Some astronomers have suggested that we can travel through a black hole and reappear instantaneously in another universe...
Nothing, not even light can escape from a black hole. But some astrophysicists believe that an astronaut falling into one might suddenly, almost instantaneously, reappear in a strange and alien universe quite unlike the one he left only moments before: a universe existing not only in another dimension of time, but one where the fundamental laws of nature are reversed.
Of course, no one knows for certain what would happen if an astronaut were to fall into a black hole. But modern astronomy, based on Einstein's theory of relativity, has pieced together an account of what could happen.
Take the case of an astronaut approaching a simple, static black hole of the type described by Karl Schwarzschild in 1916 (see diagram 1). Not everything that strays into the outer fringes of a black hole is inevitably sucked in. A spaceship, for example, could journey to the outer limits, despatch an astronaut, then safely excape by firing its rocket motors or by going into orbit: letting centrifugal force balance the spaceship's gravitational pull.
As the astronaut begins to fall towards the brightly lit outer fringe of the black hole- called the event horizon- strange things begin to happen. Relativity theory says that time runs more slowly in regions of strong gravitation. And the gravitation exerted by a black hole is more powerful than anywhere else in the Universe. As we watch the astronaut fall closer to the event horizon, time for him runs slower and slower. His watch slows down to such an extent that, to us observing him, he never seems to reach it. If we calculate, for example, that he's just one second from falling in, then the last second on his slow-running watch will be stretched to an infinite amount of time. However long we wait, he'll never quite make it. But to him, his watch keeps perfectly steady time. Although to us his fall to the event horizon takes an infinite time, to him it passes in only a matter of seconds. And once beyond the event horizon we cannot see or hear from him again.
Crushed to Oblivion
Inside a Schwarzschild black hole, our astronaut would be dragged to the single point of infinite density at the centre, called the 'singularity'. Within a fraction of a second- to him- he too, becomes part of that infinitely compressed point, crushed to oblivion. Few astronomers now think, however, that static black holes of the type described by Schwarzschild actually exist. Every star in the Universe rotates- indeed our whole Universe may itself rotate- and black holes are probably no exception. Indeed, black holes may well rotate more rapidly than most stars. And for an astronaut approaching a rotating black hole, his fate is not as grim as the certain oblivion he would meet in a static hole.
To help us visualise what would happen inside a rotating black hole, Roger Penrose has devised a set of space-time diagrams. To plot a course through a Penrose diagram the basic rule is that we can only travel through space in a generally upward direction, that is, forward in time. In diagram 2, light rays travelling at a speed of 670 million mph (1000 million km/h) mark the speed limit of possible space travel beyondwhich we can never, according to Einstein, travel. Our entire Universe, out into infinity of space, from its birth to eventual extinction along the time scale, is reduced to a diamond shape in diagram 3. The hour-glass shape (at 45 degrees to the vertical) represents the event horizon and the singularity is at the top. As our astronaut begins to fall towards the event horizon of a rotating black hole, he does not fall straight inwards. Because of the rapid rotation of the black hole, he would be dragged sideways, no matter how powerful his rockets.
Astronomers working on black hole theory are agreed upon the sequence of events so far. But from this point, the picture becomes slightly more complex. Some astronomers claim that not only do black holes rotate, they are also electrically charged. And this fact makes a difference to our astronaut as he is about to enter the hole.
Within an electrically charged black hole, there is a second 'inner' event horizon, lying inside the 'outer' horizon which seals the hole from the rest of the Universe. Here space and time change roles again. So although an astronaut falling through the outer event horizon must pursue a one way path inwards, this now takes him only as far as the inner event horizon. And once inside the inner horizon, our astronaut can manouevre how he likes, although he cannot know where he will end up. According to simple calculations, he may re-emerge in another universe, or at the same instant, elsewhere in this Universe.
Into Another Universe
We can trace the course of the journeys an astronaut could possibly take... An astronaut travelling near vertically can enter the tunnel through the outer event horizon, travel through the inner event horizon and into a totally different universe... A rotating, non-electrically charged black hole also has an inner event horizon through which we could, theoretically reach another universe. But here the singularity is not a single point, but a ring. If an astronaut could aim through this ring, he might find himself not only in a universe that was different from our own, but one where gravity, for example, instead of being a force that attracts two objects, is a force that repels. He might find himself, that is, in a 'negative universe'. Leaving aside this 'negative universe', some writers have proposed that future space travellers will journey through such tunnels within rotating black holes and hop from universe to universe.
But there are problems with this proposal. An adventurous astronaut would encounter his first problem before he even entered the black hole. Suppose the astronaut is falling feet first. His feet, being nearer to the hole, would feel a stronger gravitational pull than his head. The difference in gravitational pull along his body would stretch him out as he falls inwards. Physicists Charles Misner, Kip Thorne and John Wheeler have carried this gruesome calculation through, and according to their results the astronaut would be stretched to breaking point still hundreds of miles from the black hole.
Strangely enough, this problem is worse for smaller black holes where the force of gravitation changes more sharply with distance. For the smaller holes, the effect is so marked that gravitation is gradually shredding the structure of space itself and creating particles of matter outside the event horizon. These particles stream off into space gradually robbing the hole of its strength, and indirectly, its matter. These have been described by Stephen Hawking as 'evaporating' and 'exploding' black holes.
At the Heart of the Galaxy
An astronaut could, however, safely enter a very heavy black hole, where the gravitation changes more gradually with distance from the hole. Such holes may form at the centre of galaxies, where matter was tightly packed at the time of their birth. Some astronomers believe that our own Galaxy, the Milky Way, harbours a black hole five million times the Sun's weight at its centre.
There's an even better indication of a black hole a thousand times more massive still, in the heart of the galaxy M87. Although it's invisible, this black hole's immense gravitational pull distorts the paths of nearby stars in the galaxy. And most astronomers now believe that the distant, very bright quasars are simply hot discs of gas surrounding massive black holes at the centre of far off galaxies. So massive black holes probably do exist and, in theory at least, could be used as gateways to other universes.
A disk of hot gas at the centre of the galaxy M87. The rotation speed of the particles suggests a black hole.
In this Universe, the traveller would emerge through a white hole - the exact opposite of a black hole. Just as anything inside a black hole falls inwards, so anything inside a white hole must travel outwards, and must emerge through the event horizon. If white holes exist, they are 'cosmic gushers', spewing matter and light out in a seemingly inexhaustible fountain.
White holes are present in the Penrose diagrams already discussed. In diagram 2, for example, the lower half of the 'hour-glass' is a white hole, while the top half is a black hole. Remembering that all objects must travel more or less vertically in the diagram, it's obvious that everything within this lower triangle must eventually shoot out of the event horizon. And for the more complicated case of the rotating black hole (see diagram 3), the upper of the two diamonds making up the tunnel is, in fact, a white hole opening into the upper universe.
If white holes do exist, they would certainly be fascinating to peer into. The view into a rotating black hole would reveal not just its central singularity, but also two other universes (see diagram 2). One of these universes is the 'negative universe' seen through the centre of the ring-shaped singularity; surrounding this is the singularity itself, and the rest of the white hole would contain a distorted view of another universe, which is seen reflected within the white hole.
But there are several theoretical difficulties with white holes. For a start, despite their 'gushing' nature they have strong gravitational fields around them. And white holes can't form in space as black holes can: if they exist they must have been around since the beginning of our Universe, 15,000 million years ago. Most astronomers now accept that our Universe began with a 'Big Bang'. Any white holes that did form would have trapped so much radiation around themselves that the sheer mass of radiation would have created a black hole around the white hole. The white hole would have been swallowed up in the black hole it created around itself.
The tearing effects of the changing gravitational force within a white hole also means that the central singularity of a white hole is unstable. And effects of both these kinds are likely to happen at the inner event horizon. So although there's still a lot of detailed calculation to be done, scientists are now distinctly dubious about about universe hopping through black/white holes.
Even so, if the space-time diagrams shown here are correct, interesting questions arise as to the nature of the 'other universes'. They may be totally disconnected from our own; or they may be different regions or different times of our Universe. If so, a black hole could be a time machine - and a journey through one could end up in the age of the dinosaurs or far in the future.
- LMC X-1, is thought to be a normal and
- compact star orbiting each other. Motion
- in the binary system suggests the
- compact star is a black hole.
But while this aspect of black hole research seems to have come to a full stop, at least in terms of conventional science, observational astronomers are gathering more and more evidence that black holes do exist in our Universe, both in double star pairs like Cyg X-1 and in the centres of galaxies like M87 and the quasars. No doubt black holes have many more surprises - both observational and theoretical - in store for us. And given Man's pioneering urge it can only be a matter of time before manned spaceships are in orbit about a black hole. And someone may just be brave enough to find out what's inside.
This article was written by Nigel Henbest, and originally appeared in The Unexplained Mysteries of Time and Space, Volume 1, Issue 3.
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