Review of Special Relativity

1. Review of Special Relativity At the end of the 19th century it became clear that Maxwell’s formulation of electrodynamics was hugely successful. The theory predicted the existence of electromagnetic waves, which were eventually discovered by Hertz. In Lecture 5, eqns 17 and 18, we see that in source free regions of space the scalar and vector potential obey a wave equation. Wave equations were already known to the classical physicists in, for example, sound waves. These classical wave equations could be understood on the basis of Newtonian mechanics. Some medium was disturbed from equilibrium and the resulting disturbance propagates at a speed characteristic of the medium. If the medium was in motion relative to an observer, then the apparent speed of the disturbance to the observer was simply the vector sum of the velocity of the medium plus the inherent velocity of propagation in the medium. The speed of a sound wave relative to an observer, for example, depends on the speed of sound in air and the wind velocity. The Michelson-Morley experiment was an attempt to measure the motion of the earth through the aether, a substance hypothesized to be the disturbed medium for electromagnetic waves. The null result of the Michelson-Morley experiment, and all its successors, forced physicists to come to terms with the non-invariance of electromagnetic theory with Galilean Relativity. 1

2. Galilean Relativity Newtonian mechanics is invariant with respect to Galilean transformations. These are transformations between reference frames O and O’ given by eqns (1). P x’ = x – vt’ (1a) t’ = t (1b) z’ = z (1c) y’ = y (1d) (x,y,z,t) , (x’,y’,z’,t’) v O’ x’ x Time is assumed to be a universal parameter, independent of the reference frame. O The coordinates of point P transform according to equations 1. O’ moves to the right with a velocity v with respect to O. Invariance of physical laws with respect to transformations of inertial reference frames was a long held and justifiable assumption. We assume that this invariance is a property of space and time. Observations by all competent observers are equally valid. In the case of sound waves we could say that a reference frame moving with the wind velocity is a preferred frame, for in this frame the equations are the most simple. 2

3. In the absence of the aether there is no natural preferred reference frame for electromagnetic theory. We still conclude that all inertial reference frames are equally valid and hence the wave equations must have the same form in all inertial reference frames. However, it is straightforward to show that the wave equation does not satisfy Galilean relativity. Consider the transformation of the wave equation for a one dimensional wave V(x,t). In the O system, In general the transformation from one coordinate system to another is given by, We will use eqns 1 to transform this into the O’ system. 3

4. (4a) , 4(b) Applying eqns 4 a second time gives (5) So we see from eqn (5) that the wave equation is not a Galilean invariant. Equations 1 must be modified so that the wave equation is invariant in transforming from one inertial frame to another. The coordinates (y,z) perpendicular to v do not change. We must consider a more general transformation for the x and t coordinates. It makes sense to try a symmetrical representation of the transformation. In eqns 6 we choose coefficients a’s and b’s to be dimensionless. We now use equations 6 in equations 3 and derive for the wave equation 4

5. (7) Inorder to ensure invariance w.r.t. coordinate transformation we need We can try to find a solution to 7 that is symmetric, namely try a1=b1 , and a0=b0. Both 7a and 7b give the same result. 5

6. The counterpart to the Galilean transformations ( eqns 1), which makes the wave equation invariant is called the Lorentz transformation. The inverse transformation from x’ to x simply requires changing the sign of v. • Inherent in this derivation are two assumptions. • The first is that the speed of light, c, is the same in the O’ and O reference frames. This is actually an experimental fact. • The second is that the laws of physics have the same form in all inertial reference frames. There is, in fact, nothing special about electromagnetism other than in the vacuum the waves propagate at a universal speed, c. Any wave disturbance that travels at this speed will also require the Lorentz transformation. One point of view is that the Lorentz transformation says something about how space-time is constructed. 6

7. We would also have discovered the inadequacy of the Galilean transformation if physicists had had access to high speeds before the discovery of electromagnetism. Lorentz Invariants If we can frame our laws in such a way that they are Lorentz invariant then we have satisfied the requirements of Special Relativity. Consider the following Invariant interval, ds2 From eqns. 9 Then we can show that Eqn.11 is true for macroscopic intervals too. Time Dilation Suppose that in frame O’ we keep a clock fixed in space, dx’=0. We measure a time interval then. This is called the ‘proper’ time, dt. From eqn 11 we conclude 7

8. The observer in frame O will see the time interval dt to be larger than dt in O’. We can solve for dx in eqn 12 using eqns 10. Equation 13 expresses the time dilation phenomenon. Length Contraction Suppose the observer O wants to measure the length of an object, which he knows in the O’ frame has a length dx’. In order for O to make the measurement of length he must do so at a fixed time, so that dt = 0. From eqn 11 8

9. Equation 14 expresses the length contraction phenomenon. There are some quantities that do not depend on space-time, like the total charge on an object. The total charge should be invariant. However, the charge density is not an invariant quantity. Consider the cylinder of uniform charge below as observed by observers O and O’. Q = r’L’A = rLA L’ r ‘ O’ v O The cylinder has cross section area A and length L’ in O’, where it is at rest. 9

10. The observer in O sees a modified charge density. In fact, the charge density is increased by the factor g, which is reminiscent of the time dilation which also has the same factor g. Moreover, the observer in O also sees a current density J associated with the moving rod. In eqn (16) we note that c2r’2 must be an invariant quantity with respect to Lorentz transformations. The observer O is completely arbitrary. Another observer with a different relative velocity with respect to O’ would come to the same conclusion if the quantity on the right hand side of (16) were formed. Thus if there are two observers in reference frames 1 and 2 we can write 10

11. Notice the close parallels between eqns (11), (12), and (17). The charge density in the frame in which the charge distribution is at rest, O’, is the counterpart to the proper time in that frame. The current density is the counterpart to the position x. We can rewrite equations (9) by multiplying eqn (9b) by c. 11

12. If we change our notation from (t,x,y,z) for the time and space coordinates to Then we can conclude for two separate inertial reference frames O and O’ The quantity xmis called the space-time four-vector. Its Lorentz transformation properties are given by eqns (19). 12

13. REFERENCES 1) “Classical Electrodynamics”, 2nd Edition, John David Jackson, John Wiley and Sons, 1975 2) “Electrodynamics”, Fulvio Melia, University of Chicago Press, 2001 3) “Introduction to Electrodynamics”, 2nd Edition, David J. Griffiths, Prentice Hall, 1989