by Tasneem Zehra Husain
I'm a total pushover when it comes to stories of connection. I am delighted by accounts of barriers breaking down and disparate people uniting in purpose, of ideas coalescing and theories fusing to reveal the common threads that underlie diversity. As I look back upon the history of physics, what reaches out and grabs me are the moments of unification when strands long thought separate are suddenly braided together in a whole that is stronger and more beautiful than the sum of its parts. Sometimes we uncover hidden affinities by exploring a motif repeated in apparently unrelated contexts; at other times, we are compelled by circumstance to form alliances with those we may have neglected, to put our heads together and come up with a solution acceptable to all. The conundrum of dark matter falls solidly in the latter category.
For several decades, cosmologists and astronomers had been growing progressively distant from their particle physics colleagues. As one group craned their necks further out into uncharted space, the other crawled deeper into the recesses of the atom. The disciplines began to seem as divergent as the scales upon which they operate, but there is a surprising resonance between the minute and the colossal. Even objects of cosmic proportions are built from subatomic particles. The discovery of dark matter was a reminder that no part of the universe can be completely understood by those who turn their backs on the rest.
Discussions of dark matter (and dark energy) are often front-ended by a startling admission of ignorance: the entire gamut of matter particles we conventionally study – quarks and leptons combined – forms less than 5% of the known universe. There is about five times as much dark matter out there, we are told, while the rest of the universe is made up of dark energy. But, since neither dark matter nor dark energy can be seen, how do scientists justify this shocking claim? An analogy might help. The mechanism of human vision is such that we see objects only when they reflect light. But if you find yourself in a pitch dark room, you don't immediately conclude that just because nothing is visible, the room must be empty. You simply realize that sight is no longer a reliable guide under these circumstances, and you must lean on sounds and smells, and touch (and taste?) to probe your surroundings. For lifeforms less dependent on vision, the darkness is multi-textured and alive with variety. Consider bats, for instance. Where we rely on light hitting objects and bouncing back, bats bank on sound. They emit high frequency calls, inaudible to human ears, and use the resulting echoes to construct a sonic map of their surroundings (the further an object is, the longer it takes for the echo to come back). The moral of the story is this: as long as there is a way for you to interact with an object, you can “sense” its presence.


