Does a magnetic field really exist?

[BradP]

Electrons only generate a magnetic field when they are moving, right? Well suppose there is a piece of plastic charged with a static negative charge, and it sits in a moving boxcar.

If someone inside the boxcar holds a magnet up to it, it should not deflect at all, because the static electrons create no magnetic field. However, if the boxcar drives past someone holding a magnet to the edge of the train, the electrons in the plastic are moving relative to that magnet. Then they supposedly have a magnetic field wrapped around their velocity vector, which interacts with the held magnet and creates a slight force on them.

However, if there is a magnetic field around the electrons, it should exist no matter what someone's perspective.

 

[stevenb]

Electrons only generate a magnetic field when they are moving, right? Well suppose there is a piece of plastic charged with a static negative charge, and it sits in a moving boxcar.

If someone inside the boxcar holds a magnet up to it, it should not deflect at all, because the static electrons create no magnetic field. However, if the boxcar drives past someone holding a magnet to the edge of the train, the electrons in the plastic are moving relative to that magnet. Then they supposedly have a magnetic field wrapped around their velocity vector, which interacts with the held magnet and creates a slight force on them.

However, if there is a magnetic field around the electrons, it should exist no matter what someone's perspective.

Yes, but magnetic and electric field components are not independent components of 2 rank 1 tensors (i.e. two vectors), but are actually the 6 independent components of one antisymmetric Rank 2 tensor (i.e. a 2-form).

If you look up the covariant formulation of Maxwell's equations, you can get to grips with this pretty quickly.

http://en.wikipedia.org/wiki/Covariant_formulation_of_classical_electromagnetism

 

[facenian]

No the magnetic and electric fields depends on the state of motion of the observer. In fact you touch upon a very interesting question that trubled physicist at then end of the 19th century and beginings of 20th resolved by relativity theory

 

[Dale]

I don't know exactly what it means for something to "really exist", but stevenb is correct. Although even the 6 components of the tensor are not really independent.

Regarding your example. Don't forget the electric field. When you boost the magnetic field you get an electric field. The electrostatic force from this electric field will cancel out the magnetic force from the moving electrons. Both frames will agree that there is no net force.

 

[dgOnPhys]

electrons have intrinsic magnetic moment so they will generate a magnetic field and be subject to torque and force in a magnetic field even if static