# Gravity: The why and the how:

Discussion in 'Astronomy, Exobiology, & Cosmology' started by paddoboy, Oct 24, 2015.

1. ### Farsight

Messages:
3,492
Good stuff Schmelzer.

There are two interpretations, which Kevin Brown mentions in The Formation and Growth of Black Holes. After reading Einstein talking about the speed of light varying with position, I conclude that the "other" interpretation is the correct one.

That's what people say. But note that the force of gravity at any one location relates to the gradient in the coordinate speed of light at that location. If you dotted optical clocks throughout a slice through the Earth and the surrounding space, then plotted the clock rates, you'd be plotting out the Riemann curvature depiction above. However the coordinate speed of light at the event horizon is zero, and it can't go any lower than that. So there's no gradient in the coordinate speed of light. And so no force of gravity.

The moot point that this just can't happen in a place where the coordinate speed of light is zero. There's no falling down, and there's no light moving to define those coordinates. It's a strange place all right.

I don't think there's much of an issue with the GR spacetime interpretation myself. I think the issue is with "all coordinate systems are valid". IMHO people take this too far, and invent fantasy coordinate systems that just don't match Einstein's description of what a gravitational field is. Take a look at the picture below from MTW. On the left is a depiction of Schwarzschild coordinates. Most people take this for granted, but if you look closely, you'll appreciate that the vertical axis is the time axis, and that the infalling body goes to the end of time and back and is in two places at once. Hence Susskind's elephant, which I can confidently say is nonsense.

The Kruskal-Szekeres coordinates on the right effectively do a hop skip and a jump over the end of time. They "use a new time coordinate" which effectively puts a stopped observer in front of a stopped clock and claims he sees it ticking normally in his own frame. IMHO this is a schoolboy error. I am reminded of the dead parrot sketch.

Last edited: Nov 19, 2015

3. ### Farsight

Messages:
3,492
You abusive idiot. Einstein described space as the "ether" of general relativity. That's why arXiv is littered with papers that refer to Einstein-aether. Jesus H Christ, spare us from popscience trolls who believe in woo and who reject all education and references that challenge it.

Messages:
27,534
All cosmologists are doing is modelling the gravitational field according to observations. If the model makes successful predictions and is not falsified, then you writing it off as hypothetical is rather silly.
You dont believe the gravity field we all exist in is real?
When you misinterpret what Professor Hamilton has said, maybe you should also consider that the "No Hair theorem" is overwhelmingly accepted by mainstream cosmology. The "No Hair Theorem" of course is that a BH can have only three properties....that being mass, spin and charge, and both the latter two are negated over time.

Any supposed consensus you speak of applies to sciforum.com only. It will not be a consensus that is embraced by mainstream cosmology in general.
In fact the recently concerted effort by known anti mainstream posters of various degrees of crankdom, makes any consensus here totally unlikely as you suggest. One even has claimed to have a TOE.
Your claim re arguments directed at posters rather than a specific issue is probably also valid, but again you need to remember that whatever doubt exists about mainstream "speculative" claims [which you prefer to highlight far more often than any opposing alternative claims] is the incumbent solution until something better comes along.
Your own place in all of this in my opinion appears to be having 2 bob each way, or sitting on the fence, and coupling that with your self confessed mission to be hard on me, could be interpreted as an agenda.

Please consider what I have said and examine the contents of your posts and your claims around certainty or lack thereof with regards to myself.
And please remember what past Professors have said, which you yourself have appreciated. Scientists/cosmologists are allowed to make reasonable assumptions based on already acquired knowledge and data.
That is done all the time, and this forum and I, do not really need you to keep reminding people of that.
Considering that with the facts that scientific theories do grow in certainty over time, perhaps you need to direct your doubts in the direction of the alternative brigade.

Last edited: Nov 19, 2015

Messages:
27,534
And yet still its you that has been banned in numerous forums, and it is also you that has claimed to have a TOE. That says it all. The truth hurts.
Oh, and please calm down before you have a coronary.

Messages:
27,534
Well then do it appropriatley and submit a paper pointing out what you believe are errors.
At least it has had a number of citations as distinct from your own ether paper.

9. ### rpennerFully WiredValued Senior Member

Messages:
4,833
Same space-time, two metrics under discussion means two coordinate systems. Each must be addressed separately.

In the Gullstrand-Painlevé metric (written in SI units): $d\tau^2= \left(1-\frac{2GM}{c^2 r} \right)\, c^2 dt_r^2- 2\sqrt{\frac{2GM}{c^2 r}} c dt_r dr - dr^2-r^2 \, d\theta^2-r^2\sin^2\theta \, d\phi^2$, if we want to know the coordinate speed of light in a certain direction, we solve for $d\tau^2 = 0$, consistent with that direction.
Thus in the radial direction, $d \theta = d \phi = 0$, and we have $dr = v dt_r$.
Thus we solve $0 = d\tau^2= \left(1-\frac{2GM}{c^2 r} \right)\, c^2 dt_r^2- 2\sqrt{\frac{2GM}{c^2 r}} c v dt_r^2 - v^2 dt_r^2$ which gives a quadratic equation for v:
$v^2 + 2\sqrt{\frac{2GM}{c^2 r}} c v - \left(1-\frac{2GM}{c^2 r} \right)\, c^2 = 0$
Thus $v = \left( - \sqrt{\frac{2GM}{c^2 r}} \pm 1 \right) c$ and when $r = \sqrt{\frac{2GM}{c^2}}$ the outbound radial coordinate speed is zero while the inbound coordinate speed is -2c.
For $0 < r < \sqrt{\frac{2GM}{c^2}}$ there is no outbound radial coordinate speed, just two different inbound coordinate speeds, which makes sense if you think of black holes as places light can't escape from.

In the 1916 Schwarzschild metric, $d\tau^{2} = \left(1-\frac{2GM}{c^2r} \right) c^2\, dt^2 -\frac{dr^2}{ \left(1-\frac{2GM}{c^2r} \right)}- r^2 \, d\theta^2-r^2\sin^2\theta \, d\phi^2$ we only have to solve $\left(1-\frac{2GM}{c^2r} \right) c^2 = \frac{v^2}{ \left(1-\frac{2GM}{c^2r} \right)}$ to get $v = \pm \left(1-\frac{2GM}{c^2r} \right) c$ in the radial direction. Thus the inbound and outbound speeds of light always have the same coordinate speed and thus when $r = \sqrt{\frac{2GM}{c^2}}$, both are equal to zero, therefore the criterion Farsight used to disparage the Gullstrand-Painlevé metric applies doubly to his chosen metric, thus must be rejected as an inconsistent special pleading.

If Farsight wishes to object that the Gullstrand-Painlevé metric exhibits different coordinate speeds of light in different directions, then may I preemptively point out the same holds for the 1916 Schwarzschild metric. Assuming $dr=0, \, d\phi = 0, \, d\theta =\frac{v}{r} dt, d\tau^{2} = 0$ we get the speed of light perpendicular to the radial direction from:
$\left(1-\frac{2GM}{c^2r} \right) c^2 = v^2$ or $v = \pm \sqrt{1-\frac{2GM}{c^2r}} c$ which makes the 1916 solution anisotropic with respect to the coordinate speed of light.

Neither anisotropy in perpendicular or opposite directions is a reason to reject a proposed set of space-time coordinates. Any reasoning to reject must come from the geometry and comparison to Einstein's curvature equation at the heart of GR.

Indeed, the reason the Schwarzschild metric must have a singularity at the event horizon is that it is incapable of saying that infinitesimal motion inward isn't similar to the same motion outward therefore it blows up at the horizon to avoid the question of trapping light and objects.

Last edited: Nov 19, 2015
10. ### SchmelzerValued Senior Member

Messages:
5,003
Arxiv does not make any peer-review, it may reject some stuff out of prejudice, but that's all, an "acceptance" means nothing. Then, learn to read, I have not questioned at all the mathematics of the paper. And learn to read the arxiv site, it mentions publications of the articles in journals. The paper has been accepted and published by AJP. Which is a journal for popular articles, with teachers and their pupil as the auditory, thus, not really scientific, but nonetheless some peer-review (contrary to arxiv itself).
First, arxiv ist not a "publisher", but an eprint server. Then, I do not care about any bandwagons, but criticize an article.

I have written a more detailed critique, and have even a response of A. Hamilton. Which contains, in particular, the following: "The paper is quite critical of our paper Hamilton & Lisle published in AJP in 2008. I accept those criticisms;" See http://ilja-schmelzer.de/papers/river.php for my article as well as the full response.

As I have already explained, I do not expect citations for ether papers during the next years. We have to wait until string theory has been really given up, and people really start to search for alternatives, without prejudices.

Farsight likes this.

Messages:
27,534
Please, enough with the dramatic conspiracies!
That's nice and is what I said about being validated.
Criticise all you like, obviously as I have shown, you do have somewhat of an agenda and are consequently biased in your opinion.

I'm not concerned about any critique of your paper, I'm speaking of Professor Hamilton's mathematically validated river/water fall model that has been cited.

So you have no prejudices against string theory?
Science/cosmology are always searching for alternatives where required, and improvements/modifications of existing models. That will almost certainly be from mainstream.

12. ### SchmelzerValued Senior Member

Messages:
5,003
I prefer my own ether interpretation too, but it would be stupid to criticize the others for inconsistency if they are, however stupid, consistent.

It is zero in one direction, in the other direction quite large. At least in coordinates which are non-degenerated. Of course, degenerated coordinates are irrelevant.

Not all of them are valid, they may degenerate, and degenerated coordinates are not valid. Otherwise, all coordinates are equivalent mathematically, because one has formulas to connect them.

The description given by Einstein does not require much. There should be a symmetric tensor field in the four-dimensional spacetime, with signature (1,3), that means three spatial directions and one time-like one.

13. ### SchmelzerValued Senior Member

Messages:
5,003
What conspiracy? Arxiv.org is not peer-reviewed, this is simply an information about a well-known fact. It has never been claimed to be peer-reviewed.
So I have, in the article, criticized Hamilton's paper. And Hamilton has written a review of my critique, and accepted the main points. My paper, as well as his reply, is, of course, also about some other things, namely about the Lorentz ether interpretation of the Einstein equations of GR, which I have suggested as an improvement of his river model.
Of course I have. String theory is not even wrong. With this characterization of string theory, I simply follow some other scientists, who have written books about this.

But, given that I simply follow others here, without studying string theory myself, this is even quite correctly characterized as "prejudice", and in this sense similar to all your opinions about whatever theories discussed here.

Messages:
27,534
I'm not referring to that.
Are you now being intellectually dishonest?

That's nice, yet it still stands as a mathematically validated often cited paper.
I have never said that string theory is fully accepted by mainstream...It is still debatable and will be until we are able to observe at those scales.
I'm not prejudiced like yourself...I do not have an agenda like yourself, and I have no ego to bruise. In my capacity I accept what works and is theorised, and consider reasonably logical speculative scenarios such as the Superforce.
That capacity to understand properly may certainly be lower than yours, but by the same token, I am not in anyway inhibited by an agenda, which is obvious in your case, judged on what you post.

15. ### OnlyMeValued Senior Member

Messages:
3,914
Anytime the model includes an event horizon or anything inside of an event horizon it is no longer modeling anything that has been observed. It is extending what limited information we have based on observation and theory to conclusion beyond observation.

That does not makes sense. GR was developed to explain gravitation that included the earth, sun and other planets.., and it does that very well. Solutions that predict singularities and infinities die a death of their own creation and it does not have anything to do with gravity in a weak field. It is not even certain that they (any solution to EFE), can be trusted to acuartely predict what a gravitational field would be like at or inside of an event horizon, because once that point is reached the singularity and infinities are unavoidable, within the context of those solutions...

That was a very limited simplified restatement of a quote from the link. I left the links (not active) to the Waterfall analogy and what is Realistic, on purpose. The later further supports the understanding that the charged black hole, is a theoretically based hypothetical that does not exist... It is not realistic.

BTW it could be argued that the No Hair Theorem is a theorem in name only and based on speculation and belief. Remember no one has seen a black hole after all.

You do understand that a large part of cosmology today is inherently speculative. It is based on many many unconfirmed assumptions and an extension of weak field observations about gravitation, to extremes of both distance and.., the singularities few if any believe actually exist.

If the singularity does not exist you cannot with any certainty, know were a solution to EFE that predicts a singularity, begins to fail, as in begins to describe a gravitational field that cannot exist in our reality. All we can say is that it does a good job of describing what we can observe which stops well outside of any event horizon. Observations that support the existence of black holes is limited to not being able to see things that seem to be the center of strong gravitational fields. Nothing in what we can observe tells us exactly what we cannot see... That is the domain of theory and speculation and will remain so for a very long time.

None of the above should be interpreted to mean that I don't believe black holes exist. I just don't believe there are any singularities.

We cannot know that what we believe are black holes today, are not just massive compact objects that no longer have any atomic structure. I did not say no subatomic structure! Most of the EM radiation we are able to detect and measure is dependent on atoms. Essentially no atoms no light. With or without an event horizon. The gravitational field outside of the Schwarzchild radius, would be indistinguishable!

And I am not saying that is what is. I only offered that as an example to demonstrate that when we talk about black holes, as soon as we reach an event horizon, we are speculating. And it does not matter whether you are Einstein, Steven Hawking or their next door neighbor, it is still speculation.

After all of this, if the discussion is about a particular solution, it's predictions, even how it compares to another.., or how it compares to what can be observed, fine just stay aware that just because a solution is mathematically valid, does not make it a accurate description of reality.

Messages:
27,534
Yes, by learned men educated in the discipline of cosmology, and quite reasonable and ethical to do. Certainly not some BNS nonsense by some "would be if he could be" nor any of your usual hackneyed repetitive stuff you go on with like a stuck record. Stop avoiding the point OnlyMe.

It makes perfect sense for the reasons already stated along with the general scientific methodology.
GR in predicting its own downfall, allows cosmologists to "theorise speculate reasonably" right up to the limitations of itself.
We can reasonably predict inside the EH, at least up to the singularity.

So the no hair theorem does not exist now? 80 years of cosmology and theoretical cosmology by the likes of John Wheeler can be confined to the dustbin now?
Really, in your little game to be hard on me, you are actually sounding more like the god every day.

Do better.

More usual nonsense. Perhaps you need to study the scientific method and what is reasonably accepted on indirect evidence.
This continued fence sitting is not going to do your anatomy any good.
And if you believe an argument can be fabricated to invalidate the no hair theorem, then go ahead. Should prove interesting.
I can only conclude OnlyMe that you are being totally intellectually dishonest, that which you have often accused the god of.
Yes a large part of cosmology is speculative, speculation by learned men educated in the field that are able to come up with reasonable assumptions, which now you want to dismiss in your evangelistic like mission.
And again no one has said anything about any physical real singularity, but up to it, cosmologists are able to logically speculate, as other professors have informed you. Tough titty if that ruffles your doubting thomas feathers somewhat. That's the way it is and they are doing OK.
Rubbish! And the reality of the situation and the reputable replies we have had on various issues support what I am saying and discredit your cynical unreal approach.
2 bob each way again?
I'll tell you something OnlyMe, I was surprised somewhat the other day when I smelt a rather religious overtones in a post from the god. Seriously, you are sounding the same.
I mean how often do we hear their old catch cry "Ahaa, but it's only a theory!"

No, we don't observe BH's directly, but we do have convincing evidence of their existence...We don't see DM either, but are nearly just as confident they exist due to indirect evidence.....DE also....and who can forget the BB? We never see that either but the indirect evidence is so overwhelming that even the Catholic church now accept it.
Now don't get me wrong OnlyMe.....I have nothing at all against religious folk as long as they stick to their religion and dont try and deride the sciences.
There we have a dog's breakfast of intellectual dishonesty, cynical raving, and teaching granny how to suck eggs.

Your efforts in trying to project a sense of fairness and fence sitting has reached the stage of crankdom OnlyMe....sorry about that.

Last edited: Nov 20, 2015
17. ### OnlyMeValued Senior Member

Messages:
3,914
Following is a quote of the full text of my post the one you just responded to. I don't think you find anywhere that I even mentioned you. I spoke only to the issues.

And here is an redacted quote of your post responding to the above... Or rather that portion that was directed personally at me rather than the subject.

Do you think that you can ever have a civil discussion with anyone who does not see things exactly as you do... Even when you are discussing an area of theoretical physics that not all of the authorities agree on?

Please go back read your post the one I initially responded to and then my response again. Your responses did not always make sense. Keep in mind there are many solutions to EFE and some of them are mutually exclusive... None of those solutions are the heart of GR, the weak field limit of EFE that can and has been confirmed is. The failure of any or all solutions are not the downfall of GR.

18. ### The GodValued Senior Member

Messages:
3,546
Generally this is the signature style of Paddoboy. Arguing (and abusing too) without proper understanding of the point in hand.

It is ridiculous to suggest that some background Universe (which is a flat space time metric ) is observing the 'river metric' in foreground. You missed the whole point of the paper.....Model for lay persons.

Last edited: Nov 20, 2015

Messages:
27,534
:shrug: Really OnlyMe, whatever!
All your posts directed at me are ever saying is we don't know with 100% certainty. Then you get all indignant when I call you out and accuse you of intellectual dishonesty?
I mean how many times to I have to say that I know what is and is not 100% certain? Yet you keep on with that line.
Now go back and read your own posts, and instead of telling me "we don't know for sure" start recognising how logical and reasonable some of these assumptions are. Forget your mission! It has no effect on me!

Messages:
27,534

That's coming from a lay person who can't even recognise the fact that GR predicts total collapse once the Schwarzchild radius is breached, or that BH's do really exist in the first place.
Please support what you say, otherwise you are pissing in the wind.
And with regards to abuse, I would look at yourself first my friend.

21. ### The GodValued Senior Member

Messages:
3,546
In high school Physics, river flow numericals are taken up. Say the stream is moving at a speed Vr in some direction and a swimmer is attempting to swim at a speed Vb in some direction, then finding out travel time, resultant velocity etc etc is taught and taken up.

Probably this is what prompted Hamilton to give an analogical model about BH dynamics inside EH as he though people with less mathematical rigour can understand this. Thus the river model. It is no metric and it has no value added contribution to scientific pursuit, and neither Prof Hamilton claims so.

Analogy is simple and straightforward for non spinning neutral BH. The Schwarzschild Metric in Schwarzchild coordinates give problem even at EH (division by zero), but this problem can be eliminated if we consider Gullstrand-Painlev coordinates. Thats the starting point for Prof Hamilton. He very simply says that river starts flowing from an origin point of a very high mountain (r = infinity and Vr = 0) and acquires speed Vr = c at EH and this keeps increasing and approaches infinity (?) at r = 0. Any object irrespective of his speed Vb (as constrained by SR) gets dragged by this river flow too (simple High School Maths to calculate travel time etc). To make life simpler he attempts to say that assume that this river (space flow) flows against a backdrop of flat spacetime. Inward journey is complete.

In the outward direction since at the EH, the stream speed is c and inside it is faster than c, suggesting that nothing not even light can be seen from outside once it has crossed irrespective of direction of Vb. He further says that by chance if the object speed Vb is c at EH, then it appears to be standstill with resultant speed being zero for an outside observer.

The problem with this model is

a. River Flow is Energy.
b. Anything at r = non infinity will have a drag towards the centre of BH...ultimately collapsing inside.

But no one bothers about these apparent problems, because it is of no value to science, it is just a model like that raisin expansion model.

22. ### brucepValued Senior Member

Messages:
4,098
He's still claiming the remote bookkeeper coordinates is preferred over local proper coordinatess?We've been over this with Farsight and RJ Berry. Several times. He's pulling your chain. This nonsense leads to both them claiming black holes don't form in a finite time. And for Farsght he uses that nonsense to say the local speed of light varies in the local gravitational field. He actually claims that's what Einstein had in mind.

23. ### The GodValued Senior Member

Messages:
3,546

Your problem is Paddoboy, most of the time you do not even understand what is Mainstream, and you argue and abuse to defend the incorrect notion you have about science.