[Note: This is taken from the text used by Professor Hawking's speech synthesizer. While most of the spelling and punctuation peculiarities required by the computer translator have been corrected, some may still exist (especially names). -Unknown]How can a black hole give off radiation. How can anything get out through the event horizon of a black hole. The answer is, the Uncertainty Principle, allows particles to travel faster than light, for a small distance. This enables particles and radiation, to get out through the event horizon, and escape from the black hole. Thus, it is possible for things to get out of a black hole. However, what comes out of a black hole, will be different from what fell in. Only the energy will be the same.
As a black hole gives off particles and radiation, it will lose mass. This will cause the black hole to get smaller, and to send out particles more rapidly. Eventually, it will get down to zero mass, and will disappear completely. What will happen then to the objects, including possible spaceships, that fell into the black hole. According to some recent work of mine, the answer is that they go off into a little baby universe of their own. A small, self-contained universe branches off from our region of the universe. This baby universe may join on again to our region of spacetime. If it does, it would appear to us to be another black hole, which formed, and then evaporated. Particles that fell into one black hole, would appear as particles emited by the other black hole, and vice versa.
This sounds just what is required to allow space travel through black holes. You just steer your space ship into a suitable black hole. It better be a pretty big one, or the gravitational forces will tear you into spaggetti, before you get inside. You would then hope to re-appear out of some other hole, though you wouldn't be able to choose, where.
However, there's a snag in this intergalactic transportation scheme. The baby universes, that take the particles that fell into the hole, occur in what is called, imaginary time. Imaginary time may sound like science fiction, but it is a well defined mathematical concept. It seems essential, in order to formulate Quantum Mechanics, and the Uncertainty Principle properly. However, it is not our subjective sense of time, in which we feel ourselves as getting older, with more gray hairs. Rather, it can be thought of as a direction of time, that is at right angles to what we call, `real', time.
In real time, an astronaut who fell into a black hole, would come to a sticky end. He would be torn apart, by the difference between the gravitational force on his head and his feet. Even the particles that made up his body, would not survive. Their histories, in real time, would come to an end, at a singularity. However, the histories of the particles, in imaginary time, would continue. They would pass into the baby universe, and would re- emerge as the particles emited by another black hole. Thus, in a sense, the astronaut would be transported to another region of the universe. However, the particles that emerged, would not look much like the astronaut. Nor, might it be much consolation to him, as he ran into the singularity in real time, to know that his particles will survive in imaginary time. The motto for anyone who falls into a black hole must be: Think Imaginary.
What determines where the particles re-emerge. The number of particles in the baby universe, will be equal to the number of particles that fell into the black hole, plus the number of particles that the black hole emits, during its evaporation. This means that the particles that fall into one black hole, will come out of another hole of about the same mass. Thus, one might try to select where the particles would come out, by creating a black hole of the same mass, as that which the particles went down. However, the black hole would be equally likely to give off any other set of particles with the same total energy. Even if the black hole did emit the right kinds of particles, one could not tell if they were actually the same particles that went down the other hole. Particles do not carry identity cards: all particles of a given kind, look alike.
What all this means, is that going through a black hole, is unlikely to prove a popular and reliable method of space travel. First of all, you would have to get there by travelling in imaginary time, and not care that your history in real time came to a sticky end. Second, you couldn't really choose your destination. It would be a bit like travelling on some airlines I could name, but won't, because I would be sued.
Although baby universes may not be much use for space travel, they have important implications for our attempt to find a complete unified theory that will describe everything in the universe. Our present theories contain a number of quantities, like the size of the electric charge on a particle. The values of these quantities can not be predicted by our theories. Instead, they have to be chosen to agree with observations. However, most scientists believe that there is some underlying unified theory that will predict the values of all these quantities.
There may well be such an underlying theory. Many people think it is the theory of super strings. This does not contain any numbers whose values can be adjusted. One would therefore expect that this unified theory, should be able to predict all the values of quantities, like the electric charge on a particle, that are left undetermined by our present theories. Even though we have not yet been able to predict any of these quantities from super string theory, many people believe that we will be able to do so, eventually.
However, if this picture of baby universes is correct, our ability to predict these quantities will be reduced. This is because we can not observe, how many baby universes exist out there, waiting to join on to our region of the universe. There can be baby universes that contain only a few particles. These baby universes are so small that one would not notice them joining on, or branching off. However, by joining on, they will alter the apparent values of quantities, like the electric charge on a particle. Thus, we will not be able to predict what the apparent values of these quantities will be, because we don't know how many baby universes are waiting out there. There could be a population explosion of baby universes. However, unlike the human case, there seem to be no limitting factors, such as, food supply, or standing room. Baby universes exist in a realm of their own. It is a bit like asking: how many angels can dance on the head of a pin.
For most quantities, baby universes seem to introduce a definite, although fairly small, amount of uncertainty in the predicted values. However, they may provide an explanation of the observed value of one very important quantity, the so-called cosmological constant. This is a quantity that would give the universe, an in-built tendency to expand, or contract. On general grounds, one might expect it to be very large. Yet we can observe, how the expansion of the universe is varying with time, and determine that the cosmological constant is very small. Up to now, there has been no good explanation for why the observed value should be so small. However, having baby universes branching off and joining on, will affect the apparent value of the cosmological constant. Because we don't know how many baby universes there are, there will be different possible values for the apparent cosmological constant. However, a nearly zero value, will be by far the most probable. This is fortunate, because it is only if the value of the cosmological constant is very small, that the universe would be suitable for beings like us.
To sum up: it seems that particles can fall into black holes, which then evaporate, and disappear from our region of the universe. The particles go off into baby universes, which branch off from our universe. These baby universes can then join back on somewhere else. They may not be much good for space travel, but their presence means that we will be able to predict less than we expected, even if we do find a complete unified theory. On the other hand, we now may be able to provide explanations for the measured values of some quantities, like the cosmological constant. In the last year, this has become a very active and exciting area of research. I'm itching to get on with it.
A longer version and other similar essays can be found in Professor Hawking's book, Black Holes and Baby Universes and Other Essays, Bantam Books, 1993, ISBN: 0553095234