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38 Scientific American October 2009 © 2009 SCIENTIFIC AMERICAN, INC. EUROPEAN SPACE AGENCY, NASA AND FELIX MIRABEL French Atomic Energy Commission, Institute for and Space Physics/Conicet of Argentina B gions re- These observers. distant reach can signal there can be regions of that from predict which no unambiguously equations These . as know we force the producing gy, the time controls motion of the and ener- and elastic howof curvature space- the resulting of made were it if as spacetime distort energy ativity. The theory describes how all matter and which are at the of heart his of theory general rel- class of solutions of the Einstein eld equations, ingests. it all destroys irretrievably and within, falls that anything or anyone for cape of no es- “eventis its admits that horizon,” tain cur- the behind mystery unfathomable harbors hole a fears: primal our of some on play nants of collapsed seem almost designed to Trek BY cArlOS BArcelÓ, StefAnO liBerAti, SeBAStiAnO SOneGO AnD mAtt ViSSer mAtt AnD SOneGO SeBAStiAnO liBerAti, StefAnO BArcelÓ, cArlOS stars BY black called entities dense to forming from instead holes rise give black and true prevent may effects Quantum can escape. A conceptual boundary, the event event the boundary, conceptual A escape. can gravitation from which nothing, not even light, extreme of zone empty an by surrounded ty”) “singulari- (a in nity approach matter www.ScientificAmerican.com www.ScientificAmerican.com To theoretical physicists, black holes are a are holes black physicists, Totheoretical STARS — ing ing a rolecentral in the plot of this year’s now,play- for decades recently most ture lack holes have been a part of cul- popular black holes black movie. No wonder. These dark rem- dark No wonder. These movie. H LES LES H H LES LES H BLACK — consist of a location where where location a of consist NOT NOT

© 2009 SCIENTIFIC AMERICAN, INC. AMERICAN, © SCIENTIFIC 2009 nite. Presumably fails by not fails relativity general Presumably nite. is in- some that quantity predicts when a theory case the usually is as location, at that fails ory sides inside every suggests that re- the the- singularity a that prediction relativity’s eral gen- particular, In holes. black it describes that to theory itself is not entirely observations satisfactory The in the way certainly date but the well, quite theory tthe relativity? general by dicted pre- holes black the really observe astronomers for black holes ity’sof those . predictions millions of kilometers million a over determine, range from well only to several kilometers to few a suns, their diameters, just as best astrophysicists from can ranging have objects dark these masses own.Although their of radiation other or light no essentially some contain compact extremely bodies that emit does universe the that indicates observations cal astrophysi- high-quality of variety wide A ality? for a in black hole diameter of the ’s . the the event is horizon a sphere case, simplest the In spacetime. of rest the from horizon, separates the zone of intense gravitation Yet are these dark and compact bodies that that bodies compact Yet and dark these are So So much for ction and theory. What about re- , — matching matching general relativ- — just six kilometers

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Contrary to physicists’ physicists’ to Contrary quantum Approximate theoretical are holes Black a “black star” instead. create and forming hole a grow large enough to stop may polarization vacuum called effect quantum a wisdom, conventional to describe black holes. gravity of theory quantum consistent a full, seeking still are Physicists way. cal paradoxi- a in albeit rate, black holes slowly evapo- calculations predict that side its . in- passing after gravity hole’s black a escape can Nothing relativity. general of theory the by predicted structures in spacetime

— The The Editors 39 [BASICS] would be blocked from taking the final plunge Black Holes in Brief to infinite and from becoming enveloped in an event horizon. The black star would be A black hole is a region of curved spacetime with such intense gravity that noth- supported by something not normally considered ing can escape. Its defining feature is its event horizon: the boundary of the to be a sturdy construction material: space itself. region of no escape. A black hole is mostly empty, its mass apparently collapsed to a location with infinite density—a “singularity”—deep inside the horizon. The Weight of Quantum Nothingness A black hole with three times the mass of the sun Far away from large We derive our conclusions by applying a venera­ would have a diameter of about 18 kilometers, masses, a flash of light ble approach known as semiclassical gravity, but comparable to the length of Manhattan. spreads out symmetrical- without making all the same assumptions about ly in all directions ●1 . 18 kilometers the collapsing matter that previous studies have made—to see if we might avoid the paradoxical ●1 Light wave one territory arrived at by those studies. In the ab­ second later Flash on riz sence of a full-fledged theory of , ho t n e 2 theorists have resorted to semiclassical gravity v ● E 3 ● over the past 30 -odd years to analyze how quan­ tum mechanics alters black holes. This method Just outside a black hole’s partially incorporates aspects of quantum phys­ event horizon, the gravity captures most of a flash● 2 . ics—in particular, quantum theory—into classical Einsteinian gravity. Singularity Some light escapes, just ●3 . describes each kind of ●4 ) fundamental particle—the , the , If a flash occurs anywhere , you name it—in terms of a field that fills inside an event horizon, all the light is drawn into the space, much like the electromagnetic field. Quan­ black hole’s singularity ●4 . tum field theory’s equations are usually set up in

flat spacetime, that is, in the absence of gravity. dust disk around black hole (

Semiclassical gravity uses quantum field theory as formulated in curved spacetime. In the broadest terms, the strategy of semiclas­ sical gravity goes as follows: a collection of mat­

 In practice, black holes can be ter in some configuration would, according to Washington of Univerity observed via the material orbiting and Black hole at center falling into them. The image at the right, (not visible) classical general relativity, produce some specific taken in 1998 by the Hubble Space Tele- curved spacetime. Yet the curvature of spacetime scope, shows a vast disk of gas and dust believed to have a modifies the energy of the quantum fields. This at its center. Strictly speaking, however, modified energy, according to classical general such observations inform scientists only relativity, changes spacetime’s curvature. And so that an extremely compact, heavy object andFrank denboschvan c.

emitting little or no light of its own is Disk of gas and dust on, iteration after iteration. present; they do not provide absolute The goal is to obtain a self-consistent solu­ proof that the object is a black hole. tion—a curved spacetime containing a configu­ ration of quantum fields whose energy generates

taking into account quantum effects, which mat­ that same curvature. That kind of self-consistent Institute Telescope Space ter and energy exhibit at the microscopic scale. solution ought to be a good approximation to The search for a modified theory that incorpo­ how reality behaves in many situations involving rates , generically called quantum effects and gravity even though gravity quantum gravity, is a powerful engine driving a itself has not been described by a quantum theo­ lot of activity in research. ry. Semiclassical gravity thus incorporates quan­ roeland p. van derroelandmarelvan p. This need for a quantum theory of gravity rais­ tum corrections into general relativity in a “min­ , es fascinating questions: What would quantum- imal” way, taking into account the quantum be­ ); nasa corrected black holes be like? Would they be radi­ havior of matter but still treating gravity (that is,

cally different from classical black holes, or would spacetime curvature) classically. illustration their classical description remain a good approx­ This approach, however, immediately runs imation? The four of us have shown that certain into an embarrassing problem in that the straight­ quantum effects may well prevent black holes forward calculation of the quantum fields’ low­ from forming at all. Instead a kind of object we est possible (or “zero point”) energy—the energy

have named a black star could arise. A black star when no particles of any kind are present, the en­ lucy reading-ikkanda (

40 Scientific American October 2009 © 2009 SCIENTIFIC AMERICAN, INC. ergy of the vacuum—produces an infinite result. Black hole density in flat spacetime. This assumption makes This problem actually comes up already with or­ categories for a consistent semiclassical vacuum: the ener­ dinary quantum field theory (that is, in flat space, gy density is zero everywhere, for which general no gravity). Fortunately for theorists wishing to General relativity predicts that a relativity predicts flat spacetime. black hole is completely defined by predict particle physics phenomena that do not If some matter is present, spacetime is curved, just three quantities: mass, angular involve gravity, the particles behave in ways that momentum and . It which alters the quantum fields’ zero-point en­ depend on only the energy differences between makes no difference what went into ergy density, which means the zero-point energy states, so the value of the quantum vacuum ener­ the hole—matter, or is no longer exactly canceled. The excess amount gy plays no role. Careful subtraction schemes energy, or all three combined. is said to be caused by vacuum polarization, by known as renormalization take care of the infini­ analogy with the effect of an electric charge po­ Astronomers have observed holes in ties, allowing the energy differences to be com­ larizing a medium [see box on next page]. three mass classes: Holes of about puted with extremely high precision. five to 15 solar masses are formed We have described these features of semiclas­ With gravity in the picture, however, the vac­ from dying stars. Many sical gravity in terms of mass and energy density, uum energy . An infinite energy density harbor a hole of millions to billions but in general relativity it is not only those quan­ would seem to produce an extremely large cur­ of solar masses at their core. Holes tities that produce spacetime curvature. The mo­ of a few thousand solar masses vature of spacetime —that is, even “empty” space mentum density and the pressures and stresses have been detected in the center of would harbor an intense gravitational force, globular star clusters. associated with a specific gravitating substance which is not remotely compatible with the uni­ also do so. A single mathematical-physics object, verse that we actually observe. Astronomical ob­ known as the stress energy tensor (SET), de­ servations over the past decade indicate that the scribes all these curvature-producing quantities. net zero-point contribution to the universe’s to­ Semiclassical gravity assumes that the quantum tal energy density is extremely tiny. The semi­ fields’ zero-point contributions to the total SET classical gravity approach does not attempt to are exactly canceled in flat spacetime. The math­ solve this problem. Instead it is customary to as­ ematical-physics object obtained applying such sume that whatever the solution is, it exactly a subtraction procedure to the SET is called the cancels the zero-point contribution to the energy renormalized stress energy tensor (RSET).

[PARADOX] The Trouble with Quantum Black Holes The classical (that is, nonquantum) equations of general relativity tions that predicted black holes would randomly emit particles at forbid anything emerging from inside a black hole’s event horizon. a very low rate (left panel). The randomness created a paradoxical Yet in the 1970s Stephen W. Hawking carried out quantum calcula- scenario (right panel) known as the information problem. Information Is Lost is emitted Pair Annihilation Even in empty space, a quantum process creation Matter that falls into a black hole carries constantly produces pairs of so-called with it a vast quantity of information. Random virtual particles and antiparticles, which particle immediately annihilate each other.

Corresponding Near a black hole’s event horizon, one antiparticle may be captured by the black hole, and the second may escape. The escaped particle carries away posi- Thus, if nothing falls into the black hole, tive mass, and the captured one takes its mass and its event horizon gradually negative mass into the black hole— shrink. This evaporation process speeds thereby reducing the hole’s mass. up as the hole becomes smaller.

Escaped particle Hawking’s finding indicates that a black hole can evaporate all the way to zero mass, but the random orizon Event h Event particles it emits carry horizon almost no information. shrinks The apparent loss of information violates a Captured particle fundamental feature of quantum mechanics called unitarity. This contradiction begs for resolution. lucy reading-ikkanda lucy

www.ScientificAmerican.com SCIENTIFIC AMERICAN 41 © 2009 SCIENTIFIC AMERICAN, INC. [QUAnTUM PRIMER] wHAt emPtineSS cAn DO In classical general relativity, spacetime is dynamic, its curvature producing gravity. A quantum effect known as vacuum polarization provides another way that empty space can play an active role in the universe.

ELECTRIC AnALogY VACUUM PoLARIZATIon In a medium, a charged object’s electric fi eld (left) polarizes nearby (center), In general relativity, the role of electric charge is reducing the total electric fi eld (right). Quantum fi eld theory reveals that even a vacuum played by mass and energy and that of the electric can be polarized, because an electric fi eld polarizes virtual particle/antiparticle pairs. fi eld by curved spacetime, or gravity. The vacuum polarization produces an energy defi cit (in effect Effective cloud a cloud of negative energy) and a repulsive force. Positively charged particle of negative charge

+ + + Mass

- Electric + Effective cloud Repulsion fi e l d Atoms of negative mass

When applied in curved spacetime, the sub­ tion is negligible. Thus, the sun is far from form­ traction scheme still succeeds in canceling the ing a black hole according to the classical equa­ SET’s divergent part but leaves a fi nite, nonzero tions, and quantum corrections do not alter this value for the RSET. The end result is the follow­ picture. Indeed, astrophysicists can safely ignore ing iterative process: classical matter curves quantum gravity effects when analyzing the sun spacetime via Einstein’s equations, by an amount and most other astronomical objects. determined by the matter’s classical SET. This The quantum corrections can become signifi ­ curvature makes the quantum vacuum acquire cant, however, if a star is not much larger than its a fi nite nonzero RSET. This vacuum RSET be­ gravitational radius. In 1976 David G. Boulware, comes an additional source of gravity, modify­ now at the University of Washington, analyzed ing the curvature. The new curvature induces in the case of such a when the star is turn a different vacuum RSET, and so on. stationary (that is, not collapsing). He showed that the closer the star is to its gravitational radi­ Quantum-Corrected Black Holes us, the larger the vacuum RSET near its surface With the approach of semiclassical gravity becomes, increasing to infi nite energy density. spelled out, the question becomes: How do these This result implies that semiclassical gravity the­ quantum corrections affect predictions about ory does not permit a stationary black hole black holes? In particular, how do the correc­ (meaning one whose event horizon remains con­ [THE AUTHoRS] tions alter the process of forming a black hole? stant in size) as a solution of its equations. Carlos Barceló, Stefano Libera- The simplest black hole of some mass (say, M Boulware’s result, however, does not tell us ti, Sebastiano Sonego and times the ) is one that is not rotating what to expect in the case of a star undergoing a Matt Visser have been collaborat- and not electrically charged. Such a hole has a collapse that would lead to a black hole accord­ ing in various combinations and permutations since the new millen- radius R that works out to be 3M kilometers. ing to classical general relativity. Stephen W. nium. Barceló is professor of The radius R is called the gravitational radius or Hawking had already tackled this situation a theoretical physics and a vice for that mass. If for any year earlier, using somewhat different tech­ director at the Institute of Astro- reason some matter has collapsed to occupy a re­ niques, to show that a classical black hole formed physics of Andalusia in Spain. gion smaller than its gravitational radius, it has by collapse emits random particles. More precise­ Liberati is assistant professor of at the International formed a black hole; it has disappeared inside its ly, the particles have a distribution of energies School for Advanced Studies in own event horizon. characteristic of thermal radiation; the black hole Trieste, Italy. Sonego is professor The sun, for instance, has a 700,000 ­kilome­ has a temperature. He conjectured that quantum­ of mathema tical physics at the ter radius, which is much larger than its gravita­ corrected black holes would be essentially classi­ University of Udine in Italy. Visser tional radius (three kilometers). The relevant cal black holes subject to slow evaporation via is professor of mathematics at Victoria University of Wellington semiclassical gravity equations make it clear that this radiation. A black hole of one solar mass has

in new Zealand. the RSET of the quantum vacuum in this situa­ a temperature of 60 nanokelvins. The corre­ lucyreading-ikkanda

42 Scientific AmericAn October 2009 © 2009 SCIENTIFIC AMERICAN, INC. sponding evaporation rate is so slow that absorp­ scribed by classical general relativity and subse­ tion of cosmic background radiation would com­ Quantum quently undergo slow quantum evaporation via pletely overwhelm the evaporation and the hole matter always Hawking radiation. would grow in size. An evaporating black hole of such a mass would be indistinguishable from a seems to The Information Problem classical black hole in practice because the evap­ fi nd new ways Hawking’s discovery of black hole evaporation, oration would be immeasurably small. along with earlier results by Jacob D. Bekenstein Considerable effort by theorists in the decade of delaying of the Hebrew University of Jerusalem uncovered after Hawking’s paper, including the approxi­ gravitational a deep—and as yet not fully understood—rela­ mate calculation of the RSET in collapsing con­ tion among gravity, quantum physics and ther­ fi gurations, reinforced this picture as being the collapse. modynamics. At the same time, it opened up new correct one. Today the standard view in the phys­ problems. Perhaps the most important is known ics community is that black holes form as de­ as the information problem, which is closely related to the question of the fi nal outcome of [THE AUTHoRS’ PRoPoSAL] black hole evaporation. Take the example of a large star undergoing A BlAck StAr iS BOrn . The star embodies a vast A black hole forms when some matter collapses under its own weight and amount of information in the positions and ve­ 55 no force can stop it. Physicists’ conventional wisdom is that quantum effects locities and other properties of its more than 10 cannot be large enough to stop such a collapse. The authors disagree. particles. Suppose the star forms a black hole but then, gradually over the aeons, evaporates by emitting Hawking radiation. A black hole’s tem­ FAST CoLLAPSE Free-falling g E ra ve matter vi n perature is inversely proportional to its mass, and IS noT HALTED ta t t h io o n r thus an evaporating black hole becomes hotter The vacuum polarization i a z o l

is negligible for free- n r and evaporates faster as its mass and radius

a

falling matter, even d i

u shrink. A huge explosion ejects the last of the when the matter gets s dense enough to form black hole’s mass. But what remains afterward? an event horizon and Does the hole completely vanish, or does some become a black hole. kind of small remnant remain? In either case, what has happened to all the information of the SLoWER CoLLAPSES MAY BE DELAYED FoREVER star? According to Hawking’s calculation, the If the matter’s fall is slowed, The repulsion further slows The collapse is delayed particles radiated by the hole carry essentially no vacuum polarization may the collapse, which allows from ever forming an information about the star’s initial state. Even if grow, producing repulsion. the polarization to intensify. event horizon. some kind of black hole remnant remains, how could such a small object contain all the infor­ mation that was in the original star? The disappearance of information matters because one of the most fundamental pillars of quantum theory is that quantum states evolve in a manner that is called unitary, one consequence of which is that no information ought to ever be Vacuum Repulsion truly obliterated. Information may be inaccessi­ polarization ble in practice, such as when an encyclopedia Black star burns up, but in principle the information re­ B L A C k S T A R mains in the swirling smoke and ashes. The result is a black star. The gravitational fi eld around it is identical to that around a black Because the calculations that predict Hawk­ Mass-fi lled hole, but the star’s interior is full of matter and interior ing radiation rely on semiclassical gravity, no event horizon forms. A black star could emit Hawking-like radiation, but this radiation physicists cannot be sure if information loss carries the information that went into the is an artifact of the approximations involved black star, preserving unitarity. If a black star Material or a feature that will remain when we dis­ could be peeled layer by layer like an onion, at surface each stage the remaining core would be a cover how to compute the process exactly. smaller black star, also emitting radiation. If the evaporation process does destroy in­ Small black holes emit more radiation and have higher temperatures than larger ones, and so a Highest formation, the correct full quantum gravity temperature black star is increasingly hot toward its center. equations must violate the unitary nature of

lucyreading-ikkanda quantum mechanics as we know it. Converse­

www.ScientificAmerican.com Scientific AmericAn 43 © 2009 SCIENTIFIC AMERICAN, INC. ly, if information is preserved and a complete the­ prediction that black holes form from gravita­ ory of quantum gravity will reveal where it is in tional collapse even when quantum effects are the radiation, either general relativity or quan­ considered depends on several technical and tum mechanics seems to need modifi cation. often unstated assumptions. In particular, the old calculations assume A Radically Different Alternative that collapse proceeds very rapidly, taking about The information problem and related puzzles the same time as would be needed for material have motivated us (and others) to revisit the line at the star’s surface to free­fall to the star’s cen­ of reasoning that led physicists in the 1970s to ter. We found that for a slower collapse, quan­ the picture of evaporating almost classical black tum effects may produce a new kind of very holes. We have found that the old semiclassical compact object that does not have an event ho­ [ALTERnATIVE BoDIES] rizon and is thus much less problematic. As we have already mentioned, the RSET of OtHer wAYS Out Of A HOle the quantum vacuum in a spacetime curved by a typical star is negligible everywhere. When Many researchers have proposed more or less exotic objects that could serve the star starts to collapse, the RSET might as alternatives to the conventional (but apparently paradoxical) idea of an change. Nevertheless, the old conclusion that evaporating black hole and account for the dark, compact bodies observed by astronomers. The common feature of these proposals (and our own black star the RSET remains negligible continues to hold hypothesis) is that the new object would lack an event horizon. if the collapse is about as fast as free falling. Yet if the collapse proceeds significantly slower than free falling, the RSET can acquire The spacetime geometry around a “gravitational vacuum star” would be indistin- arbitrarily large and negative values in the re­ guishable from that of a black hole down to about 10–35 meter away from the gion near the Schwarzschild radius—where the spherical region where the classical black hole horizon would have been located. classical event horizon would have formed. A The horizon would be replaced by a shell of matter and energy a mere 10–35 meter negative RSET produces a repulsion, which fur­ thick (known as the —the length scale at which quantum gravity effects are expected to become large). The ’s interior would be empty ther slows the collapse. The collapse might come space with a large vacuum polarization, which would produce a repulsion that to a complete halt just short of forming a hori­ prevents the matter shell from collapsing any further. In a variant of the gravastar zon, or it might continue forever at an ever slow­ proposal, the classical notions of geometry break down in the region separating er pace, becoming ever closer to forming a hori­ the interior and exterior. zon but never actually producing one. This result, however, does not make it impos­ BLACk HoLE CoMPLEMEnTARITY In conventional quantum mechanics, complementarity refers to the idea that an sible for black holes to form. A perfectly homo­ observation may reveal either the particle nature of an object or the wave nature, geneous spherical cloud of matter of, say, 100 but not both. Similarly, the quantum mechanics of black holes might embody a million solar masses falling freely under its own new kind of complementarity. An observer who remains outside a black hole may weight would surely produce an event horizon. have one description of the observable geometry (for instance, imagining a Such a large cloud would have a density compa­ membrane having certain physical properties in place of the event horizon), rable to that of water when it became compact whereas an observer who falls into the hole must use a different description. enough to form a horizon. At such a low density FUZZBALLS the RSET cannot become large enough to pre­ Proponents of “fuzzballs” contend that the horizon would be a transition region vent the horizon from forming. But we know between the exterior classical geometry and a quantum interior where no defi - that what happened in the universe did not fol­ nite notion of spacetime could be specifi ed. The interior would be describable by low this script. The vast, nearly homogeneous theory and would not have a singularity (right). Each clouds of matter that emerged from the early exterior geometry (say, the geometry of a black hole of stages of the did not collapse to form exactly 1030 kilograms) could have any one of an expo- black holes. Instead a sequence of structures nentially large number of such stringy quantum developed. states as its interior. The semiclassical view of a First, stars formed, the heat of their nu­ black hole—with an event horizon, an enormous clear reactions delaying the collapse for a entropy, a temperature and emission of ther- Classical long time. When a star largely exhausts mal Hawking radiation—would amount to a description statistical average over all the possible interi- breaks down its nuclear fuel, it may develop into a ors, analogous to a description of a volume of dwarf or, if massive enough, ex­ gas that disregards the exact positions and one of 1035 plode as a , leaving behind a possible motions of the individual atoms. quantum star (a sphere made of —C.B., S.L., S.S. and M.V. string states that is only somewhat larger than the

star’s gravitational radius). In either case, lucyreading-ikkanda

44 Scientific AmericAn October 2009 © 2009 SCIENTIFIC AMERICAN, INC. it is actually a purely quantum effect—the Pauli what’s next stops, we have shown that a black star could exclusion principle—that prevents further col­ emit particles with a so-called Planckian energy lapse. The neutrons in the cannot Future work on the black star scenar- spectrum (which is very similar to a thermal io must show specific physical sys- enter the same quantum state, and the resulting tems for which vacuum polarization spectrum), at a temperature very slightly smaller pressure resists the gravitational collapse. A succeeds in halting a collapse accord- than the Hawking temperature. By having no similar story for ions and explains ing to semiclassical gravity. horizon, the black star cannot lock away any in­ why a is stable. formation. Instead the emitted particles and If the neutron star acquires more mass, even­ By describing quantum black holes whatever matter remains behind with the black tually the crushing gravitational load over­ as bundles of fundamental entities star carry all the information. Standard quan­ called , string theorists have whelms the neutrons, and further collapse oc­ reproduced predictions of semiclassi- tum physics would describe the formation and curs. We do not know for certain what happens cal gravity for certain special cases. evaporation process. Black stars do not com­ next (although the conventional view says a They hope to extend these results to pletely solve the information problem, however, black hole forms). Scientists have suggested a va­ all kinds of black holes. as long as ways remain for event horizons to riety of objects that might form—such as so- form somewhere in the universe. called stars, strange stars, stars and A definitive resolution of the informa- These evaporating objects could be called tion problem and of the fate of — Q-balls that would be stable at pressures too collapsing matter will most likely quasi black holes because when viewed from the great for a neutron star. Physicists must develop require development of a complete outside they would have approximately the same a better understanding of how matter behaves at quantum theory of gravity. thermodynamic properties as evaporating black densities well beyond that of neutrons to know holes. Their interiors, however, would harbor a which conjecture, if any of them, is correct. rainbow of temperatures, rising to a maximum Thus, experience tells us that matter follow­ near the center. If you imagine the body as an ing the laws of quantum mechanics always onionlike structure of concentric shells, each seems to find new ways of delaying gravitational shell would be slowly shrinking, never quite collapse. Although any of these roadblocks may compact enough for the combined mass of the be overcome (a typical stable configuration can shell and everything inside it to form a horizon. always be made unstable by adding enough mat­ Each shell would be prevented from collapsing ter), each process that delays collapse provides by the vacuum RSET that we predict will devel­ additional time for the quantum vacuum’s nega­ op where the conditions for a horizon are ap­ tive RSET to pile up and become significant. proached slowly enough. The deeper shells This RSET could take over the task of counter­ would have higher temperatures, just like small­ balancing the gravitational pull, and because its er-mass black holes do. We do not yet know repulsion may increase without limit, it can stop ➥ More To whether these appealing objects show up natu­ the matter’s collapse to a black hole forever. Explore rally or whether they are exceptional. Fate of Gravitational Collapse Black Stars in Semiclassical Gravity. Carlos Over the Horizon The resulting bodies would be the new kind of Barceló, Stefano Liberati, Sebastiano Study of black holes has always provoked a great object we have named black stars. Because of Sonego and Matt Visser in Physical variety of reactions from researchers. On the one Review D, Vol. 77, No. 4; February 19, their extremely small size and high density, they 2008. hand, it is exciting to think that they hide within would share many observable properties with them the door to unforeseeable new possibilities black holes, but conceptually they would be rad­ Small, Dark, and Heavy: But Is It a in physics, albeit only for those who dare to enter. ically different. They would be material bodies, Black Hole? Matt Visser, Carlos Bar- On the other hand, implications of black holes celó, Stefano Liberati and Sebastiano with a material surface and an interior filled have long disturbed some physicists—the quest Sonego in Proceedings of Black Holes with dense matter. They would be extremely in General Relativity and String Theo- for alternatives to black holes, often motivated dim because light emitted from their surface ry; August 2008. Available at http:// by distaste for one or another of their features, would be very redshifted —the light wave greatly arxiv.org/abs/0902.0346 is as old as the idea of black holes themselves. stretched—in traveling from the intensely curved Our black star proposal and other research­ space near the black star to distant astronomers. The Fuzzball Proposal for Black ers’ black hole alternatives all have the common Holes. K. Skenderis and M. Taylor in In principle, astronomers could conduct com­ Physics Reports, Vol. 467, No. 4–5, theme that the spacetime around them is essen­ plete astrophysical studies of black stars because pages 117–171; October 2008. tially identical to that around a classical black no event horizon would present an obstacle. http://arxiv.org/abs/0804.0552 hole, down to extremely close to where the hori­ Within the family of bodies of black star type, zon would have formed. Although the secret some might resemble evaporating black holes by The Black Hole War: My Battle door leading to an understanding of how quan­ with to Make emitting radiation similar to Hawking radia­ the World Safe for Quantum tum physics merges with gravity remains out of tion. For the specific case in which collapse ap­ Mechanics. . our sight, it may not be shielded from us by the proaches formation of a horizon but never quite Little Brown, 2008. impenetrable fortress of an event horizon. ■ www.ScientificAmerican.com SCIENTIFIC AMERICAN 45 © 2009 SCIENTIFIC AMERICAN, INC.