If I am standing still, and a train passes me going 100km/h I perceive the train as moving at 100km/h.
But if I am in a train going 100km/h with another train running along side also going 100km/h the other train appears to me to be standing still with me. If a third train pulls up on the other side of my train going 110km/h it appears to me that this third train is moving at 10 km/h because relative to me the difference in speed is 10km/h.
Now imagine I am in a train going 5 km/h and the through some mix up there is a train on the same track going to opposite direction at 5km/h. We are going to collide and to me it looks like the other train is moving towards me at 10km/h. 10km/h because the net of our two speeds in opposite directions means we are coming together at 10km/h.
This does not hold true for light though.
If I were moving at 95% the speed of light towards a bright lamp post and I measured the speed of light coming at me from the lamp post, I would find it was still moving at the speed of light, not 195% the speed of light like the net of our two speeds would suggest. Light cannot move any faster or slower than the speed of light.
Here is my theory:
Because light is incapable of changing speed, to conserve energy light must change its frequency relative to the objects velocity.
In other words if I was again moving at 95% the speed of light towards a lamp post, our net speed would be 195% the speed of light. But because light can only go 100% the speed of light it would have to dump this extra 95% of my forward progress into its frequency. Thus the light from the lamp post wouldn't be visible anymore it would be converted into gamma rays which would melt my face off. Since all light is on the electromagnetic spectrum the light would blueshift into gamma rays
Thus I would only be able to see the lamp post in visible light if it were also emitting radio waves which would shift up to visible light.
What does this say for the objects behind me though? Based on this reasoning if I were heading to the lamp post at 95% the speed of light, away from a large gamma ray gun behind me, looking back I would suddenly perceive the gamma rays as visible light.
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