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Paradoxes II: Instructor's Guide

Main Ideas

  1. Most “paradoxes” result from poorly-posed questions.
  2. All observers must agree!
  3. Special relativity changes our understanding of matter.

Students' Task

Estimated Time: 20 minutes, including wrap-up

  • Warmup: Have the class as a whole discuss the first two paradoxes on the handout.
  • Have each group select one of the next two paradoxes.
  • Half the group should draw a spacetime diagram of what is observed from one reference frame; the other half from the other reference frame.
  • Each group should reconcile their two diagrams.
  • If time permits, repeat with one of the other paradoxes on the handout.

Prerequisite Knowledge

  • basic understanding of spacetime diagrams
  • the ability to calibrate spacetime diagrams using hyperbolas would be helpful



  • Compare the spacetime diagrams from different groups, and for different paradoxes.
  • Discuss the rules for drawing spacetime diagrams, along the lines of §6.5 of the text.
  • See also the discussion of spacetime diagrams for the pole and barn paradox, which can be found here.

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