.com/q/822/2451 and links therein. Jul 22 '18 at 18:23

When experiments started measuring the microcosm of particles it became necessary to invent quantum mechanics in order to build descriptive and predictive mathematical models. The model we have now is called the standard model . The elementary particles are postulated as point particles and the model has been validated innumerable times, by all the available data, including for the existence of spin and magnetic moment for the point particles in the table.

Basic "axiomatic" conservation laws ( because of the mathematical format)for this quantum mechanical model, are energy conservation, momentum conservation and angular momentum conservation. In addition special relativity with its four vectors is basic in the standard model.

The reason the particles in the table are assigned a spin is because of angular momentum conservation in particle interactions. If there were only orbital angular momentum and no intrinsic angular momentum for the particle the angular momentum would not be conserved. With the assignment of a specific spin for each particle the conservation of angular momentum holds ( is validated) for all existing data.

What is spinning in a point particle with no volume? It is the same as asking :"where is the charge on a zero volume particle?".

The exact answer is : nothing is spinning, and the charge is concentrated on a point. Mathematically everything is consistent by assigning spin and charge to an electron or any of the particles in the table.

It does not sit well with classical intuition, but most everything in quantum mechanics does not sit well with classical intuition, it is a different framework, based on experimental numbers and a sophisticated mathematical model.

Intrinsic spin is a quantum number necessary for conservation of angular momentum. Intrinsic charge is a quantum number necessary for conservation of charge, and the particles are labeled by the values.

• "Nothing is spinning, and the charge is concentrated on a point." This answer is not convincing. If nothing is spinning, why is there intrinsic spin? Where does the intrinsic spin come from? You can't explain it just by saying "Intrinsic spin is a quantum number necessary for conservation of angular momentum." This does not explain the physical cause of the intrinsic spin.
– Shen
Jul 22 '18 at 13:38
• In my opinion, it is more reasonable to assume that the electron has volume, and its intrinsic spin comes from the rotation of its volume.
– Shen
Jul 22 '18 at 13:41
• I just explained in my answer that the spin comes from conservation of angular momentum and in the standard model it is just a label . We are not doing physics by beliefs and handwaving in this day and age, but by fitting data and predicting successfully new situations. An electron with volume does not fit the data as fitted by the standard model. Jul 22 '18 at 16:37
• @anna v Angular momentum is an observable, not just a label. Jul 22 '18 at 21:23
• @my2cts absolutely, angular momentum is an observable. Spin is a scientific/ mathematical "invention" . assigning a real number ( that is why I call spin a label) to each elementary particle so that conservation of angular momentum should hold in the quantum mechanical microcosm of particle physics . And the assignment works within the standard model Jul 23 '18 at 4:22

Here, I would like to present a rigorous point of view for the concept of spin. The phenomenological motivations are already given in anna v's answer and akhmeteli's answer.

First of all, spin isn't about structures, or volumes. It is about statistics (behaviors) of a field/particle, regardless of whether it has structure/volume or not.

Spin is not only unique to fermions, photon has spin, as well, just in terms of polarization. Just like polarization states of electromagnetic field, a fermion has up or down spin states. Both of them are intrinsic properties, just different statistics (behavior).

By saying spin is an intrinsic property, we mean that, considering the electron field for example, for each coordinate on spacetime there is another topological space that describes the spin behavior. Just as we describe spacetime in 4 real dimensions, this pointwise-attached spaces have 4 extra complex dimensions, each one is called fiber and the whole thing (the theory) is called fiber bundle (associated with a symmetry group - it is $SU(2)_L \times SU(2)_R$ in this case, where these symmetries are related to conservation of angular momentum). It's like each field at each position, besides the kinematic interactions, also has an inner world that describes their intrinsic behaviors.

So, instead of thinking there is a spacetime manifold and just particle fields at each point, we think there is a spacetime fiber bundle where each fiber is a spin space with its own coordinates. Fermions have components living in those coordinates, and that's why they are denoted as $\Psi_a (x)$, where $a=1,2,\dot1,\dot2$ are spinor index for left-handed particle, left-handed anti-particle, right-handed particle, and right-handed anti-particle, respectively. Another example, Dirac gamma matrices, $\gamma^\mu_{ab}$, which are the representation of fermion spin, have both spacetime and spinor components.

Actually we are already familiar with this kind of thinking when we first encounter electric and magnetic fields.

At each point, there is an electromagnetic field, $\mathbf{E}(x)$ and $\mathbf{B}(x)$ (or $F_{\mu\nu}$ in the covariant formulation), where they have different set of values for each direction. They are called the curvature of the fiber space. So, every point on the spacetime has another space that even has a curvature, and what we actually see as an electromagnetic field is the interaction of the curvature of these fiber spaces to the charged particles, which are source of the curvature.

• Spin is a form of angular momentum. If a photon with spin is absorbed then a torque is measured (Beth 1935). It is the extra angular momentum of th eelecron that causes the ground state of hydrogen to have J=1/2 instead of J=L=0 and the resulting centrifugal potential is the origin of the difference in energy between the Dirac and the Klein-Gordon equation. There is a connection with statistics, but spin is more than just statistics. Jul 22 '18 at 21:27

"An electron is assumed to be a point particle that does not have structure and volume."

This popular statement should not be accepted uncritically.

If you look at the papers offering strong experimental limitations on the size of electron (such as a review by the Nobel Prize winner H. Dehmelt in Physica Scripta. Vol. T22, 102-110, 1988), you will see that "point electron" there actually means "an electron that is described by the Dirac equation and QED with extremely high accuracy" (this is not a quote, this is just my summary; the actual quote confirming my summary is "an elementary Dirac particle, such as the electron, is the closest laboratory approximation of a point particle." See also Fig. 8 in the quoted article).

Thus, now we have a different question: does the Dirac equation describe a point particle? This is not obvious, as the Dirac equation is an equation of quantum theory, therefore, the uncertainty principle is valid for this equation. The Dirac electron can have a well-defined coordinate, but only at the expense of a poorly defined momentum. Therefore, you cannot say that an electron is "a point (without volume)" when we speak about spin (angular momentum). The uncertainty of angular momentum is of the order of the Planck constant, so there is no contradiction with the existence of spin.

Another thing. How do you define the size of the electron? The electron carries the Coulomb field. Is this electric field a part of the electron or not? (Cf. the notion of "dressed" electrons in QED). Remember that the radius of the Coulomb field is infinite.

Pointlikeness of electron is an inclusive picture. It is as pointlike as the Moon: when we content ourselves with a very simplified "image" - a point.

An electron is coupled to the electromagnetic field and it is felt far far away, as if it were extended. In QED it is indeed extendend in an elastic picture (see paper by T. Welton and by me).

On the electron spin, there is a paper of Ohanian (google it). It shows what is rotating in the QM description of electron.

• The fact that the electron can interact at a distance does not mean that the electron itself is "extended", does it? For example, the magnetic field outside a magnet is created by charges moving inside the magnet, not outside of it. Jul 22 '18 at 19:23
• @safesphere: "Outside the magnet" if you look at magnets in our visible spectrum. Take longer wavelengths and the magnets will be seen differently. Jul 23 '18 at 4:54

An electron is assumed to be a point particle that does not have structure and volume.

Perhaps the existence of the electrons electric charge could be explained by a pointlike electron. But for the electrons magnetic dipole moment the electron has to have some volume or extension. So nothing prevents me to imagine the electron as a gaseous volume. Rutherfords imagination about point like electrons could be obsolet after 100 years.

Why does it have spin? Does this imply that the electron has volume? It is hard to imagine that a point (without volume) can spin.

The electrons magnetic dipole gets aligned under motion and an external magnetic field in a way that it gets deflected (Lorentz force). It is a cyclical process of alignment, of emission of photons with deflection and disalignmt again, alignment, emission ... and so on until the kinetic energy gets exhausted (in emitted photons) and the electron stands still.

• This is not accepted physics as far as I know. Your post reads like a private theory. Do you have any peer reviewed paper to back up your assertions? Jul 22 '18 at 9:13
• @my2cts Yes, I afford to read the originals and think for myself. Jul 22 '18 at 11:43
• So you are saying the spin of the electron can have any value depending on its rotational kinetic energy? Jul 22 '18 at 13:00
• @Aaron The magnetic dipole has an unambiguous value, its an intrinsic (independent from any circumstances) property. The direction of the magnetic dipole of a free electron can be directed in any direction. In atoms the electrons magnetic dipoles are aligned. Not surprisingly exactly 2 and 8 magnetic dipoles around a nucleus are in equilibrium. Jul 22 '18 at 13:29
• @holger still you don't give any references. This forum is not for discussing unpublished work or views. This should be put on hold Jul 22 '18 at 16:20