Download A Brief History of Time PDF By Stephen Hawking. stephen hawking A Brief History of Time: From the Big Bang to Black Holes is a popular-science book on cosmology (the study of the universe) by British physicist Stephen Hawking. It was first published in 1988. Hawking wrote the book for nonspecialist readers with no prior knowledge of scientific theories.
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 manne
In the ten years since its publication in 1988, Stephen Hawking’s classic work has become a landmark volume in scientific writing, with more than nine million copies in forty languages sold worldwide. That edition was on the cutting edge of what was then known about the origins and nature of the universe. But the intervening years have seen extraordinary advances in the technology of observing both the micro- and the macrocosmic worlds.
These observations have confirmed many of Professor Hawking’s theoretical predictions in the first edition of his book, including the recent discoveries of the Cosmic Background Explorer satellite (COBE), which probed back in time to within 300,000 years of the universe’s beginning and revealed wrinkles in the fabric of space-time that he had projected.
Eager to bring to his original text the new knowledge revealed by these observations, as well as his own recent research, Professor Hawking has prepared a new introduction to the book, written an entirely new chapter on wormholes and time travel, and updated the chapters throughout.
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Summary of A Brief History of Time PDF
The Universe in Our Eyes (Chapter 1)
Ptolemy’s Earth-centric model of the planets, stars, and Sun’s positions
Hawking explores the history of astronomical studies in the first chapter, including Aristotle’s and Ptolemy’s views. Unlike many of his contemporaries, Aristotle believed that the Earth was spherical. He arrived to this conclusion from watching lunar eclipses, which he believed were produced by the Earth’s round shadow, as well as an increase in the altitude of the North Star from observers further north. Aristotle also believed that the Sun and stars revolved in complete circles around the Earth for “mystical causes.” Ptolemy, a second-century Greek astronomer, pondered the placements of the Sun and stars in the Universe and devised a planetary model that more fully described Aristotle’s beliefs.
Today, we know that the Earth revolves around the Sun, not the other way around. A sequence of discoveries in the 16th, 17th, and 18th centuries challenged Aristotelian and Ptolemaic views regarding the position of the stars and Sun. Nicholas Copernicus, a Polish clergyman, was the first to provide a thorough argument that the Earth rotates around the Sun in 1514. Galileo Galilei, an Italian scientist, and Johannes Kepler, a German scientist, researched how the moons of several planets moved in the sky nearly a century later, and used their findings to back up Copernicus’ theories.
Kepler offered an elliptical orbit model instead than a circular one to fit the observations. Isaac Newton used complex mathematics to support Copernicus’ idea in his book Principia Mathematica, published in 1687. Newton’s concept also implied that stars, such as the Sun, were not fixed but rather moving things from afar. Newton, on the other hand, felt that the Universe was made up of an endless number of stars that were mostly stationary. Many of his contemporaries disagreed with him, including German philosopher Heinrich Olbers.
Over the millennia, the genesis of the Universe has been a fascinating subject of research and controversy. Early philosophers such as Aristotle believed the Universe had always existed, whereas theologians such as St. Augustine believed it was created at a certain point in time. Time, according to St. Augustine, is a concept that was born with the creation of the Universe. Immanuel Kant, a German philosopher who lived more than 1000 years later, contended that time had no beginning.
Most galaxies are moving away from each other, according to astronomer Edwin Hubble, who discovered this in 1929. This could only be explained if the Universe itself was expanding in size. As a result, sometime between ten and twenty billion years ago, they were all crammed into a single, incredibly congested location. This revelation pushed the concept of the Universe’s genesis into the realm of science. Today, scientists utilize two theories to partially explain the workings of the Universe: Albert Einstein’s general theory of relativity and quantum mechanics. Scientists are still searching for a full Grand Unified Theory that would explain everything that exists in the Universe. Hawking believes that while the discovery of a complete unified theory may not aid our species’ survival or even have an impact on our way of life, humanity’s deepest desire for knowledge justifies our continued search, and that our goal is nothing less than a complete description of the Universe we live in. [4]
Space and Time (Chapter 2)
Following the arrival of Newtonian mechanics, Stephen Hawking shows how Aristotle’s conception of absolute space came to an end. Whether an object is ‘at rest’ or ‘in motion’ depends on the observer’s inertial frame of reference; an object may be ‘at rest’ as viewed by an observer moving in the same direction at the same speed, or ‘in motion’ as viewed by an observer moving in a different direction and/or at a different speed. There is no such thing as absolute’rest.’ Furthermore, Galileo Galilei refuted Aristotle’s thesis that heavier bodies fall faster than lighter bodies. He demonstrated this experimentally by monitoring the motion of objects of various weights and concluding that unless an external force acts on them, all objects will fall at the same velocity and reach the bottom at the same time.
Absolute time was a belief held by Aristotle and Newton. They believed that if two precise clocks in different states of motion were used to measure an event, they would agree on the amount of time that had passed (today, this is known to be untrue). The notion that light has a finite speed was first demonstrated by Danish scientist Ole Rmer, who observed Jupiter and one of its moons, Io. Because the distance between Earth and Jupiter fluctuates over time, he noticed that Io appears at different times as it circles around Jupiter.
James Clerk Maxwell studied the real propagation of light, concluding that light travels in waves travelling at a constant pace. The Michelson–Morley experiment contradicted Maxwell’s and many other physicists’ claims that light must travel through a hypothetical fluid termed aether. Later, Einstein and Henri Poincaré argued that assuming there is no absolute time, there is no need for aether to explain light motion. This is the basis of the special theory of relativity, which claims that light travels at a finite speed regardless of the observer’s speed. Furthermore, the speed of light is the quickest at which any data can move.
The famous equation E=mc2E = mc2 describes how an infinite amount of energy is required for any object with mass to travel at the speed of light. The speed of light was used to create a new way of defining a metre. Light cones, a spacetime graphical representation that restricts what occurrences are allowed and what are not based on the past and future light cones, can also be used to depict “events.” There’s also a four-dimensional spacetime described, in which’space’ and ‘time’ are inextricably intertwined. The way an object moves through space has an inextricable effect on how it perceives time.
In contrast to Newton’s approach, which portrayed gravity as a force that matter exerts on other matter, Einstein’s general theory of relativity describes how the course of a beam of light is impacted by ‘gravity,’ which, according to Einstein, is an illusion generated by the warping of spacetime. Light always travels in a straight route in 4-dimensional “spacetime,” yet due to gravitational influences, it may appear to curve in 3-dimensional space. Geodesics are straight-line pathways with no bends. The twin paradox, a special relativity thought experiment involving identical twins, proposes that identical twins can age differently if they move at different speeds relative to one other or if they lived in different regions with uneven spacetime curvature. Special relativity is predicated on events taking place in arenas of space and time, but general relativity is dynamic, with forces changing spacetime curvature and causing the Universe to expand. Hawking and Roger Penrose worked on this and eventually used general relativity to show that if the Universe had a beginning, it had to have an end as well.
The Expanding Universe (Chapter 3)
Since the Big Bang, the universe has expanded.
Hawking begins this chapter by explaining how physicists and astronomers determined the distance between stars and the Earth. Sir William Herschel confirmed the positions and distances of several stars in the night sky in the 18th century. Edwin Hubble established a method for calculating distances using the brightness of Cepheid variable stars as seen from Earth in 1924. A simple mathematical formula connects the luminosity, brightness, and distance of these stars. He estimated the distances of nine distinct galaxies using all of these. We dwell in a spiral galaxy with a large number of stars, similar to our own.
Because the stars are so far away from us, we can only see their one distinguishing feature: their light. A spectrum is created when this light passes through a prism. Every star has its own spectrum, and because each element has its own spectra, we may use the light spectra of a star to determine its chemical makeup. To determine the temperature of stars, we use their thermal spectra. When scientists looked at the spectra of different galaxies in 1920, they saw that some of the star’s characteristic lines were pushed towards the red end of the spectrum. The Doppler effect revealed the ramifications of this phenomena, and it was evident that many galaxies were traveling away from us.
Because certain galaxies are red shifted, it was assumed that some galaxies would also be blue shifted. Redshifted galaxies, on the other hand, outweighed blueshifted galaxies by a large margin. According to Hubble, the quantity of redshift is related to relative distance. He deduced from this that the Universe is expanding and had a beginning. Despite this, into the twentieth century, the belief of a static Universe prevailed. Einstein was so convinced of the existence of a static universe that he devised the cosmological constant and incorporated ‘anti-gravity’ forces to allow for the existence of an infinitely old cosmos. With one prominent exception, Russian physicist Alexander Friedmann, many astronomers tried to ignore the ramifications of general relativity and persisted with their static Universe.
Friedmann proposed two simple assumptions: the Universe is similar everywhere we are, i.e. homogeneity, and in any direction we look, i.e. isotropy. His findings demonstrated that the Universe is not static. His theories were eventually proven correct when two Bell Labs physicists, Arno Penzias and Robert Wilson, discovered unexpected microwave radiation not only from one area of the sky but from all over the sky and in approximately equal amounts. As a result, Friedmann’s first premise was proven correct.
Robert H. Dicke and Jim Peebles were also working on microwave radiation at the same time. They claimed that the light of the early Universe should be seen as background microwave radiation. Wilson and Penzias had already accomplished this when they received the Nobel Prize in 1978. Furthermore, because our location in the Universe is not unique, we should see the Universe as being roughly the same as any other area of space, which supports Friedmann’s second assumption. Until Howard Robertson and Arthur Walker created similar models, his work remained mostly ignored.
Friedmann’s concept spawned three different types of models for the Universe’s evolution. First, the Universe would expand for a set period of time, and if the expansion rate is less than the density of the Universe (resulting in gravitational attraction), the Universe would eventually collapse. Second, the Universe would expand, and if the expansion rate and density of the Universe reached a point where they were equal, the Universe would expand slowly and stop, resulting in a rather static Universe. Third, if the density of the Universe is less than the critical amount required to balance the expansion rate of the Universe, it will continue to expand indefinitely.
The first model displays the Universe’s space curving inwards. The space in the second model would result in a flat structure, while the third model would result in a negative’saddle shaped’ curvature. Even if we compute it, the current expansion rate exceeds the Universe’s critical density, which includes dark matter and all stellar masses. The initial model contained a Big Bang from a space of infinite density and zero volume known as’singularity,’ which is also where the general theory of relativity (on which Friedmann’s solutions are based) fails.
Because it supports the biblical premise that the universe has a beginning in time rather than being everlasting, this concept of the beginning of time (introduced by Belgian Catholic priest Georges Lemaître) seems to be originally inspired by religious views.
[5] In order to compete with the Big Bang idea, Hermann Bondi, Thomas Gold, and Fred Hoyle proposed the “steady state theory.” Its projections likewise matched the existing structure of the Universe. The failure of this idea and widespread acceptance of the Big Bang Theory was due to the fact that radio wave sources near us are significantly less than those in the distant Universe, and there were far more radio sources than there are now. Evgeny Lifshitz and Isaak Markovich Khalatnikov both tried, but failed, to come up with an alternative to the Big Bang idea.
Roger Penrose utilized light cones and general relativity to show that a collapsing star may produce a Black Hole, a region of zero size with infinite density and curvature. Hawking and Penrose jointly demonstrated that the Universe should have arisen from a singularity, which Hawking himself refuted once quantum effects were considered.
The Uncertainty Principle (Chapter 4)
According to the uncertainty principle, a particle’s speed and position cannot be accurately determined. Scientists beam light at a particle to determine its location. If a high-frequency light is utilized, the light can more precisely locate the particle’s position, but the particle’s speed is less clear (because the light will change the speed of the particle). If a lower frequency is employed, the light can more precisely determine the particle’s speed, but the position of the particle will be less clear. The uncertainty principle refuted the idea of a deterministic theory, or one that could foresee everything that would happen in the future.
A light wave is depicted here.
The behavior of light as a wave–particle duality is also examined in this chapter. The properties of light (and all other particles) are both particle-like and wave-like.
Many colors arise as a result of light interference.
There are crests and troughs in light waves. The crest of a wave is its highest point, and the trough is its lowest point. More than one of these waves can sometimes interfere with one another. When light waves collide, they act as a single wave with qualities that are distinct from the separate light waves.
Nature’s Elementary Particles and Forces (Chapter 5)
This chapter is about quarks and other fundamental particles.
The majority of matter in the universe is made up of quarks, which are basic particles. Up, down, weird, charm, bottom, and top are the six various “flavors” of quarks. Red, green, and blue are the three “colors” of quarks. Antiquarks, on the other hand, vary from quarks in some ways.
To make a particle of spin 1 seem like this arrow again, it must be completely spun around.
Spin is a feature that all particles (such as quarks) have. The spin of a particle shows us how a particle appears from various angles. A spin 0 particle, for example, appears the same in all directions. Unless the particle is spun entirely around, a spin 1 particle appears different in every direction (360 degrees). An arrow is Hawking’s example of a spin 1 particle. To make a particle of spin two look the same, it must be rotated halfway (or 180 degrees).
A double-headed arrow is used as an illustration in the book. In the Universe, there are two types of particles: those with a spin of 1/2 (fermions) and those with a spin of 0, 1, or 2. (bosons). The Pauli exclusion principle applies only to fermions. The Pauli exclusion principle asserts that fermions cannot share the same quantum state (formulated by Austrian physicist Wolfgang Pauli in 1925). (for example, two “spin up” protons cannot occupy the same location in space). Complex structures could not exist if fermions did not follow this law.
A proton is made up of three quarks, each of which is colored differently due to color confinement.
The exclusion principle does not apply to bosons with spins of 0, 1, or 2. Virtual gravitons and virtual photons are two instances of these particles. The force of gravity is carried by virtual gravitons, which have a spin of two. This means that when gravity interacts with two objects, virtual gravitons are exchanged. The electromagnetic force that keeps atoms together is carried by virtual photons, which have a spin of one.
There are weak and strong nuclear forces in addition to gravity and electromagnetic forces. Radioactivity is caused by the weak nuclear force. The weak nuclear force mostly affects fermions. The strong nuclear force joins quarks to form hadrons, which are typically neutrons and protons, as well as neutrons and protons to form atomic nuclei. The gluon is the particle that conveys the strong nuclear force. Quarks and gluons are never discovered on their own (unless at extremely high temperatures) and are always ‘imprisoned’ within hadrons due to a phenomenon known as color confinement.
The electromagnetic force and the weak nuclear force combine to form a single electroweak force at extremely high temperatures. The electroweak and strong nuclear forces are projected to operate as a single force at much greater temperatures. Grand Unified Theories are theories that seek to describe the behavior of this “combined” force, and they may help us understand many of the physics puzzles that scientists have yet to answer.
Black Holes (Chapter 6)
A black hole, as seen through gravitational lensing, distorts its background image.
A black hole is an area of spacetime in which gravity is so powerful that nothing can escape. The vast majority of black holes are created when massive stars collide towards the end of their lives. To collide with a black hole, a star must be at least 25 times massive than the Sun. The event horizon is the barrier around a black hole beyond which no particle may escape to the rest of spacetime.
Spherical symmetry exists in black holes that do not rotate. Only axisymmetry exists in those who have rotating angular momentum.
Astronomers have a hard time finding black holes since they don’t emit any light. When a star is consumed, one can be found. When this happens, the falling stuff emits intense X-rays that telescopes can see.
In this chapter, Hawking discusses his famous bet with fellow physicist, Kip Thorne, in 1974. Hawking claimed there were no black holes, but Thorne claimed they did. New evidence confirmed that Cygnus X-1 was actually a black hole, hence Hawking lost the bet.
Hawking Radiation (Chapter 7)
This chapter delves into a new element of black hole behavior revealed by Stephen Hawking.
According to previous theories, black holes can only get larger and never shrink since nothing that enters a black hole can escape. However, in 1974, Hawking published a novel theory which stated that black holes can “leak” radiation. He imagined what would happen if two virtual particles appeared at a black hole’s edge. Virtual particles steal energy from spacetime for a brief period of time, then annihilate with each other, returning the borrowed energy and disappearing. One virtual particle may be captured by the black hole near the edge of a black hole, while the other escapes. Particles are ‘forbidden’ from obtaining energy from the vacuum due to the second rule of thermodynamics. As a result, instead of drawing energy from the vacuum, the particle absorbs it from the black hole and escapes as Hawking radiation.
Black Holes, according to Hawking’s theory, must slowly diminish over time as a result of this radiation, rather than continuing to exist indefinitely as scientists previously assumed. Though his idea was met with suspicion at first, it was quickly accepted as a scientific breakthrough, garnering Hawking widespread acclaim in the scientific world.