If you and your friend are at rest relative to each other, you'll share the same realm of spacetime. If there is relative motion between the two of you, you won't.
Work Step by Step
This is discussed on page 664.
You can help us out by revising, improving and updating this answer.Update this answer
After you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.