5 ms·

The Discovery of Superconductivity (2010)

59 points by segfaultbuserr 2y ago

ThrowawayTestr 2y ago
Superconductivity is like anti-matter in that it really does like science fiction when you first hear about it.
- segfaultbuserr 2y 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 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.
  - tagrun 2y 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.
    - segfaultbuserr 2y 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.
      - tagrun 2y ago
        To the parent and its sibling comments: There is no atomic or subatomic current that can explain ferromagnetism in any approximation. You read some Wikipedia pages and Feynman lectures of physics. I'm a physicist who has done well over a decade of research in magnetic materials. In understanding of ferromagnetism, many incorrect theories have been proposed. By connecting ferromagnetism to circulating currents (i.e, paramagnetism and diamagnetism), you just repeated the same mistake. You're trying to bend the words to avoid being wrong. Physics is not philosophy or debate club. There is no approximation in physics in which electron is a ball with some radius, or its spin is due to a circulating current in physics. Any such explanation attempt fails spectacularly if you actually try to do the math (which gives an electron surface that is moving faster than speed of light, as Uhlenbeck/Goudsmit who proposed this incorrect idea quickly found out), so it doesn't even work as an approximation of any kind. > Yet the concept of spin in quantum mechanics was originally developed using macroscopic rotations as an analogy, Who developed this theory in quantum mechanics, where and when? Pauli, who first introduced it into quantum mechanics and the namesake of spin 1/2 matrices, insisted that it is purely quantum mechanical with no classical analogue. And regardless of who said what over 100 years ago, today, it is well understood that spin has nothing to with electric charges that move or rotate in space. More importantly, the reason ferromagnetism develops in the first place is due to exchange interaction (as I wrote above) between magnetic moments, which is due to Pauli exclusion principle and also has nothing to do with movement of charges. Furthermore, such magnetic moments (called magnetic impurities in that context) ruin the superconducting order by breaking the time-reversal symmetry, so trying to make a connection to ferromagnetism in the context of superconductivity is even worse.
eternauta3k 2y ago
I wonder what discoveries are taking place now which will give people in 100 years the chills to read about.
- pie420 2y ago
  VR porn
  - dang 2y 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 https://news.ycombinator.com/newsguidelines.html and taking the intended spirit of the site more to heart, we'd be grateful.
- freeqaz 2y ago
  Transformer models perhaps?

ThrowawayTestr 2y ago

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

segfaultbuserr 2y 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 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.
- tagrun 2y 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.
  - segfaultbuserr 2y 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.
    - tagrun 2y ago
      To the parent and its sibling comments: There is no atomic or subatomic current that can explain ferromagnetism in any approximation. You read some Wikipedia pages and Feynman lectures of physics. I'm a physicist who has done well over a decade of research in magnetic materials. In understanding of ferromagnetism, many incorrect theories have been proposed. By connecting ferromagnetism to circulating currents (i.e, paramagnetism and diamagnetism), you just repeated the same mistake. You're trying to bend the words to avoid being wrong. Physics is not philosophy or debate club. There is no approximation in physics in which electron is a ball with some radius, or its spin is due to a circulating current in physics. Any such explanation attempt fails spectacularly if you actually try to do the math (which gives an electron surface that is moving faster than speed of light, as Uhlenbeck/Goudsmit who proposed this incorrect idea quickly found out), so it doesn't even work as an approximation of any kind. > Yet the concept of spin in quantum mechanics was originally developed using macroscopic rotations as an analogy, Who developed this theory in quantum mechanics, where and when? Pauli, who first introduced it into quantum mechanics and the namesake of spin 1/2 matrices, insisted that it is purely quantum mechanical with no classical analogue. And regardless of who said what over 100 years ago, today, it is well understood that spin has nothing to with electric charges that move or rotate in space. More importantly, the reason ferromagnetism develops in the first place is due to exchange interaction (as I wrote above) between magnetic moments, which is due to Pauli exclusion principle and also has nothing to do with movement of charges. Furthermore, such magnetic moments (called magnetic impurities in that context) ruin the superconducting order by breaking the time-reversal symmetry, so trying to make a connection to ferromagnetism in the context of superconductivity is even worse.

eternauta3k 2y ago

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

pie420 2y ago
VR porn
- dang 2y 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 https://news.ycombinator.com/newsguidelines.html and taking the intended spirit of the site more to heart, we'd be grateful.
freeqaz 2y ago
Transformer models perhaps?