Theory behind how expansion moves mass but doesn't interfere with inertia?

[Rodney-Believes]

If expansion is moving objects away from each other in space, does that imply an interface of some sort between space and mass, a connection, or friction? If so, how is it that mass can drift through space and not eventually come to a stop? If moving space exerts a force on mass, why wouldn't moving mass exert a force on space?

What is the theory behind how expansion moves mass but doesn't interfere with inertia?

 

[Ibix]

If expansion is moving objects away from each other in space,

It isn't really. It's kind of hard to explain in ordinary language because it doesn't have terms for this kind of thing. Probably the best way to describe it is that now there is more distance between us and distant objects than there was in the past. Nothing is being dragged along in any sense.

A useful metaphor would be to draw the universe on an infinite piece of paper. Then draw it again, with everything slightly further apart, then again and again. Stack the sheets on top of each other. What you think of as "the universe now" is one sheet of paper. The maths of general relativity simply describes the whole stack. So nothing is growing. You are simply describing the next sheet of paper as "the universe now", then the next, then the next.

 

[Rodney-Believes]

Thank you. But that approach to understand implies that mass is a very part of space. The distance between objects is increasing but only because space is "growing"? This "stretching" of space yields exactly the right shift in the frequency of light to conform always to the Hubble formula? Yet space growing everywhere would be an exponential growth of space between all objects. Has it been shown that this motion will conform to the Hubble formula into the future?

 

[PeterDonis]

If expansion is moving objects away from each other in space

This is backwards. "Expansion" or "space" isn't a separate thing that is moving objects. Objects are moving away from each other, and we call that "expansion".

that approach to understand implies that mass is a very part of space

No, it doesn't. In General Relativity, "space" is part of spacetime, and is described by the metric. "Mass" is part of the stress-energy tensor. They're different things.

Also, spacetime does not "move" in GR; it's just curved.

This "stretching" of space

Space is not "stretching". Objects are moving apart.

 

[russ_watters]

If expansion is moving objects away from each other in space, does that imply an interface of some sort between space and mass, a connection, or friction? If so, how is it that mass can drift through space and not eventually come to a stop? If moving space exerts a force on mass, why wouldn't moving mass exert a force on space?

Even setting aside how the expansion of space works, objects don't spontaneously stop moving, because of Newton's 1st Law.

 

[phinds]

@Rodney-Believes I recommend the link in my signature.

 

[Rodney-Believes]

Even setting aside how the expansion of space works, objects don't spontaneously stop moving, because of Newton's 1st Law.

I agree and that is exactly my point. Likewise, how is expansion causing objects to move apart? Newton's law must apply to expansion. By Newton's law, there most be a friction to cause the motion, especially since the motion is reflecting the expansion.

 

[Rodney-Believes]

It isn't really. It's kind of hard to explain in ordinary language because it doesn't have terms for this kind of thing. Probably the best way to describe it is that now there is more distance between us and distant objects than there was in the past. Nothing is being dragged along in any sense.

A useful metaphor would be to draw the universe on an infinite piece of paper. Then draw it again, with everything slightly further apart, then again and again. Stack the sheets on top of each other. What you think of as "the universe now" is one sheet of paper. The maths of general relativity simply describes the whole stack. So nothing is growing. You are simply describing the next sheet of paper as "the universe now", then the next, then the next.

"Nothing is getting dragged along in any sense" (Ibix), and yet, "objects are moving away from each other and we call that expansion" (PeterDonis). What is moving the objects apart then? If it's not space growing, what is it? Between the two sheets of stacked papers, representing space growing, there is a "growth" of some kind. And yet that growth did not move the mass apart, and yet the mass is moving in response to that growth? Do you see my confusion?

 

[PeterDonis]

how is expansion causing objects to move apart?

It isn't. Expansion is objects moving apart. It is not some separate thing causing objects to move apart.

What is moving the objects apart then?

Inertia. They were moving apart in the past, so they are moving apart now. (But actually, this is a matter of interpretation; see below.)

If it's not space growing, what is it?

The geometry of spacetime.

that growth did not move the mass apart, and yet the mass is moving in response to that growth?

No, the mass is not "moving in response to that growth". The 4-dimensional geometry of spacetime, and the way the worldlines of the masses are laid out in that geometry, means that when you cut 3-dimensional slices out of that spacetime in a particular way (the way that is most "natural"), the masses are further apart in each successive slice.

One way to interpret this is, as I noted above, inertia: the masses are moving apart now because they were moving apart in the past.

Another way to interpret this is, as I also noted above, the geometry of spacetime: the worldlines happen to have a particular property because of the way they are laid out in the 4-dimensional geometry.

Neither of these interpretations requires "space" to be causing anything to move.

I understand that all of this seems highly counterintuitive to you. However, it works, so the only solution is to retrain your intuitions. Curved spacetime in GR simply does not work the same as your intuitions based on Newtonian physics are telling you things should work.

 

[Rodney-Believes]

@Rodney-Believes I recommend the link in my signature.

I read the article. Thank you. It does not answer my question. If objects are observed moving apart, and that is what "we call expansion" (PeterDonis), then what is this motion? It is obviously an expansion, but what is the nature of it? Is it mass drifting away from each other (inertia), or is it a reflection of a force acting on the mass?

If a force, what is the force? Acceleration certainly suggests a force, if there is an acceleration. If a force, is the force coming from space itself, it's expansion? Is it a repulsion of all mass by all other mass, an antigravity? If so, how could Hubble's Law describe it? It would not be the same in all directions for every object? This observed motion has to be space growing and that growth has to be "dragging" the objects. The motion is a reflection of the expansion. It's too uniform for the force to be originating from the objects themselves or be a function of the objects characteristics, like mass. Hubbles Law (v = H * d) makes this independent of the objects themselves, yet it describes their relative motions.

 

[Drakkith]

It is obviously an expansion, but what is the nature of it? Is it mass drifting away from each other (inertia), or is it a reflection of a force acting on the mass?

Neither. It's something wholly different. It's just the result of geometry, which is the math that describes spaces and the relationships of objects within these spaces. The geometry of a space (I'm using space in a more general sense than just 'outer space') can cause things to happen without required that forces act on objects. This is exactly what gravity is in general relativity. Instead of being a force, gravity is just the effect that the geometry of spacetime has on us.

If a force, what is the force? Acceleration certainly suggests a force

Not when you include geometrical effects, which don't require forces.

Is it a repulsion of all mass by all other mass, an antigravity?

It's more like gravity's cousin. Both result from the same rules in GR.

 

[PeterDonis]

what is this motion?

The kind of motion that causes light from distant objects to be redshifted when we receive it.

You seem to think that "motion" is something absolute. It's not. That is one of the intuitions you need to retrain. You should instead be focusing on the things we actually observe. We don't directly observe distant objects "moving". We only see light coming from them and can measure things like its redshift, its brightness, and the angular size of the object as we see it.

is it a reflection of a force acting on the mass?

No. All of the objects involved are in free fall, feeling no force. Gravity is not a force in GR (yet another intuition you need to retrain).

 

[Rodney-Believes]

The kind of motion that causes light from distant objects to be redshifted when we receive it.

You seem to think that "motion" is something absolute. It's not. That is one of the intuitions you need to retrain. You should instead be focusing on the things we actually observe. We don't directly observe distant objects "moving". We only see light coming from them and can measure things like its redshift, its brightness, and the angular size of the object as we see it.

No. All of the objects involved are in free fall, feeling no force. Gravity is not a force in GR (yet another intuition you need to retrain).

Actually, that is what I am trying to do, "focusing on things we actually observer." It seems like there are strange explanations struggling to explain strange observations. We know that Dr. Hubble saw distant objects moving away at speeds related to their distance from us. That is what is observed. From that "the universe has no center." Surely there is a better explanation, science has just not found it.

 

[Rodney-Believes]

Neither. It's something wholly different. It's just the result of geometry, which is the math that describes spaces and the relationships of objects within these spaces. The geometry of a space (I'm using space in a more general sense than just 'outer space') can cause things to happen without required that forces act on objects. This is exactly what gravity is in general relativity. Instead of being a force, gravity is just the effect that the geometry of spacetime has on us.
Not when you include geometrical effects, which don't require forces.
It's more like gravity's cousin. Both result from the same rules in GR.

If just geometry, that implies that observations are an illusion within relative motions caused by unobserved actual motions. How much effort has been put into finding hidden motions behind the observed? What if the observations are caused by a simple motion of all mass away from a center? What observation has precluded that?

 

[PeterDonis]

We know that Dr. Hubble saw distant objects moving away at speeds related to their distance from us. That is what is observed.

No, that is not what is observed. What is observed is that light from distant objects is redshifted, and that the redshift is related in a specific way to other characteristics of the light from the objects, specifically the brightness and the angular size of the object. We do not directly observe speeds or distances; those are calculated from observations using particular models, and different models will give different answers for speeds and distances. The current best-fit model cosmologists have of our universe was built up by testing different possible models against thousands of observations of distant objects, looking for the model that best predicts the observed fit between the directly observed characteristics that I gave above.

Surely there is a better explanation, science has just not found it.

Before going off looking for a better explanation, you should first be sure you understand the current one. You don't yet have that understanding.

What if the observations are caused by a simple motion of all mass away from a center? What observation has precluded that?

The fact that such a model predicts a relationship between the directly observed characteristics that I gave above, that is very different from the relationship we actually observe.

Ned Wright's cosmology tutorial, though somewhat dated now, is still a good resource for getting a better understanding of the basics of our current best-fit model of the universe:

http://www.astro.ucla.edu/~wright/cosmolog.htm

Pay particular attention to the "Many Distances" section in Part 2, and his explanations of luminosity distance and angular size distance and how they are related to the "distance" that cosmologists use for convenience when they are dealing with general rules like the Hubble Law.

 

[Drakkith]

If just geometry, that implies that observations are an illusion within relative motions caused by unobserved actual motions.

I wouldn't call it an 'illusion', just an effect you must account for to properly understand how objects behave.

How much effort has been put into finding hidden motions behind the observed?

A great deal. It's covered under Special and General Relativity, along with the theories and models built from them.

What if the observations are caused by a simple motion of all mass away from a center? What observation has precluded that?

Basically all of the observations supporting modern cosmology. The exact details of way the universe is arranged and how it behaves, such as the isotropy and homogeneity (uniformity) at the largest scales, the fact that the CMB is also nearly perfectly isotopic, the fact that the redshift laws don't change as you look towards different regions of the sky, etc.

Scientists are smart people. If much of cosmology could be explained by the movement of objects away from a center point then they wouldn't have developed a much more complicated model that is far less intuitive to undestand.

 

[Drakkith]

Surely there is a better explanation, science has just not found it.

If by 'better' you mean 'more intuitive', then no, there is not. The current model of the universe, which includes expansion, is the least complicated model, the model that makes the fewest assumptions, and also the one that explains the most observations with the highest accuracy and precision. All other alternatives are more complicated, make more assumptions, and/or explain fewer observations.

 

[Rodney-Believes]

Thank you so much! I read some of the tutorials but not sure if you had a specific one in mind.

Anyway, I found this statement...

"Thus if we saw a quadratic velocity vs. distance law, then an observer in a different galaxy would see a different law -- and one that would be different in different directions. Thus if we saw v(sq), then B would see much higher radial velocities in the "plus" direction than in the "minus" direction. This effect would allow one to locate the "center of Universe" by finding the one place where the redshift-distance law was the same in all directions. Since we actually see the same redshift-distance law in all directions, either the redshift-distance law is linear or else we are at the center which is anti-Copernican. "

Is the reason therefore that we are of the opinion that the Universe has no center because an expansion away from a true center would be "anti-Copernic

 

[PeterDonis]

Is the reason therefore that we are of the opinion that the Universe has no center because an expansion away from a true center would be "anti-Copernic

This line of reasoning shows that the simplest model of a universe in which we see the same redshift-distance law in all directions is one with a linear Hubble law, so that the law is the same everywhere; a model with a "center" that was special (and where we would have to be located) would be more complicated since it would have to explain what makes the "center" different and why we happen to be located there. "Anti-Copernican" is really just another way of saying that. But this is not the only line of reasoning leading to our current best-fit model of the universe, because the linear Hubble law is only one feature of that model.

 

[Rodney-Believes]

That is what I am exploring, a model that meets all the observations explained to me so far. It is an expansion out from a center. I am seeking to understand observations from others to discredit it, but I have found none that do. All the observations you suggested are predicted by my model, so far.

I am not a Cosmologist but developed a simulation based on a very simple motion from center. The model spat out expected observations, which I did not know were so until I researched them. There were no assumptions, I just did math against this motion, and the math was simple (I could do it). The motion is not complicated, but illusions within are amazing, and every observation I know of is offered. But, I know so little. I would like someone stronger to look at it.
By the way, this model can be steady state, accelerating, or it can alternate. It can be just expansion under inertia, and still offer all observations I know of, including acceleration presented to observers within when either the observer or the objects being observed get out of sync with the pattern (which gravity would do). It offers the illusion of center to every observer. It offers that the more distant an object, the more visible the expansion motion. It offers local expansion motion becoming increasingly difficult to discern as expansion ages. And more. The math showed me these things and I did not know these things were so in our Universe. It is a simple expansion pattern away from a true center. It offers that the Universe could be a string, a disk, or a sphere. I believe it is a sphere.
That is why I am asking questions. I want to understand what is known, observed. I am searching everywhere I can until someone offers something to discredit it.

 

[phinds]

... I would like someone stronger to look at it.

Not likely to happen since PF specifically does not debunk fallacious models and any model that posits a center for the expansion is prima facie fallacious.

 

[PeterDonis]

That is what I am exploring, a model that meets all the observations explained to me so far. It is an expansion out from a center.

PF is not for discussion of personal theories or for doing original research.

I am seeking to understand observations from others to discredit it, but I have found none that do. All the observations you suggested are predicted by my model, so far.

I strongly doubt that, because you have not demonstrated the kind of knowledge of the details of the actual observations that would be needed to test your model against them. Prediction doesn't mean predicting a general qualitative trend: it means predicting the actual quantitative details of all of the observations, accurately. Cosmologists have been working on models to do that for decades, and have continued to refine them as more and more detailed data comes in. Models of the type you describe were rejected some time ago because they could not match observations. I strongly doubt that you have discovered a model that was not already considered and rejected.

But in any case, as noted above, PF is not for discussion of personal theories or for doing original research. Thread closed.