I think I can safely say that nobody understands quantum mechanics-- Richard Feynman
The macro world that we know around us works on some simple set of rules and principles, that have been deeply inscribed in our intuition. When we push or pull an object, it tends to move. Throw a stone upwards and it returns to the Earth. Move towards a wall and try to walk through it and you cannot do it. These are familiar and accepted pictures of our day-to-day life. But as soon as we glance into the atomic and sub-atomic world (micro) the picture completely changes. As more and more observations were made it was clear that these microparticles followed some laws which were really very queer when compared to our accepted laws of the classical world. By the end of the 19th century, it was quite clear that classical mechanics will not work at micro levels.
Black body radiation
It began with the problem of black body radiation. A perfectly black body is one that absorbs all the electromagnetic radiation that falls on it. It is a perfectly idealized system. The radiation spectrum of such a body could not be explained by the classical law of Rayleigh-Jeans. Max Planck adopted a mathematical trick to get a solution for this problem. He assumed that light is not a continuous wave, but is made up of discrete packets of energy called quanta. This means that energy can only exist as integral multiples of some small units of energy (quanta) and not in any arbitrary amount.
Planck himself was very confused about this and did not believe it, calling it an act of sheer desperation. But with the assumption, the equations worked perfectly. With this adjustment, Planck proposed the basic form of quantum theory in 1900. It took some time for the people to get adjusted to such ideas but slowly it did happen. In 1905, Albert Einstein discovered the photoelectric effect where he considered the discrete quanta of light as discrete particles called photons. He was awarded the Nobel prize in physics for this contribution in 1921. Planck's theory coupled with Einstein's photoelectric effect is considered as the "old quantum theory".
Dawn of the era of quanta
With the advent of this concept of energy quanta, it seemed that since the building blocks of matter follow this strange law, there is an obligation to explain the entire physical world based on this conceptualization. Using the concept of energy quanta Niels Bohr finally pulled off his model of the atomic structure, which is the accepted model till date. He argued that the electrons revolving around the nucleus in different orbits can possess energy, which are integral multiples of the energy quanta, and transition from one orbit to another is associated with absorption or liberation of such energy quanta. As time went by the scientists looked for a quantum description of the fundamental forces of nature like the electromagnetic force. Richard Feynman played the most significant role in quantizing the electromagnetic force through his theory of Quantum Electrodynamics (QED). Till now we are searching for a proper quantum description of the gravitational force, which has been termed the theory of quantum gravity. String theory, Loop quantum gravity, gravity's rainbow, etc. are some of the leading contenders, but none have been able to provide a proper flawless quantum picture of gravity.
Wave-particle duality: How it really got bizarre!!
To describe the physical properties of sub-atomic particles it was seen that not only the accepted picture of classical mechanics failed but also our intuition regarding the very nature or identity of the particles needed a serious revision. This was evident when it was seen that in order to explain the physics of the micro world we needed to adopt a dual picture of wave and particle of all the matter present around us. It is actually a bizarre scenario where any matter can exist in both states depending on its state of being observed. To be more precise, a particle when not observed by the observer exists as a delocalized wave, but as soon as the observer lays his/her eyes on it, there is a complete collapse of the wave and it exhibits a pure particle nature. Equivalently it can be stated that when the properties of a particle are measured, there is a simultaneous collapse of the wave function. This is one of the fundamental features of the Copenhagen agreement of quantum mechanics between Niels Bohr and Werner Heisenberg. It was such an exotic and unbelievable concept, that Albert Einstein wrote, "It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality. Separately neither of them fully explains the phenomena of light, but together they do". He actually disliked the idea and said, "God does not play the dice with the universe". Although many scientists like Max Planck, Albert Einstein, Niels Bohr, Warner Heisenberg, Erwin Schrodinger, Arthur Compton, etc. were involved in the development of this concept, this idea is often attributed to the French physicist Louis De Broglie after he experimentally demonstrated the wave-like behavior of matter in 1927. De-Broglie was awarded the Nobel prize in physics for this effort in 1929.
Niels Bohr involved in a discussion with Albert Einstein
Further Developments
As the wave-particle duality of matter gained more and more acceptance, a picture of uncertainty at the micro level hovered in front of the eyes of physicists. This is evident when we consider the delocalization of the particle in the form of a wave. Such uncertainties will bring the mathematical theory of probability into the picture since we are no longer dwelling in our well-known deterministic world of classical objects. In order to set up a proper mathematical theory of this interpretation, we needed a wave function and a principle that well defines the uncertainty at the quantum level.
Werner Heisenberg, a young German theoretical physicist published his Uncertainty Principle in 1927, where he stated that the velocity and the position of a quantum particle cannot be measured simultaneously. He stated this mathematically through a set of inequalities asserting a fundamental limit to the accuracy with which certain pairs of physical quantities of a particle may be predicted from the initial conditions. Heisenberg also developed the matrix formulation of quantum mechanics. In 1932 he was awarded the Nobel prize in physics for "the creation of quantum mechanics".
Erwin Schrodinger was an Austrian-Irish physicist, who developed the wave mechanics of quantum mechanics. In 1926, Schrodinger published his famous wave equation that mathematically determines the wave function. He firstly derived it for time-independent systems and showed that it gave the correct energy eigenvalues for a hydrogen-like atom. He later published the dynamic solution characterizing the time dependence of the wave function. With complex solutions to Schrodinger's wave equation, quantum mechanics shifted from real numbers to complex numbers. Schrodinger was very disturbed by the probabilistic interpretation of quantum theory and the entire matrix mechanics. To ridicule the Copenhagen interpretation of quantum mechanics, he conceived the famous thought experiment known as Schrodinger's cat paradox. He kept himself completely aloof from the uncertainty principle and probabilistic aspects of quantum theory, as he did not believe in them. Regarding this, he said, "I don't like it and I am sorry I ever had anything to do with it". Nevertheless, his wave equation is universally celebrated as one of the most important achievements of the twentieth century and created a revolution in most areas of quantum mechanics. He won the Nobel prize in physics in 1933 for his extensive work on quantum mechanics.
Paul Dirac was an English theoretical physicist who had a profound impact on the development of quantum mechanics through his theory of relativistic quantum mechanics. His greatest contribution was the formulation of the Dirac equation which describes the behavior of fermions. This was the very equation that predicted the existence of antimatter. He shared the 1933 Nobel prize in physics with Schrodinger for his contributions to quantum mechanics. He also played a significant role in the reconciliation of quantum mechanics with general relativity, which was Albert Einstein's dream till death.
The development of quantum mechanics continued throughout the twentieth century and is even continuing today. Richard Feynman, an American physicist, through his development of quantum electrodynamics made a significant contribution. It reconciled quantum mechanics with electromagnetism. He received the Nobel prize in physics in 1965 jointly with Julian Schwinger and Shinichiro Tomonaga. Feynman was very famous for his Feynman diagrams which is a pictorial representation scheme for mathematical expression describing the behavior of sub-atomic particles.
The fifth Solvay conference on physics was held in 1927. The conference was dedicated to various aspects of quantum theory. Considered to be the image with the highest IQ in history.
Implications of the theory
It is believed that the bizarre nature of the theory is due to our lack of enough knowledge about it. Some of the implications of quantum theory like quantum entanglement are really baffling!! Quantum entanglement says that two particles separated by large spatial distances can interact with each other and link together to attain similar properties. It is as if we have two copies of the same particle separated by a large spatial distance. Just amazing!! This concept was addressed in a 1935 paper by Albert Einstein, Boris Podolsky, and Nathan Rosen, which came to be known as the EPR paradox. They considered this phenomenon to be an impossible event as it directly violated the causality principle.
Quantum teleportation is a procedure of transportation of quantum information from a sender to a receiver spatially separated from each other. Experimentally it has been seen that quantum teleportation is possible for quantum information in the form of photons, atoms, electrons, etc. But in the realm of teleportation, the actual challenge will be to transfer physical objects from one location to another location, which seems to be almost impossible at present.
Quantum computing is an area of study where we study computer-based technologies using the principles of quantum mechanics. In this realm, we actually use the intricacies of quantum laws to solve very complicated classical problems conveniently. To facilitate these quantum computers have been developed which are far more complex and completely different from their classical counterparts. Problems with a high degree of complexity are efficiently solved using these machines, which use quantum laws as the basic ingredient.
Quantum computer
Quantum mechanics is the theory of nature working at the most fundamental level and it is obvious that nature will not unfold its secrets so easily. The theory is still considered to be in its infancy and we do not know how long we will have to wait to know enough so that we have the complete picture at the quantum level. Over the years we have looked towards quantizing various theories of physics by reconciling them with quantum mechanics. This is felt necessary because a quantum theory works at the fundamental level and all the other theories must obey these laws at the quantum level. Currently, the biggest puzzle of modern physics is to reconcile quantum mechanics with gravitation giving a satisfactory theory of quantum gravity. Many stalwarts including Einstein have worked towards achieving it, but we are yet to get a satisfactory result. The singularity inside a black hole is believed to be the perfect laboratory for testing quantum gravity. There have been significant advances in the field of black hole physics and so we are hopeful of achieving something fruitful in near future. Perhaps we are waiting for the next Heisenberg or Schrodinger or Dirac to come along and show us the path.
By
Prabir Rudra
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