Uses

Outer space

Effects on humans and animals

See also: Human adaptation to space

Humans exposed to vacuum will lose consciousness after a few seconds and will die within minutes from asphyxiation, but the symptoms are not nearly as graphic as commonly shown in pop culture. Robert Boyle was the first to show that vacuum was lethal to small animals. Blood and other body fluids do boil (the medical term for this condition is known as ebullism) and the vapour pressure may be expected to bloat the body to twice its normal size and slow down circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid.^[5][6] Swelling and ebullism can be reduced by containment in a flight suit. Shuttle astronauts wear a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at vacuums of 15 Torr (2 kPa).^[7] However, even if ebullism is prevented, simple evaporation can cause the bends and gas embolisms. Rapid evaporation cooling of the skin will create frost, particularly in the mouth, but this is not a significant hazard.

Historical interpretation

Historically, there has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers did not like to admit the existence of a vacuum, asking themselves "how can 'nothing' be something?". Plato found the idea of a vacuum inconceivable. He believed that all physical things were instantiations of an abstract Platonic ideal, and could not imagine an "ideal" form of a vacuum. Similarly, Aristotle considered the creation of a vacuum impossible—nothing could not be something. Later Greek philosophers thought that a vacuum could exist outside the cosmos, but not inside it.

Quantum-mechanical definition

Even an ideal vacuum, thought of as the complete absence of anything, will not in practice remain empty. One reason is that the walls of a vacuum chamber emit light in the form of black-body radiation: visible light if they are at a temperature of thousands of degrees, infrared light if they are cooler. If this soup of photons is in thermodynamic equilibrium with the walls, it can be said to have a particular temperature, as well as a pressure. Another reason that perfect vacuum is impossible is the Heisenberg uncertainty principle which states that no particle can ever have an exact position. Each atom exists as a probability function of space, which has a certain non-zero value everywhere in a given volume. Even the space between molecules is not a perfect vacuum.

Pumping

Outgassing

Quality

The quality of a vacuum is indicated by the amount of matter remaining in the system. Vacuum is primarily measured by its absolute pressure, but a complete characterization requires further parameters, such as temperature and chemical composition. One of the most important parameters is the mean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As the gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70 nm, but at 100 mPa (~1×10^-3 Torr) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such as vacuum tubes. The Crookes radiometer turns when the MFP is larger than the size of the vanes.

Examples

Vacuum cleaner approximately 80 kPa (600 Torr) liquid ring vacuum pump approximately 3.2 kPa (24 Torr) freeze drying 100 to 10 Pa (1 to 0.1 Torr) rotary vane pump 100 Pa to 100 mPa (1 Torr to 10^−3 Torr) Incandescent light bulb 10 to 1 Pa (0.1 to 0.01 Torr) Thermos bottle 1 to 0.1 Pa (10^−2 to 10^−3 Torr) Near earth outer space approximately 100 µPa (10^−6 Torr) Cryopumped MBE chamber 100 nPa to 1 nPa (10^−9 Torr to 10^−11 Torr) Pressure on the Moon approximately 1 nPa (10^−11 Torr) Interstellar space approximately 1 fPa (10^−17 Torr)

Measurement

Properties

Many properties of space approach non-zero values in a vacuum that approaches perfection. These ideal physical constants are often called free space constants. Some of the common ones are as follows:

Notes

Find out more about Vacuum on Wikipedia