Talk:Special relativity/Archive 13

message: :Image:SphereAberration01.gif
(Linkified animation in response to complaints about load time. --Christopher Thomas 04:30, 30 August 2006 (UTC)
replies: text: Image:XYCoordinates.gif
(Linkified animation in response to complaints about load time. --Christopher Thomas 04:30, 30 August 2006 (UTC)

text: That's much better. I cleaned up the text a little for Relativistic Doppler effect, but it looks fine now. It would be nice if you added a summary of how you created the images to the image description pages. Other than that, keep up the good work! —Keenan Pepper 02:44, 25 August 2006 (UTC)
replies: text: I'm very appreciated for the text improvement. Thank you. I've started to add the summary . It is not finished yet. I think to make a few new images to illustrate the long formulas.--TxAlien 04:26, 28 August 2006 (UTC)

text: I think I've found the right image in the end. Now, I have to find the shortest formulas and the right words. It is :Image:Lcprojection01.jpg --TxAlien 06:52, 30 August 2006 (UTC)

text:  I am about to finish the text How this image was created I'll be glad if those images will be useful to anyone and thankful for any comments. --TxAlien 21:37, 4 September 2006 (UTC)
thread_title:  New image 

message: In "Reference frames, coordinates and The Lorentz transformation" section the reference frame is defined as "a point in space at rest, or in uniform motion...". There is no absolute rest, so I would suggest omitting the word and change to "a point in uniform (non-accelerating) motion..." --Michagal 12:03, 9 August 2006 (UTC)
thread_title:  reference frame definition 

message: An editor made an edit changing "Lorentz contraction" to "Fitzgerald contraction".  I have never heard this effect called the "Fitzgerald contraction".  Admitedly Fitzgerald published it first , but it was Lorentz who promoted it and got people to notice it.  That is how it became known as the "Lotentz contraction".  The other common title for it is the "Lorentz-Fitzgerald contraction".  Even the "Fitzgerald-Lorentz contraction" in the reference web article is an anachronism.
If this editor wants to push this change, then proper csources need to be cited.  The best that I can offer is to refer to length contraction instead. --EMS | Talk 18:51, 15 August 2006 (UTC)
replies: text: That is OK. I will not fight about it. I just remembered that it was called the "FitzGerald contraction" when I learned about it, but that was many decades ago and I cannot find the books that I learned it from. I thought that "Lorentz" was a mistake, but I see that the article on Lorentz-FitzGerald contraction hypothesis says that Lorentz also discovered it independently shortly after FitzGerald. JRSpriggs 03:23, 16 August 2006 (UTC)
thread_title:  Lorentz-Fitzgerald contraction 

message: I do not like the placement of "c" in this section. Using "ct" instead of "t" as the time coordinate leads to equations which are hard to understand because the time coordinate is given in meters (even though proper-time is still measured in seconds). There is no point in doing this because one is forced to distinguish between contravariant and covariant anyway due to the minus sign. Please allow me to rewrite the section to change the placement of "c" so that we are using International System of Units consistently. JRSpriggs 08:31, 19 August 2006 (UTC)
replies: text: First, I will make some fixes which do not affect the units or "c"s. If no one objects here in the next few days, I will move on to change the units. JRSpriggs 05:03, 21 August 2006 (UTC)
replies: text: The editing will take several days. So when I am changing the units, the section will be inconsistent. That is some formulas will have the SI units and some will still have the units which are there now where time is measured in meters. JRSpriggs 05:51, 21 August 2006 (UTC)
replies: text: OK. Now I have changed the units of time from meters to seconds in the "Physics in spacetime" section. I have also made a number of other changes to try to use contravariant and covariant indices correctly. Please take a look at it and say what you think. Especially check the equations of electromagnetism which I think may be in a form superior to those in the special relativity section of the article "Maxwell's equations". JRSpriggs 06:06, 25 August 2006 (UTC)
thread_title:  '''c''' in the section [[Special relativity#Physics in spacetime]] 

message: ::Photons are deflected by gravity, but they do this because the follow geodesics.  There are not any "forces" involved.  This is as it should be, for the concept of force depends on the frame used, and a photon has no frame.  Matter does change the directions of photons, but this has to be explained in the matter frame, again, photons don't have a frame.
::For starters, try "Griffiths, Introduction to Electrodynamics", where four-forces are called Minkowski forces.  Also, Misner, Thorne, Wheeler, "Gravitation", which uses them a lot, and explains the concept of "geometric objects" on page 48, of which four-forces are an example.   Jackson, "Classical Electrodynamics", uses 4-forces to derive the Faraday tensor (see Force, Lorentz, Covariant form) though otherwise he doesn't use them very much.
::Geometric objects, of which four-forces are an example are quite useful to relativity, i.e the following quote from "Gravitation"/
:::(References) have expounded the mathematical theory of geometric objects.  But to understand or do research in geometrodynamics, one need not master this elegant and beautiful subject.  One need only know that geometric objects in spacetime are entities that exist independently of coordinate systems or reference frames.  A point in spacetime ("event") is a geometric object.  The arrow linking two neighboring events ("vector") is a geometric object in flat spacetime, and its generalization, the "tangent vector", is a geometric object even when spacetime is curved.  The "metric" (machine for producing the squared length of any vector) is a geometric object.  No coordinates are needed to define any of these concepts.
::Four forces are the expression of the concept of force as a geometric object.  It's that simple, ultimately.
::I'm pretty sure Taylor & Wheeler "Spacetime Physics" uses four-forces, but I don't own a copy to double-check.  Most of my SR books are really E&M books - or GR books.
::Return the challenge - name a reference that supports your POV? 
::In the same vein, Re "Density of force". It would be helpful here as well to quote a reference.  I have never seen a reference to "density of force" outside the concept of electromagnetism.  Do you have some reference which supports your usage here?
::The density of energy and momentum expressed as a geometric object is not a 4-vector, but a rank 2 tensor, the stress-energy tensor.  The geometric representation of a volume element is not a scalar, but a vector - the dual vector to a 3-form.  The 3-form represents the volume element, so does its dual.  Basically, a volume can be defined by its magnitude, and the direction of the "time vector" orthogonal to the volume.   Thus "energy / momentum per unit volume" is covariantly expressed as (rank 2 tensor) * (vector representing volume) = (four-vector energy and momentum contained in volume).  This is explained in MTW's "Gravitation".
::One can take the divergence of this stress tensor to come up with a vector.  This turns out to be zero in general \nabla_a T^{ab} = 0, where T is the stress-energy tensor.  So this isn't a very interesting vector, it's zero.  It represents the differential conservation laws of energy and momentum.
::The concept of force density, as I have seen it derived, comes from breaking up the stress-energy tensor Tab into two parts, a part representing the stress-energy tensor of the electromagnetic field, and a part representing the stress-energy tensor of the particles only.    The equation says that the divergence of the field part is equal to the negative of the divergence of the particle part.  This defines the "force density".  To define it in this manner comprehensibly, one must first introduce the stress-energy tensor.  Again, if you have a reference that says differently, it might help clarify what you are trying to say to quote it. Pervect 18:19, 3 September 2006 (UTC)
replies: text: \frac{dp_\mu}{dt} = \Gamma^{\nu}_{\mu\alpha} p_{\nu} \frac{dx^\alpha}{dt} + q F_{\mu\nu} \frac{dx^\nu}{dt} + \operatorname{other}\,\operatorname{forces}
where pμ is the 4-momentum at time t and \Gamma^{\mu}_{\nu\alpha} is the Christoffel symbol.  The Christoffel symbol is the force-field of inertial forces, such as gravity, centrifugal force, and Coriolis force. It is not a tensor. One can say that these forces are a fiction which disguises the curvature of space-time, but this might well be true of all other forces also. The equivalence principle implies that curvature and inertial force are equivalent descriptions of reality.
If you had bothered to read to the bottom of my subsection on electromagnetism, you would see that I mention the stress-energy tensor and show how it is related to the force density. They are two different things. The stress-energy tensor gives the flux of momentum and energy thru a THREE dimensional boundary. While the force density gives the amount of momentum and energy changing form (from electromagnetic to material form, say) within a FOUR dimensional body. You are confusing them and then blaming me for your confusion. JRSpriggs 06:11, 4 September 2006 (UTC)
thread_title: Four force, part 2

message: The relativity of simultaneity article is in very bad need of attention.  It is currently poorly written, and the last half is an anti-Einstein diatribe which belongs in relativity priority disputes if it belongs anywhere in Wikipedia.  I made an attempt to reduce this article to a stub, but other editors objected and reverted it back.
This is an important sub-topic of relativity, and it is a disgrace to have this important article in the hideous shape that it is now in.  I am hoping that others can help me to correct it.  Otherwise, I will attempt to have it deleted.  --EMS | Talk 02:36, 8 September 2006 (UTC)
replies: text: The last half of the page is not an anti-relativity diatribe, but a calm presentation of the pre-history of relativity of simultaneity. It may be out of place, or irrelevant, or many other things, but hardly a diatribe. E4mmacro 06:37, 9 September 2006 (UTC)

text: An article with such a general title and scope should not be treated as a sub-topic of Special relativity theory - instead it is (or should be!) much wider in scope, as discussed on its Talk page. Harald88 13:21, 9 September 2006 (UTC)
thread_title:  relativity of simultaneity 

message: Looking over the items which Agne27 marked with "citation needed", I have to agree that most of them are common knowledge among physicists. However, there is one (second paragraph of "Status") which I doubt, namely "Because of the freedom one has to select how one defines units of length and time in physics, it is possible to make one of the two postulates of relativity a tautological consequence of the definitions, but one cannot do this for both postulates simultaneously, as when combined they have consequences which are independent of one's choice of definition of length and time.". Can someone justify this (the first clause)? JRSpriggs 05:55, 29 September 2006 (UTC)
thread_title: Did she get one right?