Why you age slower on a plane (and other incredibly strange effects of relativity)

Chris Lintott is a professor of astrophysics at the University of Oxford and a lecturer at Gresham College. He Chris Lintott informs us that “a core principle of Einstein’s theory of relativity, which is that time is not universal. The faster you travel, the slower time will pass for you”. He then says that scientists have proven this to be true in the real world i.e. “take a transatlantic flight from London to New York and your watch will be a ten-millionth of a second behind one left on the ground – but nonetheless you’ll have aged a fraction more slowly than if you’d stayed at home.”

Mr Lintott then says that another prediction emanating from the Theory of Relativity is “that gravity has an effect too. Get further from the Earth’s gravitational pull, and time will speed up. This affects our own bodies: it means your head is ever so slightly older than your feet. Once again, the effect is incredibly small, but at greater distances from Earth, it becomes important. The GPS system that we all depend on to navigate, with its satellites 20,000km (12,400 miles) above the Earth, needs to take this into account in order to work properly.”

Then Mr Lintott joins all of this to help us understand some remarkable things about how time would function if we were near a black hole: “…imagine falling toward a black hole… As you fall, you wouldn’t notice any difference in time for you or your close surroundings. Looking at your watch, or feeling your pulse, you would perceive the same steady beat as second after second you approach near-certain doom. If your spacecraft’s instruments let you look back to observe the Universe outside the black hole, though, you might notice something odd – events out there would appear to you to be speeding up. If you could watch Earth through a telescope, you would see the future of our planet and species play out for your enjoyment, running like a sped-up movie. If you could pick up a television signal, then you could watch the rest of humanity’s broadcasts until the Sun’s evolution into a red giant swallows the planet, albeit at speed.

Now switch perspective. Imagine you’re on a space station orbiting at some safe distance from the black hole, watching your brave or unfortunate colleague fall in. The edge of the black hole, such as it is, is the event horizon, the point at which even things travelling at the speed of light cannot escape, and it seems reasonable to expect our receding friend to reach this point and then disappear. What you would actually see is stranger – if they are waving to us, we will see them wave slower and slower as they fall deeper into the black hole’s gravitational well. A clock mounted on the outside of their spacecraft will seem to run slower compared to one safely installed on our station.

This effect is exploited in the film Interstellar, where astronauts who have explored a planet near a black hole emerge to find a changed Universe that has moved on without them. As the film makes clear, it makes no sense to ask whether the time passing near the black hole or far away is the “correct” time; relativity tells us that there’s no such thing.”

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