The most energetic physical processes in the universe (shy of the Big Bang itself) are astrophysical jets. These are relativistic beams of ions and radiation that shoot out across intergalactic space, emitting nearly the full spectrum of electromagnetic radiation, seen as quasars (quasi-stellar objects) that are thought to originate from supermassive black holes at the center of distant galaxies. The most powerful jets emit more energy than the light from a thousand Milky Way galaxies.
Where can such astronomical amounts of energy come from?
Black Hole Accretion Disks
The potential wells of black holes are so deep and steep, that they attract matter from their entire neighborhood. If a star comes too close, the black hole can rip the hydrogen and helium atoms off the star’s surface and suck them into a death spiral that can only end in oblivion beyond the Schwarzschild radius.
However, just before they disappear, these atoms and ions make one last desperate stand to resist the inevitable pull, and they park themselves near an orbit that is just stable enough that they can survive many orbits before they lose too much energy, through collisions with the other atoms and ions, and resume their in-spiral. This last orbit, called the inner-most stable circular orbit (ISCO), is where matter accumulates into an accretion disk.
Fig. 1 Artist’s rendering of a black hole pulling matter from a near-by star where it accumulates in the accretion disk just outside the black hole Schwarzschild radius. (Credit: Wikipedia)
Fig. 2 The famous first image of the black hole in M87 galaxy made by the Event Horizon Telescope collaboration. The bright ring surrounding the “shadow” is the light emitted from the accretion disk.
Fig. 3 Explanation of the image of the accretion disk around a black hole. (You have to watch the simulations at NASA.)
The Innermost Stable Circular Orbit (ISCO)
At what radius is the inner-most stable circular orbit? To find out, write the energy equation of a particle orbiting a black hole with an effective potential function as
where the effective potential is
The first two terms of the effective potential are the usual Newtonian terms that include the gravitational potential and the repulsive contribution from the angular momentum that normally prevents the mass from approaching the origin. The third term is the GR term that is attractive and overcomes the centrifugal barrier at small values of r, allowing the orbit to collapse to the center. This is the essential danger of orbiting a black hole—not all orbits around a black hole are stable, and even circular orbits will decay and be swallowed up if too close to the black hole.
To find the conditions for circular orbits, take the derivative of the effective potential and set it to zero
This is a quadratic equation that can be solved for r. There is an innermost stable circular orbit (ISCO) that is obtained when the term in the square root of the quadratic formula vanishes when the angular momentum satisfies the condition
which gives the simple result for the inner-most circular orbit as
Therefore, no particle can sustain a circular orbit with a radius closer than three times the Schwarzschild radius. Inside that, it will spiral into the black hole.
A single trajectory solution to the GR flow [1] is shown in Fig. 4. The particle begins in an elliptical orbit outside the innermost circular orbit and is captured into a nearly circular orbit inside the ISCO. This orbit eventually decays and spirals with increasing speed into the black hole. Accretion discs around black holes occupy these orbits before collisions cause them to lose angular momentum and spiral into the black hole.
Fig. 4 Orbital simulation for a particle falling starting in an elliptical orbit near a black hole. In these units, Rs = 0.15 and ISCO = 0.44. A particle that begins with an ellipticity settles into a nearly circular orbit near the ISCO, after which it spirals into the black hole. (Reprinted from Introduction to Modern Dynamics)
The gravity of black holes is so great, that even photons can orbit black holes in circular orbits. The radius or the circular photon orbit defines what is known as the photon sphere. The radius of the photon sphere is RPS = 1.5RS, which is just a factor of 2 smaller than the ISCO.
Binding Energy of a Particle at the ISCO
So where does all the energy come from to power astrophysical jets? The explanation comes from the binding energy of a particle at the ISCO. The energy conservation equation including angular momentum for a massive particle of mass m orbiting a black hole of mass M is
where the term on the right is the kinetic energy of the particle at infinity, and the second and third terms on the left are the effective potential
Solving for the binding energy at the ISCO gives
Therefore, 6% of the rest energy of the object is given up when it spirals into the ISCO. Remember that the fusion of two hydrogen atoms into helium gives up only about 0.7% of its rest mass energy. Therefore, the energy emission per nucleon for an atom falling towards the ISCO is TEN times more efficient than nuclear fusion!
This incredible energy resource is where the energy for galactic jets and quasars comes from.
[1] These equations apply for particles that are nonrelativistic. Special relativity effects become important when the orbital radius of the particle approaches the Schwarzschild radius, which introduces relativistic corrections to these equations.
… GR combined with nonlinear synchronization yields the novel phenomenon of a “synchronization cascade”.
Imagine a space ship containing a collection of highly-accurate atomic clocks factory-set to arbitrary precision at the space-ship factory before launch. The clocks are lined up with precisely-equal spacing along the axis of the space ship, which should allow the astronauts to study events in spacetime to high accuracy as they orbit neutron stars or black holes. Despite all the precision, spacetime itself will conspire to detune the clocks. Yet all is not lost. Using the physics of nonlinear synchronization, the astronauts can bring all the clocks together to a compromise frequency—locking all the clocks to a common rate. This blog post shows how this can happen.
Fig.1 The high-precision space ship with a line of clocks.
Synchronization of Oscillators
The simplest synchronization problem is two “phase oscillators” coupled with a symmetric nonlinearity. The dynamical flow is
where ωk are the individual angular frequencies and g is the coupling constant. When g is greater than the difference Δω, then the two oscillators, despite having different initial frequencies, will find a stable fixed point and lock to a compromise frequency.
Taking this model to N phase oscillators creates the well-known Kuramoto model that is characterized by a relatively sharp mean-field phase transition leading to global synchronization. The model averages N phase oscillators to a mean field where g is the coupling coefficient, K is the mean amplitude, Θ is the mean phase, and ω-bar is the mean frequency. The dynamics are given by
The last equation is the final mean-field equation that synchronizes each individual oscillator to the mean field. For a large number of oscillators that are globally coupled to each other, increasing the coupling has little effect on the oscillators until a critical threshold is crossed, after which all the oscillators synchronize with each other. This is known as the Kuramoto synchronization transition, shown in Fig. 2 for 20 oscillators with uniformly distributed initial frequencies. Note that the critical coupling constant gc is roughly half of the spread of initial frequencies.
Fig. 2 Entrainment graph of the Kuramoto transition for evenly distributed clock frequencies. N = 20.
The question that this blog seeks to answer is how this synchronization mechanism may be used in a space craft exploring the strong gravity around neutron stars or black holes. The key to answering this question is the metric tensor for this system
where the first term is the time-like term g00 that affects ticking clocks, and the second term is the space-like term that affects the length of the space craft.
Kuramoto versus the Neutron Star
Consider the space craft holding a steady radius above a neutron star, as in Fig. 3. For simplicity, hold the craft stationary rather than in an orbit to remove the details of rotating frames. Because each clock is at a different gravitational potential, it runs at a different rate because of gravitational time dilation–clocks nearer to the neutron star run slower than clocks farther away. There is also a gravitational length contraction of the space craft, which modifies the clock rates as well.
Fig. 3 The space ship orbiting a neutron star. Each identical clock is at a different gravitational potential, causing them to run at different rates.
The analysis starts by incorporating the first-order approximation of time dilation through the component g00. The component is brought in through the period of oscillations. All frequencies are referenced to the base oscillator that has the angular rate ω0, and the other frequencies are primed. As we consider oscillators higher in the space craft at positions R + h, the 1/(R+h) term in g00 decreases as does the offset between each successive oscillator.
The dynamical equations for a system for only two clocks, coupled through the constant k, are
These are combined to a single equation by considering the phase difference
The two clocks will synchronize to a compromise frequency for the critical coupling coefficient
Now, if there is a string of N clocks, as in Fig. 3, the question is how the frequencies will spread out by gravitational time dilation, and what the entrainment of the frequencies to a common compromise frequency looks like. If the ship is located at some distance from the neutron star, then the gravitational potential at one clock to the next is approximately linear, and coupling them would produce the classic Kuramoto transition.
However, if the ship is much closer to the neutron star, so that the gravitational potential is no longer linear, then there is a “fan-out” of frequencies, with the bottom-most clock ticking much more slowly than the top-most clock. Coupling these clocks produces a modified, or “stretched”, Kuramoto transition as in Fig. 4.
Fig. 4 The “stretched” Kuramoto transition for N = 20 clocks near a neutron star. The bottom-most clock is just above the surface of the neutron star (left) and at twice that height (right). The spatial separation of the clocks in these examples is RS/20, and R0 is the radial position of the bottom-most clock.
In the two examples in Fig. 4, the bottom-most clock is just above the radius of the neutron star (at R0 = 4RS for a solar-mass neutron star, where RS is the Schwarzschild radius) and at twice that radius (at R0 = 8RS). The length of the ship, along which the clocks are distributed, is RS in this example. This may seem unrealistically large, but we could imagine a regular-sized ship supporting a long stiff cable dangling below it composed of carbon nanotubes that has the clocks distributed evenly on it, with the bottom-most clock at the radius R0. In fact, this might be a reasonable design for exploring spacetime events near a neutron star (although even carbon nanotubes would not be able to withstand the strain).
Kuramoto versus the Black Hole
Against expectation, exploring spacetime around a black hole is actually easier than around a neutron star, because there is no physical surface at the Schwarzschild radius RS, and gravitational tidal forces can be small for large black holes. In fact, one of the most unintuitive aspects of black holes pertains to a space ship falling into one. A distant observer sees the space ship contracting to zero length and the clocks slowing down and stopping as the space ship approaches the Schwarzschild radius asymptotically, but never crossing it. However, on board the ship, all appears normal as it crosses the Schwarzschild radius. To the astronaut inside, there is is a gravitational potential inside the space ship that causes the clocks at the base to run more slowly than the upper clocks, and length contraction affects the spacing a little, but otherwise there is no singularity as the event horizon is passed. This appears as a classic “paradox” of physics, with two different observers seeing paradoxically different behaviors.
The resolution of this paradox lies in the differential geometry of the two observers. Each approximates spacetime with a Euclidean coordinate system that matches the local coordinates. The distant observer references the warped geometry to this “chart”, which produces the apparent divergence of the Schwarzschild metric at RS. However, the astronaut inside the space ship has her own flat chart to which she references the locally warped space time around the ship. Therefore, it is the differential changes, referenced to the ships coordinate origin, that capture gravitational time dilation and length contraction. Because the synchronization takes place in the local coordinate system of the ship, this is the coordinate system that goes into the dynamical equations for synchronization. Taking this approach, the shifts in the clock rates are given by the derivative of the metric as
where hn is the height of the n-th clock above R0.
Fig. 5 shows the entrainment plot for the black hole. The plot noticeably has a much smoother transition. In this higher mass case, the system does not have as many hard coupling transitions and instead exhibits smooth behavior for global coupling. This is the Kuramoto “cascade”. Contrast the behavior of Fig. 5 (left) to the classic Kuramoto transition of Fig. 2. The increasing frequency separations near the black hole produces a succession of frequency locks as the coupling coefficient increases. For comparison, the case of linear coupling along the cable is shown in Fig. 5 on the right. The cascade is now accompanied with interesting oscillations as one clock entrains with a neighbor, only to be pulled back by interaction with locked subclusters.
Fig. 5 The Kuramoto cascade for R0 = 1RS for global coupling (left) and linear coupling (right).
Now let us consider what role the spatial component of the metric tensor plays in the synchronization. The spatial component causes the space between the oscillators to decrease closer to the supermassive object. This would cause the oscillators to entrain faster because the bottom oscillators that entrain the slowest would be closer together, but the top oscillators would entrain slower since they are a farther distance apart, as in Fig. 6.
Fig. 6 The space ship experiencing gravitational length contraction that changes the separations among the clocks and further changes their respective gravitational potentials and clock rates.
In terms of the local coordinates of the space ship, the locations of each clock are
These values for hn can be put into the equation for ωn above. But it is clear that this produces a second order effect. Even at the event horizon, this effect is only a fraction of the shifts caused by g00 directly on the clocks. This is in contrast to what a distant observer sees–the clock separations decreasing to zero, which would seem to decrease the frequency shifts. But the synchronization coupling is performed in the ship frame, not the distant frame, so the astronaut can safely ignore this contribution.
As a final exploration of the black hole, before we leave it behind, look at the behavior for different values of R0 in Fig. 7. At 4RS, the Kuramoto transition is stretched. At 2RS there is a partial Kuramoto transition for the upper clocks, that then stretch into a cascade of locking events for the lower clocks. At 1RS we see the full cascade as before.
Fig. 7 The Kuramoto transition stretches into a cascade as the radius approaches the event horizon.
Note from the Editor:
This blog post by Moira Andrews is based on her final project for Phys 411, upper division undergraduate mechanics, at Purdue University. Students are asked to combine two seemingly-unrelated aspects of modern dynamics and explore the results. Moira thought of synchronizing clocks that are experiencing gravitational time dilation near a massive body. This is a nice example of how GR combined with nonlinear synchronization yields the novel phenomenon of a “synchronization cascade”.
Bibliography
Cheng, T.-P. (2010). Relativity, Gravitation and Cosmology. Oxford University Press.
Keeton, C. (2014). Principles of Astrophysics. Springer.
Marmet, P. (n.d.). Natural Length Contraction Due to Gravity. Newton Physics – Links to Papers, Books and Web Sites. Retrieved April 27, 2021, from https://newtonphysics.on.ca/gravity/index.html
Nolte, D. D. (2019). Introduction to Modern Dynamics (2nd ed.). Oxford University Press, USA.
This Blog Post is a Companion to the undergraduate physics textbook Modern Dynamics: Chaos, Networks, Space and Time, 2nd ed. (Oxford, 2019) introducing Lagrangians and Hamiltonians, chaos theory, complex systems, synchronization, neural networks, econophysics and Special and General Relativity.
The
physics of a path of light passing a gravitating body is one of the hardest
concepts to understand in General Relativity, but it is also one of the
easiest. It is hard because there can be
no force of gravity on light even though the path of a photon bends as it
passes a gravitating body. It is easy,
because the photon is following the simplest possible path—a geodesic equation
for force-free motion.
This blog picks up where my last blog left off, having there defined the geodesic equation and presenting the Schwarzschild metric. With those two equations in hand, we could simply solve for the null geodesics (a null geodesic is the path of a light beam through a manifold). But there turns out to be a simpler approach that Einstein came up with himself (he never did like doing things the hard way). He just had to sacrifice the fundamental postulate that he used to explain everything about Special Relativity.
Throwing Special Relativity Under the Bus
The fundamental postulate of Special Relativity states that the speed of light is the same for all observers. Einstein posed this postulate, then used it to derive some of the most astonishing consequences of Special Relativity—like E = mc2. This postulate is at the rock core of his theory of relativity and can be viewed as one of the simplest “truths” of our reality—or at least of our spacetime.
Yet as soon as Einstein began thinking how to extend SR to a more general situation, he realized almost immediately that he would have to throw this postulate out. While the speed of light measured locally is always equal to c, the apparent speed of light observed by a distant observer (far from the gravitating body) is modified by gravitational time dilation and length contraction. This means that the apparent speed of light, as observed at a distance, varies as a function of position. From this simple conclusion Einstein derived a first estimate of the deflection of light by the Sun, though he initially was off by a factor of 2. (The full story of Einstein’s derivation of the deflection of light by the Sun and the confirmation by Eddington is in Chapter 7 of Galileo Unbound (Oxford University Press, 2018).)
The “Optics” of Gravity
The invariant element for a light path moving radially in the Schwarzschild geometry is
The apparent speed of light is
then
where c(r) is always less than c, when observing it from
flat space. The “refractive index” of
space is defined, as for any optical material, as the ratio of the constant speed
divided by the observed speed
Because the Schwarzschild metric has the property
the effective refractive index of warped space-time is
with a divergence at the Schwarzschild
radius.
The refractive index of warped space-time in the limit of weak gravity can be used in the ray equation (also known as the Eikonal equation described in an earlier blog)
where the gradient of the refractive index of space is
The ray equation is then a four-variable flow
These equations represent a 4-dimensional flow for a light ray confined to a plane. The trajectory of any light path is found by using an ODE solver subject to the initial conditions for the direction of the light ray. This is simple for us to do today with Python or Matlab, but it was also that could be done long before the advent of computers by early theorists of relativity like Max von Laue (1879 – 1960).
The Relativity of Max von Laue
In the Fall of 1905 in Berlin, a young German physicist by the name of Max Laue was sitting in the physics colloquium at the University listening to another Max, his doctoral supervisor Max Planck, deliver a seminar on Einstein’s new theory of relativity. Laue was struck by the simplicity of the theory, in this sense “simplistic” and hence hard to believe, but the beauty of the theory stuck with him, and he began to think through the consequences for experiments like the Fizeau experiment on partial ether drag.
Armand Hippolyte Louis Fizeau (1819 – 1896) in 1851 built one of the world’s first optical interferometers and used it to measure the speed of light inside moving fluids. At that time the speed of light was believed to be a property of the luminiferous ether, and there were several opposing theories on how light would travel inside moving matter. One theory would have the ether fully stationary, unaffected by moving matter, and hence the speed of light would be unaffected by motion. An opposite theory would have the ether fully entrained by matter and hence the speed of light in moving matter would be a simple sum of speeds. A middle theory considered that only part of the ether was dragged along with the moving matter. This was Fresnel’s partial ether drag hypothesis that he had arrived at to explain why his friend Francois Arago had not observed any contribution to stellar aberration from the motion of the Earth through the ether. When Fizeau performed his experiment, the results agreed closely with Fresnel’s drag coefficient, which seemed to settle the matter. Yet when Michelson and Morley performed their experiments of 1887, there was no evidence for partial drag.
Even after the exposition by Einstein on relativity in 1905, the disagreement of the Michelson-Morley results with Fizeau’s results was not fully reconciled until Laue showed in 1907 that the velocity addition theorem of relativity gave complete agreement with the Fizeau experiment. The velocity observed in the lab frame is found using the velocity addition theorem of special relativity. For the Fizeau experiment, water with a refractive index of n is moving with a speed v and hence the speed in the lab frame is
The difference in the speed of light between the stationary and the moving water is the difference
where the last term is precisely the Fresnel drag coefficient. This was one of the first definitive “proofs” of the validity of Einstein’s theory of relativity, and it made Laue one of relativity’s staunchest proponents. Spurred on by his success with the Fresnel drag coefficient explanation, Laue wrote the first monograph on relativity theory, publishing it in 1910.
Fig. 1 Front page of von Laue’s textbook, first published in 1910, on Special Relativity (this is a 4-th edition published in 1921).
A Nobel Prize for Crystal X-ray Diffraction
In 1909 Laue became a Privatdozent under Arnold Sommerfeld (1868 – 1951) at the university in Munich. In the Spring of 1912 he was walking in the Englischer Garten on the northern edge of the city talking with Paul Ewald (1888 – 1985) who was finishing his doctorate with Sommerfed studying the structure of crystals. Ewald was considering the interaction of optical wavelength with the periodic lattice when it struck Laue that x-rays would have the kind of short wavelengths that would allow the crystal to act as a diffraction grating to produce multiple diffraction orders. Within a few weeks of that discussion, two of Sommerfeld’s students (Friedrich and Knipping) used an x-ray source and photographic film to look for the predicted diffraction spots from a copper sulfate crystal. When the film was developed, it showed a constellation of dark spots for each of the diffraction orders of the x-rays scattered from the multiple periodicities of the crystal lattice. Two years later, in 1914, Laue was awarded the Nobel prize in physics for the discovery. That same year his father was elevated to the hereditary nobility in the Prussian empire and Max Laue became Max von Laue.
Von Laue was not one to take risks, and he remained conservative in many of his interests. He was immensely respected and played important roles in the administration of German science, but his scientific contributions after receiving the Nobel Prize were only modest. Yet as the Nazis came to power in the early 1930’s, he was one of the few physicists to stand up and resist the Nazi take-over of German physics. He was especially disturbed by the plight of the Jewish physicists. In 1933 he was invited to give the keynote address at the conference of the German Physical Society in Wurzburg where he spoke out against the Nazi rejection of relativity as they branded it “Jewish science”. In his speech he likened Einstein, the target of much of the propaganda, to Galileo. He said, “No matter how great the repression, the representative of science can stand erect in the triumphant certainty that is expressed in the simple phrase: And yet it moves.” Von Laue believed that truth would hold out in the face of the proscription against relativity theory by the Nazi regime. The quote “And yet it moves” is supposed to have been muttered by Galileo just after his abjuration before the Inquisition, referring to the Earth moving around the Sun. Although the quote is famous, it is believed to be a myth.
In an odd side-note of history, von Laue sent his gold Nobel prize medal to Denmark for its safe keeping with Niels Bohr so that it would not be paraded about by the Nazi regime. Yet when the Nazis invaded Denmark, to avoid having the medals fall into the hands of the Nazis, the medal was dissolved in aqua regia by a member of Bohr’s team, George de Hevesy. The gold completely dissolved into an orange liquid that was stored in a beaker high on a shelf through the war. When Denmark was finally freed, the dissolved gold was precipitated out and a new medal was struck by the Nobel committee and re-presented to von Laue in a ceremony in 1951.
The Orbits of Light Rays
Von Laue’s interests always stayed close to the properties of light and electromagnetic radiation ever since he was introduced to the field when he studied with Woldemor Voigt at Göttingen in 1899. This interest included the theory of relativity, and only a few years after Einstein published his theory of General Relativity and Gravitation, von Laue added to his earlier textbook on relativity by writing a second volume on the general theory. The new volume was published in 1920 and included the theory of the deflection of light by gravity.
One of the very few illustrations in his second volume is of light coming into interaction with a super massive gravitational field characterized by a Schwarzschild radius. (No one at the time called it a “black hole”, nor even mentioned Schwarzschild. That terminology came much later.) He shows in the drawing, how light, if incident at just the right impact parameter, would actually loop around the object. This is the first time such a diagram appeared in print, showing the trajectory of light so strongly affected by gravity.
Fig. 2 A page from von Laue’s second volume on relativity (first published in 1920) showing the orbit of a photon around a compact mass with “gravitational cutoff” (later known as a “black hole:”). The figure is drawn semi-quantitatively, but the phenomenon was clearly understood by von Laue.
Python Code: gravlens.py
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
gravlens.py
Created on Tue May 28 11:50:24 2019
@author: nolte
D. D. Nolte, Introduction to Modern Dynamics: Chaos, Networks, Space and Time, 2nd ed. (Oxford,2019)
"""
import numpy as np
import matplotlib as mpl
from mpl_toolkits.mplot3d import Axes3D
from scipy import integrate
from matplotlib import pyplot as plt
from matplotlib import cm
import time
import os
plt.close('all')
def create_circle():
circle = plt.Circle((0,0), radius= 10, color = 'black')
return circle
def show_shape(patch):
ax=plt.gca()
ax.add_patch(patch)
plt.axis('scaled')
plt.show()
def refindex(x,y):
A = 10
eps = 1e-6
rp0 = np.sqrt(x**2 + y**2);
n = 1/(1 - A/(rp0+eps))
fac = np.abs((1-9*(A/rp0)**2/8)) # approx correction to Eikonal
nx = -fac*n**2*A*x/(rp0+eps)**3
ny = -fac*n**2*A*y/(rp0+eps)**3
return [n,nx,ny]
def flow_deriv(x_y_z,tspan):
x, y, z, w = x_y_z
[n,nx,ny] = refindex(x,y)
yp = np.zeros(shape=(4,))
yp[0] = z/n
yp[1] = w/n
yp[2] = nx
yp[3] = ny
return yp
for loop in range(-5,30):
xstart = -100
ystart = -2.245 + 4*loop
print(ystart)
[n,nx,ny] = refindex(xstart,ystart)
y0 = [xstart, ystart, n, 0]
tspan = np.linspace(1,400,2000)
y = integrate.odeint(flow_deriv, y0, tspan)
xx = y[1:2000,0]
yy = y[1:2000,1]
plt.figure(1)
lines = plt.plot(xx,yy)
plt.setp(lines, linewidth=1)
plt.show()
plt.title('Photon Orbits')
c = create_circle()
show_shape(c)
axes = plt.gca()
axes.set_xlim([-100,100])
axes.set_ylim([-100,100])
# Now set up a circular photon orbit
xstart = 0
ystart = 15
[n,nx,ny] = refindex(xstart,ystart)
y0 = [xstart, ystart, n, 0]
tspan = np.linspace(1,94,1000)
y = integrate.odeint(flow_deriv, y0, tspan)
xx = y[1:1000,0]
yy = y[1:1000,1]
plt.figure(1)
lines = plt.plot(xx,yy)
plt.setp(lines, linewidth=2, color = 'black')
plt.show()
One of the most striking effects of gravity on photon trajectories is the possibility for a photon to orbit a black hole in a circular orbit. This is shown in Fig. 3 as the black circular ring for a photon at a radius equal to 1.5 times the Schwarzschild radius. This radius defines what is known as the photon sphere. However, the orbit is not stable. Slight deviations will send the photon spiraling outward or inward.
The Eikonal approximation does not strictly hold under strong gravity, but the Eikonal equations with the effective refractive index of space still yield semi-quantitative behavior. In the Python code, a correction factor is used to match the theory to the circular photon orbits, while still agreeing with trajectories far from the black hole. The results of the calculation are shown in Fig. 3. For large impact parameters, the rays are deflected through a finite angle. At a critical impact parameter, near 3 times the Schwarzschild radius, the ray loops around the black hole. For smaller impact parameters, the rays are captured by the black hole.
Fig. 3 Photon orbits near a black hole calculated using the Eikonal equation and the effective refractive index of warped space. One ray, near the critical impact parameter, loops around the black hole as predicted by von Laue. The central black circle is the black hole with a Schwarzschild radius of 10 units. The black ring is the circular photon orbit at a radius 1.5 times the Schwarzschild radius.
Photons pile up around the black hole at the photon sphere. The first image ever of the photon sphere of a black hole was made earlier this year (announced April 10, 2019). The image shows the shadow of the supermassive black hole in the center of Messier 87 (M87), an elliptical galaxy 55 million light-years from Earth. This black hole is 6.5 billion times the mass of the Sun. Imaging the photosphere required eight ground-based radio telescopes placed around the globe, operating together to form a single telescope with an optical aperture the size of our planet. The resolution of such a large telescope would allow one to image a half-dollar coin on the surface of the Moon, although this telescope operates in the radio frequency range rather than the optical.
Fig. 4 Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun.
Arthur Eddington was the complete package—an observationalist with the mathematical and theoretical skills to understand Einstein’s general theory, and the ability to construct the theory of the internal structure of stars. He was Zeus in Olympus among astrophysicists. He always had the last word, and he stood with Einstein firmly opposed to the Schwarzschild singularity. In 1924 he published a theoretical paper in which he derived a new coordinate frame (now known as Eddington-Finkelstein coordinates) in which the singularity at the Schwarzschild radius is removed. At the time, he took this to mean that the singularity did not exist and that gravitational cut off was not possible [1]. It would seem that the possibility of dark stars (black holes) had been put to rest. Both Eddington and Einstein said so! But just as they were writing the obituary of black holes, a strange new form of matter was emerging from astronomical observations that would challenge the views of these giants.
Something wonderful, but also a little scary, happened when Chandrasekhar included the relativistic effects in his calculation.
White Dwarf
Binary star systems have always held a certain fascination for astronomers. If your field of study is the (mostly) immutable stars, then the stars that do move provide some excitement. The attraction of binaries is the same thing that makes them important astrophysically—they are dynamic. While many double stars are observed in the night sky (a few had been noted by Galileo), some of these are just coincidental alignments of near and far stars. However, William Herschel began cataloging binary stars in 1779 and became convinced in 1802 that at least some of them must be gravitationally bound to each other. He carefully measured the positions of binary stars over many years and confirmed that these stars showed relative changes in position, proving that they were gravitational bound binary star systems [2]. The first orbit of a binary star was computed in 1827 by Félix Savary for the orbit of Xi Ursae Majoris. Finding the orbit of a binary star system provides a treasure trove of useful information about the pair of stars. Not only can the masses of the stars be determined, but their radii and densities also can be estimated. Furthermore, by combining this information with the distance to the binaries, it was possible to develop a relationship between mass and luminosity for all stars, even single stars. Therefore, binaries became a form of measuring stick for crucial stellar properties.
Comparison of Earth to a white dwarf star with a mass equal to the Sun. They have comparable radii but radically different densities.
One of the binary star systems that Hershel discovered was the pair known as 40 Eridani B/C, which he observed on January 31 in 1783. Of this pair, 40 Eridani B was very dim compared to its companion. More than a century later, in 1910 when spectrographs were first being used routinely on large telescopes, the spectrum of 40 Eridani B was found to be of an unusual white spectral class. In the same year, the low luminosity companion of Sirius, known as Sirius B, which shared the same unusual white spectral class, was evaluated in terms of its size and mass and was found to be exceptionally small and dense [3]. In fact, it was too small and too dense to be believed at first, because the densities were beyond any known or even conceivable matter. The mass of Sirius B is around the mass of the Sun, but its radius is comparable to the radius of the Earth, making the density of the white star about ten thousand times denser than the core of the Sun. Eddington at first felt the same way about white dwarfs that he felt about black holes, but he was eventually swayed by the astrophysical evidence. By 1922 many of these small white stars had been discovered, called white dwarfs, and their incredibly large densities had been firmly established. In his famous book on stellar structure[4], he noted the strange paradox: As a star cools, its pressure must decrease, as all gases must do as they cool, and the star would shrink, yet the pressure required to balance the force of gravity to stabilize the star against continued shrinkage must increase as the star gets smaller. How can pressure decrease and yet increase at the same time? In 1926, on the eve of the birth of quantum mechanics, Eddington could conceive of no mechanism that could resolve this paradox. So he noted it as an open problem in his book and sent it to press.
Subrahmanyan Chandrasekhar
Three years after the publication of Eddington’s book, an eager and excited nineteen-year-old graduate of the University in Madras India boarded a steamer bound for England. Subrahmanyan Chandrasekhar (1910—1995) had been accepted for graduate studies at Cambridge University. The voyage in 1930 took eighteen days via the Suez Canal, and he needed something to do to pass the time. He had with him Eddington’s book, which he carried like a bible, and he also had a copy of a breakthrough article written by R. H. Fowler that applied the new theory of quantum mechanics to the problem of dense matter composed of ions and electrons [5]. Fowler showed how the Pauli exclusion principle for electrons, that obeyed Fermi-Dirac statistics, created an energetic sea of electrons in their lowest energy state, called electron degeneracy. This degeneracy was a fundamental quantum property of matter, and carried with it an intrinsic pressure unrelated to thermal properties. Chandrasekhar realized that this was a pressure mechanism that could balance the force of gravity in a cooling star and might resolve Eddington’s paradox of the white dwarfs. As the steamer moved ever closer to England, Chandrasekhar derived the new balance between gravitational pressure and electron degeneracy pressure and found the radius of the white dwarf as a function of its mass. The critical step in Chandrasekhar’s theory, conceived alone on the steamer at sea with access to just a handful of books and papers, was the inclusion of special relativity with the quantum physics. This was necessary, because the densities were so high and the electrons were so energetic, that they attained speeds approaching the speed of light.
Something wonderful, but also a little scary, happened when Chandrasekhar included the relativistic effects in his calculation. He discovered that electron degeneracy pressure could balance the force of gravity if the mass of the white dwarf were smaller than about 1.4 times the mass of the Sun. But if the dwarf was more massive than this, then even the electron degeneracy pressure would be insufficient to fight gravity, and the star would continue to collapse. To what? Schwarzschild’s singularity was one possibility. Chandrasekhar wrote up two papers on his calculations, and when he arrived in England, he showed them to Fowler, who was to be his advisor at Cambridge. Fowler was genuinely enthusiastic about the first paper, on the derivation of the relativistic electron degeneracy pressure, and it was submitted for publication. The second paper, on the maximum sustainable mass for a white dwarf, which reared the ugly head of Schwarzschild’s singularity, made Fowler uncomfortable, and he sat on the paper, unwilling to give his approval for publication in the leading British astrophysical journal. Chandrasekhar grew annoyed, and in frustration sent it, without Fowler’s approval, to an American journal, where “The Maximum Mass of Ideal White Dwarfs” was published in 1931 [6]. This paper, written in eighteen days on a steamer at sea, established what became known as the Chandrasekhar limit, for which Chandrasekhar would win the 1983 Nobel Prize in Physics, but not before he was forced to fight major battles for its acceptance.
The Chandrasekhar limit expressed in terms of the Planck Mass and the mass of a proton. The limit is approximately 1.4 times the mass of the Sun. White dwarfs with masses larger than the limit cannot balance gravitational collapse by relativistic electron degeneracy.
Chandrasekhar versus Eddington
Initially there was almost no response to Chandrasekhar’s paper. Frankly, few astronomers had the theoretical training needed to understand the physics. Eddington was one exception, which was why he held such stature in the community. The big question therefore was: Was Chandrasekhar’s theory correct? During the three years to obtain his PhD, Chandrasekhar met frequently with Eddington, who was also at Cambridge, and with colleagues outside the university, and they all encouraged Chandrasekhar to tackle the more difficult problem to combine internal stellar structure with his theory. This could not be done with pen and paper, but required numerical calculation. Eddington was in possession of an early electromagnetic calculator, and he loaned it to Chandrasekhar to do the calculations. After many months of tedious work, Chandrasekhar was finally ready to confirm his theory at the 1934 meeting of the British Astrophysical Society.
The young Chandrasekhar stood up and gave his results in an impeccable presentation before an auditorium crowded with his peers. But as he left the stage, he was shocked when Eddington himself rose to give the next presentation. Eddington proceeded to criticize and reject Chandrasekhar’s careful work, proposing instead a garbled mash-up of quantum theory and relativity that would eliminate Chandrasekhar’s limit and hence prevent collapse to the Schwarzschild singularity. Chandrasekhar sat mortified in the audience. After the session, many of his friends and colleagues came up to him to give their condolences—if Eddington, the leader of the field and one of the few astronomers who understood Einstein’s theories, said that Chandrasekhar was wrong, then that was that. Badly wounded, Chandrasekhar was faced with a dire choice. Should he fight against the reputation of Eddington, fight for the truth of his theory? But he was at the beginning of his career and could ill afford to pit himself against the giant. So he turned his back on the problem of stellar death, and applied his talents to the problem of stellar evolution.
Chandrasekhar went on to have an illustrious career, spent mostly at the University of Chicago (far from Cambridge), and he did eventually return to his limit as it became clear that Eddington was wrong. In fact, many at the time already suspected Eddington was wrong and were seeking for the answer to the next question: If white dwarfs cannot support themselves under gravity and must collapse, what do they collapse to? In Pasadena at the California Institute of Technology, an astrophysicist named Fritz Zwicky thought he knew the answer.
Fritz Zwicky’s Neutron Star
Fritz Zwicky (1898—1874) was an irritating and badly flawed genius. What made him so irritating was that he knew he was a genius and never let anyone forget it. What made him badly flawed was that he never cared much for weight of evidence. It was the ideas that mattered—let lesser minds do the tedious work of filling in the cracks. And what made him a genius was that he was often right! Zwicky pushed the envelope—he loved extremes. The more extreme a theory was, the more likely he was to favor it—like his proposal for dark matter. Most of his colleagues considered him to be a buffoon and borderline crackpot. He was tolerated by no one—no one except his steadfast collaborator of many years Ernst Baade (until they nearly came to blows on the eve of World War II). Baade was a German physicist trained at Göttingen and recently arrived at Cal Tech. He was exceptionally well informed on the latest advances in a broad range of fields. Where Zwicky made intuitive leaps, often unsupported by evidence, Baade would provide the context. Baade was a walking Wikipedia for Zwicky, and together they changed the face of astrophysics.
Zwicky and Baade submitted an abstract to the American Physical Society Meeting in 1933, which Kip Thorne has called “…one of the most prescient documents in the history of physics and astronomy” [7]. In the abstract, Zwicky and Baade introduced, for the first time, the existence of supernovae as a separate class of nova and estimated the total energy output of these cataclysmic events, including the possibility that they are the source of some cosmic rays. They introduced the idea of a neutron star, a star composed purely of neutrons, only a year after Chadwick discovered the neutron’s existence, and they strongly suggested that a supernova is produced by the transformation of a star into a neutron star. A neutron star would have a mass similar to that of the Sun, but would have a radius of only tens of kilometers. If the mass density of white dwarfs was hard to swallow, the density of a neutron star was billion times greater! It would take nearly thirty years before each of the assertions made in this short abstract were proven true, but Zwicky certainly had a clear view, tempered by Baade, of where the field of astrophysics was headed. But no one listened to Zwicky. He was too aggressive and backed up his wild assertions with too little substance. Therefore, neutron stars simmered on the back burner until more substantial physicists could address their properties more seriously.
Two substantial physicists who had the talent and skills that Zwicky lacked were Lev Landau in Moscow and Robert Oppenheimer at Berkeley. Landau derived the properties of a neutron star in 1937 and published the results to great fanfare. He was not aware of Zwicky’s work, and he called them neutron cores, because he hypothesized that they might reside at the core of ordinary stars like the Sun. Oppenheimer, working with a Canadian graduate student George Volkoff at Berkeley, showed that Landau’s idea about stellar cores was not correct, but that the general idea of a neutron core, or rather neutron star, was correct [8]. Once Oppenheimer was interested in neutron stars, he kept going and asked the same question about neutron stars that Chandrasekhar had asked about white dwarfs: Is there a maximum size for neutron stars beyond which they must collapse? The answer to this question used the same quantum mechanical degeneracy pressure (now provided by neutrons rather than electrons) and gravitational compaction as the problem of white dwarfs, but it required detailed understanding of nuclear forces, which in 1938 were only beginning to be understood. However, Oppenheimer knew enough to make a good estimate of the nuclear binding contribution to the total internal pressure and came to a similar conclusion for neutron stars as Chandrasekhar had made for white dwarfs. There was indeed a maximum mass of a neutron star, a Chandrasekhar-type limit of about three solar masses. Beyond this mass, even the degeneracy pressure of neutrons could not support gravitational pressure, and the neutron star must collapse. In Oppenheimer’s mind it was clear what it must collapse to—a black hole (known as gravitational cut-off at that time). This was to lead Oppenheimer and John Wheeler to their famous confrontation over the existence of black holes, which Oppenheimer won, but Wheeler took possession of the battle field [9].
Derivation of the Relativistic Chandrasekhar Limit
White dwarfs are created from the balance between gravitational compression and the degeneracy pressure of electrons caused by the Pauli exclusion principle. When a star collapses gravitationally, the matter becomes so dense that the electrons begin to fill up quantum states until all the lowest-energy states are filled and no more electrons can be added. This results in a balance that stabilizes the gravitational collapse, and the result is a white dwarf with a mass density a million times larger than the Sun.
If the electrons remained non-relativistic, then there would be no upper limit for the size of a star that would form a white dwarf. However, because electrons become relativistic at high enough compaction, if the initial star is too massive, the electron degeneracy pressure becomes limited relativistically and cannot keep the matter from compacting more, and even the white dwarf will collapse (to a neutron star or a black hole). The largest mass that can be supported by a white dwarf is known as the Chandrasekhar limit.
A simplified derivation of the Chandrasekhar limit begins by defining the total energy as the kinetic energy of the degenerate Fermi electron gas plus the gravitational potential energy
The kinetic energy of the degenerate Fermi gas has the relativistic expression
where the Fermi k-vector can be expressed as a function of the radius of the white dwarf and the total number of electrons in the star, as
If the star is composed of pure hydrogen, then the mass of the star is expressed in terms of the total number of electrons and the mass of the proton
The total energy of the white dwarf is minimized by taking its derivative with respect to the radius of the star
When the derivative is set to zero, the term in brackets becomes
This is solved for the radius for which the electron degeneracy pressure stabilizes the gravitational pressure
This is the relativistic radius-mass expression for the size of the stabilized white dwarf as a function of the mass (or total number of electrons). One of the astonishing results of this calculation is the merging of astronomically large numbers (the mass of stars) with both relativity and quantum physics. The radius of the white dwarf is actually expressed as a multiple of the Compton wavelength of the electron!
The expression in the square root becomes smaller as the size of the star increases, and there is an upper bound to the mass of the star beyond which the argument in the square root goes negative. This upper bound is the Chandrasekhar limit defined when the argument equals zero
This gives the final expression for the Chandrasekhar limit (expressed in terms of the Planck mass)
This expression is only approximate, but it does contain the essential physics and magnitude. This limit is on the order of a solar mass. A more realistic numerical calculation yields a limiting mass of about 1.4 times the mass of the Sun. For white dwarfs larger than this value, the electron degeneracy is insufficient to support the gravitational pressure, and the star will collapse to a neutron star or a black hole.
By David D. Nolte, Jan. 7, 2019
[1] The fact that Eddington coordinates removed the singularity at the Schwarzschild radius was first pointed out by Lemaitre in 1933. A local observer passing through the Schwarzschild radius would experience no divergence in local properties, even though a distant observer would see that in-falling observer becoming length contracted and time dilated. This point of view of an in-falling observer was explained in 1958 by Finkelstein, who also pointed out that the Schwarzschild radius is an event horizon.
[2] William Herschel (1803), Account of the Changes That Have Happened, during the Last Twenty-Five Years, in the Relative Situation of Double-Stars; With an Investigation of the Cause to Which They Are Owing, Philosophical Transactions of the Royal Society of London 93, pp. 339–382 (Motion of binary stars)
[3] Boss, L. (1910). Preliminary General Catalogue of 6188 stars for the epoch 1900. Carnegie Institution of Washington. (Mass and radius of Sirius B)
[4] Eddington, A. S. (1927). Stars and Atoms. Clarendon Press. LCCN 27015694.
[5] Fowler, R. H. (1926). “On dense matter”. Monthly Notices of the Royal Astronomical Society 87: 114. Bibcode:1926MNRAS..87..114F. (Quantum mechanics of degenerate matter).
[6] Chandrasekhar, S. (1931). “The Maximum Mass of Ideal White Dwarfs”. The Astrophysical Journal 74: 81. Bibcode:1931ApJ….74…81C. doi:10.1086/143324. (Mass limit of white dwarfs).
[7] Kip Thorne (1994) Black Holes & Time Warps: Einstein’s Outrageous Legacy (Norton). pg. 174
[8] Oppenheimer was aware of Zwicky’s proposal because he had a joint appointment between Berkeley and Cal Tech.
Galileo Unbound 