Minkowski space explained

In physics, Minkowski space (or Minkowski spacetime) (^[1]) is the main mathematical description of spacetime in the absence of gravitation. It combines inertial space and time manifolds into a four-dimensional model.

The model helps show how a spacetime interval between any two events is independent of the inertial frame of reference in which they are recorded. Mathematician Hermann Minkowski developed it from the work of Hendrik Lorentz, Henri Poincaré, and others said it "was grown on experimental physical grounds".

Minkowski space is closely associated with Einstein's theories of special relativity and general relativity and is the most common mathematical structure by which special relativity is formalized. While the individual components in Euclidean space and time might differ due to length contraction and time dilation, in Minkowski spacetime, all frames of reference will agree on the total interval in spacetime between events.^[2] Minkowski space differs from four-dimensional Euclidean space insofar as it treats time differently than the three spatial dimensions.

In 3-dimensional Euclidean space, the isometry group (maps preserving the regular Euclidean distance) is the Euclidean group. It is generated by rotations, reflections and translations. When time is appended as a fourth dimension, the further transformations of translations in time and Lorentz boosts are added, and the group of all these transformations is called the Poincaré group. Minkowski's model follows special relativity, where motion causes time dilation changing the scale applied to the frame in motion and shifts the phase of light.

Spacetime is equipped with an indefinite non-degenerate bilinear form, called the Minkowski metric, the Minkowski norm squared or Minkowski inner product depending on the context.^[3] The Minkowski inner product is defined so as to yield the spacetime interval between two events when given their coordinate difference vector as an argument.^[4] Equipped with this inner product, the mathematical model of spacetime is called Minkowski space. The group of transformations for Minkowski space that preserves the spacetime interval (as opposed to the spatial Euclidean distance) is the Poincaré group (as opposed to the Galilean group).

History

Complex Minkowski spacetime

See also: Four-dimensional space. In his second relativity paper in 1905, Henri Poincaré showed^[5] how, by taking time to be an imaginary fourth spacetime coordinate, where is the speed of light and is the imaginary unit, Lorentz transformations can be visualized as ordinary rotations of the four-dimensional Euclidean sphere. The four-dimensional spacetime can be visualized as a four-dimensional space, with each point representing an event in spacetime. The Lorentz transformations can then be thought of as rotations in this four-dimensional space, where the rotation axis corresponds to the direction of relative motion between the two observers and the rotation angle is related to their relative velocity.

To understand this concept, one should consider the coordinates of an event in spacetime represented as a four-vector . A Lorentz transformation is represented by a matrix that acts on the four-vector, changing its components. This matrix can be thought of as a rotation matrix in four-dimensional space, which rotates the four-vector around a particular axis.$$x^2\; +\; y^2\; +\; z^2\; +\; (ict)^2\; =\; \backslash text.$$

Rotations in planes spanned by two space unit vectors appear in coordinate space as well as in physical spacetime as Euclidean rotations and are interpreted in the ordinary sense. The "rotation" in a plane spanned by a space unit vector and a time unit vector, while formally still a rotation in coordinate space, is a Lorentz boost in physical spacetime with real inertial coordinates. The analogy with Euclidean rotations is only partial since the radius of the sphere is actually imaginary, which turns rotations into rotations in hyperbolic space (see hyperbolic rotation).

This idea, which was mentioned only briefly by Poincaré, was elaborated by Minkowski in a paper in German published in 1908 called "The Fundamental Equations for Electromagnetic Processes in Moving Bodies".^[6] He reformulated Maxwell equations as a symmetrical set of equations in the four variables combined with redefined vector variables for electromagnetic quantities, and he was able to show directly and very simply their invariance under Lorentz transformation. He also made other important contributions and used matrix notation for the first time in this context.From his reformulation, he concluded that time and space should be treated equally, and so arose his concept of events taking place in a unified four-dimensional spacetime continuum.

Real Minkowski spacetime

In a further development in his 1908 "Space and Time" lecture,^[7] Minkowski gave an alternative formulation of this idea that used a real-time coordinate instead of an imaginary one, representing the four variables of space and time in the coordinate form in a four-dimensional real vector space. Points in this space correspond to events in spacetime. In this space, there is a defined light-cone associated with each point, and events not on the light cone are classified by their relation to the apex as spacelike or timelike. It is principally this view of spacetime that is current nowadays, although the older view involving imaginary time has also influenced special relativity.

In the English translation of Minkowski's paper, the Minkowski metric, as defined below, is referred to as the line element. The Minkowski inner product below appears unnamed when referring to orthogonality (which he calls normality) of certain vectors, and the Minkowski norm squared is referred to (somewhat cryptically, perhaps this is a translation dependent) as "sum".

Minkowski's principal tool is the Minkowski diagram, and he uses it to define concepts and demonstrate properties of Lorentz transformations (e.g., proper time and length contraction) and to provide geometrical interpretation to the generalization of Newtonian mechanics to relativistic mechanics. For these special topics, see the referenced articles, as the presentation below will be principally confined to the mathematical structure (Minkowski metric and from it derived quantities and the Poincaré group as symmetry group of spacetime) following from the invariance of the spacetime interval on the spacetime manifold as consequences of the postulates of special relativity, not to specific application or derivation of the invariance of the spacetime interval. This structure provides the background setting of all present relativistic theories, barring general relativity for which flat Minkowski spacetime still provides a springboard as curved spacetime is locally Lorentzian.

Minkowski, aware of the fundamental restatement of the theory which he had made, said

Though Minkowski took an important step for physics, Albert Einstein saw its limitation:

For further historical information see references, and .

Causal structure

See main article: Causal structure.

Where is velocity,,, and are Cartesian coordinates in 3-dimensional space, is the constant representing the universal speed limit, and is time, the four-dimensional vector is classified according to the sign of . A vector is timelike if, spacelike if, and null or lightlike if . This can be expressed in terms of the sign of, also called scalar product, as well, which depends on the signature. The classification of any vector will be the same in all frames of reference that are related by a Lorentz transformation (but not by a general Poincaré transformation because the origin may then be displaced) because of the invariance of the spacetime interval under Lorentz transformation.

The set of all null vectors at an event^[8] of Minkowski space constitutes the light cone of that event. Given a timelike vector, there is a worldline of constant velocity associated with it, represented by a straight line in a Minkowski diagram.

Once a direction of time is chosen,^[9] timelike and null vectors can be further decomposed into various classes. For timelike vectors, one has

1. future-directed timelike vectors whose first component is positive (tip of vector located in causal future (also called the absolute future) in the figure) and
2. past-directed timelike vectors whose first component is negative (causal past (also called the absolute past)).

Null vectors fall into three classes:

1. the zero vector, whose components in any basis are (origin),
2. future-directed null vectors whose first component is positive (upper light cone), and
3. past-directed null vectors whose first component is negative (lower light cone).

Together with spacelike vectors, there are 6 classes in all.

An orthonormal basis for Minkowski space necessarily consists of one timelike and three spacelike unit vectors. If one wishes to work with non-orthonormal bases, it is possible to have other combinations of vectors. For example, one can easily construct a (non-orthonormal) basis consisting entirely of null vectors, called a null basis.

Vector fields are called timelike, spacelike, or null if the associated vectors are timelike, spacelike, or null at each point where the field is defined.

Properties of time-like vectors

Time-like vectors have special importance in the theory of relativity as they correspond to events that are accessible to the observer at (0, 0, 0, 0) with a speed less than that of light. Of most interest are time-like vectors that are similarly directed, i.e. all either in the forward or in the backward cones. Such vectors have several properties not shared by space-like vectors. These arise because both forward and backward cones are convex, whereas the space-like region is not convex.

Scalar product

The scalar product of two time-like vectors and is$$\backslash eta\; (u\_1,\; u\_2)\; =\; u\_1\; \backslash cdot\; u\_2\; =\; c^2\; t\_1\; t\_2\; -\; x\_1\; x\_2\; -\; y\_1\; y\_2\; -\; z\_1\; z\_2\; .$$

Positivity of scalar product: An important property is that the scalar product of two similarly directed time-like vectors is always positive. This can be seen from the reversed Cauchy–Schwarz inequality below. It follows that if the scalar product of two vectors is zero, then one of these, at least, must be space-like. The scalar product of two space-like vectors can be positive or negative as can be seen by considering the product of two space-like vectors having orthogonal spatial components and times either of different or the same signs.

Using the positivity property of time-like vectors, it is easy to verify that a linear sum with positive coefficients of similarly directed time-like vectors is also similarly directed time-like (the sum remains within the light cone because of convexity).

Norm and reversed Cauchy inequality

The norm of a time-like vector is defined as$$\backslash left\backslash |\; u\; \backslash right\backslash |\; =\; \backslash sqrt\; =\; \backslash sqrt$$

The reversed Cauchy inequality is another consequence of the convexity of either light cone.^[10] For two distinct similarly directed time-like vectors and this inequality is$$\backslash eta(u\_1,\; u\_2)\; >\; \backslash left\backslash |\; u\_1\; \backslash right\backslash |\; \backslash left\backslash |\; u\_2\; \backslash right\backslash |$$or algebraically,$$c^2\; t\_1\; t\_2\; -\; x\_1\; x\_2\; -\; y\_1\; y\_2\; -\; z\_1\; z\_2\; >\; \backslash sqrt$$

From this, the positive property of the scalar product can be seen.

Reversed triangle inequality

For two similarly directed time-like vectors and, the inequality is^[11] $$\backslash left\backslash |\; u\; +\; w\; \backslash right\backslash |\; \backslash ge\; \backslash left\backslash |\; u\; \backslash right\backslash |\; +\; \backslash left\backslash |\; w\; \backslash right\backslash |,$$where the equality holds when the vectors are linearly dependent.

The proof uses the algebraic definition with the reversed Cauchy inequality:^[12]

The result now follows by taking the square root on both sides.

Mathematical structure

It is assumed below that spacetime is endowed with a coordinate system corresponding to an inertial frame. This provides an origin, which is necessary for spacetime to be modeled as a vector space. This addition is not required, and more complex treatments analogous to an affine space can remove the extra structure. However, this is not the introductory convention and is not covered here.

For an overview, Minkowski space is a -dimensional real vector space equipped with a non-degenerate, symmetric bilinear form on the tangent space at each point in spacetime, here simply called the Minkowski inner product, with metric signature either or . The tangent space at each event is a vector space of the same dimension as spacetime, .

Tangent vectors

In practice, one need not be concerned with the tangent spaces. The vector space structure of Minkowski space allows for the canonical identification of vectors in tangent spaces at points (events) with vectors (points, events) in Minkowski space itself. See e.g. or These identifications are routinely done in mathematics. They can be expressed formally in Cartesian coordinates aswith basis vectors in the tangent spaces defined by$$\backslash left.\backslash mathbf\; e\_\backslash mu\backslash right|\_p\; =\; \backslash left.\backslash frac\backslash right|\_p\; \backslash text\; \backslash mathbf\; e\_0|\_p\; =\; \backslash left(\backslash begin\; 1\; \backslash \backslash \; 0\; \backslash \backslash \; 0\; \backslash \backslash \; 0\backslash end\backslash right)\; \backslash text.$$

Here, and are any two events, and the second basis vector identification is referred to as parallel transport. The first identification is the canonical identification of vectors in the tangent space at any point with vectors in the space itself. The appearance of basis vectors in tangent spaces as first-order differential operators is due to this identification. It is motivated by the observation that a geometrical tangent vector can be associated in a one-to-one manner with a directional derivative operator on the set of smooth functions. This is promoted to a definition of tangent vectors in manifolds not necessarily being embedded in . This definition of tangent vectors is not the only possible one, as ordinary n-tuples can be used as well.

A tangent vector at a point may be defined, here specialized to Cartesian coordinates in Lorentz frames, as column vectors associated to each Lorentz frame related by Lorentz transformation such that the vector in a frame related to some frame by transforms according to . This is the same way in which the coordinates transform. Explicitly,

This definition is equivalent to the definition given above under a canonical isomorphism.

For some purposes, it is desirable to identify tangent vectors at a point with displacement vectors at, which is, of course, admissible by essentially the same canonical identification. The identifications of vectors referred to above in the mathematical setting can correspondingly be found in a more physical and explicitly geometrical setting in . They offer various degrees of sophistication (and rigor) depending on which part of the material one chooses to read.

Metric signature

The metric signature refers to which sign the Minkowski inner product yields when given space (spacelike to be specific, defined further down) and time basis vectors (timelike) as arguments. Further discussion about this theoretically inconsequential but practically necessary choice for purposes of internal consistency and convenience is deferred to the hide box below. See also the page treating sign convention in Relativity.

In general, but with several exceptions, mathematicians and general relativists prefer spacelike vectors to yield a positive sign,, while particle physicists tend to prefer timelike vectors to yield a positive sign, . Authors covering several areas of physics, e.g. Steven Weinberg and Landau and Lifshitz (and respectively) stick to one choice regardless of topic. Arguments for the former convention include "continuity" from the Euclidean case corresponding to the non-relativistic limit . Arguments for the latter include that minus signs, otherwise ubiquitous in particle physics, go away. Yet other authors, especially of introductory texts, e.g., do not choose a signature at all, but instead, opt to coordinatize spacetime such that the time coordinate (but not time itself!) is imaginary. This removes the need for the explicit introduction of a metric tensor (which may seem like an extra burden in an introductory course), and one needs not be concerned with covariant vectors and contravariant vectors (or raising and lowering indices) to be described below. The inner product is instead affected by a straightforward extension of the dot product in to . This works in the flat spacetime of special relativity, but not in the curved spacetime of general relativity, see (who, by the way use). MTW also argues that it hides the true indefinite nature of the metric and the true nature of Lorentz boosts, which are not rotations. It also needlessly complicates the use of tools of differential geometry that are otherwise immediately available and useful for geometrical description and calculation  - even in the flat spacetime of special relativity, e.g. of the electromagnetic field.

Terminology

Mathematically associated with the bilinear form is a tensor of type at each point in spacetime, called the Minkowski metric.^[13] The Minkowski metric, the bilinear form, and the Minkowski inner product are all the same object; it is a bilinear function that accepts two (contravariant) vectors and returns a real number. In coordinates, this is the matrix representing the bilinear form.

For comparison, in general relativity, a Lorentzian manifold is likewise equipped with a metric tensor, which is a nondegenerate symmetric bilinear form on the tangent space at each point of . In coordinates, it may be represented by a matrix depending on spacetime position. Minkowski space is thus a comparatively simple special case of a Lorentzian manifold. Its metric tensor is in coordinates with the same symmetric matrix at every point of, and its arguments can, per above, be taken as vectors in spacetime itself.

Introducing more terminology (but not more structure), Minkowski space is thus a pseudo-Euclidean space with total dimension and signature or . Elements of Minkowski space are called events. Minkowski space is often denoted or to emphasize the chosen signature, or just . It is an example of a pseudo-Riemannian manifold.

Then mathematically, the metric is a bilinear form on an abstract four-dimensional real vector space, that is,$$\backslash eta:V\backslash times\; V\backslash rightarrow\; \backslash mathbf$$where has signature, and signature is a coordinate-invariant property of . The space of bilinear maps forms a vector space which can be identified with

M ^{* ⊗} M ^{*} , and may be equivalently viewed as an element of this space. By making a choice of orthonormal basis \{e _{\mu\}} , M: = (V, η ) can be identified with the space R ^{1,3} : = ( R ^{4} , η _{\mu\nu} ) . The notation is meant to emphasize the fact that and R ^{1,3} are not just vector spaces but have added structure. η _{\mu\nu} = diag ( -1 , +1 , +1 , +1 ) .

An interesting example of non-inertial coordinates for (part of) Minkowski spacetime is the Born coordinates. Another useful set of coordinates is the light-cone coordinates.

Pseudo-Euclidean metrics

See main article: Pseudo-Euclidean space and Lorentzian manifolds. The Minkowski inner product is not an inner product, since it is not positive-definite, i.e. the quadratic form need not be positive for nonzero . The positive-definite condition has been replaced by the weaker condition of non-degeneracy. The bilinear form is said to be indefinite.The Minkowski metric is the metric tensor of Minkowski space. It is a pseudo-Euclidean metric, or more generally, a constant pseudo-Riemannian metric in Cartesian coordinates. As such, it is a nondegenerate symmetric bilinear form, a type tensor. It accepts two arguments, vectors in, the tangent space at in . Due to the above-mentioned canonical identification of with itself, it accepts arguments with both and in .

As a notational convention, vectors in, called 4-vectors, are denoted in italics, and not, as is common in the Euclidean setting, with boldface . The latter is generally reserved for the -vector part (to be introduced below) of a -vector.

The definition ^[14] $$u\; \backslash cdot\; v\; =\; \backslash eta(u,\backslash ,\; v)$$yields an inner product-like structure on, previously and also henceforth, called the Minkowski inner product, similar to the Euclidean inner product, but it describes a different geometry. It is also called the relativistic dot product. If the two arguments are the same,$$u\; \backslash cdot\; u\; =\; \backslash eta(u,\; u)\; \backslash equiv\; \backslash |u\backslash |^2\; \backslash equiv\; u^2,$$the resulting quantity will be called the Minkowski norm squared. The Minkowski inner product satisfies the following properties.

Linearity in the first argument

η (au + v, w) = a η (u, w) + η (v, w), \forall u, v \in M, \forall a \in \R

Symmetry

η (u, v) = η (v, u)

Non-degeneracy

η (u, v) = 0 , \forall v \in M ⇒ u = 0

The first two conditions imply bilinearity. The defining difference between a pseudo-inner product and an inner product proper is that the former is not required to be positive definite, that is, is allowed.

The most important feature of the inner product and norm squared is that these are quantities unaffected by Lorentz transformations. In fact, it can be taken as the defining property of a Lorentz transformation in that it preserves the inner product (i.e. the value of the corresponding bilinear form on two vectors). This approach is taken more generally for all classical groups definable this way in classical group. There, the matrix is identical in the case (the Lorentz group) to the matrix to be displayed below.

Two vectors and are said to be orthogonal if . For a geometric interpretation of orthogonality in the special case, when and (or vice versa), see hyperbolic orthogonality.

A vector is called a unit vector if . A basis for consisting of mutually orthogonal unit vectors is called an orthonormal basis.^[15]

For a given inertial frame, an orthonormal basis in space, combined with the unit time vector, forms an orthonormal basis in Minkowski space. The number of positive and negative unit vectors in any such basis is a fixed pair of numbers equal to the signature of the bilinear form associated with the inner product. This is Sylvester's law of inertia.

More terminology (but not more structure): The Minkowski metric is a pseudo-Riemannian metric, more specifically, a Lorentzian metric, even more specifically, the Lorentz metric, reserved for -dimensional flat spacetime with the remaining ambiguity only being the signature convention.

Minkowski metric

From the second postulate of special relativity, together with homogeneity of spacetime and isotropy of space, it follows that the spacetime interval between two arbitrary events called and is:^[16] $$c^2\backslash left(t\_1\; -\; t\_2\backslash right)^2\; -\; \backslash left(x\_1\; -\; x\_2\backslash right)^2\; -\; \backslash left(y\_1\; -\; y\_2\backslash right)^2\; -\; \backslash left(z\_1\; -\; z\_2\backslash right)^2.$$This quantity is not consistently named in the literature. The interval is sometimes referred to as the square root of the interval as defined here.^[17]

The invariance of the interval under coordinate transformations between inertial frames follows from the invariance of$$c^2\; t^2\; -\; x^2\; -\; y^2\; -\; z^2$$provided the transformations are linear. This quadratic form can be used to define a bilinear form$$u\; \backslash cdot\; v\; =\; c^2\; t\_1\; t\_2\; -\; x\_1\; x\_2\; -\; y\_1\; y\_2\; -\; z\_1\; z\_2$$via the polarization identity. This bilinear form can in turn be written as$$u\; \backslash cdot\; v\; =\; u^\backslash textsf\; \backslash ,\; [\backslash eta]\; \backslash ,\; v,$$where is a

4 x 4 matrix associated with . While possibly confusing, it is common practice to denote with just . The matrix is read off from the explicit bilinear form asand the bilinear form$$u\; \backslash cdot\; v\; =\; \backslash eta(u,\; v),$$with which this section started by assuming its existence, is now identified.

For definiteness and shorter presentation, the signature is adopted below. This choice (or the other possible choice) has no (known) physical implications. The symmetry group preserving the bilinear form with one choice of signature is isomorphic (under the map given here) with the symmetry group preserving the other choice of signature. This means that both choices are in accord with the two postulates of relativity. Switching between the two conventions is straightforward. If the metric tensor has been used in a derivation, go back to the earliest point where it was used, substitute for, and retrace forward to the desired formula with the desired metric signature.

Standard basis

A standard or orthonormal basis for Minkowski space is a set of four mutually orthogonal vectors such that$$-\backslash eta(e\_0,\; e\_0)\; =\; \backslash eta(e\_1,\; e\_1)\; =\; \backslash eta(e\_2,\; e\_2)\; =\; \backslash eta(e\_3,\; e\_3)\; =\; 1$$and for which $$\backslash eta(e\_\backslash mu,\; e\_\backslash nu)\; =\; 0$$ when $\backslash mu\; \backslash neq\; \backslash nu\backslash ,.$

These conditions can be written compactly in the form$$\backslash eta(e\_\backslash mu,\; e\_\backslash nu)\; =\; \backslash eta\_.$$

Relative to a standard basis, the components of a vector are written where the Einstein notation is used to write . The component is called the timelike component of while the other three components are called the spatial components. The spatial components of a -vector may be identified with a -vector .

In terms of components, the Minkowski inner product between two vectors and is given by

$$\backslash eta(v,\; w)\; =\; \backslash eta\_\; v^\backslash mu\; w^\backslash nu\; =\; v^0\; w\_0\; +\; v^1\; w\_1\; +\; v^2\; w\_2\; +\; v^3\; w\_3\; =\; v^\backslash mu\; w\_\backslash mu\; =\; v\_\backslash mu\; w^\backslash mu,$$and$$\backslash eta(v,\; v)\; =\; \backslash eta\_\; v^\backslash mu\; v^\backslash nu\; =\; v^0v\_0\; +\; v^1\; v\_1\; +\; v^2\; v\_2\; +\; v^3\; v\_3\; =\; v^\backslash mu\; v\_\backslash mu.$$

Here lowering of an index with the metric was used.

There are many possible choices of standard basis obeying the condition

η (e _{\mu,} e _{\nu)} = η _{\mu} . Any two such bases are related in some sense by a Lorentz transformation, either by a change-of-basis matrix Λ ^{\mu} _{\nu} , a real matrix satisfying$$\backslash Lambda^\backslash mu\_\backslash rho\backslash eta\_\backslash Lambda^\backslash nu\_\backslash sigma\; =\; \backslash eta\_.$$or, a linear map on the abstract vector space satisfying, for any pair of vectors,,$$\backslash eta(\backslash Lambda\; u,\; \backslash Lambda\; v)\; =\; \backslash eta(u,\; v).$$

Then if two different bases exist, and,

e _{\mu'} = e ^{\nu} _{\mu} can be represented as e _{\mu'} = e ^{\nu} _{\mu} or e _{\mu'} = Λ e _{\mu} . While it might be tempting to think of Λ ^{\mu} _{\nu} and as the same thing, mathematically, they are elements of different spaces, and act on the space of standard bases from different sides.

Raising and lowering of indices

See main article: Raising and lowering indices and tensor contraction. Technically, a non-degenerate bilinear form provides a map between a vector space and its dual; in this context, the map is between the tangent spaces of and the cotangent spaces of . At a point in, the tangent and cotangent spaces are dual vector spaces (so the dimension of the cotangent space at an event is also). Just as an authentic inner product on a vector space with one argument fixed, by Riesz representation theorem, may be expressed as the action of a linear functional on the vector space, the same holds for the Minkowski inner product of Minkowski space.^[18]

Thus if are the components of a vector in tangent space, then are the components of a vector in the cotangent space (a linear functional). Due to the identification of vectors in tangent spaces with vectors in itself, this is mostly ignored, and vectors with lower indices are referred to as covariant vectors. In this latter interpretation, the covariant vectors are (almost always implicitly) identified with vectors (linear functionals) in the dual of Minkowski space. The ones with upper indices are contravariant vectors. In the same fashion, the inverse of the map from tangent to cotangent spaces, explicitly given by the inverse of in matrix representation, can be used to define raising of an index. The components of this inverse are denoted . It happens that . These maps between a vector space and its dual can be denoted (eta-flat) and (eta-sharp) by the musical analogy.

Contravariant and covariant vectors are geometrically very different objects. The first can and should be thought of as arrows. A linear function can be characterized by two objects: its kernel, which is a hyperplane passing through the origin, and its norm. Geometrically thus, covariant vectors should be viewed as a set of hyperplanes, with spacing depending on the norm (bigger = smaller spacing), with one of them (the kernel) passing through the origin. The mathematical term for a covariant vector is 1-covector or 1-form (though the latter is usually reserved for covector fields).

One quantum mechanical analogy explored in the literature is that of a de Broglie wave (scaled by a factor of Planck's reduced constant) associated with a momentum four-vector to illustrate how one could imagine a covariant version of a contravariant vector. The inner product of two contravariant vectors could equally well be thought of as the action of the covariant version of one of them on the contravariant version of the other. The inner product is then how many times the arrow pierces the planes. The mathematical reference,, offers the same geometrical view of these objects (but mentions no piercing).

The electromagnetic field tensor is a differential 2-form, which geometrical description can as well be found in MTW.

One may, of course, ignore geometrical views altogether (as is the style in e.g. and) and proceed algebraically in a purely formal fashion. The time-proven robustness of the formalism itself, sometimes referred to as index gymnastics, ensures that moving vectors around and changing from contravariant to covariant vectors and vice versa (as well as higher order tensors) is mathematically sound. Incorrect expressions tend to reveal themselves quickly.

Coordinate free raising and lowering

Given a bilinear form

η :M x M → R , the lowered version of a vector can be thought of as the partial evaluation of η , that is, there is an associated partial evaluation map$$\backslash eta(\backslash cdot,\; -):M\backslash rightarrow\; M^*;\; v\; \backslash mapsto\; \backslash eta(v,\backslash cdot).$$

The lowered vector

η (v, ⋅ )\in M ^{*} is then the dual map u\mapsto η (v,u) . Note it does not matter which argument is partially evaluated due to the symmetry of η .

Non-degeneracy is then equivalent to injectivity of the partial evaluation map, or equivalently non-degeneracy indicates that the kernel of the map is trivial. In finite dimension, as is the case here, and noting that the dimension of a finite-dimensional space is equal to the dimension of the dual, this is enough to conclude the partial evaluation map is a linear isomorphism from

M to M ^{*} . This then allows the definition of the inverse partial evaluation map,$$\backslash eta^:M^*\backslash rightarrow\; M,$$which allows the inverse metric to be defined as$$\backslash eta^:M^*\backslash times\; M^*\; \backslash rightarrow\; \backslash mathbf,\; \backslash eta^(\backslash alpha,\backslash beta)\; =\; \backslash eta(\backslash eta^(\backslash alpha),\backslash eta^(\backslash beta))$$where the two different usages of η ^{-1} can be told apart by the argument each is evaluated on. This can then be used to raise indices. If a coordinate basis is used, the metric is indeed the matrix inverse to .

Formalism of the Minkowski metric

The present purpose is to show semi-rigorously how formally one may apply the Minkowski metric to two vectors and obtain a real number, i.e. to display the role of the differentials and how they disappear in a calculation. The setting is that of smooth manifold theory, and concepts such as convector fields and exterior derivatives are introduced.

A full-blown version of the Minkowski metric in coordinates as a tensor field on spacetime has the appearance$$\backslash eta\_\; dx^\backslash mu\; \backslash otimes\; dx^\backslash nu\; =\; \backslash eta\_\; dx^\backslash mu\; \backslash odot\; dx^\backslash nu\; =\; \backslash eta\_\; dx^\backslash mu\; dx^\backslash nu.$$

Explanation: The coordinate differentials are 1-form fields. They are defined as the exterior derivative of the coordinate functions . These quantities evaluated at a point provide a basis for the cotangent space at . The tensor product (denoted by the symbol) yields a tensor field of type, i.e. the type that expects two contravariant vectors as arguments. On the right-hand side, the symmetric product (denoted by the symbol or by juxtaposition) has been taken. The equality holds since, by definition, the Minkowski metric is symmetric. The notation on the far right is also sometimes used for the related, but different, line element. It is not a tensor. For elaboration on the differences and similarities, see

Tangent vectors are, in this formalism, given in terms of a basis of differential operators of the first order,$$\backslash left.\backslash frac\backslash right|\_p,$$where is an event. This operator applied to a function gives the directional derivative of at in the direction of increasing with fixed. They provide a basis for the tangent space at .

The exterior derivative of a function is a covector field, i.e. an assignment of a cotangent vector to each point, by definition such that$$df(X)\; =\; Xf,$$for each vector field . A vector field is an assignment of a tangent vector to each point . In coordinates can be expanded at each point in the basis given by the . Applying this with, the coordinate function itself, and, called a coordinate vector field, one obtains$$dx^\backslash mu\backslash left(\backslash frac\backslash right)\; =\; \backslash frac\; =\; \backslash delta\_\backslash nu^\backslash mu.$$

Since this relation holds at each point, the provide a basis for the cotangent space at each and the bases and are dual to each other,$$\backslash left.\; dx^\backslash mu\; \backslash right|\_p\; \backslash left(\backslash left.\backslash frac\backslash right|\_p\backslash right)\; =\; \backslash delta^\backslash mu\_\backslash nu.$$at each . Furthermore, one has$$\backslash alpha\; \backslash otimes\; \backslash beta(a,\; b)\; =\; \backslash alpha(a)\backslash beta(b)$$for general one-forms on a tangent space and general tangent vectors . (This can be taken as a definition, but may also be proved in a more general setting.)

Thus when the metric tensor is fed two vectors fields,, both expanded in terms of the basis coordinate vector fields, the result is$$\backslash eta\_\; dx^\backslash mu\; \backslash otimes\; dx^\backslash nu(a,\; b)\; =\; \backslash eta\_\; a^\backslash mu\; b^\backslash nu,$$where, are the component functions of the vector fields. The above equation holds at each point, and the relation may as well be interpreted as the Minkowski metric at applied to two tangent vectors at .

As mentioned, in a vector space, such as modeling the spacetime of special relativity, tangent vectors can be canonically identified with vectors in the space itself, and vice versa. This means that the tangent spaces at each point are canonically identified with each other and with the vector space itself. This explains how the right-hand side of the above equation can be employed directly, without regard to the spacetime point the metric is to be evaluated and from where (which tangent space) the vectors come from.

This situation changes in general relativity. There one has$$g(p)\_\; \backslash left.\; dx^\backslash mu\; \backslash right|\_p\; \backslash left.\; dx^\backslash nu\; \backslash right|\_p(a,\; b)\; =\; g(p)\_\; a^\backslash mu\; b^\backslash nu,$$where now, i.e., is still a metric tensor but now depending on spacetime and is a solution of Einstein's field equations. Moreover, must be tangent vectors at spacetime point and can no longer be moved around freely.

Chronological and causality relations

Let . Here,

1. chronologically precedes if is future-directed timelike. This relation has the transitive property and so can be written .
2. causally precedes if is future-directed null or future-directed timelike. It gives a partial ordering of spacetime and so can be written .

Suppose is timelike. Then the simultaneous hyperplane for is . Since this hyperplane varies as varies, there is a relativity of simultaneity in Minkowski space.

Generalizations

See main article: Super Minkowski space. A Lorentzian manifold is a generalization of Minkowski space in two ways. The total number of spacetime dimensions is not restricted to be (or more) and a Lorentzian manifold need not be flat, i.e. it allows for curvature.

Complexified Minkowski space

Complexified Minkowski space is defined as .^[19] Its real part is the Minkowski space of four-vectors, such as the four-velocity and the four-momentum, which are independent of the choice of orientation of the space. The imaginary part, on the other hand, may consist of four pseudovectors, such as angular velocity and magnetic moment, which change their direction with a change of orientation. A pseudoscalar is introduced, which also changes sign with a change of orientation. Thus, elements of are independent of the choice of the orientation.

The inner product-like structure on is defined as for any . A relativistic pure spin of an electron or any half spin particle is described by as, where is the four-velocity of the particle, satisfying and is the 4D spin vector,^[20] which is also the Pauli–Lubanski pseudovector satisfying and .

Generalized Minkowski space

Minkowski space refers to a mathematical formulation in four dimensions. However, the mathematics can easily be extended or simplified to create an analogous generalized Minkowski space in any number of dimensions. If, -dimensional Minkowski space is a vector space of real dimension on which there is a constant Minkowski metric of signature or . These generalizations are used in theories where spacetime is assumed to have more or less than dimensions. String theory and M-theory are two examples where . In string theory, there appears conformal field theories with spacetime dimensions.

de Sitter space can be formulated as a submanifold of generalized Minkowski space as can the model spaces of hyperbolic geometry (see below).

Curvature

As a flat spacetime, the three spatial components of Minkowski spacetime always obey the Pythagorean Theorem. Minkowski space is a suitable basis for special relativity, a good description of physical systems over finite distances in systems without significant gravitation. However, in order to take gravity into account, physicists use the theory of general relativity, which is formulated in the mathematics of a non-Euclidean geometry. When this geometry is used as a model of physical space, it is known as curved space.

Even in curved space, Minkowski space is still a good description in an infinitesimal region surrounding any point (barring gravitational singularities).^[21] More abstractly, it can be said that in the presence of gravity spacetime is described by a curved 4-dimensional manifold for which the tangent space to any point is a 4-dimensional Minkowski space. Thus, the structure of Minkowski space is still essential in the description of general relativity.

Geometry

See main article: Hyperboloid model. The meaning of the term geometry for the Minkowski space depends heavily on the context. Minkowski space is not endowed with Euclidean geometry, and not with any of the generalized Riemannian geometries with intrinsic curvature, those exposed by the model spaces in hyperbolic geometry (negative curvature) and the geometry modeled by the sphere (positive curvature). The reason is the indefiniteness of the Minkowski metric. Minkowski space is, in particular, not a metric space and not a Riemannian manifold with a Riemannian metric. However, Minkowski space contains submanifolds endowed with a Riemannian metric yielding hyperbolic geometry.

Model spaces of hyperbolic geometry of low dimension, say 2 or 3, cannot be isometrically embedded in Euclidean space with one more dimension, i.e.

R ^{3} or R ^{4} respectively, with the Euclidean metric \overline{g} , preventing easy visualization.^[22] By comparison, model spaces with positive curvature are just spheres in Euclidean space of one higher dimension. Hyperbolic spaces can be isometrically embedded in spaces of one more dimension when the embedding space is endowed with the Minkowski metric η .

Define

H ^{1 (n)} _{R} \subset M ^{n+1} to be the upper sheet (ct > 0 ) of the hyperboloid$$\backslash mathbf\; H\_R^\; =\; \backslash left\backslash$$in generalized Minkowski space M ^{n+1} of spacetime dimension n + 1 . This is one of the surfaces of transitivity of the generalized Lorentz group. The induced metric on this submanifold,$$h\_R^\; =\; \backslash iota^*\; \backslash eta,$$the pullback of the Minkowski metric η under inclusion, is a Riemannian metric. With this metric H ^{1 (n)} _{R} is a Riemannian manifold. It is one of the model spaces of Riemannian geometry, the hyperboloid model of hyperbolic space. It is a space of constant negative curvature -1 /R ^{2} . The 1 in the upper index refers to an enumeration of the different model spaces of hyperbolic geometry, and the for its dimension. A 2 ( 2 ) corresponds to the Poincaré disk model, while 3 (n) corresponds to the Poincaré half-space model of dimension n.

Preliminaries

In the definition above

\iota: H ^{1 (n)} _{R} → M ^{n+1} is the inclusion map and the superscript star denotes the pullback. The present purpose is to describe this and similar operations as a preparation for the actual demonstration that H ^{1 (n)} _{R} actually is a hyperbolic space.

Hyperbolic stereographic projection

In order to exhibit the metric, it is necessary to pull it back via a suitable parametrization. A parametrization of a submanifold of a manifold is a map whose range is an open subset of . If has the same dimension as, a parametrization is just the inverse of a coordinate map . The parametrization to be used is the inverse of hyperbolic stereographic projection. This is illustrated in the figure to the right for . It is instructive to compare to stereographic projection for spheres.

Stereographic projection and its inverse are given bywhere, for simplicity, . The are coordinates on and the are coordinates on .

Pulling back the metric

One has$$h\_R^\; =\; \backslash eta|\_\; =\; \backslash left(dx^1\backslash right)^2\; +\; \backslash cdots\; +\; \backslash left(dx^n\backslash right)^2\; -\; d\backslash tau^2$$and the map$$\backslash sigma^:\backslash mathbf^n\; \backslash rightarrow\; \backslash mathbf\_R^;\backslash quad\; \backslash sigma^(\backslash mathbf)\; =\; (\backslash tau(\backslash mathbf),\backslash ,\; \backslash mathbf(\backslash mathbf))\; =\; \backslash left(R\backslash frac,\backslash ,\; \backslash frac\backslash right).$$

The pulled back metric can be obtained by straightforward methods of calculus;$$\backslash left.\backslash left(\backslash sigma^\backslash right)^*\; \backslash eta\backslash right|\_\; =\; \backslash left(dx^1(\backslash mathbf\; u)\backslash right)^2\; +\; \backslash cdots\; +\; \backslash left(dx^n(\backslash mathbf\; u)\backslash right)^2\; -\; \backslash left(d\backslash tau(\backslash mathbf\; u)\backslash right)^2.$$

One computes according to the standard rules for computing differentials (though one is really computing the rigorously defined exterior derivatives),and substitutes the results into the right hand side. This yields$$\backslash left(\backslash sigma^\backslash right)^*\; h\_R^\; =\; \backslash frac\; \backslash equiv\; h\_R^.$$

This last equation shows that the metric on the ball is identical to the Riemannian metric in the Poincaré ball model, another standard model of hyperbolic geometry.

See also

• Hyperbolic quaternion
• Hyperspace
• Introduction to the mathematics of general relativity
• Minkowski plane

References

• Corry. L.. Hermann Minkowski and the postulate of relativity. Arch. Hist. Exact Sci.. 51. 4. 1997. 10.1007/BF00518231. 273 - 314. 27016039. 0003-9519.
• Book: Catoni. F.. The Mathematics of Minkowski Space-Time . 2008. Frontiers in Mathematics. Birkhäuser Verlag. 1660-8046. Basel. 978-3-7643-8613-9. 10.1007/978-3-7643-8614-6. etal.
• Book: Galison, P. L.. Minkowski's Space–Time: from visual thinking to the absolute world. Historical Studies in the Physical Sciences. 10. 10.2307/27757388. R McCormach. Johns Hopkins University Press. 1979. 85 - 121. 27757388. etal.
• Giulini D The rich structure of Minkowski space, https://arxiv.org/abs/0802.4345v1.
• Book: D.. Kleppner. Daniel Kleppner. R. J.. Kolenkow. Robert J. Kolenkow. An Introduction to Mechanics. 1978. 1973. 978-0-07-035048-9. McGraw-Hill. London.
• Book: Landau. L.D.. Lev Landau. Lifshitz. E.M.. Evgeny Lifshitz. The Classical Theory of Fields. Course of Theoretical Physics. 2. 4th. Butterworth - Heinemann. 0-7506-2768-9. 2002. 1939.
• Book: Lee, J. M.. Introduction to Smooth manifolds. 2003. Springer Graduate Texts in Mathematics. 978-0-387-95448-6. 218.
• Book: Lee, J. M.. Introduction to Smooth manifolds. 2012. Springer Graduate Texts in Mathematics. 978-1-4419-9981-8.
• Book: Lee, J. M.. Riemannian Manifolds  - An Introduction to Curvature. 1997. Springer Verlag. New York · Berlin · Heidelberg. Springer Graduate Texts in Mathematics. 176. 978-0-387-98322-6.
  • • Published translation: Carus. Edward H.. Space and Time. The Monist. 1918. 28. 288. 288–302. 10.5840/monist19182826 .
  • Wikisource translation: The Fundamental Equations for Electromagnetic Processes in Moving Bodies
• Various English translations on Wikisource: Space and Time.
• .
• Book: Naber, G. L.. The Geometry of Minkowski Spacetime. 1992. Springer-Verlag. New York. 978-0-387-97848-2.
• J.. Nash. John Forbes Nash. The Imbedding Problem for Riemannian Manifolds. Annals of Mathematics. 1956. 63. 1. 20 - 63. 10.2307/1969989. 0075639. 1969989.
• Book: Penrose, Roger. Roger Penrose. 2005. Road to Reality : A Complete Guide to the Laws of the Universe. 18 Minkowskian geometry. Alfred A. Knopf. 9780679454434.
• Wikisource translation: On the Dynamics of the Electron
• Robb A A: Optical Geometry of Motion; a New View of the Theory of Relativity Cambridge 1911, (Heffers). http://www.archive.org/details/opticalgeometryoOOrobbrich.
• Robb A A: Geometry of Time and Space, 1936 Cambridge Univ Press http://www.archive.org/details/geometryoftimean032218mbp.
• Book: Sard, R. D.. Relativistic Mechanics - Special Relativity and Classical Particle Dynamics. registration. 1970. W. A. Benjamin. New York. 978-0805384918.
• Book: Shaw, R.. 1982. Linear Algebra and Group Representations. registration. § 6.6 Minkowski space, § 6.7,8 Canonical forms pp 221 - 242. Academic Press. 978-0-12-639201-2.
• Book: Walter, Scott A.. The Expanding Worlds of General Relativity . Minkowski, Mathematicians, and the Mathematical Theory of Relativity . Goenner, Hubert. 1999 . Birkhäuser . Boston . 978-0-8176-4060-6 . 45 - 86 . http://scottwalter.free.fr/papers/1999-mmm-walter.html . etal.
• .

External links

• visualizing Minkowski space in the context of special relativity.
• The Geometry of Special Relativity: The Minkowski Space – Time Light Cone
• Minkowski space at PhilPapers

Notes and References

1. http://www.dictionary.com/browse/minkowski"Minkowski"
2. This makes spacetime distance an invariant.
3. Consistent use of the terms "Minkowski inner product", "Minkowski norm" or "Minkowski metric" is intended for the bilinear form here, since it is in widespread use. It is by no means "standard" in the literature, but no standard terminology seems to exist.
4. Book: Independent Axioms for Minkowski Space–Time . illustrated . John W. . Schutz . CRC Press . 1977 . 978-0-582-31760-4 . 184–185 . Extract of page 184
5. Wikisource translation: On the Dynamics of the Electron
6. *Wikisource translation:
7. Various English translations on Wikisource: "Space and Time"
8. Translate the coordinate system so that the event is the new origin.
9. This corresponds to the time coordinate either increasing or decreasing when the proper time for any particle increases. An application of flips this direction.
10. See Schutz's proof p 148, also Naber p. 48
11. Schutz p. 148, Naber p. 49
12. Schutz p. 148
13. For comparison and motivation of terminology, take a Riemannian metric, which provides a positive definite symmetric bilinear form, i. e. an inner product proper at each point on a manifold.
14. Giulini 2008 pp. 5, 6
15. Book: The Geometry of Minkowski Spacetime: An Introduction to the Mathematics of the Special Theory of Relativity . Gregory L. Naber . illustrated . Courier Corporation . 2003 . 978-0-486-43235-9 . 8 . 2022-12-26 . 2022-12-26 . https://web.archive.org/web/20221226231018/https://books.google.com/books?id=pNfRHzwdVZ0C . live . Extract of page 8
16. Book: Spacetime and Geometry . Sean M. Carroll . illustrated, herdruk . Cambridge University Press . 2019 . 978-1-108-48839-6 . 7 .
17. Minkowski,
18. . One point in Lee's proof of the existence of this map needs modification (Lee deals with Riemannian metrics.). Where Lee refers to positive definiteness to show the injectivity of the map, one needs instead appeal to non-degeneracy.
19. Y. Friedman, A Physically Meaningful Relativistic Description of the Spin State of an Electron, Symmetry 2021, 13(10), 1853; https://doi.org/10.3390/sym13101853
20. Jackson, J.D., Classical Electrodynamics, 3rd ed.; John Wiley \& Sons: Hoboken, NJ, US, 1998
21. This similarity between flat space and curved space at infinitesimally small distance scales is foundational to the definition of a manifold in general.
22. There is an isometric embedding into according to the Nash embedding theorem, but the embedding dimension is much higher, for a Riemannian manifold of dimension .