Glossary

Vacuum energy

In quantum mechanics, the vacuum of space is not completely empty, but is a seething mass of virtual particles and antiparticles that are constantly being created then destroyed in time spans smaller than Planck time. This results in a net energy that is nonzero, even in an empty vacuum.

Exotic matter

Also known as negative energy, exotic matter has properties not normally found in normal matter, such as having negative mass. Negative pressure density, produced by the Casimir effect, is the closest known representation of exotic matter.

Antimatter

Antimatter is matter with its quantum number (such as spin or charge) negated. It annihilates on contact with normal matter, producing a burst of energy equivalent to the sum mass multiplied by the speed of light squared. The most commonly known form of antimatter is the positron, used in PET scans, whose normal matter partner is the electron.

Relativity

Einstein's two theories of relativity describe how the invariance of the speed of light and equivalence of physical laws in different inertial frames of reference affect the geometry of the universe. The special theory of relativity is concerned with the effects frames travelling at different but constant relative velocities experience. The general theory is concerned with acceleration and gravity, describing them as equivalent, and details how gravitational fields curve the fabric of space and time.

Strong nuclear force

Also known as the strong interaction or colour force, the strong force primarily binds quarks together into hadrons (for example, protons and neutrons). The residual effect is also responsible for keeping atomic nuclei bound together, and is in tension with the electrostatic force between protons in a nucleus.

Weak nuclear force

The weak force or weak interaction is an force allowing subatomic particles such as quarks and leptons to exchange their energy, mass, electric charge and flavour, effectively changing into each other. It is most commonly seen in radioactive beta decay, where a neutron is converted into a proton and an electron.

Electostatic force

The electrostatic force is the force arising between static, electrically charged particles, such as protons and electrons. Electric charge can be negative or positive, causing particles with the same sign to be repelled (e.g. two protons) and those with opposite sign to be attracted (e.g. protons and electrons). This force is responsible for keeping electrons in orbit around atomic nuclei.

Magnetic force

The magnetic force is due to the quantum mechanical spin of subatomic particles, called the spin dipole magnetic moment. It is also inducted by the movement of an electric charge, e.g. by the orbit of an electron around a nucleus or an electric current passing along a wire.

Gravitational force

Gravity occurs wherever there is any kind of matter. The strength of the gravitational force between two masses is inversely proportional to the square of the distance between them and directly proportional to their mass. In general relativity, gravity is the means by which matter and energy cause space and time to be curved.

Electromagnetism

Described by Maxwell's equations, electromagnetism is the unification of electrostatic and magnetic fields. A moving magnetically charged particle induces a moving electric charge, and an electric current has the same effect, inducing a magnetic field. In fact, electric and magnetic fields are self-propagating in this manner, and electromagnetic waves - of which light is part of the spectrum - are the result of this propagation.