# I What are magnetic and electric fields?

1. Feb 16, 2016

### John_RB

I hesitate to ask this but what are electric and magnetic fields? We are told that the electromagnetic force is carried by the photon (which consists of oscillating electric and magnetic fields) but the photon does not cause (?) or interact with external magnetic and electic fields. Presumably quantum mechanics will make things crystal clear! Incidentally isn't the prediction of the existence of electro-magnetic waves by Maxwell one of the most amazing in the history of science – coupled with the calculation of the speed of these postulated waves.

2. Feb 16, 2016

### Staff: Mentor

They are not separate - its one field - the EM field. Its a quantum field that exists everywhere just like quantum fields exist for all fundamental particles.

You have been reading popularisations. Generally speaking they are wrong. The photon is an excitation of the quantised EM field where exactly what excitation means is detailed in the theory. But just to get a feel its like the states of a quantum harmonic oscillator:
https://en.wikipedia.org/wiki/Quantum_harmonic_oscillator

https://www.amazon.com/Fields-Color-theory-escaped-Einstein/dp/0473179768

I personally believe people should start their lay journey into QM learning about QFT - it resolves a lot of issues - of course others replace it but they are easier for a beginner to grasp IMHO

Thanks
Bill

3. Feb 16, 2016

### Hornbein

Charged particles exert forces upon one another. Fields are a mathematical construct that enable us to predict the effects of such forces.

Let's look at gravity for a moment. The amazing thing about that is that Newton's laws predict as accurately as they do, even though they are based on the false assumption that the forces are communicated instantly. Electromagnetism is the same way: assume that the force is instantaneous and you may get a pretty good result, though not as good as with gravity. The correction in the result is called the "magnetic field." Gravity has a correction like that too called the gravitomagnetic field, but it is so weak it is not of practical significance here on Earth.

It is often useful to mentally split the energy of the charge attraction into electric and magnetic fields. This is partially of historical significance: magnets and electricity were discovered separately. It is useful in designing motors and so forth. The concept is that magnetic force depends on how the affected particle is moving, while the electric force doesn't. (Or at least, not so obviously.)

Photons are for people who don't like fields. Fields are for people who don't like photons. Take your choice. Physicists discuss this endlessly. My advice is don't worry about it.

Photons can be affected by magnetic fields, but only in a medium, not in a vacuum.

Yes, Maxwell's work is most impressive.

4. Feb 18, 2016

### John_RB

To be honest I don't feel much wiser - the Quantum Harmonic Oscillator went way over my head. I suppose I am asking the impossible - an explanation of QM without maths. I have looked at the array of popular books on QM that I have and found that they make no mention of QFT or a passing reference at most. It seems that the Fields of Color book may be my best hope but I don't use Amazon as they only pay UK tax in one on the other of the many-worlds and I have not found another supplier yet. I have started to read this http://plato.stanford.edu/entries/quantum-field-theory/ although I note it makes the following remark "the interpretation of QFT is particularly obscure" when answering the question "What is QFT?"
I asked Google if light is affected by magnetism and the answer was no although a discussion on the Physics forum in 2009 mentioned Delbrück scattering but the effect is so small as to be unmeasurable.

5. Feb 18, 2016

### Staff: Mentor

There is a well known experiment that shows it is:

Unfortunately you cant understand QFT at the lay level except in a very basic way like the book I mentioned. I think it will help you doing that - but its the best that an be done without delving into the mathematics.

Thanks
Bill

6. Feb 19, 2016

### Hornbein

Alternative realities are paying us tax? That has real potential.

7. Mar 6, 2016

### John_RB

We often hear about Faraday in the UK (on the history or development of science) but I never heard of his experiment showing the effect of magnetismon the polarisation of light. 1845! Obviously I must have nodded off in during the lecture on the Faraday effect at school or university. I once went to a public lecture at the Royal institution in London just to go there. The recent Cosmos series was not shown in the UK unlike the original starring Carl Sagan
I found this lecture and Q&A by Sean Carroll very interesting.
http://chycho.blogspot.co.uk/2014/05/best-explanation-of-quantum-field.html From this we discover that particles actually don't exist; wave (functions?) are the reality but when we make observations then we call the results of the observations "particles" with certain properties (like position) but sometimes the results don't have the properties of particles and we call them waves and then we have puzzles and paradoxes.
Although I remain one of the 99% of people in the world who do not understand what this means - "the Higgs Boson gives particles their mass" - I now know it is because I can't do the maths of QFT.
However I still have not "got it" about magnetic and electric fields and photons

8. Mar 6, 2016

### John_RB

I rather garbled my sentence. I meant to say that of the potentially infinite number of universes of the many-world interpretation of QM the one in which we exist where Amazon employs 7000 people in the UK (mostly on the minimum wage) they pay no UK tax.