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mfukar t1_j9or9az wrote

Reply to comment by Ethan-Wakefield in What does it mean for light to be an excitation in the electromagnetic field? by Ethan-Wakefield

An antenna is one or more conductive elements electrically connected to a receiver or transmitter. What does a black hole have to do with anything?

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Ethan-Wakefield OP t1_j9oz4l5 wrote

Suppose the following:

I throw a permanent magnet (a chunk of iron or something) into a rotating black hole. The black hole has enormous mass, and it has to conserve angular momentum. So, as the chunk of iron falls into it, it should rotate. And if the magnet is very small, it should rotate very quickly (again, to conserve angular momentum).

My questions are:

  1. As the magnet rotates, does it emits EM radiation?

  2. If the magnet is emitting EM radiation, is it an antenna? Is it more-or-less a broadcast antenna that's powered by the black hole?

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numatter t1_j9q6jl3 wrote

A stationary magnet has a static (fixed) "field" around it. In a perfect vaccuum, it doesnt emit any photons, because theres nothing interacting with it that would cause it to become "excited" and emit EM radiation.

A rotating magnet, regardless of being near a black hole, produces a dynamic (changing) magnetic field, and since those interactions contain information (quanta), we're all familiar with something heating up - which are photons in a spectrum we can't see with the human eye. Given enough time, in theory, the entire mass of the magnet would eventually be irradiated outward as light energy.

Lenz's law provides details on extracting energy from this very thing. You can spin the magnet around an electrically conductive material, like an iron nail. The nail becomes excited by the photons, and electrons start moving around (electricity). Alternatively, you can shake or spin the iron nail around a permanent magnet and produce the same thing via induction.

Your question in regards to a black hole is quite intriguing. Yes, a rotating magnet can act as a broadcast antenna, but its not very efficient to do so because magnets and all ferromagnetic materials have the uncanny ability of picking up and amplifying extra signal "noise" (ingress). What's fascinating to think about, is that a magnet in orbital motion around a black hole would indeed act as a "pickup" in exactly the same way a guitar pickup works. All these frequencies being thrown around together would be picked up and turned into the electrical signal version of the EMFs, then be induced back into the magnet in a Jimi Hendrix-esque feedback loop.

So when you're saying a "magnet" is spinning around a black hole, don't imagine a handheld rectangle magnet. Imagine instead that it's a magnetar... I mean... I can't even fathom the magnetic power of one of these alone, much less being involved in a stellar dance off with the energy source of a black hole. If something like a pulsar/magnetar were to be energetically involved with each other, theoretically you could place the magnetar in an energy jet stream of the black hole, and it would emit a "fucking huge" broadcast signal of their blended frequencies... may I say, "spacetime modulation?" Hmm...

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sticklebat t1_j9y9092 wrote

> Given enough time, in theory, the entire mass of the magnet would eventually be irradiated outward as light energy.

I don’t think this would happen. I think the spinning magnet would preferentially emit light with polarizations that would slow down the magnet’s rotation over time, until it’s no longer spinning. I think hardly any of the magnet’s mass would be converted into light in this process.

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numatter t1_j9z1zz3 wrote

Thank you for that. I was thinking in terms of Newtons 1st law of motion in a perfect vacuum (and over eons of time) and didn't consider that light itself has inertia and would affect its angular momentum as its being radiated. But, isn't it still conceivable that even down to the last atom of the magnet, there's a mathematical improbability that the spin would be zero, considering entropy? Or maybe the opposite is true, that entropy was working toward bringing the very last atom down to the lowest energy state possible, essentially converting any remaining angular momentum into light so that it can achieve the lowest energy state possible. I could see it going both ways, maybe even being in an entangled state of both outcomes until an observation is made.

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sticklebat t1_j9z3v8i wrote

Wait long enough and every system will tend toward its highest entropy and typically lowest energy state. But then we’re not really talking about the effect of the magnet’s magnetic field anymore so that’s a whole different conversation that depends on things like the stability of atoms and protons.

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