Is the twin-paradox caused by acceleration?

Answer 1

Acceleration is one factor in the Twin Paradox, which investigates why a moving reference frame experiences time at a different rate from a stationary frame (and why this effect only works in one direction). The effect of special relativity "slows down" time on an object accelerated and maintained at increased velocity with respect to its original location. However, general relativity also affects time passage and will "speed up" time on an object accelerated to a different gravitational potential (for example, an orbiting satellite). Experiments with atomic clocks in 1972 and 1997 established that clocks moving at increased altitude (in any direction) will run faster because of their increased distance from the Earth's gravitational center. Clocks travelling westward also run faster because they are moving more slowly than the Earth spins. Clocks travelling eastward were moving faster than the Earth's spin and this caused them to run more slowly, as predicted by special relativity. Clocks at the poles will also run faster than clocks at the equator for the same reason, but the difference is measured in billionths of a second.