Dark Matter, Part I: Dark Semantics

Rashied Figure 2Poor reporting is not unique to the Internet. While almost expected for political events, it is now commonplace to science reporting. Poor science reporting is what compels me to write this column. March of last year I was startled to read the following headlines: “Dark Matter Possibly Found by $2 Billion Space Station Experiment,”Strong hints of dark matter detected by space station, physicists say,” “Cosmic ray detector confirms hints of dark matter,” and “Scientists indirectly detect the existence of dark matter.”

Why startled? These headlined articles come from publications that appear to be legitimate, but don’t acknowledge the otherwise known chronology of dark matter’s discovery. Dark matter was discovered when astronomers observed that the rotation of galaxies cannot be explained with the amount of matter, as contained in stars and gas visible across the electromagnetic spectrum. This finding was published in 1980, after many years of attempting to explain the phenomena in other ways.

Of course, what underlies this confusion is a semantic distinction, which some articles care to make. Even beyond the semantic distinction is a scientific one wherein dark matter was not directly detected as reported in these article titles, but a signal that may indicate dark matter interactions. In a feeble attempt to curb the misinformation and delineate the semantic issue, let me begin by explaining the history of dark matter.

The story of dark matter begins with a discovery in the 1930s. Fritz Zwicky, a Swiss astronomer and physicist at Caltech, noted that the rate at which galaxies of the Coma galaxy cluster moved in the night sky could not be explained by their visible mass. See F Zwicky, “On the Masses of Nebulae and of Clusters of Nebulae.” Astrophysical Journal, vol. 86, p.217, October 1937. (The paper in which this was published makes one of the first references to “gravitational lensing,” as wel.) At its simplest, the luminosity of the galaxies is proportional to the amount of stars—and thus their mass. Masses attract and pull themselves together according to Newton’s law of universal gravitation—so he would have expected a certain rate of movement based on visible mass. But the rate was faster—implying at first glance there was some invisible mass or Newton’s theory of gravity was incorrect. Given all other tests of Newton’s gravity seemed accurate, Zwicky sought other explanations for the missing mass. It was possible, Zwicky wrote, that gas clouds invisible to an optical telescope could either occlude additional starlight while containing substantial mass themselves. (Coincidentally, it was about that time when Karl Jansky invented the radio telescope, which looks at the sky not for visible light but radio waves which have less energy than visible light. Janky’s telescope was thus capable of detecting the radio signal of these cool interstellar clouds.) Whatever the nature of the invisible matter, Zwicky labeled it dunkle Materie, or “dark matter,” for lack of a better description.

Several years later, Horace Babcock, a graduate student and later notable astronomer, develop his dissertation on the rotation curve of the nearest spiral galaxy—the Andromeda. The rotation curve demonstrated the rate at which stars rotated about the center of the galaxy as the distance from the center of the galaxy varied. Based on the visible light, symbolic of the disk-like mass distribution of the galaxy, one would expect a relationship like that of A, but what was observed was B, as seen in the figure at top of this post. This is an example of galactic rotation curve, where radial velocity is plotted against Distance (from galactic center) for expected (A) verse observed (B) observations of stars orbiting a galactic center. Babcock shared Zwicky’s belief that this was attributable to the light absorbing effect of gasses, though now within a galaxy itself and not just occluding stellar light.

It took about 40 years for a study to definitively establish the darkness of this matter as people sought to understand the results of Zwicky and Babcock. Vera Rubin, et al published a landmark paper in 1980 that surveyed 21 spiral galaxies (Sc type) in not just optical frequencies, but radio where cool gas clouds can be detected. After a very thorough examination they also concluded that it “is inescapable that non-luminous matter exists beyond the optical galaxy.” This “non-luminous” matter not only must lie beyond the plane of the spiral galaxies, but also is itself several times the mass of the visible galaxy itself for its rotation rate to be explained!

Now is it just, or even sensible, that these articles listed describe the potential discovery of dark matter? It seems a trifle to explain what “discovery” really means as far as the press is concerned. At the very least, it is precisely the semantic distinction I noted earlier. From Zwicky on, “dark matter” stands for a measurable but unobservable mass. It is simply dark because it is dark. Dark not just in the visible band of light but even in radio waves, X-rays, and all across the seemingly all-encompassing electromagnetic spectrum. The effect of this darkness lets us conclude that it uses gravitation, which is universally attractive, and is matter. Dark matter. Dark like a shadow with an object unseen; matter because it’s there, acting like any other lump of well-behaved matter. Indeed we have discovered dark matter many decades ago but we have only theories about its identity.

One of the most disheartening issues about these articles is scant mention of the many scientists who have worked on understanding the nature of dark matter. Specifically, the AMS-2 results reported by these news articles was a follow-up of several previous studies conducted by AMS-1, Pamela, and Fermi missions looking at the same positron excess (which will be described in a later post). I have only stumbled upon one article that respected the work of scientists behind earlier observations of the same phenomena. It would be unfair to say that popular press reports of scientific work have always been accurate or fair, with a notable example of The New York Times lambasting Robert H. Goddard’s work on early rocketry (“A Severe Strain on Credulity,” Jan. 13, 1920) while completely misunderstanding the laws of physics.

With new media the task of a scientist to break through the wall of misinformation is harder. The volume of news is greater than the context behind events further hidden behind links, misleading titles, and other noise. If indeed there is meaningful news to be read, it is a very weak signal amid a galaxy of noise. Like the experimental scientist, the goal is always to improve the ratio of signal-to-noise. I hope this column does the same.