The Ideas of Special and General Relativity

By Chris McGinlay

A qualitative overview of the foundational principles of relativity, including historical development of concepts of relativity from the time of Galileo.

This book is aimed at anyone with an interest in the history of development of special and general relativity. It should provide a working knowledge of the main ideas, able to stand on its own, or form the basis for further study. The content discussed could support the Scottish Qualifications Authority's Higher and Advanced Higher Physics courses, although it is not meant to be a perfect match for assessable learning outcomes.

• Language: English
• Publisher: Chris McGinlay
• Release date: Feb 29, 2020
• ISBN: 9781393348764

Chris McGinlay

Chris McGinlay has been teaching physics, and a variety of other technical subjects for over twenty years. Following up on the many questions asked by students has guided his forays into the history of the development of scientific and mathematical knowledge and the scientists who made it all happen.

Book preview

Introduction

AS YOU MIGHT IMAGINE, the General Theory of Relativity (GR) is a complicated subject. To give the subject a rigorous treatment requires mathematical techniques taught at university undergraduate mathematics or physics courses.

This book attempts to give a qualitative overview of the origins of GR and an introduction to its foundational principles. It will also describe some of the interesting consequences of GR. The material is essentially presented with a non-mathematical approach, other than where a few snippets of notation are included for interest and to illustrate the discussion.

General relativity describes advanced concepts of motion. This could be the motion of particles, planets or photons of light. GR describes how this motion is influenced by the presence of mass, superseding Newtonian ideas of gravity and motion. Since the time of its publication in 1915, general relativity has enjoyed over a century of experimental verification.

Galileo Galilei 1564-1642

THE CONCEPT OF RELATIVITY pre-dates Einstein by quite some time - about three hundred years in fact! The first known description of a relativistic understanding of motion was made by the Italian polymath Galileo Galilei in his 1632 book titled Dialogue Concerning the Two Chief World Systems.  At this time, the geocentric view of the universe with a stationary Earth at the centre was still widely thought to be true, as opposed to Copernicus’s heliocentric system. There were objections to the idea of a moving Earth; people claimed for example, that an object dropped from a great height would fall behind its nadir due to the motion of the Earth’s surface. Some people even claimed to have observed this deflection.

Galileo did away with the idea that there was a state of absolute rest or absolute motion. Instead he proposed that the velocity of an object could only be defined with reference to another object in relative motion to the first.

An Italian polymath who initially studied medicine before developing an interest in mathematics and natural philosophy. Galileo is noted for improving the recently invented telescope and using it to observe craters on the Moon, discovering the four brightest Jovian moons and observing the phases of Venus. His heliocentric views brought him into conflict with other astronomers as well as with the Church.

Galileo’s Relativity Hypothesis

IN MODERN TERMS WE can describe Galileo’s relativity hypothesis as follows:

If two observers are moving with constant velocity relative to each other, both observers will obtain the same results for all mechanical experiments in their respective frames of reference.

Galileo used the example of being aboard a ship moving steadily on a perfectly calm sea. Unless you can look out of a window, there’s no experiment which can prove you are on the moving ship and not in a room in harbour. Even the sound of water flowing over the hull could be due to tidal movement of water whilst the ship lies at anchor!

Galileo was thinking of mechanics experiments with rolling balls, fish swimming in a bowl, butterflies in a box. He argued that the fish swimming in their bowl do not find it any more difficult to swim in the direction of motion of the ship than they do to swim against it – the fish do not get ‘left behind’ by the motion of the ship. It therefore follows that the Earth could well be in a state of motion, without having a perceptible effect on motions on the surface. Galileo’s opponents however  knew that a rotating Earth should cause a Coriolis effect, with a sideways deflection of projectiles. The lack of any observed