In physics, the Lorentz transformations are a six-parameter family of linear transformations from a coordinate frame in spacetime to another frame that moves at a constant velocity relative to the former. The respective inverse transformation is then parameterized by the negative of this velocity. The transformations are named after the Dutch physicist Hendrik Lorentz.
The most common form of the transformation, parametrized by the real constant <math>v,</math> representing a velocity confined to the Template:Mvar-direction, is expressed as<ref>Template:Cite book Equation 6-3.24, page 210</ref><ref>Template:Harvnb</ref> <math display="block">\begin{align} t' &= \gamma \left( t - \frac{vx}{c^2} \right) \\ x' &= \gamma \left( x - v t \right)\\ y' &= y \\ z' &= z \end{align}</math> where Template:Math and Template:Math are the coordinates of an event in two frames with the spatial origins coinciding at Template:Math, where the primed frame is seen from the unprimed frame as moving with speed Template:Mvar along the Template:Mvar-axis, where Template:Mvar is the speed of light, and <math display="block"> \gamma = \frac{1}{\sqrt{1 - v^2 / c^2 }}</math> is the Lorentz factor. When speed Template:Mvar is much smaller than Template:Mvar, the Lorentz factor is negligibly different from 1, but as Template:Mvar approaches Template:Mvar, <math>\gamma</math> grows without bound. The value of Template:Mvar must be smaller than Template:Mvar for the transformation to make sense.
Expressing the speed as a fraction of the speed of light, <math display="inline"> \beta = v/c,</math> an equivalent form of the transformation is<ref>Template:Harvnb</ref> <math display="block">\begin{align} ct' &= \gamma \left( c t - \beta x \right) \\ x' &= \gamma \left( x - \beta ct \right) \\ y' &= y \\ z' &= z. \end{align}</math>
Frames of reference can be divided into two groups: inertial (relative motion with constant velocity) and non-inertial (accelerating, moving in curved paths, rotational motion with constant angular velocity, etc.). The term "Lorentz transformations" only refers to transformations between inertial frames, usually in the context of special relativity.
In each reference frame, an observer can use a local coordinate system (usually Cartesian coordinates in this context) to measure lengths, and a clock to measure time intervals. An event is something that happens at a point in space at an instant of time, or more formally a point in spacetime. The transformations connect the space and time coordinates of an event as measured by an observer in each frame.<ref group=nb>One can imagine that in each inertial frame there are observers positioned throughout space, each with a synchronized clock and at rest in the particular inertial frame. These observers then report to a central office, where all reports are collected. When one speaks of a particular observer, one refers to someone having, at least in principle, a copy of this report. See, e.g., Template:Harvtxt.</ref>
They supersede the Galilean transformation of Newtonian physics, which assumes an absolute space and time (see Galilean relativity). The Galilean transformation is a good approximation only at relative speeds much less than the speed of light. Lorentz transformations have a number of unintuitive features that do not appear in Galilean transformations. For example, they reflect the fact that observers moving at different velocities may measure different distances, elapsed times, and even different orderings of events, but always such that the speed of light is the same in all inertial reference frames. The invariance of light speed is one of the postulates of special relativity.
Historically, the transformations were the result of attempts by Lorentz and others to explain how the speed of light was observed to be independent of the reference frame, and to understand the symmetries of the laws of electromagnetism. The transformations later became a cornerstone for special relativity.
The Lorentz transformation is a linear transformation. It may include a rotation of space; a rotation-free Lorentz transformation is called a Lorentz boost. In Minkowski space—the mathematical model of spacetime in special relativity—the Lorentz transformations preserve the spacetime interval between any two events. They describe only the transformations in which the spacetime event at the origin is left fixed. They can be considered as a hyperbolic rotation of Minkowski space. The more general set of transformations that also includes translations is known as the Poincaré group.
HistoryEdit
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Many physicists—including Woldemar Voigt, George FitzGerald, Joseph Larmor, and Hendrik Lorentz<ref>Template:Harvnb</ref> himself—had been discussing the physics implied by these equations since 1887.<ref>Template:Harvnb</ref> Early in 1889, Oliver Heaviside had shown from Maxwell's equations that the electric field surrounding a spherical distribution of charge should cease to have spherical symmetry once the charge is in motion relative to the luminiferous aether. FitzGerald then conjectured that Heaviside's distortion result might be applied to a theory of intermolecular forces. Some months later, FitzGerald published the conjecture that bodies in motion are being contracted, in order to explain the baffling outcome of the 1887 aether-wind experiment of Michelson and Morley. In 1892, Lorentz independently presented the same idea in a more detailed manner, which was subsequently called FitzGerald–Lorentz contraction hypothesis.<ref>Template:Harvnb</ref> Their explanation was widely known before 1905.<ref>Template:Harvnb</ref>
Lorentz (1892–1904) and Larmor (1897–1900), who believed the luminiferous aether hypothesis, also looked for the transformation under which Maxwell's equations are invariant when transformed from the aether to a moving frame. They extended the FitzGerald–Lorentz contraction hypothesis and found out that the time coordinate has to be modified as well ("local time"). Henri Poincaré gave a physical interpretation to local time (to first order in Template:Math, the relative velocity of the two reference frames normalized to the speed of light) as the consequence of clock synchronization, under the assumption that the speed of light is constant in moving frames.<ref>Template:Harvnb</ref> Larmor is credited to have been the first to understand the crucial time dilation property inherent in his equations.<ref> Template:Harvnb</ref>
In 1905, Poincaré was the first to recognize that the transformation has the properties of a mathematical group, and he named it after Lorentz.<ref>The reference is within the following paper:Template:Harvnb</ref> Later in the same year Albert Einstein published what is now called special relativity, by deriving the Lorentz transformation under the assumptions of the principle of relativity and the constancy of the speed of light in any inertial reference frame, and by abandoning the mechanistic aether as unnecessary.<ref>Template:Harvnb</ref>
Derivation of the group of Lorentz transformationsEdit
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An event is something that happens at a certain point in spacetime, or more generally, the point in spacetime itself. In any inertial frame an event is specified by a time coordinate Template:Math and a set of Cartesian coordinates Template:Mvar, Template:Mvar, Template:Mvar to specify position in space in that frame. Subscripts label individual events.
From Einstein's second postulate of relativity (invariance of [[Speed of light|Template:Mvar]]) it follows that: Template:NumBlk in all inertial frames for events connected by light signals. The quantity on the left is called the spacetime interval between events Template:Math and Template:Math. The interval between any two events, not necessarily separated by light signals, is in fact invariant, i.e., independent of the state of relative motion of observers in different inertial frames, as is shown using homogeneity and isotropy of space. The transformation sought after thus must possess the property that: Template:NumBlk where Template:Math are the spacetime coordinates used to define events in one frame, and Template:Math are the coordinates in another frame. First one observes that (Template:EquationNote) is satisfied if an arbitrary Template:Math-tuple Template:Mvar of numbers are added to events Template:Math and Template:Math. Such transformations are called spacetime translations and are not dealt with further here. Then one observes that a linear solution preserving the origin of the simpler problem solves the general problem too: Template:NumBlk (a solution satisfying the first formula automatically satisfies the second one as well; see polarization identity). Finding the solution to the simpler problem is just a matter of look-up in the theory of classical groups that preserve bilinear forms of various signature.<ref group=nb>The separate requirements of the three equations lead to three different groups. The second equation is satisfied for spacetime translations in addition to Lorentz transformations leading to the Poincaré group or the inhomogeneous Lorentz group. The first equation (or the second restricted to lightlike separation) leads to a yet larger group, the conformal group of spacetime.</ref> First equation in (Template:EquationNote) can be written more compactly as: Template:NumBlk where Template:Math refers to the bilinear form of signature Template:Math on Template:Math exposed by the right hand side formula in (Template:EquationNote). The alternative notation defined on the right is referred to as the relativistic dot product. Spacetime mathematically viewed as Template:Math endowed with this bilinear form is known as Minkowski space Template:Mvar. The Lorentz transformation is thus an element of the group Template:Math, the Lorentz group or, for those that prefer the other metric signature, Template:Math (also called the Lorentz group).<ref group=nb>The groups Template:Math and Template:Math are isomorphic. It is widely believed that the choice between the two metric signatures has no physical relevance, even though some objects related to Template:Math and Template:Math respectively, e.g., the Clifford algebras corresponding to the different signatures of the bilinear form associated to the two groups, are non-isomorphic.</ref> One has: Template:NumBlk which is precisely preservation of the bilinear form (Template:EquationNote) which implies (by linearity of Template:Math and bilinearity of the form) that (Template:EquationNote) is satisfied. The elements of the Lorentz group are rotations and boosts and mixes thereof. If the spacetime translations are included, then one obtains the inhomogeneous Lorentz group or the Poincaré group.
GeneralitiesEdit
The relations between the primed and unprimed spacetime coordinates are the Lorentz transformations, each coordinate in one frame is a linear function of all the coordinates in the other frame, and the inverse functions are the inverse transformation. Depending on how the frames move relative to each other, and how they are oriented in space relative to each other, other parameters that describe direction, speed, and orientation enter the transformation equations.
Template:AnchorTransformations describing relative motion with constant (uniform) velocity and without rotation of the space coordinate axes are called Lorentz boosts or simply boosts, and the relative velocity between the frames is the parameter of the transformation. The other basic type of Lorentz transformation is rotation in the spatial coordinates only, these like boosts are inertial transformations since there is no relative motion, the frames are simply tilted (and not continuously rotating), and in this case quantities defining the rotation are the parameters of the transformation (e.g., axis–angle representation, or Euler angles, etc.). A combination of a rotation and boost is a homogeneous transformation, which transforms the origin back to the origin.
The full Lorentz group Template:Math also contains special transformations that are neither rotations nor boosts, but rather reflections in a plane through the origin. Two of these can be singled out; spatial inversion in which the spatial coordinates of all events are reversed in sign and temporal inversion in which the time coordinate for each event gets its sign reversed.
Boosts should not be conflated with mere displacements in spacetime; in this case, the coordinate systems are simply shifted and there is no relative motion. However, these also count as symmetries forced by special relativity since they leave the spacetime interval invariant. A combination of a rotation with a boost, followed by a shift in spacetime, is an inhomogeneous Lorentz transformation, an element of the Poincaré group, which is also called the inhomogeneous Lorentz group.
Physical formulation of Lorentz boostsEdit
Coordinate transformationEdit
Top: frame Template:Mvar moves at velocity Template:Mvar along the Template:Mvar-axis of frame Template:Mvar.
Bottom: frame Template:Mvar moves at velocity −Template:Mvar along the Template:Mvar-axis of frame Template:Mvar.<ref>Template:Harvnb</ref>
A "stationary" observer in frame Template:Mvar defines events with coordinates Template:Mvar, Template:Mvar, Template:Mvar, Template:Mvar. Another frame Template:Mvar moves with velocity Template:Mvar relative to Template:Mvar, and an observer in this "moving" frame Template:Mvar defines events using the coordinates Template:Mvar, Template:Mvar, Template:Mvar, Template:Mvar.
The coordinate axes in each frame are parallel (the Template:Mvar and Template:Mvar axes are parallel, the Template:Mvar and Template:Mvar axes are parallel, and the Template:Mvar and Template:Mvar axes are parallel), remain mutually perpendicular, and relative motion is along the coincident Template:Math axes. At Template:Math, the origins of both coordinate systems are the same, Template:Math. In other words, the times and positions are coincident at this event. If all these hold, then the coordinate systems are said to be in standard configuration, or synchronized.
If an observer in Template:Mvar records an event Template:Mvar, Template:Mvar, Template:Mvar, Template:Mvar, then an observer in Template:Mvar records the same event with coordinates<ref>Template:Harvnb</ref>
where Template:Mvar is the relative velocity between frames in the Template:Mvar-direction, Template:Mvar is the speed of light, and <math display="block"> \gamma = \frac{1}{ \sqrt{1 - \frac{v^2}{c^2}}}</math> (lowercase gamma) is the Lorentz factor.
Here, Template:Mvar is the parameter of the transformation, for a given boost it is a constant number, but can take a continuous range of values. In the setup used here, positive relative velocity Template:Math is motion along the positive directions of the Template:Math axes, zero relative velocity Template:Math is no relative motion, while negative relative velocity Template:Math is relative motion along the negative directions of the Template:Math axes. The magnitude of relative velocity Template:Mvar cannot equal or exceed Template:Mvar, so only subluminal speeds Template:Math are allowed. The corresponding range of Template:Mvar is Template:Math.
The transformations are not defined if Template:Mvar is outside these limits. At the speed of light (Template:Math) Template:Mvar is infinite, and faster than light (Template:Math) Template:Mvar is a complex number, each of which make the transformations unphysical. The space and time coordinates are measurable quantities and numerically must be real numbers.
As an active transformation, an observer in Template:Mvar notices the coordinates of the event to be "boosted" in the negative directions of the Template:Math axes, because of the Template:Math in the transformations. This has the equivalent effect of the coordinate system Template:Mvar boosted in the positive directions of the Template:Math axes, while the event does not change and is simply represented in another coordinate system, a passive transformation.
The inverse relations (Template:Mvar, Template:Mvar, Template:Mvar, Template:Mvar in terms of Template:Mvar, Template:Mvar, Template:Mvar, Template:Mvar) can be found by algebraically solving the original set of equations. A more efficient way is to use physical principles. Here Template:Mvar is the "stationary" frame while Template:Mvar is the "moving" frame. According to the principle of relativity, there is no privileged frame of reference, so the transformations from Template:Mvar to Template:Mvar must take exactly the same form as the transformations from Template:Mvar to Template:Mvar. The only difference is Template:Mvar moves with velocity Template:Math relative to Template:Mvar (i.e., the relative velocity has the same magnitude but is oppositely directed). Thus if an observer in Template:Mvar notes an event Template:Mvar, Template:Mvar, Template:Mvar, Template:Mvar, then an observer in Template:Mvar notes the same event with coordinates
and the value of Template:Mvar remains unchanged. This "trick" of simply reversing the direction of relative velocity while preserving its magnitude, and exchanging primed and unprimed variables, always applies to finding the inverse transformation of every boost in any direction.<ref>Template:Cite book Extract of page 102</ref><ref>Template:Cite book Extract of page 70</ref>
Sometimes it is more convenient to use Template:Math (lowercase beta) instead of Template:Mvar, so that <math display="block">\begin{align}
ct' &= \gamma \left( ct - \beta x \right) \,, \\ x' &= \gamma \left( x - \beta ct \right) \,, \\
\end{align}</math> which shows much more clearly the symmetry in the transformation. From the allowed ranges of Template:Mvar and the definition of Template:Mvar, it follows Template:Math. The use of Template:Mvar and Template:Mvar is standard throughout the literature. In the case of three spatial dimensions [ct,x,y,z], where the boost <math>\beta</math> is in the x direction, the eigenstates of the transformation are Template:Math with eigenvalue <math>\sqrt{(1-\beta)/(1+\beta)}</math>, Template:Math with eigenvalue <math>\sqrt{(1+\beta)/(1-\beta)}</math>, and Template:Math and Template:Math, the latter two with eigenvalue 1.
When the boost velocity <math>\boldsymbol{v}</math> is in an arbitrary vector direction with the boost vector <math>\boldsymbol{\beta}=\boldsymbol{v}/c</math>, then the transformation from an unprimed spacetime coordinate system to a primed coordinate system is given by<ref>Template:Cite book Extract of page 124</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
<math display="block">\begin{bmatrix} ct' \vphantom{-\gamma\beta _x} \\ x' \vphantom{1+\frac{\gamma^2}{1+\gamma}\beta _x^2} \\ y' \vphantom{ \frac{\gamma^2}{1+\gamma}\beta _x \beta _y } \\ z' \vphantom{\frac{\gamma^2}{1+\gamma}\beta _y\beta _z} \end{bmatrix} = \begin{bmatrix}
\gamma & -\gamma\beta _x & -\gamma\beta _y & -\gamma\beta _z \\
-\gamma\beta _x & 1+\frac{\gamma^2}{1+\gamma}\beta _x^2 & \frac{\gamma^2}{1+\gamma}\beta _x \beta _y& \frac{\gamma^2}{1+\gamma}\beta _x\beta _z \\
-\gamma\beta _y & \frac{\gamma^2}{1+\gamma}\beta _x \beta _y & 1+\frac{\gamma^2}{1+\gamma}\beta _y^2 & \frac{\gamma^2}{1+\gamma}\beta _y\beta _z \\
-\gamma\beta _z & \frac{\gamma^2}{1+\gamma}\beta _x\beta _z & \frac{\gamma^2}{1+\gamma}\beta _y\beta _z & 1+\frac{\gamma^2}{1+\gamma}\beta _z^2 \\
\end{bmatrix} \begin{bmatrix} ct \vphantom{-\gamma\beta _x} \\ x \vphantom{1+\frac{\gamma^2}{1+\gamma}\beta _x^2} \\ y \vphantom{ \frac{\gamma^2}{1+\gamma}\beta _x \beta _y } \\ z \vphantom{\frac{\gamma^2}{1+\gamma}\beta _y\beta _z} \end{bmatrix}, </math>
where the Lorentz factor is <math>\gamma =1/\sqrt{1 - \boldsymbol{\beta}^2} </math>. The determinant of the transformation matrix is +1 and its trace is <math>2(1+\gamma)</math>. The inverse of the transformation is given by reversing the sign of <math>\boldsymbol{\beta}</math>. The quantity <math> c^2t^2-x^2-y^2-z^2</math> is invariant under the transformation: namely <math>(ct'^2-x'^2-y'^2-z'^2)=(ct^2-x^2-y^2-z^2)</math>.
The Lorentz transformations can also be derived in a way that resembles circular rotations in 3-dimensional space using the hyperbolic functions. For the boost in the Template:Mvar direction, the results are
where Template:Mvar (lowercase zeta) is a parameter called rapidity (many other symbols are used, including Template:Mvar, Template:Mvar, Template:Mvar, Template:Mvar, Template:Mvar, Template:Mvar). Given the strong resemblance to rotations of spatial coordinates in 3-dimensional space in the Cartesian Template:Math, Template:Math, and Template:Math planes, a Lorentz boost can be thought of as a hyperbolic rotation of spacetime coordinates in the xt, yt, and zt Cartesian-time planes of 4-dimensional Minkowski space. The parameter Template:Mvar is the hyperbolic angle of rotation, analogous to the ordinary angle for circular rotations. This transformation can be illustrated with a Minkowski diagram.
The hyperbolic functions arise from the difference between the squares of the time and spatial coordinates in the spacetime interval, rather than a sum. The geometric significance of the hyperbolic functions can be visualized by taking Template:Math or Template:Math in the transformations. Squaring and subtracting the results, one can derive hyperbolic curves of constant coordinate values but varying Template:Mvar, which parametrizes the curves according to the identity <math display="block"> \cosh^2\zeta - \sinh^2\zeta = 1 \,. </math>
Conversely the Template:Math and Template:Mvar axes can be constructed for varying coordinates but constant Template:Mvar. The definition <math display="block"> \tanh\zeta = \frac{\sinh\zeta}{\cosh\zeta} \,, </math> provides the link between a constant value of rapidity, and the slope of the Template:Math axis in spacetime. A consequence these two hyperbolic formulae is an identity that matches the Lorentz factor <math display="block"> \cosh\zeta = \frac{1}{\sqrt{1 - \tanh^2\zeta}} \,. </math>
Comparing the Lorentz transformations in terms of the relative velocity and rapidity, or using the above formulae, the connections between Template:Mvar, Template:Mvar, and Template:Mvar are <math display="block">\begin{align}
\beta &= \tanh\zeta \,, \\
\gamma &= \cosh\zeta \,, \\
\beta \gamma &= \sinh\zeta \,. \end{align}</math>
Taking the inverse hyperbolic tangent gives the rapidity <math display="block"> \zeta = \tanh^{-1}\beta \,.</math>
Since Template:Math, it follows Template:Math. From the relation between Template:Mvar and Template:Mvar, positive rapidity Template:Math is motion along the positive directions of the Template:Math axes, zero rapidity Template:Math is no relative motion, while negative rapidity Template:Math is relative motion along the negative directions of the Template:Math axes.
The inverse transformations are obtained by exchanging primed and unprimed quantities to switch the coordinate frames, and negating rapidity Template:Math since this is equivalent to negating the relative velocity. Therefore,
The inverse transformations can be similarly visualized by considering the cases when Template:Math and Template:Math.
So far the Lorentz transformations have been applied to one event. If there are two events, there is a spatial separation and time interval between them. It follows from the linearity of the Lorentz transformations that two values of space and time coordinates can be chosen, the Lorentz transformations can be applied to each, then subtracted to get the Lorentz transformations of the differences;
<math display="block">\begin{align} \Delta t' &= \gamma \left( \Delta t - \frac{v \, \Delta x}{c^2} \right) \,, \\ \Delta x' &= \gamma \left( \Delta x - v \, \Delta t \right) \,, \end{align}</math> with inverse relations <math display="block">\begin{align} \Delta t &= \gamma \left( \Delta t' + \frac{v \, \Delta x'}{c^2} \right) \,, \\ \Delta x &= \gamma \left( \Delta x' + v \, \Delta t' \right) \,. \end{align}</math>
where Template:Math (uppercase delta) indicates a difference of quantities; e.g., Template:Math for two values of Template:Mvar coordinates, and so on.
These transformations on differences rather than spatial points or instants of time are useful for a number of reasons:
- in calculations and experiments, it is lengths between two points or time intervals that are measured or of interest (e.g., the length of a moving vehicle, or time duration it takes to travel from one place to another),
- the transformations of velocity can be readily derived by making the difference infinitesimally small and dividing the equations, and the process repeated for the transformation of acceleration,
- if the coordinate systems are never coincident (i.e., not in standard configuration), and if both observers can agree on an event Template:Math in Template:Mvar and Template:Math in Template:Mvar, then they can use that event as the origin, and the spacetime coordinate differences are the differences between their coordinates and this origin, e.g., Template:Math, Template:Math, etc.
Physical implicationsEdit
A critical requirement of the Lorentz transformations is the invariance of the speed of light, a fact used in their derivation, and contained in the transformations themselves. If in Template:Mvar the equation for a pulse of light along the Template:Mvar direction is Template:Math, then in Template:Mvar the Lorentz transformations give Template:Math, and vice versa, for any Template:Math.
For relative speeds much less than the speed of light, the Lorentz transformations reduce to the Galilean transformation:<ref>Template:Cite book Extract of page 367</ref><ref>Template:Cite book Extract of page 113</ref> <math display="block">\begin{align} t' &\approx t \\ x' &\approx x - vt \end{align}</math> in accordance with the correspondence principle. It is sometimes said that nonrelativistic physics is a physics of "instantaneous action at a distance".<ref>Template:Harvnb</ref>
Three counterintuitive, but correct, predictions of the transformations are:
- Relativity of simultaneity
- Suppose two events occur along the x axis simultaneously (Template:Math) in Template:Mvar, but separated by a nonzero displacement Template:Math. Then in Template:Mvar, we find that <math>\Delta t' = \gamma \frac{-v\,\Delta x}{c^2} </math>, so the events are no longer simultaneous according to a moving observer.
- Time dilation
- Suppose there is a clock at rest in Template:Mvar. If a time interval is measured at the same point in that frame, so that Template:Math, then the transformations give this interval in Template:Mvar by Template:Math. Conversely, suppose there is a clock at rest in Template:Mvar. If an interval is measured at the same point in that frame, so that Template:Math, then the transformations give this interval in Template:Mvar by Template:Math. Either way, each observer measures the time interval between ticks of a moving clock to be longer by a factor Template:Mvar than the time interval between ticks of his own clock.
- Length contraction
- Suppose there is a rod at rest in Template:Mvar aligned along the Template:Mvar axis, with length Template:Math. In Template:Mvar, the rod moves with velocity Template:Math, so its length must be measured by taking two simultaneous (Template:Math) measurements at opposite ends. Under these conditions, the inverse Lorentz transform shows that Template:Math. In Template:Mvar the two measurements are no longer simultaneous, but this does not matter because the rod is at rest in Template:Mvar. So each observer measures the distance between the end points of a moving rod to be shorter by a factor Template:Math than the end points of an identical rod at rest in his own frame. Length contraction affects any geometric quantity related to lengths, so from the perspective of a moving observer, areas and volumes will also appear to shrink along the direction of motion.
Vector transformationsEdit
Left: Standard configuration. Right: Inverse configuration.
The use of vectors allows positions and velocities to be expressed in arbitrary directions compactly. A single boost in any direction depends on the full relative velocity vector Template:Math with a magnitude Template:Math that cannot equal or exceed Template:Mvar, so that Template:Math.
Only time and the coordinates parallel to the direction of relative motion change, while those coordinates perpendicular do not. With this in mind, split the spatial position vector Template:Math as measured in Template:Mvar, and Template:Math as measured in Template:Mvar, each into components perpendicular (⊥) and parallel ( ‖ ) to Template:Math, <math display="block">\mathbf{r}=\mathbf{r}_\perp+\mathbf{r}_\|\,,\quad \mathbf{r}' = \mathbf{r}_\perp' + \mathbf{r}_\|' \,, </math> then the transformations are <math display="block">\begin{align}
t' &= \gamma \left(t - \frac{\mathbf{r}_\parallel \cdot \mathbf{v}}{c^2} \right) \\
\mathbf{r}_\|' &= \gamma (\mathbf{r}_\| - \mathbf{v} t) \\
\mathbf{r}_\perp' &= \mathbf{r}_\perp \end{align}</math> where Template:Math is the dot product. The Lorentz factor Template:Mvar retains its definition for a boost in any direction, since it depends only on the magnitude of the relative velocity. The definition Template:Math with magnitude Template:Math is also used by some authors.
Introducing a unit vector Template:Math in the direction of relative motion, the relative velocity is Template:Math with magnitude Template:Mvar and direction Template:Math, and vector projection and rejection give respectively <math display="block">\mathbf{r}_\parallel = (\mathbf{r}\cdot\mathbf{n})\mathbf{n}\,,\quad \mathbf{r}_\perp = \mathbf{r} - (\mathbf{r}\cdot\mathbf{n})\mathbf{n}</math>
Accumulating the results gives the full transformations,
Template:Equation box 1{c^2} \right) \,, \\
\mathbf{r}' &= \mathbf{r} + (\gamma-1)(\mathbf{r}\cdot\mathbf{n})\mathbf{n} - \gamma t v\mathbf{n} \,.
\end{align}</math> |cellpadding |border = 1 |border colour = black |background colour=white}}
The projection and rejection also applies to Template:Math. For the inverse transformations, exchange Template:Math and Template:Math to switch observed coordinates, and negate the relative velocity Template:Math (or simply the unit vector Template:Math since the magnitude Template:Mvar is always positive) to obtain
Template:Equation box 1{c^2} \right) \,, \\
\mathbf{r} &= \mathbf{r}' + (\gamma-1)(\mathbf{r}'\cdot\mathbf{n})\mathbf{n} + \gamma t' v\mathbf{n} \,,
\end{align}</math> |cellpadding |border = 1 |border colour = black |background colour=white}}
The unit vector has the advantage of simplifying equations for a single boost, allows either Template:Math or Template:Math to be reinstated when convenient, and the rapidity parametrization is immediately obtained by replacing Template:Mvar and Template:Math. It is not convenient for multiple boosts.
The vectorial relation between relative velocity and rapidity is<ref>Template:Harvnb</ref> <math display="block"> \boldsymbol{\beta} = \beta \mathbf{n} = \mathbf{n} \tanh\zeta \,,</math> and the "rapidity vector" can be defined as <math display="block"> \boldsymbol{\zeta} = \zeta\mathbf{n} = \mathbf{n}\tanh^{-1}\beta \,, </math> each of which serves as a useful abbreviation in some contexts. The magnitude of Template:Math is the absolute value of the rapidity scalar confined to Template:Math, which agrees with the range Template:Math.
Transformation of velocitiesEdit
Defining the coordinate velocities and Lorentz factor by
- <math>\mathbf{u} = \frac{d\mathbf{r}}{dt} \,,\quad \mathbf{u}' = \frac{d\mathbf{r}'}{dt'} \,,\quad \gamma_\mathbf{v} = \frac{1}{\sqrt{1-\dfrac{\mathbf{v}\cdot\mathbf{v}}{c^2}}}</math>
taking the differentials in the coordinates and time of the vector transformations, then dividing equations, leads to
- <math>\mathbf{u}' = \frac{1}{ 1 - \frac{\mathbf{v}\cdot\mathbf{u}}{c^2} }\left[\frac{\mathbf{u}}{\gamma_\mathbf{v}} - \mathbf{v} + \frac{1}{c^2}\frac{\gamma_\mathbf{v}}{\gamma_\mathbf{v} + 1}\left(\mathbf{u}\cdot\mathbf{v}\right)\mathbf{v}\right] </math>
The velocities Template:Math and Template:Math are the velocity of some massive object. They can also be for a third inertial frame (say Template:Mvar), in which case they must be constant. Denote either entity by Template:Mvar. Then Template:Mvar moves with velocity Template:Math relative to Template:Mvar, or equivalently with velocity Template:Math relative to Template:Mvar, in turn Template:Mvar moves with velocity Template:Math relative to Template:Mvar. The inverse transformations can be obtained in a similar way, or as with position coordinates exchange Template:Math and Template:Math, and change Template:Math to Template:Math.
The transformation of velocity is useful in stellar aberration, the Fizeau experiment, and the relativistic Doppler effect.
The Lorentz transformations of acceleration can be similarly obtained by taking differentials in the velocity vectors, and dividing these by the time differential.
Transformation of other quantitiesEdit
In general, given four quantities Template:Mvar and Template:Math and their Lorentz-boosted counterparts Template:Mvar and Template:Math, a relation of the form <math display="block">A^2 - \mathbf{Z}\cdot\mathbf{Z} = {A'}^2 - \mathbf{Z}'\cdot\mathbf{Z}'</math> implies the quantities transform under Lorentz transformations similar to the transformation of spacetime coordinates; <math display="block">\begin{align}
A' &= \gamma \left(A - \frac{v\mathbf{n}\cdot \mathbf{Z}}{c} \right) \,, \\
\mathbf{Z}' &= \mathbf{Z} + (\gamma-1)(\mathbf{Z}\cdot\mathbf{n})\mathbf{n} - \frac{\gamma A v\mathbf{n}}{c} \,.
\end{align}</math>
The decomposition of Template:Math (and Template:Math) into components perpendicular and parallel to Template:Math is exactly the same as for the position vector, as is the process of obtaining the inverse transformations (exchange Template:Math and Template:Math to switch observed quantities, and reverse the direction of relative motion by the substitution Template:Math).
The quantities Template:Math collectively make up a four-vector, where Template:Mvar is the "timelike component", and Template:Math the "spacelike component". Examples of Template:Mvar and Template:Math are the following:
| Four-vector | Template:Mvar | Template:Math |
|---|---|---|
| Position four-vector | Time (multiplied by Template:Mvar), Template:Math | Position vector, Template:Math |
| Four-momentum | Energy (divided by Template:Mvar), Template:Math | Momentum, Template:Math |
| Four-wave vector | angular frequency (divided by Template:Mvar), Template:Math | wave vector, Template:Math |
| Four-spin | (No name), Template:Math | Spin, Template:Math |
| Four-current | Charge density (multiplied by Template:Mvar), Template:Math | Current density, Template:Math |
| Electromagnetic four-potential | Electric potential (divided by Template:Mvar), Template:Math | Magnetic vector potential, Template:Math |
For a given object (e.g., particle, fluid, field, material), if Template:Mvar or Template:Math correspond to properties specific to the object like its charge density, mass density, spin, etc., its properties can be fixed in the rest frame of that object. Then the Lorentz transformations give the corresponding properties in a frame moving relative to the object with constant velocity. This breaks some notions taken for granted in non-relativistic physics. For example, the energy Template:Mvar of an object is a scalar in non-relativistic mechanics, but not in relativistic mechanics because energy changes under Lorentz transformations; its value is different for various inertial frames. In the rest frame of an object, it has a rest energy and zero momentum. In a boosted frame its energy is different and it appears to have a momentum. Similarly, in non-relativistic quantum mechanics the spin of a particle is a constant vector, but in relativistic quantum mechanics spin Template:Math depends on relative motion. In the rest frame of the particle, the spin pseudovector can be fixed to be its ordinary non-relativistic spin with a zero timelike quantity Template:Math, however a boosted observer will perceive a nonzero timelike component and an altered spin.<ref>Template:Harvnb</ref>
Not all quantities are invariant in the form as shown above, for example orbital angular momentum Template:Math does not have a timelike quantity, and neither does the electric field Template:Math nor the magnetic field Template:Math. The definition of angular momentum is Template:Math, and in a boosted frame the altered angular momentum is Template:Math. Applying this definition using the transformations of coordinates and momentum leads to the transformation of angular momentum. It turns out Template:Math transforms with another vector quantity Template:Math related to boosts, see relativistic angular momentum for details. For the case of the Template:Math and Template:Math fields, the transformations cannot be obtained as directly using vector algebra. The Lorentz force is the definition of these fields, and in Template:Mvar it is Template:Math while in Template:Mvar it is Template:Math. A method of deriving the EM field transformations in an efficient way which also illustrates the unit of the electromagnetic field uses tensor algebra, given below.
Mathematical formulationEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Template:Further
Throughout, italic non-bold capital letters are Template:Math matrices, while non-italic bold letters are Template:Math matrices.
Homogeneous Lorentz groupEdit
Writing the coordinates in column vectors and the Minkowski metric Template:Mvar as a square matrix <math display="block"> X' = \begin{bmatrix} c\,t' \\ x' \\ y' \\ z' \end{bmatrix} \,, \quad \eta = \begin{bmatrix} -1&0&0&0\\ 0&1&0&0 \\ 0&0&1&0 \\ 0&0&0&1 \end{bmatrix} \,, \quad X = \begin{bmatrix} c\,t \\ x \\ y \\ z \end{bmatrix} </math> the spacetime interval takes the form (superscript Template:Math denotes transpose) <math display="block"> X \cdot X = X^\mathrm{T} \eta X = {X'}^\mathrm{T} \eta {X'} </math> and is invariant under a Lorentz transformation <math display="block">X' = \Lambda X </math> where Template:Math is a square matrix which can depend on parameters.
The set of all Lorentz transformations <math>\Lambda </math> in this article is denoted <math>\mathcal{L}</math>. This set together with matrix multiplication forms a group, in this context known as the Lorentz group. Also, the above expression Template:Math is a quadratic form of signature (3,1) on spacetime, and the group of transformations which leaves this quadratic form invariant is the indefinite orthogonal group O(3,1), a Lie group. In other words, the Lorentz group is O(3,1). As presented in this article, any Lie groups mentioned are matrix Lie groups. In this context the operation of composition amounts to matrix multiplication.
From the invariance of the spacetime interval it follows <math display="block">\eta = \Lambda^\mathrm{T} \eta \Lambda </math> and this matrix equation contains the general conditions on the Lorentz transformation to ensure invariance of the spacetime interval. Taking the determinant of the equation using the product rule<ref group=nb>For two square matrices Template:Mvar and Template:Mvar, Template:Math</ref> gives immediately <math display="block">\left[\det (\Lambda)\right]^2 = 1 \quad \Rightarrow \quad \det(\Lambda) = \pm 1 </math>
Writing the Minkowski metric as a block matrix, and the Lorentz transformation in the most general form, <math display="block">\eta = \begin{bmatrix}-1 & 0 \\ 0 & \mathbf{I}\end{bmatrix} \,, \quad \Lambda=\begin{bmatrix}\Gamma & -\mathbf{a}^\mathrm{T}\\-\mathbf{b} & \mathbf{M}\end{bmatrix} \,, </math> carrying out the block matrix multiplications obtains general conditions on Template:Math to ensure relativistic invariance. Not much information can be directly extracted from all the conditions, however one of the results <math display="block">\Gamma^2 = 1 + \mathbf{b}^\mathrm{T}\mathbf{b}</math> is useful; Template:Math always so it follows that <math display="block"> \Gamma^2 \geq 1 \quad \Rightarrow \quad \Gamma \leq - 1 \,,\quad \Gamma \geq 1 </math>
The negative inequality may be unexpected, because Template:Math multiplies the time coordinate and this has an effect on time symmetry. If the positive equality holds, then Template:Math is the Lorentz factor.
The determinant and inequality provide four ways to classify Lorentz Transformations (herein LTs for brevity). Any particular LT has only one determinant sign and only one inequality. There are four sets which include every possible pair given by the intersections ("n"-shaped symbol meaning "and") of these classifying sets.
| Intersection, ∩ | Antichronous (or non-orthochronous) LTs
|
Orthochronous LTs
|
|---|---|---|
Proper LTs
|
Proper antichronous LTs
|
Proper orthochronous LTs
|
Improper LTs
|
Improper antichronous LTs
|
Improper orthochronous LTs
|
where "+" and "−" indicate the determinant sign, while "↑" for ≥ and "↓" for ≤ denote the inequalities.
The full Lorentz group splits into the union ("u"-shaped symbol meaning "or") of four disjoint sets <math display="block"> \mathcal{L} = \mathcal{L}_{+}^\uparrow \cup \mathcal{L}_{-}^\uparrow \cup \mathcal{L}_{+}^\downarrow \cup \mathcal{L}_{-}^\downarrow </math>
A subgroup of a group must be closed under the same operation of the group (here matrix multiplication). In other words, for two Lorentz transformations Template:Math and Template:Mvar from a particular subgroup, the composite Lorentz transformations Template:Math and Template:Math must be in the same subgroup as Template:Math and Template:Mvar. This is not always the case: the composition of two antichronous Lorentz transformations is orthochronous, and the composition of two improper Lorentz transformations is proper. In other words, while the sets <math>\mathcal{L}_+^\uparrow </math>, <math>\mathcal{L}_+</math>, <math>\mathcal{L}^\uparrow</math>, and <math>\mathcal{L}_0 = \mathcal{L}_+^\uparrow \cup \mathcal{L}_{-}^\downarrow</math> all form subgroups, the sets containing improper and/or antichronous transformations without enough proper orthochronous transformations (e.g. <math>\mathcal{L}_+^\downarrow </math>, <math>\mathcal{L}_{-}^\downarrow </math>, <math>\mathcal{L}_{-}^\uparrow </math>) do not form subgroups.
Proper transformationsEdit
If a Lorentz covariant 4-vector is measured in one inertial frame with result <math>X</math>, and the same measurement made in another inertial frame (with the same orientation and origin) gives result <math>X'</math>, the two results will be related by <math display="block">X' = B(\mathbf{v})X</math> where the boost matrix <math>B(\mathbf{v})</math> represents the rotation-free Lorentz transformation between the unprimed and primed frames and <math>\mathbf{v}</math> is the velocity of the primed frame as seen from the unprimed frame. The matrix is given by<ref>Template:Cite journal</ref> <math display="block">B(\mathbf{v}) = \begin{bmatrix}
\gamma &-\gamma v_x/c &-\gamma v_y/c &-\gamma v_z/c \\
-\gamma v_x/c&1+(\gamma-1)\dfrac{v_x^2} {v^2}& (\gamma-1)\dfrac{v_x v_y}{v^2}& (\gamma-1)\dfrac{v_x v_z}{v^2} \\ -\gamma v_y/c& (\gamma-1)\dfrac{v_y v_x}{v^2}&1+(\gamma-1)\dfrac{v_y^2} {v^2}& (\gamma-1)\dfrac{v_y v_z}{v^2} \\ -\gamma v_z/c& (\gamma-1)\dfrac{v_z v_x}{v^2}& (\gamma-1)\dfrac{v_z v_y}{v^2}&1+(\gamma-1)\dfrac{v_z^2} {v^2} \end{bmatrix} = \begin{bmatrix}
\gamma & -\gamma \vec{\beta}^T \\
-\gamma \vec{\beta} & I + (\gamma-1)\dfrac{\vec{\beta}\vec{\beta}^T}{\beta^2} \end{bmatrix},</math>
where <math display="inline">v=\sqrt{v_x^2+v_y^2+v_z^2}</math> is the magnitude of the velocity and <math display="inline">\gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}</math> is the Lorentz factor. This formula represents a passive transformation, as it describes how the coordinates of the measured quantity changes from the unprimed frame to the primed frame. The active transformation is given by <math>B(-\mathbf{v})</math>.
If a frame Template:Mvar is boosted with velocity Template:Math relative to frame Template:Mvar, and another frame Template:Mvar is boosted with velocity Template:Math relative to Template:Mvar, the separate boosts are <math display="block">X = B(\mathbf{v})X' \,, \quad X' = B(\mathbf{u})X </math> and the composition of the two boosts connects the coordinates in Template:Mvar and Template:Mvar, <math display="block">X = B(\mathbf{v})B(\mathbf{u})X \,. </math> Successive transformations act on the left. If Template:Math and Template:Math are collinear (parallel or antiparallel along the same line of relative motion), the boost matrices commute: Template:Math. This composite transformation happens to be another boost, Template:Math, where Template:Math is collinear with Template:Math and Template:Math.
If Template:Math and Template:Math are not collinear but in different directions, the situation is considerably more complicated. Lorentz boosts along different directions do not commute: Template:Math and Template:Math are not equal. Although each of these compositions is not a single boost, each composition is still a Lorentz transformation as it preserves the spacetime interval. It turns out the composition of any two Lorentz boosts is equivalent to a boost followed or preceded by a rotation on the spatial coordinates, in the form of Template:Math or Template:Math. The Template:Math and Template:Math are composite velocities, while Template:Math and Template:Math are rotation parameters (e.g. axis-angle variables, Euler angles, etc.). The rotation in block matrix form is simply <math display="block">\quad R(\boldsymbol{\rho}) = \begin{bmatrix} 1 & 0 \\ 0 & \mathbf{R}(\boldsymbol{\rho}) \end{bmatrix} \,, </math> where Template:Math is a Template:Math rotation matrix, which rotates any 3-dimensional vector in one sense (active transformation), or equivalently the coordinate frame in the opposite sense (passive transformation). It is not simple to connect Template:Math and Template:Math (or Template:Math and Template:Math) to the original boost parameters Template:Math and Template:Math. In a composition of boosts, the Template:Mvar matrix is named the Wigner rotation, and gives rise to the Thomas precession. These articles give the explicit formulae for the composite transformation matrices, including expressions for Template:Math.
In this article the axis-angle representation is used for Template:Math. The rotation is about an axis in the direction of a unit vector Template:Math, through angle Template:Mvar (positive anticlockwise, negative clockwise, according to the right-hand rule). The "axis-angle vector" <math display="block">\boldsymbol{\theta} = \theta \mathbf{e}</math> will serve as a useful abbreviation.
Spatial rotations alone are also Lorentz transformations since they leave the spacetime interval invariant. Like boosts, successive rotations about different axes do not commute. Unlike boosts, the composition of any two rotations is equivalent to a single rotation. Some other similarities and differences between the boost and rotation matrices include:
- inverses: Template:Math (relative motion in the opposite direction), and Template:Math (rotation in the opposite sense about the same axis)
- identity transformation for no relative motion/rotation: Template:Math
- unit determinant: Template:Math. This property makes them proper transformations.
- matrix symmetry: Template:Mvar is symmetric (equals transpose), while Template:Mvar is nonsymmetric but orthogonal (transpose equals inverse, Template:Math).
The most general proper Lorentz transformation Template:Math includes a boost and rotation together, and is a nonsymmetric matrix. As special cases, Template:Math and Template:Math. An explicit form of the general Lorentz transformation is cumbersome to write down and will not be given here. Nevertheless, closed form expressions for the transformation matrices will be given below using group theoretical arguments. It will be easier to use the rapidity parametrization for boosts, in which case one writes Template:Math and Template:Math.
The Lie group SO+(3,1)Edit
The set of transformations <math display="block"> \{ B(\boldsymbol{\zeta}), R(\boldsymbol{\theta}), \Lambda(\boldsymbol{\zeta}, \boldsymbol{\theta}) \} </math> with matrix multiplication as the operation of composition forms a group, called the "restricted Lorentz group", and is the special indefinite orthogonal group SO+(3,1). (The plus sign indicates that it preserves the orientation of the temporal dimension).
For simplicity, look at the infinitesimal Lorentz boost in the Template:Mvar direction (examining a boost in any other direction, or rotation about any axis, follows an identical procedure). The infinitesimal boost is a small boost away from the identity, obtained by the Taylor expansion of the boost matrix to first order about Template:Math, <math display="block"> B_x = I + \zeta \left. \frac{\partial B_x}{\partial \zeta } \right|_{\zeta=0} + \cdots </math> where the higher order terms not shown are negligible because Template:Mvar is small, and Template:Math is simply the boost matrix in the x direction. The derivative of the matrix is the matrix of derivatives (of the entries, with respect to the same variable), and it is understood the derivatives are found first then evaluated at Template:Math, <math display="block"> \left. \frac{\partial B_x }{\partial \zeta } \right|_{\zeta=0} = - K_x \,. </math>
For now, Template:Math is defined by this result (its significance will be explained shortly). In the limit of an infinite number of infinitely small steps, the finite boost transformation in the form of a matrix exponential is obtained <math display="block"> B_x =\lim_{N\to\infty}\left(I-\frac{\zeta }{N}K_x\right)^{N} = e^{-\zeta K_x} </math> where the limit definition of the exponential has been used (see also characterizations of the exponential function). More generally<ref group="nb">Explicitly, <math display="block"> \boldsymbol{\zeta} \cdot\mathbf{K} = \zeta_x K_x + \zeta_y K_y + \zeta_z K_z </math> <math display="block"> \boldsymbol{\theta} \cdot\mathbf{J} = \theta_x J_x + \theta_y J_y + \theta_z J_z </math> </ref>
<math display="block">B(\boldsymbol{\zeta}) = e^{-\boldsymbol{\zeta}\cdot\mathbf{K}} \, , \quad R(\boldsymbol{\theta}) = e^{\boldsymbol{\theta}\cdot\mathbf{J}} \,. </math>
The axis-angle vector Template:Math and rapidity vector Template:Math are altogether six continuous variables which make up the group parameters (in this particular representation), and the generators of the group are Template:Math and Template:Math, each vectors of matrices with the explicit forms<ref group=nb>In quantum mechanics, relativistic quantum mechanics, and quantum field theory, a different convention is used for these matrices; the right hand sides are all multiplied by a factor of the imaginary unit Template:Math.</ref>
<math display="block">\begin{alignat}{3}
K_x &= \begin{bmatrix}
0 & 1 & 0 & 0 \\
1 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 \\
\end{bmatrix}\,,\quad &
K_y &= \begin{bmatrix}
0 & 0 & 1 & 0\\
0 & 0 & 0 & 0\\
1 & 0 & 0 & 0\\
0 & 0 & 0 & 0
\end{bmatrix}\,,\quad &
K_z &= \begin{bmatrix}
0 & 0 & 0 & 1\\ 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 0\\ 1 & 0 & 0 & 0
\end{bmatrix}
\\[10mu]
J_x &= \begin{bmatrix}
0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 \\
0 & 0 & 0 & -1 \\
0 & 0 & 1 & 0 \\
\end{bmatrix}\,,\quad &
J_y &= \begin{bmatrix}
0 & 0 & 0 & 0 \\
0 & 0 & 0 & 1 \\
0 & 0 & 0 & 0 \\
0 & -1 & 0 & 0
\end{bmatrix}\,,\quad &
J_z &= \begin{bmatrix}
0 & 0 & 0 & 0 \\
0 & 0 & -1 & 0 \\
0 & 1 & 0 & 0 \\
0 & 0 & 0 & 0
\end{bmatrix}
\end{alignat}</math>
These are all defined in an analogous way to Template:Math above, although the minus signs in the boost generators are conventional. Physically, the generators of the Lorentz group correspond to important symmetries in spacetime: Template:Math are the rotation generators which correspond to angular momentum, and Template:Math are the boost generators which correspond to the motion of the system in spacetime. The derivative of any smooth curve Template:Math with Template:Math in the group depending on some group parameter Template:Mvar with respect to that group parameter, evaluated at Template:Math, serves as a definition of a corresponding group generator Template:Mvar, and this reflects an infinitesimal transformation away from the identity. The smooth curve can always be taken as an exponential as the exponential will always map Template:Mvar smoothly back into the group via Template:Math for all Template:Mvar; this curve will yield Template:Mvar again when differentiated at Template:Math.
Expanding the exponentials in their Taylor series obtains <math display="block"> B({\boldsymbol {\zeta }})=I-\sinh \zeta (\mathbf {n} \cdot \mathbf {K} )+(\cosh \zeta -1)(\mathbf {n} \cdot \mathbf {K} )^2</math> <math display="block">R(\boldsymbol {\theta })=I+\sin \theta (\mathbf {e} \cdot \mathbf {J} )+(1-\cos \theta )(\mathbf {e} \cdot \mathbf {J} )^2\,.</math> which compactly reproduce the boost and rotation matrices as given in the previous section.
It has been stated that the general proper Lorentz transformation is a product of a boost and rotation. At the infinitesimal level the product <math display="block"> \begin{align} \Lambda &= (I - \boldsymbol {\zeta } \cdot \mathbf {K} + \cdots )(I + \boldsymbol {\theta } \cdot \mathbf {J} + \cdots ) \\ &= (I + \boldsymbol {\theta } \cdot \mathbf {J} + \cdots )(I - \boldsymbol {\zeta } \cdot \mathbf {K} + \cdots ) \\ &= I - \boldsymbol {\zeta } \cdot \mathbf {K} + \boldsymbol {\theta } \cdot \mathbf {J} + \cdots \end{align} </math> is commutative because only linear terms are required (products like Template:Math and Template:Math count as higher order terms and are negligible). Taking the limit as before leads to the finite transformation in the form of an exponential <math display="block">\Lambda (\boldsymbol{\zeta}, \boldsymbol{\theta}) = e^{-\boldsymbol{\zeta} \cdot\mathbf{K} + \boldsymbol{\theta} \cdot\mathbf{J} }.</math>
The converse is also true, but the decomposition of a finite general Lorentz transformation into such factors is nontrivial. In particular, <math display="block">e^{-\boldsymbol{\zeta} \cdot\mathbf{K} + \boldsymbol{\theta} \cdot\mathbf{J} } \ne e^{-\boldsymbol{\zeta} \cdot\mathbf{K}} e^{\boldsymbol{\theta} \cdot\mathbf{J}},</math> because the generators do not commute. For a description of how to find the factors of a general Lorentz transformation in terms of a boost and a rotation in principle (this usually does not yield an intelligible expression in terms of generators Template:Math and Template:Math), see Wigner rotation. If, on the other hand, the decomposition is given in terms of the generators, and one wants to find the product in terms of the generators, then the Baker–Campbell–Hausdorff formula applies.
The Lie algebra so(3,1)Edit
Lorentz generators can be added together, or multiplied by real numbers, to obtain more Lorentz generators. In other words, the set of all Lorentz generators <math display="block">V = \{ \boldsymbol{\zeta} \cdot\mathbf{K} + \boldsymbol{\theta} \cdot\mathbf{J} \} </math> together with the operations of ordinary matrix addition and multiplication of a matrix by a number, forms a vector space over the real numbers.<ref group=nb>Until now the term "vector" has exclusively referred to "Euclidean vector", examples are position Template:Math, velocity Template:Math, etc. The term "vector" applies much more broadly than Euclidean vectors, row or column vectors, etc., see linear algebra and vector space for details. The generators of a Lie group also form a vector space over a field of numbers (e.g. real numbers, complex numbers), since a linear combination of the generators is also a generator. They just live in a different space to the position vectors in ordinary 3-dimensional space.</ref> The generators Template:Math form a basis set of V, and the components of the axis-angle and rapidity vectors, Template:Math, are the coordinates of a Lorentz generator with respect to this basis.<ref group=nb>In ordinary 3-dimensional position space, the position vector Template:Math is expressed as a linear combination of the Cartesian unit vectors Template:Math which form a basis, and the Cartesian coordinates Template:Math are coordinates with respect to this basis.</ref>
Three of the commutation relations of the Lorentz generators are <math display="block">[ J_x, J_y ] = J_z \,,\quad [ K_x, K_y ] = -J_z \,,\quad [ J_x, K_y ] = K_z \,, </math> where the bracket Template:Math is known as the commutator, and the other relations can be found by taking cyclic permutations of Template:Mvar, Template:Mvar, Template:Mvar components (i.e. change Template:Mvar to Template:Mvar, Template:Mvar to Template:Mvar, and Template:Mvar to Template:Mvar, repeat).
These commutation relations, and the vector space of generators, fulfill the definition of the Lie algebra <math>\mathfrak{so}(3, 1)</math>. In summary, a Lie algebra is defined as a vector space V over a field of numbers, and with a binary operation [ , ] (called a Lie bracket in this context) on the elements of the vector space, satisfying the axioms of bilinearity, alternatization, and the Jacobi identity. Here the operation [ , ] is the commutator which satisfies all of these axioms, the vector space is the set of Lorentz generators V as given previously, and the field is the set of real numbers.
Linking terminology used in mathematics and physics: A group generator is any element of the Lie algebra. A group parameter is a component of a coordinate vector representing an arbitrary element of the Lie algebra with respect to some basis. A basis, then, is a set of generators being a basis of the Lie algebra in the usual vector space sense.
The exponential map from the Lie algebra to the Lie group, <math display="block">\exp \, : \, \mathfrak{so}(3,1) \to \mathrm{SO}(3,1),</math> provides a one-to-one correspondence between small enough neighborhoods of the origin of the Lie algebra and neighborhoods of the identity element of the Lie group. In the case of the Lorentz group, the exponential map is just the matrix exponential. Globally, the exponential map is not one-to-one, but in the case of the Lorentz group, it is surjective (onto). Hence any group element in the connected component of the identity can be expressed as an exponential of an element of the Lie algebra.
Improper transformationsEdit
Lorentz transformations also include parity inversion <math display="block"> P = \begin{bmatrix} 1 & 0 \\ 0 & - \mathbf{I} \end{bmatrix} </math> which negates all the spatial coordinates only, and time reversal <math display="block"> T = \begin{bmatrix} - 1 & 0 \\ 0 & \mathbf{I} \end{bmatrix}</math> which negates the time coordinate only, because these transformations leave the spacetime interval invariant. Here Template:Math is the Template:Math identity matrix. These are both symmetric, they are their own inverses (see involution (mathematics)), and each have determinant −1. This latter property makes them improper transformations.
If Template:Math is a proper orthochronous Lorentz transformation, then Template:Math is improper antichronous, Template:Math is improper orthochronous, and Template:Math is proper antichronous.
Inhomogeneous Lorentz groupEdit
Two other spacetime symmetries have not been accounted for. In order for the spacetime interval to be invariant, it can be shown<ref>Template:Harvnb</ref> that it is necessary and sufficient for the coordinate transformation to be of the form <math display="block">X' = \Lambda X + C </math> where C is a constant column containing translations in time and space. If C ≠ 0, this is an inhomogeneous Lorentz transformation or Poincaré transformation.<ref>Template:Harvnb</ref><ref>Template:Harvnb</ref> If C = 0, this is a homogeneous Lorentz transformation. Poincaré transformations are not dealt further in this article.
Tensor formulationEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Template:For
Contravariant vectorsEdit
Writing the general matrix transformation of coordinates as the matrix equation <math display="block">\begin{bmatrix} {x'}^0 \\ {x'}^1 \\ {x'}^2 \\ {x'}^3 \end{bmatrix} =
\begin{bmatrix}
{\Lambda^0}_0 & {\Lambda^0}_1 & {\Lambda^0}_2 & {\Lambda^0}_3 \vphantom{{x'}^0} \\
{\Lambda^1}_0 & {\Lambda^1}_1 & {\Lambda^1}_2 & {\Lambda^1}_3 \vphantom{{x'}^0} \\
{\Lambda^2}_0 & {\Lambda^2}_1 & {\Lambda^2}_2 & {\Lambda^2}_3 \vphantom{{x'}^0} \\
{\Lambda^3}_0 & {\Lambda^3}_1 & {\Lambda^3}_2 & {\Lambda^3}_3 \vphantom{{x'}^0} \\
\end{bmatrix}
\begin{bmatrix} x^0 \vphantom{{x'}^0} \\ x^1 \vphantom{{x'}^0} \\ x^2 \vphantom{{x'}^0} \\ x^3 \vphantom{{x'}^0} \end{bmatrix}</math>
allows the transformation of other physical quantities that cannot be expressed as four-vectors; e.g., tensors or spinors of any order in 4-dimensional spacetime, to be defined. In the corresponding tensor index notation, the above matrix expression is <math display="block">{x'}^\nu = {\Lambda^\nu}_\mu x^\mu,</math>
where lower and upper indices label covariant and contravariant components respectively,<ref>Template:Harvnb</ref> and the summation convention is applied. It is a standard convention to use Greek indices that take the value 0 for time components, and 1, 2, 3 for space components, while Latin indices simply take the values 1, 2, 3, for spatial components (the opposite for Landau and Lifshitz). Note that the first index (reading left to right) corresponds in the matrix notation to a row index. The second index corresponds to the column index.
The transformation matrix is universal for all four-vectors, not just 4-dimensional spacetime coordinates. If Template:Mvar is any four-vector, then in tensor index notation <math display="block"> {A'}^\nu = {\Lambda^\nu}_\mu A^\mu \,.</math>
Alternatively, one writes <math display="block"> A^{\nu'} = {\Lambda^{\nu'}}_\mu A^\mu \,.</math> in which the primed indices denote the indices of A in the primed frame. For a general Template:Mvar-component object one may write <math display="block">{X'}^\alpha = {\Pi(\Lambda)^\alpha}_\beta X^\beta \,, </math> where Template:Math is the appropriate representation of the Lorentz group, an Template:Math matrix for every Template:Math. In this case, the indices should not be thought of as spacetime indices (sometimes called Lorentz indices), and they run from Template:Math to Template:Mvar. E.g., if Template:Mvar is a bispinor, then the indices are called Dirac indices.
Covariant vectors
There are also vector quantities with covariant indices. They are generally obtained from their corresponding objects with contravariant indices by the operation of lowering an index; e.g., <math display="block">x_\nu = \eta_{\mu\nu}x^\mu,</math> where Template:Mvar is the metric tensor. (The linked article also provides more information about what the operation of raising and lowering indices really is mathematically.) The inverse of this transformation is given by <math display="block">x^\mu = \eta^{\mu\nu}x_\nu,</math> where, when viewed as matrices, Template:Math is the inverse of Template:Math. As it happens, Template:Math. This is referred to as raising an index. To transform a covariant vector Template:Math, first raise its index, then transform it according to the same rule as for contravariant Template:Math-vectors, then finally lower the index; <math display="block">{A'}_\nu = \eta_{\rho\nu} {\Lambda^\rho}_\sigma \eta^{\mu\sigma}A_\mu.</math>
But <math display="block">\eta_{\rho\nu} {\Lambda^\rho}_\sigma \eta^{\mu\sigma} = {\left(\Lambda^{-1}\right)^\mu}_\nu,</math>
That is, it is the Template:Math-component of the inverse Lorentz transformation. One defines (as a matter of notation), <math display="block">{\Lambda_\nu}^\mu \equiv {\left(\Lambda^{-1}\right)^\mu}_\nu,</math> and may in this notation write <math display="block">{A'}_\nu = {\Lambda_\nu}^\mu A_\mu.</math>
Now for a subtlety. The implied summation on the right hand side of <math display="block">{A'}_\nu = {\Lambda_\nu}^\mu A_\mu = {\left(\Lambda^{-1}\right)^\mu}_\nu A_\mu</math> is running over a row index of the matrix representing Template:Math. Thus, in terms of matrices, this transformation should be thought of as the inverse transpose of Template:Math acting on the column vector Template:Math. That is, in pure matrix notation, <math display="block">A' = \left(\Lambda^{-1}\right)^\mathrm{T} A.</math>
This means exactly that covariant vectors (thought of as column matrices) transform according to the dual representation of the standard representation of the Lorentz group. This notion generalizes to general representations, simply replace Template:Math with Template:Math.
Tensors
If Template:Mvar and Template:Mvar are linear operators on vector spaces Template:Mvar and Template:Mvar, then a linear operator Template:Math may be defined on the tensor product of Template:Mvar and Template:Mvar, denoted Template:Math according to<ref>Template:Harvnb</ref>
From this it is immediately clear that if Template:Mvar and Template:Mvar are a four-vectors in Template:Mvar, then Template:Math transforms as
The second step uses the bilinearity of the tensor product and the last step defines a 2-tensor on component form, or rather, it just renames the tensor Template:Math.
These observations generalize in an obvious way to more factors, and using the fact that a general tensor on a vector space Template:Mvar can be written as a sum of a coefficient (component!) times tensor products of basis vectors and basis covectors, one arrives at the transformation law for any tensor quantity Template:Mvar. It is given by<ref>Template:Harvnb</ref>
Template:Equation box 1_\mu {\Lambda^{\beta'}}_\nu \cdots {\Lambda^{\zeta'}}_\rho
{\Lambda_{\theta'}}^\sigma {\Lambda_{\iota'}}^\upsilon \cdots {\Lambda_{\kappa'}}^\zeta
T^{\mu\nu \cdots \rho}_{\sigma\upsilon \cdots \zeta},
</math> Template:Spaces Template:EquationRef
|cellpadding= 6 |border |border colour = #0073CF |bgcolor=#F9FFF7
}}
where Template:Math is defined above. This form can generally be reduced to the form for general Template:Mvar-component objects given above with a single matrix (Template:Math) operating on column vectors. This latter form is sometimes preferred; e.g., for the electromagnetic field tensor.
Transformation of the electromagnetic field
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Template:Further
Lorentz transformations can also be used to illustrate that the magnetic field Template:Math and electric field Template:Math are simply different aspects of the same force — the electromagnetic force, as a consequence of relative motion between electric charges and observers.<ref>Template:Harvnb</ref> The fact that the electromagnetic field shows relativistic effects becomes clear by carrying out a simple thought experiment.<ref>Template:Harvnb</ref>
- An observer measures a charge at rest in frame Template:Mvar. The observer will detect a static electric field. As the charge is stationary in this frame, there is no electric current, so the observer does not observe any magnetic field.
- The other observer in frame Template:Mvar moves at velocity Template:Math relative to Template:Mvar and the charge. This observer sees a different electric field because the charge moves at velocity Template:Math in their rest frame. The motion of the charge corresponds to an electric current, and thus the observer in frame Template:Mvar also sees a magnetic field.
The electric and magnetic fields transform differently from space and time, but exactly the same way as relativistic angular momentum and the boost vector.
The electromagnetic field strength tensor is given by <math display="block">
F^{\mu\nu} = \begin{bmatrix}
0 & -\frac{1}{c}E_x & -\frac{1}{c}E_y & -\frac{1}{c}E_z \\
\frac{1}{c}E_x & 0 & -B_z & B_y \\
\frac{1}{c}E_y & B_z & 0 & -B_x \\
\frac{1}{c}E_z & -B_y & B_x & 0
\end{bmatrix} \text{(SI units, signature }(+,-,-,-)\text{)}.
</math> in SI units. In relativity, the Gaussian system of units is often preferred over SI units, even in texts whose main choice of units is SI units, because in it the electric field Template:Math and the magnetic induction Template:Math have the same units making the appearance of the electromagnetic field tensor more natural.<ref>Template:Harvnb</ref> Consider a Lorentz boost in the Template:Mvar-direction. It is given by<ref>Template:Harvnb</ref> <math display="block">
{\Lambda^\mu}_\nu = \begin{bmatrix}
\gamma & -\gamma\beta & 0 & 0\\
-\gamma\beta & \gamma & 0 & 0\\
0 & 0 & 1 & 0\\
0 & 0 & 0 & 1\\
\end{bmatrix}, \qquad
F^{\mu\nu} = \begin{bmatrix}
0 & E_x & E_y & E_z \\
-E_x & 0 & B_z & -B_y \\
-E_y & -B_z & 0 & B_x \\
-E_z & B_y & -B_x & 0
\end{bmatrix} \text{(Gaussian units, signature }(-,+,+,+)\text{)},
</math> where the field tensor is displayed side by side for easiest possible reference in the manipulations below.
The general transformation law Template:EquationNote becomes <math display="block">F^{\mu'\nu'} = {\Lambda^{\mu'}}_\mu {\Lambda^{\nu'}}_\nu F^{\mu\nu}.</math>
For the magnetic field one obtains <math display="block">\begin{align}
B_{x'} &= F^{2'3'}
= {\Lambda^2}_\mu {\Lambda^3}_\nu F^{\mu\nu}
= {\Lambda^2}_2 {\Lambda^3}_3 F^{23}
= 1 \times 1 \times B_x \\
&= B_x, \\
B_{y'} &= F^{3'1'}
= {\Lambda^3}_\mu {\Lambda^1}_\nu F^{\mu \nu}
= {\Lambda^3}_3 {\Lambda^1}_\nu F^{3\nu}
= {\Lambda^3}_3 {\Lambda^1}_0 F^{30} + {\Lambda^3}_3 {\Lambda^1}_1 F^{31} \\
&= 1 \times (-\beta\gamma) (-E_z) + 1 \times \gamma B_y
= \gamma B_y + \beta\gamma E_z \\
&= \gamma\left(\mathbf{B} - \boldsymbol{\beta} \times \mathbf{E}\right)_y \\
B_{z'} &= F^{1'2'}
= {\Lambda^1}_\mu {\Lambda^2}_\nu F^{\mu\nu}
= {\Lambda^1}_\mu {\Lambda^2}_2 F^{\mu 2}
= {\Lambda^1}_0 {\Lambda^2}_2 F^{02} + {\Lambda^1}_1 {\Lambda^2}_2 F^{12} \\
&= (-\gamma\beta) \times 1\times E_y + \gamma \times 1 \times B_z
= \gamma B_z - \beta\gamma E_y \\
&= \gamma\left(\mathbf{B} - \boldsymbol{\beta} \times \mathbf{E}\right)_z
\end{align}</math>
For the electric field results <math display="block">\begin{align}
E_{x'} &= F^{0'1'}
= {\Lambda^0}_\mu {\Lambda^1}_\nu F^{\mu\nu}
= {\Lambda^0}_1 {\Lambda^1}_0 F^{10} + {\Lambda^0}_0 {\Lambda^1}_1 F^{01} \\
&= (-\gamma\beta)(-\gamma\beta)(-E_x) + \gamma\gamma E_x
= -\gamma^2\beta^2(E_x) + \gamma^2 E_x
= E_x(1 - \beta^2)\gamma^2 \\
&= E_x, \\
E_{y'} &= F^{0'2'}
= {\Lambda^0}_\mu {\Lambda^2}_\nu F^{\mu\nu}
= {\Lambda^0}_\mu {\Lambda^2}_2 F^{\mu 2}
= {\Lambda^0}_0 {\Lambda^2}_2 F^{02} + {\Lambda^0}_1 {\Lambda^2}_2 F^{12} \\
&= \gamma \times 1 \times E_y + (-\beta\gamma) \times 1 \times B_z
= \gamma E_y - \beta\gamma B_z \\
&= \gamma\left(\mathbf{E} + \boldsymbol{\beta} \times \mathbf{B}\right)_y \\
E_{z'} &= F^{0'3'}
= {\Lambda^0}_\mu {\Lambda^3}_\nu F^{\mu\nu}
= {\Lambda^0}_\mu {\Lambda^3}_3 F^{\mu 3}
= {\Lambda^0}_0 {\Lambda^3}_3 F^{03} + {\Lambda^0}_1 {\Lambda^3}_3 F^{13} \\
&= \gamma \times 1 \times E_z - \beta\gamma \times 1 \times (-B_y)
= \gamma E_z + \beta\gamma B_y \\
&= \gamma\left(\mathbf{E} + \boldsymbol{\beta} \times \mathbf{B}\right)_z.
\end{align}</math>
Here, Template:Math is used. These results can be summarized by <math display="block">\begin{align}
\mathbf{E}_{\parallel'} &= \mathbf{E}_\parallel \\
\mathbf{B}_{\parallel'} &= \mathbf{B}_\parallel \\
\mathbf{E}_{\bot'} &= \gamma \left( \mathbf{E}_\bot + \boldsymbol{\beta} \times \mathbf{B}_\bot \right) = \gamma \left( \mathbf{E} + \boldsymbol{\beta} \times \mathbf{B} \right)_\bot,\\
\mathbf{B}_{\bot'} &= \gamma \left( \mathbf{B}_\bot - \boldsymbol{\beta} \times \mathbf{E}_\bot \right) = \gamma \left( \mathbf{B} - \boldsymbol{\beta} \times \mathbf{E} \right)_\bot,
\end{align}</math> and are independent of the metric signature. For SI units, substitute Template:Math. Template:Harvtxt refer to this last form as the Template:Math view as opposed to the geometric view represented by the tensor expression <math display="block">F^{\mu'\nu'} = {\Lambda^{\mu'}}_\mu {\Lambda^{\nu'}}_\nu F^{\mu\nu},</math> and make a strong point of the ease with which results that are difficult to achieve using the Template:Math view can be obtained and understood. Only objects that have well defined Lorentz transformation properties (in fact under any smooth coordinate transformation) are geometric objects. In the geometric view, the electromagnetic field is a six-dimensional geometric object in spacetime as opposed to two interdependent, but separate, 3-vector fields in space and time. The fields Template:Math (alone) and Template:Math (alone) do not have well defined Lorentz transformation properties. The mathematical underpinnings are equations Template:EquationNote and Template:EquationNote that immediately yield Template:EquationNote. One should note that the primed and unprimed tensors refer to the same event in spacetime. Thus the complete equation with spacetime dependence is <math display="block">
F^{\mu' \nu'}\left(x'\right) =
{\Lambda^{\mu'}}_\mu {\Lambda^{\nu'}}_\nu F^{\mu\nu}\left(\Lambda^{-1} x'\right) =
{\Lambda^{\mu'}}_\mu {\Lambda^{\nu'}}_\nu F^{\mu\nu}(x).
</math>
Length contraction has an effect on charge density Template:Mvar and current density Template:Math, and time dilation has an effect on the rate of flow of charge (current), so charge and current distributions must transform in a related way under a boost. It turns out they transform exactly like the space-time and energy-momentum four-vectors, <math display="block">\begin{align}
\mathbf{j}' &= \mathbf{j} - \gamma\rho v\mathbf{n} + \left( \gamma - 1 \right)(\mathbf{j} \cdot \mathbf{n})\mathbf{n} \\
\rho' &= \gamma \left(\rho - \mathbf{j} \cdot \frac{v\mathbf{n}}{c^2}\right),
\end{align}</math>
or, in the simpler geometric view, <math display="block">j^{\mu'} = {\Lambda^{\mu'}}_\mu j^\mu.</math>
Charge density transforms as the time component of a four-vector. It is a rotational scalar. The current density is a 3-vector.
The Maxwell equations are invariant under Lorentz transformations.
SpinorsEdit
Equation Template:EquationNote hold unmodified for any representation of the Lorentz group, including the bispinor representation. In Template:EquationNote one simply replaces all occurrences of Template:Math by the bispinor representation Template:Math,
The above equation could, for instance, be the transformation of a state in Fock space describing two free electrons.
Transformation of general fieldsEdit
A general noninteracting multi-particle state (Fock space state) in quantum field theory transforms according to the rule<ref>Template:Harvnb</ref> Template:NumBlk
\left( \sum_{\sigma_1'\sigma_2' \cdots} D_{\sigma_1'\sigma_1}^{(j_1)}\left[W(\Lambda, p_1)\right] D_{\sigma_2'\sigma_2}^{(j_2)}\left[W(\Lambda, p_2)\right] \cdots \right)
\Psi_{\Lambda p_1 \sigma_1' n_1; \Lambda p_2 \sigma_2' n_2; \cdots},
\end{align}</math>
| Template:EquationRef
}} where Template:Math is the Wigner's little group<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and Template:Math is the Template:Nowrap representation of Template:Math.
See alsoEdit
FootnotesEdit
NotesEdit
ReferencesEdit
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Further readingEdit
External linksEdit
Template:Wikisource portal Template:Sister project Template:Wikiversity
- Derivation of the Lorentz transformations. This web page contains a more detailed derivation of the Lorentz transformation with special emphasis on group properties.
- The Paradox of Special Relativity. This webpage poses a problem, the solution of which is the Lorentz transformation, which is presented graphically in its next page.
- Relativity Template:Webarchive – a chapter from an online textbook
- Warp Special Relativity Simulator. A computer program demonstrating the Lorentz transformations on everyday objects.
- Template:YouTube visualizing the Lorentz transformation.
- MinutePhysics video on YouTube explaining and visualizing the Lorentz transformation with a mechanical Minkowski diagram
- Interactive graph on Desmos (graphing) showing Lorentz transformations with a virtual Minkowski diagram
- Interactive graph on Desmos showing Lorentz transformations with points and hyperbolas
- Lorentz Frames Animated from John de Pillis. Online Flash animations of Galilean and Lorentz frames, various paradoxes, EM wave phenomena, etc.