Nonrelativistic strings
Barton Zwiebach · Cambridge University Press eBooks · 2011
A full appreciation for the subtleties of relativistic strings requires an understanding of the basic physics of nonrelativistic strings. These strings have mass and tension. They can vibrate both transversely and longitudinally. We study the equations of motion for nonrelativistic strings and develop the Lagrangian approach to their dynamics. Equations of motion for transverse oscillations We will begin our study of strings with a look at the transverse fluctuations of a stretched string. The direction along the string is called the longitudinal direction, and the directions orthogonal to the string are called the transverse directions. We consider, for notational simplicity, the case when there is only one transverse direction – the generalization to additional transverse directions is straightforward. Working in the (x, y) plane, let the classical nonrelativistic string have its endpoints fixed at (0, 0), and ( a , 0). In the static configuration the string is stretched along the x axis between these two points. In a transverse oscillation, the x -coordinate of any point on the string does not change in time. The transverse displacement of a point is given by its y -coordinate. The x direction is longitudinal, and the y direction is transverse. To describe the classical mechanics of a homogeneous string, we need two pieces of information: the tension T 0 and the mass per unit length μ 0 . The total mass of the string is then M = μ 0 a .