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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

One of the first people to embrace Einstein's ideas was his former teacher, Hermann Minkowski (1864-1909). He realized that although different observers experience the same events, they will describe them differently because they disagree about the nature of space and the nature of time. On the other hand space and time taken together form a more robust entity:

'Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.'
Hermann Minkowski, Space and Time in A. Einstein et al. (1952), The Principle of Relativity, New York, Dover Publications.

The union of space and time of which Minkowski spoke is now generally referred to as space-time. It represents a kind of melding together of space and time, and since space is three-dimensional, and time is one-dimensional, space-time is four-dimensional. Any particular observer, such as you or I, will divide space-time into space and time, but the way in which that division is made may differ from one observer to another and will crucially depend on the relative motion of the observers.

A very rough attempt at representing diagrammatically this change of attitude towards space and time is shown in

Figure 1.23
figure 1.23a, The pre-Einsteinian view of space and time(a) The pre-Einsteinian view of space and time. Not only are space and time separate and distinct they are also absolute. All observers agree on what constitutes space and what constitutes time, and they also agree about what it means to speak of 'the whole of space at a particular time'.
Click here for larger image (15.24kb)
figure 1.23b, The post-Einsteinian view in which space and time are seen as aspects of a unified space-time(b) The post-Einsteinian view in which space and time are seen as aspects of a unified space-time. Different observers in uniform, relative motion will each slice space-time into space and time, but they will do so in different ways. Each observer knows what it means to speak of 'the whole of space at a particular time', but different observers no longer necessarily agree about what constitutes space and what constitutes time.
Click here for larger image (10.67kb)
Before Einstein introduced special relativity, the phrase 'the whole of space at a particular time' was thought to have exactly the same meaning for all observers. After Einstein's work it was felt that each observer would understand what the phrase meant, but that different observers would disagree about what constituted the whole of space at a particular time. All observers would agree on what constituted space-time, but the way in which it was sliced up into space and time would differ from one observer to another, depending on their relative motion. No observer had the true view; they were all equally valid even though they might be different.

In retrospect, special relativity can be seen as part of a gradual process in which the laws of physics attained universal significance. The earliest attempts to understand the physical world placed Man and the Earth firmly at the centre of creation. Certain laws applied on Earth, but different laws applied in the heavens. Copernicus overturned this Earth-centred view and Newton proposed laws that claimed to apply at all places, and at all times. Special relativity continues this process by insisting that physical laws should not depend on the observer's state of motion - at least so long as that motion is uniform. It is therefore not surprising that Einstein was led to ask if physical laws could be expressed in the same way for all observers, even those who were moving non-uniformly. This was the aim of his general theory of relativity.
Continue on to relativity space, time and gravity, part 4 of 4

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Index

S207 The Physical World
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