Since light is a wave, the speed at which light waves travel does not change even if the light source moves.
If the observer moves, the speed at which the light wave travels should change, but no change could be found.
The fact that the speed at which light waves travel does not change even when the observer moves cannot be explained on the basis of time.

The concept of time is related to the distance traveled by light, but because the speed of light is the same both inside and outside a vehicle moving at high speed, time is used in a vehicle moving at high speed. Will slow down.
In other words, in a fast-moving vehicle, time advances slowly when viewed from the outside.

Time travels slower in a fast-moving vehicle than outside, so if you measure the distance from inside to outside the vehicle, the distance outside will be shorter.
From the inside of the vehicle, the outside is moving at high speed, so the outside distance that moves at high speed is shorter.
In other words, the length of a fast-moving object shrinks.

In a fast-moving vehicle, time advances slowly when viewed from the outside, so the speed of a fast-moving object and its vertical movement slow down.
When observing a completely inelastic collision with the same mass and diagonal collision at the same speed while moving at one lateral speed, it can be considered that the mass has increased according to the law of conservation of momentum.
The mass of a fast-moving object increases.

The mass of a fast-moving object will increase, and the total energy of the object is proportional to its mass.Derivation of E=mc².