If a man goes on a 50 year voyage and returns, he will not have travelled in a straight line, like a planet zooming through space would. He would have to accele

If a man goes on a 50 year voyage and returns, he will not have travelled in a straight line, like a planet zooming through space would. He would have to accelerate and change directions, which brings in the effects of general relativity which complicate things. So these two are not equivalent cases.