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Message: <blockquote data-quote="freyar" data-source="post: 6713142" data-attributes="member: 40227">Depends on the definition of "year" you want to use.  Astronomers use several different "years" to talk about the orbit of the earth (see <a href="https://en.wikipedia.org/wiki/Year" target="_blank">wikipedia</a>).  The main ones I know of are the Julian year which is exactly 356.25 days (at 24 hrs of 60 mins of 60 s) and the sidereal year, which is a bit longer and describes how long the earth takes to return to a set position with respect to "fixed stars."A lightyear is c times a Julian year.Incidentally, c is defined at a fixed value, not measured.  We measure the length of a second in terms of radiation from a cesium atom and then multiply by c to get the length of a meter.Well, I think rather than defining an amount of fuel in terms of the time it would last, you'd probably define the amount of fuel by what kind of trip you can take with it.  But for food, you're right.  For the people in the rocket, time really does pass more slowly, so they only experience the shorter length of time and therefore eat less food.  In this example, it would be the amount of food they'd normally eat in 7 years.  And, yes, there'd be a whole new generation of people on earth when they returned.Well, you'd remove the gravitational contribution to the time difference.  But that's very small for earth's gravity in comparison to the effect of the speed.Gravitational time dilation and time dilation due to orbital speed are both about the same size for a satellite around the earth, and both effects have to be included in calculations of position by GPS.  So there are real world applications of all this stuff!OK, the ship is moving at 0.9c compared to earth. So each year that passes on earth, the ship gets 0.9 lightyears farther away.  And vice-versa: according to someone on the ship, the ship is sitting still, and the earth is moving away at 0.9c, so the earth gets 0.9 lightyears farther away each year that passes on the ship.  But note that a year on the ship is not a year on earth, and a lightyear measured from the ship is not the same as a lightyear measured from earth either!Just working from the point of view of earth, if the earth sends out a radio signal 1 year after the ship leaves, the radio signal has to travel both the 0.9 lightyears plus however far the ship travels (according to earth) until the radio signal catches up!  With the ship moving that fast, it actually takes 9 years for the radio signal to reach the ship (meaning, it reaches the ship 10 years after the ship leaves earth, as according to earth).  And that's 9 lightyears away from earth, as measured by earth. This is one of those situations where you have to think carefully about which reference system you're using.</blockquote>

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