Bob and Alice are twins so of course, for many years, they share their birthday celebrations. Then one day Alice leaves immediately after their 20th birthday party and boards a spaceship to a space station which is 6 light years away. The ship travels at 0.6c so the outward journey takes 10 years according to Bob, but the ship's clock only registers 8 years. She returns at the same speed, just in time to share a party again. However, Bob is now 40 while Alice is only 36.
Purple dots - the twins share birthday parties.
Blue dots - Bob parties alone.
Red dots - Alice parties alone.
Grey horizontal grid lines are 1 year apart and vertical lines are 1 light year apart in our coordinates.
Move the slider or enter a value to see the location and time coordinates of each party as measured by a passing alien. Set to 0 for the alien to share Bob's rest frame, +0.6 for the alien's values if he shares Alice's frame during the outward leg and -0.6 for her return leg.