“Complementarity. This duality might leave you feeling a bit unsettled. “At the end of the day,” you might
ask, “which is it really, fundamentally?” Is your body a unitary entity or is it a phenomenon arising
from its smaller parts, the interacting cells? The answer is, of course, both, equally and unequivocally.
This kind of doubling of reality is a form of what quantum physicists call a complementarity. Perhaps
the most famous example of complementarity is embedded in the now well-known, if perhaps not well-
understood, notion that “light is both a wave and a particle.” Complementarity was originally framed in
regard to the “double-slit” experiment,4 which showed that streams of light behave like beams of
individual particles if observed in one way, but behave like continuous, undulating waves if observed in
a different way. This dependence on the experimental setup, on the method of observation, for whether
light appeared as waves or particles was called wave-particle duality. It became clear that either
description, on its own, was incomplete, insufficient to describe the nature of light in its totality. These
two partial descriptions—waves and particles—complemented each other. Only together could they
capture the full nature of light, each view providing information the other excluded. Their relationship
was recognized as a complementarity. Niels Bohr, one of the founders of quantum mechanics, thought
the most deeply about this concept after he announced it in 1928. It had become clear that no single
experiment could ever demonstrate both aspects of wave-particle duality at the same time. All agreed
that, at the quantum level, the impossibility of capturing both states at once was a fundamental
principle of the nature of existence. Bohr, however, went further, asserting that complementarity was
fundamental not just for describing existence at the incredibly minute scales of the quantum realm but
for describing living beings at our normal everyday scale as well.1 Furthermore, Bohr saw
complementarity as a fundamental property of existence at every scale. It was so central to his thinking
that, when he was awarded Denmark’s highest honor, the Order of the Elephant, he designed a coat of
arms for himself that featured a perfect symbol for complementarity, the yin-yang. Alas, perhaps due to
the increasing subspecialization across all fields of science as the twentieth century rolled forward,
these ideas about generalized complementarity were explored only in small corners of philosophy and
science. Nonetheless, they remain very much alive. The coat of arms of Niels Bohr. “Contraria sunt
complementa” means “Opposites are complementary.” Here is another way to envision
complementarity, the classic black-and-white image of two profiles viewed in silhouette and the space
between them looking like a vase. Which is it? Two faces? Or a vase? Of course, it is both, equally.
Neither view describes the whole image, each one leaving out something essential. A complete
description requires both opposite views to be united in a single complementarity. In just the same way,
whether a body is a singular entity in itself or a phenomenon arising from the nimble interactions of
cells is a question easily answered. It is a complementarity as well. It is both, equally, though which of
these it appears to be depends on your observational stance. Are you seeing it at the everyday scale or at
the microscopic scale? At the everyday scale your body is a unitary whole. At the microscopic scale, that
whole disappears into its parts—the ceaseless, dynamic cellular dance; cells in cooperation with other
cells, in space and in time.”