**4 The intangible Universe**

4.2 Relativity,space,time and gravityPart 1 of 4 | Part 2 | Part 3 | Part 4

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Einstein realized that many of the effects of non-uniform motion are similar to the effects of gravity. (Perhaps you have experienced the sensation of feeling heavier in a lift that is accelerating upwards.) With unerring instinct he treated this as a vital clue: any theory of general relativity would also have to be a theory of gravity. After more than ten years of struggle, the new theory was ready. According to general relativity, a large concentration of mass, such as the Earth, significantly distorts space-time in its vicinity. Bodies moving through a region of distorted space-time move differently from the way they would have moved in an undistorted space-time.

For example, meteors coming close to the Earth are attracted to it and deviate from uniform, steady motion in a straight line. Newton would have had no hesitation in saying that these deviations are due to gravitational forces. In Einstein's view, however, *there is no force*. The meteors move in the simplest way imaginable, but through a distorted space-time, and it is this distortion, generated by the presence of the Earth, that provides the attraction. This is the essence of general relativity, though the mathematics required to spell it out properly is quite formidable, even for a physicist.

The central ideas of general relativity have been neatly summarized by the American physicist John Archibald Wheeler. In a now famous phrase Wheeler said:

**'Matter tells space how to curve.**

**Space tells matter how to move.'**

Purists might quibble over whether Wheeler should have said 'space-time' rather than 'space', but as a two-line summary of general relativity this is hard to beat.

^{ Figure 1.24A highly schematic diagram showing space-time curvature near the Sun and indicating the way in which this can lead to the bending of starlight as it grazes the edge of the Sun. (The bending has been hugely exaggerated for the sake of clarity.) The observation of this effect in 1919, during a total eclipse of the Sun, did much to make Einstein an international celebrity.Click here for larger image (15.24kb)} |

If you tried to summarize Newtonian gravitation in the same way all you could say is: 'Matter tells matter how to move'; the contrast is clear.

General relativity is a field theory of gravity. At its heart are a set of equations called the Einstein field equations. To this extent general relativity is similar to Maxwell's field theory of electromagnetism. But general relativity is a very unusual field theory. Whereas electric and magnetic fields exist *in* space and time, the gravitational field essentially is space and time. Einstein was well aware of the contrast between gravity and electromagnetism, and spent a good deal of the later part of his life trying to formulate a **unified field theory** in which gravity and electromagnetism would be combined into a single 'geometric' field theory. In this quest he was ultimately unsuccessful, but general relativity remains a monumental achievement.

**Question 1.5 ****Answer**

Would it be fair to say that special relativity has the effect of leaving each observer completely free to make his or her own decision about what constitutes time?

Continue on to 5 The uncertain Universe