Standard Model of Physics

Elementary particles of the Standard Model.

The Standard Model of Physics is a comprehensive theory that describes three of the four fundamental forces (electromagnetic force, strong nuclear force, weak nuclear force, but not gravity) in the universe; it attempts to classify all known elementary particles. It has its origins in the late 20th century CE, finalized ~1970s CE when quarks were confirmed by experiment. Since then, additional proofs have been discovered for the top quark (1995 CE), the tau neutrino (2000 CE), and the Higgs boson (2012 CE), all of which lend further credence to the model's accuracy.

Despite its accuracy and repeated confirmation via experiment, it fails to adequately explain all physical phenomena (and so can't be considered a unified Theory of Everything in any real sense).

Fundamental forces

Electromagnetism (Electric Charge)

Electromagnetic force (or electromagnetism) describes the interaction between particles with electric charges via electromagnetic fields; it is the dominant force in the interactions between atoms and molecules and can be thought of as a combination of electrostatics and magnetism. The electrostatic attraction between atomic nuclei and their electrons holds atoms together, and electric forces also allow different atoms to bond into molecules, including macromolecules such as proteins. Meanwhile, magnetic interactions between the spin and angular momentum magnetic moments of electrons also play a role in chemical reactivity.

Electrostatics studies slow-moving or stationary electric charges. An electric charge (q) is a fundamental physical property of matter that causes it to experience a force when placed in an electromagnetic field; it can be positive (+) or negative (-), and an object with no net charge in either direction is called electrically neutral.

Electric charge is a conserved property; the net charge of an isolated system, the quantity of positive charge minus the quantity of negative charge, cannot be altered. Electric charge is carried by many subatomic particles; in macroscopic matter, negative charge is carried by electrons, and positive charge is carried by protons in the nuclei of atoms. If there are more electrons than protons in a piece of matter, it will have a negative charge, if there are fewer it will have a positive charge, and if there are equal numbers it will be neutral.

Electric charge is quantized; it comes in integer multiples of individual small units called the elementary charge (e, ~1.602×10⁻¹⁹ C, the smallest charge that can exist freely). Particles called quarks have smaller charges, multiples of 1/3e, but they are found only combined in particles that have a charge that is an integer multiple of e. In the Standard Model, charge is an absolutely conserved quantum number. The proton has a charge of +e, and the electron has a charge of −e. The coulomb (C) is the unit of electric charge in the International System of Units (SI). It is equal to the electric charge delivered by a 1 ampere current in 1 second.

Electric field induced by a positive electric charge (left) and a field induced by a negative electric charge (right).

Electric charges produce electric fields; a moving charge also produces a magnetic field. The interaction of electric charges with an electromagnetic field (a combination of an electric and a magnetic field) is the source of the electromagnetic force. Charged particles exert attractive forces on each other when their charges are opposite and repulse each other when their charges are the same.

Because these forces are exerted mutually, two charges must be present for the forces to take place. The electric field of a single charge (or group of charges) describes their capacity to exert such forces on another charged object. These forces are described by Coulomb's law, which says that the greater the magnitude of the charges, the greater the force, and the greater the distance between them, the weaker the force.

A magnetic field describes the magnetic influence on electric currents (moving electric charges), and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field. A permanent magnet's magnetic field pulls on ferromagnetic materials such as iron and attracts or repels other magnets. In addition, a nonuniform magnetic field exerts minuscule forces on 'nonmagnetic' materials by three other effects: paramagnetism, diamagnetism, and antiferromagnetism, although these forces are usually so small they can only be detected by laboratory equipment. Magnetic fields surround magnetized materials, electric currents, and electric fields varying in time. Since both strength and direction of a magnetic field may vary with location, it is described mathematically by a function assigning a vector to each point of space, called a vector field (more precisely, a pseudovector field).

Classes of Elemental Particles

Moving from the largest constituents to the smallest, the first division within elementary particles is between fermions and bosons.

Fermions

Fermions are particles of half-integer spin - the standard model includes 12.

Bosons

What Gaps?

• Baryon asymmetry
• gravitation
• accelerating expansion of Universe
• no dark matter / energy
• no accounting for neutrino oscillations, non-zero masses
• quantum mechanics
• hierarchy problem
• 19 parameters - origin of values?
• Strong CP Problem