Isn't space expansion logically required?

[Gerinski]

If spacetime is a single entity, shouldn't it be simply logical that space expands?

We know that spacetime is one single entity, there is no space AND time, but just spacetime.

As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the more time there is from the Big Bang. Time expands as the future unfolds.

So, is it not simply logical and unavoidable that also space must expand, so that the more we move away from the Big Bang there is not just more time but also more space?

Expecting otherwise would be destroying the unity of space and time, we should not expect only the time dimension of spacetime to grow and the space dimensions to stay the same, should we?

 

[phinds]

If spacetime is a single entity, shouldn't it be simply logical that space expands?

We know that spacetime is one single entity, there is no space AND time, but just spacetime.

As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the more time there is from the Big Bang. Time expands as the future unfolds.

So, is it not simply logical and unavoidable that also space must expand, so that the more we move away from the Big Bang there is not just more time but also more space?

Expecting otherwise would be destroying the unity of space and time, we should not expect only the time dimension of spacetime to grow and the space dimensions to stay the same, should we?

Since space is just geometry, what's your point? Space-time grows and things on a cosmological scale get farther apart but that's just geometry and since space isn't "something", it's just distance, there's no need to "create" more of it.

 

[Gerinski]

Since space is just geometry, what's your point? Space-time grows and things on a cosmological scale get farther apart but that's just geometry and since space isn't "something", it's just distance, there's no need to "create" more of it.

Space is more than just geometry, space is where fields reside. Are you saying that the space fields occupied 5 billion years ago had the same dimensional extension as the space which fields occupy at the present?

 

[phinds]

Space is more than just geometry, space is where fields reside. Are you saying that the space fields occupied 5 billion years ago had the same dimensional extension as the space which fields occupy at the present?

No, I'm saying space is just the framework in which the fields and matter exist. Space isn't a "thing", it's just dimensions. Stuff, on a cosmological scale, gets father apart but that doesn't mean more space is "created" in between the stuff ... it's just stuff changing position in the framework

EDIT: I see that I'm just repeating myself, so I guess I'm not explaining it in a way that is helpful to you. I'd suggest a forum search for something like "space time geometry" and/or "metric expansion" and check out the links at the bottom of this page.

 

[Jimster41]

I'm a bit taken aback by the statement space-time is just dimensions, like coordinates we wrote on stuff, and so, not really a thing.

Doesn't GR show that really geometry is the fundamental thing, in terms of our experience. I mean to say the traveling twin just had some run-ins with some of his or someone else's, "notions of geometry"... doesn't help his now older brother. And just because our approach of writing coordinates on real stuff to figure out what the principles of how stuff behaves are, ends up causing fits of confusion doesn't means it's just our coordinate writing that is confused. It actually means the geometry of our world, is a thing, and it is weird.

 

[Gerinski]

Stuff, on a cosmological scale, gets father apart but that doesn't mean more space is "created" in between the stuff

Well I guess you are one the few ones believing that space does not expand. If you say that stuff gets farther apart but space does not expand (no new space gets created), then you believe that space does truly stretch. At any rate, 1 Km eventually becomes 2 Km, no matter which way you think of it. I can agree that spacetime is only a set of relationships governing how spacetime events can relate to each other, but the properties of these relationships hold true, call it whatever you like. The magnitude of spacetime dimensions grow larger with time, you like it or not.

 

[phinds]

Well I guess you are one the few ones believing that space does not expand. If you say that stuff gets farther apart but space does not expand (no new space gets created), then you believe that space does truly stretch. At any rate, 1 Km eventually becomes 2 Km, no matter which way you think of it. I can agree that spacetime is only a set of relationships governing how spacetime events can relate to each other, but the properties of these relationships hold true, call it whatever you like. The magnitude of spacetime dimensions grow larger with time, you like it or not.

Yes, the magnitude of the dimensions get larger, as I have said all along, but no space is "created", things just get farther apart in the framework of space and in that sense there is definitely an expansion but this "fabric" and "stretching" stuff is crap propagated by pop-science.

 

[rootone]

I think the problem is in thinking of space as a substance.
Space is not any kind of substance, so it can't be 'created'.
Space can contain things which are substantial, ie things which have a mass.
Space can also contain various kinds of fields, (energy essentially).
Space itself though is not a form of matter or energy, It is Phinds pointed out, a 'framework' we use to describe things like distances, velocity, position and so on.

 

[PeroK]

I think the problem is in thinking of space as a substance.
Space is not any kind of substance, so it can't be 'created'.
Space can contain things which are substantial, ie things which have a mass.
Space can also contain various kinds of fields, (energy essentially).
Space itself though is not a form of matter or energy, It is Phinds pointed out, a 'framework' we use to describe things like distances, velocity, position and so on.

One can create confusion, yet confusion isn't a substance.

 

[rootone]

Sure, the term 'created' can be applied to abstract entities, but I think the OP is envisiging that expansion of space implies that something physical is being created.

 

[PeroK]

Sure, the term 'created' can be applied to abstract entities, but I think the OP is envisiging that expansion of space implies that something physical is being created.

One could argue that distance is physical rather than abstract.

 

[rootone]

I won't disagree with that.
Yes you can say that expansion means distance is being created, it's a valid statement, though it sounds like odd way of saying 'distances are increasing'
I had the impression though I could be wrong, that the OP envisages 'Stuff; that is either a form of matter or energy coming newly into existence..

 

[Jimster41]

Spacetime is just a container. It's geometry labels, It's measure merely abstract... Unbelievable.

By what thing do you move and age? Something else, something real?

Quit calculating for a second, and look around...it's not just puzzles, this is the stuff the world is made of.

 

[rootone]

Yes spacetime is a geometric framework, (well actually there a number of geometric frameworks that can describe space.)
Yes I am moving and ageing within the context of the framework, How is that unbelievable?

 

[Jimster41]

OMG.

Honestly though I love nerds. I aspire to be a real one.

 

[phinds]

Spacetime is just a container. It's geometry labels, It's measure merely abstract... Unbelievable.

By what thing do you move and age? Something else, something real?

Quit calculating for a second, and look around...it's not just puzzles, this is the stuff the world is made of.

No, it is the framework in which exists "the stuff the world is made of". 99+% of atoms are space but that doesn't make space "stuff", it's just distance. Quarks and electrons and photons, etc, are stuff.

 

[Chalnoth]

If spacetime is a single entity, shouldn't it be simply logical that space expands?

Nope. There is no such requirement. It is perfectly possible for space to either expand or contract. Doing neither is only possible in perfectly empty space with no cosmological constant (if there's any matter or other energy such as photons, then a universe that is neither expanding nor contracting is unstable: it will very soon start doing one or the other).

As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the more time there is from the Big Bang. Time expands as the future unfolds.

This isn't an accurate representation of spacetime in General Relativity. In General Relativity, the entire past and future is part of the same manifold. This is more or less required by the fact that different observers disagree on what the definition of "now" is.

If I'm standing here on Earth in the Milky way, and I have a clock that reads 12:01 today, and another person is standing on some other planet off in the Andromeda galaxy but that clock reads 12:03 on the same day according to one observer, then another observer could look at both clocks and see the same time, or see that my clock is faster than than the clock of the person on Andromeda. There's no way to say who has the right answer.

To bring it all together, if we can't agree on "now", then we can't agree on what times at different locations are in the future. And if it's not possible to agree upon which times at different locations are in the future, it makes no sense to say that the future is newly-created.

 

[Jimster41]

No, it is the framework in which exists "the stuff the world is made of". 99+% of atoms are space but that doesn't make space "stuff", it's just distance. Quarks and electrons and photons, etc, are stuff.

Not so fast friend. I know of what you speak but I'm far enough into Susskind's QM for dummies to know that just whether or not matter and energy, or geometry comes first, is not a matter entirely settled.

 

[phinds]

Not so fast friend. I know of what you speak but I'm far enough into Susskind's QM for dummies to know that just whether or not matter and energy, or geometry comes first, is not a matter entirely settled.

I'm not following you. What do you mean "comes first" and why does it matter? The discussion is about whether or not space has a material existence.

 

[Jimster41]

Nope. There is no such requirement. It is perfectly possible for space to either expand or contract. Doing neither is only possible in perfectly empty space with no cosmological constant (if there's any matter or other energy such as photons, then a universe that is neither expanding nor contracting is unstable: it will very soon start doing one or the other).This isn't an accurate representation of spacetime in General Relativity. In General Relativity, the entire past and future is part of the same manifold. This is more or less required by the fact that different observers disagree on what the definition of "now" is.

If I'm standing here on Earth in the Milky way, and I have a clock that reads 12:01 today, and another person is standing on some other planet off in the Andromeda galaxy but that clock reads 12:03 on the same day according to one observer, then another observer could look at both clocks and see the same time, or see that my clock is faster than than the clock of the person on Andromeda. There's no way to say who has the right answer.

To bring it all together, if we can't agree on "now", then we can't agree on what times at different locations are in the future. And if it's not possible to agree upon which times at different locations are in the future, it makes no sense to say that the future is newly-created.

And yet the overall a(t) of the universe, it's expansive shape, has to also be accounted for when considering the relativity of simultaneity right?

 

[Jimster41]

I'm not following you. What do you mean "comes first" and why does it matter? The discussion is about whether or not space has a material existence.

Well if matter and energy are emergent properties of space-time geometry (there are papers over in BTSM) then the question of which is a more capable primary description is open. It can be argued, I believe, that geometry may be "primary". In other words the primary "material" is quanta of space-time: "Vacuum state" properties, and algorithms.

Clearly that's 'out there' thinking, but it's not mine, I'm not making it up.
Aren't 'strings' an example? Aren't they a fundamental geometric object, from which matter and energy, and vacuum are made?

Bottom line, to my thinking re the material aspect of raw space-time. One twin gets old, one doesn't. They are both made of matter, but we describe what happened to them in terms of the geometry of space-time their paths took. And what happens to them is pretty material if you ask me. So what's the difference between that and the case of expansion? In neither case do we really know what is going on QM wise, between matter and changing geometry. The temperature or energy density of the CMB decreased because someone forgot to move our temperature framework labels, or because "space grew"? Seems fair to say "space grew", and mean it.

 

[rootone]

In neither case do we really know what is going on QM wise, between matter and changing geometry

You don't need QM to understand that if you have an expanding volume of 'stuff', then less 'stuff' exists in one cubic metre of it.
Less stuff means less energy, so not surprisingly the temperature is less.

 

[Jimster41]

You don't need QM to understand that if you have an expanding volume of 'stuff', then less 'stuff' exists in one cubic metre of it.
Less stuff means less energy, so not surprisingly the temperature is less.

And yet if you ask "how" and then how that, then how that... it's turtles all the way down. And so I would contend that, we actually do need QM to truly understand any thing...fully.

Not that we can't make iPhones and cars and the internet and rockets and pizzas and gravity boots and blow ourselves up without truly understanding everything fully. Though we probably will need QM for the gravity boots...

 

[phinds]

Well if matter and energy are emergent properties of space-time geometry (there are papers over in BTSM) then the question of which is a more capable primary description is open. It can be argued, I believe, that geometry may be "primary". In other words the primary "material" is quanta of space-time: "Vacuum state" properties, and algorithms.

Clearly that's 'out there' thinking, but it's not mine, I'm not making it up.
Aren't 'strings' an example? Aren't they a fundamental geometric object, from which matter and energy, and vacuum are made?

Bottom line, to my thinking re the material aspect of raw space-time. One twin gets old, one doesn't. They are both made of matter, but we describe what happened to them in terms of the geometry of space-time their paths took. And what happens to them is pretty material if you ask me. So what's the difference between that and the case of expansion? In neither case do we really know what is going on QM wise, between matter and changing geometry. The temperature or energy density of the CMB decreased because someone forgot to move our temperature framework labels, or because "space grew"? Seems fair to say "space grew", and mean it.

I don't follow your logic at all, but I do get that you seem to believe space "grows" in the same sense that flowers grow and I completely disagree. I think things just get farther apart and I don't see how any of the above contravenes that.

 

[rootone]

And yet if you ask "how" and then how that, then how that... it's turtles all the way down.

Unless string theory or something like that proves to be a real 'theory of everything', then you are right.
Science is in the meanwhile a method of uncovering the layers of turtles and discovering what the nature of the turtles are in each layer.
This sometimes produces useful new technologies.
(I would definitely consider anti gravity boots to be useful technology, but the operating manual might be hard to understand)

 

[tom aaron]

If spacetime is a single entity, shouldn't it be simply logical that space expands?

We know that spacetime is one single entity, there is no space AND time, but just spacetime.

As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the more time there is from the Big Bang. Time expands as the future unfolds.

So, is it not simply logical and unavoidable that also space must expand, so that the more we move away from the Big Bang there is not just more time but also more space?

Expecting otherwise would be destroying the unity of space and time, we should not expect only the time dimension of spacetime to grow and the space dimensions to stay the same, should we?

What exists has to exist. It exists because it is the rational way that matter and energy act. Not that we understand how it all works. What you are describing are observations (some debatable). Everything is the way it is because that is the reality of existence.

We don't actually 'know' much of anything other than models of space time that sort of act the way they do. Until we get some handle on what gravity is, what dark energy is, some relationship between QM and General Relativity...we don't actually know much of anything.

Someone mention Leonard Susskind...perhaps it's all some Holographic matrix.

 

[Jimster41]

Unless string theory or something like that proves to be a real 'theory of everything', then you are right.
Science is in the meanwhile a method of uncovering the layers of turtles and discovering what the nature of the turtles are in each layer.
This sometimes produces useful new technologies.
(I would definitely consider anti gravity boots to be useful technology, but the operating manual might be hard to understand)

Hey man you want to start a gravity boots thread, I'm there.

 

[rootone]

Hey man you want to start a gravity boots thread, I'm there.

Well anti/gravity boots was your idea, but I am sure that if either of us started such a thread it would consumed by the local black hole within an instant.

 

[Gerinski]

This is going to places I did not intend (although surely interesting!). I think what I asked was pretty simple.

For my question, it doesn't matter whether spacetime is 'physical' or purely informational, whether it is 'material' or not. It doesn't matter if the future exists already or not. Neither space not time needs to 'be created'. All that is rather irrelevant to my basic question.

I just said, spacetime is believed to be one single entity. At the event of 'me now', there is more time extension since the Big Bang than there was at the event when the solar system formed, and even more than at the event when the first galaxies formed. I don't care whether also the future 'already exists'. The extension of the time dimension is larger 'in my now' than in my past, and smaller now than in the future.

Similarly, the spatial extension of spacetime is larger in my here and now than it was in the past, and it will be even larger in the future. It doesn't matter whether you think it gets created or it has always been there. I just say that its dimensional magnitude extends in both the time and the spatial dimensions. As it gets older it gets bigger.

So now the question was, is it not kind of logical, given that it is a single entity spacetime?

If you think not so, it means that you defend that space is completely arbitrary from time. As the extension of the time dimension gets larger, space can get smaller or larger or stay the same, there is no relationship whatsoever between space and time magnitudes. If so that's fine, I guess this is the orthodox viewpoint, but I was just wondering it there could be some relationship between time extension and spatial extension, since they are both aspects of the same entity.

 

[PeroK]

This is going to places I did not intend (although surely interesting!). I think what I asked was pretty simple.

For my question, it doesn't matter whether spacetime is 'physical' or purely informational, whether it is 'material' or not. It doesn't matter if the future exists already or not. Neither space not time needs to 'be created'. All that is rather irrelevant to my basic question.

I just said, spacetime is believed to be one single entity. At the event of 'me now', there is more time extension since the Big Bang than there was at the event when the solar system formed, and even more than at the event when the first galaxies formed. I don't care whether also the future 'already exists'. The extension of the time dimension is larger 'in my now' than in my past, and smaller now than in the future.

Similarly, the spatial extension of spacetime is larger in my here and now than it was in the past, and it will be even larger in the future. It doesn't matter whether you think it gets created or it has always been there. I just say that its dimensional magnitude extends in both the time and the spatial dimensions. As it gets older it gets bigger.

So now the question was, is it not kind of logical, given that it is a single entity spacetime?

If you think not so, it means that you defend that space is completely arbitrary from time. As the extension of the time dimension gets larger, space can get smaller or larger or stay the same, there is no relationship whatsoever between space and time magnitudes. If so that's fine, I guess this is the orthodox viewpoint, but I was just wondering it there could be some relationship between time extension and spatial extension, since they are both aspects of the same entity.

"Logical" means that one thing necessarily follows from another. There is no logical imperative that demands that space and time are equivalent in any or all respects.

If it were logical that space must somehow expand in the same way as time, then you would be able to take the conclusion further:

1) Each spatial dimension would "pass" in the way time passes.

2) Spatial dimensions would pass in one direction only: from the past to the future.

3) Or, time would be a spatial-like dimension, expanding in both directions - but not "passing".

4) It might also be logical that there be three time dimensions.

The expansion of space and the passing of time are very different phenomena. In fact, if you take the "flat" spacetime metric of special relativity, then the distance between two points in spacetime is given by:

$d^2 = (\Delta x)^2 + (\Delta y)^2 + (\Delta z)^2 - c^2 (\Delta t)^2$

In any case, in the theory of spacetime, there is a very clear and fundamental distinction between the three spatial dimensions and the time dimension, as you can see from the spacetime metric above.

 

[Jimster41]

"Logical" means that one thing necessarily follows from another. There is no logical imperative that demands that space and time are equivalent in any or all respects.

If it were logical that space must somehow expand in the same way as time, then you would be able to take the conclusion further:

1) Each spatial dimension would "pass" in the way time passes.

2) Spatial dimensions would pass in one direction only: from the past to the future.

3) Or, time would be a spatial-like dimension, expanding in both directions - but not "passing".

4) It might also be logical that there be three time dimensions.

The expansion of space and the passing of time are very different phenomena. In fact, if you take the "flat" spacetime metric of special relativity, then the distance between two points in spacetime is given by:

$d^2 = (\Delta x)^2 + (\Delta y)^2 + (\Delta z)^2 - c^2 (\Delta t)^2$

In any case, in the theory of spacetime, there is a very clear and fundamental distinction between the three spatial dimensions and the time dimension, as you can see from the spacetime metric above.

And yet doesn't that very metric describe a conservation law that constrains their relation to one another, in a real sense asserting they are "part of the same entity", whatever system it is that enforces that complicated symmetry?

Also, doesn't the standard FLRW model of the universe define distance as a function of time?

-{ c }^{ 2 }d{ \tau }^{ 2 }=-cdt^{ 2 }+a{ (t) }^{ 2 }d{ \Sigma }^{ 2 }

Marcus' threads here on that model and others like it are really good at giving one (me at least) a sort of shocked sensation about the connection between space and time, when describing the spatial universe as having a history we can know...

My sense of what the OP is noticing is that they are deeply connected, inextricably tied, an in a real sense aspects of the same entity. IMHO this is a deep and exciting revelation that should be noticed. It certainly defines a contrast between walking around looking at clocks and rulers, when you don't know anything about Relativity, and looking at them when you do.

I'm not trying to say they are "the same", just that getting a bit stupefied by their apparently deep connection, is a good place surely, from which to ask, what is the difference between them? But then I like that feeling of being stupefied and shocked by relationships in physics. It makes me remember them.

Not to imply you are stupefied @Gerinski. Sort of the opposite.

 

[Chalnoth]

And yet the overall a(t) of the universe, it's expansive shape, has to also be accounted for when considering the relativity of simultaneity right?

I don't understand what you're saying. You need to understand the geometry between observer A and observer B to translate one time at observer A with another time at observer B, but there is still no single unambiguous way to translate between two observers at different locations and/or traveling at different velocities.

 

[Jimster41]

I think I understand. But maybe not.
You have to do it. But there is no specific way that is more correct in terms of reference frame.

I think my point is that just because which way you run the labels doesn't matter doesn't mean the "framework" is invented. Rather I think it highlights just how physically real the thing the "framework" describes is. It is always required for perspective but agnostic as to how any observer defines it (coordinate label-wise). Which seems pretty... 'wow' to me. It seems as easy to be impressed by the universality of the requirement for some "metric" that connects space and time, as it is to be unimpressed somehow by that same fact because it doesn't care how the coordinates of the metric are chosen.

But isn't it correct to say that the notion and operation of that framework (a framework required, but agnostic with respect to coordinate labeling) is actually constrained... to look something like the GR field eq? In other words it has to represent exactly the constraint system mass places on space and time. Any smart alien would have to have discovered something that notices this. The important thing is not the flexibility of coordinate perspective, but rather the universality and specificity of the constraints it places on the relationship between things (like the traveling twins).

Question I've wondered about: Is it correct that the FRLW use that t to mean "proper time of an observer comoving with the CMB". So it picked the CMB as the reference frame?

 

[Chalnoth]

Yes, the reference frame for FLRW is the CMB rest frame.

 

[SeanS6]

I presume that the OP meant that the state space of the universe is increasing. That is the set of possible configurations everything in universe could potentially be in is increasing. That seems like a reasonable idea.

 

[Jimster41]

I presume that the OP meant that the state space of the universe is increasing. That is the set of possible configurations everything in universe could potentially be in is increasing. That seems like a reasonable idea.

That was one of the first questions I asked when I got here. Whether expansion was the cause of entropy (not precisely the same question but close). I was told it was wrong to think so, but even now, as you say, it seems like a reasonable idea.

One alternative is that the number of possible configurations, the "Liouville space" was set "on day one" or was "always set", and the actual state has been moving through that space over time in the direction of increasing probability and entropy . That idea has always seemed more awkward to me.

I think the problem with the idea that the phase space of the universe is growing, is that it implies that the universe isn't "The Universe". What could be "feeding" that growth, cause by definition it ain't the universe feeding itself.

The problem with the "was always" is that it seems almost equally illogical in terms of the "antinomy of cosmogenesis", and it leaves the driver of change un-addressed, and the idea of expansion (arguably the most important discovery of science) as a possible source of change itself, somehow - ignored

As I understand it is possible to suggest what we experience as space-time (which is expanding) is not necessarily synonymous with "The Universe". From there both notions seem a lot easier to think about! Or rather space-time seems easier to think about, as fed by something, or "set " by something. "The Universe" still defies thought at some point. IMHO

 

[PeterDonis]

Doesn't GR show that really geometry is the fundamental thing

Yes, but it's spacetime geometry. Spacetime does not expand; it just is. The spacetime geometry of our universe happens to admit a slicing into "space" and "time" such that the scale factor of "space" increases with "time"; but there's no requirement that you have to view it using such a slicing, or even view it as split into "space" and "time" at all.

the spatial extension of spacetime

Which depends on how you slice up spacetime into "space" and "time". You don't have to do that. We do it because of the limitations of our cognitive faculties, not because of any limitation inherent in the physics. But even given that, as Chalnoth said, there is no requirement that spacetime must admit a slicing such that the scale

 

[Jimster41]

Yes, but it's spacetime geometry. Spacetime does not expand; it just is. The spacetime geometry of our universe happens to admit a slicing into "space" and "time" such that the scale factor of "space" increases with "time"; but there's no requirement that you have to view it using such a slicing, or even view it as split into "space" and "time" at all.
Which depends on how you slice up spacetime into "space" and "time". You don't have to do that. We do it because of the limitations of our cognitive faculties, not because of any limitation inherent in the physics. But even given that, as Chalnoth said, there is no requirement that spacetime must admit a slicing such that the scale

Is there an example of such an alternative relational model? I thought GR was arguably a fundamental observation. And that even if an alien had different words for things like "distance" and "getting old" things for which she would have to have words, once we got those figured out we would recognize that she had identified the same operational mechanics of the geometry we share. For instance would the signature difference between the temporal and spatial dimensions (or some identifiable dual of it) at least be an expected shared constraint? Or could she have ignored that altogether? Likewise on the scale factor, could any meaningful model ignore the observation of cosmological red-shift, or CMB distribution, and the question of orientation of energy density gradient, the second law of thermodynamics, and QM superposition. These would have to have duals, and consistent interactions, in any relational theory of physics and space-time?

It seems like we are discussing the difficulty of simultaneously knowing your description of something is idiosyncratic to your own experience, while also having some real faith that it refers to something that exists, and has real symmetry with respect to that description. Absent the latter we seem like an odd gathering of busy-body solipsists.

I'm not suggesting that abstract things like geometry (strings) are most real by the way (I am not a Platonist), rather that we may at present be otherwise entirely sightless w/respect to what the physical thing they describe really is, but that something that looks like we think it does, is really there.

 

[Finny]

My own favorite description of space and time:
"Time is what keeps everything from happening at once; Space is what keeps everything from happening to me."

Space is more than just geometry, space is where fields reside.

Thats most likely a simplistic view. A place to start, perhaps, but probably not to conclude your thinking about space...distance...spacetime...time...Following are some contradictory views by some well known physicsts... some things to consider...And if you read between the lines, issues arising between GR and QFT...

Carlo Rovelli:
"Special relativity weakens the notion of absolute time; general relativity weakens it further. Relativity shows time is not constant...and varies between observers due to relative speed and or differences in gravitational potential. This means space-time is a dynamical field...we learn from GR that spacetime is a dynamical field and we learn from QM that all dynamical fields are quantized..."

[The first part probably reflects the change in time in varying gravitational potentials [GR] and between observers in relative motion.]

"...Conventional QFT relies ….on the existence of a non–dynamical background spacetime metric..[but]…with GR we have understood that there is no such non–dynamical background spacetime metric in nature….
http://arxiv.org/abs/gr-qc/0604045
Unfinished revolution

[Yet in the wonderful Wheeler-Dewitt quantum mechanical equation there is no time variable!]

Lee Smolin
Abstract: There are a number of arguments in the philosophical, physical and cosmological literatures for the thesis that time is not fundamental to the description of nature. According to this view, time should be only an approximate notion which emerges from a more fundamental, timeless description only in certain limiting approximations. ... The view that time is real and not emergent is, I will argue, supported by considerations arising from all these issues It leads finally to a need for a notion of law in cosmology which replaces the freedom to choose initial conditions with a notion of laws evolving in time. The arguments presented here have been developed in collaboration with Roberto Mangabeira Unger .
-- http://pirsa.org/08100049/


"Forget time"Authors: Carlo Rovelli
(Submitted on 23 Mar 2009 (v1), last revised 27 Mar 2009 (this version, v3))
Abstract: Following a line of research that I have developed for several years, I argue that the best strategy for understanding quantum gravity is to build a picture of the physical world where the notion of time plays no role. I summarize here this point of view, explaining why I think that in a fundamental description of nature we must "forget time", and how this can be done in the classical and in the quantum theory. The idea is to develop a formalism that treats dependent and independent variables on the same footing. In short, I propose to interpret mechanics as a theory of relations between variables, rather than the theory of the evolution of variables in time. http://arxiv.org/abs/0903.3832

 

[PeterDonis]

Is there an example of such an alternative relational model?

I don't understand what you mean by this. I wasn't talking about any "alternative" to GR; I was talking about GR.

I thought GR was arguably a fundamental observation.

I don't understand what you mean by this either. GR is a theory, not an observation. Tidal gravity is a fundamental observation, but identifying tidal gravity with spacetime curvature, which is what GR does, is not; it's a theory, which can only be judged by the accuracy of its predictions. No observation will ever tell you directly that spacetime curvature exists; you have to adopt a theory that tells you what observations indicate spacetime curvature (as GR says that observations of tidal gravity indicate spacetime curvature).

would the signature difference between the temporal and spatial dimensions (or some identifiable dual of it) at least be an expected shared constraint?

Since we measure "temporal dimensions" with clocks, and "spatial dimensions" with rulers, they would seem to be fundamentally different things, physically, so we would expect any valid theory to have to include that difference.

on the scale factor, could any meaningful model ignore the observation of cosmological red-shift, or CMB distribution, and the question of orientation of energy density gradient, the second law of thermodynamics, and QM superposition. These would have to have duals, and consistent interactions, in any relational theory of physics and space-time?

Most of these are observations, but you've mixed in some interpretations too. For example, "cosmological red shift" presupposes that the red shift we observe in light from distant galaxies is of cosmological origin--i.e., that it's due to the expansion of the universe. That's a theoretical conclusion, not an observation (but the red shifts themselves are observations). Similarly, the second law of thermodynamics is not really an observation: entropy is not something we directly observe, it's a theoretical construct.

Also, I don't understand what you mean by "orientation of energy density gradient".

 

[Jimster41]

By "alternative relational theory" I was trying to imagine an alternative description of reality, one that didn't view it as "split into slice into space and time". As I think you say is natural, or needed, later. The fact that GR admits any set of slices, seems subtle but as I argued earlier I don't think it implies "un-reality" rather the opposite.

Your second statement, the question of difference between observation, theory and knowing is exactly what was interesting about the discussion.

I'm paraphrasing Polanyi from a long time ago here - At some point we "indwell" in knowledge that at first can only be explicit, starting with raw observation, from that theory, sensitivity and a map, then at some point, through repetition, it becomes "tacit", unconsidered, sensory, and we are poised at a new height, for new raw observation. Like the observation that my car goes left when I turn the wheel left, and the theory that my steering wheel is mechanically connected to my tires, and the front end of my car... I'm never thinking of that when driving. I'm thinking about this d@$#% forum.

I can imagine at some distant future point a sentient creature or entity tied to a gravitational wave sensing apparatus, looking out at an asteroid field "seeing" the beautiful contours of raw spacetime distortion. To that being your statement that no observation will tell you that spacetime exists seems solipsistic - how are her instruments different from our eyes or fingers?

Clearly this is off into the philosophy of science... which certainly can be a rat hole.

 

[PeterDonis]

By "alternative relational theory" I was trying to imagine an alternative description of reality, one that didn't view it as "split into slice into space and time".

In GR, you don't have to view spacetime as split into space and time. That's a convenience for us humans, not a necessary part of the theory. GR is perfectly capable of describing all the physics without ever splitting spacetime into space and time.

 

[Gerinski]

Spacetime does not expand; it just is.

Don't you agree that the extension of the spatial dimensions of spacetime do expand (get larger) as the extension of the time dimension increases?

 

[Gerinski]

Carlo Rovelli:
"Special relativity weakens the notion of absolute time; general relativity weakens it further. Relativity shows time is not constant...and varies between observers due to relative speed and or differences in gravitational potential. This means space-time is a dynamical field...we learn from GR that spacetime is a dynamical field and we learn from QM that all dynamical fields are quantized..."

"...Conventional QFT relies ….on the existence of a non–dynamical background spacetime metric..[but]…with GR we have understood that there is no such non–dynamical background spacetime metric in nature….

This reminds me of this reasoning:"Newton believed that physical objects and phenomena have a local and objective existence in a 'canvas' of absolute space and time. To make it more clear, let's split this in 2 statements:

1. things exist locally and objectively
2. space and time are absolute

Einstein with his General Relativity (GR) showed that space and time are not absolute, they are flexible and subjective, different observers will perceive them differently, but GR still assumes that objects exist locally and objectively in that spacetime.

Quantum Theory (QT) on the other hand showed that existence is neither local nor objective, it's all a bunch of probability waves, non-locality was confirmed, observation and information is what defines the properties of objects and phenomena (for example path information yes/no defines the outcome of the famous double slit experiment), but QT still assumes a physical, objectively existing spacetime background.

So you see, GR showed that postulate 2 of Newton was false but kept postulate 1, while QT showed that postulate 1 was false but kept postulate 2.

So the ultimate theory, if it exists, needs to be one which gets rid of both postulates at the same time. Existence is neither local nor objective, and spacetime is not an objective physical entity. Everything is the outcome of information processing, and spacetime is only a set of relationships governing how information events can relate to each other and which appearance they must take when perceived by a consciousness. Physical existence is a sort of illusion, although we of course must take it as very real for ourselves, but it is not material in the sense that we assume, its 'material appearance' is only a consequence of the rules governing how the information behaves as perceived by any consciousness able to process it."Debate is welcome :-)

 

[PeterDonis]

Don't you agree that the extension of the spatial dimensions of spacetime do expand (get larger) as the extension of the time dimension increases?

No, because I don't understand what this even means. I think you need to take a step back and think about what my statement "spacetime does not expand; it just is" really means.

 

[PeterDonis]

Einstein with his General Relativity (GR) showed that space and time are not absolute, they are flexible and subjective, different observers will perceive them differently

Be careful; the way you are putting this implies a contradiction with this:

GR still assumes that objects exist locally and objectively in that spacetime.

How can objects exist "locally and objectively" in something that is "flexible and subjective"? "Space" and "time" may be "flexible and subjective", but that just means you should not be looking at "space" and "time" separately; you should be looking at spacetime. Spacetime is not "flexible and subjective" in GR; it's objective and invariant. What it isn't, in GR, is independent of the matter and energy content of the universe. In Newtonian physics, space and time are each, separately, constant and independent of anything else. In SR, "space" and "time" can no longer be considered separately, but spacetime is still constant and independent of anything else. GR makes spacetime, as Rovelli says, "a dynamical field", i.e., its geometry is now dependent on the matter and energy content of the universe. But that geometry is still objective and invariant; different observers do not measure different spacetime geometries.

The "flexible and subjective" part is that there is no unique way to split up spacetime into "space" and "time". But that split is itself not a necessary part of GR; it's just a convenience for us humans, because we find it difficult to let go of our intuitive concept of "space" and "time" as separate things. You don't need to make that split to model any physics or compute any observables. So if you are trying to understand the fundamentals, the best thing I can recommend is to simply throw away the whole idea of splitting spacetime into "space" and "time", as well as anything else that is flexible and subjective. Einstein commented that the name "relativity" was a misnomer; his theory should have been called the "theory of invariants", because the whole point was supposed to be to emphasize that all of the physics is contained in things that are not "flexible and subjective", but objective and invariant.

 

[Finny]

The OP:

At the event of 'me now', there is more time extension since the Big Bang than there was at the event when the solar system formed, and even more than at the event when the first galaxies formed. I don't care whether also the future 'already exists'. The extension of the time dimension is larger 'in my now' than in my past, and smaller now than in the future.

It's not a bad question. But of course one needs to define 'larger' and smaller' time...We can agree space and time seem different, whatever they are. And keep in mind everyone's 'reality' is local. Yours is not the same as something many light years distant; nor it is the same as someone who is caually disconnected...out of the reach of light.

In our universe it seems like despite vastly different gravitational and spatial backgrounds [from moments after the big bang to a large ,cold, dead universe at the end] with slowly evolving entropy and informational conditions, as far as I can tell local time plods along at a steady pace. I don't know of any theory that requires 'an extension of the time dimension'.

You can also consider anti particles...don't they move backward in time in our models? I don't think we should describe that as a 'smaller' time dimension.

 

[rootone]

This reminds me of this reasoning:
Existence is neither local nor objective, and spacetime is not an objective physical entity. Everything is the outcome of information processing, and spacetime is only a set of relationships governing how information events can relate to each other and which appearance they must take when perceived by a consciousness. Physical existence is a sort of illusion, although we of course must take it as very real for ourselves, but it is not material in the sense that we assume, its 'material appearance' is only a consequence of the rules governing how the information behaves as perceived by any consciousness able to process it."Debate is welcome :-)

It sounds a lot like the 'We are a simulation' proposal.
For me that doesn't work because a simulation has to simulate something, so is that 'something' the real something?, or is it another simulation, ad infinitum.
... not to mention what is the ACTUAL physical thing which does the simulating?

 

[Gerinski]

No, because I don't understand what this even means. I think you need to take a step back and think about what my statement "spacetime does not expand; it just is" really means.

I can guess that you take the view of block time, but even so, that block spacetime would not be like a constantly thick slice bread as it is sometimes depicted in popular science books, it would be more like a cone bread, getting larger in its space dimensions as it gets larger in its time dimension.
If that's not the case, kindly enlighten me.

TX

 

[PeterDonis]

You can also consider anti particles...don't they move backward in time in our models?

The short answer is "no". A longer answer really belongs in the quantum physics forum.