What are magnetic monopoles and how do they work?

What are magnetic monopoles and how do they work?

This is actually not a simple question. The short answer is that magnetic monopoles would be very interesting if they did exist; but despite a decades long, very serious hunt, none have ever been found.

You may have heard of protons, individual particles that are positively charged; and electrons, individual particles that are negatively charged. If you've ever petted a cat on a dry day or put on a skirt fresh out of the dryer, you may even have built up an electric field, which might have made the cat's fur stand on end, and you might have felt a funny tingling in your finger tips or had your skirt show static cling. This is the kind of thing that can happen when you get an excess of positive or negative electric field charges on an object; like charges repel, so if each cat hair is positively charged, it tends to repel its positively-charged neighbor hairs.

If you recall an old CRT TV (cathode-ray tube); well, the cathode ray is a beam of electrons, each one carrying a negative charge; constantly hitting the screen. If you ever touched an old CRT screen while it was on, you felt that static charge; that is a consequence of the fact that the glass of the screen was saturated with an excess of negatively charged electrons from the cathode-ray beam!

So we understand that electric charges can be on particles, all by their lonesome, and you can beam these particles around, and the charges transfer to objects, and so objects can be charged up with positive or negative electrical charges.

Now a magnetic field is intimately associated with this kind of electric field. In fact, a magnetic field is always generated when an electric field is changing over time. But wherever we find a place in this field that is a magnetic 'north', we find that there must be a magnetic 'south' somewhere nearby. We just don't find any magnetic 'norths' without a 'south' connected to it.

Metal bar and U magnets are a special case of the behavior of electromagnetic fields in certain metal media; they are really peculiar things and if Feynman can't adequately explain them to an adult then I certainly don't think I could do better for a 5 year old.

So we've observed that we just don't find 'norths' floating around in space, or anywhere else; the way that we do in fact find negatively charged electrons floating around in space. Mathematically this is expressed as one of Maxwell's equations which are four equations that describe very, very fundamental statements about the physical reality of our universe. (The particular equation in question, Gauss' law of magnetism, basically is a fancy mathematical way of saying that magnetic monopoles do not exist, can never exist.)

Why do we even talk about magnetic monopoles, then? Well, it's because it's deeply unsatisfying for a lot of math and physics people that the laws describing electric fields are not symmetric to the laws describing magnetic fields. If in fact isolated magnetic 'norths' and 'souths' - monopoles - really did exist, then the equations describing electric fields would be exactly symmetric to the equations describing magnetic fields, and that would make a lot of other equations much more 'clean' and easy to deal with, for the math-y and physics-y people who like to describe the world by dealing with those type of equations.

Another reason we still talk about monopoles is that they're not forbidden by all the laws of physics that we've managed to discover. We know nothing can travel faster than c, the speed of light. We know that mass-energy can't be created or destroyed. We know an object in motion tends to stay in motion and an object at rest tends to stay at rest. We know a heck of a lot of things about the real world.

None of the things that we know seem to say that magnetic monopoles can't exist! It's just an observation that's been made. And the law that came out of this observation, Gauss' law for magnetism, is an incredibly important foundation block for a lot of physics that came after it.

For all we know, magnetic monopoles could exist. In that case, a lot of smart physicists have gotten a lot of things totally wrong in the Standard Model of physics - it would change everything. But very clever people have devised very clever experiments to hunt for magnetic monopoles - the hunt has been on for nearly 100 years - and none have ever been found.

Every so often, by the way, some quirky low-temperature quantum physics experimentalist will do something with a flux tube or a Bose-Einstein condensate and a journalist will pick up the story and write about magnetic monopoles. To explain like you're five: these journalists are idiots who don't know what they're talking about. They ought not to write such articles; they are muddling up two completely different topics in a very confusing way.