[Physics FAQ] - [Copyright]

The Twin Paradox

This hardy perennial seems to generate endless running threads on the sci.physics.* hierarchy.  This contrasts sharply with the situation among mainstream physicists/hidebound reactionaries (take your choice), where the "paradox" has been no more than an entertaining (and educational) exercise since it first saw the light of day.

It's too much to hope that this FAQ entry will change any minds, except maybe an "undecided" or two.  But perhaps it can reduce the traffic on this topic.

Read the sections as whim and mouse-clicks take you, though you might want to read the Introduction first.

Introduction

Where we introduce our heroes, Earth-bound Terence and star-bound Stella, and settle on a few things, like terminology.  Also, the "acceleration" explanation of the twin paradox.

The Doppler Shift Explanation

Suppose Stella and Terence each film the other's clock through a telescope throughout the trip; when does Stella see Terence's clock run fast?

The Spacetime Diagram Explanation

Where we take an Olympian view, and gaze with the Fates upon the world lines of Terence and Stella.

The "General Relativity" Explanation

Gravitational time dilation, and some matters of terminology.

The Time Gap Objection

Is some time simply missing from Terence's life, according to Stella?

The Distance Dependence Objection

If Stella travels twice as far, does Terence age twice as fast during her Turnaround?  Action at a distance!

Too Many Explanations: a Meta-Objection

Or, the parable of the broken vase.