# Warm-Up Exercise 38

## Due 12:15 pm, Mon 26 Nov 2012

Physics 123, Fall 2012

Last class period we briefly discussed the famous "twin paradox". What is the seeming paradox, and how is it resolved?

The paradox is that, to a twin on a fast-moving rocket the twin on earth seems to have time move slowly, but to the twin on earth the twin on the rocket seems to have time move slowly. The paradox is resolved in that in order to meet back together, the twin on the rocket must *accelerate* (as he turns around). During the period of acceleration, the twin on earth seems to have time move much more rapidily.

What is the "rest length" of an object?

It's the length someone at rest relative to the object would measure. That's the longest possible measurement of its length.

What is the "proper time" of a journey?

That's the time interval someone undergoing the journey would measure. That's the *shortest* possible measurement of the time interval.

Was there anything from the reading assignment that you would like me to spend extra time on?