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why is the speed of light constant for all observers
Throw a ball at thirty metres per second from a train doing twenty, and the ground sees fifty. Speeds add. Shine light from that train, though, and it never adds.
This isn't a mysterious property of light alone. Maxwell's equations give one speed, one over root mu-zero epsilon-zero, with no frame in it. Michelson and Morley found no ether wind in 1887.
Here is what has to give. Build a clock from light bouncing between two mirrors, one metre apart. Each tick takes two L over c, about six point seven nanoseconds.
Now slide the clock past you at point six c. You see the light zigzag, a longer path. Same c, longer path, so each tick takes longer: time slows.
So c is constant because speed is distance over time, and nature keeps c fixed by bending both. It is the universe's speed limit on cause and effect.
Light's speed does not bend to us; instead our clocks and rulers bend, so that everyone measures the same c.