Light beam path from a moving light source

Neddy Bate

Neddy Bate

Valued Senior Member
photon_trajectory.PNG


A laser is moving quickly to the right relative to us, the observers. It is emitting a beam perpendicular to the line of its motion.

Case A and Case B show two different interpretations of possible light beam paths. Case A has the photons totally independant of the motion of the light source, while Case B shows a form of dependancy. This might be interpreted as the light source imparting some of its momentum to the photons, but I am not stipulating that as the cause. The angle shown in Case A is not important; only that there is some angle in Case A, and there is no angle in Case B.

I have my own concepts of which case would be the better approximation for various applications, but I am more interested in whether anyone would be kind enough to offer their interpretation. I just thought it might be fun to think about. Thank you.
 
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Something to think about:

Which reference frame are your pictures supposed to apply to? Are they drawn from the point of view of somebody watching the emitter fly past, or from the point of view of somebody sitting on the emitter? Or is one of them for one case and one for the other?
 
James R said:
Something to think about:

Which reference frame are your pictures supposed to apply to? Are they drawn from the point of view of somebody watching the emitter fly past, or from the point of view of somebody sitting on the emitter? Or is one of them for one case and one for the other?
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I stipulated that "A laser is moving quickly to the right relative to us, the observers," but are you implying that both cases could be correct for a single laser beam?
 
I think that, if the light beam is being sent out at 90 degrees to the direction of motion in the observer frame, then B would be the correct picture.
 
James R said:
I think that, if the light beam is being sent out at 90 degrees to the direction of motion in the observer frame, then B would be the correct picture.
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James, would you care to elaborate?

PS:
I will edit the original post to include the 90 degree stipulation.
 
This is like throwing a ball out the window of a moving car. The ball has two components to its velocity: one in the direction it is thrown, and one which it picks up from the car's motion.
 
James R said:
This is like throwing a ball out the window of a moving car. The ball has two components to its velocity: one in the direction it is thrown, and one which it picks up from the car's motion.
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James,
You make it so clear to understand, thank you. Perhaps this concept could be a teaching tool, because it helps me. My scepticism of SR initially stemmed from this very misunderstanding.

Now I will share my concept of Case A and Case B:

Case A would be appropriate for accelerated frames.
Case B would be appropriate for inertial frames.


Fairly neat, would you agree? I have heard of Case A referred to as light 'curving', under acceleration, but here we see that it remains a straight line, afterall.
 
Neddy Bate said:
James,
You make it so clear to understand, thank you. Perhaps this concept could be a teaching tool, because it helps me. My scepticism of SR initially stemmed from this very misunderstanding.

Now I will share my concept of Case A and Case B:

Case A would be appropriate for accelerated frames.
Case B would be appropriate for inertial frames.


Fairly neat, would you agree? I have heard of Case A referred to as light 'curving', under acceleration, but here we see that it remains a straight line, afterall.
Click to expand...

Now all you have to do is justify why light should have an invariance to the motion of its source in the projected beam vector but have motion induced laterally to the projected beam by the motion of the source. :D
 
It is "A" in either case, constant velocity, acceleration, or whatever. The light will continue to travel perpenticular to the motion of the emitter (it does not travel at 30 degrees as the picture makes it appear), but a "beam of light" is not a solid. What has been emitted (the photons) will appear to come from an emitter at the location the emitter was in when the photon was emitted. Without this being true time would not exist, information would travel instantaniously and the universe would have been fried by the amount of photons that take up every point in space.

- KitNyx
 
MacM said:
Now all you have to do is justify why light should have an invariance to the motion of its source in the projected beam vector but have motion induced laterally to the projected beam by the motion of the source. :D
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Hi Mac,
You imply that you prefer Case A. I can appreciate that, and maybe even for UNIKEF, this might be an helpful teaching tool. Every theory needs clear, incontravertable ways of establishing its claims.
 
KitNyx said:
It is "A" in either case, constant velocity, acceleration, or whatever.
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Your confidence is most impressive. I had thought about that for a long time, but now I will have to reconsider the whole thing.

KitNyx said:
The light will continue to travel perpenticular to the motion of the emitter (it does not travel at 30 degrees as the picture makes it appear),
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It is already stipulated that the angle does not matter -- any non-zero angle would qualify.

KitNyx said:
but a "beam of light" is not a solid. What has been emitted (the photons) will appear to come from an emitter at the location the emitter was in when the photon was emitted.
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Yes, you have it exactly right. The path of the photon will also determine where the source appears to be for anyone who happens to perceive the light.

KitNyx said:
Without this being true time would not exist, information would travel instantaniously and the universe would have been fried by the amount of photons that take up every point in space.

- KitNyx
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I'm not sure if that last part is true, but it has got to be the coolest response I ever could have hoped for. Thanks :) .
 
superluminal said:
Does this turn anyones crank?

photons.gif
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It is beautiful. SuperL, could you please remind me where I can down load that animation program?

Aside from that, can you elaborate?
 
Is there someone here who does not subscribe to case A? Just curious...sometimes it is hard to tell exactly what people are trying to say (including myself I am sure).

- KitNyx
 
I think the drawings are misleading as they present light "rays" appearing instantaneously.

What do you think?
 
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