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<blockquote data-quote="freyar" data-source="post: 6710537" data-attributes="member: 40227">I'm late to the party, but...Just about, but we have to be a bit more careful, actually (I'd have to mark off a bit in my class <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f609.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=";)" title="Wink&nbsp; &nbsp; ;)"&nbsp; data-smilie="2"data-shortname=";)" /> ).&nbsp; What we need to know is what the .9c speed of the rocket is measured with respect to.&nbsp; People generally seem to be assuming that the rocket is moving at 0.9c with respect to the earth, but then that means the earth wouldn't move IF the earth weren't accelerating.&nbsp; In other words, all that time passes on the earth, but the earth is sitting still.But the earth's motion around the sun accelerates, so we don't really want to use that as a benchmark. It's better to say that the rocket moves at 0.9c with respect to the sun, while the earth orbits the sun.&nbsp; That's a much better approximation, since the sun accelerates very little in its motion around the galaxy. Of course, from the point of view of the earth, that 0.9c rocket travel is modulated just a bit by the motion of the earth around the sun.Or we could say that the rocket moves at 0.9c with respect to the galaxy.&nbsp; Then the sun is moving through the galaxy, too, which sounds closest to what you've described.&nbsp; But then, according to the sun, the rocket isn't moving quite at 0.9c either on the way out or the way back.Of course, the motion of the sun around the galaxy and the earth around the sun are at very small speeds compared to c, so these are all very minor corrections to the big picture everyone's talked about.&nbsp; But usually in relativity we need to be pretty clear about the question we're asking as the answer can come out quite differently in slightly different situations.</blockquote>

[QUOTE="freyar, post: 6710537, member: 40227"] I'm late to the party, but... Just about, but we have to be a bit more careful, actually (I'd have to mark off a bit in my class ;) ). What we need to know is what the .9c speed of the rocket is measured with respect to. People generally seem to be assuming that the rocket is moving at 0.9c with respect to the earth, but then that means the earth wouldn't move IF the earth weren't accelerating. In other words, all that time passes on the earth, but the earth is sitting still. But the earth's motion around the sun accelerates, so we don't really want to use that as a benchmark. It's better to say that the rocket moves at 0.9c with respect to the sun, while the earth orbits the sun. That's a much better approximation, since the sun accelerates very little in its motion around the galaxy. Of course, from the point of view of the earth, that 0.9c rocket travel is modulated just a bit by the motion of the earth around the sun. Or we could say that the rocket moves at 0.9c with respect to the galaxy. Then the sun is moving through the galaxy, too, which sounds closest to what you've described. But then, according to the sun, the rocket isn't moving quite at 0.9c either on the way out or the way back. Of course, the motion of the sun around the galaxy and the earth around the sun are at very small speeds compared to c, so these are all very minor corrections to the big picture everyone's talked about. But usually in relativity we need to be pretty clear about the question we're asking as the answer can come out quite differently in slightly different situations. [/QUOTE]

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