Anecdote about a woman who challenges a famous astronomer, suggesting that the world is a flat disc carried on the back of a giant turtle, with the universe being "turtles all the way down." Hawking reflects on why people now find this idea silly and explains how ancient Greek philosopher Aristotle provided evidence that the Earth is round. Aristotle observed that the Earth's shadow on the full moon during an eclipse is round, and as one moves south, the North Star appears lower in the sky. Greek mariners also noticed that the sails of approaching ships became visible before their hulls, supporting the round Earth concept.
Aristotle believed that the Earth was at the center of the universe, with the sun and stars revolving around it. The text includes Figure 1.1, illustrating Ptolemy's model of the universe, which featured the Earth at the center and transparent, concentric spheresrepresenting each planet, including the sun, with the stars located in the outermost sphere. Although Ptolemy's model was flawed, it gained widespread acceptance due to its compatibility with Christian scripture, as it allowed for the existence of heaven and hell beyond the observable universe.
The next significant development discussed is the proposal made by Polish priest and mathematician Nicholas Copernicus in 1514. Copernicus suggested a simpler model where the Earth and other planets revolve around the sun. However, this model did not gain serious consideration until much later. It wasn't until the observations of Italian astronomer Galileo Galilei, who discovered that Jupiter had its own orbiting moons using a newly invented telescope in 1609, that the Copernican theory gained support. German astronomer Johannes Kepler further corrected Copernicus's theory by demonstrating that planetary orbits are elliptical rather than perfectly circular, as predicted by
Copernicus. Kepler's findings disappointed him since his theory involved magnetic forces that required circular orbits.
In 1687, English polymath Sir Isaac Newton published his work "Philosophiae Naturalis Principia Mathematica," which Hawking describes as probably the most important single work ever published in the physical sciences. Newton's theory of gravity explained elliptical orbits by stating that larger bodies exert more gravity and that closer bodies have a stronger gravitational pull than those farther away. Newton's theoryalso rendered Ptolemy's celestial sphere model obsolete, introducing the concept of an infinite and moving universe. According to Newton, if the stars were stationary, they would be gravitationally pulled toward a central point, which is impossible in an infinite universe. Hawking points out that although Newton's notion of an infinite universe was a common belief at the time, scientists now understand that an infinite and static universe is impossible. However, instead of completely replacing Newton's theories, scientists made efforts to adjust them to fit the new understanding of the universe.
The text then introduces Heinrich Olbers, who, in 1823, published arguments for a universe in motion. Olbers raised an intriguing question: If the universe were both infinite and static, then in any direction we look, we should see an infinite collection of stars, resulting in a night sky as bright as the day. Olbers proposed that matter between the stars absorbs this light energy, preventing the sky from being overly bright. However, if this were the case, the matter absorbing the light would also heat up and radiate light. Therefore, to explain the dark night sky, Olberssuggested that the stars are not all "turned on" simultaneously, so their light does not reach the Earth at the same time. The challenge remained in understanding why or how the stars are "turned on."
Before the 20th century, there were two widely accepted beliefs about the origins of the universe: either it had always existed and would continue infinitely, or there was a specific event of creation that brought the universe into its current state. Philosophers tended to lean towards the idea of an eternal universe, while religious figures believed in a creation event.
Edwin Hubble's groundbreaking discovery in 1929 played a crucial role in understanding the nature of the universe. Hubble observed that galaxies located far from Earth's solar system were receding, indicating that the universe is expanding. This led scientists to infer that everything
in the universe was closer together in the past. Based on these observations, scientists estimate that around 10 to 20 billion years ago, all matter in the universe existed in one infinitely dense and infinitesimally small mass. This primordial mass then underwent a rapid expansion known as the Big Bang, setting the universe's observable motions into motion. According to Hawking, the laws of physics came into existence at the moment of the Big Bang, rendering any events preceding it observationally irrelevant since time and space originated from that point. He humorously remarks that an expanding universe does not eliminate the possibility of a creator but places limitations on when such a creator could have performed the job.
The text continues by discussing how scientific theories are developed and refined. Often, a new theory builds upon or revises a flawed yet promising older theory. For example, Newton's theory of gravity accurately predicts planetary motions but has slight errors when applied to the motion of Mercury. Albert Einstein's theory of relativity, on the other hand, accounts for Mercury's motion and provides a more accurate description. Nevertheless, in most cases involving larger-scale phenomena, Newton's simpler theory is sufficient and continues to be used.
Hawking then explains the ultimate goal of physical science, which is to discover a single theory that can explain and describe the entire universe. This goal is typically divided into two objectives. The first objective is to explain the present state of the universe and make predictions about its future. The second objective is to explain the initial state of the universe. To achieve this, scientists oftenfocus on studying smaller parts of the whole, aiming to develop accurate partial theories that can eventually be integrated into a larger theory.
The text highlights two partial theories that have made significant contributions to understanding the universe: the general theory of relativity and quantum mechanics. Hawking refers to these as the "great intellectualachievements of the first half of the 20th century." However, these two theories are not consistent with each other, presenting a challenge for scientists. The general theory of relativity describes gravity and large-scale aspects of the universe, while quantum mechanics explains the behavior of subatomic particles and the small-scale aspects of the universe, such as atoms. The primary goal of physics is to find a unified theory that can combine these two theories into a single
framework known as the quantum theory of gravity.
Hawking acknowledges that the pursuit of a unified theory may seem unnecessary in practical terms. After all, partial theories like the ones mentioned have already led to advancements such as nuclear power and microelectronics. However, he argues that humanity's thirst for knowledge is justification enough to continue this quest for a unified theory.
In summary, the text provides a detailed retelling of Stephen Hawking's forward to the 1996 edition of "A Brief History of Time." It mentions the unexpected success of the book, updates made in the new edition, and the inclusion of a chapter on wormholes and the concept of "dualities" in physics. The text then covers the content of Chapter 1, discussing historical perspectives on the shape of the Earth, advancements in astronomy by figures like Aristotle, Copernicus, Galileo, Kepler, and Newton. It further delves into concepts such as an infinite and moving universe, Olbers' paradox, the Big Bang theory, and the quest for a unified theory of physics. Hawking emphasizes the importance of understanding the universe as a whole and humanity's intrinsic desire for knowledge.
Before Galileo and Newton, most scientists understood the laws of motion through Aristotle's lens. Aristotle believed that objects at rest tend to stay at rest unless acted upon by a force, and he also proposed that heavier objects fall to the ground faster than lighter ones. However, Galileo's experiments contradicted Aristotle's ideas by demonstrating that bodies in motion remain in motion unless acted upon and that objects of different weights fallat the same rate, except for the effect of air resistance. Galileo's observations led to a shift in the understanding of physics, emphasizing the importance of experiments alongside reasoning.
Newton expanded on Galileo's work and introduced his laws of motion. The first law states that an object at rest will remain at rest, and an object in motion will continue in motion in a straight line unless acted upon by an external force. The second law explains how the acceleration of an object depends on the applied force and the mass of the object. Newton also formulated the laws that describe gravity. According to his
theory, every object attracts other objects with a force proportional to their masses. The force of gravity also weakens with distance, explaining the shape of planetary orbits and why objects of different weights fall at the same rate.
Newton's laws challenged the notion of absolute rest proposed by Aristotle. Newton's laws demonstrated that there is no absolute rest, and whether an object is considered at rest or in motion depends on the frame of reference. This idea troubled Newton due to his strong belief in an absolute God, and he was reluctant to accept what his laws indicated.
Newton believed that time between two events could be objectively measured and that all observers, regardless of their perspective, would record the same amount of time passing. However, in 1676, Danish astronomer Ole Christensen Roemer's observations of Jupiter's moons showed that light takes time to travel, and its speed was estimated at 140,000 miles per second. This discovery laid the foundation for understanding the relationship between light, time, and motion.
James Clerk Maxwell, nearly 200 years later, unified the existing theories of electricity and magnetism, leading to the establishment of the electromagnetic spectrum. This spectrum includes visible light and is divided based on wave frequency. The speed of light was thought to require a reference frame called the ether, but Albert Michelson and Edward Morley's experiments in 1887 proved that the speed of light remains constant regardless of the observer's motion. Albert Einstein later demonstrated that absolute time does not exist and that time warps at high speeds, leading to his theory of relativity. This theory unified Newton's laws with Maxwell's theory and explained the constancy of the speed of light for all observers.
Einstein's theory of relativity had profound consequences for physics. It showed that mass and energy are equivalent, asexpressed by the famous equation E=mc^2. The theory also revealed that as an object approaches the speed of light, its mass increases, making acceleration more difficult. Additionally, the theory abolished the concept of absolute time and established that space and time are interconnected, forming a four-dimensional entity known as space-time. Space-time diagrams were introduced to represent this concept visually.
In 1915, Einstein developed the theory of general relativity, which explained gravity as a consequence of space-time's multi-dimensional nature. According to this theory, large bodies such as suns and planets
cause space to curve around them, creating gravitational effects. Einstein's inclusion of gravity in his theory of relativity led to the term "general relativity," which encompasses both special relativity and gravity.
It discusses the shift from Aristotle's understanding of motion to Galileo andNewton's laws of motion. The chapter covers the concepts of inertia, the effect of forces on objects, and the law of universal gravitation. It also delves into the implications of Einstein's theory of relativity, including the interconnectedness of space and time, the constancy of the speed of light, and the warping of space by massive objects. The chapter emphasizes the shift from absolute notions of rest and time to a more relativistic understanding of the universe.
Highlighting the presence of numerous points of light in the night sky, including stars and neighboring planets. It introduces Edwin Hubble's discovery in 1924 that the Milky Way is just one of many galaxies, leading to the acceptance that Earth exists in a spiral galaxy.
Hubble aimed to determine the distances of these galaxies from Earth. Since distant galaxies do not appear to move, Hubble used indirect measuring methods to calculate their distances. He observed that stars of the same type in nearby galaxies had the same luminosity, allowing him to deduce their distances based on their apparent brightness.
The chapter explores the concept of the Doppler effect, which causes the spectra emitted by stars in faraway galaxies to appear redder relative to their distance from Earth. Hubble's meticulous cataloging of galaxies revealed that the majority of galaxies experienced a redshift, indicating that they are moving away from Earth. He also discovered that the farther a galaxy is, the faster it moves away fromEarth, leading to the realization of an expanding universe.
The text discusses the contrasting beliefs regarding the nature of the universe prior to Hubble's work. Newton's laws implied that a static universe would contract due to the force of gravity, and even Einstein adjusted his calculations to accommodate the idea of a static universe. However, Russian physicist Alexander Friedmann theorized that the universe should appear the same from any point of observation, which
predicted Hubble's discovery.
The chapter mentions the confirmation of Friedmann's assumptions through the detection of consistent microwave noise, which was found to originate beyond the Milky Way. This discovery supported the hypothesis that the universe had a beginning and was extremely hot and dense. The existence of this cosmic microwave background radiation provided further evidence against alternative theories like the steady-state theory.
The text also covers the estimation of the rate of expansion of the universe and the density of the universe, including the presence of dark matter. Despite all the matter in the known universe, there is insufficient mass to halt the expansion, indicating that the universe will likely continue expanding indefinitely.
Friedmann's predictions suggest that the universe was once contracted into a singularity, where the density and curvature of space-time were infinite. As current methods cannot predict what happened before the Big Bang, it is assumed that time began at that moment. This idea raises questions about the origin of the universe and the role of divine intervention, with the Catholic Church aligning the Big Bang model with Scripture.
Explores alternative theories such as the steady-state theory, which proposes continuous galaxy formation, and a contraction theory without a singularity. However, these theories were ultimately disproven or considered less likely than the Big Bang theory.
Physicist Roger Penrose's work on black holes and singularities, combined with Stephen Hawking's research, provided evidence that the Big Bang singularity must have occurred according to the theory of general relativity and current estimates of the universe's matter content.
Introducing the deterministic viewof the universe proposed by the Marquis de Laplace in the early 1800s. This view suggests that if the state of all components of the universe were known at a single moment, scientific laws could predict every event in the universe. Lord Rayleigh and Sir James Jeans made calculations based on this assumption, indicating that a body emitting heat and light would do so infinitely.
In 1900, Max Planck introduced the concept of quanta, discrete units in
which light is released. Each quantum contains a specific amount of energy, and as the frequency of the waves increases, the energy of the quantum also increases. This led to the realization that there would be a frequency beyond which the available energy would not be sufficient to release a quantum, resulting in a finite loss of energy.
In 1926, Werner Heisenberg formulated the uncertainty principle, using Planck's ideas to challenge determinism. Heisenberg proposed that to predict the position of a particle in the future, one must accurately know its current position and velocity. However, the act of measuring a particle's position and velocity using light waves disturbs the particle's behavior in unpredictable ways. The uncertainty principle states that it is impossible to know both the position and velocity of a sub-atomic particle simultaneously, refuting determinism and introducing inherent uncertainty into scientific observations.
Heisenberg, along with Erwin Schr?dinger and Paul Dirac, developed quantum mechanics in the 1920s. Quantum mechanics suggests that particles exist in a quantum state, which is a combination of position and velocity. Rather than making single, specific observations, scientists make observations that result in various potential outcomes, each with a predicted likelihood. This introduces randomness and uncertainty to observations at the sub-atomic level.
Despite being one of the early contributors to quantum mechanics, Albert Einstein disliked the idea of randomness in the fundamental materials of the universe. He famously stated, "God does not play dice." However, most scientists now accept and utilize quantum science, and modern technology, biology, and chemistry rely on its principles. Nevertheless, incorporating quantum theory into the study of gravity and the large-scale structure of the universe has not been successfully achieved.
Explains the duality of particles and waves, wherein particles exhibit wave-like properties and waves can be treated as particles. This duality leads to interference phenomena when waves interact with each other. Figure 4.1 illustrates the interference pattern observed when photons(light particles) pass through two slits in a partitionand create bands of light and dark on a screen. The uncertainty principle allows even a single light particle to travel through both slits and interfere with itself, forming an interference pattern. This phenomenon applies to all sub-atomic particles and has significant implications for understanding
atoms.
Niels Bohr proposed partial theories in the 1920s to explain the behavior of electrons orbiting the nucleus of atoms at certain distances. Quantum mechanics provided a full explanation by suggesting that particles also exhibit wave behavior. Electrons do not appear where interference causes wavelengths to cancel out, as demonstrated in the two-slit experiment.
Einstein's general theory of relativity, while successful in explaining the universe on large scales, is considered a classical theory that does not account for the uncertainty principle inherent in quantum mechanics. General relativity adequately explains gravity in weak force situations but is predicted to become problematic in singularities such as black holes, where gravity becomes much stronger. Reconciling the infinite density of black holes with the predictions of general relativity requires the incorporation of quantum mechanics. Although no unified theory has been developed to reconcile general relativity and quantum theory, scientists understand the necessary features such a theorywould have. The chapter concludes by highlighting that a successful unified theory would play a crucial role in understanding black holes and the Big Bang, which will be explored in later chapters.
Discussing the historical debate between Aristotle's view of continuous matter and Democritus' theory of atoms. It wasn't until John Dalton observed atoms clumping together in molecules that physical evidence supported the atomist school of thought. Einstein's discovery of Brownian motion in 1905 further confirmed the existence of atoms.
The internal structure of atoms was revealed by J. J. Thomson, who discovered electrons in the late 19th century, and Ernest Rutherford, who demonstrated that atoms have a nucleus and orbiting electrons in 1911. Protons and neutrons were later discovered as components of the nucleus, and in the mid-1960s, physicist Murray Gell-Mann named the fundamental particles within protons and neutrons as quarks. Quarks come in six flavors and have three different colors.
The size of the basic building blocks of the universe is uncertain, as
quantum mechanics dictates that particles must be measured with a wavelength smaller than themselves. Scientists initially used electricfields to charge electrons with enough energy for measurement, but technological advancements have allowed for higher levels of energy to be achieved. Although smaller particles are not impossible, current theoretical reasons suggest that the smallest unit of particles has already been discovered.
All particles in the universe can be understood as either matter or forces. Particles have characteristic spins, with spin 1/2 particles constituting matter and spin 0, 1, and 2 particles serving as the sources of forces that act on matter. Wolfgang Pauli discovered the exclusion principle, which prevents two matter particles from existing in the same state and explains why matter particles do not collapse under the influence of force particles.
Paul Dirac's theory of quantum mechanics merged with general relativity and predicted the existence of antimatter, including the positron. Matter and antimatter particles annihilate each other upon contact. When matter particles touch, they emit force-carrying particles, causing changes in velocity and resulting in the effect of a force between the particles. Researchers aim to develop a unified theory that combines all forces into a single force behaving in different ways. Three of the forces - the weak nuclear force, strong nuclear force, and electromagnetic force - have been unified, while the gravitational force is discussed in a later chapter.
Gravity, despite being the weakest force, acts over large distances and has a significant impact on massive collections of particles such as stars. Electromagnetic force interacts with particles carrying an electrical charge, while the weak nuclear force is responsible for radioactivity and interacts with particles of spin 1/2. The strong nuclear force, conducted by the gluon, binds quarks together to form protons and neutrons, and ultimately, the nucleus of an atom. There is ongoing progress towards a Grand Unified Theory that encompasses all basic particles and forces.
At the beginning of the universe, there may have been an equal number of particles and antiparticles, but a slight asymmetry in the laws of physics allowed for a small excess of quarks and electrons over anti-quarks and anti-electrons. Most of the particle-antiparticle pairs annihilated each other, leaving behind the matter observed in the present universe.
Mentioning John Michell's theory in 1783, which proposed the existence of stars so massive that their gravity would prevent light from escaping. This idea was initially overshadowed by thebelief that light is composed of waves, but it regained significance in the 1900s with the understanding that particles can also exhibit wave-like behavior. These incredibly dense and light-trapping stars came to be known as black holes.
The formation of stars involves the coalescence of gas clouds. As the gas ball grows denser and larger, the atoms within it collide more frequently, generating heat and light. This controlled fusion process prevents the star from collapsing further under its own gravity. Large stars burn hot and quickly consume their fuel within millions of years, while smaller stars, such as the Sun, have longer lifespans lasting billions of years.
When a star exhausts its fuel, it undergoes a collapse and transforms into a compact and extremely dense object. Smaller stars become white dwarfs, about 1,000 miles in diameter, where the collapse is halted by the repulsion of electrons within closely packed atoms. Stars larger than about 1.5 times the size of the Sun reach a critical point known as the Chandrasekhar limit. Beyond this limit, the star continues to collapse until protons and neutrons are compressed together, forming a neutron star with a diameter of about 10 miles and an incredibly high density.
In 1939, Robert Oppenheimer demonstrated that stars even more massive than the Chandrasekhar limit would collapse to such an extent that nothing, not even light, could escape their gravitational pull. The boundary beyond which light cannot escape is called the event horizon, and the region within is known as a black hole. Time near a massive gravity field slows down, so an astronaut traveling towards a black hole would appear to slow down from the perspective of an observer outside the black hole. Hawking and Roger Penrose showed in the late 1960s that these collapsing stars would ultimately result in an infinitely dense point where the laws of science break down. This event is likened to the reverse of the Big Bang. However, objects outside the event horizon are protected from the black hole's internal physics. Despite variations in individual stars, the laws of relativity dictate that all black holes end up being identical, except for their size and rotation rate.
The first evidence of black holes was discovered in 1963 by Maarten Schmidt, who identified distant objects emitting tremendous amounts of energy. These objects, known as quasars or quasi-stellar objects, are black holes containing the collapsed mass of entire galactic centers. Furtherevidence for black holes comes from observing stars that orbit around invisible objects, which pull gas from the visible star and accelerate it to emit X-rays. The orbital characteristics of the visible star indicate that the invisible companion is too large to be a neutron star. Hence, black holes reveal themselves through their effects on surrounding objects.
It is speculated that there may be more black holes than stars, which could account for the additional mass required to explain the observed rotation of galaxies. The centers of galaxies are believed to harbor extremely large black holes, with even larger ones powering the bright cores of quasars. Small black holes might have formed in the early universe under high pressure conditions, while the explosion of a super-large hydrogen bomb could also potentially create a black hole.
Discussing the merging of two black holes and how the area of their event horizon increases as they combine. The size of this new event horizon represents the total entropy or disorder of the black hole. A visual analogy is provided with two boxes, one filled with oxygen and the other with nitrogen, which, when joined and the separating wall removed, mix together and become less organized. The merging of black holes is illustrated with a diagram resembling a pair of pants, where the smaller tubes representing the paths of the black holes combine to form a larger tube, symbolizing the merger.
The text then introduces the concept of black hole evaporation, which occurs due to the creation of virtual particles near the event horizon. These particles manifest as pairs, with one particle having positive energy and the other having negative energy. While both particles may fall into the black hole, occasionally the positive-energy particle escapes, while the weak negative-energy particle is absorbed. The negative energy of the absorbed particle results in a reduction in the total mass-energy of the black hole. From an external perspective, it appears that the positive-energy particle escapes from the black hole. This slow
process of particle emission causes black holes to gradually lose mass and energy, eventually evaporating. Smaller black holes evaporate faster and get hotter until their remaining mass is released in a final burst of energy. The time it takes for black holes to completely evaporate is incrediblylong, with larger black holes taking billions of years to disappear.
It is suggested that small black holes may have formed during the Big Bang. These primordial black holes, with masses of around a billion tons each, are warmer and evaporate at a faster rate. They can potentially evaporate within 10 to 20 billion years, and some may have already vanished. The production of gamma rays by black holes is mentioned, but the total cosmic background gamma radiation indicates that the combined contribution of all primordial black holes is minimal, accounting for only a small fraction of the universe's total matter.
The chapter concludes by noting the detection of gamma-ray bursts by nuclear test-ban satellites, which are primarily associated with events occurring outside the solar system. While some of these bursts may be attributed to evaporating primordial black holes, they are often linked to other extraordinary phenomena such as collisions between neutron stars.
Describing the chaotic nature of the universe in its initial stages, where particles collided and annihilated each other. Over time, as the universe cooled down, protons and neutrons started to come together and form the lightest atoms, including helium. As the universe expanded, areas of slightly greater density began to clump together under the influence of gravity, leading to the formation of galaxies. Within these galaxies, gas clouds continued to contract and form stars, which underwent nuclear fusion and radiated energy, preventing further collapse. Larger stars burned hotter and created heavier elements. When stars exhausted their nuclear fuel, they contracted further, with the most massive stars exploding and releasing mass into space, contributing to the formation of new stars. The remnants of these giant stars also formed the heavier elements that eventually coalesced into planets like Earth. Life began on
Earth with the emergence of macromolecules capable of self-replication, and over time, life forms that were better at surviving and reproducing became dominant. The presence of life led to changes in the atmosphere, making it suitable for more complex species.
Highlights the remarkable fine-tuning of the universe. Even small deviations in the rate of expansion after the Big Bang would have resulted in a collapse or a drastically different universe. The specific conditions required to support life are not predicted by the Standard Modelof physics and remain a mystery. The anthropic principle is proposed as an explanation, suggesting that the universe must contain the necessary laws and conditions for intelligent life to exist. One version of the anthropic principle, the strong anthropic principle, argues that the universe itself is dependent on the existence of intelligent life. Another version, the weak anthropic principle, posits the existence of multiple universes with different laws of physics, and humans exist in the universes compatible with their existence.
The concept of inflation is introduced, which refers to a rapid expansion of the universe that occurred within a fraction of a second after the Big Bang. During this phase, the irregularities or "wrinkles" in the density of the early universe were smoothed out. The explanation for this smoothing process is still a topic of speculation, with possibilities such as phase transitions or a supercooled state being considered. The idea of the universe having regions of varying energy, with one region becoming the observable universe, aligns well with the observed cosmic background radiation and allows for a range of initial conditions that can lead to the existence of our universe.
Hawking's application of quantum mechanics to gravity, known as quantum gravity, yields two significant ideas. Firstly, it suggests that there may not have been a singularity at the beginning of the universe. Secondly, it proposes the concept of the universe having "no boundary," analogous to a ship sailing across the seas without encountering an edge. According to this idea, the universe can expand and contract endlessly without requiring an act of creation at the Big Bang. The chapter includes a diagram, Figure 8.1, illustrating the expansion and contraction of the universe using the analogy of a globe, with the north pole representing the big bang and the south pole representing the "big crunch." The mathematical calculations involved in understanding these concepts often employ imaginary numbers, which facilitate calculations
in quantum gravity.
Highlights how quantum mechanics explains the slight variations in the cosmic microwave background radiation and the presence of denser areas and galaxies. These variations are consistent with the uncertainties in the initial conditions of the universe and support Hawking's proposition of the no boundary condition.
Highlighting the intriguing nature of time, as most physical lawsoperate symmetrically in both forward and backward directions. However, certain observations, such as a broken teacup never spontaneously repairing itself, indicate that time has a preferred direction. Hawking introduces three arrows of time to further understand this phenomenon.
The first arrow discussed is the thermodynamic arrow of time, which is associated with entropy. Entropy is a measure of disorder, and the second law of thermodynamics states that the entropy of a closed system tends to increase over time. Hawking explains that there are significantly more states of disorder than states of order, leading to a natural progression towards greater disorder. This concept aligns with our everyday experience of objects becoming more disordered with time.
The second arrow is the psychological arrow of time, which pertains to our subjective perception of events unfolding in a forward direction. Humans remember the past but not the future, creating a distinct flow of time in our consciousness. While the psychological arrow is related to the thermodynamic arrow, it is a separate concept based on human experience.
The third arrow is the cosmological arrow of time, which is tied to the expansion and contraction of the universe. As the universe expands, the arrow points forward, but during contraction, it reverses. The contraction phase is purely speculative and suggests a reversal of the thermodynamic and psychological arrows, wherein people would remember the future and things would become more orderly.
Hawking's no-boundary proposal comes into play, suggesting that after the early moments of cosmic inflation, the universe transitions from
a state of mild disorder to one of increasing entropy. The universe then continues to expand for an extensive duration until all matter decays into energy, resulting in a state of near-perfect disorder. Subsequently, the universe starts collapsing, but during this contraction phase, no form of life can exist since life relies on energy processing, which necessitates a degree of orderliness.
Ultimately, Hawking argues that the thermodynamic, psychological, and cosmological arrows of time always align in the same direction. Humans can only exist in a universe that is expanding and undergoing an increase in entropy. This connection between the arrows of time and the conditions for human existence suggests a fundamental interplay between the nature of time and the existence of life.
Hawking discusses the limitations of conventional ships powered byengines and explains that as a ship approaches the speed of light, its mass increases infinitely, making faster-than-light travel impossible through conventional means.
However, Hawking proposes that a potential solution lies in the creation of wormholes, which are hypothetical tunnels that connect distant regions of space. The key to creating a wormhole lies in harnessing "negative" energy, which exists in specific circumstances. One example of negative energy is found in the space between closely held metal plates. Due to the limitation of possible wavelengths of virtual particles between the plates, fewer virtual particles can exist there compared to outside the plates. This imbalance creates more pressure from virtual particles outside the plates, resulting in negative energy between the plates. The existence of negative energy suggests that it might be possible to utilize it to create a wormhole.
The existence of wormholes would have profound implications for time travel. It would allow for the potential to travel to distant locations, such as Alpha Centauri, which is four light-years away, and return home before one even embarked on the journey. Moreover, it would open up the possibility of going back in time and altering events, creating paradoxes where one never went through the wormhole in the first place. To resolve these contradictions, Hawking suggests that time travel would need to occur before the individual actually enters the wormhole.
This self-consistency principle ensures that the universe is protected from the potential chaos caused by time travel.
On a microscopic scale, time travel is already observed to occur. Hawking explains that a particle moving forward in time is equivalent to an anti-particle moving backward in time. In the vicinity of black holes, virtual particles form at their edges. When one of these particles, the anti-particle, falls into the black hole, the other particle can be perceived as traveling backward in time as it escapes the black hole.
While the concept of wormholes and time travel presents intriguing possibilities, Hawking acknowledges that these ideas are largely theoretical and require further scientific exploration and understanding.Highlights the captivating nature of these concepts and the potential they hold for our understanding of the universe and the nature of time.
Hawking points out thatthese theories, although successful in their respective domains, are considered "partial theories" as they do not fully fit together and contain arbitrary values that cannot be predicted by the theories themselves.
Gravity, in particular, is seen as a classical theory that does not have a quantum-based foundation like the other three forces. To achieve a unified theory, Hawking highlights the importance of combining general relativity with the uncertainty principle, which is a fundamental principle of quantum mechanics. However, the task of incorporating gravity into a quantum framework is challenging. The presence of virtual particles, with their associated mass, poses a significant issue as the calculations would suggest that the universe would curl up into an infinitesimally small point. Various techniques, such as "renormalization," have been employed to circumvent these problems, but they often rely on estimates and arbitrary values. Hawking emphasizes the need for a more accurate and comprehensive theory, known as a Grand Unified Theory of Everything.
Hawking illustrates the concept of strings and their movement through time using diagrams. Open strings, depicted in Figure 11.1, resemble ordinary pieces of string, and their upward movement through time is
represented by a strip of paper known as the "world-sheet." Closed strings, shown in Figure 11.2, take the form of loops, and their upward travel through time generates a world-sheet that resembles a cylinder.
According to string theory, strings can join together or separate from each other. The merging of strings is demonstrated in Figures 11.3 and 11.4, where two open strings or two closed-loop strings combine to form a single, larger string. Conversely, Figure 11.5 depicts the separation of strings, where one string leaves another and joins a third. These illustrations provide visual representations of the behavior and interactions of strings in string theory.
To accommodate the mathematical framework of string theory, the universe is required to have at least 10 dimensions. However, most of these dimensions are toosmall to be observable or noticeable. Uncurling some of these additional dimensions would significantly alter the behavior of the universe, affecting phenomena like gravity and electron behavior. The anthropic principle suggests that human life is restricted to universes with only three unfurled physical dimensions, as deviations from this configuration may render life impossible.
Hawking explains that string theory has evolved over time, and while it is compatible with a multitude of possible configurations of spatial dimensions, many of these configurations ultimately lead to the four known dimensions of space-time. Furthermore, p-brane theory expands upon string theory by introducing particles with two to nine dimensions. Each theory, whether it be particle theory, string theory, or p-brane theory, has its own advantages and limitations, much like needing multiple maps to describe the Earth completely.
Hawking concludes by emphasizing that scientific theories will never be able to predict the complete course of the universe due to the inherent limitations imposed by the uncertainty principle and the increasing complexity of calculations involving interactions among multiple objects. However, through continued experimentation and theorizing, humanity can gain a deeper understanding of the universe and its fundamental laws.
Ancient theories proposed that spirits resided within natural elements
such as rivers, mountains, the sun, and the moon, and offerings were made to these gods to seek their favor and blessings, such as bountiful harvests. However, as careful observations and record-keeping revealed the precise and predictable motions of celestial bodies, it became apparent that offerings had no impact on these cosmic functions. Science, in more recent times, has uncovered laws that explain the movements of the universe and its constituents. The need for personification to understand the workings of the cosmos diminished, but the origin of the universe remained an enigma.
Hawking mentions the Marquis de Laplace, who proposed the idea of a deterministic universe where the laws of physics could be discovered, but their ultimate origin was attributed to adivine creator. However, scientific discoveries, particularly the uncertainty principle, introduced an element of unpredictability to the universe. According to quantum mechanics, it is impossible to precisely determine both the position and velocity of a particle. Instead, particles are seen as a set of waves propagating over time, and their history can be described, encompassing all possible future states. Hawking suggests that the notions of position and velocity may be biases that science can eventually overcome, much like the anthropomorphic beliefs in spirits and mythologies.
In conclusion, Hawking reflects on the shift from ancient personifications and religious explanations to the development of scientific theories and laws that illuminate the workings of the universe. The mystery of the universe's creation persists, but the progress made in theoretical physics encourages further exploration and invites humanity to expand its understanding of the cosmos and its origins.
During World War I, he campaigned for peace in Germany, and later on, he became a supporter of Zionism, advocating for the return of Jews to the "Holy Land" as a means to escape anti-Semitism. However, Einstein's views on war shifted when he secretly campaigned for the United States government to develop an atomic bomb before Germany could. After World War II, he became a staunch advocate for nuclear disarmament.
In 1952, Einstein was offered the presidency of Israel, but he declined
the position, choosing to prioritize his scientific work. He famously remarked, "Politics is for the present, but an equation is something for eternity," emphasizing his dedication to the timeless pursuit of scientific knowledge and understanding.
Galileo's pivotal role as the first modern scientist. He believed in the power of careful observation to learn about the workings of the world. Galileo'ssupport for the Copernican theory, which proposed that the Earth revolves around the sun, clashed with the views of the Catholic Church, resulting in condemnation of his ideas as heretical. The pope forbade Galileo from promoting the heliocentric theory.
Despite the opposition he faced, Galileo managed to convince a later pope to allow him to write a book presenting both the Copernican and Earth-centered theories in an impartial manner. The resulting publication, titled "Dialogue Concerning the Two Chief World Systems"(1632), received widespread acclaim and played a significant role in persuading people that the sun, rather than the Earth, lay at the center of the solar system.
However, Galileo's convincing arguments led to his house arrest and a public recantation of his beliefs. Nonetheless, he continued his scientific endeavors in secret, smuggling his next book, "Two New Sciences," to Holland for publication. This work had an even greater impact on European readers and played a crucial role in advancing the field of modern physics.
Newton is renowned for writing one of the most significant physics books in history, titled "Principia Mathematica." His contributions to the field of physics were so groundbreaking that he was appointed as the president of the scientific Royal Society and became the first scientist to be knighted.
However, Newton's life was not without controversies and quarrels. He engaged in a feud with John Flamsteed, the Astronomer Royal, who initially provided crucial data for Newton's Principia but later refused to
collaborate with him. Despite Flamsteed's refusal, Newton acquired the information he needed but faced legal constraints preventing him from publishing it.
One of Newton's notable achievements was the invention of calculus, a mathematical framework used to study rates of change and accumulation. Interestingly, the German philosopher Gottfried Leibniz also independently developed calculus around the same time. However, Leibniz published his findings before Newton, sparking a significant feud between the two scholars.
The Appendix of the book is divided into six sections, each covering different topics. The first section explores "Dark Energy and the Accelerating Expansion of the Universe." It discusses how astronomers discovered that the universe is expanding at an increasing rate, ruling out the possibility of a "big crunch." The concept of a "cosmological constant" that counters gravity, initially proposed by Einstein, is revisited in light of this discovery, and the term "dark energy" is usedas a placeholder to explain the accelerating expansion.
According to the anthropic principle, humanity exists in a universe that has a tolerable value of anti-gravity "dark energy," allowing the universe to remain stable and not fly apart.
"Microwave Background Radiation and the No Boundary Proposal" is the third section, which highlights how the discovery of microwave background noise in 1965 provided evidence for the early universe being very hot. The principles of inflation, expansion, and the uncertainty principle are discussed in relation to Hawking's no-boundary proposal. The concept that space and time are meaningless before the Big Bang is emphasized.
The fourth section discusses the search for "gravitational waves" predicted by Einstein's General Theory of Relativity. The LIGO Collaboration's detection of gravitational waves in 2016 from the collision of black holes is mentioned, and it is noted that studying gravitational waves provides valuable insights into black holes and the extreme regions of the universe.
Next, the "Information Paradox" is explored. It addresses the preservation of information in the universe and the challenges posed by black holes. While black holes absorb particles and their information, it is uncertain whether the information can be retrieved when the black holes eventually evaporate.
The final section, titled "Outlook," reflects on the progress made since the second edition of the book. It acknowledges the unexpected discoveries, such as dark energy, and mentions the idea of a multiverse, which may be difficult to accept but is similar to humanity's acceptance of their existence in a vast universe.
Overall, the Appendix covers various topics, including dark energy, inflation, gravitational waves, and the preservation of information in the universe, offering insights into these fascinating areas of physics.