Quantum Physics and Magnetism

[BigMacnFries]

Is there I quantum physical explanation for a magnetic or electric field. I read Richard Feynman's Quantum Electrodynamic book and he says all electrodynamic phenonomen can be fundamentally explained by electrons and photons. Is this true, and if so how are magnetic and electric fields created.

[fo3nix]

this is sort of true yes, but he neglects some other things. it's a good book though, very well explained overall.

there are two fundamental classifications of matter:

leptons, quarks.

this is what makes up everything that's made out of matter. then we have 4 fundamental forces:

strong, weak, electromagnetic, gravity

this is basically whats called the Standard Model of particles. they each have their own "exchange particles":

strong: gluon
weak: W+, W-, Z0
electromagnetic: photon
gravity: graviton (not yet found).

so as you may have now guessed, magnetic and electric fields (two sides of the same coin, if you will), are composed of photons. though its often easier to think of them as waves.

an electromagnetic field can be created by radiating photons. this can happen for example when an electron goes from one high energy level to a lower energy level.

[BigMacnFries]

Yes thank you for that, to some degree I understand electromagnetic radiation through feynam diagrams and double split experiments with single photons. What I am confused about is static electric fields and static magnetic fields. For example are photons streaming out of a bar magnet or a charged sphere? how is it that photons create the static field?

[fo3nix]

there are two major types of field: scalar fields and vector fields. then as a separate group, you can also have static fields.

now, you must learn what a scalar and a vector is. a scalar is just a number, a quantity like mass. you are x kilogrames. a vector is something with a size (like a scalar), and a direction. an example of a vector is velocity. you travel at 10mph (the scalar bit).. going east (the direction bit). so vectors are like arrows, of a certain length. the length gives them the scalar bit, and the direction of the arrow is obviously the direction of the vector.

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going onto another post...

[fo3nix]

....

imagine a farmer's field, where he's laying seeds to grow. every metre or so, he drops 10 seeds in a clump. and he does this all over his field. now, this is fine for 2-dimensional space, as the field is 2D (just a plane). but our space is 3D (we perceieve it that way anyway). So imagine that this field also extends upwards too... like many, many fields lying on top of each other, up to the sky. this is now a 3D field (you can go up-down, left-right, forwards-back).

now realise that this is a discrete field, as it has discrete values in space. i.e. there are gaps where the seeds are. but space is continuous (so we think...)... i mean, when moving your arm, you can move it any small amount you like. you dont have to move it in increments of 5cm at a time or anything. so because of this property of space, you now need to imagine that there are no real gaps between the seeds. in fact, there may be gaps, but we cant detect them as they're so small.

you've now just imagined a scalar field. this is because at each point in space, there is a scalar (10 seeds, its just a number.. a scalar). of course, the number of seeds at each point in space can be different, they dont have to all be 10.

a vector field is one which at each point in space, instead of there being 10 seeds, there is now a vector, an arrow of a certain length pointing in a certain direction. but remember that this arrow can point in a different direction and have a different length at each point in space. this is a vector field.

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[fo3nix]

....

incidentally, a field (vector or scalar) with the same vector or scalar number at each point in space (like 10 seeds at ech point in space, or the same vector at each point in space, with same length and direction), is called a uniform field.

so these fields depend on position: x, y, z. we call this "r".
so a field f = f(r). i.e. the field "f" depends on "r".

a field can of course vary in time too. at one time, there could be 10 seeds. a second later, a bird could have flown down and now there's only 8 seeds at that point. the field varies in time: f = f(r,t).

now i answer your question: a static field is one which does not vary in time. it is time-independant.

[BigMacnFries]

That is very interesting, however you didn't use the word photon once. I am still perplexed as to how, on a particle physics level, static electric and magnetic fields are realised by photons.

[fo3nix]

well wherever you can imagine a wave propogating through space, imagine a photon moving through instead.

[jake-the-peg]

I think that things are a little more complex than is being said here.

These carier bosons (so called because they transmit the forces) are virtual particles

http://www.herts.ac.uk/astro_ub/aV_ub.html#Virtualp

[fo3nix]

yes, thats true. i think in summary.. its not best to think of electromagnetism in terms of feynman diagrams and electrons and photons. it can be done but will overcomplicate things (hence the use of semi-real fields).

Feynman diagrams are just a type of book-keeping device that Feynman used while working out low-level particle interactions. They're very useful for that kind of thing, but not sure how useful it is for electromagnetism.

[BigMacnFries]

Let me ask the question in a different way. Suppose you have two hydrogen atoms close to one another. One of them can absorb a photon, it's electron moves to a higher energy state, then returns and releases a photon of the same energy. This photon can be absorbed by the other hydrogen atom and the same thing can happen. Scientists can detect the energy or frequency of the photon and corrospondingly the energy states of the electron.

  If you move these two hydrogen atoms closer together they will repel each other. Being an electric force assume something to do with photons is going on here. I also assume that something slightly different is going on than in the first paragraph above as I have never heard of electric fields having wavelength properties and these photons do. My question is what is happening between these two hydrogen atoms as they approach each other?

[jake-the-peg]

This is pushing at the boundries of my knowlege but...

The two hydrogen atoms cannot be thought of as "classical" atoms with solid nuclei and electrons wizzing around in orbits about them.

The electron shells are probability distributions and as the shells get closer to each other there is a greater probability of interaction.

This has to be treated in a quantum mechanical manner and that can be thought of as an exchange of virtual photons.

Virtual particles can pop in and out of existance as long as they do so for a short enough time to to violate the uncertainty principle which can be expressed as:

E.t > h/2 (E, Energy t, time, h, =Plank' s constant)

The following is a good link but to really understand this you probably need a good grasp of Quantum Mechanics

http://math.ucr.edu/home/baez/physics/Quantum/virtual_pa rticles.html

[fo3nix]

jake-the-peg: you're right :)

bigmac: the electromagnetic field is just an exchange of photons between the two atoms. so basically the EM field is just what you've described, but its going on very often, and so with lots of photons.

it can be shown that the equations for electromagnetic fields (the Maxwell equations) take the form of the wave equation in free space, and thus are waves. Thus they do have a wavelength property.