Does light really travel in same frame of reference

Main Question or Discussion Point

Hi, I have a simple question in my mind, I could not find the answer anywhere so I came here. Here is my question,
Assume a source of light and observer are five light seconds apart, and are travling parllel to each other at a constant speed of say 200 meters per second. Now if the source of light sends a narrow beem (only one centimeter in diameter) of light towards the observer for one second, will the observer ever see that beem?
Thank you in anticipation.

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Hi, I have a simple question in my mind, I could not find the answer anywhere so I came here. Here is my question,
Assume a source of light and observer are five light seconds apart, and are travling parllel to each other at a constant speed of say 200 meters per second. Now if the source of light sends a narrow beem (only one centimeter in diameter) of light towards the observer for one second, will the observer ever see that beem?
Thank you in anticipation.
If there is a reference frame in which both the source of light and the observer are both at rest and if the narrow beam hits the observer in that rest frame, then in any other reference frame the the beam will hit the observer and in that case, the answer is yes.

pervect
Staff Emeritus
Hi, I have a simple question in my mind, I could not find the answer anywhere so I came here. Here is my question,
Assume a source of light and observer are five light seconds apart, and are travling parllel to each other at a constant speed of say 200 meters per second. Now if the source of light sends a narrow beem (only one centimeter in diameter) of light towards the observer for one second, will the observer ever see that beem?
Thank you in anticipation.
It rather depends on what you mean by "towards the observer". Relativistic aberration is an effect that has to be accounted for. See for instance http://en.wikipedia.org/w/index.php?title=Relativistic_aberration&oldid=376034269

I'll provide a short quote to illustrate the point of interest:

Relativistic aberration is described by Einstein's special theory of relativity, and in other relativistic models such as Newtonian emission theory. It results in aberration of light when the relative motion of observer and light source changes the position of the light source in the field of view of the observer. The effect is independent of the distance between observer and light source.

WOW! two replies.. both YES, seems like I have to agree, I alway thought that light travels in a straight line, and it is indipendent of its sources motion, I never knew that light rays get tilted like normal objects does for example A stone thrown out from a moving train etc.

Now keeping this tilt thing in mind, let us go back to orignal senario and this time have the source send constant streem of light pulses(In the form of binary) towards the observer(A Photo detector).
Please Note: Light source and photo detector are 5 light seconds apart and they will accelerate and stop together like they are mounted on a very long bar.

Situation 1. If both the source and observer are stationary, I know observer will receive all the pulses send by source. No doubt about it.

Situation 2. Now if they both decide to accelerate for some time and then come to a constant speed. What will happen? Will the observer miss the pulses that were send while they were accelerating? And resume to receive the pulses only after they start travling at a constent speed.?

Situation 3. They are traveling at a constant speed and come to a a sudden stop(within no time). Will the light pulses overshoot the observer, because they got tilted from the sources motion. Or will the photons know (somehow) that the source has stopped and they should change there cource to hit the observer?

Yuiop and Pervect, Thank you for your time and sharing your knowledge. I bet if all the people on this fourm can get together and share there views and knowledge, The Time machine will no longer be a dream :)

WOW! two replies.. both YES, seems like I have to agree, I alway thought that light travels in a straight line, and it is indipendent of its sources motion, I never knew that light rays get tilted like normal objects does for example A stone thrown out from a moving train etc.
Yep, that is about right. Congratulations for catching on so fast! We have had very long disputes about this with "newcomers" the past. One inportant aspect is that whatever the angle the photon is emitted in, its velocity is still c relative to any reference frame. This means that unlike a stone, if the photons is emitted directly forward or backwards from the high speed train, its velocity is still c relative to an observer on the track and relative to an observer on the train. If the photon is emitted at right angles from the train (as far as the observer on the train is concerned) then its path will appear to be tilted forward as far an observer on the track is concerned, but the velocity it gains in the forward direction is cancelled out the velocity it loses in the direction at right angles so its total velocity is still c. To the observer that is at rest with the track, the velocity of the photon parallel to the track is always the same as the velocity of the emitter on the train if the train velocity is constant the the photon is emitted at right angles in the train frame.

Now keeping this tilt thing in mind, let us go back to orignal senario and this time have the source send constant streem of light pulses(In the form of binary) towards the observer(A Photo detector).
Please Note: Light source and photo detector are 5 light seconds apart and they will accelerate and stop together like they are mounted on a very long bar.

Situation 1. If both the source and observer are stationary, I know observer will receive all the pulses send by source. No doubt about it.

Situation 2. Now if they both decide to accelerate for some time and then come to a constant speed. What will happen? Will the observer miss the pulses that were send while they were accelerating? And resume to receive the pulses only after they start travling at a constent speed.?
O.K. just to be clear, the acceleration happens at right angles to the connecting bar. Now assuming that the pulses are fired from a directional source such as a laser, then yes, the observer miss the pulses that were sent while they were accelerating and also yes they resume to receive the pulses only after they start travelling at a constant speed. If the constant speed after the acceleration ceases, is such that the gamma factor is 2, then there will be a delay of about 2.5 seconds between the observer resuming constant velocity and the observer resuming to receive light pulses due to the light travel time and the time dilation factor.
Situation 3. They are traveling at a constant speed and come to a a sudden stop(within no time). Will the light pulses overshoot the observer, because they got tilted from the sources motion. Or will the photons know (somehow) that the source has stopped and they should change there cource to hit the observer?
When the observer comes to a sudden stop he will have missed the 2.5 seconds worth of signals sent from the source (according to a clock moving with the source) and there will be a gap of about 5 seconds before the stopped observer starts to resume receiving pulses again, assuming the source continues to emit pulses after it stops. The lost signals will be due to the overshoot you described in your first option. The photons do not have any knowledge of the observers motion while in travel and do not compensate in any way. The path of the photons is fully determined at the time they are emitted.

Saw
Gold Member
WOW! two replies.. both YES, seems like I have to agree, I alway thought that light travels in a straight line, and it is indipendent of its sources motion, I never knew that light rays get tilted like normal objects does for example A stone thrown out from a moving train etc.[/B].
A usual explanation is that light does not share the speed of the source but does share its direction of motion. In my opinion, that is simply descriptive of what happens. I think it is a little more explanatory to add that the light hits a target at rest with the source because the latter (eg, a laser gun) is pointing in that direction. Hence the photons that come out of the mouth of the instrument are only those that have already acquired, within the length of the instrument, the appropriate direction, the one leading to the target.

Situation 2. Now if they both decide to accelerate for some time and then come to a constant speed. What will happen? Will the observer miss the pulses that were send while they were accelerating? And resume to receive the pulses only after they start travling at a constent speed.?

Situation 3. They are traveling at a constant speed and come to a a sudden stop(within no time). Will the light pulses overshoot the observer, because they got tilted from the sources motion. Or will the photons know (somehow) that the source has stopped and they should change there cource to hit the observer?
Whenever the target is accelerated (backwards or forwards) after the light was emitted (in other words, if the target accelerates while light is "in transit"), the photon will miss its target. Whether the source has accelerated or not is irrelevant in this respect. Answering your question: light is not a sentient being, it does not know whether the source or the target have accelerated.

I bet if all the people on this fourm can get together and share there views and knowledge, The Time machine will no longer be a dream :)
Well, it is not that I have much knowledge to contribute, but don't count on me to spend time trying to invent a Time machine...

Well explained! No doubt left for this one,
It means that if me and my wife are standing in opposite corners of a bus this wide (5 light seconds) and smiling at each other, We will loose each others sight if this imaganary bus suddenly starts accelerating. Just joking, We always sit closer, so no worries. lol.
Any way can I ask another question?

Okay, I have heard that universe is expanding at the speed of light. Is it true? Does it mean that our Earth is headed towards something at light speed?

ghwellsjr