What's wrong with these pictures?

[nitsuj]

It took me a while to finally see where the following scenario is wrong.

I found it a "fun one" to consider.Scenario
Observer A moving at 0.5c compared to observer B.

Observer A sends out an idealized light pulse in a perfect circle. Perhaps by using a single light source & through diffraction of sorts is redirected in a perfect circle, expanding outwards at c.

Image one is from the PoV of observer B who I'll say is at rest.

Image two is from the PoV of A who is moving at 0.5c compared to B.

two

Ignore the difference in diameter (timing of the image).

I had always been confused how observer A could determine that B measures them self at the centre of the circle. Given observer A sees them near the "top" of the circle, I was able to explain the "contracted" length that is "seen" near to top of the circle, but had no clue what explains the "elongation" at the bottom.

After sometime, I finally realize that my scenario is a fallacy. ( at least I think it is)

 

[yuiop]

I am not sure why you are referring to elongation. Each observer would see themselves as being at the centre of an expanding perfect circle of light. One way to demonstrate this is with the following thought experiment. Imagine A has a circle of mirrors attached to him with a radius of 1 light second and B is attached to a similar circle of mirrors. When they pass close by each other at a relative velocity of 0.5c a static electricity spark between them causes a flash of light to be emitted. Exactly 2 seconds later (by A's clock), A will see the reflected flash return simultaneously from all A's mirrors and B will also see a flash reflected back from all B's mirrors simultaneously, (exactly 2 seconds later as measured by B's clock). The situation is perfectly symmetrical.

 

[nitsuj]

The ruler is contracted in the direction of motion. Please explain how observer A measures themselves in the centre.

 

[robphy]

It might help to draw a spacetime diagram of the lightcone of flash event,
and each inertial observer's worldline (through that event)
and that observer's [hyper]plane of simultaneity.

On each observer's plane of simultaneity,
the lightcone events on that plane are equidistant from that observer's worldline event on that plane.

 

[nitsuj]

Are you saying there is nothing wrong with the above scenario & pictures?

How is it both observer A & B would measure the idealized light pulse as a perfect circle?

 

[robphy]

Can you see your pictures in my avatar?

 

[nitsuj]

yea, It's a good one! In the "rest frame" image below I can "see" length contraction (A). That is I know, since c is constant Observer A would measure themselves in the centre of the light pulse circle, I can understand that due to a contracted ruler observer A measures a longer distance to the light pulse in front then the "contracted" space seen from the "rest" FoR image.

How can observer B account for observer A's measure of length to the "rear" of the circle (B) that would place them in the centre? (this is practically giving up the fallacy of the scenario)

These images are like spacetime diagrams. These images have spatial axis, numerous time axis (depending on how you "cut it") and an event. The image below is the boosted/primed one. Actually the more I think of it this is very similar to a light cone, this image is from the top looking down on the event (which is an observer in this case, the event was the pulse of light which happens to originate with the observer). Simply treat the Circle of the light pulse as the null line (which it is).
150493[/ATTACH]"]

 

[nitsuj]

I am not sure why you are referring to elongation. Each observer would see themselves as being at the centre of an expanding perfect circle of light.

Kinda misunderstood the scenario, A is in motion and only A generates the pulse.

The first image is how observer B sees the scenario (this is why I can say observer B is at rest)

The second image is how Observer A measures the scenario.

How can you account for the measure of the "bottom" length as performed by observer A; who measures them self in the centre of the circle.

Just to be crystal clear the letters A & B in the above image are just for identifying two measurements; not the two observers.

 

[ghwellsjr]

Just to be crystal clear the letters A & B in the above image are just for identifying two measurements; not the two observers.

None of your images are crystal clear. I have no idea what you are trying to portray. Why do you think anyone looking at your drawings and with the help of you explanations would have any idea what you are trying to convey?

For example, in your first and last images, there is a faint line going up from the bottom of the picture into the circle but not in your second image. What does it represent and why is it not in the second image?

Next to the top of this faint line, there is something that looks like text in a font so tiny that it is unreadable. What is that? And why in the second image does similar feature appear a little below and to the right of the center of the circle?

In your last image you explain that A and B are not the observers called A and B. Can you understand why this could be confusing? In any case, it's still confusing because in your first post, you said the first image, which looks like something is moving, is for B, who is at rest and the second image, which looks stationary, is for A, who is moving. Then in your last image, it looks like you have added detail to the first image, but again, this is for B, who is not moving.

You said in your first post that this issue is a "fun one". I can help you maximize your fun by directing you step by step how to arrive at the correct images and conclusions but it will take a lot of work on your part and it will take a lot of time, but it will be a lot of fun and you will learn a lot. Or I could just show you the bottom line (image) and it will be all over in one post. Which do you prefer?

 

[nitsuj]

I preffer you just tell me why you think the above scenario is not correct. I already know why it is not correct.

The main thing to consider with this scenario is that both observer A & B see the image as a perfect circle.


"Why do you think anyone looking at your drawings and with the help of you explanations would have any idea what you are trying to convey?"

You an ***. How about because it is clear what the scenario is. What about it confuses you? the irrelevant line in one image but not the other, the fact the two different measures & two different observers are both identified with A / B. yea that's super complicated.

The scenario is describe with two sentences. And what each observer measures is in two images. Info overload!

 

[ghwellsjr]

I preffer you just tell me why you think the above scenario is not correct.

Your first image looks to me like it is for observer A who is traveling, not observer B. That's one thing wrong.

Your second image looks to me like it is for observer B who is at rest. That's two things wrong.

You indicated that each observer is determining that the other observer is at the center of the circle of light. That's wrong. Each observer determines that he himself is at the center of the circle of light, he has no opinion about the other observer.

You indicated that one of the observers concludes that he is near the top of the circle and far from the bottom of the circle but you have not explained how he makes that determination. It's wrong for you to make such statements without telling us how the observer is supposed to figure this out.

Justin, I'm trying to help you. Please cooperate with me.

 

[Michael C]

Kinda misunderstood the scenario, A is in motion and only A generates the pulse.

It makes no difference who generates the pulse. The pulse could be generated by A, by B, or by somebody else moving relative to both of them.

It's also misleading to say that A is in motion and B is at rest: A is in motion relative to B and B is in motion relative to A.

The situation is, as others have remarked symmetric: each observer sees himself as the centre of the circle (or sphere, is we are in a world with 3 spatial dimensions) and each observer sees the other observer moving away from the centre.

In order to understand why this is so, you need to think about relativity of simultaneity. Imagine that observer B is standing at the midpoint between two trees, one "below" and one "above" him in your diagram. He sees the light hit both trees at the same time. Observer A, moving relative to B and the trees, does not see the light hitting the two trees at the same time.

 

[nitsuj]

k, so only Observer A makes a light pulse. Both images are of Observer A.

Image one is what Observer B sees when looking at A. This is because A is moving, B sees A in the position shown in the first image. I hear you ghwellsjr about the pictures not being very clear. In the first image, observer B is seen near the top of the circle, at the end of the blue line that indicates the motion of A.


Image two, is how Observer A would see them self.


In the second image A observes them self in the centre of the circle.



For you to spot the "fallacy" in the scenario, you have to understand which frame each image is observed from. Is it clear now?

 

[nitsuj]

It makes no difference who generates the pulse. The pulse could be generated by A, by B, or by somebody else moving relative to both of them.

It's also misleading to say that A is in motion and B is at rest: A is in motion relative to B and B is in motion relative to A.

Actually it does matter, what a lame response. How about you understand from the description of the scenario that only A makes the pulse, relative to the event of generating the pulse A is in motion. B relative the event of generating the pulse is stationary.

So tell me why it doesn't matter who is moving? That doesn't even makes sense in this scenario. Your analysis of this scenario is way off.

 

[nitsuj]

In order to understand why this is so, you need to think about relativity of simultaneity. Imagine that observer B is standing at the midpoint between two trees, one "below" and one "above" him in your diagram. He sees the light hit both trees at the same time. Observer A, moving relative to B and the trees, does not see the light hitting the two trees at the same time.

In order to see the fallacy in the scenario you need to think about relativity of simultaneity.

 

[Michael C]

Actually it does matter, what a lame response. How about you understand from the description of the scenario that only A makes the pulse, relative to the event of generating the pulse A is in motion. B relative the event of generating the pulse is stationary.

How can you define "in motion" or "stationary" relative to an event?

 

[robphy]

It makes no difference who generates the pulse. The pulse could be generated by A, by B, or by somebody else moving relative to both of them.

That is correct for special relativity.
The speed of light is independent of the source.

In other words, the light cone of the flash event is a property of the event... not of any worldline meeting that event.

It's best to simply say that there is a flash event when the two objects meet.

 

[nitsuj]

That is correct for special relativity.
The speed of light is independent of the source.

In other words, the light cone of the flash event is a property of the event... not of any worldline meeting that event.

It's best to simply say that there is a flash event when the two objects meet.

I'm gunna point out the "fallacy" with the scenario as presented.

yes micheal c is right regarding relative motion, but it does matter as far as the shape of the light pulse. This is because of RoS.

Because A is in motion & is the one who emits the light pulse to make a circular light pulse image, it would not be seen as circular to any other observer in relative motion with A.

The fallacy of the scenario is in both FoR the image is presented as a circle, which is incorrect due to RoS. Interestingly, there is no spatial separation or two separate events in this demonstration of RoS.

To the point that's been made here already, c is invariant.

 

[ghwellsjr]

Well, I can see it is not going to be productive to try to lead you in the right direction. I'm just going to show you the bottom line:

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

The red guy is your observer A and the green guy is your observer B. When A meets B, a flash of light is emitted (as MichaelC said, it doesn't matter who emits it). Each of the observers carries with them their own set of mirrors that they have measured to be in a perfect circle but because of length contraction along the direction of motion, A's mirrors form an ellipse. Neither one of them can see the circle of light as it is expanding, they will only be able to tell its presummed position away from them when they see the reflections from all their own mirrors collapsing on them simultaneously. Note how they each see a different set of reflections (but only at the moment of collapse) and B's form a circle centered on the original location of the flash while A's form a displaced circle that coincides with the future location of A when he arrives at the location of the collapse at the moment of the collapse.

All of this is from the Frame of Reference in which B is stationary. Remember, Frames of Reference do not provide the observers in them with any more knowledge or insight into what is happening with the propagation of light or the motion of other observers, rather, they provide us with instantaneous knowledge of where the light and both observers are throughout the scenario because we define the propagation of light and the motion of the observers to be as such.

You need to watch this over and over again and then pretend that you are observer A (the red guy) and see if his experience is any different than the experience of observer B (the green guy.

By the way, this is exactly the scenario that yuiop asked you to imagine in post #2.

 

[nitsuj]

Well, I can see it is not going to be productive to try to lead you in the right direction. I'm just going to show you the bottom line:

That's cute Hmmm... well I can see it is not going to be productive for me to try and lead you to understand this very simple scenario, I am just gunna tell you the bottom line.


Because of RoS, the two observers would not see the shape of the light pulse the same.

The scenario misrepresented this; the light pulse being circular from perspective of each FoR, it could not be.

Yes I have seen your annimation before, even reffered others to it. It's a really cool one. Mine is two still images of nearly the same scenario.

 

[Michael C]

I'm gunna point out the "fallacy" with the scenario as presented.

yes micheal c is right regarding relative motion, but it does matter as far as the shape of the light pulse. This is because of RoS.

Because A is in motion & is the one who emits the light pulse to make a circular light pulse image, it would not be seen as circular to any other observer in relative motion with A.

No, that cannot be. c is a contant, the same for all observers. For any observer, the light emanating from a single flash will be seen as a growing circle (or sphere).

 

[nitsuj]

No, that cannot be. c is a contant, the same for all observers. For any observer, the light emanating from a single flash will be seen as a growing circle (or sphere).

Cool, how does A measure themselves in the centre of the circle? (c being constant is another "reason" why the issue with the scenario is misrepresentation of RoS)

 

[ghwellsjr]

Cool, how does A measure themselves in the centre of the circle?

The same way B does--with mirrors placed a measured distance away in a circular pattern. Both A and B are doing exactly the same thing as far as they are concerned. It only looks different to us because we are viewing both of them from a single Frame of Reference. But it won't matter which FoR we decide to use, it will still illustrate the same experience for them.

 

[Michael C]

Cool, how does A measure themselves in the centre of the circle?

A measures himself at the centre of the circle, but sees B moving away from this centre. B measures himself at the centre of the circle, but sees A moving away from this centre.

Look at the animation that was just posted. Read ghwellsjr's explanation.

 

[Michael C]

You still need to answer this: how can you define "in motion" or "stationary" relative to an event?

 

[nitsuj]

You still need to answer this: how can you define "in motion" or "stationary" relative to an event?

Very clearly, as seen in the first image, givin the details of the scenario clearly B is at "rest" relative to the event, since he sees the light pulse as circular. ( as I pointed out now, A would not see the light pulse as circular.

I'll post an image showing what A sees, if B sees the light pulse as a circle. It might make it more clear.

 

[nitsuj]

A measures himself at the centre of the circle, but sees B moving away from this centre. B measures himself at the centre of the circle, but sees A moving away from this centre.

Look at the animation that was just posted. Read ghwellsjr's explanation.

Yes & length contraction accounts for the measure from the "top" of the light pulse circle, how is the measure to the bottom of the light pulse circle accounted for in the first image?

Note intervals are invariant. The interval between observer A and the bottom of the light circle is not the same as the interval with A in the centre of the circle.

Alright, so if A immits a light pulse in a perfect circle from their PoV, then here is the shape of the light pulse circle from B's PoV.



Note because of the invariance of c, "length contraction" is visable as the shape of the light pulse, of course this not actually length contraction but merely shows that A did not fire the light pulse as one event from B's PoV, from A's PoV it was a single event.

[PLAIN]http://i40.photobucket.com/albums/e204/tl01magic/aabb.jpg[/PLAIN]

The suggested image below from B's PoV is wrong.

[PLAIN][PLAIN]http://i40.photobucket.com/albums/e204/tl01magic/a.png

 

[Michael C]

Very clearly, as seen in the first image, givin the details of the scenario clearly B is at "rest" relative to the event, since he sees the light pulse as circular. ( as I pointed out now, A would not see the light pulse as circular.

You're assuming that A doesn't see the light pulse as circular and using this to produce your "definition". It doesn't make sense. An event has no velocity, it doesn't define an inertial frame. You can define motion relative to a particle, but not relative to an event.

Try this: A and B both carry a light source. When A and B coincide, both A and B send a light pulse. Two light pulses are sent, at the same time and from the same place. How can A see the light from his own source expanding in a circle and the light from the other source not expanding in a circle? That would mean that A sees the light from one source moving at a different speed to that of the light from the other source.

 

[nitsuj]

What I thought was going to give it up for sure was A not being in the centre of the circle for both images. as clearly would be the case since there is no spatial separation and it is a "single" event. the only other thing to account for RoS is simply a disagreement on when the light was emitted from the souce, which in turn effects the shape of the light pulse.

you're assuming they both actually see a circle, because of RoS and invariant c, it cannot be the case.

 

[Michael C]

Yes & length contraction accounts for the measure from the "top" of the light pulse circle, how is the measure to the bottom of the light pulse circle accounted for?

Note intervals are invariant. The interval between observer A and the bottom of the light circle is not the same as the interval with A in the centre of the circle.

The length contraction formula is for rigid objects. The light circle is not a rigid object: the "bottom of the circle" and the "top of the circle" do not happen at the same times for A and for B. If B is surrounded by a circular mirror (as in the animation), he sees the light hit the "top" and the "bottom" of the mirror at the same time (in the animation, it's the right side and the left side of the mirror). For A, these two events are not simultaneous: A sees the light hit the top of B's mirror before it hits the bottom if B's mirror. 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.

you're assuming they both actually see a circle, because of RoS and invariant c, it cannot be the case.

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

 

[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 "seperation"*, 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 excercise 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 excercise 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