Swot up on your physics wi th our handy glossary, by popular science writer
Central to the second law of thermodynamics, entropy is a measure of the disorder in a system. It reflects the number of different ways the components of a system can be rearranged. The letters making up the words on this page have low entropy – there’s only one way to arrange them (assuming each individual a,b, c, etc. is unique) to produce the text you’re reading. But if you scramble the letters, it will have higher entropy, as there are lots of ways to arrange them jumbled up. The second law of thermodynamics reflects that it’s easier to go from an ordered page to scrambled letters than it is to go from a pile of letters to the contents of this magazine. Similarly, it’s easier to break an egg than to unbreak it.
FUNDAMENTAL FORCES OF NATURE
Physics recognises four fundamental forces: electromagnetism, which deals with interactions in matter and light; the strong nuclear force, which holds the particles of atomic nuclei together; the weak nuclear force, which is involved in nuclear decay; and gravity. All except gravity fit with quantum theory.
The General Theory of Relativity, published by Einstein in 1915, explains how mass warps space and time, and how these warps influence the way that matter moves. It provides equations that give us a precise description of gravity, indirectly predicting phenomena like black holes, gravitational waves and the Big Bang.
Einstein’s General Relativity predicts that massive objects warp space enough to make passing light curve around them. This means that large cosmic structures like galaxies can act like lenses. Light coming from behind the galaxy is bent around it towards the viewer, bringing distant bodies into focus.
This stands for Modified Newtonian Dynamics – a theory that expands on Newton’s laws of motion. It offers a potential explanation for the unexpected behaviour of spiral galaxies and galactic clusters usually attributed to dark matter. It is based on the idea that the effect of gravity behaves in a subtly different manner on a vast scale. Even so, it still doesn’t explain all the observed oddities – but then neither does dark matter.
German physicist Max Planck mathematically derived the Planck length, a unit of distance around 100 billion billion times smaller than the nucleus of an atom, using constants of nature such as the speed of light. If space is not continuous but made up of quanta – the minimum amount of a physical property that can be interacted with – it has been suggested that its quanta might be a Planck length across (see below for more on quantum theory). Below this distance, measurement would not be possible. A Planck area is a Planck length squared. In black hole theory, when a black hole absorbs a single bit of information, its event horizon – the boundary around it from which not even light can escape – expands by one Planck area.
This theory describes the behaviour of light and matter on a very small scale – that of individual particles such as atoms, electrons and photons. The theory takes its name from its central idea that phenomena are not continuous in nature but are instead broken down into tiny indivisible chunks or packets called quanta. In classical mechanics, objects always exist in a specific place at a specific time. But in quantum theory we can only determine the probability of an object being in a certain place at a certain time. This seems counterintuitive, but the theory is incredibly successful in explaining the interactions of light and matter.
String theory was devised to explain inconsistencies in particle physics. It is a leading approach in the attempt to produce the so-called Theory of Everything. In string theory, particles are replaced with vibrating strings, but for the maths to work there need to be nine spatial dimensions rather than the three we observe.
Originally developed to provide a theoretical basis for the design and operation of steam engines, thermodynamics – literally the movement of heat – is now a fundamental area of study in physics. It has four laws, of which the most important are the first ‘energy is always conserved’, and the second ‘heat always moves from a hotter to a colder body’. The second law also shows that, on average, in a system that’s isolated from its surroundings, entropy stays the same or increases – to decrease it requires energy.
The amount of light energy emitted by a spiral galaxy such as the Milky Way is roughly proportional to its speed of rotation. The faster the galaxies spin, the brighter they are. This is known as the Tully- Fisher relation, named after the astronomers Brent Tully and Richard Fisher who discovered it.