A K Chakrabarti
“Aalo, aamar alo ogo, aalo bhuvan bhara..” ( Light, Oh Light, The world is vivid with light) – with these few lines in verse, Tagore described the celebration of life, illuminated by the wonderful rays of light. The universe is full of light, and of energy.
Tagore imagined the ‘dancing light waves’ – a phenomenon that had puzzled the scientists for over more than two centuries. Whether the light danced like sea wave as Tagore had observed, or was ‘fluttering and dancing’ discreetly like Wordsworth’s daffodils – it attaches a great meaning to the quest of the scientists since the 18thCentury. The great duality in about physics whether light is a wave or discrete particles, is yet to be resolved. Tracing the path of light a group of scientists is struggling to solve the mystery of creation through the theories of the ‘Big-Bang’, and ‘Unification of fundamental forces’, while the other group is very actively engaged in new inventions which turn “Fantasy into Reality”- destined to make a huge impact on the course of human civilization in 21st Century and beyond.
Part I : Brief on Quantum Mechanics
The colour of light amazed the observers for centuries. Rainbow with seven distinct colours, and the same through the prism, red-hot iron, white- hot electric bulb are the very common examples of light in different colours.
Since early 1800 scientists believed that light is a wave ‘dancing around’, and proved it experimentally. But questions about the cause behind the separation of seven colours in a rain-bow or why don’t we see a gradual change from violet to red, have opened up opportunities for revolutionary discoveries. The manifestation of light in distinctly separate colours made the scientist view the light spectrum as multiples of whole number. If this assumption is true then no light will exist between the gap of two whole numbers which indicates the existence of distinctly different colours.
What is Light? Light is an electro-magnetic radiation within a certain portion of electromagnetic spectrum. Three types of lights are known – Visible Light, Ultra- Violet and Infra-red. Visible light is between infra-red ( longer wave length) and ultra- violet ( shorter wave length).
The Great Break Through:
This very elementary question bothered a great physicists Max Planck who was seriously advised by his mentors to take up any subject other than physics , as many thought that every problem of physics has been solved. It is simply the good fortune of mankind that Planck continued his studies with physics. Exactly at the dawn of 20th Century, Planck published the revolutionary theory describing that electromagnetic energy could be emitted only in quantize form and energy could be multiple of an elementary unit. Nobody could guess at that moment the significance of it, even perhaps the inventor himself. Helge Kragh mentioned about this in his article ‘ Max Planck , the Reluctant Revolutionary’, in Physics World Magazine (written in 2000). He wrote “ If a revolution occurred on December 1900, nobody seemed to notice it. Planck was no exception…” Planck created an yet simple but path breaking equation like that of Einstein’s E = mc2, ε = h v
ε (epsilon) is a discrete energy element of a radiating atomic system, v is the frequency of radiation, h is Planck’s constant. Planck was awarded Nobel Prize in 1918.
Arrival of a Genius
The research in light is almost 200 years old. Scientists observed through spectroscopy the light emitted and absorbed by various elements, known as Spectral Lines. In 1888, physicist Johannes Rydberg described the spectral lines by mathematical equations. Nobody knew how the equation worked till it was modified by Niels Bohr in 1913 using quantum theory. In 1905 Albert Einstein published four path breaking articles in Annalen der Physik. The Annus Mirabilis papers ( Latin annus mirabilis means “ extra-ordinary year”) on Photo-electric effect, Special theory of relativity, Mass-energy ( E=mc2)equivalence and Brownian motion considered as the foundation of modern physics.
In the first paper Einstein postulated that the light itself consists of localized particles, the ‘Photons’- the name was introduced by Gilbert N Lewis in 1926. Einstein published this postulate titled, “Concerning an Heuristic Point of View towards Emission and Transformation of Light”. ‘Light is wave or particle’- duality still exists. It is still not clear how the particles are related to waves.
It is very interesting that the path breaking new concepts in physics in Europe in early 20th century came from the continent only to reach city of Calcutta to ignite the minds of two young minds – S N Bose and M N Saha, at the University of Calcutta. Satyendra Nath Bose, the noted physicist, made significant contribution in quantum science. While teaching in the physics department of the University of Dhaka, Bose wrote a paper titled, ‘Planck’s law and the Hypothesis of light Quanta’. Bose sent the paper to his ‘Guru’ Eistein who translated it into German and got it published in “Zeitschrift fur Physik” in 1824.
Bose derived the Plank’s quantum radiation law without any reference to classical physics and by using a new concept of counting states of individual particle. Bose’s new concept was later known as Quantum statistics. Interpretation of Bose is now known as Bose-Einstein Statistics. Einstein extended the new concept to study of atom which led to the prediction of existence of new phenomena, known as ‘Bose-Einstein Condensate’, a dense collection of particles Boson, named after Bose who will be remembered ever for his contribution in particle physics, leading to the discovery of Higgs boson, the God Particle. Bose missed the Nobel, but many scientists were awarded Nobel working with Boson.
Brief History of Quantum Mechnics
By 1930s quantum mechanics reached its peak, and then it slowed down till 70s when new theories emerged for strong and weak interactions. Though the intricacies of this advanced field is very difficult to fathom, but the history of development of quantum bymechanics is amazingly interesting. The story goes like this: 1838 – Discovery of cathode ray by Michael Farady, 1859 – Explanation on Black body radiation by Gaustav Kirchhoff, 1877 – Statement of energy state by Ludig Boltzman 1887- Photo-electric effect by Henrich Hertz, 1900 – Quantum Hypothesis by Max Planck 1905 – Explaining Photo electric effect by Einstein through the concept of Photon, 1913 Spectral lines of Hydrogen atom by Weils Borg 1923 Theory of matter Wave by Louise de Broglie, 1926 Wave mechanics by Schrodinger 1927 Heisenberg uncertainty theory 1927 Unifying quantum mechanics with Special Relativity by Paul Dirac 1927 Quantum Field Theory by Paul Dirac et al 1960 Quantum Chromodynamics 1965 Theory of quantum electrodynamics by Richard Feynman 1975 Weak nuclear force and quantum electrodynamics by Abdus Salem
The list indicates some major milestones, but surely not exhaustive and complete. Many other renowned physicists made significant contribution which will someday solve the great mystery of creation.
Unified Field Theory
Eisntein worked for three decades for a Unified Field Theory to combine the four fundamental forces, namely
1. Strong Interaction– Holding neutrons and protons together to form atomic nuclei.
2. Electro-magnetic Interaction – Acts on electrically charged particle. Photon is the exchange particle for this force.
3. Weak Interaction- a short range interaction responsible for some form of radio-activity, that acts on electrons, neutrino and quark.
4. Gravitational Interaction- a long range attractive interaction that acts on all particle .Einstein could not combine all the forces. However, in 1960s physicists could combined three fundamental force, except gravitational, to a new model, known as Standard model. Abdus Salam, Sheldon Glashow, Steven Weinberg, and others pioneered to build the Standard Model. The QuantumTheory of Gravity – the last of the four is to be resolved.
Theory of Everything and a brilliant Indian scientist.
A new concept which emerged over last few decade is the String Theory, combining quantum mechanics and relativity.
In this theory the particles are represented by one-dimensional object called, ‘String’. Sting theory is expected to be complete, unified and consistent description of the fundamental structure of the universe, known as Theory of Everything. Ashoke Sen ,FRS, distinguished Professor of Physics, Harish-Chandra Research Institute, Allahabad is one of the leading scientists of String Theory, and the winner of $ 3 million- inaugural Fundamental Physics Prize, 3 times that of Nobel. That concludes, very briefly, the story of Quantum Mechanics.
Part II : The Fantasies and the Reality
Homo sapiens are gifted with the power of imagination. Absorbing stories in our great mythologies were created by brilliant imaginative minds. In mythological stories characters with super natural power can appear anywhere and vanish, appear but not visible, and many such combinations. Nobody knew that Mind Wave and Matter-Wave will converge some day to transform those fantasies into reality.
Vanish and Reappear : Quantum Entanglement
This is very common in mythological stories. While making Star Trek , the creator Genne Roddenberry was running out of time to prepare a set for landing of Starship. Instead, he created a ‘transporter’ with special effect of ‘Fading-out’ and Fading in. This fictional transporter is the Tele-transporter of Quantum Mechanics Tele-transportation is a theoretical transfer of matter or energy from point A to point B, irrespective of the distance between them and without travelling the physical space between them. This happens due to Quantum Entanglement.
Quantum Entanglement is a physical phenomenon that occurs when pair or group of particle generated in such a way that quantum state of each particle cannot be Star Trek Transporter described independently of the other. Entanglement is an extremely strong correlation between two or more particles to bring them into a perfect unison, even if separated by great distance.
Machine generates the quantum replica of a person, to be located anywhere, even the other side of the universe. It means that the person, entering into a quantum machine reaches instantly on the other end of the universe without losing any identity. Enter a machine in Hyderabad and fraction of a second get out from a machine in New York – absolutely hassle free travels, hoping the immigration is friendly.
Alice and Bob are the fictional character commonly used in Cryptology. When the two are in Classical Channel, Eve the eavesdropper can easily enter and crack the code and listen to their talk When Alice-Bob use the Quantum channel Eve gets frustrated . Quantum communication works on fusing two or more particles in complementary state. No particle can be described independently. The particles that exist in shared quantum state will collapse when observed by an eavesdropper. Quantum communication is highly secured, can’t be cracked. China has launched satellite with quantum communication technology.
Quantum Computer : Quantum Superposition
The processing power of Classical computers increases with doubling the number transistors every two years as was observed by Gordon Moore , co-founder of Fairchild Semiconductors. His statement is known as Moore’s law. But processing power of classical computers will still have limit on performance due to many other physical constraints, such as rise in temperature. Quantum computer has no such limitations and will have unimaginable performance when a practical system is demonstrated.
Quantum computer works on the principal of quantum superposition which permits the quantum system to exists in multiple states at the same time- here, there, left, right, up, down and everywhere Classical computer works on binary system – the ‘bits,’ combination ones and zeros. Quantum computer functions with ‘quibits’ – a combination ones and Quibits – mix of photns,electrons etc zeros as in a binary system with added advantage of superposition of ones and zeros, creating much larger combinations of the binary arrangement. Tasks which are impossible to be achieved by conventional computers can be done easily and efficiently by Quantum computer.
Quibits are made of photons, atoms, electrons, molecules and some other particles. Such particles are notoriously difficult to manipulate. Hence scientists have to find ways and means to make quibits cooperate. As of now, two most promising uses of quantum computing are Quantum Serach and Quantum Factoring. Though a practical quantum computer is still years away, experimental computer with few quibits have been built.
Quantum Clock :
If watch goes slow by 1 second per day, who cares. Get ready for a shock. The smart phone in our hand, loses 1 second in 1 million year. The clock which controls the GPS signal from the satellite needs such accuracy to help us navigate accurately on a city road. The clock in satellite is called ‘Atomic Clock’. It works on a principle to measure the frequency of electromagnetic radiation, similar lines as we count a simple pendulum swings. The radiation occurs when an electron is made to change its orbit, either by absorbing or emitting energy. Caesium 133 atom is used for Atomic Clock, having Atomic Clock accuracy of 1 sec. in 1 million year.The Quantum Clock is embarrassingly more accurate. Just guess – 1 second in a billion year. It will be used in Radio-astronomy and space research.
Energy Harvester: Quantum dots
Energy harvester takes energy from the environment and convert into other form of energy. It harvest heat, e.g., from a hot computer chip and convert it back to electricity. Quantum dots, sometime referred to as artificial atoms, are very small size manufactured particles. Quantum dots allow to carry transport electrons at specific energy level to produce electricity from heat . In the figure on left, electrons move from source to drain(to tap) through a pair of left and right dots. Presently, efficiency of quantum dot solar cell is around 7%, expected to be much higher in future. Quantum Well is another energy harvester working on similar principle. Quantum Dots and Wells will be major source of energy, alternate to fossil fuel.
Quantum Wires: It is like quantum dots stretched out in one direction. In some application, such narrow conduit which is 10,000 time thinner than human hair, can be a very good conductor of electricity. Carbon nano-tubes are used to make quantum wires.
Being invisible : Metamaterial.
Science now can make any object invisible like what an expert magician does. This is possible with manipulating the path of light. We know about refractive index which drifts the image when an object is put in a different medium, say a rod in a glass of water as shown. A rod in the tumbler on the left of the picture appears shifted, maintaining the same direction as in the air. But the rod, made of Metamaterial, on the right in the picture is seen shifted with changed direction.
This phenomenon is due to negative refractive index of Metamaterial.
Metamaterial is not a natural material. It is manufactured with its internal elements separated by a distance less than the wave length of light. In the next picture, on left, the light rays rolls over the sphere to reach the other end unobstructed. Hence the presence of the ball is not felt by the viewers standing on the either end of the sphere which is coated with Metamaterial. We can see through a person wearing a cloak made of metamaterial, as the Castle is seen through Harry Potter, in the picture on the right.
The imagination of J K Rowling is now a reaility.
List of new innovations in quantum science is very long. Many other applications in chemistry, biology, drug discovery will be the hallmark of 21st science and technology. Feynman said, “There’s is plenty of room at the bottom”. By bottom Feynman meant the sub-atomic level.
Half a century back, we studied steam and IC engines, valve version of electronic amplifiers etc. in our engineering courses. To-day, it is an amazingly new world of engineering with atoms and sub-atomic particles.
God said: “Let there be light, and light was there’. Along with light, came the Quantum Magic. 21st Century will be century of light.
Source; Quantum Computing 101 Univ of Waterloo. Wikipedia.