The Dark Matter Mystery

The Dark Matter Mystery.

I’ve been thinking about “dark matter” problems for years now, but only recently did it dawn on me how deep the quest for dark matter penetrates into philosophy and even theology. Today’s dark-matter problems arise when we study the motion of stars and galaxies, the bending of light from very distant galaxies, or how the expansion of the universe changes with time. In all those cases, when we work back from the observed motions to their causes, we find discrepancies. There’s too much gravity. The objects we can see, using all the tools of astronomy, just don’t have enough mass to generate the gravitational forces we infer. Everything can be accounted for if there’s a new form of matter, so far unidentified, which interacts very feebly with ordinary matter (and with itself), and if space has a small density. That two-pronged “solution” might seem desperate, but the quest for things inferred, yet invisible, has a glorious pedigree.

In the 19th century, precise calculations of the orbit of Uranus disagreed with accurate observations. In 1846 Urbain Le Verrier and John Couch Adams proposed that the influence of another planet, as yet unseen, might cause the discrepancy. Le Verrier was able to tell observers where they should point their telescopes. He nailed it. They looked—and discovered Neptune. Around the same time, Friedrich Bessel proposed that jittery movements of two stars, Sirius and Procyon, occurred because each had a companion invisible to the telescopes of the time. Only decades later did astronomers develop sufficiently powerful tools to see the stars’ partners—members of a very dense, Earth-sized class of stars, the white dwarfs.

In 1930, Wolfgang Pauli postulated new subatomic particles, neutrinos, as a kind of dark (that is, unseen) matter. They could account for “missing” energy and momentum in radioactive decays. At the time, Pauli said that he had done something very bad by proposing “a particle that cannot be detected.” But in 1956 neutrinos were detected, and today their study is an industry in experimental physics. Long before these advances, in 1692, Isaac Newton had posed the deepest dark-matter question. He wrote that the idea “that one body may act upon another at a distance thro’ a Vacuum, without the Mediation of anything else…is to me so great an Absurdity” that no competent thinker could fall for it. For Newton, space could not be a void. There had to be something, yet undetermined, to support the forces between bodies. Centuries later, the theory of electric and magnetic “fields,” entities that fill all space, vindicated Newton’s intuition. The Scots physicist James Clerk Maxwell, whose 1864 equations epitomised the new understanding, rhapsodised, “The vast interplanetary and interstellar regions will no longer be regarded as waste places in the universe, which the Creator has not seen fit to fill…so full, that no human power can…produce the slightest flaw in its infinite continuity.”

In Einstein’s general relativity theory, space itself becomes, even more tangible, a material medium. Space can bend: That’s how general relativity accounts for gravity. And, as this year’s Nobel Prize celebrates, space can “ring” like a bell (reverberating with gravitational waves)! So it’s not unreasonable, as Einstein anticipated, that space has non-zero density, as astronomers recently discovered. By dumping a lot of energy into a small volume, at high-energy accelerators, we can shatter space and see what it’s made of. We’ve unearthed a lot of things that contribute, but how they conspire to give the density we observe remains deeply mysterious. Physicists also have some promising ideas about what the hypothetical “dark matter” particles might be. They’re designing fantastically sensitive new kinds of instruments to observe them. Effects without apparent causes inspire us to look at the world in new ways. Thus do we render darkness visible.

Credit: Frank Wilczek for The Wall Street Journal, 26 October 2017.