Can magnetism exist independently of current or charge?

[Antiphon]

A pair of opposite charges annihilate releasing a pair of gamma ray photons.

The photons' trajectories are bent into large circles by a distribution of neutron stars.

The electric and magnetic fields (as photons) travel around the same neighborhood of neutron stars for 20 billion years after the charges have ceased to exist.

I hope this is thought experiment enough to convince anyone that there can be magnetism long after the source charge has gone.

 

[Drakkith]

I don't think the argument was that magnetic fields cannot exist after their sources were gone, but that magnetism needed no charge as a source. At least that was what I got. Two charges that annihilate still have a magnetic and electric field that is extending outwards at c, but as the fields no longer have an existing source, no further field is created beyond what already existed before the annihilation.

 

[Khashishi]

Magnetism can exist independently from charges in the form of electromagnetic waves. Electromagnetic waves can be formed by annihilation of matter/antimatter, presumably without needing charges to exist.

 

[Drakkith]

Magnetism can exist independently from charges in the form of electromagnetic waves. Electromagnetic waves can be formed by annihilation of matter/antimatter, presumably without needing charges to exist.

EM Waves are made up of electric AND magnetic fields. I believe they are both the source of each other?

 

[Antiphon]

I don't think the argument was that magnetic fields cannot exist after their sources were gone, but that magnetism needed no charge as a source. At least that was what I got. Two charges that annihilate still have a magnetic and electric field that is extending outwards at c, but as the fields no longer have an existing source, no further field is created beyond what already existed before the annihilation.

Ah, this is a much more interesting question.

If you start off in a universe with zero fields and zero charges you still have zero-point magnetic fields. But let's ignore that.

There's no obvious way to create any EM fields under these conditions.

But there may be an unobvious way- via Hawking radiation. A charge-neutral black hole in an empty universe will radiate EM fields without the presence of charge.

 

[Drakkith]

Ah, this is a much more interesting question.

If you start off in a universe with zero fields and zero charges you still have zero-point magnetic fields. But let's ignore that.

There's no obvious way to create any EM fields under these conditions.

But there may be an unobvious way- via Hawking radiation. A charge-neutral black hole in an empty universe will radiate EM fields without the presence of charge.

I don't know much about black holes, but wouldn't it only be charge neutral when equal amounts of + and - charges have fallen in? (Ignoring the problem of electric fields propagating from a black hole, if that is a problem)

Also, isn't hawking radiation the result of the creation of real particles from virtual particles? Which are charged?

 

[kcdodd]

I have also asked myself if sources can exist without any fields. It seems clearly they cannot. One cannot have an electron without any electric or magnetic field. It is intrinsic to what an electron is. However, one can have EM fields without electrons. A charged particles does not seem intrinsic to EM fields the way EM fields is intrinsic to charged particles. Also, field energy can give rise to the creation of charged particles via pair production, so it really begs the question of which is more fundamental; source or field.

 

[Drakkith]

I have also asked myself if sources can exist without any fields. It seems clearly they cannot. One cannot have an electron without any electric or magnetic field. It is intrinsic to what an electron is. However, one can have EM fields without electrons. A charged particles does not seem intrinsic to EM fields the way EM fields is intrinsic to charged particles. Also, field energy can give rise to the creation of charged particles via pair production, so it really begs the question of which is more fundamental; source or field.

Is the field produced by a charged particle and the alternating fields in EM radiation different or the same? (If that makes any sense)

I would say that both the EM fields in a photon, and the fields from a charged particle both require SOME source. What produced the initial radiation from the beginning of the universe? Was there a "source"?

 

[kcdodd]

I am not quite sure if one can distinguish fields like that. Anywhere there is no source then the field automatically obeys the source-free maxwell equation, whether or not one considers the field to be "sourced" somewhere else. Classically one might suppose that a constant field must be due to sources somewhere since there would be no other way for it to arise. However, that all depends on the time-scale one is considering and what they consider "constant". Clearly no arrangement of atoms one can create can create a truly constant field, since they just created them. How would one tell the difference between a constant field and a field that just happens to have a very low frequency and/or very long wavelength. QED wise the fields break down to photons anyway.

As to the beginning of the universe; who knows. I think current theory holds that matter (aka "sources") came after the radiation. Or at least was radiation dominant. Before radiation I don't think we know, because there is no radiation from that time to look at.

 

[vanhees71]

I guess somebody has already answered in this direction within this thread, but let me try again:

From a fundamental point of view all matter consists of elementary particles. The sources of the electromagnetic field are electric charge distributions, current densities, and magnetic moments of the elementary particles. Of course, these particles form larger compounds like atomic nuclei, atoms, molecules, and finally condensed matter as we know it, and the macroscopic properties are given as the appropriate averages over all these microscopic degrees of freedom.

This leads to an effective description of the macroscopic electromagnetic properties with sources given by the charge distributions, current-density distribution, and the magnetization of matter. The magnetization can formally be subsumed into the effective current-density distribution:

\vec{j}_{\text{eff}}=\vec{j}_{\text{free}}+<br /> \vec{\nabla} \times \vec{M}.

The Maxwell equation (in Heaviside-Lorentz aka. rationalized Gaussian units), related to this reads

\vec{\nabla} \times \vec{H}-\frac{\partial \vec{D}}{c \partial t}=\vec{j}_{\text{eff}}.

 

[Antiphon]

I don't know much about black holes, but wouldn't it only be charge neutral when equal amounts of + and - charges have fallen in? (Ignoring the problem of electric fields propagating from a black hole, if that is a problem)

Also, isn't hawking radiation the result of the creation of real particles from virtual particles? Which are charged?

Actually in principle you could create a black hole by aggregating enough uncharged particles. Neutrons come to mind but if you want to be really severe about it, make a black hole using only Neutrinos. A LOT of Neutrinos.

Hawking radiation can produce all kinds of particles but let's just work with the photons here.

So you start with an uncharged universe filled with an immense quantity of neutrinos. They colocate by random chance so that a black hole forms. This black hole then emits photons by Hawking radiation, and Never was a single charge present.