Knowledge in a Nutshell: Quantum Physics

The complete guide to quantum physics, including wave functions, Heisenberg's uncertainty principle and quantum gravity

Sten Odenwald

• Publisher: Arcturus
• Year: 2020
• ISBN:
  9781839404023
• Topic: Ciencias físicas
• Subtopic: Teoría cuántica

Quantum theory is at the heart of modern physics, but how does it actually work? NASA scientist and communicator Sten Odenwald demystifies the subject and makes this crucial topic accessible to everyone. Featuring topics such as Schrodinger's cat, the wave-particle duality and the newly emerging theories of quantum gravity, as well as the personalities behind the science, such as Max Planck, Neils Bohr, Werner Heisenberg, Richard Feynman and many more, Knowledge in a Nutshell: Quantum Physics provides an essential introduction to cutting edge science. Presented in an easy-to-understand format, with diagrams, illustrations and simple summary sections at the end of each chapter, this new addition to the 'Knowledge in a Nutshell' series brings clarity to some of the great mysteries of physics. ABOUT THE SERIES: The 'Knowledge in a Nutshell' series by Arcturus Publishing provides engaging introductions to many fields of knowledge, including philosophy, psychology and physics, and the ways in which human kind has sought to make sense of our world.

Chapter 1

The Dawn of Atomic Physics

The idea that the things around us, such as rocks, air and water, are themselves made from other ingredients more elementary than the physical world visible to us is one that was long in coming. From the dawn of the written word – fashioned from Sumerian cuneiform, Egyptian hieroglyphics, or even ancient Chinese pictograms – there is scarcely a hint that our very distant ancestors considered such ideas noteworthy. Of course, humans at all times could distinguish among the many diverse substances: from rocky compounds and elementary gold to the bewildering variety of biological matter in trees, plants and animals. But the idea that there were still more elementary substances behind the ones we commonly see was inconceivable. So far as recorded history can tell us, this all changed during the last few centuries bce, when ancient Indian philosophers and ancient Greek proto-scientists came up with the idea of the ‘smallest particles of matter’. The Indian idea is credited to Acharya Kanad, who lived sometime between the 6th and 2nd centuries bce in Prabhas Kshetra (near Dwaraka) in Gujarat, India. Beginning with grains of rice, he developed the idea of the smallest particles of matter, which he later called Parmanu. Kanad later founded the Vaisheshika school of philosophy, where he taught his ideas about the atom and the nature of the universe. Virtually identical ideas about the nature of matter were developed, either independently or by the natural diffusion of ideas, by the ancient Greeks.

The Atomic Universe

The theory of Atoms (from the Greek ἄτομος, atomos, meaning indivisible), proposed by Democritus and Leucippus in the 5th century bce was a dramatic departure from prevailing ideas around the Mediterranean and in the Far East, in which matter was a continuum characterized by a handful of distinct properties. The Atomists made the innovative proposal that there were an infinite number of atoms varying in shape, and that these shapes determined the properties of the matter they combined to create. For example, iron atoms had tiny hooks that linked together at room temperature, making it a strong solid. Atoms were so small they could not be seen, and like marbles in a half-empty box, they could rearrange themselves to form new substances.

Democritus and Leucippus developed the ancient Greek idea of the atom.

This meant that in addition to atoms there must also exist a completely empty Void, otherwise these atoms would be locked together into a vast immovable mass for eternity. Only the Void made atoms a workable construct, and so from this time forward, the properties of empty space and atoms were intimately linked together. In 55 bce, Lucretius, in his On the Nature of Things, described the shapes of atoms as follows:

‘Since atoms are what they are by nature, and not cut by hand to a single predetermined pattern, some of them must have shapes unlike some others. By reasoning, we may readily comprehend why lightning-fire is much more penetrating than ours that comes from torches here on earth. Or one may say that the lightning fire is finer and made in smaller shapes, and thus can pass through opening that our fire cannot, spring as it is of wood and torch…. Things which are hard and dense must be composed of particles hooked and barbed and branch-like, intertwined and tightly gripped.’

(Lucretius, Epicurean and Poet, by John Masson 1907)

Meanwhile, several hundred years after the Atomists established their fledgling school of thought in the West, Aristotle (384–322 bce) espoused the existence of only five essences: Earth, Air, Fire, Water and Aether. In fact, he found the idea of the Atomist’s empty Void abhorrent, and as one of the most influential philosophers of his time he was able to mount many criticisms of the Atomists’ school. Relatively quickly the theory of Atoms went into eclipse as a theory of matter. Nevertheless, by the 2nd century ce, alchemists, metallurgists and jewellers knew of a great number of elemental species of Aristotle’s Earth element and recognized that some of these, such as gold, were unable to mix with other compounds, making them more elementary than other forms of Earth. There were strong dividing lines between what the Aristotelian theoreticians were saying, and what was known by alchemists experimentally. By the 14th century, there was a revival of interest in the Atomist school of thought, but because atomism was associated with the philosophy of Epicureanism, which contradicted orthodox Christian teachings, belief in atoms was not considered acceptable.

Aristotle’s four elements, earth, air, fire and water held sway as the basic elements of matter until the 14th century.

Another long period of time elapsed before chemist Robert Boyle (1627–1691) and Sir Isaac Newton (1642–1727) defended the concept of atomism. Once again this idea of the structure of matter reappeared within the growing scientific community, nearly 2,000 years after it had first been proposed. In the Philosophiae Naturae Principia Mathematica, Newton claimed ‘…the least parts of bodies to be—all extended, and hard and impenetrable, and moveable, and endowed with their proper inertia’. Newton also went on to propose that atoms interacted among themselves through non-gravitational forces. However, his atoms were rather featureless and did not account for any of the chemical properties of atoms, only in their densities being different to account for water being lighter than rock. So, while tremendous advances were being made in studying the movement of bodies under the influence of gravity, our basic understanding of the nature of matter remained extremely primitive. The fleshing out of the atomic model proceeded very rapidly once chemists became more analytical about how atoms and molecules combined and transformed in a variety of reactions they could observe at the laboratory bench.

Antoine Lavoisier discovered the chemistry of oxygen.

Analytical Chemistry and the Properties of Atoms

Alchemy and what we now call chemistry were a cottage industry in both the Eastern and Western worlds as people struggled to synthesize gold from baser compounds. To find just the right mixture and process to make gold appear at the bottom of their crucibles required enormous amounts of time, and careful notations about what was tried and what failed. Along the way, certain compounds and substances began to appear rather regularly out of these alchemical syntheses. Some alchemists ignored these results and continued with their fevered search for various life-extending elixirs and gold, while others became increasingly intrigued by how some specific combinations led to specific end results. In 1777, Carl Scheele (1742–1786) was the first to recognize that the ordinary air described by Aristotle was itself a compound substance consisting of foul air (nitrogen) and fire air (oxygen). No doubt, foul air had trace impurities of nitrogen sulphide in it, which gave it the rotten eggs aroma. The English chemist Joseph Priestly (1733–1804), meanwhile, had succeeded in producing a variety of gases under laboratory conditions. One of these was called dephlogisticated air, which we now know as oxygen.

In France, Antoine Laurent Lavoisier (1743–1794) announced in 1783 that combustion was a chemical process involving the combination of Priestly’s newly discovered oxygen with the combustible substance. Following Lavoisier’s investigations, Earth, Air, Fire, Water and Aether were no longer regarded as the fundamental building blocks of nature, and it was understood that they could, themselves, be composed of yet more fundamental substances. Lavoisier’s Traité élémentaire de chimie (Elementary Treatise of Chemistry), published in 1789 a few years before his beheading at the height of the French Revolution, is regarded as the first modern textbook of chemistry and included the first systematic listing of some 33 elementary substances known by his time.

Lavoisier’s list of elements c. 1789.

By the end of the 18th century we had therefore entered a period where atoms were being considered for their pragmatic efficiency in helping to classify elementary substances, though there was still hardly much detail as to what they were in and of themselves. The English chemist John Dalton (1766–1884) is usually credited with proposing the atomic theory for the elements and their chemical reactions. His basic ideas were:

1. Elements are made of extremely small particles called atoms.

2. Atoms of a given element are identical in size, mass and other properties; atoms of different elements differ in size, mass and other properties.

3. Atoms cannot be subdivided, created or destroyed.

4. Atoms of different elements combine in simple, whole-number ratios to form chemical compounds.

5. In chemical reactions, atoms are combined, separated or rearranged.

Dalton may have been influenced by the Irish chemist Bryan Higgins (1741–1818), who was one of the first chemists to inquire as to the internal structure of atoms. He came u...