A Brief History of Time: From the Big Bang to the Present Day by Stephen Hawking is a popular book about the history of the universe.
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In Philosophy, the study of time is called temporality. Time has been a major subject of philosophical speculation since at least the Pre-Socratics.
Most people today think of time as a linear progression from past to future, but that is not how most cultures have thought about time throughout history. For example, the indigenous Maya civilization of Mesoamerica believed in a cyclical conception of time. This means that, for them, time did not move from past to future in a straight line, but rather in cycles. This is why their calendar was based on cycles of thirteen months rather than twelve.
The linear progression of time is a relatively modern concept that came about with the development of Newtonian physics in the seventeenth century. In Newtonian physics, time is absolute and it flows at the same rate everywhere in the universe. This is what we mean when we say that time is a fourth dimension.
Newton’s laws of motion and his law of gravity are both expressed in terms of mathematical equations that describe how objects move through space and how they are affected by forces such as gravity. These equations are symmetrical with respect to time, which means that they treat past and future in exactly the same way. This symmetry implies that time is absolute and objective in Newtonian physics.
The Big Bang
In the beginning, there was the Big Bang. Scientists believe that our universe began with a massive explosion some 14 billion years ago. All of the matter and energy in the universe was condensed into a tiny point, or singularity. Then, in an instant, it all exploded outward and has been expanding ever since.
This theory is supported by observations of the cosmic microwave background radiation—the faint afterglow of the Big Bang that can be detected throughout the universe. As this radiation has cooled over time, it has enabled us to look further and further back in time, ultimately giving us a glimpse of the very beginning of our universe.
The Formation of Galaxies
It is helpful to think of the universe as a giant balloon that is being inflated. As it does so, the distance between any two points on the surface of the balloon increases. Similarly, the distance between any two galaxies also increases as the universe expands. But because gravity is pulling all of the matter in the universe together, this increase is not uniform; rather, it is slower in regions where the gravity is stronger.
Formation of Galaxies
The first galaxies formed about a billion years after the Big Bang, when the extra-galactic gas that had been mostly smooth and uniform up to that point began to clump together under its own gravity. These clumps then became dense enough to collapse further and form stars. The most massive clumps became galaxy clusters, while smaller clumps became individual galaxies.
The Formation of Stars
The familiar element hydrogen is by far the most abundant in the Universe, making up about 75% of its visible mass. Helium, the second most abundant element, makes up about 23%. The remainder is made up of a wide variety of heavier elements, including carbon, nitrogen, and oxygen, which are essential to life on Earth.
Stars are born when great clouds of hydrogen gas collapse under their own gravity. As the clouds collapse, they begin to spin faster and faster. Eventually, they flatten out into spinning discs with central regions that are extremely dense and hot. In these regions, hydrogen atoms are forced so close together that they fuse together to form helium atoms—a process that releases enormous amounts of energy. This energy prevents the collapse from continuing and creates tremendous pressure that pushes outward against the gravitational pull inward. The end result is a star—a massive ball of gas held together by gravity and heated by nuclear fusion in its core.
The Formation of Planets
Most of the mass in the solar system is in the sun. The planets are much less massive; for example, Jupiter has only one thousandth of the mass of the sun. Nevertheless, there are enough heavy elements in the planets to make them interesting places.
The theory of how planets form is now quite well understood. It starts with the nebular hypothesis, first proposed by Kant and Laplace in the eighteenth century. According to this hypothesis, a rotating cloud of gas and dust collapses to form a disk around a central star. (The word “nebula” comes from the Latin word for “cloud.”) As the cloud collapses, it conserves its angular momentum, so it spins faster and faster.
The Evolution of Life
The first living things on Earth were simple single-celled organisms called prokaryotes. These were followed by more complex single-celled eukaryotes. The first multicellular organisms appeared about 2 billion years ago, and the first animals appeared about 600 million years ago. The first land plants appeared about 400 million years ago, and the first land animals appeared about 100 million years later.
The history of life can be divided into four major stages:
1. The prebiotic stage, during which the chemical precursors of life formed.
2. The biochemical stage, during which the first living things appeared.
3. The cellular stage, during which the first cells appeared.
4. The multicellular stage, during which the first multicellular organisms appeared.
The Evolution of Humans
The evolution of humans is the long process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits characteristic of all people originated from apelike ancestors and evolved over a period of millions of years.
One of the earliest defining human traits was bipedalism — the ability to walk on two legs. Bipedalism freed the hands for tasks such as tool use and carrying food and other supplies. Other important human characteristics evolved in response to changing climates, such as the development of sweat glands, which helped regulate body temperature in hot climates, and the growth of body hair, which offered protection from the sun and other elements.
Over time, humans developed larger brains and learned to use more sophisticated tools and weapons. They also developed language skills, which allowed them to communicate with each other and pass on their knowledge to future generations.
As humans evolved, they adapted to different environments and became specialized in certain activities. For example, some groups of humans became hunters while others became farmers. Still others became herders or fishermen. As humans continued to evolve, they diversified into even more specialized occupations.
Today, there are an estimated seven billion people living on Earth — each with their own unique set of skills and abilities that have been shaped by their ancestors’ long journey of evolution.
In the years since its publication in 1988, Stephen Hawking’s A Brief History of Time has become an international bestseller, translated into forty languages. Of all the books on cosmology and astrophysics written for a general audience, it is probably the most widely read and influential. In these revised and refreshed editions, with a new introduction by the author, Professor Hawking tells us about the latest discoveries in space-time research. He takes us on a fascinating journey to black holes and neutron stars, to the Big Bang and a new theory of eternal inflation. He discusses the search for extraterrestrial intelligent life, including his own work with SETI (the Search for Extraterrestrial Intelligence), and assesses the odds of success. With characteristic lucidity he explains recent discoveries that have modified our understanding of space-time: such as wormholes connecting different parts of space-time or “branes” that form parallel universes. And he gives his own views on what happened before the Big Bang, putting forward his idea that time may not exist at all at very early stages in the history of our universe. Finally he brings us back to our own time—the present day—and raises fundamental questions about human values and our future survival in a universe that is expanding and cooling….