There is no better established fact of physical reality than that nothing, neither matter, radiation nor information, propagates through empty space at a speed greater than the speed of light in a vacuum, about 300,000 km per second (or 186,000 miles per second, in units left over from the Stone Age), as seen by any non-accelerating observer.

Yet astronomers who view very distant, extremely active young galaxies (known as quasars) often observe gigantic jets of material ejected from the cores of these very unstable systems. These jets travel at speeds very close to that of light, up to about 97% of light speed. What is interesting is that when one of the jets is directed approximately toward earth, it appears to move at a speed many times the speed of light! This "superluminal jet velocity" is a very simple consequence of the fact that the speed of light is finite, and I have not found anywhere on the Internet a clear explanation of how it happens. Often the faster-than-light jet speed is called an "optical illusion." Nothing of the kind. What's going on? Let's choose a simple example where the numbers are easy for me to type in and for you to read and manipulate in your head.

Suppose an alien space ship is about 100 light years from earth and coasting at nearly the speed of light when it test-fires its engines in a great flare that can easily be viewed by earth astronomers. Also suppose its direction of motion is almost (but not quite) directly toward earth. Wait 50 years... the space ship is nearly 50 light years closer to earth, and the light its engines emitted is precisely 50 light years closer. To be a bit more specific, suppose, viewed from earth, the light is about a year ahead of the space ship. At this time, imagine the still-coasting space ship test-fires its engines again. A new burst of light is emitted. This new burst of light takes about 50 years to reach the earth, but the first burst already has a one year head start, and arrives first by one year. So what do earth astronomers see? They see two bursts of light from the same object. The bursts come from points in space about 50 light years apart, but they are seen at times just one year apart. The object from which they were emitted obviously appears to have travelled about 50 light years in one year, and thus to be moving 50 times faster than light.

Note that when the space ship or galactic jet is moving nearly directly away from observer, the reverse situation occurs. Even though the ship or jet is moving at just a few percent less than the speed of light, it would appear to be standing almost still! Since light always takes a finite time to cover a finite distance, when we observe any object, it is always seen at a position it occupied in the past, not the position it occupies "now". In everyday life, the discrepancy is so tiny we could not notice. But when distances involve light years and speeds are close to the speed of light, the discrepancy is enormous and unless we are aware of it, what we see will often seem impossible or incomprehensible.

For movies of such quasar jets, click right here. By the way, it is worth mentioning that the visionary British author Olaf Stapledon imagined such jets in the early 20th Century, long before they were observed by astronomers. In picturing the entire history of intelligent life in the universe, he imagined that the very earliest galaxies to form were intelligent beings, and that they battled one another by ripping rivals apart with immense particle beams generated at their dynamo-like centers. See his incredible novel STARMAKER for more details.

By the way, astronomers are never baffled by what the actual speed of an object relative to earth is, because moving objects seen by astronomers invariably consist of 90% ionized hydrogen... that's what most visible matter in the universe is. Since the spectrum of the object at rest is as well known as one's childhood back yard, the Doppler shift of the spectrum that is observed instantly supplies the actual speed of the object when the light left it, relative to earth. It is also worth mentioning that relativistic jets are emitted by some fairly nearby individual stars, not just some early, extremely distant galaxies.