What Is the Physical Reality of Space?

[justwondering]

For space to expand is it just able to 'stretch' to a larger size or must newly created space units be added?

 

[bcrowell]

All the presently available cosmological data are well described by models that are based on general relativity. In GR, space is continuous, not granular.

 

[marcus]

All the presently available cosmological data are well described by models that are based on general relativity. In GR, space is continuous, not granular.

Except for the fact that the continuum model has a singularity where it breaks down and fails to give meaningful numbers.

With that reservation, I agree whole-heartedly with your point of view: there is a huge amount of data and the fit is amazing (except for right around the start of expansion.)

So it could be added that all the presently available cosmological data (by fitting the model so well) insistently points back to something that is NOT well described by general relativity.

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Justwondering,

Since space is not a substance, points in space don't have objective existence as far as we know, it might be preferable to ask the question about GEOMETRY.
General Relativity is really a theory of geometry, how it is influenced by matter and how it evolves.
Geometry can be quantized and then it may seem neither continuous nor granular. Geometric measurments can have discrete values (like the energy levels of an atom) but they can have a quantum uncertainty and the discreteness does not have to correspond to "space being made up of little bits".

That's work in progress, an ongoing research program. To get a quantum version of the equations that cosmologists use to model the universe----that recovers standard GR cosmology at normal energy and density, consistent with the available data---but which also does not break down at very high density around the start of expansion.

Since there is no material called "space"---in cosmology at least, it does not make sense to think of "space expanding" by analogy with a material. I try never to say "space expands".

Instead I say that distances expand. Distances are part of geometry. Geometry can change.
You don't have to "make new space", nothing has to be stretched or have new stuff pumped in . Geometry simply changes. Distances can increase or decrease. That is what spacetime curvature is about.

So the question of is "space" a continuous or granular material never comes up. It is not a physical substance.

 

[bcrowell]

Good explanation, marcus :-)

 

[marcus]

Good explanation, marcus :-)

Thanks, Ben, :-) your approval carries additional weight with me since you've published books explaining physics/astronomy at layman and college level.

 

[Andrew Mason]

Perhaps we can ask the question another way.

We have learned that space is not empty. Quantum physics tells us that the "vacuum" is full of phantom particles coming into existence and disappearing all the time. So space has a physical reality - it is not "nothing".

So perhaps we could ask: "is there a discreteness to the vacuum - a minimum volume of space needed for the laws of quantum physics to operate?".

Would the Planck unit of length^3 (10^-60 m^3) not be a minimum volume of space?

AM

 

[justwondering]

And since "cosmologists deny any existence of anything including an empty vacuum prior to the Big Bang." it sure seems convincing that space is far from being nothing.

 

[marcus]

And since "cosmologists deny any existence of anything including an empty vacuum prior to the Big Bang." it sure seems convincing that space is far from being nothing.

heh heh wacky quote Where did that quote come from?

Could it be slightly disingenuous? Or out-dated, e.g. from before 2005?

There is a branch of cosmology called quantum cosmology (QC) that develops models of the BB where there are prior conditions---events leading up to it that can influence the outcome in potentially detectable/measurable ways.

These folks certainly are not "denying the existence" of prior stuff happening. I thought this was widely known.

I'll get a link to some hundreds of QC research papers that have appeared since 2008.
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=dk+quantum+cosmology+and+date+%3E+2007+&FORMAT=WWW&SEQUENCE=citecount%28d%29
I see this currently gives 326 papers all from 2008 or later.

 

[bcrowell]

So perhaps we could ask: "is there a discreteness to the vacuum - a minimum volume of space needed for the laws of quantum physics to operate?".

Would the Planck unit of length^3 (10^-60 m^3) not be a minimum volume of space

The short answer is that we don't have a theory of quantum gravity, so we don't know. I'll leave the long answer up to Marcus :-)

 

[justwondering]

"often it is suggested that spacetime might have a discrete or foamy structure at Planck length scale."

Space may well have components, yes?

 

[bcrowell]

"often it is suggested that spacetime might have a discrete or foamy structure at Planck length scale."

Space may well have components, yes?

It's a generic feature of attempts at a theory of quantum gravity that something happens at the Planck scale. Since we don't have a working theory of quantum gravity, we don't really know what that "something" is.

 

[marcus]

"often it is suggested that spacetime might have a discrete or foamy structure at Planck length scale."

Space may well have components, yes?

Please start giving links to the source of your quotes. I don't think your conclusion follows from either "discrete structure" or "foamy structure". But I need to know who said that and when, and more context of what else they said, in order to understand what they mean.

A continuum---one connected whole---can be given a discrete structure in the sense that the geometric observables are quantized and take on discrete values. This is what one sees in one of the most common approaches.

So disconnected components does not logically follow.

But mainly we need source links for your quotes. Have to know where you are coming from and what the larger context is, in order to attempt a reasonable response.

 

[dm4b]

Perhaps we can ask the question another way.

We have learned that space is not empty. Quantum physics tells us that the "vacuum" is full of phantom particles coming into existence and disappearing all the time. So space has a physical reality - it is not "nothing".

AM

Since many deny the existence of virtual particles, which you seem to be describing here, is this demoting space back to being "nothing"

If not, and folks continue to deny virtual particles actually exist, what is the "something" that constitutes space?

Hopefully, that made sense?