"Theoretical Physics is one of the most important of fundamental sciences. It won't be an exaggeration to say that in fact it is the most significant original and basic science because it deals with the "underlying" laws behind most of Physics and the whole of Chemistry and Biological Sciences. In the words of Dirac who is one of the greatest theoretical physicists of all time--alongside Maxwell, Einstein and Schrodinger: “The general theory of quantum mechanics is now almost complete. The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known."
Fundamental means something which comprises or involves a foundation. So, what are the foundations of Physics? The foundations of Physics are the most fundamental entities that make up the universe and the most fundamental laws governing their existence and interactions. By definition the most fundamental laws of nature must also be the most general, having the greatest scope and thus generate the most accurate results. These “fundamental laws” of nature are contained in just two theories: Einstein’s General Theory of Relativity and Glashow, Salam, Weinberg’s Standard Model of Particle Physics. If we want to go beyond that—still that will be an extension/unification of these two theories. Because, they are parts of Theoretical Physics—therefore it’s the most fundamental of all sciences.
Our nature itself is a complex and diversified system. But we believe that in its core it is made up of much more simple or fundamental stuffs. The goal of Physics as a discipline was always the understanding of these basic constituents of nature by reducing it to the interactions of it’s parts or to simpler or more “fundamental” things and to a few simple fundamental laws. Newton thus reduced all celestial and terrestrial phenomenon in terms of a single fundamental entity—that is gravity and gave us the inverse square law. However, it’s the special case of something more generic and in that sense Einstein’s General Theory of Relativity is much more fundamental and the most accurate theory of gravity currently known. It can explain all gravitational phenomena that Newton’s law can and goes beyond that. Similarly, Maxwell unified electricity, magnetism and light—previously thought to be separate entities. Now we know that it is part of a more general theory and that theory is the most successful and “most” accurately tested scientific theory in the whole history of the human civilization.
All known atomic interactions are electro-magnetic in nature and, no matter how accurate the equipment one can devise, the predictions made by renormalized QED theory hold true. In some cases, it gives predictions which are accurate to more than six significant figures!!! No science exists—whose predictions can encompass the predictions of QED. This this exactly the nature of anything that can be the most fundamental. Any physical phenomena can be explained completely in terms of relations between other more fundamental phenomena and if it is the part of Physics that deals with the later—then there can’t be anything more fundamental than that. In fact we know that the complete theory governing a large part of Particle Physics and the whole of Condensed Matter Physics as well as Chemistry and Complex Biological Systems are contained within a single mathematical construct that is the Lagrangian of Quantum Electrodynamics. If we include Nuclear Physics to it then all known physics (except gravity) is contained within the Lagrangian of the Standard Model.
The scope of Theoretical Physics is probably the biggest in anything that a man could do in his life-time. It surpasses every other cerebral subject that man has ever contemplated. For example, Biology deals only with life and living organisms; Chemistry is all to do with elements, compounds as well as the changes they undergo during chemical reactions but Physics in general and Theoretical Physics in particular has something or the other to do with everything as it seeks to understand the whole universe. It tries to comprehend and explain how everything from the DNA in an organism to the stars and galaxies, are made up from the same fundamental constituents. Even Mathematics seems like a pillar and the tool and language of something greater, which is Theoretical Physics. A key difference between Physics and Mathematics is that since Physics is ultimately concerned with accounts of the material world, it assesses its theories by comparing the predictions of its theories with data obtained from observations and experimentations, whereas Mathematics is concerned with abstract patterns, not restricted by those observed in the real world. The distinction, however, is not always clear-cut. There is a large area of research intermediate between Physics and Mathematics, known as Mathematical Physics.
If something is more fundamental then its domain will be extremely wide compared to something less fundamental. In fact, the range of the scope of Theoretical Physics outmatches any known discipline in the history of human knowledge. It covers a remarkable range of magnitude of physical quantities. At one end, it studies phenomena at the very small scale of length (10 to the power -14 meter or even less) comprising of electrons, protons, etc.; at the other end, it deals with astronomical phenomena at the scale of galaxies or even the entire universe whose magnitude is of the order of 10 to the power 26 meter. The two length scales differ by a factor of 10 to the power 40 or yet more!!! The whole observable universe according to our current understanding is made of just 12 fundamental entities and their interactions. Which part of Physics deals with that? If we want to go even beyond that—which part of Physics we need to study?
So, Quantum Electrodynamics, Standard model and General Theory of Relativity are the most fundamental of all known physics and they are part of what we call Theoretical Physics."
The Department of Theoretical Physics, University of Dhaka was established in 1975, but although very much successful, the Department had to stop its activities after only three years due to necessary faculty and infrastructural facilities. This was a very unfortunate event. Professor A.M. Harun-or-Rashid was the founder Chairman of the department. Dr.Golam Mohammad Bhuiyan, Dr.Arshad Momen and Dr.Md.Kamrul Hassan took the primary initiative for its revival and they had the major role in the re-establishment of this Department (thanks to them). Realizing the importance of the study of fundamental science in the 21st century, the University authorities decided to revive the Department. At last with the earnest will of the Department of Physics and the University authorities it was re-established in 2007 (thanks to them too).