Physics

From Natural Philosophy to the Enigma of Dark Matter

Anne Rooney

• Publisher: Arcturus
• Year: 2020
• ISBN:
  9781398800328
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

Physics is the science which ties all others together. Aiming to explain the universe, from the smallest subatomic particles to vast galaxies, it represents human intellectual endeavor at its most ambitious. This illustrated guide traces the development of physics, from the natural philosophers of the ancient world to cutting-edge experiments in quantum mechanics. These complex ideas are expressed in clear and easy-to-understand language, providing a wonderful introduction to the vast subject. Topics include:
• The nature of matter - first theorized as atoms 2, 500 years ago
• Light and optics
• Masses and forces
• The bizarre subatomic world of quantum mechanics
• Our fascination with the stars
• The universe around us, from the Big Bang to distant galaxies ABOUT THE SERIES: The Arcturus Fundamentals Series explains fascinating and far-reaching topics in simple terms. Designed with retro, tactile covers and filled with dynamic illustrations and fact boxes, these books will help you quickly get to grips with complex topics that effect our day-to-day living.

CHAPTER 1

Mind over matter

It’s difficult to imagine, when looking at a solid object, that it is composed of many very tiny particles and a lot of empty space. It’s even stranger when we pause to think that the particles themselves are more space than matter. The idea that matter is not continuous, and even that it contains a lot of empty space – which is a fair description of modern atomic theory – was first suggested around 2,500 years ago. Even so, atomic theory has only been accepted by a majority of scientists for a little over a century. For much of the intervening time, the concept was discredited, and even ridiculed.

The first physicist?

The origins of ‘natural philosophy’ – or science, as we call it today – probably lie, as with so much else that underpins Western culture, in ancient Athens. The first person we can call by name a physicist is Anaxagoras who flourished in the 5th century BC. At a time when logic was in its infancy, he tried to fit his myriad observations and the results of his experimentation into a logical framework that would enable him to understand and explain the nature of the world. Anaxagoras sought a view of the material universe in which superstition or divine intervention need play no part, a scheme in which everything was explicable to the rational mind – a truly scientific model. In limiting himself to types of matter that could be perceived, Anaxagoras set a pattern for physicists for dealing with the visible, physical world that was to last for nearly 2,500 years.

THE SEEDS OF MATTER

For Anaxagoras, the central feature of the natural world was change. He saw everything in constant movement, one thing shifting into another in an endless cycle. Matter, he said, could neither come into existence from nothing nor cease to exist, a belief he shared with earlier thinkers Thales of Miletus and Parmenides (c.515–c.445BC). This same belief was presented much later by the French chemist Antoine Lavoisier (1743–94) in the law of the conservation of mass (see page 37). Further, he claimed that all matter was composed of the same fundamental ingredients – essential properties, and perhaps ‘seeds’ of basic substances. The properties always existed in pairs that were polar opposites, such as hot–cold, dark–light and sweet–sour. There would always be the same quantity of each property in total. The seeds were principally of organic matter (blood, flesh, bark, fur).

Anaxagoras believed that any portion of matter, irrespective of how small it is, contains all possible properties (or materials). This means it must be capable of infinite division. The properties that predominate are evident and give the substance its observable characteristics, while others are latent. So a tree has more bark than fur, but it still has some of each – it just doesn’t have enough fur to manifest ‘furriness’. This explains how one substance can be made from another, as it simply requires taking different proportions of all the properties (or materials) to form the new substance.

‘Nothing will come of nothing.’

King Lear, Act I, Scene 1

Anaxagoras (c.500–c.430BC)

Born in Ionia, on the western coast of what is now Turkey, Anaxagoras moved to Athens at the age of 20 where he immediately entered the highest intellectual circles. He became the close companion and instructor of Pericles, political ruler of Athens at the height of the city’s power (454– 431BC). Anaxagoras taught and wrote a treatise on natural philosophy that was later used by the Greek philosopher Socrates (469–399BC). His fame spread far and wide, his zeal for the intellectual life and disregard of all fleshly and social pleasures becoming as famous as his teachings. Anaxagoras was so devoted to the life of the mind that he neglected everything else and allowed his sub stantial inheritance to waste away.

Despite being the leading intellectual figure in Athens, he moved away from the city after about 30 years, and little is known of his later life. He died at Lampsacus on the coast of the Dardanelles aged around 70, but his influence continued for a century after his death.

MIND ANIMATING MATTER

Anaxagoras had an additional ingredient to throw into the melting pot, and this was mind, or nous. He did not believe mind to be present in all matter, but only in animate (living or conscious) things. Mind had an additional role, though: at the start of all things, matter was not distinguished into different substances, but a homogenous pile of particles or slurry that became sorted into ‘proper’ matter by the principle of mind. This sounds horribly like creation by a divine entity, though, and Anaxagoras was adamant that he wanted no superstition or religion in his account of the world. His ‘mind’ was not an intelligent creator but some kind of inspiring element that set in motion the physical forces that whirled elemental matter around, causing it to separate, differentiate and form bodies such as the Earth and Sun. It is difficult to be precise about the role of mind as Anaxagoras’ complete text has not survived. However, Plato reports that Socrates bought a copy of Anaxagoras’ work because he thought it contained an explanation involving a designing intelligence and was disappointed.

ALL CHANGE

Anaxagoras had a model in which matter could not be created or destroyed, but in which the mutability of the world around us is explained by changing the position of matter over time. If a tree is cut down and the wood made into a boat, the matter has been moved and rearranged, but is of the same type and quantity (counting the boat, off-cuts and sawdust) as before. Other changes require more substantial rearrangement: setting fire to a tree, for example, produces ash, water vapour and smoke that do not appear to be at all similar to wood. As every object contains, in different proportions, all the possible types of matter and qualities, there is always the potential for each type of matter to be derived from any object – so a plant could grow from soil, for instance, by rearranging or extracting types of matter.

Anaxagoras realized that to make this work, the constituent parts of matter (seeds) must be extremely small, as otherwise the kinds of changes we see every day would not be possible. The requirement for the components of matter to be infinitesimally small was to present insurmountable problems for the model.

UNCUTTABLE PORTIONS

The word ‘atom’ comes from the Classical Greek word ‘atomos’ meaning uncuttable or indivisible. The suggestion that everything is made from very tiny, indivisible particles has its origins in the 5th century BC with the work of Leucippus and then his pupil Democritus. Far more is known about Democritus (c.460–c.370BC) than about Leucippus, to the extent that the Greek philosopher Epicurus (341–270BC) doubted Leucippus even existed. It is impossible to tell what portion of the atomic model came from Leucippus. Atomism holds that the universe comprises matter made from tiny, indivisible particles that exist in a void. The atoms of any particular substance are all the same size and shape and made of the same material.

If atoms are tiny, homogenous (homoiomerous) particles there is an obvious question – why can’t they be further divided? If Democritus had an answer, it has not survived. It may have been that atoms, being homogenous, have no internal void (whereas larger chunks of matter have space between the atoms), and this alone means they cannot be divided.

There is an innate paradox, too, in a model of matter made up of infinitesimal particles. What Anaxagoras meant by infinitesimal was that the particles were smaller than any arbitrarily small measure, but larger than zero. Even so, he believed that every object held an infinite number of particles, as no matter how small a portion he took there would always be some of every type of matter. If atoms or seeds have no extension in space (zero size), then even an infinite number of them could not make up matter of finite size. This dilemma presented insuperable problems for later Greek thinkers, and led the atomic model into the doldrums from which it did not emerge for 2,000 years.

Homoiomeries

Anaxagoras and later Greek thinkers distinguished between substances that were homoiomerous (homogenous) and those that were not. A homoiomerous substance is one in which all parts are like the whole. So a lump of gold is homoiomerous because no matter how small a chunk of it is taken, it still has the properties of a large chunk of gold. A tree or a ship is not homoiomerous as it can be broken into parts that have different characteristics. To modern eyes, homoiomeries are the elements and pure chemical compounds.

THINGS AND NOT-THINGS

So far, atomism sounds very similar to Anaxagoras’s model yet he had all matter floating in the air or aether (see page 28), which is a physical substance, while the atomists had particles of matter existing in a void. Democritus (or Leucippus) was the first to postulate a void, though it was clearly needed if matter is to move: in a universe jam-packed full of matter every bit of space would already be occupied so it could not be occupied by something else moving into it. When something moves, it not only shifts into an empty space or nudges something else into empty space, it also leaves empty space behind it. While earlier thinkers had denied there is a void (‘what is not’), Democritus relied on the evidence of our senses – we know that things move – to establish the void as a valid concept. Further, we can see that the universe is made up of many things (it has plurality), whereas if there were no empty space, all matter would be continuous. Plurality and change both require a void.

Atomic and elemental matter

To the modern mind, atoms and elements are part of the same model of the universe. The elements are the pure chemical substances, each made up of identical atoms, so all of gold is gold atoms, and all of hydrogen is hydrogen atoms. Compounds, on the other hand, contain atoms of two or more elements, so carbon dioxide comprises carbon and oxygen atoms, for instance. In ancient theories of matter, though, atoms and elements belong to different models.

FOUR – OR FIVE – ELEMENTS

Empedocles (c.490–c.430BC) taught that everything is made up from four ‘roots’: earth, air, water and fire. This model was reworked and championed by Aristotle, perhaps the greatest and most influential thinker in the history of the West.

Plato renamed the four roots ‘elements’ and Aristotle used this term. Each element is characterized by two properties from the natural contraries – hot–cold and wet–dry.

So, therefore, earth is cold and dry, water is cold and wet, air is hot and wet, and fire is hot and dry. These properties also formed the basis of the model of health and illness based on the four humours proposed by Hippocrates (c.460–c.377BC) or his school, which endured well into the 19th century.

According to elemental theory, all matter naturally occupied a realm that was associated with its elements and matter is drawn towards its natural realm. Earth occupied the lowest position, fire the highest, with water and air between them. This explained some types of movement in the physical world: heavy objects fall to earth because earth is their principal element; smoke comprises fire and air, which occupy the upper realms, so it rises. Once an element is in its natural place, it will not move unless something causes it to do so.

In addition to the four elements, there is a very different fifth element (or ‘quintessence’), called the ‘aether’. The concept of an ‘aether’ (or ‘ether’) never quite went away, though it went in and out of favour over thousands of years (see page 28).

Although Democritus’ atomist model was in fact much closer to reality as understood today, it was the idea favoured by Empedocles, Plato and Aristotle, of a world made up of four elements, that proved most popular. When the Arab thinkers of the early Middle Ages reinvigorated and developed the thinking...

Table of contents

• Cover
• Title
• Contents
• Introduction: The book of the Universe
• Chapter 1: Mind over matter
• Chapter 2: Making light work — optics
• Chapter 3: Mass in motion — mechanics
• Chapter 4: Energy fields and forces
• Chapter 5: Into the atom
• Chapter 6: Reaching for the stars
• Chapter 7: Space-time continuing
• Chapter 8: Physics for the future
• Copyright