Let's say there is a virtual digital pet, who is aging, as a computer simulation. And there is one flying computer, launched from far way, which is now close to the earth and is moving away from the earth to a star. There is another flying computer moving toward the earth from the star. Those 2 flying computers both are in 0.9c speed.

The earthlings upload this digital pet to the computer moving away from the earth, when it is close to the earth. Then it flies 100 LY away from the earthling's view. At that place, it encounters the other computer moving toward the earth. The virtual pet, now who is a little aged, is copied to the computer moving toward the earth. Sometime later, this computer finally gets close to the earth again. The earthlings download the virtual pet, and check its age.

Now, who is older? The earthlings or the virtual pet? It's easy to say, the computers flied fast and their time delayed. But from the virtual pet's view, the earthlings always moved at very fast speed, though the direction changed. Did the earthlings' time delay? How can we explain this version of twin paradox? I think this captures the essence of twin paradox. There is no acceleration and no gravity.

I know this is really similar to 2 clocks and synchronizing version of twin paradox. But in this way, I think we can clearly see the traveling one also as another independent observer.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – ACuriousMind Sep 1 '17 at 12:28

Youu are right: This completely captures the essence of the twin paradox; in fact it is identical to the usual twin paradox. You might as well assume that there is one virtual dog (call him Fido) initally on the outbound ship and another virtual dog (call him Rover) initally on the inbound ship, and they change places when the ships cross. Here is the amount of time that passes between events according to the outbound dog, the inboud dog, and their earthbound friend:

enter image description here

You can see that the picture is perfectly symmetric in every plausible way.

If Fido and Rover are the same age when they trade places (as is the case both in your scenario and in the usual story where the Outbound Twin suddenly becomes the Inbound Twin), then the dog that returns to earth (two hours later by the earth clock) is one hour older than the dog that left.

  • $\begingroup$ This looks nice. And it makes me feel like Fido experiences a sort of time leap when he is transferred to Rover's computer. But is it real? Instant time collapse to the future? If it is real and feels natural, is there good explanation why one should experience this at that moment? I think this experience should be quite different from a time machine in movie. $\endgroup$ – Jaesik Kwak Aug 29 '17 at 9:46
  • $\begingroup$ @JaesikKwak: I don't know what a "time leap" is, but I'm quite confident that Fido experiences no such thing. $\endgroup$ – WillO Aug 29 '17 at 13:53

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