# What happens in the twin paradox if the ship doesn't return?

What happens if the twin in the spaceship doesn't return? Would he still be younger than his other twin? Is the symmetry broken simply by accelerating out of earth? If it is still symmetrical when he doesn't return, why do satellites have a different time than the time on earth if they didn't return?

• Have you tried drawing a spacetime diagram for your setup? All seemingly confusing setups involving modifications of the twin paradox can be resolved by simply looking at the spacetime diagram. Jan 25 '21 at 8:36

What happens if the twin in the spaceship doesn't return? Would he still be younger than his other twin?

It's really a moot point, because you can't compare clocks. There is no absolute time! You can't say, "What's each twin's age at this instant?" because "this instant" depends on the observer.

Is the symmetry broken simply by accelerating out of earth?

That and accelerating back.

If it is still symmetrical when he doesn't return, why do satellites have a different time than the time on earth if they didn't return?

Satellites have a different time than on earth because of weaker gravity. They also experience orbital acceleration which breaks symmetry from us.

• Also we receive time signals from the satellites, not the other way around, so even without gravity and acceleration there is still a broken symmetry.
– user10851
Dec 9 '14 at 22:42
• Good point - we can never know what time it is at a satellite "right now", only what time it was when it sent the signal we just received. Dec 9 '14 at 22:55
• If the twin looks through a telescope that lets him see the twin though, would he see the old twin or the new one? Also, why does accelerate break symmetry? Doesn't earth accelerate the other way for him? Sorry for the basic questions, I'm still learning. Dec 9 '14 at 22:58
• "Why does acceleration break symmetry?". Because the twins experience different things. The travelling twin experiences acceleration - just as you do when a car accelerates or breaks - but the stay at home twin does not. Every observer - including the stay at home twin, the travelling twin, and visiting Klingon spaceships whizzing by - will see one twin as having accelerated at some point and the other twin as not accelerating. Mar 1 '15 at 17:37

To keep things simple, let us assume that there is no acceleration outside of the self-propulsion that the rocket ship uses to travel. If the ship returns, the twin who was on it will definitely be younger because of general relativity reasons. However, if the rocket doesn't come back, then from the point when the rocket reaches a constant velocity after first accelerating during take-off to however many years after, both twins are viewed as being in inertial reference frames and general relativity no longer applies. In this case, if the rocket were to keep going at constant velocity for however many years after that point, and, at the end of those years, you somehow waved a magic wand so that the two twins instantaneously appeared before you, they would appear to you to have both aged the same amount since that point.

• Wouldn't it depend on which twin was stationary in your frame of reference? Jan 25 '21 at 8:39
• By "instantaneously appeared before you", I meant that they would be magically plucked from their reference frames and placed into yours instantaneously. In other words, they would be in your living room having coffee with you after you waved your wand Jan 25 '21 at 8:57
• If I was already in the living room with twin A I would have expected B to be younger. Conversely if I was on the rocket with B I would have expected twin A to be younger. Jan 25 '21 at 9:02
• Perhaps imagining a more specific version of the above scenario will help: Since the point mentioned above, you have been in your own inertial reference frame (your frame can be any inertial reference frame), and have not seen the twins (let's name them Mike and Ike) for what you perceive to be years since that point. Sitting alone in your living room, you have one photograph of each twin taken at that point. You wave your wand and--poof--the two twins are now magically (instantaneously) sitting with you in your living room, in your inertial reference frame. Jan 26 '21 at 18:56
• Now with the three of you all together, you see that Mike has aged (more wrinkles, bald, etc.) compared to the photograph of Mike just as much as Ike has aged compared to the photograph of Ike. Of course, I am assuming that the only difference between the twins for all the years since the point the photographs were taken has been their inertial reference frames (i.e., smoking status, radiation exposure, quality of diet, etc. were the same) Jan 26 '21 at 19:24