magnetic, electric fields and motion

[Perpetual Student]

A moving magnetic field creates an electric field and a moving electric field gives rise to a magnetic field. Now, since motion is relative, if I had an electric field before me and I began to move around, that would create a magnetic field. So, how does my motion cause the electric field to create a magnet field? Obviously, the same question arises about my moving around a magnetic field to create an electric field.
It would appear that the two fields are there all along and that my motion somehow causes the other field to be revealed. Does that make any sense?

[sol invictus]

A moving magnetic field creates an electric field and a moving electric field gives rise to a magnetic field. Now, since motion is relative, if I had an electric field before me and I began to move around, that would create a magnetic field. So, how does my motion cause the electric field to create a magnet field? Obviously, the same question arises about my moving around a magnetic field to create an electric field.

That's a very good question. Fortunately, some guy already came along and answered (http://www.pro-physik.de/Phy/pdfs/ger_890_921.pdf) it.

It would appear that the two fields are there all along and that my motion somehow causes the other field to be revealed. Does that make any sense?

Kind of. In relativity, E and B fields are related in a way similar the way space and time are related. Just as one cannot think of space and time separately, and should think of "spacetime", E and B fields fit into a second-rank Lorentz tensor known as the electromagnetic field strength. In a sense, they are as closely related as "in front of me" is to "behind me" - a rotation (in this case, a boost or change in velocity) can turn one into the other.

[Perpetual Student]

Vielen Dank für die Quelle. Ich reagiere auf Sie nach der Dekade, die ich sie lesen muss.

[Zeuzzz]

That's a very good question. Fortunately, some guy already came along and answered (http://www.pro-physik.de/Phy/pdfs/ger_890_921.pdf) it.
Vielen Dank für die Quelle. Ich reagiere auf Sie nach der Dekade, die ich sie lesen muss.


Am I missing something here? Or can everyone but me speake fluent german?!?

Can U speak German Mr Invictus?

[sol invictus]

Vielen Dank für die Quelle.

Bitte.

Am I missing something here? Or can everyone but me speake fluent german?!?

Can U speak German Mr Invictus?

Poorly (although physics papers are pretty easy). Here's (http://en.wikisource.org/wiki/On_the_Electrodynamics_of_Moving_Bodies) an English translation.

[Ziggurat]

It would appear that the two fields are there all along and that my motion somehow causes the other field to be revealed. Does that make any sense?

In case you don't want to wade through Sol's reference (basically, Einstein's seminal paper on special relativity), basically both electricity and magnetism are part of a single electromagnetic tensor (represented by a matrix) in special relativity. There are four dimensions (three space and one time), which gives us a 4x4 matrix with 16 elements. However, the matrix is antisymmetric, which means the 4 diagonal elements must be zero, and the other 12 elements must be mirrors of each other, meaning 6 independent parameters. Which is exactly how many you have with two 3D vectors. So you can combine your two 3D vectors for magnetic and electric fields into a single 4D electromagnetic tensor.

Now, when you change reference frames, you need to transform quantities like position and velocity (vector quantities), and you can do that with matrix multiplication, where the relevant matrix is your Lorentz transformation. Well, you use this same matrix to transform your electromagnetic tensor too. And just like the various components of a position or velocity vector get shuffled around when you do that transformation, the various components of your electromagnetic tensor do too. The result is that in some frame you may have only the electric field field components, but in another you have both electric and magnetic field components, just like a velocity vector could have only x components in one frame but both x and y in another frame.

[sol invictus]

To amplify what Zig said, here's a very useful mnemonic. Just as rotations preserve lengths of vectors (like x^2+y^2+z^2), Lorentz transformations also preserve certain quantities. In the case of E and B, there are two: $\vec E \cdot \vec E - \vec B \cdot \vec B~~\& ~~ \vec E \cdot \vec B$ (I've left off a few unit-dependent constants in the first expression).

So if in one frame there's an E field but no B field, you'll never be able to boost to a frame where B is bigger than E in magnitude - because that would flip the sign of that first expression. And for the same reason, if E and B are perpendicular in one frame, they are in all.

[Perpetual Student]

To amplify what Zig said, here's a very useful mnemonic. Just as rotations preserve lengths of vectors (like x^2+y^2+z^2), Lorentz transformations also preserve certain quantities. In the case of E and B, there are two: $\vec E \cdot \vec E - \vec B \cdot \vec B~~\& ~~ \vec E \cdot \vec B$ (I've left off a few unit-dependent constants in the first expression).

So if in one frame there's an E field but no B field, you'll never be able to boost to a frame where B is bigger than E in magnitude - because that would flip the sign of that first expression. And for the same reason, if E and B are perpendicular in one frame, they are in all.

Fascinating! What would a response to my question have been in Maxwell's day? -- or anytime before SR?

[sol invictus]

Fascinating! What would a response to my question have been in Maxwell's day? -- or anytime before SR?

Some of the ideas of SR pre-dated Einstein - Lorentz transformations are named after Lorentz, after all. But I don't know the answer to your question in any detail. Maybe someone else can comment.

[Ziggurat]

Fascinating! What would a response to my question have been in Maxwell's day? -- or anytime before SR?

One answer (which is wrong, but seemed reasonable at the time) would have been that there's a proper reference frame from which to do E&M calculations, namely the reference frame of the medium (the aether) in which electromagnetic waves propagate. If you shift to another reference frame, your calculations of fields will be wrong, but it's a happy coincidence that (to first order) the calculated forces on any charges work out the same, so let's not worry too much about it.

[ben m]

A moving magnetic field creates an electric field and a moving electric field gives rise to a magnetic field. Now, since motion is relative, if I had an electric field before me and I began to move around, that would create a magnetic field. So, how does my motion cause the electric field to create a magnet field? Obviously, the same question arises about my moving around a magnetic field to create an electric field.
It would appear that the two fields are there all along and that my motion somehow causes the other field to be revealed. Does that make any sense?

I do want to say, Perpetual Student, that this was a really great question, and indeed (insofar as it prompted Einstein to find Special Relativity) one of the most important questions ever asked in the history of physics. In this day and age, it's unusual to find anyone who would bother to ask "what about moving observers" except for people who already know Special Relativity which highlights this question so clearly. Kudos to you for getting it the other way round.

[Tim Thompson]

What would a response to my question have been in Maxwell's day? -- or anytime before SR?
I think the answer would have been something on the order of "beats the hell out of me, son".

Electricity & magnetism were thought to be completely separate and independent forces, until Ørsted (http://en.wikipedia.org/wiki/Hans_Christian_Ørsted) & Faraday (http://en.wikipedia.org/wiki/Michael_Faraday) discovered the generation of magnetic fields by electric currents. But it was Maxwell (http://en.wikipedia.org/wiki/James_Clerk_Maxwell) who nailed it all together in the 1860's and established the science of electromagnetism. Indeed, Maxwell was the first physicist to truly unify forces (electricity & magnetism) which had previously been thought to be mutually independent.

Fundamental physics in the late 19th century was dominated by the terrible conflict that the physics of Newton & Maxwell were both unquestionably correct, and equally unquestionably incompatible with each other, because the speed of light in Maxwell's electromagnetism is not relative to anything, as Newton would demand, but an absolute constant. There were lots of inventive attempts to solve this problem, but the one that finally worked was Einstein (http://en.wikipedia.org/wiki/Albert_Einstein)'s, which was probably the least "inventive" of all. He simply went with Maxwell, and the rest is history, as they say.

[Sol88]

Perpetual Student
magnetic, electric fields and motion
A moving magnetic field creates an electric field and a moving electric field gives rise to a magnetic field. Now, since motion is relative, if I had an electric field before me and I began to move around, that would create a magnetic field. So, how does my motion cause the electric field to create a magnet field? Obviously, the same question arises about my moving around a magnetic field to create an electric field.
It would appear that the two fields are there all along and that my motion somehow causes the other field to be revealed. Does that make any sense?

Great work there Son!

Keep asking them questions, they are spot on relevant to a post you made here in Plasma cosmology woo or not 1735 (http://forums.randi.org/showpost.php?p=4558579&postcount=1735)

And you will have an epiphany!

Because Ziggurat asked Those points don't address what's driving your proposed charge separation. And it can't be electricity: electricity always opposes the separation of charges. So what else is there? Magnetism? Well, that won't work either, because you're claiming that charge separation drives the currents which create those magnetic fields, so you don't have a magnetic field until after you get charge separation. Unless you think it works like a galaxy-sized perpetual motion machine, but if you believe that, there's no point in any further discussion. So I'll assume you just didn't think far enough to realize that you don't actually have a mechanism to drive your charge separation.

[Perpetual Student]

Great work there Son!

Keep asking them questions, they are spot on relevant to a post you made here in Plasma cosmology woo or not 1735 (http://forums.randi.org/showpost.php?p=4558579&postcount=1735)

And you will have an epiphany!

Because Ziggurat asked

Sorry, I don't understand your point.

[Ziggurat]

Sorry, I don't understand your point.

I don't think anyone does. He's dragging in a quote from me in a different thread, for reasons I cannot fathom. Not that he was making any sense in that other thread.