Last week I took a little break from work to participate in Virtual Drinking Skeptically, which is a fun video chat with cool skeptical people from all over the world. Someone, and I’m sorry because I can’t remember who it was as I was still fiddling with the software and dealing with a slow network… brought up jets in galaxies that seem to be moving faster than the speed of light. I don’t think I did a very good job of explaining it, and I couldn’t find a terribly good illustration right away, so I’ll try again here!
Most galaxies have a supermassive black hole (that’s millions of times the mass of the sun) in the center, and if this black hole has gas and other material falling onto it, you get a bright, energetic active galactic nucleus (AGN). Sometimes, this AGN gives off powerful radio radiation in the form of a jet of material that blasts away from the nucleus at nearly the speed of light.
Boom, baby. (Click to embiggenate. Courtesy NRAO/AUI)
In some of the jets, when you zoom in to the very small scales with a telescope like the Very Long Baseline Array, you can track “blobs” of luminous material as it moves away from the black hole at the center. And in some of these cases, the blobs appear to be moving across the sky at a speed greater than the ultimate, cosmic speed limit… the speed of light!
3C279, how fast can you go? via NRAO/AUI.
To find out what’s really going on, you need to take the angle of the jet into account. The jet of 3C279, and other such AGNs that appear to show superluminal motion, are lined almost along the line of sight between the AGN and Earth.
Let’s take an example where a blob of material moves at about 0.96 times the speed of light… fast, but not impossible for this blob of ionized material in a jet. It moves 25 light years (that’s a measure of distance, don’t forget!) in a span of 26 years along a path that is just a few degrees away from our line of sight. We take observations of this blob at the beginning of this track and again at the end of it, so we are seeing the light emitted from the blob at each of the plotted positions.
Ignoring my imprecise angles, you can make a right triangle where you break the distance it has traveled into its components along and perpendicular to our light of sight.
At the first position, the blob’s light is given off to be detected by a telescope on Earth. 26 years later, light is give off from a different position that is now 24 light years closer to Earth. Therefore, the light is received only two year after the observation of the first position!
To an earthling, it appears that they blob has moved horizontally 7 light years in just 2 years, making its motion 3.5 times the speed of light! But in reality, it was just going 0.96 times light speed.
Apparent superluminal motion is a fun little concept, and it is a great example of astronomical objects that show change well within a human lifetime. And it shows us that you don’t want to call Einstein wrong just at the first glance of the observations. Radio jets point at or away from us at all angles, so naturally we will see some show this phenomenon. (More than you would think because of relativistic beaming, but that’s another story.)
This also has nothing to do with the concept of the “edge of the visible universe” in which galaxies are receding away from us at faster than the speed of light, so we will never see them, because of the rapid expansion of spacetime. This also does not violate the cosmic speed limit since space itself is expanding and just dragging the galaxies along, and space is not subject to the same constraints as matter.
Lest you ever forget… the universe is a weird and awesome place…