Anatomy of an atom In the second half of my story, the three strands of relativity, quantization and symmetry will be braided together. Before we embark on this journey, let us take a rest and consider some ways in which the properties of the small-scale quantum world govern our world at large. Most people wonder about the relevance of subatomic studies for our understanding of ourselves. Quite a few popular accounts of particle physics contain a heavy dose of reflections upon Larger Issues and related Deep Thoughts. I will try to avoid that, but it cannot be denied that the connections between the subatomic scale and our daily world are strong and direct, albeit invisible to our large-scale senses. Let us first consider atoms, the building blocks that have the most direct significance in our daily lives (in the remaining chapters we will discuss the structure of the particles of which atoms are made). For simplicity and definiteness, and because we have encountered it in previous discussions, I will mainly speak of the hydrogen atom. Its nucleus is a proton, having one unit of positive electric charge. A neutral hydrogen atom is formed when an electron, bearing one negative unit of electric charge, binds to it. The binding makes the atom electrically neutral.
Anatomy of an atom In the second half of my story, the three strands of relativity, quantization and symmetry will be braided together. Before we embark on this journey, let us take a rest and consider some ways in which the properties of the small-scale quantum world govern our world at large. Most people wonder about the relevance of subatomic studies for our understanding of ourselves. Quite a few popular accounts of particle physics contain a heavy dose of reflections upon Larger Issues and related Deep Thoughts. I will try to avoid that, but it cannot be denied that the connections between the subatomic scale and our daily world are strong and direct, albeit invisible to our large-scale senses. Let us first consider atoms, the building blocks that have the most direct significance in our daily lives (in the remaining chapters we will discuss the structure of the particles of which atoms are made). For simplicity and definiteness, and because we have encountered it in previous discussions, I will mainly speak of the hydrogen atom. Its nucleus is a proton, having one unit of positive electric charge. A neutral hydrogen atom is formed when an electron, bearing one negative unit of electric charge, binds to it. The binding makes the atom electrically neutral.
The Quantum theory deals with the microscopic structure of the Universe that describes the finer details of nature at the subatomic domain.To a physicist, quantum mechanics is one of the three great pillars supporting our intuition of evocative, bewildering and fascinating natural World as we continuously struggle with the inescapable strangeness of the subatomic domain as if something strange is always afoot.It is well accepted fact that quantum mechanics is a physical theory that describes the way things behave at subatomic level which is quite dazzling in its precision and explanatory power.A test of quantum electrodynamics, the oldest and best understood of the modern quantum theory that involves measuring the way an electron behaves in the vicinity of a magnetic force. Many of the technological and social changes that have revolutionized our lives have actually risen out of the fundamental theoretical research carried out with only motivation to understand the World in a better way than ever. For example, the invention of transistor actually came with the fundamental theoretical research on the Uncertainty Principle which states that the uncertainty of the position of a particle times the uncertainty of its velocity is always a constant. If we double the certainty of the position of the particle, we have to make half the certainty of its velocity and vice versa. Quantum theory is perhaps the prime example of the infinitely esoteric in the sense that it describes a World in which a particle really can be in several places at once and it moves from one place to another by exploring the entire Universe simultaneously-the theory of simultaneity. The diverse and complex Universe is actually made of a few tiny particles according to the rules of quantum theory which is so simple that can be summarized on the back of an envelope. The more we understand about the elemental nature of the Universe, the simpler it looks. We have already discovered that everything in the diverse and complex Universe is nothing but the different assemblies of atoms, and the wide variety of atoms are constructed using the three building blocks at the sub-atomic domain i.e. subatomic particles: protons, neutrons and electrons. We have also discovered that protons and neutrons ate made up of combination of three entities called quarks. And that is where things stop as far as recent discovery is concerned. But the recent progress in particle physics inspiring the quest for the ultimate unified theory of physics that can describe everything in the Universe in one go. The discovery of Boson particle certainty will revolutionaries the Newton's mass concept and gravity into whole new looks. To an optimistic point of view, we must hope the best out of the progress in physics.
The hydrogen atom is the Rosetta stone of the early twentieth century atomic physics. The attempt to decipher its structure and properties led to the development of quantum mechanics and the unraveling of many of the mysteries of atomic, molecular, and solid state physics, and a good deal of chemistry and modern biology. Unlike the various one-dimensional model problems that we have been studying in the previous chapters, the hydrogen atom is a real physical system in three dimensions. It consists of an electron moving in a spherically symmetric potential well due to the Coulomb attraction of the positively charged nucleus. In three dimensions, the electron is not constrained to move linearly. It can execute orbital motions and, thus, has angular momentum. Not only is the total energy of the electron in the atom quantized, its angular momentum also has interesting and unexpected quantized properties that cannot possibly be understood on the basis of classical mechanics and electrodynamics. They are, however, the natural and necessary consequences of the basic postulates of quantum mechanics, as will be shown in this chapter. According to classical mechanics and electrodynamics, it is not possible to have a stable structure consisting of a small positively charged nucleus at the center of an electrically neutral atom with an electron sitting in its vicinity. For the electron not to be attracted into the positive charge, it must be orbiting around the nucleus so that the centrifugal force will counter the Coulomb attraction of the nucleus and maintain a constant electron orbit.
Those defile the purity of mathematical and philosophical truthy who confound real quantities with their relations and sensible measures. —ISAAC NEWTON, Principia Before we say anything about quantum mechanics, let us take a quick look at the Newtonian (or ‘classical’) view of the physical universe. According to that view, nature consists of an enormous number of particles. When Newtonian physics is combined with the theory of the electromagnetic field, it becomes convenient to think of these particles as dimensionless (even if there is a kind of conceptual strain involved in trying to think of something as having a mass but not any size ), and as possessing electrical properties – negative charge, or positive charge, or neutrality. This leads to the well-known ‘solar system’ view of the atom – with the electrons whirling around the nucleons (neutrons and protons) just as the planets whirl around the sun. Out of atoms are built molecules; out of molecules, macroscopic objects, scaling from dust motes to whole planets and stars. These latter also fall into larger groupings – solar systems and galaxies – but these larger structures differ from the ones previously mentioned in being held together exclusively by gravitational forces. At every level, however, one has trajectories (ultimately that means the possibility of continuously tracing the movements of the elementary particles) and one has causality (ultimately that means the possibility of extrapolating from the history of the universe up to a given time to its whole sequence of future states).
Introduction I give in this chapter the quantum-mechanical description of an atom. I look in some detail at the hydrogen atom, which is the simplest atom we have, with just one proton and one electron. From the picture that emerges, we shall be able to see how to describe all other atoms. Before continuing, I need to explain what I mean when I say that the proton and electron have equal and opposite electric charges. It is because of these charges that the atom is stable, and does not spontaneously break up into the two separate particles. Electric charge and potential energy If I take a ball in my hand and then let go, it drops to the floor. This is because of the gravitational interaction between the ball and the earth. The ball had gravitational potential energy when I held it, and when I release it, this energy is progressively converted to kinetic energy as it falls. Gravitation makes two bodies move towards each other, and not away from each other: the gravitational force is always attractive. As a result of many experiments done over the years, we find that there is another kind of force in nature, somewhat analogous to, but in one aspect different from, gravitation. I take two idealized examples, and use them to show the features of this force. First I consider two electrons that I imagine to be pinned down a distance r apart.
An atom consists of a nucleus, containing protons and neutrons, with various electrons swarming around the outskirts. The electrons are held in the vicinity of the nucleus as they are negatively charged and hence attracted by the positively charged protons. Although atoms are complicated objects, there are certain situations where they can be treated as simple lumps of matter. Their internal structure has no relevance to the details of what is happening. This sort of situation is exemplified by experiments in which atoms are deliberately smashed into one another at comparatively low energy and as such provide an ideal start in a study of the quantum physics of identical particles. When we are dealing with atoms, there is something more to being ‘identical’ than we can appreciate with larger-scale objects. Atoms of the same type are really identical, measurably identical if you like, which can make a considerable difference in some physically important situations.
Introducing quantum mechanics immediately after considering the issue of cell division and cell-cell recognition, with their associated complex structures and structural changes, is rather shocking. “Continuum” normally refers to the continuous allowed values of energy of a quantum particle once its energy becomes positive. Electrons bound to atoms have negative total energies, indicating they are in bound states. Stationary states of relevance in biophysics usually occur in cases of electrons or protons bound to atoms or molecules. The chapter examines some interesting and apparently complicated cases in photosynthesis and heme proteins. Symmetry is an important concept in both quantum mechanics and in biosystems. Collisions of atoms constantly occur in gases, liquids, and solids. Particle collisions are the bread and butter of particle physics. The discovery of the “Rutherford atom” was made by collision experiments, Rutherford scattering, and first showed that an atom had a small, heavy nucleus surrounded by light electrons.