BLACK HOLES

A slightly different kind of supernova explosion occurs when even larger, hotter stars (blue giants and blue supergiants) reach the end of their short, dramatic lives. These stars are hot enough to burn not just hydrogen and helium as fuel, but also carbon, oxygen and silicon. Eventually, the fusion in these stars forms the element iron (which is the most stable of all nuclei, and will not easily fuse into heavier elements), which effectively ends the nuclear fusion process within the star. Lacking fuel for fusion, the temperature of the star decreases and the rate of collapse due to gravity increases, until it collapse completely on itself, blowing out material in a massive supernova explosion.

In the huge compressed mass remaining, even neutrons are crushed and the core of the star collapses completely into a gravitational singularity, a single point containing all the mass of the entire star, and creating what has become known as a black hole. The largest blue stars may skip even the supernova stage and even their outer shells become incorporated into the singularity. Black holes are, almost by definition, where gravity has overwhelmed all over forces. Thus, the gravity of a body just a few times smaller and denser than a neutron star would result in it further collapsing to a black hole, effectively trapping all the light in its field of gravity and completely “closing up” the space around it.

(Click for a larger version) Simulated black hole in front of the Milky Way (Source: Space Time Travel: http://www.spacetimetravel.org/ galerie/galerie.html - Credit: Ute Kraus)

A black hole actually exerts no more gravitational pull on the objects around it than the original star from which it was formed, and any objects orbiting the original star (and which survived the supernova blast) would now orbit a black hole instead. However, its gravitational effects are different from those of a star in that as a body approaches a black hole its gravitational pull gets stronger and stronger, whereas a star’s gravity is strongest at its surface.

Although the centre of a black hole is infinitely dense, the black hole itself is not necessarily huge, as is sometimes assumed. A black hole with the mass of our Sun, for example, would have a radius of about three kilometres (roughly two hundred million times smaller than the Sun), while one with the mass of the Earth would fit in the palm of your hand! It can grow bigger over time, however, as its gravity attracts more and more matter and even other black holes, and some do become massive.

We can not observe black holes directly, but they can be detected by the gravitational effect they exert on other bodies or on light rays (this is especially easy to spot in the case of binary star systems where an ordinary star is orbiting around a black hole). We know that, in our own Milky Way galaxy alone, there are many millions of black holes of about ten solar masses each. We also know, from the speed with which the stars close the centre of Milky Way are orbiting, that there is a supermassive black hole (called Sagittarius A) with a mass of around 4 million times that of our Sun. Even larger black holes lurk in the centres of other galaxies, which we can infer from observations of the intense radiation of gases swirling around them at close to the speed of light, some weighing as much as several billion suns and as large as our entire Solar System.

(Click for a larger version) Spinning and non-spinning black holes (Source: Chandra X-Ray Observatory: http://chandra.harvard.edu/ photo/2003/bhspin/)

The simplest type of black hole, in which the core does not rotate and just has a singularity and an event horizon, is known as a Schwarzschild black hole after the German physicist Karl Schwarzschild who pioneered much of the very early theory behind black holes in the 1910s, along with Albert Einstein. In 1958, David Finkelstein published a paper, based on Einstein and Schwarzschild’s work, describing the idea of a “one-way membrane” which triggered a renewed interest in black hole theory (although the phrase itself was not coined until a lecture by John Wheeler in 1967). In particular, it caught the imagination of the young Roger Penrose, and the calculations of Penrose and Wheeler showed that any non-rotating star, however complicated its initial shape and internal structure, would end up after gravitational collapse as a perfectly spherical black hole, whose size would depend solely on its mass.

In 1963, the New Zealander Roy Kerr discovered a solution to Einstein’s field equations of general relativity which described a spinning object, and suggested that anything which collapsed would eventually settle down into a spinning black hole. It spins because the star from which it formed was spinning, and it is now thought that this is actually likely to be the most common form in nature. A rotating black hole would bulge outward near its equator due to its rotation (the faster the spin, the more the bulge).

Although it may seem a very complex, peculiar and perhaps counter-intuitive object, a black hole can essentially be described by just three quantities: how much mass went into it, how fast it is spinning (its angular momentum) and its electrical charge. ”, after John Wheeler’s comment that “black holes have no hair”, by which he meant that any other information about the matter which formed a black hole (for which "hair" is a metaphor) remains permanently inaccessible to external observers within its event horizon, and is all but irrelevant.

Brandon Carter and Stephen Hawking proved the No-Hair Theorem mathematically in the early 1970s, showing that the size and shape of a rotating black hole would depend only on its mass and rate of rotation, and not on the nature of the body that collapsed to form it. They also proposed four laws of black hole mechanics, drawing an analogy with thermodynamics and, in 1974, Hawking calculated that black holes should thermally create and emit sub-atomic particles, known today as Hawking radiation, until they exhaust their energy and evaporate completely.

A corollary of this, though, is the so-called “Information Paradox” or “Hawking Paradox”, whereby physical information (which roughly means the distinct identity and properties of particles going into a black hole) appears to be completely lost to the universe, in contravention of the accepted laws of physics. Hawking vigorously defended this paradox against the arguments of Leonard Susskind and others for thirty years, until famously retracting his claim in 2004. His latest line of reasoning is that the information is in fact conserved, although not in our observable universe but in the multiverse as a whole.

It is also theoretically possible that "primordial" or "mini" black holes could have been created in the conditions during the early moments after the Big Bang, possibly in huge numbers. No such mini black holes have ever been observed, however - indeed, they would be extremely difficult to spot - and they remain largely speculative. It is anyway likely that all but the largest of them would have already evaporated by now as they leak away Hawking radiation.