Is Empty Space Actually Empty? Nothing Is Harder Than It Sounds

A vacuum is a region with less gas in it than the surroundings, and because removing every molecule is unattainable the term describes a range rather than a state. Pressure is the measure, and the useful range spans an enormous span, from a rough vacuum a domestic pump achieves down through the high vacuum required for most laboratory work to the extremely high vacuum needed in particle accelerators and surface science. Each decade of improvement is harder than the last, requiring different pumps, different materials and different techniques, and the practical limit at the extreme is set by gas seeping out of the container walls themselves rather than by anything remaining in the space.

Obstacles change as the pressure falls and each stage has its own:

• •Leaks through seals and joints, which dominate at moderate pressures
• •Water and other molecules adsorbed on the interior surfaces, which release slowly for days
• •Baking the whole system at high temperature to drive those off, which is standard practice
• •Gas dissolved within the metal of the walls, diffusing outward continuously
• •Materials chosen for low outgassing, which excludes most plastics and lubricants
• •Pumps that trap or freeze molecules rather than moving them, since ordinary pumps stop working

The applications are more numerous than the obvious ones. Vacuum removes the air that would scatter or react with anything travelling through it, which is why particle beams, electron microscopes and the deposition of thin coatings all require it. It removes the convection that would carry heat, which is how a vacuum flask works and why spacecraft thermal design is unlike anything terrestrial. It lowers the boiling point of liquids, which allows freeze drying and the distillation of substances that would decompose at their ordinary boiling temperature. It removes oxygen and moisture, which preserves food and prevents oxidation during welding and metallurgy. And it supplies a force, since atmospheric pressure acting against an evacuated space lifts and holds things.

Whether a vacuum could exist at all was disputed for centuries, with the dominant position holding that nature would not permit one. The question was settled experimentally in the seventeenth century. Torricelli filled a tube with mercury, inverted it in a dish and observed the column fall to a fixed height leaving a space above it, which nothing had entered, and correctly argued that the column was supported by the weight of the atmosphere. Pascal had the experiment repeated at different altitudes and found the height varied, which confirmed the explanation. Von Guericke built a pump and demonstrated in 1654 that two evacuated hemispheres could not be pulled apart by teams of horses. That sequence established both that vacuum exists and that air has weight.

Even a region containing no particles at all is not nothing according to modern physics, which is a genuinely strange result rather than a figure of speech. Quantum field theory describes fields present everywhere, which retain a lowest possible energy state that is not zero, and fluctuations in those fields produce measurable effects. Two uncharged plates placed extremely close together in vacuum experience an attractive force arising from those fluctuations, which was predicted in 1948 and has been measured. The vacuum affects the energy levels of atoms in ways that are measured to high precision and match calculation. Space also contains radiation from the early universe, stray particles and fields. Emptiness in the everyday sense turns out not to be available even in principle.

A vacuum is a matter of degree rather than a state, spanning an enormous range of pressures with each decade harder than the last. The limit at the extreme comes from gas seeping out of the walls, which is why systems are baked and why most plastics are excluded. A region with no particles still contains fields whose lowest energy is not zero, producing forces that have been measured.