A few comments on Gravity

My memory may be faulty - I said it was either there (my best bet) or in Principles - only two of Newtons books I have read. I'm not very good at searching but thank you for this link. I'll open it and see if it refreshes ~50 year old memories.

 

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Here from optics is the version of gravity there:
"Qu. 21. Is not this Medium much rarer within the dense Bodies of the Sun, Stars, Planets and Comets, thajn in the empty celestial Spaces between them? And in passing from them to great distances, doth it not grow denser and denser perpetually, and thereby cause the gravity of those great Bodies towards one another, and of their parts towards the Bodies; every Body endeavouring to go from the denser parts of the Medium towards the rarer? For if this Medium be rarer within the Sun's Body than at its Surface, and rarer there than at the hundredth part of an Inch from its Body, and rarer there than at the fiftieth part of an Inch from its Body, and rarer there than at the Orb of Saturn; I see no reason why the Increase of[Pg 351] density should stop any where, and not rather be continued through all distances from the Sun to Saturn, and beyond. And though this Increase of density may at great distances be exceeding slow, yet if the elastick force of this Medium be exceeding great, it may suffice to impel Bodies from the denser parts of the Medium towards the rarer, with all that power which we call Gravity."

There is no mention of "negative momentum" but the greater density of the "medium" on side of earth farther from the sun instead pushes (accelerates, in more modern terms) Earth towards the sun. Perhaps as I knew the was no radially increasing density I converted this to negative moment, or Newton himself may have discussed it that way in Principles. My overwhelm take-away form principles lasting for 50+ years was the power of geomentric proofs, but thinking about it now , as it is mainly about gravitational interactions, there is a good change Newton does speculate therein about the cause of gravity.

In the next Qu.22 Newton does grapple with the problem of how the planets pass thru this dense medium with no resistance. That may be with 50+ years of simmering in my brain / memory why I thought he had a mass less particle field coming from the sun. It late foe me and I'm tired so will go to bed and give a second look tomorrow. Again thanks for the link.

 

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Mod Hat

brucep, Dr_Toad, be nice...

 

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Post 24's video is useful, but some what a false conflict as newton never spoke of the "speed of gravity" or even its source. In post 22 I tell what he suggested gravity was. For newton, and every scientist for ~200 years after him space was filled by an Aether. Newton in Optice had it much like a "density" (but "concentrations" would have been a better term as it had no mass or viscosity). Yet all objects with mass "ratified"* this concentration (Newton's density) in proportion to their mass (I think is at least implied if not specifically stated) - quite similar to mass warping space POV of gravity. Also only strongly implied in the text of optics quoted in post 22, is the greater concentration on the side of Earth, etc. away from the sun, a concetration gradient held the planets in their orbit - sort of like a car on a circular banked roadway.

At the end of principles of mathematics Newton very clearly states he does not know the cause of gravity (other than perhaps "god's will" as the main point of that brief final chapter "General Scholium," GS, Is that god made and rules the universe. Near end of the second paragraph of the GS is: "This most beautiful system of the sun,plantes,and comets, could only proceed from the counsel and dominion of an intelligent and powerful Being." (I. N. did capitalize "Being" not I in this quote.) In the fifth paragraph of the GS, after its assertion that gravity come from each particle of the sun outward as inverse square in strength, I.N. says: "I have not been able to deduce the cause of gravity from phenomena and frame no hypothesis; ..."

Although the first edition of Mathematical Principles is dated 8 May 1686 and that of Optics is dated 1 April 1704 this is far from the order in which they were written. Most of Optics was sent to the Secretary of the Royal Society and read at their meeting in 1675 and the rest added 12 years later. Others were circulating in letters parts of it not always accurate version. In the preface of 1st edition I. N. said he had delayed publication and still would have, "had not the importunity of friends prevail upon me."

Optics is DESCRIPTIVE mainly of his experiments. (even the sin law of refraction is, just that, not derived); but Mathematical Principles starts with definitions and derives all from there - is analytical. That work was not even started util 1684. Others had guessed the nature of gravity but lack the skills to show how that lead to Kepler's laws and other things then know by astrophysical observations. So I. N. turned his attention to doing that with mathematical rigor. Surely almost immediately he realized that is POV I quoted in post 22 was wrong. In modern terms: the gradient of "concentration" is inverse cube but the force required is in verse square. That "concentration" gravity could explain tides, but not orbits.

Although I may have read some where that I.N. viewed gravity as a flux on "negative momentum" mass less particles, or I subconsciously converted the false "concentration gradient" of gravity into one that could be valid during the 50+ years it "cooked" in my subconscious brain - I. N. never said that and firmly cancelled his earlier, but published later POV.

* Sucking up nearby Higgs partiles, no doubt.

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I think what's notable is that Newton describes light being curved and refracted much like sonar. Much like Ned Wright's deflection and delay, and much like Einstein, who said a curvature of rays of light can only occur when the speed of light varies with position. You hear people saying light curves because spacetime is curved, but Einstein never said that, and actually it's wrong on various levels. One way it's wrong is that curved spacetime relates to the tidal force which is related to the second derivative of potential, but light curves due to the first derivative of potential which you can think of as the "spacetime tilt" or better still the "spatial inhomogeneity".

My pleasure.

 

I watched the video. Not only did it create a false conflict about the speed of gravity, it said Einstein described gravity as curved spacetime. But when you read what Einstein actually said, you appreciate that he never said that. See http://arxiv.org/abs/physics/0204044 where Peter M Brown says this: "The interpretation of gravity as a curvature in space-time is an interpretation Einstein did not agree with." Einstein never ever said light curves because spacetime is curved. That's a popscience myth.

Note that there's another popscience myth when it comes to aether. Einstein is said to have done away with the aether, but it just isn't true. See his Leyden Address where he refers to space as the aether of general relativity, and see arXiv where there's umpteen papers with aether in the title.

 

Farsight, there is a difference in the English language between the words pop science and modern interpretation. GR cannot today be solely defined or described by einsteins words from nearing 100 years ago.

And then here we go again! Will you never even begin to understand what the context of historical statements has on, what should be their historical interpretation? And by context you know from past discussion I am talking about both the background of his audience and the conceptual transition, he was ushering in with the introduction of SR and GR... Or will you continue to insist on read the past, based on your personal understanding today? I don't think you have ever tried to understand what it was any of your historical quotes was intended to convey.., when they were actually made.

You continue to raise the issue of what Einstein said in 1920 to a group of students and university staff members, likely not all of whom were students or instructors of physics, let alone relativity.., and whose background was rooted in an understanding of physics that included the luminiferous aether. Do you really believe that in the five years between his introduction of GR and that address, GR had been fully incorporated into university curriculum? Einstein was presenting a lay oriented talk, about a relatively new way of looking at theoretical physics and gravity.., and he used analogy to draw his audience out of a model of reality based on the luminiferous aether, to a new and better way of describing existing observation and formalizing predictions that could at the time only be found in the math and imagination.

 

And yet his famous field equations include the (usually) non-zero Ricci curvature tensor. What was he thinking of, one wonoders, if not the (local) curvature of his 4-manifold when the energy-matter tensor is non-zero?

Pete :Brown aka pmb aka PhyMan aka Physicist da da da is an unreliable source and has been banned from this and all other math/physics sites I know

 

Newton actually describes light much different from sonar, since Newton's theory of light is corpuscular, not wavelike.

This is a basic mistake in understanding.

Again, obviously not, since Newton's details are substantially different.

Again, obviously different, since the details of Einstein's work differ substantially from the corpuscular theory espoused by Newton.

It is foolish to look only to the grossest properties of light, where of course the theories must approximately match, and ignore the details where the real science is.

Except that the "curved space" that most people talk about is exactly the spacetime geometry that Einstein used. Again, it is important to look at the detail of the science rather than a fuzzy picture or made up straw men.

It can relate, but does not necessarily.

It would be incorrect to say that anyone ever produces a theory of gravity that uses "spatial inhomogeneity".

It is important to note that despite Farsight saying for some time that "spatial inhomogeneity" is the cause of gravity, he has been unable to show how this is the case in any example.

Reasonable beliefs should have some evidence.

 

Once again, Farsight tries to argue by google.

Just show us one example, Farsight. It is truly insane to offer a way of doing physics that can't actually do physics. Since we presume that you are not insane, then you are merely mean-spirited to never show us how to do a physics problem properly.

 

Action at a distance means the force of gravity is transmitted instantaneously over distance. That's what Newton's physics said when he wrote down his model and what it means now. Why don't you actually learn the physics and quit spewing nonsense about stuff that you clearly know very little about. It has to be that way for his theoretical model to work. It's the reason Newtons Law Of Gravitation has been falsified. It's one of the reasons why Einsteins model is a LOCAL THEORY of GRAVITY which has not been falsified.

 

Oh, my god, that's the same guy, isn't brucep?

Newton's theory assumed that gravitation acts instantaneously, regardless of distance.

http://plato.stanford.edu/archives/win2008/entries/qm-action-distance/#ActDisCoExiNonSepHol

 

That's as may be, but if GR today is defined or described via something that contradicts Einstein, there's a problem. And if there's also a false conflict concerning Newton, there's a further problem.

I'm not "interpreting" Einstein based on my personal understanding today. You are. I just give the quotes and the references and point out that Einstein said what he said. Or in this case, I point out that Einstein never actually spoke about curved spacetime. If you'd like to dig up a quote to demonstrate that I'm wrong, please do.

I've looked at them very carefully. If you had done likewise, you'd appreciate that there are differences between what Einstein is supposed to have said, and what he actually said.

Because it's totally relevant. Billy referred to aether, thinking Einstein did away with it. But he didn't. That's a popscience myth, see arXiv.

Look at arXiv, don't dismiss Einstein's aether just because you aren't familiar with it. And note that GR wasn't mainstream until the sixties. See Wikipedia:

"Kip Thorne identifies the "golden age of general relativity" as the period roughly from 1960 to 1975 during which the study of general relativity,[19] which had previously been regarded as something of a curiosity, entered the mainstream of theoretical physics."

 

He was thinking in a curvature in the metric, wherein "metric" relates to measurement. For example, imagine you’ve placed an array of parallel-mirror light-clocks in an equatorial slice through the Earth and the surrounding space. When you plot all the clock rates, your plot resembles the Riemann curvature depiction:

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GNUFDL image by Johnstone, see wikipedia

The curvature you can see here relates to curved spacetime and the tidal force. But whilst it's a curvature in your metric, it isn't curved space. Instead it's a curvature in your plot of the metrical properties of inhomogeneous space. And note that clocks don't go slower when they're lower because your plot of clock rates is curved. They don't go slower when they're lower because spacetime is curved. They go slower when they're lower because the space down there is different. Because a concentration of energy in the guise of a massive star "conditions" the surrounding space, altering its metrical properties, and this effect diminishes with distance.

As I said in my first post, Newton did not believe in action at a distance. Nor did he say gravity is transmitted instantaneously. And as I said a few posts later, in Opticks Newton referred to light bending due to a density gradient. That's local.

Trooper: Newton was scathing of action-at-a-distance. So you should be suspicious of "there seems to be no reason to exclude the possibility of action at a distance in the quantum realm". There is. Gravity is local.

 

GR is the geometrical description of gravity as detailed by Albert Einstein.
As its originator, I find it stupid in the extreme to infer that Einstein did not actually encompass spacetime curvature.
And I'm 100% sure that he would at one time or another spoke of it.

Einstein did not accept the aether as originally thought of, and as searched for by Michelson and Morley.......

see....
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http://www-history.mcs.st-andrews.ac.uk/Extras/Einstein_ether.html

Albert Einstein gave an address on 5 May 1920 at the University of Leiden. He chose as his topic Ether and the Theory of Relativity. He lectured in German but we present an English translation below. The lecture was published by Methuen & Co. Ltd, London, in 1922.

:EXTRACT:
Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.

 

So now you are telling us how he thought?

 

I repeat myself......

Here is the best and most robust quote from Einstein

\(T_{\alpha \beta}= R_{\alpha \beta}-g_{\alpha \beta}R\)

where the \(\alpha,\,\, \beta\) indices range over 0,1,....,3 and \(R_{\alpha \beta}\) reprents the scalar components of the Ricci curvature tensor, \(R\) its trace (viewed as a matrix), and \(g_{\alpha \beta}\) the scalar components of the metric tensor.

And if, ACCORDING TO EINSTEIN, \( T_{\alpha \beta}=0\) then perforce \(R_{\alpha \beta}=0\)

That is.....spacetime is "flat" in the absence of a gravitational source (whatever that might be), and not otherwise

Your man had rno eason to say more - the theory says it all

 

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http://www-history.mcs.st-andrews.ac.uk/Extras/Einstein_ether.html
It was Newton's theory of gravitation that first assigned a cause for gravity by interpreting it as action at a distance, proceeding from masses. Newton's theory is probably the greatest stride ever made in the effort towards the causal nexus of natural phenomena. And yet this theory evoked a lively sense of discomfort among Newton's contemporaries, because it seemed to be in conflict with the principle springing from the rest of experience, that there can be reciprocal action only through contact, and not through immediate action at a distance.
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I fail to see as an interested layman, how any other Interpretation could logically be deduced from that.
Good stuff.