Contents

## INTRODUCTION

The light deflection by a Kerr’s black hole, that is in an axially symmetric gravitational field created by a spinning black hole, is a phenomenon predicted by general relativity (Kerr’s metric).

This metric generalizes the spherical symmetry of a stationary black hole (Schwarzschild’s metric). Refer to appendix for detailed calculations and additional information.*Note: to avoid any confusion, the writing simplification \(c=G=1\) is not used in the present document and all equations are written explicitly.*

## GENERAL RELATIVITY – KERR’S METRIC

The elementary displacement of the photon is a like-light vector and its scalar product is zero.^{1}

Assuming that the gravitational field is axially symmetrical and applying the Kerr’s metric (see its limits in the conclusion), the scalar product of the elementary displacement \(\overrightarrow{ds}\) \((cdt, dr, d\varphi, d\theta)\) in Boyer-Lindquist’s coordinates can be written:

\(ds^2=(1-\frac{2GM}{c^2\Sigma})c^2dt^2-\frac{4GMar\sin^2{\theta}}{c^2\Sigma}cdtd\varphi+\frac{\Sigma}{\Delta}dr^2+\Sigma d\theta^2\)

\(+(r^2+a^2+\frac{2GMa^2r\sin^2{\theta}}{c^2\Sigma})\sin^2{\theta}d\varphi^2=0\)^{2}.

with \(G\) gravitational constant, \(c\) speed of light in a vacuum, \(M\) mass of the black hole, \(a=\frac{J}{cM}\) with \(J\) spin angular momentum of the black hole, \(\Delta=r^2-\frac{2GM}{c^2}r+a^2\) and \(Σ=r^2+a^2\cos^2{\theta}\).

The coefficients of the metric are independent of \(t\) and \(\varphi\): the geometry of Kerr spacetime is therefore stationary and axially symmetrical.

Note: in the asymptotic region \(r \gg \frac{2GM}{c^2}\), the coordinate \(r\) is interpreted as the physical distance between the photon and the center of the black hole.

### Parametric equations of motion

The invariance of the energy \(\varepsilon\), the angular momentum component \(l_z\) on the spin axis of the black hole and the Carter’s constant \(Q\) enables to get the four parametric equations of motion of the photon and to calculate the light deflection by Kerr’s black holes:

\(\left(\frac{dr}{d\lambda}\right)^2=\left(\left(r^2+a^2-ac\frac{l_z}{\varepsilon}\right)^2-\Delta\left(\left(a-c\frac{l_z}{\varepsilon}\right)^2+c^2\frac{Q}{\varepsilon^2}\right)\right)\frac{\varepsilon^2}{ c^2\Sigma^2}\)

\(\left(\frac{d\theta}{d\lambda}\right)^2=\left(c^2\frac{Q}{\varepsilon^2}+\cos^2\theta\left(a^2-c^2\frac{l_z^2}{\varepsilon^2\sin^2\theta}\right)\right)\frac{\varepsilon^2}{ c^2\Sigma^2}\)

\(\frac{d\varphi}{d\lambda}=\left(2mar+(\Sigma-2mr)c\frac{l_z}{\varepsilon\sin^2\theta}\right)\frac{\varepsilon}{c\Delta\Sigma}\)

\(\frac{dct}{d\lambda}=\left((r^2+a^2)^2-\Delta a^2\sin^2\theta-2mar\ c\frac{l_z}{\varepsilon}\right)\frac{\varepsilon}{c\Delta\Sigma}\)

with \(r\) radial coordinate, \(\theta\) colatitude, \(\varphi\) longitude, \(t\) time measured by a static observer, \(\lambda\) an affine parameter and \(m=\frac{GM}{c^2}\) homogeneous to the metre.

Note that the coordinate system is undefined at the poles \(\theta=0\) and \(\theta=\pi\).

In the following, the value \(R_s=2m\) and the dimensionless values \(\bar{r}=\frac{r}{m}\) and Kerr’s parameter \(\bar{a}=\frac{a}{m}\) will be used.

\(\)

When \(\bar{a}>0\), the spin of the black hole is trigonometric and when \(\bar{a}<0\), its spin is clockwise.

It is assumed that \(|\bar{a}|\) lies between \(0\) and \(1\), limits included, except for the over extreme Kerr’s spacetime described briefly before the conclusion.

### Photon trajectories

In the general case, photon trajectories near a spinning black hole can be found by integration of each of the 4 parametric equations, according to the affine parameter \(\lambda\).

The initial values to be taken into account are \(r_0\), \(\theta_0\), \(\varphi_0\), \(t_0\), and the signs of \(\frac{dr}{d\lambda}_0\) and\(\frac{d\theta}{d\lambda}_0\).

The trajectory of the photon is fully determined by the constants \(M\), \(a\), \(\frac{l_z}{\varepsilon}\) and \(\frac{Q}{\varepsilon^2}\).

For a given value of \(\frac{l_z}{\varepsilon}\), there is a critical value of \(\frac{Q}{\varepsilon^2}\):

– if \(\frac{Q}{\varepsilon^2}>\frac{Q_{crit}}{\varepsilon^2}\) the photon coming from \(\infty\) will be deflected by the black hole and continue towards \(\infty\),

– if \(\frac{Q}{\varepsilon^2}<\frac{Q_{crit}}{\varepsilon^2}\) the photon coming from \(\infty\) will be absorbed by the black hole,

– if \(\frac{Q}{\varepsilon^2}=\frac{Q_{crit}}{\varepsilon^2}\) the photon coming from \(\infty\) will be captured by the black hole on an orbit.

### Photon orbits

A constant radial coordinate \(r\) is given by annulling the potential of the first parametric equation and its derivative with respect to \(r\), which leads after calculation to \(\bar{r}\) being a root of the polynomial

\(q(\bar{r})=\bar{r}^5-3\bar{r}^4+2\bar{a}^2\bar{r}^3\sin^2i-2\bar{a}^2\bar{r}^2+\bar{a}^4\bar{r}\sin^2i+\bar{a}^4\sin^2i\)

\(+2\bar{a}\bar{r}\cos i\sqrt{3\bar{r}^4+(1-3\sin^2i)\bar{a}^2\bar{r}^2-\bar{a}^4\sin^2i}\)

\(i\) being the constant angle of inclination of the angular momentum \(\overrightarrow{l}\) with respect to the spin axis of the black hole.

For given \(m\), \(a\) and \(i\), there are at least one root \(\bar{r}\) between 0 and 4 giving an orbit for the photon.

If \(i\in[0,\pi/2[\) the orbit is prograde (same direction of spin as the black hole), and if \(i\in ]\pi/2,\pi]\) the orbit is retrograde (opposite direction of spin to the black hole).

The roots of the polynomial \(q(\bar{r})\) are difficult to calculate analytically except in the following cases:

– equatorial prograde orbit (\(\cos i=1\)) \(\Rightarrow\bar{r}_{prograde}=2\left (1+\cos\left (\frac{2}{3}\arccos\left(-\bar{a}\right)\right)\right)\),

– equatorial retrograde orbit (\(\cos i=-1\)) \(\Rightarrow\bar{r}_{retrograde}=2\left (1+\cos\left (\frac{2}{3}\arccos\left(\bar{a}\right)\right)\right)\),

– polar orbit (\(\sin^2i=1\)) \(\Rightarrow\bar{r}_{polar}=1+2\sqrt{1-\frac{\bar{a}^2}{3}}\cos\left(\frac{1}{3}\arccos\left(\frac{1-\bar{a}^2}{\left(1-\frac{\bar{a}^2}{3}\right)^\frac{3}{2}}\right)\right)\)

The event horizon of a spinning black hole has the dimensionless radial coordinate \(\bar{r_h}=1+\sqrt{1-\bar{a}^2}\).

Note: when \(\bar{a}=0\), the above formulas lead to the special case of the Schwarzschild’s metric \(\Rightarrow\bar{r}_{prograde}=\bar{r}_{retrograde}=\bar{r}_{polar}=3\) that is \(r=3m=\frac{3}{2}R_s\) and \(r_h=2m=R_s\) with \(R_s=\frac{2GM}{c^2}\).

Each orbit is defined by its constant dimensionless radial coordinate \(\bar{r}\) and by the inclination \(i\) (of the angular momentum \(\overrightarrow{l}\) of the photon) associated with this value.

**General**

Each orbit is defined by its constant dimensionless radial coordinate value \(\bar{r_c}\) and by the constant inclination \(i\) of the angular momentum of the photon \(\overrightarrow{l}\), associated with this value.

There are therefore an infinite number of photon « spheres » with constant dimensionnless radial coordinates \(\bar{r}\in[0,4]\), the bound 4 being reached for \(\bar{a}=\pm\ 1\).

Furthermore, the geometric shape of each orbit is not really a sphere, but an ellipsoid of radius \(\sqrt{r^2+a^2}\sin\theta\) (in Cartesian Boyer-Lindquist’s coordinates) and colatitude \(\theta\) between a value \(\theta_{lim}\) and a value \(\pi-\theta_{lim}\), a function of \(\bar{a},\bar{r_c}\) and \(\sin^2i\).

**Parametric equations**

With \(l_z=l\cos i\), \(Q=l^2\sin^2i\) and defining the impact parameter \(b_{crit}=c\frac{l}{\varepsilon}\), the equations of the trajectory are:

\(r=r_c\)

with the 3 parametric equations which become:

\(\left(\frac{d\theta}{d\lambda}\right)^2=\left(b_{crit}^2\left(1-\frac{\cos^2i}{\sin^2\theta}\right)+a^2\cos^2\theta\right)\frac{\varepsilon^2}{\Sigma^2 c^2}\)

\(\frac{d\varphi}{d\lambda}=\left(2mar_c+\left(\Sigma-2mr_c\right)b_{crit}\frac{\cos i}{\sin^2\theta}\right)\frac{\varepsilon}{\Delta\Sigma c}\)

\(\frac{cdt}{d\lambda}=\left(\left(r_c^2+a^2\right)^2-\Delta a^2 \sin^2\theta-2mar_cb_{crit}\cos i\right)\frac{\varepsilon}{\Delta\Sigma c}\)

The value of the critical impact parameter can be calculated using the formula:

\(b_{crit}=m\sqrt{\frac{3\bar{r_c}^4+\bar{a}^2\bar{r_c}^2}{\bar{r_c}^2-\bar{a}^2\sin^2i}}\).

### Animated trajectories

Examples of photon trajectories with near-capture by an extreme Kerr black hole

Examples of photon orbits with different \(\bar{a}\) and \(\frac{cl_z}{m\varepsilon}\)

## DESCRIPTION OF BLACK HOLE REGIONS, SINGULARITIES AND SHADOW

A Kerr’s black hole mathematically has four centered regions, each included in the other and defined by mathematical hypersurfaces. From largest to smallest:

– outer ergosphere \(r_{ergoext}=m (1+\sqrt{1-\bar{a}^2\cos^2\theta})\)

– event horizon \(r_h=m (1+\sqrt{1-\bar{a}^2})\)

– Cauchy’s horizon \(r_{Cauchy}=m (1-\sqrt{1-\bar{a}^2})\)

– inner ergosphere \(r_{ergoint}=m (1-\sqrt{1-\bar{a}^2\cos^2\theta})\).

\(r_{ergoint}\) and \(r_{ergoext}\) are the roots of the equation \(\Sigma-2mr=0\) and \(r_{Cauchy}\) and \(r_h\) are the roots of the equation \(\Delta=0\).

For \(|\bar{a}|\in ]0,1[\), the four regions are distinct, and for \(\bar{a}=\pm\ 1\), the event horizon and Cauchy’s horizon are merged.

\(\bar{a}=0\) corresponds to the Schwarzschild’s black hole, where the event horizon and outer ergosphere are merged, and there is no Cauchy’s horizon or inner ergosphere.

The hypersurfaces delimiting ergospheres are stationarity limits and infinite redshift surfaces.

Note: once it has crossed the event horizon, any object (particle or photon) can return to it, but can never cross it in the other direction.

### Presence of regions

The central body is by definition a Kerr’s black hole, so the two regions defined by the outer ergosphere and the event horizon (merged with the Cauchy’s horizon for an extreme Kerr black hole) physically exist.

The other regions (defined by the Cauchy’s horizon for a non-extreme Kerr’s black hole and by the inner ergosphere) can only exist if the physical body of the black hole is « inside » them.

### Singularities

The parametric equations seen above show that a zero value of either \(\Delta\) or \(\Sigma\) does not give a definition of the motion of the photon.

\(\Delta=r^2-\frac{2GM}{c^2}r+a^2= 0\) occurs when the photon crosses the event horizon or the Cauchy’s horizon: it is a simple singularity of the Boyer-Lindquist’s coordinates, which generalizes the singularity of the Schwarzschild’s coordinates in \(r=R_s\)^{3}.

The singularity in \(r\) such as \(Σ=r^2+a^2\cos^2{\theta}=0\) is a true singularity, just as the singularity in \(r=0\) of the Schwarzschild’s metric^{4}.

This is the circle of Cartesian radius \(|a|\) whose center is that of the black hole, located in its equatorial plane. This circle borders the inner ergosphere.

### Apparent image or shadow

For a given inclination angle \(i\), no photon with an impact parameter below \(b_{crit}\) can reach an outside observer.

If this observer is located at a great distance from the black hole and in its equatorial plane, the apparent outline of a Kerr black hole can be determined by the 2 coordinates^{5}:

\(\alpha=-c\frac{l_z}{\varepsilon}\) and \(\beta=\pm c\frac{\sqrt{Q}}{\varepsilon}\), that is:

\(\frac{\alpha}{m}=\frac{\bar{r_c}^3-3\bar{r_c}^2+\bar{a}^2\bar{r_c}+\bar{a}^2}{\bar{a}(\bar{r_c}-1)}\) and \(\frac{\beta}{m}=\pm\sqrt{\frac{-\bar{r_c}^3(\bar{r_c}^3-6\bar{r_c}^2+9\bar{r_c}-4\bar{a}^2)}{\bar{a}^2(\bar{r_c}-1)^2}}\)

with \(\bar{r_c}\) dimensionless radial coordinates of photon orbits varying between a value \(\bar{r}_{c_{min}}\) and a value \(\bar{r}_{c_{max}}\).

### Over extreme Kerr’s spacetime

When \(|\bar{a}|>1\), the Kerr’s spacetime is said to be over extreme and \(\Delta\) has no root, so there is no event horizon or Cauchy’s horizon, implying that the massive object is not a black hole. It has a naked singularity (circle of Cartesian radius \(|a|\)) with adjacent outer and inner ergospheres that form a kind of open torus.

It is a mathematical object whose physical existence is currently highly unlikely.

## CONCLUSION

As most celestial objects rotate on themselves, the axially symmetric Kerr’s metric provides an absolutely accurate representation of the countless black holes that populate the universe, the Schwarzschild’s metric being a special case, obtained with a zero Kerr’s parameter.

The structure of a spinning black hole is extremely simple: just two real numbers, *m *and *a*, are needed to describe it fully.

The light deflection by Kerr’s black holes and the trajectories or orbits of photons can be precisely calculated using the Kerr’s metric.

Note that this metric does not apply to a spinning star: its metric cannot be described by just a few scalar parameters, even outside the star. It depends on the distribution of mass and momentum inside the star.

- https://luth.obspm.fr/~luthier/gourgoulhon/fr/master/relatM2.pdf ↩︎
- https://luth.obspm.fr/~luthier/gourgoulhon/fr/master/relatM2.pdf ↩︎
- https://luth.obspm.fr/~luthier/gourgoulhon/fr/master/relatM2.pdf ↩︎
- https://luth.obspm.fr/~luthier/gourgoulhon/fr/master/relatM2.pdf ↩︎
- https://arxiv.org/pdf/2105.07101 ↩︎