Superluminal motion

In astronomy, superluminal motion is the apparently faster-than-light motion seen in some
radio galaxies, quasars and recently also in some galactic sources called microquasars. All of these sources are thought to contain a black hole, responsible for the ejection of mass at high velocities.

When first observed in the early 1970s, superluminal motion was taken to be a piece of evidence against quasars having cosmological distances. Although a few astrophysicists still argue for this view, most believe thatapparent velocities greater than the velocity of light are
optical illusions and involve no physics incompatiblewith the theory of special relativity.

Explanation

The explanation can be given in a fairly straightforward way as a light travel time effect. Imagine a body of matter starting at the center of a galaxy and moving quickly towards the observer, nearly head-on but not exactly.

When the body is at the center of the galaxy, it emits some light towards the observer. After it has moved, and again emits light towards the observer, this light will have a shorter travel time since the object is now closer to Earth. An observer ignoring the movement towards Earth and only noticing the perpendicular movement will underestimate the true time interval (for their inertial reference frame), and so will overestimate the object's speed; this overestimated speed can be many times the speed of light.

This explanation depends on the jet making a sufficiently narrow angle with the observer's line-of-sight to explain the degree of superluminal motion seen in a particular case. [See http://www.mhhe.com/physsci/astronomy/fix/student/chapter24/24f10.html for a graph of angle versus apparent speeds for two given actual relativistic speeds.]

Superluminal motion is often seen in two opposing jets, one moving away and one toward Earth. If Doppler shifts are observed in both sources, the velocity "and" the distance can be determined independently of other observations.

ome contrary evidence

As early as 1983, at the "superluminal workshop" held at Jodrell Bank, referring to the seven then-known superluminal jets,

Schilizzi ... presented maps of arc-second resolution [showing the large-scale outer jets] ... which ... have revealed outer double structure in all but one (3C 273) of the known superluminal sources. An embarrassment is that the average projected size [on the sky] of the outer structure is no smaller than that of the normal radio-source population. [(R Porcas, "Superluminal motion: Astronomers Still Puzzled", "Nature", vol.302, no.28, April 1983, p.753)]

In other words the jets are evidently not, on average, close to our line-of-sight. (Their apparent length would appear much shorter if they were.)

In 1993, Thomson et al suggested that the (outer) jet of the quasar 3C 273 is nearly perpendicular to our line-of-sight. Superluminal motion of up to ~9.6c has been observed along the (inner) jet of this quasar. [R D Thomson, C D Mackay and A E Wright, "Internal structure and polarization of the jet of the quasar 3C273", "Nature", vol.365, 9 Sept. 1993, p.135 (cf. p.134); T J Pearson et al, " [http://www.nature.com/nature/journal/v290/n5805/abs/290365a0.html Superluminal expansion of quasar 3C273] ", "Nature", vol.290, 2 April 1981, p.365-; Davis, Unwin, Muxlow, " [http://www.nature.com/nature/journal/v354/n6352/abs/354374a0.html Large-scale superluminal motion in the quasar 3C273] ", "Nature", vol.290, 5 Dec. 1991, pp.374-6.]

Superluminal motion of up to 6c has been observed in the inner parts of the jet of M87. To explain this in terms of the "narrow-angle" model, the jet must be no more than 19° from our line-of-sight. [J A Biretta et al, " [http://www.nature.com/nature/journal/v401/n6756/full/401891a0.html Formation of the radio jet in M87 at 100 Scwarzchild radii from the central black hole] ", "Nature", vol.401, 28 October 1999 (pp.891-2), p.* ; Biretta, W B Sparks, F Maccheitto, " [http://adsabs.harvard.edu/abs/1999ApJ...520..621B Hubble Space Telescope Observations of Superluminal Motion in the M87 Jet] ", "Astrophysical Journal", vol.520, pp.621-6, 1 August 1999.] But evidence suggests that the jet is in fact at about 43° to our line-of-sight. [Biretta, Zhou, Owen, "Detections of Proper Motions in the M87 Jet", "Astrophys. Jnl", vol.447, 1995, p.582.]

Suggestions of turbulence and/or "wide cones" in the inner parts of the jets have been put forward to try to counter such problems, and there seems to be some evidence for this. [See, e.g., J A Biretta et al, " [http://www.nature.com/nature/journal/v401/n6756/full/401891a0.html Formation of the radio jet in M87 at 100 Scwarzchild radii from the central black hole] ", "Nature", vol.401, 28 October 1999 (pp.891-2).]

History

In 1966 Martin Rees predicted (Nature 211, 468) that "an object moving relativistically in suitable directions may appear to a distant observer to have a transverse velocity much greater than the velocity of light".

A few years later (in 1970) such sources were indeed discovered as very distant astronomical radio sources, such as radio galaxies and quasars. They were called superluminal (lit. "above light") sources. The discovery was a spectacular result of a new technique called Very Long Baseline Interferometry, which allowed astronomers to determine positions better than milli-arcseconds and in particular to determine the change in positions on the sky, called proper motions in a timespan of typically years. The apparent velocity is obtained by multiplying the observed proper motion by the distance and could be up to 6 times the speed of light.

In 1994 a galactic speed record was obtained with the discovery of a superluminal source in our own galaxy, the cosmic x-ray source GRS 1915+105. The expansion occurred on a much shorter timescale. Several separate blobs were seen (I.F. Mirabel and L.F. Rodriguez, "Nature" 371, 48, "A superluminal source in the Galaxy") to expand in pairs within weeks by typically 0.5 arcsec. Because of the analogy with quasars, this source was called a microquasar.

Notes

External links

* [http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/Superluminal/superluminal.html A more detailed explanation]

* [http://www.physics.purdue.edu/MOJAVE/superluminal.html Superluminal motion Flash Applet]