3.5.6  Fundamental Interactions

There are four fundamental forces in nature, which can not be reduced like the other forces, and they result from the various interactions between fermions as they are attracting and repelling each other. These four forces are also called the fundamental interactions.

Two of these fundamental forces are long-range, and the other two operate only on the subatomic distances. The gravitational and electromagnetic forces can be observed directly in everyday life, while the strong and weak forces govern nuclear interactions. Some scientists speculated the existence of a fifth force, referred to as “quintessence”, but this has not been proven yet. It is worth to mention here that the Duality of Time predicts at least five forces, but the fifth is the magnetic force itself, though it only manifests in nature coupled at least with the electric force, thus it is called electromagnetic. We shall come back to this issue in chapters V and VI.

As we have seen in section 3, Einstein explained the gravitational force in terms of the curvature of space-time, while the other three are discrete quantum fields, and their interactions are mediated by elementary particles described by the Standard Model of particle physics, that will be discussed further in section 5.18. It is believed that gravity should be also quantized, but all the theories and models of Quantum Gravity have not been fruitful yet. The essence of the problem is that most of these theories are trying to implement some kind of quantization in a continuous space-time background, which makes any prospective theory of gravity non-re-normalizable, thus impossible to test because it cannot make any meaningful predictions.

The two serious approaches to this critical problem are Strings theory, described in section 7, and Loop Quantum Gravity, described in section 8. The first one tries to develop an effective Quantum Field Theory of gravity at low energies, by postulating strings instead of point particles, while Loop Quantum Gravity uses spin networks to obtain granular space that evolves with time. This makes the latter closer to General Relativity that considers space-time to be dynamic, but here the smooth background is replaced by nodes. Therefore, while Strings theory still depends on the background continuum, Loop Quantum Gravity tries to be independent of the background by quantizing space-time itself.

The strong interaction is carried by a particle called the gluon, which is responsible for the binding of quarks together to form hadrons, such as protons and neutrons. As a residual effect, it creates the nuclear force that binds the latter particles to form atomic nuclei.

The weak interaction is carried by particles calledandbosons, and also acts on the nucleus of atoms, mediating radioactive decay.

The electromagnetic force, carried by the photon, creates electric and magnetic fields, which are responsible for chemical bonding and electromagnetic waves, including visible light, and forms the basis for electrical technology.

Gravity is also expected to be carried by a particle called the graviton, but there is physical confirmation of such a theoretical particle.

Table 3.1: The four fundamental interactions of nature

Although the electromagnetic force is far stronger than gravity, it tends to cancel itself out within large objects, so over the largest distances, on the scale of planets and galaxies, gravity tends to be the dominant force.

All four fundamental forces are believed to be related, and to unite into a single force at high energies on a minuscule scale, the Planck scale, but particle accelerators cannot produce the enormous energies required to experimentally probe this.

The weak and electromagnetic forces have already been unified with the electroweak theory of Sheldon Glashow, Abdus Salam, and Steven Weinberg, for which they received the 1979 Nobel Prize in physics.

Progress is currently being made in uniting the electroweak and strong fields within a Grand Unified Theory.

The bigger challenge, however, is to find a way to quantize the gravitational field, resulting in a theory of Quantum Gravity, which would unite gravity in a common theoretical framework with the other three forces.

In the conceptual model of fundamental interactions, matter consists of fermions, which carry properties called charges and spin. They attract or repel each other by exchanging bosons.

The exchange of bosons always carries energy and momentum between the fermions, thereby changing their speed and direction. The exchange may also transport a charge between the fermions, changing the charges of the fermions in the process, which turns them from one type of fermions to another.

Since bosons carry one unit of angular momentum, the fermion’s spin direction will flip from, or vice versa, during such an exchange, in units of the reduced Planck’s constant.