# Is space a thing or a relationship between existents?

1. Mar 16, 2012

### steve watson

Is "space" a thing or a relationship between existents?

When I think of "space", I think of the distance between things rather than "space" being a thing like me or a planet or some other existent. If "space" is a a thing and not "nothing" can it be described like any other thing that exist, e.g., a pencil?

2. Mar 16, 2012

### Jano L.

Re: Is "space" a thing or a relationship between existents?

Hello Steve,

welcome at PF. Newton introduced absolute space and ascribed to it the origin of some forces, like the centrifugal force. He also made an experiment to demonstrate that the absolute space does influence material objects: water in a jar that is at rest has a flat surface, but in rotating jar it has curved surface even when it is still with respect to the jar. I have read somewhere that he saw imperfections of absolute space, but he decided that still it is the best thing he can think of and went on with developing his mechanics.

Einstein's general theory of relativity introduced a different view on space (and melded it with time), but confirmed that it plays an important role. In this theory, the space has properties which differ in each its point (like curvature) and now even the space itself is influenced by the material objects.

These theories were very successful. However, the concept of absolute space was criticized by Mach. The basic idea of Mach is the following. He pointed out that without external objects, the water in the jar would have no way to tell that the jar is rotating. Hence it is possible that the water would be always flat. This hints that maybe they are the external objects that are responsible for the centrifugal force on the water.

Machism is a very attractive idea (it was to Einstein as well). It basically says we should not be content with the theory we have but search for better and simpler explanation. I think it would be great, if we could explain properties of absolute space - such things as centrifugal force, existence of inertial frames, or even the fact that there are three dimensions - in terms of relations between objects.

There are some attempts at building theory without the absolute space, relying only on relations between material objects. It is called relational mechanics - it is about relations between objects, in distinction to relations between objects and space, which is in general use. Unfortunately, these attempts probably were not as successful as the ordinary theory.

3. Mar 16, 2012

### steve watson

Re: Is "space" a thing or a relationship between existents?

Hello back, Jano L.
Tks for the welcome and tks for the reply.
I'm not sure I understand all that you wrote. I think my first problem is that I am not clear on what you mean by "absolute space" ... I'm puzzled why "space" has to be described as "absolute" rather than simply by "space". Conversely, can "space" be described as being "inabsolute"?

4. Mar 16, 2012

### Jano L.

Re: Is "space" a thing or a relationship between existents?

As I understand it, the adjective " absolute " refers to the fact that the water in the jar is rotating with respect to space absolutely. This means every possible observer will conclude there is mutual motion between the jar and the space.

This stands in contrast to inertial motion, like that of a moving train, which is only relative. This means that all motions happen the same way and the same laws hold with respect to train as if it was at rest.

5. Mar 16, 2012

### steve watson

Re: Is "space" a thing or a relationship between existents?

But how else can the water in the jar be rotating if not "with respect to the
"space 'absolutely' "? It cannot be rotating "inabsolutely". Seems to me the water is either roating in the jar or not roatating in the jar, and the adjectivie "absolutely" describes nothing or is redundant. And every possible observer should be observing the same thing, save for the power of their eyes and/or the distances from which they percieve this.

I'm not really clear about the train example.

6. Mar 17, 2012

### Jano L.

Re: Is "space" a thing or a relationship between existents?

But it could.

For example, it is possible to imagine that the jar together with water was rotating with respect to Earth in such a way that the surface of the water was planar. Then we could jump into reference frame R centered at the jar and rotating together with it and we would see water standing still in the jar. There would be no sign of rotation in the behaviour of the jar with water. Hence we would describe the things as follows: the rotation is with respect to Earth, and is not present with respect to the reference frame R.

If only some observers concluded there is rotating motion of the water, but others did not, then it would be natural to say the rotation of the water is relative (need another object to be related to).