Editing Relativistic beaming (section)

{{short description|Change in luminosity of a moving object due to special relativity}}
{{Redirect|Doppler beaming|other uses named for Christian Doppler|Doppler (disambiguation)}}
{{Redirect|Beaming|beaming in music|Beam (music)|the album by Tony Oxley|Beaming (album)}}
{{refimprove|date=March 2014}}
{{expert needed|physics|reason=to provide an accessible overview of the physics and the governing equations|date=November 2024}}

[[File:M87 jet.jpg|thumb|upright=0.88|Only a single jet is visible in [[Messier 87|M87]].]]
[[File:Radio galaxy 3C31.png|thumb|upright=1|Two jets are visible in [[NGC 383|3C 31]].]]

In [[physics]], '''relativistic beaming''' (also known as '''Doppler beaming, Doppler boosting,''' or the '''headlight effect''') is the process by which [[Special Relativity|relativistic effects]] modify the apparent luminosity of emitting matter that is moving at speeds close to the [[speed of light]]. In an astronomical context, relativistic beaming commonly occurs in two oppositely-directed [[relativistic jet]]s of [[Plasma (physics)|plasma]] that originate from a central [[Compact star|compact object]] that is [[Accretion disc|accreting]] matter. Accreting compact objects and relativistic jets are invoked to explain [[X-ray binary|x-ray binaries]], [[gamma-ray burst]]s, and, on a much larger scale, [[Active galactic nucleus|(AGN) active galactic nuclei]] (of which [[quasar]]s are a particular variety).

Beaming affects the apparent brightness of a moving object. Consider a cloud of gas moving relative to the observer and emitting electromagnetic radiation. If the gas is moving towards the observer, it will be brighter than if it were at rest, but if the gas is moving away, it will appear fainter. The magnitude of the effect is illustrated by the [[Active galactic nucleus|AGN]] jets of the galaxies [[Messier 87#Jet|M87]] and [[NGC 383|3C 31]] (see images at right). M87 has twin jets aimed almost directly towards and away from Earth; the jet moving towards Earth is clearly visible (the long, thin blueish feature in the top image at right), while the other jet is so much fainter it is not visible.<ref>{{Cite journal
| volume = 355
| pages = 804–806
| last = Sparks
| display-authors = etal
| first = W. B.
| title = A counterjet in the elliptical galaxy M87
| journal = Nature
| date = 1992
| doi = 10.1038/355804a0
| bibcode=1992Natur.355..804S
| issue=6363
| s2cid = 4332596
}}</ref> In 3C 31, both jets (labeled in the lower figure at right) are at roughly right angles to our line of sight, and thus, both are visible. The upper jet points slightly more in Earth's direction and is therefore brighter.<ref>{{Cite journal |last1=Laing |first1=R. |last2=Bridle |first2=A. H. |date=2002 |title=Relativistic models and the jet velocity field in the radio galaxy 3C 31 |journal=Monthly Notices of the Royal Astronomical Society |volume=336 |issue=1 |pages=328–352 |arxiv=astro-ph/0206215 |bibcode=2002MNRAS.336..328L |doi=10.1046/j.1365-8711.2002.05756.x |s2cid=17253191 |doi-access=free}}</ref>

Relativistically, moving objects are beamed due to a variety of physical effects. [[Relativistic aberration|Light aberration]] causes most of the photons to be emitted along the object's direction of motion. The [[Relativistic Doppler effect|Doppler effect]] changes the energy of the photons by red- or blue shifting them. Finally, time intervals as measured by clocks moving alongside the emitting object are different from those measured by an observer on Earth due to [[time dilation]] and photon arrival time effects. How all of these effects modify the brightness, or apparent luminosity, of a moving object is determined by the equation describing the [[relativistic Doppler effect]] (which is why relativistic beaming is also known as Doppler beaming).