Pressure is caused by collisions between particles. Scientists use the term “vacuum” when there are few particles, and thus few collisions. Air in our atmosphere is dense with particles; atmospheric pressure is very high compared to lab vacuum or the vacuum of space. Scientists use vacuum in many ways. Vacuums were used in lightbulbs and vacuum tubes (such as the old CRT or cathode ray tubes of old TVs and computers). Vacuums are used for depositing materials in clean environments, such as on silicon wafers for microcircuitry. Vacuums are used for separating liquids that have different evaporation points. In scientific labs, we can produce pressures billions of times lower than atmospheric pressure, but the pressure in space is still lower.

Atmospheric pressure: Every cubic centimeter (also called a milliliter) of air contains 2.5 x 10¹⁹  air molecules. That’s 25,000,000 trillion molecules, where the US debt is roughly $12 trillion, and a terabyte (TB) hard-drive holds a trillion bytes of information. That is a lot of particles causing a lot of collisions. The average particle travels only 66 nanometers before colliding with another particle. That’s only about 200 times the size of a nitrogen molecule.

On top of Mount Everest: Pressure is roughly 1/3 of the pressure at sea level, and there are 8 x 10¹⁸ molecules of air per cubic centimeter. The average particle travels 280 nm before colliding with another particle.

Incandescent light bulb: The pressure inside a lightbulb is 1 to 10 Pascals (pressure at sea level is 100,000 Pascals). There are still about 10¹⁴ molecules/cm³, or 100 trillion molecules. The average particle travels a mm to a cm before a collision. This pressure is too low for plants or animals to survive.

Ultra high lab vacuum: The most sophisticated lab vacuum equipment can produce pressures of 10^-7 to 10^-9 Pascals, yielding about 10,000,000 to 100,000 molecules/cm³, respectively. Particles travel an average distance of 100 to 10,000 km before colliding with another particle. Such extreme vacuums require highly specialized equipment, including specialized pumps and chambers. Only certain materials can be used; paint, many plastics and certain metals can release gases at very low pressures, making them unsuitable.

Space vacuum: The vacuum of space depends on what part of space you mean. The pressure on the moon is 10^-9 Pa, or roughly our highest lab vacuum, with 400,000 particles/cm³. The pressure in interplanetary space (within the solar system) is lower yet, with only about 11 particles/cm³. It is estimated that there is only about 1 particle per meter cubed in the space between galaxies. Still, some microorganisms have survived exposures of days to space vacuum by forming a protective glass around themselves.

Going the other way, there are pressures much higher than the pressure of our atmosphere.

At the bottom of the Mariana trench: Pressure is about 1.1 x 10⁸ Pa, or about 1100 atmospheres. A variety of life has been observed in the Mariana trench.

At the center of the sun: Pressure is about 2.5 x 10¹⁶ Pa, or 2.5 x 10¹¹ atmospheres, or about 100,000 times the pressure at the core of the earth. This pressure is sufficient to fuel the fusion process of the sun, where hydrogen is combined to form helium.

At the center of a neutron star: Pressure is about 10³⁴ Pa, or 10¹⁸ times the pressure at the center of the sun. Here, pressure is so high that normal atoms with electrons around a core of protons and neutrons cannot exists. Nuclei cannot exist in the core of a neutron star.

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