Editing Quadrupole (section)

=== Gravitational quadrupole ===
The mass quadrupole is analogous to the electric charge quadrupole, where the charge density is simply replaced by the mass density and a negative sign is added because the masses are always positive and the force is attractive. The gravitational potential is then expressed as:
<math display="block">V_\text{q}(\mathbf{R}) = -\frac{G}{2|\mathbf{R}|^3} \sum_{i,j} Q_{ij}\, \hat{R}_i \hat{R}_j\ .</math>

For example, because the Earth is rotating, it is oblate (flattened at the poles). This gives it a nonzero quadrupole moment. While the contribution to the Earth's gravitational field from this quadrupole is extremely important for artificial satellites close to Earth, it is less important for the Moon because the <math>{1}/{|\mathbf{R}|^3}</math> term falls quickly.

The mass quadrupole moment is also important in [[general relativity]] because, if it changes in time, it can produce [[gravitational radiation]], similar to the electromagnetic radiation produced by oscillating electric or magnetic dipoles and higher multipoles. However, only quadrupole and higher moments can radiate gravitationally. The mass monopole represents the total mass-energy in a system, which is conserved—thus it gives off no radiation. Similarly, the mass dipole corresponds to the center of mass of a system and its first derivative represents momentum which is also a conserved quantity so the mass dipole also emits no radiation. The mass quadrupole, however, can change in time, and is the lowest-order contribution to gravitational radiation.<ref>{{Cite journal| last = Thorne | first = Kip S. | journal = Reviews of Modern Physics | title = Multipole Expansions of Gravitational Radiation |date=April 1980 | volume = 52 | issue = 2 | pages = 299–339 | doi = 10.1103/RevModPhys.52.299 | bibcode=1980RvMP...52..299T| url = http://authors.library.caltech.edu/11159/1/THOrmp80a.pdf }}</ref> Because only the mass multipole moments above the dipole moment can contribute to gravitational radiation, gravitational radiation is significantly weaker in magnitude to electromagnetic radiation.<ref>{{Cite book |last=Kennefick |first=Daniel |title=Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves |date=2016 |publisher=Princeton University Press |isbn=978-1-4008-8274-8 |location=Princeton, NJ}}</ref> The magnitude of the gravitational radiation, or gravitational wave, can be described by [[Quadrupole formula|Einstein's quadrupole formula]].<ref>{{Cite journal |last=Chruściel |first=Piotr T. |date=2019 |title=Elements of General Relativity |url=https://link.springer.com/book/10.1007/978-3-030-28416-9 |journal=Compact Textbooks in Mathematics |language=en |doi=10.1007/978-3-030-28416-9 |issn=2296-4568|url-access=subscription }}</ref>

The simplest and most important example of a radiating system is a pair of mass points with equal masses orbiting each other on a circular orbit, an approximation to e.g. special case of binary [[black hole]]s. Since the dipole moment is constant, we can for convenience place the coordinate origin right between the two points. Then the dipole moment will be zero, and if we also scale the coordinates so that the points are at unit distance from the center, in opposite direction, the system's quadrupole moment will then simply be
<math display="block">Q_{ij} = M\left(3x_i x_j - |\mathbf{x}|^2 \delta_{ij}\right) </math>

where '''M''' is the mass of each point, <math>x_i</math> are components of the (unit) position vector of one of the points, and <math>\displaystyle \delta_{ij}</math> is the Kronecker delta.<ref>{{Cite book |last=Miller |first=M. Coleman |title=Gravitational waves in physics and astrophysics: an artisan's guide |last2=Yunes |first2=Nicolás |date=2021 |publisher=IOP Publishing |isbn=978-0-7503-3051-0 |series=AAS-IOP astronomy. [2021 collection] |location=Bristol [England] (Temple Circus, Temple Way, Bristol BS1 6HG, UK)}}</ref> As they orbit, this '''x'''-vector will rotate, which means that it will have a non-zero first, and also a non-zero second time derivative (this is of course true regardless the choice of the coordinate system). Therefore, the system will radiate gravitational waves. Energy lost in this way was first observed in the changing period of the [[Hulse–Taylor binary]], a pulsar in orbit with another neutron star of similar mass.

Just as electric charge and current multipoles contribute to the electromagnetic field, mass and mass-current multipoles contribute to the gravitational field in general relativity, causing the so-called [[gravitomagnetism|gravitomagnetic]] effects. Changing mass-current multipoles can also give off gravitational radiation. However, contributions from the current multipoles will typically be much smaller than that of the mass quadrupole.<ref>{{Cite book |last=Miller |first=M. Coleman |title=Gravitational waves in physics and astrophysics: an artisan's guide |last2=Yunes |first2=Nicolás |date=2021 |publisher=IOP Publishing |isbn=978-0-7503-3051-0 |series=AAS-IOP astronomy. [2021 collection] |location=Bristol [England] (Temple Circus, Temple Way, Bristol BS1 6HG, UK)}}</ref>