The Kerr Metric for Rotating, Electrically Neutral Black Holes:: The Kerr Metric for Rotating, Electrically Neutral Black Holes: The Most Common Case of Black Hole Geometry Ben Criger and Chad Daley
Assumptions: Assumptions Non-zero angular momentum
Insignificant charge
Axial symmetry
No-Hair Theorem
Derivation (Abridged): Derivation (Abridged) Null Tetrad: Any set of four vectors (one timelike and three spacelike such as m) for which the null condition (defined below) is met. Frame Metric: Where is our metric of choice, and is one of the null vectors in our tetrad.
But What’s the Point?: But What’s the Point? To represent any metric in null tetrad / frame metric form.
Meaningful Example: Schwarzschild Metric
Slide5: We perform an ingenious substitution of co-ordinates and obtain the following null vectors as valid for a new metric: We use the process detailed in the previous slides to find the metric for these null vectors. Drumroll please . . . .
The Kerr Metric in Boyer-Lindquist Co-ordinates: The Kerr Metric in Boyer-Lindquist Co-ordinates We present the Kerr Metric in Boyer-Lindquist co-ordinates (first, we present BL co-ordinates here in comparison with spherical co-ordinates): Where and
However. . . : However. . . We don’t have any physical intuition at this point about the metric!
We will have to prove (or convince ourselves) that a is an angular momentum parameter, etc.
We start by setting this parameter a = 0 and seeing what happens. Now, we can say with confidence that a represents angular momentum (and has dimensions of radius).
Nothing Succeeds Like Success: Nothing Succeeds Like Success Now, we try removing m from the equation, and leaving a fixed a. This metric may look deceptively complex, but this is simply the expression of flat space in Boyer-Lindquist Co-ordinates.
Here, we have confirmed that m and a are what they appear to be, and that our metric (chosen through a convenient, if unintuitive method) is a valid solution for rotating black holes.
Just one more thing. . .: Just one more thing. . . We need to prove that the metric is flat at infinity. We recover flat space in spherical co-ordinates. We have effectively argued that this metric is valid, and we can apply this method to the Reissner-Nordstrom metric to obtain. . .
The Kerr-Newman Metric: The Kerr-Newman Metric Represents a rotating, charged, black hole
Can devolve to any of the Schwarzschild, Kerr, or Reissner-Nordstrom metrics. We use the following definitions:
Singularities and Horizons: Singularities and Horizons 2 categories, essential and coordinate
Schwarzschild Solution
Essential singularity at r = 0
Event horizon at Schwarzschild radius, r = 2m
Reissner-Nordström Solution
Retain essential singularity at r = 0
0 - 2 coordinate singularities at
Horizons and Singularities of the Kerr Metric: Horizons and Singularities of the Kerr Metric Looking at our metric we find an essential singularity for;
Remembering the definitions of our co-ordinates we find;
This corresponds to a ring of radius a
Horizons and Singularities for a2 < M2: Horizons and Singularities for a2 < M2 A surface of infinite gravitational red shift can be determined by;
Setting a = 0, or θ = π/2 these reduce to;
Horizons and Singularities Con’t…: Horizons and Singularities Con’t… We can also recover two event horizons setting the radial coefficient to zero
In the case of a = 0, these surfaces reduce to:
Summary of Kerr Geometry (a2 < m2): Summary of Kerr Geometry (a2 < m2) Essential ring singularity at:
Two surfaces of infinite
red shift at :
Two event horizons
at: Particle
Possible Energy Source?: Possible Energy Source? Equip yourself with a large mass
Within the ergophere throw the mass against the rotation
Upon exiting the ergosphere you will have gained energy
A word about cases of a2 ≥ m2: A word about cases of a2 ≥ m2 For a2 > m2 we find only the essential singularity at r = 0
This naked singularity violates Penrose’s cosmic censorship hypothesis
The solution for a2 = m2 is unstable
Observed Black Holes: Observed Black Holes Cygnus X-1 widely accepted as the first observed black hole candidate
Jets observed companioning black holes termed quasars
Slide19: Fin.