Relativistic submarine, up or down..?
Suppose a submarine, floating at some depth in the ocean, starts traveling horizontally at relativistic speeds.
From the point of view of an outside observer, the sub will be shorter (length contraction), displace less water and it will therefore (less buoyancy) move down....
From the point of view of the sailors on the sub however, the water is moving towards them, each volume element of water will be length contracted and therefore the density of the water will be higher than when they where at relative rest. This increased density of the water will increase the buoyancy and will cause the submarine to move up... .
What is is, up or down?
@trueproblem Your answer does not resolve the paradox, but merely states that nothing happens just to get rid of the paradox. The answer is that the sub will sink. I leave it to you to figure out how to include gravity in this problem...