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Amplification of electromagnetic fields by a rotating body

(www.nature.com)
142 points keepamovin | 9 comments | 13 Oct 24 07:23 UTC | HN request time: 0.001s | source | bottom
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mikewarot ◴[17 Oct 24 20:34 UTC] No.41873484[source]▶
So, if you use eddy currents to delay the phase of an exciting field long enough that the object those eddy currents are inside of can spin more than 90 degrees, the response eddy current fields now AID instead of opposing the original field?

This sounds quite a bit like what Steorm[1] was doing years ago. If ultraconductors[2] worked, you could actually build a mechanical device that had losses low enough to actually gain energy once a critical speed were obtained.

[1] https://en.wikipedia.org/wiki/Steorn

[2] https://patents.google.com/patent/US5777292A/en

(Claim 7 is for material with a conductivity of 10^11 S/cm, which is 150,000 times better than copper)

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Terr_ ◴[17 Oct 24 21:20 UTC] No.41873927[source]▶
> If ultraconductors[2] worked, you could

Not familiar with that idea, but this construction sounds a bit like: "If only you had an (infinitely) rigid rod, you could push one end to communicate faster than lightspeed."

Or in balder terms: "If only we had a subtly impossible component, we could make a blatantly impossible machine."

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1. MadnessASAP ◴[17 Oct 24 21:54 UTC] No.41874203[source]▶
I am somewhat curious where the math on a perfect rigid rod breaks down (that is, where does the 0 end up under the line)
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2. kstrauser ◴[17 Oct 24 22:15 UTC] No.41874347[source]▶
My gut instinct is you’d really need perfect incompressibility such that pushing one end of the rod would propagate the pressure wave to other end instantaneously. In other words, you have to make the speed of sound faster than the speed of light. I have no idea what the physical construction would look like. Maybe a string of singularities lined up and touching (hey, there’s your 0!) without tearing spacetime apart?
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3. TeMPOraL ◴[17 Oct 24 22:53 UTC] No.41874631[source]▶
I think this is where the math breaks down:

> you’d really need perfect incompressibility such that pushing one end of the rod would propagate the pressure wave to other end instantaneously

I.e. by pushing an abstraction of "perfect incompressibility" and "instantenous propagation of pressure waves" to the point stops corresponding to reality. Those ideas are descriptive simplifications, abstracting away the underlying process of matter / fields interacting sequentially, an interaction that propagates at the speed of light.

It's the same kind of thing like assumption that array access is O(1). It is, until the array gets so large the process of finding the right place in memory becomes visibly O(n).

Or, on a more basic level, arithmetic on numbers seems to be O(1) with respect to the values of the numbers. Almost all programming practices and popular algorithms depend on that assumption, but it only holds for numbers that the hardware can process in one go. Adding 64-bit numbers is constant-time. Adding 64000-bit numbers isn't.

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4. kstrauser ◴[17 Oct 24 23:06 UTC] No.41874711{3}[source]▶
Right. "Let's assume force propagates instantaneously--" "Let me stop you there."
5. 00N8 ◴[17 Oct 24 23:12 UTC] No.41874752[source]▶
I think the math is fine, but a perfectly rigid rod cannot exist in our universe, so there's no paradox, even though it would let you send messages faster than light. It's like calculating how strong of a hydraulic press you'd need to compress matter into neutronium in your garage, or claiming that if a spaceship could travel at 2x light speed, then it could fly its way back out of a black hole.
6. Tuna-Fish ◴[17 Oct 24 23:26 UTC] No.41874828[source]▶
There simply are no such things as rigid bodies in this universe. Everything is composed of particles that act on each other with forces, which take time to transmit the information that something is pushing on the other side.
7. Terr_ ◴[18 Oct 24 00:32 UTC] No.41875250[source]▶
> Maybe a string of singularities lined up and touching (hey, there’s your 0!) without tearing spacetime apart?

At that point it's starting to sound less like a rigid rod and more like changing spacetime itself so that the two locations are closer together. :p

8. Junk_Collector ◴[18 Oct 24 02:04 UTC] No.41875759[source]▶
Force cannot transmit through the material faster than the speed of sound and because fundamentally at a subatomic level, mechanical force is the interaction of electromagnetic and gravitational fields between molecules, the speed of sound cannot exceed the speed of light in a material. The rod will compress or physics breaks down. Solid particles are an abstraction.
9. CamperBob2 ◴[18 Oct 24 03:10 UTC] No.41876045[source]▶
My guess is that the Pauli exclusion principle would ultimately come into play, the same thing that keeps stuff from falling through other stuff. Electrons will shove other electrons out of the way, which isn't a lossless process or an instantaneous one. So a perfectly rigid rod would run afoul of special relativity if you pushed on one end.