A Brief History Of Time

“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?”

Hawking offers some of the fundamental questions about reality. While some, like time travel, seem to be the realm of science fiction, scientists take these questions seriously and have, over the past several decades, made great strides in answering them. The book provides a summary of those discoveries, and Hawking entices the reader’s curiosity by introducing these existential and provocative considerations early on.

“When asked: ‘What did God do before he created the universe?’ Augustine didn’t reply: ‘He was preparing Hell for people who asked such questions.’”

The point of Hawking’s quip is that it’s futile to speculate on what happened before the Big Bang, since “before” doesn’t exist outside the universe. The nature of anything beyond observable or deducible existence is thus forever closed off to humans. Instead, science starts with what it knows, goes back as far as it can, and halts at the moment when all knowledge about the universe disappears at the moment of its creation.

“I shall take the simpleminded view that a theory is just a model of the universe, or a restricted part of it, and a set of rules that relate quantities in the model to observations that we make. It exists only in our minds and does not have any other reality (whatever that might mean). 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.”

Hawking takes a view common to most scientists: A theory must be simple and explain a lot, and a theory works if it fits the available data and can predict new data accurately. Hawking points out that theories are mental constructs and don’t “exist” outside our minds in any tangible sense; since his work deals with the nature of existence, Hawking is very careful and intentional about how he defines reality.

“No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory.”

Scientists are cautious about embracing new theories and test them rigorously. Science welcomes data that lead to new and better theories, even if the new information overturns cherished, older theories. By emphasizing the ease with which most theories are disproved, Hawking simultaneously argues for The Need for Humility in the Process of Scientific Discovery and lends authority to the theories that have maintained their usefulness for decades or centuries.

“The eventual goal of science is to provide a single theory that describes the whole universe.”

The more general a theory is, the more power it has. The most powerful theories are often the simplest: They replace complexities with simple, over-arching ideas that explain what partial theories cannot. A single idea that explains all the forces and particles that make up the universe would greatly improve humans’ ability to explore reality.

“It turns out to be very difficult to devise a theory to describe the universe all in one go. Instead, we break the problem up into bits and invent a number of partial theories. Each of these partial theories describes and predicts a certain limited class of observations, neglecting the effects of other quantities, or representing them by simple sets of numbers. It may be that this approach is completely wrong. If everything in the universe depends on everything else in a fundamental way, it might be impossible to get close to a full solution by investigating parts of the problem in isolation. Nevertheless, it is certainly the way that we have made progress in the past.”

Having established the goal of science, Hawking describes the current methods in achieving a Grand Unified Theory. Especially at first, several theories may be needed to explain natural phenomena. Physics currently needs two theories to explain the universe—relativity and quantum mechanics—and there’s no guarantee that these theories will ever be reconciled as long as they are studied and applied separately. By describing the processes, challenges, and limitations of modern theoretical physics, Hawking introduces the reader to the ideologies that undergird current scientific processes.

“[O]ur goal is nothing less than a complete description of the universe we live in.”

The quest of physics isn’t merely to know how the Earth works; it’s to know the basic rules of the entire universe. Hawking’s work to understand how the cosmos began and how parts of it condense to solitary points called black holes is meant to advance human knowledge of all of reality. It’s ambitious, and there is no guarantee that such a solution is possible, but scientists already have made great strides toward reaching that goal.

“[O]ne cannot really argue with a mathematical theorem.”

Hawking and his colleague Penrose proved that the universe began as a singular, infinitely dense point and exploded outward to become the cosmos we see today. Many scientists hated the idea of such a singularity, but Hawking stuck to his theory because the equations matched the data and because reality doesn’t change just because humans object to it. However, Hawking later came to disagree with his own work, and suggested alternate solutions, again emphasizing the importance of humility in science.

“According to some accounts, a journalist told Eddington in the early 1920s that he had heard there were only three people in the world who understood general relativity. Eddington paused, then replied, ‘I am trying to think who the third person is.’”

Eddington—whose 1919 expedition to observe light warping around the sun’s gravity field during a solar eclipse helped prove Einstein’s theory of General Relativity—explained and popularized Einstein’s work. Eddington speculated correctly that stars generate heat and light from fusing hydrogen into helium in their cores. As such, Eddington was known as one of the few people on Earth who completely understood Einstein’s theories of relativity. His reaction to the reporter’s comment shows that he also had a sense of humor; Hawking offers a sense of the personalities of the scientists whose work he discusses to engage readers and help the layperson feel connected to the scientific community. Hawking’s motive is to make extremely difficult science accessible and enjoyable.

“[M]aybe what we call imaginary time is really more basic, and what we call real [time] is just an idea that we invent to help us describe what we think the universe is like […] a scientific theory is just a mathematical model we make to describe our observations: it exists only in our minds. So it is meaningless to ask: which is real, ‘real’ or ‘imaginary’ time? It is simply a matter of which is the more useful description.”

Hawking uses quantum theory to try to describe the behavior of gravity in extreme situations like inside black holes or at the beginning of the universe. In classical relativity theory, the laws of physics break down inside intense gravitational fields. By using quantum theory, which contains imaginary numbers, the law-breaking singularities don’t exist. Hawking’s discoveries alter not only the way scientists think about black holes but how they think about reality and the nature of existence. His science contains deeply philosophical implications.

“So long as the universe had a beginning, we could suppose it had a creator. But if the universe is really completely self-contained, having no boundary or edge, it would have neither beginning nor end: it would simply be. What place, then, for a creator?”

Evidence supports the idea that the universe might have no border—it bends back on itself so that no one can escape it—and it oscillates between expansion and contraction. Hawking proposes that this may remove the need for a divine creator because the universe always existed. Here, Hawking models how scientific theories inform existential philosophy and shape ideologies. 

“Some people never admit that they are wrong and continue to find new, and often mutually inconsistent, arguments to support their case […]. Others claim to have never really supported the incorrect view in the first place or, if they did, it was only to show that it was inconsistent.”

Hawking posits that arrogance interferes with intellectual objectivity. The relentless search for the right answer is more important to science than any single theory or person being correct.

“The reason we say that humans have free will is because we can’t predict what they will do.”

People are fundamentally unpredictable, in part because the universe itself is fundamentally unpredictable due to the limits of the uncertainty principle. Certain grand effects of the universe can be predicted—that the sun will rise in the east, for example—but small details are harder to corral. Their random effects add up over time, and their interactions with other small details quickly lead to nearly infinite possible future paths. Hawking makes a distinction here between what is discernable and what is real; just because science cannot yet predict human behavior does not necessarily prove that it cannot be predicted at all.

“[T]he possibility of time travel remains open. But I’m not going to bet on it. My opponent might have the unfair advantage of knowing the future.”

Hawking’s research doesn’t prohibit time travel, but he’s concerned that it would be inconsistent with the physical universe as it’s understood. His characteristic, ironic sense of humor emphasizes his point; if he were certain, there would be no hypothetical opponent to bet against him.

“But the idea that God might want to change his mind is an example of the fallacy, pointed out by St. Augustine, of imagining God as a being existing in time: time is a property only of the universe that God created.”

To create the universe, one must, in some sense, be outside it. Hawking’s point is that time itself, intimately connected to space by the theory of relativity, doesn’t exist separately from the universe that humanity occupies. Outside the universe, there is no time; here, Hawking emphasizes the difficulty of conceiving of an existence that operates differently from how humans fundamentally experience reality.

“What would it mean if we actually did discover the ultimate theory of the universe? […] [W]e could never be quite sure that we had indeed found the correct theory, since theories can’t be proved. But if the theory was mathematically consistent and always gave predictions that agreed with observations, we could be reasonably confident that it was the right one. It would end a long and glorious chapter in the history of humanity’s intellectual struggle to understand the universe.”

No matter what theory of everything scientists produce, it won’t make the uncertainty principle go away, which limits how much can be predicted about the future. Scientists also can never be sure that they have enough data to prove conclusively that any unified theory is correct. A Grand Unified Theory, if found, will be located at the border of humans’ ability to reason. Because of this, Hawking predicts that a widely accepted Grand Unified Theory would radically shift worldview and represent a watershed transition in science and human history.

“Only a few people can keep up with the rapidly advancing frontier of knowledge, and they have to devote their whole time to it and specialize in a small area. The rest of the population has little idea of the advances that are being made or the excitement they are generating.”

It’s vital for scientists to interpret their discoveries so that others can understand them. Hawking’s books, especially A Brief History of Time, are a contribution to that effort. He wants people to understand what he’s learned about how the universe got its start. Such knowledge might not translate directly into immediate benefits for humanity, but the doors it opens to the mysteries of the cosmos might bring knowledge about how to build a better future.

“[E]ven if we do find a complete set of basic laws, there will still be in the years ahead the intellectually challenging task of developing better approximation methods, so that we can make useful predictions of the probable outcomes in complicated and realistic situations. A complete, consistent, unified theory is only the first step: our goal is a complete understanding of the events around us, and of our own existence.”

Humans have known Newton’s Law of Gravity for centuries, yet even today scientists can’t fully calculate the gravitational interactions between three bodies, let alone the thousands that make up Earth’s solar system. Knowing the principle isn’t the same as applying it, and, in this universe, the complexities will continue to challenge scientists’ ability to compute and predict, even after a Grand Unified Theory is found. 

“Einstein once asked the question: ‘How much choice did God have in constructing the universe?’ If the no boundary proposal is correct, he had no freedom at all to choose initial conditions.”

The no-boundary theory of space and time generates a universe much like ours, whose initial conditions must be very precisely controlled, or life becomes impossible. Hawking believes that his ideas restrict an intelligent creator’s freedom of action. He proposes that if the origin of the universe is, in some sense, inevitable, there’s no longer a need for a god to create it. In this way, Hawking applies scientific principles to the question of why humanity exists.

“Why does the universe go to all the bother of existing? Is the unified theory so compelling that it brings about its own existence? Or does it need a creator, and, if so, does he have any other effect on the universe? And who created him?”

Science still has many questions to answer about the nature of reality. Even a complete theory of the universe might not explain why there is something rather than nothing, and the debates about that question—and the other questions raised because of it—will likely continue for a very long time.

“[I]f we do discover a complete theory, it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason—for then we would know the mind of God.”

Should a Grand Unified Theory of Everything be produced, Hawking hopes scientists will make that theory’s principles understandable to everyone. Science uses math and experiments and complex machines to learn about reality, but its conclusions—beyond all the experiments and equations—ought to be available to all, since they have major implications for how humanity conceives of its own existence. The final phrase of the quote is also the last phrase of the original main text, and it’s one of the most famous things Hawking ever wrote.

“When a book was published entitled 100 Authors Against Einstein, he retorted, ‘If I were wrong, then one would have been enough!’”

Einstein ruffled feathers in Germany with his anti-war activities and support for Zionism. His comment about the 100 Authors book summarizes an attitude toward the truth held by most scientists: Facts aren’t determined by authority, and power doesn’t decide what’s true. A single piece of evidence can overturn a theory, no matter how fervently believed or protected by politics.

“[O]ne can never be too sure of an idea until it is tested in nature.”

The theory of black holes promoted by Hawking works on paper but must be confirmed by empirical evidence. Gravitational wave detectors now in operation will help with that. Hawking’s attitude is typical of good scientists, in that it’s cautious about any ideas that haven’t been proven in practice. Even the grandest, most logical theories can collapse if real-world testing disproves them. Ideas, no matter how compelling, are only viable if they reflect measurable conditions.