 According
to general relativity, the space-time of special relativity - the
common space-time of unaccelerated observers - is also the kind of
space-time that exists in the absence of gravity. In the presence
of gravity, in the vicinity of a massive object such as the Earth,
say, space-time becomes distorted or ‘curved’. Because of this,
moving objects will behave differently in the presence of such massive
bodies. The curvature of space-time causes the moving object to behave
as though it is being pulled towards the massive body by some kind
of force. This, in essence, is Einstein’s explanation of gravity:
there is not really any such thing as a gravitational ‘force’, simply
the appearance of a force due to the response of moving objects to
the distortion of space time. Near the Earth, the curvature of space-time
is fairly slight, so Newton’s seventeenth century theory of gravity
is sufficiently accurate for most purposes. But where space-time curvature
is much greater, near to a black hole for instance, the shortcomings
of Newton’s theory become obvious and Einstein’s theory has to be
used in its place. The highly schematic diagram shows space-time curvature
near the Sun, indicating the way in which this can lead to the bending
of starlight as it grazes the edge of the Sun. |
