Can magnetic field expand faster than light?

[Phrak]

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.

I have no idea what a c-switch might be, or what it could mean to turn something off at c.
c is a velocity. How can something be turned off at a velocity?

 

[Phrak]

Tominator, as Dave says, there's no mystery here. Shin a flashlight at the sky. Turn it off. The beam keeps on going. As the little dashed of the beam might hit something, that something will react even with the flashlight off.

 

[Dale]

Tominator, the effect of any change in any electric or magnetic field propagates at c. The End. Stop. Fin.

You have been answered and answered and answered. If you do not understand this answer then please learn more about Maxwell's equations. They fully describe the result of all of the scenarios you posed.

 

[Raap]

So in short, the magnet would bounce off the field when it reached it, even if the generator was turned off.

 

[Tominator]

Thanks,
I understand that the change is propagating at c and there is no "mystery" in it.
But you have missed my point. So it is likely that I have not explained it well.
I will try my best:
I was asking about a special case. (described below)

Conditions
1) We have a generator of a stable magnetic field and electromagnet, which is somewhere in that field, but very far from the generator. The field is on (everywhere), but the electromagnet is yet off.

"Experiment"
2)Then we turn off the generator. This change propagates at c towards the point, where electromagnet is, as you all have said.
But the field, in the point where the electromagnet is, is still on, because the change has not yet arrived there.
If we turn the electromagnet on, before the change arrives, would it bounce off the magnetic field, which is still on in that point?
(assuming the change is still miles away and the field is strong enough even at that distant point)
If yes, how would momentum conserve here?

 

[pallidin]

I have no idea what a c-switch might be, or what it could mean to turn something off at c.
c is a velocity. How can something be turned off at a velocity?

Phrak, what I am referring to here is very important for the OP to understand because the "effect" he is after is theoretically possible, but is not currently practical(to my knowledge)

When an electromagnet is "turned-off" by either mechanically or electronically opening the circuit, the electron flow in that electromagnet does not come to a halt instantaneously, of course.

Most importantly, the electron flow does not stop at the rate of c. The rate at which the electron flow stops(with a degrading power curve of course) is much, much slower due to the aspect of an electrons momentum.

In other words, for a short time AFTER opening the circuit from it's power source, the electromagnet will continue to generate an EM field(degrading) completely absent of any continuing, external power source.

Can we agree on that?

If so, his experiment has a problem.

 

[Dale]

2)Then we turn off the generator. This change propagates at c towards the point, where electromagnet is, as you all have said.
But the field, in the point where the electromagnet is, is still on, because the change has not yet arrived there.
If we turn the electromagnet on, before the change arrives, would it bounce off the magnetic field, which is still on in that point?
(assuming the change is still miles away and the field is strong enough even at that distant point)

I am not going to answer this yet again. You use your own brain, apply a little knowledge and reason, and answer the question yourself. Are there any terms in Maxwells equations or the Lorentz force law that would lead to a dependency on what is happening at distant points? You have already realized that there is still a magnetic field at the point where the electromagnet is located, despite the fact that the generator is off. Given what you know about the laws and the fields you tell me, how does an electromagnet act when it is turned on in a magnetic field?

If yes, how would momentum conserve here?

You have already been answered for this in post 11 by Phrak and post 23 by me. The fields themselves carry momentum. When you include the momentum of the fields momentum is always conserved.

 

[Tominator]

I am not going to answer this yet again. You use your own brain, apply a little knowledge and reason, and answer the question yourself. Are there any terms in Maxwells equations or the Lorentz force law that would lead to a dependency on what is happening at distant points? You have already realized that there is still a magnetic field at the point where the electromagnet is located, despite the fact that the generator is off. Given what you know about the laws and the fields you tell me, how does an electromagnet act when it is turned on in a magnetic field?

You have already been answered for this in post 11 by Phrak and post 23 by me. The fields themselves carry momentum. When you include the momentum of the fields momentum is always conserved.

Turns out that I did not understand your answer.
Well I do not know much about mexwell's equations yet. Based on my little knowledge of Lorentz force law, and on that what you are saying, I would say, the electromagnet would bounce off or attract to the point, where generator is. (If we are considering the time between turning off the generator and the change reaching the electromagnet)
So if I understood this correctly, by connecting the generator with the electromagnet, a propusion system can be created, as I have proposed in post number 19 (at the end of the post). Can it?

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.

I still do not understand how field can carry a momentum. Don't you mean the change? I can not imagine a momentum of a stable, non-fluctuating magnetic field, because it does not have any speed (p=m.v).

 

[Dale]

So if I understood this correctly, by connecting the generator with the electromagnet, a propusion system can be created, as I have proposed in post number 19 (at the end of the post). Can it?

Since the fields carry momentum and momentum is conserved at all times, what do you think?

I still do not understand how field can carry a momentum. Don't you mean the change? I can not imagine a momentum of a stable, non-fluctuating magnetic field, because it does not have any speed (p=m.v).

The momentum density of the EM field is the http://en.wikipedia.org/wiki/Poynting_vector" divided by c².

 

[Tominator]

Since the fields carry momentum and momentum is conserved at all times, what do you think?

I think, this does not violate the law of conservation of momentum, so it should work.
For example: If we have a lightbulb, which radiates light to all directions, there is no change in its momentum. But if we connect the lightbulb with a mirror, then the whole "system" (lightbulb and mirror) would accelerate in the direction from lightbulb to mirror. (this depends on the angle between axis of mirror and vector - from lightbulb to mirror).
This system would not even lift itself from the ground, but the system I am proposing is a bit different, but still does not violate the law of conservation of momentum. For more info read the post 19

The momentum density of the EM field is the http://en.wikipedia.org/wiki/Poynting_vector" divided by c².

Uff, for naw I will accept this as fact, because I am not yet capable to visualise these equations and so I do not understand them.

 

[jtbell]

I still do not understand how field can carry a momentum.

The total mechanical momentum of a system of charges interacting via electromagnetic forces is not conserved, in general.

However, using the $\vec E$ and $\vec B$ produced by the charges, we can define the quantity $\vec g = \epsilon_0 \vec E \times \vec B$, which has an interesting property. Its integral over all space varies varies exactly oppositely to the total mechanical momentum of the charges. That is, the sum of the total mechanical momentum and the integral of $\vec g$ is constant. Proving this requires some fancy footwork with vector calculus.

Therefore, we find it convenient to call the integral of $\vec g$ the "momentum of the electromagnetic field."

I can not imagine a momentum of a stable, non-fluctuating magnetic field, because it does not have any speed (p=m.v)

When charged particles are moving (and therefore have momentum), they don't produce a stable, non-fluctuating electromagnetic field.

 

[Dale]

I think, this does not violate the law of conservation of momentum, so it should work.
For example: If we have a lightbulb, which radiates light to all directions, there is no change in its momentum. But if we connect the lightbulb with a mirror, then the whole "system" (lightbulb and mirror) would accelerate in the direction from lightbulb to mirror. (this depends on the angle between axis of mirror and vector - from lightbulb to mirror).
This system would not even lift itself from the ground, but the system I am proposing is a bit different, but still does not violate the law of conservation of momentum.

Correct. Your electromagnet system is different from the lightbulb system in the sense that it doesn't use reflection to direct the EM momentum. But they are the same in the sense that the propulsion is due to conservation of momentum between the device and the EM fields.

 

[Tominator]

Correct. Your electromagnet system is different from the lightbulb system in the sense that it doesn't use reflection to direct the EM momentum. But they are the same in the sense that the propulsion is due to conservation of momentum between the device and the EM fields.

WOW
First time, my idea was verified.
I taught that if it works in GHz frequencys, it should produce much more thrust than the lightbulb and mirror. Because then, the electromagnet and generator (electromagnet too) could be only few centimeters (or maybe even less) from each other.
Are there any electromagnets able to work on such a frequencys?
Hasn't this already been invented?
If yes, why NASA doesn't use it?

 

[Dale]

WOW
First time, my idea was verified.

Hehe, one of the benefits of thinking through something step by step!

I taught that if it works in GHz frequencys, it should produce much more thrust than the lightbulb and mirror. Because then, the electromagnet and generator (electromagnet too) could be only few centimeters (or maybe even less) from each other.

The magnitude of the momentum of light is given by p = E/c, so it is independent of the frequency and depends only on the energy. It is much more important to have a high energy source and to have it very tightly collimated. In this sense a laser is probably your best bet.

Are there any electromagnets able to work on such a frequencys?
Hasn't this already been invented?
If yes, why NASA doesn't use it?

There are lots of very smart guys at NASA who understand this principle, and several different designs that are based on it. Why do you think it isn't used? How much momentum can you get by shining a light off the back of a rocket vs. how much momentum can you get by throwing rocket exhaust gasses off the back?

 

[Tominator]

The magnitude of the momentum of light is given by p = E/c, so it is independent of the frequency and depends only on the energy. It is much more important to have a high energy source and to have it very tightly collimated. In this sense a laser is probably your best bet.

The lightbulb and mirror was only an example, far away from what I am talking about. This "electromagnet system" as you have named it, is not based on EM reflection, as you have already said. The electromagnet here bounces of "residual field" of generator, which was shut down a moment before. And the electromagnet is turned off again before the change caused by shutting down the generator reaches it.

There are lots of very smart guys at NASA who understand this principle, and several different designs that are based on it. Why do you think it isn't used? How much momentum can you get by shining a light off the back of a rocket vs. how much momentum can you get by throwing rocket exhaust gasses off the back?

You probably mean solar sail and such a things.
But force produced by electromagnets is far more greater, than a force produced by beam of light of lightbulb, or laser. If these two electromagnets are only few centimeters from each other, the "residual field" is very powerful. This short distance could be achieved by turning both the generator and the other electromagnet on and off, in GHz frequencys. The wavelenght at GHz frequencys is few centimeters, so the magnetic force here is far superior to force caused by laser or light of a lightbulb.
In my opinion it could be similar to force produced by exhaust gasses of rocket, although this depends on power source and distance between electromagnets.
The question is, wheather the changes in a field at GHz frequencys can be considered not only as EM waves, but also as a propagating fields. So the electromagnet would be able to bounce off them as if it bounced off a field of permanent magnet. Would it?

 

[Dale]

But force produced by electromagnets is far more greater, than a force produced by beam of light of lightbulb, or laser. ... the magnetic force here is far superior to force caused by laser or light of a lightbulb.

That is certainly true, but not relevant for propulsion in space. You can generate large forces that will cause a lot of stress in the structure of your device, but the only way to actually propel your device in space is by throwing momentum off the back. All that matters is how much momentum you can toss off and how tightly you can collimate it towards the back. For EM the momentum is given by E/c and so the only remaining question is the focus, which would be far better for a laser than for your electromagnet.

The question is, wheather the changes in a field at GHz frequencys can be considered not only as EM waves, but also as a propagating fields.

EM waves are the same thing as propagating fields.

 

[Tominator]

That is certainly true, but not relevant for propulsion in space. You can generate large forces that will cause a lot of stress in the structure of your device, but the only way to actually propel your device in space is by throwing momentum off the back. All that matters is how much momentum you can toss off and how tightly you can collimate it towards the back. For EM the momentum is given by E/c and so the only remaining question is the focus, which would be far better for a laser than for your electromagnet.

If I set frequencys and distance between the two electromagnets properly, I should be able to get focused force instead of the "lot of stress". Because as you have said:

Your electromagnet system is different from the lightbulb system in the sense that it doesn't use reflection to direct the EM momentum. But they are the same in the sense that the propulsion is due to conservation of momentum between the device and the EM fields.

EM waves are the same thing as propagating fields.

I want the electromagnet to bounce off EM waves, but as long as the EM waves can be considered a propagating field, the electromagnet should gain the same momentum, as if it bounced off the field of the generator itself in the same point. Shouldn't it?
It can even bounce off the field of the generator, but till the change caused by this bouncing off gets to the generator, the generator has to be turned off (for this, GHz frequencys are needed). So there will be no change in momentum of the generator, but there will be significant change in momentum of the electromagnet.

 

[Dale]

Tominator, you have a really annoying habit of repeatedly asking the same question over and over and over and somehow thinking that the answer will change.

If I set frequencys and distance between the two electromagnets properly, I should be able to get focused force instead of the "lot of stress". ... So there will be no change in momentum of the generator, but there will be significant change in momentum of the electromagnet.

What do you think dynamic stress is besides a change in momentum of one part of a structure and not another?

Go ahead and do the math. I have already told you the conclusion, but you obviously need to work it out for yourself. Again, it is not force that you need to focus, it is momentum.

 

[Tominator]

Tominator, you have a really annoying habit of repeatedly asking the same question over and over and over and somehow thinking that the answer will change.What do you think dynamic stress is besides a change in momentum of one part of a structure and not another?

Sorry Dalespam, but it is because I still have some doubts about your conclusion that it will be capable only of acceleration simmilar to acceleration gained by laser.
Because if the "primary" electromagnet bounces off the field of the "secondary" one (in close proximity), the force is far greater, than force produced by laser. And because the "secondary" electromagnet is switched off before the "primary" could apply a force on it (the "primary" will actually bounce off "secondary's" field, not knowing, that the "secondary" electromagnet was turned off), it will produce acceleration in one direction. Am I wrong? If yes, where exactly?
What is dynamic stress?

Go ahead and do the math. I have already told you the conclusion, but you obviously need to work it out for yourself. Again, it is not force that you need to focus, it is momentum.

Well, I am not so good at maths, instead of doing math, I try to visualize it, that is why I like physics much more.
If I apply a force on something in free space, it will accelerate and also gain momentum.
I do not need to care about momentum, because in this case it will be conserved thanks to the EM fields.

 

[Dale]

Sorry Dalespam, but it is because I still have some doubts about your conclusion that it will be capable only of acceleration simmilar to acceleration gained by laser. ... Well, I am not so good at maths

That's fine. There is no reason to trust me, but you cannot have it both ways. You need to make the (several years of) mental effort to learn this stuff (including the math) yourself. Otherwise your ideas are nothing more than wishful thinking or daydreaming.

What is dynamic stress?

Stress (actually strain) is a deformation of a structure from its "resting" shape. Dynamic strain is simply strain that is not constant in time. So, if we have a spring it starts out unstressed, if we suddenly squeeze it with a certain force then it will start changing shape over time (dynamic strain), eventually it will settle down to a new shorter length that is constant over time (static strain, or just strain). If all parts of a structure move at the same velocity then there is no change in the structure's shape (no dynamic strain), but if different parts of a structure are moving at different velocities then the structure's shape is changing (dynamic strain).

 

[Tominator]

That's fine. There is no reason to trust me, but you cannot have it both ways. You need to make the (several years of) mental effort to learn this stuff (including the math) yourself. Otherwise your ideas are nothing more than wishful thinking or daydreaming.

Stress (actually strain) is a deformation of a structure from its "resting" shape. Dynamic strain is simply strain that is not constant in time. So, if we have a spring it starts out unstressed, if we suddenly squeeze it with a certain force then it will start changing shape over time (dynamic strain), eventually it will settle down to a new shorter length that is constant over time (static strain, or just strain). If all parts of a structure move at the same velocity then there is no change in the structure's shape (no dynamic strain), but if different parts of a structure are moving at different velocities then the structure's shape is changing (dynamic strain).

Well trusting you is the easyer and probably also wiser way. And after thinking for a while, I realized that you were right.
Or I would produce small acceleration by using current pulses, or dynamic stress by using normal AC currrent. But still I want to learn a bit more about these things.
If an EM wave interfere with oscilating mag field of EM oscilator on the same frequency, in the way that they would be canceled, could it affect the EM oscilator? (its momentum)
Or is the oscilating field in close proximity from the EM oscilator considered EM wave?

 

[map19]

dalespam. I've got news for you. Your statement
"The magnitude of the momentum of light is given by p = E/c, so it is independent of the frequency"
Wrong, wrong ! E = hf. Energy of a photon is directly proportional to frequency.

 

[azzkika]

sorry to alter the tone of this topic, but i have had problems posting a new topic.

Could someone explain what electromagnetic waves are? what they are made of, as it appears to me they are made of nothing, but obviously that cannot be right. It is what the actual wave itself is made of, that i am curious to understand.

 

[Tominator]

sorry to alter the tone of this topic, but i have had problems posting a new topic.

Could someone explain what electromagnetic waves are? what they are made of, as it appears to me they are made of nothing, but obviously that cannot be right. It is what the actual wave itself is made of, that i am curious to understand.

Never mind I have been asking similar question only months ago.
EM waves are waves caused by alternation of magnetic and electric fields. When there is change in magnetic field there is also change in electric field and vice versa.
If the change is fast enough, it produces a wave capable of self propagating - EM wave.

 

[azzkika]

yes i understand how waves are made, but not what they actually are made of. eg, a wave in the sea is made of water, or water is what propagates the wave energy. i suppose what I'm getting at, is what is the physical property of EM waves, or are they made of nothing?

 

[Mapes]

If the change is fast enough, it produces a wave capable of self propagating - EM wave.

There is no speed requirement. Any change in electric or magnetic field, no matter how slow, propagates as an EM signal with speed c (with a very long wavelength).

 

[Tominator]

yes i understand how waves are made, but not what they actually are made of. eg, a wave in the sea is made of water, or water is what propagates the wave energy. i suppose what I'm getting at, is what is the physical property of EM waves, or are they made of nothing?

The nature of every wave is energy. The EM waves are made of energy as well. The water waves needs water as environment to propagate, but again, they are nothing else than energy. It is said that EM waves does not need any environment to propagate. But on the first page of this forum DaveC said that electric and magnetic field extend to infinity. So then the enviroment, in which EM waves propagate would be everywhere. But EM waves are not like the water waves, they are composed of photons. Photon is something between wave and particle. But I still do not understand it so well to teach you, there are better explanations of EM waves all around the forum, you only need to do a bit of searching.

There is no speed requirement. Any change in electric or magnetic field, no matter how slow, propagates as an EM signal with speed c (with a very long wavelength).

I have heard, that EM waves with frequency under 50Hz are considered oscilations of the field.

 

[Mapes]

Never heard of a cutoff at 50 Hz. Can you provide a reference so's I might learn more?

 

[Tominator]

Never heard of a cutoff at 50 Hz. Can you provide a reference so's I might learn more?

By " I have heard " I really meant heard, so sorry but I can't provide you with any references. I meant it more as a question, because I was not and I am not sure about it. But it is something I have heard at school, so I expected it to be right. Maybe my memory messed it up somehow with the equation of progressive Em wave. You might post a new thread with this question if you want.

 

[Dale]

dalespam. I've got news for you. Your statement
"The magnitude of the momentum of light is given by p = E/c, so it is independent of the frequency"
Wrong, wrong ! E = hf. Energy of a photon is directly proportional to frequency.

I am not wrong, but I guess I should have been more clear. I was thinking specifically in terms of a space propulsion system. So if you have a vehicle that carries a finite quantity of fuel then your E is fixed. Assuming that you could convert your fuel to light of two different frequencies, then for the lower frequency you would generate more photons of lower energy each than for the higher frequency. The total momentum of all the photons would be the same at both frequencies.

 

[Tominator]

Hmmm
But isn't the light propulsion extremely inefficient? Because if you compare the energy (power) used to "throw off" light, to the momentum gained, it looks to me that it is extremely inefficient. If you use the same energy to push off a magnet by electromagnet, the momentum gained is far more greater.

 

[Dale]

I already answered that several times Tominator.

 

[Tominator]

I already answered that several times Tominator.

Yes sorry, I only wanted to take it step by step, as you have suggested.
Well, if something is inefficient, but working, there is always possibility to make it work as efficiently, as technology will let us. Because waves are actually energy, the light propulsion is gaining momentum directly by "throwing off" energy. That seemingly violates the law of conservation of momentum, but as far as I know, law of conservation of energy is superior to law of conservation of momentum.
doesnt the law of conservation of momentum apply only on closed systems? (no energy out or in)
If my assumptions are correct, than it is not a physical non-sense to run around the law of conservation of momentum, by using non-closed system and law of conservation of energy. Thus creating propulsion system, capable of creating force in one direction directly from energy. (?)

 

[Klockan3]

Hmmm
But isn't the light propulsion extremely inefficient? Because if you compare the energy (power) used to "throw off" light, to the momentum gained, it looks to me that it is extremely inefficient. If you use the same energy to push off a magnet by electromagnet, the momentum gained is far more greater.

Actually light is the by far most efficient thing to throw off.

First you know the formula E=mc^2, right? It means that energy=mass.
Then you also got from special relativity that mass is just energy with its momentum vector along the time axis, and that energy is the length of these momentum vectors, and that no matter the direction the object always move in it with the speed of c. Of course we use the general coordinates of ct being the time axis.

Then you see that you get the most momentum per energy by using pure light, since it got no mass which gives 0 momentum per energy and everything in between light and mass is just a mix.

Anyway the reason we don't propagate things using light is because we aren't that good at transforming mass to energy.


Anyway, your thing about electomagnets, the problem with your analysis is that when you look at electro magnets strength you look at stable fields. Stable fields never give out any momentum, its just oscillating fields. Oscillating magnet fields becomes electro magnetic radiation in the same way oscillating electric fields creates electro magnetic radiation, they are the same phenomenon.

If you want to compare anything you should compare the magnetic field with the electric field, and then the magnetic field is weaker unless you move at the speed of light in which case they are equal. The only reason you notice magnetic fields more is because a wire can have charges move relative to each other while still have a total charge of 0 meaning that you get magnetic fields without electric fields, however if you would isolate just a few grams of purely charged matter you would get extreme forces.

 

[Dale]

Because waves are actually energy, the light propulsion is gaining momentum directly by "throwing off" energy. That seemingly violates the law of conservation of momentum, but as far as I know, law of conservation of energy is superior to law of conservation of momentum.

No, conservation of energy and conservation of momentum are on equal footing. They are the timelike and spacelike parts of the http://en.wikipedia.org/wiki/Four-momentum#Conservation_of_four-momentum" respectively. This is what Klockan3 is referring to in his second paragraph.

doesnt the law of conservation of momentum apply only on closed systems? (no energy out or in)

Yes, same with the conservation of energy.

If my assumptions are correct, than it is not a physical non-sense to run around the law of conservation of momentum, by using non-closed system and law of conservation of energy. Thus creating propulsion system, capable of creating force in one direction directly from energy. (?)

You cannot run around either conservation law. All you can do is understand them and use them to accomplish your design goal. TANSTAAFL