[Physics] Is the magnetic field/force just a relativistic electric field/force or is there a fundamental difference

electromagnetismelectrostaticsmagnetic fieldsvirtual-particles

I have read these questions:

Difference between electric and magnetic field (relating to EEG & MEG)

Can someone please explain magnetic vs electric fields?

Explain the origin of magnetic fields in layman terms

Mechanism by which electric and magnetic fields interrelate

where it says:

The simplest explanation I know of requires only one test charge and two reference frames with a relative velocity between them.
Frame 1: The charge is at rest. It is the source of a (purely) electric field.
Frame 2: The charge is moving. It is a current, and the source of a magnetic field.

Understanding the relationship between electricity and magnetism

What's the core difference between the electric and magnetic forces?

where it says:

Electric forces are created by and act on, both moving and stationary charges; while magnetic forces are created by and act on only moving charges.

Can magnetic force really be just electric force in a different frame?

Magnetic field as effect of relativity?

E field:

  1. charge is stationary to the observer

  2. monopole

  3. lines of firce radially converge

  4. easy to block

  5. mediated by virtual photons

M field:

  1. charge is moving to the observer

  2. dipole

  3. can't be blocked

  4. mediated by virtual photons

We know that electrons have E charge and M dipole moment too. But a magnet has usually no E charge.

Electric forces can act on different materials like statically charged plastics too.

Magnetic forces can usually only act on metals but not on plastics.

None of these questions says whether there is one fundamental force, the EM force, and it has different ways of acting on different materials and depending on if the charge is moving relative to the observer, or there is an electric force, and a magnetic force, and they are both mediated by virtual photons, but they act on different materials and differently depending on whether they are moving relative to the observer.

They also say that with the magnetic field, the charge is moving relative to the observer. But if you look at a magnet, it has a magnetic field, and magnetic force, and is not moving relative to the observer.

Is there a fundamental difference or is the M field/force just the E field/force in relativity (moving)?

Question:

  1. What is the fundamental difference between an E and a M field? Is it just that one is stationary the other is moving relative to the observer?

  2. Is there just one force, EM force, or is there E force and M force as they can act differently on different material? Is the M force just an E force in relativity?

Best Answer

While I agree with the answer posted by lesnik, it's worth pointing out that not every magnetic field can be thought of as arising from an electric field. This isn't even true for uniform fields!

The reason is that there are two invariants of the electromagnetic field, $$E^2 - B^2 \quad \text{and} \quad \mathbf{E} \cdot \mathbf{B}$$ which don't change under Lorentz transformations. If you start with only a magnetic field, there's no way to transform it into only an electric field. We say $\mathbf{E}$ and $\mathbf{B}$ are unified because they transform freely into each other, but there are limitations. It's like how space and time are still distinct, despite being unified into spacetime; a spacelike interval cannot be transformed into a timelike one.

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