# What Direction is Light Observed From a Moving Source.

1. Jul 15, 2013

### T0mr

An object is an isotropic light emitting source and the object moves very near the speed of light c. I am an observer on a planet without an atmosphere and I see the path of the object as a line that is perpendicular to the normal from the planet's surface where I am standing. Now take a line segment of that objects path that is the same length on each side of the intersection point with the normal and call it AB. I am at point C and lets say points ABC make an equalaterial triangle. If at point A, a light pulse is emitted from the object and it is traveling at c, along which direction will I observe the beam? Will it be along the direction AC or along some other direction.

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I am thinking that the pulse emitted at A will reach C almost along the line BC. Does this make sense within the framework of Relativity? If so what is this phenomenon called? Has it been experimentally verfied and what are the experiments? Thanks.

Last edited by a moderator: May 6, 2017
2. Jul 15, 2013

### pervect

Staff Emeritus
3. Jul 15, 2013

### T0mr

Thanks for your reply. It does look like I was wrong in my assumption.

I looked at the link, specifically the gif of the observer and the source. If the light pulse appears to come from A (it would actually need to be emitted from a point before A) in my reference frame, then wouldn't that also be a case of abberation? Wouldn't only the relative velocity between the source and observer matter? Then the only way light would be seen to come from the point A would be if the relative velocity between the two frames was zero. Is that right?

Does aberration happen because of the relative velocity between the source and object or does the observer really need to be the one that moves?

Last edited: Jul 15, 2013
4. Jul 16, 2013

### ghwellsjr

I think maybe what you are getting confused about is that when you see the light being emitted by the object at A, the object is almost at B and so the light that appears to come from A is indeed emitted from a point before the location of the object at the time you see the light from A.

The second postulate of Special Relativity states that the propagation of light "is independent of the state of motion of the emitting body", so the answer is no.

No. Light always comes from the point it appears to come from (assuming no mirrors or gravitational issues are involved).

You have specified in your reference frame that light comes from a point. The fact that you are watching the object as it is moving has nothing to do with the question you asked since you are only concerned about a single "light pulse" that was emitted when the object was at point A (according to your reference frame). Whatever the object did before or after that single emission of light is of no consequence (to your questions).

Since we cannot say that the observer is really the one that moves (this is relativity), aberration is only concerned with the relative velocity between the source and the object but it also involves a continual emission of light, not just the light from a single point, which is what you asked about.

Last edited: Jul 16, 2013
5. Jul 16, 2013

### T0mr

Yes. I did have this point wrong. If the light appears to come from A it must have come from A regardless of where the object currently is on its path. I did get confused here.

I am having trouble understanding this point. I do not want to try to add the velocity of light and the object or anything like that, and I think I understand the meaning of the two postulates of special relativity fairly well. I thought that we are limited to the use of relative velocities in SR, and that I cannot really know which object has the velocity or if it is a combination of the two objects velocities. But that a prefered frame for the velocity is not something to consider. This is what I get out of Pervects post above. I interpret his post to be saying that only when I am moving does abberation play a role. But like you say later to what I thought was the same question:

...Since we cannot say that the observer is really the one that moves (this is relativity), aberration is only concerned with the relative velocity between the source and the object but it also involves a continual emission of light, not just the light from a single point, which is what you asked about. [/QUOTE]

So we will consider only the relative velocity between the objects. This makes sense to me.

But the part about abberation requiring the continual emission of light seems odd to me. Wouldn't a short pulse of light act the same as continuous light?

Last edited: Jul 16, 2013
6. Jul 16, 2013

### Staff: Mentor

Even a short pulse of light is continuous for as long as it's on; there's no such thing as a pulse of light of zero duration

So when we talk about a "short pulse" or a "single flash", we mean that the duration is small enough that the movement of the source during that time is negligible and we don't see, can't measure, don't need to worry about aberration and related effects. But those effects would be there if we looked hard enough for them with instruments of sufficient sensitivity.

7. Jul 16, 2013

### T0mr

Thanks Nugatory.

That is my understanding too. Maybe it is was unnessassary to describe the original problem using a pulse of light. Which to me is something very brief and continuous in space for that short amount of time but still detectable. Einstein uses the term light signal, and I think that is fine to use here. I am really only concerned with the wave front of that light signal (Which would be like a light pulse). So in my reference frame we can detect the instant the light signal is emitted from point A and when the wavefront appears to me and at what angle I will see it at.

8. Jul 16, 2013

### T0mr

Ahh! So this effect would be called Light-Time Correction not Aberration of Light as I was thinking. It took me a while to make the distinction. But it does seem like these two phenomena are two sides of the same coin. The wikipedia page for Aberation of Light (link by Pervect) says that Aberration is specifically when the observer frame is considered to be moving and the source is considered at rest. The light-time correction effect is when the source is moving and observer frame is considered at rest. This is from wikipedia:
I was thinking light-time correction and aberration of light were similar but apparently they are not. This is what wikipedia says about the two:
I think I understand the Light-time correction better than aberration. Wikipedia gives this analogy for aberration:

This kind of reminds me of the thought experiment where an elevator is flying very fast through space. It has a hole in it for light from a distant source to go through but the light beam hits the other side of the inside of the elevator higher than the hole because of the motion.

9. Jul 17, 2013

### ghwellsjr

Yes, that's exactly right.

That's not a good way of saying it. You should say Aberration is when the observer is moving in a frame and the source is at rest.

And similarly you would say the light-time correction effect is when the source is moving in a frame and the observer is at rest.

What you should understand is that in the field of astronomy where you are actually looking at objects through telescopes, if it is a distant star, you use the frame where you are moving and the star is at rest and you call it aberration.

If you are looking at a planet, you'd like to use the frame in which you are stationary and the planet is moving and you would call it light-time correction but it is more practical to use the frame in which the sun is stationary and both you and the planet are moving and you call it planetary aberration which is a combination of both "effects".

These issues are real factors for astronomers where the speeds are very slow and continually repeatable due to orbits and the effects are very small but big enough that they can't be ignored when aiming a telescope.

Your thought experiment of someone observing a continually light-emitting object traveling at near light speed would never be an issue for an astronomer because he would never have enough warning of where to look for such an object so it's really not important in my opinion whether you label it the one effect or the other.

In Special Relativity, we generally regard this issue as part of the Relativistic Doppler effect, specifically for motion in an arbitrary direction and we call it Relativistic Aberration.

The important thing to keep in mind is that you can take any scenario such as this one or the one you first described (except with a continual light source) and you can transform it using the Lorentz Transformation process from the frame in which the observer is stationary into the frame in which the source is stationary. Then what you call Light-Time Correction in the first frame becomes Aberration in the second frame. Or if you really want to mix the effects, you can have two observers who carry light sources and have both effects going on at the same time. Then if you want to really, really mix the effects, you can transform to a third frame in which both observers are moving, say, in opposite directions at the same speed (or any other frame).

However, in spite of all this fiddling with nomenclature, you should be aware that transforming to different frames has no bearing on what each observer actually sees.

10. Jul 17, 2013

### T0mr

You are right. This wording makes more sense. In Aberration the example talks about being in the suns frame of reference. I guess we can consider a distant star like Polaris to be at rest with respect to the sun. While on Earth the observer would be moving.

Thanks for taking the time to explain this idea.

11. Jul 18, 2013

### ghwellsjr

You're welcome.