HPS 0410 Einstein for Everyone Spring 2015

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# Assignment 1. Principle of Relativity

For submission

According to the principle of relativity, no experiment conducted within a laboratory can reveal its uniform (=inertial) motion; all that can be revealed is the uniform motion of the laboratory with respect to other bodies.

1. Special relativity tells us that moving rods shrink and moving clocks slow down. The page shows you how to calculate how big these effects are. Two rows for 10,000 mi/sec and 93,000 mi/sec have been left blank. Fill in the blanks.

Why do these attempts to detect rod shrinking and clock slowing fail?
If they did not fail, why would your success at measuing rods shrinking and clocks slowing amount to a violation of the principle of relativity.

For discussion in the recitation.

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. Two spaceships pass a planet, moving in opposite directions. A planet observer judges each to be moving at 100,000 miles per second. An observer on one of the spaceships measures the speed of the other spaceship.

(a) According to classical physics, what speed will that spaceship observer measure for the other spaceship? Is this speed faster than light?

(b) According to relativity theory, what speed will that spaceship observer measure for the other spaceship? Is this speed faster than light?

E. 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?

F. 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)?