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They say it’s impossible for the human mind to think about nothing at all, but apparently we think about it a lot.1 For example, the Zen kōan, with its impossible question or illogical juxtaposition, is designed to disrupt the continuous buzzing of the active mind and send the practitioner into a relaxed, passive, receptive state. This is why meditation is so refreshing: it is like the darkness of deep sleep before the nightly pageantry of dreamtime begins.
But you don’t have to be a Zen master to contemplate emptiness. Quantum physicists attempt to understand the void of creation all the time. After all, empty space makes up the largest fraction of the universe. For example, it’s a common metaphor that, if the nucleus of an atom—any atom from hydrogen to plutonium—were blown up to the size of a baseball, then the electrons in their various energy shells surrounding it would be like flies buzzing around inside the space of a cathedral. If you could stop their motion, then you could sweep the dead electrons and the nucleus itself up with a brush and dustpan, leaving a cathedral-sized nothing behind. And if a molecule is a group of atoms linked by sharing their electrons, then molecules are simply a concatenation of cathedral-sized empty spaces. And even in the most densely packed material, like that brick of plutonium, the space between the molecules would be even emptier.
Outside the densely packed substance of the Earth and its atmosphere, in interplanetary space, the most prolific form of matter is particles of the solar wind. Depending on the state of the Sun and its recurring coronal mass ejections, these particles occur at a density of between four and ten per cubic centimeter.2 And most of them are not intact atoms from the Sun’s store of hydrogen and helium but instead their ions—that is, uncoupled atomic fragments like protons and electrons. Thin soup indeed! Interstellar space, beyond the boundary of the Sun’s heliosphere, is even emptier.3
And yet, in the mind of the physicist, the empty space between atoms and particles, even the space between the planets and between the stars, is laced with the fields that are associated with dynamic particles. These fields include the electromagnetic field accompanying the photons4 flying outward from the sun and from any other release of energy, or the Higgs field accompanying the long-sought Higgs boson5 that enables all the other particles in the grand vision of quantum mechanics to have mass. So “empty” space is full of—well, let’s call a field the “potential” for things to happen if the right amounts of matter and energy are present. So empty space has structure—or at least the possibility of structure—based on the presence and number of those nano-sized baseballs, dead flies, and other bits of matter or energy, on how much mass each one contains, and on how fast it’s moving.
Science fiction writers have taken this idea of the structure of empty space to absurd but imaginatively useful limits. For example, the empty space of the physical universe is envisioned as folded and crumpled in dimensions more numerous than the three—x, y, and z—coordinates we use for defining the space in which we normally move around. The idea goes that, if you could focus enough energy at a particular point in normal space, you could break through that folded structure and instantaneously arrive at another place that might be light-years away in your frame of reference but just around the corner in that multidimensional crumple.
Another useful fiction is that, with the application of enough energy, the structure of space itself can be pulled and pushed around like a lump of taffy. This give rise to the Star Trek warp drive. Using this hypothetical propulsion system, a starship can move faster than light while not exceeding the speed of light, c, the universal speed limit, because its “warp field” collapses the space in front of the ship and expands the space behind it. This is rather like being able to walk along at a hundred miles an hour, rather than the usual human pace of four miles per hour, because the sidewalk bunches up—in the example here, at the rate of twenty-five feet for every step—before your front foot hits the ground, and then it smooths out as you lift your back foot for the next step. You walk in a bubble of collapsing and expanding space and never exceed your normal walking pace. What the warp field does to the ship itself, the passengers, and the empty spaces inside their molecules and atoms is another question.
Some theoretical physicists, taking their ideas from the pixilation of a digital image or an LED television screen, propose that empty space is actually just a field of unfilled holes waiting to be occupied by matter and energy. In this view, space is like a giant honeycomb and, rather than moving through it haphazardly, particles and objects simply transition from one invisible cell to the next, blinking into and out of existence in an orderly fashion. For me, that’s a great mind game, but it doesn’t tell you more about the rules behind matter and energy than simply imagining particles and their associated waves flying through empty space.
Finally, because the movements of stars in the spiral galaxies that we can observe do not seem to match the masses and corresponding gravitational fields of those galaxies,6 physicists believe the universe has an unseen component called “dark matter.” This is not only matter we cannot see, but also matter we cannot detect with any of our instruments because it doesn’t interact with the atoms, energies, and fields—except for gravity—that compose the universe we live in. Based on the stellar movements we can observe,7 physicists think that “normal” or “baryonic” matter—that is, particles with known masses like protons and neutrons, the stuff we’re made of—composes only about five percent of the universe, while this dark matter makes up approximately twenty-seven percent.
It gets worse. The galaxies we can see are moving away from each other—and not just moving but accelerating, moving faster and faster—rather than collapsing inward under the gravity of all the matter we can see and detect, plus any contribution from the mass of all that dark matter. Since the outward fling imparted by the universe’s supposed origin in the Big Bang would be at a steady velocity—or even gradually decelerating, as gravity began to take over—something else must be pushing the galaxies apart. Again, whatever this “something” might be is invisible to our senses and undetectable by our instruments, and so it is called “dark energy.” Based on the observed acceleration of the galaxies, this energy is thought to constitute approximately sixty-eight percent of the matter and energy in the visible universe.
And we haven’t a clue about the nature of either dark matter or dark energy. Physicists attribute the former to objects called WIMPs—weakly interacting massive particles—and MACHOs—massive astrophysical compact halo objects. These are clever names that cloak a bit of an idea but essentially translate as “I don’t know.” And dark energy is sometimes attributed to “vacuum energy,” which is giving some structure or property to the empty space between those atomic baseballs and dead flies. Some theories propose that this energy comes from virtual pairs of particles—one of matter, the other antimatter—that randomly pop into existence in empty space and immediately annihilate each other without leaving behind any visible or audible “pop.” So the whole action is invisible to us. The amount of vacuum energy or the number of virtual-pair annihilations can be adjusted to account for the universe’s dark energy requirement. But hey, when you’re summoning pixies or counting angels dancing on pinheads, any number will suffice.8
So, while we can debate whether a glass is half-full or half-empty, we can also fill up that empty place with all sorts of imaginative particles, fields, and structures. For some of us, all this “nothing” seems to be our favorite subject.
1. You knew this one was going to be weird, right?
2. When I write “cubic centimeter,” think of a sugar cube—back in the days when sugar came in little cubes in a box that you poured into a bowl, instead of measured packets of white powder that is usually not real sugar.
3. What a concept is “emptier”! More empty than empty. Perhaps the construction should be “less filled up”—until we get to the something that is really, totally nothing.
4. I don’t count photons among the particles in the solar wind because the photon only has apparent mass—and so physical existence—because it’s traveling at the speed of light. If you stop it in its tracks, it transfers that energy into something else and simply disappears. Physics is complicated stuff.
5. See “What exactly is the Higgs boson? Have physicists proved that it really exists?” from Scientific American.
6. From the vantage point of Earth, all we can see are the stars in other galaxies. We know that they must also contain an amount of nonluminous matter like planets, asteroids, comets, and loose dust and gases. But since those quantities in our own local neighborhood are such a tiny fraction of the mass of the Sun itself, we discount them in computing the mass of any galaxy.
7. Based on the masses we can see, we would expect the stars closer to the center of the galaxy to move faster than those out on the rim, like wood chips circling inside a tornado or whirlpool. Instead, the stars appear to move in a relatively fixed pattern, as if they were painted on a spinning disk. To achieve this effect, you would need more mass in the system than you can account for by the stars we can see.
8. See also Three Things We Don’t Know About Physics (I) from December 30, 2012, and (II) from January 6, 2013.