Lorentz Transformation

( given x, y, z and x',y',|v| -- find out t, t', z' . In this case,out of x',y',z' , one can arbitrarily choose 2 co-ordinates only and not 3. Here we have chosen x',y')

Description :

Lorentz transformation is the transformation of spatial and temporal co-ordinates(time) from one inertial frame of reference to another which is moving at a uniform velocity V with reference to the other. Lorentz transformation is a special case of Poincare transformation w.r.t. linear uniform motion & rotation of co-ordinates.

* Suppose one observer is in frame S and the other in frame S', the co-ordinate system being Cartesian in Euclidean space.

* Clocks of both frames are synchronized at t=0 i.e. when both the frames coincide.

* S' is moving at a uniform velocity V in the direction of positive x-axis w.r.t. S.We take V as +Ve for motion along positive x/y/z direction. For simplicity, we assume that velocity has no component in y & z direction. V_x= V and angle Ψ =0°, angle Φ =90°, angle θ =90°.  However in general it can have V_{y ,} V_z components.

* An event is recorded in frame S'. Its co-ordinates are (x', y', z', t').Co-ordinates of the origin of S' are s'(x), s'(y), s'(z) and s'(t)

* If the same event is recorded in S and is having co-ordinates (x, y, z, t) , what are the transformation laws between S and S' for x,y,z,t ?

* The transformation law that measures (x',y',z', t') in terms of (x, y, z, t) or Vice Versa between two inertial frames of reference is called the Lorentz transformation.

* Had this been Galilean transformation ,it would have been

    x'= x±vt; ( +vt when v is -Ve and -vt when V is positive )

   y'= y

   z'= z

   t'= t

* The essence of Lorentz transformation lies in treatment of time. In Galilean transformation, events are simultaneous in both the frames and so t'=t.. Not so in Lorentz transformation because event can be perceived only when the signal (light or any communication signal) reaches the observer and the time of propagation of signal to each observer is not instantaneous but is a function of the distance between the observer and the event and hence in general time =f(x,y,z) for each observer. So time t' of an event perceived in S' frame is not the same as time t perceived in frame S but is a function of x, y, z and t.

In general x' =A₁₁x +A₁₂y+A₁₃z+A₁₄t ;

               y' =A₂₁x +A₂₂y+A₂₃z+A₂₄t ;

            z' =A₃₁x +A₃₂y+A₃₃z+A₃₄t ;

            t' =A₄₁x +A₄₂y+A₄₃z+A₄₄t ;

i.e. Each co-ordinate can be expressed as the linear combination of all the co-ordinates in other frame. The combination is assumed to be linear because if it is in any other power of x,y,z or t,then  x1'-x' which is the length of an event would be different in different frames which is not tenable. Hence vector representation is maintained.

Since there is no projection of y-axis on x-axis or other axes by the Orthogonality condition of the axes,  A₁₁,A₂₂ , A₃₃ ,A₄₄ are not equal to zero and all other co-efficientare zero. A_ij =0 for all i ≠j except   A₂₄ ,A₃₄ ,  A₄₂  &  A₄₃  i.e. co-efficients associated with t.

Therefore 

            x' =A₁₁x +A₁₄t ;

               y' =A₂₂y+A₂₄t ;

            z' =A₃₃z+A₃₄t ;

            t' =A₄₁x +A₄₂y+A₄₃z+A₄₄t ;

  In the instant case, since we have restricted movement of S' frame in x-axis, A₂₂₌  A₃₃ =1  & A₂₄₌  A₄₂ =A₄₃=  A₃₄=0

            x' =A₁₁x +A₁₄t  

               y' =y ;

            z' =z;

            t' =A₄₁x +A₄₄t  

 Again x'=x-vt which implies that A₁₄=-A₁₁v  ( this part has to be more elaborately explained?)

            x' =A₁₁(x -vt)  .......(1)

               y' =y ;

            z' =z;

            t' =A₄₁x +A₄₄t ......(2)

     Since the event can be recorded only on communication of the signal, if velocity of signal is c ,then

     x²+y²+z²=c²t²      ......   or x²+y²+z² - c²t²         =0   (3)

     x^'2+y^'2+z^'2=c²t^'2  ......  or x^'2+y^'2+z^'2 - c²t^'2   =0    (4)

    or x^'2+y^'2+z^'2 - c²t^'2   = x²+y²+z² - c²t²      or  x^'2- c²t^'2   = x² - c²t²  ..........(5)
    Substituting the value of x' and t' from (1) & (2) respectively in (5) and rearranging we,get,

   x²(A²₁₁ -A²₄₁c²) + t²( A²₁₁v²-A²₄₄c²) -2xt ( A²₁₁v+A₄₁A₄₄c²) = x² - c²t²   ..........(6)

  Comparing the co-efficients of x²,t²  and  xt , we get

  A²₁₁ -A²₄₁c² =1  ........    (7)

 A²₁₁v²-A²₄₄c² = -c^2.......(8)

A²₁₁v+A₄₁A₄₄c² = 0 ...... (9)

There are 3 equations and 3 unknowns. Solving them we have

 A₁₁ = 1/√(1-v²/c²) ..............(10)

A₄₁ = (- v/c²)* (1/√(1-v²/c²)) ..............(11)

A₄₄ = 1/√(1-v²/c²)    ..............(12)

Putting them in (1) and (2),

x' =1/√(1-v²/c²) *(x -vt)  .......(1)

y' =y ;

z' =z;

t' =(t -vx/c²)*1/√(1-v²/c²)  ......(2)

When v has x,y ,z component, A₄₁ =- ((v_x x+ v_y y  + v_z z)/c²)* (1/√(1-v²/c²))

Some Important Observations on Lorentz Transformation

1) γ is known as the Lorentz factor. γ =f(v). When v=0, γ =1

                                                            when v-->c, γ--> infinity without bound

      1/γ is called reciprocal Lorentz Factor.

2) Elements of Lorentz transformation form a Group. Basically 4 conditions are to be satisfied for a set of objects to form a group. These are

    a) If A, B are elements of the group, then C=A. B must also be an element of the group where . is a binary operation.

   b) If A,B,C are elements of the group, then A(BC) =(AB)C must be true which is associative law.

   c) There exists an identity element I such that AI=IA=A

   d) Every element has an inverse which is also an element of the group.

   Lorentz transformation obeys all these. The group contains 3 matrices, identity,forward,inverse transformation matrix.

  3) Lorentz transformation occurs in Pseudo-Euclidean space because in x²+y²+z² - c²t²   the last term is negative in stead of being positive and the same term is dependent upon space co-ordinates unlike in Euclidean space where all terms (x,y,z) are independent of each other. Here space, time are combined into a single manifold.