Comment by shawn_w

Ah, the joys of trusting a LLM for story IDs...

This one actually exists (a rarity in my experience), and the plot largely matches the description up to the last sentence (though the speed isn't exactly reduced; it's made a constant)

>Humans discover they can modify the speed of light but find they can only make it slower.

That doesn't happen in the story. It ends with the aliens disabling the device (the drive of a derelict starship) generating the field that caused experiments to suggest the speed of light is constant, and then leaving, destroying "the whole basis of their physics, but that is just a fantasy anyways."