Question on special relativity

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Discussion Overview

The discussion revolves around the implications of special relativity, particularly focusing on the scenario of an observer moving towards two light sources. Participants explore the apparent contradiction regarding the simultaneity of light rays reaching the observer and the effects of relative motion on their perception of these events.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • kknull presents a scenario where an observer moves towards one of two light sources and questions how the light rays meet at the observer's position.
  • Some participants argue that the perception of simultaneity depends on the frame of reference, suggesting that the rays meet at the midpoint from the sources' perspective but not from the observer's perspective.
  • Others assert that the observer will encounter the light from the nearer source first due to their motion, challenging kknull's interpretation of the situation.
  • There is a discussion about the relativity of simultaneity, with some participants stating that the rays were not emitted simultaneously from the observer's frame of reference.
  • One participant uses a classical mechanics analogy involving balls to illustrate that if the observer moves towards one source, they will receive the light from that source before the other.
  • Another participant emphasizes that the speed of light remains constant at c, regardless of the observer's motion, but acknowledges that the observer's position changes over time.
  • Some participants express confusion about the implications of the relativity of simultaneity and how it leads to a circular argument regarding the meeting point of the light rays.
  • There is a suggestion to disregard the sources and focus solely on the light rays, arguing that their relative speed remains c regardless of the observer's motion.

Areas of Agreement / Disagreement

Participants do not reach a consensus. There are multiple competing views regarding the simultaneity of the light rays and the implications of the observer's motion, leading to ongoing debate and confusion.

Contextual Notes

Participants express uncertainty about the relativity of simultaneity and its implications for the scenario. The discussion reveals a lack of agreement on the interpretation of the observer's experience versus the sources' experience.

  • #31
understood. thank you! So my mistake was to consider my question as a paradox, while it was simply an empirical demonstration that simultaneity is relative.
 
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  • #32
Exactly!..
 
  • #33
Again, from the Einstein train example, the staionary observer sees te light flashed at the same time. The one in motion catches the lead flash before the rear flash.

Question is: what is the mathematics behind this?

Assume the train is 150,000 km in lngth which places the train observer 17000 km in the middle. It will take the light from the front flash 0.5 sec to reach the stationary observer. If the train is moving at 0.5 c, how long will it take the front falsh light to reach the moving observer on the train - DO THE MATH! Not just words - DO THE MATH

I have a feeling it is still 0.5 seconds but it shouldn't be
 
  • #34
stevmg said:
Assume the train is 150,000 km in lngth which places the train observer 17000 km in the middle. It will take the light from the front flash 0.5 sec to reach the stationary observer. If the train is moving at 0.5 c, how long will it take the front falsh light to reach the moving observer on the train - DO THE MATH! Not just words - DO THE MATH
How long will it take according to whom? The observer on the train or the observer on the platform? What's the proper length of the train? It the train is 150,000 km long, the middle would be 75,000 km from each end.

Since you use a time of 0.5 seconds, perhaps you meant the train to be 300,000 km long? If so, then the light from each flash would take 0.5 sec to reach the middle of the train according to train observers.

According to the platform observer, it would take less time since the length is shorter and the train moves towards the incoming light. The length of the train would be shorter by a factor of γ = 1.155. So if the proper distance between the middle and ends of the train is L = 0.5 light-seconds, that distance would be L/γ according to the platform observer. To find the time for the front flash to reach the middle of the train, use:
L/γ - vt = ct

Solving for time, you get:
t = L/[γ(c+v)] ≈ 0.29 sec
 

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