A tennis court appears to have a length (24 metres) and a tennis match appears to have a duration (say three hours). Likewise, a shoe appears to have a definite size, and the wink of an eye seems to take less than a second. The key question is about these distances and durations. Bodies appear … Read more
EINSTEIN IN A NUTSHELL Everyone now agrees that special relativity is well confirmed by experiment. But there remains stark disagreement about why length contraction and time dilation occur – about what is going on behind the scenes to produce such startling effects. This may come as a surprise. Einstein’s theory is 100 years old. Surely … Read more
SPACE, TIME AND EINSTEIN Before Einstein, physicists thought that time flows at the same rate everywhere. There was supposed to be, we might say, a “universal Tuesday”: if it were Tuesday here on Earth, it was Tuesday throughout the entire universe. That is, it was believed that one and the same instant of time occurred … Read more
SPACE, TIME AND EINSTEIN Using his two principles, the constancy of the speed of light and relativity, Einstein made a second, astonishing, prediction. As Jill’s spaceship speeds up, earth-bound Jack will find that its length shrinks. If the spaceship had the shape of a long sausage with fins when it blasted off, at high speeds … Read more
Einstein’s central idea is that there is democracy among all inertial measurements. Any measurement made by a set of rulers and clocks moving at a steady speed in a fixed direction is equally as good as a measurement made by any other set. Suppose that there are two sets of rulers and clocks moving relatively … Read more
Space, Time and Einstein J. B. Kennedy PART I – Einstein’s revolution The lazy ship When Einstein published his ideas about time dilation in 1905, he limited his predictions to a special case: to special sorts of measure- ments. As mentioned above, special relativity is special because it is limited to special cases. To understand … Read more
Space, Time and Einstein J. B. Kennedy PART I – Einstein’s revolution Faster speeds, longer hours Someone might mistakenly think that the constancy of the speed of light leads to contradictions, and therefore cannot be true. For example, suppose there are two rockets travelling through space in the same direction but at different speeds, and … Read more
Space, Time and Einstein J. B. Kennedy PART I – Einstein’s revolution The speed of light is constant The central mystery is light. It is, first of all, astonishingly fast. With a flick of a switch, light floods a room. Before the rise of modern science, it was sometimes thought that light leapt magically across … Read more
Space, Time and Einstein J. B. Kennedy PART I – Einstein’s revolution CHAPTER 2.1 Einstein in a nutshell – Two theories of relativity There are two Einsteins. For most of the world, Einstein (1879–1955) is a cult figure: the pre-eminent icon of genius. With his wispy, wild grey hair, missing socks and other-worldly idealism, he … Read more
Space, Time and Einstein J. B. Kennedy PART I – Einstein’s revolution CHAPTER 1: From Aristotle to Hiroshima Cup your hands together and peer down between your palms. What is between them? One answer is “air”. But we think of air as composed of separate molecules, like isolated islands. What lies between the molecules? Nothing? … Read more