Pascal principle with light pressure Hello, Supose you have an U tube, in which the inner walls are made absolutelly reflective. Now, put a termical state (temperature T) for the radiation field inside the tube and close it with reflective discs which can slide without friction. Supose further that de diameter of the tube in its terminations are not equal. For the sake of definiteness, lets assume 2 sq meters cross section in one side and 10 sq meters in the other. My question is: Taking into account light pressure effects , is the Pascal principle valid in this context ?