[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]
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