# fields and photons?

#### interested

If photons travel in straight lines and do not interact with electric or magnetic fields. How can magnetic fields and electric fields be propagated by photons???

ie magnetic fields can both repel and attract depending on polarisation. Electric fields can depending polarity attract or repel. Photons dont attract or repel.

#### topsquark

Forum Staff
If photons travel in straight lines and do not interact with electric or magnetic fields. How can magnetic fields and electric fields be propagated by photons???
The Maxwell equations can be manipulated into predicting a self-reinforcing state that travels at a speed of $$\displaystyle c \equiv \dfrac{1}{ \sqrt{ \epsilon _0 \mu _0 }}$$
where $$\displaystyle \epsilon _0$$ is the permittivity of empty space and $$\displaystyle \mu _0$$ is the permeability of free space.

Also, the Maxwell equations (which predict the speed of light) deal with waves, not particles. Electromagnetic fields are linear so if there are other E and B fields in the region we can separate out the self-reinforcing state.

-Dan

#### Woody

Yes, but what is it?

Would it be reasonable to describe your "self-reinforcing state" photon,
as a distortion of the electromagnetic field?

So (for example) an electron releases energy into the electromagnetic field
creating a disturbance which travels (at the speed of light) until it encounters another electron
(which is at a suitable energy state) to accept the package of energy carried by the electromagnetic disturbance.

1 person

#### benit13

If photons travel in straight lines and do not interact with electric or magnetic fields. How can magnetic fields and electric fields be propagated by photons???
Short answer: they aren't.

A real photon is a fluctuating electromagnetic field that just exists in its own right. It can be observed directly by some kind of absorption process.

A virtual photon is a fluctuating electromagnetic field that represents the electromagnetic interaction between two charged particles. It cannot be observed directly.

ie magnetic fields can both repel and attract depending on polarisation. Electric fields can depending polarity attract or repel. Photons dont attract or repel.
Correct. EM fields affect charged particles. Photons do not have a charge.

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#### interested

Short answer: they aren't.

A virtual photon is a fluctuating electromagnetic field that represents the electromagnetic interaction between two charged particles. It cannot be observed directly.
Am I correct in thinking radio waves are transmitted by virtual photons with polarizable properties, which can be affected by other fields?
And Maxwells equations describe virtual photons not real photons? If so, do you have a good reference.

Edit furthermore what are magnetic field lines made up of? are they dressed electrons? virtual particles or what? They bend, and travel through solid material, that would absorb or reflect photons.

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#### benit13

Am I correct in thinking radio waves are transmitted by virtual photons with polarizable properties, which can be affected by other fields?
Nope. Like I said earlier, virtual photons are responsible for exchanging energy and momentum between two charged particles. Photons (whether virtual or not) are not directly affected by fields.

If you have some system of charged particles and then impose another electric field on the system by adding another charged particle, then the charged particles will interact with the new one as well as each other. Consequently, the number of virtual photons flying around to exchange energy and momentum among the charged particles is higher.

Radio waves are EM radiation. EM radiation can either be described using waves (which sometimes works but sometimes does not), particles (i.e. real photons, which also only sometimes works) or quantum particles (which always works, but often the mathematics is not conveniently tractable for macroscopic bodies).

And Maxwells equations describe virtual photons not real photons? If so, do you have a good reference.
Maxwell's equations describe electromagnetism. If you want a framework to adequately describe virtual particles, you probably need to look at Quantum Electrodynamics (QED) and/or gauge theory. If you want a framework to adequately describe real photons, look at particle physics, quantum mechanics (QM) and QED. I studied particle physics and QM at University, so I can help with those to a limited extent, but I know absolutely nothing about QED or gauge theory..

Furthermore what are magnetic field lines made up of? Are they dressed electrons? virtual particles or what? They bend, and travel through solid material, that would absorb or reflect photons.
Field lines, for all fields, are an abstract concept used to characterise fields and, consequently, how objects with certain properties are able to seemingly interact without touching each other. They're not made up of anything.

There's a philosophical debate to be had about whether "fields" really exist. I'm sure there are people who really believe that fields are out there in the material Universe and can be considered as "stuff", but, if so, they are unique and not composed from other things... certainly not virtual particles.

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#### Woody

What is it (2)

It could be said that fields are mathematical constructs that allow predictions to be made of observed phenomena.
How the mathematical constructs relate to actual physical "things" in the universe is unclear.

The gravitational "field" is (since Einstein) related to the physical geometry of 4 dimensional space-time.
The electromagnetic field does not (as far as I know) have a similar physical description.

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#### topsquark

Forum Staff
The gravitational "field" is (since Einstein) related to the physical geometry of 4 dimensional space-time.
The electromagnetic field does not (as far as I know) have a similar physical description.
The Einstein field equations are a set of 10 non-linear simultaneous partial differential equations. Most of the time they are truly ugly. Together they define what is called a "metric" that usually depends on the point in space you are at. The metric tells us how space-time "bends."

The Maxwell equations give us a metric as well, but it is much simpler. In this case there are no spacial or temporal changes when going from point to point. It is in this sense that the Minkowski metric is "constant."

-Dan

#### interested

Nope.
Field lines, for all fields, are an abstract concept used to characterise fields and, consequently, how objects with certain properties are able to seemingly interact without touching each other. They're not made up of anything.

There's a philosophical debate to be had about whether "fields" really exist. I'm sure there are people who really believe that fields are out there in the material Universe and can be considered as "stuff", but, if so, they are unique and not composed from other things... certainly not virtual particles.
Surely a magnetic field can be demonstrated to exist by repulsion and attraction between magnets and they can be visualized easily by sprinkling iron filings on a piece of paper, with a magnet under it?

Are you suggesting that fields perhaps do exist outside of normal space time dimensions? Or maybe more simply fields are better represented as distortions in space time, caused by particles? Those fields mirroring perhaps the particles alignment, movement and shape in some way. This would require an aether of sorts to transmit the field, rather than particles virtual or otherwise.

#### benit13

Surely a magnetic field can be demonstrated to exist by repulsion and attraction between magnets and they can be visualized easily by sprinkling iron filings on a piece of paper, with a magnet under it?
I think you're missing my point...

In particle physics, there is a way of describing particles and their collisions, which is not based on fields but is instead based on particle interactions and interaction probabilities. There are four possible mechanisms for interaction: the EM interaction, the strong nuclear interaction, the weak nuclear interaction and gravity. In this model, there are no fields: in this model, particles with certain properties have a probability of interacting with each other based on their state and proximity. When they interact, they exchange virtual particles (called gauge bosons) which only exist for a short time, but are responsible for exchanging energy and momentum. The outcome of the interaction could just be a change in energy and momentum, or it could be something else, like pair-production, annihilation, or any one of a whole host of different processes.

So... the key point is that there is more than one way of describing particles and particle behaviours.

Are you suggesting that fields perhaps do exist outside of normal space time dimensions? Or maybe more simply fields are better represented as distortions in space time, caused by particles? Those fields mirroring perhaps the particles alignment, movement and shape in some way. This would require an aether of sorts to transmit the field, rather than particles virtual or otherwise.