This thread is another extending from the `Schwarzschild Metric` and `Climbing out of a Black Hole`. I hope the members of the physics forum will not object to me testing out some ideas against their wider knowledge in this subject area. Anybody reading about relativity bumps into the Twin paradox sooner or later. It relates to two twins, one takes a return trip to a star at near light speed, while his sibling remains on Earth. If everything is relative, then either twin could claim that the other was moving relative to them and therefore should experience time dilation due to velocity, i.e. SR. The paradox arises as both twin are arguing that the other will be younger at the end of the trip. The standard response to the paradox is that only one twin `feels the force` of acceleration and deceleration with respect to a common starting point on Earth. As such, the twin who travels to the star is younger because he can translate the acceleration into a higher relative velocity with respect to the stay at home twin. QED. However, if you extend the twin paradox to a triplet paradox, things get a bit more interesting. One of the triplets stays at home, while the other two head off towards two stars (A & B). These stars are in the opposite direction to each other but on returning, each will pass Earth at near light speed and go on to the other star before eventually returning to Earth. We can clarify their paths as follows: Triplet-1: Earth (E) Triplet-2: E-A-E-B-E Triplet-3: E-B-E-A-E Having visited the first star, the two space-faring triplets speed past each other, as well as their Earth bound sibling on the way to the second star. At this point, all three could argue that the others are moving at near light speed with respect to them . What is the relative age of each triplet on finally returning to Earth? I would argue that the two space-faring triplets have to be the same age, but younger than their Earth bound brother, otherwise you just create another paradox. However, this answer raises an interesting issue, at least, I thought so. Time dilation due to special relativity is not just about relative velocity; it has to account for the relative starting point. If so, two galaxies that are speeding away from each other at near light speed, along radial paths, from a common point in spacetime, they must experience time and space against a common tick of the clock. Would this mean that time is relative to the expansion of the universe? Again, would be interested in any other thoughts.