A Brief History of Time – Stephen Hawking

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The image of wheelchair-bound Stephen Hawking expounding on the exotic realities of a vast universe has become so indelibly etched into popular culture that it’s easy to forget that Hawking achieved a spot in the ranks of history’s great thinkers long before he became a scientific icon. It isn’t every day that you get to read a book whose author helped to shape his field in a fundamental way. Even though he is considered the most brilliant theoretical physicist since Albert Einstein, Hawking has managed to produce an accessible and enjoyable book that can be understood by laymen as well as scientists.

In this summary, you will learn

  • How breakthroughs by thinkers like Isaac Newton, Albert Einstein and Werner Heisenberg changed contemporary notions of how the universe works,
  • Why scientists now seek a single unifying theory that will explain the workings of the entire universe, and
  • How the discovery of such a theory would affect modern theology.


  • The ultimate goal of science is to provide a single theory to explain the universe.
  • Modern science builds on the work of Aristotle and Isaac Newton.
  • AlbertAlbert Einstein’s theory of relativity explains the behavior of large bodies in the universe. Quantum mechanics explains the behavior of objects on a very small scale.
  • Inconsistencies between the two have helped to spark the quest for a unifying theory.
  • Newton’s theories suggested that space was relative. Einstein showed that time was relative as well.
  • Werner Heisenberg’s uncertainty principle revolutionized thinking about how the universe works and established the validity of quantum mechanics.
  • The universe is constantly expanding.
  • Physicists believe that the universe was created in a single moment – the big bang – which set in motion this process of expansion.
  • The big bang theory is based on the behavior of collapsing stars and black holes.
  • The discovery of a unifying theory would have profound theological implications.


The Turtle Theory

During one long exposition on the nature of the universe, a little old lady arose at the back of the lecture hall. “What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise,” she declared.

The don who was speaking smiled and replied, “But what is the tortoise standing on?” Not to be dissuaded, she shot back, “You’re a very clever young man, very clever. But it’s turtles all the way down!”

“Our goal is nothing less than a complete description of the universe we live in.”

As much as her perspective might be at variance with the facts, the little old lady had one thing going for her: She had a picture of the universe. Where did the universe come from? What was it made of? How old was it? The picture of the universe envisioned by our greatest thinkers has changed over time.

“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.”

“Our Picture of the Universe”

In 340 B.C., the Greek philosopher Aristotle wrote that the Earth was probably round, as opposed to the commonly held view that it was flat. He also believed that the Earth was fixed in the cosmos, with the sun and planets orbiting it. It took almost 1,800 years for a Polish priest named Nicholas Copernicus to suggest that the sun was actually at the center of things, with the Earth and planets moving about it in a circular fashion.

In 1687, Sir Isaac Newton published his Principia, probably the most important single work ever published in the physical sciences. He put forward a theory of how bodies move through space and time, as well as the complex mathematics needed to analyze those motions. He also postulated a law of universal gravitation, according to which every body in the universe is attracted toward ever other entity by a force that becomes stronger as mass increases and distance diminishes.

“The old idea of an essentially unchanging universe that could have existed, and could continue to exist, forever was replaced by the notion of a dynamic, expanding universe that seemed to have begun a finite time ago, and that might end at a finite time in the future.”

“The Expanding Universe”

In 1929, Edwin Hubble made a landmark observation – wherever you gaze in the heavens, distant galaxies are moving rapidly away from you. This discovery suggested that the universe was (and is) expanding. The discovery that the universe was expanding also provided the first suggestion of the big bang – the point at which the expansion began.

“The progress of the human race in understanding the universe has established a small corner of order in an increasingly disordered universe.”

Today, scientists use two basic theories – both of them incomplete – to describe the universe. One is the general theory of relativity, which describes the force of gravity and the large-scale structure of the universe. The other is quantum mechanics, which deals with phenomena on an extremely small scale – a millionth of a millionth of an inch. Because the outcomes of these two theories are somewhat inconsistent, they cannot both be correct. One of the major initiatives in the physical sciences today is to find a new theory that will incorporate both – often called a quantum theory of gravity.

Does Anybody Really Know What Time It Is?

Aristotle believed that an object moved only if something made it move. Galileo Galilei and Newton came along and changed that view, however. First, Galileo demonstrated that gravity was a constant force regardless of the weight of the object being affected. His measurements were used by Newton to help determine the basic laws of motion.

“A theory is a good theory if it satisfies two requirements. It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations.”

Newton’s first law says that an object in motion would continue its motion unless acted upon by an outside force. The second law states that a body will accelerate or change its speed at a rate proportional to the force applied. Newton also determined that the farther away two objects were, the smaller the force of attraction. Whereas Aristotle saw the universe in a preferred state of rest, Newton saw no fixed standard. One could say A was at rest and B was moving, but it was equally accurate to say A was moving and B was at rest, a conclusion that implied that absolute perspective was illusory. This denial of absolute space was troubling to Newton, who believed strongly in the notion of an absolute God. Like Aristotle, Newton found comfort in the one force that he thought could be measured in unshakably absolute terms – time.

“The fundamental postulate of the theory of relativity, as it was called, was that the laws of science should be the same for all freely moving observers, no matter what their speed.”

Enter British physicist James Clerk Maxwell. In 1865, he succeeded in predicting the existence of wavelike disturbances in the electromagnetic field. These disturbances traveled like ripples in a pond. They varied in size. Those that measured a meter or more were dubbed radio waves. Microwaves spanned just a few centimeters, while infrared waves measured less than one ten-thousandths of a centimeter. Maxwell’s theory stated that all waves should travel at a fixed speed – a presumption that contradicted Newton’s laws of motion. Scientists created the notion of a mythical ether that somehow could account for the discrepancy, but such an explanation was clearly lacking.

Albert Einstein and his theory of general relativity required no mythical ether. Einstein made the revolutionary suggestion that gravity is not like other forces. Rather, he said, it is a consequence of the fact that space-time is not flat, but rather is curved or warped by the distribution of mass and energy within it. Where Newton’s laws killed the idea of an absolute position in space, Einstein’s theory meant the end of absolute time. Now, when a body moved, it affected the curvature of space and time. It was an idea that would revolutionize physics.

“Heisenberg’s uncertainty principle is a fundamental, inescapable property of the world.”

Expanding Vision

The nearest star to the Earth, Proxima Centauri, is about four light-years or about 23 million million miles away from our planet. Most of the stars visible to the naked eye are within a few hundred light-years of Earth. The sun, by comparison, is just eight light-minutes away. There are some hundred thousand million galaxies in the universe, and each of those contains some hundred thousand million stars. Hubble’s great discovery was that these stars were all moving away from us, and, indeed, that those stars farthest from ours were moving away the fastest. Therefore the universe cannot be static but is, in fact, expanding. It has been determined that the universe is expanding by between five and ten percent every billion years.

“At the start of the 1970s we were forced to turn our search for an understanding of the universe from our theory of the extraordinarily vast to our theory of the extraordinarily tiny.”

The notion that the universe is expanding inevitably raises the question: “When, where and how did it all start?” In an attempt to provide an answer, physicists Stephen Hawking and Roger Penrose applied some of Penrose’s discoveries about the behavior of collapsing stars to the universe as a whole. Penrose had published his work in 1965, building on the previous discovery of singularities – commonly known as black holes – which formed when stars collapsed. Applying Einstein’s theory of relativity, Penrose and Hawking in 1970 expanded this process to the workings of the universe.

By reversing the mathematics of the collapsing stars, they concluded that, given the universe’s current rate of expansion, it must have begun as a singularity. Although this concept has been widely adopted, Hawking has since reversed course to posit that there was no singularity at the beginning of time and space.

“In general, quantum mechanics does not predict a single definite result for an observation. Instead, it predicts a number of different possible outcomes and tells us how likely each of these is.”

Uncertain Times

There is a saying among physicists that uncertainty is a fact of life and certainty is a fact of 20th-century physics. In 1926, German scientist Werner Heisenberg formulated his famous uncertainty principle. Heisenberg realized that while it was possible to determine the position and velocity of a sub-atomic particle, doing so would disturb the particle and change its velocity. The measuring mechanism itself – be it light, electron or radio wave – will alter the object being observed.

“Black holes are not really black after all: They glow like a hot body, and the smaller they are, the more they glow.”

Heisenberg demonstrated that the uncertainty in the particle’s position times the uncertainty in its velocity times the mass of the particle is never smaller than a certain quantity, known as Planck’s constant.

This discovery forced scientists to reformulate their vision of the sub-atomic world into what became known as quantum mechanics. In quantum mechanics, particles no longer have separate, well-defined positions and velocities, but rather posses a quantum state that combines both position and velocity. In effect, Heisenberg taught that it is sometimes helpful to think of particles as waves, while at other times it is better to think of waves as particles.

“The existence of radiation from black holes seems to imply that gravitational collapse is not as final and irreversible as we once thought.”

Einstein, by the way, was never comfortable with the element of randomness brought on by quantum mechanics, proclaiming ultimately that: “God does not play dice.”

“At the big bang itself the universe is thought to have had zero size, and so to have been infinitely hot.”

The disagreement between Einstein and Heisenberg goes far beyond any personal or theoretical dispute. Einstein’s theory of relativity appears to govern the workings of the large-scale universe, and quantum mechanics seems to explain the behavior of subatomic particles and waves. But the theory of relativity does not take into account the uncertainty principle that underlies quantum mechanics. How then, can we reconcile the two? The quest for a single, unifying theory has become the preoccupation of many scientists around the world.

Black Holes

In 1928, an Indian graduate student, Subrahmanyan Chandrasekhar, calculated that when a star had expended the fuel that pushed outward as it burned, gravity would make it collapse back in on itself if the star was of sufficient size – about one and a half times the size of our sun. A star smaller than the Chandrasekhar limit could become a white dwarf – with a radius of a few thousand miles and a density of hundreds of tons per cubic inch – or could collapse in more spectacular fashion, creating a neutron star with a radius of just 10 miles or so and a density of hundreds of millions of tons per cubic inch.

“One possible answer is to say that God chose the initial configuration of the universe for reasons that we cannot hope to understand.”

Stars above the limit had an even stranger fate. They could explode and throw off enough matter to avoid a catastrophic collapse. Or, the calculations showed, they could shrink to a single point in space – a singularity – that would be incredibly dense.

Sir Arthur Eddington and Einstein opposed this idea as preposterous.

After his work on the Manhattan project, Robert Oppenheimer thought about the effects of singularities. Light, he said, bends slightly inward near the surface of a star. As the star contracts, the gravitational field gets stronger and the light is bent more and more. Finally, light and everything else can no longer escape and is dragged back by the gravitational field. This field is a black hole, and the point beyond which nothing can escape is called the event horizon. Although they cannot be seen, the existence of black holes in galaxies like Cygnus X1 has been proven by their effects on other orbital bodies.

The Theory of Everything

The discovery of a single unifying theory in the world of physics would have profound theological implications. Einstein once wondered, “How much choice did God have in constructing the universe?”

To date, most physicists have been too busy trying to describe the “what” of the universe to give much thought to the question of “why.” Philosophers who devote their lives to the question of “why” haven’t proven particularly adept at keeping up with the march of science. It wasn’t too many centuries ago that philosophers considered all areas of human knowledge – physics, chemistry, art and religion – as part of their field of study. In the 19th and 20th centuries, however, science became too technical, mathematical and specialized for all but a few experts to truly understand. The hope is that a unified theory will simplify things enough for a broad range of humanity to grasp, thereby providing them with an introduction to nothing less than the very mind of God.

About the Author

It is one of the ironic footnotes to history that Stephen Hawking, who has done so much to popularize the physical sciences, was born in 1942 on the anniversary of the death of Galilei Galileo. He holds the Isaac Newton chair as Lucasian Professor of Mathematics at the University of Cambridge. Hawking is generally considered to be the most brilliant theoretical physicist since Albert Einstein. He has written numerous scientific papers and books, and is also the author of Black Holes and Baby Universes.

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