Vacuum

• 1. Introduction

  ›

  Vacuum is, by definition, nothing at all. Yet for more than two thousand years, philosophers, scientists, and engineers have argued, experimented, and built entire industries around the question of what that nothing actually is.

  The word itself comes from the Latin vacuus, meaning simply "empty". It is one of the few words in the English language containing two consecutive instances of the vowel u. That quirk of spelling hints at the strangeness of the concept it names.

  A perfect vacuum, a region entirely free of matter, has never been achieved in a laboratory. Even the furthest reaches of intergalactic space hold the equivalent of just a few hydrogen atoms per cubic meter. Yet partial vacuums, regions where pressure falls far below the surrounding atmosphere, have become indispensable tools. They preserve the filaments in light bulbs, allow surgeons to perform delicate procedures, and make semiconductor manufacturing possible.

  What the ancient Greeks dismissed, what medieval scholars argued violated the laws of God and nature, and what a 17th-century Italian glassblower first captured in a tube of mercury has become one of the most versatile conditions in modern science and engineering. The story of vacuum is a story about absence, and what astonishing things that absence can accomplish.

• 2. Two Thousand Years of Argument

  ›

  Lucretius, writing in the first century BC, argued that the void must exist. Without empty space, he reasoned, nothing could move, because there would be nowhere for anything to go. His contemporary Hero of Alexandria tried to put that argument into physical practice, but failed to produce an artificial vacuum.

  Aristotle had rejected the void entirely. In his Physics, book IV, he offered a pointed objection: motion through a medium with no impediment could continue forever with no reason ever to stop. A void would make physics incoherent. His view shaped Western thought for centuries.

  In the 10th-century medieval Muslim world, the physicist Al-Farabi wrote a treatise rejecting the vacuum on similar grounds, concluding that air simply expands to fill any space available. His contemporary Abu Rayhan al-Biruni took a more cautious position, stating that there is no observable evidence ruling out the possibility of a vacuum. The suction pump, described by the Arab engineer Al-Jazari in the 13th century, gave engineers practical experience with negative pressure without resolving the philosophical question.

  European scholars Roger Bacon, Blasius of Parma, and Walter Burley gave the problem sustained attention in the 13th and 14th centuries. The commonly held view that nature abhors a vacuum was given the name horror vacui. Jean Buridan reported in the 14th century that teams of ten horses could not pull open sealed bellows. The debate turned briefly theological: there was speculation that even God could not create a vacuum, and the 1277 Paris condemnations of Bishop Etienne Tempier, which required that no restriction be placed on the powers of God, pushed scholars toward the conclusion that a supernatural void might exist beyond the cosmos.

  Rene Descartes, almost two thousand years after Plato, proposed a geometrically based alternative to atomism that avoided the dichotomy of void and atom altogether. His coordinate system quietly redefined empty space as a quantified extension of volume, separating the philosophical from the measurable. That separation would make empirical work possible.

• 3. Mercury, Hemispheres, and Pumps

  ›

  Evangelista Torricelli produced the first laboratory vacuum in 1643. He filled a tall glass container closed at one end with mercury, then inverted it in a bowl. The mercury column dropped, leaving a space at the sealed top that contained almost nothing. The Torricellian vacuum was born.

  Blaise Pascal's experiments in the same period confirmed the finding and helped establish that what held the mercury column up was atmospheric pressure pushing down on the open bowl, not some force sucking mercury upward from above.

  Otto von Guericke went further in 1654. He invented the first vacuum pump and staged one of the most theatrical experiments in the history of science. Two hollow metal hemispheres were placed together and the air pumped out. Teams of horses were then hitched to each hemisphere and driven in opposite directions. They could not pull the halves apart. Atmospheric pressure, pressing inward from outside, held them together with a force no team of horses could overcome. Robert Boyle saw the result, improved von Guericke's pump design, and continued the work with the help of Robert Hooke.

  Research then paused for roughly two centuries before August Toepler invented the Toepler pump in 1850 and Heinrich Geissler invented the mercury displacement pump in 1855, reaching a partial vacuum of about 10 Pa, or 0.1 Torr. At that pressure level, a number of electrical properties became observable for the first time, renewing interest in the physics of low pressure environments. The 19th century also produced the luminiferous aether hypothesis, which held that space was filled with a rigid invisible medium through which light propagated. By 1912, astronomer Henry Pickering was still writing that the interstellar absorbing medium "is characteristic of a gas, and free gaseous molecules are certainly there." Luminiferous aether was subsequently discarded.

• 4. What the Quantum Vacuum Actually Is

  ›

  Paul Dirac proposed in 1930 that the vacuum is not empty but is instead an infinite sea of particles with negative energy, which he called the Dirac sea. The theory helped refine the predictions of his earlier Dirac equation and successfully predicted the existence of the positron, which was confirmed two years later.

  Werner Heisenberg had already complicated the picture in 1927 with his uncertainty principle, which placed a fundamental limit on how precisely both position and momentum, or both energy and time, can be known simultaneously. That limit means energy can fluctuate briefly even in a region that contains no particles.

  In quantum electrodynamics, QED vacuum is defined as the state with the lowest possible energy, containing no matter particles and no photons. But QED vacuum is not truly empty. The electric and magnetic fields within it have zero average values, but their variances are not zero. Virtual particles hop into and out of existence in a phenomenon called vacuum fluctuations. Experimentally verified effects of these fluctuations include spontaneous emission and the Lamb shift, a tiny but measurable displacement in the energy levels of the hydrogen atom.

  In quantum chromodynamics, QCD vacuum, multiple vacuum states can coexist. Transitions between different vacuum states are thought to have driven the beginning and end of cosmological inflation. String theory is believed to have a vast number of possible vacuum states, a landscape of solutions so large that it has challenged the idea of a single unified physics.

  The strictest classical definition holds that a true vacuum is a region where all components of the stress-energy tensor equal zero. Even then, in general relativity, a vacuum does not mean flat space-time. Gravitational waves and tidal forces can exist in a region that contains no matter at all. A black hole with zero electric charge is an example of a region entirely occupied by vacuum while still exhibiting extreme curvature.

• 5. Grades, Gauges, and Measurement

  ›

  Vacuum quality is not a single condition but a spectrum defined by the standards body ISO 3529-1:2019, which divides the range into low, medium, high, ultra-high, and extreme-high vacuum. Each level demands different materials, seals, and pumping technologies.

  At the low end, a typical vacuum cleaner reduces air pressure by around 20%. At the high end, ultra-high vacuum chambers used in chemistry and physics operate below one trillionth of atmospheric pressure, or 100 nPa, and reach around 100 particles per cubic centimetre. Outer space pushes further still: intergalactic space holds the equivalent of just a few hydrogen atoms per cubic meter, though it is never truly empty.

  Measuring these conditions requires instruments matched to the range. Hydrostatic gauges, such as the mercury column manometer, work well from 1 Torr down to above atmospheric. The McLeod gauge isolates a known volume, compresses it, and reads the amplified pressure difference; it reaches as low as 10 to the power of negative 6 Torr, which is the lowest direct pressure measurement possible with current technology. Thermal conductivity gauges, including the Pirani gauge, use a platinum filament heated by a known current; the filament's temperature indicates the rate at which surrounding gas carries heat away, and thus the pressure. Ionization gauges, which measure the current produced when gas molecules are struck by electrons, are used in ultrahigh vacuum down to 10 to the power of negative 10 Torr for the hot cathode variety.

  The mean free path of residual gas molecules is often as important as raw pressure. At atmospheric pressure, air molecules travel only 70 nm between collisions. At 100 mPa, the same air at room temperature has a mean free path of roughly 100 mm. When that distance exceeds the dimensions of a chamber or pump, fluid mechanics no longer applies and gas must be treated as individual particles. The Crookes radiometer, a familiar classroom demonstration, turns precisely because the mean free path of its surrounding gas is larger than its vanes.

• 6. Industrial and Scientific Uses

  ›

  Vacuum's first mass-market application was the incandescent light bulb, where it protected the filament from chemical degradation that would quickly destroy it in air. That application alone placed vacuum technology inside homes across the world.

  Industrial uses multiplied rapidly. Chemical vapor deposition, physical vapor deposition, and dry etching, all essential to fabricating semiconductors and optical coatings, depend on high to ultra-high vacuum to let particle beams deposit or remove materials without contamination. Vacuum arc processes are used to produce certain grades of steel and other high-purity materials. Electron microscopes and cathode-ray tubes depend on the electrical properties of vacuum.

  Freeze drying, used to preserve foods and pharmaceuticals, uses deep vacuum to lower the boiling point of water so it sublimates directly from ice without passing through the liquid phase. Distillation, metallurgy, and adhesive preparation also exploit that drop in boiling point. Thermos bottles use the reduction of convection in a vacuum layer to provide thermal insulation.

  Vacuums also drive machines directly. The Newcomen steam engine used vacuum rather than pressure to drive its piston. In the 19th century, Isambard Kingdom Brunel experimented with an atmospheric railway that used vacuum for traction. Vacuum brakes were once standard on trains in the United Kingdom before being replaced by air brakes. Some aircraft instruments, including the Attitude Indicator and Heading Indicator, are typically vacuum-powered as protection against total electrical failure.

  Outgassing is the shadow side of vacuum technology. Every material releases a small amount of vapour into the surrounding space, and when the vacuum pressure drops below that material's vapour pressure, the outgassing becomes a serious limitation. Water absorbed by chamber walls is the most common problem. Ultra-high vacuum systems are usually baked under vacuum to drive off outgassing materials, then cooled. Some systems are cooled with liquid nitrogen to suppress residual outgassing further and to cryopump the system simultaneously.

• 7. Vacuum and the Human Body

  ›

  Robert Boyle was the first to demonstrate in 1660 that vacuum is lethal to small animals. What happens to larger animals, including humans, turns out to be different from what popular culture typically depicts.

  A human exposed to vacuum will lose consciousness after a few seconds from hypoxia. Death would follow within minutes. But the body does not explosively decompress. The elastic pressure of blood vessels keeps the boiling point of blood above the internal body temperature of 37 degrees Celsius, so blood does not boil. Gas bubbles do form in bodily fluids under reduced pressure, a condition called ebullism. The body can bloat to roughly twice its normal size, and circulation slows, but tissues are elastic and porous enough to prevent rupture.

  Animal experiments provide the most systematic data. A NASA study on eight chimpanzees found that all of them survived two-and-a-half minute exposures to vacuum and recovered rapidly. Full-body exposures beyond 90 seconds are fatal, and resuscitation has never succeeded in those cases. Limbs can endure much longer exposure without harm, provided breathing is not impaired.

  Shuttle astronauts wore the Crew Altitude Protection Suit, a fitted elastic garment abbreviated CAPS, which restrains swelling and ebullism at pressures as low as 2 kPa, or 15 Torr. Most spacesuits use only 20 kPa of pure oxygen, enough to prevent ebullism while keeping the suit flexible. Decompression sickness and gas embolisms remain risks at those pressures if decompression rates are not managed carefully.

  Rapid decompression can be more dangerous than sustained vacuum exposure. A pressure drop of 13 kPa, which causes no symptoms when gradual, can be fatal if it occurs suddenly. Alveoli in the lungs may rupture before pressure can vent through the windpipe. These injuries are classified as barotrauma. Tardigrades, among the most durable known organisms, can survive vacuum conditions for days or weeks, a fact that has made them subjects of interest in astrobiology.