What is Physics?

Human Curiosity and the Birth of Science

Humans have always been curious about the world around them. The night sky with its bright celestial objects has fascinated humans since time immemorial. The regular repetitions of the day and night, the annual cycle of seasons, the eclipses, the tides, the volcanoes, and the rainbow have always been a source of wonder.

The inquiring and imaginative human mind has responded to the wonder and awe of nature in different ways. One kind of response from the earliest times has been to observe the physical environment carefully, look for any meaningful patterns and relations in natural phenomena, and build and use new tools to interact with nature. This human endeavour led, in course of time, to modern science and technology.

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The Meaning of Science

The word 'Science' originates from the Latin verb Scientia, meaning ‘to know’. The Sanskrit word Vijñãn and the Arabic word Ilm convey a similar meaning, namely ‘knowledge’.

Science, in a broad sense, is as old as the human species. Early civilisations of Egypt, India, China, Greece, Mesopotamia and many others made vital contributions to its progress. From the sixteenth century onwards, great strides were made in science in Europe. By the middle of the twentieth century, science had become a truly international enterprise, with many cultures and countries contributing to its rapid growth.

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Defining Science and the Scientific Method

Science is a systematic attempt to understand natural phenomena in as much detail and depth as possible, and use the knowledge so gained to predict, modify and control phenomena. Science is exploring, experimenting and predicting from what we see around us. The curiosity to learn about the world and unravel the secrets of nature is the first step towards the discovery of science.

The Scientific Method involves several interconnected steps:

• Systematic observations: Observing phenomena in a careful and structured manner.
• Controlled experiments: Performing experiments under controlled conditions to test specific ideas.
• Qualitative and quantitative reasoning: Analysing the results using both descriptive and mathematical logic.
• Mathematical modelling: Creating mathematical equations or frameworks to describe the phenomena.
• Prediction and verification or falsification of theories: Using the model to predict new outcomes and testing if those predictions are true.

Speculation and conjecture also have a place in science, but ultimately, a scientific theory, to be acceptable, must be verified by relevant observations or experiments.

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The Dynamic Nature of Science

The interplay of theory and observation (or experiment) is basic to the progress of science. Science is ever dynamic. There is no ‘final’ theory in science and no unquestioned authority among scientists. As observations improve in detail and precision or experiments yield new results, theories must account for them, if necessary, by introducing modifications.

Sometimes the modifications may not be drastic. For example, when Johannes Kepler (1571-1630) examined the extensive data on planetary motion collected by Tycho Brahe (1546-1601), the planetary circular orbits in the heliocentric theory imagined by Nicolas Copernicus (1473–1543) had to be replaced by elliptical orbits to fit the data better.

Occasionally, however, the existing theory is simply unable to explain new observations. This causes a major upheaval in science. For instance:

• In the beginning of the twentieth century, it was realised that Newtonian mechanics, a very successful theory until then, could not explain some of the most basic features of atomic phenomena.
• Similarly, the then-accepted wave picture of light failed to explain the photoelectric effect properly.

This led to the development of a radically new theory, Quantum Mechanics, to deal with atomic and molecular phenomena.

This relationship between theory and experiment is a two-way street:

• Experiment influencing Theory: The experiment of scattering of alpha particles by gold foil in 1911 by Ernest Rutherford (1871–1937) established the nuclear model of the atom. This model then became the basis of the quantum theory of the hydrogen atom given in 1913 by Niels Bohr (1885–1962).
• Theory influencing Experiment: The concept of the antiparticle was first introduced theoretically by Paul Dirac (1902–1984) in 1930. This idea was confirmed two years later by the experimental discovery of the positron (antielectron) by Carl Anderson.

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Defining Physics

Physics is a basic discipline in the category of Natural Sciences, which also includes other disciplines like Chemistry and Biology. The word Physics comes from a Greek word meaning nature. Its Sanskrit equivalent is Bhautiki, which is used to refer to the study of the physical world.

A precise definition of this discipline is neither possible nor necessary. We can broadly describe physics as a study of the basic laws of nature and their manifestation in different natural phenomena.

There are two principal thrusts in physics: unification and reduction.

1. Unification

In Physics, we attempt to explain diverse physical phenomena in terms of a few concepts and laws. The effort is to see the physical world as a manifestation of some universal laws in different domains and conditions.

Example: The same law of gravitation (given by Newton) describes the fall of an apple to the ground, the motion of the moon around the earth and the motion of planets around the sun. Similarly, the basic laws of electromagnetism (Maxwell’s equations) govern all electric and magnetic phenomena.

2. Reductionism

This is a related effort to derive the properties of a bigger, more complex, system from the properties and interactions of its constituent simpler parts. This approach, called reductionism, is at the heart of physics.

Example: The subject of thermodynamics, developed in the nineteenth century, deals with bulk systems in terms of macroscopic quantities such as temperature, internal energy, entropy, etc. Subsequently, the subjects of kinetic theory and statistical mechanics interpreted these quantities in terms of the properties of the molecular constituents of the bulk system. In particular, the temperature was seen to be related to the average kinetic energy of molecules of the system.

Scope and Excitement of Physics

We can get some idea of the scope of physics by looking at its various sub-disciplines. Basically, there are two domains of interest: macroscopic and microscopic.

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Domains of Physics

1. Macroscopic Domain

The macroscopic domain includes phenomena at the laboratory, terrestrial and astronomical scales. The branch of physics that deals mainly with these phenomena is called Classical Physics. It includes subjects like:

• Mechanics: Founded on Newton’s laws of motion and the law of gravitation, it is concerned with the motion (or equilibrium) of particles, rigid and deformable bodies, and general systems of particles.

  Examples: The propulsion of a rocket by a jet of ejecting gases, propagation of water waves or sound waves in air, the equilibrium of a bent rod under a load.

• Electrodynamics: Deals with electric and magnetic phenomena associated with charged and magnetic bodies. Its basic laws were given by Coulomb, Oersted, Ampere and Faraday, and encapsulated by Maxwell in his famous set of equations.

  Examples: The motion of a current-carrying conductor in a magnetic field, the response of a circuit to an AC voltage (signal), the working of an antenna, the propagation of radio waves in the ionosphere.

• Optics: This deals with the phenomena involving light.

  Examples: The working of telescopes and microscopes, colours exhibited by thin films.

• Thermodynamics: In contrast to mechanics, it does not deal with the motion of bodies as a whole. Rather, it deals with systems in macroscopic equilibrium and is concerned with changes in internal energy, temperature, entropy, etc., of the system through external work and transfer of heat.

  Examples: The efficiency of heat engines and refrigerators, the direction of a physical or chemical process.

2. Microscopic Domain

The microscopic domain of physics deals with the constitution and structure of matter at the minute scales of atoms and nuclei (and even lower scales of length) and their interaction with different probes such as electrons, photons and other elementary particles. Classical physics is inadequate to handle this domain and Quantum Theory is currently accepted as the proper framework for explaining microscopic phenomena.

Recently, the domain intermediate between the macroscopic and the microscopic (the so-called mesoscopic physics), dealing with a few tens or hundreds of atoms, has emerged as an exciting field of research.

You can now see that the scope of physics is truly vast. It covers a tremendous range of magnitude of physical quantities like length, mass, time, energy, etc.

• Length: At one end, it studies phenomena at the very small scale of length ($10^{-14}$ m or even less) involving electrons, protons, etc.; at the other end, it deals with astronomical phenomena at the scale of galaxies or even the entire universe whose extent is of the order of $10^{26}$ m. The two length scales differ by a factor of $10^{40}$ or even more.
• Time: The range of time scales can be obtained by dividing the length scales by the speed of light: from $10^{–22}$ s to $10^{18}$ s.
• Mass: The range of masses goes from, say, $10^{–30}$ kg (mass of an electron) to $10^{55}$ kg (mass of known observable universe).

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The Excitement in Physics

Physics is exciting in many ways.

• To some people, the excitement comes from the elegance and universality of its basic theories, from the fact that a few basic concepts and laws can explain phenomena covering a large range of magnitude of physical quantities.
• To some others, the challenge in carrying out imaginative new experiments to unlock the secrets of nature, to verify or refute theories, is thrilling.
• Applied physics is equally demanding. Application and exploitation of physical laws to make useful devices is the most interesting and exciting part and requires great ingenuity and persistence of effort.

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Factors Behind Physics' Progress

What lies behind the phenomenal progress of physics in the last few centuries? Great progress usually accompanies changes in our basic perceptions.

1. Importance of Quantitative Measurement

First, it was realised that for scientific progress, only qualitative thinking, though no doubt important, is not enough. Quantitative measurement is central to the growth of science, especially physics, because the laws of nature happen to be expressible in precise mathematical equations.

2. Universality of Basic Laws

The second most important insight was that the basic laws of physics are universal — the same laws apply in widely different contexts.

3. Strategy of Approximation

Lastly, the strategy of approximation turned out to be very successful. Most observed phenomena in daily life are rather complicated manifestations of the basic laws. Scientists recognised the importance of extracting the essential features of a phenomenon from its less significant aspects.

A good strategy is to focus first on the essential features, discover the basic principles and then introduce corrections to build a more refined theory of the phenomenon.

Example: A stone and a feather dropped from the same height do not reach the ground at the same time. The reason is that the essential aspect of the phenomenon, namely free fall under gravity, is complicated by the presence of air resistance. To get the law of free fall under gravity, it is better to create a situation wherein the air resistance is negligible. We can, for example, let the stone and the feather fall through a long evacuated tube. In that case, the two objects will fall almost at the same rate, giving the basic law that acceleration due to gravity is independent of the mass of the object.

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Hypothesis, Axioms, and Models

One should not think that everything can be proved with physics and mathematics. All physics, and also mathematics, is based on assumptions, each of which is variously called a hypothesis, axiom or postulate, etc.

• A hypothesis is a supposition without assuming that it is true. It would not be fair to ask anybody to prove the universal law of gravitation, because it cannot be proved. It can be verified and substantiated by experiments and observations.
• An axiom is a self-evident truth.
• A model is a theory proposed to explain observed phenomena.

For example, the universal law of gravitation proposed by Newton is an assumption or hypothesis. Before him, there were several observations on the motion of planets, the moon, and falling bodies. What the universal law of gravitation says is that, if we assume that any two bodies in the universe attract each other with a specific force, then we can explain all these observations in one stroke and also predict the results of future experiments.

Similarly, Bohr’s model of the hydrogen atom is based on certain rules (postulates) that Bohr assumed. He did this because there was a large amount of spectroscopic data which no other theory could explain. By assuming the atom behaves in a certain manner, all the data could be explained at once.

Einstein’s special theory of relativity is also based on two postulates: the constancy of the speed of light and the validity of physical laws in all inertial frames of reference.

In mathematics too, we need axioms and hypotheses. Euclid’s statement that parallel lines never meet is a hypothesis. If we assume this statement, we can explain many properties of geometric figures. If you don’t assume it, you are free to use a different axiom and get a new geometry.

Physics, Technology and Society

The connection between physics, technology and society can be seen in many examples. The relationship is not one-sided; sometimes technology gives rise to new physics, and at other times physics generates new technology.

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The Interplay between Physics and Technology

1. Technology inspiring Physics

Sometimes, the need to improve an existing technology drives the development of a new branch of physics.

Example: The discipline of thermodynamics arose from the need to understand and improve the working of heat engines. The steam engine, as we know, is inseparable from the Industrial Revolution in England in the eighteenth century, which had a great impact on the course of human civilisation.

2. Physics generating Technology

More frequently, a fundamental discovery in physics leads to the creation of entirely new technologies.

Example: An example of this is the wireless communication technology that followed the discovery of the basic laws of electricity and magnetism in the nineteenth century.

The applications of physics are not always easy to foresee. As late as 1933, the great physicist Ernest Rutherford had dismissed the possibility of tapping energy from atoms. But only a few years later, in 1938, Hahn and Meitner discovered the phenomenon of neutron-induced fission of uranium, which would serve as the basis of nuclear power reactors and nuclear weapons.

Yet another important example of physics giving rise to technology is the silicon ‘chip’ that triggered the computer revolution in the last three decades of the twentieth century.

A most significant area to which physics has and will contribute is the development of alternative energy resources. The fossil fuels of the planet are dwindling fast and there is an urgent need to discover new and affordable sources of energy. Considerable progress has already been made in this direction (for example, in conversion of solar energy, geothermal energy, etc., into electricity), but much more is still to be accomplished.

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Key Physicists and their Contributions

This table lists some of the great physicists, their major contribution and the country of origin. You will appreciate from this table the multi-cultural, international character of the scientific endeavour.

Name	Major contribution/discovery	Country of Origin
Archimedes	Principle of buoyancy; Principle of the lever	Greece
Galileo Galilei	Law of inertia	Italy
Christiaan Huygens	Wave theory of light	Holland
Isaac Newton	Universal law of gravitation; Laws of motion; Reflecting telescope	U.K.
Michael Faraday	Laws of electromagnetic induction	U.K.
James Clerk Maxwell	Electromagnetic theory; Light-an electromagnetic wave	U.K.
Heinrich Rudolf Hertz	Generation of electromagnetic waves	Germany
J.C. Bose	Ultra short radio waves	India
W.K. Roentgen	X-rays	Germany
J.J. Thomson	Electron	U.K.
Marie Sklodowska Curie	Discovery of radium and polonium; Studies on natural radioactivity	Poland
Albert Einstein	Explanation of photoelectric effect; Theory of relativity	Germany
Victor Francis Hess	Cosmic radiation	Austria
R.A. Millikan	Measurement of electronic charge	U.S.A.
Ernest Rutherford	Nuclear model of atom	New Zealand
Niels Bohr	Quantum model of hydrogen atom	Denmark
C.V. Raman	Inelastic scattering of light by molecules	India
Louis Victor de Broglie	Wave nature of matter	France
M.N. Saha	Thermal ionisation	India
S.N. Bose	Quantum statistics	India
Wolfgang Pauli	Exclusion principle	Austria
Enrico Fermi	Controlled nuclear fission	Italy
Werner Heisenberg	Quantum mechanics; Uncertainty principle	Germany
Paul Dirac	Relativistic theory of electron; Quantum statistics	U.K.
Edwin Hubble	Expanding universe	U.S.A.
Ernest Orlando Lawrence	Cyclotron	U.S.A.
James Chadwick	Neutron	U.K.
Hideki Yukawa	Theory of nuclear forces	Japan
Homi Jehangir Bhabha	Cascade process of cosmic radiation	India
Lev Davidovich Landau	Theory of condensed matter; Liquid helium	Russia
S. Chandrasekhar	Chandrasekhar limit, structure and evolution of stars	India
John Bardeen	Transistors; Theory of superconductivity	U.S.A.
C.H. Townes	Maser; Laser	U.S.A.
Abdus Salam	Unification of weak and electromagnetic interactions	Pakistan

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Technology and its Underlying Physics Principles

This table lists some important technologies and the principles of physics they are based on. Obviously, these tables are not exhaustive. We urge you to try to add many names and items to these tables with the help of your teachers, good books and websites on science. You will find that this exercise is very educative and also great fun. And, assuredly, it will never end. The progress of science is unstoppable!

Technology	Scientific principle(s)
Steam engine	Laws of thermodynamics
Nuclear reactor	Controlled nuclear fission
Radio and Television	Generation, propagation and detection of electromagnetic waves
Computers	Digital logic
Lasers	Light amplification by stimulated emission of radiation
Production of ultra high magnetic fields	Superconductivity
Rocket propulsion	Newton’s laws of motion
Electric generator	Faraday’s laws of electromagnetic induction
Hydroelectric power	Conversion of gravitational potential energy into electrical energy
Aeroplane	Bernoulli’s principle in fluid dynamics
Particle accelerators	Motion of charged particles in electromagnetic fields
Sonar	Reflection of ultrasonic waves
Optical fibres	Total internal reflection of light
Non-reflecting coatings	Thin film optical interference
Electron microscope	Wave nature of electrons
Photocell	Photoelectric effect
Fusion test reactor (Tokamak)	Magnetic confinement of plasma
Giant Metrewave Radio Telescope (GMRT)	Detection of cosmic radio waves
Bose-Einstein condensate	Trapping and cooling of atoms by laser beams and magnetic fields.

Fundamental Forces in Nature

Understanding Force

We all have an intuitive notion of force. In our experience, force is needed to push, carry or throw objects, deform or break them. We also experience the impact of forces on us, like when a moving object hits us or we are in a merry-go-round.

Going from this intuitive notion to the proper scientific concept of force is not a trivial matter. Early thinkers like Aristotle had wrong ideas about it. The correct notion of force was arrived at by Isaac Newton in his famous laws of motion. He also gave an explicit form for the force for gravitational attraction between two bodies.

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Derived Forces in the Macroscopic and Microscopic Worlds

In the macroscopic world, besides the gravitational force, we encounter several kinds of forces:

• Muscular force
• Contact forces between bodies
• Friction (which is also a contact force parallel to the surfaces in contact)
• The forces exerted by compressed or elongated springs and taut strings and ropes (tension)
• The force of buoyancy and viscous force when solids are in contact with fluids
• The force due to pressure of a fluid
• The force due to surface tension of a liquid
• Forces involving charged and magnetic bodies.

In the microscopic domain again, we have electric and magnetic forces, nuclear forces involving protons and neutrons, interatomic and intermolecular forces, etc.

A great insight of twentieth-century physics is that these different forces occurring in different contexts actually arise from only a small number of fundamental forces in nature. For example, the elastic spring force arises due to the net attraction/repulsion between the neighbouring atoms of the spring when the spring is elongated/compressed. This net attraction/repulsion can be traced to the (unbalanced) sum of electric forces between the charged constituents of the atoms.

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The Four Fundamental Forces

At the present stage of our understanding, we know of four fundamental forces in nature, which are described in brief here:

1. Gravitational Force

• It is the force of mutual attraction between any two objects by virtue of their masses.
• It is a universal force. Every object experiences this force due to every other object in the universe.
• All objects on the earth, for example, experience the force of gravity due to the earth.
• It governs the motion of the moon and artificial satellites around the earth, motion of the earth and planets around the sun, and, of course, the motion of bodies falling to the earth.
• It plays a key role in the large-scale phenomena of the universe, such as the formation and evolution of stars, galaxies and galactic clusters.

2. Electromagnetic Force

• Electromagnetic force is the force between charged particles.
• In the simpler case when charges are at rest, the force is given by Coulomb’s law: attractive for unlike charges and repulsive for like charges.
• Charges in motion produce magnetic effects and a magnetic field gives rise to a force on a moving charge. Electric and magnetic effects are, in general, inseparable – hence the name electromagnetic force.
• Like the gravitational force, electromagnetic force acts over large distances and does not need any intervening medium.
• It is enormously strong compared to gravity. The electric force between two protons, for example, is $10^{36}$ times the gravitational force between them, for any fixed distance.
• Since the electromagnetic force is so much stronger than the gravitational force, it dominates all phenomena at atomic and molecular scales. It governs the structure of atoms and molecules, the dynamics of chemical reactions and the mechanical, thermal and other properties of materials.
• It underlies the macroscopic forces like ‘tension’, ‘friction’, ‘normal force’, ‘spring force’, etc.
• Gravity is always attractive, while electromagnetic force can be attractive or repulsive. This is because mass comes only in one variety (there is no negative mass), but charge comes in two varieties: positive and negative charge.

Albert Einstein (1879-1955), born in Ulm, Germany in 1879, is universally regarded as one of the greatest physicists of all time. His astonishing scientific career began with the publication of three path-breaking papers in 1905. In the first paper, he introduced the notion of light quanta (now called photons). In the second paper, he developed a theory of Brownian motion. The third paper gave birth to the special theory of relativity. He also created the general theory of relativity, which is the modern theory of gravitation. Some of his other significant contributions include the notion of stimulated emission, the static model of the universe, quantum statistics of a gas of massive bosons, and a critical analysis of the foundations of quantum mechanics.

3. Strong Nuclear Force

• The strong nuclear force binds protons and neutrons in a nucleus.
• It is evident that without some attractive force, a nucleus will be unstable due to the electric repulsion between its protons. This attractive force cannot be gravitational since the force of gravity is negligible compared to the electric force.
• The strong nuclear force is the strongest of all fundamental forces, about 100 times the electromagnetic force in strength.
• It is charge-independent and acts equally between a proton and a proton, a neutron and a neutron, and a proton and a neutron.
• Its range is, however, extremely small, of about nuclear dimensions ($~10^{–15}$m). It is responsible for the stability of nuclei.
• The electron, it must be noted, does not experience this force.
• Recent developments have, however, indicated that protons and neutrons are built out of still more elementary constituents called quarks.

Satyendranath Bose (1894-1974), born in Calcutta in 1894, is among the great Indian physicists who made a fundamental contribution to the advance of science in the twentieth century. In 1924, in a brilliant flash of insight, Bose gave a new derivation of Planck’s law, treating radiation as a gas of photons and employing new statistical methods. The key new conceptual ingredient in Bose’s work was that the particles were regarded as indistinguishable. It was soon realised that the new Bose-Einstein statistics was applicable to particles with integers spins. Particles with integer spins are now known as bosons in honour of Bose.

4. Weak Nuclear Force

• The weak nuclear force appears only in certain nuclear processes such as the $\beta$-decay of a nucleus.
• In $\beta$-decay, the nucleus emits an electron and an uncharged particle called a neutrino.
• The weak nuclear force is not as weak as the gravitational force, but much weaker than the strong nuclear and electromagnetic forces.
• The range of weak nuclear force is exceedingly small, of the order of $10^{–16}$ m.

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Summary of Fundamental Forces

Name	Relative strength	Range	Operates among
Gravitational force	$10^{–39}$	Infinite	All objects in the universe
Weak nuclear force	$10^{–13}$	Very short, Sub-nuclear size ($\sim 10^{-16}$m)	Some elementary particles, particularly electron and neutrino
Electromagnetic force	$10^{–2}$	Infinite	Charged particles
Strong nuclear force	1	Short, nuclear size ($\sim 10^{-15}$m)	Nucleons, heavier elementary particles

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Towards Unification of Forces

We remarked in section 1.1 that unification is a basic quest in physics. Great advances in physics often amount to the unification of different theories and domains.

• Newton unified terrestrial and celestial domains under a common law of gravitation.
• The experimental discoveries of Oersted and Faraday showed that electric and magnetic phenomena are in general inseparable.
• Maxwell unified electromagnetism and optics with the discovery that light is an electromagnetic wave.
• Einstein attempted to unify gravity and electromagnetism but could not succeed in this venture. But this did not deter physicists from zealously pursuing the goal of unification of forces.

Recent decades have seen much progress on this front. The electromagnetic and the weak nuclear force have now been unified and are seen as aspects of a single ‘electro-weak’ force. Attempts have been (and are being) made to unify the electro-weak and the strong force and even to unify the gravitational force with the rest of the fundamental forces. Many of these ideas are still speculative and inconclusive.

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Milestones in Unification of Forces

Name of physicist(s)	Year	Achievement in unification
Isaac Newton	1687	Unified celestial and terrestrial mechanics; showed that the same laws of motion and the law of gravitation apply to both the domains.
Hans Christian Oersted, Michael Faraday, James Clerk Maxwell	19th Century	Unified electricity, magnetism and optics; showed that light is an electromagnetic wave.
Albert Einstein	1905; 1915	Unified space-time and gravity and developed the general theory of relativity.
Sheldon Glashow, Abdus Salam, Steven Weinberg	1979	Showed that the 'weak' nuclear force and the electromagnetic force could be viewed as different aspects of a single electro-weak force.

Nature of Physical Laws

Exploring Physical Laws

Physicists investigate the universe, from subatomic particles to distant stars, using scientific processes based on observation and experimentation. Beyond discovering facts, they strive to find laws that summarise these facts, often in mathematical equations.

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Conservation Laws of Nature

A remarkable aspect of physical phenomena is that while many quantities change over time, some special physical quantities remain constant. These are known as conserved quantities, and understanding their conservation principles is crucial for quantitatively describing observed phenomena.

1. Conservation of Mechanical Energy

For motion under an external conservative force (like gravity), the total mechanical energy (sum of kinetic energy and potential energy) of a body remains constant. For example, during the free fall of an object, its kinetic and potential energies continuously change, but their sum stays fixed. If released from rest, initial potential energy converts entirely into kinetic energy just before impact.

This specific law for conservative forces should not be confused with the more general law of conservation of energy.

2. General Law of Conservation of Energy

This is a fundamental principle, considered valid across all domains of nature (microscopic to macroscopic), and is the basis of the First Law of Thermodynamics. When all forms of energy (heat, mechanical, electrical, etc.) are accounted for, the total energy of an isolated system is conserved.

Example: In the falling object example, if air resistance and the heat/sound generated upon impact are included, the total mechanical energy is not conserved. However, the initial potential energy is transformed into heat and sound, and the total energy of the system (stone + surroundings) remains unchanged.

The total energy of the universe, considered the most ideal isolated system, is also believed to remain unchanged. This law is routinely applied in analysing atomic, nuclear, and elementary particle processes.

3. Conservation of Mass (and Mass-Energy Equivalence)

Before Einstein's theory of relativity, the law of conservation of mass was considered a basic conservation law, as matter was thought to be indestructible. It remains important in chemical reactions, where atoms are merely rearranged, meaning the total mass of reactants equals the total mass of products. The small changes in binding energy in chemical reactions are not measurable as mass changes.

According to Einstein's theory of relativity, mass ($m$) is equivalent to energy ($E$) via the relation:

$\qquad E = mc^2$

where $c$ is the speed of light in vacuum. This means mass can be converted to energy (and vice-versa), as seen in nuclear processes like power generation and explosions. In such cases, the separate conservation of mass and energy does not hold; instead, mass-energy is conserved.

Sir C.V. Raman (1888-1970), born in Thiruvanaikkaval, graduated from Presidency College, Madras. While working for the Indian Government's financial services, he pursued his interest in physics at the Indian Association for Cultivation of Science. His research focused on vibrations, musical instruments, ultrasonics, and diffraction. He became a Professor at Calcutta University in 1917, was elected a 'Fellow' of the Royal Society of London in 1924, and received the Nobel Prize in Physics in 1930 for his discovery, the Raman Effect. This effect describes the inelastic scattering of light by molecules excited to vibrational energy levels, opening new research avenues. He spent his later years at the Indian Institute of Science and the Raman Research Institute in Bangalore, inspiring generations of young students.

4. Conservation of Linear Momentum and Angular Momentum

Energy is a scalar quantity. However, not all conserved quantities are scalars. The total linear momentum and the total angular momentum (both vector quantities) of an isolated system are also conserved. These laws can be derived from Newton's laws of motion but extend beyond mechanics, serving as fundamental conservation laws in all domains where Newton's laws might not be valid.

Conservation laws are invaluable even when the full dynamics of a complex problem (involving multiple particles and forces) cannot be solved. For example, during a car collision, momentum conservation can help predict or rule out possible outcomes without knowing the intricate forces involved.

In nuclear and elementary particle physics, conservation laws are critical analytical tools. For instance, in 1931, Wolfgang Pauli predicted the existence of the neutrino (an uncharged particle emitted in $\beta$-decay along with an electron) using the conservation laws of energy and momentum for $\beta$-decay.

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Conservation Laws and Symmetries of Nature

Conservation laws have a deep connection with the symmetries of nature:

• Conservation of Energy: Linked to the symmetry of nature with respect to translation in time. The laws of nature do not change with time; an experiment yields the same results today as it would a year later under identical conditions.
• Conservation of Linear Momentum: Related to the symmetry of nature with respect to translation in space. Space is homogeneous, meaning there's no intrinsically preferred location in the universe, and the laws of nature are the same everywhere. (Note: Phenomena may differ due to varying conditions, e.g., gravity on the Moon vs. Earth, but the underlying law is the same.)
• Conservation of Angular Momentum: Underlies the isotropy of space, meaning there's no intrinsically preferred direction in space.

Conservation laws for charge and other attributes of elementary particles are also related to certain abstract symmetries. These symmetries of space, time, and other abstract concepts play a central role in modern theories of fundamental forces.

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Nature of Conservation Laws

Conservation laws are hypotheses based on observations and experiments. It's crucial to remember that they cannot be proven; they can only be verified or disproven by experiments.

• An experiment consistent with a law verifies or substantiates it, but does not prove it.
• A single experiment that contradicts the law is sufficient to disprove it.

The law of conservation of energy, for example, is an outcome of centuries of experience and has been found valid across all fields of physics (mechanics, thermodynamics, electromagnetism, optics, atomic and nuclear physics). Observing that mechanical energy is constant for a body falling under gravity is a verification, not a proof, of the law.

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