Branches of Physics

Physics, in its beginning was known as, and is still known in some places as 'Natural Philosophy'. Physics then, had not specialized so much, as to be split into hundreds of, even more specialized branches. Philosophy is man's attempt in making sense of all that happens in nature and to find the root cause behind everything. I still choose to call it 'Natural Philosophy', as for me, physics and philosophy are not two different things. This article will give you a summary of all the main branches of physics and their definition.

As physicists went on exploring nature at a deeper and deeper level, different physics branches evolved, as specialized fields, with a coherent theoretical structure. To describe and provide rationale, for all that happens, from scales lesser than size of a nucleus, to scales larger than galaxies, there is a different branch of physics, at every level. That is because, every branch is based on a set of principles or approximations that best describe the phenomena at that level. The division of physics into different branches is artificial and is made for our own convenience of understanding. Nature does not compartmentalize this world, as its dynamics at every level, are all connected.

The branches of physics can be broadly divided into two types which are 'Theoretical Physics' and 'Experimental Physics'. The reason for this is best understood in the words of one of the greatest physicist ever, Prof. Richard Feynman. I quote his words from the 'Feynman Lectures on Physics':

"The principle of science, the definition almost, is the following: 'The test of all knowledge is experiment'. Experiment is the sole judge of scientific 'truth.'. . . Experiment itself. . Gives us hints. . But also needed is 'imagination' to create from these hints the great generalizations - to guess at the wonderful, simple, but very strange patterns beneath them all. . And then to experiment to check again, whether we have made the right guess. This imagination process is so difficult, that there is a division of labor in physics: there are theoretical physicists who imagine, deduce and guess at new laws, but do not experiment; and then there are experimental physicists who experiment, imagine, deduce and guess."

I think this completely explains why there is a 'division of labor' in physics into the two branches. The fields are complementary to each other. Every branch of physics has a theoretical discipline and an experimental. Theory cannot be validated without experiment and experiments cannot be performed without theoretical basis and direction.

Physics Branches
Here is a list of main physics branches, along with a summary of what is studied in that particular branch. Every branch of physics is further divided into smaller sub-branches. As explained before, every one of these branches except mathematical physics, has an experimental and theoretical sub-division. The classification of these branches of physics is artificial and these branches overlap onto each other to create further specialized fields.

Classical Mechanics
This is the oldest branch of physics which analytically describes motion of all objects on the macroscopic scales. It describes everything from, why large objects like balls bounce, why pendulum swings to why planets revolve around the Sun! It describes 'mechanics' of all kinds on the large scale and its classical, because it cannot explain motion at atomic level. Fluid mechanics is one specialized sub-branch of classical mechanics, which describes the physics of all types of fluids.

Mathematical Physics
This is the branch of physics, which gives theoretical physics its tools of analysis. Mathematics is the language of nature and therefore if one wants to understand nature, one must understand mathematics. Mathematics brings precision to physics. It is the branch which is an overlap of pure mathematics and physics. Mathematical physics techniques form the toolbox of a physicist. Just like a workman must use the right kind of tools to get his job done, so must a physicist use the right mathematical tools to solve a problem! The more and more deeply we explore nature, every new law discovered can only be expressed in a new form of mathematics.

Classical Electrodynamics
This field is the most broadly applied of all the branches of physics. Classical electrodynamics is based on Maxwell's laws of electromagnetism, which describes all kinds of electromagnetic phenomena from atomic to global scales. It is the theoretical basis of optics, telecommunication and many other sub-fields. Its domain extends over all of nature, as the 'Electromagnetic Force' is all pervading and we live in an electromagnetic world.

Quantum Mechanics
This branch describes a new kind of mechanics, which can explain phenomena at the sub-atomic level, which classical mechanics fails to describe. It provides the clearest picture of nature at the sub-atomic scales. Quantum physics, is based on the principle of uncertainty, and predicts all phenomena in terms of probabilities. It describes a weird sub-atomic world, which is totally different from the world at macroscopic scales. Studying quantum physics requires quite a bit of mathematical expertise and it is the theoretical basis of all branches of physics, that describe phenomena at atomic or sub-atomic scales. For more on this read the basics of quantum mechanics for dummies.

Thermodynamics and Statistical Mechanics
Thermodynamics and statistical physics is one of the core branches of physics, which gives a theoretical mechanism to describe the motion of and phenomena in multi-particle systems. Even though a single particle motion can be analyzed by quantum mechanics, it cannot describe multi-particle systems analytically, as the variables of calculation there are too many. So, a statistical approach is needed that describes motion of matter in bulk. Thermodynamics is a predecessor of statistical mechanics. Statistical mechanics combined with quantum mechanics, forms quantum statistical mechanics.

Condensed Matter Physics
Condensed Matter Physics is a sub-branch of quantum physics and statistical mechanics, which describes all phenomena that occur in matter, which is in condensed form. This includes everything from liquids, solid and gases. The physics of semiconductor devices, which make today's age of information technology possible, is a result of research developments in condensed matter physics. It describes all phenomena in bulk matter like ferromagnetism, superfluidity and superconductivity.

Nuclear Physics
Nuclear physics describes all the phenomena that occur at the level of the atomic nucleus. It deals with and explains phenomena like radioactivity, nuclear fission and nuclear fusion. Developments in nuclear physics led to the production of nuclear weapons like the atom bomb, the Hydrogen bomb and made nuclear energy source available to mankind. For more on this, refer the list of radioactive elements.

Quantum Field theory
This is the physics which describes the physics of particles, which are very small and very fast. It is also known as particle physics. It is based on the three theoretical foundations of quantum mechanics, special theory of relativity and the concept of fields. It is based on the unification of all these three foundations and it describes the physics of fundamental particles of matter. It is one of the most difficult physics branches, which describe the ultimate building blocks of nature.

Non-Linear Dynamics
This is a sub-field of classical mechanics, which solves the problems on macroscopic scales, which cannot be solved by classical mechanics. It is an advanced branch of mathematics, which attempts to solve non-linear differential equations of motion, which are not amenable to a solution by conventional techniques. A greater part of it is also known as 'Chaos Theory', which delves in to the organized chaos that exists in the macroscopic world. It is the most happening branch of physics currently. For more on this read this introduction to chaos theory.

Astronomy and Astrophysics
Astronomy is the observational study of the universe in all its manifestations and astrophysics (a confluence of all branches of physics), is the theoretical basis, which can explain all those phenomena. It is the most all encompassing of all the physics branches, which has a singular goal of explaining every phenomenon that occurs in the universe.

General Theory of Relativity and Cosmology
The general theory of relativity is the correct theory, which describes gravitation at all scales. It interprets gravity not as a force, but as a consequence of the curvature of space-time. Space around massive objects actually gets warped and bent. Gravity is the result of this warping of space time. Special relativity unifies space and time in to 'Spacetime' and general relativity makes 'Spacetime' interact with matter. How much space warps, depends on the content of matter and energy in it. In simple words, general relativity is described by, 'Matter tells space how to bend, space tells matter how to move!' For more refer this article - Does the Fourth Dimension of Time Exist.

The power of the principle of general relativity is such, that it gave birth to the new science of cosmology, which provides the framework, that can describe the evolution of the Universe in totality! For the first time, thanks to cosmology, physics can provide an answer to the question, how was the universe created.

This was a whirlwind tour of all the physics branches. There are many more specialized sub-branches of physics like 'Biophysics' (a confluence of biology and physics), which describes the physics involved in biological systems. Hope this article has given you an idea of the vastness of physics and the power it provides at your disposal!