Question

Consider two people born on Earth at the same time (like twins). Alice gets on a spaceship and travels to Alpha Centauri, 4.4 light years away, and back at an average velocity of 0.5c. Bob (her fraternal twin, I guess) stays on Earth. Who is older when they reunite, and why?

Answer 1

Here Bop will be older than Alice.

Because Alice is travilling is space.

When a space-like interval separates two events, not enough time passes between their occurrences for there to exist a causal relationship crossing the spatial distance between the two events at the speed of light or slower. Generally, the events are considered not to occur in each other's future or past. There exists a reference frame such that the two events are observed to occur at the same time, but there is no reference frame in which the two events can occur in the same spatial location.

This whole therory is explained by Einstien , Space time relation . in which we can see that in space time get slower.