According to Einstein's Theory of Special Relativity, clocks on rapidly moving spacecraft will advance more slowly than stationary clocks. This applies to all "clocks" (mechanical, atomic, biological).

For example, a clock on board a spacecraft moving at 99% of the speed of light will appear to be advancing 7 times more slowly than a clock at rest (see formula on page 472).

This means that astronauts age more slowly than do we who remain on Earth ("the Twin Paradox"). Example: Trip to Alpha Centauri (4.3 light-years) at 99% of the speed of light would take 8.7 Earth years, but the astronauts would only age by a year and 3 months!

If humans were willing to return to an Earth populated by their descendants then voyages across the Galaxy might be possible.