Lorentz Force (magnetic force term)

[TheVat]

Yeah, it's confusing when folks think about Doppler, since that does give a sense of apparent difference on the two directions. The universe is contracted in both directions, but you still get a red shift on your backside. It only seems paradoxical, since you are really talking about two distinct effects.

[Faradave]

Right. Doppler shifts will be in addition to and typically more pronounced, than effects attributable to length contraction.

[bangstrom]

Right. Doppler shifts will be in addition to and typically more pronounced, than effects attributable to length contraction.

I agree the effect I attributed to SR was really a Doppler effect but the problem remains that electromagnetism can not be explained by how it appears to an outsider. Observers in motion at different velocities may have different relativistic views of what is happening but none of their observations have any effects that are relevant to what is happening at the local level where the only considerations that apply are the physical properties of the wire, current, and the environment. Things like relativistic length contraction are irrelevant as an explanation for electromagnetism.

[DJ_Juggernaut]

Things like relativistic length contraction are irrelevant as an explanation for electromagnetism.

I agree.

Side note: By definition length contraction cannot be observed because you have to be in the moving frame to detect it. This is why length contraction is only indirectly inferred. So far no experiment has made a direct observation. And you cannot, actually. This applies to all relativistic effects. Even time dilation for eg. If a train moves relative to you, the train frame concludes that you are moving and your clocks are slowing down. You say, no, my clocks are fine.

So how do you measure time dilation? By knowing who is really moving. In which case, it's not really relativity, it's one of many earlier aether frame theories. Then you can measure time dilation. If you use SR, no clock slows down, because they are all mutually at rest.

[DJ_Juggernaut]

Richard Feynman
http://www.feynmanlectures.caltech.edu/II_01.html#Ch1-S5

Because they are moving, they will behave like two currents and will have a magnetic field associated with them (like the currents in the wires of Fig. 1–8). An observer who was riding along with the two charges, however, would see both charges as stationary, and would say that there is no magnetic field.

When you move at the same speed as the current (electrons for eg), the protons in the wire appear to move in the opposite direction, producing a magnetic field in the wire. (Biot Savart Law). Therefore, the magnetic field is not going anywhere.

PS. In this chapter, Feynman discusses what a magnetic field is; and alludes to Maxwell's idle-wheels, I think.

[Faradave]

You're not alone.

These are the same objections raised by many beginners in Relativity. Length contraction is indeed difficult to verify directly, as we are not yet able to accelerate measurably large objects to significant fractions of lightspeed.

Time dilation is another matter, exhaustively verified with great accuracy and complete agreement with Relativity. (Muon experiments are the prototype.) Time dilation implies length contraction from velocities, which necessarily incorporate contracted path lengths.

"their half-life, which itself is modified by the relativistic corrections of two quantities: a) the mean lifetime of muons and b) the length between the upper and lower atmosphere (at Earth's surface). This allows for a direct application of length contraction upon the atmosphere at rest in inertial frame S, and time dilation upon the muons at rest in S′."

Keep reading. Feynman is wonderful but if not your cup of tea, there is abundant other literature.

P.S. In principle, as muons are charged particles, their travel constitutes a current with the requisite magnetic effects.

[TheVat]

Muons are just overweight and unstable electrons. (are we experiencing a Muon Presidency?) They emit less bremsstrahlung, which allows them to penetrate the earth's atmosphere easily. What confuses me is that they have 207 times the mass of an electron, yet they decay only to an electron and a couple of wimpy neutrinos. Then I remember that they are spawned by high-energy cosmic rays striking the earth's upper atmosphere. Without their relativistic velocity, they would decay over a distance of something like 450 meters. Just to give you an idea of how fast they are going - that's quite a Lorentzian contraction of a hundred kilometers of atmosphere, from the muon's POV.

[DJ_Juggernaut]

These are the same objections raised by many beginners in Relativity.

Let side notes be side notes. Okay, a quick reply won't hurt, I guess. Didn't want to derail this thread. But...

Length contraction is indeed difficult to verify directly, as we are not yet able to accelerate measurably large objects to significant fractions of lightspeed.

Even if you did accelerate an object at high speeds, you won't be able to measure length contraction, directly. Because you have to put your detector on the moving body. When you do, it's not moving relative to the body. Hence you can't detect length contraction, directly. You can only indirectly infer it.

Here's the rest of what I said:

Even time dilation for eg. If a train moves relative to you, the train frame concludes that you are moving and your clocks are slowing down. You say, no, my clocks are fine.

So how do you measure time dilation? By knowing who is really moving. In which case, it's not really relativity, it's one of many earlier aether frame theories. Then you can measure time dilation. If you use SR, no clock slows down, because they are all mutually at rest.

[DJ_Juggernaut]

Feynman is wonderful but if not your cup of tea, there is abundant other literature.

Not my cup of tea.

[DJ_Juggernaut]

Time dilation is another matter, exhaustively verified with great accuracy and complete agreement with Relativity.

I think you missed what I said. Time dilation is a pre-relativity effect.
More here: https://en.wikipedia.org/wiki/Time_dilation#History

[bangstrom]

Time dilation is another matter, exhaustively verified with great accuracy and complete agreement with Relativity. (Muon experiments are the prototype.) Time dilation implies length contraction from velocities, which necessarily incorporate contracted path lengths.

"their half-life, which itself is modified by the relativistic corrections of two quantities: a) the mean lifetime of muons and b) the length between the upper and lower atmosphere (at Earth's surface). This allows for a direct application of length contraction upon the atmosphere at rest in inertial frame S, and time dilation upon the muons at rest in S′."

Time dilation and length contraction are not observed at the local level of the object “in motion.” Instead, the object in motion finds the outside world in an altered condition so the changes are observer dependent. Another way of looking at this is to say that neither has changed but both have experienced a rotation through 4-D space relative to each other so they no longer share the same time-line.

[Faradave]

you won't be able to measure length contraction, directly. Because you have to put your detector on the moving body. When you do, it's not moving relative to the body. Hence you can't detect length contraction, directly. You can only indirectly infer it.

That's not how it would be done.

With you at rest in your own frame, hold up a LARGE* ruler also at rest.

A rocket comes rushing past in the direction of the rulers length. You note the rocket's length on the stationary ruler.

Later, when the rocket lands in your rest frame, you measure its length with the same ruler and find it to be longer.

Lincoln goes to some trouble to address your concern below, by measuring in the moving frame and translating back. But this seemingly simple presentation requires an understanding of (or at least agreement with) the relativity of simultaneity.


*If you don't have such a ruler, angular assessments can be substituted.

[DJ_Juggernaut]

Even if you're using a ruler, you have to see it. Seeing implies, light reflects or is emitted off the surface of the moving body. If the reflections or emissions come from the moving frame, it's as if the emissions came from a stationary frame since in the moving frame, the frame is not moving. Hence you won't see any contraction.

Another interpretation might be, simultaneity prevents you from seeing the contraction. Even if they were emitted simultaneously, you won't see the contraction since the emissions are coming from the moving frame, where you are the one that is moving.

[DJ_Juggernaut]

But this seemingly simple presentation requires an understanding of (or at least agreement with) the relativity of simultaneity.

I hope you've been able to figure out why you can't measure or see length contraction, directly. If not, hint: simultaneity.

Side Note: In Newton's theory, where length's don't contract, the train-ruler-experiment would be in agreement with SR. You can't detect it because lengths don't contract. Whereas, SR would assert, lengths do contract, but you can't detect it because of simultaneity.

[Faradave]

A camera can be placed to simultaneously (in its rest frame) catch light emitted form both ends of a moving object. Allowing adequate sensitivity, it would find the object length contracted in the direction of motion. That does not mean the light was emitted simultaneously in the frame of the moving object (quite the contrary).

I think we're talking past each other.

At the end of Lincoln's video (above), he relates the popular paradox of a moving 40 foot pole fitting in a 20 foot barn - with both barn doors closed!
That is, both doors closed simultaneously in the barn's (and our) rest frame. Thus, the pole is contracted to 20 feet long (as measured in the barn's frame). It's not a trick.

For someone riding on the pole, it remains 40 feet long, while the barn is contracted to only 10 feet long! However the rider sees the entry door close after the pole is completely past it. And the exit door reopens just when he gets to it. The rider never sees both doors closed simultaneously (while he's inside) in his frame.


Agree or not, the point is that your objection is not new. It's been raised again & again and carefully dealt with. Many (some quite intelligent) folks who originally agreed with you, now agree with Relativity. Very few (if any) go the other way.

[DJ_Juggernaut]

Allowing adequate sensitivity, it would find the object length contracted in the direction of motion. That does not mean the light was emitted simultaneously in the frame of the moving object (quite the contrary).

The above two sentences are talking past each other. A camera can't detect contraction.

[DJ_Juggernaut]

Agree or not, the point is that your objection is not new.

I know. Thanks for bringing it to my attention.

Many (some quite intelligent) folks who originally agreed with you, now agree with Relativity. Very few (if any) go the other way.

I think we are talking past each other.

[Faradave]

A camera can't detect contraction.

No one is asking it to. It just has to measure length (NASA cameras do this all the time, often with a reference standard in view).

Given adequate sensitivity, a picture of an object in motion would be contracted, compared to a picture of the object at rest in the camera's rest frame.

The camera could, for instance, take a picture of Lincoln's 20 foot barn at the moment its two doors are closed, with the moving pole inside it. Another photo of the pole at rest shows it to be 40 feet long!

This example is given in many forms. Lots of length contraction videos use a train in a tunnel.

I expect your convictions will carry you through till this becomes clear. At that point, it will be of value to note exactly what it was that changed your mind. The insight could be of great help as this issue is raised by newcomers to Relativity discussions all over the web.

[DJ_Juggernaut]

No one is asking it to. It just has to measure length

Take an object measure its length via a camera. You get an image based on the photon distribution with simultaneity being valid for both. Now, move the object, it is contracted. Take a picture again, due to relativity of simultaneity, its emissions occur sequentially, thus artificially broadening its emissions. These broader emissions will be equivalent to the emissions that occurred when the object was stationary. Therefore you can't see the contraction when it is moving. You see it's true length again.

Of course, I'm keeping the scenario very simple here. Relativistic beaming not taken into account. And you shouldn't bring that in, given we are only interested in knowing an object's length. Two lightning strikes as a measure of length of a train is enough here. Each time, you will get the same length, whether it is moving or not.

[Faradave]

[moving]object... is contracted. Take a picture again, due to relativity of simultaneity, its emissions occur sequentially, thus artificially broadening its emissions.

By "broadening", I take it that you mean the distance between the events of their emissions. That would depend on the order of the sequence.

If, relative to the direction of motion, a photon is emitted from the leading edge first, then a photon is emitted from the rear edge - after it has traveled forward some distance - you can see how the image would show a contracted length, if both photons arrive simultaneously, in the camera's rest frame.

Relativity of simultaneity changes the timing (and even the order) of events, depending on their separation, and the relative velocity. While speed c is invariant, photons simultaneously received in the camera's reference frame, were not simultaneously emitted in the moving objects frame.

All this (including time dilation and relative mass-energy) derives from accepting the invariance of universal speed limit c. As we don't yet have the ability to measure length contraction directly with great accuracy, it can be derived from time dilation (as with the muon experiment). Similarly, in particle accelerators, the path length experienced by fast moving particles is shorter than that in the accelerator's rest frame (Ehrenfest paradox, Born-rigidity). This must be taken into account (as per Lorentz transforms) for the experiments to work.

[TheVat]

[DJ_Juggernaut]

By "broadening", I take it that you mean the distance between the events of their emissions.

Correct. The emissions are sequential. If they were simultaneous, your camera would capture a contracted rod. Here's the scenario modified slightly so you are on the same page.

A moving rod can be thought of as a series of LEDs. You take a picture when the rod is stationary to you. Assume the lights turn on simultaneously. You get an indirect length of the rod. Next you move the rod, it contracts. You turn on the LEDs, with the same mechanism, the LEDs do not light up simultaneously; but they light up sequentially, due to relativity of simultaneity. Coupled with their motion and the sequential emission of light, the moving picture of the rod and the stationary picture of the rod would be, identical. Are you able to follow?

[Faradave]

It's important to understand that the rod (with LEDs) can't escape its own rest frame. No matter how we see it move, it is always in its own rest frame (same as you and I are trapped in ours). Thus, the LEDs light simultaneously for the rod no matter how fast its moving. The rod's rest frame defines its simultaneity (all of its space at a given moment).

If we are at rest with respect to the rod, we share its simultaneity, including any lengths we care to measure. If the rod moves relative to us and turns on its row of lights simultaneously (in its own frame), they will light not light simultaneously in our rest frame. The camera will only catch light it views as simultaneous in its rest frame.

[TheVat]

Haven't got time atm to find it, but our old thread on the Two Rockets problem might help clarify the Lorentz situation.

[DJ_Juggernaut]

The camera will only catch light it views as simultaneous in its rest frame.

You are now up to speed.

[DJ_Juggernaut]

Of course, I'm keeping the scenario very simple here. Relativistic beaming not taken into account. And you shouldn't bring that in, given we are only interested in knowing an object's length.

I think the camera not being able to see a length contraction can be extended to relativistic beaming as well. Can you see a non-spherical distribution of photons due to relative velocity? I don't think so; due to relativity of simultaneity.

Take a stationary point source that emits light and keep your detector curved to register simultaneous emissions. You get spherical distribution of photons. Then you move the source and take another photo. Would the two photos look the same? I think so.

Side note. "It was shown by several authors such as Roger Penrose and James Terrell that moving objects generally do not appear length contracted on a photograph. For instance, for a small angular diameter, a moving sphere remains circular and is rotated."

https://en.wikipedia.org/wiki/Length_co ... al_effects

[Faradave]

I think the camera not being able to see a length contraction...

We don't have cameras sensitive enough or fast moving targets large enough to reliably make such measurements. But the person in this diagram could be replaced by a camera (the eye is a bio-camera). Thus, in principle, length contraction is detectable by a camera.


The point is that light simultaneously received from both ends of the relatively moving object was not simultaneously emitted in the rest frame of the object. Any other measure of length, including momentarily closing two barn doors on the length contracted train, would in principle work as well.

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.

[DJ_Juggernaut]

It was shown by several authors such as Roger Penrose and James Terrell that moving objects generally do not appear length contracted on a photograph. For instance, for a small angular diameter, a moving sphere remains circular and is rotated.

https://en.wikipedia.org/wiki/Length_contraction#Visual_effects

[DJ_Juggernaut]

Thus, in principle, length contraction is detectable by a camera.

You are wrong! But you are free to have your opinion. Maybe you should read a little while you take a break:

https://www.tempolimit-lichtgeschwindig ... sphere.pdf
https://journals.aps.org/pr/abstract/10 ... v.116.1041

Cheers.

[DJ_Juggernaut]

That's not to concede any point on the reality of length contraction. I would assume the same of your contrary position.

I would change my position the day one can "see" length contraction.