## Lorentz Force (magnetic force term)

Discussions on classical and modern physics, quantum mechanics, particle physics, thermodynamics, general and special relativity, etc.

### Re: Cameraman

Faradave » July 13th, 2018, 9:18 pm wrote:
As discussion is going in circles, I may take a break. That's not to concede any point on the reality of length contraction. I would assume the same of your contrary position.

My money is with Juggernaut. The only “instant” of importance is the instant of the camera click and we can forget the other simultaneities. The light from the nearest point on the train will reach the camera first and light from the more distant parts will take longer. If the train is approaching, light from the front of the train will reach the camera first and light from the rear of the train will follow. The camera will record light from the front of the train closest to the moment of the click and, in the same instant, it will record slightly older light from the rear of the train. Light from the rear of the train will have been emitted first when the train was more distant so the rear of the train train will appear farther down the tracks than estimated by Lorentz contraction. There will be no apparent contraction to the length of the train.

There is a problem with your diagram because it does not depict the Doppler effect which would effect the fast moving train but not the slow train.
bangstrom
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### Re: Small Issue

Wikipedia wrote:For instance, for a small angular diameter, a moving sphere remains circular and is rotated.

Faradave wrote:We don't have cameras sensitive enough or fast moving targets large enough to reliably make such measurements.

Yes, for small angular diameter. Note, the article in no way denies length contraction. It also describes relativistic electromagnetism exactly as by the videos above.

You'll find lots of this, if you search "Length contraction in particle accelerators":
"a charged particle, like an electron, traveling at relativistic velocity has electric field lines that are compressed along the direction of motion as seen by a stationary observer. (See [link].) As the electron passes a detector, such as a coil of wire, its field interacts much more briefly, an effect observed at particle accelerators such as the 3 km long Stanford Linear Accelerator (SLAC). In fact, to an electron traveling down the beam pipe at SLAC, the accelerator and the Earth are all moving by and are length contracted. The relativistic effect is so great than the accelerator is only 0.5 m long to the electron. It is actually easier to get the electron beam down the pipe, since the beam does not have to be as precisely aimed to get down a short pipe as it would down one 3 km long. This, again, is an experimental verification of the Special Theory of Relativity."

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I posted this earlier. This paper discusses the sun. No contraction to be seen. While you're on a break, read the other paper I posted as well.
https://www.tempolimit-lichtgeschwindig ... sphere.pdf
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### Re: Gamma Game

Interesting! The large object is calculated to be "render the geometric apparent shape effectively invisible."

Note the author's frequent use of gamma and reference to the Lorentz transforms. Though the article anticipates some apparent visual phenomena, it says nothing to contradict length contraction (nor in any way addresses the OP).

Once you accept the postulates of SR (invariance of physical laws, universal speed limit c), you can't avoid gamma. Thus, relativistic mass-energy, time dilation, relativistic frequency shift and length contraction (among others, including above article) follow.

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### Re: Lorentz Force (magnetic force term)

[ Post violating forum guidelines was removed at moderator's discretion. Advising other members who engage in good faith discourse to "take a break" from posting is rude and condescending. Please keep it civil. ]

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### Re: Picture This

bangstrom wrote:My money is with Juggernaut. The only “instant” of importance is the instant of the camera click ... There will be no apparent contraction to the length of the train.

Let's concede that there may be visual effects which render an object invisible at relativistic speed. What you should ask yourself is: Can a camera can take a picture of a barn sharing the camera's rest frame? (hint: It's done all the time.) Then ask if it can take a picture of a 20' wide barn with the doors open on each end. And then ask if it can take a picture of the same barn with both doors closed. (same hint applies)

So, in principle, if a 40' pole passes through the barn at > .866c, all that is required to prove length contraction is to snap a picture of a resting barn with its doors closed (while the pole is momentarily inside). It's an angular assessment (by camera) of a linear assessment (by barn).
Anyone (on advice) can snap a picture of a resting (thus, quite visible) barn!

That's the point Lincoln was making in his video on length contraction (at time 9:20).

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### Re: Small Issue

Faradave » July 14th, 2018, 10:04 am wrote:

You'll find lots of this, if you search "Length contraction in particle accelerators":
"a charged particle, like an electron, traveling at relativistic velocity has electric field lines that are compressed along the direction of motion as seen by a stationary observer. (See [link].) As the electron passes a detector, such as a coil of wire, its field interacts much more briefly, an effect observed at particle accelerators such as the 3 km long Stanford Linear Accelerator (SLAC). In fact, to an electron traveling down the beam pipe at SLAC, the accelerator and the Earth are all moving by and are length contracted. The relativistic effect is so great than the accelerator is only 0.5 m long to the electron. It is actually easier to get the electron beam down the pipe, since the beam does not have to be as precisely aimed to get down a short pipe as it would down one 3 km long. This, again, is an experimental verification of the Special Theory of Relativity."

It might be said from this example of Lorentz contraction that an electron “sees” less distance between itself and its environment when the velocity between them is extremely great but, when the velocity between them is close to zero, they see more distance. That was Amperes understanding of magnetism the days prior to relativity.

When a current in two closely parallel wires runs in the same direction or when you have two or more loops in a coil of wire, the wires will attract but, if the current is in opposite directions, the wires will repel. Also, when you have electrons with their spins all aligned in the same direction, as with an electric current or a permanent magnet, you will have a magnetic field.

Ampere concluded that electrons flowing in one wire “see” the electrons flowing in the other wire “edge on.” So, if the electrons in both wires are moving in the same direction, they will see their nearest sides spinning in the opposite direction at a great velocity (like gears grinding) and the wires will attract because the electrons see less distance between them. But, if the currents are in opposite directions, the electrons will see their nearest sides spinning in the same direction (like gears meshing) and they will repel.

Ampere explained electromagnetic attraction as an electron to electron attraction or repulsion depending on the direction of the current as opposed to the video that explained electromagnetism as a special relativistic effect between the current and the wire. Ampere’s explanation makes more sense to me because it is not observer dependent and it applies equally well to a long wire or a double loop while the video explanation only works with a long wire. I also don’t see how the video explanation could apply in the case of the wireless current in a CRT.
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### Re: Time for a Ride

bangstrom wrote:Ampere's explanation makes more sense to me...

Then stick with that.

A watch is used to tell the time and a car is a means of transportation. When I was a kid, both were primarily mechanical. Thus, despite clearly different functions, each could be understood in terms of mechanics (gears and such). They were thus "unified" conceptually and it was satisfying to know that a great many other gadgets could be understood by taking them apart and looking at them.

Times have changed watches, cars and ourselves. Looking doesn't seem to cut it anymore. BUT it doesn't really change the reality of what we knew before. There is truth in saying that we could safely travel to Mars using only Newton's Laws. Relativity is a great and welcome advance but it doesn't diminish Newton.

Ampere's treatment works as well as it always did in most circumstances. There's no compelling reason that everyone needs to unify electric and magnetic interaction into electromagnetism (EM). Similarly, though electro-weak unification has been accomplished, I would dare to say it is sufficiently cumbersome that most physicists still treat EM and Weak interaction separately as a practical matter.

Physicists seek the satisfaction of a basis with upon which to unify the four known interactions (a ToE) and hope that if it's fundamental enough, it might shed light on some unknowns as well (dark matter, dark energy, indeterminism, entanglement, etc.). I hope they succeed. In any case, life goes on...*

*Interestingly, I happened to dine with Chris Burke last evening. Still quite a character!

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### Re: Lorentz Force (magnetic force term)

I will stick with Amperes’ explanation because by good guessing or intuition he came up an explanation for magnetism that later turned out to be consistent with special relativity just as Lorentz did with his contraction explanation for the M-M experiment.

I still think the video explanation for electromagnetism is a misapplication of relativity. It might make sense narrowly with a long wire example but I don’t see how it applies beyond that.
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