,Instructor Solutions Manual: Modern General Relativity
Mike Guidry
This document gives the solutions for all problems at the ends of chapters
for the first edition of Modern General Relativity: Black Holes, Gravitational
Waves, and Cosmology by Mike Guidry (Cambridge University Press, 2019).
Unless otherwise indicated, literature references, equation numbers, figure ref-
erences, table references, and section numbers refer to the print version of that
book.
,1 Introduction
1.1 From Eq. (1.2), the value of γ is infinite if v = c, so there is no Lorentz transformation
to an inertial frame corresponding to a rest frame for light.
1.2 Since E = mγ , for a 7 TeV proton,
E 7 × 1012 eV
γ= = = 7460.
m 938.3 × 106 eV
Then from the definition of γ ,
s
v 1
= 1− = 0.999999991.
c γ2
This is a speed that is only about 3 meters per second less than that of light.
1.3 This question is ambiguous, since it does not specify whether the curvature is that of
the surface itself (which is called intrinsic curvature) or whether it is the apparent curvature
of the surface seen embedded in a higher-dimensional euclidean space (which is called
the extrinsic curvature). In general relativity the curvature of interest is usually intrinsic
curvature. Then the sheet of paper can be laid out flat and is not curved, the cylinder is
also flat, with no intrinsic curvature, because one can imagine cutting it longitudinally
and rolling it out into a flat surface, but the sphere has finite intrinsic curvature because it
cannot be cut and rolled out flat without distortion. The reason that the cylinder seems to
be curved is because the 2D surface is being viewed embedded in 3D space, which gives
a non-zero extrinsic curvature, but if attention is confined only to the 2D surface it has no
intrinsic curvature. This is a rather qualitative discussion but in later chapters methods will
be developed to quantify the amount of intrinsic curvature for a surface.
1
, Coordinate Systems and
2
Transformations
2.1 Utilizing Eq. (2.31) to integrate around the circumference of the circle,
s 2
Z +R
dy
I I
2 2 1/2
C = ds = (dx + dy ) = 2 dx 1 + ,
−R dx
subject to the constraint R2 = x2 + y2 , where the factor of two and the limits are because x
ranges from −R to +R over half a circle. The constraints yield dy/dx = −(R2 − x2 )−1/2 x,
which permits the integral to be written as
s
R2
Z R
C=2 dx .
−R R 2 − x2
Introducing a new integration variable a through a ≡ x/R then gives
Z +1
da
C = 2R √ = 2π R,
−1 1 − a2
−1
since the integral is sin a. In plane polar coordinates the line element is given by Eq.
(2.32) and proceeding as above the circumference is
I I
C= ds =(dr2 + r2 d ϕ 2 )1/2
s
Z 2π 2 Z 2π
dr
= d ϕ r2 + =R d ϕ = 2π R,
0 dϕ 0
where r = R has been used, implying that dr/d ϕ = 0.
2.2 Under a Galilean transformation x ′ = x − vt and t ′ = t it is clear that the acceleration
a and the separation vector r = ∆xx between two masses are unchanged. Thus the second
law F = maa and the gravitational law F = Gm1 m2 r̂/r2 are invariant under Galilean trans-
formations.
2.3 Our solution follows Example 1.2.1 of Foster and Nightingale [88]. The tangent and
dual basis vectors, and the products for gi j = g ji = e i ·ee j , were worked out in Example 2.3.
The elements for gi j = g ji = e i ·ee j can be obtained in a similar fashion. For example,
g12 = g21 = ( 21 i + 12 j )·( 12 i − 12 j ) = 41 − 14 = 0,
where the orthonormality of the cartesian basis vectors has been used. Summarizing the
results,
2 1
4v + 2 4uv 2v 2 0 −v
gi j = 4uv 4u2 + 2 2u gi j = 0 1
2 −u
2v 2u 1 2
−v −u 2u + 2v + 1 2
2
Mike Guidry
This document gives the solutions for all problems at the ends of chapters
for the first edition of Modern General Relativity: Black Holes, Gravitational
Waves, and Cosmology by Mike Guidry (Cambridge University Press, 2019).
Unless otherwise indicated, literature references, equation numbers, figure ref-
erences, table references, and section numbers refer to the print version of that
book.
,1 Introduction
1.1 From Eq. (1.2), the value of γ is infinite if v = c, so there is no Lorentz transformation
to an inertial frame corresponding to a rest frame for light.
1.2 Since E = mγ , for a 7 TeV proton,
E 7 × 1012 eV
γ= = = 7460.
m 938.3 × 106 eV
Then from the definition of γ ,
s
v 1
= 1− = 0.999999991.
c γ2
This is a speed that is only about 3 meters per second less than that of light.
1.3 This question is ambiguous, since it does not specify whether the curvature is that of
the surface itself (which is called intrinsic curvature) or whether it is the apparent curvature
of the surface seen embedded in a higher-dimensional euclidean space (which is called
the extrinsic curvature). In general relativity the curvature of interest is usually intrinsic
curvature. Then the sheet of paper can be laid out flat and is not curved, the cylinder is
also flat, with no intrinsic curvature, because one can imagine cutting it longitudinally
and rolling it out into a flat surface, but the sphere has finite intrinsic curvature because it
cannot be cut and rolled out flat without distortion. The reason that the cylinder seems to
be curved is because the 2D surface is being viewed embedded in 3D space, which gives
a non-zero extrinsic curvature, but if attention is confined only to the 2D surface it has no
intrinsic curvature. This is a rather qualitative discussion but in later chapters methods will
be developed to quantify the amount of intrinsic curvature for a surface.
1
, Coordinate Systems and
2
Transformations
2.1 Utilizing Eq. (2.31) to integrate around the circumference of the circle,
s 2
Z +R
dy
I I
2 2 1/2
C = ds = (dx + dy ) = 2 dx 1 + ,
−R dx
subject to the constraint R2 = x2 + y2 , where the factor of two and the limits are because x
ranges from −R to +R over half a circle. The constraints yield dy/dx = −(R2 − x2 )−1/2 x,
which permits the integral to be written as
s
R2
Z R
C=2 dx .
−R R 2 − x2
Introducing a new integration variable a through a ≡ x/R then gives
Z +1
da
C = 2R √ = 2π R,
−1 1 − a2
−1
since the integral is sin a. In plane polar coordinates the line element is given by Eq.
(2.32) and proceeding as above the circumference is
I I
C= ds =(dr2 + r2 d ϕ 2 )1/2
s
Z 2π 2 Z 2π
dr
= d ϕ r2 + =R d ϕ = 2π R,
0 dϕ 0
where r = R has been used, implying that dr/d ϕ = 0.
2.2 Under a Galilean transformation x ′ = x − vt and t ′ = t it is clear that the acceleration
a and the separation vector r = ∆xx between two masses are unchanged. Thus the second
law F = maa and the gravitational law F = Gm1 m2 r̂/r2 are invariant under Galilean trans-
formations.
2.3 Our solution follows Example 1.2.1 of Foster and Nightingale [88]. The tangent and
dual basis vectors, and the products for gi j = g ji = e i ·ee j , were worked out in Example 2.3.
The elements for gi j = g ji = e i ·ee j can be obtained in a similar fashion. For example,
g12 = g21 = ( 21 i + 12 j )·( 12 i − 12 j ) = 41 − 14 = 0,
where the orthonormality of the cartesian basis vectors has been used. Summarizing the
results,
2 1
4v + 2 4uv 2v 2 0 −v
gi j = 4uv 4u2 + 2 2u gi j = 0 1
2 −u
2v 2u 1 2
−v −u 2u + 2v + 1 2
2
