Solution

ID: past-exam-of-the-physics-1-paper-of-the-university-of-oxford/2016-resit/11/c/solution

Replace the frame speed by , requiring . With ,
The outbound leg is now the longer one and the inbound leg the shorter one. Doubling the speed also changes the Lorentz factor; it is not enough to swap the two original leg times. A proposed doubled speed at or above does not define an inertial observer frame.

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