# Modern Physics

Modern physics is a branch of physics that deals with the fundamental nature of the universe with post-Newtonian concepts. In the early twentieth century, some experimental results could not be matched with the predictions of classical physics, which describes the physical phenomena at an ordinary scale. Modern physics gradually took birth from these theories. The two pillars of modern physics are quantum theory and the theory of relativity. Quantum theory explains the physical phenomena at a short scale whereas the theory of relativity describes large-scale physics and gravity. The results of classical theory can be approximated from both theories.

### Father of Physics

Physics is the study of all natural phenomena from both theoretical and experimental viewpoints. The developments of the subject have been made by numerous scientists. Considering the most important contributions, the title “Father of Physics” is given to three scientists at different times. Galileo Galilei is called the Father of Observational Physics for his contributions to Astrophysics.

Sir Issac Newton gave the laws of motion and gravitation. Classical physics is based on his theory, which works fine on an ordinary scale. He also gave the theory of calculus in mathematics. For the remarkable contributions, Newton is known as the Father of Physics.

Albert Einstein is considered the Father of Modern Physics. He gave the special theory of relativity and the general theory of relativity. These theories govern the behavior of objects at high speeds (close to the speed of light) and gravity. He was awarded the Nobel prize for the explanation of the photoelectric effect.

### The advent of Quantum Theory

Classical physics failed to explain the experimental results of black body radiation, photoelectric effect, and the phenomena of interference of electrons, the stability of an atom. Classical physics considers waves and particles as different notions. In 1900, Max Planck hypothesized that light consists of packets or quanta of energy, called photons. Each photon has energy

E = hv

Here, is the frequency of light and h is Planck’s constant. Although it contradicts the classical theory that considers light as an electromagnetic wave, the black body radiation phenomenon could be described by this hypothesis. Later in 1905, Einstein successfully explained the photoelectric effect, considering light as a swarm of photons (quanta of energy).

On the other hand, the interference of electrons and the stability of an atom could only be described if electrons were considered as waves. De Broglie hypothesized that every particle behaves as a wave, having wavelength:

$\lambda = \frac{h}{p}$

Here, p is its momentum. Everyday objects have very short wavelengths, such that classical theory works at an ordinary scale but the wavelengths of subatomic particles like electrons are comparable with their dimensions.

To describe the physics at small scales (e.g. atomic scale), quantum theory was found to be necessary. In this theory, energy, angular momentum, and other quantities of a bound system are quantized. Many physicists including Bohr, Heisenberg, Schrödinger, Pauli, and Dirac formulated the theory from a mathematical point of view. In the late twentieth century, Quantum Field Theory emerged through the works of scientists like Jordan, Hawking, Weinberg, Feynman.

### Origin of the Theory of Relativity

Einstein realized that space and time are not different concepts. Any observation depends on a frame of reference so that space and time are relative. Newtonian physics considers time as a constant that does not depend on the observer. The classical theory failed to explain Mercury’s precision and time difference of satellites. The theory of relativity could explain these phenomena. Einstein introduced the idea of “spacetime”. A massive object can wrap the fabric of spacetime and gravity is its consequence. Einstein also realized that mass and energy are equivalent concepts. The equivalent energy E corresponding to a mass m is,

$E = mc^{2}$

Here, c denotes the speed of light in vacuum.

### Concepts of modern physics

The key concepts of quantum theory are,

• Wave - Particle Duality : Light behaves as both wave and particle. Light consists of photons or quanta of energy. Particles have a wave nature. Particles are delocalized in space

• Uncertainty Principle : It is not possible to measure the precise position and momentum of a particle simultaneously.

• Measurement Problem : Performing a measurement or observing a system, changes its state.

The concepts of relativity are,

• No massive object can have a speed greater than that of light. The laws of physics always remain invariant for all observers.

• Mass causes curvature in spacetime.

• When an object approaches the speed of light, its length reduces (length contraction). A moving clock slows down (time dilation).

• The sequence of events or the cause-effect structure (causality) remains preserved.

• Gravitational and inertial masses are equivalent.

### Did You Know?

• Gravity can bend light. It causes gravitational lensing, which is the phenomenon of bending of light near a massive object.

• Time slows down near a massive object.

• Gravitational attraction is the consequence of the bending of spacetime.

• An accelerating mass can create ripples in spacetime, which is referred to as gravitational waves. In 1915, gravitational waves were detected.

• Classical physics can be retrieved from modern physics by taking appropriate limits.

• Interference of electron, photoelectric effect, hydrogen spectrum, blackbody radiation are verifications of quantum physics.

• Anomalies in the orbits of planets, time gaps in satellites, gravitational waves match the predictions of relativity.

• There are four fundamental forces in nature namely gravitational force, electromagnetic force, strong and weak forces. The last three forces are described by the Standard model.

• Scientists are trying to incorporate quantum theory and the theory of relativity through the conception of a more general theory, often referred to as the “theory of everything”.