Lense–Thirring precession explained

S .

The difference between de Sitter precession and the Lense–Thirring effect is that the de Sitter effect is due simply to the presence of a central mass, whereas the Lense–Thirring effect is due to the rotation of the central mass. The total precession is calculated by combining the de Sitter precession with the Lense–Thirring precession.

According to a 2007 historical analysis by Herbert Pfister,^[1] the effect should be renamed the Einstein–Thirring–Lense effect.

The Lense–Thirring metric

The gravitational field of a spinning spherical body of constant density was studied by Lense and Thirring in 1918, in the weak-field approximation. They obtained the metric^{[2] [3]}

$$\backslash mathrm\; ds^2=\backslash left(1-\backslash frac\backslash right)c^2\backslash ,\backslash mathrm\; dt^2-\backslash left(1+\backslash frac\backslash right)\backslash ,\backslash mathrm\; d\backslash sigma^2+4G\backslash epsilon\_S^k\; \backslash frac\; c\; \backslash ,\backslash mathrm\; dt\backslash ,\backslash mathrm\; dx^j,$$where the symbols represent:

ds ^{2} the metric,d\sigma ^{2 =} dx ^{2} + dy ^{2} + dz ^{2 =} dr ^{2} + r ^{2d\theta} ^{2} + r ^{2\sin} ^{2\thetad\varphi} ^{2} the flat-space line element in three dimensions,

• $r\; =\; \backslash sqrt$ the "radial" position of the observer,

c the speed of light,G the gravitational constant,\epsilon _{ijk} the completely antisymmetric Levi-Civita symbol,

• $M\; =\; \backslash int\; T^\; \backslash ,\backslash mathrm\; d^3x$ the mass of the rotating body,
• $S\_k\; =\; \backslash int\; \backslash epsilon\_x^l\; T^\; \backslash ,\backslash mathrm\; d^3x$ the angular momentum of the rotating body,

T ^{\mu\nu} the energy–momentum tensor.

The above is the weak-field approximation of the full solution of the Einstein equations for a rotating body, known as the Kerr metric, which, due to the difficulty of its solution, was not obtained until 1965.

The Coriolis term

The frame-dragging effect can be demonstrated in several ways. One way is to solve for geodesics; these will then exhibit a Coriolis force-like term, except that, in this case (unlike the standard Coriolis force), the force is not fictional, but is due to frame dragging induced by the rotating body. So, for example, an (instantaneously) radially infalling geodesic at the equator will satisfy the equation^[2] $$r\backslash frac+2\backslash frac\backslash frac=0,$$where

t is the time,\varphi is the azimuthal angle (longitudinal angle),J = \Vert S \Vert is the magnitude of the angular momentum of the spinning massive body.

The above can be compared to the standard equation for motion subject to the Coriolis force:

$$r\backslash frac+2\backslash omega\backslash frac\; =\; 0,$$

where

\omega is the angular velocity of the rotating coordinate system. Note that, in either case, if the observer is not in radial motion, i.e. if dr/dt = 0 , there is no effect on the observer.

Precession

The frame-dragging effect will cause a gyroscope to precess. The rate of precession is given by^[3] $$\backslash Omega^k\; =\; \backslash frac\; \backslash left[S^k\; -\; 3\; \backslash frac\{(S\; \backslash cdot\; x)\; x^k\}\{r^2\}\backslash right],$$where:

\Omega is the angular velocity of the precession, a vector, and \Omega _{k} one of its components,S _{k} the angular momentum of the spinning body, as before,S ⋅ x the ordinary flat-metric inner product of the position and the angular momentum.

That is, if the gyroscope's angular momentum relative to the fixed stars is

L ^{i} , then it precesses as$$\backslash frac\; =\; \backslash epsilon\_\; \backslash Omega^j\; L^k.$$

The rate of precession is given by$$\backslash epsilon\_\; \backslash Omega^k\; =\; \backslash Gamma\_,$$where

\Gamma _{ij0} is the Christoffel symbol for the above metric. Gravitation by Misner, Thorne, and Wheeler^[3] provides hints on how to most easily calculate this.

Gravitomagnetic analysis

It is popular in some circles to use the gravitomagnetic approach to the linearized field equations. The reason for this popularity should be immediately evident below, by contrasting it to the difficulties of working with the equations above. The linearized metric

h _{\mu\nu} = g _{\mu\nu} - η _{\mu\nu} can be read off from the Lense–Thirring metric given above, where ds ^{2} = g _{\mu\nu} dx ^{\mu} dx ^{\nu} , and η _{\mu\nu} dx ^{\mu} dx ^{\nu} = c ^{2} dt ^{2} - dx ^{2} - dy ^{2} - dz ^{2} . In this approach, one writes the linearized metric, given in terms of the gravitomagnetic potentials \phi and \vec{A} is$$h\_\; =\; \backslash frac$$and$$h\_\; =\; \backslash frac,$$where$$\backslash phi\; =\; \backslash frac$$is the gravito-electric potential, and$$\backslash vec\; =\; \backslash frac\; \backslash vec\; \backslash times\; \backslash vec$$is the gravitomagnetic potential. Here \vec{r} is the 3D spatial coordinate of the observer, and \vec{S} is the angular momentum of the rotating body, exactly as defined above. The corresponding fields are$$\backslash vec\; =\; -\backslash nabla\backslash phi\; -\; \backslash frac$$for the gravito-electric field, and$$\backslash vec\; =\; \backslash vec\; \backslash times\; \backslash vec$$is the gravitomagnetic field. It is then a matter of substitution and rearranging to obtain$$\backslash vec\; =\; -\backslash frac\; \backslash left[\backslash vec\{S\}\; -\; 3\; \backslash frac\{(\backslash vec\{S\}\backslash cdot\backslash vec\{r\})\; \backslash vec\{r\}\}\{r^2\}\backslash right]$$as the gravitomagnetic field. Note that it is half the Lense–Thirring precession frequency. In this context, Lense–Thirring precession can essentially be viewed as a form of Larmor precession. The factor of 1/2 suggests that the correct gravitomagnetic analog of the gyromagnetic ratio is (curiously!) two. This factor of two can be explained completely analogous to the electron's g-factor by taking into account relativistic calculations.

The gravitomagnetic analog of the Lorentz force in the non-relativistic limit is given by$$\backslash vec\; =\; m\; \backslash vec\; +\; m\; \backslash frac\; \backslash times\; \backslash vec,$$where

m is the mass of a test particle moving with velocity \vec{v} . This can be used in a straightforward way to compute the classical motion of bodies in the gravitomagnetic field. For example, a radially infalling body will have a velocity \vec{v} = - \hat{r} dr/dt ; direct substitution yields the Coriolis term given in a previous section.

Example: Foucault's pendulum

To get a sense of the magnitude of the effect, the above can be used to compute the rate of precession of Foucault's pendulum, located at the surface of the Earth.

For a solid ball of uniform density, such as the Earth, of radius

R , the moment of inertia is given by 2 MR ^{2/5,} so that the absolute value of the angular momentum S is \Vert S\Vert = 2 MR ^{2\omega/5,} with \omega the angular speed of the spinning ball.

The direction of the spin of the Earth may be taken as the z axis, whereas the axis of the pendulum is perpendicular to the Earth's surface, in the radial direction. Thus, we may take

\hat{z} ⋅ \hat{r} = \cos\theta , where \theta is the latitude. Similarly, the location of the observer r is at the Earth's surface R . This leaves rate of precession is as$$\backslash Omega\_\backslash text\; =\; \backslash frac\; \backslash frac\; \backslash cos\backslash theta.$$

As an example the latitude of the city of Nijmegen in the Netherlands is used for reference. This latitude gives a value for the Lense–Thirring precession$$\backslash Omega\_\backslash text\; =\; 2.2\; \backslash cdot\; 10^\; \backslash text/\backslash text.$$

At this rate a Foucault pendulum would have to oscillate for more than 16000 years to precess 1 degree. Despite being quite small, it is still two orders of magnitude larger than Thomas precession for such a pendulum.

The above does not include the de Sitter precession; it would need to be added to get the total relativistic precessions on Earth.

Experimental verification

The Lense–Thirring effect, and the effect of frame dragging in general, continues to be studied experimentally. There are two basic settings for experimental tests: direct observation via satellites and spacecraft orbiting Earth, Mars or Jupiter, and indirect observation by measuring astrophysical phenomena, such as accretion disks surrounding black holes and neutron stars, or astrophysical jets from the same.

The Juno spacecraft's suite of science instruments will primarily characterize and explore the three-dimensional structure of Jupiter's polar magnetosphere, auroras and mass composition.^[4] As Juno is a polar-orbit mission, it will be possible to measure the orbital frame-dragging, known also as Lense–Thirring precession, caused by the angular momentum of Jupiter.^[5]

Results from astrophysical settings are presented after the following section.

Astrophysical setting

A star orbiting a spinning supermassive black hole experiences Lense–Thirring precession, causing its orbital line of nodes to precess at a rate^[6] $$\backslash frac=\; \backslash frac\; =\; \backslash frac,$$where

• a and e are the semimajor axis and eccentricity of the orbit,
• M is the mass of the black hole,
• χ is the dimensionless spin parameter (0 < χ < 1).

The precessing stars also exert a torque back on the black hole, causing its spin axis to precess, at a rate^[7] $$\backslash frac\; =\; \backslash frac\backslash sum\_j\; \backslash frac,$$where

• L_j is the angular momentum of the j-th star,
• a_j and e_j are its semimajor axis and eccentricity.

A gaseous accretion disk that is tilted with respect to a spinning black hole will experience Lense–Thirring precession, at a rate given by the above equation, after setting e = 0 and identifying a with the disk radius. Because the precession rate varies with distance from the black hole, the disk will "wrap up", until viscosity forces the gas into a new plane, aligned with the black hole's spin axis (the "Bardeen–Petterson effect").^[8]

Astrophysical tests

The orientation of an astrophysical jet can be used as evidence to deduce the orientation of an accretion disk; a rapidly changing jet orientation suggests a reorientation of the accretion disk, as described above. Exactly such a change was observed in 2019 with the black hole X-ray binary in V404 Cygni.^[9]

Pulsars emit rapidly repeating radio pulses with extremely high regularity, which can be measured with microsecond precision over time spans of years and even decades. A 2020 study reports the observation of a pulsar in a tight orbit with a white dwarf, to sub-millisecond precision over two decades. The precise determination allows the change of orbital parameters to be studied; these confirm the operation of the Lense–Thirring effect in this astrophysical setting.^[10]

It may be possible to detect the Lense–Thirring effect by long-term measurement of the orbit of the S2 star around the supermassive black hole in the center of the Milky Way, using the GRAVITY instrument of the Very Large Telescope.^[11] The star orbits with a period of 16 years, and it should be possible to constrain the angular momentum of the black hole by observing the star over two to three periods (32 to 48 years).^[12]

See also

• Gravity Probe B

External links

• (German) explanation of Thirring-Lense effect Has pictures for the satellite example.

Notes and References

1. Pfister . Herbert . On the history of the so-called Lense–Thirring effect. General Relativity and Gravitation . November 2007. 39. 11. 1735–1748. 10.1007/s10714-007-0521-4 . 2007GReGr..39.1735P . 10.1.1.693.4061 . 22593373 .
2. Book: Ronald Adler . Maurice Bazin . Menahem Schiffer . Introduction to General Relativity . registration . 1965 . McGraw-Hill Book Company . 0-07-000423-4 . Section 7.7.
3. Book: Charles W. Misner . Kip S. Thorne . John Archibald Wheeler . Gravitation . 1973 . W. H. Freeman . 0-7167-0334-3 . Chapter 19.
4. Web site: Juno Science Objectives . October 13, 2008 . . https://web.archive.org/web/20081016083427/http://juno.wisc.edu/science.html . October 16, 2008.
5. Iorio . L. . August 2010 . Juno, the angular momentum of Jupiter and the Lense–Thirring effect . . 15 . 6 . 554–560 . 0812.1485 . 2010NewA...15..554I . 10.1016/j.newast.2010.01.004.
6. Book: Merritt, David . David Merritt . Dynamics and Evolution of Galactic Nuclei . 2013 . . Princeton, NJ . 169 . 978-1-4008-4612-2 .
7. Merritt . David . David Merritt . Vasiliev . Eugene . Spin evolution of supermassive black holes and galactic nuclei . Physical Review D . November 2012 . 86 . 10 . 102002 . 2012PhRvD..86j2002M . 1205.2739 . 10.1103/PhysRevD.86.022002. 118452256 .
8. Bardeen . James M. . Petterson, Jacobus A. . The Lense–Thirring Effect and Accretion Disks around Kerr Black Holes . The Astrophysical Journal Letters . January 1975 . 195 . L65 . 1975ApJ...195L..65B . 10.1086/181711. free .
9. James C. A. Miller-Jones . Alexandra J. Tetarenko . Gregory R. Sivakoff . Matthew J. Middleton . Diego Altamirano . Gemma E. Anderson . Tomaso M. Belloni . Rob P. Fender . Peter G. Jonker . Elmar G. Körding . Hans A. Krimm . Dipankar Maitra . Sera Markoff . Simone Migliari . Kunal P. Mooley . Michael P. Rupen . David M. Russell . Thomas D. Russell . Craig L. Sarazin . Roberto Soria . Valeriu Tudose . 29 April 2019 . A rapidly changing jet orientation in the stellar-mass black-hole system V404 Cygni . Nature . 569 . 7756 . 374–377 . 10.1038/s41586-019-1152-0. 1906.05400 . 31036949 . 2019Natur.569..374M . 139106116 .
10. News: Space-time is swirling around a dead star, proving Einstein right again . 2020-01-30 . Space.com.
11. Eisenhauer . Frank . etal . GRAVITY: Observing the Universe in Motion . The Messenger . March 2011 . 143 . 16–24 . 2011Msngr.143...16E .
12. Grould . Marion . Vincent . Frédéric H. . Paumard . Thibaut . Perrin . Guy . Detection of relativistic effects on the S2 orbit with GRAVITY . Proceedings of the International Astronomical Union . Cambridge University Press (CUP) . 11 . S322 . 2016 . 1743-9213 . 10.1017/s174392131601245x . 25–30. free .