VIRIAL THEOREM

In mechanics, the 'virial theorem' provides a general equation relating the average total kinetic energy $leftlangle\; T$
ight
angle of a system with its average total potential energy $leftlangle\; V\_\{TOT\}$
ight
angle, where angle brackets represent the average of the enclosed quantity. Mathematically, the 'virial theorem' states
:$$
2 leftlangle T
ight
angle = -sum_{k=1}^{N} leftlangle mathbf{F}_{k} cdot mathbf{r}_{k}
ight
angle

where 'F'_''k'' represents the force on the ''k^th'' particle, which is located at position 'r'_''k''. The word "virial" derives from ''vis'', the Latin word for "force" or "energy", and was given its technical definition by Clausius in 1870.^[1]. Fritz Zwicky was the first to use the virial theorem to deduce the existence of unseen matter, what is now called dark matter.
The significance of the virial theorem is that it allows the average total kinetic energy to be calculated even for very complicated systems that defy an exact solution, such as those considered in statistical mechanics; this average total kinetic energy is related to the temperature of the system by the equipartition theorem. However, the virial theorem does not depend on the notion of temperature and holds even for systems that are not in thermal equilibrium. The virial theorem has been generalized in various ways, most notably to a tensor form.
If the force between any two particles of the system results from a potential energy ''V(r)''=''αr ⁿ'' that is proportional to some power ''n'' of the inter-particle distance ''r'', the virial theorem adopts a simple form
:$$
2 langle T
angle = n langle V_{TOT}
angle

Thus, twice the average total kinetic energy $leftlangle\; T$
ight
angle equals ''n'' times the average total potential energy $leftlangle\; V\_\{TOT\}$
ight
angle. Whereas ''V(r)'' represents the potential energy between two particles, ''V_TOT'' represents the total potential energy of the system, i.e., the sum of the potential energy ''V(r)'' over all pairs of particles in the system. A common example of such a system is a star held together by its own gravity, where ''n'' equals -1. As another example of its many applications, the virial theorem has been used to derive the Chandrasekhar limit for the stability of white dwarf stars.
Although the virial theorem depends on averaging the total kinetic and potential energies, the presentation here postpones the averaging to the last step; please be patient!

Contents

Definitions of the virial and its time derivative

Connection with the potential energy between particles

Special case of power-law forces

Time averaging and the virial theorem

Generalizations of the virial theorem

Inclusion of electromagnetic fields

Virial radius

See also

References

Additional reading

Definitions of the virial and its time derivative

For a collection of $N$ point particles, the scalar moment of inertia ''I'' about the origin is defined by the equation
:$$
I = sum_{k=1}^{N} m_{k} mathbf{r}_{k}^{2} = sum_{k=1}^{N} m_{k} r_{k}^{2}

where ''m''_''k'' and 'r'_''k'' represent the mass and position of the ''k''^''th'' particle. The scalar 'virial' ''G'' is defined by the equation
:$$
G = sum_{k=1}^{N} mathbf{p}_{k} cdot mathbf{r}_{k}

where 'p'_''k'' is the momentum vector of the ''k''^''th'' particle. Assuming that the masses are constant, the 'virial' ''G'' is the time derivative of this moment of inertia
:$$
G = rac{1}{2} rac{dI}{dt} = sum_{k=1}^{N} m_{k} rac{dmathbf{r}_{k}}{dt} cdot mathbf{r}_{k} = sum_{k=1}^{N} mathbf{p}_{k} cdot mathbf{r}_{k}

In turn, the time derivative of the virial ''G'' can be written
:$$
rac{dG}{dt} =
sum_{k=1}^{N} mathbf{p}_{k} cdot rac{dmathbf{r}_{k}}{dt} +
sum_{k=1}^{N} rac{dmathbf{p}_{k}}{dt} cdot mathbf{r}_{k}

::$$
= sum_{k=1}^{N} m_{k} rac{dmathbf{r}_{k}}{dt} cdot rac{dmathbf{r}_{k}}{dt} + sum_{k=1}^{N} mathbf{F}_{k} cdot mathbf{r}_{k}

or, more simply,
:$$
rac{dG}{dt} = 2 T + sum_{k=1}^{N} mathbf{F}_{k} cdot mathbf{r}_{k}.

Here $m\_\{k\}$ is the mass of the $k^\{mathrm\{th\}\}$ particle, is the net force on that particle and $T$ is the total kinetic energy of the system
:$$
T = rac{1}{2} sum_{k=1}^{N} m_{k} v_{k}^{2} =
rac{1}{2} sum_{k=1}^{N} m_{k} rac{dmathbf{r}_{k}}{dt} cdot rac{dmathbf{r}_{k}}{dt}.

Connection with the potential energy between particles

The total force $mathbf\{F\}\_\{k\}$ on particle $k$ is the sum of all the forces from the other particles $j$ in the system
:$$
mathbf{F}_{k} = sum_{j=1}^{N} mathbf{F}_{jk}

where $mathbf\{F\}\_\{jk\}$ is the force applied by particle $j$ on particle $k$. Hence, the force term of the virial time derivative can be written
:$$
sum_{k=1}^{N} mathbf{F}_{k} cdot mathbf{r}_{k} =
sum_{k=1}^{N} sum_{j=1}^{N} mathbf{F}_{jk} cdot mathbf{r}_{k}.

Since no particle acts on itself (i.e., $mathbf\{F\}\_\{jk\}\; =\; 0$ whenever $j=k$), we have
:$$
sum_{k=1}^{N} mathbf{F}_{k} cdot mathbf{r}_{k} =
sum_{k=1}^{N} sum_{j sum_{k=1}^{N} sum_{j>k} mathbf{F}_{jk} cdot mathbf{r}_{k} =
sum_{k=1}^{N} sum_{j ight).

where we have assumed that Newton's third law of motion holds, i.e., $mathbf\{F\}\_\{jk\}\; =\; -mathbf\{F\}\_\{kj\}$ (equal and opposite reaction).
It often happens that the forces can be derived from a potential energy $V$ that is a function only of the distance $r\_\{jk\}$ between the point particles $j$ and $k$. Since the force is the gradient of the potential energy, we have in this case
:$$
mathbf{F}_{jk} = -
abla_{mathbf{r}_{k}} V =
- rac{dV}{dr} left( rac{mathbf{r}_{k} - mathbf{r}_{j}}{r_{jk}}
ight),

which is clearly equal and opposite to $mathbf\{F\}\_\{kj\}\; =\; -$
abla_{mathbf{r}_{j}} V, the force applied by particle $k$ on particle $j$, as may be confirmed by explicit calculation. Hence, the force term of the virial time derivative is
:$$
sum_{k=1}^{N} mathbf{F}_{k} cdot mathbf{r}_{k} =
sum_{k=1}^{N} sum_{j ight) =
-sum_{k=1}^{N} sum_{j ight)^2}{r_{jk}} =
-sum_{k=1}^{N} sum_{j
Thus, we have
:$$
rac{dG}{dt} = 2 T +
sum_{k=1}^{N} mathbf{F}_{k} cdot mathbf{r}_{k} = 2 T -
sum_{k=1}^{N} sum_{j

Special case of power-law forces

In a common special case, the potential energy $V$ between two particles is proportional to a power ''n'' of their distance ''r''
:$$
V(r_{jk}) = lpha r_{jk}^{n},

where the coefficient α and the exponent ''n'' are constants. In such cases, the force term of the virial time derivative is given by the equation
:$$
-sum_{k=1}^{N} mathbf{F}_{k} cdot mathbf{r}_{k} =
sum_{k=1}^{N} sum_{j sum_{k=1}^{N} sum_{j
where $V\_\{TOT\}$ is the total potential energy of the system
:$$
V_{TOT} = sum_{k=1}^{N} sum_{j
Thus, we have
:$$
rac{dG}{dt} = 2 T +
sum_{k=1}^{N} mathbf{F}_{k} cdot mathbf{r}_{k} = 2 T - n V_{TOT}

For gravitating systems and also for electrostatic systems, the exponent ''n'' equals -1, giving the 'Lagrange's identity'
:$$
rac{dG}{dt} = rac{1}{2} rac{d^{2}I}{dt^{2}} = 2 T + V_{TOT}

which was derived by Lagrange and extended by Jacobi.

Time averaging and the virial theorem

The average of this derivative over a time $au$ is defined as
:$$
leftlangle rac{dG}{dt}
ight
angle_{ au} = rac{1}{ au} int_{0}^{ au} rac{dG}{dt},dt = rac{1}{ au} int_{0}^{ au} dG = rac{G( au) - G(0)}{ au},

from which we obtain the exact equation
:$$
leftlangle rac{dG}{dt}
ight
angle_{ au} =
2 leftlangle T
ight
angle_{ au} + sum_{k=1}^{N} leftlangle mathbf{F}_{k} cdot mathbf{r}_{k}
ight
angle_{ au}.

The 'virial theorem' states that, 'if'
ight
angle_{ au} = 0, then
:$$
2 leftlangle T
ight
angle_{ au} = -sum_{k=1}^{N} leftlangle mathbf{F}_{k} cdot mathbf{r}_{k}
ight
angle_{ au}.

There are many reasons why the average of the time derivative might vanish, i.e.,
ight
angle_{ au} = 0. One often-cited reason applies to ''bound systems'', i.e., systems that hang together forever. In that case, the virial $G^\{mathrm\{bound\}\}$ is usually bounded between two extremes, $G\_min$ and $G\_max$, and the average goes to zero in the limit of very long times $au$
:$$
lim_{ au
ightarrow infty} left| leftlangle rac{dG^{mathrm{bound}}}{dt}
ight
angle_{ au}
ight| =
lim_{ au
ightarrow infty} left| rac{G( au) - G(0)}{ au}
ight| le
lim_{ au
ightarrow infty} rac{G_max - G_min}{ au} = 0.

Even if the average of the time derivative
ight
angle_{ au} pprox 0 is only approximately zero, the virial theorem holds to the same degree of approximation.
For power-law forces with an exponent ''n'', the general equation holds
:$$
langle T
angle_{ au} = -rac{1}{2} sum_{k=1}^{N} langle mathbf{F}_{k} cdot mathbf{r}_{k}
angle_{ au} = rac{n}{2} langle V_{TOT}
angle_{ au}.

For gravitational attraction, ''n'' equals -1 and the average kinetic energy equals half of the average negative potential energy
:$$
langle T
angle_{ au} = -rac{1}{2} langle V_{TOT}
angle_{ au}.

This general result is useful for complex gravitating systems such as solar systems or galaxies.
A simple application of the Virial theorem
concerns galaxy clusters. If a region of space is unusually full of galaxies, it is safe to assume that they have been together for a long time, and the Virial theorem can be applied. Doppler measurements give lower bounds for their relative velocities, and the Virial theorem gives a lower bound for the total mass of the cluster, including any dark matter.
The averaging need not be taken over time; an ensemble average can also be taken, with equivalent results.
Although derived for classical mechanics, the virial theorem also holds for quantum mechanics.

Generalizations of the virial theorem

Lord Rayleigh published a generalization of the virial theorem in 1903.^[2] Henri Poincaré applied a form of the virial theorem in 1911 to the problem of determining cosmological stability.^[3] A variational form of the virial theorem was developed in 1945 by Ledoux.^[4] A tensor form of the virial theorem was developed by Parker,^[5] Chandrasekhar^[6] and Fermi.^[7]

Inclusion of electromagnetic fields

The virial theorem can be extended to include electric and magnetic fields. The result is^[8]
:$$
rac{1}{2}rac{d^2}{dt^2}I
+ int_Vx_krac{partial G_k}{partial t}d^3r
= 2(T+U) + W^E + W^M - int x_k(p_{ik}+T_{ik})dS_i,

where ''I'' is the moment of inertia, ''G'' is the momentum density of the electromagnetic field, ''T'' is the kinetic energy of the "fluid", ''U'' is the random "thermal" energy of the particles, ''W^E'' and ''W^M'' are the electric and magnetic energy content of the volume considered. Finally, ''p_ik'' is the fluid-pressure tensor expressed in the local moving coordinate system
:$$
p_{ik}
= Sigma n^sigma m^sigma langle v_iv_k
angle^sigma
- V_iV_kSigma m^sigma n^sigma
,
and ''T_ik'' is the electromagnetic stress tensor,
:$$
T_{ik}
= left( rac{arepsilon_0E^2}{2} + rac{B^2}{2mu_0}
ight)
- left( arepsilon_0E_iE_k + rac{B_iB_k}{mu_0}
ight).

A plasmoid is a finite configuration of magnetic fields and plasma. With the virial theorem it is easy to see that any such configuration will expand if not contained by external forces. In a finite configuration without pressure-bearing walls or magnetic coils, the surface integral will vanish. Since all the other terms on the right hand side are positive, the acceleration of the moment of inertia will also be positive. It is also easy to estimate the expansion time τ. If a total mass ''M'' is confined within a radius ''R'', then the moment of inertia is roughly ''MR''², and the left hand side of the virial theorem is ''MR''²/τ². The terms on the right hand side add up to about ''pR''³, where ''p'' is the larger of the plasma pressure or the magnetic pressure. Equating these two terms and solving for τ, we find
:$au,sim\; R/c\_s,$
where ''c_s'' is the speed of the ion acoustic wave (or the Alfven wave, if the magnetic pressure is higher than the plasma pressure). Thus the lifetime of a plasmoid is expected to be on the order of the acoustic (or Alfven) transit time.

Virial radius

In astronomy, the term ''virial radius'' is sometimes used to refer to the radius of a sphere, centered on a galaxy or a galaxy cluster, within which the average density is greater, by a specified factor, than the critical density $$
ho_{crit}=rac{3H^2}{8pi G}. (Here, ''H'' is the Hubble parameter and ''G'' is the gravitational constant.) A common choice for the factor is 200, in which case the virial radius is designated as $r\_\{200\}$.

See also

★ Virial stress

★ Equipartition theorem

References

1. On a Mechanical Theorem Applicable to Heat, , RJE, Clausius, Philosophical Magazine, Ser. 4, 1870
2. Unknown, , , Lord Rayleigh, , 1903
3. Lectures on Cosmological Theories, , H, Poincaré, Hermann, ,
4. On the Radial Pulsation of Gaseous Stars, , P., Ledoux, Ap. J., 1945
5. Tensor Virial Equations, , E.N., Parker, Physical Review, 1954
6. The Potentials and the Superpotentials of Homogeneous Ellipsoids, , S, Chandrasekhar, Ap. J., 1962
7. Problems of Gravitational Stability in the Presence of a Magnetic Field, , S, Chandrasekhar, Ap. J., 1953
8. George Schmidt, ''Physics of High Temperature Plasmas'' (Second edition), Academic Press (1979), p.72

Additional reading

★ Classical Mechanics, , H, Goldstein, Addison-Wesley, 1980, ISBN 0-201-02918-9

★ The Virial Theorem in Stellar Astrophysics, , GW, Collins, Pachart Press, 1978,