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HISTORY     OCTOBER 17, 2023

The History of Time: From Ancient Mysteries to Modern        Physics.

by John Yedru

             Introduction to the Universe
Time, a fascinating concept that permeates every aspect of our lives, has been a subject of intrigue for countless generations. From ancient civilizations striving to comprehend the cyclical nature of the celestial bodies to modern physicists grappling with the enigmatic space-time continuum, the understanding of time has evolved dramatically over the course of human history. In this article, we embark on an extensive exploration to unravel the captivating and complex tapestry of temporal understanding.

              Ancient Ponderings

 Time in Ancient Cultures: Delving into the Mesopotamian, Egyptian, and Mayan perspectives on time, understanding their celestial observations, and their development of calendars.

 Greek Philosophers' Contributions: Investigating the philosophical concepts of time proposed by thinkers like Heraclitus, Parmenides, and Plato, and how they laid the foundations for future inquiry.

 Indian and Chinese Traditions: Analyzing the cyclic conception of time in Indian mythology and the profound Chinese understanding of the cosmos and its interweaving with time.

 Biblical Perspectives: Exploring the Hebrew and Christian interpretations of time as evidenced in religious texts and their impact on Western thought.

            Early Explorations.

Classical Antiquity: Unraveling Aristotle's ideas, including his notions of the eternal and motion, and how time was perceived during the Hellenistic period.

 Islamic Scholars: Investigating the contributions of prominent Islamic scholars like al-Biruni, alFarabi, and Avicenna, whose works on time and its nature laid the groundwork for later developments.

 Medieval Europe: Exploring how thirteenth-century philosopher Thomas Aquinas merged

Aristotelian and Christian perspectives, and the subsequent impact on the understanding of time. 2.4 Renaissance and Early Modern Thought: Analyzing the transformation of time through the works of Nicolaus Copernicus, Galileo Galilei, and Johannes Kepler.

               The Scientific Revolution

Newtonian Revolution: Examining Sir Isaac Newton's laws of motion and their implications on time as an absolute entity unaffected by external influences.

 Industrial Revolution and the Concept of Clock Time: Discussing the societal transition to clock time and the profound impact it had on economies and daily life.

 19th-century Transformations: Exploring advancements in fields such as thermodynamics, evolutionary biology, and the electromagnetic spectrum that shaped new conceptions of time.

 The Emergence of the Theory of Relativity: Analyzing Albert Einstein's groundbreaking theory and its profound implications on time, space, and the nature of the universe.

             Modern Approaches

Quantum Time: Examining the intersection of quantum mechanics and time, exploring the nature of causality, superposition, and temporal paradoxes.

Temporal Perception and Consciousness: Investigating how our subjective experience of time influences our understanding of reality and the ongoing debates in cognitive science.

Cosmological Time: Unraveling the concepts of cosmic time, the Big Bang, and how our universe has evolved through billions of years.

The Future of Time: Exploring the frontiers of time research, including interdisciplinary studies incorporating neurobiology, quantum gravity, and philosophical inquiries.

 After navigating this extensive journey through the history of time, one can appreciate the richness of human thought and our enduring quest to comprehend one of the most fundamental dimensions of existence. From ancient mysticism to modern physics, our understanding has evolved exponentially, continually pushing the boundaries of knowledge and raising profound questions that pique our curiosity. As time continues to shape our lives, our ever-evolving understanding will undoubtedly remain the subject of endless fascination and exploration.

              The Expanding Universe

The expansion of the universe since the Big Bang

In this chapter, S. 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.

The first model depicts the space of the Universe to be curved inwards. In the second model, the space would lead to a flat structure, and the third model results in negative 'saddle shaped' curvature. 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 the general theory of relativity (Friedmann's solutions are based in it) also breaks down.

This concept of the beginning of time (proposed by the Belgian Catholic priest Georges Lemaître) seemed originally to be motivated by religious beliefs, because of its support of the biblical claim of the universe having a beginning in time instead of being eternal.[3] So a new theory was introduced, the "steady state theory" by Hermann Bondi, Thomas Gold, and Fred Hoyle, to compete with 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 the failure of this theory and universal acceptance of the Big Bang Theory. Evgeny Lifshitz and Isaak Markovich Khalatnikov also tried to find an alternative to 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.

A representation of a light wave

Hawking then discusses how Heisenberg's uncertainty principle implies the wave–particle duality behavior of light (and particles in general).

Light interference causes many colors to appear.

He then describes the phenomenon of interference where multiple light waves interfere with each other to give rise to a single light wave with properties different from those of the component waves, as well as the interference within particles, exemplified by the two-slit experiment. Hawking writes how interference refined our understanding of the structure of atoms, the building blocks of matter. While Danish scientist Niels Bohr's theory only partially solved the problem of collapsing electrons, quantum mechanics completely resolved it. According to Hawking, American scientist Richard Feynman's sum over histories is a nice way of visualizing the wave-particle duality. Finally, Hawking mentions that Einstein's general theory of relativity is a classical, non-quantum theory which ignores the uncertainty principle and that it has to be reconciled with quantum theory in situations where gravity is very strong, such as black holes and the Big Bang.

           Elementary Particles and Forces of Nature

In this chapter, Hawking traces the history of investigation about the nature of matter: Aristotle's four elements, Democritus's notion of indivisible atoms, John Dalton's ideas about atoms combining to form molecules, J. J. Thomson's discovery of electrons inside atoms, Ernest Rutherford's discovery of atomic nucleus and protons, James Chadwick's discovery of neutrons and finally Murray Gell-Mann's work on even smaller quarks which make up protons and neutrons. Hawking then discusses the six different "flavors" (up, down, strange, charm, bottom, and top) and three different "colors" of quarks (red, green, and blue). Later in the chapter he discusses anti-quarks, which are outnumbered by quarks due to the expansion and cooling of the Universe.

A particle of spin 1 needs to be turned around all the way to look the same again, like this arrow.

Hawking then discusses the spin property of particles, which determines what a particle looks like from different directions. Hawking then discusses two groups of particles in the Universe based on their spin: fermions and bosons. Fermions, with a spin of 1/2, follow the Pauli exclusion principle, which states that they cannot share the same quantum state (for example, two "spin up" protons cannot occupy the same location in space). Without this rule, complex structures could not exist.

A proton consists of three quarks, which are different colours due to colour confinement.

Bosons or the force-carrying particles, with a spin of 0, 1, or 2, do not follow the exclusion principle. Hawking then gives the examples of virtual gravitons and virtual photons. Virtual gravitons, with a spin of 2, carry the force of gravity. Virtual photons, with a spin of 1, carry the electromagnetic force. Hawking then discusses the weak nuclear force (responsible for radioactivity and affecting mainly fermions) and the strong nuclear force carried by the particle gluon, which binds quarks together into hadrons, usually neutrons and protons, and also binds neutrons and protons together into atomic nuclei. Hawking then writes about the phenomenon called color confinement which prevents the discovery of quarks and gluons on their own (except at extremely high temperature) as they remain confined within hadrons.

Hawking writes that at extremely high temperature, the electromagnetic force and weak nuclear force behave as a single electroweak force, giving rise to the speculation that at even higher temperatures, the electroweak force and strong nuclear force would also behave as a single force. Theories which attempt to describe the behaviour of this "combined" force are called Grand Unified Theories, which may help us explain many of the mysteries of physics that scientists have yet to solve.

             The Arrow of Time.

In this chapter Hawking talks about why "real time", as Hawking calls time as humans observe and experience it (in contrast to "imaginary time", which Hawking claims is inherent to the laws of science) seems to have a certain direction, notably from the past towards the future. Hawking then discusses three "arrows of time" which, in his view, give time this property. Hawking's first arrow of time is the thermodynamic arrow of time: the direction in which entropy (which Hawking calls disorder) increases. According to Hawking, this is why we never see the broken pieces of a cup gather themselves together to form a whole cup. Hawking's second arrow is the psychological arrow of time, whereby our subjective sense of time seems to flow in one direction, which is why we remember the past and not the future. Hawking claims that our brain measures time in a way where disorder increases in the direction of time – we never observe it working in the opposite direction. In other words, he claims that the psychological arrow of time is intertwined with the thermodynamic arrow of time. Hawking's third and final arrow of time is the cosmological arrow of time: the direction of time in which the Universe is expanding rather than contracting. According to Hawking, during a contraction phase of the universe, the thermodynamic and cosmological arrows of time would not agree.

Hawking then claims that the "no boundary proposal" for the universe implies that the universe will expand for some time before contracting back again. He goes on to argue that the no boundary proposal is what drives entropy and that it predicts the existence of a well-defined thermodynamic arrow of time if and only if the universe is expanding, as it implies that the universe must have started in a smooth and ordered state that must grow toward disorder as time advances. He argues that, because of the no boundary proposal, a contracting universe would not have a well-defined thermodynamic arrow and therefore only a Universe which is in an expansion phase can support intelligent life. Using the weak anthropic principle, Hawking goes on to argue that the thermodynamic arrow must agree with the cosmological arrow in order for either to be observed by intelligent life. This, in Hawking's view, is why humans experience these three arrows of time going in the same direction.

          Our Picture of the Universe

Ptolemy's Earth-centric model about the location of the planets, stars, and SunIn 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.

          Black Holes

A black hole, showing how it distorts its background image through gravitational lensing

In this chapter, Hawking discusses black holes, regions of space-time where extremely strong gravity prevents everything, including light, from escaping from within them. Hawking describes how most black holes are formed during the collapse of massive stars (at least 25 times heavier than the Sun) approaching end of life. He writes about the event horizon, the black hole's boundary from which no particle can escape to the rest of space-time. Hawking then discusses non-rotating black holes with spherical symmetry and rotating ones with axis-symmetry. Hawking then describes how astronomers discover a black hole not directly, but indirectly, by observing with special telescopes the powerful X-rays emitted when it consumes a star. Hawking ends the chapter by mentioning his famous bet made in 1974 with American physicist Kip Thorne in which Hawking argued that black holes did not exist. Hawking lost the bet as new evidence proved that Cygnus X-1 was indeed a black hole.