Natural Sciences

Understanding Natural Phenomena 12: The Four Interactions

Editor’s Note: This article is a part of the series Understanding Natural Phenomena.

There are four types of forces or interactions in our universe. The first is the gravitational interaction, or the gravitational force field. It is very weak, but is always present between any two particles or bodies. It is proportional to the product of the masses of the objects interacting. Since most of the celestial bodies are very massive, the gravitational force becomes very significant for them. Your weight is the gravitational force with which the Earth attracts you towards its centre.

Like charges repel, and unlike charges attract. Similarly, like magnetic poles (north-north or south-south) repel, and unlike magnetic poles (north-south) attract. Research showed that the electric interaction and the magnetic interaction are really two aspects of the same underlying phenomenon, so the term electromagnetic interaction was coined. This is the second of the four interactions.

The third is the ‘weak nuclear’ interaction. It is operative, for example, inside the nuclei of radioactive materials, and is responsible for the emission of alpha-particles, beta-particles, etc. from inside such nuclei.

Lastly we have the ‘strong nuclear’ interaction, which is very strong but very short-ranged, and is responsible for the large binding energies of nuclei: A rather large amount of energy is required for extracting a proton or a neutron from inside the nucleus of an atom.

Maxwell’s theory of the electromagnetic interaction was a classical theory. So also was Einstein’s theory of the gravitational interaction, namely the general theory of relativity. Quantum effects become very significant at sub-atomic length scales. Moreover, the early universe (at and immediately after the Big Bang) was also of very small dimensions. We therefore need a quantum formulation for all the four interactions.

In quantum theory, not only are the elementary particles quanta of mass/energy, even the force fields or interactions among the particles are mediated by quanta. For example, when two electrons interact, the fundamental particle which mediates the interaction is the photon. One electron emits a (‘virtual’) photon and recoils in the process. The other electron absorbs the photon and also recoils. This back and forth exchange of photons constitutes the electromagnetic interaction. Quantum field theories have to be formulated for all the four interactions.

Historically, the electromagnetic interaction was the first to be cast in a quantum-mechanical form, and this subject goes by the name of quantum electrodynamics (QED). Richard Feynman, who played a major role in the development of QED, had also formulated a sum-over-histories version of quantum mechanics (see Part 4). He used his famous path-integrals for working out the details of QED. But the QED theory ran into a conceptual problem. The summation over the infinitely many possible histories resulted in an infinite mass and charge for the electron, which was an absurd result. Feynman got over this problem by a procedure called renormalization, but I shall not go into its technical details here.

Why are there four different fundamental interactions, and not just one? The fact is that there was indeed only one interaction to start with, but as the universe expanded and cooled, symmetry-breaking transitions occurred, resulting in the successive appearance of different and less symmetric interactions. Attempts continue to be made for ‘unifying’ the four interactions to see what kind of a theory emerges when this has been achieved. Such efforts run parallel to those for obtaining quantum-field-theoretic formulations for the four interactions.

The first to be unified were the electromagnetic interaction and the weak-nuclear interaction, resulting in what is called the electroweak interaction. A bonus point of this unification was that the renormalization procedure could be successfully carried out for the unified interaction for obtaining its quantum field theory (without encountering the ‘infinities’ problem mentioned above), whereas it was not achievable for the weak interaction separately.

The quantum field theory which successfully achieved renormalization for the quantum version of the strong nuclear interaction is called quantum chromodynamics (QCD). In this theory the proton and the neutron, as also some other particles, are envisioned as made up of a more fundamental set of particles called quarks. Quarks come in three ‘colours’ (nothing to do with the usual meaning of colour): red, green, and blue, along with the respective ‘anticolours’. The quarks cannot exist as free, stable particles. Only those combinations of them can exist as free particles which do not have a net colour. For example, a colour and its anticolour cancel, giving a neutral net colour. Composite particles in which this occurs are the particles called mesons.

Another possibility is that all three colours (one each), or all three anticolours, occur together in a composite particle. The name for such a composite particle is ‘baryon‘. Protons and neutrons are examples of baryons.

In addition to colour, quarks have quantum parameters like ‘up’ (u), ‘down’ (d); ‘charm’, ‘strangeness’; and ‘top’, ‘bottom’ (do not pay attention to the literal meanings of such words; they are just labels, with no literal meaning). Two up quarks and a down quark make a proton, and two down quarks and one up quark make a neutron.

The information given in this post is part of what is called the STANDARD MODEL of particle physics. In it, the electromagnetic interaction and the weak-nuclear interaction have been unified into the electroweak interaction, and a quantum version for it has been established. In addition, there is a quantum-mechanical formulation for the strong nuclear interaction (QCD), but no entirely satisfactory unification with any of the other interactions. The gravitational interaction has been neither quantized, nor properly unified with other interactions. I shall return to the Standard Model in the next post.

A total unification of all the four interaction was a distant dream till recently. Now ‘string theory’ and ‘M-theory’ have emerged as solutions to some of the pending problems in particle physics and cosmology. I shall discuss them in a future post.

About the author

Vinod Wadhawan

Dr. Vinod Wadhawan is a scientist, rationalist, author, and blogger. He has written books on ferroic materials, smart structures, complexity science, and symmetry. More information about him is available at his website. Since October 2011 he has been writing at The Vinod Wadhawan Blog, which celebrates the spirit of science and the scientific method.


  • Dr. Wadhawan,

    A total unification of all the four interaction was a distant dream till recently.

    You seem to sound optimistic about unification. I wonder what is your opinion of the views expressed in this book ( where the author (Marcelo Gleiser) argues that scientists are misguided is their search for a grand unified theory. He equates this search to the misguided search for a monotheistic God.

    His simple layman explanation goes something like this. If there was a grand unified theory then there will be no imperfections. But the universe is filled with imperfections, so a grand unified theory should not exist.

    Does the author have a point? Or do you think that it is the author who is misguided?

    • 1. If you are not happy with the word ‘grand’, it can be dropped. But the quest for a unification of the four interactions is based solidly on a huge number of experimental facts. The theory is only our best available attempt at making sense of a large number of observations. That is how science progresses.

      2. At the heart of the theory is the idea that as the universe cooled, it underwent a series of symmetry-lowering phase transitions, each resulting in the emergence of a new interaction. That is how we have four of them now.

      3. I admit that we are still some way from having a unified theory. But even when we succeed, what is the link between ‘perfection’ and the availability of a good theory?

    • His simple layman explanation goes something like this. If there was a grand unified theory then there will be no imperfections.

      I haven’t read the book and this is a layperson’s question prompted only by the question above. Ascribing perfection or other such evaluative attributes to the universe per se appears to make no sense because it assumes preferences, whereas a description of the universe maybe called ‘perfect’ if it is a true account of every possible observation. Couldn’t the apparent ‘imperfection’ (if we treat discrepancy between predictions and observations as the working definition of imperfection) in the observable universe be simply due to application of the wrong theory that yields wrong predictions? How is this an argument against the existence of a theory that will yield predictions that perfectly describe the observable universe?

      • I think I used the wrong word. Assymetries would be a better word than imperfection. The book’s thesis is that it is the assymetries that creates things like galaxies and stars. If a grand unified theory exists such assymetries will not exist and the universe we know will not exist.

        I read the book a while ago and that is the message I remember. May be I am oversimplifying the book. I will consult the book again to see if that is an accurate enough description.

        • “The book’s thesis is that it is the asymetries that creates things like galaxies and stars. If a grand unified theory exists such asymetries will not exist and the universe we know will not exist.”

          Wrong thesis. Asymmetry decreases as we go up the temperature scale across the phase transitions I mentioned. At the present temperature, the universe is as symmetric as possible. Before the first phase transition occurred, the temperature was extremely high and there were no galaxies and stars. But the present average temperature is not that temperature; it is much lower.

  • Interesting reading. I do have the following comment to offer: While talking about quantum electrodynamics, the article mentions that the solution to the problem of infinities was found by Feynman. In reality Quantum Electrodynamics was independently discovered by Feynman, Schwinger and Tomonaga and each one of them independently figured out how to avoid infinities. The credit should not be given to Feynman alone.

    • I tried to choose my words carefully. Here is what I have written: “Richard Feynman, who played a major role in the development of QED, …”. My experience as a scientist has been that it is not enough to just do or discover something new. You have to keep working in that field for enough time to make a mark for yourself. Nevertheless, I agree with your basic point.

      • I am sorry. Not sure I have understood. Do you mean to say Schwinger and Tomanaga have not worked in the field enough time?

Leave a Comment