Space and time used to be so simple. You trundled around reasonably freely in the three dimensions of the one, and experienced occasional heartache at the remorseless forward march of the other. C’est la vie.
Or is it? Einstein revolutionised our perceptions a century ago when, in his theories of relativity, he first forbade anything in the cosmos from travelling faster than the speed of light, and then bundled both space and time into one unified space-time that can be warped by gravity (see “How to think about… Space-time“). The contortions introduced by Einstein’s special and general theories make intervals in both space and time dependent on where we measure them from. Two observers with flashlights in fast-moving trains might both measure the other to have flashed their flashlight first – and both be right from their own point of view.
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The recent blockbuster Interstellar is based on premises that Einstein made technically plausible, if not (yet) technologically feasible: that by travelling close to the speed of light, or moving in an intense gravitational field such as that of a black hole, we age more slowly than those we leave behind on Earth (see diagram). We don’t need to travel that far to see less dramatic effects of relativity in action. Astronauts on the International Space Station age a little less because of the velocity at which they travel, and a little more for enjoying less of the gravity of mothership Earth. The effects don’t quite cancel out. Velocity wins, leaving each ISS astronaut who completes a six-month tour of duty .
For most everyday purposes, such effects matter not a jot. But for physicists like Sean Carroll of the California Institute of Technology in Pasadena, who peer deep into the cosmos, relativity is a crucial consideration. He often resorts to drawing a diagram. “As far as special relativity goes, it’s very natural to think in terms of pictures,” he says. Relativity can seem full of paradoxes if we don’t first think carefully about how our own motion affects our perception of how time is passing for others – but also how others might see our time passing differently, too.
Carroll has a few rules of thumb to guide his own perceptions. “Basically, time is kind of like space, but not exactly,” he says. The main difference is that whereas in space a straight line is the shortest distance between two points, in time it is the longest. The way to minimise the time you experience between two events that occur at the same point in space is to move as far and as fast as you can in the interim. “If you zoom off near the speed of light, then zoom back, you will experience less time than someone who simply sits still,” says Carroll. So time passes slowly when you’re having fun.
Read more: “Get your head around the 13 boldest ideas in science”
This article appeared in print under the headline “How to think about relativity”
