# Reflecting light seen by two moving persons, explanation already known?

## Main Question or Discussion Point

When two persons meet each other in one point, one standing still (can be moving too) and the other moving (compared with the other) and in that point a light wave has started 90 degrees up and reflects (mirror 0 degrees), both persons will see the light wave back, the moving person in another location. This is right I think (otherwise movement cound be detected).

Is there already theory how this is possible, because when there would be only 1 lightwave, what is happening after the relection. It is not an illusion for the moving person, he really sees the light wave back.

Maybe I know now (suddenly I see) it's explained because of time delation, during the movement time stands still (compared to the other) so he sees the same light wave, right (hmm, not sure with 1 lightwave) ?

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DaveC426913
Gold Member
I'm sorry I can't make heads or tails of what you're describing.

You're talking about time dilation. How fast are these people moving?

ghwellsjr
Gold Member
I already showed you an animation of how this works. If you need more explanation of what's going on, please ask.

I'm sorry I can't make heads or tails of what you're describing.

You're talking about time dilation. How fast are these people moving?
(Hi Gwh, yes I have seen already the animation, but my question is now one step further),

Hi Dave,

You suprise me that you can't follow me.

Am I right that both persons see the same light wave reflecting ( A standing still and B moving, light wave started at meeting point) ? Forget time dilation, is there a theory already that both persons see the same light wave or is something wrong in my understanding because you don't follow me ?

I already showed you an animation of how this works. If you need more explanation of what's going on, please ask.

I suppose I am right what I see in your animation (I see that the moving person sees it some moments later becasue of the walking distance)? Now the theory what explains it, if existing already ? Suppose exact 1 light wave in thought ?

Matterwave
Gold Member
If you emit the light from a laser, so it's a concentrated source, then whether an observer sees that laser or not depends on where it gets reflected to (obviously), but if you emit the light isotropically like a light bulb, then ghwellsjr's animation should suffice as an explanation.

Dale
Mentor
The theory is Maxwells equations and special relativity.

DaveC426913
Gold Member
You suprise me that you can't follow me.
English is not your first language. It is very difficult to follow with so little context.

But I have not had the benefit of other posts on the subject, which it seems others have.

The theory is Maxwells equations and special relativity.
So at first am I right that I see this correct, it is a vertical reflection (only 1 light wave) but both persons sees the light wave back or only the person standing still (mirror constructed also standing still compared to the person standing still) ?.

If correct what is explained with special relativity ?

ghwellsjr
Gold Member
I suppose I am right what I see in your animation (I see that the moving person sees it some moments later becasue of the walking distance)? Now the theory what explains it, if existing already ?
Prior to Einstein, scientists had already discovered the fact that two observers moving with respect to each other would both measure the round-trip speed of light to be the same as each other. But they believed that light really only traveled at a constant speed in the absolute rest state of the luminiferous ether. So they came up with the idea that the lengths of everything moving through the ether would be contracted along the direction of motion and that moving clocks would slow down. This allowed them both to get the same answer when they measured the speed of light.

The green man is depicted as stationary with respect to the ether. The red man is depicted as moving at one half the speed of light through the ether.

So they believed that only the green man was in the center of the expanding circle of light and since his set of mirrors form a perfect circle, the collapsing circle of reflected light would reach him simultaneously from all directions.

They believed that when the red man placed his mirrors in what he thought was a perfect circle, the mirrors actually formed an ellipse and so when the expanding circle of light hit the mirrors as shown in the animation, it caused a different collapsing circle of light that just so happened to collapse on the red man just when he arrived at its center. They also believed that since his clock was running slower, he would get the same measurement for the round-trip speed of light that the green man got, even though, it is obvious that the light took much longer and traveled much farther. They realized that the red man could not tell what the one-way speed of light was, that is, he could not really tell if he was in the center of the expanding circle of light. But then, neither could the green man.

The theory that they developed was called the Lorentz Ether Theory and it explained everything that was happening except they couldn't identify the rest state of the ether.

Einstein came along and suggested that the red man could assume that he really was in the center of the expanding circle of light, that is, he could assume that he was stationary in the ether and that the green man was the one that was moving through the ether, and no one would know any different. Now this of course meant that the idea that there really was an absolute rest state of the luminiferous ether was irrelevant and there was no point in considering it any longer. He showed how it was possible for any inertial (non-accelerating) observer to define the one-way speed of light to be the same as the measured two-way speed of light and from this to establish the idea of a Frame of Reference in which the three-dimensions of space and the one-dimension of time form a four-dimensional space-time of "events" which could be transformed from one Frame of Reference to another Frame of Reference moving with respect to the first one. And that's what the Theory of Special Relativity is all about.
Suppose exact 1 light wave in thought ?
I have no idea what this question means. Please rephrase it.

ghwellsjr
Gold Member
So at first am I right that I see this correct, it is a vertical reflection (only 1 light wave) but both persons sees the light wave back or only the person standing still (mirror constructed also standing still compared to the person standing still) ?.

If correct what is explained with special relativity ?
In my animation, there is one expanding light wave that forms a circle. Each person has his own set of mirrors that are standing still compared to each person. Since we see the red man moving, we see his mirrors moving with him. He thinks he is stationary and that his mirrors are stationary.

Why are you asking about a vertical reflection? Do you mean just a spot of light that moves upward? Obviously if the spot of light is smaller than the mirror, then only one spot will reflect back to one observer. That's why I drew the mirrors as having gaps between them. If two observers were to actually do this experiment, they would have to arrange the mirrors so they don't crash into each other, but this is a thought experiment where we can ignore those (and many other) minor problems that have no bearing on the point of the experiment.

Dale
Mentor
it is a vertical reflection (only 1 light wave)
I am not sure what you mean by this. The animation shows one spherical wave, but I don't know why you would describe it as vertical. It proceeds spherically in all directions.

In my animation, there is one expanding light wave that forms a circle. Each person has his own set of mirrors that are standing still compared to each person. Since we see the red man moving, we see his mirrors moving with him. He thinks he is stationary and that his mirrors are stationary.

Why are you asking about a vertical reflection? Do you mean just a spot of light that moves upward? Obviously if the spot of light is smaller than the mirror, then only one spot will reflect back to one observer. That's why I drew the mirrors as having gaps between them. If two observers were to actually do this experiment, they would have to arrange the mirrors so they don't crash into each other, but this is a thought experiment where we can ignore those (and many other) minor problems that have no bearing on the point of the experiment.
Hi Gwh/DaleSpan,

I see now that's difficult for me to explain my problem.

I try again (perhaps it is a language problem, English is not my native language).

I think that my topic is wrong, only the man standing still sees the light wave back and the moving man not.

So I try again (maybe it is more the topic "detecting if you are moving", the subject light is still a problem for me, I understand SRT, but that's found because of the constant speed of light).

--------------------------

Light has a constant speed for any observer but we know light can also not be changed in other directions (normal conditions, no bending space etc.) through objects (e.g. light source) with another speed.

So we have now a fixed construction in thought: a light source (with a detector for reflecting light waves) and a mirror for reflecting a light wave from the source (mirror 0 degrees or horizontally position). That whole construction we send into space, so in vacuum and no acceleration, so a constant speed. We trigger on distance (radio controlled) a light wave from the source and probably it will be detected by our detector after reflection independed from the value of the constant speed.

It seems for me, that our light wave is going with the whole construction in it's direction too, otherwise you should think the light wave is sending into space and misses the mirror.

Now I have described my problem very well, if I understand now your answers I am further in understanding ...

Sorry my question of course ... how is it explained that my light wave is not misssing the mirror ? (mabe you say, also the light wave's amplitude is going smaller, 1/γ because of the SRT and that's why it misses not the mirror as in a situation "standing still" in the SRT meaning) ..

Dale
Mentor
Sorry my question of course ... how is it explained that my light wave is not misssing the mirror ?
The light wave follows Maxwell's equations in all inertial frames. This causes it to hit the mirror in all inertial frames if things are lined up right or miss in all frames if things are not lined up right. You will never get that Maxwell's equations have it hit in one frame and miss in another.

I have no idea what this question means. Please rephrase it.
Thanks Gwh for this answer, but it is not quite what I meant but that's my problem with the language (and physics of course) ... I have heard now the name Maxwell, one of my stages I guess to get the complete picture .. questions enough (in time) ...

The light wave follows Maxwell's equations in all inertial frames. This causes it to hit the mirror in all inertial frames if things are lined up right or miss in all frames if things are not lined up right. You will never get that Maxwell's equations have it hit in one frame and miss in another.
So you tell me that the speed of the frame is not important, suprisely for me (in fact confused).

But suppose there is no mirror and my construction has an open roof, what will be the route for the light wave compared to Earth (if somebody could follow the light wave back in thought to it's source if possible, where would he/she ends) ?

I had a new topic in thought about time dilation but I wait now until I understand this ... I have read Maxwell on Wiki, not easy to follow but I focuse me now only on conclusions (from others of course) .. very clever from phycisists to come so far ..

Dale
Mentor
So you tell me that the speed of the frame is not important, suprisely for me (in fact confused).

But suppose there is no mirror and my construction has an open roof, what will be the route for the light wave compared to Earth (if somebody could follow the light wave back in thought to it's source if possible, where would he/she ends) ?
You would end at the location of the emitter at the time of emission, again, in all frames.

You would end at the location of the emitter at the time of emission, again, in all frames.
Thanks DaleSpam, this is real new information for me and gives another view (really relatively, Einstein got his idea's from Maxwell and Lorenz).

So you (or Maxwell) says, a light wave is a relative something, absolute to it's frame but relative to other frames ? Other frames can see this light wave but with a SRT effect.

Dale
Mentor
Thanks DaleSpam, this is real new information for me and gives another view (really relatively, Einstein got his idea's from Maxwell and Lorenz).
Yes. The Lorentz transform is closely linked to Maxwell's equations because the Lorentz transform preserves the form of Maxwell's equations. Einstein invented neither Maxwell's equations nor the Lorentz transform. What he did is to explain the Lorentz transform in a completely novel and particularly simple way.

So you (or Maxwell) says, a light wave is a relative something, absolute to it's frame but relative to other frames ?
I am not sure exactly what you mean here, but it sounds wrong. If you have some distribution of charges and currents it generates an EM field according to Maxwell's equations. If you Lorentz transform those charges and currents into some other frame moving wrt the first then it also generates an EM field according to Maxwell's equations.

ghwellsjr
Gold Member
Light has a constant speed for any observer but we know light can also not be changed in other directions (normal conditions, no bending space etc.) through objects (e.g. light source) with another speed.

So we have now a fixed construction in thought: a light source (with a detector for reflecting light waves) and a mirror for reflecting a light wave from the source (mirror 0 degrees or horizontally position). That whole construction we send into space, so in vacuum and no acceleration, so a constant speed. We trigger on distance (radio controlled) a light wave from the source and probably it will be detected by our detector after reflection independed from the value of the constant speed.

It seems for me, that our light wave is going with the whole construction in it's direction too, otherwise you should think the light wave is sending into space and misses the mirror.

Now I have described my problem very well, if I understand now your answers I am further in understanding ...

Sorry my question of course ... how is it explained that my light wave is not misssing the mirror ? (mabe you say, also the light wave's amplitude is going smaller, 1/γ because of the SRT and that's why it misses not the mirror as in a situation "standing still" in the SRT meaning) ..
This same issue was brought up recently in another thread. Please study this to see if it helps:

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Yes. The Lorentz transform is closely linked to Maxwell's equations because the Lorentz transform preserves the form of Maxwell's equations. Einstein invented neither Maxwell's equations nor the Lorentz transform. What he did is to explain the Lorentz transform in a completely novel and particularly simple way.

I am not sure exactly what you mean here, but it sounds wrong. If you have some distribution of charges and currents it generates an EM field according to Maxwell's equations. If you Lorentz transform those charges and currents into some other frame moving wrt the first then it also generates an EM field according to Maxwell's equations.
Ok so I understand that the startpoint of a light wave in all frames is a crossing point, a light wave can have an x,y,z speed (like a base ball in a train) because of the speed in direction x,y,z. Than was my view of light not already correct (because I did not know the x,y,z speed, I know the speed in it's own direction is always c).

I saw in mind space consisting of points were light waves are started, the rest is moving.
But that view is not correct anymore (if it was) because light can have an x,y,z speed, so light moves too (so photons with a mass 0 can have an x,y,z speed). You know the speed in it's own direction but not the x,y,z speed, so you can't track a light wave back (if it was possible in thought, and it's not because of it's speed) to it's starting point.