Here's my scenario:

One person with a clock on planet A, one person with a clock on planet B. Planet A's mass is greater than that of planet B.

From what I understand, from each person's point of view, the other planets clock appears to be moving slower (even though one planets mass is greater than the other), correct? And an observer from a 3rd location, say location C, which is very far away from both planets A and B, would see the clock on planet A slower than that of planet B's, correct?

However, imagine the people on planet A and B leave their planets and take there clocks with them. They start to approach a location in between the two planets, but is still ver far away from either planet. As observer A gets closer to observer B, would they see person B's clock getting faster until it got ahead of their own clock? And would person B see person A' clock stay the same or get even slower?

  • $\begingroup$ That depends on who of them actually decelerates. If they just pass by each other, they'll never see anyones clock go faster. I don't see how this isn't just another variation on the twin paradox. $\endgroup$
    – ACuriousMind
    Jan 13, 2016 at 14:30
  • $\begingroup$ Are the first two statements I made correct? if both people meet in the middle, as in stop, is what I meant. I apologize if what I'm saying doesn't make any sense $\endgroup$
    – Brainless
    Jan 13, 2016 at 14:53

1 Answer 1


From what I understand, from each person's point of view, the other planets clock appears to be moving slower

You are confusing time dilation of special relativity with gravitational time dilation. If two spaceships in vacuum are moving in different directions near light speed, each of them observes the other going slower. But this is time dilation of special relativity which depends on their relative velocity.

Gravitational time dilation does not depend on movements. The factor of time dilation is a direct effect of gravitation. So, if you observe two people on mars and on Jupiter, their respective clock is faster/ slower, and this fact will not change if you move (as long as they are not moving).

  • $\begingroup$ So the person on the more massive planet would see the smaller planet's clock moving faster? $\endgroup$
    – Brainless
    Jan 13, 2016 at 21:50
  • $\begingroup$ Yes. The twin of the smaller planet would age a little bit faster than the twin on the big planet. Gravity is dilating time, it makes clocks go a bit slower. $\endgroup$
    – Moonraker
    Jan 14, 2016 at 17:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.