Imagine you’re casually flying your spaceship through the solar system. You zoom past Neptune, the final planet, and keep going until you reach the edge of the Milky Way galaxy. But your journey doesn’t stop here. Your engines continue blazing and soon you’re engulfed by the intergalactic medium. Far away from any star, planet, galaxy or galactic cluster.

It’s dark, very dark. The blackness is all around you and seemingly endless. You decide to look around and scout this tenebrous pocket of the universe. You search for anything to indicate that this vast pit of inky black sky contains something, rather than nothing. Despite trying to shed some light on this puzzling conundrum, there seems to be one key question bouncing around in your mind; is there such a thing as truly empty space?

Does the concept of empty space really exist, or is it just that; a concept? Picture yourself standing outside on a clear night, not a cloud in sight and all you can see are the stars glimmering against the blanket of the night sky. The space that fills the gap between us and those stars, is perceived by us to be empty. From a conceptual standpoint, this is a perfectly valid definition, yet from a scientific perspective we can do better.

We could define emptiness as a volume of space which contains ‘nothing’ in the truest sense of the word. No atoms, no elements, molecules, protons, neutrons, quarks, electrons, you get the picture. If this does actually exist in the Universe, it would provide us with a more rigorous and concrete meaning to the words ‘empty space’. We know that the sky on a dark night contains billions of atoms, molecules and dust because we can see them. If we shine a torch out into the sky, at just the right angle, we can see with our own eyes these tiny little dust particles floating around. Now you might be thinking; “What about between the dust particles? What about between the atoms in the sky? Or even between the space inside the atom?” In fact, it is the answer to these questions that will unveil the reasons behind why empty space doesn’t exist, from a scientific perspective!

So if empty space isn’t really ‘empty’, then what is it? To uncover this mystery we need to understand what exists in the space between the tiniest of particles, especially between the most fundamental particles of nature. So what’s inside one of the smallest building blocks of ordinary matter, the proton? If you zoom in on the proton you will be able to see three quarks. Quarks are fundamental particles which means they have no internal substructure (that we currently know of!) so we can’t look inside them and find more particles. Quarks, together with three other types of particles, make up the Standard Model of Particle Physics. This – still incomplete – theory contains all the fundamental particles we have in the Universe. In protons there are two ‘up’ quarks, and one ‘down’ quark jittering around inside. If you zoom in even further, to the space between the quarks, you would see the force that keeps these quarks together, called the strong force. The strong force is carried by eight particles called gluons and a unique property of these gluons is that they can interact with themselves. Consequently, if you tried to look at just one, you would see it popping in and out of existence due to it’s self-interactions (sounds crazy, but it’s true, I promise). This popping in and out happens at an incomprehensibly fast rate, creating a kind of bubbling soup in the space between the elementary quarks.

Not only are there gluons between quarks but they also exist in all of the empty space we can think of; between particles, between molecules in the sky, between galaxies. This sea of gluons coming in and out of the vacuum is known as quantum vacuum fluctuations, and is absolutely crucial to our existence. We can see the effects of the presence of the gluons, through observing their effects on other particles, which is how we can tell that they are actually there. So what can we take from all of this? While it is true that most of the atom, and therefore matter, is (what we initially thought of as) empty space, it is also true that this space contains quantum vacuum fluctuations; particles popping in an out of existence and interacting with the particles all around us, therefore rendering it not really empty after all.

Despite your best efforts, you won’t find a volume of space where these quantum fluctuations don’t exist. We can fly far across the universe away from any planet, star, galaxy, into the dark depths of the Cosmos, but all we will find within the space we thought of as nothingness, is a continuum of tiny quantum fluctuations. So next time you look out your window at night and see a shimmering star, remember that all of the space between you and that star is a swarming sea of fluctuating particles. Can you see them?