I have been following this conversation for a while now:

https://qoto.org/@zeccano/103205964102778422

@freemo, problem is that @zeccano does not understand what "relative to" and the "frame of reference" are or mean.
Without understanding this, it's all total waste of time.

It's also painfully obvious, when you bring up the bus and the ball.
I am not sure why zeccano cant (or refuses) to understand that if I send you a photon, while we both move, I see it travelling in a straight line from me to you and you see the same.
Now, for someone OUTSIDE of our frame, the photon moves diagonally relative to his reference point.
Exactly what happens if I was standing on a bridge, looking down at the bus and those 2 kids towing a ball to each other.

This is where people screw up. They mix the frames of reference where events occur and where they are - outside of it.

It's like they refuse to understand you can have a frame inside a frame.

This is also causes the confusion about why the laws of physics remain the same in all inertial frames of reference.

@CCoinTradingIdeas @freemo
Your illustration involving a ball tossed between two people, will be seen as a diagonal when viewed from a differnet moving perspective.
But its not like this with light.
According to every physicist including einstein, Light is the only thing that is absolute, its own self is the ONLY absolute frame of reference, which is why light is invariably always c.
Because physicists are saying that lights frame is the preferred frame. (the only absolute frame)
So in your scenario you have done the impossible, you have set the observer who sees the diagonal as if he were in the absolute preferred frame of light!
He cannot be in that frame of reference. Its absolute.

The two guys trying to toss the ball between each other wont have any problem if its a ball, which gains the inertia of the guys, as the ball has mass it CAN gain the inertia of the guys, but light cant, as its without mass.
If you try to reverse it, and claim that the guys are not moving, they are just tossing the ball back and forth, its the observer that is moving past, so he will see the diagonal, then still it only can work for a ball, not light? Why? Because in this scenerio, with the moving observer, you now have him AND light in the same absolute frame, again its not possible.
Anyway, what are you going to do with Einstein and every other physicist who say flat out, that light is NOT dependent on the motion of the source?
So move the guy who tosses the photon or the guy trying to catch it, and they will NOT stay in the same frame as the photon, as my video shows.
You guys are talking around in circles, contracting your own claims with weak logic.
A photon has no mass, therefore no inertia and cant have any momentum, relativistic or not.