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Author Topic: How would two attracting magnets interact near a black hole?  (Read 1176 times)

Offline chris

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If two magnets feeling a force between them approached a black hole, what, if anything, would happen to the magnetic force felt between them?


 

Offline evan_au

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The Electromagnetic force is much stronger than the Gravitational force, in that the magnetic attraction of two magnets is much greater than the Gravitational attraction between the two magnets.

However, the gravitational attraction of a stellar-mass black hole (with a range of millions of km) is far greater than the magnetic attraction of two permanent magnets (with a human-detectable range of perhaps cm to a meter).

Part of this difference in range is that the gravitational force has an inverse square law; masses are a gravitational monopole. However, permanent magnets are a dipole field, which declines in strength much faster than an inverse square law; from a distance much larger than the size of the magnet, the two poles almost cancel each other out.

So if the gravitational force of the black hole 100x stronger than the magnetic attraction between the magnets, you can almost ignore the impact of the magnetism.

If the two magnets started off "at rest" relative to the black hole, but one was slightly closer to the black hole, the extreme gravitational gradient near the black hole would pull them far apart as they approached the event horizon. (It would then proceed to rip each of the magnets into little pieces: spaghettification)

Is there some particular scenario you would like to consider in more detail?
 
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Offline Thebox

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If two magnets feeling a force between them approached a black hole, what, if anything, would happen to the magnetic force felt between them?


The magnets will remain ''stuck'' together and be pulled into the BH, the gravitational effect of the BH having no force applied to separate the magnets, however if the magnets were to approach a star, a curie point will be reached and the magnets will lose their attractive force to each other.

 

Offline chris

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Let's assume we're considering two bar magnets with the long axis pointing into the black hole; as the magnet closer to the hole goes in, it should stretch and become very long and thin. So what will that do to the magnetic dipoles inside? Would the magnet nearer the black hole weaken?
 

Offline jeffreyH

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Time is an important consideration and how its dilation affects the magnetic field. Fields have force carriers whose speed varies on a coordinate basis in a gravitational field. It is like saying the force is dilated in the dimension of time. It basically takes longer for the force to affect its surroundings. Is this actually a weakening or just a diluted distribution?

It is unlikely to be diluted if length contraction were at work. However space is expanding rather than contracting so this simply amplifies the effect exponentially. Taking this view then the force must weaken.
 

Offline jeffreyH

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One more point. Force carriers trying to leave the vicinity of the event horizon may be slowed but what about those approaching the EH from the outer magnet? They cannot be traveling at superluminal speeds. That would violate relativity. So what exactly are they doing? If they can't go faster than c then they either maintain a speed of c or they slow down. If spacetime is actually stretching then this may cancel out and they may APPEAR to an outside observer to be traveling at superluminal speeds. These are the only 3 options.
 

Offline evan_au

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Quote from: Chris
as the magnet closer to the hole goes in, it should stretch and become very long and thin.
Most permanent magnets are crystalline structures, and rather brittle.
While tidal stretching would occur with organics like humans, spaghettification of a hard crystalline material would result in it shattering into smaller and smaller crystals, as the rapidly increasing tidal forces exceed the tensile strength of the magnet.
The cloud of shattered magnet fragments will then become long and thin as they plunge into the black hole.
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So what will that do to the magnetic dipoles inside? Would the magnet nearer the black hole weaken?
Each individual crystal would retain some of the original magnetic field, but each smaller fragment would exert a smaller external magnetic field.

As Jeffrey states, near the event horizon, time dilation & distortion of spacetime will start to affect the shape & strength of the magnetic field. For an unfortunate observer falling with the pieces of magnet, the results would be similar to what you would see with a similar-sized magnet on your desk. But a distant observer would see a slowing of time.

As TheBox says, an increase in temperature (eg due to an accretion disk, or tearing the magnet into small pieces) or even strong vibration (eg due to shattering the magnet) will erase the residual magnetic field.

It is thought that many black holes are surrounded by a very strong magnetic field, and this could slightly divert the path of the magnets, or reorient them to align with the external field as they plunge towards the black hole.
 
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