Simple Nature - Light and Matter

7.2.7 ⋆ Four-vectors and the inner productExample 7 makes it natural that we define a type of vector withfour components, the first one relating to time and the others beingspatial. These are known as four-vectors. It’s clear how we shoulddefine the equivalent of a dot product in relativity:A · B = A t B t − A x B x − A y B y − A z B zThe term “dot product” has connotations of referring only to threevectors,so the operation of taking the scalar product of two fourvectorsis usually referred to instead as the “inner product.” Thespacetime interval can then be thought of as the inner product of afour-vector with itself. We care about the relativistic inner productfor exactly the same reason we care about its Euclidean version; bothare scalars, so they have a fixed value regardless of what coordinatesystem we choose.ac / Example 10.The twin paradox example 10Alice and Betty are identical twins. Betty goes on a space voyageat relativistic speeds, traveling away from the earth and then turningaround and coming back. Meanwhile, Alice stays on earth.When Betty returns, she is younger than Alice because of relativistictime dilation (example 1, p. 391).But isn’t it valid to say that Betty’s spaceship is standing still andthe earth moving? In that description, wouldn’t Alice end upyounger and Betty older? This is referred to as the “twin paradox.”It can’t really be a paradox, since it’s exactly what was observedin the Hafele-Keating experiment (p. 381).Betty’s track in the x-t plane (her “world-line” in relativistic jargon)consists of vectors b and c strung end-to-end (figure ac). Wecould adopt a frame of reference in which Betty was at rest duringb (i.e., b x = 0), but there is no frame in which b and c are parallel,so there is no frame in which Betty was at rest during both b andc. This resolves the paradox.We have already established by other methods that Betty agesless that Alice, but let’s see how this plays out in a simple numericalexample. Omitting units and making up simple numbers, let’ssay that the vectors in figure ac area = (6, 1)b = (3, 2)c = (3, −1) ,where the components are given in the order (t, x).experienced by Alice is then√|a| = 6 2 − 1 2 = 5.9 ,The time406 Chapter 7 Relativity