In special relativity and in quantum mechanics, the four-gradient is used to define the properties and relations between the various physical four-vectors and tensors.
There are alternate ways of writing four-vector expressions in physics:
is a four-vector style, which is typically more compact and can use vector notation, (such as the inner product "dot"), always using bold uppercase to represent the four-vector, and bold lowercase to represent 3-space vectors, e.g. . Most of the 3-space vector rules have analogues in four-vector mathematics.
is a Ricci calculus style, which uses tensor index notation and is useful for more complicated expressions, especially those involving tensors with more than one index, such as .
The Latin tensor index ranges in {1, 2, 3}, and represents a 3-space vector, e.g. .
The Greek tensor index ranges in {0, 1, 2, 3}, and represents a 4-vector, e.g. .
In SR physics, one typically uses a concise blend, e.g. , where represents the temporal component and represents the spatial 3-component.
Tensors in SR are typically 4D -tensors, with upper indices and lower indices, with the 4D indicating 4 dimensions = the number of values each index can take.
The tensor contraction used in the Minkowski metric can go to either side (see Einstein notation):[1]: 56, 151–152, 158–161
The strong equivalence principle can be stated as:[4]: 184
"Any physical law which can be expressed in tensor notation in SR has exactly the same form in a locally inertial frame of a curved spacetime." The 4-gradient commas (,) in SR are simply changed to covariant derivative semi-colons (;) in GR, with the connection between the two using Christoffel symbols. This is known in relativity physics as the "comma to semi-colon rule".
So, for example, if in SR, then in GR.
On a (1,0)-tensor or 4-vector this would be:[4]: 136–139
On a (2,0)-tensor this would be:
Usage[]
The 4-gradient is used in a number of different ways in special relativity (SR):
Throughout this article the formulas are all correct for the flat spacetime Minkowski coordinates of SR, but have to be modified for the more general curved space coordinates of general relativity (GR).
As a 4-divergence and source of conservation laws[]
Divergence is a vector operator that produces a signed scalar field giving the quantity of a vector field's source at each point. Note that in this metric signature [+,−,−,−] the 4-Gradient has a negative spatial component. It gets canceled when taking the 4D dot product since the Minkowski Metric is Diagonal[+1,−1,−1,−1].
This is the equivalent of a conservation law for the EM 4-potential.
The 4-divergence of the transverse traceless 4D (2,0)-tensor representing gravitational radiation in the weak-field limit (i.e. freely propagating far from the source).
: Transverse condition
is the equivalent of a conservation equation for freely propagating gravitational waves.
The conservation of energy (temporal direction) and the conservation of linear momentum (3 separate spatial directions).
It is often written as:
where it is understood that the single zero is actually a 4-vector zero .
When the conservation of the stress–energy tensor () for a perfect fluid is combined with the conservation of particle number density (), both utilizing the 4-gradient, one can derive the relativistic Euler equations, which in fluid mechanics and astrophysics are a generalization of the Euler equations that account for the effects of special relativity.
These equations reduce to the classical Euler equations if the fluid 3-space velocity is much less than the speed of light, the pressure is much less than the energy density, and the latter is dominated by the rest mass density.
In flat spacetime and using Cartesian coordinates, if one combines this with the symmetry of the stress–energy tensor, one can show that angular momentum (relativistic angular momentum) is also conserved:
where this zero is actually a (2,0)-tensor zero.
As a Jacobian matrix for the SR Minkowski metric tensor[]
The Lorentz transformation is written in tensor form as[4]: 69
and since are just constants, then
Thus, by definition of the 4-gradient
This identity is fundamental. Components of the 4-gradient transform according to the inverse of the components of 4-vectors. So the 4-gradient is the "archetypal" one-form.
By applying the 4-gradient again, and defining the 4-current density as one can derive the tensor form of the Maxwell equations:
where the second line is a version of the Bianchi identity (Jacobi identity).
As a way to define the 4-wavevector[]
A wavevector is a vector which helps describe a wave. Like any vector, it has a magnitude and direction, both of which are important: Its magnitude is either the wavenumber or angular wavenumber of the wave (inversely proportional to the wavelength), and its direction is ordinarily the direction of wave propagation
The 4-wavevector is the 4-gradient of the negative phase (or the negative 4-gradient of the phase) of a wave in Minkowski Space:[6]: 387
This is mathematically equivalent to the definition of the phase of a wave (or more specifically a plane wave):
where 4-position , is the temporal angular frequency, is the spatial 3-space wavevector, and is the Lorentz scalar invariant phase.
with the assumption that the plane wave and are not explicit functions of or .
The explicit form of an SR plane wave can be written as:[7]: 9
A general wave would be the superposition of multiple plane waves:
Again using the 4-gradient,
or
, which is the 4-gradient version of complex-valuedplane waves
As the d'Alembertian operator[]
In special relativity, electromagnetism and wave theory, the d'Alembert operator, also called the d'Alembertian or the wave operator, is the Laplace operator of Minkowski space. The operator is named after French mathematician and physicist Jean le Rond d'Alembert.
As it is the dot product of two 4-vectors, the d'Alembertian is a Lorentz invariant scalar.
Occasionally, in analogy with the 3-dimensional notation, the symbols and are used for the 4-gradient and d'Alembertian respectively. More commonly however, the symbol is reserved for the d'Alembertian.
Some examples of the 4-gradient as used in the d'Alembertian follow:
In the Klein–Gordon relativistic quantum wave equation for spin-0 particles (ex. Higgs boson):
As a component of the 4D Gauss' Theorem / Stokes' Theorem / Divergence Theorem[]
In vector calculus, the divergence theorem, also known as Gauss's theorem or Ostrogradsky's theorem, is a result that relates the flow (that is, flux) of a vector field through a surface to the behavior of the vector field inside the surface. More precisely, the divergence theorem states that the outward flux of a vector field through a closed surface is equal to the volume integral of the divergence over the region inside the surface. Intuitively, it states that the sum of all sources minus the sum of all sinks gives the net flow out of a region. In vector calculus, and more generally differential geometry, Stokes' theorem (also called the generalized Stokes' theorem) is a statement about the integration of differential forms on manifolds, which both simplifies and generalizes several theorems from vector calculus.
or
where
is a 4-vector field defined in
is the 4-divergence of
is the component of along direction
is a 4D simply connected region of Minkowski spacetime
is its 3D boundary with its own 3D volume element
is the outward-pointing normal
is the 4D differential volume element
As a component of the SR Hamilton–Jacobi equation in relativistic analytic mechanics[]
The Hamilton–Jacobi equation (HJE) is a formulation of classical mechanics, equivalent to other formulations such as Newton's laws of motion, Lagrangian mechanics and Hamiltonian mechanics. The Hamilton–Jacobi equation is particularly useful in identifying conserved quantities for mechanical systems, which may be possible even when the mechanical problem itself cannot be solved completely. The HJE is also the only formulation of mechanics in which the motion of a particle can be represented as a wave. In this sense, the HJE fulfilled a long-held goal of theoretical physics (dating at least to Johann Bernoulli in the 18th century) of finding an analogy between the propagation of light and the motion of a particle
The generalized relativistic momentum of a particle can be written as[1]: 93–96
where and
This is essentially the 4-total momentum of the system; a test particle in a field using the minimal coupling rule. There is the inherent momentum of the particle , plus momentum due to interaction with the EM 4-vector potential via the particle charge .
The relativistic Hamilton–Jacobi equation is obtained by setting the total momentum equal to the negative 4-gradient of the action.
The temporal component gives:
The spatial components give:
where is the Hamiltonian.
This is actually related to the 4-wavevector being equal the negative 4-gradient of the phase from above.
To get the HJE, one first uses the Lorentz scalar invariant rule on the 4-momentum:
which is just the 4-gradient version of the wave equation for complex-valuedplane waves
The temporal component gives:
The spatial components give:
As a component of the covariant form of the quantum commutation relation[]
In quantum mechanics (physics), the canonical commutation relation is the fundamental relation between canonical conjugate quantities (quantities which are related by definition such that one is the Fourier transform of another).
corresponds to SU(2) invariance = (3) weak force gauge bosons (i = 1, …, 3)
corresponds to SU(3) invariance = (8) color force gauge bosons (a = 1, …, 8)
The coupling constants are arbitrary numbers that must be discovered from experiment. It is worth emphasizing that for the non-abelian transformations once the are fixed for one representation, they are known for all representations.
These internal particle spaces have been discovered empirically.[3]: 47
Derivation[]
In three dimensions, the gradient operator maps a scalar field to a vector field such that the line integral between any two points in the vector field is equal to the difference between the scalar field at these two points. Based on this, it may appearincorrectly that the natural extension of the gradient to 4 dimensions should be:
incorrect
However, a line integral involves the application of the vector dot product, and when this is extended to 4-dimensional spacetime, a change of sign is introduced to either the spatial co-ordinates or the time co-ordinate depending on the convention used. This is due to the non-Euclidean nature of spacetime. In this article, we place a negative sign on the spatial coordinates (the time-positive metric convention ). The factor of (1/c) is to keep the correct unit dimensionality, [length]−1, for all components of the 4-vector and the (−1) is to keep the 4-gradient Lorentz covariant. Adding these two corrections to the above expression gives the correct definition of 4-gradient:
Regarding the use of scalars, 4-vectors and tensors in physics, various authors use slightly different notations for the same equations. For instance, some use for invariant rest mass, others use for invariant rest mass and use for relativistic mass. Many authors set factors of and and to dimensionless unity. Others show some or all the constants. Some authors use for velocity, others use . Some use as a 4-wavevector (to pick an arbitrary example). Others use or or or or or , etc. Some write the 4-wavevector as , some as or or or or or . Some will make sure that the dimensional units match across the 4-vector, others do not. Some refer to the temporal component in the 4-vector name, others refer to the spatial component in the 4-vector name. Some mix it throughout the book, sometimes using one then later on the other. Some use the metric (+ − − −), others use the metric (− + + +). Some don't use 4-vectors, but do everything as the old style E and 3-space vector p. The thing is, all of these are just notational styles, with some more clear and concise than the others. The physics is the same as long as one uses a consistent style throughout the whole derivation.[7]: 2–4
^ abThe Cambridge Handbook of Physics Formulas, G. Woan, Cambridge University Press, 2010, ISBN978-0-521-57507-2
^ abcdefghijkKane, Gordon (1994). Modern Elementary Particle Physics: The Fundamental Particles and Forces (Updated ed.). Addison-Wesley Publishing Co. ISBN0-201-62460-5.
^ abcdeShultz, Bernard F. (1985). A first course in general relativity (1st ed.). Cambridge University Press. ISBN0-521-27703-5.
^ abcdCarroll, Sean M. (2004). An Introduction to General Relativity: Spacetime and Geometry (1st ed.). Addison-Wesley Publishing Co. ISBN0-8053-8732-3.