What's wrong with these pictures?

[nitsuj]

Note that A sees B's circular mirror as an oval (length contraction) but A sees the light from the flash emanating in a circle.


Once more, any observer must see the light from a flash emanating in a circle, precisely because c is invariant.

1. And vice versa. But A emitts the light and has determind what makes a circle, B disagrees, says its an oval that A made. This is because the light pulse that A emmits is not simultaneous from B's perspective.


Length contraction "is for" measuremnt. Distance can be measured as length, for example the "contracted" distance infront & behind A in the direction of motion. And, the non contracted length that is perpendicular to the direction of motion.


Once more no they don;t micheal. The creation of a circular light pulse is observer dependent. Don't forget that A is in motion and is the one that determines the circular light pulse. Because A's measure of length / time is different from B's it will not look like a circle to B. Both measure the speed of the light pulse as c, that is the shape never changes.

 

[robphy]

As Michael C says... "The length contraction formula is for rigid objects. "

More precisely, "length contraction" essentially refers to the apparent spatial-separation between two parallel [in spacetime] timelike-worldlines (like two ends of a meterstick)... not the spatial-separation between a timelike worldline and a lightlike line (which, if you are about to emit another light flash, is more like [but not quite] a wavelength ).

 

[robphy]

Once more no they don;t micheal. The creation of a circular light pulse is observer dependent. Don't forget that A is in motion and is the one that determines the circular light pulse. Because A's measure of length / time is different from B's it will not look like a circle to B. Both measure the speed of the light pulse as c, that is the shape never changes.

Can you clarify something for me?

Suppose observers Alice and Bob each carry a flashlight.
When Alice and Bob meet at event O, both momentarily turn on their flash lights.
On a spacetime diagram,
are the events on the light-cone of Alice's flash
the same events as those on the light-cone of Bob's flash?

 

[nitsuj]

lol, oh okay, yes it is for Rigid objects, like a ruler, that measures non rigid things like distance. A great point that clairifies...

So Rob, you're saying that if A is traveling at 0.5 c and emits a circular light pulse, that "at rest" observer B sees it as a circle as well?

 

[robphy]

lol, oh okay, yes it is for Rigid objects, like a ruler, that measures non rigid things like distance. A great point that clairifies...

So Rob, you're saying that if A is traveling at 0.5 c and emits a circular light pulse, that "at rest" observer B sees it as a circle as well?

If a flash is emitted at the meeting event,...

Alice will say that the set of events reached by the flash 1 sec after the meeting looks like a circle to her.
However, Bob will say that those events are not simultaneous [to him] and will [eventually] trace out an ellipse* in his reference frame.
(Note that I am referring to events that Alice say occurred 1 sec after the meeting.)

in accordance with the principle of relativity...
Bob will say that the set of events reached by the flash 1 sec after the meeting looks like a circle to him.
However, Alice will say that those events are not simultaneous [to her] and will [eventually] trace out an ellipse* in her reference frame.
(Note that I am referring here to events that Bob say occurred 1 sec after the meeting.)

Each observer is making their own cut of the light-cone of the meeting event with their own distinct plane of simultaneity.

*You could see this in the video posted by ghwellsjr, if those reflections events were marked.
Note that the emission event and the reception event in that video are at the foci of the ellipse.

 

[nitsuj]

Exactly, were not in any disagreement here that I can see.

Observer A sees a circle, observer B sees an oval (when looking at A's Light pulse).

Consider that A emmits (and in turn determines what timing makes a light pulse a circle shape) for A that is an instant emmision of the light.

B merely observes this, and sees and oval shape. To B, the light pulse was not an instant emmision of light. To the point of these comparisons being semetrical, image one in the first post does not follow that rule, it is not semetric ( a give away of it being incorrect, the "fallacy" in the scenario)

The fallacy in the scenario I posted was the light pulse being circular from the perspective of both FoRs. Which is the "fallacy" I was hoping would be spotted in a less cumbersome way.

I was going to answer you question as; "if bob and alice are in relative motion they do not have the same measure of length / time and would in turn assign different coordinates to the events."

I would not know how to draw it other then with one FoR with time / length orthogonal, and the other with less then 90 degrees "separation"*, in turn showing their "spacetime" coordinates not being in line with each other. (specifically less then 45 degrees of separation from a null path)

Consider my scenario opposite to the video. instead of receiving the light pulse, the observer in motion emitts it. To them it's a circle, to the at rest observer it is an "oval" shape.

This wasn't very fun :(

 

[ghwellsjr]

Justin, there is no theory that claims that a flash of light propagates at anything other than a perfect circle (or sphere). It does not make sense that light could create an oval shape as it propagates outward. But you cannot watch the shape of propagating light. The only thing you can do is put reflectors out in space at measured distances and then watch for the reflections after they get back to you. You measure these distances with rigid rulers. The rulers are what get contracted along your direction of motion in the selected Reference Frame, not the shape of the light. The shape of the mirrors form an oval, not the light. The light always propagates as either an expanding circle (or sphere) or a collapsing circle (or sphere) or some of each. Because the mirrors for moving observer A are themselves moving and in the shape of an ellipse (or oval), the light does not strike A's mirrors simultaneously which means that part of the light is still expanding while another part is starting to collapse. After all the reflections are done, the circle of light is all collapsing and eventually arrives at A's future location.

Observer A can have no awareness of the light hitting his mirrors at different times. All he knows is that the original flash occurred at his location when he was colocated with observer B and then later, all the reflections arrived simultaneously from all the mirrors. When he measures how far away the mirrors are, he believes they form a perfect circle. So he concludes that he is in the center of the expanding circle of light. There is nothing to indicate otherwise to him.

And we can say the same thing for observer B, can't we? Both observers do exactly the same thing and come to exactly the same conclusion.

 

[nitsuj]

Justin, there is no theory that claims that a flash of light propagates at anything other than a perfect circle (or sphere). It does not make sense that light could create an oval shape as it propagates outward. But you cannot watch the shape of propagating light. The only thing you can do is put reflectors out in space at measured distances and then watch for the reflections after they get back to you. You measure these distances with rigid rulers. The rulers are what get contracted along your direction of motion in the selected Reference Frame, not the shape of the light. The shape of the mirrors form an oval, not the light. The light always propagates as either an expanding circle (or sphere) or a collapsing circle (or sphere) or some of each. Because the mirrors for moving observer A are themselves moving and in the shape of an ellipse (or oval), the light does not strike A's mirrors simultaneously which means that part of the light is still expanding while another part is starting to collapse. After all the reflections are done, the circle of light is all collapsing and eventually arrives at A's future location.

Observer A can have no awareness of the light hitting his mirrors at different times. All he knows is that the original flash occurred at his location when he was colocated with observer B and then later, all the reflections arrived simultaneously from all the mirrors. When he measures how far away the mirrors are, he believes they form a perfect circle. So he concludes that he is in the center of the expanding circle of light. There is nothing to indicate otherwise to him.

And we can say the same thing for observer B, can't we? Both observers do exactly the same thing and come to exactly the same conclusion.

Thanks for still replying ghwellsjr.


to the part I bolded; I agree, that is why I am saying if B sees the light pulse as a circle it means A didn't emmit the light pulse in all directions simultaneously. (the "fallacy of my scenario is it ignore RoS and shows the image as a circle in both FoRs)

I agree with everything you said, how come we are not agreeing on this scenario of A & B not agreeing on the shape of the light pulse?

I totally agree the imagery would be symetrical. So in my scenario only A emitts a light source, no need to consider B doing the same, it would be symetrical. B is only in my scenario to observe A.

K, so as far as B is concerned A's measure of time & length are different from hers.

Observer A devises an apparatus that emmits a light pulse in a circular shape. That shape to observer B is an oval, because c is invariant it shows A's attempt to make a perfect circle, actual produces a circle contracted in the direction of motion. (from B's PoV)

This has been more an exercise in my communication skills than my understanding of SR.

 

[Michael C]

Consider my scenario opposite to the video. instead of receiving the light pulse, the observer in motion emitts it. To them it's a circle, to the at rest observer it is an "oval" shape.

No observer can see the light pulse as an oval! If an observer sees the light emanating in oval form, it means that, for that observer, the light that is traveling along the long axis of the oval must be going faster than the light traveling along the short axis. That isn't possible - the speed of light is a constant!

 

[ghwellsjr]

Thanks for still replying ghwellsjr.


to the part I bolded; I agree, that is why I am saying if B sees the light pulse as a circle it means A didn't emmit the light pulse in all directions simultaneously. (the "fallacy of my scenario is it ignore RoS and shows the image as a circle in both FoRs)

I agree with everything you said, how come we are not agreeing on this scenario of A & B not agreeing on the shape of the light pulse?

I totally agree the imagery would be symetrical. So in my scenario only A emitts a light source, no need to consider B doing the same, it would be symetrical. B is only in my scenario to observe A.

K, so as far as B is concerned A's measure of time & length are different from hers.

Observer A devises an apparatus that emmits a light pulse in a circular shape. That shape to observer B is an oval, because c is invariant it shows A's attempt to make a perfect circle, actual produces a circle contracted in the direction of motion. (from B's PoV)

This has been more an exercise in my communication skills than my understanding of SR.

You keep saying that there is something wrong with the shape of the expanding flash of light in your images but there's nothing wrong with them. They are circles, just like in my animation. The problem with your images is that they don't have any mirrors.

 

[nitsuj]

No observer can see the light pulse as an oval! If an observer sees the light emanating in oval form, it means that, for that observer, the light that is traveling along the long axis of the oval must be going faster than the light traveling along the short axis. That isn't possible - the speed of light is a constant!

Of course, that's why I am saying A didn't emit the light pulse simultaneously from B's perspective. i.e. the light farthest from the centre was emitted before the light closer to the centre.

 

[ghwellsjr]

Observer A doesn't have to do anything special to emit light in a circle. All he has to do is set off a flash bulb or an explosion that happens at a single point in time. In fact, since A and B are traveling at 0.5c with respect to each other and the flash occurs when they are colocated, that automatically insures that it can only occur at a point in time. So we have one event for the time and location of A, B and the flash. And that's one reason why everyone is saying that it doesn't matter who emits the flash.

 

[nitsuj]

The light pulse emitter from B's perspective would be length contracted right?

If B were watching that emitter to measure how the light leaves the emitter what would she see.

To be a perfect circle would the light pulse not have to leave the contracted axis of the emitter first? The light pulse being emitted not simultaneously, but sequentially until reaching the non contracted axis at which point the emission is done and you have a perfect circle. Okay so I agree now B would see a perfect circle. But it is an issue of RoS as I felt, (relying on invariance of c).

The timing of the light pulse emission is oval shape for Observer B*, observer A sees the light pulse emission as being simultaneous; this is RoS.

In the context of true to SR images, the first one in the first post does not have symmetry. I didn't realize this and was confused by it because it obeys other concepts of SR, confused by the image because I wasn't able to account for how A measures them self in the centre of... the mirrors.

As far as the description I gave (one way speed of light / your mirrors) that first image is wrong.

You can look my claim the first image in the thread is wrong this way--> I was trying to come up with an animation like yours, while trying to think of the shape for the moving mirrors I came up with that first image, and thought hey those intervals don't look symmetrical relative to the observers position, something isn't right here.

Thanks for helping resolve this (if it is) I think I was confusing the one way & two way speed of light & how each would be coordinated in each FoR, thinking observer B would measure an oval shaped light pulse, but that is just the one way speed and is not measureable, two way speed it's symmetrical and what's measurable.

 

[Michael C]

Of course, that's why I am saying A didn't emit the light pulse simultaneously from B's perspective. i.e. the light farthest from the centre was emitted before the light closer to the centre.

The light pulse is a single event. How can a single event not be simultaneous with itself?

The light pulse emitter from B's perspective would be length contracted right?

The light pulse is emitted at a single point. A point cannot be length contracted.

 

[robphy]

I think this thread suffered from lots of miscommunication... especially with regard to terminology.

Looking back, I think "light pulse" means "wavefront" ( an observer dependent set of simultaneous events on an event's lightcone ).

 

[harrylin]

[..]
The timing of the light pulse emission is oval shape for Observer B*, observer A sees the light pulse emission as being simultaneous; this is RoS. [..]

The animation that ghwellsjr provided looks correct to me; do you disagree?

https://www.youtube.com/watch?v=dEhvU31YaCw \

If you want to present an "oval" light shape because of RoS, you could present something like Einstein in his 1905 paper, in section 8:

The spherical surface—viewed in the moving system—is an ellipsoidal surface

- http://www.fourmilab.ch/etexts/einstein/specrel/www/

And by symmetry, a spherical surface according to S' that encloses all the light at a certain time t', appears as an ellipsoidal surface to S - like the moving mirror at the time that the light reaches its surface in the animation. The time that the light hits that mirror surface corresponds to different times t.

Does that help?

Harald

 

[pervect]

I think this thread suffered from lots of miscommunication... especially with regard to terminology.

Looking back, I think "light pulse" means "wavefront" ( an observer dependent set of simultaneous events on an event's lightcone ).

We can imagine or even create experimentally a very short light pulse. Probably the most straightforwards way to model how it spreads is to use an affine parameterization to describe how it spreads. If you have a monochromatic beam, the affine parameterization will be related to the wavelength of the emitted light and will mark out equal intervals along the light beam.

A rest frame can still be defined for such a pulse, it's the unique frame in which the light remains monochromatic as it expands. In a non-rest frame, you'll see some of the light red-shifted and some blue-shifted, so it won't be monochromatic through the whole sphere.

I believe that the expansion will be spherical only when it's monochromatic, due to the affine parameterization argument - we can say that at some particular instant, the pulse wavefront will be N wavelengths away from the origin.

While it's simplest to understand if the light pulse only contains one frequency in the rest frame, I don't think it's vital to the argument.

 

[robphy]

My point was that
there seems to be a confusion in terms
because "light pulse"
seemed, in some places, to refer to "the point-event of the flash"
and, in other places, to "an observer-dependent wavefront of simultaneous events".

 

[yoron]

Sort of interesting.

I started to play with it in terms of 'what if' nitsujs idea would work :) Then we would have a way of defining who's moving relative who, also we would have had a way of 'observing' a Lorentz contracted space from both observers..

Ah well, nice discussion.

 

[nitsuj]

Think I may sorted my self out, thanks to all that told me where I was wrong.

Observer A's FoR
They measure them self in the centre of the light pulse, only they emit a light pulse.

The relative motion between observers A & B is 0.5c.

Using observer A's "plane of simultaneity" & observer B's measure of proper time / length, observer B would coordinate the "event" of the leading edge of the light pulse similar to (i don't know the math to properly contract the image):

The image above is merely a "calculation" that is two separate physical "realities" superimposed. Observer B using their own "plane of simultaneity" & measure of proper time / length would coordinate the leading edge of the light pulse similar to:

In the context of this scenario, a reason for length contraction being only for objects is because the object as a whole "exists" simultaneously. (robphy pointed this out & pretty clearly, my head was a little thick at the time)

This is apparent in the mentioned details of image 2, where observer B's measure of proper time / length is superimposed on observer A's plane of simultaneity, in turn the light pulse in the image is "contracted".

the contracted mirrors in ghwellsjr animation illustrates that the measure of length between arbitrary points "a" & "b" is a simultaneous "event". That is the "position" of points "a" & "b" are simultaneous events.

Pretty sure the above is right, doesn't seem to contradict what has been mentioned in the thread.

The point made by micheal c regarding contraction only applying to "rigid objects" was my biggest misunderstanding I think. Now it is clear why this is the case. Thanks micheal c for pointing it out, sorry for not subscribing at the time.

This answers my original question* of why there is no symmetry in the last image, only simultaneous "positions" contract (this may not be worded right).

*which I thought I knew the answer to when starting this thread. yay physics forum! again