Q: We hear of humans going to...

Q: We hear of humans going to distant planets in the future. But if some planets are light-years away, how could an astronaut live long enough to get there?

A: The solution, according to Caltech theoretical physicist Teviet Creighton, is provided by relativity. The faster a person travels, the more slowly time seems to pass for that individual--a phenomenon called time dilation. Imagine an astronaut on a spaceship that continually accelerates at 1 G--the acceleration due to Earth’s gravity. The astronaut’s perceived travel times to a few interesting destinations are:

nearest star (4.3 light-years)...................3.6 years

center of our galaxy (30,000 light-years).........21 years

Andromeda galaxy (2 million light-years)..........29 years

edge of known universe (15 billion light-years)...47 years

In a round-trip to Andromeda, an astronaut would therefore age 60 years. But people on Earth, aging at the normal rate, would have aged 4 million years.