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The Discovery of Superconductivity (2010)

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ThrowawayTestr5 months ago

Superconductivity is like anti-matter in that it really does like science fiction when you first hear about it.

segfaultbuserr5 months ago

Knowing superconductivity makes magnets less mysterious. Once you accept that physics absolutely allows the creation of a static magnetic field from a circulating current that flows forever in a zero-resistance inductor coil, then the existence of ferromagnetism is no stranger than that - to a first approximation, it also comes from circulating currents, "just" on a subatomic scale. [1] It's kind of surprising that the Atomic Current Hypothesis of ferromagnetism was already proposed by Ampere back then. Following the same heuristics, the fact also becomes clear that the energy in an inductor coil can't really be "spent" to do useful work forever without de-energizing it, and the same is true for permanent magnets. [2]

[1] https://www.feynmanlectures.caltech.edu/II_36.html

[2] This intuition debunks many types of incorrect "infinite energy of magnets" ideas that lead to perpetual motion. Although it can't debunk the "perpetual motion solely from an uneven static (electromagnetic or gravitational) field" idea, which is even older.

tagrun5 months ago

Ferromagnetism has nothing to do with currents, it is due to aligned spins of partially filled shells. Below a certain temperature (Curie temperature of the material), exchange interaction (which penalizes any misalignment, in the case of ferromagnetic exchange interaction) between electrons leads to this alignment.

Spin is a type of intrinsic angular momentum that is not associated with any spatial motion.

The Feynman lecture you linked to is an explanation why currents fail to explain ferromagnetism. You need to read the next chapter, but being a lecture for undergrads, it doesn't go deep into the subject anyway. If you're really interested, any modern book on magnetism would be much helpful.

segfaultbuserr5 months ago

You said,

> Ferromagnetism has nothing to do with currents

This is why I said ferromagnetism is circulating current in the sense of "to a first approximation" and "heuristically". Wiktionary defines "heuristic" to be:

> a practical method [...] not following or derived from any theory, or based on an advisedly oversimplified one.

I think that if you ask Feynman, he would probably agree or sympathize with the naive idea of "atomic currents" as a heuristic argument in the introduction of this topic... which is nothing new anyway, and has been a heuristic argument used in electromagnetism for a long time, at least before QM.

In Feynman's own words,

> These days, however, we know that the magnetization of materials comes from circulating currents within the atoms—either from the spinning electrons or from the motion of the electrons in the atom. It is therefore nicer from a physical point of view to describe things realistically in terms of the atomic currents [...] sometimes called “Ampèrian” currents, because Ampère first suggested that the magnetism of matter came from circulating atomic currents.

You said,

> Spin is a type of intrinsic angular momentum that is not associated with any spatial motion.

Yet the concept of spin in quantum mechanics was originally developed using macroscopic rotations as an analogy, although today we know that spin is an intrinsic property of subatomic particles (thus the joke, "Imagine a ball that is spinning, except it is not a ball and it is not spinning.") In the same sense that Ampère's concept of "atomic currents" was developed using circulating electric current as an analogy.

> The Feynman lecture you linked to is an explanation why currents fails to explain ferromagnetism. You need to read the next chapter.

Of course, "The actual microscopic current density in magnetized matter is, of course, very complicated." This is surely explained in the next chapter. I could've mentioned "atomic currents" without citing any link, but I included it to allow anyone who's interested to read the whole thing in context.

+1
tagrun5 months ago
adrian_b5 months ago

What you say is correct only when you adopt certain specific narrow definitions of the words, which you have not explained.

In its original sense, an electric current is any kind of movement of electric charge. In this wide sense, it also applies to the source of ferromagnetism.

Its meaning can be restricted to refer to the translational movement of electrically charged particles. With this narrower sense, there is still no need to use quantum mechanics to explain ferromagnetism. Even in classical electromagnetism, with the narrower-defined current, the sources of magnetic fields are decomposed into distributions of electric current densities and of magnetic moment densities, where the latter are the source of ferromagnetism. If necessary, it is possible to also use distributions of higher-order moment densities and the series of moments when the "electric current" is used in the narrow sense (of a first order moment) corresponds to the "electric current" used in its original, wide sense.

The isolated sentence "Spin is a type of intrinsic angular momentum that is not associated with any spatial motion" is logically contradictory (because, by definition, angular momentum is a characteristic of moving bodies). It can be correct only when you first specify that by "spatial motion" you mean only a certain kind of spatial motion.

The joke mentioned by another poster "Imagine a ball that is spinning, except it is not a ball and it is not spinning" is just a joke, because there is no doubt that the elementary particles are spinning.

Even when you model the elementary particles in the standard way, as point-like bodies (and it is debatable whether this is a good model), you cannot say that they are not rotating, because this would be the same mistake as saying that a delta distribution has a null value in the origin.

On the contrary, while you cannot say other things about the value of a delta distribution in the origin, what you can say with certainty is that it is not null.

In the same way, while you cannot say anything about characteristics of an electron like radius, mass density, angular velocity, electric current density and so on, you can say with certainty the values of various integral quantities (which integrate the corresponding delta distributions), like mass, electric charge, angular momentum and magnetic moment, so you can say with certainty that any electron is rotating (i.e. it has a non-null angular momentum).

superposeur5 months ago

As other commenters have said, whether or not an electron’s magnetic moment is “to do with currents” is a little open to interpretation.

I’ll add that the Dirac equation (governing electron field) correctly predicts magnetic moment given the inputs of charge and mass. * I interpret this as indicating that magnetic moment is a derived phenomenon just as it would be in the classical picture of a spinning ball of charge; I.e. the quantum picture refines but does not totally discard the classical understanding.

* Well, technically, sympathetic vibrations with all the other standard model fields also make tiny contributions to the magnetic moment.

eternauta3k5 months ago

I wonder what discoveries are taking place now which will give people in 100 years the chills to read about.

freeqaz5 months ago

Transformer models perhaps?

pie4205 months ago

VR porn

dang5 months ago

Could you please stop posting unsubstantive comments and flamebait? You've unfortunately been doing it repeatedly. It's not what this site is for, and destroys what it is for.

If you wouldn't mind reviewing https://news.ycombinator.com/newsguidelines.html and taking the intended spirit of the site more to heart, we'd be grateful.