This is an extension of a discussion I'm having with MacM in the thread "Relativity Question" I decided to make a new thread to avoid confusion with the other thread, and because this post can stand on its own as a new discussion. Feel free to join in. Mac, You and I are in a spaceship each. You're floating through the universe, speed unknown, age 20. You have two 100 year stopwatches on board. You look ahead, and see another ship (it's mine!) drawing nearer at high speed... about 0.8 c. Thoughts flicker through your head... "Am I still, and he moving toward me? Am I moving toward him? Are we both moving?" You dismiss these thoughts as unanswerable for now, because I'm almost on you. As I pass, you start both stopwatches, activate the teleporter and transfer one of the stopwatches (labelled Pete) from your ship to mine. Instantly. No acceleration at all. "I love thought experiments!" you think to yourself, and feel a momentary resonance from the mind on the other ship before it sweeps past and is lost in the empty vastness behind. Time passes. Same ship, 10 years later. You see another ship approaching... This one seems to be going faster than the one you gave your timepiece to. "Maybe I can get my stopwatch back" you think, as you gather your belongings (including the other stopwatch, labelled Mac), prepare yourself for teleportation, then transfer instantly to the other ship. Oddly, it looks much the same as your old ship... someone must be trying to bore you to death? Time passes. Your new ship, 10 years later. There's my ship again, coming up fast at about 0.8c again. "There it is!" you say. "I hope he hasn't been fiddling with my stopwatch!". You grab your stopwatch and towel, and as you draw level with my ship you teleport over and grab the stopwatch from where it has been lying, untouched. You stop both stopwatches and see... ============================================ In reality What would the stopwatch you kept with you read? What would the stopwatch that took a ride with me read? Special Relativity says that in my reference frame... What time has passed between our meetings? At what time did you change ships? How would I calculate the time that you experienced on your first ship? How would I calculate the time that you experienced on your second ship? What does this mean for the times I see on the stopwatches? Special Relativity says that in your reference frame... What time has passed between our meetings? How would you calculate the time that I experienced during your first ten year trip? How would you calculate the time that I experienced during your second ten year trip? What does this mean for the times you see on the stopwatches?