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The Restless universe
Introduction to The restless Universe

1 The lawful Universe

2 The clockwork Universe

3 The irreversible Universe

4 The intangible Universe

4.1 Electromagnetism and fields 1/4

4.1 Electromagnetism and fields 2/4

4.1 Electromagnetism and fields 3/4

4.1 Electromagnetism and fields 4/4

4.2 Relativity, space, time and gravity 1/4

» 4.2 Relativity, space, time and gravity 2/4

4.2 Relativity, space, time and gravity 3/4

4.2 Relativity, space, time and gravity 4/4

5 The uncertain Universe

6 Closing items

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Other titles in the Physical World series

Describing motion

Predicting motion

Classical physics of matter

Static fields and potentials

Dynamic fields and waves

Quantum physics: an introduction

Quantum physics of matter

4 The intangible Universe

4.2 Relativity,space,time and gravity

Part 1 of 4 | Part 2 | Part 3 | Part 4

For a printable version of 'The intangible Universe' click here

The answer is that a lot of physics had been discovered between the time of Galileo and that of Einstein. Most notably Maxwell's theory of electromagnetism had achieved the feat of predicting the speed of light using fundamental constants of electromagnetism, constants that could be measured using simple laboratory equipment such as batteries, coils and meters. Now, if the principle of relativity were extended to cover Maxwell's theory, the fundamental constants of electromagnetism would be the same for all uniformly moving observers and a very strange conclusion would follow: all uniformly moving observers would measure the same speed of light. Someone running towards a torch would measure the same speed of light as someone running away from the torch. Who would give credence to such a possibility?

Einstein had the courage, self-confidence and determination to reassert the principle of relativity and accept the consequences. He realized that, if the speed of light were to remain the same for all uniformly moving observers, space and time would have to have unexpected properties, leading to a number of startling conclusions, including the following:

Moving clocks run slow. If I move steadily past you, you will find that my wrist watch is ticking slower than yours. Our biological clocks are also ticking, and you will also find that I am ageing less rapidly than you.

Moving rods contract. If an observer on a platform measures the length of a passing railway carriage, he or she will measure a shorter length than that measured by a passenger who is sitting inside the carriage.

Simultaneity is relative. Suppose you find two bells in different church towers striking at exactly the same time (i.e. simultaneously). If I move steadily past you, I will find that they strike at different times (i.e. not simultaneously). It is even possible for you to find that some event A happens before some other event B and for me to find that they occur in the opposite order.

The speed of light in a vacuum is a fundamental speed limit. It is impossible to accelerate any material object up to this speed.

If these consequences seem absurd, please suspend your disbelief. It took the genius of Einstein to realize that there was nothing illogical or contradictory in these statements, but that they describe the world as it is. Admittedly we donít notice these effects in everyday life but that is because we move slowly: relativistic effects only become significant at speeds comparable with the speed of light (2.998 times 108 metres per second). But not everything moves slowly. The electrons in the tube of a TV set are one example, found in most homes, where relativistic effects are significant.
Continue on to relativity, space, time and gravity, part 3 of 4

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Index

S207 The Physical World
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