As per standard model, there are certain elementary particles and interactions that are at the heart of every process in nature:
What are Interactions?
Loosely speaking, these interactions are nothing but “forces” as we know them in rest of the physics (such as electromagnetic force, gravity etc.) although it is not exactly so. In the domain of subatomic particles interactions are mediated by another set of “particles” known as carrier particles. e.g. if a positive charge comes nearby to another positive charge, it does not get repelled immediately. There is a certain(albeit tiny) amount of time before the incoming charge “feels” the force.
We assume that something leaves the stationary charge and arrives at the incoming charge in a finite interval of time to interact with it. It is only then that it is said to have “felt” the force of repulsion. The source(stationary) charge also feels the force by interacting with a particle emitted by the test charge. This carrier particle is(did you guess it?) nothing but a photon:
All electromagnetic interactions can be modeled by photons of different energies. That means every force we encounter in daily life(such as friction, tension in a string, normal force or even getting punched in your stomach) can be ultimately understood as nothing but the interaction of photons with matter, because all of these are electromagnetic interactions at the most basic level(except gravity).
If electromagnetic force and gravity are all there is, what holds the nucleus together? Gravity is too weak at the subatomic scale so the electromagnetic repulsion among protons should be able to pull any nucleus with more than one proton apart, right?
But this is not observed. You and I wouldn’t exist if the nuclei couldn’t hold themselves together. Hence, there must be something we are missing. We called it the “strong nuclear force” because it is stronger than electromagnetic force and gravity, and acts inside the nucleus. Unlike gravity and electromagnetism, the strong force does not act at large distances.
Shortly after the discovery of neutrons, it was observed that they have many similarities to protons. Infact, someone suggested that we treat them as different quantum bound states of the same particle. Strange as it seems, it was later realized to be quite logical. The strong force does not distinguish between them. Strong force between two protons is the same as that between two neutrons or that between a neutron and a proton. The carrier particles for strong interaction are called gluons. There is a lot more to it, but let’s save that for another post.
Strong force explains the stability of a nucleus. If there were only strong and electromagnetic forces at play, nuclei would never decay. However, we know that certain isotopes are radioactive and emit particles. For example, the radioactive isotope 14C decays into 14N(the basis for Radiocarbon dating):
In this decay, a Carbon-14 nucleus decays to a Nitrogen-14 nucleus, emitting an electron and an antineutrino. We can understand the underlying reaction as:
i.e. a neutron inside the nucleus decays into a proton, electron and antineutrino. The proton stays in the nucleus but electron and antineutrino leave. This process is called beta decay. This process cannot be explained unless we assume yet another kind of interaction, known as the weak interaction that concerns Leptons(electrons, muons, tau and their corresponding antiparticles, neutrinos and antineutrinos). For this reason, the electron is sometimes called a beta-particle.
The weak force is called weak because it is weaker than both electromagnetic and strong interactions. It is stronger than gravity though it is small range as well and mostly associated with decay processes.
The weakest of all, gravity has been the most elusive. Everyone is familiar with gravity and we have the beautiful theory of general relativity to explain it, yet it feels like we don’t know much about gravity. The most peculiar thing about it, is its weakness as compared to the other three. It has an inverse square relationship and acts over large distances just like electromagnetism but the similarities stop at that. The carrier particle for gravity is hypothesized to be a particle of spin 2, called Graviton. However, there are many theoretical difficulties in explaining the working of graviton, if it is real.
We can summarize the properties of all four fundamental interactions as follows:
Unification of forces
When thermodynamics was first developed, the gas laws were discovered empirically. Later Kinetic theory of gases explained these laws in terms of molecular interactions in the gas and derived ideal gas law from Newtonian mechanics. e.g. Temperature of a gas can be explained in terms of internal energy of the gas molecules quite easily. In this way, the domains of thermodynamics and mechanics were combined into one. A similar unification took place when electricity and magnetism were discovered to be the different aspects of the same thing, and the field of electromagnetism was conceived. Later, light was discovered to be a type of electromagnetic radiation and the fields of optics and electromagnetism became one.
This unification of different aspects of nature is, thus a central theme in physics and many efforts have been made to simplify and explain laws of nature as manifestations of the same thing. The holy grail of Physics is to explain everything in terms of a single interaction. We are looking for what can be called as the theory of everything which would unite all forces into one. So far, a single theory of Electroweak force has been established which combines electromagnetic and weak interaction. The grand unification theory combines the strong force with electroweak force. However, it has been unfruitful so far to combine gravity with remaining three. Attempts are being made towards the theory of everything, but no one has ever succeeded. Theories like superstring theory and M-theory are beautiful, but difficult to verify (See Unified forces by CERN for more information).