Again consider two twin sisters, Gwen and Jackie, who are both 20 years old in the year 2020. This time, assume that Jackie makes the round-trip to Vega, which is 25 light-years away, at an average speed of 0.9999c
Which of the following correctly describes the situation when Jackie returns to Earth? (Ages in this problem refer to biological ages—that is, how much time has passed for each sister since she was born.) A) It's the year 2070. Gwen (who stayed home) is 70 years old, but Jackie is only a little older than her age of 20 when she left.
B) It's the year 2070 and both twin sisters are 70 years old.
C) It's the year 2070. Gwen is 70 years old. Jackie is 19.
D) According to Gwen, Jackie returns to Earth in the year 2070, but according to Jackie, she returns in the year 2021.
A
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