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## Main Question or Discussion Point

This is a "twins paradox" related question, but I am not interested in the ageing of either twin, per se.

One of our closer neighbours is a star called Delta Pavonis. It is interesting for two reasons, firstly it is close to a nice round 20 light years away and secondly because it is on the verge of becoming a red giant.

Say one of a pair of twins (Mary) travels off to visit Delta Pavonis at 0.8c, performs a series of observations (say for a year), then travels back to Earth at 0.8c.

How far does Mary travel? (0.8c * 50year = 40 ly) or (0.8c * 30year = 24 ly)?

The reason I ask is that the ship must expend some fuel to get up to 0.8c, so Mary will know she is cruising at that speed (relative to the Earth at blast off). 15 years of shipboard time later she arrives at Delta Pavonis so a quick calculation will show her that she has travelled 12 ly. (The same applies on the way back, hence the total apparent distance travelled is 24 ly.)

How is this resolved in a real universe example? It seems unlikely that the universe shrinks when Mary is in motion relative to the Earth. Has she somehow used a variant of "warp speed"? (If Mary mixes frames it will seem to be so, she knows that Delta Pavonis is 20 ly away before blasting off, then she blasts off and travels for 15 years of shipboard time before arriving at Delta Pavonis. 20ly / 15 years = 1.3333c. "Warp factor 1.3333. Engage!")

cheers,

neopolitan

One of our closer neighbours is a star called Delta Pavonis. It is interesting for two reasons, firstly it is close to a nice round 20 light years away and secondly because it is on the verge of becoming a red giant.

Say one of a pair of twins (Mary) travels off to visit Delta Pavonis at 0.8c, performs a series of observations (say for a year), then travels back to Earth at 0.8c.

How far does Mary travel? (0.8c * 50year = 40 ly) or (0.8c * 30year = 24 ly)?

The reason I ask is that the ship must expend some fuel to get up to 0.8c, so Mary will know she is cruising at that speed (relative to the Earth at blast off). 15 years of shipboard time later she arrives at Delta Pavonis so a quick calculation will show her that she has travelled 12 ly. (The same applies on the way back, hence the total apparent distance travelled is 24 ly.)

How is this resolved in a real universe example? It seems unlikely that the universe shrinks when Mary is in motion relative to the Earth. Has she somehow used a variant of "warp speed"? (If Mary mixes frames it will seem to be so, she knows that Delta Pavonis is 20 ly away before blasting off, then she blasts off and travels for 15 years of shipboard time before arriving at Delta Pavonis. 20ly / 15 years = 1.3333c. "Warp factor 1.3333. Engage!")

cheers,

neopolitan