Imagine two twins: one orbits close to a massive black hole while the other stays on Earth watching via livestream. After many Earth years, who will actually have aged less?

Near a black hole, the orbiting twin’s clock runs slower

If one twin spends time in a sufficiently close, stable orbit around a black hole while the other remains far away on Earth, the orbiting twin can experience less time. When they later compare ages, the twin who stayed on Earth will have aged more.

The cause is gravitational time dilation: clocks deeper in a gravitational field tick more slowly relative to clocks far from it. A black hole’s field can make that difference especially large. The orbiting twin’s motion also affects the comparison—moving clocks run slow relative to stationary observers—so the exact amount depends on the orbit and the black hole. But in the scenario posed, the close orbit is chosen so that the orbiting twin accumulates less time overall.

The livestream would show a distorted view

The Earth twin would not see the orbiting twin’s life as a perfectly synchronized live feed. Light escaping the black hole’s strong gravity is stretched toward lower frequencies, an effect called gravitational redshift. Signals can also take a long time to travel outward. As a result, the transmitted image may look slowed and reddened compared with events on Earth.

That view is not the same thing as the twins’ eventual age comparison. Each twin’s own clock measures time along their path through spacetime; the comparison becomes clear when they reunite or otherwise compare clocks after accounting for the signal’s journey.

A close orbit still has limits

The twin cannot simply hover anywhere near the black hole. Orbits must be stable, and sufficiently close in, objects can no longer maintain a lasting orbit. The black hole’s mass matters, too: near a very massive black hole, tidal forces—the stretching caused by gravity being stronger on one side of an object than the other—can be less severe at the horizon than near a smaller one.

The striking consequence is that the twins can share a starting point and later differ in age not because either clock malfunctioned, but because they followed paths through spacetime that allowed different amounts of time to pass.