What is Physics?

The meaning of the word “Physics”, from Ancient Greek: φυσική , literally means “knowledge of nature”.

According to Oxford dictionary:  

“Physics is the branch of science concerned with the nature and properties of matter and energy”. 

But this definition is too general. Moreover, for those who are not familiar with physics, this definition is not clear. And for those who are familiar, it is inaccurate.

Let me try to define it: 

Physics is an attempt of humans to explore and explain the most fundamental laws of nature. The methodology is to create a mathematical model that describes experimentally observed phenomena. If this model adequately describes observed phenomena and makes predictions which are confirmed experimentally, we are calling it Physical Theory or Physical Law.  These laws are found to be an adequate approximation of nature.

While physics aims to discover universal laws, its theories lie in explicit domains of applicability. Loosely speaking, the laws of classical physics accurately describe systems whose important length scales are greater than the atomic scale and whose motions are much slower than the speed of light. Outside of this domain, observations do not match predictions provided by classical mechanics. Albert Einstein contributed the framework of special relativity, which replaced notions of absolute time and space with spacetime and allowed an accurate description of systems whose components have speeds approaching the speed of light. Max Planck, Erwin Schrödinger, and others introduced quantum mechanics, a probabilistic notion of particles and interactions that allowed an accurate description of atomic and subatomic scales. Later, quantum field theory unified quantum mechanics and special relativity. General relativity allowed for a dynamical, curved spacetime, with which highly massive systems and the large-scale structure of the universe can be well-described. General relativity has not yet been unified with the other fundamental forces; several candidate theories of quantum gravity are being developed, such as string theory and loop quantum gravity.

Physics covers a wide range of phenomena, from elementary particles (such as quarks, neutrinos, and electrons) to the largest superclusters of galaxies. Included in these phenomena are the most basic objects composing all other things. Therefore, physics is sometimes called the “fundamental science”. Physics aims to describe the various phenomena that occur in nature in terms of simpler phenomena. Thus, physics aims to both connect the things observable to humans to root causes, and then connect these causes together.

The main branches of physics could be represented by the following scheme:

But it’s not correct to think that physics explains everything. There are a lot of fundamental, still unsolved problems in physics. Some of the major unsolved problems in physics are theoretical, meaning, that existing theories seem incapable of explaining a certain observed phenomenon or experimental result.

A few examples [Johan Hansson, The 10 Biggest Unsolved Problems in Physics, International Journal of Modern Physics and Applications Vol. 1, No. 1, 2015, pp. 12-16]: 

• There are still some deficiencies in the Standard Model of Elementary Particles (the theory describing three of the four of the fundamental forces of nature – Electro-Magnetic interaction, Weak and Strong nuclear interactions), such as:
  • The origin of mass. Why do elementary particles have such masses? There is no unambiguous explanation – in the existing theory, to explain the masses of elementary particles, one has to introduce many numerical parameters that are not related to fundamental physical constants – an indicator that the theory is not complete. 
  • The strong CP problem. That is, in the theory of strong interactions (force between quarks), there exists a certain symmetry, which theoretically can be violated. But experimentally the CP-violation was not observed. This is an indicator that the theory is not complete. 
  • Neutrino mass. According to the standard model, neutrinos should not have mass. However, it has been experimentally discovered that neutrinos have a small mass (neutrino oscillation).
  • Matter–antimatter asymmetry.  In the standard model, there is no advantage of matter over antimatter. Why then does most of the observable matter in the universe exist in the form of matter?
  • The nature of dark matter and dark energy. Dark matter is responsible for the dynamics of stars in galaxies and for the dynamics of super-large masses, such as clusters of galaxies. Dark energy is responsible for the expansion of the universe. Depending of it density, the universe may expand, or contract, or to be static. But their origin is still unknown. 

The last two are most concerning, since 74% of total energy in the universe is the dark energy, and 21% of total energy is the dark matter, which physics still don’t understand what it is, and only 4% is an ordinary matter, which is adequately described by known physical theories.   

• Arrow of time (e.g. entropy’s arrow of time): Why does time have a direction (from past to the future)? Why did the universe have such low entropy (the measure of disorder)  in the past, and time correlates with the universal (but not local) increase in entropy, from the past and to the future, according to the second law of thermodynamics? (Simple example: Someone reading scientific paper brings more “order” to his head (entropy in the head decreases). But the brain, trying to understand that paper, emits a lot of heat to the environment, heating the air around. Thus, globally (head + environment), the entropy increases, because rising of the gas temperature leads to increasing of “chaos” in motion of gas molecules) Why are CP violations (CP-symmetry – it’s a certain symmetry in theories of elementary particles, responsible after indistinguishability of physical laws under reverse of direction of time) observed in certain weak force decays, but not elsewhere? Are CP violations somehow a product of the second law of thermodynamics, or are they a separate arrow of time? Are there exceptions to the principle of causality? Is there a single possible past? Is the present moment physically distinct from the past and future, or is it merely an emergent property of consciousness? What links the quantum arrow of time (in elementary particles) to the thermodynamic arrow (for all Universe)? 
• Although much progress has been made in high-energy, quantum, and astro-physics, many everyday phenomena involving complexity, chaos, or turbulence are still poorly understood. Complex problems that seem like they could be solved by a clever application of dynamics and mechanics remain unsolved. For example, the kind of chaos inherent in turbulence – in both time and space – is still a mystery. And for the complex systems (like climate, earthquakes, granular materials (sand), glasses etc.) the unanswered question is: although the four known fundamental forces of nature (gravity, electromagnetism, strong nuclear and weak nuclear) relatively simple, almost impossible on their basis to predict in detail the behavior of even moderately complex systems. Is this a real aspect of nature, or just a result of our theories so far being formulated in non-ideal ways? 

But most important in Physics is it spirit. The spirit of curiosity, open mind, creative thinking and desire to understand the word surrounding us.

“The important thing is not to stop questioning. Curiosity has its own reason for existence. One cannot help but be in awe when he contemplates the mysteries of eternity, of life, of the marvelous structure of reality. It is enough if one tries merely to comprehend a little of this mystery each day.

― Albert Einstein “Old Man’s Advice to Youth: ‘Never Lose a Holy Curiosity.'” LIFE Magazine (2 May 1955) p. 64”