The passage of time often seems like the most malleable thing in the world. A minute may feel much longer when you’re asked to hold a plank position in a workout class, and an hour may fly by when you’re chatting with a friend. In these cases, your perception of time is altered.
But real, physical time dilation also happens. According to Albert Einstein’s special theory of relativity, clocks that are closer to Earth’s centre of gravity, say at sea level, tick more slowly than those at the top of a mountain. A similar slowing happens when a clock is physically travelling extremely fast.
Why banishing irrational numbers could trigger a revolution in quantum theory Undeniably, this sounds bizarre, but decades’ worth of experiments haven’t managed to disprove it. Yet in what exact sense time dilation is real – and what it means for a physical effect to be real at all – has inspired philosophical debates ever since Einstein first formulated his theory in 1905. In fact, in June, at the Foundations of Physics conference in Irvine, California, I heard Marco Giovanelli discuss how time dilation is both real and apparent at once.
It sounds paradoxical, but when I called Giovanelli, who is a historian of philosophy and science at the University of Turin in Italy, to talk through it, I started to understand that it reflects the facets of our physical world that Einstein’s work first uncovered. For Einstein, it was all about rods and clocks, explains Giovanelli. Suppose that you and a friend each had an identical clock and a metre-long rod.
You stand still, while your friend gets into an incredibly fast spaceship and sets off on a journey. Compared with yours, their clock will tick less quickly and their rod will contract in size. If you want to know by how much, Einstein’s got your back: he worked out those calculations more than a century ago.
If you happen to have a second friend who has a slightly slower spaceship and their own clock-and-rod set, the same trick will work again, except that their clock will slow and their rod contract a little less than your speedier friend. Finally, if you also acquire a spaceship and catch up with either of your friends, your clocks will start to tick at the same rate again, and your metre-long rods will again look the same length. To discuss these effects, physicists use the language of “inertial frames”, similar to comparing a room at rest, or a rest frame, with a spaceship travelling at a constant speed, or a moving frame.
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