So why can't we use high-intensity lights powered by a nuclear reactor
as a source of space propulsion?


They can. But the F is small, so you need a lot of t to reach much of a v, as
long as you are far enough from a g.

Such systems have only been developed in science fiction however because we are
not yet doing enough interstellar travel, and people are uncomfortable with
nuclear reactors in space.

Jose

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