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Chapter 1 - Our Picture of the Universe
Chapter 2 - Space and Time
Chapter 3 - The Expanding Universe
Chapter 4 - The Uncertainty Principle
Chapter 5 - Elementary Particles and the Forces of Nature
Chapter 6 - Black Holes
Chapter 7 - Black Holes Ain't So Black
Chapter 8 - The Origin and Fate of the Universe
Chapter 9 - The Arrow of Time
Chapter 10 - Wormholes and Time Travel
Chapter 11 - The Unification of Physics
Chapter 12 - Conclusion
Glossary
Acknowledgments & About The Author
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.
FOREWARD
I didn’t write a foreword to the original edition of A Brief History of Time. That was done by Carl Sagan. Instead,
I wrote a short piece titled “Acknowledgments” in which I was advised to thank everyone. Some of the
foundations that had given me support weren’t too pleased to have been mentioned, however, because it led to
a great increase in applications.
I don’t think anyone, my publishers, my agent, or myself, expected the book to do anything like as well as it did.
It was in the London Sunday Times best-seller list for 237 weeks, longer than any other book (apparently, the
Bible and Shakespeare aren’t counted). It has been translated into something like forty languages and has sold
about one copy for every 750 men, women, and children in the world. As Nathan Myhrvold of Microsoft (a
former post-doc of mine) remarked: I have sold more books on physics than Madonna has on sex.
The success of A Brief History indicates that there is widespread interest in the big questions like: Where did
we come from? And why is the universe the way it is?
I have taken the opportunity to update the book and include new theoretical and observational results obtained
since the book was first published (on April Fools’ Day, 1988). I have included a new chapter on wormholes
and time travel. Einstein’s General Theory of Relativity seems to offer the possibility that we could create and
maintain wormholes, little tubes that connect different regions of space-time. If so, we might be able to use
them for rapid travel around the galaxy or travel back in time. Of course, we have not seen anyone from the
future (or have we?) but I discuss a possible explanation for this.
I also describe the progress that has been made recently in finding “dualities” or correspondences between
apparently different theories of physics. These correspondences are a strong indication that there is a complete
unified theory of physics, but they also suggest that it may not be possible to express this theory in a single
fundamental formulation. Instead, we may have to use different reflections of the underlying theory in different
situations. It might be like our being unable to represent the surface of the earth on a single map and having to
use different maps in different regions. This would be a revolution in our view of the unification of the laws of
science but it would not change the most important point: that the universe is governed by a set of rational laws
that we can discover and understand.
On the observational side, by far the most important development has been the measurement of fluctuations in
the cosmic microwave background radiation by COBE (the Cosmic Background Explorer satellite) and other
collaborations. These fluctuations are the finger-prints of creation, tiny initial irregularities in the otherwise
smooth and uniform early universe that later grew into galaxies, stars, and all the structures we see around us.
Their form agrees with the predictions of the proposal that the universe has no boundaries or edges in the
imaginary time direction; but further observations will be necessary to distinguish this proposal from other
possible explanations for the fluctuations in the background. However, within a few years we should know
whether we can believe that we live in a universe that is completely self-contained and without beginning or
end.
Stephen Hawking
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CHAPTER 1
OUR PICTURE OF THE UNIVERSE
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A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He
described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast
collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and
said: “What you have told us is rubbish. The world is really a flat plate supported on the back of a giant
tortoise.” The scientist gave a superior smile before replying, “What is the tortoise standing on.” “You’re very
clever, young man, very clever,” said the old lady. “But it’s turtles all the way down!”
Most people would find the picture of our universe as an infinite tower of tortoises rather ridiculous, but why do
we think we know better? What do we know about the universe, and how do we know it? Where did the
universe come from, and where is it going? Did the universe have a beginning, and if so, what happened before
then? What is the nature of time? Will it ever come to an end? Can we go back in time? Recent breakthroughs
in physics, made possible in part by fantastic new technologies, suggest answers to some of these
longstanding questions. Someday these answers may seem as obvious to us as the earth orbiting the sun – or
perhaps as ridiculous as a tower of tortoises. Only time (whatever that may be) will tell.
As long ago as 340 BC the Greek philosopher Aristotle, in his book On the Heavens, was able to put forward
two good arguments for believing that the earth was a round sphere rather than a Hat plate. First, he realized
that eclipses of the moon were caused by the earth coming between the sun and the moon. The earth’s
shadow on the moon was always round, which would be true only if the earth was spherical. If the earth had
been a flat disk, the shadow would have been elongated and elliptical, unless the eclipse always occurred at a
time when the sun was directly under the center of the disk. Second, the Greeks knew from their travels that
the North Star appeared lower in the sky when viewed in the south than it did in more northerly regions. (Since
the North Star lies over the North Pole, it appears to be directly above an observer at the North Pole, but to
someone looking from the equator, it appears to lie just at the horizon. From the difference in the apparent
position of the North Star in Egypt and Greece, Aristotle even quoted an estimate that the distance around the
earth was 400,000 stadia. It is not known exactly what length a stadium was, but it may have been about 200
yards, which would make Aristotle’s estimate about twice the currently accepted figure. The Greeks even had a
third argument that the earth must be round, for why else does one first see the sails of a ship coming over the
horizon, and only later see the hull?
Aristotle thought the earth was stationary and that the sun, the moon, the planets, and the stars moved in
circular orbits about the earth. He believed this because he felt, for mystical reasons, that the earth was the
center of the universe, and that circular motion was the most perfect. This idea was elaborated by Ptolemy in
the second century AD into a complete cosmological model. The earth stood at the center, surrounded by eight
spheres that carried the moon, the sun, the stars, and the five planets known at the time, Mercury, Venus,
Mars, Jupiter, and Saturn.
Figure 1:1
The planets themselves moved on smaller circles attached to their respective spheres in order to account for
their rather complicated observed paths in the sky. The outermost sphere carried the so-called fixed stars,
which always stay in the same positions relative to each other but which rotate together across the sky. What
lay beyond the last sphere was never made very clear, but it certainly was not part of mankind’s observable
universe.
Ptolemy’s model provided a reasonably accurate system for predicting the positions of heavenly bodies in the
sky. But in order to predict these positions correctly, Ptolemy had to make an assumption that the moon
followed a path that sometimes brought it twice as close to the earth as at other times. And that meant that the
moon ought sometimes to appear twice as big as at other times! Ptolemy recognized this flaw, but nevertheless
his model was generally, although not universally, accepted. It was adopted by the Christian church as the
picture of the universe that was in accordance with Scripture, for it had the great advantage that it left lots of
room outside the sphere of fixed stars for heaven and hell.
A simpler model, however, was proposed in 1514 by a Polish priest, Nicholas Copernicus. (At first, perhaps for
fear of being branded a heretic by his church, Copernicus circulated his model anonymously.) His idea was that
the sun was stationary at the center and that the earth and the planets moved in circular orbits around the sun.
Nearly a century passed before this idea was taken seriously. Then two astronomers – the German, Johannes
Kepler, and the Italian, Galileo Galilei – started publicly to support the Copernican theory, despite the fact that
the orbits it predicted did not quite match the ones observed. The death blow to the Aristotelian/Ptolemaic
theory came in 1609. In that year, Galileo started observing the night sky with a telescope, which had just been
invented. When he looked at the planet Jupiter, Galileo found that it was accompanied by several small
satellites or moons that orbited around it. This implied that everything did not have to orbit directly around the
earth, as Aristotle and Ptolemy had thought. (It was, of course, still possible to believe that the earth was
stationary at the center of the universe and that the moons of Jupiter moved on extremely complicated paths
around the earth, giving the appearance that they orbited Jupiter. However, Copernicus’s theory was much
simpler.) At the same time, Johannes Kepler had modified Copernicus’s theory, suggesting that the planets
moved not in circles but in ellipses (an ellipse is an elongated circle). The predictions now finally matched the
observations.
As far as Kepler was concerned, elliptical orbits were merely an ad hoc hypothesis, and a rather repugnant one
at that, because ellipses were clearly less perfect than circles. Having discovered almost by accident that
elliptical orbits fit the observations well, he could not reconcile them with his idea that the planets were made to
orbit the sun by magnetic forces. An explanation was provided only much later, in 1687, when Sir Isaac Newton
published his Philosophiae Naturalis Principia Mathematica, probably the most important single work ever
published in the physical sciences. In it Newton not only put forward a theory of how bodies move in space and
time, but he also developed the complicated mathematics needed to analyze those motions. In addition,
Newton postulated a law of universal gravitation according to which each body in the universe was attracted
toward every other body by a force that was stronger the more massive the bodies and the closer they were to
each other. It was this same force that caused objects to fall to the ground. (The story that Newton was inspired
by an apple hitting his head is almost certainly apocryphal. All Newton himself ever said was that the idea of
gravity came to him as he sat “in a contemplative mood” and “was occasioned by the fall of an apple.”) Newton
went on to show that, according to his law, gravity causes the moon to move in an elliptical orbit around the
earth and causes the earth and the planets to follow elliptical paths around the sun.
The Copernican model got rid of Ptolemy’s celestial spheres, and with them, the idea that the universe had a
natural boundary. Since “fixed stars” did not appear to change their positions apart from a rotation across the
sky caused by the earth spinning on its axis, it became natural to suppose that the fixed stars were objects like
our sun but very much farther away.
Newton realized that, according to his theory of gravity, the stars should attract each other, so it seemed they
could not remain essentially motionless. Would they not all fall together at some point? In a letter in 1691 to
Richard Bentley, another leading thinker of his day, Newton argued that this would indeed happen if there were
only a finite number of stars distributed over a finite region of space. But he reasoned that if, on the other hand,
there were an infinite number of stars, distributed more or less uniformly over infinite space, this would not
happen, because there would not be any central point for them to fall to.
This argument is an instance of the pitfalls that you can encounter in talking about infinity. In an infinite
universe, every point can be regarded as the center, because every point has an infinite number of stars on
each side of it. The correct approach, it was realized only much later, is to consider the finite situation, in which
the stars all fall in on each other, and then to ask how things change if one adds more stars roughly uniformly
distributed outside this region. According to Newton’s law, the extra stars would make no difference at all to the
original ones on average, so the stars would fall in just as fast. We can add as many stars as we like, but they
will still always collapse in on themselves. We now know it is impossible to have an infinite static model of the
universe in which gravity is always attractive.
It is an interesting reflection on the general climate of thought before the twentieth century that no one had
suggested that the universe was expanding or contracting. It was generally accepted that either the universe
had existed forever in an unchanging state, or that it had been created at a finite time in the past more or less
as we observe it today. In part this may have been due to people’s tendency to believe in eternal truths, as well
as the comfort they found in the thought that even though they may grow old and die, the universe is eternal
and unchanging.
Even those who realized that Newton’s theory of gravity showed that the universe could not be static did not
think to suggest that it might be expanding. Instead, they attempted to modify the theory by making the
