6. Black Holes - DAMTP
6. Black HolesBlack holes are among the most enigmatic objects in the universe. They are describedby deceptively simple solutions to the Einstein equations, yet hold a host of insightsand surprises, from the meaning of causal structure, to connections to thermodynamicsand, ultimately, quantum gravity. The purpose of this section is to begin to uncoversome of the mysteries of these wonderful objects.6.1 The Schwarzschild SolutionWe have already met the simplest black hole solution back in Section 1.3: this is theSchwarzschild solution, with metric 12GM2GM22ds 1dt 1dr2 r2 (d 2 sin2 d 2 )(6.1)rrIt is not hard to show that this solves the vacuum Einstein equations Rµ 0. Indeed,the calculations can be found in Section 4.2 where we first met de Sitter space. TheSchwarzschild solution is a special case of the more general metric (4.9) with f (r)2 1 2GM/r and it’s simple to check that this obeys the Einstein equation which, aswe’ve seen, reduces to the simple di erential equations (4.10) and (4.11).M is for MassThe Schwarzschild solution depends on a single parameter, M , which should be thoughtof as the mass of the black hole. This interpretation already follows from the relation toNewtonian gravity that we first discussed way back in Section 1.2 where we anticipatedthat the g00 component of the metric should be (1.26)g00 (1 2 )with the Newtonian potential. We made this intuition more precise in Section 5.1.2where we discussed the Newtonian limit. For the Schwarzschild metric, we clearly have GMrwhich is indeed the Newtonian potential for a point mass M at the origin.The black hole also provides an opportunity to roadtest the technology of Komarintegrals developed in Section 4.3.3. The Schwarzschild spacetime admits a timelikeKilling vector K @t . The dual one-form is then 2GMK g00 dt 1dtr– 232 –
Following the steps described in Section 4.3.3, we can then construct the 2-formF dK 2GMdr dtr2which takes a form similar to that of an electric field, with the characteristic 1/r2fall-o . The Komar integral instructs us to compute the mass by integratingZ1MKomar ?F8 G S2where S2 is any sphere with radius larger than the horizon r 2GM . It doesn’t matterwhich radius we choose; they all give the same answer, just like all Gaussian surfacesoutside a charge distribution give the same answer in electromagnetism. Since the areaof a sphere at radius r is 4 r2 , the integral givesMKomar Mfor the Schwarzschild black hole.There’s something a little strange about the Komar mass integral. As we saw inSection 4.3.3, the 2-form F dK obeys something very similar to the Maxwell equations, d ? F 0. But these are the vacuum Maxwell equations in the absence of anycurrent, so we would expect any “electric charge” to vanish. Yet this “electric charge”is precisely the mass MKomar which, as we have seen, is distinctly not zero. What’shappening is that, for the black hole, the mass is all localised at the origin r 0, wherethe field strength F diverges.We might expect that the Schwarzschild solution only describes something physicallysensible when M 0. (The M 0 Schwarzschild solution is simply Minkowski spacetime.) However, the metric (6.1) is a solution of the Einstein equations for all valuesof M . As we proceed, we’ll see that the M 0 solution does indeed have some ratherscrewy features that make it unphysical.6.1.1 Birkho ’s TheoremThe Schwarzschild solution (6.1) is, it turns out, the unique spherically symmetric,asymptotically flat solution to the vacuum Einstein equations. This is known as theBirkho theorem. In particular, this means that the Schwarzschild solution does notjust describe a black hole, but it describes the spacetime outside any non-rotating,spherically symmetric object, like a star.– 233 –
Here we provide a sketch of the proof. The first half of the proof involves settingup a useful set of coordinates. First, we make use of the statement that the metric isspherically symmetric, which means that it has an SO(3) isometry. One of the morefiddly parts of the proof is to show that any metric with such an isometry can be writtenin coordinates that make this isometry manifest,ds2 g ( , )d 2 2g ( , )d d g ( , ) d 2 r2 ( , ) d 22Here and are some coordinates and d 22 is the familiar metric on S2d 22 d 2 sin2 d2The SO(3) isometry then acts on this S2 in the usual way, leaving and untouched.This is said to be a foliation of the space by the spheres S2 .The size of the sphere is determined by the function r( , ) in the above metric. Thenext step in the proof is to change coordinates so that we work with and r, ratherthan and . We’re then left with the metricds2 g ( , r)d 2 2g r ( , r)d dr grr ( , r) dr2 r2 d 22In fact there’s a subtlety in the argument above: for some functions r( , ), it’s notpossible to exchange for r. Examples of such functions include r constant andr . We can rule out such counter-examples by insisting that asymptotically thespacetime looks like Minkowski space.Our next step is to introduce a new coordinate that gets rid of the cross-term g r .To this end, consider the a coordinate t̃( , r). Then 2 2@ t̃@ t̃ @ t̃@ t̃22dt̃ d 2d dr dr2@ @ @r@rWe can always pick a choice of t̃( , r) so that the cross-term g r vanishes in the newcoordinates. We’re then left with the simpler looking metric,ds2 f (t̃, r) dt̃2 g(t̃, r) dr2 r2 d 22Where we’ve now included the expected minus sign in the temporal part of the metric,reflecting our chosen signature. This is as far as we can go making useful coordinatechoices. To proceed, we need to use the Einstein equations. As always, this involvessitting down and doing a fiddly calculation. Here we present only the (somewhatsurprising) conclusion: the vacuum Einstein equations require thatf (r, t̃) f (r)h(t̃) and g(r, t̃) g(r)– 234 –
In other words, the metric takes the formds2 f (r)h(t̃)dt̃2 g(r)dr2 r2 d 22But we can always absorb that h(t̃) factor by redefining the time coordinate, so thath(t̃)dt̃2 dt2 . Finally, we’re left with the a metric of the formds2 f (r)dt2 g(r)dr2 r2 d 22(6.2)This is important. We assumed that the metric was spherically symmetric, but madeno such assumption about the lack of time dependence. Yet the Einstein equationshave forced this upon us, and the final metric (6.2) has two sets of Killing vectors. Thefirst arises from the SO(3) isometry that we originally assumed, but the second is thetimelike Killing vector K @t that has emerged from the calculation.At this point we need to finish solving the Einstein equations. It turns out that theyrequire f (r) g(r) 1 , so the metric (6.2) reduces to the simple ansatz (4.9) that weconsidered previously. The Schwarzschild solution (6.1) is the most general solution tothe Einstein equations with vanishing cosmological constant.The fact that we assumed only spherical symmetry, and not time independence,means that the Schwarzschild solution not only describes the spacetime outside a timeindependent star, but also outside a collapsing star, providing that the collapse isspherically symmetric.A Closer Look at Time IndependenceThere are actually two, di erent meanings to “time independence” in general relativity.A spacetime is said to be stationary if it admits an everywhere timelike Kililng vectorfield K. In asymptotically flat spacetimes, we usually normalise this so that K 2 ! 1asymptotically.A spacetime is said to be static if it is stationary and, in addition, is invariant undert ! t, where t is a coordinate along the integral curves of K. In particular, this rulesout dt dX cross-terms in the metric, with X some other coordinate.Birkho ’s theorem tells us that spherical symmetry implies that the spacetime isnecessarily static. In Section 6.3, we’ll come across spacetimes that are stationary butnot static.– 235 –
6.1.2 A First Look at the HorizonThere are two values of r where the Schwarzschild metric goes bad: r 0 and r 2GM .At each of these values, one of the components of the metric diverges but, as we willsee, the interpretation of this divergence is rather di erent in the two cases. We willlearn that the divergence at the point r 0 is because the spacetime is sick: this pointis called the singularity. The theory of general relativity breaks down as we get closeto the singularity and to make sense of what’s happening there we need to turn to aquantum theory of spacetime.In contrast, nothing so dramatic happens at the surface r 2GM and the divergencein the metric is merely because we’ve made a poor choice of coordinates: this surfaceis referred to as the event horizon, usually called simply the horizon. Many of thesurprising properties of black holes lie in interpreting the event horizon.There is a simple diagnostic to determine whether a divergence in the metric is dueto a true singularity of the spacetime, or to a poor choice of coordinates. We build ascalar quantity that does not depend on the choice of coordinates. If this too divergesthen it’s telling us that the spacetime itself is indeed sick at that point. If it does notdiverge, we can’t necessarily conclude that the spacetime isn’t sick because there maybe some other scalar quantity that signifies there is a problem. Nonetheless, we mightstart to wonder if perhaps nothing very bad happens.The simplest scalar is, of course, the Ricci scalar. But this is necessarily R 0 forany vacuum solution to the Einstein equation, so is not helpful in detecting the natureof singularities. The same is true for Rµ Rµ . For this reason, the simplest curvaturediagnostic is the Kretschmann scalar, Rµ Rµ . For the Schwarzschild solution it isgiven byRµ Rµ 48G2 M 2r6(6.3)We see that the Kretschmann scalar exhibits no pathology at the surface r 2GM ,where Rµ Rµ 1/(GM )4 . This suggests that perhaps this divergence in themetric isn’t as worrisome as it may have first appeared. Note moreover that, perhapscounter-intuitively, heavier black holes have smaller curvature at the horizon. We seethat this arises because such black holes are bigger and the 1/r6 factor beats the M 2factor.In contrast, the curvature indeed diverges at the origin r 0, telling us that thespacetime is problematic at this point. Of course, given that we have still to understand– 236 –
the horizon at r 2GM , it’s not entirely clear that we can trust the Schwarzschildmetric for values r 2GM . As we will proceed, we will see that the singularity atr 0 is a genuine feature of the (classical) black hole.The Near Horizon Limit: Rindler SpaceTo understand what’s happening near the horizon r 2GM , we can zoom in and lookat the metric in the vicinity of the horizon. To do this, we writer 2GM where we take 2GM . We further take 0 which means that we’re looking atthe region of spacetime just outside the horizon. We then approximate the componentsof the metric as2GM 1 and r2 (2GM )2 (2GM )2r2GMTo this order, the Schwarzschild metric becomesds2 2GM 2dt2 d (2GM )2 d 222GM The first thing that we see is that the metric has decomposed into a direct product ofan S2 of radius 2GM , and a d 1 1 dimensional Lorentzian geometry. We’ll focuson this 2d Lorentzian geometry. We make the change of variables 2 8GM after which the 2d metric becomes 2 2ds dt d 24GMThis rather simple metric is known as Rindler space. It is, in fact, just Minkowski spacein disguise. The disguise is the transformation ttT sinhand X cosh(6.4)4GM4GM2 after which the metric becomesds2 dT 2 dX 2(6.5)We’ve met something very similar to the coordinates (6.4) previously: they are thecoordinates experienced by an observer undergoing constant acceleration a 1/4GM ,where t is the proper time of this observer. (We saw such coordinates earlier in (1.25)which di er only by a constant o set to the spatial variable .) This makes sense:an observer who sits at a constant value, corresponding to a constant r value, mustaccelerate in order to avoid falling into the black hole.– 237 –
Tr 2GMXr 2GMFigure 43: The near horizon limit of a black hole is Rindler spacetime, with the null linesX T corresponding to the horizon at r 2GM . Also shown in red is a line of constantr 2GM outside the black hole.We can now start to map out what part of Minkowski space (6.5) corresponds to theoutside of the black hole horizon. This is 0 and t 2 ( 1, 1). From the changeof variables (6.4), we see that this corresponds the region X T .We can also see what becomes of the horizon itself. This sits at r 2GM , or 0.For any finite t, the horizon 0 gets mapped to the origin of Minkowski space,X T 0. However, the time coordinate is degenerate at the horizon since g00 0.If we scale t ! 1, and ! 0 keeping the combination e t/4GM fixed, then we seethat the horizon actually corresponds to the lines:r 2GM)X TThis is our first lesson. The event horizon of a black hole is not a timelike surface, likethe surface of a star. Instead it is a null surface. This is depicted in Figure 43.Although our starting point was restricted to coordinates X and T given by (6.4),once we get to the Minkowski space metric (6.5) there’s no reason to retain this restriction. Indeed, clearly the metric makes perfect sense if we extend the range of thecoordinates to X, T 2 R. Moreover, this metric makes it clear that nothing fishy ishappening at the horizon X T . We see that if we zoom in on the horizon, thenit’s no di erent from any other part of spacetime. Nonetheless, as we go on we willlearn that the horizon does have some rather special properties, but you only get tosee them if you look at things from a more global perspective.6.1.3 Eddington-Finkelstein CoordinatesAbove we saw that, in the near-horizon limit, a clever change of variables allows us– 238 –
to remove the coordinate singularity at the horizon and extend the spacetime beyond.Our goal in this section is to play the same game, but now for the full black hole metric.Before we proceed, it’s worth commenting on the logic here. When we first metdi erential geometry in Section 2, we made a big deal of the fact that a single set ofcoordinates need not cover the entire manifold. Instead, one typically needs di erentcoordinates in di erent patches, together with transition functions that relate the coordinates where the patches overlap. The situation with the black hole is similar, butnot quite the same. It’s true that the coordinates of the Schwarzschild metric (6.1)do not cover the entire spacetime: they break down at r 2GM and it’s not clearthat we should trust the metric for r 2GM . But rather than finding a new set ofcoordinates in the region beyond the horizon, and trying to patch this together withour old coordinates, we’re instead going to find a new set of coordinates that workseverywhere.Our first step is to introduce a new radial coordinate, r? , defined bydr?2 12GMr 2dr2The solution to this di erential equation is straightforward to find: it is r 2GMr? r 2GM log2GM(6.6)(6.7)We see that the region outside the horizon 2GM r 1 maps to 1 r? 1 inthe new coordinate. As we approach the horizon, the change in r is increasingly slowas we vary r? (since dr/dr? ! 0 as r ! 2GM .) For this reason it is called the tortoisecoordinate. (It is also sometimes called the Regge-Wheeler radial coordinate.)The tortoise coordinate is well adapted to describe the path of light rays travellingin the radial direction. Such light rays follow curves satisfying dr2GMdr?2ds 0 ) 1) 1dtrdtWe see that null, radial geodesics are given byt r? constantwhere the plus sign corresponds to ingoing geodesics (as t increases, r? must decrease)and the negative sign to outgoing geodesics.– 239 –
Next, we introduce a pair of null coordinatesv t r?and u tr?In what follows we will consider the Schwarzschild metric written first in coordinates(v, r), then in coordinates (u, r) and finally, in Section 6.1.4, in coordinates (u, v).Ingoing Eddington-Finkelstein CoordinatesAs a first attempt to extend the Schwarzschild solution beyond the horizon, we replacet with t v r? (r). We havedt dv dr? dv12GMr 1drMaking this substitution in the Schwarzschild metric (6.1), we find the new metric2ds 2GMr1 dv 2 2dv dr r2 d 22(6.8)This is the Schwarzschild black hole in ingoing Eddington-Finkelstein coordinates. Wesee that the dr2 terms have now disappeared, and so there is no singularity in themetric at r 2GM . However, the dv 2 term vanishes at r 2GM and, moreover, flipssign for r 2GM . You might worry that this means that the coordinates still go badthere, or even that the signature of the metric changes as we cross the horizon. Toallay such worries, we need only compute the determinant of the metric0BBdet(g) det BB@2GM)r(1100100000r20000 r2 sin2 1CCC CAr4 sin2 We see that the dv dr cross-term stops the metric becoming degenerate at the horizonand the signature remains Lorentzian for all values of r. (The metric is still degenerateat the 0, but these are simply the poles of the S2 and we know how to deal withthat.)This, then, is the advantage of the ingoing Eddington-Finkelstein coordinates: the rcoordinate can be continued past the horizon, all the way down to the singularity atr 0.– 240 –
The original Schwarchild metric (6.1) was time independent. Mathematically, thisfollows from the statement that the metric exhibits a timelike Killing vector K @t .This Killing vector also exists in the Eddington-Finkelstein extension, where it is nowK @v . The novelty is that this Killing vector is no longer everywhere timelike.Instead, it remains timelike outside the horizon where gvv 0, but becomes spacelikeinside the horizon where gvv 0. In other words, the full black hole geometry is nottime independent! We’ll learn more about this feature as we progress.The Finkelstein DiagramTo build further intuition for the geometry, we can look at the behaviour of light rayscoming out of the black hole. These follow paths given byu tr? constantEliminating t, in preference of the null coordinate v t r? , outgoing null geodesicssatisfy v 2r? constant. The solutions to this equation have a di erent naturedepending on whether we are outside or inside the horizon. For r 2GM , we can usethe original definition (6.7) of the tortoise coordinate r? to get r 2GMv 2r 4GM log constant2GMClearly the log term goes bad for r 2GM . However, it is straightforward to writedown a tortoise coordinate that obeys (6.6) on either side of the horizon: we simplyneed to take the modulus of the argumentr? r 2GM logr2GM2GMThis means that r? is multi-valued: it sits in the range r? 2 ( 1, 1) outside thehorizon, and in the range r? 2 ( 1, 0) inside the horizon, with the singularity atr? 0. Outgoing geodesics inside the horizon then obey 2GM rv 2r 4GM log constant(6.9)2GMIt remains to find the outgoing null geodesic at the horizon r 2GM . Here the dv 2term in the metric (6.8) vanishes, and one can check that the surface r 2GM is itselfa null geodesic. This agrees with our expectation from Section 6.1.2 where we saw thatthe horizon is a null surface.– 241 –
t * v rrr 2GMFigure 44: The Finkelstein diagram in ingoing coordinates. Ingoing null geodesics andshown in red, outgoing in blue. Inside the horizon at r 2GM , outgoing geodesics do not goout.We can capture this information in a Finkelstein diagram. This is designed so thatingoing null rays travel at 45 degrees. This is simple to do if we label the coordinatesof the diagram by t and r? . However, as we’ve seen, r? isn’t single valued everywherein the black hole. For this reason, we will label the spatial coordinate by the originalr. We then define a new temporal coordinate t? by the requirementv t r? t ? rSo ingoing null rays travel at 45 degrees in the (t? , r) plane, where t? v r.
Black Holes Black holes are among the most enigmatic objects in the universe. They are described by deceptively simple solutions to the Einstein equations, yet hold a host of insights and surprises, from the meaning of causal structure, to connections to thermodynamics and, ultimately, quantum gravity. The purpose of this section is to begin to uncover some of the mysteries of these wonderful .
BLACK HOLES IN 4D SPACE-TIME Schwarzschild Metric in General Relativity where Extensions: Kerr Metric for rotating black hole . 2 BH s GM c M BH 32 BH 1 s BH M rM Uvv Jane H MacGibbon "The Search for Primordial Black Holes" Cosmic Frontier Workshop SLAC March 6 - 8 2013 . BLACK HOLES IN THE UNIVERSE SUPERMASSIVE BLACK HOLES Galactic .
mass black holes, no credible formation process is known, and indeed no indications have been found that black holes much lighter than this \Chandrasekhar limit" exist anywhere in the Universe. Does this mean that much lighter black holes cannot exist? It is here that one could wonder about all those fundamental assumptions that underly the theory of quantum mechanics, which is the basic .
However, in addition to black holes formed by stellar collapse, there might also be much smaller black holes which were formed by density fluctua-202 S. W. Hawking tions in the early universe [9, 10]. These small black holes, being at a higher temperature, would radiate more than they absorbed. They would therefore pre- sumably decrease in mass. As they got smaller, they would get hotter and .
Black holes have entropy S. Black holes have Hawking temperature T H, consistent with thermodynamic relation between energy, entropy and temperature. Thermodynamics S A 4 where Ais the area of the event horizon. T H 2 ˇ where in the surface gravity of the black hole. Luke Barclay Durham, CPT luke.barclay@durham.ac.uk Supervisor: Ruth GregoryBlack Holes, Vortices and Thermodynamics. Path .
Sep 27, 2015 · Beams - Straight Beams are measured by counting the number of holes. Beams come in odd numbers when counting the holes, with one exception. Beams start with15 holes and go down in size by two holes to the 3 hole beam and include one even-numbered beam with 2 holes. The number of holes corresponds to the length
Gap edge vortex-pair Kida vortex pair M-K. Lin (DAMTP) The stability of self-gravitating gaps November 30, 2010 14 / 27. . Edge disturbance acts as external forcing on smooth part of the disc. Edge disturbance is like a planet. Does its potential vary on global scale? M-K. Lin (DAMTP) The stability of self-gravitating gaps November 30, 2010 .
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The American Revolution This French snuffbox pictures (left to right) Voltaire, Rousseau, and colonial states-man Benjamin Franklin. Enlightenment and Revolution641 Americans Win Independence In 1754, war erupted on the North American continent between the English and the French. As you recall, the French had also colonized parts of North America through-out the 1600s and 1700s. The conflict .
