As organisms made of water on a watery planet, it makes sense that we humans have always likened the universe to a vast sea.
Our understanding of the universe has always been fluid.
Eras of human history are marked by waves of change in our worldview, like the Copernican revolution that ousted earthlings from our presumed place at the centre of the universe, or later research by Hubble (the scientist and the telescope) demonstrating our Earth one speck among trillions awash in a universe getting bigger, faster every moment like an endless tsunami.
It’s little wonder, then, that we humans have always described the universe with allusions and metaphors connected to water.
As far back as 5,000 years ago, many of our ancestors envisioned the night sky to be a vast and dark ocean. In Egyptian cosmography, the world started with a single hill rising from an ocean of chaos named Nun. The two regions — the Earth and the chaotic watery Nun — are separated by the goddess of the sky, Nut, whose body is covered in stars.
In Sumeria, the world from the primordial ocean Abzu. In Zoroastrianism, the heavenly sea is named Vourukasha; in Hinduism, its churning waters are described in the Samudra Manthana. This type of origin story is so common that the “cosmic ocean” has become its own mythological motif in disparate cultures around the world.
Sky myths also often reference a river. In Greek mythology, the world is encircled by Oceanus, a mythical river from which the sun Helios rises and sets. The modern name of our own galaxy — the Milky Way — comes from envisioning its trail of stars in the night sky as a river of spilled milk.
The Romans called it “via lactea” (road of milk), inspired by the previous Greek name “galaxias kyklos” (milky circle). In many parts of East Asia it is still known by its Chinese names that translate to Silver River or the Sky River.
Humankind’s watery fixation with the stars perseveres to modern times, as is evident in our word for a human space traveler — astronaut — a compound of the Greek words for “star” and “sailor.” It’s not so difficult to intuit why water is such a persistent metaphor. On Earth, water is the most common and most important fluid; in the sky, it’s something else entirely.
On Earth, the molecule H20 exists in three states: solid (ice), liquid (water), and gas (steam). Plasma is the fourth state of matter, one which shows up in such extreme conditions that all matter melts into a stew of positively charged ions and negatively charged electrons.
Understanding plasma is essential to understanding huge swaths of matter in the universe because so much of it consists of one of two types of plasma. If the fluid-like nature of the stew is stronger than the electromagnetic pull of the charged particles, the plasma is referred to as weakly charged.
Weak plasmas exist in the cosmic in-betweens, such as the interstellar medium (material between stars) and the intracluster medium (the medium between galaxies living in a cluster). The physics governing weak plasmas is understood through solving fluid equations, often with the help of so-called hydrodynamic or magnetohydrodynamic codes.
Strong plasmas (plasmas in which the electromagnetic force between the particles dominates over its “fluidity”) describe some of the more extreme environments in the universe, like the insides of white dwarfs and neutron stars.
Even much earlier in the history of the universe — before there were stars or even the cosmic microwave background, deep in the proverbial waters of Nun — the entire cosmos was made of plasma.
Roughly a nanosecond to a microsecond after the big bang, the universe was so hot that all matter boiled down to some of its most basic components and existed only in the form of a quark-gluon soup. When scientists at CERN and similar laboratories smash heavy ions in their colliders, they recreate these soup-like conditions of the nascent fluid universe.
Even though plasma fluids don’t share all of their characteristics with liquid water, we encounter water so commonly that it makes a tempting metaphor not just for plasmas, but other phenomena as well. Physics teachers rely on this, because it’s through the metaphor of a wave in water that we can begin to imagine a pressure wave or a sound wave. It’s through the same metaphor that we can imagine waves in a physical field as representations of particles.
And it’s also through this metaphor that physicists profoundly (mis)understood light for centuries.
The wave theory of light implies that light waves, much like other waves, must be traveling through something, like a swell in a stormy sea. Physicists first described light-bearing or luminiferous aether, named after the Greek primordial god of the bright upper atmosphere.
Much ink was spilled over the properties of the aether, and many experiments devised to demonstrate its presence and measure its properties. In 1887, the Michelson-Morley experiment measured the speed of light in perpendicular directions. The idea was as follows: if light moves at a constant speed through the aether, and the Earth is also moving through the invisible aether, then light should get a bump in speed in the direction of the Earth’s movement, but not in perpendicular directions. Their result, as it turned out, was null, suggesting that the speed of light was always the same and casting doubt on the aether’s existence.
It wasn’t until Einstein’s theory of special relativity some 18 years later that the idea of the aether was dealt its final blow: it had never existed at all and, unlike sound, light is perfectly capable of moving through the vacuum of empty space.
The cosmic ocean metaphor, like all metaphors, does have limits. To the cosmos, water is just one of many molecules it can produce, less than 0.1% of its matter.
But we humans are beings mostly made of water, gazing out at the universe from a planet mostly made of water. It’s little wonder, then, that we equate the experience of gazing up at the stars to standing on the shore of a vast and wonderful ocean and we feel compelled to explore its uncharted depths.
Luna Zagorac is a particle cosmologist at Perimeter Institute for Theoretical Physics in Waterloo, Canada.
