Magnetic monopoles are beyond me

[PeterPeter]

How can you possibly have a magnetic monopole? The field lines must terminate somewhere, surely!

I heard about them on a cosmology video on YouTube.

 

[Nugatory]

That's a good reason not to get your physics from videos on YouTube.

But if you are going to indulge, at least give us the link so that we can tell you whether that particular video is any good. In this case, I'm skeptical because there are no magnetic monopoles.

 

[DrDu]

How can you possibly have a magnetic monopole? The field lines must terminate somewhere, surely!

field lines from a magnetic monopole may end on a magnetic monopole of opposite magnetic charge.

 

[PeterPeter]

It was sometime ago so I don't recall the exact video. I did attempt to watch the early Cosmology Lectures by Leonard Susskind so it was probably mentioned in one of those. Sorry, I can't be more specific.

 

[dauto]

That's a good reason not to get your physics from videos on YouTube.

But if you are going to indulge, at least give us the link so that we can tell you whether that particular video is any good. In this case, I'm skeptical because there are no magnetic monopoles.

Not so fast. The standard model predicts the existence of magnetic monopoles. Look up "T hooft Polyakov monopole"

 

[Nugatory]

Not so fast. The standard model predicts the existence of magnetic monopoles. Look up "T hooft Polyakov monopole"

OK, OK, if the video in question is discussing this hypothetical possibility I'm not so skeptical.

However, I stand by the heuristic value of my comment about not getting one's physics from youtube videos

 

[Khashishi]

Magnetic field lines don't terminate. They go in loops.
Magnetic monopoles are possible in the sense that they aren't ruled out by our known laws, but we haven't found them so they probably don't exist.

 

[DrDu]

Magnetic field lines don't terminate. They go in loops.

No, magnetic field lines can start and end on magnetic monopoles as $\nabla \cdot B=\rho_\mathrm{m}$, where $\rho_\mathrm{m}$ is the magnetic monopole density. Only if the latter is zero, the magnetic field is divergence free.

 

[dauto]

Magnetic field lines don't terminate. They go in loops.
Magnetic monopoles are possible in the sense that they aren't ruled out by our known laws, but we haven't found them so they probably don't exist.

"We haven't found them so they probably don't exist" isn't very rigorous logic. In fact, based on theoretical arguments, they probably do exist but are very rare and too massive to be produced in colliders.

 

[PeterPeter]

Still Puzzled!

No, magnetic field lines can start and end on magnetic monopoles.

1. Wouldn't this mean that there are as many north monopoles as there are south monopoles? In other words they come in pairs?

2. Suppose then there were just two monopoles in the (multi)universe, wouldn't this mean that the field would look identical to that of a (gigantic) bar magnet?

 

[DrDu]

1. Wouldn't this mean that there are as many north monopoles as there are south monopoles? In other words they come in pairs?

If the universe is closed, yes.

2. Suppose then there were just two monopoles in the (multi)universe, wouldn't this mean that the field would look identical to that of a (gigantic) bar magnet?

Yes, up to the flux inside the magnetic bar.

 

[PeterPeter]

1. So, if the universe is closed, there would seem to be no point in talking about monopoles. We could just talk about gigantic magnets?

2. And if the Universe is not closed, presumably one pole could move so far away from us that a light signal couldn't reach us from it (because of the finite age of the universe) and so we'd be left with just a single pole?

 

[DrDu]

1. No, there is still a difference between a usual, although gigantic, magnet and a monopole, namely that there is some coil or iron bar joining the two poles in a magnet while the monopoles are isolated objects.
2. Even in a closed universe, there may be monopoles so far away that light from them hasn't reached us.
The point is that with a closed universe and an unbalanced number of monopoles the field strength would become infinite as the field lines emanating from one uncompensated monopole could end nowhere and would therefore fill all space.