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What are physics constant in Kerr metric?

  1. Jan 10, 2011 #1
    1. What are the value of physics constant in Kerr metric, including G, M, c, a, r, or others?
    I expect to simplify Gamma

    2. why g_compts[1,4] has element and not [4,1]?

    3. Some book assume G = c = 1, what is the meaning of this setting?

    4. Different material have different metric, are there a metric table for element table?

    5. What is theta in Kerr metric?

    ************** Kerr metric *****************
    t r theta phi
    t
    r
    theta
    phi

    with(tensor):
    coord := [t, r, theta, Phi]:

    g_compts:=array(sparse,1..4,1..4):

    G := 6.67*10^(-11)

    triangle := r^2 - 2*G*M*r/c^2 + a^2:
    p2 := r^2 + ((cos(theta))^2)*a^2:
    A := (r^2+a^2)^2 - (a^2)*triangle*(sin(theta))^2:

    g_compts[1,1]:= (triangle - (a^2)*(sin(theta))^2)*(c^2)/p2:
    g_compts[1,4]:= 4*G*M*a*r*(sin(theta))^2/(c*p2):
    g_compts[2,2]:= -p2/triangle:
    g_compts[3,3]:= -p2:
    g_compts[4,4]:= -A*(sin(theta)^2)/(p2):

    g1 := create([-1,-1], eval(g_compts)):
    g1_inv := invert( g1, 'detg' ):

    D1g := d1metric( g1, coord ):

    Cf1_1 := Christoffel1(D1g):
    Cf2_1 := Christoffel2(g1_inv, Cf1_1):
    displayGR(Christoffel2,Cf2_1):
     
  2. jcsd
  3. Jan 19, 2011 #2

    stevebd1

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    Gold Member

    G is the gravitational constant in units m^3/(kg s^2) (1 in geometric units)

    m is mass in kg where M is the geometric unit for mass (M=Gm/c^2) in metres

    c is the speed of light in m/s (or 1 in geometric units)

    a is the geometric units for angular momentum in metres (a=J/mc where J is angular momentum in SI units)

    r is radius in metres

    Delta (or triangle as you call it) is the radial parameter in m^2.

    when writing delta, you have written delta=r^2-2*G*m*r/c^2+a^2. If geometric units are used, you can simply write delta=r^2-2M+a^2 where M=*G*m*r/c^2, the answers are the same.

    g_compts[1,4] does include for [4,1], they've just substituted the 2*(2*.. with a 4*.., it can be rewritten-

    g_compts[1,4]=2*(2*M*a*r*(sin(theta))^2/(p2)), [1,4] & [4,1] being the same, another way of writing it is 2*g_compts[1,4].

    theta is the latitude approach, 90 degrees (or pi/2) at the equator and 0 at the poles.

    You may also find this web page useful-
    http://www.astro.ku.dk/~milvang/RelViz/000_node12.html
     
  4. Jan 20, 2011 #3

    stevebd1

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    Gold Member

    The text above relating to delta should read-

    'when writing delta, you have written delta=r^2-2*G*m*r/c^2+a^2. If geometric units are used, you can simply write delta=r^2-2Mr+a^2 where M=G*m/c^2, the answers are the same.'
     
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