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; this is the charge of a single proton, and its negative value is the charge of a single electron). 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).

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

Electric current is measured in amperes (i). An ampere of current represents the passage of one coulomb of charge per second, or 6.2 × 10¹⁸ electrons per second. A current is positive when it is in the direction of the flow of positive charges; its direction is opposite to the flow of negative charges.

An ordinary flashlight battery delivers a current that provides a total charge flow of approximately 5,000 coulombs, which corresponds to more than 10²² electrons, before it is exhausted.

Electric charges produce electric fields; a changing charge also produces a magnetic field (similarly, a changing magnetic field creates an electric 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.

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).

The magnetic field does not point along the direction of the source of the field; instead, it points in a perpendicular direction. In addition, the magnetic force acts in a direction that is perpendicular.

These fields are fundamental in nature and can exist in space far from the charge or current that generated them. Remarkably, electric fields can produce magnetic fields and vice versa, independent of any external charge. Electric and magnetic fields travel together through space as waves of electromagnetic radiation, with the changing fields mutually sustaining one another.

The term field denotes a property of space, so that the field quantity has a numerical value at each point of space. These values may also vary with time. The value of the electric or magnetic field is a vector (a quantity having both magnitude and direction). The value of the electric field at a point in space equals the force that would be exerted on a unit charge at that position in space.

Electromagnetic field activity absent corresponding matter can be seen with radio and television waves, microwaves, infrared rays, visible light, ultraviolet light, X-rays, and gamma rays, all of which travel at the velocity of light (roughly 300,000 kilometres, or 186,000 miles, per second). They differ from each other only in the frequency at which their electric and magnetic fields oscillate.

According to Coulomb, the electric force for charges at rest has the following properties:

1. Like charges repel each other, and unlike charges attract. Thus, two negative charges repel one another, while a positive charge attracts a negative charge.
2. The attraction or repulsion acts along the line between the two charges.
3. The size of the force varies inversely as the square of the distance between the two charges. If the distance between the two charges is doubled, the attraction or repulsion becomes weaker, decreasing to one-fourth of the original value. If the charges come 10 times closer, the size of the force increases by a factor of 100.
4. The size of the force is proportional to the value of each charge. The unit used to measure charge is the coulomb (C). If there were two positive charges, one of 0.1 coulomb and the second of 0.2 coulomb, they would repel each other with a force that depends on the product 0.2 × 0.1. If each of the charges were reduced by one-half, the repulsion would be reduced to one-quarter of its former value.

Static cling is a practical example; garments made of synthetic material collect a charge, especially in dry winter air. A plastic or rubber comb passed quickly through hair also becomes charged and will pick up bits of paper. The synthetic fabric and the comb are insulators; charge on these objects cannot move easily from one part of the object to another. Similarly, an office copy machine uses electric force to attract particles of ink to paper.

Electromagnetic force is enormously strong compared with gravitational force. The absence of one electron out of every billion molecules in two 70-kilogram people standing two metres apart would repel them with a 30,000-ton force.

Naturally occurring minerals exhibit magnetic properties and have magnetic fields, which result from the motion of electrons in the atoms of the material. They also result from a property of electrons called the magnetic dipole moment, which is related to the intrinsic spin of individual electrons. In most materials, little or no field is observed outside the matter because of the random orientation of the various constituent atoms. In some materials such as iron, atoms within certain distances tend to become aligned in one particular direction.

Weak Nuclear Force

The weak nuclear force, also known as the weak interaction, is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms: The weak interaction participates in nuclear fission and nuclear fusion. The theory describing its behavior and effects is best known as electroweak theory (EWT).

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