Key Concepts for the 21st Century

'Key Concepts for the 21st Century', brings together a selection of leading Scientific concepts concerning the formation of the universe. Introduced here, they summarise the incredible progress achieved through Scientific development. These advancements have made an impact on all our lives and have challenged the way we look at the world.

Newton's Mechanics

The theory of motion presented by Sir Isaac Newton in his great Principia (1686). It consists of a set of mathematical laws describing the rigidly deterministic motion of objects under the action of forces against the backdrop of an absolute space and absolute time. Newtonian mechanics governed the way in which scientists described the physical world for more than two centuries, until it was overthrown by experimental and theoretical developments in the early part of the 20th Century.

Quantum Theory

Quantum theory describes the behaviour of matter on very small scales. The quantum world essentially comprises two distinct notions. One of these is that matter and energy are not smoothly distributed but are to be found in discrete packets called quanta. The other is that the behaviour of these quanta is not predictable as in Newton's theory, but that only probabilities can be calculated.

The Big Bang

The Big Bang is a term, originally coined by Sir Fred Hoyle, that describes the standard picture of the cosmos and how it evolves. Currently expanding and cooling, the universe was hotter and denser in the past. Clues to its high-energy phase can be found in its expansion, in the relic radiation that pervades all space, and in the trace quantities of light atoms cooked in the primordial nuclear furnace. The early stages of the Big Bang are used by particle cosmologists to study the character of the fundamental forces of nature. The Big Bang model breaks down at the very beginning of space and time because of the existence of a singularity. It is therefore seriously incomplete, and will remain so unless and until a quantum theory of gravity has been worked out.

Black Holes

Black holes are regions of space-time where the effect of gravity is so strong that light cannot escape. Black holes are thought to exist in nature, but though the evidence for them is compelling, it remains circumstantial. For theorists, black holes provide natural test cases in which to try to explore the consequences of fitting Einstein's general theory of relativity together with the principles of quantum mechanics. Hawking himself showed that quantum effects can allow black holes to radiate, so that they are not entirely black.

Relativity

Albert Einstein developed the theory of relativity in a series of monumental papers in the early part of the 20th century, beginning with the publication of the special theory of relativity in 1905 and culminating in the general theory of 1915. Relativity theory is a theory of space and time. It deprived physics of the absolute meaning of these concepts that was embedded in Newtonian mechanics. Dealing not with space and time separately, but with a hybrid concept called space-time (which can be curved and warped), relativity replaced Newton's law of gravity with a theory of how space can be distorted by the presence of mass.

Singularities

A singularity is a point or region of space-time where the mathematical equations of a theory break down because some quantity becomes infinite. The centre of a black hole is an example of such a singularity in the general theory of relativity, as is the origin of the universe in the Big Bang model. Penrose and Hawking have proved a number of theorems about the nature and occurrence of these singularities. Their existence in Einstein's theory suggests that general relativity may be incomplete. A quantum theory of gravity is required to describe the properties of matter at the enormous densities that pertain at the Big Bang or in a black hole.

Unified Theories

As physics has grown through the 20th century, it has brought more and more disparate phenomena within the scope of unified theories. The first major step in this programme was the unification of the theories of electricity and magnetism by James Clerk Maxwell, to produce a theory of electromagnetism. Theories now exist in which electromagnetism and the nuclear forces can be described in terms of a single set of mathematical formulae. Physicists would like to include the one force missing from this treatment so far - gravity - but this force has so far eluded attempts to include it. If and when gravity is unified, a 'Theory of Everything' would be the result.

Quantum Gravity

The 'missing link' in the chain of reasoning leading to a Theory of Everything is a mathematical description that combines the general theory of relativity with the ideas of quantum mechanics. Although much effort has been expended in the search for such a theory, formidable mathematical difficulties have defeated many attempts. Only in a few special cases have gravity and quantum theory been combined in an intelligible way.