When a famous scientist (possibly Bertrand Russell) gave a public astronomy lecture, he described the orbits of the planets in the solar system and how the sun orbits the center of our galaxy. After he finished, an old lady at the back told him he was talking nonsense, as the world is flat and sits on the back of a tortoise. When he asked what the tortoise stands on, she replied it is tortoises all the way down.
Hawking opens his book about mankind's great scientific progress to date with an anecdote of a stubborn old lady who is determined to hang on to her superstitions despite informed individuals' best attempts to help her access the latest understanding of the universe's make up. Some people, it seems, just can't be taught—but Hawking shows he's going to try anyway.
Although the old lady’s image is ridiculous, do scientists really know better? New technologies are helping to offer answers to age-old questions about the universe and where humans came from. Maybe one day the answers will seem as obvious as the earth’s orbit, or as ridiculous as the image of the tortoises. Time (whatever it is) will tell.
Hawking does not side with the old lady, but he does stop to ask how it is that scientists can say they have better ideas than she does. Just stating a worldview does not mean that it is correct. This comparison shows that all people long to understand the universe and humanity's place in it.
Greek philosopher Aristotle gave two good arguments for the earth being a sphere instead of flat. First, a lunar eclipse must be the earth blocking the sun’s light, and the shadow is always round, not elongated as it would be if the earth was flat. Second, the Greeks saw that the North Star (which lies over the North Pole) is more central in the sky the further north you sail, and closer to the equator the further south you travel. From this, Aristotle could even make an educated guess about the distance around the earth. Another point the Greeks noticed is that when ships came over the horizon, one always sees the sails first, and later the hull.
Famed classical philosopher Aristotle could apply logic to everyday phenomena, such as the position of the north star, to deduce that the earth was round. This did not require deep scientific knowledge, but simply using logic to follow up his curiosity. This was true not only for Aristotle but also for his compatriots, showing that curiosity about the world is not confined to intellectuals. Everyone has questions about why the world works the way it does; it is a natural part of being human.
Yet Aristotle believed the earth was fixed in place at the center of the universe, and all the other heavenly bodies moved around it in perfect circular orbits. Ptolemy took this idea further in the 2nd century AD, creating a cosmological model consisting of eight spheres—one for the moon, sun, stars, and the five known planets. Each moved on their own complicated paths in these spheres while the fixed stars remained in the same formation at the outer limit, rotating together across the sky. Anything beyond that limit was unknown.
Even the wise Aristotle could not see past his own biases. While he could accept that the earth was round, he could not overcome his baseless conviction that the earth was at the center of the universe. Ptolemy, a multispecialist Greco-Roman thinker, had the same stubbornnes, and created an overly complex model to fit previous assumptions.
Ptolemy’s model could fairly accurately predict the movement of each heavenly body. But for it to be correct, the moon would have to pass by the earth twice as close as normal every now and then, something that bugged Ptolemy, as it ought to have appeared twice as big as normal at those times. But the model was generally accepted anyway, including by the Christian church, as there was lots of space outside the model for heaven and hell.
Ptolemy directly overlooked obvious flaws in his model because he was determined to prove his own ideas were correct. His ideas were popular because they agreed with how people saw the world, and their place in it. They did not challenge the church or its teachings, so the model was easily accepted.
Nicholas Copernicus, a Polish priest, proposed a simpler model in 1514, publishing anonymously at first to avoid being called a heretic. It took nearly a century for his idea, that the sun sat stationary at the center of the planets, to be taken seriously. German astronomer Johannes Kepler and Italian Galileo Galilei backed his theory, even though it was not perfect based on the observable movements in the cosmos.
Copernicus, working in anonymity for fear of reprisals from an obstinate church, discovered a truth that was too important to ignore. The fact that it took decades for his work to be respected demonstrates the difficulties scientists can face when promoting new ideas. Nevertheless, because of the accuracy of his model, the idea finally stuck.
The final blow came to the ancient model with the invention of the telescope. Galileo observed Jupiter and found it had several satellites, meaning not everything orbited the earth. Those moons could still primarily orbit the earth and have very complicated journeys that also cause them to appear to orbit Jupiter—but Copernicus’s idea was simpler. Kepler added the idea that orbits could be elongated, not perfectly circular, and finally the theory worked with the observable movement of the heavenly bodies.
Galileo offered clear and simple evidence that backed Copernicus’s model, showing that the truth will out when it comes to matters of science. The key here is that Galileo and Kepler could match their models with what was actually observed. Ptolemy, in contrast, had seen the moon move in ways contrary to what his model suggested, but did not adapt his ideas—a critical failure.
Kepler didn’t like the idea of elliptical (elongated) orbits as much as perfect circles, but the theory seemed to work well in practice. Now the problem was that this didn’t seem to work with the idea that magnetic forces controlled all this movement. It wasn’t until Sir Isaac Newton’s Philosophiae Naturalis Principia Mathematica came out in 1687 that an explanation was offered.
Although he had earlier backed a relatively new idea, that the earth orbited the sun, Kepler still could not let go of his assumption that magnetic forces drove the movement of the heavenly bodies. Even the best and brightest, who can see the errors in others' judgment, cannot be as objective with their own work.
Newton’s work offered the math to back up his ideas about how things move in space and time. His law of universal gravitation suggested everything is attracted to everything else, with the force being stronger when those things are closer together and bigger. That’s why things fall to the ground. Newton mentioned the idea coming to him as an apple fell to the ground, though the idea that the apple hit him on the head was probably added by others later. Regardless, his theory showed the moon moves in an elliptical orbit around the earth, while the planets have an elliptical orbit around the sun.
Newton created his laws on the basis of two crucial foundations: evidence and observation. He explained, mathematically, why apples fall to the ground and why the planets' orbits are not perfectly circular, the latter being an idea that obsessed Aristotle and Kepler, blinding them from the path to greater progress.
The idea of a natural boundary to the universe was thrown out along with Ptolemy’s celestial spheres, replaced with the Copernican model. Thus, the new assumption was that the fixed stars were not so fixed after all, but very far away and hard to measure. In fact, given his idea of gravity, these stars should all be moving around each other, and at some point should fall together. If there are finite stars in a finite universe, the stars would fall into each other, Newton wrote in a letter to a friend in 1691. But infinite stars spread uniformly across an infinite universe would not, as there would be no center, he reasoned.
Newton's revolutionary discovery of gravity, however, only led to more questions. If every star and planet was attracting every other star and planet, the universe ought to be collapsing in on itself. Newton was unsure how to account for this, as the law of gravity seemed correct in and of itself. He focused on the question of whether the universe was finite or infinite to try to get to the bottom of this quandry.
This is one of many snares when talking about the infinite. In an infinite universe every point is the center because every point has infinite stars either side. These days, it is now thought the finite model must be correct. Adding more stars beyond the limit of that boundary (i.e. picturing a bigger universe with more stars) makes no difference—all the stars will still fall in on each other at the same pace. It is now known there cannot be a model of an infinite universe where all the bodies are always attracting each other.
Newton was following the wrong path to try to solve the problems his laws seemed to raise, but Hawking does not present him as stubbornly sticking to unfounded assumptions. He was tackling new concepts, and it is always easier to make judgments in hindsight, with knowledge of centuries of subsequent scientists' work.
Before the twentieth century, no one had suggested the universe was expanding or contracting, which reveals the way people were thinking back then. Everyone either thought the universe had always existed in its current state, or that it was created at a certain point in the same state it is now. This could have been because of people’s belief in eternal truths, or perhaps the comfort of the idea of an unchanging universe, eternal even after their own deaths.
The time it took for humanity to notice that the universe is expanding shows the deep, inbuilt stubborness people must overcome to see through their assumptions, especially because it is hard to realize that they are assumptions in the first place. It had simply never occurred to anyone that the universe was not static. This illustrates people's way of thinking, as the form of the universe implies humanity's role within it. Up-ending one's understanding of the world has direct implications for one's sense of self, and one's destiny.
Even though Newton’s theory showed that the universe was not static, people did not immediately consider that it might be expanding. Instead they toyed with the idea that at great distances gravity could be repulsive, rather than attractive. It allowed the stars to remain in equilibrium. But now such a model is considered unstable, as movements in either direction would create increasingly strong repulsive or attractive forces.
Newton and his contemporaries could not see what they could not see. Unaware of their own inflexible perspective, they again tried to manipulate the theory of gravity into their preconceived image of the universe. Their attempts simply would not work, because they were not correct. Yet this offers further evidence that humanity will continue to ask more questions even after new answers come to light.
German philosopher Heinrich Olbers wrote in 1823 that in an infinite static universe, every line of sight would end on a star. Others had made similar arguments, even at the same time as Newton, but Olbers’s objection to Newton’s concept of an infinite, static universe was the first to be widely noted. Thus, the night sky ought to be as bright as daylight. The only way to explain the night sky was that each star was created at a finite time. If so, the light from those stars might not have reached us yet. But this, in turn, raised the question of when the stars came to be.
Olbers reasoned, scientifically, that the universe must be finite, answering Newton's previous question. But, as ever, this only raised more questions. If the world is finite, and the stars had not been around forever, the question now was when they began. Olbers took prior knowledge and applied it to observation to draw logical conclusions. He was one in a long line of people to do so, who together create the history of scientific progress.
The idea of a beginning to the universe was not new. Religious thought had already put the beginning at a not too distant time in the past. One line of reasoning for the beginning was a “First Cause,” which caused everything else in a connected line of causality. St. Augustine put Creation—as per the book of Genesis in the Bible—at around 5,000 BC. That’s not that far off the end of the last Ice Age in around 10,000 BC, when civilization took off.
Every civilization has been curious about the universe and humanity's place in it. One key subject in that discussion is how this all came to be. As such, religious and scientific thinkers all deal with the same topics, albeit approaching the matter from different angles. Nevertheless, Hawking consistently places the scientific approach in a higher position than any other.
But Greek philosophers, including Aristotle, did not like the idea of a beginning because it sounded like divine intervention. They thought people and the world had existed and will exist forever. They had also considered the ideas of cultural and scientific progress toward greater understanding, but argued that large disasters had always put the human race back to square one.
Aristotle and his counterparts saw civilization as cyclical—catastrophic natural disasters would reset humanity's progress, and the cultural and scientific machines would start back up again, endlessly. This contrasted with viewpoints such as St. Augustine's, which reasoned the progress we see shows time is linear. Yet Hawking suggests the Greek philosophers discounted the idea of a beginning simply because it did not agree with their ideas about religion, rather than objectively looking for an answer.
Philosopher Immanuel Kant later considered the question of whether the universe had a beginning in time and if it is limited in space, in his Critique of Pure Reason, published in 1781. He called the questions antinomies, meaning contradictions, of pure reason, because both ideas—that the universe had a beginning, and was eternal—had compelling arguments.
Kant took a similar approach to Aristotle, by seeking answers to questions about the universe by applying logic. He found, essentially, that one could reason either case just as logically. Hawking uses this example to show the limited success that logic can achieve—ultimately, one must apply that logic to observation to prove a point.
Kant reasoned that if the universe did not have a beginning, the time before any event was infinite, which seems ridiculous. If the universe started at a particular time, the time before that was infinite, so why would it start at any specific time. Both arguments are really the same—they assume time moves back forever, whether or not the universe exists. But really, the concept of time itself did not exist before the beginning. This is an idea St. Augustine used, when asked what God did before the beginning. He stated time is a concept only within Creation, and did not exist before it.
Hawking describes, and criticizes, Kant's work to illustrate another approach humans take to understanding the universe. Hawking shows again that religious and scientific thought are not mutually exclusive. St. Augustine was on the right track when he said that time is a property of the universe, and so has no meaning or bearing before any beginning. This is something Kant did not grasp, though he was asking the right questions.
The question of the beginning was largely related to metaphysics or theology back when everyone thought of the world as static and unchanging. The world looked much the same from both ends of the argument. That changed in 1929 when Edwin Hubble saw that all distant galaxies are moving rapidly away from each other. As such, at some point, possibly 10 billion to 20 billion years ago, all the matter in the universe must have been in one tiny place of zero size, meaning the density of the universe was infinite. This realization made the question of beginnings one about science.
Hubble's landmark discovery that every galaxy in every direction is rapidly moving away from every other galaxy turned the question of beginnings on its head. The idea suggested a definite point in time and space where everything came into being. Now that a better model for the universe had been found, the question became how to measure its history—to understand how the universe came into its current state and where it was heading. It was no longer a matter of theologizing, but calculating.
Hubble’s discovery created the idea of the big bang, a time when the universe was tiny and infinitely dense before rapidly expanding. At that time, all laws of science would break down, meaning time had its beginning in the big bang, because any previous times would no longer have any bearing.
The big bang agrees with St. Augustine's argument that time is purely a property of the universe and has no meaning outside its boundary in space and time. Anything that existed or happened before would have no effect on anything existing or happening now.
This new idea of the creation of time is completely different to any that preceded it. In an unchanging universe, some outside power determines the start, and there is no physical need for a beginning. But if the universe is expanding there could be physical reasons behind the need for a beginning. An expanding universe does not rule out the existence or involvement of God, but it does determine when time started.
The big bang does not place religion and science at opposite ends of possibility. But understanding more about the physics of a beginning does complicate the idea of God creating the world. This shows that understanding the “how” can sideline God, in turn showing that humans use deities to explain what they do now yet know.
To discuss all these questions, first one needs to know what a scientific theory is. A theory is a model of the universe, or one part of it, and rules that link aspects of that model to what we can observe. It exists only in our minds (whatever that means). Theories are good if they can explain observations with a few factors, and can accurately predict outcomes in future observations. For example, Empedocles’s idea that the four elements were earth, air, fire, and water is simple, but cannot make any predictions. By contrast, Newton’s theory of gravity, which is determined by mass and distance, is even simpler, but can accurately predict the movements of the stars.
Hawking definitively states the specific characteristics of the modern scientific approach to differentiate it from the others outlined previously. Ideally, scientists should be objective and results-focused. This should rule out personal agenda, ego, or stubbornness. The focus is on finding the laws that govern the world, not finding complicated mechanisms by which one's assumptions can be transplanted onto reality.
Any physical theory cannot be proved entirely. Even if every test has backed up the theory so far, one cannot prove that the next test will not disprove it. Even one single piece of evidence contradicting the theory can disprove it. Philosopher of science Karl Popper said that confidence in any theory grows with each accurate prediction, but that theory must be cast aside or adapted if even one test outcome or observation contradicts it. (Although in reality, one can always question the competence of the observer.)
Scientists must be totally objective, ready to drop or adjust any theory where it does not match observations. This is a stark contrast from the earlier approaches Hawking outlined. No worldview is considered absolute or untouchable. Everything can change in an instant.
Usually, new theories are largely extensions of previous theories. For example, Mercury’s movement diverged slightly from predictions made by applying Newton’s law of gravity. Albert Einstein’s slightly different prediction, via his general theory of relativity, matched with what was seen, a critical confirmation of his new theory. Newton’s theory is still used in most cases, as the differences are so tiny and no difference is visible in day-to-day usage. Newton’s theory is also much simpler to use.
Theories build on other theories, or adjust previous theories, to gradually build up an ever-more accurate and dependable set of laws with which to measure and assess the universe. Yet, as is shown by the continued use of Newton's less precise theory, different theories can fit different uses, revealing a patchwork of ideas and rules available for understanding the universe.
Science’s ultimate goal is to offer one theory for the entire universe—a theory of everything. But usually scientists deal with it in one of two ways. First, they apply the laws that explain how things move through time to make predictions. Second, there is the question of the initial state of the universe. Some think only the first question is strictly science; the second is metaphysical or do to with religion. They say God can do whatever he likes. While that could be true, he made the universe in a way that is governed by certain laws, meaning there are also laws determining the beginning.
Hawking introduces a key theme of this book and of modern scientific endeavor: the hunt for a unifying theory of everything. The idea is that one set of rules can explain how the universe came to be and how it all works. People approach this question from different angles, and indeed religion represents one such approach. The main point is to keep asking questions, as the universe can be understood.
It is tricky to offer one theory for the whole universe right now. Instead there are numerous partial theories. This could be the wrong approach. If everything in the universe fundamentally depends on everything else, only looking at certain parts cannot reveal the whole picture. But, that is how progress has been made so far. For example, gravity depends only on mass, not the content of an object, so we do not need a theory on the construction of the sun to predict its movement.
All the progress made so far is contributing to this mission to find one unifying theory, whether intentionally or not. While not ideal, according to Hawking, these partial theories have brought a certain degree of progress, and have individual worth.
Scientists now describe the universe in terms of the general theory of relativity and quantum mechanics—both great achievements of the first half of the 20th century. The first relates to gravity and large to really large-scale structures of the universe. The second relates to miniscule matter a billionth of an inch wide. But, the two do not relate and cannot both be correct. What is needed is a quantum theory of gravity, but it might be some time until we have one. Many of the aspects and predictions of that theory are already known, though.
Hawking sets out the two key theories that the book will address, as well as the fact the next great step in finding a unifying theory is to unify these central concepts. Scientists have discovered truths about the largest and smallest structures in the universe, and now the task is to find how these can be used together to answer the last remaining questions.
If the universe is not chaotic, but rather is governed by laws, all the partial theories must fit into one overarching theory of everything. But there is a fundamental contradiction in that search. We’ve assumed so far that we are rational beings that can know the world, which would mean we can progress to such knowledge. But if there really is such a theory, it would determine our own actions too, meaning the theory would determine its own discovery. Why would discovery be the ultimate conclusion, rather than a wrong conclusion, or no conclusion?
As Hawking has already shown, science shares many borders and overlaps with philosophy. If this unified theory really could predict everything, that would include human behavior and intelligence, meaning the theory would predict its own discovery. So, the question Hawking posits is whether the theory would necessitate its own discovery.
Charles Darwin’s theory of natural selection might offer an answer. He stated that genetic differences occur in any group of self-reproducing beings, and that certain differences will result in strengths that cause those beings to be more likely to survive. So far in history, intelligence and science have indicated a survival advantage. Today, our discoveries could well kill us all. Also, a unified theory might not affect our likelihood of survival. But, given the regular evolution of the universe, our logical reasoning as developed by natural selection means we should be able to make the right conclusions.
Hawking reveals his optimism that humanity can and will uncover a unified theory of everything. Although it might not directly improve the species' likelihood of survival—for example, it might not assist food production or might improve weapon functionality and lead to extinction—nevertheless humanity's intellect has led it well so far, meaning it should be possible for humans to discover this unifying theory.
The theories we have so far work for the wide majority of cases. As such, searching for the ultimate theory becomes hard to justify. Then again, people argued this about relativity and quantum mechanics, which eventually gave us nuclear power and microelectronics. Thus, the search for a theory of everything might not help us survive, or ever change our lives all that much, but it does tackle questions we have asked for millennia. People want to understand the world and its order, where we came from and why we’re here. This deep longing is the justification for this mission, a mission that asks for a complete description of the world we live in.
Hawking justifies his quest, perhaps his calling, to uncover a unified theory of everything, not on the basis of the technology it might create (though that is a possibility too), but instead on the fact that humanity has an innate desire to understand the universe and our role within in. Therefore, finding such a one-stop rule that unlocks the deeper truths of existence pays direct service to the inner longing that has gripped humanity since its earliest days