Fundamental forces

**Fundamental forces** are the interactions in physical systems that cannot be reduced to lower interactions.

## Basics

There are four conventionally accepted fundamental interactions in our universe —gravitational, electromagnetic, strong nuclear, and weak nuclear.

### Gravitational

The gravitational force is the weakest fundamental force, with effects only observed on the large scale. It is purely attractive, and acts between all objects that have mass. It is theorised to be mediated by the graviton, a massless tensor boson with a spin of 2.

The current theory that best describes gravitation is general relativity. In general relativity, gravity emerges from the effects of time dilation gradients around massive objects. Since objects closer to massive bodies are perceived as having a slower internal clock than objects on the outskirts of these bodies, the atoms on the 'bottom' (where bottom is measured relative to the gravitational field in question) of the object will undergo less time than those at the top. Similar to a truck that gets slowed down from its tires entering a mud puddle, this difference produces a drag effect which accelerates the object through space at the expense of time. All things travel at the speed of light in Einstein's theory of general and special relativity, given that speed is measured over not only distance but also *time*.

The universal gravitational constant is 6.67408 × 10-11 m3\*kg-1\*s-2. The exact value is hard to determine, as multiple studies have produced differing results for the value. The difficulty lies in the fact that all experiments which can be practically conducted are within Earth's gravitational field and the force of gravitation is exceedingly weak compared to the other three main forces.

In an extension of quantum mechanics called loop quantum gravity, it is hoped this force can be reconciled with the other three for a theory of everything, as was Stephen Hawking's aim.

### Electromagnetic

   The electromagnetic force occurs between particles with a charge, and corresponds to the gauge group U(1). It can be attractive or repulsive depending on the charges (similar charges repel, opposite charges attract). It is mediated by the photon, a massless, chargeless boson with a spin of 1.

### Weak Nuclear

The weak nuclear force acts between particles with a weak isospin, and corresponds to the gauge group SU(2). The weak nuclear force is responsible for radioactive decay, which plays an essential role in nuclear fission. It is carried by three types of boson, the massive W+, W- and Z bosons with a spin of 1.

The two types of weak interaction are the charged-current interaction and the neutral current interaction.

The charged current interaction is mediated by the charged W+ and W- bosons, hence the name. It allows a charged lepton to absorb a W+ boson and become a corresponding neutrino, and also allows a down-type quark to absorb a W+ boson or emit a W- boson and become an up-type quark.

The neutral current interaction is mediated by the neutral Z boson, hence the name. It allows a quark or lepton to emit or absorb a Z boson.

### Strong Nuclear

The strong nuclear force acts between particles with a colour charge, and corresponds to the gauge group SU(3). It is the strongest fundamental force, and holds quarks together inside hadrons and holds nucleons together inside nuclei. It is carried by the eight types of gluon, which are massless, have a spin of 1, and each have a different colour charge.

## Unification

At high energies, the fundamental forces become unified.

### Electroweak

The electroweak force is the unification of the electromagnetic and weak nuclear forces above the electroweak scale (246 GeV), acting on particles with a weak isospin or a weak hypercharge, and corresponds to the gauge group SU(2) × U(1). It is carried by four types of boson: the W1, W2 and W3 bosons, and the B boson, all massless with spins of 1.

At low energies, the W3 boson and the B boson mix to become the Z boson and the photon, and the W1 and W2 bosons mix to become the W+ boson and the W- boson.