Vacuum

A **classical** **vacuum** is a region of space without energy, momentum, particles (matter) and physical fields such as electromagnetism.

### Types of vacuums:

* A QED (quantum electrodynamics) vacuum is the lowest energy state of the electromagnetic field when the fields are quantized. This quantum vacuum is therefore space without matter but with quantized fields that have zero-point energy (ZPE) fluctuations called the *quantum foam* which provides it with vacuum energy, a special case of ZPE. When Planck's constant approaches zero, the QED vacuum is converted to a classical vacuum. QED vacuums have continuously appearing and disappearing *virtual particles* which "borrow" energies from the vacuum and appear for Planck times, called the vacuum fluctuation.

* A QCD (quantum chromodynamics) vacuum is a state characterized by non-vanishing condensates such as the gluon condensate and the quark condensate. In a superconductor, electric charges and magnetic flux are compressed. In a QCD vacuum dual superconductor, chromomagnetic monopoles and chromoelectric flux are compressed, sometimes resulting in magnetic monopoles.

* A perfect vacuum with nothing in it is attainable only with the new laws of physics, such as absolute zero. A classical vacuum emits light in the form of black body radiation and may contain gravitons, dark energy, virtual particles, and other aspects of a quantum vacuum. A perfect vacuum is also impossible with the old laws of physics due to the Heisenberg uncertainty principle which states that no particles can ever have an exact position.

* Deep space has physical approximations of vacuums with a few hydrogen atoms per cubic meter, large numbers of photons and neutrinos, and the cosmic background radiation. Outer space was once considered to either be an empty (perfect) vacuum, or consist of "aether".

* Artificial vacuums range from 1×105 Pa to 1×10-10 Pa of pressure. Deep or outer space is generally much more empty than any artificial vacuum, ranging from 1×10-4 Pa to 1×10-15 Pa of pressure. These are used in vacuum cleaners, incandescent light bulbs, thermos bottles, welding, etc.

### Casimir effect

The Casimir effect refers to a weird attractive force that appears between two uncharged parallel plates in a vacuum. A vacuum is full of fluctuating electromagnetic waves in all possible wavelengths, implying that it has a certain amount of zero-point energy. If the two parallel plates are brought very close together (to a few nanometers apart), longer waves will not fit between them, and the total amount of vacuum energy between the plates will be smaller than outside of the plates, thus causing the plates to attract each other. This attraction was first predicted in 1948 by physicist Hendrik Casimir.

Casimir forces can also be interpreted using other approaches without reference to zero-point energy. A vacuum is actually a sea of ghostly virtual particles springing in and out of existence. From this viewpoint, one can understand the Casimir effect by realizing that fewer virtual photons exist between the plates because some wavelengths are forbidden. The excess pressure of photons outside the plates squeezes the plates together.

Theoretical applications of the Casimir effect have been proposed, ranging from using its negative energy density for propping open traversable wormholes between different regions of space and time, to its use for developing levitating devices due to theories that the Casimir effect can give rise to repulsive forces.