In Einstein’s theory of gravity, mass warps space-time, creating an effect known as gravitational time dilation. This means that an elapsed span of time is measured differently for observers in different gravitational potentials. So, as you get closer to a black hole, the flow of time slows down, compared to the flow of time far from the black hole.
According to an observer far from the black hole, an object falling into a black hole freezes in time at what is known as the ‘event horizon’ (the edge of a black hole, the point of no return).
Nothing ever appears to cross the event horizon. However, the observer falling into the black hole doesn’t experience time stopping at the event horizon. They witness time passing normally, but will see time speeding up far from the black hole. The closer to the event horizon they approach, the faster time will appear to progress far from the black hole.
Gravitational time dilation can actually be measured. In 1976, NASA launched an atomic clock into space to measure the rate at which time passes at an altitude of 10,000km (6,214 miles), compared to that on Earth’s surface. The measurement precisely matched that expected from Einstein’s theories.
At sea level, time progresses by one-billionth of a second less per year than at the summit of Mount Everest.
This article is an answer to the question (asked by Sean Roberts, via email) 'What happens to time at the event horizon of a black hole?'
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