By Stephen Hawking
Science's greatest mysteries explained simply.
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Themes
An elderly woman at a public lecture interrupts to explain that the world rests on a giant tortoise. Asked what the tortoise stands on, she replies it is turtles all the way down. The story opens the deepest question in physics: not just what the universe is made of, but what counts as an explanation at all.
An elderly woman at a public lecture interrupts to explain that the world rests on a giant tortoise. Asked what the tortoise stands on, she replies it is turtles all the way down. The story opens the deepest question in physics: not just what the universe is made of, but what counts as an…
When it appeared in 1988, A Brief History of Time occupied an unusual position in the culture of popular science. It was not the first attempt to explain physics to a general audience, but it was among the most ambitious, and its commercial success transformed expectations about what such books…
Physics has never possessed a single theory that explains everything. It has operated through partial theories, each valid within a specific range of conditions, each breaking down when pushed beyond its domain. Understanding why this is so, and what it means, is one of the central concerns running…
Until the twentieth century, space and time were considered separate and absolute. Space was the stage on which events occurred; time was the clock that measured their duration. Both were assumed to be the same for all observers, regardless of where they were or how fast they were moving. Einstein…
For most of the nineteenth century, physicists assumed that the universe was in principle completely knowable. Given sufficiently precise measurements, everything that would ever happen could be calculated in advance. Laplace stated this explicitly, imagining a supreme intelligence that, knowing the…
The question of whether the universe had a beginning was, for most of human history, a matter for theology and philosophy. Science had nothing useful to say about it. That changed in the twentieth century, when observations and mathematics together transformed it into an empirical question with…
From the earliest attempts to explain the movements of stars and planets to the construction of quantum field theories and models of the cosmos, the history of science is a record of humanity's expanding capacity to comprehend the universe. This progress has not been smooth or linear. It has…
An elderly woman interrupts a scientist's public lecture on astronomy to declare that the universe rests on a giant tortoise. When asked what supports the tortoise, she answers without hesitation: it is turtles all the way down. The story captures an enduring human impulse to build a picture of the…
Until 1924, astronomers assumed the Milky Way was the entire universe. Edwin Hubble ended that assumption by measuring the distances to faint cloudy objects called nebulae and showing they were independent galaxies, millions of light-years beyond our own. The observable universe turned out to…
Aristotle thought matter was infinitely divisible. The atomists disagreed, proposing that everything was made of indivisible particles. John Dalton settled the argument in 1803 by demonstrating that chemical compounds always combine in fixed proportions, a result that only makes sense if matter…
In November 1970, while preparing for bed, Hawking realises something fundamental about the boundary of a black hole. Light rays forming the event horizon can never approach one another: if they did, they would fall inward and cease to be part of the boundary. The area of the event horizon can…
The laws of physics treat past and future identically. Run a collision film backward and the equations still hold. Yet a broken cup never leaps off the floor and reassembles itself on a table. The gap between symmetric laws and asymmetric experience requires explanation. Three arrows of time all…
Physics rests on two great partial theories. General relativity governs gravity and the large-scale structure of the universe. Quantum mechanics governs the subatomic world. Each is extraordinarily accurate within its own domain, and each breaks down completely where the other must apply: at the big…
An elderly woman at a public lecture interrupts to explain that the world rests on a giant tortoise. Asked what the tortoise stands on, she replies it is turtles all the way down. The story opens the deepest question in physics: not just what the universe is made of, but what counts as an explanation at all.