Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [76]
COPERNICAN PRINCIPLE The idea that there is nothing special about our position in the Universe, in either space or time. This is a generalised version of Copernicus’s recognition that Earth is not in a special position at the centre of the solar system but is just another planet circling the Sun.
COSMIC BACKGROUND RADIATION The “afterglow” of the Big Bang fireball. Incredibly, it still permeates all of space 13.7 billion years after the event, a tepid radiation corresponding to a temperature of –270 degrees Celsius.
COSMIC RAYS High-speed atomic nuclei, mostly protons, from space. Low-energy ones come from the Sun; high-energy ones probably come from supernovas. The origin of ultra-high-energy cosmic rays, particles millions of times more energetic than anything we can currently produce on Earth, is one of the great unsolved puzzles of astronomy.
COSMOLOGY The ultimate science. The science whose subject matter is the origin, evolution, and fate of the entire Universe.
COSMOS Another word for Universe.
DARK ENERGY Mysterious “material” with repulsive gravity. Discovered unexpectedly in 1998, it is invisible, fills all of space and appears to be pushing apart the galaxies and speeding up the expansion of the Universe. Nobody has much of a clue what it is.
DARK MATTER Matter in the Universe that gives out no light. Astronomers know it exists because the gravity of the invisible stuff bends the paths of visible stars and galaxies as they fly through space. There is between 6 and 7 times as much dark matter in the Universe as ordinary, light-emitting matter. The identity of the dark matter is the outstanding problem of astronomy.
DECOHERENCE The mechanism that destroys the weird quantum nature of a body—so that, for instance, it appears localised rather than in many different places simultaneously. Decoherence occurs if the outside world gets to “know” about the body. The knowledge may be taken away by a single photon of light or an air molecule that bounces off the body. Since big bodies like tables are continually struck by photons and air molecules and cannot remain isolated from their surroundings for long, they lose their ability to be in many places at once in a fantastically short time—far too short for us to notice.
DEGENERACY PRESSURE The bee-in-a-box-like pressure exerted by electrons squeezed into a small volume of space. A consequence of the Heisenberg uncertainty principle, it arises because a microscopic particle whose location is known very well necessarily has a large uncertainty in its velocity. The degeneracy pressure of electrons prevents white dwarfs from shrinking under their own gravity, whereas the degeneracy pressure of neutrons does the same thing for neutron stars.
DENSITY The mass of an object divided by its volume. Air has a low density, and iron has a high density.
DIMENSION An independent direction in space-time. The familiar world around us has three space dimensions (east–west, north–south, up-down) and one of time (past-future). Superstring theory requires the Universe to have six extra space dimensions. These differ radically from the other dimensions because they are rolled up very small.
DOUBLE SLIT EXPERIMENT Experiment in which microscopic particles are shot at a screen with two closely spaced, parallel slits cut in it. On the far side of the screen, the particles mingle, or “interfere,” with each other to produce a characteristic “interference pattern” on a second screen. The bizarre thing is that the pattern forms even if the particles are shot at the slits one at a time, with long gaps between—in other words, when there is no possibility of them mingling with each other. This result, claimed Richard Feynman, highlighted the “central mystery” of quantum theory.
ELECTRIC CHARGE A property of microscopic particles that comes in two types—positive and negative. Electrons, for instance, carry a negative charge and protons a positive charge. Particles with the same