Appendix A: Special Relativity

1.1. The two postulates of special relativity and the tension between them. When Einstein first presented what came to be known as special relativity, he based the theory on two postulates or principles, called the “relativity postulate” or “relativity principle” and the “light postulate.” Both postulates are supported by a wealth of experimental evidence. The combination of the two, however, appears to lead to contradictions. To avoid such contradictions, Einstein argued, we need to change some of our fundamental ideas about space and time. Einstein formulated the relativity postulate as follows: “The same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good” (Einstein 1905r, 891). Such frames of reference are called inertial frames and an observer at rest in one of them is called an inertial observer. A few examples will suffice to understand both the concept of an inertial frame and the meaning of the relativity postulate. First consider a plane which starts out sitting on the tarmac, proceeds to fly through clear skies, and eventually hits turbulence. All the while a passenger is nursing a cup of coffee. Sipping coffee without spilling is easy during the smooth portion of the flight. This is because the laws governing the behavior of the coffee in the frame of reference of the plane flying at constant velocity are the same as in the frame of reference of the airport.1 In fact, these same laws hold in any frame moving uniformly (i.e., with constant velocity) with respect to the frame of the airport. Drinking coffee without spilling when the plane ride gets bumpy is much harder. The laws for the coffee in noninertial frames, such as the frame of a plane encountering turbulence, are more complicated than in inertial frames. As a second example consider a cruise ship that sets out from its port of origin, sails smoothly on a calm sea, and eventually is caught in a storm. All the while two passengers engage in a drawn-out tennis match on the ship’s upper deck..
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Michel Janssen (2012). The Twins and the Bucket: How Einstein Made Gravity Rather Than Motion Relative in General Relativity. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 43 (3):159-175.

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