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Since the dawn of our species, mankind’s relentless search for knowledge has led us to ponder the universe. How did it come into being, and did it even have a beginning? Does it have an end? Why is it the way it is? What is our role in it?

With leaps in our knowledge of quantum physics, we now know more about the true nature of the universe than ever. Although the theoretical physicist Stephen Hawking, author of A Brief History of Time, was confined to a wheelchair and deprived of the powers of speech by the motor neurone disease ALS, he was able to advance our understanding of the universe, the nature of time, and our role within existence itself more than almost any other person who ever lived.

This summary will cover a wide range of topics, from Newton’s laws of gravity to special and general relativity, the Big Bang, black holes, the possibilities of time travel, and quantum theory. These are all complicated and highly counterintuitive topics that push the limits of what humans can conceive. Accordingly, we will discuss each of them at a high level, exploring just the main concepts and theoretical frameworks for each.

The Nature of Theories

We try to make sense of our universe by testing different theories about why we observe the phenomena we observe. The ultimate goal of all scientific endeavors is to produce a unified theory that completely explains the universe—a detailed roadmap of existence. A good theory accurately describes events, with few exceptions or changes; predicts future observations; and has the ability to be disproven in the future. Even one observation that contradicts the theory can negate it. In this sense, we never “prove” theories—we only grow more confident in them with every accurate prediction they make.

One theory of the universe that once had wide acceptance was the geocentric model of the universe, in which celestial bodies revolve around the Earth in fixed circular orbits. But this theory failed to predict the movement of the Moon accurately. Thus, it was discarded in favor of a heliocentric model, in which the Earth and the planets revolve around the sun—which lined up far better with observed reality. Subsequent theories and discoveries have enhanced our understanding.

Everything Is in Motion

Other theories that have enhanced our understanding of the universe include Newton’s Laws of Gravity. Newton’s Laws of Gravity state that force doesn’t set things in motion—everything is already in motion. There is no absolute standard of rest, because objects are always moving in relation to one another. The concepts of “rest” and “motion” are entirely relative. A stationary train may appear to be at rest, but it’s not—because it is sitting upon a moving Earth.

This relationship between the two objects (the train and the Earth) would be the same if the Earth was at rest and the train was moving. Because there’s no absolute standard of motion or rest, there’s also no absolute standard of space. It’s impossible to determine whether two events occurring at different times occurred in the same space, because everything is moving relative to everything else.

The Relativity of Time

It’s not just space—time is also relative. We know that light moves at a constant and finite speed to all observers, regardless of their position in space. But this could only be reconciled with Newton’s laws if time, too, was relative. This was the basis of Albert Einstein’s special theory of relativity, which argued that the laws of science should apply universally, regardless of the differing speed of the observers. Thus, if the speed of light is constant for everyone looking at it, then time must also be relative. ** This was expressed in Einstein’s famous equation e=mc2, with “e” standing for energy, “m” standing for mass, and “c” standing for the speed of light.

Thus, time is not a dimension separate from space: it is inherently interwoven with it, in the fabric of space-time . Einstein’s general theory of relativity, proposed in 1916, incorporated gravity into this framework. Under this theory, gravity is a special force that exists because of the curvature of space-time itself.

Space-time, according to this theory, is not flat. It is analogous to placing an object on a stretched-out piece of fabric. The weight of this object will cause the fabric to sink—this is the same mechanism by which gravity warps the curvature of space-time.

Universal Gravitation

In addition to his laws of motion, Newton’s laws describing gravity have also changed the way we understand the universe. Newton’s theory of universal gravitation states that bodies are attracted to one another by a force (gravity) that grows stronger in proportion to their mass and inversely proportional to their distance . But gravitational attraction, left unchecked, would eventually cause all the matter to collapse in on itself—the only way this wouldn’thappen would be if the universe was dynamic and expanding, a notion that was totally at odds with religious and theological concepts of the time.

The Big Bang

In 1823, the German astronomer Heinrich Wilhelm Olbers observed that if the universe were truly infinite, static, and eternal, the sky would be blinding white light—because in an infinite universe, there would be an infinite number of stars. Every line of sight would end at a star. This paradox pointed the way toward an understanding of the universe as finite, expanding, and with a definite beginning point in time. The reason we aren’t blinded by light when we gaze at the sky is because the light from some stars hasn’t reached Earth yet. And if this is the case, then the universe must be finite and there must have been some point at which the stars “turned on” and the universe began. This beginning is what we now know as the Big Bang.

In 1929, the American astronomer Edwin Hubble confirmed through observation what Olbers’ paradox had suggested. Hubble discovered that the universe was expanding by observing the “red-shifting” of distant galaxies—the movement of their light toward longer, red wavelengths, indicating movement away from us.

The Beginning of Time

The theory that the universe is expanding explained what happened after the Big Bang. But what happened before?

Events “before” the Big Bang are inconsequential because they are, by definition, unobservable and unable to affect anything that came “after” the Big Bang. Indeed, the very concept of time itself can be said to have come into existence at the Big Bang.

But did general relativity and the existence of an expanding universe require a Big Bang event? The British mathematician and physicist Roger Penrose sought to answer this in 1965. Reasoning from general relativity and the principle that gravity is always attractive, Penrose theorized that when a star died and collapsed under the weight of its own massive gravity, it would be compressed to a space of zero surface and volume—a singularity, whose conditions would be much like those before the Big Bang. The great insight of Stephen Hawking, a colleague of Penrose’s, was to put Penrose’s theorem in reverse—If all stars ended up as singularities, then an expanding universe must have begun with a singularity.

Star Death

Stars are massive bodies with enormous gravitational pull. What keeps them from collapsing in on themselves during the majority of their lifetimes are the nuclear reactions that are occurring at their cores—this “outward” pressure balances the “inward” gravitational pressure.

Eventually, however, the star exhausts its supply of hydrogen and nuclear fuel. At this point, the star begins to cool, causing it to contract. Stars that are more than one and a half times the mass of our sun collapse into a singularity of infinite density—a black hole. A black hole has infinite gravity, such that not even light can escape. Under such conditions, all scientific predictability breaks down. “Space” and “time” have no meaning, just like the conditions before the Big Bang. **

Black holes can’t be seen, but we know they exist, because they exert gravitational influence over other objects. Scientists have detected them in the distant Cygnus X-1 system and possibly at the center of our own galaxy. Because of the age of the universe, there are probably more black holes than observable stars, as many stars must have undergone collapse since the Big Bang. Black holes could also have been created under the high energy and density conditions of the early universe.

The State of the Universe

Although there’s evidence the universe is currently expanding, does general relativity actually predict an eventual gravitational collapse of the entire universe, also known as a “Big Crunch” event? Are our universe’s days numbered? For most of the 20th century, the “Hot Big Bang” model, a variant of the Big Bang theory, has been used to tell the story of the origin and development of the universe and answer these questions.

According to the Hot Big Bang model, after the initial Big Bang explosion, the universe was incredibly hot (hence the name), meaning that particles were moving too quickly to merge together to form protons, neutrons, atoms, and molecules. But as the universe expanded, it began to cool, and the particles slowed down. This led to a series of nuclear fusions, in which stars formed, as well as more complex elements like hydrogen and helium (this all would have taken place within mere milliseconds after the Big Bang). After just a few hours, most of the hydrogen and helium in our universe today was created, concentrated in enormous clouds. These then became the building blocks of the other elements and molecules that form all the matter in our universe (including Earth and human beings).

But it all seems too perfect. The universe seems too uniform (at least on a large scale) to be the product of an explosion like the Big Bang. Moreover, it’s curious that the rate of expansion is just large enough to avoid gravitational collapse. After all, if the initial rate of expansion after the Big Bang had been only infinitesimally smaller, the universe would have already collapsed.

Alan Guth at MIT suggested an “inflationary” hypothesis to explain this puzzling state of the universe, without reverence to a deity or intelligent being carefully setting such conditions.

This theory tells us that the expansion after the Big Bang was so rapid that the universe ballooned to a million million million million million times its radius in less than a second.This expansion would have supercooled the universe much faster than in the traditional “Hot” Big Bang model and produced a great smoothing and uniformity across the universe on a large scale, even if it didn’t start out uniform and smooth.

Entropy and Backwards Time Travel

We’ve already established that time is relative. But this just raises another major question about our universe—iftime is relative, in what direction does it point? Can it possibly move backwards?

Unfortunately for those who might wish to journey to the past, there are three reasons to suggest that this is an unlikely possibility.

Reason #1: The second law of thermodynamics says that, within a closed system, total entropy, or disorder, can only increase or remain stable overtime—it can never decrease. But the backwards movement of time would represent an inversion of this law. Under such conditions, we would witness the decreasing of entropy over time—a broken cup, for example, becoming an intact cup. We don’t observe the spontaneous reconstitution of broken cups into whole ones, thus the thermodynamic arrow of time moves only in a forward direction.

Reason #2: Time also moves only in a forward direction in a psychological sense—we don’t remember events in the future and always perceive ourselves as moving toward the unknown future and away from the known past. This also happens to be the direction in which entropy increases. This is the psychological arrow of time.

Reason #3: Time moves only in a forward direction because entropy only increases in the same direction as the expansion of the universe. We can only exist in the expanding phase of the universe, when entropy is increasing. This is the cosmological arrow of time.

Paradoxes of Backwards Time Travel

To travel back in time, one would need to be able to travel faster than the speed of light. Imagine you were in a rocket travelling from point A to point B. Under the laws of general relativity, all observers would be able to agree that your rocket was at point A before it reached point B—provided that it traveled below the speed of light. But if it were travelling faster than the speed of light, your rocket would outpace its own light being transmitted to observers at point A. Once you arrived at Point B, you would be able to observe your past self leaving Point A. You would be at your destination before you left—in other words, you would have traveled backwards in time.

But physics seems to disallow this, because, according to Einstein’s famous equation e=mc2, mass increases with speed, and you would need more and more energy to power a rocket past the speed of light, and there will...

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A Brief History of Time Summary Chapter 1: The Quest for Understanding

Since the dawn of our species, mankind’s relentless search for knowledge has led us to ponder the universe. How did it come into being, and did it even have a beginning? Does it have an end? Why is it the way it is? What is our role in it? With leaps in our knowledge of quantum physics, we now know more about the true nature of the universe than ever.

While it may seem vast and unknowable, far beyond the limits of human comprehension, the universe is actually governed by rational laws that we can observe, predict, and understand.Throughout our existence, rational observation has been the tool we’ve used to change and update our knowledge. We’ll see how humankind revised its early, primitive understanding of the Earth and the universe by making observations, testing those observations against existing theoretical frameworks, and adjusting or discarding those frameworks based on how well observation lined up with prediction.

What Is a Theory?

Before we dive into our exploration of the universe and the forces that govern it, we need to answer a simple question: What is a theory?. A good theory satisfies three criteria. It must:

  1. Accurately describe events based on a model, with very few exceptions or changes
  2. Accurately predict future observations
  3. Be able to be falsified or disproved through observation

For example, Newton’s theory of universal gravitation was a good theory because it met all these criteria: it explained the movement of celestial bodies, predicted what they would do in the future, and was testable.

Even the best theories are inherently vulnerable, as only one instance of falsification is needed to disprove a theory. A single observation that directly clashes with a theory’s predictions invalidates the theory. Every experiment that produces results that conform to the theory strengthens it, but you can never definitively prove that a theory is true.

The ultimate goal of all scientific endeavors is to produce a unified theory that completely explains the universe—a detailed roadmap of existence. This is extraordinarily difficult and might be altogether impossible. But scientists, philosophers, and theologians have never wavered in their quest to achieve this understanding, though they do so by quite different means. In general, scientists attempt to break the theory of the universe up into partial theories, each dealing only with certain categories of observation.

This approach, of course, might be problematic—if the universe is an interconnected system, with everything dependent upon everything else, you can’t establish a grand unified theory by studying it piecemeal. But at least for now, this is the best scientists can do.

There are two main partial theories of the universe. The first is the theory of relativity, which deals with phenomena on an extraordinarily large scale (for example, bodies a million million million million miles apart).The second is quantum theory, which deals with the infinitesimally small (particles that measure one millionth of a millionth of an inch). Unfortunately, these theories contradict each other: it’s impossible for both of them to be completely correct (we’ll explore why in Chapter 3).

Still, why try to arrive at some grand unified theory of the universe? If it can’t improve day-to-day human experience, does any of this stuff matter? The answer is, yes, it does. First of all, fields like relativity and quantum mechanics that were once thought to be entirely abstract have, in fact, been found to have enormously consequential practical applications (like nuclear energy and microelectronics).

But perhaps more importantly, it is human nature to question our world and the broader universe in which we live, and to attempt to understand the laws that order and govern our universe. It is the quest to ultimately satisfy mankind’s unquenchable thirst for knowledge.

New Observations, New Theories

Humans have been theorizing about the heavens for centuries. In the 4th century BCE, the Greek philosopher Aristotle posited the idea of a round Earth (contrary to popular notions of the time), because he observed three things: 1) that eclipses entirely obscured the sun; 2) that the position and angle in the sky of the North Star changed relative to the position of the observer; and 3) that one saw the sails of an incoming sailboat before the boat itself. None of these would happen if the Earth were flat.

But he also wrongly believed in a geocentric model of the universe, in which celestial bodies revolve around the Earth in fixed circular orbits. This later influenced Ptolemy’s cosmological model. This theory had a flaw, in that it did not predict the movement of the Moon accurately. In order to make it work, Ptolemy had to create an exception within his theory, making the assumption that the Moon followed a path that sometimes brought it closer to Earth, rather than orbiting it in a perfect circle. Clearly, the model did not elegantly explain observed phenomena(although it did, centuries later, become medieval Church dogma, because it left room outside the stars for heaven and hell).

In the 16th century CE, Polish astronomer Nicolaus Copernicus proposed that the sun was stationary and that the other bodies revolved around it, a position later bolstered by the work of the Italian astronomer Galileo Galilei. Galileo observed through a telescope that Jupiter had its own moons that revolved around it—clearly, not everything revolved around the Earth. The geocentric model was becoming a flimsier and flimsier theory to explain observed phenomena, requiring complicated exceptions.

The German Johannes Kepler expanded upon the work of those who came before him, putting forward the idea that orbits were not circular, but elliptical. This finally brought prediction into line with observation—a heliocentric solar system with bodies...

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A Brief History of Time Summary Chapter 2: It’s All Relative

This lack of absolute space was distressing to European thought in the 17th century (including for Newton himself) because it did not square with then-dominant Judeo-Christian ideas of an absolute God. But an even more revolutionary discovery was in the offing: There was also no absolute concept of time. Time, too, was relative.That is, time moves at different speeds for different observers.

Abandoning Absolute Time

Newton’s contemporary, the Danish astronomer Ole Rømer, discovered that light moved at a constant and finite speed. He was the first person to calculate a finite speed for light, though his initial numbers were quite far off the mark (he believed it moved at 140,000 miles/second, when modern calculations have it at 186,000 miles/second). Still, his observation about the uniformity of the speed of light held true—no matter the positions of different observers, and no matter how fast those observers were travelling, light would always appear to be travelling at 186,000 miles/second.

But if this was the case, how did that reconcile with Newton’s law stipulating that there was no state of absolute rest? Light traveled at a constant speed, but relative to what? People at the time understood that speed was equal to distance divided by time, represented by the formula s=d/t. If you’re on a flight travelling 500 miles/hour, you know you’ll be 500 miles closer to your destination in one hour. The relationship between time and space is clear—your distance in space relative to your destination is decreasing with time. If either the d or t variables in the s=d/t equation were to change, then so would the speed of your airplane.

But if your airplane was travelling at the speed of light, this logic would lead you to a very odd conclusion. Because light is the same speed everywhere, the s variable of the equation is constant, even as the d variable also changes (you’re still moving closer to your destination as you travel at the speed of light). Mathematically, this is impossible if the denominator t variable remains a constant. The only way for the equation to remain valid and for the speed of light to be constant is for t to be variable, so that the proportional relationship of d/t always yields the same value. Accordingly, time would have to slow down as one approached the speed of light.

This was how Albert Einstein and Henri Poincaré (working simultaneously but independently) discovered that Newton’s theory could stand, provided that one abandoned the idea of absolute time. This was one of the cornerstones of the theory of relativity. It postulated that the laws of science should apply universally and that time must also be relative in order for observers at different points in space to observe the same speed of light. Thus, observers will record different times for the same events and time would slow down as one approached the speed of light.Thus, one could depart in a spaceship traveling close to the speed of light and return to Earth having barely aged at all, while one’s relatives would have aged by decades. **

This was expressed in Einstein’s famous equation e=mc2, with “e” standing for energy, “m” standing for mass, and “c” standing for the speed of light. The equation also states that nothing can travel faster than the speed of light. This is because an object increases its mass as its energy rises and its speed increases. And, as the mass of an object increases, it requires more energy to move it. As it approaches the speed of light, it would approach infinite mass and require infinite energy, which is impossible. Objects can approach the speed of light, but never quite reach it.

Space-Time and Light Cones

Time is not a dimension separate from space: it is inherently interwoven with it, in the fabric of space-time. All events must be plotted according to a particular point in time and a particular point in space, relative to the position and motion of the observer. Thus, we use a four-dimensional coordinate to map events in space-time: the three dimensions of space, and the fourth dimension of time.

It’s easier to visualize this if we think about future and past light-cones. Since light is a wave, it expands in a widening curve through space-time as it travels from its point of origin, like ripples on a pond when you toss a rock onto its surface. The light spreading out from event A after its occurrence is that event’s future cone. Observers can only see event A when they cross into the path of the future light cone. The past light cone is the set of events whose light pulses or emissions can reach event A, generating their future light cones in the other direction. The visual below helps to illustrate this concept.

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The future light cone is the “audience” that can see event A. The past light cone is everything that can be seen by event A. There are events, however, that lie outside both the future and past light cones of a given event. These are said to lie in an event’s “elsewhere.” For example, there are stars whose light has not yet reached Earth. Thus, such a star is invisible to us, because we have not yet crossed into its future light cone. Thus, when we look at the starry sky, we are seeing a snapshot of the universe as it was in the past—the light from the stars whose future light cone we have crossed into.

General Relativity

Einstein’s theory of relativity greatly enhanced understanding of the interplay between the speed of light and the passage of time. But it was still missing a crucial element—gravity. Gravitational effects ought to be immediate, meaning that gravity travels at infinite velocity (it shouldn’t take time for gravity to exert its effects). But how could this square with the idea that nothing could travel faster than the...

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A Brief History of Time Summary Chapter 3: Black Holes

In 1783, the English scientist John Michell wrote a paper arguing that if a star were of sufficient mass, its gravity field would be so strong that not even light could escape from it. These massive stars would be invisible, detectable only through their gravitational effects on other objects. This was the earliest sketch of what we now know of as a black hole.

How do stars become black holes? Let’s trace the life cycle of a star. Stars are first formed when free-floating hydrogen atoms begin to cluster together due to gravitational attraction. Eventually, the heat and energy from these interactions causes nuclear reactions within those atoms. Those reactions then form other elements, mostly helium. Eventually, these atoms cluster together through the force of gravitational attraction to form stars. The “outward” energy produced by the nuclear reactions that are occurring at the core of the star balances out the “inward” gravitational pressure and the star reaches a state of stability.

Eventually, however, the star exhausts its supply of hydrogen and nuclear fuel. The star begins to cool, causing it to contract. At this point, the attractive force of gravity becomes dominant and the star begins to collapse in on itself.

The Chandrasekhar Limit

Scientists once thought that, per the exclusion principle (which prevents similar particles from having the same position and velocity), the different velocities of the particles would cause the star to continue expanding, counterbalancing the attractive force of gravity. In other words, that star collapse due to gravity was impossible.

But Subrahmanyan Chandrasekhar proved that this was not the case. Chandrasekhar was an Indian graduate student who was working in the 1920s at Cambridge University. He discovered that the exclusion principle would not apply to stars that had reached a certain critical mass. He estimated that stars more than one and a half times the mass of our sun would become too dense after they had exhausted their nuclear fuel to be able to withstand the force of their own gravity. This threshold is now known as the Chandrasekhar limit.

Those stars below this limit would shrink into what are known as white dwarfs or neutron stars. In these cases, the exclusion principle would act on either the electrons or the protons and neutrons in the star to prevent total collapse, and the once-massive star would condense to a tiny fraction of its former mass—with a radius of perhaps a few thousand miles for a white dwarf and a mere ten miles for a neutron star.

But what about stars with masses above the limit? Chandrasekhar argued that such stars would collapse into a singularity of infinite density—a black hole. This prediction met with noted hostility by many prominent figures in the scientific community at the time, who refused to accept the existence of black holes.

The Point of No Return

The American theoretical physicist Robert Oppenheimer took the theory of black holes further, arguing that contracting stars would impact the light cones emanating from them. A light cone, if you’ll recall, is the path of light through space-time from an event occurring at a single point in space and a single moment in time.

Oppenheimer theorized that the gravitational pull from a condensing star would bend light cones at the surface inward, making them appear dimmer and redder (because of red-shift “stretching” the wavelengths) to observers in the future light cone of the event.

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At a certain point in a massive star’s collapse, the gravity makes it impossible for light to escape at all. And if light can’t escape,nothing can, because nothing can travel faster than the speed of light. Such a gravitational singularity is a black hole. The boundary of a black hole, the point past which light just fails to escape, is called the event horizon.

Hawking and Penrose’s work suggested that black holes must be infinitely dense and infinitely curved in space-time. Under such conditions, all scientific predictability would break down. “Space” and “time” would have no meaning, just like the conditions before the Big Bang.Penrose argued that singularities can’t be seen, because nothing can be observed behind the event horizon (where light cannot escape). Objects can enter the singularity but nothing can exit.

Seeing the Unseeable

So can we actually observe black holes? Because nothing can escape from them to reach us, it would seem that we could only observe their effects. In 1967, scientists at Cambridge first detected radio waves coming from a distant neutron star. This was the first absolute evidence that neutron stars were real, instead of just being predicted mathematically.

Neutron stars are objects of extremely high density, almost as dense as black holes. Remember, the exclusion principle only just prevents them from becoming a black hole singularity. Once their existence was proven, the notion of black holes suddenly wasn’t so far-fetched.

Black holes can’t be seen, but they still exert gravitational influence over other objects. This brings us to the strange case of the Cygnus X-1 system. This system consists of a star orbiting around an unseen object. Based on the star’s orbit, we know that the unseen object must have great density. The mass of the star is six times greater than the sun— too large for the unseen object it is orbiting to be anything other than a black hole.

Other likely black holes have been identified throughout the observable universe, including one possibly at the center of our own galaxy. Because of the age of the universe, there are probably more black holes than observable stars, as many stars must have undergone collapse since the Big Bang.

**Low-mass stars could also form black holes, though not...

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A Brief History of Time Summary Chapter 4: The Building Blocks of the Universe

As we explored, general relativity tells us that all physical theories break down at the Big Bang, including general relativity itself. The time immediately after though, during the infancy of the universe, requires us to study quantum mechanics—the study of extraordinarily small particles.

The German physicist Max Planck suggested that waves (like light, radio, and microwaves) emit energy not randomly or arbitrarily, but instead, in discrete quantities which he called “quanta.” The amount of energy released in a quantum is proportional to the wave frequency of the radiation type in question. Wave frequency is simply the number of waves that pass through a fixed point in a given unit of time.

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In quantum theory, the higher the frequency, the more energy that is released. This means that, at very high frequencies, the release of the quantum requires more energy than is actually available. In such a scenario, bodies lose energy at a finite rate. This theory had the advantage of accurately predicting and describing the observed radiation of extremely hot bodies.

The Uncertainty Principle and Quantum Mechanics

We touched on the uncertainty principle briefly in our exploration of particle emissions from black holes, but we’ll explore the concept in greater detail here. Werner Heisenberg reasoned from Planck’s theory that the very act of shining a quantum of high frequency light upon a particle (which was the only way to measure its position and velocity) would, in and of itself, disturb the particle and disrupt its velocity. Exactly how it would be disrupted, however, was entirely unpredictable. The very attempt at observation made it impossible to accurately gauge particle position and speed. This was the uncertainty principle.

This was the nail in the coffin for the principle of scientific determinism, which held that we should be able to predict everything about the universe, provided that we have perfect and complete information about it at any one time. The uncertainty principle, however, held that it was impossible to measure the precise state of the universe at any one point in time—therefore, it was impossible to make a 100 percent accurate prediction for any future point in time. This was the beginning of the new field of quantum mechanics.

Quantum mechanics takes a probabilistic approach rather than an absolute approach. Obeying the uncertainty principle, it does not seek to establish a single, definitive interpretation or result for quantum mechanics and the uncertainty principle tell us that it’s not as useful to think of the physical world as being composed of waves (like light and radio) and particles (like the atoms out of which all matter is made). It is better to think of our observations of the world in terms of waves and particles—depending on the phenomenon, sometimes it’s more useful to think of particles as waves, and sometimes it’s more useful to think of waves as particles.

These theories that argue for the varying behavior of particles and waves have had a profound impact on our understanding of the structure of atoms and molecules. While we can never predict the movement of electrons within atoms with 100 percent certainty, we can predict the probabilities of different events, within the constraints imposed by the uncertainty principle. any observation. It predicts a range of possible outcomes, and tries to work out probabilities for each.

Quantum Mechanics vs. Relativity

Quantum mechanics is incompatible with the predictions of relativity—even though both, paradoxically, are well-tested theories that make accurate predictions about the universe. The predictions of relativity, which deal with phenomena on the massive, universal scale, break down when applied to the subatomic level of quantum mechanics. So much of what we think we know about physics simply breaks down at the quantum level, where particles behave in strange and—thanks to the uncertainty principle—unpredictable ways.Uniting quantum mechanics and relativity is one of the great challenges to establishing a complete theory of the universe.

Quarks

To understand how the universe works on a large scale, we need to grasp how it works on a small scale. We know that atoms are the basic building blocks of matter in the universe. In 1803, the English chemist John Dalton discovered that atoms combined together in specific configurations to create molecules. But were the atoms themselves divisible, composed of smaller units? It turned out that there were indeed subatomic particles. By the early 20th century, scientists had worked out the basic atomic structure: a positively charged nucleus (consisting of a positive proton and a neutral neutron) orbited by a number of negatively charged electrons.

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In the 1960s, scientists conducted experiments in which they collided these subatomic particles together. The collisions revealed yet another layer, smaller particles which made up the protons and neutrons. These particles were called quarks, of which there are six types or “flavors” —up, down, bottom, top, strange, and charmed. In turn, each flavor had three possible “colors”—red, green, and blue (quarks don’t literally have flavors or colors, these are just the labels given to them by particle physicists). Protons and neutrons are made of different color combinations of both up and down quarks. In studying quarks, we are looking at the most elemental building blocks of our universe. And it turns out that our knowledge of quarks is the key we can use to unlock a great deal about our universe.

Quarks have a property called “spin.” They’re not literally spinning, but it’s the best way to describe their...

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A Brief History of Time Summary Chapter 5: Past, Present, and Future

For most of the 20th century, the “Hot Big Bang” model has been used to tell the story of the origin and development of the universe. The Hot Big Bang theory states that, after the initial Big Bang explosion, the universe was incredibly hot (hence the name), meaning that particles were moving too quickly to merge together to form protons, neutrons, atoms, and molecules. But as the universe expanded, it began to cool, and the particles slowed down. This led to a series of nuclear fusions, in which stars were formed, as well as more complex elements like hydrogen and helium (this all would have taken place within mere milliseconds of the Big Bang). After just a few hours, most of the hydrogen and helium in our universe today were created, concentrated in enormous clouds.

Eventually, as the universe continued expanding and cooling over the next few million years, these massive clouds collapsed under their gravity, catalyzing a new series of nuclear reactions. Moreover, the first generation of stars began to suffer gravitational collapse within roughly 100 million years after their creation, throwing off mass and energy as they died. Through these combined effects, some of the heavier and more complex elements produced in the cores of this first generation of stars were blown off into space in explosions called supernovae. Through gravitational attraction, the fallout from the supernovae formed second-, third-, and fourth-generation stars like our own, as well as planets like Earth.

Eventually, elements like oxygen were formed on the cooling Earth, combining with the abundant hydrogen to create water and the other necessities of intelligent life. From there, astrophysics yields to evolutionary biology and human history to tell the rest of the story of how we got here.

The Hot Big Bang model explains a lot about the observable universe, but not everything.

  • It merely assumes its core premise—that the early universe was hot—without explaining why.
  • It also fails to account for the seeming uniformity of the universe on a large scale at all points of space-time, perhaps best exemplified by the omnipresent background microwave radiation that we explored earlier in the summary.
  • Its rate of expansion for the universe is too perfect—it postulates a rate of expansion just large enough to avoid collapse, when, if the initial rate of expansion after the Big Bang had been only infinitesimally smaller, the universe would have already collapsed.
  • The model doesn’t account for the small variations in density that led to the formation of regional phenomena, like stars and galaxies, forming where they did.

Chaotic Boundary Conditions

One could just go the theological route to answer these questions about the state of the universe at the time of the Big Bang, the very beginning of space-time itself. Maybe God just created these conditions for unknowable reasons that mankind can never hope to uncover. This explanation, however, is not consistent with the history of science. Human scientific pursuit is defined by the discovery of rational and consistent laws for what were once thought to be inexplicable acts of divinity. We know that the universe today obeys concrete scientific laws, so why shouldn’t it have done so before the Big Bang?

These properties of the universe can be explained through the theory of chaotic boundary conditions. Chaotic boundary conditions assume either an infinitely vast single universe or an infinite number offiniteuniverses. Under these limitless conditions, we would eventually expect to see a universe that happens to exhibit the smooth and uniform properties we observe in our universe (given infinity, all conditions, no matter how unlikely, will eventually occur).

Anthropic Principles

So which of these conditions is true? To answer that, we need to explore what are known as the anthropic principles. There are actually two versions of them—a weak one and a strong one.

The weak anthropic principle states that we see the universe as smooth and uniform because those are the only regions of the universe where intelligent life capable of observing it (like us) can exist.

The strong anthropic principle states that the laws of science simply aren’t the same in different regions of the universe or across universes.

These theories are appealing because it’s hard to explain why everything in our universe appears to be so perfectly calibrated to create the conditions we observe. Even slight variations in the charges of subatomic particles would have created something vastly different from what we see. It seems too precise to be just a coincidence. Thus, we look either to 1) a divine hand or 2) an infinite set of universes, where we just happen to be living in one that matches the observations we can see.

Stephen Hawking had difficulty accepting these ideas, especially the strong anthropic principle. It seems to be more of a cop-out, a dismissal of the ability of science to accurately model the universe. This is once again at odds with scientific history, relying on near-magical explanations to hand-wave away hypothetical problems. Still, we need to find some way to explain the very precise and extraordinarily unlikely conditions of the universe without recourse to the supernatural or metaphysical.

Inflationary Hypothesis

Alan Guth at MIT suggested an “inflationary” hypothesis to explain this puzzling uniformity of the universe without reverence to a deity or intelligent being carefully setting such conditions. This theory tells us that the expansion after the Big Bang was so rapid that the universe ballooned to a million million million million million times its radius in less than a second.

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This expansion would have supercooled the universe much...

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Table of Contents

  • 1-Page Summary
  • Chapter 1: The Quest for Understanding
  • Chapter 2: It’s All Relative
  • Exercise: Understand the Dynamic Universe
  • Chapter 3: Black Holes
  • Exercise: Understanding Black Holes
  • Chapter 4: The Building Blocks of the Universe
  • Chapter 5: Past, Present, and Future