# What is Space?

1. Dec 7, 2004

### cAm

First off, sorry if this is the wrong forum, obviously i'm new here, so not sure quite how things go

Anyway, I've been thinking a bit, and wondering exactly What space is, or what the current theory on it is. From what i have thought about myself, it seems that space would have to have a certain density because it has to have volume (doesn't it?), so also, would its density have to be constant? Would it have any relationship to fluids? Most stuff i read doesn't have that much to say about 'space' but it IS there, so it has to be dealt with. From what i've heard about string theory (i haven't read much, so sorry if i'm wrong on this) strings might be considered the building block of Everything, but if that is so, what is BETWEEN the strings? There has to be something between them, because if there was Nothing between things, then they wouldn't be seperated...

thats all i have for now, please, destroy this theory, and replace it w/ a better one :tongue2:

and sorry if this isn't too comprehensible, my brain is shot, just finished phys c2 test and digi elec tests :tongue2:

2. Dec 7, 2004

### Rob Woodside

The short answer is no one knows. People have been dealing with "space" since they were throwing rocks at small animals for food and wondering at the night sky. It wasn't until the Sumerians and Egyptians, concerned with land ownership, started tabulating the sides of right angle triangles that the notion of space began to appear. The sumerians had a different unit for vertical distance than horizontal distance to account for the fact that things fall down and not sideways. The Greeks axiomatized the early results into Euclidean geometry that we now call flat space. This was the first axiomatic theory and it gives a geometry that is different from our visual space. Visually, railroad tracks bend and join at the horizon. If you want to build things you can't just trust your eyes! Some 1500 years later Newton built a theory of motion of physical bodies on an arena of flat space. This space was immutable and unaffected by the presence of matter. In the nineteenth century mathematicians realized there were alternative geometries that were logically independant from Euclidean geometry. Gauss and Riemann measured the angles in a triangle made by three mountain tops and allowing for earth's curvature found the angles to sum to 180 degrees, just what Euclid would have suspected. Early last century Einstein realised that the arena for Newtonian mechanics was different than the arena for electromagnetism. To resolve the difference Einstein had to weld space and time together to make space-time and the theory of special relativity. Then thinking deeply about gravitation, Einstein came up with General Relativity. In this theory the distance between any two points depends slightly on the presence of nearby matter. Although one talks of relativity "theory", both special and general are experimentally well justified. More recent ideas of a discrete space-time and spin networks etc do not share that luxury.

So what is space? Is it a real thing or is it a conceptual map that allows us to get about? Is it a material substance with odd properties or a mental construct that lives in our heads and mirrors relations between physical objects? Mathematicians have the same problem with numbers. I would argue that reality consists of physical objects AND the relations between them. People who talk of the "fabric of space-time" are telling you space is a material substance. This raises the possibility of cutting and pasting it to make wormholes like trouser legs. Sadly I doubt this. My personal belief is that space is purely relational.

Last edited by a moderator: Apr 22, 2005
3. Dec 7, 2004

### pmb_phy

The container of objects, i.e. That which contains all objects.

That's the short version. There is some physics literature on this. Two noted texts are Concepts of Space, by Max Jammer and Space & Time, Hans Reichenbach.

Einstein wrote on this some in his relativity texts. He also spoke a bit on it in the foreword of Jammer's text referenced above. Basically I think his definition was "That which contains all objects" but that summary probably doesn't do it justice.

Pete

4. Dec 7, 2004

### marcus

you are in good company
a long history of this idea
from aristotle to descartes, including Leibnitz

space does not have a separate existence, it is not an absolute thing that exists in and of itself
but consists of the relations between things, between events

next to, between this and that, where that happened

In Rovelli book Quantum Gravity (last year's draft is downloadable free fromhis site just google the name "rovelli") he has a discussion of
the history of the idea of space, and different ideas of space (and time)
in physics today.

Particle theorists, quantum field theorists, tend to treat space as an absolute-----some kind of space exists on its own and things happen in it.
Relativists (including quantum gravitists) tend to treat space as relational.

Newton broke with the tradition of relational space and invented an absolute space and time (Rovelli has some interesting quotes, Newton had some misgivings about what he was doing).
Flat Minkowski space is the descendant of newtonian absolute space.

In 1915 Einstein went back to a relational approach and realized the flat space of his 1905 theory as one possible solution of the GR equation, a solution where there is no matter to curve the space.

In GR points have no physical reality-----there are only things, events, relations between them----rovelli has some interesting quotes from Einstein about this too.

It is a fascinating subject.

I suppose we will understand space better when we understand why matter bends it.

there is a sense in which space is nothing other than the gravitational field
(and the gravitational field can be defined in such a way that it does not require any absolute space in which to be defined)

5. Dec 8, 2004

### pmb_phy

This is related to Mach's Principle and the notion that in the absence of matter there is an absence of space (no metric). It also is related to the specific interpretation of Mach's Principle. I've seen some claim that space exists even in the absence of matter. But I've never agreed with that interpretation.

Pete

6. Dec 8, 2004

### Rob Woodside

Quote from Marcus:
...there is a sense in which space is nothing other than the gravitational field (and the gravitational field can be defined in such a way that it does not require any absolute space in which to be defined)

Quote from Pete:

Reply: I think it more of a perspective than a definition. If you do not see this you are only considering a physical field living in material spacetime. What Marcus is meaning by absolute space is just such a substantial space that exists as a thing in itself. This absolute space is essentially independent from other kinds of matter, but possibly influenced by them. However, spacetime may be more subtle than a physical object or material substance. Reality may consist of physical objects and the patterns they follow or relations they have. Thus the electromagnetic field, current densities and vector potential (up to a guage transformation) are physical objects and Maxwell's equations are just as real, but not physical objects. They are the real patterns and relations that the physical objects enjoy. Taking fields as physical objects one can imagine some special fields that were non vanishing everywhere and every when. Such physical objects could be the metric (up to guage transformations) or curvature, then spacetime manifolds are just some of the real relations or patterns enjoyed by the curvature or metric.

Last edited: Dec 8, 2004
7. Dec 8, 2004

### QuantumTheory

I believe space is nothing more than a fabric of time itself. It is a void, where there is matter, dark matter, and anti-particles. We will never figure out EVERYTHING about the universe; there will always be questions.

Everything in this world, on our earth, and in the universe as a whole has a relationship to one another.

For example, to understand why plantets orbit, why blackholes work, etc, and to understand space as "a fabric", do this little experiment:

Take a light fabric of some type, and put a somewhat heavy ball in the middle. Then let one person hold onto the fabric, while you put the ball in the middle. Then, take a smaller ball, at least 1/2 size of the larger one, and you'll see that the lighter ball will spin around the heavier one, around and around, until it finally meets it.

Space is the sheet, or fabric, and the ball is any massive object in space. Thus, space and time.

Another fun way to try a fun experiment to have simulate black holes, is to do this:

Take a cup of coffee. Put a spoon in it, stir the coffee ALOT with the spoon. You'll notice the motion from the spoon is direcetd into the coffee, making it whirl around in circles. Then take a small object, like a pea, and put it in the coffee. The pea will whirl around, and around, and around, until it gets to the center, and then dip down into the middle of the "black hole".

In this case, the swirling coffee is the black hole, while the pea is some lighter object ( star, etc) .

8. Dec 8, 2004

### Rob Woodside

So Quantum Theory thinks that space is merely a fabric of time, a void, the sheet, or fabric. No wonder Einstein thought that Quantum Theory was confused.

When I stir a coffee cup with something floating on the surface, I find that it settles on the rim and not at the centre when the rotation stops. Similarly something like tea leaves that rest on the bottom collect at the centre and not the rim as the rotation stops. Can Quantum Theory explain this contradiction?

9. Dec 8, 2004

### pmb_phy

This doesn't make sense to me. It doesn't define space and it doesn't yield to a definition that I saw here of "space". Whether this thing exists will depend on what it was that Marcus was speaking of. Its really to vauge for me to see what you and Marcus are talking about.
This I don't understand, i.e. why you call them physical objects. They describe physical objects. They are not physical objects in themelves. To me that'd be like calling velocity a physical object.

Pete

10. Dec 9, 2004

### RawThinkTank

If had space made of something then it should have been of smallest particles because otherwise nothing could have moved through it so easily. BUT if so then what is inbetween those particles ? So what do U think, Or Can U ?

11. Dec 9, 2004

### Gamish

what if space is actually made of something, such as dark matter? If space can be thickened and thinned, according to SR and GR, then it must have a fabric. This is what I say to it all.

"Matter, energy, Space, Time, Us, and God, we all exist, but we will never understand how or why, until it is revealed to us. certantly not in this life. We see the effects of the entities of the universe, but we dont see how it could have been. God does. The fact is that in the beginning there was something, and if there was nothing, then nothing that exist could have come into existence that exist, and if something did actual come from nothing, then that nothing was really something in the first place. But what is this something, a being who created all, or just random energy or matter?"

12. Dec 10, 2004

### Rob Woodside

Sure, velocity is a property of physical object. It is a relation that an object has relative to another object. I would claim that it is just as real as the object, but as you point out, it is very different You seem to say that an electromagnetic field is a property of charges and currents and these in turn are properties of physical objects. If so, you are saying reality consists of physical objects and their relations. Presumably we both agree that the relations are just as real, though different from physical objects. We differ on just which are the relations and which are the objects. For me the electromagnetic field, vector potential (up to a gauge transformation) and current densities are real physical objects which can be singled out by their electromagnetic relations (Maxwell Equations) and may have other properties as well, such as energy tensors etc. The fact that we can't agree on whether or not the electromagnetic field is a physical object or a relation is exactly the same situation as with space. Is it a physical object like a fabric or merely a set of relations?

Gamish, Don't bring God into this. It is confused enough already. As Laplace said to Napoleon, when questioned about not even mentioning God in the Mechanique Celeste, "Sire, I had no need for the hypothesis."

Last edited: Dec 10, 2004
13. Dec 10, 2004

### donjennix

This is really the essence of the answer and I have little to add; I would observe that the current cosmological models (by current I mean circa 2000, 2001 popularizations) seem to have "space" expanding faster than c (Lawrence Krauss, "Quintescence") -- an interpretation that was confirmed by a recent PhD in physics from Cal Tech. Exactly what that means is something I tend to ignore.

14. Dec 10, 2004

### yogi

I will take issue with most of the above. While space does not appear to be made of anything that we can identify with matter or material particles - it does have important properties. It has a capacitance per unit lenght and and inductance per unit length (like a tranmission line) Together these properties determine the velocity of radio waves, and its characteristic impedance - antenna designers know they must match the characteristic impedance 377 ohms to the impedance of antennas in order to get maximum power transfer. There have always been two opposing views of space - like those of Libnetz and berkely that space was only a framework reference - a sideless box as Berkely called it - but on the other side there is Einstein, Dirac and Newton. The most convincing evidence to me is in the concept of inertia - accelerate a mass wrt space and you get an instantaneous reactionary force.

15. Dec 10, 2004

### Janitor

Some time back I was looking at a website that discussed this idea. If I can find it again, I will link to it. The author claimed that as Einstein developed General Relativity, he hoped it would be purely relational, in line with Mach's thoughts. But when the dust settled, Einstein realized that his theory was not purely relational. The author of the website was doubtful that any truly 'purely' relational scheme could ever describe our actual universe. He did mention in passing that a couple of people had written articles (books?) that pushed relationalism as far as it could go. If I had to try to come up with names, it may have been Barbour and Bertotti. Anybody heard of them?

16. Dec 10, 2004

### Loren Booda

Relative to us are an infinity of spaces that follow their own physics - some accessible to us, and others not.

17. Dec 11, 2004

### force5

My view

Right or wrong, this is the direction my research has taken me;

Space is an extention of its source. The source is like one side of the coin and the space is the other side. When energy radiates away from its source, the total mass of the object is constantly being transformed into space. This will result in a change in either the volume or the density of the space in question. If this is the case, then, our solar system would extend out about 5 billion LY's in all directions and have some gravitational affect on any mass within that radius. Anyway, I havn't found a better explanation.

Just my thoughts.........John A.

18. Dec 11, 2004

### Rob Woodside

Aristotle thought that space was relational and a space devoid of objects a nonsense. "Nature abhor's a vacuum"

In 1600 Giordano Bruno was burnt at the stake for suggesting that space was infinite. (A satisfying, but unethical method of solving academic disputes)

Newton physically interpreted Descartes analytic geometry as inertial frames. He was aware of Galileo's principle of relativity, but took the frame of the fixed stars as an absolute space. This was the sensorium of God and Newton populated it with aether, a remarkably penetrable substance.

Maxwell, as Yogi says, saw space as not just geometry, but with the properties of a dielectric. But was that space or the aether?

As Laplace dealt with the concept of God, so Einstein dealt with the concept of aether. In support of Mach's ideas, Einstein saw an object "kicking" space by changing the nearby curvature with its mass and space "kicking back" by endowing the object with mass. I have never understood this, but it is treating space as an object or material substance. Certainly one way of spotting a physical object is: if you kick it, it kicks back.

Paradoxically quantum theory treats the vacuum as the plenum. Is it space that is frothing or is it the vacuum fields?

Certainly the understanding of a vacuum will be pivotal in unifying Quantum and Relativistic physics.

19. Dec 11, 2004

### Rob Woodside

If they are not accessible, how do you know they are there?

20. Dec 11, 2004

### Loren Booda

By scientific faith that measurable physics will eventually discover more vectors of communication than just the photon - e. g., the Higgs boson.

Moreover, by statistical argument, an infinitude of spaces exhibits limitless configurations (whether we can observe them or not), assuming a boundless cosmology.