If time changes as a result of the speed of space travel, do the cells in an astronaut鈥檚 body still multiply at the same rate?
Ron Dippold
San Diego, California, US
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It is all relative. The simple answer is yes for the astronaut, no for someone (relatively) stationary watching them. To the astronaut and anyone with them, their cells are still growing at the usual rate. Their hair and nails need to be cut just as often.
To an observer just sitting in a chair on Earth, though, the astronaut鈥檚 cells will appear to be growing slower. The International Space Station (ISS) is travelling at about 7700 metres per second relative to us 鈥� the people up there are ageing 0.01 seconds slower per year than those of us down here because of that. But 0.01 seconds doesn鈥檛 really register mentally, so let鈥檚 kick it up a bit.
If the ISS were orbiting at 90 per cent the speed of light (not possible, but let鈥檚 pretend), the Lorentz factor, or time difference, would be much higher. At this speed, the Lorentz factor would be 2.3, which means the astronauts would be ageing 43 per cent as fast as the people on Earth. At 99 per cent the speed of light, it would be 14 per cent as fast. To us, their hair and nails would be growing one-seventh as fast and they would be moving in slow motion. But to them, if they managed to avoid getting squashed against the walls or being too dizzy to eat, their hair and nails would be growing at the usual rate.
If you had 20-year-old twins and one of them somehow went up to the 99 per cent speed of light ISS and spent 10 years (for them) there before returning, their twin would be 90. This is known as the twin paradox 鈥� which isn鈥檛 really a paradox, it is just Albert Einstein鈥檚 special relativity at work. Even at slower speeds, this has been repeatedly measured and GPS satellites need corrections for it. It鈥檚 real!
Alan Dix
Aberaeron, Ceredigion, UK
It is a common misconception that time 鈥渟lows鈥� as one moves faster.
Einstein said that time is what it says on the clock, or perhaps the astronaut鈥檚 wristwatch. The core of special relativity is that there is no universal clock against which things run slower or faster. Time is individual and so we may get out of sync with our clocks depending on the route we take through space-time. The astronaut鈥檚 cells will multiply at the same rate by their own clock, but, when they return to Earth, will have done so less than the cells of colleagues who stayed behind, because they took a 鈥渟horter鈥� route.
Nick Canning
Coleraine, County Londonderry, UK
Yes: consider Ann and Bea, identical twin microbiologists each with an identical clone of an E. coli bacterium cell, kept under identical conditions. Ann stays at home and measures the rate of cell division of her sample. Bea takes her sample on a space trip near the speed of light. She also measures the rate of cell division in her sample.
At the end of Bea鈥檚 round trip, the twins meet up again and compare measurements. They both find that they recorded the same rate of cell division, but Ann鈥檚 cells (both the E. coli and her own) have divided more times than Bea鈥檚.
For Bea, less time has passed than for Ann; she has aged less between their meetings than Ann has due to the relativistic time dilation effect. This is a real phenomenon confirmed by many different experiments 鈥� for example, the half-life of radioactive particles in motion is longer than when they are at rest, as seen in accelerator experiments or in cosmic ray showers.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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