How empty is space?

Image credit: NASA

The laws of physics put a limit on how empty a region in space can be. A true vacuum is theoretically impossible, but space comes fairly close, at least when compared to something as dense as the earth’s atmosphere. But how empty is space really?

The universe on the whole is far from empty. It contains billions of galaxies, which each have billions of stars, planets, asteroids, comets, and other loose chunks of rock. Besides these, there are huge gas clouds that stretch across trillions of kilometres called nebulae. There’s also blackholes, neutron stars, pulsars, and other remnants of dead stars. In this sense, space is obviously not empty. But what about the space between all of these things?

Is the solar system empty?

Is the space between the sun and the earth empty? Again, not really. There’s comets, asteroids, Mercury and Venus, as well as the satellites and other space junk we leave behind. Even the regions that do seem empty contain cosmic dust, which can be seen as a faint glow before sunrise due to the scattering of light. These particles are quite far apart from each other, with only 1 particle present in every 100~ cubic metres. Cosmic dust is present throughout the solar system, and even outside it.

Besides cosmic dust, space also contains protons, neutrons, electrons and ions spewed out by the sun during coronal mass ejections, which are massive expulsions of plasma that often accompany solar flares. On average, these ejections expel 1.6 ✖ 10¹⁶ kg of plasma, and depending on the solar cycle, can happen between once every five days to three times a single day! This continuous stream of charged particles is called the solar wind, and has an average density of 5 to 10 protons / cm³.

Is intergalactic space empty?

In the empty regions between galaxies, things get weird. These regions are seemingly empty, but have been shown to contain large amounts of dark matter, which is a mysterious substance that does not interact with matter or light, but exerts a gravitational pull like normal matter. There is also dark energy, a force that is associated with the expansion of the universe. However, we can neither see nor physically interact with either dark matter or dark energy.

Overall, when we only consider normal matter, the average density of the universe comes out to be only around 4.5 × 10^-31 grams per cubic centimetre, or one proton in every four cubic metres of space, but this is a misleading figure. Most of the matter in the universe is concentrated in stars, which have a much higher density, while the rest of space is even emptier than what this figure suggests.

All of the matter and energy, and even the dark matter and dark energy, in the universe is clumped up in small regions, and the space between these regions contains close to nothing. All of the matter in the universe occupies around 0.0000000000000000000042% of the total volume of the universe, and the volume occupied by energy, dark matter and dark energy, though difficult to calculate, may not be much more than this. In conclusion, the universe is pretty goddamn empty!