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Does Time Have A Beginning? Know More In: A Brief History Of Time. By Stephen Hawking.

Colouredboii
By Colouredboii | self meida writer
Published 7 months ago - 13 views

In A Brief History of Time, Stephen Hawking attempts to explain a range of subjects in cosmology, including the Big Bangblack 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


PICTURE OF THE UNIVERSE


Hawking discussed the history of astronomical studies, including the ideas of Aristotle and Ptolemy. Aristotle, unlike many other people of his time, thought that the Earth was round. He came to this conclusion by observing lunar eclipses, which he thought were caused by the Earth's round shadow, and also by observing an increase in altitude of the North Star from the perspective of observers situated further to the north. Aristotle also thought that the Sun and stars went around the Earth in perfect circles, because of "mystical reasons". Second-century Greek astronomer Ptolemy also pondered the positions of the Sun and stars in the Universe and made a planetary model that described Aristotle's thinking in more detail.

Today, it is known that the opposite is true: the Earth goes around the Sun. The Aristotelian and Ptolemaic ideas about the position of the stars and Sun were disproved in 1609. The first person to present a detailed argument that the Earth revolves around the Sun was the Polish priest Nicholas Copernicus, in 1514. Nearly after a century later, Galileo Galilei, an Italian scientist, and Johannes Kepler, a German scientist, studied how the moons of some planets moved in the sky, and used their observations to validate Copernicus's thinking.

To fit the observations, Kepler proposed an elliptical orbit model instead of a circular one. In his 1687 book on gravity, Principia MathematicaIsaac Newton used complex mathematics to further support Copernicus's idea. Newton's model also meant that stars, like the Sun, were not fixed but, rather, faraway moving objects. Nevertheless, Newton believed that the Universe was made up of an infinite number of stars which were more or less static. Many of his contemporaries, including German philosopher Heinrich Olbers, disagreed.

The origin of the Universe represented another great topic of study and debate over the centuries. Early philosophers like Aristotle thought that the Universe has existed forever, while theologians such as St. Augustine believed it was created at a specific time. St. Augustine also believed that time was a concept that was born with the creation of the Universe. More than 1000 years later, German philosopher Immanuel Kant thought that time goes back forever.

In 1929, astronomer Edwin Hubble discovered that galaxies are moving away from each other. Consequently, there was a time, between ten and twenty billion years ago, when they were all together in one singular extremely dense place. This discovery brought the concept of the beginning of the Universe within the province of science. Today, scientists use two partial theories, Albert Einstein's general theory of relativity and quantum mechanics, to describe the workings of the Universe. Scientists are still looking for a complete unified theory that would describe everything in the Universe. Hawking believes that the discovery of a complete unified theory may not aid the survival of our species, and may not even affect our life-style, but that humanity's deepest desire for knowledge is justification enough for our continuing quest, and that our goal is nothing less than a complete description of the Universe we live in.


SPACE AND TIME

Stephen Hawking talks about how the Aristotle theory of absolute space came to an end following the Newtonian theory that 'rest' and 'motion' can be the same state if an observer sees the event while at rest or if he moves at the same speed as the event. Therefore, 'rest' cannot be the standard position. Moreover, Galileo Galilei also disproves Aristotle theory that heavier bodies falls more quickly than the lighter ones just because of their mass. He experimentally proves it by sliding objects of different weights and even concludes that both these objects would fall at the same rate and would reach the bottom at the same time unless an external force acts on them.

Aristotle and Newton believed in absolute time. They believed that if an event is measured using two different clocks at a different state of motion, they'll have to agree on the same time if the clocks used are synchronized, which by now we know it isn't. But the fact that the light travels with a finite speed was first explained by the Danish scientist Ole Rømer, by his observation of Jupiter and one of its moons Io. He observed that Io appeared sometimes quicker and sometimes later when it revolves around Jupiter because the distance between Earth and Jupiter changes every time because of their orbital motion around the Sun.

The actual propagation of light was published by James Clerk Maxwell who concluded that light travels with a fixed speed. Later, many argued that light must travel through a hypothetical fluid called aether, which was disproved by the Michelson–Morley experiment. Einstein and Henri Poincaré later on argued that there's no need for aether provided one has to abandon absolute time. The Special Theory of Relativity is based on this, that light travels with a finite speed no matter what the speed of the observer is. Moreover, the speed of light is assumed to be the ultimate speed.

Mass and energy are also related by the famous equation {\displaystyle E=mc^{2}}E = mc^2, and so it would require infinite energy to get to the speed of light. A new way of defining a metre using speed of light is also developed. "Events" can also be described by using the light cones, a spacetime graphical representation that restricts what events are allowed and what are not based on the past and the future light cones. The new 4-dimensions is also described, how different the path is seen when one changes reference from 3D to 4D or from 3D to 2D.

The General Theory of Relativity explains how the path of a ray of light is affected by 'gravity', which according to Einstein is a mere illusion in contrast to Newton's views. It is spacetime curvature, where light moves in a straight path in 4D but is seen as a curve in 3D. 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 relatively different speeds to each other, or even at different places where spacetime curvature is different. 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 the expanding Universe. Hawking and Roger Penrose worked upon this and later proved using general relativity that if the Universe had a beginning then it also must have an end.


THE EXPANDING UNIVERSE


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 brightness of the stars. The luminosity, brightness and distance are related by a simple mathematical formula. Using all these, he fairly calculated distances of nine different galaxies. We live in a spiral galaxy just like other galaxies containing vast numbers of stars.

The stars are very far away from us, so we 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 know a star's composition. We use thermal spectra of the stars to know their temperature. In 1920, when scientists were examining spectra of different stars, they found that some of the characteristic lines of the star spectrum was shifted towards the red end of the spectrum. The implications of this phenomenon was given by the Doppler effect, and it was clear that some stars were moving away from us.

It was assumed that, since some stars are red shifted, some stars would also be blue shifted. When found, none of them were blue shifted. Hubble found that the amount of redshift is directly proportional to relative distance. So, it was clear that the Universe is expanding. Despite this, the concept of a static Universe persisted until the 20th century. Einstein was so sure of a static Universe that he developed the 'cosmological constant' and introduced 'anti-gravity' forces to persist with the earlier claim. Moreover, many astronomers also tried to avoid the face value implications of general relativity and stuck with their static Universe, with one notable exception, the Russian physicist Alexander Friedmann.

Friedmann made two very simple assumptions: the Universe is identical in every direction, i.e. Homogeneity, and that this would be true wherever we look from, i.e. Isotropy. His results showed that the Universe is non-static. His assumptions were later proved when two physicists at Bell LabsArno Penzias and Robert Wilson, found extra microwave radiation noise 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 Noble Prize in 1978. In addition, our place in the Universe is not exceptional, so we should see the Universe as the same from any other part of space, which proves 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 model 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 become equal, it would expand slowly and stop at infinite time, 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.

The first model depicts the space of the Universe to be curved inwards, a somewhat Earth-like structure. In the second model, the space would lead to a flat structure, and the third model results in negative curvature, or saddle shaped. Even if we calculate, the current expansion rate is more than the critical density of the Universe including the dark matter and all the stellar masses. The first model included the beginning of the Universe as a big-bang from a space of infinite density and zero volume known as 'singularity', a point where General Theory of Relativity (Friedmann's solutions are based in it) also breaks down.

This concept of the beginning of time was against many religious beliefs, so a new theory was introduced, "Steady state theory" by Hermann BondiThomas Gold, and Fred Hoyle, to tackle the Big Bang theory. Its predictions also matched with the current Universe structure. But the fact that radio wave sources near us are far fewer than from the distant Universe, and there were numerous more radio sources than at present, resulted in failure of this theory and everybody finally supported the Big Bang theory.

Evgeny Lifshitz and Isaak Markovich Khalatnikov also tried to avoid the Big Bang theory but also failed. Roger Penrose used light cones and general relativity to prove that a collapsing star could result in a region of zero size and infinite density and curvature called a Black Hole. Hawking and Penrose proved together that the Universe should have arisen from a singularity, which Hawking himself disproved once Quantum effects are taken into account.

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