Merged Relativity+ / Farsight

[Kwalish Kid]

I didn't say that. I've said previously that there's issues with FLRW models and black hole point singularities. But I've also said GR is one of the best-tested theories we've got.

This is clearly a disingenuous dodge of the question. On the one hand, you claim that you are not changing the equations of relativity. On the other, you claim that the equations of relativity that are used in cosmology and in galaxy dynamics are wrong. You have offered no alternatives. You have not shown where the mistakes are.

You offer insults and dogma.

Please show us your equations that deal with galaxy dynamics.

Yes. Because GR doesn't start with the the assumption of homogeneity and isotropy of space, the Friedman Lemaitre Robertson Walker metric does.

As did Einstein with his cosmological models.

Why do you blindly follow Einstein everywhere except in his cosmological models?

 

[edd]

This is a fairly general post I'm making, not directed too much at Farsight, except where it obviously is.

There's a problem in cosmology (there's lots actually, which is a major reason why I find it interesting, but I'm going to talk about one). Cosmologists like all other scientists want to use Occam's Razor and find the simplest model to deal with things.

In General Relativity, the simplest model is a completely empty universe. This is trivially falsifiable. I don't think that needs further explanation.

The next step up, arguably, is the FRW cosmology, where everything is homogeneous and isotropic. That too is trivially falsifiable, but with the critical point that it is actually a very very good approximation to what is observed, in a way that the empty universe is not. We like it as a result, and use it a lot, but it is without question not exactly right.

We don't know how to model the universe exactly right though. It's big and messy, but on the largest scales the mess can be averaged over. General relativity, thanks to it being deliciously non-linear (that's where all the fun comes from perhaps), is very hard to compute solutions to, and there are few that work well except the FRW one. The 'Swiss Cheese' cosmology basically takes an FRW universe and peppers it with black holes, and that's a start, but it's not the real thing.

First point I want to make, is that this doesn't mean the FRW universe model is useless, nor is it lacking in explanatory value.

Secondly, it may not have a gravitational field in it as such (at no point can you assign a vector to the gravitational field) but it certainly has what are arguably gravitational effects that control the evolution of the universe. Since GR perhaps does away with the idea of gravitational fields as the things inducing forces (by doing away with 'gravitational force' altogether) this isn't such a big deal I'd say.

Thirdly, it's unclear at best how the expansion of the universe acts on systems where the FRW model is clearly breaking down, such as inside galaxies and absolutely for you sitting in your chair reading this. I'd say space is not expanding in such regions, and I'd expand (excuse the pun) on this further if my laptop battery were not nearly dead and I might do so later/tomorrow, but when Farsight talks of changing energy densities inside galaxies ('how long has the universe has been expanding whilst this galaxy has been gravitationally bound') it really isn't clear what the situation is, and that's without what looks to me like Farsight conflating energies with different equations of state as well. I'll probably be asking an expert tomorrow on some things I'd never considered before in such circumstances, actually.

Anyway, TLDR: FRW is a good and useful practical and pedagogical solution.

 

[ben m]

Not so. The above point is quite crucial. The FLRW metric starts with the assumption of homogeneity and isotropy of space.

More standard crackpot technique: read about something that seems to be an assumption, stop reading at the "Astro For Poets" level, and pretend that everything you read is an unquestioned dogma that's waiting for an out-of-the-box thinker to upend it.

The "assumption" you talk about is not an dogma, it's a conclusion derived from the data:

http://adsabs.harvard.edu/abs/1995ApJ...443....1S

We talk about the Universe as almost-homogenous and almost-isotropic because the data indicates that it's almost homogenous and isotropic.

The differences between an "exact" FLRW metric and a "not quite" FLRW metric have been well-studied for fifty years:

http://adsabs.harvard.edu/abs/1967ApJ...147...73S
http://adsabs.harvard.edu/abs/1968Natur.217..511R
http://adsabs.harvard.edu/abs/1983AnPhy.150..487E
http://adsabs.harvard.edu/abs/1989PhRvD..40.1804E
http://adsabs.harvard.edu/abs/2011CQGra..28p4006W
http://adsabs.harvard.edu/abs/2013arXiv1303.5079P

Oh, yeah, and: please note that the FLRW metric is large scale, i.e. cosmology. Complaining about FLRW is not a plausible way to wriggle out of your actual claim: you want to pretend you can explain galaxies without dark matter, saying:

Every galaxy has this halo or shell of inhomogeneous space

which has nothing to do with the FLRW metric. Nothing at all. I repeat my question: do you actually pretend that you can use ordinary GR---the standard, tested, Einstein-written and MTW-propagated equations---to model a galaxy, accurately, without dark matter? Please answer by (at least) pretending to know something about the actual "halo or shell of inhomogenous space" that you were previously pretending to find in your GR work, which you pretend is absent in mainstream work, and which you pretend explains the dark matter problem.

Please do not dodge the question by pretending to have an "crucial" objection to a completely different calculation.

 

[W.D.Clinger]

Farsight also rejects the meaning of "field" that Einstein relied upon in his mathematics, saying it is inconsistent with the "state of space" Einstein used in his Leiden address...

The field is a state of space comes from Einstein's 1929 field theory article.

I stand corrected. Neither of those popular lectures actually contains the phrase "state of space". Neither lecture actually defines a field as a state of space. Both lectures trace the historical development in which fields that were once conceived as states of an ether came to be regarded as states of empty space. Both lectures go on to suggest the gravitational field can be identified with the g_μν, which describe the local geometry of spacetime. If the g_μν are interpreted as coordinate-dependent components, which is the historically plausible reading, then they are fully equivalent to a coordinate-dependent state of space that varies over the corresponding coordinate-dependent dimension of time.

Because of those similarities between the two popular lectures, it doesn't really matter which one you've been using as your sacred proof-text.

...As becomes clear from what Einstein wrote following the words I quoted, what Einstein actually means here is that any free-falling observer is free to select a coordinate system for spacetime in which, at every point on the observer's spacetime world line, the ten coordinate-dependent components of the pseudo-metric tensor gμν coincide with those of the Minkowski metric in the standard basis. That means the free-falling observer can always imagine himself to be free from the influence of gravity, attributing the rapidly accelerating approach of the earth (for example) to acceleration of the earth rather than acceleration of the observer.

That's wrong too. Einstein was clear that you cannot transform away a special (=real) gravitational field. It's in section 20 of Relativity: the Special and General Theory, and I quote:

"We might also think that, regardless of the kind of gravitational field which may be present, we could always choose another reference-body such that no gravitational field exists with reference to it. This is by no means true for all gravitational fields, but only for those of quite special form. It is, for instance, impossible to choose a body of reference such that, as judged from it, the gravitational field of the earth (in its entirety) vanishes".

There is no contradiction between the passage you quoted and what I wrote (or what Einstein wrote in The Meaning of Relativity). Einstein and I were both careful to say a choice of coordinate system that makes the gravitational field disappear along the free-falling observer's world line makes that gravitational field disappear only in the sense that the components of the pseudometric tensor coincide with those of the Minkowski pseudometric at each point of the world line. Neither Einstein nor I claimed the observer's choice of coordinate system could make that happen at points not on that world line, and we didn't even come close to claiming that the entire gravitational field could be made to vanish.

I was actually a little more careful than Einstein. Einstein spoke of "an infinitesimally small region of the space-time continuum", while I spoke of points. I was also careful to say "coordinate system" instead of "coordinate patch", but it would take some time to explain that distinction even to the physicists. I'm not quite so foolish as to try to explain a truly subtle mathematical distinction to Farsight.

 

[Reality Check]

Still unanswered, Farsight:
Farsight: (29 July 2014) What is the scientific definition of space?

Farsight: (29 July 2014) What is the scientific definition of a field in physics? And the follow-on: How does this make What a field in physics really is by W.D.Clinger wrong?

 

[Reality Check]

Farsight: Information about dark matter that you seem to be ignorant about

No. It tells you not to presume that dark matter is comprised of WIMPs.

No, Farsight - the mere existence of General Relativity which (charitably ) is what you are talking about tells us nothing about the nature of dark matter.
No one is presuming that dark matter is comprised of WIMPs - that is what the evidence suggests.

That there is strong evidence for dark matter means that over the last ~80 years since it was first proposed in 1933 to explain the dynamics of galaxies in galaxy clusters by Zwicky we have to find sources for that dark matter. From 1933 to the late 1960s, the obvious source was visible matter that had not been detected before, e.g. emitting outside of the spectrum that we could detect. But the gaps in that were steadily closed with new instruments.

Vera Rubin in 1975 found the evidence for dark matter in the rotation curves of spiral galaxies. Spiral galaxies are studied extensively across the spectrum - we have a good account of their visible matter. in the almost 40 years since then we have got even better at detecting visible matter - still not enough .

What General Relativity actually tells us when matched with observations of the CMB is: According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[2][3].
As an aside: We cannot even detect all of that 4.9% ordinary matter - the last I heard we were missing about half which is the 'missing mass' problem.

What simulations of the universe shows us is that this 26.8% of the total mass–energy of the known universe that is dark matter modeled as particles is needed to explain the large scale structure of the universe.

Dark matter as particles works down to the scale of galaxies.

Attempts to explain dark matter by modifying Newtonian dynamics (MOND) fail except for galaxies. Attempts to explain dark matter by modifying GR, e.g. TeVeS, fail except for galaxies.

From 2006: A direct empirical proof of the existence of dark matter

We present new weak lensing observations of 1E0657-558 (z=0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8-sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen.

(my emphasis added)

And a

 

[Reality Check]

But he is talking about aether

Farsight: He is taking an "Aether" (sometimes in quotes as if he did not believe it existed ) which one of the several uses of the word aether:

Do note that "space is dark" is an insanely ignorant statement since is reveals your ignorance about what space is.
Farsight: (29 July 2014) What is the scientific definition of space?

Do note that Clinger's field definition is consistent with the actual modern definition of field. Not that you have any idea what that is, Farsight:
Farsight: (29 July 2014) What is the scientific definition of a field in physics? And the follow-on: How does this make What a field in physics really is by W.D.Clinger wrong?

Farsight: (1 August 2014) Information about dark matter that you seem to be ignorant about !

Gibberish about a "halo or shell of inhomogeneous space" existing somewhere in your head has been snipped !
This may the fantasy that galaxy halos are caused by your imaginary form of expansion of the universe.

 

[Farsight]

Post deleted.

 

[ctamblyn]

No, but what is right about it, is that there's no overall gravitational field when space is homogeneous. A homogeneous universe is a flat universe.

Not true. Both parts are wrong, in fact. A sphere is homogeneous (and isotropic), but not flat. Hyberbolic space is homogeneous (and also isotropic), but not flat. You can also have anisotropic homogeneous cosmologies, such as the Mixmaster universe, in which tidal effects are present (i.e. there exists, by any reasonable standard, a gravitational field).

 

[edd]

You've fallen at the first hurdle. See Einstein's 1920 Leyden address. Note this: "recognition of the fact that 'empty space' in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gmn), has, I think, finally disposed of the view that space is physically empty". Space is not nothing

I didn't say space was nothing. I was talking just about a particular case where space had nothing in it.

 

[Farsight]

I repeat my question: do you actually pretend that you can use ordinary GR---the standard, tested, Einstein-written and MTW-propagated equations---to model a galaxy, accurately, without dark matter?

You'll doubtless respond with some jibe, so you'll excuse me if I don't.

Reality Check: I've answered your questions.

I didn't say space was nothing. I was talking just about a particular case where space had nothing in it.

Then forget what I said.

You know, I am really getting hacked off with stuff like this:

This is clearly a disingenuous dodge of the question...

You offer insults and dogma...

More standard crackpot technique...

 

[W.D.Clinger]

You know, I am really getting hacked off with stuff like this:

This is clearly a disingenuous dodge of the question...

You offer insults and dogma...

Sorry to hear about your personal feelings, but you do indeed have a well-documented history of offering insults and dogma in lieu of substantive responses. In this particular case you managed to avoid insults while repeating some of your personal dogma, but Kwalish Kid was entirely correct to say you were dodging the question. In the part you snipped, Kwalish Kid explained why your response was a dodge:

This is clearly a disingenuous dodge of the question. On the one hand, you claim that you are not changing the equations of relativity. On the other, you claim that the equations of relativity that are used in cosmology and in galaxy dynamics are wrong. You have offered no alternatives. You have not shown where the mistakes are.

ben m did an even better job of explaining why your response was a dodge:

Complaining about FLRW is not a plausible way to wriggle out of your actual claim: you want to pretend you can explain galaxies without dark matter, saying:

Every galaxy has this halo or shell of inhomogeneous space

which has nothing to do with the FLRW metric. Nothing at all.

What ben m said there is entirely correct. Having explained why your previous response was an irrelevant dodge, ben m repeated his question:

I repeat my question: do you actually pretend that you can use ordinary GR---the standard, tested, Einstein-written and MTW-propagated equations---to model a galaxy, accurately, without dark matter? Please answer by (at least) pretending to know something about the actual "halo or shell of inhomogenous space" that you were previously pretending to find in your GR work, which you pretend is absent in mainstream work, and which you pretend explains the dark matter problem.

Please do not dodge the question by pretending to have an "crucial" objection to a completely different calculation.

That's a perfectly legitimate question. Its phrasing expressed some exasperation, but in a way that should have helped you to understand what's wrong with your previous responses. Your response:

You'll doubtless respond with some jibe, so you'll excuse me if I don't.

That's another dodge.

When you are accused of dodging the question, and you confirm those accusations by continuing to dodge the question, telling us you're "getting hacked off" strengthens the impression you've already given of not being able to back up your claims with substantive reasoning.

 

[edd]

Farsight, can I ask if at this point you still think a homogeneous universe must be flat?

 

[Kwalish Kid]

You know, I am really getting hacked off with stuff like this:

I am not surprised that you do not like the truth. I am also not surprised that you cherry-pick the parts of posts that you choose to comment on. You completely ignore, as you have for years, the idea that you might have to produce evidence (as you describe it) for your own claims other than the textual analysis (selective references and quotations) that you always give.

Either produce the relationship between GR and rotation curves that everyone has missed or concede the point.

 

[Farsight]

Sorry to hear about your personal feelings, but you do indeed have a well-documented history of offering insults and dogma in lieu of substantive responses...

Not so. I have a well-documented history of offering substantive responses while receiving a continual tirade of insults from dogmatic posters who seek to spoil discussions. All you have to do to substantiate that is read the thread. The rest of your post is yet more aspersion, and I've had enough of it.

Farsight, can I ask if at this point you still think a homogeneous universe must be flat?

Yes I do. Let's set aside the expansion of the universe for a moment, because it muddies the waters. We have space that is homogeneous on the large scale. Shine a light beam through it, and this light beam goes straight. There is nothing to make the light beam deviate from straight. To the left and to the right, above and below, everything is the same. The light beam goes straight as an arrow. And there is no evidence of any "intrinsic" curvature or any mechanism that can make that light beam loop back round on itself. People like ctamblyn will refer to a sphere and say it's homogeneous in that it has a constant curvature, but there is no evidence of any higher dimensions.

As for the expansion, as I've said before, the universe can't be infinite, because an infinite universe can't expand. Which means it has to have some kind of edge. And there is nothing beyond it. No void, no space, nothing. Get yourself a stress-ball. Squeeze it down in your fist. Let go, and watch.

 

[steenkh]

You'll doubtless respond with some jibe, so you'll excuse me if I don't.

You must realise that this response to a perfectly valid question must be seen as an concession that your bluff was called.

 

[edd]

Yes I do. Let's set aside the expansion of the universe for a moment, because it muddies the waters. We have space that is homogeneous on the large scale. Shine a light beam through it, and this light beam goes straight. There is nothing to make the light beam deviate from straight. To the left and to the right, above and below, everything is the same. The light beam goes straight as an arrow. And there is no evidence of any "intrinsic" curvature or any mechanism that can make that light beam loop back round on itself. People like ctamblyn will refer to a sphere and say it's homogeneous in that it has a constant curvature, but there is no evidence of any higher dimensions.

I am about to quote a paper. You are welcome to guess who wrote it. As a clue, it had to be translated from the original German.

In my original investigation, I proceeded from the following assumptions:
1. All locations in the universe are equivalent; in particular the locally averaged
density of stellar matter should therefore be the same everywhere. (edd note - this means homogeneous)
2. Spatial structure and density should be constant over time.
At that time, I showed that both assumptions can be accounted for with a non-zero mean density ρ, if the so-called cosmological term is introduced into the field equations of the general theory of relativity such that these read:
( R_ik -1/2 g_ik R ) + λ g_ik = - κT_ik ..... (1)
These equations can be satisfied by a spatially spherical static world of
radius √ (2/κρ) where ρ is the (pressure-free) mean density of matter.

Two light beams originating from the same point in such a universe will diverge and then converge back, meeting again.

 

[Farsight]

You must realise that this response to a perfectly valid question must be seen as an concession that your bluff was called.

Nope.

I've responded to edd. It's useless trying to respond to ben m.

I am about to quote a paper. You are welcome to guess who wrote it. As a clue, it had to be translated from the original German.

I've spoken before about Einstein proposing a hypersphere universe, and how IMHO his intuition somehow failed him when it came to cosmology. As if he didn't believe in his own theory. I've said things like "Oh Albert, lose the dust". Then he would have been left with just space, which he knew was something rather than nothing. The energy-pressure diagonal in the stress-energy-momentum tensor should have told him space has an innate pressure and just has to expand. The shear stress term is a reminder that it's elastic, like the stress ball. Maybe part of it is that he didn't do much in the way of memorable physics after 1920.

 

[ctamblyn]

I've said things like "Oh Albert, lose the dust". Then he would have been left with just space, which he knew was something rather than nothing. The energy-pressure diagonal in the stress-energy-momentum tensor should have told him space has an innate pressure and just has to expand.

If you "lose the dust" and really are left with "just space", you're talking about solving the vacuum field equations, surely? In that case, the diagonal elements in the energy-momentum tensor (in fact, all the elements in the energy-momentum tensor) are zero.

Also, it looks to me like you're mixing up the meaning of the left and right hand sides of the equation G_ab = 8πT_ab

 

[edd]

Credit for the translation:
C. O’Raifeartaigh and B. McCann, arxiv:1312.2192