Black hole

"After the nuclear fuel is used up, the star goes into a state of gravitational collapse. All parts of the star fall more or less freely inward... [Y]ou would imagine that the freefall could not continue... because the falling material would... arrive at the center... But Einsteins equations have the peculiar consequence... permanent freefall without ever reaching the bottom... what we call a black hole. ...[T]he space ...is so strongly curved that space and time become interchanged... time becomes space and... space becomes time. More precisely, if you observe... from the outside, you see... motion slow down and stop because the direction of time inside... is perpendicular to the direction of time as seen from the outside. The collapsing star can continue to fall freely forever..."

"Schwarzschilds solution"—revealed a stunning implication of general relativity. He showed that if the mass of a star is concentrated in a small enough spherical region, so that its mass divided by its radius exceeds a particular critical value, the resulting space-time warp is so radical that anything, including light, that gets too close to the star will be unable to escape its gravitational grip. ...John Wheeler ...called them black holes—black because they cannot emit light, holes because anything getting too close falls into them, never to return. The name stuck."

"The subject of this book is the structure of space-time on length-scales from 10-13 cm, the radius of an elementary particle, up to 1028 cm, the radius of the universe. ...we base our treatment on Einsteins General Theory of Relativity. This theory leads to two remarkable predictions about the universe: first, that the final fate of massive stars is to collapse behind an event horizon to form a black hole which will contain a singularity; and secondly, that there is a singularity in our past which constitutes, in some sense, a beginning to the universe."

"Even though a black hole is practically invisible, astronomers can infer its presence from the effects it has on spacetime itself. ...Andrea Ghez... uses s to study the motions of stars near the center of our galaxy. By watching how these stars move, she is really measuring the curvature of spacetime—the strength of gravity—in the heart of the Milky Way. ...Ghez realized that the stars are wheeling about an invisible, supermassive object that weighs more than two and a half million times as much as our sun. The black hole... dubbed ... cannot be seen directly, but Ghez was able to find it because of the effect it has on spacetime, on the stars orbiting it. Ghezs technique is quite similar to what Vera Rubin did when she made the first compelling case for ."

"[F]or a physicist, the upper limit to entropy... is a critical, almost sacred quantity. ...the Bekenstein and Hawking result tells us that a theory that includes gravity is, in some sense, simpler than a theory that doesnt. ...If the maximum entropy in any given region of space is proportional to the regions surface area and not its volume, then perhaps the true, fundamental degrees of freedom—the attributes that have the potential to give rise to that disorder—actually reside on the regions surface and not within its volume. Maybe... the universes physical processes take place on a thin, distant surface that surrounds us, and all we see and experience is merely a projection of those processes. Maybe... the universe is rather like a hologram."

"[A]round 1967, Wheeler became very interested in the gravitationally collapsed objects that had described in 1917. At the time they were called black stars or dark stars. ...Wheeler began calling them black holes. At first the name was blackballed by the... . ...the term ...was deemed obscene! But John fought it... Amusingly, Johns next coinage was the saying "Black holes have no hair." ...he was making a very serious point about black hole horizons. ...[Each a] smooth ...perfectly regular, featureless sphere. Apart from their mass and rotational speed, every black hole was exactly like every other. Or so it was thought."

"A star does not evolve over its lifetime through each spectral type, as Russell once thought; rather, each star experiences its own distinct history, based on its mass at birth. Smaller stars, such as tiny s, will never reach the red-giant stage but just dully burn away like red-hot ovens. Stars that are born with appreciably more mass than our Sun, such as the white-hot O and B stars, will burn swiftly and eventually blow up, leaving behind a city-sized or even a black hole, a gravitational pit from which no light or matter can escape. ...the term black hole wasnt even coined until 1968. Yet the first tentative steps toward understanding this great metamorphosis, the distinct and striking stages in a stars life, were taken at the turn of the century. The elements in the stars themselves were telling the tale in the spectral messages they were telegraphing throughout the cosmos."

"Black holes have the universes most inscrutable poker faces. ...When youve seen one black hole with a given mass, charge, and spin (though youve learned these thing indirectly, through their effect on surrounding gas and stars...) youve definitely seen them all. ...black holes contain the highest possible ...a measure of the number of rearrangements of an objects internal constituents that have no effect on its appearance. ...Black holes have a monopoly on maximal disorder. ...As matter takes the plunge across a black holes ravenous , not only does the black holes entropy increase, but its size increases as well. ...the amount of entropy ...tells us something about space itself: the maximum entropy that can be crammed into a region of space—any region of space, anywhere, anytime—is equal to the entropy contained within a black hole whose size equals the region in question."

"A natural guess is that... a black holes entropy is... proportional to its volume. But in the 1970s and Stephen Hawking discovered that this isnt right. Their... analyses showed that the entropy... is proportional to the area of its event horizon... less than what wed naïvely guess. ...Berkenstein and Hawking found that... each square being one by one Planck length... the black holes entropy equals the number of such squares that can fit on its surface... each Planck square is a minimal unit of space, and each carries a minimal, single unit of entropy. This suggests that there is nothing, even in principle, that can take place within a Planck square, because any such activity could support disorder and hence the Planck square could contain more than a single unit of entropy... Once again... we are led to the notion of an elemental spatial entity."

"There is no shortage of candidates for... baryonic . It may come in many forms—clouds of gas or dust, large planetlike objects, various forms of degraded stars, and black holes. ...MACHOS could include black holes and burned-out stars, such as s or s... Black holes are perhaps the most intriguing, and the most difficult to detect and quantify. As far back as the eighteenth century, scientists speculated about worlds so massive that nothing escaped their gravitational grip, not even light. In the early twentieth century, J. Robert Oppenheimer used Einsteins general theory of relativity to explain how a black hole might form: The black hole would warp adjacent space so deeply that the would exceed the speed of light... hence nothing... could leave... The center of the Milky Way emits intense gamma radiation—the death cry, perhaps, of stars falling into a black hole. Black holes may also be distributed in galactic halos, where they might constitute a substantial fraction of baryonic dark matter."