"Do not take the lecture too seriously . . . just relax and enjoy it. I am going to tell you what nature behaves like. If you will simply admit that maybe she does behave like this, you will find her a delightful, entrancing thing. Do not keep saying to yourself "But how can it be like that?" because you will get . . . into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that."

(Richard Feynman introducing a lecture about quantum theory)

Natural sciences always had a great influence on philosophy and on the way we see the world. Until the age of the Renaissance there was no clear distinction between philosophy and science. Speculations about physics and astronomy were among the favourite topics of the natural philosophers of antiquity and continued to flourish until the time of Copernicus. The desire to explore the starry heavens and to reveal its secrets is probably as old as mankind itself. However, notable advances in this discipline were made only fairly recently, after the invention of the telescope in the 17th century. This section deals with the accomplishments of 20th century physics in the world of the largest structures, such as galaxies and stars, and that of the smallest structures, such as atoms and particles. We take a closer look at Relativity and Quantum Physics in particular, both of which have given us amazing new insights into what we call creation.

Newton: the three laws of motion.

In the eyes of physics, the world used to be a predictable place. Aristotle and Ptolemy laid the foundation for the scientific understanding of the universe, which remained authoritative for one-and-a-half thousand years. Until the time of Galileo, the Greeks were undisputed in natural science and astronomy. Galileo, Copernicus, and Newton changed this. Isaac Newton (1642-1727) revolutionised physics with his proposition that all bodies are governed by the three laws of motion. The first law of motion states that a body continues in a state of rest or continues to be moving uniformly in a straight line unless a force is applied to the object. The second law states that the force applied to an object is proportional to its mass multiplied by acceleration (F=ma). The third law states that for every action there is an equal opposite reaction.

With these three simple laws, Newton created a whole new model of the universe, superseding Ptolemy's model of epicycles. Eighty years before, Galileo (1564-1642) had pointed out that the Earth rotates around the Sun. The mechanics developed by Newton and Galileo provided the basis for 17th to 19th century cosmology. In this view, planets revolved in well-defined orbits around stars, where the rotational force is balanced by the gravitational force. According to the universal law of gravitation, bodies attract each other with a force F=m1*m2/r², which means that the force increases with mass and decreases (squared) with distance.

Laplace: the mechanistic universe.

Given these natural laws, mankind derived a picture of the universe that accounts neatly for mass, position, and the motion of the celestial bodies while it interprets the latter as dynamic elements of a celestial apparatus, not unlike that of a mechanical apparatus. It is therefore called the mechanistic worldview. It was elaborated in its purest form by Marquis de Laplace (1749-1827) in his writing Mécanique Céleste. The mechanistic view sees the universe as an arrangement in which stars and planets interact with each other like springs and cogs in a clockwork, while God is watching from above. If the initial positions and states of all objects in a mechanically determined universe are known, all events can be predicted until the end of time, simply by applying the laws of mechanics. It was further thought that this kind of knowledge is available only to an omniscient God.

The mechanistic view does not make any statements about the creation of the universe. Things were taken as preestablished by the creator. From a mechanistic standpoint, solar systems like our own are in a delicate balance, because only a slight increase or decrease in mass or velocity of the planets would let the planets either spiral into the Sun or wander into outer space. There had to be a construction plan. There was a necessity for a creator God who initially put balance into the universe. Needless to say that the church was comfortable with this theory, despite the earlier quarrels with Galileo, and in spite of the fact that it generally viewed scientific progress with great suspicion.

Discovery of the speed of light.

In 1676 the Danish astronomer Ole Roemer (1644-1710) announced a remarkable discovery. He observed seasonal variations in the disappearances of Jupiter's moons behind Jupiter. Because the distance between Earth and Jupiter varies with the seasons, while the Earth travels on its path around the Sun, this means that the light from Jupiter's moons travels either shorter or longer distances throughout the year. The changes in Roemer's observation corresponded with the distances between Earth and Jupiter, which implied that the speed of light is finite. Roemer's observation did, however, not directly contradict the mechanistic worldview. In the mechanistic view, light waves travel through the ether, just as sound waves travel through air. - Yet, there was a problem with the concept of "ether". Its existence could never be detected.

At the end of the 19th century, the mechanistic view was in trouble. Astronomers noticed that Mercury's perihelion (the closest point to the Sun in its orbit) changed slightly with every orbit. This observation shattered the notion of immutable orbits. Astronomers tried to solve this problem by predicting a mystery planet they called Vulcan, which would account for the observed gravitational variations. Needless to say that it was never found.

The American physicists Michelson and Morley brought the mechanistic worldview into even more trouble. In an experiment, which was designed to measure the velocity of the earth, they found that the speed of light is constant, contrary to what they had expected. They found this characteristic of light to be in disagreement with the Galilean velocity addition formula v'=v1+v2, which means their observation contradicted classical mechanics.

Einstein changes everything.

At the beginning of the 20th century, a formerly unknown clerk of the Swiss patent office by the name of Albert Einstein thought to himself: "Falling objects don't feel gravity." He imagined what it would be like to ride through space on a beam of light and came to the conclusion that space and time can be visualised as coordinate systems, or "reference frames", relative to the observer. This was the basis for his Relativity Theory. At about the same time, other physicists pondered on equally fundamental problems, which concerned interactions of matter and radiation, but came to totally different conclusions than Einstein. The result of their collective thought, quantum theory, explained the behaviour of subatomic particles.

With this being written in the year 1999 it is safe to say that Relativity was the single most influential physical theory of the 20th century for the way it has changed our view of the universe. Not that other discoveries in physics were less significant, but few of them have been so well received by the general public. Relativity has grabbed people's imagination and sparked discussions in philosophy and religion which last until the present day. Quantum physics, although perhaps more pertinent to daily life, is a close second.

Is causality questioned by modern physics?

Relativity and Quantum Theory have implications on cosmology, epistemology, and metaphysics. We only begin to understand their impact on our traditional ways of seeing the world. How does God fit into our new picture of the universe? Can the stuff the world is made of be explained by physics alone? What is space and time? Does quantum physics contradict causality? To find out more about these questions and to learn about the findings of Einstein, Heisenberg, and others, take a closer look at the fascinating world of modern physics.

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