How ageing on Earth mimics the effects of space travel

One of the most famous thought experiments in physics is the twin paradox, first proposed by Albert Einstein in his 1905 paper on special relativity, and later expanded on by physicist Paul Langevin. It goes something like this. An astronaut leaves Earth and spends a few months travelling through space at almost the speed of light. He has a twin brother who stays on Earth. When the astronaut returns, he finds that his brother has aged decades while he is still young. I don’t claim to understand why this would happen, but it has something to do with the relative passage of time experienced by a fast-moving traveller compared with someone on Earth. Or something. It doesn’t really matter: this is a column about ageing.

It is impossible to travel at such speeds, so the twin paradox will remain a thought experiment for the foreseeable future. But something quite similar actually happens in the real world, only in this case, it is the space traveller who ages faster. A twin who goes into space for six months will, by some measures, age 40 times faster than their earthbound sibling.

The vast majority of us will never have to worry about this, but the factors driving accelerated ageing in orbit are increasingly common on terra firma. The good news is that attempts to protect astronauts against these could benefit us all.

As of now, depending on where you draw the boundary, some 781 people have been into space. Some of them spent only hours or days up there, but around half lived on a space station for months. A trip to the International Space Station (ISS) – which has been undertaken by nearly 300 astronauts – usually entails spending six months or more in space.

NASA has long been concerned that extended time in space can harm astronauts’ health and, mindful that lengthy missions to Mars and beyond are on the drawing board, has done extensive research on the