7 General Relativity
121
Sarah turned to him. “But there are no black holes near the
earth. … Surely.”
Jim shrugged. It wasn’t his field. “Not unless someone decided
to make one.”
In the next instant, Jim’s world vanished. He felt himself
falling into a black silence, so intense that it swallowed even the
beating of his heart, where seconds and centuries, metres and
kilometres swirled together in a cosmic potpourri.
∗ ∗ ∗
We defer our discussion of black holes until later in this Chapter
(Sect. 7.5). By then the reader will be in a better position to decide whether
the story of Jim and Sarah is fantasy or prophecy.
7.2 A 20th Century Cathedral
As we have seen in the previous Chapter, in 1905 Einstein proposed new
concepts of space and time that overturned the credos of centuries. Two
hitherto different quantities, space and time, were combined into the single
entity: space–time. Profound physical consequences were found to arise from
one basic hypothesis: that the speed of light in a vacuum is the same for all
non-accelerating observers.
Immediately, the restrictive nature of this hypothesis raises a question:
what happens when the observer is in an accelerating frame of reference? We
have intimated, when discussing the twin paradox in the last Chapter, that
consequences do arise, which are not at all obvious. The other outstanding
issue, untreated in Special Relativity, was gravity, probably the most ubiquitous source of acceleration throughout the cosmos.
It was clear to Einstein that Newton’s theory of gravity 2 was incompatible
with Special Relativity, and in need of modification or replacement. Newton’s
equations contain no reference to time: the gravitational attraction between
two objects depends only on their mass and their separation. According to
Newton, a movement of one mass results in an instantaneous change in the
force being felt by the other. This perturbation in the force propagates at
an infinite velocity, which violates the limit of c imposed by Special Relativity. Einstein worked on these problems for a decade, and finally in 1916
2 When Newton propounded his theory of Gravity to the Royal Society in 1686, Robert Hooke, a
contemporary of his and a gifted scientist in many different fields, immediately claimed that Newton
had plagiarised the law from him. The controversy continues to this day.
121
Sarah turned to him. “But there are no black holes near the
earth. … Surely.”
Jim shrugged. It wasn’t his field. “Not unless someone decided
to make one.”
In the next instant, Jim’s world vanished. He felt himself
falling into a black silence, so intense that it swallowed even the
beating of his heart, where seconds and centuries, metres and
kilometres swirled together in a cosmic potpourri.
∗ ∗ ∗
We defer our discussion of black holes until later in this Chapter
(Sect. 7.5). By then the reader will be in a better position to decide whether
the story of Jim and Sarah is fantasy or prophecy.
7.2 A 20th Century Cathedral
As we have seen in the previous Chapter, in 1905 Einstein proposed new
concepts of space and time that overturned the credos of centuries. Two
hitherto different quantities, space and time, were combined into the single
entity: space–time. Profound physical consequences were found to arise from
one basic hypothesis: that the speed of light in a vacuum is the same for all
non-accelerating observers.
Immediately, the restrictive nature of this hypothesis raises a question:
what happens when the observer is in an accelerating frame of reference? We
have intimated, when discussing the twin paradox in the last Chapter, that
consequences do arise, which are not at all obvious. The other outstanding
issue, untreated in Special Relativity, was gravity, probably the most ubiquitous source of acceleration throughout the cosmos.
It was clear to Einstein that Newton’s theory of gravity 2 was incompatible
with Special Relativity, and in need of modification or replacement. Newton’s
equations contain no reference to time: the gravitational attraction between
two objects depends only on their mass and their separation. According to
Newton, a movement of one mass results in an instantaneous change in the
force being felt by the other. This perturbation in the force propagates at
an infinite velocity, which violates the limit of c imposed by Special Relativity. Einstein worked on these problems for a decade, and finally in 1916
2 When Newton propounded his theory of Gravity to the Royal Society in 1686, Robert Hooke, a
contemporary of his and a gifted scientist in many different fields, immediately claimed that Newton
had plagiarised the law from him. The controversy continues to this day.