Recitation 1. Principle of Relativity

A. What is inertial motion? An inertial observer? Accelerated motion? Absolute motion? Relative motion? A light clock?

B. You are in a uniformly moving spaceship that enters an asteroid field. You observe the asteroids of the field rushing past your window (and fear a collision with one). Does this observation constitute an experiment that violates the principle of relativity? Explain.

C. You are inside an airplane drinking coffee. The airplane strikes turbulent air. Your stomach falls and the coffee flies out of the cup. You have no doubt now that you are moving. Does this observation constitute an experiment that violates the principle of relativity? Explain.

D. It follows from the principle of relativity and the light postulate that a light clock slows when set in motion. If the light clock uses rods aligned with the direction of motion, it also follows that they shrink in length. But isn't a light clock an odd sort of clock that no one really uses? Why should conclusions about fanciful light clocks also hold for ordinary, real clocks? (Hint: What might be possible if those same conclusions did not hold for ordinary, real clocks?)

E. The relativistic slowing of clocks and shrinking of rods is invisible to observers moving with the clocks and the rods. How can we observe these effects?

F. Imagine that you have a gun that can fire a particle at 100,000 miles per second. You are in a spaceship moving at 100,000 miles per second with respect to the earth. You point the gun in the direction of your motion and fire. Would an earthbound observer judge the particle to travel at 200,000=100,000+100,000 miles per second? Show that the earthbound observer could not since that would violate the principle of relativity, when that principle is combined with the light postulate. How rapidly would you (the spaceship observer) judge the particle to be moving?

Spaceship

G. The arguments we have investigated show that relativity theory prohibits us accelerating an object past the speed of light. Do any of them rule out objects that have always been traveling faster than light (or, possibly, were created initially already moving faster than light)?