Talk:Length contraction

Wheel Illustration of length contraction

The wheel picture should be a rather different (probably an egg-like) shape. The centre of curvature of the (stationary) base should be at the height of the original centre (= half way to the top), and the wheel should be getting narrower with increaing height at this half way point.

(Special relativity actually shows this to be an oversimplification, as the wheel would be bent out-of plane). PhysicistQuery (talk) 17:48, 18 March 2021 (UTC)

In the rest frame of its center, the wheel is a circle. Since a Lorentz transformation changes a circle into an ellipse, the moving wheel appears like an ellipse. When the wheel spins, its circumference is contracted, but not its spokes (because they move perpendicularly to their length). Hence the wheel shall be bent out of plane. I supposed that the elasticity of the spokes is much greater than the elasticity of the circumference to avoid such a bending. TD (talk) 05:41, 19 March 2021 (UTC)

Magnetic Forces section

Magnetic forces are caused by relativistic contraction when electrons are moving relative to atomic nuclei. The magnetic force on a moving charge next to a current-carrying wire is a result of relativistic motion between electrons and protons.

This is utter rubbish! It implies that an electron outside and stationary to the wire would be repelled because in that case the electrons would be contracted.

Hilariously enough, this concept is due to the relative velocity inside the wire of electrons to the nuclei, because their relative motion is what generates the changing E fields in the wire and along its direction, thus inducing the curl B. (Displacement current).

The Feynman citation and the other (assuming its based on the same thing) ought to be flushed ASAP! Byron Forbes (talk) 05:44, 23 May 2023 (UTC)

In 1820, André-Marie Ampère showed that parallel wires having currents in the same direction attract one another. In the electrons' frame of reference, the moving wire contracts slightly, causing the protons of the opposite wire to be locally denser. As the electrons in the opposite wire are moving as well, they do not contract (as much). This results in an apparent local imbalance between electrons and protons; the moving electrons in one wire are attracted to the extra protons in the other. The reverse can also be considered. To the static proton's frame of reference, the electrons are moving and contracted, resulting in the same imbalance. The electron drift velocity is relatively very slow, on the order of a meter an hour but the force between an electron and proton is so enormous that even at this very slow speed the relativistic contraction causes significant effects.

LOL. Really? How about, above and below the wires we have B fields attracting each other. Do we really need to stink up everything with Relativity? — Preceding [[Wikipedia:Signatures|Byron Forbes (talk) 07:15, 23 May 2023 (UTC)]] comment added by [[User:Hosh1313|Byron Forbes (talk) 07:15, 23 May 2023 (UTC)]] (talk • contribs) 05:50, 23 May 2023 (UTC)

It's as if Maxwell never happened, right?

It's so unfortunate this explanation is so common even on youtube. This needs to change.

Here's my suggestion on how to change the paragraph:

"It's commonly taught that length contraction is responsible for the appearance of magnetic fields in wires. In this view, magnetic forces are caused by relativistic contraction when electrons are moving relative to atomic nuclei. The magnetic force on a moving charge next to a current-carrying wire would be a result of relativistic motion between electrons and protons. This explanation originated form Edward Purcell's 1965 book Electricity and Magnetism , and it's been recycled by Feynman and Landau.

The problem with this explanation is that length contraction happens in any frame that is not at rest with the observer, which would be the case in the scenario where a stationary charge is placed near a wire with a current. From the point of view of the test charge electrons in the wire are subject to length contraction due to their motion, so the average distance between electrons is larger than the distance between the positive nuclei. This ultimately results in a net positive electric field (due to the higher charge density of nuclei) which would act to repel or attract the test charge, depending on its sign.

In experiments this is not observed: the effect of the magnetic field generated by the current applies only on charges moving with respect to the wire. The reason for this is that frame-dependent electrostatic forces are exactly balanced by frame-dependent magnetostatic forces.: thus, the implication that magnetic fields are fundamentally electric fields seen from the "wrong" reference frame is erroneous, as in reality both fields have an independent existence as different aspects of a unified electromagnetic field ." Leureka (talk) 09:01, 15 August 2023 (UTC)

References

Can't do that. Properly sourced content folllowed by an entirely wp:unsourced analysis is standard wp:original research. By design, Wikipedia sticks to properly sourced content, as indeed is the case in the current section Length contraction#Magnetic forces. - DVdm (talk) 09:19, 15 August 2023 (UTC)

Isn't that simply a matter of sourcing the theory of length contraction and the experiment with wires? It's not hard to do. I'll see if I have the time, other users could do it as well. Leureka (talk) 10:16, 15 August 2023 (UTC)

Please indent your talk page messages as outlined in wp:THREAD and wp:INDENT — See Help:Using talk pages. Thanks.

What I mean, is that the part of what you wrote, starting with "The problem with this explanation is...", is unsourced, original research and thus unfit for Wikipedia. - DVdm (talk) 11:58, 15 August 2023 (UTC)

So if an incorrect explanation is sourced, it's fine to be left there?

Anyway, I added the sources. Leureka (talk) 15:14, 15 August 2023 (UTC)

To the question: Almost, someone saying that a sourced explanation is incorrect, is not sufficient to remove it.

To the added sources: First, not being published peer reviewed work or established text book content, they can't qualify as reliable sources. Second, they don't directly state the thesis, and combining them with the other sources to bring a new thesis, is an example of wp:SYNTH, another instance of original research. Tricky Wikipedia . - DVdm (talk) 23:02, 15 August 2023 (UTC)

The sources ARE published and peer reviewed. An older version of kampen'S article can be found here[1]. The second source is from a blog of West Texas A&M University, by a tenured professor, whose credentials you can check here. More established than that, I don't know. From what I read wikipedia doesn't require articles from journals, only articles that can be verified and come from authorities in the subject.

The thesis is: a magnetic field is NOT simply the result of length contraction. Kampen's article does state the thesis, in the form of equations: first he states clearly what the situation is in both cases where the test particle is stationary and is moving, then he derives the forces on the test charge. He finds the paradox in the rest frame of electrons. Then he derives that the only reason there are no forces acting on charged stationary particles is because magnetic and electric forces acting on it are equal and opposite. You couldn't have magnetic forces in this situation if those were only the result of lorentz transformations on electrostatic fields. Hence, the two fields exist independently of each other. I didn't combine sources to get here. The second source is simply a rephrasing for clarity.

For the part "The problem with this explanation is that length contraction happens in any frame that is not at rest with the observer, which would be the case in the scenario where a stationary charge is placed near a wire with a current." I don't even bother adding a source, since what length contraction does is explained in the same page.

If you still find this unsatisfactory, I'd urge you and everyone else reading to actively participate in the correction of the article, since as it stands it gives a completely erroneous idea of electromagnetism. Leureka (talk) 00:40, 16 August 2023 (UTC)

The citeseerx source above produces "502 Bad Gateway", and a blog is not a reliable source. Check wp:RS - DVdm (talk) 09:43, 16 August 2023 (UTC)

I can download the PDF just fine. It's just a problem on your end.

Just look it up on google at this point.

"A blog is not a reliable source" and yet in this same page there is a link like this and this, which are both blog-like entries in an academic site (.edu).

In any case, this argument is pointless, since that blog is far from the only source available saying the same thing. Here's another, also .edu site. It's simply impossible to transform a pure electrostatic field into a pure magnetostatic field, because E^2 - B^2 is a Lorentz invariant.Leureka (talk) 09:22, 19 August 2023 (UTC)