Time and Space

One of the key assumptions that Newton felt he had to introduce was the concepts of time and space. He defines time as

Absolute, true and mathematical time, of itself, and from its own nature flows equably without regard to anything external, and by another name is called duration: relative, apparent and common time, is some sensible and external (whether accurate or unequable) measure of duration by the means of motion, which is commonly used instead of true time ...

and space as

Absolute space, in its own nature, without regard to anything external, remains always similar and immovable. Relative space is some movable dimension or measure of the absolute spaces; which our senses determine by its position to bodies: and which is vulgarly taken for immovable space ... Absolute motion is the translation of a body from one absolute place into another: and relative motion, the translation from one relative place into another ..

both space and time exist independent of all people, all observers, and all matter. Space for him is of course also Euclidean space (the laws and axions of Euclid are all satisfied). He also clearly distiguishes between absolute and relativie motion and space.

While absolute time is more acceptable, absolute space is in direct contradiction to Galilean Relativty. Newton used rotation to argue that one can tell the difference between a rotating body and one that is not rotating. But of course, a rotating body does not obey the law of inertia. The motion of the pieces of the rotating body must have forces on them because they have centrigugal accelerations, while the pieces of a body which is itself moving uniformly in a given direction do not need any forces because they are moving naturally. Part of this might be still a holdover from Aristotle, or from Descartes whose "plenum" could be said to provide a natural frame defining the natural velocity of any frame. However, Newton's law state that this is all irrelevant. There is not natural rest frame. All of Newton's laws are still the same if we were to move everything at a uniform velocity. Uniform velocity plus uniform velocity is also uniform velocity, so Newton's first law is satisfied. Uniform velocity has no acceleration, so does not change the right side of F=ma. The forces (especially gravity) depend only on the difference in the distances between two bodies, and those distances do not change if both objects are moving in the same way, so the left side does not change so Newton's second law does not notice uniform motion. And the equal and opposite does not change if the forces are moving.

This caused long long running argument between Newton and Leibnitz (who thought that both time and space must be derived quanlities which arose only because of the relationships of pieces of matter to each other), and a dispute which goes on even today between the relationists and absolutists.

copyright W Unruh (2018)