Faster than Light Superluminal Motions

As we move in close to the core of a radio loud galaxy, we often find something even more incredible. Radio-emitting material blasts outward in bursts that have been tracked by very high-resolution interferometers with a resolving capability of a thousandth of a second of arc. The hot spots often appear to be moving faster than the speed of light! This occurs not just in the jet but very close to the core,

A stunning discovery about tire bright radio cores is that they all show year-to-year movement at the smallest observable scales. Because the distance to a given quasar or radio galaxy is known from the redshift of its optical counterpart, it is possible to calculate how fast such blobs are moving. Depending on which source is being studied, they are found to travel between 3 and 20 times the speed of light! This conclusion, however, is at odds with one of the best known laws of physics—nothing can travel faster than light.

The discovery of this apparently superluminal (faster-than-light) motion in 1971 threw the astronomical community into a temporary tizzy. Relative order was restored by the realization that one can get the illusion of superluminal motion through a peculiar projection effect. If a double radio source is pointed nearly at us, then we obviously see the nearside material directed toward us and the farside material moving away. This has several consequences, some related to predictions made by relativity theory. The gas on the near side is blueshifted (because it is moving toward us) and that at the far side is red shifted (moving away). However, if the ejection velocity is close to the speed of light the emission on our side becomes greatly intensified due to relativistic effects, whereas the farside material would become so faint as to almost disappear. The apparent superluminal motion is a peculiar consequence of the fact that the material ejected toward us is traveling almost as fast as any light it emits toward us. A radio signal (traveling at the speed of light) cannot leave its source very far behind, and therefore two bursts of radio emission separated by a year could appear to us to be separated by a month, say, depending on the speed in the jet and on the angle between the jet and our line of vision. Theref ore, when we see movement in the jets of the radio sources our initial estimate of the velocity of material could be completely wrong. This effect allows us to avoid the faster-than-light dilemma, but then another one pops up! If the large, straight jets are related to the small core jets, they too may be directed toward us, in which case they would be physically much longer than estimated. Therefore the jets may be far larger, and must be far longer-lived, than first suspected.

On the other hand, since the intensity of the emission from a core jet pointed nearly at us is highly dependent on subtle relativity effects, the energy we think it is emitting may be far less than originally estimated!

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