Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [81]
PARTICLE PHYSICS The quest to discover the fundamental building blocks and fundamental forces of nature.
PAULI EXCLUSION PRINCIPLE The prohibition on two microscopic particles (fermions) sharing the same quantum state. The Pauli exclusion stops electrons, which are fermions, from piling on top of each other and, consequently, explains the existence of different atoms and of the variety of the world around us.
PHOTOCELL A practical device that exploits the photoelectric effect. The interruption of an electric current when a body breaks the light beam falling on a metal is used to control something—for instance, an automatic door at the entrance to a supermarket.
PHOTOELECTRIC EFFECT The ejection of electrons from the surface of a metal by photons striking the metal.
PHOTON Particle of light.
PHYSICS, LAWS OF The fundamental laws that orchestrate the be-havior of the Universe.
PLANCK ENERGY The superhigh energy at which gravity becomes comparable in strength to the other fundamental forces of nature.
PLANCK LENGTH The fantastically tiny length scale at which gravity becomes comparable in strength to the other fundamental forces of nature. The Planck length is a trillion trillion times smaller than an atom. It corresponds to the Planck energy. Small distances are synonymous with high energies because of the wave nature of matter.
PLASMA An electrically charged gas of ions and electrons.
POSITRON Antiparticle of the electron.
PRECESSION OF THE PERIHELION OF MERCURY The fact that the orbit of Mercury, the planet closest to the Sun, does not follow a straightforward elliptical orbit but rather an elliptical orbit whose nearest point to the Sun gradually moves around the Sun, resulting in the planet tracing out a rosettelike pattern. The explanation is that the gravity of the Sun weakens with distance from the Sun more slowly than in the case of Newtonian gravity, which uniquely predicts elliptical orbits. It weakens more slowly because, in the Einsteinian picture, gravity itself is a source of more gravity.
PRINCIPLE OF EQUIVALENCE The idea that gravity and acceleration are indistinguishable.
PROTON One of the two main building blocks of the nucleus. Protons carry a positive electrical charge, equal and opposite to that of electrons.
PULSAR A rapidly rotating neutron star that sweeps an intense beam of radio waves around the sky much like a lighthouse.
QED See Quantum Electrodynamics.
QUANTUM The smallest chunk into which something can be divided. Photons, for instance, are quanta of the electromagnetic field.
QUANTUM COMPUTER A machine that exploits the fact that quantum systems such as atoms can be in many different states at once to carry out many calculations at once. The best quantum computers can manipulate only a handful of binary digits, or bits, but in principle such computers could massively outperform conventional computers.
QUANTUM ELECTRODYNAMICS Theory of how light interacts with matter. The theory explains almost everything about the everyday world, from why the ground beneath our feet is solid to how a laser works, from the chemistry of metabolism to the operation of computers.
QUANTUM INDISTINGUISHABILITY The inability to distinguish between two quantum events. These may be indistinguishable, for instance, because they involve identical particles or simply because the events are not observed. The crucial thing, however, is that the probability waves associated with indistinguishable events interfere. This leads to all manner of quantum phenomena.
QUANTUM NUMBER A number that specifies a microscopic property that comes in chunks such as the spin or orbital energy of an electron.
QUANTUM PROBABILITY The chance, or probability, of a microscopic event. Although nature prohibits us from knowing things with certainty, it nevertheless permits us to know the probabilities with certainty.
QUANTUM