Does light really travel in same frame of reference

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Discussion Overview

The discussion centers around the behavior of light in relation to a moving observer and light source, specifically whether an observer can see a beam of light emitted from a source while both are in motion. The scope includes conceptual exploration of relativistic effects, such as relativistic aberration, and the implications of acceleration and constant speed on the reception of light pulses.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that if there is a reference frame where both the source and observer are at rest, the observer will see the beam of light, suggesting that this holds true in other frames as well.
  • Others argue that the concept of "towards the observer" is complicated by relativistic aberration, which alters the apparent position of the light source from the observer's perspective.
  • A participant raises scenarios involving the emission of light pulses while both the source and observer accelerate, questioning whether the observer will miss any pulses during acceleration.
  • Another scenario discusses the implications of the observer coming to a sudden stop, questioning if light pulses will overshoot the observer due to the motion of the source.
  • Some participants note that while the angle of emitted light may change due to motion, the speed of light remains constant relative to all observers, regardless of their motion.
  • There is a suggestion that if the observer and source are moving at a constant speed, the observer may miss signals sent during acceleration, and there may be a delay in receiving light pulses after resuming constant speed.

Areas of Agreement / Disagreement

Participants generally agree that light behaves differently in moving frames and that relativistic effects must be considered. However, there are multiple competing views regarding the specifics of how motion affects the reception of light, and the discussion remains unresolved on several points.

Contextual Notes

Limitations include assumptions about the nature of acceleration, the definition of "towards the observer," and the effects of relativistic aberration. The discussion also does not resolve the mathematical implications of time dilation and light travel time in the proposed scenarios.

NoDoubt
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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.
 
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NoDoubt said:
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.
 
NoDoubt said:
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:approve:, 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 scenario 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 :)
 
NoDoubt said:
WOW! two replies.. both YES, seems like I have to agree:approve:, 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. :wink: 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 canceled 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.

NoDoubt said:
Now keeping this tilt thing in mind, let us go back to orignal scenario 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 traveling 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.
NoDoubt said:
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.
 
NoDoubt said:
WOW! two replies.. both YES, seems like I have to agree:approve:, 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.

NoDoubt said:
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.

NoDoubt said:
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...:smile:
 
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?
 
In an expanding universe, everything is moving away from everything else, not towards anything else.
 
ghwellsjr said:
In an expanding universe, everything is moving away from everything else.

Everything is moving away from everthing? does Earth moving away from Sun? no. or everything keeping he same distance between each other but all togather moving away from the center of universe? Or is it just the boundary of our universe expanding at the speed of light?
 

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