Four-momentum explained

In special relativity, four-momentum (also called momentum–energy or momenergy^[1]) is the generalization of the classical three-dimensional momentum to four-dimensional spacetime. Momentum is a vector in three dimensions; similarly four-momentum is a four-vector in spacetime. The contravariant four-momentum of a particle with relativistic energy and three-momentum, where is the particle's three-velocity and the Lorentz factor, is$$p\; =\; \backslash left(p^0,\; p^1,\; p^2,\; p^3\backslash right)\; =\; \backslash left(\backslash frac\; E\; c,\; p\_x,\; p\_y,\; p\_z\backslash right).$$

The quantity of above is ordinary non-relativistic momentum of the particle and its rest mass. The four-momentum is useful in relativistic calculations because it is a Lorentz covariant vector. This means that it is easy to keep track of how it transforms under Lorentz transformations.

Minkowski norm

Calculating the Minkowski norm squared of the four-momentum gives a Lorentz invariant quantity equal (up to factors of the speed of light) to the square of the particle's proper mass:$$p\; \backslash cdot\; p\; =\; \backslash eta\_\; p^\backslash mu\; p^\backslash nu\; =\; p\_\backslash nu\; p^\backslash nu\; =\; -\; +\; |\backslash mathbf\; p|^2\; =\; -m^2\; c^2$$whereis the metric tensor of special relativity with metric signature for definiteness chosen to be . The negativity of the norm reflects that the momentum is a timelike four-vector for massive particles. The other choice of signature would flip signs in certain formulas (like for the norm here). This choice is not important, but once made it must for consistency be kept throughout.

The Minkowski norm is Lorentz invariant, meaning its value is not changed by Lorentz transformations/boosting into different frames of reference. More generally, for any two four-momenta and, the quantity is invariant.

Relation to four-velocity

For a massive particle, the four-momentum is given by the particle's invariant mass multiplied by the particle's four-velocity,$$p^\backslash mu\; =\; m\; u^\backslash mu,$$where the four-velocity is$$u\; =\; \backslash left(u^0,\; u^1,\; u^2,\; u^3\backslash right)\; =\; \backslash gamma\_v\; \backslash left(c,\; v\_x,\; v\_y,\; v\_z\backslash right),$$and$$\backslash gamma\_v\; :=\; \backslash frac$$is the Lorentz factor (associated with the speed

v ), is the speed of light.

Derivation

There are several ways to arrive at the correct expression for four-momentum. One way is to first define the four-velocity and simply define, being content that it is a four-vector with the correct units and correct behavior. Another, more satisfactory, approach is to begin with the principle of least action and use the Lagrangian framework to derive the four-momentum, including the expression for the energy. One may at once, using the observations detailed below, define four-momentum from the action . Given that in general for a closed system with generalized coordinates and canonical momenta,$$p\_i\; =\; \backslash frac\; =\; \backslash frac,\; \backslash quad\; E\; =\; -\backslash frac\; =\; -\; c\; \backslash cdot\; \backslash frac,$$it is immediate (recalling,,, and,,, in the present metric convention) that$$p\_\backslash mu\; =\; -\backslash frac\; =\; \backslash left(-\backslash mathbf\; p\backslash right)$$is a covariant four-vector with the three-vector part being the (negative of) canonical momentum.Consider initially a system of one degree of freedom . In the derivation of the equations of motion from the action using Hamilton's principle, one finds (generally) in an intermediate stage for the variation of the action,$$\backslash delta\; S\; =\; \backslash left.\; \backslash left[\backslash frac\{\backslash partial\; L\}\{\backslash partial\; \backslash dot\; q\}\backslash delta\; q\backslash right]\backslash right|\_^\; +\; \backslash int\_^\; \backslash left(\backslash frac\; -\; \backslash frac\; \backslash frac\backslash right)\backslash delta\; q\; dt.$$

The assumption is then that the varied paths satisfy, from which Lagrange's equations follow at once. When the equations of motion are known (or simply assumed to be satisfied), one may let go of the requirement . In this case the path is assumed to satisfy the equations of motion, and the action is a function of the upper integration limit, but is still fixed. The above equation becomes with, and defining, and letting in more degrees of freedom,$$\backslash delta\; S\; =\; \backslash sum\_i\; \backslash frac\backslash delta\; q\_i\; =\; \backslash sum\_i\; p\_i\; \backslash delta\; q\_i.$$

Observing that$$\backslash delta\; S\; =\; \backslash sum\_i\; \backslash frac\backslash delta\; q\_i,$$one concludes$$p\_i\; =\; \backslash frac.$$

In a similar fashion, keep endpoints fixed, but let vary. This time, the system is allowed to move through configuration space at "arbitrary speed" or with "more or less energy", the field equations still assumed to hold and variation can be carried out on the integral, but instead observe$$\backslash frac\; =\; L$$by the fundamental theorem of calculus. Compute using the above expression for canonical momenta,$$\backslash frac\; =\; \backslash frac\; +\; \backslash sum\_i\; \backslash frac\backslash dot\_i\; =\; \backslash frac\; +\; \backslash sum\_i\; p\_i\backslash dot\_i\; =\; L.$$

Now using$$H\; =\; \backslash sum\_i\; p\_i\; \backslash dot\_i\; -\; L,$$where is the Hamiltonian, leads to, since in the present case,$$E\; =\; H\; =\; -\backslash frac.$$

Incidentally, using with in the above equation yields the Hamilton - Jacobi equations. In this context, is called Hamilton's principal function.----

The action is given by$$S\; =\; -mc\backslash int\; ds\; =\; \backslash int\; L\; dt,\; \backslash quad\; L\; =\; -mc^2\backslash sqrt,$$where is the relativistic Lagrangian for a free particle. From this,The variation of the action is$$\backslash delta\; S\; =\; -mc\backslash int\; \backslash delta\; ds.$$

To calculate, observe first that and that$$\backslash delta\; ds^2\; =\; \backslash delta\; \backslash eta\_dx^\backslash mu\; dx^\backslash nu\; =\; \backslash eta\_\; \backslash left(\backslash delta\; \backslash left(dx^\backslash mu\backslash right)\; dx^\backslash nu\; +\; dx^\backslash mu\; \backslash delta\; \backslash left(dx^\backslash nu\backslash right)\backslash right)\; =\; 2\backslash eta\_\; \backslash delta\; \backslash left(dx^\backslash mu\backslash right)\; dx^\backslash nu.$$

So$$\backslash delta\; ds\; =\; \backslash eta\_\; \backslash delta\; dx^\backslash mu\; \backslash frac\; =\; \backslash eta\_\; d\backslash delta\; x^\backslash mu\; \backslash frac,$$or$$\backslash delta\; ds\; =\; \backslash eta\_\; \backslash frac\; \backslash fracd\backslash tau,$$and thus$$\backslash delta\; S\; =\; -m\backslash int\; \backslash eta\_\; \backslash frac\; \backslash fracd\backslash tau\; =\; -m\backslash int\; \backslash eta\_\; \backslash frac\; u^\backslash nu\; d\backslash tau\; =\; -m\backslash int\; \backslash eta\_\; \backslash left[\backslash frac\{d\}\{d\backslash tau\}\; \backslash left(\backslash delta\; x^\backslash mu\; u^\backslash nu\backslash right)\; -\; \backslash delta\; x^\backslash mu\backslash frac\{d\}\{d\backslash tau\}u^\backslash nu\backslash right]\; d\backslash tau$$which is just$$\backslash delta\; S\; =\; \backslash left[-mu\_\backslash mu\backslash delta\; x^\backslash mu\backslash right]\_^\; +\; m\; \backslash int\_^\; \backslash delta\; x^\backslash mu\backslash fracds$$----$$\backslash delta\; S\; =\; \backslash left[-mu\_\backslash mu\backslash delta\; x^\backslash mu\backslash right]\_^\; +\; m\backslash int\_^\backslash delta\; x^\backslash mu\backslash fracds\; =\; -mu\_\backslash mu\backslash delta\; x^\backslash mu\; =\; \backslash frac\backslash delta\; x^\backslash mu\; =\; -p\_\backslash mu\backslash delta\; x^\backslash mu,$$

where the second step employs the field equations,, and as in the observations above. Now compare the last three expressions to find$$p^\backslash mu\; =\; -\backslash partial^\backslash mu[S]\; =\; -\backslash frac\; =\; mu^\backslash mu\; =\; m\backslash left(\backslash frac,\; \backslash frac,\; \backslash frac,\; \backslash frac\backslash right),$$with norm, and the famed result for the relativistic energy,

where is the now unfashionable relativistic mass, follows. By comparing the expressions for momentum and energy directly, one has

that holds for massless particles as well. Squaring the expressions for energy and three-momentum and relating them gives the energy–momentum relation,

Substituting$$p\_\backslash mu\; \backslash leftrightarrow\; -\backslash frac$$in the equation for the norm gives the relativistic Hamilton - Jacobi equation,

It is also possible to derive the results from the Lagrangian directly. By definition,which constitute the standard formulae for canonical momentum and energy of a closed (time-independent Lagrangian) system. With this approach it is less clear that the energy and momentum are parts of a four-vector.

The energy and the three-momentum are separately conserved quantities for isolated systems in the Lagrangian framework. Hence four-momentum is conserved as well. More on this below.

More pedestrian approaches include expected behavior in electrodynamics. In this approach, the starting point is application of Lorentz force law and Newton's second law in the rest frame of the particle. The transformation properties of the electromagnetic field tensor, including invariance of electric charge, are then used to transform to the lab frame, and the resulting expression (again Lorentz force law) is interpreted in the spirit of Newton's second law, leading to the correct expression for the relativistic three- momentum. The disadvantage, of course, is that it isn't immediately clear that the result applies to all particles, whether charged or not, and that it doesn't yield the complete four-vector.

It is also possible to avoid electromagnetism and use well tuned experiments of thought involving well-trained physicists throwing billiard balls, utilizing knowledge of the velocity addition formula and assuming conservation of momentum.^[2] This too gives only the three-vector part.

Conservation of four-momentum

As shown above, there are three conservation laws (not independent, the last two imply the first and vice versa):

• The four-momentum (either covariant or contravariant) is conserved.
• The total energy is conserved.

p = \left(p ^{1,} p ^{2,} p ^{3\right)} is conserved (not to be confused with the classic non-relativistic momentum m v ).

Note that the invariant mass of a system of particles may be more than the sum of the particles' rest masses, since kinetic energy in the system center-of-mass frame and potential energy from forces between the particles contribute to the invariant mass. As an example, two particles with four-momenta and each have (rest) mass 3GeV/c² separately, but their total mass (the system mass) is 10GeV/c². If these particles were to collide and stick, the mass of the composite object would be 10GeV/c².

One practical application from particle physics of the conservation of the invariant mass involves combining the four-momenta and of two daughter particles produced in the decay of a heavier particle with four-momentum to find the mass of the heavier particle. Conservation of four-momentum gives, while the mass of the heavier particle is given by . By measuring the energies and three-momenta of the daughter particles, one can reconstruct the invariant mass of the two-particle system, which must be equal to . This technique is used, e.g., in experimental searches for Z′ bosons at high-energy particle colliders, where the Z′ boson would show up as a bump in the invariant mass spectrum of electron–positron or muon–antimuon pairs.

If the mass of an object does not change, the Minkowski inner product of its four-momentum and corresponding four-acceleration is simply zero. The four-acceleration is proportional to the proper time derivative of the four-momentum divided by the particle's mass, so$$p^\backslash mu\; A\_\backslash mu\; =\; \backslash eta\_\; p^\backslash mu\; A^\backslash nu\; =\; \backslash eta\_\; p^\backslash mu\; \backslash frac\; \backslash frac\; =\; \backslash frac\; \backslash frac\; p\; \backslash cdot\; p\; =\; \backslash frac\; \backslash frac\; \backslash left(-m^2c^2\backslash right)\; =\; 0\; .$$

Canonical momentum in the presence of an electromagnetic potential

For a charged particle of charge, moving in an electromagnetic field given by the electromagnetic four-potential:$$A\; =\; \backslash left(A^0,\; A^1,\; A^2,\; A^3\backslash right)\; =\; \backslash left(A\_x,\; A\_y,\; A\_z\backslash right)$$where is the scalar potential and the vector potential, the components of the (not gauge-invariant) canonical momentum four-vector is$$P^\backslash mu\; =\; p^\backslash mu\; +\; q\; A^\backslash mu.$$

This, in turn, allows the potential energy from the charged particle in an electrostatic potential and the Lorentz force on the charged particle moving in a magnetic field to be incorporated in a compact way, in relativistic quantum mechanics.

Four-momentum in curved spacetime

In the case when there is a moving physical system with a continuous distribution of matter in curved spacetime, the primary expression for four-momentum is four-vector with covariant index: ^[3]

P _{\mu} = \left( (E)/(c) , - P \right).

Four-momentum

P _{\mu} is expressed through the energy E of physical system and relativistic momentum P . At the same time, the four-momentum P _{\mu} can be represented as the sum of two non-local four-vectors of integral type:P _{\mu} = p _{\mu} + K _{\mu.}

Four-vector

p _{\mu} is the generalized four-momentum associated with the action of fields on particles; four-vector K _{\mu} is the four-momentum of the fields arising from the action of particles on the fields.

Energy

E and momentum P , as well as components of four-vectors p _{\mu} and K _{\mu} can be calculated if the Lagrangian density l{L} = l{L} _{p} + l{L} _{f} of the system is given. The following formulas are obtained for the energy and momentum of the system:E = \int _{V} (\partial)/(\partial v) \left( (l{L)/(_{p}) }{u ^{0}} \right) ⋅ v u ^{0} \sqrt { - g} dx ^{1} dx ^{2} dx ^{3} - \int _{V} \left ( l{L} _{p} + l{L} _{f} \right ) \sqrt { - g} dx ^{1} dx ^{2} dx ^{3} + \sum ^{N} _{n =1} \left( v _{n} ⋅ (\partial L _{f})/(\partial v _{n}) \right ) . P = \int _{V} (\partial)/(\partial v) \left( (l{L)/(_{p}) }{u ^{0}} \right) u ^{0} \sqrt { - g} dx ^{1} dx ^{2} dx ^{3} + \sum ^{N} _{n =1} (\partial L _{f})/(\partial v _{n}) .

Here

l{L} _{p} is that part of the Lagrangian density that contains terms with four-currents; v is the velocity of matter particles; u ^{0} is the time component of four-velocity of particles; g is determinant of metric tensor; L _{f} = \int _{V} l{L} _{f} \sqrt { - g} dx ^{1} dx ^{2} dx ^{3} is the part of the Lagrangian associated with the Lagrangian density l{ L} _{f} ; v _{n} is velocity of a particle of matter with number n .

See also

• Four-force
• Four-gradient
• Pauli–Lubanski pseudovector

References

• Book: Goldstein, Herbert. Classical mechanics. 1980. Addison–Wesley Pub. Co.. Reading, Mass.. 978-0201029185. 2nd .
• Book: Landau. L. D.. Lev Landau. Lifshitz. E. M.. E. M. Lifshitz. Mechanics. 1975. 3rd. 1939. 978-0-7506-28969. Elsevier. Amsterdam. Translated from Russian by J. B. Sykes and J. S. Bell..
• Book: Landau. L.D.. E.M.. Lifshitz. The classical theory of fields. 2000. Butterworth Heinemann. Oxford. 9780750627689 . 4th rev. English edition, reprinted with corrections; translated from the Russian by Morton Hamermesh.
• Book: Rindler, Wolfgang . Introduction to Special Relativity . 2nd . Oxford . Oxford University Press . 1991 . 978-0-19-853952-0 . registration .
• Book: Sard, R. D.. Relativistic Mechanics - Special Relativity and Classical Particle Dynamics. 1970. W. A. Benjamin. New York. 978-0805384918. registration.
• G. N.. Lewis. Gilbert N. Lewis. R. C.. Tolman. Richard C. Tolman. The Principle of Relativity, and Non-Newtonian Mechanics. Phil. Mag.. 6. 18. 106. 10.1080/14786441008636725. 510 - 523. 1909. Wikisource version

Notes and References

1. Book: Taylor . Edwin . Wheeler . John . Spacetime physics introduction to special relativity . 1992 . W. H. Freeman and Company . New York . 978-0-7167-2327-1 . 191.
2. Wikisource version
3. Fedosin . Sergey G. . 268967902 . 2024-04-18 . What should we understand by the four-momentum of physical system? . Physica Scripta . 99. 5. 055034. en . 10.1088/1402-4896/ad3b45 . 2024PhyS...99e5034F .