Twin Paradox

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Contents
Scenario
The Paradox
The Paradox using Proper Time
Legitimate Use of Δ𝛕 = Δt /γ
A scenario where each of two passing spaceships legitimately uses Δ𝛕 = Δt /γ to calculate proper time intervals on the other.
Back to the Paradox
Spaceship’s Worldline in Earth’s Reference Frame
Calculating the Proper Time Interval for the Spaceship’s Worldline
Problem with the Calculation
Two Solutions
Extending Δ𝛕 = Δt /γ to Flat Spacetime

Sean Carroll, Spacetime and Geometry: an Introduction to General Relativity

Don Koks, Where is the Boundary between Special and General Relativity?

Using Multiple Inertial Reference Frames
Upshot