Image Courtesy: CERN
Quantum Mechanics and the Theory of Relativity are two pillars of human knowledge generation during the 20th century, causing two revolutions in the history of science and technology. Quantum Mechanics describes the microcosm involving atoms and molecules, while the Theory of Relativity opens up a window to the vast spread of the universe to understand the origin of the universe involving millions of galaxies and stars. The 20th century opened its eyes to the Quantum Theory of Max Planck in 1900, describing light-matter interaction through energy quantization. The year 1905 was a magical year when Albert Einstein developed the Theory of Relativity to fuse space and time to space-time continuum, Brownian motion as an experimental proof for the existence of atoms and molecules and the Theory of Photo Electricity by hypothesizing the existence of light particles or photons. A majority of scientists including Planck did not accept the concept of photons. Even Einstein, in his inner heart, had a doubt on the reality of photons.
The confirmation of the reality of photons was made by two famous Indian scientists, Prof Satyendra Nath Bose and Prof CV Raman. Prof Raman got the Nobel Prize in Physics acknowledging the importance of Raman Effect, which employed light scattering in a medium considering light as a stream of photons. Using photons as identical particles, Prof Bose was able to re-derive Planck’s famous formula describing the spectral behaviour of black body radiation by introducing the concept of energy quantisation, commonly known as the old quantum theory.
BOSONS AND BOSE-EINSTEIN STATISTICS
When Prof Raman created a new branch of studies in Physics, namely Raman Spectroscopy, Prof Bose collaborated with Albert Einstein to develop a subject called Quantum Statistics, to analyse the number of boson particles which can occupy a single quantum state. There is no limit in occupation number. This is due to the fact that photons do not obey Pauli’s exclusion principle according to which particles like electrons can occupy only one particle in one energy level. When Bose submitted a paper on the statistics of photon number distributions in different energy states, the British Journal, The Philosophical Magazine did not accept it for publications since its reviewers did not believe the idea of Bose. Instead of going for an appeal, Bose sent the manuscript to Einstein, requesting him to submit it to the German journal Zeits Fur Physik. Einstein immediately understood the importance of Bose’s paper and submitted it to Zeits Fur Physik with a comment that this paper had immense applications in atomic physics about which he would write a paper for publication. Einstein generalised Bose’s idea to calculate probability distributions of indistinguishable particles in their energy states without obeying Pauli’s principle. This led to the discovery of Bose-Einstein statistics (creating a new field of quantum statistics) and such indistinguishable particles are called bosons. Bose-Einstein statistics predicted a special type of condensation of bosons, which can be brought to the lowest energy state. But, the experimental confirmation of BE condensation had to wait till 1990s, when physicists were able to trap BE condensates at extremely low temperature of mille Kelvin (close to -273 degree centigrade) Alternate to Bose-Einstein statistics, Fermi and Dirac described Fermi-Dirac statistics for material particles like electrons and protons which satisfy Pauli’s principle.
Image Courtesy: Internet
British scientist Peter Higgs predicted the existence of the boson particle, known as Higgs boson, interactions of which with particles will give the property of mass to the interacting particles depending on whether the speed of interacting particles changes their speed or not. For example, gravitons and photons do not change their speed during their interactions with Higgs bosons so they have zero mass, whereas electrons and other particles suffer speed reduction and hence they acquire mass. Experiments to detect the Higgs boson were carried out at CERN (Conseil Européen pour la Recherche Nucléaire (in French) or the European Council for Nuclear Research) in Geneva, Switzerland, which concluded with the discovery of the Higgs boson. The detection of the Higgs boson brought the Nobel Prize to Peter Higgs in 2013. He died as the most satisfied scientist by witnessing his theoretical prediction getting confirmed by experiments at CERN.
Force carrying field particles like photons and gravitons are bosons and material particles like electrons, protons, neutrons, etc., are fermions. In other words, we can say that this universe is made up of two classes of particles called bosons and fermions.
A unique member in the class of boson particles to understand the Universe is Higgs boson which is the field particles or quanta of strong interaction field between quarks predicted by British physicist Peter Higgs. The Higgs boson was discovered during an experiment conducted at a CERN facility called Large Hadron Collider (LHC) in Europe. Higgs particles help particles get them the property mass which is one of the important fundamental properties of particles. Interaction between the Higgs boson and a particle results in a change in the speed of the particles like electrons, protons, etc., thereby, they will get their mass which is the measure of the quantity of matter. If there is no change in speed of the particle, the mass of the particle will be zero. This is the case for field particles like photons, gravitons, gluons, etc. In this aspect, Higgs bosons can be treated as the important partner in the threshold of the creation of the universe. It was an inspiring and unforgettable event when the leader of the LHC experiments announced the discovery of Higgs bosons by announcing, “we did it”. It is as though standing behind the Higgs boson and looking over its shoulders, we witness the universe in the process of getting created.
BUILDING MATERIALS OF THE UNIVERSE
From time immemorial, the widespread presence of the cosmos and millions of stars, planets and satellites have remained sources of wonder to the human mind, creating a quest to know their secrets. Our universe contains multitudes of different objects which make our everyday life happy and comfortable. Scientists explored the possibilities of matter, consisting of a few elementary particles, knowledge of which will be useful to physicists and chemists. Till the beginning of 1930s, there were only three such particles, namely electrons orbiting around the nucleus, and protons and neutrons inside the nucleus. However, as methods of theoretical physics and instrumentation techniques improved, newer particles emerged and it was found that there are only two classes of particles, called leptons and quarks. Nuclear particles protons and neutrons are made up of two up (u) and one down(d) as well as two down and one up quarks to make their resulting charges +1 and 0 respectively. According to standard model, universe is made up of six leptons (electrons, mu meson or muons, tau lepton, electron neutrino, muon neutrino and tau neutrino) and six quarks (named as up, down, strange, charm, beauty (bottom), top (truth)) along with their anti-particles. In short, all materials of the universe are made up of 12 types of particles and their antiparticles along with three field particles namely, photons, gravitons and gluons. Quarks inside nucleons are under the influence of a special type of strong force field with gluons as the field particle in which force between quarks increases with their separation and gets maximum when they reach the boundaries of nucleons. Scientists have modeled this type of interaction using string theory.
Images Courtesy: S N Bose National Centre for Basic Sciences
LARGE HADRON COLLIDER
The LHC experiment at CERN is the largest international collaborative experimental set up in nuclear and particle physics with 176 research and university institutions from 38 countries involving about 3000 scientists, 1000 research scholars and a large number of technical assistants. Discoveries in LHC experiments by tracking down the Higgs boson generated immense interest all over the world due to the belief that man had reached the threshold to watch the creation of the universe. The LHC laboratory is about 100 m below the earth’s surface with a circular vacuum metal tube of 27 km circumference. The basic principle of the LHC experiment is that head-on collisions between particles at higher energies will get them smashed to reveal the internal structure of the particles, if any. Initially, cosmic rays were the only source of elementary particles to study their interactions and particle collisions. By photographing the cosmic ray showers from the atmosphere, scientists were able to discover new particles including anti particles like anti electrons called positrons predicted by Dirac.
Scientists developed different types of particle accelerators to increase the collision energies of particles. Particles in circular motion in an evacuated metal tube at clockwise and anticlockwise directions in the LHC facility reveal several head-on collision events which can be photographed by specially designed cameras. Through a series of different steps of acceleration, circulating proton beams in clockwise and anticlockwise directions get head-on collisions at very high energies of TeV magnitude (1000GeV energy). Through an untiring and lengthy series of experiments and data deduction, scientists were able to confirm the presence of field particles predicted by Peter Higgs. Since they are the force carrying field particles, they should be bosons and hence the world witnessed the presence of the Higgs boson as predicted by the standard model.
Clockwise from left: Bose delivering a speech in the company of Prasanta Chandra Mahalanobis and Ronald Aylmer Fisher; a stamp released by Govt of India on Bose’s centenary in 1994
In the conference room at the LHC facility, Peter Higgs was present in the front row to witness the historic declaration of the discovery of Higgs bosons. He was so emotional that he said, “It is so gratifying to be present and hear the success of the theoretical prediction through experimental confirmation. I did not even dream of the situation of discovery of the Higgs boson in my lifetime.” Unfortunately, since the posthumous Nobel prize cannot be awarded, Prof Satyendra Nath Bose missed the Nobel Prize.
Images Courtesy: S N Bose National Centre for Basic Sciences
CONCLUSION
The Indian contribution in the prediction of boson particles has still more importance in the history of science. When Prof SN Bose described the light particle photons, he attributed them as particles with integer spin 1 with two coponentsn+1 and -1 for clockwise and anticlockwise polarised light beams. When Einstein modified the manuscript of the paper of Bose, he did not approve the photon spin as described in the paper, corresponding to RH and LH polarized light beams. The paper was published without mentioning the photon spin. If Einstein had not struck off the photon spin from the paper, Bose could have been the physicist to be credited for the introduction of particle spin in quantum mechanics instead of Pauli to whom the discovery of particle spin property is attributed, followed by exclusion principle. Had time supported Prof Bose, he could have been the discoverer of the exclusion principle, and the history of Quantum Mechanics would have Bose’s exclusion principle instead of Pauli’s exclusion principle, renaming Fermion particles as half integer Bose particles. One should accept the fate of the history of science just as the fate of the human species.
On this edition of the National Science Day, one should remember Prof Satyendra Nath Bose who could have brought the second Nobel Prize in Physics to India. But fate has its own design!
*The writer is a Visiting Scientist, Cochin University of Science and Technology, Kochi. He can be reached at nampoori@gmail.com.
