The void is full of something. Can you see it?
We all know the aphorism that one can see a glass filled to the middle with water as half-full or half-empty, the optimistic versus the pessimistic views. What about an empty glass? Of course, everyone knows that it’s filled with air. Can we, however, contemplate total emptiness? And if we can, does it really exist? Is there such a thing as a complete void in Nature, the absence of everything? This question has occupied the minds of philosophers and physicists for a very long time. KC Cole, a past member of 13.7, wrote a wonderful book about the topic.
What we mean by “nothing” has radically changed over time. And of course, the meaning of nothing is deeply related to our notion of reality, a topic we’ve been exploring for the past few weeks. Total emptiness, if it exists, has foreboding undertones. Nothingness is scary. We want to fill it up with something. And it appears that we are in luck.
We tend to organize perceived reality in terms of opposites: day-night, light-dark, hot-cold, male-female, before-after, good-evil, life-death and so forth. The opposite of nothing is everything. In the dictionary, the proper word for a completely empty space is void. So, it is to the void that we turn as the complete absence of matter.
Imagine that we had a super-pump, capable of sucking out every single air molecule from a bottle. (Of course, this has to be an imaginary perfectly rigid bottle, for otherwise it would implode.) Is there a point where nothing would remain inside?
Already in ancient Greece the question incited passionate debate. Parmenides, a pre-Socratic philosopher from southern Italy, would say that “eon” permeates all that exists, representing immutable Being in its most abstract form. According to his view, the void would be an impossibility, since it would mean a place without eon and hence with non-Being. Against Parmenides and his disciples, Atomists such as Democritus and later Lucretius would say that everything is composed of atoms moving in the void. So, matter and no-matter coexist, and where atoms are missing there is emptiness.
Aristotle strongly disagreed with the "atomists." To him, “Nature abhorred a vacuum”: the void was impossible. His logic went something like this: drop a marble on a glass containing water and into one containing honey. The marble’s falling speed is larger in the glass with water. Since the speed of an object moving in a material medium (air, water, honey, etc.) is inversely proportional to the medium’s density, Aristotle extrapolated that in a medium which is completely empty (zero density) the speed of an object would be infinite. That, to Aristotle, was absurd and hence no medium could be completely empty. He thus postulated the existence of the aether, an inert and eternal substance that filled all of space above the Earth.
In the early 17th century, Descartes suggested that the cosmos was filled with an aether-like substance that was able to sustain vortices similar to hurricanes. He used this notion to explain the orbits of planets around the Sun and of the Moon around the Earth (they would be carried around by the vortex like corks spiraling around an emptying drain). In his masterpiece the Principia, Newton proved Descartes wrong, arguing that if there was such a substance in space, it would cause so much friction that the planets would fall onto the Sun; orbits would be unstable. Space was empty after all.
But not for long.
When James Clerk Maxwell showed in the mid-1800s that light was an electromagnetic wave propagating at 300,000 kilometers per second. He also concluded that, as any good wave should, light should be waving on something material. But what? It couldn’t be dense at all, as Newton had shown. It had to be transparent, since we can see light from distant stars. It had to be rigid to allow for the propagation of fast waves. And it had to be weightless. So, the aether made a come back, now dressed in the more precise language of science. When Michelson and Morley failed to see the aether in their famous 1887 experiment, a quiet sense of desperation spread through the physics community. Strange ideas were proposed to salvage the aether. Only in 1905 did Einstein prove that the aether was not needed to sustain the propagation of electromagnetic waves: they could propagate in vacuo, in the void.
Side by side with this emptiness, atoms were also seen as mostly empty: electrons orbited protons at huge distances. A famous image is that if we amplify a proton to the size of a cherry, an electron would be circling at a radius equivalent to that of the Dome of Saint Peter’s Basilica in the Vatican. More precisely, the size difference is of about 100,000. So, the vision of the ancient atomists had a grain of truth: atoms do move in the void. Or do they?
As the 20th century progressed, the image of particles as little billiard balls scattering off each other went through a deep revision. The fundamental change in approach wasn’t very new, dating back to the work of Faraday and Maxwell in the 19th century. It was the notion that fields permeate all of space, being connected to specific sources. For example, any massive body such as yourself creates a gravitational field that “spreads” throughout space and falls with the square of the distance. Similarly, electric charges also create their fields. Soon enough, fields were linked to all matter particles and to the forces connecting them (such as gravity and electromagnetism). In fact, particles began to be seen as excitations of fields, just as waves are excitations of a material medium. (Like a rock tossed on a pond.)
Physical reality is now seen as consisting of fundamental fields and their excitations. There is no such thing as empty space. Nobel laureate Frank Wilczek wrote an excellent popular book on this view, The Lightness of Being.
And just to make things even more interesting, in 1998 astronomers discovered that the whole of space is filled up with a mysterious entity which, for lack of a better name, is called dark energy. We don’t know what it is but know what it does: it accelerates the expansion of space itself, rushing galaxies away from each other. So, in a sense, Aristotle was right after all. Dark energy could be thought as a kind of aether, although we don’t know if this substance (if we can call it a substance) is eternal and immutable. It may very well spring from the fields that permeate space, its origins related to the quantum fuzziness that dictates atomic behavior. Whatever it is, it seems that the void is no more.