Introduction
You want to understand the origin of the cosmos, the nature of time, and the physics of black holes without getting lost in impenetrable equations. Stephen Hawking guides you through complex modern physics while sharing his personal journey with amyotrophic lateral sclerosis and his daily life as a theoretical physicist.
A collection of essays exploring the origins of the universe, quantum gravity, black hole evaporation, and the personal reality of living with ALS.
The book covers seven major areas:
- Early life influences
- Living with motor neurone disease
- The philosophy of scientific models
- Particle physics and quantum gravity
- The geometry of time and space
- The mechanics of black holes
- The ultimate fate of the universe
You will discover answers to several fundamental questions as you read:
- How can the universe have a beginning without a creator?
- What happens when a black hole evaporates completely?
- Why does time always move forward?
- Can you predict the future if you know the laws of physics?
1. Early Life and Academic Pursuits
Childhood and Background
Stephen Hawking's childhood fascination with building working models and complex games laid the groundwork for his later drive to understand and control the universe through cosmology. You find that this urge to master how systems operate guided his early play.
His father brought him an American train complete with a cowcatcher and a figure-eight track from the United States on the Queen Mary. He spent his youth building model airplanes, boats, and trains. He also invented elaborate manufacturing, war, and feudal games. One war game used a board containing four thousand squares.
Childhood mechanics and games fostered a drive to understand cosmology, which Hawking chose over elementary particles because general relativity provided a rigorous framework.
Academic Pursuits
Hawking chose to pursue theoretical cosmology over elementary particles because general relativity offered a well-defined existing theory, whereas particle physics lacked a proper theoretical framework at the time. When entering postgraduate research, he considered the very large and the very small.
He rejected elementary particles because scientists could only arrange them into botanical-like families without a proper theory. Cosmology, by contrast, was anchored by Einstein's general theory of relativity. He initially applied to do his Ph.D. with Fred Hoyle at Cambridge, but was ultimately assigned to Denis Sciama.
2. Living with ALS and Technological Aids
Living with ALS
Shortly after moving to Cambridge, you learn that Stephen Hawking was diagnosed with amyotrophic lateral sclerosis at age twenty-one with no cure offered. Believing he would not live long enough to finish his Ph.D., he initially lacked motivation. Getting engaged to Jane Wilde forced him to secure a job, which required finishing his Ph.D., thereby igniting his dedication to work.
An ALS diagnosis shifted his life trajectory, while specialized computer systems and speech synthesizers enabled him to continue writing and lecturing.
While hospitalized for tests, he saw a boy die of leukemia in the opposite bed, which put his own condition into perspective.
Following a tracheostomy in 1985, Hawking communicated via a computer program called Equalizer created by Walt Woltosz, allowing him to select words from screen menus via a hand switch. David Mason later mounted a personal computer and speech synthesizer to his wheelchair. He used his wheelchair-mounted system to write books and scientific papers, communicating at a rate of up to fifteen words a minute.
After contracting pneumonia in Geneva in 1985, a tracheostomy saved his life but removed his voice. Walt Woltosz sent Hawking a computer program he originally developed to help his mother-in-law, who had the same condition.
You operate the system through specific steps:
- Move a cursor across a computer screen using hand movements
- Select desired words to print out on the lower half of the screen
- Send constructed sentences to a speech synthesizer or save them on disk
Following these procedures, he managed twenty-four hours of daily nursing care required after his tracheostomy. The resulting synthesized speech operated at one-tenth the speed of normal speech.
3. Cosmology and the Philosophy of Physics
Philosophy of Science
You cannot know what reality is independent of a theory. Your perception of reality is conditioned by the models you subscribe to.
A physical theory should be viewed merely as an elegant mathematical model that describes observations and makes predictions, rather than as a direct description of objective reality.
You reject the search for model-independent concepts of reality. You champion a positivist approach that values a theory based on its elegance, its broad descriptive power, and its predictive success.
Methodology of Physics
Major theories are rarely advanced purely on the basis of experiment. They originate from the desire to make disparate parts of theoretical frameworks fit together into a consistent mathematical whole, which then generates testable predictions.
In theoretical physics, the search for logical self-consistency and mathematical elegance has historically driven major advances more than raw experimental results.
The Michelson-Morley experiment in 1887 showed the speed of light was constant regardless of observer motion, which Lorentz and Fitzgerald tried to reconcile using ad hoc adjustments before Einstein introduced space-time in 1905.
4. Standard Model and Quantum Gravity
Physical interactions are divided phenomenologically into four categories by strength: strong nuclear forces, electromagnetism, weak nuclear forces, and gravity. Matter particles are split into hadrons composed of quarks and leptons, obeying the Pauli exclusion principle with half-integer spins. Interactions are mediated by integer-spin fields.
Fundamental forces and particle families require a quantum theory of gravity to resolve singularities at the big bang and inside black holes.
The Salam-Weinberg theory unified electromagnetic and weak interactions, utilizing spontaneous symmetry breaking, later confirmed by observations of W and Z particles at the CERN laboratory in Geneva in 1983. Gerard 't Hooft showed the Salam-Weinberg model is renormalizable in 1971. The Nobel Prize in physics was awarded to Salam, Weinberg, and Glashow in 1979.
Standard Model and Unification
Strong interactions unify with weak and electromagnetic interactions at an energy scale of 10^15 GeV. Grand unified theories predict a lifetime of a proton of 10^31 years. Macroscopic systems requiring statistical methods contain 10^23 particles.
Quantum Gravity
General relativity predicts singularities at the big bang and inside black holes where classical physics breaks down. Near black holes, intense gravitational fields cause the creation and emission of particle-antiparticle pairs, imparting an extra degree of unpredictability that reduces our ability to make definite predictions to mere probabilities. Space-time acquires a foamlike structure at a length scale of 10^-33 cm, and the high energy gravitational cutoff sits at 10^28 eV.
Future of Physics
N = 8 extended supergravity is a leading candidate for a complete unified theory because it incorporates gravity, operates in four dimensions, and may be finite without infinite subtractions. Hawking discusses supergravity as a way to unify matter and interactions and cancel out infinities. Because the N = 8 theory contains a specific set of particle fields whose numbers are insufficient to account for all observed particles, many observed particles like quarks and gluons must be composite bound states rather than truly elementary.
The N = 8 theory contains:
- 8 gravitonos
- 28 spin-1 particles
- 56 spin-1/2 particles
- 70 spin 0 particles
The anthropic principle explains that the physical parameters and initial conditions of our universe must be conducive to intelligent life, because otherwise no one would be around to observe them. Hawking examines why initial conditions and parameters like particle masses do not appear arbitrary. If values such as the proton-neutron mass difference or the initial expansion rate had been slightly different, stable nucleides and stars could not have formed.
5. Relativity, Quantum Mechanics, and Time
Relativity and Gravity
Einstein formulated the general theory of relativity in 1915. Space and time are not a fixed background, but dynamic quantities curved by mass and energy.
Because mass and energy are always positive, gravity always attracts, causing space-time to curve back on itself. Objects and light follow the straightest possible paths through this warped space-time, which appears to us as a bending caused by a gravitational field.
A British expedition observed the solar eclipse of 1919 and confirmed that light from distant stars was bent as it passed near the sun, verifying general relativity.
Quantum Mechanics
Heisenberg's uncertainty principle establishes that you can never precisely measure both the position and velocity of a particle simultaneously, introducing fundamental randomness into physics.
To locate a particle, you must shine light on it using at least one packet, or quantum. This packet disturbs the particle, meaning the product of the uncertainties in its position and speed will always exceed a minimum amount, preventing exact predictions of future states.
Einstein's explanation of the photoelectric effect proved that light comes in discrete packets called quanta, which won him the Nobel Prize in 1922.
Richard Feynman's sum over histories proposes that a physical system does not follow a single path, but takes every possible history in space-time.
The probability of a particle traveling from point A to point B is calculated by adding up numbers associated with all possible paths it could take, where paths nearly straight contribute the most.
This behavior resembles ink spreading through blotting paper along every possible path, where even if a straight line is blocked, the ink finds a way around.
Origin of the Universe
Applying the uncertainty principle to black holes shows that they are not completely black and can leak particles and radiation. Primordial black holes formed in the early universe could be smaller than an atomic nucleus while possessing the mass of Mount Fuji and emitting massive amounts of energy.
Using imaginary time allows physicists to treat the time direction as a spatial dimension, smoothing out singularities and mathematical infinities. Imaginary time is at right angles to real and ordinary time. In imaginary time, space and time directions can meet up to form a closed surface without boundaries or edges, behaving like the surface of the earth.
Just as humanity adjusted to the earth being round rather than flat, imaginary time is an intellectual leap that makes physical laws workable at cosmic boundaries.
The no-boundary proposal suggests that the universe is finite in extent yet has no boundaries, edges, or beginning singularities in imaginary time. Proposed by Jim Hartle and Stephen Hawking in 1983, this model states that the quantum state of the universe is defined by a sum over nonsingular, closed histories, allowing the laws of science to determine everything completely.
Hawking tested the concept physically by traveling around the earth and verifying that you do not fall off a bounded edge.
The early universe underwent a period of extreme inflationary expansion, creating all observed matter out of nothing using gravitational energy debt. During inflation, the universe doubled in size rapidly, borrowing heavily from its negative gravitational energy debt to finance the creation of matter-antimatter pairs, a process analogous to Keynesian economics.
Germany after the First World War experienced extreme economic inflation where a loaf of bread rose to millions of marks, which serves as a human analogy for cosmic expansion scales.
Edwin Hubble's discovery of the expanding universe and the detection of cosmic microwave background radiation disproved the steady state theory. Arno Penzias and Robert Wilson discovered background microwave radiation in 1964 with a temperature of 2.7 degrees above absolute zero, providing the final blow to the steady state theory.
6. Black Holes and Baby Universes
Physics of Black Holes
When a star exhausts its nuclear fuel, outward pressure ceases and gravity causes it to collapse. Once the radius drops to the Schwarzschild radius, light can no longer escape, forming a black hole bounded by an event horizon.
For a star with a mass ten times that of the sun, the Schwarzschild radius is about thirty kilometers. The velocity of light reaches 300,000 kilometers per second, which exceeds the escape velocity of normal celestial bodies.
Properties of Black Holes
Black holes preserve almost no details of the objects that formed them, satisfying the no-hair theorem.
When gravitational collapse creates a black hole, it rapidly settles down to a state characterized by only three parameters: mass, angular momentum, and electric charge. All other information about the collapsed matter is irretrievably lost.
Quantum Effects in Black Holes
Quantum mechanics allows black holes to emit thermal radiation and particles, causing them to eventually evaporate.
Virtual particle-antiparticle pairs materialize near the event horizon. If one member falls into the black hole, the other can escape as radiation. Because the black hole emits particles, it loses mass, heats up, and eventually radiates itself out of existence.
A primordial black hole weighing a billion tons would have a temperature of some 120 billion degrees Kelvin and release energy at the rate of 6,000 megawatts. That billion-ton primordial black hole is comparable to a mountain and features a radius of 10^-13 centimeter, releasing energy at a rate equivalent to six large nuclear power plants. A black hole with the mass of the sun takes 10^66 years to last before evaporating.
Cosmology and Baby Universes
Particles falling into evaporating black holes can pass into self-contained baby universes. As a black hole evaporates down to zero mass, the objects and particles that fell into it can transition into a little baby universe in imaginary time that may later branch off or join our region of space-time.
7. Determinism, Chaos, and the Fate of the Cosmos
The fundamental laws of physics may determine everything, but quantum uncertainty generates a family of different histories for the universe. Although a grand unified theory may dictate the evolution of the universe from its initial state, quantum uncertainty in the early universe means there are multiple possible histories rather than a single deterministic path.
There is a history in which the Nazis won the Second World War, though the probability is low.
Determinism
Equations in physics often exhibit chaos, making long-term predictions impossible even when the basic laws governing a system are fully known. Chaos means systems are unstable where a tiny initial change leads to completely different outcomes later. While we know the laws of chemistry and biology, chaotic behavior in brain equations prevents predicting human behavior or weather far ahead.
Slightly changing how a roulette wheel is spun alters the winning number unpredictably, preventing physicists from making a fortune at casinos.
Evolution and Intelligence
Human ability to draw correct conclusions about physical laws is a product of Darwinian natural selection. Random genetic errors and natural selection favored organisms whose central nervous systems correctly recognized environmental data and took life-preserving actions, eventually leading to language and scientific inquiry. It took 3 billion years to evolve up to the human race and 10,000 years over which written language was developed.
Free Will and Responsibility
Free will and moral responsibility serve as an effective theory because predicting human behavior from fundamental equations is impossible.
Because the human brain contains about 10^26 particles and is sensitive to initial conditions, solving fundamental equations to predict human behavior is completely impossible. Therefore, you must adopt the effective theory of free will.
Limits of Scientific Prediction
The cosmic microwave background radiation is remarkably uniform in every direction, indicating that the large-scale universe is simple and non-chaotic. Observed by the Cosmic Background Explorer satellite, temperature variations in the microwave background are extremely small, showing that average density and expansion rates are uniform across the universe.
A small percentage of the flecks seen on an untuned television screen are microwaves left over from the hot early universe, which raises food to 2.7 degrees above absolute zero. The microwave background is 99,999 parts out of 100,000 identical in every direction, with maximum variation in temperatures across the sky of one part in a hundred thousand.
Fate of the Universe
The ultimate fate of the universe depends entirely on its average density relative to a critical value.
If average density exceeds the critical value, gravitational attraction will halt expansion and cause a recollapse ending in a singularity or big crunch. If density is lower, the universe expands forever.
Visible stars and gas clouds account for one percent of critical density, while observed dark matter in galaxies and clusters provides an estimated density of ten percent. The universe will continue expanding for a minimum of ten billion years before any potential recollapse, having already been expanding for approximately fifteen billion years.
The universe's density is so precisely tuned to the critical value that scientists invoke either the anthropic principle or the theory of inflation to explain it. The anthropic principle posits the universe has this density because otherwise, intelligent observers wouldn't exist to question it. Alternatively, inflation theory proposes massive early expansion that drives the universe onto the exact critical edge, avoiding the amount of one part in a thousand billion by which initial density one second after the big bang would have caused premature recollapse or emptiness.
10 Key Ideas
- Childhood mechanics and games fostered a drive to understand cosmology, which Hawking chose over elementary particles because general relativity provided a rigorous framework.
- An ALS diagnosis shifted his life trajectory, while specialized computer systems and speech synthesizers enabled him to continue writing and lecturing.
- A physical theory should be viewed merely as an elegant mathematical model that describes observations and makes predictions, rather than as a direct description of objective reality.
- In theoretical physics, the search for logical self-consistency and mathematical elegance has historically driven major advances more than raw experimental results.
- Fundamental forces and particle families require a quantum theory of gravity to resolve singularities at the big bang and inside black holes.
- The probability of a particle traveling from point A to point B is calculated by adding up numbers associated with all possible paths it could take, where paths nearly straight contribute the most.
- Using imaginary time allows physicists to treat the time direction as a spatial dimension, smoothing out singularities and mathematical infinities.
- Black holes preserve almost no details of the objects that formed them, satisfying the no-hair theorem.
- Quantum mechanics allows black holes to emit thermal radiation and particles, causing them to eventually evaporate.
- Free will and moral responsibility serve as an effective theory because predicting human behavior from fundamental equations is impossible.