HPS 0410 Einstein for Everyone Spring 2010

Back to main course page

# Assignment 4: Origins of Special Relativity

For submission Mon. Feb. 1, Tues. Feb 2.

Read the introduction and first two sections of Einstein's paper "On the electrodynamics of moving bodies." Read it slowly and reverently. This text is to modern physics what Genesis is to modern Judeo-Christianity and the Declaration of Independence is to US history.

1. Compare what is moving with respect to what in the magnet and conductor thought experiment in the two accounts you have: the one Einstein gives in his paper and the one in the chapter section, Magnet and Conductor. How do the two accounts differ?

2. What is the "definition of simultaneity" that Einstein describes in the first section of his paper? That is, what must be stipulated by definition according to Einstein if we are to be able to compare the timing of events at a point A and a point B of space?

For discussion in the recitation.

A. In the introduction, what is established by the magnet and conductor thought experiment? How do ether current experiments enter the discussion? What is "apparently irreconcilable" and why is it so? How is Einstein suggesting that he will solve the problem?

B. In Section 2, how does Einstein establish that observers in relative motion may disagree on the lengths of rods and the synchrony of clocks?

C. If the synchrony of different clocks is set by a definition, presumably freely chosen, then it would seem that any velocities measured by them are also a matter of freely chosen definition. So how can Einstein at the end of Section 1 say that the constancy of the speed of light is a universal constant "in agreement with experience"?

Here are some questions about E=mc2:

D. What does the law of conservation of mass say? What does the law of conservation of energy say? In classical physics, these are two separate laws. What becomes of them in relativity physics?

E. When an electric battery is charged, what happens to its mass?
When a hot body cools, what happens to its mass?
When a spring is compressed what happens to its mass?
Inside a completely isolated spacestation, an electric battery is used to warm the hands of an astronaut and to run a motor that winds a spring. What happens to the total energy of the spaceship? What happens to the total mass of the spaceship?

F. When an atom of Uranium-235 undergoes fission and breaks into parts, the total mass of the parts is less than the mass of the original atom. What happens to the missing mass? Why is this missing mass important in modern life? What does the law of conservation of mass say? What does the law of conservation of energy say? In classical physics, these are two separate laws. What becomes of them in relativity physics?