A Brief History Of Time Full Book
Cecelia Seiner
A Brief History of Time: From the Big Bang to Black Holes is a book on theoretical cosmology by the physicist Stephen Hawking. It was first published in 1988. Hawking wrote the book for readers who had no prior knowledge of physics.
In A Brief History of Time, Hawking writes in non-technical terms about the structure, origin, development and eventual fate of the Universe, which is the object of study of astronomy and modern physics. He talks about basic concepts like space and time, basic building blocks that make up the Universe (such as quarks) and the fundamental forces that govern it (such as gravity). He writes about cosmological phenomena such as the Big Bang and black holes. He discusses two major theories, general relativity and quantum mechanics, that modern scientists use to describe the Universe. Finally, he talks about the search for a unifying theory that describes everything in the Universe in a coherent manner.
Early in 1983, Hawking first approached Simon Mitton, the editor in charge of astronomy books at Cambridge University Press, with his ideas for a popular book on cosmology. Mitton was doubtful about all the equations in the draft manuscript, which he felt would put off the buyers in airport bookshops that Hawking wished to reach. With some difficulty, he persuaded Hawking to drop all but one equation.[2] The author himself notes in the book's acknowledgements that he was warned that for every equation in the book, the readership would be halved, hence it includes only a single equation: E = m c 2 \displaystyle E=mc^2 . The book does employ a number of complex models, diagrams, and other illustrations to detail some of the concepts that it explores.
In A Brief History of Time, Stephen Hawking explains a range of subjects in cosmology, including the Big Bang, black holes and light cones, to the non-specialist reader. His main goal is to give an overview of the subject, but he also attempts to explain some complex mathematics. In the 1996 edition of the book and subsequent editions, Hawking discusses the possibility of time travel and wormholes and explores the possibility of having a Universe without a quantum singularity at the beginning of time. The 2017 edition of the book contained twelve chapters, whose contents are summarized below.
In the first chapter, Hawking discusses the history of astronomical studies, particularly ancient Greek philosopher Aristotle's conclusions about spherical Earth and a circular geocentric model of the Universe, later elaborated upon by the second-century Greek astronomer Ptolemy. Hawking then depicts the rejection of the Aristotelian and Ptolemaic model and the gradual development of the currently accepted heliocentric model of the Solar System in the 16th, 17th, and 18th centuries, first proposed by the Polish priest Nicholas Copernicus in 1514, validated a century later by Italian scientist Galileo Galilei and German scientist Johannes Kepler (who proposed an elliptical orbit model instead of a circular one), and further supported mathematically by English scientist Isaac Newton in his 1687 book on gravity, Principia Mathematica.
In this chapter, Hawking also covers how the topic of the origin of the Universe and time was studied and debated over the centuries: the perennial existence of the Universe hypothesised by Aristotle and other early philosophers was opposed by St. Augustine and other theologians' belief in its creation at a specific time in the past, where time is a concept that was born with the creation of the Universe. In the modern age, German philosopher Immanuel Kant argued again that time had no beginning. In 1929, American astronomer Edwin Hubble's discovery of the expanding Universe implied that between ten and twenty billion years ago, the entire Universe was contained in one singular extremely dense place. This discovery brought the concept of the beginning of the Universe within the province of science. Currently scientists use Albert Einstein's general theory of relativity and quantum mechanics to partially describe the workings of the Universe, while still looking for a complete Grand Unified Theory that would describe everything in the Universe.
In this chapter, Hawking describes the development of scientific thought regarding the nature of space and time. He first describes the Aristotelian idea that the naturally preferred state of a body is to be at rest, and which can only be moved by force, implying that heavier objects will fall faster. However, Italian scientist Galileo Galilei experimentally proved Aristotle's theory wrong by observing the motion of objects of different weights and concluding that all objects would fall at the same rate. This eventually led to English scientist Isaac Newton's laws of motion and gravity. However, Newton's laws implied that there is no such thing as absolute state of rest or absolute space as believed by Aristotle: whether an object is 'at rest' or 'in motion' depends on the inertial frame of reference of the observer.
Einstein's general theory of relativity explains how the path of a ray of light is affected by 'gravity', which according to Einstein is an illusion caused by the warping of spacetime, in contrast to Newton's view which described gravity as a force which matter exerts on other matter. In spacetime curvature, light always travels in a straight path in the 4-dimensional "spacetime", but may appear to curve in 3-dimensional space due to gravitational effects. These straight-line paths are geodesics. The twin paradox, a thought experiment in special relativity involving identical twins, considers that twins can age differently if they move at different speeds relative to each other, or even if they lived in different locations with unequal spacetime curvature. Special relativity is based upon arenas of space and time where events take place, whereas general relativity is dynamic where force could change spacetime curvature and which gives rise to a dynamic, expanding Universe. Hawking and Roger Penrose worked upon this and later proved using general relativity that if the Universe had a beginning a finite time ago in the past, then it also might end at a finite time from now into the future.
In this chapter, Hawking first describes how physicists and astronomers calculated the relative distance of stars from the Earth. In the 18th century, Sir William Herschel confirmed the positions and distances of many stars in the night sky. In 1924, Edwin Hubble discovered a method to measure the distance using the brightness of Cepheid variable stars as viewed from Earth. The luminosity, brightness, and distance of these stars are related by a simple mathematical formula. Using all these, he calculated distances of nine different galaxies. We live in a fairly typical spiral galaxy, containing vast numbers of stars.
The stars are very far away from us, so we can only observe their one characteristic feature, their light. When this light is passed through a prism, it gives rise to a spectrum. Every star has its own spectrum, and since each element has its own unique spectra, we can measure a star's light spectra to know its chemical composition. We use thermal spectra of the stars to know their temperature. In 1920, when scientists were examining spectra of different galaxies, they found that some of the characteristic lines of the star spectrum were shifted towards the red end of the spectrum. The implications of this phenomenon were given by the Doppler effect, and it was clear that many galaxies were moving away from us.
It was assumed that, since some galaxies are red shifted, some galaxies would also be blue shifted. However, redshifted galaxies far outnumbered blueshifted galaxies. Hubble found that the amount of redshift is directly proportional to relative distance. From this, he determined that the Universe is expanding and had a beginning. Despite this, the concept of a static Universe persisted into the 20th century. Einstein was so sure of a static Universe that he developed the 'cosmological constant' and introduced 'anti-gravity' forces to allow a universe of infinite age to exist. Moreover, many astronomers also tried to avoid the implications of general relativity and stuck with their static Universe, with one especially notable exception, the Russian physicist Alexander Friedmann.
Friedmann made two very simple assumptions: the Universe is identical wherever we are, i.e. homogeneity, and that it is identical in every direction that we look in, i.e. isotropy. His results showed that the Universe is non-static. His assumptions were later proved when two physicists at Bell Labs, Arno Penzias and Robert Wilson, found unexpected microwave radiation not only from the one particular part of the sky but from everywhere and by nearly the same amount. Thus Friedmann's first assumption was proved to be true.
At around the same time, Robert H. Dicke and Jim Peebles were also working on microwave radiation. They argued that they should be able to see the glow of the early Universe as background microwave radiation. Wilson and Penzias had already done this, so they were awarded with the Nobel Prize in 1978. In addition, our place in the Universe is not exceptional, so we should see the Universe as approximately the same from any other part of space, which supports Friedmann's second assumption. His work remained largely unknown until similar models were made by Howard Robertson and Arthur Walker.
Friedmann's model gave rise to three different types of models for the evolution of the Universe. First, the Universe would expand for a given amount of time, and if the expansion rate is less than the density of the Universe (leading to gravitational attraction), it would ultimately lead to the collapse of the Universe at a later stage. Secondly, the Universe would expand, and at some time, if the expansion rate and the density of the Universe became equal, it would expand slowly and stop, leading to a somewhat static Universe. Thirdly, the Universe would continue to expand forever, if the density of the Universe is less than the critical amount required to balance the expansion rate of the Universe.
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