By Brian Greene
String theory's quest to unify physics
Subject
Themes
Modern physics rests on two towering pillars that cannot both be right as currently formulated. Einstein's general relativity governs the cosmos on the scale of stars and galaxies, describing gravity as the warping of space and time by mass and energy. Quantum mechanics governs the realm of atoms and subatomic particles, where Heisenberg's uncertainty principle ensures that even empty space seethes with fleeting activity. Each theory has been confirmed to extraordinary precision within its own domain, yet whenever both are needed at once, inside a black hole or at the first instant of the big bang, their combined equations collapse into nonsense, producing impossible answers such as infinite probabilities.
Modern physics rests on two towering pillars that cannot both be right as currently formulated. Einstein's general relativity governs the cosmos on the scale of stars and galaxies, describing gravity as the warping of space and time by mass and energy. Quantum mechanics governs the realm of atoms…
Published in 1999, this account of superstring theory arrived at a distinctive moment in the history of theoretical physics, one shaped by roughly three decades of failed attempts to unite quantum mechanics with general relativity and by a very recent surge of confidence following the mid-1990s…
At the heart of modern physics sits an uncomfortable fact: the two theories that between them describe essentially everything in the observable universe cannot both be correct as currently formulated. General relativity governs the realm of the very large, predicting with exquisite accuracy how…
A central tension runs through the account of string theory's development: a theory can be mathematically elegant, internally consistent, and capable of resolving deep theoretical puzzles, yet still lack the one thing that has traditionally validated scientific ideas, direct experimental…
One of the most disorienting ideas explored is that the three dimensions of space familiar from everyday experience, left and right, forward and back, up and down, may not be the whole story. The suggestion that the universe contains additional spatial dimensions, curled up so tightly that they have…
Perhaps the most unsettling thread running through the whole account is the repeated demonstration that space and time, the concepts that feel like the most basic and unquestionable backdrop of everyday experience, are not fixed absolutes at all. Special relativity establishes the first crack in…
The progress traced from Einstein's early twentieth century breakthroughs to the string theory revolutions of the 1980s and 1990s does not unfold as a single triumphant march toward a finished theory. Instead, it advances through a distinctive rhythm: bursts of dramatic insight followed by long…
Modern physics rests on two towering pillars that, as currently formulated, cannot both be right. Einstein's general relativity governs the universe on the scale of stars, galaxies and the cosmos as a whole, while quantum mechanics governs atoms, electrons and quarks. Each has been confirmed to…
Special relativity's ban on faster-than-light travel clashes directly with Newton's theory of gravity, which assumes gravitational influence is transmitted instantaneously across any distance. If the sun exploded, Newton's equations say the earth would instantly leave its orbit, even though light…
Quantum mechanics and general relativity each work brilliantly within their own domains, but combining them, as required inside black holes or at the big bang, produces nonsensical answers such as probabilities that come out infinite. Heisenberg's uncertainty principle implies that even seemingly…
Physical laws are symmetric in time, place and orientation, meaning the same laws apply everywhere and always, and special and general relativity extend this symmetry to all observers regardless of their motion. In 1967 Sidney Coleman and Jeffrey Mandula proved that no further such symmetries…
Edward Witten argues that string theory's inclusion of gravity through its graviton vibration counts as a postdiction rather than a confirmed prediction, since gravity was already known before string theory existed. Sharp criticism was levelled at string theory in the 1980s: Sheldon Glashow and Paul…
General relativity's Riemannian geometry forbids the fabric of space from tearing, yet speculative ideas like wormholes and the singularities inside black holes, protected by Roger Penrose's cosmic censorship hypothesis, raise the question of whether quantum gravity might permit genuine spatial…
Research in the late 1960s and 1970s by physicists including Werner Israel, Roy Kerr and Stephen Hawking established John Wheeler's no hair principle: black holes are fully characterised by just mass, force charge and spin, the same properties distinguishing elementary particles, hinting that…
Edward Witten reflects that string theory might either be near completion or merely one step on a much longer road, remarking that younger physicists well into the twenty-first century will have to judge whether a final theory has truly been found. Five open questions are surveyed for the future of…
By Brian Greene
Subject
Themes
Modern physics rests on two towering pillars that cannot both be right as currently formulated. Einstein's general relativity governs the cosmos on the scale of stars and galaxies, describing gravity as the warping of space and time by mass and energy. Quantum mechanics governs the realm of atoms and subatomic particles, where Heisenberg's uncertainty principle ensures that even empty space seethes with fleeting activity. Each theory has been confirmed to extraordinary precision within its own domain, yet whenever both are needed at once, inside a black hole or at the first instant of the big bang, their combined equations collapse into nonsense, producing impossible answers such as infinite probabilities.