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Physics: What is space?

paximperium

Penultimate Amazing
paximperium
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May 30, 2008
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So the universe is expanding. Space is still expanding as a result of the Big Bang and it is slowly being being accelerated apart by something currently termed Dark Energy.

As a layperson with only rudimentary basic college physics, I have no real idea what "space" really is.

What is it? A medium, an actual thing?
Are the 3 dimensions what compose of space or is it just a description?
As the cosmos expand, does space "dilute"? What happens to space?
 
If there was no matter in the universe, I wonder if it would be fair to say that anything actually exists (Given that forces such as gravity depend upon matter in order to exist).
 
The best answer we have currently comes from general relativity. GR describes gravitational forces by geometry - specifically, it retains Newton's 1st postulate (that bodies move in straight lines unless acted on by an external force), but instead of treating gravity as a force, it treats it as a deformation of space caused by energy* (and "straight line" becomes "geodesic" - the shortest distance between two points on a curved space, like the paths airplanes take on long flights).

So what is space in GR? It's a dynamical thing (i.e. it can change with time, in response to changing configurations of matter) described by an object called the metric tensor. So it's not fixed or rigid. It's determined by the value the metric takes in the region near you, just as the electric field is the value the electric field takes in the region near you. Physically, one measures it by studying the motion of any collection of objects, much as one studies the electric field by studying the motion of charged particles.

If one tries to go beyond GR and include quantum mechanics it's clear that this description will fail at some level. Probably what replaces it is a quantum version in which the metric we measure on large distances (via gravity) is an average, while the true metric is fluctuating randomly about that average. One of the peculiar facts about quantum gravity is that those fluctuations get stronger and larger if you measure them on shorter distances - which means below some distance (the Planck length), it is probable that notions like "length", "space", and "time" lost their meaning - the average isn't a good description when fluctuations are too large.

That's all speculation, though - no one knows for sure.


*In GR one does NOT need matter or energy to have space, as seems to be a common misconception on these forums. One needs matter or energy to have a curved space.
 
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sol invictus said:
The best answer we have currently comes from general relativity. GR describes gravitational forces by geometry - specifically, it retains Newton's 1st postulate (that bodies move in straight lines unless acted on by an external force), but instead of treating gravity as a force, it treats it as a deformation of space caused by energy* (and "straight line" becomes "geodesic" - the shortest distance between two points on a curved space, like the paths airplanes take on long flights).

So what is space in GR? It's a dynamical thing (i.e. it can change with time, in response to changing configurations of matter) described by an object called the metric tensor. So it's not fixed or rigid. It's determined by the value the metric takes in the region near you, just as the electric field is the value the electric field takes in the region near you. Physically, one measures it by studying the motion of any collection of objects, much as one studies the electric field by studying the motion of charged particles.

If one tries to go beyond GR and include quantum mechanics it's clear that this description will fail at some level. Probably what replaces it is a quantum version in which the metric we measure on large distances (via gravity) is an average, while the true metric is fluctuating randomly about that average. One of the peculiar facts about quantum gravity is that those fluctuations get stronger and larger if you measure them on shorter distances - which means below some distance (the Planck length), it is probable that notions like "length", "space", and "time" lost their meaning - the average isn't a good description when fluctuations are too large.

That's all speculation, though - no one knows for sure.


*In GR one does NOT need matter or energy to have space, as seems to be a common misconception on these forums. One needs matter or energy to have a curved space.
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Thanks for the info but that seems more like a descriptor of its characteristics as oppose to describing what it actually is. I know there may not be an answer but I'm curious if there are any well supported hypothesis(or philosophical musings) as to what it is.
 
paximperium said:
As a layperson with only rudimentary basic college physics, I have no real idea what "space" really is.
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paximperium said:
Thanks for the info but that seems more like a descriptor of its characteristics as oppose to describing what it actually is. I know there may not be an answer but I'm curious if there are any well supported hypothesis(or philosophical musings) as to what it is.
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Science doesn't answer questions like that. Science is about finding theories (i.e. sets of statements that can be used to predict the results of experiments) and then finding out how accurate the predictions are. It just isn't possible to find out what something "really is".
 
paximperium said:
Thanks for the info but that seems more like a descriptor of its characteristics as oppose to describing what it actually is. I know there may not be an answer but I'm curious if there are any well supported hypothesis(or philosophical musings) as to what it is.
Click to expand...

You know what air is, right? So why don't you answer the question "What is air?" Please make sure you don't just describe its characteristics - tell us what it actually is.
 
paximperium said:
So the universe is expanding. Space is still expanding as a result of the Big Bang and it is slowly being being accelerated apart by something currently termed Dark Energy.

As a layperson with only rudimentary basic college physics, I have no real idea what "space" really is.

What is it? A medium, an actual thing?
Are the 3 dimensions what compose of space or is it just a description?
As the cosmos expand, does space "dilute"? What happens to space?
Click to expand...

My professional opinion?

Space = Room to swing a cat.[/leaves swiftly]
 
Good point. I was just wondering if there was anything more...should I ask "What is time" now?
 
More seriously, isn't there a basic dichotomy between physicists who regard their mathematics as merely modelling an external reality, and others who regard the maths as the reality. In the latter, you can't get a better answer to "what is space" than that it is a set of numbers, where those numbers define different parameters for objects, also defined by numbers, where the differences in the parameters between one object and another has the same property as what we would call a difference in their spatial position.

I'm not sure which option I want to be true, but both hurt my head.
 
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Badly Shaved Monkey said:
In the latter, you can't get a better answer to "what is space" than that it is a set of numbers, where those numbers define different parameters for objects, also defined by numbers, where the differences in the parameters between one object and another has the same property as what we would call a difference in their spatial position.
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Actually there's a much better answer than that above. But point taken :).

More seriously, isn't there a basic dichotomy between physicists who regard their mathematics as merely modelling an external reality, and others who regard the maths as the reality.
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The history of physics - and certain theoretical considerations now - make it clear that our current models cannot be reality (because they're not self-consistent). They're either an imperfect model for some non-mathematical reality, or an imperfect model for the true mathematical "reality".

In my view, math is just a symbolic representation of the rules by which the world runs itself. If we could figure out all the rules and write them down, it would be pretty reasonable to call that theory (or its solutions) "reality". But whether that's possible is anyone's guess.
 
sol invictus said:
Actually there's a much better answer than that above. But point taken :).



The history of physics - and certain theoretical considerations now - make it clear that our current models cannot be reality (because they're not self-consistent). They're either an imperfect model for some non-mathematical reality, or an imperfect model for the true mathematical "reality".

In my view, math is just a symbolic representation of the rules by which the world runs itself. If we could figure out all the rules and write them down, it would be pretty reasonable to call that theory (or its solutions) "reality". But whether that's possible is anyone's guess.
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If one could right down the rules, do you need to 'run' them on something to generate a reality or does the reality pop out spontaneously? I think I mean this as a rhetorical question, not really expecting an answer.
 
Answering this question presents the same difficulties as asking "What is time?"

We all have an intuitive sense for space & time, but when you really get down to it, these are concepts which are so fundamental that they are - in the words of Carl Sagan - "profoundly resistant to simple definition."

:)
 
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paximperium said:
So the universe is expanding. Space is still expanding as a result of the Big Bang and it is slowly being being accelerated apart by something currently termed Dark Energy.

As a layperson with only rudimentary basic college physics, I have no real idea what "space" really is.

What is it? A medium, an actual thing?
Are the 3 dimensions what compose of space or is it just a description?
As the cosmos expand, does space "dilute"? What happens to space?
Click to expand...
Whatever you can put more stuff into is space. If you can't, it isn't.:)
 
Badly Shaved Monkey said:
More seriously, isn't there a basic dichotomy between physicists who regard their mathematics as merely modelling an external reality, and others who regard the maths as the reality. In the latter, you can't get a better answer to "what is space" than that it is a set of numbers, where those numbers define different parameters for objects, also defined by numbers, where the differences in the parameters between one object and another has the same property as what we would call a difference in their spatial position.

I'm not sure which option I want to be true, but both hurt my head.
Click to expand...

The problem with people insisting that their "mathematics is reality" is that reality doesn't necessarily correspond to that math. For instance, this point of view was common among many philosophers who argued that Euclidean geometry allowed one to deduce basic truths. Then, in the late 19th century, along came non-Euclidean geometry which, after being successfully applied to & described the world via general relativity, blew the lid off all that. So much for the "true reality" represented by Euclid's geometry.
 
Badly Shaved Monkey said:
If one could right down the rules, do you need to 'run' them on something to generate a reality or does the reality pop out spontaneously? I think I mean this as a rhetorical question, not really expecting an answer.
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Hmmm.... interesting question.

MattusMaximus said:
The problem with people insisting that their "mathematics is reality" is that reality doesn't necessarily correspond to that math. For instance, this point of view was common among many philosophers who argued that Euclidean geometry allowed one to deduce basic truths. Then, in the late 19th century, along came non-Euclidean geometry which, after being successfully applied to & described the world via general relativity, blew the lid off all that. So much for the "true reality" represented by Euclid's geometry.
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That just says that model wasn't correct - not that no correct model exists.
 
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