See if you can help me with this question:

What we know:

*Einstein's inertial invariance principle says physics behaves the same inside a moving frame as if it were a stationary frame.

*An object moving will experience time slower than an object at rest. Best example of that is the twin paradox: Bob stays on earth while his brother Bill travels to Saturn and back at near the speed of light. When Bill returns from his year in space, he finds his brother and all of earth to have experienced five years. (My numbers here, are for example only. I didn't do the math to find the exact figures this example would take, but you catch my meaning).

*Momentum (or frequency of radiation) causes gravity, not mass.

Question:

If physics is the same in the frame that is moving as the frame that is stationary, how come Billy is experiencing a difference in time?

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