Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [78]
GRAVITATIONAL LIGHT BENDING The bending of the trajectory of light that passes by a massive body. Because the space in the vicinity of such a body is warped like a valley, the light has no choice but to travel along a curved path.
GRAVITATIONAL RED SHIFT The loss of energy as light climbs out of the valley in space-time around a massive celestial body. Since the “colour” of light is related to its energy, with red light having less energy than blue light, astronomers talk of light being shifted to the red end of the spectrum or “red-shifted.”
GRAVITATIONAL WAVE A ripple spreading out through spacetime. Gravitational waves are generated by violent motions of mass, such as the merger of black holes. Because they are weak, they have not yet been detected directly.
GRAVITY See Gravitational Force.
HALF-LIFE The time it takes half the nuclei in a radioactive sample to disintegrate. After one half-life, half the atoms will be left; after two half-lives, a quarter; after three, an eighth, and so on. Half-lives can vary from the merest split-second to many billions of years.
HEISENBERG UNCERTAINTY PRINCIPLE A principle of quantum theory that there are pairs of quantities such as a particle’s location and speed that cannot simultaneously be known with absolute precision. The uncertainty principle puts a limit on how well the product of such a pair of quantities can be known. In practice, this means that if the speed of a particle is known precisely, it is impossible to have any idea where the particle is. Conversely, if the location is known with certainty, the particle’s speed is unknown. By limiting what we can know, the Heisenberg uncertainty principle imposes “fuzziness” on nature. If we look too closely, everything blurs like a newspaper picture dissolving into meaningless dots.
HELIUM Second lightest element in nature and the only one to have been discovered on the Sun before it was discovered on Earth. Helium is the second most common element in the Universe after hydrogen, accounting for about 10 per cent of all atoms.
HORIZON The Universe has a horizon much like the horizon that surrounds a ship at sea. The reason for the Universe’s horizon is that light has a finite speed and the Universe has been in existence for only a finite time. This means that we only see objects whose light has had time to reach us since the Big Bang. The observable universe is therefore like a bubble centred on Earth, with the horizon being the surface of the bubble. Every day the Universe gets older (by one day), so every day the horizon expands outwards and new things become visible, just like ships coming over the horizon at sea.
HORIZON PROBLEM The problem that far-flung parts of the Universe that could never have been in contact with each other, even in the Big Bang, have almost identical properties such as density and temperature. Technically, they were always beyond each other’s horizon. The theory of inflation provides a way for such regions to have been in contact in the Big Bang and so can potentially solve the horizon problem.
HYDROGEN The lightest element in nature. A hydrogen atom consists of a single proton orbited by a single electron. Close to 90 per cent of all atoms in the Universe are hydrogen atoms.
HYDROGEN BURNING The fusion of hydrogen into helium accompanied by the liberation of large quantities of nuclear binding energy. This is the power source of the Sun and most stars.
HYDROSTATIC EQUILIBRIUM The state in which the gravitational force trying to crush a star is perfectly balanced by the force of its hot gas pushing outwards.
INERTIA The tendency for a massive body, once set in motion,