Hi Vivian,
Fun question. Obviously, you already know that a Light Year is a distance. It's how far light can travel in a Year:
1 Light Year in distance:
About: 60x60x24x365.25x300,000 = 9,462,480,000,000 Kilometers.
or
Exactly: 9,460,730,472,580,800 Meters.
or
About: 5,878,625,000,000,000,000 Miles.
Vivian wrote:If something is four light years away, how do we say how long it would take to get there?
Depends on your speed and point of view.
If you could travel at Light Speed.. it would take 4 years as measured from home.
If you could travel at Light Speed.. it would take 0 time as measured by your watch.
See clocks and aging slow down with your speed as compared to the Speed of Light Maximum.
If you could travel at about 86% the Speed of Light:
Then your Trip would take about 4.6 years by Earth clocks.
And you would measure your trip to take about 2 years by your watch.
At about 86% the Speed of Light your aging and clocks will slow down to about 50% of normal.
http://www.emc2-explained.info/Dilation-Calc/#.WKhcEzjSMSUNote: This doesn't consider the time it takes to get going or time to slow down. That is a lot more complicated.
http://convertalot.com/relativistic_star_ship_calculator.html1 Gravity (1G) of acceleration means you will feel normal weight while always picking up speed. At 2G's you will have twice the weight for the whole trip.
At 5G's you would weight 5 times as much as normal.. so how long do you think you could stand weighing this much? Would not be very comfortable for sure.
Computing the time for speeding up and slowing down:
For 4 Light Years of distance:
At 2G's it would take 4.8 years to go the distance by Earth clocks.
At 2G's it would take 2.2 years to go the distance by your watch.
The nice thing about acceleration/deceleration (2G's) is if you wanted to go real far.. like 25 times further or 100 Light Years distance.. then it would only take about twice as long (5.1 years by your watch). Sadly, your friends back home will be about 100 years older during your 5.1 year trip there.
If your target destination was 200 Light Years away (twice as far).. then by your watch.. it would only take about 5.8 years at 2G's (pack a really big lunch). Back home about 200 years have passed for your 5.8 year trip (so don't forget to turn off the oven before you leave).
(Also, stopping for gas along the way would be a bitch.. lol.)
Hope that helps.
Best wishes,
Dave :^)