In science, a vacuum is a volume of space empty of matter, or more practically, any space where the pressure is significantly lower than normal atmospheric pressure. A perfect vacuum with absolutely zero particles has never been achieved on Earth or even observed in nature. Even the emptiest reaches of outer space contain a few stray atoms per cubic meter, making every real vacuum a partial one.
Why a Perfect Vacuum Doesn’t Exist
The air you breathe contains roughly 30 billion billion molecules per cubic centimeter. Interstellar space, the void between stars, gets that number down to about 1 atom per cubic centimeter. That’s extraordinarily empty, but it’s not zero. Intergalactic space is even emptier, yet still contains a handful of atoms per cubic meter. So when scientists talk about a vacuum, they’re almost always talking about degrees of emptiness rather than absolute nothingness.
This is why scientists categorize vacuums by pressure. Atmospheric pressure at sea level is about 1,013 millibars. A “low vacuum” might bring that down by a factor of a thousand. An “ultra-high vacuum,” like the ones used in advanced physics experiments, pushes pressure down to a trillionth of an atmosphere or less. The closer you get to zero, the harder and more expensive it becomes to remove the remaining molecules.
How Scientists First Proved a Vacuum Could Exist
For centuries, philosophers debated whether empty space was even possible. Aristotle famously argued that “nature abhors a vacuum.” That idea held for nearly 2,000 years until 1643, when Italian physicist Evangelista Torricelli built the first mercury barometer and, in doing so, created the first known vacuum.
Torricelli filled a glass tube with mercury, sealed one end, and inverted it into a dish of mercury. The mercury column dropped to about 76 centimeters, leaving a gap at the top of the tube. He reasoned that this gap contained nothing: it was a vacuum. Critics argued some invisible force in that empty space was pulling the mercury upward, but Torricelli disproved this by changing the shape of the tube’s sealed end. If a mysterious vacuum force were responsible, different tube shapes should produce different mercury heights. They didn’t. The heights stayed the same.
His conclusion was elegant and correct: “We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight.” It was the weight of the atmosphere pressing down on the mercury in the dish that supported the column, not any force from the vacuum above it. This insight gave birth to the science of atmospheric pressure and opened the door to vacuum research.
The Quantum Vacuum: Not Truly Empty
Classical physics treats a vacuum as simply empty space. Quantum physics tells a stranger story. Even if you could remove every single particle from a region of space, that space would still contain energy. This is called zero-point energy: the minimum energy a quantum system can possess, which is never zero.
At the quantum level, empty space is constantly buzzing with brief fluctuations. Pairs of particles and their antimatter counterparts pop into existence and annihilate each other almost instantly. These “virtual particles” exist for such short durations that you can’t directly observe them, but their effects are measurable.
The most famous demonstration of this is the Casimir effect. Place two uncharged metal plates extremely close together in a vacuum, just nanometers apart, and they experience a tiny but real force pushing them together. This happens because the quantum fluctuations between the plates are restricted (fewer wavelengths fit in the gap), while the fluctuations outside the plates are unrestricted. The imbalance creates a net inward push. The Casimir effect has been confirmed experimentally many times and stands as physical proof that a vacuum is not truly “nothing.”
The Speed of Light and Other Vacuum Constants
A vacuum serves as the baseline for some of the most fundamental measurements in physics. The speed of light in a vacuum is exactly 299,792,458 meters per second. This isn’t an approximation. Since 1983, it has been the defined value used to calibrate the meter itself.
Light slows down when passing through glass, water, or air, but its speed in a vacuum is the absolute cosmic speed limit. Sound, on the other hand, cannot travel through a vacuum at all. Sound requires a medium (air, water, solid material) to propagate as a pressure wave. With no molecules to vibrate, there’s no sound. This is why the tagline “In space, no one can hear you scream” is scientifically accurate.
How Vacuums Are Created
Creating a vacuum means physically removing gas molecules from an enclosed space. Different types of pumps handle different levels of emptiness. For moderate vacuums, mechanical pumps do the job. Rotary vane pumps and rotary piston pumps are positive-displacement devices that physically trap and expel gas with each cycle, much like a piston in a car engine running in reverse.
Getting to higher vacuums requires different strategies. Turbomolecular pumps use rapidly spinning blades to transfer momentum to individual gas molecules, flinging them out of the chamber. Diffusion pumps use high-velocity jets of oil or mercury vapor to sweep gas molecules toward an exhaust. For the most extreme vacuums, ion pumps capture gas molecules and chemically bind them to surfaces, effectively trapping them rather than pumping them out.
In many advanced facilities, these pump types are used in stages. A mechanical pump brings the pressure down to a moderate vacuum, then a turbomolecular or diffusion pump takes over for the next several orders of magnitude.
Vacuums in Modern Technology
The Large Hadron Collider at CERN, the world’s most powerful particle accelerator, relies on three separate vacuum systems. The beam pipes where protons travel at near light speed are pumped down to pressures around 10⁻¹⁰ to 10⁻¹¹ millibar, roughly as empty as the surface of the Moon. Achieving this takes nearly two weeks of continuous pumping. In the arcs of the accelerator, the ultra-high vacuum is maintained by cooling the beam pipes to cryogenic temperatures, which causes stray gas molecules to condense and stick to the pipe walls.
Vacuums are equally critical in manufacturing. Semiconductor fabrication requires ultra-clean vacuum chambers to deposit atom-thin layers of material onto chips without contamination. Vacuum insulation keeps thermoses hot and cryogenic tanks cold by eliminating the air molecules that would otherwise conduct heat. Vacuum packaging extends food shelf life by removing the oxygen that bacteria need to grow.
False Vacuum Decay: A Theoretical Extreme
In theoretical physics, the word “vacuum” takes on yet another meaning. The “vacuum state” of the universe refers to the lowest energy state of all quantum fields. But there’s a haunting possibility: the vacuum we live in might not be the true lowest energy state. It could be a “false vacuum,” a state that appears stable but could, in principle, tunnel to a lower energy state.
If that transition happened, it would begin as a tiny bubble of “true vacuum” that would expand at the speed of light, fundamentally rewriting the laws of physics inside it. In particle physics, a decay of the Higgs field’s vacuum state has been described as the “ultimate ecological catastrophe” because it would alter the forces and masses that hold atoms together. This process is also thought to have played a role in the Big Bang, when transitions between vacuum states helped create space, time, and matter. No experimental evidence suggests this decay is imminent, and recent research is only beginning to test the phenomenon in controlled laboratory analogs.