If your twin flies through space for 20 years at close to the speed of light, they’ll be 20 years older when they come back.
They will. I think you mean: If your twin flies through space at close to the speed of light and arrives back 20 years later, they’ll be 20 years older when they come back. That one’s false.
To be more explicit: What is needed to make the statement interestingly wrong is for the two 20-year figures to be in different reference frames. If your twin does something for 20 years, then they will be 20 years older; but if they do something for what you experience as 20 years they may not be.
They will. I think you mean: If your twin flies through space at close to the speed of light and arrives back 20 years later, they’ll be 20 years older when they come back. That one’s false.
Reversed polarity on a few statements. Thanks.
Your first statement is still correct.
To be more explicit: What is needed to make the statement interestingly wrong is for the two 20-year figures to be in different reference frames. If your twin does something for 20 years, then they will be 20 years older; but if they do something for what you experience as 20 years they may not be.
Edited to more firmly attach “for 20 years” to the earth.
Rephrased to more explicitly place “for 20 years” in the earth’s reference frame.