Why can't there be magnetic monopoles?

[steevey]

Couldn't I just make something out of pure electrons or protons? I'm sure there's some force in the universe that can do that, There's stuff thats made out of only neutrons and its still being held together. Otherwise more classically, I understand that when you cut a magnet, the left over pieces will have north and south poles, but why always every time? And what would it take to make a monopole?

Edited December 2, 2010 by steevey

[Cap'n Refsmmat]

Couldn't I just make something out of pure electrons or protons? I'm sure there's some force in the universe that can do that, There's stuff thats made out of only neutrons and its still being held together. Otherwise more classically, I understand that when you cut a magnet, the left over pieces will have north and south poles, but why always every time? And what would it take to make a monopole?

 

You can certainly make something out of pure electrons or protons, but that'll give you an electric monopole, not a magnetic monopole. Magnetic fields are caused by moving charges, and have a north and south pole regardless of whether you use electrons or protons to make them.

[steevey]

You can certainly make something out of pure electrons or protons, but that'll give you an electric monopole, not a magnetic monopole. Magnetic fields are caused by moving charges, and have a north and south pole regardless of whether you use electrons or protons to make them.

 

What's wrong with a pure positive moving charge?

[Cap'n Refsmmat]

What's wrong with a pure positive moving charge?

 

A purely positive moving charge is just fine, but it'll generate a magnetic field like one in a wire:

 

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html

 

That is to say, you'd generate a circular magnetic field. It won't be a north or south pole by itself.

[steevey]

A purely positive moving charge is just fine, but it'll generate a magnetic field like one in a wire:

 

http://hyperphysics....tic/magcur.html

 

That is to say, you'd generate a circular magnetic field. It won't be a north or south pole by itself.

 

So then you can have a magnetic monopole?

[Mr Skeptic]

Well if you do find a magnetic monopole somewhere, I'm pretty sure I could make a perpetual motion machine from it and a magnetic dipole (which we do have). Or if you could find me a fundamental electric dipole (not one made of two monopoles). I'd say the reason these don't exist have something to do with the law of conservation of energy.

[Cap'n Refsmmat]

So then you can have a magnetic monopole?

 

No.

 

Magnetic field lines are drawn to emanate from the North pole and enter into the South pole. (Or maybe the reverse -- it depends on convention.) Look at the diagram of a bar magnet's field lines and you'll see them come out of the North, go around, and re-enter the South.

 

A magnetic monopole would be depicted as field lines coming out of a point and then going off forever, for they'd have no south pole to go to.

 

On the other hand, a current in a wire, or a single moving charge, just makes a field line that goes around in a closed loop.

[michel123456]

(...)

A magnetic monopole would be depicted as field lines coming out of a point and then going off forever, for they'd have no south pole to go to.

(...)

 

Replace the words "magnetic monopole" with the words "gravitational field", and it fits exactly.

 

P.S. I'd like to vote for your post #7 at least 10 times, but I can't.

Edited December 2, 2010 by michel123456

[ajb]

I don't think that is obvious that magnetic monopoles* do not exist when you look a Maxwell's equations. In Vacuum there is a symmetry (E, B ) -> (B,-E). This is the very well-know electromagnetic duality, or Montonen-Olive duality.

 

Now, if we have an electric charge then Maxwell's equations no longer have this symmetry, unless they are modified to include a magnetic charge.

 

Thinking pure mathematically "suggest" that magnetic monopoles should be realised.

 

A very important result is the Dirac quantisation of electric charge. This relies on magnetic monopoles. In short, if magnetic monopoles exist then electric charge is quantised. To date I know of no other explanation for the quantisation of electric charge.

 

The big problem is magnetic monopoles created in the early universe are cosmologically significant. More than this, they should have been created in huge amounts and we just don't see them. Inflationary models propose to explain this by diluting them to maybe a few per horizon.

 

We would only need one in our horizon to explain the quantisation of electric charge! I am not sure about the experimental status, such as the bounds on the density of monopoles in our horizon, their mass etc. I am sure a quick "google" would pick something up.

 

 

 

 

 

 

*we will think about them in just EM theory, various supersymmetric gauge theories also have them.

[Mr Skeptic]

Replace the words "magnetic monopole" with the words "gravitational field", and it fits exactly.

 

There's plenty of gravitational monopoles (what with there being only one gravitational charge). But that's absolutely not the same as a magnetic monopole, since you don't get an electric field from moving a gravitational monopole.

[TonyMcC]

In a sense perhaps you could say that neither north or south poles exist? Either that or each atom when aligned has its own north and south pole. What I am getting at is that the magnetic lines of force are considered continuous loops. where they are trapped inside magnetic material they have no, or little, effect but where the field breaks into the air and the effect is noticeable we say a pole exists. If I put two bar magnets in contact end to end NSNS how many poles have I actually got?