Relativity and Absolute Space: A Reconciliation?

[Epsilon Pi]

Yes, the point is that, that concept of an inertial ref. frame is such an idealization, that it does not work at all, in that real life of rotating systems... then why we insist in them?

Just wondering, wondering and wondering

My best regards

EP

To reproduce with an electric current in a conductor the magnetic field generated by a permanent magnet, we should use a solenoid. What happens in the reference frame of the moving charges inside the conductor is not obvious at all, because they are not in an inertial ref. frame; worst, they are rotating in a spiral!
It's not possible to say simply that the magnetic field wouldn't vanish.

 

[Parlyne]

VE, you are quite correct. In the reference frame of the particles in the current, there will be no magnetic field due to that current - just an electric field. However, there will still be a magnetic field due to the bar magnet. That field, however, will not be exactly the same as the one seen in the lab frame. And, in fact, there will also be an electric field associated with the bar magnet.

EP, what I have stated here is exactly the result predicted by special relativity. In fact, this is what is meant by saying that the E and B fields are only defined relative to a given reference frame. It means that the fields due to a given set of sources depend also on the velocity of the observer relative to the sources.

 

[ZapperZ]

Hey !, hello to all,

Being a layperson, I’m very excited about the arguments / counterarguments that are everywhere in these forums. They are all an immense source of new information about the physics and the people trying to understand and explain how such physics work.

So, Zz, what you are saying is that, if there is a permanent magnet lying on the same table as a lab experiment setup with moving charges in a conductor, there will be a difference of perceived resulting magnetic fields if I were to move at the speed of the moving charges and look at the table. One field will remain and the other one will vanish… is this right ?VE

The issue here, some that EP doesn't seem to understand, is the geometry of the magnetic field.

Consider the magnetic field generated by a beam of charges moving with a constant velocity v. Now solve the Maxwell Equation to get the magnetic field. This magnetic field has a very particular geometry. Now, transform yourself to the frame of reference moving with the charges. You can now look at it in two different ways:

1. You see the "source" of the fields, being the charges, at rest in your frame, and thus, you ONLY see an electric field. This is no different than you having a line of charge in the first place. Any undergrad physics student can solve for such a field.

2. You don't look at the "source", but rather act on the field. In this case, you do a tranformation of the magnetic field to the moving frame, and you will see that you get the same E-field.

This shows that 1 and 2 are consistent with each other. The principle of symmetry is one of the fundamental principles of our universe.

Now, this is where we get back to your question. If you have the same charges moving, and you have, let's say a bar magnet, what happens if you do the SAME transformation of the field? The bar magnet will NOT lose its magnetic field in the moving frame, not even by applying the relativistic Maxwell equation! Why? Becuase the geometry of the field is all "wrong" - it isn't identical to the field created by the moving charges. You will get a different geometry from the bar magnet in the moving frame depending on how it is alligned, but not the "disappearing" field that you get with the line source.

This is why I said early on that the geometry of the field is extremely important. Something like a dipole field can't be transformed away to an inertial frame. The geometry of the field won't work, and this is what you would get when applying the relativistic maxwell equation.

Zz.

 

[Epsilon Pi]

Hello my dear friend ZZ,

You wrote:

Consider the magnetic field generated by a beam of charges moving with a constant velocity v. Now solve the Maxwell Equation to get the magnetic field. This magnetic field has a very particular geometry.

In this case, I suppose you are referring to Ampere's law, that says that around a current of electrons, there is a magnetic field around. This is what you understand by solving Maxwell equation, right? If I interpret things like this I don't find any problem with the geometry of the magnetic field, and again this solution does not have anything to do with SR.

But now, you speculate,

Now, transform yourself to the frame of reference moving with the charges. You can now look at it in two different ways:

1. You see the "source" of the fields, being the charges, at rest in your frame, and thus, you ONLY see an electric field. This is no different than you having a line of charge in the first place. Any undergrad physics student can solve for such a field.

2. You don't look at the "source", but rather act on the field. In this case, you do a tranformation of the magnetic field to the moving frame, and you will see that you get the same E-field.

This shows that 1 and 2 are consistent with each other. The principle of symmetry is one of the fundamental principles of our universe.

and speculate, because here you are not making any reference to Maxwell equation but just to the principle of relativity, please note that for your argument here you are making an abstraction in the concept of charge, you are not taking or thinking in the real origin of both the charge and the magnetic field, I mean, the electron. I really cannot follow you here, please, you cannot do this with the real currents we deal everyday!

My Best regards

EP

The issue here, some that EP doesn't seem to understand, is the geometry of the magnetic field.

Consider the magnetic field generated by a beam of charges moving with a constant velocity v. Now solve the Maxwell Equation to get the magnetic field. This magnetic field has a very particular geometry. Now, transform yourself to the frame of reference moving with the charges. You can now look at it in two different ways:

1. You see the "source" of the fields, being the charges, at rest in your frame, and thus, you ONLY see an electric field. This is no different than you having a line of charge in the first place. Any undergrad physics student can solve for such a field.

2. You don't look at the "source", but rather act on the field. In this case, you do a tranformation of the magnetic field to the moving frame, and you will see that you get the same E-field.

This shows that 1 and 2 are consistent with each other. The principle of symmetry is one of the fundamental principles of our universe.

Now, this is where we get back to your question. If you have the same charges moving, and you have, let's say a bar magnet, what happens if you do the SAME transformation of the field? The bar magnet will NOT lose its magnetic field in the moving frame, not even by applying the relativistic Maxwell equation! Why? Becuase the geometry of the field is all "wrong" - it isn't identical to the field created by the moving charges. You will get a different geometry from the bar magnet in the moving frame depending on how it is alligned, but not the "disappearing" field that you get with the line source.

This is why I said early on that the geometry of the field is extremely important. Something like a dipole field can't be transformed away to an inertial frame. The geometry of the field won't work, and this is what you would get when applying the relativistic maxwell equation.

Zz.

 

[Parlyne]

In this case, I suppose you are referring to Ampere's law, that says that around a current of electrons, there is a magnetic field around. This is what you understand by solving Maxwell equation, right? If I interpret things like this I don't find any problem with the geometry of the magnetic field, and again this solution does not have anything to do with SR.

No, it doesn't have anything to do with relativity. Relativity is only necessary when we want to know what the fields that someone moving through the lab at constant speed would measure the fields to be.

and speculate, because here you are not making any reference to Maxwell equation but just to the principle of relativity, please note that for your argument here you are making an abstraction in the concept of charge, you are not taking or thinking in the real origin of both the charge and the magnetic field, I mean, the electron. I really cannot follow you here, please, you cannot do this with the real currents we deal everyday!

My Best regards

EP

Actually, he is making quite a bit of reference to the Maxwell equations. Perhaps it would help to make it more explicit.

An electrical current consists of a collection of charges with some coherent motion (generally much smaller than the random motions of the individual electrons). We can express this idea with the formula:

I = \lambda q v_d

Or, in english, current is equal to the number of charged particles per unit length multiplied by the charge of one particle and the drift speed. The drift speed is the magnitude of the average of all the particle velocities.

If, instead of just sitting in the lab frame, we were to move parallel to the wire at speed v_d, we would observe that there was no net motion of electrons in the wire and, hence, no current. We know from Ampere's law (or, if you feel like making things really ugly, the Bios-Savart law) that there should be a magnetic field due to the motion of the charges, when there is such a motion. This means that in the lab frame, a magnetic field should be observed, while in the moving frame one should not be.

If we use a relativistic description of the fields, we will find that this same result holds just by transforming between the frames, instead of having to consider what happens to the sources. And, were we to do this explicitly, we would find that not only is are the magnetic fields different in the two frames; but, the electric fields are, as well. This can be attributed to the difference in charge density due to the Lorentz-Fitzgerald contraction.

In the cases of a current loop or a bar magnet, there is no frame we could transform to in which there is no net motion of charges (or equivalent motion, in the case of the bar magnet). This means that there is no frame with 0 magnetic field. However, as ZZ pointed out, the fields will not be the same when measured in two different frames; they will just not be 0 in either.

 

[Epsilon Pi]

Thank you, Parlyne for being more explicit!

If, instead of just sitting in the lab frame, we were to move parallel to the wire at speed v_d, we would observe that there was no net motion of electrons in the wire and, hence, no current. We know from Ampere's law (or, if you feel like making things really ugly, the Bios-Savart law) that there should be a magnetic field due to the motion of the charges, when there is such a motion. This means that in the lab frame, a magnetic field should be observed, while in the moving frame one should not be.

Sorry, my friend, but I really, do not understand this, from the physical point of view, unless you are thinking in a though experiment. I've never seen a current vanished in the real life by such a relative movement, this is just an idealization, a result of applying the principle of relativity to something that is not relative: the inherent magnetic field of the electron.

Oh God, how you can say you can cancel a current by a relative movement? Do you realize the implication this would have if it were true?

My best regards

EP

 

[lightarrow]

For a current loop which is circular, IS possible to find a ref. frame in which the electrons are stationary: a frame in the center, rotating with the same electron's angular speed. Of course, it's not an inertial ref. frame, however.
I was answering to Parlyne.

 

[Parlyne]

Thank you, Parlyne for being more explicit!



Sorry, my friend, but I really, do not understand this, from the physical point of view, unless you are thinking in a though experiment. I've never seen a current vanished in the real life by such a relative movement, this is just an idealization, a result of applying the principle of relativity to something that is not relative: the inherent magnetic field of the electron.

Oh God, how you can say you can cancel a current by a relative movement? Do you realize the implication this would have if it were true?

My best regards

EP

Have you ever tried to measure the magnetic field of a wire through which no current is travelling? If not, try it. You'll find that there is no magnetic field. This is not all that different from the case of moving along a wire at the drift velocity of the electrons.

And, no one here except you is trying to apply relativity to the magnetic dipole of an electron. Why? Because the magnetic field from the current in a wire is not caused by the electrons' dipole moments.

 

[Parlyne]

For a current loop which is circular, IS possible to find a ref. frame in which the electrons are stationary: a frame in the center, rotating with the same electron's angular speed. Of course, it's not an inertial ref. frame, however.
I was answering to Parlyne.

True, but we can't use Lorentz transformations to get to that frame, so the question of what happens to the fields under Lorentz transformations can't be answered that way.

 

[Epsilon Pi]

Sorry a wire, through which no current is traveling, or a wire where there is no electrons?

An electrons' dipole moments? what is it? I know the electron has an intrinsic magnetic moment, or an inherent polarity that gives reason of that intrinsic magnetic moment, and intrinsic means it cannot be vanished by a relative movement.

My best regards

EP

Have you ever tried to measure the magnetic field of a wire through which no current is travelling? If not, try it. You'll find that there is no magnetic field. This is not all that different from the case of moving along a wire at the drift velocity of the electrons.

And, no one here except you is trying to apply relativity to the magnetic dipole of an electron. Why? Because the magnetic field from the current in a wire is not caused by the electrons' dipole moments.

 

[masudr]

A slight error in this discussion is that since a wire is electrically neutral, electrons really move relative to the positive ions; so in a frame where the electrons are at rest, there will be positive ions moving in the opposite direction.

For this reason, I recommend we use a beam of charged particles and look at that in different frames, as opposed to a wire.

 

[ZapperZ]

Sorry a wire, through which no current is traveling, or a wire where there is no electrons?

An electrons' dipole moments? what is it? I know the electron has an intrinsic magnetic moment, or an inherent polarity that gives reason of that intrinsic magnetic moment, and intrinsic means it cannot be vanished by a relative movement.

My best regards

EP

Take a stationary line charge. Now unless you are also claiming that Gauss's law is also wrong and an "idealization", find all the field due to that line charge.

Now, someone else is moving with velocity v in the direction that this line charge is oriented. This person sees instead, a moving line of charge, i.e. a current! Thus, via Maxwell equation alone, that person detects a magnetic field.

Now, which part of those two do you not understand, or don't you agree?

Thirdly, NO ONE is claiming that one can transform away a dipole field. You keep asking for the magnetic moment due to the electron spin to be transformed away. Even transforming the relativistic maxwell equations do not result in that! So stop with this nonsense already.

Lastly, and again, this has been mentioned earlier, is that you keep forgetting that we DO transform such a thing in particle accelerators. There are many instances where the dynamics, especially when the particles are undergoing several beam "acrobatics", is simpler when it is solved in the particle's reference frame! In such a condition, the beam self-energy from the magnetic field is transformed away and allows for many of the dynamics to be solved. We then tranform back to the lab frame and use the result! If such a thing is wrong or an "idealization", we would have a ton of wrong results that simply will not match our experiments.

I have asked you before to cite valid references to back your claim. You produced no such thing. I believe that we have been MORE than patience and given you more than enough opportunity for you to air your "opinion" on this matter dispite of your continuing violation of the speculative personal theory guidelines that you have agreed to.

Therefore, this thread is done, and nothing of this nature can be discussed in the main physics forum. Any further issues related to this can only be done in the IR forum.

Zz.