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Quanta Magazine's The Hidden Magnetic Universe Begins to Come Into View is a fascinating review of a rapidly evolving field in astronomy.

It contains some statements that I feel are important to understand, but I don't.

However, once a “seed” magnetic field arises from charged particles in motion, it can become bigger and stronger by aligning weaker fields with it. Magnetism “is a little bit like a living organism,” said Torsten Enßlin, a theoretical astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany, “because magnetic fields tap into every free energy source they can hold onto and grow. They can spread and affect other areas with their presence, where they grow as well.”

Ruth Durrer, a theoretical cosmologist at the University of Geneva, explained that magnetism is the only force apart from gravity that can shape the large-scale structure of the cosmos, because only magnetism and gravity can “reach out to you” across vast distances. Electricity, by contrast, is local and short-lived, since the positive and negative charge in any region will neutralize overall. But you can’t cancel out magnetic fields; they tend to add up and survive.

Questions:

  1. What does the phrase "...magnetic fields tap into every free energy source they can hold onto and grow..." refer to in the astronomical context of interstellar space?

  2. What does the phrase "...only magnetism and gravity can “reach out to you” across vast distances. Electricity, by contrast, is local and short-lived, since the positive and negative charge in any region will neutralize overall. But you can’t cancel out magnetic fields; they tend to add up and survive." refer to in the context of interstellar space? Is it the lack of free magnetic monopoles, or the dearth of polarizable ferromagnetic material?

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What does the phrase "...magnetic fields tap into every free energy source they can hold onto and grow..." refer to in the astronomical context of interstellar space?

Interstellar space is filled with charged particles/dust which have their own magnetic fields (e.g., their own dipole moment or any field induced by the charge moving). When you apply a larger, more global magnetic field to these small charges, very often an alignment occurs (e.g., the particles dipole will align with the larger field, or the motion will suddenly change to move along the field line). This causes these smaller fields to add to the larger field, making it even larger. This is a foundational assumption for academics which study light polarity in terms of dust grain alignment via Radiative Torque (RAT) - namely that the background global magnetic field of interstellar space aligns charged grains with that field, thus further contributing to the overall field.

What does the phrase "...only magnetism and gravity can “reach out to you” across vast distances. Electricity, by contrast, is local and short-lived, since the positive and negative charge in any region will neutralize overall. But you can’t cancel out magnetic fields; they tend to add up and survive." refer to in the context of interstellar space? Is it the lack of free magnetic monopoles, or the dearth of polarizable ferromagnetic material?

If you take a large, galactic magnetic field and it passes through an interstellar dust cloud that is full of charged particles, those charged particles will align with the field and overall add to it, making the galactic field stronger in that area. The only thing that could weaken the magnetic field is something which could cancel it.

Electric fields can easily be cancelled because an electric field being radiated out by a proton can easily be cancelled by the "opposite" field being radiated out by a bunch of nearby electrons (hence the concept of the Debye length). Magnetic fields are harder to screen and, as you say, because the things that could screen them just don't exist in space.

I would also mention that if monopoles even existed, it would require that you could have two separate flavors of monopoles such that they could cancel each other out much like the positive and negative charges of electrons. A logical pick for flavors would be a "north" monopole and a "south" monopole, but I could imagine one constructing a more complicated, yet valid system. Since we've never seen a monopole, any monopole-based models and purely hypothetical at this point.

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