A Brief History of Time Book Summary, by Stephen Hawking

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1-Page Summary of A Brief History of Time


Stephen Hawking’s A Brief History of Time is about the universe. It covers both stars and planets, general relativity, and atoms and subatomic particles. The reason it does this is because understanding both helps you understand how the universe works as a whole. Some theories explain how stars work but not planets; others do the opposite. There isn’t one theory that explains everything we know about astronomy or physics today.

Scientists have learned a lot about the universe. They still have a long way to go, but we can learn from their discoveries so far. Some scientists had made great contributions in understanding the universe and its laws. Einstein was one of them; he came up with special and general theories of relativity as well as quantum mechanics. Other scientists include Planck, who developed the quantum principle that says light occurs in packets; Heisenberg, who measured distance by shining light on particles; Feynman’s sum over histories theory, which says particles can take many paths to get from point A to point B; and Aristotle…

Key Takeaways

The universe is expanding. It began with the big bang, which was a huge explosion that created the universe. There are two theories about how the universe works: general relativity and quantum mechanics. General relativity explains large bodies in space, such as stars and planets; quantum mechanics explains tiny pieces of matter, like atoms and subatomic particles.

There are three arrows of time that point in the same direction. One is entropy, one is human memory, and one is the expansion of the universe.

Type 1: Reorder sentence using commas (2pts)

A star with a high density collapses into itself, creating a black hole. Black holes have such strong gravitational force that even light can’t escape them. This means that the gravitational forces of stars and black holes combine to make planets rotate in ways different from what we would expect based on the strength of star gravity alone.

The general theory of relativity, which says gravity can bend space and time, establishes the possibility of something called a wormhole. A practical problem remains: How do we create one?

Key Takeaway 1: The universe began with an explosion known as the big bang.

The universe began as a single point that exploded and expanded. It cooled down as it did so, which caused matter to come together and form stars. Planets formed from the debris of those stars, but the earth was uninhabitable at first because it was too hot and had no atmosphere. Once primitive life forms came into existence and emitted oxygen, an atmosphere developed on earth that allowed for more complex organisms to live here.

The big bang is the name given to the initial explosion that formed our universe. Scientists believe it was extremely hot and dense, with particles moving at close to the speed of light. It was so hot that atoms could not form; instead, there were only subatomic particles in a plasma state. This meant that photons—a type of particle associated with electromagnetic radiation—were unable to travel freely through space because they were constantly being absorbed by other particles. The temperature decreased as time went on, until finally it cooled enough for matter (protons) and antimatter (antiprotons) to annihilate each other and produce energy in the form of photons. For 380,000 years after the big bang, these photons traveled unimpeded through space before recombining into hydrogen atoms. After this point, stars began forming from clouds of gas. Our solar system came together about 9 billion years after the big bang.

Key Takeaway 2: The universe has been expanding since it began.

Make Analysis

The rate at which the universe expands is crucial to its sustainability. Scientific models indicate that it is expanding at a rate just fast enough to continue growing. If the expansion rate was smaller by even one part in a hundred thousand million million, the universe would recollapse into itself due to gravity.

The universe began with a big bang. Scientists thought that the force of gravity would cause the expansion rate to slow down, but it didn’t. The reverse happened; the acceleration of expansion increased five million years ago. This was due to dark energy, which is still not fully understood by scientists today. They believe that 68% of all energy in the universe is made up of this mysterious entity called dark energy.

Key Takeaway 3: At the moment, one of the two theories that explains the most about the universe is general relativity. This theory explains time and large bodies, such as stars and planets.


A general theory of relativity was developed by Albert Einstein in 1915. It is a mathematical explanation for how gravity works in the universe. A decade earlier, he came up with a special theory of relativity that contradicted Newton’s laws and contended that nothing can move faster than the speed of light, as well as space and time being part of one continuum (space-time). The general theory explains this contradiction by contending that gravitational force does not travel in a straight line but is warped by space-time. Therefore, light could indeed be the fastest moving object in the universe.

One way to think about the distortion of space-time by a massive object is to imagine a bowling ball in the middle of a trampoline. The weight of the bowling ball would cause the material (trampoline) beneath it to sag, which would distort its shape. A marble rolling around on that surface would spiral inward toward the center like water going down a drain because of gravity. This effect works in much the same way as gravitational lensing, where light bends around large objects such as black holes or quasars due to their mass warping space-time. Quasars are very bright and similar to stars; they’re 8 billion light years away and 400 million light years from Earth’s galaxy. If it weren’t for gravitational lensing, people wouldn’t be able see this quasar because our galaxy blocks out its view.

Key Takeaway 4: The other theory that explains the most about the universe is quantum mechanics, which deals with tiny pieces of the universe, such as atoms and subatomic particles.

Quantum mechanics is a series of theories that scientists have developed over time. Many of them decided to focus on this discipline because the behavior of subatomic particles differs from the behavior that general relativity says large objects in the universe exhibit. An example of this difference is that relativity states it’s possible to determine the locations of planets and other large objects in the universe fairly precisely, while quantum mechanics says position cannot be predicted with precision; only probabilities can be determined.

The uncertainty principle is a key concept of quantum mechanics. It states that the position and velocity of particles cannot be predicted with precision, which has led to other intriguing applications. One is a quantum computer, which would use qubits instead of bits in its calculations. A qubit is based on the movement of particles rather than electrical pulses. The number of particles used in a quantum computer could be huge, so it would have an extremely high capacity for computing power and speed. The same uncertainty principle can also be used to encrypt information into unbreakable code by using photons as the subatomic particle that will be used to encrypt data.

Key Takeaway 5: Scientists are searching for a unified theory of how the universe works.

Analysis Skills

Scientists have a problem: the general theory of relativity doesn’t fit with subatomic particles, while quantum mechanics does not fit with large objects. Scientists want to find one theory that fits all behaviors in the universe, a “theory of everything.”

Some scientists believe that an idea conceived in the 1960s will ultimately come to be the theory of everything. This is string theory, which posits that the world does not consist of particles but long, thin subatomic strings. The strings can vibrate in different ways. In music, differences in the vibrations of violin strings produce different notes. In string theory, the different vibrations of strings are what cause stars and planets to behave the way they do and subatomic particles to behave the way they do. “The electron is a string vibrating one way,” says physicist Brian Greene, “the up-quark is a string vibrating another way,” says theoretical physicist Michio Kaku, “and so on,” he continues. If someone shakes an electron’s (string) so it vibrates differently, it turns into another particle called a quark; shake it again and it becomes a photon (light). Shake it again and it becomes something else: gravity. Pieces of superstrings (longer than regular strings) keep joining together or splitting apart ; as they join or split away from each other force space/time around them to curl up — this causes gravity !

Key Takeaway 6: There are three arrows of time in the universe that flow in the same direction. One deals with the universe’s progression from order to disorder, one with humans remembering the past rather than the future, and one with the universe expanding rather than contracting.

The arrow of time is a concept that deals with the natural progression from order to disorder. It’s divided into three parts: thermodynamic, psychological and cosmological.

The universe was simple at the beginning. It took a long time for even basic elements to be created, let alone life. If the cosmological arrow went in the opposite direction, people would remember what happened in the future instead of recalling events from their pasts. It could be exhilarating but also scary because people would conjure up images of bad things that might happen to them in the future. The scary thing about an expanding universe is that it means humanity will eventually die out anyway, no matter what happens now or later on down the road. Hawking and other scientists predict that our universe will contract someday so we’re all doomed anyway, but at least we don’t have to worry about death for now since our universe is still expanding right now.

Key Takeaway 7: Black holes, which is the result of a star of a certain density that has collapsed into itself, have such strong gravitational force that not even light can escape them. One of the implications of this is that the gravitational force of black holes combines with the gravitational force of stars to make planets rotate in ways different from what could be expected by the gravitational force of stars alone.

Black holes cannot be seen, but they exert a gravitational force that can be detected. Black holes are so strong that they suck in other objects and keep light particles from escaping their burning mass. There may be more primordial black holes than visible stars because without them the gravitational forces of stars would make planets rotate differently than they do today. The reason for this is that black hole’s gravity combines with star gravity to produce the planet rotation patterns we see today.

Stephen Hawking is a world-renowned expert on black holes. Last year, he changed his mind about the relationship between black holes and their surroundings. He now believes that light may be able to escape from a black hole after all. Before this change of thinking, it was commonly accepted that there was an invisible boundary around a black hole called the event horizon. Matter would be sucked through it into the black hole, and light trying to escape from the black hole would get stuck in the event horizon, unable to escape. Now though, Hawking has come up with new thinking: There is no sharp boundary surrounding a black hole; instead there’s an apparent horizon. The idea of an apparent horizon fits relativity theory but not quantum mechanics — so if we want our theories to mesh together properly, then we need something more nebulous than an event horizon. One major implication of this new thinking is that light may be able to escape from a black hole through an apparent horizon rather than just getting stuck at its edges like before.

Key Takeaway 8: The general theory of relativity, which says gravity curves space-time, makes time travel theoretically possible. However, many practical problems must be overcome before time travel can actually happen.


According to the theory of relativity, gravity can bend space-time. This means that time could loop back on itself and people traveling in the loop would return to their past. However, there are a lot of practical challenges that need to be overcome before this becomes possible. One challenge is that it would have to occur at light speed. Traveling faster than light has never been done before and we don’t know if it’s even possible with today’s technology or knowledge. Another challenge is creating wormholes (tunnels connecting two locations) because they would have to be small enough for humans but large enough for them not to get stuck inside them once created. Scientists also think black hole travel might be possible because of its huge gravitational force which bends space-time near breaking point; this could theoretically create a tunnel between another black hole as well as tiny rips in each one which could connect with each other via these tunnels. These tunnels would have to be enlarged so people can pass through them, however, and keeping them open long enough will require a lot of energy. The amount needed far exceeds our current capabilities but the general theory says it is possible so scientists will continue trying until they succeed at some point in the future.

Book Structure

Hawking explains the development of science in a way that normal people can understand. He lays out the history of scientific discoveries chronologically, showing how one discovery builds on another to form a whole. Quantum mechanics is an example; it’s not so much one theory as it is many theories and rules developed over time by different scientists.

Although this book is long, it contains a lot of information. It covers many theories and laws about the universe, how they were discovered, and why they’re important. Stephen Hawking also talks about scientists who have contributed to knowledge of the universe; he mentions Aristotle, Isaac Newton, Copernicus and Galileo among others. He doesn’t talk very much about his own contributions to science but does mention a few stories about himself such as bets he made with other scientists regarding various concepts or ideas. He also makes some funny asides in the book that show off his wit which has become well known for him over time

About the Author

Stephen Hawking is a renowned physicist and cosmologist. He has been a professor at Cambridge University for decades, and he’s also known for his fight against ALS. Some of his most acclaimed work has dealt with the beginning of the universe as well as collapsed stars known as black holes. His book, A Brief History in Time was an international bestseller when it appeared in 1988, and it continues to sell today. He has written four other books about the universe for adults, two children’s books on science, several popular science articles in newspapers around the world, and dozens of scientific papers published by leading scientific journals such as The Journal of Cosmology (which he founded).

Full Summary of A Brief History of Time

Overall Summary

Where did the universe and everything in it come from? Modern science has developed new technology that allows us to answer such questions. Stephen Hawking details some of those breakthroughs, including a discussion on how our understanding has evolved over time.

Though people in ancient Greece figured out that the earth is round, thinkers like Aristotle still thought the earth was at the center of everything else. This theory wasn’t really challenged until Nicholas Copernicus showed that planets including Earth orbit the sun. In 1687, Sir Isaac Newton devised his own laws related to gravity and theorized that all stars should exert this force on one another. He also wondered if there was a time when everything was in a single tiny place. God could fit into this theory as well.

Science is looking for one theory that ties together the two main theories of relativity and quantum mechanics. Those theories deal with massive celestial bodies on a large scale, as well as tiny particles on a small scale. Scientists are trying to find something that unifies those two theories into one big idea because they want to understand how we came about.

Newton put forth the idea that objects move at a constant speed unless forces act upon them. This challenges the belief of absolute space, because if you’re on a train moving 40 miles per hour and throw something like a ping pong ball, it will travel much farther than expected by someone standing outside the train.

The idea of absolute time took longer to overcome. However, in 1865, James Clerk Maxwell discovered that light has different wavelengths and is faster than any other object. Albert Einstein later pointed out that light always moves at the same speed and is faster than anything else in his theory of relativity. The general theory of relativity also put forth that gravity warps—bends and curves—space-time. For example, time moves more slowly near objects with larger masses. Thus, though space and time affect objects’ movements, objects’ movements also affect space and time; neither is absolute.

In the mid 18th century, astronomers discovered that our galaxy is a spiral. In the 20th century, Hubble showed that there are other galaxies and that they’re moving away from us. This explains why we don’t need Einstein’s cosmological constant to explain why the universe isn’t collapsing in on itself—Einstein said it was his worst mistake.

A physicist named Friedmann found that the universe is roughly the same in all directions. He was able to prove this by measuring the universe’s uniform microwave radiation. Some of his other models included a big bang at the beginning, which became widely accepted later with Penrose’s work on black holes and Hawking’s paper on them.

In the early 1800s, Laplace said that because science was doing such a good job at predicting things, we would be able to predict everything if we knew the exact state of the universe at one point in time. But Heisenberg’s uncertainty principle proved him wrong when he tried to measure exactly where particles were with light. The more accurately Heisenberg wanted to measure this, the more it affected where particles were and their velocity. This lead scientists to create quantum mechanics.

Theories about the atom have been developed over time. Scientists discovered electrons, neutrons, and protons in the early 1900s. Murray Gell-Mann won a Nobel Prize for discovering that these were made up of quarks, which are different types of particles with specific spins. Particles also follow the Pauli exclusion principle, which states that they cannot be in exactly the same place at once because they repel each other. Further research has revealed more information about anti-particles (which occur when matter collides with antimatter) as well as force-carrying particles such as gravitational forces and electromagnetic forces.

The term ‘black hole’ was coined in 1969, but the idea existed before that. In 1783 John Michell said that if a star has enough mass it would have so much gravity that light could not escape from it. Not only can we not see black holes, but we could measure their influence on surrounding material and find them because of their effect on other objects. This happens at the end of a star’s life when its energy is all used up and it collapses under its own weight to form a black hole. Nobel Prize winner Subrahmanyan Chandrasekhar calculated how big something would need to be before collapsing into a black hole, which helped him win his prize. Most stars do collapse into black holes, however; our sun will probably just get hotter as it burns through its fuel until there is nothing left for nuclear reactions to occur with anymore—it won’t become one since it isn’t massive enough and won’t even form an object like what we think of as a neutron star (which forms when more-massive stars burn out).

Black holes are so dense that they can stop light from escaping. The boundary of a black hole is called its event horizon. At the center of a black hole is where the laws of physics break down and become infinite, which physicists call singularities.

Hawking was one of the first to note that once a black hole has formed, it can only grow larger over time. This principle is akin to the law of thermodynamics in which systems experience an increase in entropy—or disorder—over time. Bekenstein suggested that if we measure the area of the event horizon and use this as a measure for how much energy is given off by a system (such as a black hole), then we could harness this power. The challenge then is developing technology capable enough to harvest that energy without being destroyed due to radiation around these back holes. And even with such technologies, scientists won’t be able to access all available energy within these back holes due them having existed so many years ago.

Stephen Hawking attended a conference at the Vatican in 1981. The Pope told scientists not to study this topic because it was God’s work, but Hawking had recently discussed the possibility that there was no beginning to the universe, as it has no boundaries. There are many different theories about how the early universe looked like; however, in one theory called “the hot big bang model,” particles were moving very fast and eventually clumped together into galaxies and stars.

Hawking then decided to focus on black holes when he returned to Cambridge University from Geneva. He worked with Roger Penrose, who had discovered a new area of mathematics involving singularities. A black hole is formed when a star dies ; all of the matter inside becomes packed so tightly that nothing can escape its gravitational pull. This means that light cannot escape either, which is why they are invisible. It also means they have infinite density; if you compressed matter any further than this, according to Einstein’s equation E = mc2 ( energy equals mass times speed of light squared ) you would get an enormous amount of energy. In fact, there should be enough energy contained within a single black hole to boil off Earth ‘ s oceans.

Scientists need a quantum theory of gravity to figure out what happened at the beginning, but that hasn’t been discovered yet. Meanwhile, the anthropic principle is used to explain why humans are around to ask these questions; it essentially says that if there were no intelligent life on Earth, we wouldn’t be here asking these questions.

There are three arrows of time, which point in the same direction. The first arrow is due to entropy, or things getting more disordered over time. This can be seen when you leave a room and it becomes messy again; that’s an example of increasing disorder. The second arrow is psychological and refers roughly to memory formation. It means that if you remember doing something in the past, then it must have happened before now; this also represents increasing disorder because making memories takes energy, thus creating more disorder as that energy is emitted into the world around us.

The third arrow points to cosmology (the study of the universe), stating that there are no boundaries for space-time and therefore everything expands outward from one point—in other words, we live in a big bang universe with an expanding timeline. Therefore all these three arrows point forward on our timelines because they each represent increasing disorder over time; intelligent beings could not exist otherwise since order would have already been achieved by now! Even making memories increases disorder as it takes energy to make new memories so they too create more disorder as their energy is emitted into space around us…

The universe will eventually contract. It’s not known if time flows backward or forward, but it can’t flow backwards because disorder increases. The universe is only suitable for life during the expanding phase, when celestial bodies are still burning and people are still alive.

The early universe was chaotic, so time travel might have been possible. However, observations of the uniform radiation across the universe suggest that this wasn’t the case. As time is not absolute, it would be possible for space travelers to return to earth in what seems like a short time to them but thousands of years may have passed on earth’s perception of time. It would also be possible to warp space-time and create a wormhole between two points in order to travel backwards through time if an advanced civilization could stabilize them.

The quest to unify physics is ongoing. The first step is to incorporate the uncertainty principle into general relativity, and then come up with a theory that does so. String theories are possible answers, as they visualize particles as waves on one-dimensional strings in two dimensions of space-time instead of dots. Particles are then visualized as waves passing down that “string” (or string theory). However, there are problems with this approach: either 10 or 26 dimensions must be added for it to work properly.

Even though there may be a unified theory of physics, its existence will still depend on the uncertainty principle. We want to know why we exist and where we come from. If we can find that out, then we’ll also understand the mind of God.

Chapter 1

A scientist was giving a public lecture on astronomy, and he mentioned that the solar system is heliocentric. An old lady at the back of the room said his ideas were nonsense because she believed the world to be flat and sitting on top of a giant tortoise. The scientist asked what this tortoise stands upon, to which she replied “Tortoises all the way down”.

Scientists may laugh at the image of an old lady sitting on a tortoise, but do they really know more than her? New technologies are helping us understand the universe better. Maybe one day we’ll see that as silly as she looks riding a tortoise. Time will tell.

Aristotle was one of the first people to suggest that the earth is round. He had two arguments for this claim. First, lunar eclipses are caused by the shadow of the earth on the moon. The shadow cast during an eclipse is always round, so there’s no way a flat object like a disc could block out something as big as an entire sun and still have a circular shape. Second, he noticed that when ships came over the horizon, they were visible before their hulls became visible; therefore, it must be because they’re closer to us than their hulls are.

Aristotle believed that the earth was fixed at the center of the universe and all heavenly bodies revolved around it. Ptolemy further developed this by adding a system of eight spheres to account for all known celestial bodies, which moved in perfect circular orbits. The outermost sphere held the fixed stars, while everything else rotated around them. This model remained unchallenged until Copernicus came up with his heliocentric theory in 1543 AD (Copernicus).

Ptolemy’s model of the solar system was accurate in predicting the movement of each heavenly body. However, it had a flaw; every once in a while, Ptolemy noticed that the moon appeared twice as big as normal when orbiting around Earth. This didn’t make sense to him because he believed there were other planets beyond his model where heaven and hell could be located.

A Polish priest named Nicholas Copernicus came up with a simpler model of the universe in 1514. At first, he published anonymously to avoid being called a heretic. It took nearly 100 years for his idea, that the sun sat stationary at the center of planets, to be taken seriously by German astronomer Johannes Kepler and Italian Galileo Galilei.

The Ptolemaic model of the solar system was destroyed by a number of discoveries, including telescopes. The telescope revealed that Jupiter has several moons, which means not everything orbits Earth. However, those moons could still orbit Earth and have very complicated journeys around Jupiter—this is called an epicycle. Kepler added the idea that orbits are elliptical rather than perfectly circular, so now the theory works with what we observe in space.

Kepler believed that elliptical orbits were not as good as perfect circles, but they worked. The idea of magnetic forces controlling all this movement was not popular until Sir Isaac Newton’s Philosophiae Naturalis Principia Mathematica came out in 1687 and explained everything.

Newton developed the math to back up his ideas about how things move in space and time. He theorized that everything is attracted to everything else, with the force being stronger when those things are closer together and bigger. This explains why objects fall to the ground. His theory showed that the moon moves in an elliptical orbit around the earth, while planets have elliptical orbits around the sun.

The idea of a celestial sphere and an infinite universe was replaced with the Copernican model. This model states that stars are not fixed, but rather very far away. Given Newton’s law of gravity, these distant stars should fall into each other at some point. However, if there is no center to this infinite universe, then all the stars would be spread uniformly across it.

The infinite is a tricky concept. If you think about it, every point in an infinite universe will be the center because every point has stars on either side of it. However, we now know that this can’t be true, so there must be some sort of boundary to our universe (i.e., our universe may only consist of a certain number of galaxies). This means that adding more stars beyond what’s already been discovered won’t make any difference—the same number will still fall into each other at the same rate. It’s also thought that there can’t possibly exist a model for an infinite universe where all bodies attract each other equally; if they did, then they would have attracted together long ago and we wouldn’t exist!

Before the twentieth century, people had no idea whether or not the universe was expanding. This could have been because they believed in eternal truths and were comfortable with that idea of an unchanging universe, even after their own deaths.

Even though Newton’s theory showed that the universe was not static, and he had proposed his law of universal gravitation to counter this idea – it was later discovered that gravity is attractive in every direction. The only implausible factor about the model being that stars were still able to stay together and remain in equilibrium for such a period of time. Nowadays we believe that if they move in either direction then they would be charged with increasingly strong repulsive or attractive forces against each other.

German philosopher Heinrich Olbers argued that if the universe is infinite and static, then every line of sight would end on a star. This was an objection to Newton’s concept of an infinite, static universe. Therefore, there must be a finite time in which stars were created. However, this raises the question about when the stars came into being.

The idea of the universe having a beginning is not new. Religious thought had already put it at a time in the past, and one line of reasoning for that was that there must be a First Cause which caused everything else. St. Augustine put Creation—as per the book of Genesis in the Bible—at around 5,000 BC (which is not far off from 10,000 BC when civilization took off).

Aristotle was against the idea of a beginning because it sounded like divine intervention. He thought that people and the world existed forever, and cultural progress did not exist.

Philosopher Immanuel Kant considered the question of whether the universe had a beginning in time. He believed that it was possible for both arguments (that it did and didn’t) to be true, but he called those ideas antinomies because they contradicted each other.

Kant thought that if the universe did not have a beginning, then time would continue forever. He said this because he thought that the past was infinite and there was no specific moment when it began. However, Augustine argued that time is only meaningful within creation and didn’t exist before it.

Questions about the beginning of the universe used to be metaphysical, but in 1929 Edwin Hubble saw that all distant galaxies are moving away from each other. As such, at some point in time (possibly 10 billion years ago) there was a single source of matter that contained everything in the universe. That’s why it had infinite density and was infinitely small. This realization made questions about beginnings scientific ones rather than philosophical or religious ones.

Hubble’s discovery led to the idea that the universe began in a big bang. This would be an event when time and space existed, but there were no laws of science.

The idea of time is different in an expanding universe. It’s not static, so it has a beginning that could be physical and explain the need for a start. This doesn’t rule out God, but it does determine when time started.

A theory is a model of the universe, or one part of it. It predicts what will happen in future observations when certain factors are present. For example, Empedocles’ idea that there were four elements was simple but couldn’t predict anything. Newton’s theory of gravity, which is determined by mass and distance, can accurately predict the movement of stars.

No physical theory can be proven entirely. Even if every test has backed up the theory so far, one cannot prove that the next test will not disprove it. Even one piece of evidence contradicting the theory can disprove it. Philosopher Karl Popper said that confidence in any theory grows with each accurate prediction, but that a scientist must cast aside or adapt his or her theories if even one observation contradicts them.

New theories usually build on the previous ones. For example, Mercury’s movement was slightly different than what Newton predicted using his theory of gravity. Einstein’s general relativity made a different prediction that matched with what was seen. So, it confirmed Einstein’s new theory and showed its accuracy over Newton’s simpler one in certain situations.

Science has an ultimate goal of creating one theory that explains everything. However, it usually deals with the question by applying laws that explain how things move through time to make predictions or asking what was the beginning state of the universe? Some people think only questions about time are strictly science; others say God can do whatever he wants because he’s a god. While this could be true, there are also laws governing his actions, which means there must have been rules for the beginning as well.

It’s difficult to come up with a single theory that explains everything in the universe. We can only explain certain things, and we need multiple theories for those parts. For example, gravity is determined by mass rather than the content of an object; so it doesn’t matter what’s inside a star, just its mass.

Scientists now describe our universe using general relativity and quantum mechanics. Both of these theories are great achievements, but they don’t work together. General relativity relates to gravity on a large scale and deals with really big things in the universe. Quantum mechanics is about the smallest parts of matter that we know about, which are very small indeed (a billionth of an inch). But we need to combine them into one theory if we’re going to make sense of everything in between those extremes. We might not have such a theory for some time yet, though many aspects and predictions have already been discovered by scientists studying this area so far.

If the universe is governed by laws, then we should be able to discover a complete theory that explains everything. However, if such a theory exists, it would determine our actions and therefore influence its own discovery. This means that discovery itself cannot be the ultimate conclusion of this search for knowledge; instead, there must be something greater than discovery.

Charles Darwin’s theory of natural selection may explain why we are so intelligent. He believed that differences in intelligence make some people more likely to survive than others. Today, our discoveries could be the end of us all. Also, a unified theory might not affect our survival rate. However, given the evolution of the universe and its regular changes, it is logical for us to come up with conclusions about how we should live our lives.

We have theories that describe the majority of cases. However, they may not be perfect and it’s hard to justify looking for a better theory because we have nuclear power and microelectronics now. Still, many people want to understand the world in which they live, so trying to find a complete description might help us answer questions we’ve asked for thousands of years about where we came from.

Chapter 2

Forces and motion were not well understood until Galileo and Newton came along. Before them, people believed Aristotle’s theory that objects naturally stay still unless a force is applied to move them. According to his logic, heavier objects should fall faster than lighter ones when dropped from the same height. However, Galileo was the first person who bothered to test out this theory by dropping balls of different weights off the Tower of Pisa (although he actually rolled balls down a hill).

Galileo found that each ball increased its speed at the same rate, regardless of its weight. The acceleration of the balls was directly proportionate to the incline of the hill, not their different weights. If one dropped a lead ball and a feather, air resistance would slow down both balls equally because there is no air on the moon.

Newton used Galileo’s measurements of the motion of balls to formulate his three laws of motion. Newton believed that an object in motion would continue moving at a constant speed unless some force acted upon it. The only force acting on the object was its weight, which caused it to accelerate. Therefore, if there was no force acting upon an object, then it would move straight ahead at a constant speed.

Newton was the first person to propose what we now call Newton’s First Law, which forms the foundation of physics. The law states that an object will change its speed proportionally to how much force is applied, and it also affects how deceleration affects objects based on their mass—masses can be manipulated or accelerated more easily depending on their size.

Newton also discovered the law of gravity, which is the idea that every object attracts every other object proportionally to its mass; the bigger an object, the stronger its gravitational pull. The gravitational force between two objects doubles if just one object’s mass doubles. If another were to triple its original mass, then their combined gravitational pull would be six times stronger than before. This is why all objects fall (or accelerate) at a rate that can be predicted with these formulas.

The law of gravity states that the force between two massive objects is directly proportional to the product of their masses and inversely proportional to the square of their distance apart. This helps us predict how far away planets will be from stars, because if this weren’t true, they would either fall into or fly out of orbit around them.

The main difference between Aristotle’s approach and that of Galileo and Newton is the former’s idea of preferred state of rest, meaning an object would remain still if no force were acting on it. But Newton’s laws tell us there is no one standard of rest. For example, if we ignore the fact the earth is orbiting the sun, we could say a train traveling over a still earth at 90 miles per hour. But you could also equally say that the earth is moving south at this same rate; in other words you can’t deduce which object—the train or Earth—is moving at 90 miles per hour and which is at rest.

Because there is no absolute rest, it’s hard to tell if two events that took place at different times occurred in the same location. For example, a table tennis ball bounces on a moving train and someone inside the train will think that the bounces happened 40 meters apart as they continued traveling along between bounces.

According to the author, we cannot say that an event happened at a specific time and place. The two people on the train wouldn’t agree on where or when it took place.

Newton was worried that his theory of relativity implied the nonexistence of absolute space because it didn’t agree with his idea of an absolute God. He refused to accept such a notion, even though he discovered his own laws supported it. Many criticized him for this belief, including Bishop Berkeley who believed everything material was illusory. Dr. Johnson disagreed with the Bishop and kicked a rock to show how real he thought time, matter, and space were by kicking something physical in the present moment (in contrast to something illusory).

Aristotle and Newton believed that there was a definite amount of time between two events. For example, if you were to ask them how long it took for the sun to set, they would say 2 hours and 23 minutes. This means that they thought time was separate from space. However, this view has changed over the years because we now know that things are not as simple as Aristotle had thought. When dealing with objects like apples or planets moving at slower speeds than light (186,282 miles per second), Aristotelian physics works fine; however, when looking at objects traveling near or faster than the speed of light, this commonsense approach doesn’t work at all.

In 1676, Ole Christensen Roemer noticed that light had a finite speed. He deduced this by observing Jupiter’s moons and noticing the eclipses were later as Earth was further from Jupiter. This was remarkable because it came 11 years before Newton’s Principia Mathematica.

James Clerk Maxwell’s theory of the transmission of light was a major advancement in science. It unified theories about electricity and magnetism, which had previously been studied separately. He said that electromagnetic field disturbances would travel at a constant speed like ripples on water. This is known as the “electromagnetic wave” theory, where each wavelength or frequency is categorized by its length (radio waves are longer than infrared).

In the 1800s, a scientist named Fresnel came up with an idea that light traveled at different speeds depending on its wavelength. Newton’s theory of relativity had already said that everything moved relative to other objects in space, so this question raised the issue of what it was moving relative to. People thought they could answer this by saying there was something called ether that filled all space and acted as a medium for light to travel through.

For example, light should travel faster when moving in the same direction as the earth’s movement around the sun than at right angles to it. But Albert Michelson and Edward Morley tested this in 1887 and found that it wasn’t true according to observation—the speed of light was always the same.

Many people tried to explain why light travels at the same speed no matter how fast it’s going. It was not until 1905, when Albert Einstein suggested that there is no need for the idea of ether if you accept time as relative. Henri Poincaré made a similar point soon after this new idea came about called relativity, which means that all observers will measure the same speed of light regardless of their motion through space and time.

Although it seems simple, Einstein’s idea had a huge impact. Energy and mass are equivalent, as shown in his famous equation E=mc2. This means that an object’s motion-related energy will increase its mass, which makes it harder for the object to move faster.

Objects that are moving close to the speed of light have an exponentially increasing mass. It would take an infinite amount of energy to reach the speed of light. Only objects with no mass, like waves, can get to the speed of light.

Relativity has changed the way we see space and time. Under Newton’s theory, observers would agree on how long it took a beam of light to reach one place from another, but not the distance between those points because of absolute space. But relativity says that there is no absolute space; rather, observers must agree on the speed of light in order for them to measure time as well. The time taken for light to travel equals the distance traveled divided by the speed of light (which they all measure). Therefore, there is no such thing as an absolute time; each observer measures their own according to their clock and may not necessarily agree with others’ clocks.

If you use radar to record the place and time of an event, you can send a pulse to that event and receive it back. The distance is calculated by multiplying half the distance between the two events by the speed of light. A space-time diagram can be used by different observers moving at different speeds, but no measurement is more correct than any other because they are all related. So if one observer knew another’s relative velocity, he could calculate where that person would measure as being in space-time.

Today, we can accurately measure distances because we have better ways to measure time. For example, a meter is defined as the distance light travels in a fraction of a second. We can also use units called light-seconds—the distance that light travels in one second.

In the theory of relativity, we can measure distance with time and light speed. So every observer must agree on the speed of light. We don’t need a theory of ether because it’s undetectable anyway. Time is also dependent on space, which is why they’re combined in an idea called space-time. In everyday life, we use three dimensions to locate something: height above sea level or longitude and latitude for example. But this isn’t sufficient to locate the moon because it depends on multiple references points (coordinates). We could pick any point from among suns or planets but these in turn cannot locate our sun compared to other galaxies so there are many overlapping layers that determine where anything is located.

An event occurs at a certain point in space and time. It can be measured according to four coordinates: three in space, and the fourth in time. These can all be arbitrary. In space-time, there is no distinction between space and time coordinates because they are both interchangeable.

A 2-dimensional space can be easily represented by a map, because we can determine the location of any point on earth by latitude and longitude. Space-time diagrams show time increasing on one axis, while the other axis shows some dimension in space. Hawking presents a diagram where years are shown going up vertically, and miles are measured horizontally between two objects: Earth’s Sun and Alpha Centauri (a nearby star). The path of each object is a vertical line; the diagonal line connecting them is a ray of light that takes four years to travel from one object to another.

Maxwell predicted that the speed of light is constant, regardless of its source. This was proven to be true. As light spreads out from a certain point, it forms a cone similar to ripples spreading out from where a stone hits the surface of water in a lake. In this case, we take pictures of these ripples and stack them up to create what’s called the future light cone (Fig 2.4). We can also create past light cones by taking pictures at different angles and stacking those together as well (not shown here).

When you have an event, it can be seen as a point on the graph. All other events in the universe can be divided into three groups: those that are in the future of the event (those that will happen after), those that are in its past (those that happened before), and everything else.

For example, if the sun went out right now it wouldn’t affect what’s happening on Earth because it takes 8 minutes for light from the sun to reach us. After those 8 minutes, we would be in the future of that event and see a dark sky. In the same way, we don’t know what is happening right now in distant space but are seeing events as they happened in the past.

Einstein and Poincaré’s theory was the first to explain that gravity doesn’t affect light. It also explained why the speed of light is always constant, even when moving at a high velocity. This simplified Einstein’s theory by showing how it works in this case.

The theory that nothing moves faster than the speed of light is inconsistent with Newton’s laws on gravity. This implies that gravitational effects take effect instantly, which doesn’t work with relativity’s idea that nothing moves at or above the speed of light.

Albert Einstein proposed the general theory of relativity in 1915. He suggested that gravity is not like other forces, but rather the result of space-time being curved by mass and energy. A geodesic is a path taken by an object in space which minimizes its distance to travel between two points. The shortest path between two airports for airplanes is called a great circle, because it’s also the straightest line possible on Earth’s surface.

In general relativity, objects take a straight route in curved space-time. However, they appear to take a curved path when viewed from 3D space. For example, an airplane flying straight will have a shadow that appears to be curving on the ground due to the curvature of Earth’s gravity field.

The sun’s mass creates a curvature in space-time, so although the earth travels on a straight line through four dimensional space-time, it appears to follow a circular path in three dimensional space. This is why Newton’s law of gravity predicted planetary movements fairly accurately. However, Mercury was too close to the sun and had an elongated orbit that rotated one degree every ten thousand years. Einstein’s new theory accounted for this deviation and helped confirm his idea about curved space-time. These smaller deviations have been found elsewhere as well.

Light also seems to not take straight paths through three-dimensional space. Light should also be bent by gravity, according to general relativity. Light cones near the sun ought to bend slightly inward, because of the sun’s mass. Light from a distant star that passes by the sun ought to bend, making it appear where it is not. If light always passed near the sun in a predictable pattern, we would not be able to tell this effect was happening; however, since stars move relative to each other as we orbit around them and they pass behind our view of them at different times during their orbits around us (in addition to moving across our line of sight), we can see how light bends in front of massive objects like stars when viewed through telescopes on earth or even with just your own eyes if you know what you’re looking for!

It is difficult to observe the bending of light around the sun, because of its gravity as well as the brightness of its light. However, during an eclipse, we can see this effect by blocking out the sun’s light and measuring how much starlight has been bent. This effect was later observed and measured and confirmed Einstein’s theory.

According to Einstein’s theory of general relativity, time should run slower the closer you are to objects with large mass. This is why clocks on earth run more slowly than those in space. The higher a clock’s frequency (or number of waves per second), the higher its energy level and the faster it runs. Light loses energy as it travels through space toward an observer on earth, which makes its frequency slow down so that everything below appears to be moving at a slower pace than where the observer is located.

Newton’s laws ended the idea of absolute space and relativity ended the idea of absolute time. If twins separated, with one living on top of a mountain and one living by the sea, they would age differently when they met again. The twin who lived in higher elevations would be older than his brother. Twins can also experience this phenomenon if they take rides in spaceships at speeds approaching that of light (300 million meters per second). In this example, the difference is small but it gets more pronounced as you approach light speed. Really there is no such thing as an objective measure for time because everyone has their own sense of timing based on where they are or how fast they’re moving relative to someone else.

Before 1915, people thought space and time were separate and that they went on forever. But with Einstein’s theory of general relativity, we now know that both are affected by objects’ movement and forces. In turn, the movement of those forces is also affected by space-time. The universe affects everything in it, so there’s no meaning to anything outside of it—just as you can’t put a value on something without knowing its worth within the context of what surrounds it.

Stephen Hawking was influenced by Einstein’s work, which led him to the idea that the universe is dynamic and possibly finite.

Chapter 3

On a moonless night, the brightest objects are most likely Venus, Mars, Jupiter, and Saturn. There are also many stars that look similar to our sun but are farther away from us than these planets. However, they move relative to each other as we see them moving against the background of more distant stars.

The closest star to us is Proxima Centauri, which is 23 million million miles away. Other stars are within a few hundred light-years of Earth. The sun in our solar system lies 8 light minutes away from us. These stars are concentrated into the band we call the Milky Way galaxy because people thought this was so back in 1750 and astronomers confirmed it by 1900.

In 1924, Hubble discovered that our galaxy is one among many others in the universe. He measured their distance by using the fact that they appear brighter when they are closer to us. From his measurements, he found out that there are hundreds of thousands of millions of galaxies in the universe. Our own galaxy has a diameter of 100,000 light years and rotates around its center once every 200 million years.

Isaac Newton discovered that by using a prism, we can measure the different colors of light. He also used a telescope and prism to see what stars were made up of. By looking at their spectra (the rainbow pattern formed when white light is separated into its component wavelengths), he could tell what chemicals are in each star. Missing colors indicate which elements are missing from the star’s spectrum, allowing us to determine what it’s made out of.

When scientists looked at stars in other galaxies, they noticed that the light from those stars was missing some colors. The same thing happened when looking at closer stars; however, the color shift was toward red. This is because of the Doppler effect, which says that if something moves away from us we will see its waves as longer and therefore in a different color (toward red).

As Edwin Hubble observed galaxies and their distances from the earth, he noticed that most of them were moving away from us. This means that they’re getting farther away faster than those that weren’t moving. The further a galaxy is, the faster it’s moving away from us. Therefore, we can conclude that the universe must be expanding. It was such a great discovery because people wondered why no one had thought of this before (Newton should have guessed). Since the universe is expanding beyond what gravity can balance out, it could expand forever like a rocket going into outer space instead of falling back to earth.

Even though the universe is expanding, people didn’t believe it until recently. Even Einstein thought of a kind of anti-gravitational force called the cosmological constant instead.

A Russian physicist called Alexander Friedmann tackled this head on. He assumed the universe looked uniformly the same in every direction, no matter where you are in it. This alone suggested that the universe is not static, and he made this assumption before Hubble’s landmark discovery.

In the nearby space, the universe does not appear to be uniform. However, further away from Earth it appears more uniform. This was later confirmed by Arno Penzias and Robert Wilson who tested a very sensitive microwave detector that picked up a lot of background noise than expected. After checking their equipment and taking measurements in all directions as the earth traveled around the sun, they determined it came uniformly from everywhere in space. They had confirmed Friedmann’s first assumption about how the universe would look like at large distances based on his equations for general relativity.

At the same time, two other American physicists were working on a theory that stated that microwave radiation is evidence of the early universe. They said it was so long ago that energy would have red-shifted to become microwave radiation. Penzias and Wilson heard about this and they saw that they had already found evidence of this phenomenon.

If the universe looks the same in every direction, it should look the same from any other point in the universe too. We argue this on modesty grounds; we cannot prove it yet. In Friedmann’s model, both space and time are expanding equally everywhere at once (like a balloon with every point expanding from every other point), so that objects further away will be moving faster than those closer to us. This turns out to match Hubble’s findings about how fast distant galaxies are moving away from us. Despite its accuracy, however, Friedmann’s work was not known outside Russia until Howard Robertson and Arthur Walker independently came up with similar solutions for an expanding universe.

In three out of the four models, gravity plays a critical role in halting the expansion and eventually bringing about contraction. One of Friedmann’s assumptions is that universal expansion or contraction must cease (or stop slowing) to remain static. In his model, our universe seems destined for endless expansion—which falls in line (with confirmation from modern cosmological research) with current ideas of a “flat” universe expanding forever.

The first model is that the universe is finite, but does not have a boundary. Gravity bends space around itself and makes it curved, like the surface of the earth. When you combine this with quantum mechanics (which we will discuss in more detail later), space can be finite without a boundary. However, if you were to travel right around the universe back to where you started, it would collapse again before you managed it because of its curvature. You’d have to travel faster than light and since that’s impossible…

To know how the universe will expand, we need to know the rate of expansion and average density. The Doppler effect shows that galaxies are moving away from us at 5-10% every billion years. We can measure only one percent of what’s needed to stop this expansion. Even if there were more than we could see, it still wouldn’t be enough to halt the expansion. It seems that nothing will be able to stop the universe from expanding forever, but even if it did recollapse, humans would probably die out before then anyway.

All the Friedmann models start out with a beginning where the space between everything was zero—the universe was infinitely dense and curved. Laws of science break down at this point, so we can’t go back in time before that. In essence, then, time began with the big bang.

Some people dislike the big bang model because it leaves room for God. However, other scientists proposed a theory called the steady state theory, which suggests that matter spontaneously comes into being in gaps between expanding galaxies. A group of astronomers disproved this theory using radio waves from different galaxies and measurements taken by Penzias and Wilson.

Evgenii Lifshitz and Isaac Khalatnikov also tried to disprove the big bang theory. They said that as galaxies don’t move directly away from each other, perhaps they were simply nearby at the beginning, not in a singularity (a point with infinite density). They created many more models, and found that there were examples for both sides of this argument.

Roger Penrose showed that if stars collapse in on themselves, they can become singularities. Stephen Hawking took up this idea and applied it to the big bang theory. Hawking believed that if the universe is infinite and expanding too fast to recollapse, it should have started at a singularity. However, he’s since changed his mind when taking quantum mechanics into account.

Over the years, we’ve gained a better understanding of how things work. Penrose and Hawking’s research showed that Einstein’s general theory of relativity doesn’t apply to the beginning of time. It breaks down when you look at extremely small objects like particles. As such, their focus turned from large objects to tiny ones like atoms.

Chapter 4

In the 1800s, Marquis de Laplace predicted that science would eventually be able to predict everything if scientists could figure out the total makeup of the universe. However, this idea was unpopular among those who believed God should have free will.

Max Planck suggested that light was emitted in quanta. This would mean the total amount of energy released would be finite, even though it seemed infinite. However, there were still objections to this theory because it didn’t explain how these quanta worked or why they existed.

Werner Heisenberg used the theory of quantum physics to create his uncertainty principle. To measure a particle’s position and velocity, one must shine light on it because you can only see something when there is light being reflected off of it. The shorter the wavelength of that light, or higher its frequency, the more accurately you will be able to see where that object is because shorter wavelengths mean greater accuracy in seeing things. However, shining a higher frequency light means applying more energy to that object and therefore changing its position or velocity. Therefore, using high-frequency lights will result in less certainty about an object’s location or speed than if lower-frequency lights were used to observe them. This is fundamental law of nature.

Laplace’s idea of determinism was not perfect. There is still a place for God in it, but he does not help us understand how he works. Instead, we should leave out theories that are beyond human understanding.

Quantum mechanics was created by Erwin Schrodinger, Paul Dirac and Heisenberg in the 1920s. It is based on the uncertainty principle, a theory that does not predict definite outcomes but potential outcomes. This theory works on probability and randomness. Einstein said “God doesn’t play dice,” yet quantum mechanics has been shown to be very effective with observations and it underlies all of modern science, including electronic chips. The only area of science that has not yet been integrated into this theory is gravity and the larger structure of the universe.

Einstein proposed that light is both a wave and a particle. Heisenberg’s uncertainty principle stated that particles can also act like waves, spreading out their movement according to probability. This means scientist must consider the interference of these waves, where one wave cancels another out when they meet at a peak or trough. The same effect creates colors in bubbles because the waves overlap each other and strengthen or cancel each other out.

If particles can be waves, then they will cancel each other out. If light passes through two slits onto a wall behind it, the light from each slit travels different distances to reach the wall and overlap with another’s peak or trough. This creates a fringed pattern of light on the wall as if there were interference between multiple beams of light. The same happens when one particle passes through one slit at a time: it still appears as though there was interference even though only one particle passed through that slit.

This has helped scientists to understand how atoms work. Scientists initially thought that electrons orbited around the nucleus like planets orbiting a star. However, they wondered why it didn’t collapse into itself. Niels Bohr suggested in 1913 that electrons could only orbit at specific distances and could not move between orbits because of this. If the electron’s wavelength was not whole number multiple of another wave, then it would cancel out on its way around the atom.

Richard Feynman created a theory to explain how particles move. He said that they take every possible path from A to B, and then add up all the wavelengths of those paths, which cancel each other out. This allows us to predict particle movement mathematically. However, in practice it is too difficult for anything more than simple atoms or molecules.

Einstein’s general theory of relativity is considered a classical theory because it does not include quantum mechanics. This doesn’t lead to inconsistency, though, as gravitational forces are so weak compared to other forces. But gravity would be much stronger in black holes or at the big bang and needs to be integrated into quantum mechanics. Scientists also know some properties such a unified theory would have, as well as areas where it will have the greatest significance.

Chapter 5

Aristotle thought the universe was comprised of earth and water, which tended to sink, and wind and fire, which tend to rise. He believed that matter was continuous. Later on in history, Democritus came up with the idea of atoms. The debate over whether or not they existed wasn’t settled until 1803 when John Dalton discovered molecules. In 1905 Albert Einstein explained how dust particles move around in liquid by showing that it’s caused by collisions between those two objects – atoms (as shown by Thompson) and electrons (shown by Rutherford).

James Chadwick discovered the neutron, which has no charge. He later won a Nobel Prize for his discovery of this particle.

In the mid 1900s, Murray Gell-Mann discovered quarks and won a Nobel Prize for his work on them. There are six “flavors” of quark: up, down, strange, charmed, bottom and top. Each comes in three “colors”: red (or green or blue). Quarks form protons, electrons and neutrons. Scientists can create other particles from quarks but these are unstable.

The question of what the smallest things in existence are did not have a definite answer. Scientists determined that they would need to do an extensive study to find out how small they could see, because particles cannot be seen at all with larger wavelengths. If we were able to look at higher energies, though, and particles had smaller wavelengths as energies increased, scientists might figure it out sooner or later. Today’s technology is advancing rapidly enough that scientists think they already know the answer: this is what the tiniest thing in existence looks like.

Particles have a property called spin, which can be used to describe their shape. A particle with no spin is like a dot—it looks the same from any angle. A particle that has one half of a turn is an arrow and must make one full rotation before it looks the same from the original viewpoint. Particles that have two turns are like screws and must make two full rotations before they look the same again. All matter particles belong to this group, including electrons, protons, neutrons, photons etc. Forces between these particles are created by other spinning particles such as photons (light) or gluons (strong force).

Wolfgang Pauli won the Nobel Prize for his discovery of the Pauli exclusion principle, which states that two matter particles cannot exist in exactly the same space going at the same velocity. This is what gives structure to our universe and prevents it from collapsing into a soup. He also showed how spin ½ works mathematically. In addition, he predicted that electrons should have partners called positrons or anti-electrons, which later led to his Nobel Prize.

Forces are carried by force particles. Matter particles emit the force-carrying particles, which then change the particle’s course from the recoil resulting from emission. The force-carrying particle is then absorbed by another matter particle, also affecting its velocity. These force carrying particles do not obey Pauli exclusion principle and can build up to become bigger forces. Their range depends on their mass (which cannot be detected directly).

Scientists have noticed that there are forces out there. They can be seen in the form of waves, such as light or gravity. Scientists hope to combine these four types of force into one single force, which will explain everything we now see around us.

The first force is gravity. Every particle experiences it, depending on its mass and energy. It’s the weakest of the four forces, but unlike other forces, it acts over great distances. Large objects can create a larger gravitational pull than others can. A spin two particle called a graviton carries this force because it has no mass; therefore, it travels long distances with ease. The earth orbits the sun thanks to these particles that make up gravity—gravitons are virtual particles since they don’t have any mass and travel great distances easily in space.

Another force is electromagnetism. It affects only electrically charged particles, such as electrons and protons. Electromagnetic forces are stronger than gravity, and they can attract or repel each other depending on their charge. Opposite charges attract each other while matching charges repel one another. This force arises from the exchange of photons (particles of light). We see these photons in everyday life as light, which we capture with cameras to make photographs.

The third force is the weak nuclear force. It acts on matter particles, but not force-carrying ones. In 1967, Steven Weinberg and Abdus Salam both put forward ideas that linked this force with the electromagnetic one. They said three types of particle called massive vector bosons carried this weak nuclear force, and are spin-1 particles. These particles only seem different at low energies, and at high energies they all act the same; at low energies it looks like there are 37 slots to fall into but at high energies they travel in circles around those slots. This is called spontaneous symmetry breaking when these forces take on higher masses and shorter ranges as their energy levels increase; they break their symmetry from being a variety of forces acting in different ways to becoming just one unified form as their energy level increases—like a roulette ball which has 37 slots to fall into until its spinning speeds up so much that it travels around a circle instead of falling into any slot—at high enough energy levels (the speed of light) it no longer falls into any slot but travels round them instead.

There are four main forces in the universe. The strong nuclear force holds together atoms, and is carried by gluons (particles with a spin of 1). Gluons interact only with quarks and other gluons. This force exhibits confinement, meaning that particles that form an object must add up to a white color (one each of red, green, or blue) so that they can be stable. For example, mesons are unstable because they have no color—they’re made when quarks join with antiquarks—and therefore cannot be stable. Quarks and gluons can’t go around alone; otherwise they’d become unstable due to their color.

The strong nuclear force, weak nuclear force and electromagnetic force are all different types of the same phenomenon. The idea is that at high energy levels, the strong nuclear force would weaken and the other two forces would strengthen to become equal.

The grand unification energy is not known. Scientists can’t test it because an accelerator that would have the power to do so would have to be as big as the universe. However, scientists can test low-energy outcomes of this theory by seeing if protons decay into smaller particles, but they haven’t observed this yet.

The universe may have been made of anti-matter and matter, which could have annihilated each other. If that were the case, there would be a lot of radiation where particles meet anti-particles, resulting in very little matter left over. This is why we see an imbalance between matter and anti-matter today.

There are certain kinds of symmetries that the laws of physics can obey, such as symmetry C, which means that particles and anti-particles behave in the same way. Another example is symmetry P, where things look like their mirror image. The third kind is T for time reversal.

In 1956, Tsung-Dao Lee and Chen Ning Yang found that the weak nuclear force does not follow symmetry P. Chien-Shiung Wu proved this by causing radioactive atoms to spin in a magnetic field: some electrons were given off one way then another. Lee and Yang won the Nobel Prize for their idea. The weak nuclear force also does not follow C or CP symmetries, meaning it would cause a universe of anti-particles to behave differently than our own. However, it does obey combined CP symmetry—meaning an antiparticle universe would develop in the same way as our own. Any theory that obeys general relativity must also obey quantum mechanics’ CPT symmetry—swapping particles for anti-particles and taking mirror images but not reversing time like Cronin did; therefore, physics do not appear to follow T (reversing time).

As the universe expands, it cools. Forces that don’t obey symmetry cause more antielectrons to become quarks than electrons to become anti-quarks, thus creating matter. Gravity is a weak force but builds up over time and eventually causes stars to collapse into black holes. Black holes are where general relativity and quantum mechanics come together.

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Chapter 6

John Wheeler coined the term black hole in 1969 to describe an idea that had been around for centuries. People argued whether light was a particle or a wave, and how gravity would affect it. Newton’s laws of motion showed that it could be both. Roemer proved that light has a specific speed, rather than traveling infinitely fast, so gravity can have significant effects on its movement.

John Michell suggested the idea of black holes in 1783, but he didn’t use that name. He said that any star with a strong enough gravitational pull would trap light and make it impossible to see. People thought this was too ridiculous, so they ignored his theory for a long time.

Although light is affected by gravity, it doesn’t act like other matter that’s pulled in by gravity. For example, cannonballs fall to the earth because of gravity. However, unlike a cannonball or any other object that falls to the ground due to gravity, light does not change speed when it passes through different gravitational fields. Einstein’s theory of general relativity explains why this happens and how objects are affected differently than light when they travel through space with varying gravitational fields.

Stars are formed when large quantities of gas begin to collapse under their own gravity. Once this happens, the atoms collide more frequently and release energy. Eventually, it becomes so hot that helium is produced as a by-product of the collisions. This pressure balances out the gravitational pull and halts further contraction; however, after a star runs out of fuel, it collapses again because there’s no longer any pressure to offset its pull.

While traveling to England in 1928, Subrahmanyan Chandrasekhar was able to work out how big a star needs to be in order for it not collapse under its own gravity. He did this by using the Pauli exclusion principle, which states that matter cannot occupy the same space as other particles. This causes nearby particles to repel each other and balance against gravity.

The Pauli exclusion principle states that no two electrons in an atom can occupy the same quantum state. However, this only applies to subatomic particles and not to larger objects such as stars. As a result, smaller stars are supported by the exclusion principle between protons and neutrons; however, larger ones will collapse under their own gravity because they cannot be supported by the exclusion principle acting between protons and neutrons.

When a star is above the Chandrasekhar limit, it will encounter problems. It might explode or become a black hole. If it doesn’t go supernova, it will collapse into infinite density and ultimately become a black hole anyway.

In 1939, American scientist Robert Oppenheimer proposed a theory that light is affected by the gravitational pull of stars. He theorized that nothing can escape from within this area of influence and it’s called a black hole. This idea was proven at least in part in recent years with technology.

Time is relative. This means that time will move differently for different observers depending on where they’re standing and what’s happening around them. For example, if an astronaut sent signals to the distant observer every second until the star contracted past a critical radius at which nothing could escape, by 11am (the last signal before 11am), it would take an infinite amount of time for those signals to reach the distant observer because no light can escape from black holes. The redder and redder the light gets as it approaches this point, the farther back in time we are approaching. Finally, at 11am, there is no more light being emitted from this star so it appears as a black hole. However, since we know that gravity exists even after something becomes a black hole (and also because space ships still orbit these objects ), we know that everything works out just fine despite our perception of things changing drastically when something passes through its critical radius.

Because gravity is strongest at the center of a star, an astronaut would be torn apart as he or she approached the surface. Galaxies also collapse in similar ways.

Penrose and Hawking discovered that there must be a singularity of infinite density at the center of black holes. This is similar to the big bang, but it’s actually what happens inside black holes. The laws of physics break down in these places, so no one can see into them or escape from them. Penrose called this idea that breakdowns are hidden “cosmic censorship.”

Some options within general relativity allow the astronaut to travel through a wormhole and reappear elsewhere in the universe. However, these are unstable and unpredictable, probably destroying him in the process. The stronger cosmic censorship theory states that time ends when he reaches a singularity so it is always at his future because once an object crosses the event horizon of a black hole, there’s no way back out again.

According to general relativity, moving objects produce gravitational waves that bend space-time. These waves carry energy away from the object producing them and slowly make the object lose energy until they eventually stop moving.

The process of neutron stars colliding is too slow to see on earth. However, two neutron stars are orbiting each other in a system called PSR 1913 + 16. Two men were awarded the Nobel Prize for this discovery. The outcome of the collision will be that these two rapidly spinning bodies will collide in 300 million years.

When stars collapse, they do so rapidly. The shape of the final type of object is unclear. A black hole may look like a sphere and be identical for all, according to Werner Israel’s analysis of Karl Schwarzschild’s solutions to general relativity equations.

It was previously thought that black holes could only be perfectly spherical. However, Penrose and Wheeler proposed that gravitational waves from a collapsing star would make it spherical. Later, Kerr extended this to rotating black holes as well. Carter helped prove these solutions in 1970 by showing that if the rotation axis of a black hole is symmetrical, its size and shape depend only on mass and rate of rotation. Hawking later proved this for stationary rotating black holes too. This means any type or amount of matter can become a black hole, meaning there are many more than we originally thought possible.

Black holes were predicted to exist before they were found. In 1962, Maarten Schmidt discovered a quasar, which is an entire galaxy falling in on itself. In 1967, Jocelyn Bell-Burnell and her supervisor Anthony Hewish discovered pulsars, which are rotating neutron stars. These findings gave hope to the theory of black holes. However, at first people thought they might be alien signals.

It’s hard to imagine something we cannot see, being real. However, Michell (a scientist during 1883) believes it is still possible to measure a black hole’s gravitational effects on the material around it. Only one observation exists where stars are orbiting some sort of invisible object and that object has more mass than our sun can have before collapsing into a white dwarf or neutron star. That evidence points towards the possibility of a black hole being real and having enough mass to be larger than 1. 4 solar masses (the Chandrasekhar limit).

Since then, more black holes have been found. There could be as many of them as there are stars in the sky. The extra mass would explain why our galaxy rotates. There could also be a very large black hole at the center of it that’s even larger than we thought before. Even larger ones may lie at the center of quasars and create jets of particles after they consume matter from objects orbiting around them.

There could be smaller black holes that are the result of large amounts of external pressure. This would have happened in the early stages of the universe, when it was still hot and dense. Scientists can determine whether they exist by studying those areas where these black holes would have formed—they can find them if they exist, which will help scientists understand how the early universe behaved.

Chapter 7

Before 1970, Hawking’s work was mainly focused on the big bang. Around his daughter’s birth, he thought about black holes and their event horizon. He realized that trapped light in the event horizon could not cross paths or fall into a black hole. As such, all of the light at an event horizon must be moving parallel to or away from every other ray so that it can stay stationary or grow without crossing paths with other rays.

Because black holes never decrease in size, we can determine their area by using the event horizon. This sounds like entropy because it’s not decreasing. For example, gas particles will spread out of a half-full box when you remove the partition that separates them inside from outside the box. The most probable outcome is for things to spread and become more disordered.

In order to measure a black hole’s entropy, we can look at its event horizon. As matter falls into the black hole, its event horizon will expand and increase in size. The area of the event horizon plus the outside entropy will not decrease over time as more matter is pulled into it.

A scientist named Bekenstein made an interesting discovery about black holes. He noticed that they had a temperature and he theorized that this was because they were emitting radiation. However, the law of entropy states that energy can’t be created or destroyed; it merely changes forms. Black holes are meant to absorb things, not emit anything. Hawking and some other scientists wrote a paper in 1972 challenging Bekenstein’s findings as well as his misuse of some work by Stephen Hawking himself. In the end, though, it turned out Bekenstein was right about black hole emissions after all.

Hawking went to Moscow in 1973. He met with some Russian physicists, who convinced him that black holes should emit particles when they rotate. Hawking later did the math and found out that even non-rotating black holes should also emit radiation. But he didn’t want Bekenstein to find out.

Hawking finally agreed with the idea because black holes emit radiation that’s the same as any other object at a similar temperature. Other scientists have since confirmed these results and Hawking now believes that black holes obey entropy.

In fact, particles emitted by the black hole do not come from the black hole itself. Instead, they’re produced outside of the event horizon in what is supposedly empty space. This space isn’t actually empty—there are certain minimum fluctuations and uncertainty. Pairs of particles and virtual particles will appear and collide with each other, annihilating each other into a single new particle that has positive energy (if it’s real) or negative energy (if it was a virtual particle). The real particle always has positive energy under normal circumstances but if its potential energy increases beyond a threshold so does its mass-energy content; this means that it can have negative energy. A negative-energy virtual particle could fall towards the event horizon where it becomes a real particle that no longer needs to annihilate with its partner. Both might now fall into the black hole, or the former-virtual positive-mass one might escape while leaving behind only its antimatter counterpart as evidence of an emission having occurred at all. Smaller black holes emit more radiation because they’re easier to escape from than larger ones: their gravity is weaker which makes them less able to hold onto their emissions’ partners for long enough before they too must be emitted again as new pairs form outside of them

If energy is released from a black hole, it could balance the negative energy that’s falling in. This means that as more mass is lost by the black hole, its event horizon will shrink, which would reduce entropy within the black hole proportionally. As a result of this contraction and increase in entropy outside the black hole, it heats up and releases even more energy as it shrinks even further. Eventually, all of its mass will be evaporated into radiation after an explosion.

Black holes are much colder than the rest of the universe, so they will take a long time to emit more energy than they absorb.

Black holes from the early universe would have been much smaller, due to irregular pressure rather than their own size. They would also be hotter and more powerful. Today’s scientists can’t harness them yet, but in the future they could provide immense power for us to use. It would be like having an atom-sized mountain that we could tow into space and then orbit around Earth as a source of energy.

Scientists can measure background gamma radiation in the universe to determine how many early universe black holes there are. The evidence suggests that they are quite rare, so it’s unlikely we’ll find one near Pluto.

If a black hole were to explode near Pluto, it would be detectable. But since the time for such an event is 20 billion years, we don’t need to worry about one happening right now. However, if you look at around 1 light-year away from us in any direction, you’ll see that these events are common throughout the universe or just outside of our galaxy. Even though we may not actually pinpoint where they came from in space and time (the early universe), we can still learn a lot by looking at them and seeing what’s present today.

Stephen Hawking, as well as other scientists, theorized that black holes emitted radiation. However, John G. Taylor disagreed with him and said that the two great theories of general relativity and quantum mechanics could not be combined together. Eventually though, everyone agreed to the idea of combining these two theories if they were correct in their predictions about black holes emitting radiation.

A new idea about black holes suggests that they don’t disappear after absorbing all of the surrounding matter and energy. Rather, when a black hole becomes very small, it will simply disappear. It was believed that quantum theory undermined the idea of singularities, but Hawking’s work turned in that direction in the late 70s by focusing on Feynman’s sum over histories.

Chapter 8

Albert Einstein’s theory of general relativity predicts that space-time began as a singularity in the big bang, and ends in the potential big crunch singularity when everything collapses back in on itself. However, applying quantum mechanics to black holes might change our understanding of these phenomena.

Stephen Hawking was interested in the origin and fate of the universe. He had a discussion with other scientists about this topic at a conference in Vatican City. At the end, Pope Francis met with them to discuss their findings, but he didn’t know that Stephen Hawking’s talk focused on a no boundary finite universe. The Pope warned them not to ask too many questions about how things began because God created everything.

In order to understand the hot big bang model, one must first understand the Friedmann model. This is a type of universe in which matter cools as it expands, so particles have lower energy and are more likely to clump together. When they collide at high temperatures, there’s an increase in particle production. As matter cools down, fewer particles are created because they’re more likely to annihilate with their corresponding anti-particles.

When the universe was created, it was infinitely small and extremely hot. The universe would have been made up of photons, electrons, anti-particles and neutrinos at that time. As the universe cooled down, electron pairs annihilated each other faster than they were being produced. This led to more photons forming as a result of those annihilations.

One hundred seconds after the big bang, protons and neutrons would have formed into heavy hydrogen atoms. This was first proposed by George Gamow and Ralph Alpher in a 1948 paper.

Gamow and Alpher said that radiation from the first hot stage of the universe should still be present in the universe today. Penzias and Wilson found this to be true, which means that scientists can have a fairly accurate picture of what happened after the first few seconds following the big bang. After those first few hours, production of new elements would have stopped, though expansion would continue.

As the temperature of the universe decreased, it became easier for particles to stay together. This led to the formation of atoms and eventually galaxies. Galaxies stopped expanding when they reached a certain size, which made them spin faster as they contracted.

After a while, clouds of gas would form. They’d get bigger and bigger until they were dense enough to start collapsing due to gravity. As the gas collapsed, it got hotter and hotter because of all the collisions between atoms in the cloud. Eventually, some of these collisions turned hydrogen into helium through nuclear fusion reactions. This created heat and pressure that balanced out gravity’s pull on the gas clouds so they stopped contracting.

Stars are formed when huge clouds of gas and dust collapse. They can continue to burn in their core for a long time, emitting heat and light. When the fuel runs out, they contract into dense regions that become neutron stars or black holes. This is not completely understood yet because some parts of the star may be blown off, flinging heavier matter out for the next generation of stars or planets to form from it.

The earth was very hot when it first formed, but then it cooled and gases from the rocks were released into the atmosphere. These gases created an atmosphere that allowed primitive life to form in the oceans. Small errors in reproduction led to new genes that gave those organisms a better chance of survival. This process eventually led to more complex organisms, including humans.

The observable universe is what we can see today, but it doesn’t answer the question of how the early universe was so hot and why it’s so uniform. It also doesn’t explain why there are regions where galaxies exist. General relativity cannot explain these things because its laws break down when dealing with singularities (the big bang). This gives us a beginning to time at the big bang. God left us rules for creating the universe in accordance with quantum mechanics, but how did he decide on those rules? We could say we will never know his intentions for creating a set of rules that determine our existence. However, if God is all-knowing and powerful enough to create this universe from nothingness, then he should be able to understand more about our world as scientists do more research.

One explanation is that the universe is infinite. It’s hard to imagine how an irregular early universe would become more uniform, and it seems like there should be more black holes from that time period.

Even though the universe was chaotic early on, some regions of it could have smoothed out to create a more uniform place. We may just be living in one of those smooth places that support intelligent life like us. This is called the anthropic principle, where we exist because only certain parts of the universe can sustain intelligent life. Some people take this further and propose what’s called strong anthropic principle stating that it will seem like these laws were chosen on purpose for us to exist here.

There are many scientific laws that seem to be perfectly calibrated for supporting life. Scientists cannot predict them yet, but they might one day. Or perhaps our universe was created with these laws in mind so we could live here. It’s remarkable that there would only be a narrow range of possible configurations of the universe where life can form. This seems like divine purpose or the strong anthropic principle at work in our world and universe.

There are many challenges to the strong anthropic principle. First, other universes cannot be detected, so they don’t need to be part of our theories. Second, different laws in other regions of the universe would prevent us from moving between them and there is no basis for assuming that these other universes exist for us.

We need to know the make up of the early universe in order to answer some questions. The hot big bang model says that there was not a uniform temperature throughout the universe, so it seems like heat didn’t have time to move around and create an even temperature. This could be hard to explain other than saying that this is how God wanted His creation (the universe) to be.

To explain how the universe started and developed into what we see today, Alan Guth said that it could have expanded rapidly in the beginning. This idea is called inflation, which states that particles had enough energy for the strong nuclear force to be unified with electromagnetic force. As they cooled down, these forces broke their symmetry from each other.

However, Guth proposed that water could skip the freezing process and remain liquid. He suggested that maybe forces in the universe could also avoid symmetry breaking as it cooled down. This would give extra energy to these areas of space, which would expand faster than other parts of the universe due to repulsion from gravity.

Guth’s model, which states that the universe expanded faster in the past than it does now, allows for time to pass so light can travel across the universe. This means different parts of space are at the same temperature and composition. Moreover, this could explain why there is so much matter in our universe without assuming divine input into its creation. Also, Guth’s theory would account for why there is no energy in empty space because positive charges balance negative charges. All matter has positive energy and thus repels other matter; at the same time gravitational force attracts all particles towards each other as they try to escape its pull.

If the universe was twice as big, its energy would still be zero. In the inflationary model of the universe, it expands quickly and has a constant amount of energy. But in today’s expanding universe, there is less total energy available to particles because space is growing faster than matter can create new particles.

Eventually, the strong nuclear force and the weak nuclear force will become symmetrical with each other. In turn, that leads to a slower expansion rate of our universe. This explains how we have come to be in a uniform state despite chaotic beginnings.

Guth’s original theory said that bubbles of matter were moving at different speeds and would eventually slow down enough to join together. Hawking pointed out that if the bubbles moved too fast, they wouldn’t be able to join up. Andrei Linde suggested that our universe could be one of these bubbles. Hawking later showed Guth’s idea couldn’t work mathematically but encouraged Linde to pursue his research anyway.

Paul Steinhardt and Andreas Albrecht proposed similar ideas to Linde’s at a similar time, and are given credit with him for the new inflationary model. The idea was based on slow-breaking symmetry. However, the theory has been largely discredited by physicists because we shouldn’t see more differences in background radiation than what we do now. In 1983, Linde put forward his chaotic inflationary model that said there would be spin-0 fields in certain regions of the early universe that would have large values due to quantum fluctuations. This energy would have anti-gravity effects like a cosmological constant, increasing the rate of expansion of space. The energy slowly decreases over time as it is converted into matter via particle interactions (i.e., atomic nuclei). One such region could be our observable universe; this region has enough mass density so that gravity pulls all its contents back together after an expansion phase or two and then stops expanding forever (the big crunch) before starting another cycle (big bang).

A new model for working out the beginning of our universe that’s more accurate leaves room for what could have possibly happened, meaning we might not need to give up on the anthropic principle.

To understand how the universe started, we need to use quantum mechanics. However, there is no consistent theory that combines quantum mechanics and gravity. If there were a theory that combined them, it would rely on Feynmann’s sum over histories proposal which states particles move from A to B by every possible path. Scientists know how to measure this but actually doing the math requires using imaginary numbers (i). They are normal mathematical tools that allow us to multiply two unlike numbers together and get a negative number (-2 times -2 = 4; i times i = -4). The problem with real numbers is they can only go left or right on an axis (0-9), while imaginary numbers can also go up and down (3i).

To calculate the sum of something over time, we must use imaginary numbers. This device allows us to treat space and time equally. It’s a mathematical tool because it helps us solve problems in math. Therefore, this is not really used in physics but rather is just a way to do calculations that result in answers that are easier for people to understand.

Another feature of the unified theory of quantum mechanics and general relativity is that gravity is represented in a curved space-time. Applying the sum over histories to Einstein’s ideas on gravity, one adds up all the wavelengths for all possible particle histories representing that universe.

Quantum and general relativity theories are based on the idea that if we know everything about the universe at one point in time, then we can predict what will happen in the future. However, there are different possibilities with regard to how long the universe has existed. In quantum theory, it’s possible for a finite amount of time to be infinite without a beginning or end. This means that there would be no singularities (theoretical points where matter and energy become infinitely dense). There would also be no need for God because science wouldn’t break down as predicted by general relativity theory.

Stephen Hawking first presented this theory at a Vatican conference, but it was not well understood. He worked with others over the summer to develop the idea further and then presented it again in England. The theory remains a proposal because of its complexity; making predictions based on this theory is difficult.

There are many possible histories of the universe. The anthropic principle can explain why one history is right rather than others—we know we exist, so life must be involved in the model. But it would be better to know which history is most probable.

The history of the universe is not random, but rather moves in a way that can be predicted. The lines on maps are similar to the history of the universe because they get bigger and smaller depending where you start from. For example, if you look at these lines on a map going northward (going back in time), then moving toward the equator (the maximum size), and finally southward again, it’s like looking into our past as we go further back in time.

It’s possible that singularities don’t exist in imaginary time. This would be a reversal of Stephen Hawking’s earlier work. But, it was discovered that singularities only appear to be present in real time and are better explained by quantum theory. Singularities may not even exist at all, but we can say they do because our real time is just another way to explain what we see with imaginary time.

By looking at different probabilities that lead to our current universe, we can predict what characteristics of the universe were likely to happen together. For instance, it’s logical (and correct) that this “our” universe was expanding uniformally and quickly before its first moment in history when light became visible. This is backed up by Penzias and Wilson’s discovery of background radiation which proved all energy is homogeneously spread out.

There was a large uncertainty in the early universe regarding its size. It wasn’t exactly how we see it now, and some of its characteristics were determined then: galaxies, space, distance between planets. The more diverse things are, the bigger their differences are from one another – that is an important principle in our post-universe conception. People can figure out why these principles were missing back then to understand what has been done so far and predict further research needs for understanding the very beginning of all time itself.

By stating that space-time has an edge or a boundary, it seems to eliminate the role of God. Consequently, discoveries made by humans about nature and how it works don’t need a creator because they’re found naturally in this world. The question then is: if there was no beginning where did the universe come from?

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Chapter 9

Early in the 1900s, people realized that time is relative to each person. The speed of light doesn’t change based on who’s observing it, so everyone can agree when an event takes place if they have a good clock. But then physicists figured out that imaginary numbers are just as valid as real numbers and began using them in their equations. Imaginary time is like space because you can move back and forth through imaginary time but not forward or backward through real time.

Scientific laws obey the symmetry of C, P and T. The first refers to particles acting as anti-particles do. The second refers to their mirror image. T means reversing time. Scientific laws will be the same in the symmetry of C and of P, meaning that mirror-image people would live in a universe that’s a flip-side version of ours—our mirror images would live in a world identical to ours except everything is inverted left for right (a reflection).

But if you ran time backward, the laws of science would not be the same. For example, a glass falling off a table and shattering wouldn’t reassemble itself back into order. This is because entropy—or disorder in any system—would increase as time went on. A smashed cup doesn’t reassemble itself back into order.

Entropy is a concept that’s defined in the second law of thermodynamics. It directs the first arrow, which is thermodynamic and describes how energy dissipates over time. The second arrow is psychological, as it describes our sense of time passing us by. The third and final one is cosmological—it determines the direction of the universe’s expansion. These three arrows determine what we perceive to be “time.”

Hawking said that the no boundary universe model and the weak anthropic principle explain why these three arrows point in the same direction, as well as why they exist at all. The cosmological arrow will not always point in the same direction, but it does right now. When this happens, there is a suitable environment for life forms that can ask questions about these arrows pointing in a certain way (the anthropic principle).

Thermodynamics is the study of heat and energy. It involves a system in which things are more likely to become disordered over time, rather than ordered. Imagine a jigsaw puzzle box that has all the pieces in place to form an image on the front of it. The more you shake it, the more likely it is for those pieces to fall out and become disorganized. However, if disorder decreased with time instead of increased, broken glasses would repair themselves or jump back onto tables by themselves.

The human mind creates more order internally, but it also emits energy outward. The combination of these two things makes the psychological arrow of time almost trivial. Humans measure and remember time in the direction that disorder increases.

General relativity is a theory that helps describe the universe. This theory cannot tell us what happened at its beginning, because it breaks down when dealing with singularities (points where gravitational pull becomes infinite). The universe might have been smooth and ordered at first, but it also could have been lumpy and disordered. If this were the case, thermodynamics would point in the opposite direction from cosmology; however, we observe them pointing in the same direction. A quantum theory of gravity is needed to understand how everything began rather than guessing about it.

The no boundary principle says that the universe is infinite and uniform, with matter expanding throughout it. After a period of inflationary expansion, regions slow their expansion and begin to clump together into stars, galaxies, planets and people. In this way disorder increases as the universe expands.

Then, the question arises as to whether disorder decreases as the universe begins to collapse. Hawking believed that it would reverse its thermodynamic arrow.

Stephen Hawking initially thought that the big bang was a singularity, but his colleague pointed out that it could be different. Also, Raymond Laflamme discovered how the contracting phase would look very differently from when the universe expanded.

When Hawking admitted his mistake, he did so because others refused to admit theirs. Einstein gave a better example when he called the cosmological constant the greatest regret of his life.

Hawking had to admit his mistake. When Eddington opposed black holes, he did so because he could not admit a mistake. Others often pretend they had never made the mistake in the first place, and pretend it never happened. But Einstein gave a better example when he called the cosmological constant the greatest regret of his life.

Hawking wondered why the universe is expanding. If disorder always increases, then why isn’t it contracting? The answer lies in the anthropic principle, which states that there must be a point when contraction cannot happen because all stars would have burned out and life wouldn’t exist.

In the early stages of the universe, there would be no thermodynamic arrow because it was in a state of almost total disorder. However, intelligent life requires an order to break down food and create heat. The expansion doesn’t drive disorder but rather creates a condition that allows for intelligent life to exist.

The laws of science don’t distinguish between the past and future, but they do apply to time. Our minds create order out of disorder by creating memories that will help us in the future. However, our bodies generate heat that increases disorder in the world many times over.

Chapter 10

In the previous chapter, time was like a railway track. You could only move forward but never back. However, maybe it is possible to travel back in time by going through loops or branches on that line. Time travel has been portrayed in many sci-fi movies and books but has not yet become a reality; however, with recent technological advancements, it may happen soon.

Kurt Gödel suggested a new model of space-time in 1949 under general relativity. He said the whole universe was rotating in one direction. Einstein didn’t want this to be part of his theory, as it allowed for time travel. This also doesn’t match observation, as observation shows that the universe is not rotating.

Some space-times allow for time travel. These include the interior of a rotating black hole, or if two cosmic strings move past each other really fast. Cosmic strings formed in the early universe as a result of symmetry-breaking and are under such tension that they can propel vast objects at high speeds in milliseconds when they straighten out.

A solution to the Gödel theorem and cosmic string theory suggest that time travel is possible. However, there’s no evidence that God created such a chaotic reality. The universe appears uniform and flat in all directions, suggesting that it was not so curved near its beginning to permit time travel.

When it comes to space travel, time is relative. It takes less time for those traveling through space compared to those on earth. However, that’s not the best way to go about it because everyone back home would be dead by the time you returned. But if you could travel faster than light, then you could return before leaving!

If a spaceship travels from point A to point B below the speed of light, different observers would have different measurements and disagree on which happened first. It may even be possible for an observer to travel back from B before event A ever occurred.

But in order to travel faster than light, you need a lot of energy. So much so that it’s impossible for rockets to achieve the speed required. Perhaps we could use some sort of warp drive or wormhole technology to allow us to get from point A to point B much quicker; this would effectively allow time travel into the past.

Albert Einstein and Nathan Rosen were the first to suggest that wormholes could exist. They are also known as Einstein-Rosen bridges. Wormholes are unstable, but advanced civilizations might be able to stabilize them by using negative energy density.

Quantum theory allows a negative energy balance in certain areas as long as the universe’s overall balance is positive. Scientists have detected virtual particles from observing the different pressures applied to metal plates created by discrepancies of the density of virtual photons between and outside the plates. Within the plates, there are fewer photons because their wavelengths must be whole numbers or they would cancel each other out when hitting a trough or crest outside. Outside, where more photons exist than within, an inward pressure exists due to higher photon density. The cavity within has zero energy (the normal conditions), while what’s inside has negative energy (a warp in space-time). These observations show that space-time could be warped.

It’s possible to travel through time, but we haven’t met anyone from the future yet. It could be that it’s not physically possible for us to meet someone who has traveled from the past or present and changed history. This doesn’t explain why changing the past results in paradoxes because of contradictions if you can change history. One explanation is called consistent histories, which says everything must be consistent with physics laws. You would only travel into the past if you had already done so according to history books. The idea comes down to free will versus determinism—if there really is a unifying theory of physics, then human behavior might be determined by this theory and thereby negate free will.

Another explanation is that there are different histories, or timelines, in which the time traveler goes back to his or her own timeline.

Richard Feynman’s theory of sum over histories allows for time travel on a very small scale. As particles follow the C, P, and T symmetries (C symmetry refers to charge conjugation—exchanging all particles with their anti-particles; P symmetry refers to parity—swapping left with right; and T symmetry refers to time reversal), they can be considered an anti-particle traveling forward in time when going backward in time. For example, black holes emit particles as one component of a particle/anti-particle pair escapes while its partner falls into the black hole. The former appears to be created by the latter escaping from the black hole. It could also be described as an anti-particle traveling back in time out of the black hole.

One idea, called the chronology protection conjecture, suggests that space-time curves so much that it prevents time travel. It is not known if this idea holds any truth to it or if there are other ways for time travel to be possible. Scientists have not yet proven whether this idea is correct or incorrect and more research needs to be conducted on the topic.

Chapter 11

It is difficult to explain everything in the universe with one theory. There are many theories, but it would be better if there was a single theory that covered all aspects of physics and didn’t have to make up certain numbers or ideas for some things. This mission is called the unification of physics.

Einstein tried to find a theory of everything, but he didn’t have enough information about nuclear science. He also refused to accept quantum theory, despite contributing to its creation. The uncertainty principle is the basis for quantum theory and must be part of any unified theory.

When discoveries are made, they often lead to more questions being asked. For instance, Max Born said that physics was over after Paul Dirac’s discovery of the electron. However, the discovery of the neutron and nuclear forces led to even more questions. Even so, science is still progressing towards an answer.

Previous chapters have covered general relativity and incomplete gravity theories. However, these do not include the gravitational force. To incorporate it into GUTs, we need to combine them with quantum mechanics. This has already resulted in significant rethinks about black holes (e.g., they’re not entirely black) and that the universe is infinite (i.e., it doesn’t have edges). The problem is that under the uncertainty principle there are technically infinite numbers of particles, which add infinite mass to the universe, and so curve space-time into an infinitely small size.

Mathematical infinities can be canceled out by introducing other infinites. However, this means certain values have to be chosen from observation because the theories themselves don’t predict those values. The predictions match with observations only if you adjust some of their parameters.

In the 1970s, a possible solution was offered to solve this problem. It involved combining particles with different spins into one superparticle that unified matter and force particles. However, the calculations to see if there were any infinities left over were too difficult to do.

1984 Particles were thought to be points in space, but string theory changed that. String theory suggests particles are very thin lines with one dimension and can be either open or closed strings. Particles occupy a single point at any given time, while strings have two-dimensional histories called world sheets where both axes represent time and position on the string itself.

Two strings can join together at one end or from the center to form a circle for closed strings. Strings can also split apart and emit energy. This theory replaces particles with waves traveling down the string, which is how gravity works in this model. The sun and earth are linked by gravitational force, so they’re basically two pipes connected by a wave traveling between them.

String theory came about in the 1960s to describe the strong nuclear force. Atoms were described by waves on strings, and these strings would attract each other with 10 tons of tension. In the 1970s John Schwarz and Joel Scherk said string theory could also be used to explain gravity, but only if that tension was significantly higher than previously thought. The work didn’t gain much attention at first because Scherk died from diabetes, leaving Schwarz alone to continue his research.

String theory was out of fashion but came back in the 1980s. It’s still not proven, but it has led to a new version called heterotic string theory. This may be able to eliminate infinities, although this is still unproven. The biggest problem with string theories is that they require either 10 or 26 dimensions.

It is possible that we can’t see all the dimensions because they are curved up into very small spaces. We only see the three spatial dimensions that we’re used to, but they are actually flat; it just looks like there aren’t any other dimensions when you look at them from far away. If you get closer to something and look at it in more detail, then you’ll be able to identify many more points on it. In string theory, looking on a very small scale reveals ten dimensions (as opposed to four), but no room for space ships (or anything else).

The four dimensions of space and time that we can see are flat, but others could not be. Why? The anthropic principle offers an explanation. Two dimensional animals couldn’t exist because they wouldn’t be able to feed themselves.

The biggest problem with more dimensions is that gravitational forces would be weaker. This would cause instability in the earth because it would spiral away from the sun if any disturbances occurred. The sun itself might not hold together, and atoms could also be unstable.

String theory suggests that other regions of the universe have four flat dimensions, just like ours. However, there may be some intelligent beings in those other regions.

There are many theories about what the universe is made of. Some say that there are many dimensions, while others think there may be fewer or more than four. Scientists have found a way to combine some of these theories and use them effectively in their work.

Stephen Hawking suggests that there is no one formula to unify all the laws of nature. Instead, it’s more like a patchwork quilt. Each piece of the quilt provides some information about how nature works. The pieces overlap and provide a complete picture when you look at them together.

There are three possibilities: there is a theory that explains all the laws of physics; there are several partial theories, but together they explain everything (perhaps in different ways); or things happen randomly. Some people think it’s random because this would create room for God to do as he pleases. However, we know with quantum mechanics that we can’t say something is absolutely random because even then some laws hold sway.

There are two possibilities: either scientists will find a new layer of particles beyond quarks, or they’ll hit an upper limit on energy. Either way, we might discover a unifying theory within this lifetime.

Even if the unifying theory is discovered, it will still only be a theory and can later be disproven. But if its predictions are consistent with observations, scientists could have confidence in it. This would end an era where humanity strived for ultimate knowledge of the universe. It would also revolutionize how ordinary people view the universe today because they wouldn’t need to depend on specialists to understand things like physics or astronomy anymore; everyone could understand these concepts at some level.

Scientists have a theory about the universe, but it’s incomplete. They are still trying to find out more about how the universe works. The reason for this is that they can’t figure out all of the math involved in understanding everything because there’s too much uncertainty and complexity with human behavior. Scientists know what most things in the universe are like, but they don’t know why humanity exists or why we’re here.

Chapter 12

The world is confusing. People try to understand it by creating a picture of the world they see around them, whether or not it’s true. They might believe that we live on top of an infinite tower of tortoises with the flat earth on their backs, for example. It’s a nice theory but doesn’t match reality because people don’t fall off the edge when they walk around.

In the beginning, people believed that gods controlled everything. However, as time went on and regularities were observed in nature (such as the sun rising every morning), it was thought that there might still be a god controlling these things but they obeyed strict laws. In recent times, scientists have been able to discover more about these laws and are now capable of predicting things like human behavior if only they knew all the details about how our universe works at one given point in time. Although this is an impressive achievement by itself, it does not explain why those laws exist or what happened before we could observe them with science.

Laplace’s approach is now defunct because of the uncertainty principle in quantum mechanics. People used to believe that particles have well-defined positions and velocities, but this isn’t true according to quantum mechanics. Instead, they have waves with uncertain values for position and velocity. This mismatch between preconceived ideas and reality gives rise to unpredictability or randomness in nature.

The purpose of science is to identify the laws that govern the universe. Gravity has taken prominence in this book because it forms large-scale structure of the universe, despite being the weakest force. It was incompatible with previous misconceptions about an unchanging universe.

According to the theory of general relativity, there must have been a point in time when all matter was compressed into an infinitely dense space. This is called the big bang. The universe would eventually return to this state if it keeps expanding forever, which is known as a big crunch. General relativity also predicts that black holes will have singularities where the laws of physics break down and allow for God’s work to happen.

Quantum mechanics is a branch of physics that deals with the behavior of matter and light on an atomic or subatomic scale. It’s important because it explains how things work at that level, which helps us understand what we observe in our everyday lives. Quantum mechanics introduces the idea of a four-dimensional space without boundaries, but this raises questions about whether there could be such thing as God. If so, he would have had no choice but to follow the laws of science when creating the universe.

Even though we continue to make discoveries about the universe, there are still gaps in our knowledge. We have discovered many physical laws that govern nature, but those laws don’t fully explain how they work or why they exist. Scientists believe a unifying theory could provide an explanation for these phenomena and help us better understand the world around us. However, it is difficult for them to discern between what’s real and what’s not due to their inability to see beyond scientific theories and models.

If a complete theory is found, it will likely be distilled to the point where everyone can understand and discuss it. Then everyone could figure out why we exist and the universe exists. If humans could find that answer, they would know God’s mind.

A Brief History of Time Book Summary, by Stephen Hawking

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