The Cable-Car Model

A sailing ship which depends on light from the sun to accelerate it remains in that way connected to the sun; its reference is the sun, and its speed is limited to less than the speed of light, $c$, relative to the sun. Propulsive energy cannot reach a ship traveling away from the sun faster than that. It is limited to the speed of light for the same reason a cable car is limited to the speed of the cable. It is riding on, and being pulled along by, a lightbeam. Einstein wondered what it would be like to ride on a lightbeam. It may simply be like riding on a cable car. A sailing ship is limited to that speed, $c$, by design, not by some inscrutable, overarching, outranks-everything-else Law of Nature. A speed limit - an actual speed limit -- exists for it. On the other hand if the ship carried a back-up rocket it could now be lit to provide additional thrust and further acceleration.