Category Archives: Advanced topics

Particle Physics: The Fundamental Interactions

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As per the 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:

The two charges interact via exchange of photons (only one photon is shown for simplicity)

Electromagnetic Interaction

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).

Strong Interaction

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.

Weak Interaction

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):

\large _{6}^{14}\textrm{C}\rightarrow _{7}^{14}\textrm{N} +_{-1}^{0}\textrm{e} +_{0}^{0}\bar{\nu_{e}}

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:

\large _{0}^{1}\textrm{n}\rightarrow _{1}^{1}\textrm{p} +_{-1}^{0}\textrm{e} +_{0}^{0}\bar{\nu _{e}}

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.

Gravitation

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:

The four fundamental forces of nature and their relative strength

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).

Particle Physics: Particles and the Standard Model

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Particles are everywhere

We know that matter consists of atoms which in turn consists of neutrons, protons and electrons. There are also photons, that carry energy associated with the electromagnetic interaction. The electron appears to be a fundamental particle since we never see it decay into something else. However, the neutrons and protons seem to have some internal structure. A neutron, if left on its own, can disintegrate[1] into a proton and an electron in nearly 15 minutes:

$n \rightarrow p + e^{-} + \bar{\upsilon }_{e}$

The \bar{\upsilon }_{e} particle is called anti-neutrino. It is the antiparticle of neutrinos (more on this later) that are presumably massless and uncharged point-like particles which appear mostly in processes involving electrons (or antielectrons). The electrons and neutrinos do not appear to have any internal structure (unlike protons and neutrons) and are thus classified as some of the “elementary particles”.

Experiments such as the Large Hadron Collider and studies of cosmic rays have resulted in the discovery of even more elementary particles. They were named pions, muons, kaons, tau etc. until we ran out of greek letters! All of these particles present a challenge to the physicists because many of them exist for miniscule durations and at high energies which are not easily attainable. However based on what we know (and we know quite a bit), there are certain patterns in their properties and behaviour. These patterns make them easier to sort into categories.

The standard Model

The standard model is widely accepted as the most fundamental level of classification of elementary particles we have yet achieved. This model assumes that at the heart of everything there are certain elementary particles which can interact with each other according to certain conservation laws. There are basically two kinds of particles, leptons and quarks (collectively known as the Fermions). The interactions among them (forces) are modeled by carrier particles, known as Bosons1.

Thus, the standard model divides elementary particles into two categories, Bosons (e.g. photons, W and Z bosons etc.) and Fermions (electrons, quarks etc.).The Fermions are further categorized into Leptons and Quarks.

It also lists their properties and interactions. The interactions are governed by Conservation Laws. In this way, the standard model is a theory in itself[2]. Although there are certain anomalies in this model but for the sake of our discussion we assume it is correct.

You can check out particleadventure.org for details. I love this website and they have an app too. Happy learning!

Unlike Newtonian Physics, we do not treat forces as anything different from particles. Essentially all particles are considered to be the manifestation of some field. Photons are thus, equivalent to electromagnetic fields and particles have mass due to Higgs field.

Disclaimer: All of the information provided above is a simplification. Most of these topics require understanding of advanced concepts and involve mathematical difficulties in explaining them theoretically. I recommend you refer to a standard textbook such as Introduction to Elementary Particles by David Griffiths for clear and precise explanations.