[Physics Class Notes] on Quantum Mechanics Pdf for Exam

Quantum mechanics definition states that it is a basic theory in physics that offers a description of the physical attributes of nature at the scale of atoms and subatomic particles.  It is the basis of all quantum physics incorporating the following fields:

• Quantum chemistry, 

• Quantum field theory, 

• Quantum technology, and 

• Quantum information science

So, what is quantum physics? Well! Quantum physics definition illustrates how everything works: the renowned description we have of the nature of the particles that form matter and the forces with which they interact.

This page explains how quantum mechanics differ from classical physics. Also, we will get to know what quantum mechanics is and who proposed it.

How is Quantum Mechanics different from Classical Physics?

From the above text, we understood that quantum physics considers the properties of nature at the atomic and subatomic particle scale. Now, how it varies from classical physics is still a question mark. So, let us understand the same.

Classical physics:

Classical physics is one of the branches of physics that talk about the collection of theories that existed before the arrival of quantum mechanics, describing several aspects of nature at an ordinary or macroscopic scale only. It indicates that it was not sufficient for describing the properties at small (atomic and subatomic) scales. We can derive most of the theories in classical physics from quantum mechanics as an approximation valid at a large (macroscopic) scale.

But discovering the properties of nature at the atomic/subatomic level still remains a question, for which quantum mechanics came into advent. Now, let us understand the difference between classical and quantum mechanics.

Difference between Classical Physics and Quantum Mechanics

Quantum mechanics vary from classical physics in a way that energy, momentum, angular momentum, and other quantities of a bound system in quantum mechanics are limited to discrete values (quantisation), where objects have features of both particles and waves (wave-particle duality).

Also, there are restrictions to how accurately the value of a physical quantity can be assumed before its measurement, given a full set of initial conditions (the uncertainty principle).

Now, let us go through remarkable differences between classical physics and quantum mechanics:

So, this was the basic variation of classical physics from quantum mechanics. Now, let us understand more about what quantum theory is.

What is The Advent of Quantum Physics

Quantum mechanics arose progressively from theories to explain observations that couldn’t be reconciled with classical physics, Max Planck’s solution in 1900 to the black-body radiation problem, and the correspondence between energy and frequency in Albert Einstein’s 1905 paper, which explained the photoelectric effect. 

These early attempts to understand microscopic phenomena, now known as the “old quantum theory”, brought about the full development of quantum mechanics in the mid-1920s by Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Max Born, and others. The contemporary-day theory is formulated in numerous specially developed mathematical formalisms. In one of them, a mathematical entity known as the wave function provides information, in the form of probability amplitudes, approximately what measurements of a particle’s energy, momentum, and different physical properties may yield.

Now, let us understand what quantum theory is.

What is Quantum Physics?

• Do you know that the concept of the dual nature of light is the main concept of quantum mechanics and why it was brought into the physical world? Well! Quantum theory tells us that both light and matter comprise tiny particles which have wavelike properties associated with them. Light is formed of particles called photons, and matter is composed of particles called electrons, protons, neutrons.

Quantum theory deals with the behaviour of matter and light on the atomic and subatomic scale. Additionally, this theory tries to express and account for the properties of molecules and atoms and their constituents – electrons, protons, neutrons, and other more esoteric particles like quarks and gluons.

But to recognise how things work in the actual world, quantum mechanics ought to be mixed with other factors of physics – principally, Albert Einstein’s special theory of relativity, which explains what occurs whilst things move very fast – to create what are called quantum field theories.

• Three different quantum field theories deal with 3 of the 4 fundamental forces by which matter interacts: electromagnetism, which explains how atoms hold collectively; the strong nuclear force, which explains the stability of the nucleus on the heart of the atom; and the weak nuclear force, which explains why a few atoms undergo radioactive decay.

• Over the past five decades or so these three theories were added together in a ramshackle coalition called the “standard version” of particle physics. For all the impact that this model is slightly held together with sticky tape, it is the most correctly tested picture of matter’s basic working that’s ever been devised. Its crowning glory came in 2012 with the discovery of the Higgs boson, the particle that offers all other fundamental particles their mass, whose existence was predicted on the basis of quantum field theories as far back as 1964.

Conventional Theories of Quantum Mechanics

Conventional quantum field theories work nicely in describing the outcomes of experiments at high-energy particle smashers, including CERN’s Large Hadron Collider, wherein the Higgs was discovered, which probes matter at its smallest scales. But in case you need to understand how things work in many less esoteric situations – how electrons move or don’t
move via a solid cloth and make a material a metal, an insulator, or a semiconductor, for example – things get even more complex.

Some Unresolved Questions on Quantum Mechanics

• Do you know that there is no single quantum theory? There are quantum mechanics, the primary mathematical framework that underpins it all, which was first developed in the 1920s through Niels Bohr, Werner Heisenberg, Erwin Schrödinger, and others. It characterises simple things, which include how the position or momentum of a single particle or the group of a few particles changes over time.

• The billions upon billions of interactions in these crowded environments require the development of “effective field theories” that gloss over a number of gory details. The difficulty in building such theories is why many important questions in solid-state physics continue to be unresolved – for instance, why at low temperatures a few substances are superconductors that permit modern without electrical resistance, and why we can’t get this trick to work at room temperature.

• But underneath some of these practical problems lies a huge quantum mystery. To a primary degree, quantum physics predicts very strange things approximately how matter works that are absolutely at odds with how things appear to work in the actual world. Quantum particles can behave like particles located in a single place, or they can act like waves, dispensed throughout space or in numerous places at once. How they appear seems to rely upon how we choose to measure them, and earlier than we measure, they seem to have no definite properties at all – leading us to an essential conundrum about the nature of basic reality.

• This fuzziness results in obvious paradoxes, which include Schrödinger’s cat, wherein, thanks to an uncertain quantum process, a cat is left dead and alive at the same time. But that’s not all. Quantum particles also seem so that they will have an effect on each other instantaneously even when they’re some distance far from each other. This, without a doubt, the bamboozling phenomenon is called entanglement, or, in a word coined through Einstein (an incredible critic of quantum theory), “spooky action at a distance”. Such quantum powers are overseas to us but are the basis of rising technologies, including ultra-secure quantum cryptography and ultra-powerful quantum computing.

• But as to what all of it means, no one knows. However, we may simply accept that quantum physics explains the material world in terms we find impossible to square with our experience in the larger, “classical” world, while some people believe that there is a more intuitive theory available to discover.

So, we conclude that in all the discoveries of quantum mechanics, there are numerous elephants in the room. For a start, there’s a fourth essential force of nature that so far the quantum theory has been unable to give an explanation for. Gravity remains the territory of Einstein’s general idea of relativity, a firmly non-quantum theory that doesn’t even involve particles. Intensive efforts over many years to bring gravity under the quantum umbrella and so give an explanation for all of the essential physics within one “theory of everything” that have come to nothing.

Meanwhile, cosmological measurements imply that over ninety-five percent of the universe includes dark matter and dark energy, stuff for which we presently haven’t any clarification within the preferred version and conundrums which include the extent of the role of quantum physics in the messy workings of life continues to be unexplained. The world is at a few level quanta – however, whether quantum physics is the last word about the world remains an open question.

Facts on Quantum Theory in Physics

Niels Bohr and Max Planck are two of the founding fathers of Quantum Theory. Each received a Nobel Prize in Physics for their work on quanta. On the other hand, quantum theory was proposed by Max Planck or Max Karl Ernst Ludwig Planck. He is a German theoretical physicist who originated quantum theory, for which he is considered the father of quantum physics. His premier work made him win the Nobel Prize for Physics in 1918.

However, Einstein is regarded as the third founder of Quantum Theory as he explained light as quanta in his theory of the Photoelectric Effect, for which he also won the Nobel Prize in the year 1921. 

The term “quantum” is derived from the Latin for “how much” and reflects the instance that quantum models always incorporate something moving in discrete amounts. Therefore, the energy comprised in a quantum field is always in integer multiples of some fundamental energy, like we see in the image below: