Illusions in the Sky: Objects Speeding Faster Than Light

ACCORDING to Einstein's special theory of relativity, nothing can travel faster than light in a vacuum. Yet there are objects in the sky that seem to break the speed limit. Discovery of the latest speedster was reported recently in the journal Nature.

With several dozen of the so-called superluminal sources already cataloged since 1969, adding one more to the list usually is no big thrill for astronomers. But this one is different. Instead of being several billions of light-years away, it's roughly estimated to be about 40,000 light-years away in our own Milky Way galaxy.

That being so, ``the significance of the discovery must not be underestimated,'' says Galen Gisler of the Los Alamos National Laboratory in New Mexico in a commentary accompanying the research paper.

It's worth taking a second look at this news to see what the excitement is all about.

It's not about breaking Einstein's speed limit. The faster-than-light speeds are an illusion created by the way we see these things in the sky. However, something real has to be moving at a substantial fraction of light-speed to create that illusion. Astronomers believe this to be blobs of plasma ejected with tremendous energy from a powerful source. It's hard to get at the mechanism involved when the source is billions of light-years away. But as Dr. Gisler points out, a superluminal source in our own galaxy is ``the ideal object'' to help astronomers figure out what is going on.

Also, as the discoverers point out, this kind of object offers a new way of determining distances in our galaxy using ``a method that has no precedents in astronomy.'' No wonder these discoverers - I. Felix Mirable of the Centre d'Etudes de Saclay at Gif-sur-Yvette near Paris and Luis F. Rodriguez with the National Autonomous University in Mexico City - have caught astronomers' attention.

The object's central powerhouse is a well-known X-ray source designated GRS1915+105. Drs. Mirable and Rodriguez have found a pair of blobs racing away from this source - one coming toward us, one moving away. Their calculations suggest blob speeds on the order of 90 percent that of light. This makes the blob moving toward us appear to travel at 125 percent light-speed across the sky.

The illusion arises because the blob moves at an angle to our line of sight. We see it move from point A to point B on the sky. We don't realize that point A is farther away from us than is point B so that light from point A takes longer to reach us than does light from point B. The result is that we think the blob covers the distance between the two points in less time than is actually involved. Given the right geometry, this trick of perspective has made blobs in extra-galactic sources appear to move as much as 10 to 15 times faster than light.

Mirabel and Rodriguez hope to pin down their object's distance using two numbers. Measurement of the blobs' angular motion across the sky gives one number. Measuring the change in a blobs' light, infrared, or X-ray emissions due to its velocity gives a second number. From these two numbers, the distance can be calculated accurately.

Extra-galactic superluminal sources produce energy at rates that rival the output of entire galaxies. The new nearby source isn't in that energy league. But the fact that it can throw out material at near light-speed velocities suggests that common mechanisms may be involved.

If more such nearby examples are found, a new line of research will be opened.