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11 11 Einstein’s Special Theory of Relativity chapter Einstein’s Special Theory of Relativity In this chapter, you will be able to • state Einstein’s two postulates for the special theory of relativity • describe Einstein’s thought experiments demonstrating relativity of simultaneity, time dilation, and length contraction • state the laws of conservation of mass and energy, using Einstein’s mass–energy equivalence • conduct thought experiments involving objects travelling at different speeds, including those approaching the speed of light By the end of the nineteenth century, physicists were well satisfied with their under- standing of the physical world around them. Newtonian mechanics had been successful in explaining the motion of objects on Earth and in the heavens. Maxwell’s theory of electromagnetism had consolidated knowledge about the special relationship between electric and magnetic forces and had predicted the existence of electromagnetic waves with properties similar to those of light—so similar, in fact, that light itself was assumed to be an electromagnetic wave. Physics as it was known up to this time is referred to as classical physics. The puzzles that still remained—the structure of the atom and its nucleus—were expected to be solved by further applications of the currently accepted theories. Such was not to be the case, however. The solution to these puzzles required the proposal of two revolutionary and clever concepts: the theory of relativity and the quantum theory. These theories changed our understanding of the universe drastically; their develop- ment and application to the search for the structure of the atom are referred to as modern physics. This and the remaining two chapters deal with the development of these theories and how they are applied to the problems of understanding atomic and nuclear structure. 560 Chapter 11 NEL 1. Jack and Kayla are on the deck of a boat moving at 16 m/s relative to the shore. Kayla throws a ball to Jack, who is down the deck near the rail. Why is it not accurate to say, “The ball is moving at 16 m/s”? 2. What do people mean when they say “It’s all relative?” 3. The expressions “Time flies” and “This has been the longest day of my life” suggest that time does not flow equally in all situations. Can you describe any cases in which it is actually true that the flow of time is in some sense variable? 4. You are travelling on a spaceship moving, relative to Earth, at 90% the speed of light. You direct a laser beam in the same direction as the spaceship is travelling. How fast does the laser light travel relative to the ship? relative to Earth? 5. The law of conservation of mass tells us that the mass of all the reactants in a chem- ical reaction should be equal to the mass of all the products. Do the reactions inside the Sun or a nuclear reactor violate the law of conservation of mass? 6. One of the most famous of all equations is Einstein’s E mc 2 . What do the symbols represent? What does the equation imply? REFLECT on your learning
