## Let’s Delve Deeper into the Topic!

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 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 view points. 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 their 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 behaviour 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, v 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 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,

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Here, c denotes the speed of light in a vacuum.

### Black Body Radiation

Black-body radiation is the thermal electromagnetic radiation released by a black body when it is in thermodynamic equilibrium with its surroundings (an idealised opaque, non-reflective body). It has a defined spectrum of wavelengths that are inversely linked to intensity and are only dependent on the body's temperature, which is considered to be uniform and constant for the sake of calculations and theory.

Many everyday items spontaneously release thermal radiation that can be approximated as black-body radiation. Internally, a fully insulated container in thermal equilibrium includes black-body radiation, which it will release through a hole in its wall if the opening is small enough to not influence the equilibrium.

### Solid State Physics

Quantum mechanics, crystallography, electromagnetism, and metallurgy are all used in solid-state physics to explore rigid matter or solids. It is the most important subdiscipline in condensed matter physics. Solid-state physics investigates how solid materials' large-scale characteristics are derived from their atomic-scale properties. Solid-state physics is thus the theoretical foundation of materials science. It also has direct uses, such as in transistor and semiconductor technologies.

The majority of solid-state physics is centred on crystals as a generic theory. This is mostly because the periodicity of atoms in a crystal — its distinguishing feature — makes mathematical modelling easier. Similarly, crystalline materials frequently possess electrical, magnetic, optical, or mechanical characteristics that can be used in engineering applications.

### Atomic Theory

The scientific hypothesis that matter is made up of tiny bits called atoms is known as atomic theory. The origins of atomic theory may be traced back to an ancient intellectual tradition known as atomism. According to this theory, if you cut a lump of stuff into smaller and smaller bits, you would ultimately reach a point where the parts can no longer be sliced into smaller pieces. These hypothesised fundamental elements of substance were given the name ‘atomos’ by ancient Greek philosophers, which meant "uncut."

### John Dalton

John Dalton researched and expanded on this previous work, defending a new idea later known as the law of multiple proportions: if the same two elements can be combined to form several different compounds, the ratios of the two elements' masses in their various compounds will be represented by small whole numbers. This was a prevalent pattern noted by Dalton and other scientists at the time in chemical processes.

### Avogadro

Amedeo Avogadro addressed the weakness in Dalton's theory in principle in 1811. Equal volumes of any two gases, under equal temperature and pressure, contain equal numbers of molecules, according to Avogadro (in other words, the mass of a gas's particles has no bearing on the volume it occupies). By observing the volumes at which gases interacted, Avogadro's law allowed him to derive the diatomic nature of many gases. For example, when two litres of hydrogen react with one litre of oxygen to make two litres of water vapour (at constant pressure and temperature), a single oxygen molecule splits in half to produce two water particles. As a result, Avogadro was able to provide more precise estimations of the atomic mass of oxygen and other elements, as well as distinguish between molecules and atoms.

### Brownian Motion

Robert Brown, a British botanist, noticed that dust particles inside pollen grains floating in water jiggled around for no apparent cause in 1827. Albert Einstein proposed in 1905 that the Brownian motion was created by water molecules constantly pushing the grains around, and he constructed a hypothetical mathematical model to explain it. In 1908, French physicist Jean Perrin confirmed this model experimentally, offering more support for particle theory (and by extension atomic theory).

### 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 electrons, 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”.

### Conclusion

Modern physics deals with the fundamental nature of the universe with post-Newtonian concepts. Two pillars of modern physics are quantum theory and the theory of relativity. The title "Father of Physics" is given to three scientists at different times for their contributions to Astrophysics.

## FAQs on Modern Physics

**1. What are the Modern Physics Topics?**

Modern physics is based on quantum mechanics and relativity. Quantum theory explains the physical phenomena at a short scale whereas the theory of relativity describes large-scale physics and gravity. Quantum physics broadly consists of topics like atomic structure, wave nature of matter, the spin of elementary particles, uncertainty principle, radioactivity, and many more. Relativity is concerned with the mass-energy equivalence, the curvature of spacetime, and the equivalence principle.

**2. Who is the Father of Modern Physics?**

Albert Einstein is called the father of modern physics. Issac Newton is called the father of physics for his contributions to classical physics. He gave the laws of motion and calculus. Newton’s theories describe physical phenomena at ordinary scales. Galileo Galilei is called the father of observational physics for his observations in astronomy.

**3. What is the difference between modern and classical physics?**

Classical physics is concerned with everything you can see or feel in your environment. Mechanics (Newtonian, fluid, and aerodynamics) is concerned with your movements as well as those of everything you can see, such as bicycles, birds, and even the breeze.

Most of the technology you use is based on classical electrodynamics and magnetism, including your home's circuits, road lamps, and this screen.

Everything you see above is the result of modern physics. To put it another way, current physics is concerned with the very basic building components of matter, as well as the nature of the space and time in which matter exists.

Classical physics studies more fundamental things than general relativity and quantum physics, and these fundamental 'things' (which might be particles, fields, symmetries, and more) give birth to the daily events that classical physics describes simply.

**4. Why is modern physics referred to as Modern?**

It is based on two significant twentieth-century breakthroughs: relativity and quantum theory. The word "modern physics" refers to current physics. This name alludes to the breakthrough that occurred following Newton's laws, Maxwell's equations, and thermodynamics, which are all considered "classical" physics.