Physics runs on two rulebooks, and they don't agree with each other. That's the whole problem this book is about, and everything from vibrating strings to extra dimensions exists because physicists are trying to write a single rulebook that both can obey.

The first rulebook is general relativity, Einstein's 1915 theory of gravity. It says gravity isn't a force pulling objects together in the way a rope does — it's the effect of mass and energy curving spacetime itself. A planet orbits the sun not because the sun tugs on it, but because the sun bends the space around it into a curve, and the planet is simply following the straightest possible path through that curved space. General relativity is spectacularly successful at large scales: it predicts the bending of starlight around the sun, the existence of black holes, the expansion of the universe, and the slow inspiral of colliding black holes that gravitational-wave detectors picked up in 2015. It treats spacetime as smooth and continuous, with no minimum size or graininess.

The second rulebook is quantum mechanics, developed in the 1920s to describe how matter and energy behave at the scale of atoms and subatomic particles. Quantum mechanics says that at small scales, nature is fundamentally probabilistic and grainy — energy comes in discrete chunks called quanta, particles can behave like waves, and you cannot simultaneously know a particle's exact position and exact momentum (the Heisenberg uncertainty principle). Built on top of quantum mechanics is the Standard Model of particle physics, a well-tested framework that describes all the known fundamental particles — quarks, electrons, photons, and the rest — and three of the four fundamental forces: electromagnetism, the weak force (responsible for radioactive decay), and the strong force (which holds atomic nuclei together). The Standard Model has been confirmed by decades of particle-collider experiments, most famously the 2012 discovery of the Higgs boson.