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# Observational Confirmation of General Relativity Help

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By — McGraw-Hill Professional
Updated on Sep 12, 2011

## Observational Confirmation of General Relativity

When Einstein developed his general theory of relativity, some of the paradoxes inherent in special relativity were resolved. (These paradoxes have been avoided here because discussing them would only confuse you.) In particular, light rays from distant stars were observed as they passed close to the Sun to see whether or not the Sun’s gravitational field, which is quite strong near its surface, would bend the light rays. This bending would be observed as a change in the apparent position of a distant star in the sky as the Sun passes close to it (Fig. 20-10).

Fig. 20-10 . If the acceleration is great enough, the spatial curvature becomes extreme.

The problem with this type of observation was, as you might guess, the fact that the Sun is far brighter than any other star in the sky, and the Sun’s light normally washes out the faint illumination from distant stars. However, during a total solar eclipse, the Sun’s disk is occulted by the Moon. In addition, the angular diameter of the Moon in the sky is almost exactly the same as that of the Sun, so light from distant stars passing close to the Sun can be seen by earthbound observers during a total eclipse. When this experiment was carried out, the apparent position of a distant star was indeed offset by the presence of the Sun, and this effect took place to the same extent as Einstein’s general relativity formulas said it should.

More recently, the light from a certain quasar has been observed as it passes close to a suspected black hole. On its way to us, the light from the quasar follows multiple curved paths around the dark, massive object. This produces several images of the quasar, arranged more or less in the form of a “plus sign” or “cross” with the dark object at the center.

The curvature of space in the presence of a strong gravitational field has been likened to a funnel shape (Fig. 20-11), except that the surface of the funnel is three-dimensional rather than two-dimensional. The shortest distance in three-dimensional space between any two points near the gravitational source is always a curve with respect to four-dimensional space. This is impossible for most (if not all) people to envision directly without “cheating” by taking away one dimension. The mathematics is straightforward enough, though, and observations have shown that it correctly explains the phenomenon.

Fig. 20-11 . Spatial curvature in the vicinity of an object that produces an intense gravitational field.

## And So.. .

We have conducted “mind experiments” in this chapter, many of which require us to suspend reality. In real life, scenarios such as these would kill anyone attempting to make the observations. So why is relativity theory important? If space is bent and time is slowed by incredibly powerful gravitational fields, so what?

General relativity plays an important role in the development of theories concerning the geometry and evolution of the universe. On a vast scale, gravitation acquires a different aspect than on a local scale. A small black hole, such as that surrounding a collapsed star, is dense and produces gravitation strong enough to destroy any material thing crossing the event horizon. However, if a black hole contains enough mass, its density is not necessarily great. Black holes with quadrillions of solar masses can exist, at least in theory, without life-threatening forces at any point near their event horizons. If such a black hole is ever found, and if we develop space ships capable of intergalactic flight, we will be able to cross its event horizon unscathed, leave this universe, and enter another—forever.

Practice problems of these concepts can be found at: Relativity Theory Practice Test

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