# Can magnetic field expand faster than light?

• Tominator
In summary: The magnetic field of the spinning magnet does not generate the electric field of the em wave. It's the CHANGE in the magnetic field, i.e. the time derivative that generates the electric field of the em wave. In summary, the conversation discussed the possibility of a collapsing/forming magnetic field exceeding the speed of light, and whether this could be a reason why EM waves are generated at higher frequencies. It was also mentioned that changes in the field would still propagate at the speed of light and that rotating permanent magnets emit electromagnetic waves. It was clarified that the magnetic field does not attract, but exerts a force on charges, and that the EM waves generated by a rotating magnet would be ordinary, but predicting their frequency and amplitude would
Tominator
When I was reading abuth EM oscilators, a question suddenly ran through my mind:
“Is there a frequency, when the speed of a colapsing/forming magnetic field would exceed the speed of light?“
Einstain said that nothing can go faster than light. Well, ok, but if the magnetic field (generated by EM oscilator, with AC current) was for example 1km vast (from its source), with frequency higher than 75 kHz, then the magnetic field would have to travel the trajectory of 4km in one period (it would have to expand to 1km distance and diminish back, then reverse polarity and to 1km and back) faster than 300000km per s. As long as this is, according to Einstein, impossible, could it be a reason why EM waves are generated at higher frequencies?

Tominator said:
When I was reading abuth EM oscilators, a question suddenly ran through my mind:
“Is there a frequency, when the speed of a colapsing/forming magnetic field would exceed the speed of light?“
Einstain said that nothing can go faster than light. Well, ok, but if the magnetic field (generated by EM oscilator, with AC current) was for example 1km vast (from its source), with frequency higher than 75 kHz, then the magnetic field would have to travel the trajectory of 4km in one period (it would have to expand to 1km distance and diminish back, then reverse polarity and to 1km and back) faster than 300000km per s. As long as this is, according to Einstein, impossible, could it be a reason why EM waves are generated at higher frequencies?
Changes in the field would still propogate at the speed of light. They would spread outward like ripples in a pond.

So does it mean, that even if I turn the polarity of the field almost instantly, for a moment, in some distant point, the field would still have the same polariy and magnetic induction as before? What would happen to magnetic field of a toroidal permanent magnet rotating at a speed near the speed of light? To not exceed the speed of light the field lines would have to curve, how?

Tominator said:
So does it mean, that even if I turn the polarity of the field almost instantly, for a moment, in some distant point, the field would still have the same polariy and magnetic induction as before? What would happen to magnetic field of a toroidal permanent magnet rotating at a speed near the speed of light? To not exceed the speed of light the field lines would have to curve, how?

There is no physical "almost instantly" when the results move at the speed of light. No matter how fast you physically swap the polarity, it will be a snails pace when compared to the speed of light. At best you could, in principle, approach some arbitrary fraction of the speed of light.

Tominator said:
To not exceed the speed of light the field lines would have to curve, how?

To add to what DavidC was telling you,

Yes. And they do curve. From a rotating magnet, with the poles not aligned with the axis of rotation, as you imply, there are spiraling magnetic fields alternating in direction propagating outward at the speed of light (in vacuum conditions). The guys proficient in Cosmology can tell you about this.

DaveC426913 said:
There is no physical "almost instantly" when the results move at the speed of light. No matter how fast you physically swap the polarity, it will be a snails pace when compared to the speed of light. At best you could, in principle, approach some arbitrary fraction of the speed of light.

I know, but as in the example shown in the thread, if the field was vast enough, even this fraction of speed of light would be enough. In your reply to my thread, you have said:
DaveC426913 said:
Changes in the field would still propogate at the speed of light. They would spread outward like ripples in a pond.
I was asking about some distant point of the field, to which "ripples" has not yet arrived. If the field was really vast, and I turned off the field generator, would the field, in some very distant point, be able to, for example, attract some metal micrometeorite? (even though, its generator was turned off)
Phrak said:
To add to what DavidC was telling you,

Yes. And they do curve. From a rotating magnet, with the poles not aligned with the axis of rotation, as you imply, there are spiraling magnetic fields alternating in direction propagating outward at the speed of light (in vacuum conditions). The guys proficient in Cosmology can tell you about this.
If I understand this well, then the rotating permanent magnet would generate waves. would it? If yes would they be ordinary EM waves?

There are some misconceptions that you seem to have.

It will benefit you if you can stop thinking of electric and magnetic fields as isolated entities. While it is educationally pragmatic to isolate them, it is not always appropriate to do so. Particularly for considerations of 'light'. Maxwell's laws tell us that a time-varying E-field will generate a B-Field and time-varying B-field will generate an E-field.

Magnetic fields do not "attract". They exert a force on charges moving through them. So yes, if the field at a point has not yet "realized" the generator is off, then it will still exert a force on a charge moving through it.

Rotating Permanent Magnets do emit electromagnetic waves. So does accelerating anything that contains charge. The challenge is to accelerate them so intensely and coherently that the EM waves will have an amplitude observable above any background radiation. The EM waves generated by rotating a magnet would be "normal"; however, the analysis necessary to predict the frequency and amplitude of the waves would be different.

Tominator said:
If I understand this well, then the rotating permanent magnet would generate waves. would it? If yes would they be ordinary EM waves?

As MackBlanch says, yes. The funemental frequency would be the rotational frequency of the magnet. DaveC's ripples are spiraling ripples. Since we know that the alternating magnetic fields point (mostly) in the circumferal directions, the alternating electric field would be parallel with the axis of rotation.

Tominator said:
If yes would they be ordinary EM waves?

What do you consider to be "ordinary EM waves" as opposed to "extraordinary EM waves" or whatever?

jtbell said:
What do you consider to be "ordinary EM waves" as opposed to "extraordinary EM waves" or whatever?
I was just asking, because as phrak said
Phrak said:
DaveC's ripples are spiraling ripples.
I taught, these waves might be somehow different... but As Mack said I was not considering, that changing M field generates E field. My fault.

MackBlanch said:
Magnetic fields do not "attract". They exert a force on charges moving through them. So yes, if the field at a point has not yet "realized" the generator is off, then it will still exert a force on a charge moving through it.

And what about momentum? How would the change of momentum of the object slowed/accelerated by the field (with generator just turned off) manifest in the field itself? I mean, according to law of conservation of momentum, the momentum of the field would have to change somehow. Does such a magnetic field (with generator just turned off)even have weight? (so we can talk about its momentum - or change in it)

Tominator said:

I was just asking, because as phrak said
I taught, these waves might be somehow different... but As Mack said I was not considering, that changing M field generates E field. My fault.

That's OK, Tominator. I remember asking the same sort of questions about moving magnets. There's nothing wrong with this.

A magnetic field, in a region of space free of charge, and changing over distance will result in an electric field. As far as propagating magnetic fields go, they will always be associated with propagating electric fields. In the case of a spinning magnet, it is an induced electric field.

But propagating fields are not the only fields allowed in free space, The magnetic field about a stationary magnet is one example, or course. Upclose to the spinning magnet, the fields don't look like propagating fields at all.

And what about momentum? How would the change of momentum of the object slowed/accelerated by the field (with generator just turned off) manifest in the field itself? I mean, according to law of conservation of momentum, the momentum of the field would have to change somehow. Does such a magnetic field (with generator just turned off)even have weight? (so we can talk about its momentum - or change in it)

I'm not sure what object you mean. If you stop the magnet from spinning, the electromagnetic waves will still propagate outward at the speed of light (it is light), and has momentum given by the averaged Poynting vector. Like any other electromagnetic wave, it is capable of transfering momentum.

As it is mentioned above, I asked about speed of a change in really vast magnetic field. DaveC said that
DaveC426913 said:
Changes in the field would still propogate at the speed of light. They would spread outward like ripples in a pond.
Then, I was asking about some distant point of the field, to which "ripples" has not yet arrived. If the field was really vast, and I turned off the field generator, would the field, in some very distant point, be able to, for example, attract some metal micrometeorite? (even though, its generator was turned off). This question was answered by Mack
MackBlanch said:
Magnetic fields do not "attract". They exert a force on charges moving through them. So yes, if the field at a point has not yet "realized" the generator is off, then it will still exert a force on a charge moving through it.
Tominator said:
And what about momentum? How would the change of momentum of the object slowed/accelerated by the field (with generator just turned off) manifest in the field itself? I mean, according to law of conservation of momentum, the momentum of the field would have to change somehow. Does such a magnetic field (with generator just turned off)even have weight? (so we can talk about its momentum - or change in it)

I am going to restate my understanding of your question:

If the magnetic field pushes on the charge moving through it, then the charge must push back on something because "every action has an equal and opposite reaction". How is the "push back" manifested?

I am not confident in my answer, and I should probably let someone else answer, but I believe the "push back" manifests itself as an alteration in the field itself.

so in principle. if I had two magnets in a void, X distance away from each other so the fields don't overlap...

if one of those magnets were spinning the field would eventually become an elliptical pattern and push the 2nd magnet away?

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MoonKnight said:
so in principle. if I had two magnets in a void, X distance away from each other so the fields don't overlap...

Like gravity, the fields always overlap; they extend to infinity. (Unlike gravity though, the strength drops off as the cube rather than the square of the distance).

Moonknight, What is it you read?

DaveC426913 said:
Like gravity, the fields always overlap; they extend to infinity. (Unlike gravity though, the strength drops off as the cube rather than the square of the distance).

I'm sure you're correct Dave, but with a changing magnetic field there is a re-enforcing electric field, as you've said. At distances that are large compared to the rotational period, there is a propagating wave whos strength reduces as the square of the distance.

This same sort of fuzzy business comes up with say, a discrete capacitor, where the near electric fields are calculated in one way but the transmitted radiation another. How it all meshes together is a mess involving intractable and mysterious Bessel functions of some sort. Concerning the capacitor, Feynman takes so steps toward it in his "Lecture on Physics".

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From what I understand the only way to move faster than the speed of light is to literally stretch spacetime itself. And when I say stretch I mean the sort of stretching we might have seen with accelerating expansion.

Since a magnetic field would exists within this structured, yet distorted structure - my answer would be "no". Its top speed would always be relative (and smaller) than the paradigm it exists within.

-Taylor

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In one of the threads, I have posted 2 questiones and I have forgot to separate them, that might have confused you, Moonknight, sorry.

One was about a field of spinning permanent magnet (this was answered by phrak and Mack)
The other was about the speed of a change in a magnetic field. Now, we are discussing a special case: If we turned off the generator of a vast magnetic field off, it would take some time for the change to get to some distant point of the field. The question was: Can such a field (with generator just turned off) affect some metal object (metorite,or...) at the distant point?(to which the change has not yet arrived)
If yes, what would be the reaction of the field? (how would momentum conserve?)

MackBlanch said:
If the magnetic field pushes on the charge moving through it, then the charge must push back on something because "every action has an equal and opposite reaction". How is the "push back" manifested?

I am not confident in my answer, and I should probably let someone else answer, but I believe the "push back" manifests itself as an alteration in the field itself.

I taught, it might drain power from the magnetic field as a reaction.
The reaction of the field can not affect the generator, because at that time it is offline.
If the field affects the object moving through it, and the reaction does not affect the generator (because it was turned off), then this could be used as a propulsion system, if we manage to connect the object and the generator. The object would have to be large electro-magnet, which would be turned on in the same moment, the generator would be turned off. So the electro-magnet would "bounce off" the magnetic field without pushing on the generator.(which is at that moment offline)
So my question is: Would the magnetic field itself (with generator just turned off) affect (for example) some metal object.

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sorry, I posted that without looking at the whole thread...

thanks for clearing that up though

You are welcome
although my question is still unanswered: If we turned off the generator of a vast magnetic field off, it would take some time for the change to get to some distant point of the field. The question was: Can such a field (with generator just turned off) affect some metal object (metorite,or...) at the distant point?(to which the change has not yet arrived)
If yes, what would be the reaction of the field? (how would momentum conserve?)

(it was posted in previous post)

EM waves propagate at the speed of light. Turning off a generator will not instantaneously affect distant points. Momentum is conserved because the fields themselves carry momentum.

Yeah, I don't really see how your question hasn't been answered. Any change in the field will propogate at c. Full stop.

DaleSpam said:
EM waves propagate at the speed of light. Turning off a generator will not instantaneously affect distant points. Momentum is conserved because the fields themselves carry momentum.

But, either I do not understand your answer, or you have not understood my question.
The distant point has not yet been reached by the change, when it affected the object (that is how I meant the question). So the change in momentum of that object (magnet, metal,, or ...) happened before, the change reached that point.

DaveC426913 said:
Yeah, I don't really see how your question hasn't been answered. Any change in the field will propogate at c. Full stop.

Yes, but I was not sure if the field itself (with generator just turned off) could, for example, push off some electromagnet (or...), because it does not have anything to bounce from. (cos the generator is off)

Tominator said:

But, either I do not understand your answer, or you have not understood my question.
The distant point has not yet been reached by the change, when it affected the object (that is how I meant the question). So the change in momentum of that object (magnet, metal,, or ...) happened before, the change reached that point.

Yes, but I was not sure if the field itself (with generator just turned off) could, for example, push off some electromagnet (or...), because it does not have anything to bounce from. (cos the generator is off)

You're swimming in the lake. A half mile away, a boat zooms by, making waves. These waves take two minutes to reach you and buffet you.

If the boat is suddenly yanked out of the water a half mile away, do you instantly stop getting buffeted by the existing, advancing waves because they don't have the boat to "bounce from"?

No. The waves that have been generated so far continue to advance with no "knowledge" of what's happening behind them (or in front of them). Nothing can have any influence faster than c in any direction.

Tominator said:
But, either I do not understand your answer, or you have not understood my question.
The distant point has not yet been reached by the change, when it affected the object (that is how I meant the question). So the change in momentum of that object (magnet, metal,, or ...) happened before, the change reached that point.
But how is this possible? How is it possible for the distant object to be affected before the field change has reached it?

Going back to you original post:
Tominator said:
Well, ok, but if the magnetic field (generated by EM oscilator, with AC current) was for example 1km vast (from its source), with frequency higher than 75 kHz, then the magnetic field would have to travel the trajectory of 4km in one period (it would have to expand to 1km distance and diminish back, then reverse polarity and to 1km and back) faster than 300000km per s.
I'm seeing a fundamental misconception.

It seems to me, you're thinking there can be only one cycle between source and target. Thuis is tantamount to saying that there can be ever be one wave crest between boat and swimmer. Not true.

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See attached diagram.

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Kind of off-topic to this but a bit related, but with the big bang didn't space expand by like 10^75 times in a second ? =P That's a bit faster than the speed of light! But i think no matter actually did, only space

Chewy0087 said:
Kind of off-topic to this but a bit related, but with the big bang didn't space expand by like 10^75 times in a second ? =P That's a bit faster than the speed of light! But i think no matter actually did, only space
The expansion of space - both at the beginning and even present day - is not limited to the speed of light.

There are parts of the universe right now that are moving apart at greater than c.

RockyRaccoon said:
From what I understand the only way to move faster than the speed of light is to literally stretch spacetime itself. And when I say stretch I mean the sort of stretching we might have seen with accelerating expansion.

I don't know what stretching spacetime means, but if I see a map of the Earth, I know things on the flat map will appear differently than they do on a globe. The shape of continents and their relative sizes says things about the map as well as continents. The map is a part of the story.

If I apply a Gallilean inerial frame to Minkowski space, some things won't apply exactly as they would where speeds are slow. In fact, I could claim that the speed of light varies with respect to a Gallilean map.

If I map a nice, flat Minkowki space over the Universe, distant velocites tell me as much about my Minkowski map as they do about distant velocities.

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Tominator, years ago I also mused over the implications, and potential utility, of this effect.
My initial thought experiment, which I am sure many others before me have entertained, is what would happen to our Earth if our sun suddenly vanished out of existence?
Of course, the Earth would continue to both receive sunlight AND orbit for about 9 minutes as if the sun were still there.

This astounded me! I could easily understand how the Earth would still be illuminated for 9 minutes, but it was mind blowing that the Earth would, for 9-min, continue to ACT and REACT as if the sun was still there.

"Mind blowing" because the concept has implications in advanced propulsion physics with respect to properly utilizing electromagnetism. Very difficult, I know, but the potential is there.

If I might add, the issue here is with respect to switching the on-off state of an electromagnet at c.
This is not possible under classical configuration.
What's needed is a technological technique to c-switch. This might involve a radical approach, and likely very expensive.

DaveC426913 said:
You're swimming in the lake. A half mile away, a boat zooms by, making waves. These waves take two minutes to reach you and buffet you.

If the boat is suddenly yanked out of the water a half mile away, do you instantly stop getting buffeted by the existing, advancing waves because they don't have the boat to "bounce from"?

No. The waves that have been generated so far continue to advance with no "knowledge" of what's happening behind them (or in front of them). Nothing can have any influence faster than c in any direction.

I am sorry, I forgot to mention that the electromagnet would be turned on in the same time, the generator would be turned off. (although I mentioned this in one of the previous posts)

In the picture you have posted Dave, you assume, the generator was off in the biginning, but I meant it was turned on before. That is why I asked about the change, when we turn it off.
There wouldn't be any fluctuations/changes in the field before the generator was turned off.

pallidin said:
If I might add, the issue here is with respect to switching the on-off state of an electromagnet at c.
This is not possible under classical configuration.
What's needed is a technological technique to c-switch. This might involve a radical approach, and likely very expensive.

Because the distance between generator and electromagnet is long enough, it would take some time for the change to reach the electromagnet, so this would not require c-switch. Meanwile, the electromagnet would bounce off the magnetic field. Could it bounce off the field with generator just turned off? Because as long as I know, the field itself has almost no weight.

I'm sorry, I'm just not getting this whole contraption. I think you're overcomplicating it.

Any and all changes propogate at the speed of light. That's all I've got.

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