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Relativity Theory/Einstein/Feynman: every thing that has energy also has mass. Mass in the sense that it is subject to the gravitational effect. Even light that has energy, has a "mass".
Def Relativity Principle/Newton: "The movements of objects within a given space are the same in relation to one another, whether the space is at rest or whether it moves constantly on a straight line."
Feynman: the Newtonian laws appear to be the same in a uniformly moving system.
Twin Paradox/Feynman: one might think: If by the movement of one twin that one ages more slowly, that because of the relative movement of the two to each other both should actually "become younger"?
FeynmanVs: there is an asymmetry in the twin paradox: the man who experiences the acceleration at the start and at landing is the one who ages more slowly. The other one is in a completely different situation. Only the one who has moved can come back.
That is a difference between them in an absolute sense and it is also true.
Vom Wesen physikalischer Gesetze München 1993
Vorlesungen über Physik I München 2001