- Casimir effect or Casimir-Polder force: It is a physical force arising from a quantized field. The typical example is of two uncharged metallic plates in a vacuum, placed a few micrometers apart, without any external electromagnetic field. In a classical description, the lack of an external field also means that there is no field between the plates, and no force would be measured between them. When this field is instead studied using quantum mechanics, it is seen that the plates do affect the virtual photons which constitute the field, and generate a net force -either an attraction or a repulsion depending on the specific arrangement of the two plates. Because the strength of the force falls off rapidly with distance, it is only measurable when the distance between the objects is extremely small. On a submicrometre scale, this force becomes so strong that it becomes the dominant force between uncharged conductors. In fact, at separations of 10 nm -about 100 times the typical size of an atom- the Casimir effect produces the equivalent of 1 atmosphere of pressure.
- Colour force: Before the 1970s, when protons and neutrons were thought to be fundamental particles, the phrase "strong force" referred to what is today known as the strong nuclear force or the residual strong nuclear force. What were being observed were the "residual" effects of the strong force, which act on hadrons, both baryons and mesons. This force was postulated to overcome the electric repulsion between protons in the nucleus, and for its strength (at short distances) it was dubbed the "strong force". After the discovery of quarks, scientists realized that the force was actually acting upon the quarks and gluons making up the protons, not the protons themselves. After the older notion was referred to as the residual strong force, and the "new" strong interaction was called colour force.
- Electromagnetic force: It is the force that the electromagnetic field exerts on electrically charged particles. It is the electromagnetic force that holds electrons and protons together in atoms, and which hold atoms together to make molecules. The electromagnetic force operates via the exchange of messenger particles called photons and virtual photons.
- Fundamental interaction or fundamental force: It is a mechanism by which particles interact with each other, and which cannot be explained in terms of another interaction. Every observed physical phenomenon can be explained by these interactions. In modern physics, there are four fundamental interactions (forces): gravitation, electromagnetism, the weak interaction, and the strong interaction. Their magnitude and behaviour vary greatly, as described in the table below.
Interaction Current Theory Mediators Relative Strength[1]
Long-Distance Behaviour Range(m)
Strong
Quantum chromodynamics
(QCD) gluons
1038 1
(see discussion below)
10-15
Electromagnetic
Quantum electrodynamics
(QED) photons
1036
1045
Weak
Electroweak Theory
W and Z bosons
1025
10-17
Gravitation
General Relativity
(GR) gravitons
1
infinite
- Gravitation is a natural phenomenon by which all objects with mass attract each other. Gravitation endows objects with weight. It is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping the Moon in its orbit around the Earth, for the formation of tides; for convection (by which hot fluids rise); for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena that we observe. Gravitation is also the reason for the very existence of the Earth, the Sun, and most macroscopic objects in the universe; without it, matter would not have coalesced into these large masses and life, as we know it, would not exist. Modern physics describes gravitation using the general theory of relativity, but the much simpler Newton's law of universal gravitation provides an excellent approximation in most cases.
- Gravity: Gravitation and gravity are distinct. "Gravitation" is the attractive influence that all objects exert on each other, while "gravity" specifically refers to a force which all massive objects (objects with mass) are theorized to exert on each other to cause gravitation.
- Pressure gradient force: It is the force that is usually responsible for accelerating a parcel of air from a high atmospheric pressure region to a low pressure region, resulting in wind.
- Strong interaction or strong force: It represents the interactions between quarks and gluons -see theory of quantum chromodynamics (QCD). This fundamental force is responsible for the cohesion of particles in the atomic nucleus. The strong force is the fundamental force mediated by gluons, acting upon quarks, antiquarks, and the gluons themselves. Of the four fundamental forces -gravity, electromagnetism, the strong nuclear force and the weak nuclear force- the strong interaction is the most powerful. The strong force only acts directly upon elementary particles. However, a residual of the force is observed between hadrons (the best known example being the force that acts between nucleons in atomic nuclei) as the nuclear force. Here the strong force acts indirectly, transmitted as gluons which form part of the virtual pi and rho mesons.
- Weak interaction or weak force or weak nuclear force: It is one of the
four fundamental interactions of nature. In the Standard Model of particle
physics, it is due to the exchange of the heavy W and Z bosons. Its most
familiar effect is beta decay (of neutrons in atomic nuclei) and the associated
radioactivity. The word "weak" derives from the fact that the
field strength is some 1013 times less than that of the strong force.