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Stephen Hawking discusses Baby Universes, UC Berkeley 1988

Stephen Hawking discusses Baby Universes, UC Berkeley 1988


(audience applauds) – On behalf of the astronomy
and physics departments at Berkeley, it’s my pleasure
to welcome you to the second of three Hitchcock
Lectures, to be delivered by Stephen Hawking, Lucasian
Professor of Mathematics, at Cambridge University. Professor Hawking is universally regarded as the world’s foremost authority
on gravitational physics. The inheritor of the legacy
handed down by Issac Newton, and Albert Einstein. In an illustrious career,
in which he has elucidated surprising and beautiful
relationships between gravitation, thermodynamics, and quantum
physics, Professor Hawking has won many scientific
awards and honors, including the Adams Prize, the Eddington
Medal, the Heineman Prize, the Maxwell Medal, the Hughes
Medal, the Einstein Medal, and most recently, the
Wolf Prize for Physics. In 1982, he was made a
Commander of the British Empire. His numerous scientific
achievements are all the more remarkable, for having
been achieved in the face of great personal adversity. Few profiles of courage
surpass in heroism and valor that of our speaker. To commemorate his inspirational
visit to our campus, I call upon Doctor Forbes
Norris, vice president and clinical director
of the San Francisco ALS Research Foundation, and
a member of the Scientific Advisory Board of the
International ALS Foundation. Doctor Norris will make a presentation to Doctor Glenn Seaborg, representing the Lawrence Hall of Science. (audience applauds) – The ALS Foundation for
research on this disease in San Francisco is terribly
pleased to share with you the great honor of the presence in the Bay Area of Stephen Hawking. And, in commemoration of
his visit, we would like to present to the university, for
the Lawrence Hall of Science, a bust of Professor Hawking,
by the noted Irish Sculptress, Marjorie Fitzgibbon. (audience applauds) – Thank you very much. I am very proud to accept on behalf of the Lawrence Hall of Science, this bust of Sir Stephen Hawking, on this heartwarming occasion. I also want to express my
gratitude to Doctor Norris and the ALS Research
Foundation for their generosity in making this bust available. I think it’s very appropriate
that this go to the Lawrence Hall of Science, Sir Stephen Hawking symbolizes the best in our attempts to effect the public
understanding of science, to help us understand
our place in the universe and the nature of things. And the Lawrence Hall
of Science is dedicated to just these ideas. The public understanding of
science, and of its place in today’s world. I invite you all, and
not all at the same time, to come up to the Lawrence Hall
of Science and see this bust of Professor Stephen Hawking on display. Thank you very much. (audience applauds) – Well, without further
ado, I now call upon Professor Stephen Hawking to
give us his Hitchcock Lecture. The title of his talk has
been changed from the one that was originally announced. The new title is: Baby Universes,
Children of Black Holes. Ladies and gentlemen,
please join me in welcoming, once more, Professor Stephen Hawking. (audience applauds) – Can you hear me? In this lecture, I want
to talk about black holes, and their offspring, baby universes. Falling into a black hole
has become one of the horrors of science fiction. In fact, black holes can now
be said to be really matters of science fact, rather
than science fiction. As I shall describe, there are
good reasons for predicting that black holes should exist. And the observational
evidence points strongly to the presence of a number of
black holes in our own galaxy and more in other galaxies. Of course, where the
science fiction writers really go to town, is on
what happens if you do fall in a black hole. A common suggestion is that
if the black hole is rotating, you can fall through a
little hole is space-time, and out into another
region of the universe. This obviously raises great
possibilities for space travel. Indeed, we need something like this, if travel to other stars,
let alone other galaxies, is to be a practical
proposition in the future. Otherwise, the fact
that nothing can travel faster than light, means that a round trip to the nearest star will
take at least eight years. So much for a weekend
trip on Alpha Centauri. (audience laughs) On the other hand, if one could
pass through a black hole, one might reemerge
anywhere in the universe. Quite how you choose your
destination is not clear. You might set out for a holiday in Virgo, and end up in the Crab Nebula. I am sorry to disappoint
prospective galactic tourists, but this scenario doesn’t work. If you jump into a black
hole, you will get torn apart, and crushed out of existence. (audience laughs) However, there is a sense
in which the particles that make up your body, do
carry on into another universe. I don’t know if it would
be much consolation to someone being made into
spaghetti in a black hole, to know that his particles might survive. Despite a slightly flippant
tone I have adopted, this talk will be based on hard science. Most of what I shall say is
now agreed by other scientists working in this field, though
this acceptance has come only fairly recently. The last part of the
lecture, however, is based on very recent work on
which there is, as yet, no general consensus. But this work is arousing
great interest and excitement. Although the concept of what
we now call a black hole goes back more than 200
years, the name black hole was introduced only in 1967,
by the American Physicist, John Wheeler. It was a stroke of genius. The name ensured that black
holes entered the mythology of science fiction. It also stimulated scientific
research by providing a definite name for
something that previously had not had a satisfactory title. The importance in science of a good name should not be underestimated. The first person, as far as I
know, to discuss black holes was a Cambridge man called John Michell, who wrote a paper about them in 1783. His idea was this. Suppose you fire a cannonball
vertically upwards, from the surface of the earth. As it goes up, it will be slowed down by the effect of gravity. Eventually, it will stop going up, and will fall back to earth. However, if it had more than
a certain critical speed, it would never stop and fall back, but would continue to move away. This critical speed is
called the escape velocity. It is about seven miles
a second for the earth, and about 100 miles a second for the sun. Both of these velocities
are higher than the speed of a real cannonball, but
they are much smaller than the velocity of light, which
is 186,000 miles a second. This means that gravity doesn’t
have much effect on light, and light can escape without difficulty from the earth or the sun. However, Michell reasoned
that it would be possible to have a star that was
sufficiently massive, and sufficiently small in
size that its escape velocity would be greater than
the velocity of light. We would not be able to see such a star, because light from its
surface would not reach us, but would be dragged back
by its gravitational field. However, we might be able
to detect the presence of the star by the effect
that its gravitational field would have on your biomatter. It is not really consistent to
treat light like cannonballs, because according to an
experiment carried out in 1897, light always travels at
the same constant velocity. So, how then can gravity slow down light? A fully consistent theory
of how gravity affects light came in 1915, when Einstein
formulated the general theory of relativity. Even so, the implications of
this theory for old stars, and other massive bodies,
were not generally realized until the 1960’s. According to general relativity,
space and time together can be regarded as forming
a four-dimensional space, called space-time. The space is not flat, but
it is distorted or curved by the matter and energy in it. Objects try to move on straight
lines through space-time, but because it is curved,
they move on paths called geodesics, which are the
nearest thing to a straight line in a curved space. Thus, the earth tries to
move on a straight line, but because space-time is
bent by the mass of the sun, it follows a spiral path, going
in a circle around the sun, while advancing in time. Similarly, light tries to
move on a straight line, but because space-time is
curved, it appears to follow a path that is bent. We can actually observe
this bending of light during an eclipse. The moon blocks out the sun,
and allows us to observe stars that are in almost the
same direction as the sun. We find that the stars appear
to be in slightly different positions, because the
light from them is bent by the curved space-time near the sun. In the case of light passing near the sun, the bending is very small. However, if the sun were to shrink, until it was only a few miles across, the bending would be so great
that light leaving the sun would not get away, but
would be dragged back by the gravitational field. According to the theory of relativity, nothing can travel faster than light. So there would be a region from
which it would be impossible for anything to escape. This region is called a black hole. Its boundary is called the event horizon. It is formed by the light
that just fails to get away from the black hole, but
stays hovering on the edge. It might sound ridiculous to suggest that the sun could shrink. Sorry about that. (audience laughs) It might sound ridiculous to suggest that the sun could shrink, to
being only a few miles across. Surely, matter cannot
be compressed so far. The answer is, it can. The sun is the size it
is, because it is so hot. It is burning hydrogen into helium, like a controlled H-bomb. The heat released in this
process generates a pressure that enables the sun to
resist the attraction of its own gravity, which is
trying to make it smaller. Eventually, however, the sun
will run out of nuclear fuel. This will not happen for about
another five billion years, so there’s no great
rush to book your flight to another star. However, more massive stars
will burn up their fuel much more rapidly. When they finish their fuel,
they will start to lose heat, and to contract. If they are less than about
twice the mass of the sun, they will eventually stop contracting, and will settle down to a stable state. This state can be what
is called a white dwarf. These have radii of a few thousand miles, and densities of hundreds
of tons per cubic inch. Or it can be a neutron star. These have a radius of about 10 miles, and densities of millions
of tons per cubic inch. We observe large numbers of white dwarfs in our immediate
neighborhood in the galaxy. Neutron stars, however, were
not observed until 1967, when Jocelyn Bell and
Tony Hewish at Cambridge, discovered objects called
pulsars, which were emitting regular pulses of radio waves. At first, they wondered
whether they had made contact with an alien civilization. Indeed, I remember that the seminar room in which they announced their discovery, was decorated with figures
of little green men. (audience laughs) In the end, however,
they, and everyone else, came to the less romantic conclusion that they were rotating neutron stars. This was bad news for
writers of space westerns, but good news for the small number of us who believed in black holes at that time. If stars could shrink as
small as 10 or 20 miles across to become neutron stars, one
might expect that other stars could shrink even further,
to become black holes. A star with a mass more than
about twice that of the sun, cannot settle down as a white
dwarf, or a neutron star. In some cases, the star may explode, and throw off enough matter to bring its mass below the limit. But this won’t happen in all cases. Some stars will shrink so small, that their gravitational
fields will bend light so much, that it comes back towards the star. No further light, or anything
else, will be able to escape. The stars will have become black holes. We now have fairly good
observational evidence for a number of black holes. One of the best cases is Cygnus X-1. This is a system consisting
of a normal star, orbiting around an unseen companion. Matter seems to be being
blown off the normal star, and falling on the companion. As it falls towards the companion, it develops a spiral motion,
like water running out of a bath. It will get very hot, and
will give off the X-rays that are observed. The unseen companion must be
very small, a white dwarf, neutron star, or a black hole. However, one can show that
the mass of the companion must be at least six
times that of the sun. This is too much for
it to be a white dwarf, or a neutron star. So, it has to be a black hole. I once bet Kip Thorne of
the California Institute of Technology, that Cygnus
X-1 does not contain a black hole. This was not because I didn’t
believe that there really was a black hole in Cygnus X-1, rather, it was an insurance policy. I had done a lot of work on black holes, and it all would have been
wasted, if it had turned out that black holes didn’t exist. But then, at least, I would
have had the consolation of winning my bet. However, I now consider the
evidence for black holes so compelling, that I’m
going to concede the bet. I will give Kip Thorne a
subscription to Penthouse. (audience laughs) Anything that falls into a
black hole, comes into a region of space-time in which
light is bent so much, that it cannot get out. Since nothing can travel
faster than light, this means that nothing
else can get out either. So think carefully before you decide to jump into a black hole. You won’t be able to change your mind, if you don’t like what you find inside. The laws of physics are time-symmetric. So, if there are objects
called black holes, which things can fall
into, but not get out, there ought to be other objects
that things can come out of, but not fall into. One could call these white holes. One might speculate that one
could jump into a black hole, in one place, and come out
of a white hole, in another. This would be the ideal method of long-distance space
travel, mentioned earlier. All you would need would be
to find a nearby black hole. At first, this form of space
travel seemed possible. There are solutions of
Einstein’s general theory of relativity in which it is possible to fall into a black hole,
and come out of a white hole. However, later works
show that these solutions were all very unstable. The slightest disturbance,
such as the presence of a spaceship, would destroy
the wormhole, or passage, leading from the black
hole to the white hole. The spaceship would be torn apart by infinitely strong forces. Anyone care to buy a
ticket for the Titanic? After that, it seemed hopeless. Black holes might be useful
for getting rid of garbage, or even some of one’s friends. (audience laughs) But they were a country from
which no traveler returns. However, everything I
have been saying so far, has been based on
calculations using Einstein’s general theory of relativity. This theory is in excellent agreement with all the observations we have made. But we know it cannot be quite right, because it doesn’t incorporate
the uncertainty principle of quantum mechanics. The uncertainty principle says
that particles cannot have both a well-defined position,
and a well-defined velocity. The more precisely you measure
the position of a particle, the less precisely you
can measure its velocity, and vice versa. In 1973, I started
investigating what difference the uncertainty principle
would make to black holes. To my great surprise, and
that of everyone else, I found that it meant that black holes are not completely black. They would be sending out
radiation and particles at a steady rate. My results were received
with general disbelief, when I announced them at
a conference near Oxford. The chairman of the session
said they were nonsense, and wrote a paper saying so. However, when other people
repeated my calculations, they found the same effect. Please wait while I load
the rest of my lecture. Yes. 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 space-time. 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
emitted 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 spaceship
into a suitable black hole. It better be a pretty big one,
or the gravitational forces will tear you into spaghetti,
before you get inside. You would then hope to reappear
out of some other hole, though you wouldn’t be
able to choose where. However, there is 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. Thank you. (audience laughs) 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 reemerge as
the particles emitted 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 reemerge? 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 of 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 traveling 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
traveling on some airlines I could name, but won’t,
because I would be sued. (audience laughs) 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 cannot 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 of these quantities. There may well be such
an underlying theory. Many people think it is
the theory of superstrings. 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
of 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 superstring 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 cannot
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 limiting 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 the 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 am itching to get on with it. Thank you. (audience applauds) – Professor Hawking will be
willing to take some questions, we do have some time. If you have a question,
please speak loudly, so that we can hear you. – [Woman In Audience] (speaking faintly) – [Announcer] The question
is, how can radiation escape from a black hole, if light cannot? – To get out of a black
hole, you have to travel faster than light. But the uncertainty principle
allows particles to travel faster than light, for a short distance. – [Person In Audience]
Do you believe that God is the creator of science and
the universe, if not, why? (audience laughs) – I won’t repeat the question. (audience laughs) (audience members applaud) – No. (audience laughs) Does it need a creator? Maybe it just exists. Does it need a creator? Maybe it just exists. – [Person In Audience] (speaks faintly) Did the universe evolve or emerge from a (speaks faintly) singularity, if so, why? (audience laughs) – No. (audience laughs) – [Group Attendant] One more question. – [Group Member] I’d
like to as a question. – [Group Attendant] Just one more. – [Group Member] Okay, thank you. I have heard, read, that our
own universe can be defined as a black hole. In truth, it’s feasible. Or no, but it is feasible. And, the theory that
particles from the universe are slipping away into
black holes, how long then, when we find a great
number of these particles in our own universe,
how long can this go on? And, (speaks faintly) other universes, what will you get? – [Group Attendant] I
said just one question. (group laughs) I’m afraid we do, we
are running out of time. So perhaps, those of you
who have urgent questions, save them for the next
talk, which will take place on Thursday in Zellerbach,
at the same time. In the meantime. (audience applauds loudly)

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