# I don't understand how magnetism works at a distance.

Probably not what you’re after, but cooler than iron filings – ferro fluid

As long as the electrons aren’t touching, it’s not gay.

I’ll walk away now.

Does it make sense to talk about the frequency of a magnetic field? Can I make a radio antenna magnetic by sending the right signal to it?

I know that’s what your link says, but it’s wrong. The link’s field strength versus distance data doesn’t get far enough from the magnet to show this. Magnetism, because it’s always a dipole, drops off with the cube of distance. The electric field of an isolated charge, or the gravitational field, drop off with the square of distance. (Electric dipoles also drop off as the cube of distance.)

The weak and strong nuclear forces do drop off exponentially. This is because their force carriers have mass, unlike photons and gravitons which are massless .

Yes to the first, although a varying magnetic field will always have a varying electric field associated with it.

I don’t really understand the second question.

I mean turning a radio broadcasting antenna into a big magnet. If it’s all photons and magnetic fields have a frequency, then it ought to be possible.

Well, a big magnet will have a magnetic field with a frequency of zero. A radio broadcasting antenna works at AC, in the KiloHertz range for AM, MegaHertz for FM. So no, it’s not really possible.

If you take a loop antenna, and run electricity through it at DC, you can make an electromagnet.

So, dropping off with the cube of distance isn’t “exponential” enough for you? Inverse square, inverse cube, it still all 'drops off exponentially". Right?

>So, dropping off with the cube of distance isn’t “exponential” enough for you?

It isn’t exponential enough for anybody. Dropping off exponentially with distance looks like:

force = constant ^ distance

Dropping off with the cube of distance looks like:

force = constant * distance^3

Although, if you had a magnetic monopole[sup]*[/sup], it would have a 1/r[sup]2[/sup] falloff, just like electrostatics and gravity.

• As my advisor puts it, we know that magnetic monopoles exist. There might just be a very small number of them in the Universe, like zero.

So magnets are interacting by emitting low energy photons? Where do these come from? Wouldn’t the energy of the solid diminish as these photons are emitted? Did I miss something in this discussion?

“Low energy” isn’t really the right term to use. Some of them are high energy, some of them are low energy, some of them are zero or negative energy, and the average energy of all of them is zero. A negative energy photon is of course absurd, but since they’re only virtual photons, not real ones, nobody cares.

:smack:

You’re right, obviously. I was just thinking ZenBeam was comparing magnetic force to electrostatic force. After reading more carefully, I see she(he) was comparing electricity and magnetism to the weak and strong forces. If I had read right the first time, I probably wouldn’t have made that mistake.

I know what exponential means. Confusing it with “something that has an exponent in it” is one of the stupider things I’ve done on this board. Ah well. Time fixes all mistakes by making people forget them.

PS: I can’t redeem myself by nitpicking, but I’ll try anyway:
>Dropping off exponentially with distance looks like: force = constant ^ distance

That’s growing exponentially. An exponential decrease would look like:

force = constant^-distance

I know. It was a small contribution, but there ya go.

In either case, though, you need some other constant in the exponent with units of inverse length. Put that constant in, and you can just absorb the minus sign (if any) into the constant and use the same formula for growth and decay.

>one of the stupider things I’ve done on this board

>That’s growing exponentially. An exponential decrease would look like:
>force = constant^-distance
ummm… obviously, I was picturing us facing one another having this conversation, and so of course defined distance with the sign opposite to what you were using.

>there ya go.

Exactly. Exactly.

I don’t get it.

When I hold my two little magnets 6 inches apart and feel them resisting each other, you’re saying there are actually photons being exchanged at that distance?

I’m baffled by where the energy comes from. When I try pushing together the like poles of two magnets I can feel a lot of resistance pushing (I assume) the magnets apart. Are the magnets losing a small amount of mass to create that energy? It absolutely stunning to think that photons carry enough energy to push the magnets apart like that. (I totally believe it’s the work of photons–it’s still crazy.)

Not actually, virtually, but basically, yes. The strange thing about virtual particles is that they don’t appear as particles anywhere but in the mathematics; what we experience because of them is what we usually call forces or interactions.

This is what’s called a Feynman diagram. In those, outgoing legs are real particles, and connecting lines – the propagators – are virtual ones. Since they don’t ‘leave’ the diagram, they cannot be independently detected. What’s shown in this diagram is the exchange of a momentum k between the particles p[sub]1[/sub] and p[sub]2[/sub]; what you would observe in the real world is both particles changing their trajectory in accordance with Newton’s 3rd law – i.e., they have exerted a force upon each other.

Virtual particles are allowed a bit more leeway in physics than real ones, most importantly they need not lie on the mass shell, i.e. they need not satisfy the equation m[sup]2[/sup]c[sup]4[/sup] = E[sup]2[/sup] - p[sup]2[/sup]c[sup]2[/sup], which means that their kinetic energy does not have the usual relation to their velocity and is basically due to them existing only for a limited time and space and the uncertainty principle.
This, I think, is what allows them to ‘average to zero’, as Chronos already said, so the magnets don’t have to lose mass in order to attract/repulse each other.

>I’m baffled by where the energy comes from. […] Are the magnets losing a small amount of mass to create that energy? It absolutely stunning to think that photons carry enough energy to push the magnets apart like that.

There is energy in the magnetic field. In your example, since you push the magnets together, you are actually adding energy, not removing it. But in any case changes in the mechanical energy elsewhere, like letting two magnets jump together to pull a string and lift a weight, are moving some finite amount of energy around in the system. The most you could do is demagnetize the magnet, spending the last of that energy, but I don’t know how to do that simply by letting the magnet attract things. They did put some energy into it during its manufacture, to give it that field.

Not quite. As long as there isn’t any PHYSICal arousal [del] resultant[/del] resulting from the touching, it’s not gay.

And I will now** run** away! - “Jack”