How DO magnets work?

Sophia Collins, the organizer of I’m A Scientist, recently posted a blog on one of the questions we faced, which provoked a lot of discussion at the time: “how do magnets attract and repel?” (posted by lucasjacobs) . This is a perfectly reasonable question, but it turns out to be difficult to answer. You can read my answer here (comments are very welcome – I don’t think I’ve had any comments on this blog yet, so perhaps they don’t work you can see they work, below – thanks Adam). Sophia’s blog was pretty accurate – in essence we all struggled to answer the question, and after some discussion (e.g., on twitter and with colleagues) we realised that it was not just us, but that magnetism was indeed very hard to explain properly without jargon.

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However, Sophia did say this (referring to me):

“Now these scientists include a man who spends many of his days working with an fMRI scanner, containing a magnet so powerful that you have to remove any ferrous object from your person before entering the room. But still, magnetism turned out to be one of those things that he learned about years ago, and sort of assumed he understood.”

Now this is not quite true. The true part is the embarrassing fact that I don’t understand magnetism, the false part is that I had assumed I understood – in fact I was very much aware of, and troubled by, the gap in my understanding. I dropped Physics after my O-level, so perhaps it wasn’t to surprising that I had struggled. But it was also a challenge to the other scientists in my group, all of whom, I believe, have Physics PhDs. I use magnetic resonance imaging (MRI) in my research, I teach MRI at postgraduate level, and I am therefore very interested in magnets and how they work. Over the years I’ve questioned many physicists about magnetism (often to the point of irritation). Although they can generally describe many magnetic phenomena I’ve often come away with the impression that there is a “gap” in the explanation.

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In trying to understand this I turned first of all to Richard Feynman who as well as having a hand in the theories that most accurately describe magnetism, was also a great communicator. Feynman’s book “QED: the strange theory of light and matter” brilliantly explains a range of electrical and optical phenomena for a lay audience, avoiding classical physics and using only a (non-mathematical but accurate) description of quantum mechanics. Unfortunately, the QED book doesn’t say much about magnetism.

Here is a clip of Feynman trying to answer a layman’s question about magnetism:

I think the reason he struggles is because he recognizes how difficult the question is, and doesn’t want to fob the interviewer off with an incomplete or misleading answer. Feynman thought a lot about the nature of science and explanation, and he explicitly understood that renaming or describing a phenomenon did not constitute any kind of explanation. When asked to give a talk to a group of science teachers on the subject “What is Science?” he stressed this. I strongly recommend reading the whole talk, but I quote a relevant passage which shows how deeply ingrained this principle was in his psyche – he had learned the lesson from his father, as a child during a trip to the Catskill mountains:

“… this boy said to me, ‘See that bird standing on the stump there? What’s the name of it?’
 
I said, ‘I haven’t got the slightest idea.’
 
He said, ‘It’s a brown-throated thrush. Your father doesn’t teach you much about science.’

I smiled to myself, because my father had already taught me that [the name] doesn’t tell me anything about the bird. He taught me ‘See that bird? It’s a brown-throated thrush, but in Germany it’s called a halsenflugel, and in Chinese they call it a chung ling and even if you know all those names for it, you still know nothing about the bird–you only know something about people; what they call that bird. Now that thrush sings, and teaches its young to fly, and flies so many miles away during the summer across the country, and nobody knows how it finds its way,’ and so forth. There is a difference between the name of the thing and what goes on.”

To return to magnets, I never did hear a clear explanation of how magnets attract and repel one another. For me, a clear explanation would have to be consistent with other mechanistic explanations of electrical and optical phenomena. In his lecture on the topic, Feynman is emphatic that although carefully selected magnetic phenomena can be explained in classical terms, these examples are misleading, in that the intuitions that flow from them are wrong.

I can see that Maxwell’s field equations which describe electrical and magnetic fields in space and time are incredibly useful, and sufficient for many applications of magnetism – talk about fields and lines of force doesn’t really constitute an explanation. Although these concepts go far beyond merely “naming” or even describing magnetism, they don’t really explain what it going on. It is my understanding that, ideas like “fields” and “force” are nowadays thought to summarize more fundamental processes, similar to those covered in QED. From reading this and other books I’ve learned that I have to accept that there are some mechanisms in nature that cannot be understood in terms of the phenomena we encounter in everyday experience. But my understanding is that the mechanism of magnetism is well explained by current theories. It’s just that so far, no one has been able to explain it to me. Doubtless, some of the problems are at my end, but I wonder if there is any physicist out there who is willing to take this challenge seriously and try to explain how magnetism works in a self consistent way without using jargon – Feynman’s QED book suggests that this is not impossible.

The best explanations I have heard so far relate to the virtual exchange of photons (magnons?), and this is the answer I tried to give on the I’m A Scientist website. But, there is something very odd about these virtual particles, in that they clearly influence other magnets and electrons in nearby wires etc. but they do not show up in other ways (for example as light). And the magnetism will only work if they are exchanged very precisely with one another (in time and space) as if each magnet somehow knows where the other one is! Or perhaps I have misunderstood something – after all I only have O-level Physics, and perhaps it is presumptuous to even hope for an explanation I can understand. Feynman was a true genius, and perhaps I have fallen victim to the spell of his charisma; he made me think I could hope to understand some of this stuff, and I am not giving up yet.

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4 thoughts on “How DO magnets work?

  1. Excellent post – much in the spirit of Feynman. My PhD was about the popularisation of physics. I concluded that many physicists adopt the role of a ‘shaman’ – they see themselves as the intermediaries not between the spirit world and the rest of us but between the mysterious world of quantum mechanics and the rest of us. The more baffling they are the better they feel they are representing the mysteries of the universe. In contrast, other physicists (and Feynman is an excellent example here) realise that to speak about the ‘world’ is not enough by itself; you also have to speak about ‘what you can say about the world’. In QED The Strange Story of Light and Matter Feynman invents an entirely new formalism in order to speak directly to his non-physicist audience. (It’s a bit like Lego developing a new programming language to allow kids to control the microprocessor in its MindStorms products. It works, but it’s probably not the way you’d do it if you were a qualified firmware developer.)Description comes much more easily to physicists than explanation, and much of the time description is all that is required. I’ve got a degree in physics and so could probably describe how magnetic fields arise and how particles behave in magnetic fields with greater sophistication than Tom Hartley with his O-Level physics (he doesn’t look old enough to have an O-Level!) But explanations have to meet the expectations of the person to whom you are explaining. From my own experience, what counts as an explanation for a physicist does not always count as an explanation for a biologist, and vice versa.’Can you describe magnetism?’ and ‘Can you explain magnetism in a way that remains consistent with its description by physicists?’ are two different challenges. Because the latter question is audience dependent, physicists are not always going to rise to it better than others. I’m glad to see psychologists / neurologists giving it a go.

  2. Thanks for the comment Adam. I don’t have time to make a cogent reply at the moment, but its great to know that the comments work, and to have your input. Other readers, feel free to comment!

  3. Thank you for this post! I have always been fascinated by magnetism, but like Adam above, the explanations given to me have been in the context of what the effects are. I’ve never read a good explanation of what exactly a magnetic field *is*, only that the motion of charge within a material produces it. I also have to agree with you, Tom, that the virtual particles explanation is fairly difficult to accept. It feels like the “holes” and “poles” explanation given for the function of a transistor many years ago. The explanation was technically incorrect, but it worked well enough to describe the perceived effects and make some very basic predictions of its behavior.For me, as a layman who loves Physics, magnetism is just as intriguing as the relationship between velocity and time. I cannot intuitively understand how moving quickly or slowly through the Universe affects the flow of time. Even at fairly low speeds (satellites orbiting the Earth) the relative difference in time is measurable. Perhaps it’s due to my lack of knowledge in quantum mechanics, but I find these sorts of things absolutely fascinating.Thanks again for the fun read. 🙂

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