import astropy.units as u
import numpy as np
from einsteinpy import constant, utils
nonzero_christoffels_list = [
(0, 0, 1),
(0, 0, 2),
(0, 1, 3),
(0, 2, 3),
(0, 1, 0),
(0, 2, 0),
(0, 3, 1),
(0, 3, 2),
(1, 0, 0),
(1, 1, 1),
(1, 2, 2),
(1, 3, 3),
(2, 0, 0),
(2, 1, 1),
(2, 2, 2),
(2, 3, 3),
(1, 0, 3),
(1, 1, 2),
(2, 0, 3),
(2, 1, 2),
(1, 2, 1),
(1, 3, 0),
(2, 2, 1),
(2, 3, 0),
(3, 0, 1),
(3, 0, 2),
(3, 1, 0),
(3, 1, 3),
(3, 2, 0),
(3, 2, 3),
(3, 3, 1),
(3, 3, 2),
] #: Precomputed list of tuples consisting of indices of christoffel symbols which are non-zero in Kerr-Newman Metric
[docs]def charge_length_scale(
Q, c=constant.c.value, G=constant.G.value, Cc=constant.coulombs_const.value
):
"""
Returns a length scale corrosponding to the Electric Charge Q of the mass
Parameters
----------
Q : float
Charge on the massive body
c : float
Speed of light. Defaults to 299792458 (SI units)
G : float
Gravitational constant. Defaults to 6.67408e-11 (SI units)
Cc : float
Coulumb's constant. Defaults to 8.98755e9 (SI units)
Returns
-------
float
returns (coulomb's constant^0.5)*(Q/c^2)*G^0.5
"""
return (Q / (c ** 2)) * np.sqrt(G * Cc)
[docs]def rho(r, theta, a):
"""
Returns the value sqrt(r^2 + a^2 * cos^2(theta)).
Specific to Boyer-Lindquist coordinates
Parameters
----------
r : float
Component r in vector
theta : float
Component theta in vector
a : float
Any constant
Returns
-------
float
The value sqrt(r^2 + a^2 * cos^2(theta))
"""
return np.sqrt((r ** 2) + ((a * np.cos(theta)) ** 2))
[docs]def delta(
r, M, a, Q, c=constant.c.value, G=constant.G.value, Cc=constant.coulombs_const.value
):
"""
Returns the value r^2 - Rs * r + a^2
Specific to Boyer-Lindquist coordinates
Parameters
----------
r : float
Component r in vector
M : float
Mass of the massive body
a : float
Any constant
Q : float
Charge on the massive body
c : float
Speed of light
G : float
Gravitational constant
Cc : float
Coulomb's constant
Returns
-------
float
The value r^2 - Rs * r + a^2 + Rq^2
"""
Rs = utils.schwarzschild_radius_dimensionless(M, c, G)
return (r ** 2) - (Rs * r) + (a ** 2) + (charge_length_scale(Q, c, G, Cc) ** 2)
[docs]def metric(
r,
theta,
M,
a,
Q,
c=constant.c.value,
G=constant.G.value,
Cc=constant.coulombs_const.value,
):
"""
Returns the Kerr-Newman Metric
Parameters
----------
r : float
Distance from the centre
theta : float
Angle from z-axis
M : float
Mass of the massive body
a : float
Black Hole spin factor
Q : float
Charge on the massive body
c : float
Speed of light
G : float
Gravitational constant
Cc : float
Coulomb's constant
Returns
-------
~numpy.array
Numpy array of shape (4,4)
"""
m = np.zeros((4, 4), dtype=float)
rh2, dl = rho(r, theta, a) ** 2, delta(r, M, a, Q, c, G, Cc)
c2 = c ** 2
# set the diagonal/off-diagonal terms of metric
m[0, 0] = (dl - ((a * np.sin(theta)) ** 2)) / (rh2)
m[1, 1] = -rh2 / (dl * c2)
m[2, 2] = -rh2 / c2
m[3, 3] = (
(((a * np.sin(theta)) ** 2) * dl - ((r ** 2 + a ** 2) ** 2))
* (np.sin(theta) ** 2)
/ (rh2 * c2)
)
m[0, 3] = m[3, 0] = (
-a * (np.sin(theta) ** 2) * (dl - (r ** 2) - (a ** 2)) / (rh2 * c)
)
return m
[docs]def metric_inv(
r,
theta,
M,
a,
Q,
c=constant.c.value,
G=constant.G.value,
Cc=constant.coulombs_const.value,
):
"""
Returns the inverse of Kerr-Newman Metric
Parameters
----------
r : float
Distance from the centre
theta : float
Angle from z-axis
M : float
Mass of the massive body
a : float
Black Hole spin factor
Q : float
Charge on the massive body
c : float
Speed of light
G : float
Gravitational constant
Cc : float
Coulomb's constant
Returns
-------
~numpy.array
Numpy array of shape (4,4)
"""
return np.linalg.inv(metric(r, theta, M, a, Q, c, G, Cc))
[docs]def dmetric_dx(
r,
theta,
M,
a,
Q,
c=constant.c.value,
G=constant.G.value,
Cc=constant.coulombs_const.value,
):
"""
Returns differentiation of each component of Kerr-Newman metric tensor w.r.t. t, r, theta, phi
Parameters
----------
r : float
Distance from the centre
theta : float
Angle from z-axis
M : float
Mass of the massive body
a : float
Black Hole spin factor
Q : float
Charge on the massive body
c : float
Speed of light
G : float
Gravitational constant
Cc : float
Coulomb's constant
Returns
-------
dmdx : ~numpy.array
Numpy array of shape (4,4,4)
dmdx[0], dmdx[1], dmdx[2] & dmdx[3] is differentiation of metric w.r.t. t, r, theta & phi respectively
"""
Rs = utils.schwarzschild_radius_dimensionless(M, c, G)
dmdx = np.zeros((4, 4, 4), dtype=float)
rh2, dl = rho(r, theta, a) ** 2, delta(r, M, a, Q, c, G, Cc)
c2 = c ** 2
# metric is invariant on t & phi
# differentiation of metric wrt r
def due_to_r():
nonlocal dmdx
drh2dr = 2 * r
dddr = 2 * r - Rs
dmdx[1, 0, 0] = (dddr * rh2 - drh2dr * (dl - (a * np.sin(theta)) ** 2)) / (
rh2 ** 2
)
dmdx[1, 1, 1] = (-1 / (c2 * (dl ** 2))) * (drh2dr * dl - dddr * rh2)
dmdx[1, 2, 2] = -drh2dr / c2
dmdx[1, 3, 3] = ((np.sin(theta) ** 2) / (c2 * (rh2 ** 2))) * (
(
(((a * np.sin(theta)) ** 2) * dddr - 4 * (r ** 3) - 4 * (r * (a ** 2)))
* rh2
)
- (drh2dr * (((a * np.sin(theta)) ** 2) * dl - ((r ** 2 + a ** 2) ** 2)))
)
dmdx[1, 0, 3] = dmdx[1, 3, 0] = (
(-a) * (np.sin(theta) ** 2) / (c * (rh2 ** 2))
) * ((dddr - 2 * r) * rh2 - drh2dr * (dl - r ** 2 - a ** 2))
# differentiation of metric wrt theta
def due_to_theta():
nonlocal dmdx
drh2dth = -2 * (a ** 2) * np.cos(theta) * np.sin(theta)
dmdx[2, 0, 0] = (
(-2 * (a ** 2) * np.sin(theta) * np.cos(theta)) * rh2
- drh2dth * (dl - ((a * np.sin(theta)) ** 2))
) / (rh2 ** 2)
dmdx[2, 1, 1] = -drh2dth / (c2 * dl)
dmdx[2, 2, 2] = -drh2dth / c2
dmdx[2, 3, 3] = (1 / (c2 * (rh2 ** 2))) * (
(
(
(4 * (a ** 2) * (np.sin(theta) ** 3) * np.cos(theta) * dl)
- (2 * np.sin(theta) * np.cos(theta) * ((r ** 2 + a ** 2) ** 2))
)
* rh2
)
- (
drh2dth
* (((a * np.sin(theta)) ** 2) * dl - ((r ** 2 + a ** 2) ** 2))
* (np.sin(theta) ** 2)
)
)
dmdx[2, 0, 3] = dmdx[2, 3, 0] = (
(-a * (dl - r ** 2 - a ** 2)) / (c * (rh2 ** 2))
) * (
(2 * np.sin(theta) * np.cos(theta) * rh2) - (drh2dth * (np.sin(theta) ** 2))
)
due_to_r()
due_to_theta()
return dmdx
[docs]def christoffels(
r,
theta,
M,
a,
Q,
c=constant.c.value,
G=constant.G.value,
Cc=constant.coulombs_const.value,
):
"""
Returns the 3rd rank Tensor containing Christoffel Symbols for Kerr-Newman Metric
Parameters
----------
r : float
Distance from the centre
theta : float
Angle from z-axis
M : float
Mass of the massive body
a : float
Black Hole spin factor
Q : float
Charge on the massive body
c : float
Speed of light
G : float
Gravitational constant
Cc : float
Coulomb's constant
Returns
-------
~numpy.array
Numpy array of shape (4,4,4)
"""
invg = metric_inv(r, theta, M, a, Q, c, G, Cc)
dmdx = dmetric_dx(r, theta, M, a, Q, c, G, Cc)
chl = np.zeros(shape=(4, 4, 4), dtype=float)
for _, k, l in nonzero_christoffels_list[0:4]:
val1 = dmdx[l, 0, k] + dmdx[k, 0, l]
val2 = dmdx[l, 3, k] + dmdx[k, 3, l]
chl[0, k, l] = chl[0, l, k] = 0.5 * (invg[0, 0] * (val1) + invg[0, 3] * (val2))
chl[3, k, l] = chl[3, l, k] = 0.5 * (invg[3, 0] * (val1) + invg[3, 3] * (val2))
for i, k, l in nonzero_christoffels_list[8:16]:
chl[i, k, l] = 0.5 * (
invg[i, i] * (dmdx[l, i, k] + dmdx[k, i, l] - dmdx[i, k, l])
)
for i, k, l in nonzero_christoffels_list[16:20]:
chl[i, k, l] = chl[i, l, k] = 0.5 * (
invg[i, i] * (dmdx[l, i, k] + dmdx[k, i, l] - dmdx[i, k, l])
)
return chl
[docs]def em_potential(
r,
theta,
a,
Q,
M,
c=constant.c.value,
G=constant.G.value,
Cc=constant.coulombs_const.value,
):
"""
Returns a 4-d vector(for each component of 4-d space-time) containing the electromagnetic potential around a Kerr-Newman body
Parameters
----------
r : float
Distance from the centre
theta : float
Angle from z-axis
a : float
Black Hole spin factor
Q : float
Charge on the massive body
M : float
Mass of the massive body
c : float
Speed of light
G : float
Gravitational constant
Cc : float
Coulomb's constant
Returns
-------
~numpy.array
Numpy array of shape (4,)
"""
rq, rh2, c2 = charge_length_scale(Q, c, G, Cc), rho(r, theta, a) ** 2, c ** 2
vec = np.zeros((4,), dtype=float)
vec[0] = r * rq / rh2
vec[3] = ((-c2) / (rh2 * G * M)) * a * r * rq * (np.sin(theta) ** 2)
return vec
[docs]def maxwell_tensor_covariant(
r,
theta,
a,
Q,
M,
c=constant.c.value,
G=constant.G.value,
Cc=constant.coulombs_const.value,
):
"""
Returns a 2nd rank Tensor containing Maxwell Tensor with lower indices for Kerr-Newman Metric
Parameters
----------
r : float
Distance from the centre
theta : float
Angle from z-axis
a : float
Black Hole spin factor
Q : float
Charge on the massive body
M : float
Mass of the massive body
c : float
Speed of light
G : float
Gravitational constant
Cc : float
Coulomb's constant
Returns
-------
~numpy.array
Numpy array of shape (4,4)
"""
c2 = c ** 2
m = np.zeros((4, 4), dtype=float)
rh2, rq = rho(r, theta, a) ** 2, charge_length_scale(Q, c, G, Cc)
drh2dr, drh2dth = 2 * r, -2 * (a ** 2) * np.cos(theta) * np.sin(theta)
# set the tensor terms
m[0, 1] = ((-rq) / (rh2 ** 2)) * (rh2 - drh2dr * r)
m[0, 2] = r * rq * drh2dth / (rh2 ** 2)
m[3, 1] = (c2 * a * rq * (np.sin(theta) ** 2) / (G * M * (rh2 ** 2))) * (
rh2 - r * drh2dr
)
m[3, 2] = (c2 * a * rq * r / (G * M * (rh2 ** 2))) * (
(2 * np.sin(theta) * np.cos(theta) * rh2) - (drh2dth * (np.sin(theta) ** 2))
)
for i, j in [(0, 1), (0, 2), (3, 1), (3, 2)]:
m[j, i] = -m[i, j]
return m
[docs]def maxwell_tensor_contravariant(
r,
theta,
a,
Q,
M,
c=constant.c.value,
G=constant.G.value,
Cc=constant.coulombs_const.value,
):
"""
Returns a 2nd rank Tensor containing Maxwell Tensor with upper indices for Kerr-Newman Metric
Parameters
----------
r : float
Distance from the centre
theta : float
Angle from z-axis
a : float
Black Hole spin factor
Q : float
Charge on the massive body
M : float
Mass of the massive body
c : float
Speed of light
G : float
Gravitational constant
Cc : float
Coulomb's constant
Returns
-------
~numpy.array
Numpy array of shape (4,4)
"""
mcov = maxwell_tensor_covariant(r, theta, a, Q, M, c, G, Cc)
ginv = metric_inv(r, theta, M, a, Q, c, G, Cc)
# contravariant F = contravariant g X covariant F X transpose(contravariant g)
# but g is symettric
return np.matmul(np.matmul(ginv, mcov), ginv)
[docs]@u.quantity_input(mass=u.kg, Q=u.C)
def kerrnewman_time_velocity(pos_vec, vel_vec, mass, a, Q):
"""
Velocity of coordinate time wrt proper metric
Parameters
----------
pos_vector : ~numpy.array
Vector with r, theta, phi components in SI units
vel_vector : ~numpy.array
Vector with velocities of r, theta, phi components in SI units
mass : ~astropy.units.kg
Mass of the body
a : float
Any constant
Q : ~astropy.units.C
Charge on the massive body
Returns
-------
~astropy.units.one
Velocity of time
"""
_scr = utils.schwarzschild_radius(mass).value
Qc = Q.to(u.C)
g = metric(pos_vec[0], pos_vec[1], mass.value, a, Qc.value)
A = g[0, 0]
B = 2 * g[0, 3]
C = (
g[1, 1] * (vel_vec[0] ** 2)
+ g[2, 2] * (vel_vec[1] ** 2)
+ g[3, 3] * (vel_vec[2] ** 2)
- 1
)
D = (B ** 2) - (4 * A * C)
vt = (B + np.sqrt(D)) / (2 * A)
return vt * u.one
