Text of ‘Baby Universes, Children of Blackholes’ by S.W. Hawking
(Copyright 1988 Stephen W. Hawking. All rights reserved.)
[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).]
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