B

3. The total wristwatch time x from event #1 to event #3 is the sum of the wristwatch time T^ between events #1 and #2 plus the wristwatch time tg between events #2 and #3:

4. Now ask: When—at what frame time t—will the stone, following its natural path, pass the intermediate point and emit the second flash #2? Answer with the Principle of Extremal Aging: Time t will be such that the aging (t in equation [18]) is an extremum. To find this extremum set the derivative of x with respect to t equal to zero. Take the derivative of both sides of [18] and substitute from equations [15] and [17]:

Use Principle of Extremal Aging to find the time for the middle flash

Quantity whose value is the same for adjacent segments

5. The last equality in equation [19] leads to the equation

6. In expression [20] the frame time t is the time for the particle to traverse segment A. Call this time fA. The time (T -1) is the frame time for the particle to traverse segment B. Call this time iB Then equation [20] can be rewritten in the simple form