Say I get on a Sci-Fi speed Rocket Ship and leave Earth at .999999% the Speed of Light to me I travel for 21 minutes reach Mars then U-Turn back to Earth for another 21 minutes at 0.999999% the Speed of Light again. Back on Earth if I compared my watch to someone else’s would my watch be slightly ahead or slightly behind?
Like if I’m the one traveling I’d expect their watch to be slightly ahead of mine because slightly less time has passed. But at the same time from my frame of view. Earth and my bussy with the watch just shot away from me for 21 minutes and then returned and came back 21 minutes later so my watch should be ahead of theirs since they were the one traveling.
In: Physics
The premise of the twin paradox is that both frames are *inertial*, which means moving at constant velocity.
The key to dissolving the apparent paradox is acceleration. The ship is the one undergoing acceleration, not the Earth. Acceleration acts like gravity (the equivalence principle), and so the accelerating body undergoes the time dilation.
Why? Because acceleration takes you out of your inertial reference frame, which was initially the Earth’s rest frame. Even though you can never see your own clock ticking slower, the fact that you were accelerated out of Earth’s frame inexorably ties the time dilation to your clock.
Once you decelerate back to the Earth’s frame your clock *must* be behind the Earth’s clock, because Earth remained inertial while you were accelerating.
This problem is often confusing because relativity problems, unless advanced, will say that you accelerate instantaneously. Without acceleration the paradox becomes apparent.
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