TURBULENCE IN

INTERSTELLAR SPACE

by Ben Holder

Many physicists consider turbulence the greatest unsolved problem in classical physics. Although scientists have observed turbulence for hundreds of years, little is known about how it actually creates the random motions that dominate the oceans, atmosphere, and almost every fluid movement on earth. I have recently been involved in a project that is trying to understand turbulence by using space as the laboratory for investigation. Our group of astronomers have been observing the fluid jets of young, violent stars located halfway across our Galaxy. We have discovered that turbulence in space looks very similar to turbulence seen on Earth. But, our theories seem to contradict some of the recent theories that attempt to describe the workings of turbulence. It is our contention that this apparent paradox highlights the fact that turbulence is not as well understood as previously thought.

Perhaps the most familiar example of turbulence is found in airplane travel. The jolt we get by "encountering some turbulence" is a result of randomly occurring gusts of wind that throw our plane in a random direction. Chaotic activity of this sort troubles physicists because their primary scientific goal is to predict events. If a physicist drops a leaf in a familiar stream, she expects to know exactly where it will be after 10 seconds. Indeed, if the stream is flowing smoothly she will be able to make a good guess after studying the path and velocity of the water. If, however, that stream becomes turbulent, swirling eddies of changing shape and size will appear, and the physicist will have no way of knowing where the leaf might end up.

Figure 1

Our team of astronomers have observed two regions of young and active stars that have huge jets of material shooting out of them. When the jets strike the surrounding gases, they create a turbulent storm that sparks the formation of bright gas clumps, called masers (figure 1). These masers, which work in a way similar to the way lasers work on earth, allow us to see the detailed motions of these turbulent jets. By studying the movement of these masers in both regions, we have been able to test for signatures of earthly turbulent behavior. The only measurable test that has been found to describe turbulence accurately is called "Kolmogorov's law." This law is a relation between the velocity difference of two points in a turbulent fluid and their separation. In earthly turbulence, the ratio of these two quantities (or the slope of their plotted relationship -- Figure 2) is known to be one-third. Our astronomic observations of both regions show exactly the same ratio. This seems to be evidence that turbulence is a universal phenomenon -- the same physical process we might se in a country stream is also occurring in hundred-million-mile-long stellar jets.

Figure 2 -- Graph of the velocity versus the distance between each master in the region. Notice the slope of this graph is 1/3 -- the same slope that Kolmogorov discovered for turbulence on earth.

Another familiar feature of astronomical turbulence is that it does not fill space. There are many small pockets of active turbulence that are surrounded by large regions of unaffected gas. This quality, called intermittency, is well known - the bumps we experience on an airplane are interspersed with hours of smooth flying. Intermittency in earthly turbulence, however, is much less extreme than our astronomical turbulence. In the jets we observe, turbulence is concentrated into extremely small bundles of activity. If one were to fly a plane through a stellar jet, the ride would hardly ever be bumpy.

Figure 3 -- Four views of the turbulent region. The spots on the graphs indicate maser activity. Each graph is shown at a progressively smaller scale. Notice the same sort of clumpiness and large patches of inactivity at each scale.

Note****1AU=the distance from the earth to the sun =90 million miles

Although intermittency is common in earthly turbulence, its presence is considered a challenge to Kolmogorov's law. Most theorists claim that pure turbulence completely fills space and that strong intermittency should produce a noticeable change in the law. These scientists make this claim based on a number of newly proposed ideas about how turbulence actually works. But, because intermittency in of earthly turbulence is slight, all of these theories remain unproved. Our astronomical observations, however, provide an ideal laboratory for testing these intermittency theories. According to one theory, the velocity/separation ratio should change from 1/3 to nearly 2. Our observations, however, show the ration remains exactly 1/3. Our results therefore contradict the corrective theories made to account for intermittency, and call into question the physical models upon which they were based.

This is perhaps the most serious conclusion to be gleaned from my research. Models that have given scientists a clear idea of how earthly turbulence works but that put special requirements on its behavior cannot hold up to our experimental tests in space. As a physical process, turbulence seems to be much more complicated than we can yet imagine -- a chance for nature to keep physicists frowning at streams in disbelief.

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