When a ship approaches to within 90% of the speed of light, time slows down. Characters on board the ship would not notice, but if they were to make hourly reports back to their point of origin, those reports might arrive only once every hundred hours.
This creates an interesting paradox, in that if a character managed to travel at the speed of light to another star and back again, a newborn child he left behind would now be older than him—if the child hadn’t died of old age some time ago.
The actual amount of time dilation observed aboard a ship traveling near light speed increases in proportion to just how close it is to light speed. Technically, time dilation occurs at any speed, but it only becomes noticeable at relativistic speeds. The dilation is a ratio that determines how much time passes aboard the ship; it is a multiplier when determining how much time passes outside the ship.
For example, a ship moving at 70% the speed of light has a time dilation of 1.4. Ten hours of travel aboard the ship at this speed means that 14 hours (10 × 1.4) have passed outside the ship. However, if ten hours pass for those left behind, only 7.1 hours have passed aboard the ship (10 divided by 1.4).
|Table: Time Dilation|
|Starship Speed (miles/second)||AU per hour||% Speed of Light||Time Dilation|
Starship Speed: The vessel’s speed in miles per second.
AU per Hour: How many Astronomical Units (AU) a vessel traveling at this speed can cross in 1 hour. One AU equals 93,000,000 miles (the distance between the Sun and the Earth).
% Speed of Light: The percentage of the speed of light (186,000 miles per second).
Time Dilation: Divide the time traveled by this number to arrive at the amount of time that passes on board the starship.