Lorentz and Poincaré groups

Transcription

1 HAPTER VIII Lorentz and Poincaré groups onsider the four-dimensional real vector space R 4. Its vectors will generically be denoted in a sans-serif font, as e.g. x. Assuming a basis has been chosen, the components of a vector will be denoted with Greek letters, running from to 3: x µ. The components x, x 2, x 3 will often be collectively represented by a three-vector ~x. The introduction of a pseudo-euclidean metric with signature (, +, +, +), corresponding to a metric tensor with components 8 >< for µ = = µ + for µ = 2{, 2, 3} (VIII.a) >: for µ 6= turns R 4 into a pseudometric space, called Minkowski (af) spacetime and hereafter denoted M 4. It is convenient to associate with the metric tensor a 4 4 matrix defined as i.e. such that its entries are precisely the components µ. = diag(,,, ) (VIII.b) VIII. Definitions and first properties With the metric tensor (VIII.), the pseudo-distance between two infinitesimally close points with respective coordinates {x µ } and {x µ +dx µ } is the so-called line element ds 2 dx µ µ dx = (dx ) 2 +d~x 2, (VIII.2a) where d~x 2 denotes the squared Euclidean norm of the three-vector with components (dx, dx 2, dx 3 ). Introducing the (column) four-vector dx with components dx µ and the transposed row-vector dx T, one can equivalently write ds 2 =dx T dx, (VIII.2b) where is the matrix introduced in Eq. (VIII.b). VIII.. Poincaré group Theorem & Definition VIII.3. The transformations x 2 M 4! x 2 M 4 that leave the infinitesimal line element (VIII.2) invariant, i.e. the Minkowski spacetime isometries, form a group, called the Poincaré (ag) group. Requiring that the transformations should be at least twice continuously differentiable, one can show that they are necessarily affine, (45) i.e. of the form (45) AproofispresentedinAppendixVIII.Atothischapter. (af) H. Minkowski, (ag) H. Poincaré,

2 VIII. Definitions and first properties 3 x µ! x µ = µ x + a µ 8µ 2{,, 2, 3}, (VIII.4) where the real numbers {a µ } are arbitrary, while the coefficients µ obey the relation µ µ = 8, 2{,, 2, 3}. (VIII.5a) Equation (VIII.4) leads to dx µ = µ dx, which in turn gives for the infinitesimal line element ds 2 =dx µ µ dx dx µ µ dx = µ dx µ dx =dx µ µ dx. By definition this should equal ds 2 =dx dx, which is only possible for every dx µ if and only if Eq. (VIII.5a) holds. 2 Viewing the numbers µ as the entries of a 4 4 matrix, Eq. (VIII.5) is equivalent to the matrix identity T =, (VIII.5b) where was introduced in Eq. (VIII.b). This condition can directly be derived by using the infinitesimal line element in its form (VIII.2b) and writing dx = dx. Theorem VIII.6. The spacetime translations x! x = x + a, i.e. component-wise with a µ 2 R, formanormalsubgroupofthepoincarégroup. x µ! x µ = x µ + a µ 8µ 2{,, 2, 3} (VIII.6) Another important subgroup of the Poincaré group, which however is not a normal subgroup, is that introduced in the following section. VIII..2 Lorentz group Theorem & Definition VIII.7. The linear transformations x 2 M 4! x = x 2 M 4 (VIII.7a) or equivalently x µ! x µ = µ x 8µ 2{,, 2, 3} (VIII.7b) that preserve the infinitesimal line element (VIII.2) are called Lorentz (ah) transformations and form agroup,thelorentz group, denotedo(3, ). Remarks: As the notation suggests, the Lorentz group is one of the indefinite orthogonal groups introduced in V..2 d. Obviously, the Lorentz transformations are the Minkowski spacetime isometries that leave a point of M 4 invariant, i.e. such that a µ =in Eq. (VIII.4). Accordingly, the Lorentz group is a subgroup of the Poincaré group. (ah) H. A. Lorentz,

3 4 Lorentz and Poincaré groups VIII..2 a :::::::: ::::::::::::::::::::::::::::::::::::::: Examples of Lorentz transformations The matrices of the form R = R A with R 2 SO(3) (VIII.8) represent a first class of Lorentz transformations, whose action on a four-vector is a rotation of the spatial components while keeping the time-component unchanged: (x,~x)! (x = x,~x = R~x ). For instance, the rotation through around the x -axis is represented by the matrix R(~e, ) = cos sin sin cos Theorem VIII.. The matrices of the form (VIII.8) form a subgroup of the Lorentz group. A. This subgroup is actually a (faithful and reducible) representation of SO(3). (VIII.9) A second large class of Lorentz transformations consists of the so-called Lorentz boosts (or special Lorentz transformations). For such a boost with (reduced) velocity 2 ], [ along the direction with unit vector ~n 2 S 2, the corresponding transformation reads (46) ( x! x = x cosh +(~n ~x )sinh (VIII.) ~x! ~x = ~x + (cosh )(~n ~x )~n +(sinh )x ~n, where artanh 2 R is the rapidity of the boost. For a Lorentz boost along the x -direction, the associated transformation matrix is cosh sinh = Bsinh A (VIII.2) The Lorentz boosts do not form a group successive boosts along non-parallel directions do not yield a boost, but the combination of a boost and and spatial rotation. However, the Lorentz boosts along a fixed (arbitrary) direction ~n do form a subgroup of the Lorentz group, which is isomorphic to (R, +). Let us eventually mention three particular Lorentz transformations, which will help us characterize the connected components of the Lorentz group in the following paragraph. ~x ) reads in matrix representa- The space inversion transformation (x,~x)! (x = x,~x = tion P = A. (VIII.3) The time reversal transformation (x,~x)! (x = x,~x =~x ) is represented by the matrix T = A. (VIII.4) (46) The reader is invited to note the similarity between the form of this transformation and Rodrigues formula (VI.3) for rotations.

4 VIII. Definitions and first properties 5 Eventually, the composition of the previous two transformations transforms x into x = x, i.e. the corresponding matrix is the negative of the identity, P T = T P = 4. These three transformations are obviously their own inverses, and one easily checks that the matrices { 4, P, T, P T } form a subgroup which is isomorphic to the Klein group V 4. VIII..2 b ::::::::: ::::::::::::::::::::::::::::::: Structure of the Lorentz group Taking the determinant of both sides of Eq. (VIII.5b) gives (det ) 2 det =det for every matrix of the Lorentz group, that is, since det is non-zero, det = ±. (VIII.5) Theorem & Definition VIII.6. The Lorentz transformations with determinant equal to, called proper Lorentz transformations, formasubgroupofthelorentzgroupdenotedso(3, ). In contrast, the Lorentz transformations with determinant are called improper, as e.g. the spatial-parity and time-reversal transformations (VIII.3) (VIII.4). onsider now the property (VIII.5a) obeyed by the coefficients of a Lorentz transformation for the case = =: µ µ = =. On the right hand side, only the terms with µ = are relevant, and one obtains 3X 2 =+ i 2, (VIII.7) i= valid for all 2 O(3, ). Accordingly, the coefficient is either greater than or equal to +, or smaller than or equal to. More precisely, one easily shows the following two results: Theorem & Definition VIII.8. The Lorentz transformations such that, calledorthochronous Lorentz transformations, formasubgroupofthelorentzgroupdenotedo + (3, ). Theorem & Definition VIII.9. The proper, orthochronous Lorentz transformations, i.e. such that det =+and, formanormalsubgroupofthelorentzgroupdenotedso + (3, ) and called restricted Lorentz group. Theorem VIII.2. The most general element of the restricted Lorentz group SO + (3, ) is a product of a Lorentz boost (VIII.) and a rotation transformation (VIII.8). Property VIII.2. SO + (3, ) is the connected component of the identity in O(3, ). In fact, the Lorentz group like every indefinite orthogonal group consists of four connected components, which can be characterized with the help of the spatial inversion (VIII.3) and time reversal transformation (VIII.4): The connected component of the identity is the restricted Lorentz group SO + (3, ) introduced in definition (VIII.9). Note that this component contains the subgroup of rotations (VIII.8). The connected component of the space inversion transformation (VIII.3) consists of the matrices of the form = P " + with " + 2 SO+ (3, ). These matrices are precisely the orthochronous ( ) improper (det = ) Lorentz transformations. The connected component of the time reversal transformation (VIII.4) consists of the matrices of the form = T " + with " + 2 SO+ (3, ). These are the Lorentz transformations with det = and apple ( antichronous ). Eventually, the Lorentz transformations with det =and apple are of the form " + with " + 2 SO+ (3, ) and constitute the connected component of 4.

5 6 Lorentz and Poincaré groups Besides the topological meaning of the four subsets SO + (3, ), P SO + (3, ), T SO + (3, ), and P T SO + (3, ) mentioned above, one can also note that these are the cosets of the restricted Lorentz group in O(3, ), i.e. O(3, )/SO + (3, ) = { 4, P, T, P T } = V 4, (VIII.22) where the second isomorphism was already mentioned at the end of VIII..2 a. VIII.2 The Lorentz and Poincaré groups as Lie groups According to theorem (VIII.2), any element of the restricted Lorentz group SO + (3, ) can be written as the product of a Lorentz boost and a three-dimensional rotation. As the reader knows from Sec. VI., the latter are characterized by three real parameters. In turn, a general Lorentz boost (VIII.) also depends on three real parameters, namely the rapidity and the two parameters associated with the unit three-vector along which the boost takes place. All in all, the Lorentz group O(3, ) is thus a 6-parameter group. Since the rapidity can take any value in R, this is a non-compact group. Turning to the Poincaré group, one deduces from the generic form (VIII.4) of a Minkowski spacetime isometry that it is a non-compact -parameter group. Thus, 6 real parameters correspond to the Lorentz transformations ( µ ), and 4 to the spacetime translations (a µ ). In this section, we shall investigate the generators of both groups, starting with those of the (restricted) Lorentz group (Sec. VIII.2.), and dealing afterwards with the Poincaré group (Sec. VIII.2.2). Note, however, that since O(3,) is a subgroup of the Poincaré group, one may instead begin with the latter and consider the former afterwards. VIII.2. Generators of the Lorentz group VIII.2.2 Generators of the Poincaré group

6 Appendix to hapter VIII VIII.A Proof of the linearity of Minkowski spacetime isometries In this appendix, we show that the transformations x µ! x µ that preserve the infinitesimal line element ds 2 = µ dx µ dx are affine transformations. Let x µ! x µ (x ) be such a transformation. As we shall see hereafter, the functions x µ should be at least twice continuously differentiable to ensure the validity of the proof physically, this requirement is however quite reasonable. Using the chain rule, the differential of the coordinate function x reads µ dxµ, (VIII.23) which allows one to write the infinitesimal line element expressed in terms of the primed components: ds 2 dx @x dxµ dx. The condition ds 2 =ds 2 with ds 2 = µ dx µ dx thus translates into µ @x for all µ, 2{,, 2, 3}. (VIII.25) One can now differentiate both sides of this equations with respect to the coordinate x with 2{,, 2, 3}, which thanks to the product rule , µ, 2{,, 2, 3}. Exchanging the dummy indices and µ resp. and, this equation 2 = @ @x µ 8, µ,. (VIII.26b) µ 8, µ,. (VIII.26c) One can now add Eqs. (VIII.26a) and (VIII.26b) and subtract Eq. (VIII.26c). The second term of Eq. (VIII.26a) cancels out with that of Eq. (VIII.26c) while, thanks to the identity =,the first terms of Eq. (VIII.26b) and (VIII.26c) compensate. Eventually, the two remaining terms 2 µ 8, µ, 2{,, 2, 3}. This equation can be interpreted as representing the product from left to right of a row 2 x /@x (with four components =,, 2, 3) with two 4 4-matrices with respective entries 2 /@x. If the latter matrix is regular, i.e. if the transformation x µ! x µ can be inverted, one can multiply the above equation with the inverse matrix, which leads to the 2 = 8, µ, 2{,, 2, 3}. (VIII.28)

7 8 Lorentz and Poincaré groups After twofold integration, this equation leads first µ = µ with µ 2 R, then to the affine transformation x = µ x µ + a, (VIII.29) i.e. to the announced result (VIII.4).