What’s Out There?
Questions about what exists mark the starting point of science. Scientific explorers of the past reveled in voyaging to exotic lands in pursuit of animal, plant, and mineral specimens. Chemists isolated element after element, physicians dissected diseased corpses, astronomers cataloged countless stars, and physicists scrutinized unusual phenomena associated with electricity and magnetism.Even after centuries of labors, by most estimates we have identified only one or two percent of all living species on earth, have sampled only the thin outer skin of the planet, and have described only a few of the 80,000 proteins that our bodies produce. We know all 100 or so stable elements of the periodic table, but the number of possible combinations of these elements is for all intents and purposes infinite. Looking outward to space, we observe tens of billions of stars in each of tens of billions of galaxies-perhaps a trillion solar systems exist for every human. There is so much left to discover.
Skeptics would have us believe that the hundreds of thousands of scientists around the world who devote their lives to exploring these domains are like high-tech postage-stamp collectors-filling in a few blanks rather than pursuing interesting research. These skeptics are wrong. The earth, our solar system, and the universe beyond holds wonders to captivate (and profit) the human race for millennia.
Moreover, if the task of describing the tangible universe weren’t enough, it now appears that most of the mass of the universe-as much as 99 percent by some estimates-is missing, evidently consisting of strange matter unlike anything we now comprehend. Within the past two decades astronomers have discovered overwhelming evidence that the universe is littered with dark matter-seemingly invisible stuff that must be out there but can’t be found even with our most powerful telescopes.
Almost all of the universe’s matter that we know about is concentrated in galaxies, which exist on a scale almost beyond comprehension. Each galaxy holds tens to hundreds of billions of stars in a region that may exceed a 100,000 light years in diameter (a light year, the distance light travels in a year, is almost 6 trillion miles). Our own galaxy, the Milky Way, contains all the stars and constellations that are familiar to us in the night sky, but billions of other galaxies are also easily visible with the aid of telescopes.
For astronomers who want to study the nature and distribution of the universe’s mass, galaxies are the logical place to start. These scientists rely on two complementary methods to estimate a galaxy’s mass. The quick and easy way is to count the total number of visible stars (an effort simplified by image-processing computers), and then multiply that number times the average mass per star (a value painstakingly determined from observations and theory). This calculated value is known as the “visible mass” of a galaxy.
Alternatively, astronomers determine the “dynamical mass” of a galaxy by observing how stars move. Specifically, they measure the position and orbital speed of its stars or clouds of gas as they circle about the galactic center, the locus of immense gravitational forces. The more massive the galaxy, the faster its stars must travel in their galactic orbits to keep from falling in, closer to the center. Ultimately, if we have properly accounted for all of a galaxy’s variables, the visible mass should exactly match the dynamical mass.
But in the 1970s astronomers discovered that outer portions of spiral galaxies rotate two to three times faster than they should, based on the gravity produced by stars we can see. The simple equation describing orbits has only three variables: orbital distance, orbital speed, and mass. Two of these variables, orbital distance and speed, can be measured by telescopic observations, so a galaxy’s true mass can be calculated. The conclusion: estimates of mass based on visible stars are wrong; most of a galaxy’s mass is not visible. It follows that most of the matter in the universe is dark and invisible.
Speculation about the nature of dark matter abounds. The first step is to eliminate what dark matter isn’t: It can’t be made of ordinary clumps of matter like snowballs or black holes, because we could detect its effects on light arriving from more distant sources. It can’t be made of electrically charged particles like electrons or protons, because such particles emit telltale electromagnetic radiation. Indeed, the fact that we can’t presently detect dark matter in the laboratory suggests that it must pass right through ordinary collections of atoms.
Faced with these daunting constraints, scientists have postulated a number of weird possibilities for the missing mass-exotic subatomic particles such as massive neutrinos or axions, mini black holes, or clusters of quarks called quark nuggets-but no one knows for sure. Around the world, teams of physicists are struggling to design more sensitive detectors to capture the subtle signals of dark matter. It may take many decades, but researchers are not likely to give up for lack of interest.
If all our present science is based on observations and measurements of a paltry 1 percent of reality’s building blocks-everyday atomic matter-then how can physics be almost over? The search for dark matter, still in its barest infancy, is not a trivial academic pursuit. In fact, the nature and amount of missing mass is closely tied to the ultimate fate of the universe, namely whether the expanding universe’s enormous gravity will eventually cause it to slow and finally collapse back into itself. The missing mass problem thus lies at the heart of our most fundamental attempts to understand the past, present, and future state of the cosmos.
Moreover, how astounding it is to think that the stuff of which we are made and the only matter we know may constitute only a tiny fraction of what exists. We are confronted with so many questions: What is this strange stuff? How can we study it? What laws govern its behavior? And if we can confine and shape this matter to our will, what undreamed of technologies might follow?