Register to reply 
A SR experiment in which an entity exists in frame A but not frame B 
Share this thread: 
#37
Jan2713, 08:28 PM

P: 221

The details intrigue me. For example, it is not enough to calculate that the net charge transferred from terminal to terminal is the same in both frames. The progression of the electromagnetic field must be one and the same in both frames. By that I mean that there can be only one "actual" or "real" history for the em field. (Of course, the observed values of the field will be different from frame to frame at any chosen world point, according to the transformation equations. There must be one history, transformed into any number of frames.) Taking this idea one step further, the history of any individual electron must be the same in both frames. We cannot have an electron in motion relative to its neighbors in one frame, and stationary relative to its neighbors in the other. The reason for this particular stipulation will be clear in a moment. It is not clear to me that my qualitative analysis would satisfy the requirement of one reality. It, too, is likely to be overly simplistic. (I take pervect's most recent post as indicating the same: the situation is highly complex.) If we consider the em field, it might be possible to construct a common history from the two "frame narratives" in my qualitative analysis. The history of the em would have a common beginning and end in the two frames. In the beginning, the charge is flowing out of the positive terminal; in the end charge is flowing into the negative terminal. The history in the middle of the episode is a bit trickier. By middle, I mean the period in which: a) the charged plate is in contact with neither terminal in the battery frame; and b) a constant (?) current is flowing in the plate frame. The magnitude of the em field might be the same in both framesin one frame due to static charge, and in the other frame due to constant current. However, even if the magnitude of the field is the same in the two frames, can the particle history be the same, with current in one frame but not the other? 


#38
Jan2713, 08:55 PM

Mentor
P: 17,543




#39
Jan2813, 12:05 AM

PF Gold
P: 4,087




#40
Jan2813, 01:52 AM

Emeritus
Sci Advisor
P: 7,662

Which is why I say that circuit theory isn't quite the same thing as Maxwell's equations. The paper seems to have some other defintion of a "circuit" in mind, perhaps using retarded potentials. But it's difficult to see exactly what they had in mind, at least from my first reading it wasn't terribly clear. In any case, I think it's clear that charge will flow. It might help to replace the battery with a pair of charged spheres, one with a net + charge and the other with a net  charge, and analyze the resulting problem. The battery will be sort of like that, except there will be some mechanism that transports charge at some rate to attempt to keep the charge on the spheres constant. Said mechanism will not be and cannot be perfect. 


#41
Jan2813, 02:19 AM

PF Gold
P: 4,087

If the battery potential is 10v and it has 10 ohm internal resistance, when the terminals are short circuited we get a steady current of 1 amp on our am(p)meter. Every observer will agree on that reading. If we had instruments that could detect potentials in the battery rest frame, the readings on those instruments would be invariant. So in what way could we detect this LT failure ? 


#42
Jan2813, 04:31 AM

Emeritus
Sci Advisor
P: 7,662

3Current through a wire isn't any sort of Lorentz invariant, it's just the component of a 4vector. The 4vector as a whole is of course Lorentz invariant. But circuit theory doesn't use the 4vector approach, it uses the noncovariant 3vector. If you boost a current loop you get the situation in http://www.physicsforums.com/showthread.php?t=631446, in which the 3current varies from I*gamma to I/gamma. The easy situation to analyze is a transverse boost, where because of time dilation, the current drops by a factor of gamma. The case of a parallel boost gets a bit trickier  rather than get off topic I suggest refering to the original "boosting a current loop" post. BUt the point is that no, not everyone agrees that the current is equal to I in this situation. At least by my reckoning. Ohms law is another example of something used in circuit theory that is not Lorentz invariant. You can write it in tensor form, but the standard circuit theory version isn't written that way. Kirchoff's voltage law has some issues, too, I believe. The definition of "voltage around the loop" depends on then notion of simultaneity used. I believe that these isssues are highly relevant to the "paradox" in question. So there are a lot of elements of circuit theory that are not Lorentz invariant though Maxwell's equations certinaly are. Let me point out on the experimental level that it would be a rather unusual battery that had a 10 ohm internal impedance at all. frequences from 0100 Ghz. (It might be possible to design one with careful enough construction techniques  it doesn't seem fundamentally impossible like designing a battery with no internal impedance). If one is trying to actually understand the actual physics of a real battery in the situation, the DC impedance of the battery would be mostly irrelevant. The lowest frequency of interest would be 1 / (2 pi t), t being the time of contact. So if t is on the order of 1 ns (a rather long wire or battery of 1 foot), you'd be in the Ghz region already as the lowest frequency of interest. Actual circuits usually have capacitors across the battery placed at strategic locations to keep the impedance low across a range of frequencies, they don't rely on a battery having a low impedance at "high" frequencies. Usually you use a lot of such "bypass" capacitors, you scatter them around the circuit board strategically, near the source of things you need to bypass. What's really important at high frequencies is capacitance  and lead inductance. This suggests some obvious (to my mind) simplifications of the original thought experiment to avoid batteries (just use a capacitor, that's what you'd use realistcally anyway) but perhaps this is drifting away from the point (though if you wanted to actually carry out an experiment, it would be essential). I still believe that the simple resolution of the issue is to say that Maxwell's equations are Lorentz invariant, but lumped circuit theory has various and sundry issues. There might be a way to cast circuit theory in a provably Lorentz invariant form, but I'm not aware of anyone who has actually done this. 


#43
Jan2813, 05:57 AM

PF Gold
P: 4,087

Pervect, thanks for the detailed breakdown of the issues, some of which you've aired in previous posts here and elsewhere. The point I'm trying to make is that it is not relevant that noone has modelled all these phenomena in a relativistic way because relativity already contains the resolution.
What I mean is this  the physical events that take place can be thought of as a skein of timelike and null worldlines representing electrons and other involved matter, and em radiation. The configuration of these 4D curves is the physics. And it is fundamental to SR that no relativistic effect can alter this. For instance in the simple case of the electron moving relative to an electric field. Some observers see a magnetic field why ? Because otherwise the worldline of the electron would look different in that frame if it were not there. So relativistic effects are there to preserve the integrity of the fundamental configuration. So we know there can be no paradox in the batterytrack scenario, and efforts to explain this are purely academic exercises  and not being able to explain it actually means nothing at all. Although it can be fun and instructive. 


#44
Jan2813, 09:20 AM

Emeritus
Sci Advisor
P: 7,662

Unfortunately, the dedicated "paradox hunter" seems to be mostly beyond reach (perhaps someday one will surprise me though by listening and understanding.) The "paradox hunter", by focusing on evermore complex problems, manages to avoid confronting the underlying issues (mostly related to the relativity of simultaneity) that cause their problems in understanding the theory. Basically the way to actually learn a theory is to study simple examples, not complex ones. Complex examples are good for creating fear, uncertainty, and doubt, debating, and generally "blowing smoke", but aren't usually very good for learning. 


#45
Jan2813, 11:25 AM

PF Gold
P: 4,087

My post is a bit whimsical if not tautological and offtopic. However, I don't rate simultaneity as a major cause of confusion. For example, in the classic barnpole scenario, the paradox is said to arise from a simultaneity issue. But actually there is no frame in which pressing the button and the door closing can be simultaneous. The resolution is that the time between these events ( which happen in the barn frame) is hugely dilated in the pole frame. So the resolution is by looking at clock rates  not simultaneity. I'm occupied for the next 12 hours, so I won't be able to reply to a storm of protest, should one arise. 


#46
Jan2813, 04:21 PM

PF Gold
P: 4,792

I haven't read every detail of this thread but I don't think any of you have considered the fact that there is an EM wave and currents flowing in the copper plate even before the plate reaches a battery terminal which makes the analysis even more complicated.



#47
Jan2813, 05:45 PM

P: 221

I doubt that there is a simple solution to the question posed in the referenced paper, or to the question posed in the similar problem in Taylor and Wheeler. Indeed, the qualitative analysis which I presented a few posts back has the same problem as the solution offered by the authors of the referenced paper. The problem is this: In a simple solution, we assume that when (in a frame) a terminal is not in contact with the plate, no charge is transferred from that terminal to the plate. If we choose any event from the middle part of the sequence, we will find that neither terminal is touching the plate in the rest frame of the battery, and both terminals are touching the plate in the rest frame of the plate. Consequently, in any simple solution, we will find charge passing from terminal to plate in the one frame but not the other. We will thus have an em field in the neighborhood of the terminal that exists in one frame but not the other. 


#48
Jan2813, 06:02 PM

Physics
Sci Advisor
PF Gold
P: 6,260




#49
Jan2813, 09:23 PM

P: 221

It makes no sense because two spatially separated terminals cannot be at one event. The mental image that I had when I made the statement was based on the qualitative explanation several posts back. The times and positions in that explanation are from the spacetime diagram shown in that post. Even so, as I started to say what I meant in more precise terms, I found that the mental image I had formed of the process is not in agreement with the spacetime diagram. I'll need some time to think this through more carefully. 


#50
Jan2813, 09:27 PM

Emeritus
Sci Advisor
P: 7,662

The "wave" of moving charges exists in all frames. There's also the "retarded potential". I would guess that you're talking about the "em wave" in the paper, and my best guess is that they're talking about the retarded potential, so I'll answer it assuming that's what you meant, and assuming that's what the paper meant. (If you're complaining about the paper, I have to agree it's not terribly clear what it meant). If you meant something else, we can have another goaround. The retarded potential is a bit like a voltage. It's not really directly measurable (at least not classically). It's not uniquely defined because of the gauge condition, so you might regard it as a mathematical abstraction. If you regard tensors as "existing" then it exists. If you require that it be able to be measured with some instrument, then it's just a mathematical abstraction (in all frames) because no instrument can measure the gauge part of it, the gauge part can be set arbitrarily (within certain rules. There's no interpretation of the retarded potential that meets your criterion of "existing in one frame and not another" that I can see. you can regarded as "existing" or "being a mathematical abstraction" sensibly, depending on the details of what you mean by "exist", but the question of its "existence" doesn't depend on the frame in any way. In this refinement of the barnandpole paradox, we learn that lumped circuit elements don't exist. They are rather similar to rigid bodies, in that they are overly simple. A rigid body is defined by a small set of numbers  it's position, and rotation. The equations that model it are simple differential equations. A nonrigid body is defined by a "fluid". The equations that model it are partial differential equations. The lumped circuit elements are also defined by a small set of numbers  charge for a lumped capacitor, current for a lumped inductor. Their equations of lumped circuit elements, in ordinary circuit theory, are described by ordinary differntial equations. This is only an approximation. Real, physical circuit elements need to be described by fields. The equations that describe these fields are partial differential equations, Maxwell's equations. Lumped circuit elements, are, like rigid bodies, only approximations. The actual description of a bodies state requires more than a few numbers. If you start to draw off charge from a capacitor, lets say you put a discharging wire on the left side of the plate, the voltage on the right side of the plate does not jump instantaneously, faster than light. The charge has to flow across the plate, through the wire. You can try to make a "paradox" out of this. Nothing can move faster than light but in your lumped circuit model, the right side of the plate discharges instantaneously when you connect the wire to the left side of the plate. But there isn't any "paradox". There is only a model that's insufficiently advanced  a model that's trying to describe what needs to be describe by fields and partial differential equatons by "avereages" of the fields and ordinary differential equations. 


#51
Jan2813, 10:12 PM

P: 221




#52
Jan2813, 10:39 PM

Physics
Sci Advisor
PF Gold
P: 6,260

http://www.physicsforums.com/showpos...6&postcount=22 


#53
Jan2913, 05:38 AM

Emeritus
Sci Advisor
P: 7,662

http://en.wikipedia.org/w/index.php?...ldid=518131167 Also known as the "retarded potential" http://en.wikipedia.org/w/index.php?...ldid=530495062 To get the potential, you need to add up the contributing potentials for all charges. Initially, all the charges should be in the battery, so at the start the LW potetial is determined by the battery. The electric part of it varioulsy called E or [itex]\varphi[/itex] is basically a voltage. You can measure it with a voltmeter except for an additive constant. If you'll look at the definition, you'll see that because of the retarded time, the L.W. potential of a charge incorporates lightspeed propagation delays. So the position of a charge now doesn't add to the potential until "later", later being determined by the lightspeed delay in the frame you choose to do the analysis. The magnetic part of it is usually called A. I'm not sure how much to say about A,perhaps it's best to read the wiki article and see if you have any questions about it. Failing that (i.e. if the wiki is so much goobley gook as far as you're concerned) you can tell us if you know what div, grad, and curl are. If you do, we might be able to say a bit more about A. (E,A) forms a perfectly valid 4vector (i.e. a tensor). If you regard tensors as "existing", then it "exists". But I'm not sure of your philosophy here. 


#54
Jan2913, 07:17 AM

Mentor
P: 17,543

However, if someone did propose such a scenario then the charges for a) and b) are not related by a boost so we wouldn't expect the resulting EM fields or any other aspect of the scenarios to be related by a boost. That doesn't contradict my assertion above, it is just not relevant to such a scenario. 


Register to reply 
Related Discussions  
Rotating cylinder on x'axis in S' frame. Find twist per unit length in S frame  Advanced Physics Homework  1  
Defining a Lagrangian in an rotating reference frame frame  Classical Physics  4  
Natural Frequency Of Space Frame & Motorcycle Frame  General Physics  5  
Transformation of EM fields from rest frame to rotational frame  Classical Physics  0  
A frame dragging experiment in an accelerated reference frame  Special & General Relativity  5 