4.3. Superluminal Motion
Dramatic confirmation of the suspected relativistic motions came from the advancing technology of radio astronomy. Radio astronomers using conventional interferometers had shown that many sources had structure on a sub-arcsec scale. Scintillation of the radio signal from some AGN, caused by the interplanetary medium of our solar system, also implied sub-arcsec dimensions (Hewish, Scott, and Wills 1964). The compact radio sources in some AGN showed flat spectrum components and variability on timescales of months (Dent 1965; Sholomitsky 1965). The variability suggested milliarcsec dimensions on the basis of light travel time arguments. The spectral shape and evolution found explanation in terms of multiple, expanding components that were optically thick to synchrotron self-sbsorption, which causes a low frequency cutoff in the emitted continuum (Pauliny-Toth and Kellermann 1966, and references therein). Such models had interesting theoretical consequences, including angular sizes (for cosmological redshifts) as small as 10-3 arcsec, and large amounts of energy in relativistic electrons, far exceeding the energy in the magnetic field.
These inferences made clear the need for angular resolution finer than was practical with conventional radio interferometers connected by wires or microwave links. This was achieved by recording the signal from the two antennas separately on magnetic tape, and correlating the recorded signals later by analog or digital means. This technique came to be known as "very long baseline interferometry" (VLB, later VLBI). After initial difficulties finding "fringes" in the correlated signal, competing groups in Canada and the United States succeeded in observing several AGN in the spring of 1967, over baselines of roughly 200 km (see Cohen et al. 1968). The U.S. experiments typically used the 140 foot antenna at the National Radio Astronomy Observatory in Green Bank, West Virginia, in combination with increasingly remote antennas in Maryland, Puerto Rico, Massachusetts, California, and Sweden. The latter gave an angular resolution of 0.0006 arcsec. Within another year, observations were made between Owens Valley, California, and Parkes, Australia, a baseline exceeding 10,000 km or 80 percent of the earth's diameter. A number of AGN showed components unresolved on a scale of 10-3 arcsec.
On October 14 and 15, 1970, Knight et al. (1971) observed quasars at 7840 MHz with the Goldstone, California - Haystack, Massachusetts "Goldstack" baseline. 3C 279 showed fringes consistent with a symmetrical double source separated by (1.55 ± 0.03) × 10-3 arcsec. Later observations on February 14 and 26, 1971, by Whitney et al. (1971) showed a double source structure at the same position angle, but separated by a distinctly larger angle of (1.69 ± 0.02) × 10-3 arcsec. Given the distance implied by the redshift of 0.538, this rate of angular separation corresponded to a linear separation rate of ten times the speed of light! Cohen et al. (1971), also using Goldstack data, observed "superlight expansion" in 3C 273 and 3C 279. Whitney et al. and Cohen et al. considered a number of interpretations of their observations, including multiple components that blink on and off (the "Christmas tree model") and noncosmological redshifts. However, most astronomers quickly leaned toward an explanation involving motion of emitting clouds ejected from the central object at speeds close to, but not exceeding, the speed of light. Rees (1966) had calculated the appearance of relativistically expanding sources, and apparent expansion speeds faster than that of light were predicted. A picture emerged in which a stationary component was associated with the central object, and clouds were ejected at intervals of several years along a fairly stable axis. (Repeat ejections were observed in the course of time by VLBI experiments.) If this ejection occurred in both directions, it could supply energy to the extended double sources. The receding components would be greatly dimmed by special relativistic effects, while the approaching components were brightened. The two observed components are then associated with the central object and the approaching cloud, respectively. The fact that the two observed components had roughly equal luminosities found an explanation in the relativistic jet model of Blandford and Königl (1979).
Apparent superluminal motion has now been seen in a number of quasars and radio galaxies, and a possibly analogous phenomenon has been observed in connection with black hole systems of stellar mass in our Galaxy (Mirabel and Rodriguez 1994).