65 votes
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Resolving Conflicting Reports on Fermilab $g-2$ Results

This seems rather incredible that these two seemingly conflicting announcements come on the same day. The pre-print for the Nature paper by the BMW group was placed on arXiv in 2020 around the same ...
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  • 3,703
16 votes
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Could someone explain the muon $g-2$ experiment problem?

The $g$ factor describes the magnetic moment of a spinning particle. The $g$ factor for a classically spinning particle is equal to 1, but in the "basic" (ie, non-interacting) quantum field ...
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14 votes

Why Is an Inhomogenous Magnetic Field Used in the Stern Gerlach Experiment?

The simple answer is that the spins are magnetic dipoles, not monopoles. A monopole will feel a force in any field. But a dipole needs a spatially varying field, because otherwise each pole of the ...
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  • 10.8k
11 votes

Paradox about the Stern-Gerlach experiment with $B=0$, $\nabla B\ne 0$

Steve's intuition for the measurement operator is in the right direction, but it's not quite the right interaction potential. Getting that right is simple, but it requires the complex task of taking ...
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11 votes
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Why Is an Inhomogenous Magnetic Field Used in the Stern Gerlach Experiment?

In the Stern-Gerlach experiment you want the atoms to be deflected depending on the direction of their magnetic dipole moment. But you get a net force on the magnetic dipole moment only if the ...
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10 votes
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Relation between magnetic moment and angular momentum -- classic theory

The easiest way to see the equality is to use a more general formula for the magnetic dipole moment of a particle. For a flat planar loop of current, it's true that $\mu = IA$, with the direction of ...
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10 votes

How neutrons interact if not through an electromagnetic interaction?

The neutron is not an elementary particle. Its overall charge is zero, but depending on the energy available the fact that it is composed out of charged quarks becomes important. Thus both the strong ...
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  • 223k
10 votes
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Has anyone tried to incorporate the electrons magnetic dipole moment into the atomic orbital theory?

To be crystal clear: Has anyone tried to incorporate the electrons magnetic dipole moment into the atomic orbital theory? YES. They've tried and they've succeeded. The electron's spin magnetic ...
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10 votes

Why Is an Inhomogenous Magnetic Field Used in the Stern Gerlach Experiment?

@Thomas Fritsch uploaded a nice picture that provides intuition about the dynamics of the situation. I would just like to add that the force $\textbf{F}$ exerted on this infinitesimal loop pictured (...
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9 votes
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Why do we assume a neutron has no electromagnetic charge?

It is an experimental fact that neutrons have no net charge. They have a magnetic dipole moment, which points to having charged constituents (quarks), the charges of which algebraically sum to $0$. (...
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  • 39.5k
9 votes
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When does $\nabla \times B =0$?

Ampere's law says that $$\nabla \times \boldsymbol{B} = \mu_0 \boldsymbol{J} + \epsilon_0\mu_0 \frac{\partial}{\partial t} \boldsymbol{E}$$ so "far away from sources" means that the current density $\...
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  • 54k
8 votes

Magnetic moment of uncharged particles

Elementary particles have quantum mechanical spin. This induces a spin magnetic moment, independent of the presence (or, indeed, absence) of a (net) electric charge. This is how the neutron attains ...
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  • 15.8k
8 votes
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Why is a current carrying loop considered a dipole?

The current loop creates a magnetic field. This field looks just like that of a magnet, which has two poles (a dipole).
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  • 1,044
8 votes
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Electron to electron interaction

As far as we know, electrons are point particles, so you don't want to try to calculate anything when they are at zero separation. You'd just get infinite forces. Classically, the magnetostatic ...
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  • 50k
7 votes
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What determines if magnetic spins align parallel or anti parallel in a material?

There are two important interactions in magnetic materials: the dipolar interaction and the exchange interaction. The dipolar interaction corresponds to the magnetic fields from the Maxwell ...
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  • 1,091
7 votes
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Why is tauon not being probed for high accuracy $g-2$ values?

This part-per-million measurement of the muon's anomalous magnetic moment is a part-per-billion measurement of the muons total magnetic moment. Every part-per-billion measurement is hard. This one is, ...
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  • 74.5k
7 votes

How can the 'spin' of a particle point in the opposite direction of its magnetic moment?

Suppose your "particle" is actually a positively-charged sphere. Spin the sphere about some axis and the moving charges generate a magnetic field. You can use the Biot-Savart law to figure ...
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6 votes
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Relationship Between Magnetic Dipole Moment and Spin Angular Momentum

$\mathbf{S}$ is the spin operator. It is a vector operator that acts on spinors. It will have three components $(S_x, S_y, S_z)$ and for example if you take the $z$ axis as your spin measurement axis, ...
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  • 1,964
6 votes

What is the magnetic dipole moment of a pion?

It is zero. If the magnetic dipole moment $\boldsymbol \mu$ were different from zero, the particle would have a preferred direction in space, which is not possible for a scalar. Recall that, in ...
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6 votes

Is magnetic force relativistic effect or intrinsic property?

A particle with spin, such as an electron, has two different magnetic fields, one due to its motion through space and one due to its spin. (Note: The spin of an elementary particle can not be ...
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  • 50k
6 votes

Is magnetic force relativistic effect or intrinsic property?

The symmetry of spacetime in special relativity requires that if the laws of physics include the possibility of electric fields, then they must also include the possibility of magnetic fields, and ...
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6 votes
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Is the boundary between ferromagnetism and paramagnetism well-defined?

It turns out that if you work out the physics behind the magnetism of a material, the magnetisation $M$ obeys a transcendental equation of the form $$M=\tanh\left(\frac{\alpha M+\gamma H}{T}\right)$$ ...
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6 votes

Is it not necessary for electrons to move to create a magnetic field?

It is not necessary for them to move. Even an electron at rest has a magnetic field. This is related to its intrinsic angular momentum or “spin”. It is not actually spinning like a little ball, and ...
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  • 50k
6 votes
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Difference between magnetic dipole interactions and spin interactions

The origin of magnetism is an complex problem in solid physics, (may be the the most lasting discussion in condensed matter physics). My point is: if caring about interaction of spin in system, we ...
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  • 1,472
6 votes

Gluon radiation from a nucleon?

The radiation for a neutron in a magnetic field has bravely been calculated. They conclude: The calculations in this paper are mainly of theoretical interest, as good pedagogical examples in the ...
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  • 223k
6 votes
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What does the metres squared represent in the dipole moment?

First of all, let us consider only a solenoid (without iron core) of length ${l}$ and area of cross-section ${A}$. The magnetic moment of the solenoid is given by:- $${M}_{solenoid}=({n}{l}){I}{A}$$ ...
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6 votes

What is $a_\mu$ in particle physics? In terms of a muon; and its magnetic moment? What is, '$a_\mu$ × $10^9$ - 1165900 '?

The spin magnetic moment of a fundamental particle with mass $m$, charge $q$, and spin 1/2 is $$\vec\mu=g\frac{q}{2m}\vec S$$ where $\vec S$ is its spin vector. The "$g$-factor" is a ...
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  • 50k
6 votes
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Is it possible to introduce magnetic monopoles without breaking $∇ · B = 0$?

No, it is not possible. If you have a magnetic monopole then, by definition, there is some closed Gaussian surface $S$ over which $$\oint_S \vec B \cdot d\vec S \ne 0$$ which by implies that $$\int_V \...
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  • 69.3k
5 votes
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Why does Earth behave like Natural Bar Magnet?

The Earth's magnetic field is caused by eddy currents in the liquid parts of the planet's interior. We believe the field is not due to a permanent magnet because: (1) Its direction and strength ...
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