How can Newton's idea of absolute space be reconciled with Galilean relativity?

Question

I wasn't sure if this might be better suited to History of Science and Mathematics SE, but I suppose it is a bit more 'science-y' than historical.

Apparently Newton believed in absolute space and absolute time, existing independently from anything else within the universe (these beliefs were supposedly laid down in the 'Scholium' of his Principia; see here).

However I was under the impression that Galilean relativity, although supporting the idea of absolute time, did away with absolute space. And yet Newton believed in Galilean relativity and his first law- a body remaining in a state of rest or uniform motion unless acted on by a net force- is essentially a statement of Galielean relativity.

How can it be so that Newton believed in absolute space and believed in Galilean relativity? In fact, in the link it suggests that Newton believed in absolute motion as well (which would be implied from his belief in the existence of absolute space and absolute time anyway).

Perhaps I am getting confused between the notions of 'relativity' and 'absolute space/time'...

Answer 1

Galilean relativity is Galileo's observation that the laws of physics are the same in all reference frames. Specifically, he noted that if a scientist and his laboratory move at constant speed in an unvarying straight path, the scientist will have no way of knowing his reference frame is in motion. In other words, unaccelerated motion of an object has meaning only in relation to another object.

Although Newton believed that there is an absolute space and an absolute time, he also believed that humans live in relative space and relative time. He defined absolute space as a mathematical construction existing "without regard to anything external" (in his own words). But he also postulated that all motion takes place in relative space, which we incorrectly perceive to be absolute space.

Newton did not deny Galileo's principle of relativity. He supplemented it by saying that relative motion exists in a theater of absolute space and time. The idea of absolute space is not necessary to explain the laws of Newtonian mechanics. Perhaps Newton's idea of absolute space provided a way for him to reconcile relativity with the existence of an absolute God.

Answer 2

Newton's idea of absolute space simply appeared as an answer to the following question: What is an inertial system? Saying that an inertial system is one with constant velocity relative to another inertial system of course does not answer the question. To avoid such logical weakness in Newton's first law one has, at some point, to assume that there is a frame of reference, called absolute space, that - by definition - is inertial.

On the other hand Galilean relativity consists on transformations among inertial frames of reference and such relations do not forbid an absolute space. In fact, the idea is that we can use Galilean relativity transformations to relate any inertial frame to the absolute space.

Although the concept of absolute space can be removed by Mach's definition of inertial frame (given an isolated particle, there is a reference frame, called inertial, relative to which the particle has constant velocity), there is no contradiction between Galilean relativity and absolute space. Only Special Relativity can definitely rule out the existence of absolute space.

Answer 3

Ernie's answer is the correct one. But let me emphasize both the tension in what is been asked here and the answer that is given, for future readers.

The principle of relativity can be presented in the form of Galileo's ship: there is no physical experiment you could perform in an inertial reference frame that would tell you that you are moving at a particular speed or you are located in a particular place in space. You can only make relational statements, like "I'm at this distance from this other observer, or I'm moving at a relative speed of 10 m/s relative to this object". If you ignore the motion aspect of it for the moment, the principle of relativity simply tells you that there is no experiment you could ever wish to perform that would tell you your location in space. This means that absolute space is not empirically accesible and you will never get any clues about it whatever you do. So our perception of space is purely relational, relative to other things.

How could Newton believed in absolute space then if he believed in the principle of relativity? One answer could be that there is no reason why a person shouldn't hold two incompatible beliefs in his head, even if it's Newton. We do this all the time. Newtonian mechanics indeed presuposes no absolute space even if Newton personally believed in it.

But a better answer is that both beliefs are actually compatible if you consider the fact that Newton believed absolute space to be the ontology of nature (the ground truth of the universe) even if he also believed that it was empirically innacessible because this absolute space was homogeneous and the laws of physics were invariant under translations (no observer would ever come up with a good experiment to show this is true). So yeah, according to Newton absolute space exists even if it plays no role in physics and therefore it is not a testable hypothesis.

Still, it is also true that Newton played a bit with the idea of perhaps proving the existence of absolute space, and he came up with a thought experiment called the Bucket argument that would suggest there are experiments you could perform that would tell you something about the existence of absolute space. As far as I understand, if this idea was to be taken seriusly, then it would have indeed be in direct contradiction with the principle of relativity (and thus his laws of mechanics) since absolute space would be empirically accesible un thus you would have means of knowing in which inertial frame you are in absolute terms. But I might be mistaken on this. The tension generated by the Bucket argument detonated many thoughts in the XX century before the arrival of Special Relativity.