Difference between expansion of space and objects just moving apart?

[marcus]

I'm back, the stew is simmering in the oven. But it is too late to continue the other post so I'll start a new one.

...
In suggesting that the number of dimensions may change with scale, from 2 to 4, the authors seem to imply that some 'dimensions' emerge? unfold? or are born? out of quantum chaos...

=============
My attitude about the Utrecht model quantum universe is that it is one example out of a handful of quantum geometry models---one which has reached the break-out stage.
One where they have reached the stage of running computer simulations of the universe and having spacetime emerge as an epiphenomenon.

the appearance of classic smoothness (satisfying Einstein eqn.) arising from microscopic quantum roughness and confusion

spacetime as a path integral---average of all different crazy ways of getting from this spatial geometry to that. maybe 4D spacetime doesn't exist, maybe it is always just a path from this space-state to that one.

So my attitude is to watch for other approaches, like spinfoams, to get to a stage where they have a path integral and where they can run their models in computer and get smooth classic geometry to emerge as a largescale average.

And I want to see if there are OTHER approaches that, when they get to this stage, also predict fractal microstructure and a decline in dimensionality at small scale.

there have been hints of this kind of development. Freidel just posted a paper obtaining a path-integral for spinfoams (making the spinfoams approach look more like the Utrecht model---evolutionary convergence) and two papers were delivered about this at the international QG conference last week in UK.
Martin Reuter at University of Mainz has an approach which actually preceded the Utrecht people in finding hints of spacetime dimension around 2 down at very small scale, hints of microscopic fractal structure. His is yet a different approach (not spinfoams, not simplex) which in other respects is perhaps not as satisfactory, but at least confirms this point coming from a different direction.
These are just straws in the wind.

In suggesting that the number of dimensions may change with scale, from 2 to 4, the authors seem to imply that some 'dimensions' emerge? unfold? or are born? out of quantum chaos. If so, it seems possible that the relation between our time and space dimensions, as measured by the ratio of their GR metric coefficients, could change. I've often wondered if this ratio is eternal. If it were to change...

If I understand you mean an evolving speed of light. In their model they do have a shape parameter which is the ratio of the timelike edges to the spacelike edges, of the simplex. the simplex does not need to be equilateral. All simplexes are identical. they have sometimes played around with this shape ratio parameter, but it stays constant for all simplexes for the duration of the computer run. as far as I know it has always been treated as a constant. I don't know the very latest.

It doesn't matter what my hunch is, of course, because I could so easily be wrong. My hunch is that a variable speed of light will never come out of the Utrecht model. the model is too simple. As the SciAm article says, it is hard to imagine any more minimalist way to treat quantum gravity. minimal paraphernalia, minimal assumptions, simple rules.

it might be for some other computer modeling approach farther down the road to try a variable speed of light, not this approach----but I could easily be mistaken.

 

[sketchtrack]

If a galaxy can have a recession speed greater than C, then why could not a space ship? If a spaceship accelerates to .8 C relative to earth, and then stops accelerating, it will no longer be accelerating, but it will still be moving away from Earth at .8 C although it will now be at rest in its own inertial frame. What is to stop it from then later accelerating another .8 C and then stop accelerating again coming to rest in its inertial frame, but now moving away from Earth at 1.6 C?

 

[George Jones]

If a galaxy can have a recession speed greater than c, then why could not a space ship?

In Friedmann-Robertson-Walker universes, the the rate of change of proper distance between a spaceship and a planet with respect to cosmological time can sometimes be greater than c when the proper distance between them is greater than zero, but this doesn't violate any fundamental principle of relativity.

Suppose you and I are both on planet A, and planet B is accessible to A by rocket. Suppose you set off for B in a rocket. At the same moment that you set sail, I can fire a laser in a direction such that the laser light reaches B before you do. In this sense, rockets alway move more slowly than light speed.

There is another sense in which the speed of light is a speed limit. Suppose that you are deep in space. At any particular time, there may be other astronauts, as well light beams, coincident with you. If you measure the speed of any astronaut that is coincident with you, you find that this speed is less than the speed of any light pulse that is coincident with you.

 

[marcus]

If a galaxy can have a recession speed greater than C, then why could not a space ship?

It easily could have a recession speed >c. Nobody said it couldn't, that I know of.
All Special Rel implies is that the spaceship could never catch up with, and pass, a photon.

Have you read Lineweaver and Davis, "Expanding Confusion"? I used to recommend that paper to PF people a lot back in 2004-2005. I see it is still getting recommended. Basic paper.
there is also the SciAm article by the same authors. March 2005.. so important to read the basic stuff.

If a spaceship accelerates to .8 C relative to earth, and then stops accelerating, it will no longer be accelerating, but it will still be moving away from Earth at .8 C although it will now be at rest in its own inertial frame. What is to stop it from then later accelerating another .8 C

Why should it start the engines a second time? Why waste fuel? If it just WAITS long enough it will eventually be receding with a recession speed of c, or 1.5c, or 2c, or whatever.
But it will never catch up with a photon.

=================
EDIT: hi George! didn't see your response when I was writing this.

 

[sketchtrack]

It easily could have a recession speed >c. Nobody said it couldn't, that I know of.
All Special Rel implies is that the spaceship could never catch up with, and pass, a photon.

Have you read Lineweaver and Davis, "Expanding Confusion"? I used to recommend that paper to PF people a lot back in 2004-2005. I see it is still getting recommended. Basic paper.
there is also the SciAm article by the same authors. March 2005.. so important to read the basic stuff.

If a spaceship accelerates to .8 C relative to earth, and then stops accelerating, it will no longer be accelerating, but it will still be moving away from Earth at .8 C although it will now be at rest in its own inertial frame. What is to stop it from then later accelerating another .8 C

Why should it start the engines a second time? Why waste fuel? If it just WAITS long enough it will eventually be receding with a recession speed of c, or 1.5c, or 2c, or whatever.
But it will never catch up with a photon.

It is just that I always hear people saying that aliens could never make it here from distant galaxies because they are so far away, and that a spaceship cannot move faster than light.

Say I am racing a photon to a destination which is an infinite distance away, and we start at the same time same place, I am constantly accelerating, how will I never pass the photon?

 

[marcus]

It is just that I always hear people saying that aliens could never make it here from distant galaxies because they are so far away, and that a spaceship cannot move faster than light.

Say I am racing a photon to a destination which is an infinite distance away, and we start at the same time same place, I am constantly accelerating, how will I never pass the photon?

Because of special rel.

Here is the Lineweaver Davis SciAm article. You need to read this.
http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf

there are many PF threads about your confusion about recession speed (recession speed needs to be treated different from speed in your local frame) here is one possible thread tht might help
https://www.physicsforums.com/showthread.php?t=212573

 

[sketchtrack]

Wouldn't it require that the recession speed of light from the starting location depends on the recession speed of me. How would it work if there was another contender who accelerated slightly faster than me? If you measure distance from participant and starting line over time according to an observer at the starting line, then at some time I would be further so long as light never moves away from the observer faster than C.

I guess this is where we step through the looking glass.

 

[marcus]

...as light never moves away from the observer faster than C.

whoever told you that? I certainly never said that. don't know where you would have gotten that idea.

most of the light in the world that happens to be moving away from us is, in fact, doing so much faster than C

because it has its local velocity of C plus its recession speed (associated with its distance from us)

 

[sketchtrack]

because it has its local velocity of C plus its recession speed (associated with its distance from us)

Could you elaborate? What do you mean by local velocity? What other way do you measure velocity other than a function of Distance and time? Is light not traveling faster than C, and therefor has a velocity greater than C?

 

[marcus]

Is light not traveling faster than C, and therefor has a velocity greater than C?

A couple of posts back I suggested you look at this thread

here is one possible thread tht might help
https://www.physicsforums.com/showthread.php?t=212573

did you have a look, sketchtrack? Did you find anything that was helpful?

You ask is the light not traveling faster than C----if you combine both the local velocity and the recession speed together. Of course it is. Most of the photons in the universe that happen to be aimed away from us ARE going away with combined speed greater than c. often much greater.

what's the problem? I assume you know that the space of special relativity is primarily useful as a local approximation.
Special rel rules apply locally, like when one thing is catching up to something else and passing it and both are in the same local frame. have to go.
try reading something and figure out by yourself

the reference to how to get "Expanding Confusion" is on page 45 of the Lineweaver SciAm article. the link to that is in my sig if you need it. on page 45 it saysExpanding Confusion: Common Misconceptions of Cosmological
Horizons and the Superluminal Expansion of the Universe. Tamara M.
Davis and Charles H. Lineweaver in Publications of the Astronomical
Society of Australia, Vol. 21, No. 1, pages 97–109; February 2004.
astro-ph/0310808
==========
when you see something like astro-ph/0310808, change it to http//arxiv.org/abs/astro-ph/0310808
and put that in the browser window---when the page comes up click on PDF.
everything on arxiv is a free download

====================

OK I'm back. Here's something (you should read Lineweaver and Davis "Expanding confusion" for yourself but here's something I can add):
there is a sweet easy way to keep track of these motions that a lot of people use (not all but many or most) which uses the CMB as an anchor. Using the CMB as a reference gives you a clear way of defining distances and analysing overall change in distance into a local component and a longrange recession component.

It's not all that complicated. I can try to outline it to you, if you want. maybe in a separate thread. Let me know if you are interested.

 

[yuiop]

whoever told you that? I certainly never said that. don't know where you would have gotten that idea.

most of the light in the world that happens to be moving away from us is, in fact, doing so much faster than C

because it has its local velocity of C plus its recession speed (associated with its distance from us)

Lets say we observe a galaxy with redshift z=2 and launch a projectile in its direction at 0.1c. The equivalent SR velocity of the galaxy is v/c=0.8c and in SR terms it seems the projectile would never catch up with the galaxy, but in comoving terms the projectile would have its own velocity plus the local recession speed (just like the photon) and would eventually catch up with the distant galaxy because the projectile is moving at 0.1c relative to the local CMB while the galaxy it is heading towards (and all the galaxies it passes on the way) are at rest with the CMB. Does that seem valid? At no time does the rocket exceed the local velocity of light. It is clear that there are different predictions here between the SR interpretation and FWR/GR comoving interpretation.

 

[oldman]

My attitude about the Utrecht model quantum universe is ....These are just straws in the wind.

Thanks for illuminating the present state of affairs for me. I guess one must just be patient and keep a watching brief for real progress --- with a sharp eye --- which is what you seem to be doing .

If I understand you mean an evolving speed of light...

it might be for some other computer modeling approach farther down the road to try a variable speed of light, not this approach

No -- I can't bring myself to accept VSL theories --- I find myself clinging to the belief that physics is observed to be too invariant across time and space for this to be a fruitful approach. But I do sometimes wonder if it is the time dimension that is as invariant as we suppose it to be, letting the space scale factor cause all change --- altering expansion and starting, sustaining and stopping exponential inflation. Seems to me to be dangerously conservative to blame everything on varying space dimensions.

 

[ThomasT]

I hope the links and discussion have resolved the issue for you.

I think the discussion has resolved that my interpretation of what is meant by the "expansion of space" is not what is meant by mainstream cosmologists when they speak of the expansion of space -- and that what they mean by the expansion of space is simply that the distances between large scale cosmological structures are increasing.
Is this correct?

On the other hand, the issue of what's actually happening in reality (with respect to the nature of empty space, etc.) seems to me to be maybe an unresolvable problem. I'm not sure how I want to think about this yet. But the discussion has been very helpful. I hope it continues, and that some other threads branch from it. There's enough "food for thought" to keep me busy for quite a long while I think.

My own intuitive speculation with regard to the ontology of empty intergalactic space is that it's composed of all sorts of wave activity involving media of unknown structure. The problem is to fashion some sort of model that assumes hierarchical media/wave interaction that can eventually yield some testable hypotheses. I don't understand GR, but I think of the idea of "curved space" as a simplification of the wave mechanics that would more precisely describe what's actually happening regarding the gravitational behavior of cosmological structures. That is, gravitational attraction isn't really due to curved space, but is, rather, due to wave interactions within and across various media. The center of a gravitational field would be the area circumscribing the most intense wave activity of the field.

Anyway, I gather that when mainstream cosmologists speak of "space", they're simply referring to the "metric" of some theory, and not the sort of physical cauldron of wave activity that I imagine it to be.

 

[marcus]

I think the discussion has resolved that my interpretation of what is meant by the "expansion of space" is not what is meant by mainstream cosmologists when they speak of the expansion of space -- and that what they mean by the expansion of space is simply that the distances between large scale cosmological structures are increasing.
Is this correct?

I believe so

the shorthand figure of speech refers to somewhat more----to the regular pattern of increase distance expressed in Hubble Law.
that on average and at large scale the rate of increase is proportional to the distance itself----the ratio being the Hubble parameter during whatever time period is being considered.

everything in that pattern is anchored to a particular idea of now. the distance to the galaxy where it is now at this instant, the rate of increase at this instant, and that idea of now of course requires an idea of all observers being at rest, in this case with respect to the CMB

you need that in order to define the idea of distance occurring in the law----to make any meaningful statement about distance one has to be very clear about what it means operationally (there are different ideas of distance)

this is the proper distance at the present moment, or whatever moment is being considered

but that is a technicality which there is rarely time to go into in a casual conversation

so the main thing is that when one says space expands it is a shorthand for saying that Hubble Law applies---that rate of distance increase is proportional to distance at large scale

it is actually an empirical fact. Hubble Law can be seen to work! so we arent just talking theory and models here.

On the other hand, the issue of what's actually happening in reality (with respect to the nature of empty space, etc.) seems to me to be maybe an unresolvable problem. I'm not sure how I want to think about this yet. ...

that is accurate and perceptive. it is actually a question of ONTOLOGY---what is the underlying reality from which appearances arise.

Naturally it would not be something one would be apt to state in the English language. Mathematics is normally found to describe things and then the math is popularized with metaphors and analogies. And we are right now in a revolution or transition between the CLASSICAL spacetime ontology of vintage 1915 Gen Rel and a new QUANTUM ontology where there is a deeper mathematics underlying the older version, from which the older emerges at large scale.

the fundamental descriptors (traditionally called degrees of freedom) at the micro level have not been established, but for a sample of what they might look like see my favorite SciAm article in my sig. the picture of spacetime at micro might be rough chaotic fluctuating geometry in which matter and geometry are intimately involved in their very existence with each other, the macro picture that arises from that rough chaotic picture is by contrast smooth. the SciAm article uses the analogy of snow. the flakes are feathery and fractally, but the snowdrifts and ski-slopes are smooth and rounded

But the discussion has been very helpful. I hope it continues, and that some other threads branch from it. There's enough "food for thought" to keep me busy for quite a long while I think...

Anyway, I gather that when mainstream cosmologists speak of "space", they're simply referring to the "metric" of some theory, and not the sort of physical cauldron of wave activity that I imagine it to be.

You are right. And that is the old CLASSICAL ontology. All that 1915 Gen Rel gives us is the metric. That is, it gives us the gravitational field itself (the field is essentially the metric up to an equivalence). And in classical Gen Rel there is no physical reality to points of spacetime. Events exist, like the collision of two particles, and geometric relations between exist.

And Gen Rel is still the established theory of spacetime geometry! It is our best and almost only theory of gravity! So naturally when people talk they are often referring to the classical ontology based on Gen Rel. What else can they use without risking speculation? So naturally they have to say that spacetime has no physical existence---as Einstein already pointed out.

But there is a more modern quantum ontology that people are working on, and you are already speculating yourself about what it could be. You present ideas in your post.

There is a new book scheduled to come out in March 2009 by Cambridge University Press called Approaches to Quantum Gravity---Towards a New Understanding of Space, Time, and Matter. and one of the early chapters is by GERARD 'T HOOFT. It will be interesting to see what he has to say. The chapter is called The Fundamental Nature of Space and Time.

't Hooft leads the theoretical physics institute at Utrecht. He is senior to the researchers in the Utrecht group responsible for the SciAm article I mentioned, but though not part of the group he is aware of their research. He is a highly original thinker and I am looking forward to what he has to say. In fact the whole book, which includes chapters by over a dozen other top people, should be interesting. I think we are on a kind of threshold as regards the ontology of spacetime and matter.

But that is just my speculation. We can't ever know for certain what the future of fundamental physics research holds.

 

[ThomasT]

Thanks again marcus et al.

 

[Sundance]

G'day from the land of ozzzzz

Expansion of space is a theory that supports the Big Bang Theory.

Observations show us that the galaxies, forming a unit and collide with other galaxies and collect into clusters and those clusters collect into super clusters. Over 100 billion galaxies are known to exist in the observable universe 13.2 Gyrs deep field and we are told that these galaxies took only 500 million years to form. Big ask.

I have been reading all this hoo haa for the last forty years and it seems to go around in circles. Not only that you have groups of people on varies fronts and theories that hold onto their emotional ideas and theories to such an extent if people do not agree are banned from those forums. The end result science goes walking.

 

[Garth]

Sundance there are two sides to scientific investigation, theory and observation.

In the last forty years cosmology has been a science that has progressed from being largely theory to one loaded with observation, as a result the standard $\Lambda$CDM model has emerged.

There are plenty of questions to be asked of this model, however it does fit the known data very well and on that basis is well established.

We do discuss questions about the standard model on PF, such as on threads I have started, amongst others, here: Critique of Mainstream Cosmology, here: Is there an Age Problem in the Mainstream Model?, here: Cosmological Coincidences and here: The Anomalous Acceleration of the Pioneer Spacecrafts. Reasonable alternative theories are also discussed, especially where they can be tested and falsified such as here: Alternative theories being tested by Gravity Probe B.

The purpose of these Forums at PF is to discussed published theory and observation and not be a venue for wild speculative ideas. That is why people are banned, not because they want to ask questions but because they want to push crackpottery.

If you have a "Big ask" then do so and others will honestly try to answer your question, but you will also have to be prepared to listen to the answer and perhaps, if those answers do not satisfy, respond with a further question.

You will find that most serious questions have been asked already, some of these will have been answered, others may still be open. That is why the subject is so exciting.

Garth

 

[Sundance]

G'day from the land of ozzzzzzz

Hello Garth

I thank you for the links and I shall read them. I'm not a crankpot,,,,,,,smile.

Until than I have to get my head around, what you said:

Quote:
"There are plenty of questions to be asked of this model, however it does fit the known data very well and on that basis is well established".

after reading:

Confirmation of the remarkable compactness of massive quiescent galaxies at z~2.3: early-type galaxies did not form in a simple monolithic collapse
Authors: Pieter van Dokkum, Marin Franx, Mariska Kriek, Bradford Holden, Garth Illingworth, Daniel Magee, Rychard Bouwens, Danilo Marchesini, Ryan Quadri, Greg Rudnick, Edward Taylor, Sune Toft
http://arxiv.org/abs/0802.4094v1

The evolution of the morphological scale of early-type galaxies since z=2
Authors: P. Saracco, M. Longhetti, S. Andreon, A. Mignano (INAF - Osservatorio Astronomico di Brera)
http://arxiv.org/abs/0801.2269v1

The Age of Cluster Galaxies from Continuum Colors
Authors: K. Rakos (UVienna), J. Schombert (UOregon), A. Odell (NAU)
http://arxiv.org/abs/0801.3665v1