Maarg Manthan · Topic 9.1

Theories and Laws of Physics, Including the Theory of Relativity

Science & Technology › Physics – Modern Physics and Astrophysics · Topic 9.1

Physics rests on a small number of laws and theories that describe nature from the scale of atoms to the universe. Newton's laws of motion and law of universal gravitation, published in 1687, explain everyday motion and the orbits of planets. Maxwell's equations describe electricity, magnetism and light. The laws of thermodynamics govern heat and energy. Early in the twentieth century, two theories changed physics. Quantum mechanics describes atoms and particles, where energy comes in packets, and Heisenberg's uncertainty principle of 1927 says that position and momentum cannot both be known exactly. Albert Einstein's special theory of relativity of 1905 says that the speed of light is the same for all observers, and the general theory of 1915 describes gravity as the curvature of space and time caused by mass and energy.

5 min read · UPSC Prelims · Science & Technology · Physics – Modern Physics and Astrophysics

At a Glance

Newton Laws of motion and gravitation, 1687
Special relativity Einstein, 1905
General relativity Einstein, 1915
Uncertainty principle Heisenberg, 1927
Mass-energy E = mc squared
Exam link UPSC Prelims, Science & Technology: Physics – Modern Physics and Astrophysics

Where Does It Fit?

This is the first topic of the macro on modern physics and astrophysics. It gives the theories that the topics on the Big Bang, black holes and gravitational waves use. Previous topic: Types and Applications of Lasers. Next topic: Big Bang Theory and Cosmological Singularity.

What Are the Main Laws and Theories?

Theory or law	Statement and impact
Newton’s laws of motion	A body stays at rest or in uniform motion unless a force acts, force equals mass times acceleration, and every action has an equal and opposite reaction
Law of gravitation	Every mass attracts every other with a force proportional to the product of masses and inversely proportional to the square of the distance
Kepler’s laws	Planets move in ellipses with the Sun at one focus, sweep equal areas in equal times, and the square of the period is proportional to the cube of the orbit size
Maxwell’s equations	1860s. Unified electricity, magnetism and light, as in Concepts of Electromagnetism
Quantum mechanics	From Planck in 1900 and Bohr, Schrodinger, Heisenberg and Dirac. Energy is quantised and particles behave like waves
Special relativity	Time and space depend on the motion of the observer, and nothing with mass can reach the speed of light
General relativity	Mass bends spacetime, and the bending is gravity

What Does Relativity Say and How Is It Tested?

Constant speed of light: the speed of light in a vacuum is the same for all observers, whatever their motion.
Time dilation: a moving clock runs slow compared with a stationary one. Fast-moving muons from the upper atmosphere live longer, and astronauts age very slightly less.
Length contraction: objects appear shortened along the direction of motion at speeds near that of light.
Mass-energy equivalence: energy equals mass times the speed of light squared, which explains nuclear energy, where a small mass loss gives a large energy, as in Nuclear Technology and Energy.
Gravitational lensing and the 1919 eclipse: Arthur Eddington’s observation of starlight bending near the Sun in 1919 supported general relativity.
GPS: satellite clocks need relativistic corrections or the positions would drift by kilometres a day.
Gravitational waves and black holes: predicted by general relativity and observed in 2015 and 2019, as in Gravitational Waves.

What Else Should You Remember?

Quantum ideas: wave-particle duality, quantised energy levels and the uncertainty principle, the basis of lasers, transistors and quantum computers, as in Types and Applications of Lasers.
Photoelectric effect: Einstein explained it in 1905, which won him the Nobel Prize in 1921, as in ionosphere Space…">Electromagnetic Wave Behaviour.
Pauli exclusion principle: no two electrons can have the same quantum state, which explains the structure of atoms.
Standard Model: combines quantum theory and the weak, strong and electromagnetic forces, but leaves out gravity, as in Fundamental Particles and Basic Forces of Nature.
Indian contributions: Satyendra Nath Bose’s work on photon statistics led to Bose-Einstein statistics in 1924, C.V. Raman discovered the Raman effect in 1928, and Subrahmanyan Chandrasekhar found the mass limit for white dwarfs of about 1.4 solar masses, for which he received the Nobel Prize in 1983.
Conservation laws: energy, momentum, angular momentum and charge are conserved in isolated systems.
Limits of theories: Newton’s laws work well at low speeds and weak gravity, and relativity and quantum theory are needed at extreme scales. A theory of quantum gravity is still being sought.
Exam pattern: matching of scientists with laws and the idea behind E = mc squared.

Why Does It Matter for the Exam?

Scientist-theory pairs: Newton, Maxwell, Planck, Einstein, Heisenberg.
Concept questions: time dilation, curved spacetime and mass-energy equivalence.
Technology link: GPS, nuclear power, lasers and semiconductors depend on modern physics.
Current affairs link: gravitational waves, black hole images and quantum missions.
Common errors: saying that Einstein disproved Newton or that relativity says that everything is relative.

As an analytical point, each theory holds in a domain. Newton remains the working tool for engineers, while relativity and quantum mechanics extend physics to the very fast, the very large and the very small.

Why Is This Topic Important?

Facts Scientists and years asked
Concept Relativity ideas
Link GPS and nuclear energy

What Came Next?

The next topic covers the Big Bang theory and cosmological singularity. Next topic: Big Bang Theory and Cosmological Singularity.

Key People and Terms

Spacetime Combined space and time
Time dilation Moving clocks run slow
Quantum Smallest packet of energy
Uncertainty principle Limit on exact knowledge
Mass-energy E = mc squared
Gravitational lensing Bending of light by gravity

Exam Corner

Points to Remember

Special relativity: 1905; general: 1915.
E = mc squared.
Uncertainty principle: Heisenberg, 1927.
Eddington eclipse test: 1919.
Chandrasekhar limit: about 1.4 solar masses.

Do Not Confuse With

Special and general relativity No gravity versus gravity as curvature
Newtonian gravity and Einstein gravity Force versus curved spacetime
Classical and quantum physics Large scale versus atomic scale

Memory Hook

Newton for everyday motion, Maxwell for light; Planck’s quantum, Einstein five and fifteen; Heisenberg’s uncertainty, spacetime bent by mass.

Mains Angle

Questions ask about the impact of modern physics on technology. These points add depth.

Classical Newton and Maxwell
Modern Quantum, relativity
Use GPS, nuclear, lasers

A Question You May Face

An original practice question, not a past paper question.

“Explain the theory of relativity and discuss how modern physics has changed technology and our understanding of the universe.”

How to Answer

Classical Newton.
Relativity Special, general.
Quantum Uncertainty.
Use GPS, lasers.

GS Relevance

Prelims: laws and scientists, relativity predictions, quantum ideas, tests of general relativity. GS Paper 3: science and technology.

Frequently Asked Questions

What is the theory of relativity?

The theory of relativity, developed by Albert Einstein, has two parts. Special relativity of 1905 says that the speed of light is the same for all observers and that time and space depend on motion. General relativity of 1915 describes gravity as the curving of spacetime by mass and energy.

What does E equals mc squared mean?

E equals mc squared is Einstein's equation showing that mass and energy are equivalent, where E is energy, m is mass and c is the speed of light. Since c squared is a huge number, a tiny amount of mass corresponds to a very large energy, which explains the energy released in nuclear reactions and in the Sun.

What is the uncertainty principle?

The uncertainty principle, stated by Werner Heisenberg in 1927, says that certain pairs of properties of a particle, such as its position and momentum, cannot both be known exactly at the same time. The more precisely one is known, the less precisely the other can be known, which is a basic feature of quantum mechanics.

How was general relativity tested?

General relativity was tested in 1919 when Arthur Eddington measured the bending of starlight near the Sun during a solar eclipse, as the theory predicted. Later tests include the orbit of Mercury, gravitational lensing, GPS clock corrections and the detection of gravitational waves in 2015.

What is the Chandrasekhar limit?

The Chandrasekhar limit is the maximum mass, about 1.4 times the mass of the Sun, that a stable white dwarf star can have. It was calculated by the Indian-born astrophysicist Subrahmanyan Chandrasekhar, who received the Nobel Prize in Physics in 1983, and stars above it can collapse into neutron stars or black holes.

PYQ Practice — Statement Analysis

1 General relativity describes gravity as curvature of spacetime.

True

Published in 1915.

2 The uncertainty principle was proposed by Einstein.

False

By Heisenberg in 1927.

3 Starlight bending in 1919 supported general relativity.