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

[weirdoguy]

Velocity of light

Speed. Velocity (a vector) is not constant, it can have different directions in different frames.

 

[Herman Trivilino]

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?

 

[fog37]

Thank you everyone.

 

[Xforce]

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?

 

[PeterDonis]

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.