What is relativity?
Sir Isaac Newton built upon the ideas of relativity that were first developed in the 1632 publication of Galileo Galilei who first coined the term with respect to what we now call physics. Although relativity also refers to concepts in social science, it is in physics that it is best known. Galileo described the idea of the relative position, or viewpoint, of an observer as being fundamental to observable science. He came up with the idea that someone below deck on a boat that was travelling at a constant speed without rocking would have no idea if he or she were, indeed, moving. It helped him to explain why the Earth might move around the sun even though human beings cannot sense their planet whizzing through space.
What is the principle of relativity?
The principle of relativity is derived from Galileo's ideas. It is a frame of reference for describing scientific enquiries themselves which allow the laws of physics to be written and apply everywhere. For example, Newtonian mechanics are a form of the principle of relativity in action. When the work of Einstein expanded our knowledge of physics in the twentieth century, his principles had to conform with Newtonian ideas somehow so that the same frame of reference, or principle, was maintained.
Is there a difference between general and special relativity?
Otherwise known as GTR, the general theory of relativity is a concept in geometry that relates to gravitation. Einstein published on the subject of GTR in 1915, revolutionising human understanding on the topic, as previously laid out by Newton. Einstein introduced a new concept, known as space-time, which Newton had not included, that allowed him to explain why gravity varies according to its relative position to its observer. Special relativity is also attributed to Einstein, but it was a theory developed by him from the work of Hendrik Lorentz. In this theory, an attempt is made to define the relationship between time and physical space. Famously, it postulates the notion that the speed of light is a constant for all observers regardless of their relative position, so long as it is travelling in a vacuum.
What is numerical relativity?
Numerical relativity is a mathematical discipline that was developed from Einstein's GTR model. It uses numerical methods and highly powered algorithms to solve all sorts of problems, such as those involved in the study of neutron stars or black holes which cannot be observed easily by other means. This is because the nature of space-time is not yet fully understood. Usually, numerical relativity is reserved for supercomputers which carry out high-end computational functions.
How does relativity explain black holes?
According to GTR, items of sufficient mass to constitute a black hole are theoretically possible in the universe. Being things that are so high in gravity that they don't allow light to escape them, it is not possible to observe them visually. However, their bending of space-time has now been observed from Earth, lending credit to the theory of general relativity.