Draw the spacetime diagram for the motion of the twins discussed in Section 2.8. Draw light signals being emitted from each twin at annual intervals and count the number of light signals received by each twin from the other.
Strategy We shall let Mary leave Earth at the origin (x, ct) = (0, 0). She will return to Earth at x = 0, but at a later time ct = 20 ly. Her worldlines will be described by two lines of slope +c/v and -c/v, whereas Frank’s worldline remains fixed at x = 0. Frank’s and Mary’s signals have slopes of \pm 1 on the spacetime diagram. We pay close attention to when the light signals sent out by Frank and Mary reach their twin’s worldlines.
