Absolute Time
The scientific study of time really began in the 16th Century with the work of the Italian physicist and astronomer Galileo Galilei, and continued in 17th Century England with the work of Isaac Barrow and Sir Isaac Newton. In non-relativistic or classical physics (the physics of Galileo, Newton, Maxwell, etc), time has always been considered one of the fundamental scalar quantities, along with length, mass, charge, etc (a scalar quantity is one that can be described by a single real number, usually with measurement units assigned). It was also considered to be absolute and universal, i.e. the same for everyone everywhere in the universe.
Newtonian Time
According to its most famous proponent, Sir Isaac Newton, for example, absolute time (which is also sometimes known as “Newtonian time”) exists independently of any perceiver, progresses at a consistent pace throughout the universe, is measurable but imperceptible, and can only be truly understood mathematically. For Newton, absolute time and space were independent and separate aspects of objective reality, and not dependent on physical events or on each other.
Time, in this conception, was external to the universe, and so must be measured independently of the universe. It would continue even if the universe were completely empty of all matter and objects, and essentially represented a kind of container or stage setting within which physical phenomena occur in a completely deterministic way. In Newton’s own words: “absolute, true and mathematical time, of itself, and from its own nature, flows equably without relation to anything external”.
Newton’s ideas about absolute time were largely borrowed from Isaac Barrow, his predecessor at Cambridge. Barrow himself described time as a mathematical concept, analogous to a line in that it has length, is similar in all its parts, and can be looked on either as a simple addition of continuous instants, or alternatively as the continuous flow on one instant.
Although absolute time is the “real” time in Newton’s opinion, he cautioned that we mere mortals are not however capable of perceiving it directly. Instead, we are only capable of perceiving what he called “relative, apparent and common time”, which can be observed in the measurement of perceivable objects in motion (like the Moon or Sun) and the ticking of terrestrial clocks, and from which we infer the everyday passage of time. He attributed any discrepancies between absolute and apparent time to such things as irregularities in the motion of the Earth.
The Legacy of Absolute Time
It should be remembered that Newton, perhaps even more than Galileo before him, was a product of his times, and he approached physics from the point of view that God had created the laws of nature, and the job of the scientist was merely to uncover their workings. Newton’s conception of time was essentially unchanged from that of Galileo (and, for that matter, everyone else all the way back to Aristotle some 2,000 years earlier). By systematizing and formalizing it, though, Newton was able to formulate his hugely important laws of motion, and to set the stage for other developments to come.
Newton’s view of absolute time dominated during the explosion of science in the 18th and 19th Century, despite the objections of relativists like Gottfried Leibniz (who believed that time is essentially derived from events – see the section on Early Modern Philosophy for the details behind this argument). Indeed, it persisted until the revelations of Albert Einstein and his 1905 Theory of Relativity turned our conception of time on its head (see the section on Relativistic Time). Einstein once wrote “Newton, forgive me” in his memoirs.
It should be pointed out, though, that the Newtonian version is still a very good approximation of what time is and how it behaves in the world we actually we live in and experience. As we will see, relativistic time only differs from absolute time to any noticeable degree when travelling at speeds approaching the speed of light or in conditions of extremely high gravity. At everyday speeds, everyday common-sense non-relativistic physics applies.