Why Can’t We Make a Perfectly Empty Vacuum?
A perfectly empty vacuum cannot be made because gas keeps entering or leaving the system: chamber walls release water and hydrogen, seals can admit gas, and pumps only remove what reaches them. Ultra-high vacuum means extraordinarily low pressure, not nothingness. Deep space is thinner than many laboratory vacuums, yet it still contains particles, radiation, dust, and plasma.
Is a 100% vacuum possible?
No. A 100% vacuum is an ideal: no particles in a chosen volume. Physical vacuum systems are measured by pressure, which records how much force gas molecules exert on an area. The Journal of Research of the National Institute of Standards and Technology defines one pascal as one newton per square meter.
Lower pressure means fewer gas particles, not a clean switch between “air” and “empty.” At 0.1 Pa and 22 degrees C, NIST estimates that about 1013 molecules remain in each cubic centimeter. That is sparse beside room air, yet it is still a vast count of molecules in a small volume.
The labels can mislead when treated as finish lines. Ordinary and high vacuum describe progressively lower-pressure working ranges, while NIST identifies ultra-high and extreme-high vacuum applications as requiring pressures below 1.0 x 10-6 Pa. An ideal vacuum is the zero-particle thought experiment. Ultra-high vacuum is an engineered condition with a number on a gauge.
| Condition or place | What the source reports | What it means |
|---|---|---|
| 0.1 Pa chamber at 22 degrees C | About 1013 molecules per cm3 | Low pressure still leaves many molecules. |
| Ultra-high or extreme-high vacuum | Below 1.0 x 10-6 Pa | An engineering target, not absolute emptiness. |
| Best laboratory vacuum | 1,000 atoms per cm3 | Thin by laboratory standards. |
| Intergalactic space | About one atom or molecule per m3 | Far thinner, still not zero. |
Words need measurements behind them. A vacuum is a condition. Perfect emptiness is a limit.
Why can't scientists remove every atom from a vacuum chamber?
Scientists cannot remove every atom because the chamber itself remains a source of gas while the pump is running. NIST describes equilibrium pressure as a balance between gas flowing into a chamber and gas removed from it. Pumping is not a one-time act. It is a continuing contest.
Four paths feed that contest: external leaks, gas released by materials, trapped gas called virtual leaks, and gas permeating through seals. Exposure to air leaves water on internal surfaces. NIST reports that as many as 100 water monolayers can build up; nearly all but the final monolayer can be pumped away, though room-temperature removal can take days.
Then comes hydrogen. In its 2021 chamber study, NIST measured water and hydrogen outgassing from chambers made with different materials and treatments. After baking at 125 to 150 degrees C for at least 72 hours, titanium, aluminum, and vacuum-fired stainless-steel chambers reached hydrogen outgassing rates below 1 x 10-11 Pa L s-1 cm-2. Those chambers performed at least 100 times better than the study’s 304L stainless-steel chamber.
Better is not empty. Baking lowers one source. Material choice lowers another. Seals, joints, walls, and trapped volumes still define the practical floor. The gauge reports the balance, not a victory.
This is the same habit used in a guide to buying a used GMC Savana: a promising label is not the condition itself. The details decide.
Is outer space a perfect vacuum?
No. Outer space is thin, not empty, and its contents change with location. NASA Science lists a typical density near Earth within the heliosphere as 5 atoms per cubic centimeter. The Local Interstellar Cloud is listed at 0.3 atoms per cubic centimeter, and the nearby Local Bubble at less than 0.001.
NASA’s comparison is sharp. Its table gives the best laboratory vacuum as 1,000 atoms per cubic centimeter, while intergalactic space is thought to reach about one atom or molecule per cubic meter, or 10-6 per cubic centimeter. Deep space can therefore be far emptier than a laboratory chamber without being empty.
Interplanetary space carries more than sparse gas. NASA says its “vacuum” includes solar radiation, dust, gas, and the solar wind: a plasma of ions and electrons. Empty to a human eye is not empty to an instrument.
Voyager 1 makes the point with direct measurements. NASA reports that it crossed the heliosphere’s boundary on August 25, 2012, then measured its surrounding electron density with plasma-wave signals. The reported density rose roughly 40-fold from 2013 to mid-2015, with fluctuations continuing in data analyzed through early 2020. NASA’s Voyager account describes interstellar space as particles and radiation, not a blank void.
Quantum physics adds a separate meaning. A 2011 Nature paper reported the dynamical Casimir effect in a superconducting circuit, where rapid changes to a circuit’s electrical length generated real photons and two-mode squeezing. The result shows measurable quantum-field effects under time-varying boundary conditions. It does not show that a pumped chamber is full of ordinary, directly observable particles.
Space is not a box with nothing inside. It is a place where almost nothing remains.
Frequently Asked Questions
What is left in a vacuum after the air is pumped out?
Residual gas remains, including water released from surfaces and hydrogen released from chamber materials. A chamber can also receive gas through leaks, trapped-gas virtual leaks, and permeation through seals. The amount left depends on the pressure, materials, pumping, and time.
Do quantum fluctuations mean empty space is not really empty?
Quantum-field experiments show that vacuum states can produce measurable effects under controlled conditions. The dynamical Casimir experiment generated real photons by rapidly changing the electrical length of a superconducting circuit. That result does not mean an ordinary vacuum chamber contains a visible sea of particles.