A black hole is one of the theoretical remnants of a dying star – one so large that the collapsing core left behind after its explosive supernova is so gravitationally powerful that even light cannot escape. At present there are no confirmed black holes, but astronomers and theoretical physicists are developing several ways to test conclusively whether they exist and where to find them.
– Black Hole Formation and the Theory of Relativity –
When stars run out of hydrogen and can no longer sustain the fusion reactions which keep them glowing hot, they die: stars roughly the size of our Sun became glowing-hot embers called white dwarves, and slightly larger stars are too large to preserve the spaces between the atoms and compress themselves into tiny, incredibly heavy spheres called neutron stars. But massive blue giants, many times the mass of our own Sun, collapse into remnants so heavy that their gravity continues to suck in all matter and energy nearby – including light.
That these would become black holes was an insight made possible by Einstein’s theory of relativity. All stars, planets, and other celestial objects have very powerful gravity, from which one can only escape by travelling at a sufficiently high speed, known as the escape velocity. On Earth, for example, we use rockets to accelerate spacecraft until they achieve escape velocity, and will no longer fall back to Earth.
In massive stellar remnants, the escape velocity is far higher – so objects wishing to escape must travel even faster. Light, it was revealed by Einstein, is a form of radiation which travels at a specific speed: the speed of light, or 186,000 miles per second. Under Einstein’s theories, nothing can travel faster than light, and it is virtually impossible for a physical object to actually travel as fast as light; doing so would require infinite energy. The problem with sufficiently large remnants, those known as black holes, is that the escape velocity is even higher than the speed of light. As a result, everything – even light – gets pulled in by the gravity of a black hole.
– Finding Black Holes –
The problem introduced by this theory is, of course, that by its nature a black hole will be invisible: it will emit no light and therefore will never be seen in a telescope. The best option for the moment is to find black holes through the distortion of light travelling past them.
Given what physicists believe about black holes, it will be impossible to see an object lying more or less directly behind one, from our vantage point: it will simply absorb all of the light. However, if there are several large stars or galaxies behind a black hole, we may be able to infer its existence by its effect on the light from those other objects: it will be bent by the black hole, forming what appears to be an empty circular void in space surrounded by light that continues on more or less unaffected. This is similar to a process currently used to detect planets orbiting stars, called gravitational lensing.
In theory, there must by now be a large number of black holes in the universe, formed by dead stars. The most likely location of black holes, however, is at the centre of relatively large galaxies such as our own Milky Way Galaxy, where especially dense gravitational forces are believed to create supermassive black holes around which the rest of the galaxy slowly orbits.
– Life and Death of Black Holes –
Black holes, once they have formed, will continue to draw in more and more energy and matter, and grow even stronger through this process of accretion. A reverse process of very slowly leaking, or escaping, energy may also occur, and is known as Hawking radiation after its proponent, famous cosmologist Stephen Hawking. Theoretical physicists have gone to considerable extents to trace exactly what will happen to space, matter, and even time in very close quarters to a black hole; however, since no black holes have ever been found and studied, all of this work must currently be seen as tentative and abstract, subject to intense revision if and when an actual black hole is found.
If black holes exist as theorized, however, these are important questions to answer for one reason in particular: given sufficient billions of years, black holes will continue to accrete mass until they have absorbed much of the mass and energy currently used to form stars and planets. They therefore play at least some meaningful role in the development of theoretical scenarios of the physical ending of the universe.
