Light pulse path, length and shape, when bouncing between two mirrors

Click For Summary

Discussion Overview

The discussion revolves around the path and behavior of a light pulse bouncing between two mirrors, particularly from the perspectives of different inertial observers. It explores concepts in special relativity, including the nature of light paths, time dilation, and the implications of measurement in different reference frames.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the light pulse follows a straight path in the rest frame of the mirrors, while a moving observer sees a zig-zag path due to the effects of special relativity.
  • Others argue that both paths can be considered physically true, as they represent different projections of the same invariant worldline in spacetime.
  • A participant suggests that the "true" path is the four-dimensional worldline, which can be projected onto different three-dimensional surfaces, leading to different spatial descriptions.
  • There is a discussion about time dilation, with some participants questioning whether it stems from the act of measurement or if it is an inherent property of moving clocks.
  • One participant emphasizes that the ticking of a clock is the measurement itself, and that time dilation occurs regardless of the measurement process.
  • Another participant highlights that even in Galilean relativity, the path shape differs across frames, indicating that the concept of time dilation is not solely dependent on light travel time.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of the light pulse's path and the implications of time dilation. The discussion remains unresolved, with no consensus on the "true" nature of the paths or the origins of time dilation.

Contextual Notes

Participants note that the discussion involves complex concepts in relativity, including the dependence of spatial paths on the observer's frame of reference and the nuances of time measurement across different inertial frames.

  • #31
fog37 said:
Velocity of light

Speed. Velocity (a vector) is not constant, it can have different directions in different frames.
 
Physics news on Phys.org
  • #32
fog37 said:
Summary: The path of a light pulse moving bouncing between two mirrors (top and bottom) from two different inertial frames.

Question: is the path followed by the light pulse straight or zig-zag? Which one is physically true? Both, even if they are different in length and shape?

Note that you don't have to look at special relativity to address this issue. Imagine you are in a commercial airliner cruising at a steady speed of, say, 600 mi/h. As you sit in your seat you toss a ball upward, and it comes back down to land in your hand. The ball's path is straight.

Now suppose I am on top of a mountain with a telescope watching you. I will see the ball travel in a parabola.

Which is the true path of the ball, a straight line or a parabola?
 
  • Like
Likes   Reactions: fog37 and Ibix
  • #33
Thank you everyone.
 
  • #34
I’m confused by the basis of relativity, why everything, including the spacetime and everything in this universe does not have an absolute velocity, but have a relative velocity to others. Everything around us is actually constantly moving, depending on the perspective. What makes photons so special, they have a fixed, absolute value of velocity? No matter the perspective, even you are just going one Planck distance (or the diameter of singularity)/second slower than c?
 
  • #35
Xforce said:
What makes photons so special, they have a fixed, absolute value of velocity?

The fact that they are massless--they have zero invariant mass. Anything with zero invariant mass must move at the same invariant velocity in all reference frames. That's one of the basic facts of relativity.
 

Similar threads

High School Am I understanding the concept of proper frame of reference?
  • · Replies 33 ·
2
Replies
33
Views
2K
Undergrad Distance between two light pulses
  • · Replies 4 ·
Replies
4
Views
1K
Undergrad Conventionality of the One-Way Speed of Light
  • · Replies 6 ·
Replies
6
Views
1K
Undergrad Is there any way to derive the time dilation formula?
  • · Replies 31 ·
2
Replies
31
Views
4K
Undergrad Independence of speed of light and velocity of source
  • · Replies 34 ·
2
Replies
34
Views
4K
Undergrad SR flat Lorentzian manifold and Anderson simultaneity convention
  • · Replies 16 ·
Replies
16
Views
2K
Undergrad The Einstein Clock aka Light Clock
  • · Replies 34 ·
2
Replies
34
Views
3K
Undergrad What causes time dilation and length contraction
  • · Replies 10 ·
Replies
10
Views
2K
High School Light Speed In Forward and Backward Directions
  • · Replies 51 ·
2
Replies
51
Views
5K
Undergrad Why do people keep saying photons are timeless?
  • · Replies 27 ·
Replies
27
Views
5K