The nearest star being Alpha Centauri C at about 4.3 light years, I believe. I know the observer at Earth may see the space completing a round trip in 8-9 some years, but how much time the the passenger on the spacecraft feel passed?
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At that speed, the time dilation factor is about 71 (meaning time passes about 71 times more slowly). So a round trip of 8.6 years earth-time would feel like about 44 days on board the ship.
The formula for the time dilation factor is:
1/√(1-(v/c)²)
Where "v" is your speed, and "c" is the speed of light. Once your speed gets very near to "c", the dilation factor changes very dramatically in response to even slight changes in speed. For example, if you add an extra "9" (making it 99.999%), the dilation factor jumps to 224.
The formula for the time dilation factor is:
1/√(1-(v/c)²)
Where "v" is your speed, and "c" is the speed of light. Once your speed gets very near to "c", the dilation factor changes very dramatically in response to even slight changes in speed. For example, if you add an extra "9" (making it 99.999%), the dilation factor jumps to 224.