# Twin Paradox (I know not again)

1. Jun 21, 2009

### Libohove90

I am new to the forums so hello everyone.

I have spent time trying to understand the twin paradox, and I have only partially grasped it.

What I specifically don't grasp is....what really happens when the traveler changes inertial frames from leaving Earth to heading towards Earth? I know the symmetry is broken at this point.

The traveler can say the Earth's time is slower than his but when he changes inertial frames to heading towards Earth, he realizes that the Earth has already aged tremendously as he heads back. Maybe someone here can clarify clearly what happens when the traveler changes inertial frames that causes so much time on Earth to pass.

2. Jun 21, 2009

### Staff: Mentor

3. Jun 21, 2009

### ZikZak

Relativity of Simulteneity: Different sets of events are simultaneous in different frames.

Suppose that the spacecraft goes to Vega 10 light-years away at nearly c, and that there is a clock on Vega synchronized with Earth's in the Vega-Earth rest frame. Suppose that when the ship gets to Vega, its clock reads 10.1 years.

In neither of the ship's two frames are the Earth's Clock and Vega's Clock synchronized, and they are mis-synchronized in opposite senses in the two frames. In the outgoing frame, the event simultaneous with "Vega's Clock Reads 10.1 years" is "Earth's clock reads 0.01 years." In the returning frame, the event simultaneous with "Vega's Clock Reads 10.1 years" is "Earth's Clock Reads 20.19 years." And that's it.