Editing Interstellar travel (section)

==== Time dilation ====
{{Main|Time dilation}}

Physicists generally believe faster-than-light travel is impossible. 
Relativistic [[time dilation]] allows a traveler to experience time more slowly, the closer their speed is to the speed of light.<ref>{{cite book |last1=Parkinson |first1=Bradford W. |url=http://app.knovel.com.libezp.lib.lsu.edu/hotlink/pdf/rcid:kpGPSVTA03/id:kt00C9LNA2/global-positioning-system/time-dilation?kpromoter=federation. |title=18.2.2.1Time Dilation |last2=Spilker |first2=James J. Jr. |last3=Axelrad |first3=Penina |last4=Enge |first4=Per |date=2014 |publisher=American Institute of Aeronautics and Astronautics |isbn=978-1-56347-106-3 |author3-link=Penina Axelrad}}</ref> This apparent slowing becomes noticeable when velocities above 80% of the speed of light are attained. Clocks aboard an interstellar ship would run slower than Earth clocks, so if a ship's engines were capable of continuously generating around 1&nbsp;g of acceleration (which is comfortable for humans), the ship could reach almost anywhere in the galaxy and return to Earth within 40 years ship-time (see diagram). Upon return, there would be a difference between the time elapsed on the astronaut's ship and the time elapsed on Earth.

For example, a spaceship could travel to a star 32 light-years away, initially accelerating at a constant 1.03g (i.e. 10.1&nbsp;m/s<sup>2</sup>) for 1.32 years (ship time), then stopping its engines and coasting for the next 17.3 years (ship time) at a constant speed, then decelerating again for 1.32 ship-years, and coming to a stop at the destination. After a short visit, the astronaut could return to Earth the same way. After the full round-trip, the clocks on board the ship show that 40 years have passed, but according to those on Earth, the ship comes back 76 years after launch.

From the viewpoint of the astronaut, onboard clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 light years per ship-year. The universe would appear contracted along the direction of travel to half the size it had when the ship was at rest; the distance between that star and the Sun would seem to be 16 light years as measured by the astronaut.

At higher speeds, the time on board will run even slower, so the astronaut could travel to the center of the [[Milky Way]] (30,000 light years from Earth) and back in 40 years ship-time. But the speed according to Earth clocks will always be less than 1 light year per Earth year, so, when back home, the astronaut will find that more than 60 thousand years will have passed on Earth.