'Speed of Light' Thought Experiment

Click For Summary

Discussion Overview

The discussion revolves around a thought experiment involving a car moving away from a stationary observer and the implications for the time it takes for a light signal emitted from the car to reach the observer. Participants explore the effects of relative motion on the perceived travel time of light, considering different reference frames and the concept of simultaneity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that the time for the light signal to reach the observer is simply S/c, where S is the distance to the observer.
  • Others argue that the reference frame matters, and the time measured can differ depending on whether the observer or the car is considered as the frame of reference.
  • A later reply questions the assumption of simultaneity in starting the clocks for the observer and the car, suggesting that this could affect the perceived travel time.
  • Some participants assert that from the car's viewpoint, the light signal takes longer due to the relative motion, leading to a proposed formula involving the speed of light and the car's speed.
  • Another perspective is that the distance S appears contracted in the car's frame, leading to different calculations for the light travel time.
  • One participant suggests reversing the scenario, sending the light signal from the observer to the car, and argues that the travel time should remain S/c regardless of the direction of the signal.
  • There is a contention regarding whether the distance traveled by the light signal changes based on the direction of emission, with some asserting that it does while others maintain it does not affect the travel time.
  • Participants discuss the implications of the invariance of the speed of light and how it relates to the calculations of travel time in different frames.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as multiple competing views remain regarding the effects of reference frames on light travel time and the implications of simultaneity. There is ongoing debate about the correct interpretation of the thought experiment and the calculations involved.

Contextual Notes

Limitations include assumptions about simultaneity, the dependence on reference frames, and the unresolved mathematical steps regarding the calculations of light travel time in different scenarios.

  • #31
Its worse than that, Doc (edit: well, that's already a pretty fundamental misunderstanding...)-- In your last post, Thomas2, you propose using markings on a rope to measure distance. This implies that distance is invariant - it isn't. Read this next part very carfully: Two people on different ends of a rope being reeled out will not necessarily agree on how much rope is being reeled out and will not necessarily agree with what the markings on the rope say about the distance traveled. This is a consequence of SR. The use of markings on a rope is an invalid way to measure distance.

What we have here is a simple refusal to accept SR.
 
Last edited:
Physics news on Phys.org
  • #32
russ_watters said:
What we have here is a simple refusal to accept SR.
SR does not enter into this consideration at all, only the principle of the invariance of the speed of light in moving reference frames. SR is derived from similar considerations but gives results inconsistent with the above one.
You can't use the conclusions of derived theories in order to falsify the original principles from which they were derived. The only question of relevance here is if the principle of the invariance of the 'speed' of light is being interpreted consistently.
 
  • #33
You misunderstand my point - being uncomfortable with what SR says and knowing that SR is derived from the constancy of C, you're trying to attack SR by finding an error in our understanding of the constancy of C.

Caveat: time dilation and length contraction were first derived as consequences of SR, but now they have been verified experimentally. So it doesn't matter which end you look at it from (how time dilation and length contraction affect C or how C affects time dilation or length contraction), you're wrong both ways!
 
Last edited:

Similar threads

Graduate 1-Way Speed of Light
  • · Replies 42 ·
2
Replies
42
Views
3K
High School Einstein thought experiment confusion: “light clock in a moving frame”
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad Synchronizing clocks in an inertial frame if light is anisotropic
  • · Replies 67 ·
3
Replies
67
Views
4K
Undergrad One way speed of light
  • · Replies 26 ·
Replies
26
Views
568
Undergrad Conventionality of the One-Way Speed of Light
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Understanding Relativity of Simultaneity in Resnick, Part II
  • · Replies 20 ·
Replies
20
Views
3K
High School One way speed of light in one reference frame measurement
  • · Replies 5 ·
Replies
5
Views
2K
High School Thought Experiment: Behavior of shadow of object moving at speed c
  • · Replies 14 ·
Replies
14
Views
2K
Undergrad Analyzing Time Dilation in a Spaceship-Mars Scenario
  • · Replies 14 ·
Replies
14
Views
2K
High School Is there a thought experiment to show that the speed of light is constant?
  • · Replies 17 ·
Replies
17
Views
3K