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Special Relativity tells us that the faster a clock moves the slower it ticks.  Originally this was understood to mean that time was physically running slower for moving clocks than for clocks that were at rest.  But it also bought about some confusion because, since velocity is relative, it required that ‘at rest’ clocks would also have to be physically running slower than the ‘moving’ clocks.

To overcome this problem, some theorists decided that time dilation was not something physical or actual, but something apparent – in that each clocks would ‘see’, ‘observe’, or ‘measure’ the other to be running slower than itself.

This newer interpretation evokes an issue with the Twins Paradox story.  In this story, a space traveller makes a round-trip journey at high speed and returns home to find, for example, his twin had aged eleven years while he, the ‘traveller’, aged only one.  So under this new interpretation of Relativity, does this mean that the ‘home twin’ is actually ten years older or does he just appear to be ten years older?  Or does each twin ‘see’ the other as being ten years older while physically being an identical age?

Rather than answer that, let us instead modify the scenario so that the calculated difference between them becomes 200 years.  As nobody can live that long it stands to reason that one or both of them would have to be dead.  In this new scenario then, which of the following best describes the situation when the twins finally reunite:

A. The home twin will be dead.

B. The travelling twin will be dead.

C. The home twin won’t be dead but the travelling twin will see home twin as dead.

D. The travelling twin won’t be dead but the home twin will see travelling twin as dead.

E. Both twins will be alive but each will see the other as dead.

F. Both twins will be dead but each will see the other as alive.

G. It won’t be possible for them to determine which of them is dead because, due to the kinematics of spacetime and their long separation, they will be unable to agree on a shared reference frame or on a definition of relative simultaneity.  If such an experiment were to take place we would expect both twins to be alive, although each might measure themselves as being dead, and the reasons for this should be found in Einstein’s 1916 paper.

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This essay was inspired by the site TwinParadox.net, in which a group of relativity experts were asked to say where in the Twins’ journey their age difference accumulates.  The above answer G was adapted from their amusing responses.