Michael Crosiar wrote:
>
>
>>> We create a torroidal magnetic field and rotate it
>
>>Whoops your gedanken just jumped the tracks. You *can't* rotate a field.
>
>>You can rotate an object. You can rotate a frame of reference. You can
>>rotate your head trying to follow an obscure argument. But you can't
>>rotate a field, nor move it, nor do anything else which requires
>>"pushing" on it because you can't get a grip on it.
>
>
> Well, I have had to think long and hard how I want to respond. I really
> didn't want to get tied down as to the engineering of how one would
> produce such a field. But I do get your point, something must produce
> the field. If we were to take a bar magnet and rotate it at relativistic
> speeds, obviously it would fly apart.
No problem there! I wasn't concerned with practical issues, just the
issue of what, exactly, was supposed to be rotating.
So, a bar magnet it is.
If we do it the easy way, so that you've got something like this:
|
| Spin Axis
|
-------------------------------
| |
| North South|
| |
-------------------------------
|
|
|
then the answer's pretty obvious.
Start with it stationary. With it stationary, you've got a field which,
if the magnet's been there a long time, extends through all space.
Now, start the magnet spinning. As I said, you can't spin a field! So,
though one's mental model might be of the whole field spinning as a
rigid body, that's not what happens. What happens is the information
regarding the rotation propagates away from the magnet at C.
If I've pictured this right (which I may not have!), the result, in
short order, is going to be field lines twisted up in something like
spirals, with the spiral pattern expanding away from the magnet at C. A
stationary observer will see the peaks and troughs passing by at C, and
will interpret it as an EM wave.
In short, it radiates.
If you spin it about the long axis, it's a lot harder to understand what
happens, for me, at least, because the distant field is rotationally
symmetric about the long axis. In that case I don't think you get
radiation, but I think you do get an E field which wasn't there while
the magnet was stationary.
(But I could be all wet on this, I'm no E&M expert...)
> And yet it still seems that we
> should be able to imagine such a magnet that does not fly apart for the
> purposes of a thought experiment. We could talk about a stepper motor
> type configuration, but I think there would be question as to whether we
> are actually rotating a magnetic field or producing multiple fields that
> are phased in such a manner as to rotate another field (no, I'm not sure
> I said that right). And then there is the ultimate question of what a
> magnetic field is - and I'm not talking about the mathematical
> description of it.
>
> I have read - can't remember the source, and I'm probably butchering it
> - that all of physics is geometry. That is to say that once we
> understand all of the forces, they will be reduced to geometry. I don't
> know if you agree with this or not, but I do believe this will be found
> to be true. So there is the question as to whether magnetism is a
> property of the geometry of space-time itself. Yes, something does seem
> to be required to modify space-time in such a manner as to produce
> magnetism, but is that magnetism a property of that which produces it,
> or a property of the geometry of space-time itself? I say it is the
> result of the geometry of space-time itself. As such, I argue that the
> magnetic lines of force are "physical" and can be rotated relative to an
> outside frame of reference, and the thought experiment is valid.
>
> But I suspect you will insist that we must consider that which is
> producing the magnetic lines of force. Ok, I will try. But I am not an
> engineer, and know there are those much smarter than me that could
> engineer such a device. So I do this with the hope that the physics
> involved can be discussed and not my bad engineering skills. But, I will
> be brave and take a shot at this.
>
> First, there is no solid physical object that we could use to produce
> such a field and have it survive the centrifugal force involved when
> rotating at anything even close to relativistic speeds. We could use a
> stepper motor type configuration to create rotation of another existing
> magnetic field, but anything creating that field would also be subject
> to the same forces. So, we must look elsewhere. So my solution is to
> look at a plasma. We can create a rarefied plasma and align and rotate
> the magnetic domains in that plasma. I would suggest an inert rarefied
> gas be used that is ionized, perhaps Xenon. The ionized gas would be in
> a column.
>
>
> To align and rotate the magnetic domains we would use RF signals
> injected around the outside of the column, with the phases of the
> signals being shifted in much the same manner as a stepper motor. The RF
> signals injected would at first be of a low frequency and then be
> increased in frequency. The RF signals themselves would be produced by
> Back Wave Oscillators and Traveling Wave Tube Amplifiers, perhaps in a
> push-pull configuration. I do realize that there may be issues with
> harmonic oscillations that would need to be addressed.
>
>
> As the field begins to rotate I would expect it to want to collapse
> inward, so I would have a toroidal shaped shell made of Bismuth, a
> strong diamagnetic, that would surround the core with the purpose of
> forcing the magnetic lines of force outward and preventing the collapse
> of the field.
>
>
> Ok, that is my best shot. I'm sure I left out many details. In the end
> we are talking about rotating the magnetic domains in a plasma with RF
> signals to produce a magnetic field that is rotating relative to an
> outside observer. Will it work? I don't know, but I suspect that even if
> what I have described does not, someone smarter than me can think of how
> one would produce such a relativistic magnetic field.
>
>
> So, assuming such a field can be produced, would there be time dilation
> effects inside the field? Would the field acquire inertial mass because
> of the relativistic effects? Would there be a change in gravitational
> mass? If a radioactive isotope were placed inside the field, would an
> outside observer detect any change in the rate of decay?
>
>
> C. Michael Crosiar
>
>
>
>>A so-called "traveling" field, like EM radiation, is a field which
>>disappears at one point and appears at another point. If you look at
>>the equations, and think about what they're telling you, that turns out
>>to be the only thing they're saying -- there is nothing in there about
>>the fields actually "moving", nor about the "velocity" of the field
>>itself. The things that move are the maxima, minima, nodes, and
>>accompanying values. The field itself just kind of "is".
>
>>But in your gedanken, you're rotating *something*, even if it's not the
>>field. So, let's back up and ask exactly what you *are* rotating.
>>Rotation is physical, so you must rotate a physical thing. So, what
>>physical thing are you thinking of rotating?
>
>>Spell that part out and we can take it from there, and the whole thing
>>will make much more sense, I'm sure.
>
>>> at relativistic
>>> velosities, such that the inside of the torroid would be rotating at near
>>> the speed of light. The outside of the field would extand outwards
> and would
>>> have an agular velocity that would be greater, proportional to the
> increase
>>> in circumference. First, is that correct? Clearly nothing can go
> faster than
>>> the speed of light, but as we increase the speed of the rotation, the
> energy
>>> must go somewhere, yes? Would this cause the mass of the field to
> change? In
>>> other words, would it bend space-time inside the field? And could the
>>> curvature be negative or positive depending on the direction of rotation
>>> relative to the N/S pole? Would time run at a different rate inside the
>>> field versus outside the field? If we were to place a radioactive isotope
>>> inside the field, could we cause it to decay faster or slower?
>>>
>>> I'll be anxiously awaiting your insights,
>>>
>>> C. Michael Crosiar
>>>
>>>
>>
>
>
