Gravitational time dilation

Time depends on the trajectory

In general relativity, each observer measures his own time. Two identical clocks following different trajectories can meet and show different durations. In a stationary gravitational field, the clock deeper in the potential accumulates less time.

It's not an optical illusion

The effect is measured by comparing atomic clocks at different heights and observing gravitational frequency shifts. Navigation satellites need relativistic corrections: without them, the position error would grow rapidly. Traveling into the future through time dilation is proven physics, although usually the effect is small.

The extreme case

Close to a compact object, the difference can be enormous when viewed from a distance. But staying close to a black hole requires considering orbits, radiation, tidal forces, and the difference between the traveler's time and the distant observer's coordinates. Crossing a horizon does not offer a route back outside.

Gravity and speed

Special relativity adds dilation by motion. A ship traveling at a very high fraction of the speed of light could return after many more years have passed on Earth. The traveler never sees his own clock malfunction: the asymmetry arises from complete trajectories.

Bridge to wormholes

A gravitational or kinematic phase shift between two hypothetical mouths could convert a wormhole in temporary machine. Dilation is real; the stable throat that would take advantage of it remains hypothetical.