Question about gravity

[Southwind17]

Now the question is, why do you think that gravity is not well understood?

Putting aside the fact that nobody here seems capable of explaining their understanding of gravity in a concise, meaningful manner using non/pseudo-scientific language that a reasonably intelligent person can relate to and understand, this, for starters, makes me a little dubious:

Historical alternative theories:
Aristotelian theory of gravity
Le Sage's theory of gravitation (1784) also called LeSage gravity, proposed by Georges-Louis Le Sage, based on a fluid-based explanation where a light gas fills the entire universe.
Nordström's theory of gravitation (1912, 1913), an early competitor of general relativity.
Whitehead's theory of gravitation (1922), another early competitor of general relativity.

Recent alternative theories:
Brans-Dicke theory of gravity (1961)
Induced gravity (1967), a proposal by Andrei Sakharov according to which general relativity might arise from quantum field theories of matter.
Rosen bi-metric theory of gravity
In the modified Newtonian dynamics (MOND) (1981), Mordehai Milgrom proposes a modification of Newton's Second Law of motion for small accelerations.
The new and highly controversial Process Physics theory attempts to address gravity
The self-creation cosmology theory of gravity (1982) by G.A. Barber in which the Brans-Dicke theory is modified to allow mass creation.
Nonsymmetric gravitational theory (NGT) (1994) by John Moffat
Tensor-vector-scalar gravity (TeVeS) (2004), a relativistic modification of MOND by Jacob Bekenstein

But let's look at your explanation. You seem to be suggesting that gravity is better understood than photosynthesis, for example, because our understanding of gravity relies on only one axiom, whereas our understanding of photosynthesis relies on many. But let's consider this single gravitational axiom:

In the case of gravity, the only axiom is that space-time can be represented by a real four-dimensional smooth manifold with a metric tensor of Lorentzian signature that satisfies Einsteins equation.

"pace-time can be represented by a real four-dimensional smooth manifold with a metric tensor of Lorentzian signature that satisfies Einsteins equation."

It seems to me that your explanation does not rely on a single axiom, but is wholly comprised by that axiom. In other words, to use your word, it is a "magical" explanation. I wonder what the Great Randi might have to say about that?!

Do you see my point, or do you maintain that my inability to comprehend this statement results from my ignorance, and is, therefore, my problem, and that I should go read some books?!

 

[Cold one]

I think for him to even attempt to understand you need to make a web page and google link to it, without that google link it just isnt real.

 

[robinson]

General relativity. The equations demonstrating that have already been presented.

So, you can't tell us, or link to scientific paper that backs you up?

How many times do we have to tell you that? I see Ziggurat just did it again.

These pages explain it pretty well: 1,2.

No, those pages don't even contain the term "inverse square law", much less claim it does not apply to Newton's Law of Gravity.

Again, if you have any evidence, please quote it, or link to a paper that supports your claim.

 

[robinson]

Here, let me make it easy for you.

"The gravitational attraction between two massive objects, in addition to being directly proportional to the product of their masses, is inversely proportional to the square of the distance between them"

Now go find a credible source that says that isn't true, as you claimed. Shucks, at this point try finding any website of any kind that claims that.

This should be interesting.

 

[ponderingturtle]

Here, let me make it easy for you.

"The gravitational attraction between two massive objects, in addition to being directly proportional to the product of their masses, is inversely proportional to the square of the distance between them"

Now go find a credible source that says that isn't true, as you claimed. Shucks, at this point try finding any website of any kind that claims that.

This should be interesting.

On the 18th November he made a discovery about which he wrote For a few days I was beside myself with joyous excitement . The problem involved the advance of the perihelion of the planet Mercury. Le Verrier, in 1859, had noted that the perihelion (the point where the planet is closest to the sun) advanced by 38" per century more than could be accounted for from other causes. Many possible solutions were proposed, Venus was 10% heavier than was thought, there was another planet inside Mercury's orbit, the sun was more oblate than observed, Mercury had a moon and, really the only one not ruled out by experiment, that Newton's inverse square law was incorrect. This last possibility would replace the 1/d2 by 1/dp, where p = 2+ for some very small number . By 1882 the advance was more accurately known, 43'' per century. From 1911 Einstein had realised the importance of astronomical observations to his theories and he had worked with Freundlich to make measurements of Mercury's orbit required to confirm the general theory of relativity. Freundlich confirmed 43" per century in a paper of 1913. Einstein applied his theory of gravitation and discovered that the advance of 43" per century was exactly accounted for without any need to postulate invisible moons or any other special hypothesis. Of course Einstein's 18 November paper still does not have the correct field equations but this did not affect the particular calculation regarding Mercury. Freundlich attempted other tests of general relativity based on gravitational redshift, but they were inconclusive

link

 

[boooeee]

Putting aside the fact that nobody here seems capable of explaining their understanding of gravity in a concise, meaningful manner using non/pseudo-scientific language that a reasonably intelligent person can relate to and understand, this, for starters, makes me a little dubious:

Are you dubious of Andrew Wiles' proof of Fermat's Last Theorem? I don't think anybody can explain that in a "concise, meaningful manner using non/pseudo-scientific language that a reasonably intelligent person can relate to and understand".

I still don't understand what you're getting at in this thread. I don't think anybody else does either.

 

[69dodge]

Oh, well maybe you should say what you mean then.

Sorry. I didn't intend to be misleading. Before asking you, "If Newton's law of gravity is exact, how can Einstein's law of gravity be better?", I did say, "I don't understand what you're trying to say here", to show that I was trying to point out a problem with what you said rather than trying to find out the real answer to the question.

I agree with the scientist and engineers who actually make things go out into space and orbit and come back, or explore the planets with probes. I agree with the people who make things work, who design and launch space ships and space stations.

I know what formulas they use, the math they use, and I know who's name is on the Laws they follow to do this. Orbital mechanics is a very exact science. They use Kepler and Newton's Laws.

Successfully exploring planets with space probes requires a great deal of accuracy from the theory of gravity used in planning the missions, but it doesn't require infinite accuracy. So, using Newton's theory of gravity works for sending out probes, even though the theory is not exactly correct, because the answers it gives in those situations are sufficiently close to the correct answers.

 

[Ziggurat]

Successfully exploring planets with space probes requires a great deal of accuracy from the theory of gravity used in planning the missions, but it doesn't require infinite accuracy.

Quite so. Even if we could do the calculations perfectly, we can't launch perfectly, so we have to be able to do mid-course corrections anyways, and it doesn't really matter the origin of these corrections if you have to make them regardless. There's really no point in doing the calculations with greater accuracy than you can launch with, or the accuracy with which you can track your current position/velocity.

 

[shadron]

Really? The inverse square law has been proven wrong? Oh noes! We need to call somebody.

Ah, OK, I see where the "inverse square" denial on the other thread came from. So now I know why you quoted it.

You do realize, of course, that the IAU (or the Committee for Science Czars) didn't sit down one day and define "theory", "law", "fact" and all the other Aristotelian categories of science knowledge so that "Law of Gravity" means something different from "Theory of Gravity", like they did for "moon" and "planet". Generally, a law refers to some pithy, easy-to-remember one-liner (or math equation) which succinctly summarizes a theory, or part of one. "The three laws of thermodynamics: you can't win, you can't break even, and you can't leave the game." So lets let the little things go and concentrate on the meat.

 

[Fredrik]

No, those pages don't even contain the term "inverse square law", much less claim it does not apply to Newton's Law of Gravity.

Again, if you have any evidence, please quote it, or link to a paper that supports your claim.

Sometimes I wonder if you understood everything we've been trying to explain to you a long time ago, but decided to just be as annoying as possible instead of admitting you got it wrong...

I anticipated that complaint from you. That's why I also explained that the inverse square law implies that orbits are elliptical. If experiments show that orbits aren't elliptical (with corrections for those other things I mentioned), then this disproves the inverse square law. Is that really so hard to understand?

Both of these sites say that the observed perihelion precession of Mercury is 43 seconds of arc per century more than Newton's theory (i.e. the inverse square law) can explain. Did you miss that? Did you just dismiss it as a lie?

The proof that the inverse square law implies elliptical orbits can be found in just about any textbook on classical mechanics or ordinary differential equations. The proof that GR predicts corrections to the inverse square law can be found in just about any textbook on GR.

I'm quite sick of explaining this to you. You have either decided to never think rationally about this again, or to never admit that you've been wrong. This discussion is pointless either way.

 

[Fredrik]

You do realize, of course, that the IAU (or the Committee for Science Czars) didn't sit down one day and define "theory", "law", "fact" and all the other Aristotelian categories of science knowledge so that "Law of Gravity" means something different from "Theory of Gravity", like they did for "moon" and "planet". Generally, a law refers to some pithy, easy-to-remember one-liner (or math equation) which succinctly summarizes a theory, or part of one. "The three laws of thermodynamics: you can't win, you can't break even, and you can't leave the game." So lets let the little things go and concentrate on the meat.

Well said, especially the stuff I colored blue. I didn't really explain how the word "law" is used in my posts above, but you just did it perfectly.

 

[robinson]

Ah, OK, I see where the "inverse square" denial on the other thread came from. So now I know why you quoted it.

You do realize, of course, that the IAU (or the Committee for Science Czars) didn't sit down one day and define "theory", "law", "fact" and all the other Aristotelian categories of science knowledge so that "Law of Gravity" means something different from "Theory of Gravity", like they did for "moon" and "planet".

They also changed the definition of planet recently. So instead of having 12 planets, we now have 8. That we know of.

I think the problem of terms reflects the essential fact that nobody is really in charge of "science". Much less terminology.

This semantic mudpit over "wrong", "incorrect", or whatever, is like arguing over pie.

Pi is 3.141592653

No it is not! To be correct you should use
3.14159 26535 89793 23846 26433 83279 50288 4197​

No, 9 digits is enough to be accurate.

No! If you use 39 decimal places you will be more precise.​

39 digits! That is enough to compute the circumference of any circle that fits in the observable universe to a precision comparable to the size of a hydrogen atom!

Yes! So you see, it is more correct. You were wrong.​

I can't measure anywhere close to that! What would be the point?

That I am right, and you sir, were wrong!​

Pi to 12 places is more than enough for anything we would ever need.

Why don't you just admit you are wrong?​

 

[robinson]

link

Yay! Good find Turtle.

 

[sol invictus]

I don't know why I'm bothering to post this, since you have ignored it several times already - but your trolling has been effective. I find it incredible that you are willing to make yourself look this foolish, and I can't help contributing. It's probably a personality flaw of mine... a bit like rubbernecking at an accident as you drive by.

"The gravitational attraction between two massive objects, in addition to being directly proportional to the product of their masses, is inversely proportional to the square of the distance between them"

Now go find a credible source that says that isn't true, as you claimed. Shucks, at this point try finding any website of any kind that claims that.

However far we might travel through space, we should find everywhere an attenuated swarm of fixed stars of approximately the same kind and density. This view is not in harmony with the theory of Newton.
...
In order to escape this dilemma, Seeliger suggested a modification of Newton’s law, in which he assumes that for great distances the force of attraction between two masses diminishes more rapidly than would result from the inverse square law.
...
Suppose we draw lines or stretch strings in all directions from a point, and mark off from each of these the distance r with a measuring-rod. All the free end-points of these lengths lie on a spherical surface. We can specially measure up the area (F) of this surface by means of a square made up of measuring-rods. If the universe is Euclidean, then F = 4 pi r^2; if it is spherical, then F is always less than 4 pi r^2.
...
But the general theory of relativity permits of our answering it with a moderate degree of certainty, and in this connection the difficulty mentioned in Section XXX finds its solution.

http://www.bartleby.com/173/30.html

 

[Fredrik]

Putting aside the fact that nobody here seems capable of explaining their understanding of gravity in a concise, meaningful manner using non/pseudo-scientific language that a reasonably intelligent person can relate to and understand,...

You demand an explanation that doesn't mention things like curvature. This is like demanding an explanation of Newton's theory that doesn't mention the inverse square law or Newton's laws.

this, for starters, makes me a little dubious:

Historical alternative theories:
Aristotelian theory of gravity
Le Sage's theory of gravitation (1784) also called LeSage gravity, proposed by Georges-Louis Le Sage, based on a fluid-based explanation where a light gas fills the entire universe.
Nordström's theory of gravitation (1912, 1913), an early competitor of general relativity.
Whitehead's theory of gravitation (1922), another early competitor of general relativity.

This is irrelevant. It's like saying that all of medicine is nonsense because blood-letting was a common method of treatment a long time ago.

Recent alternative theories:
Brans-Dicke theory of gravity (1961)
Induced gravity (1967), a proposal by Andrei Sakharov according to which general relativity might arise from quantum field theories of matter.
Rosen bi-metric theory of gravity
In the modified Newtonian dynamics (MOND) (1981), Mordehai Milgrom proposes a modification of Newton's Second Law of motion for small accelerations.
The new and highly controversial Process Physics theory attempts to address gravity
The self-creation cosmology theory of gravity (1982) by G.A. Barber in which the Brans-Dicke theory is modified to allow mass creation.
Nonsymmetric gravitational theory (NGT) (1994) by John Moffat
Tensor-vector-scalar gravity (TeVeS) (2004), a relativistic modification of MOND by Jacob Bekenstein

None of this changes the fact that GR describes a very wide range of phenomena with a fantastic accuracy, or the fact that the theory itself is understood just about perfectly by physicists. I mentioned this earlier, and followed up by asking "If this isn't understanding, then what is?". That's the question you're going to have to answer if you're going to convince anyone that gravity isn't well understood.

People are always looking for ways to improve this theory (and every other theory) that much is true, but there's still no experimental or observational evidence that says that we need another (classical) theory of gravity. So the fact that people are trying is not a reason to believe gravity is not well understood.

But let's look at your explanation. You seem to be suggesting that gravity is better understood than photosynthesis, for example, because our understanding of gravity relies on only one axiom, whereas our understanding of photosynthesis relies on many. But let's consider this single gravitational axiom:



"pace-time can be represented by a real four-dimensional smooth manifold with a metric tensor of Lorentzian signature that satisfies Einsteins equation."

It seems to me that your explanation does not rely on a single axiom, but is wholly comprised by that axiom. In other words, to use your word, it is a "magical" explanation. I wonder what the Great Randi might have to say about that?!

Every theory contains axioms (statements that you don't try to explain), so this is not just a problem with general relativity. It's a problem that all theories have.

Do you see my point, or do you maintain that my inability to comprehend this statement results from my ignorance, and is, therefore, my problem, and that I should go read some books?!

Is your point that nothing explains the axiom? Again, that's a problem all theories have. The only thing that could explain it would be some other theory, but those theories would have axioms too.

And yes, I do maintain that your inability to understand this theory is caused by your lack of knowledge about those topics that I mentioned before: linear algebra, calculus, classical mechanics, special relativity and differential geometry. General relativity is a very difficult subject, mostly because it requires knowledge of differential geometry.

 

[sol invictus]

Putting aside the fact that nobody here seems capable of explaining their understanding of gravity in a concise, meaningful manner using non/pseudo-scientific language that a reasonably intelligent person can relate to and understand, this, for starters, makes me a little dubious:

Sorry southwind, you've been supplanted by an even bigger troll. FYI, every single one of those theories has been since ruled out by experiment. I could go down the list and tell you how, but I won't bother.

By the way, an equation that predicts the results of literally millions of experiments given one single number as an input is hardly an axiom. It's the most elegant and simple theory in the history of science, and having such a theory is the (or at least my) definition of understanding something well.

 

[Fredrik]

They also changed the definition of planet recently. So instead of having 12 planets, we now have 8. That we know of.

I think the problem of terms reflects the essential fact that nobody is really in charge of "science". Much less terminology.

This semantic mudpit over "wrong", "incorrect", or whatever, is like arguing over pie.

We've had a semantic discussion about "theory" and "law", but your claim that the inverse square law is exactly true is not a semantic issue. That claim is just false, and we have explained how we can know that.

 

[ponderingturtle]

Yay! Good find Turtle.

Note the bolded underlined part that you said no one else ever said.

 

[MattusMaximus]

Ponderingturtle answered the question about the Newtonian vs. Einsteinian view of gravity very well. As a physics & astronomy teacher I'd like to just add a couple of points:

1. When we teach about Newtonian gravity (what some here are calling the "inverse square law"), I make sure to let my students know that it is, at best, an approximation. In physics, this is sometimes called the "weak field approximation".

Newton does not, as ponderingturtle pointed out with the link, explain all observed phenomena - such as the precessional motion of Mercury's orbit about the sun. Nor does Newton's law explain other observations such as gravitational lensing effects or the gravitational redshift of light, both of which have mountains of evidence to back them up.

For many things, Newton's law works very well, at least to within the precision necessary. But for higher orders of precision, and to explain various observations (as outlined above) you have to go with the more accurate Einsteinian view of gravity, also known as general relativity.

2. There are real, practical uses Einstein's view (general relativity). For example, gravitational fields produce time dilation effects - clocks deeper in a gravity well will run slower with respect to those clocks further out of the well. For a planet like the Earth, this isn't much of an effect, but it is measurable.

Global positioning satellites (GPS) must take these gravitational time dilation effects into account, because the calculations done by the electronics are so fast that a small difference in the rate at which the ground clock versus the satellite clock ticks will cause the system to screw up, giving erroneous results.

So, Newton's view doesn't work for GPS, but general relativity a la Einstein does. The next time you use your GPS in your car, thank Einstein for his hard work. I like to tell my students to call it the General relativity Positioning System

Cheers - Mattus

 

[Southwind17]

You demand an explanation that doesn't mention things like curvature. This is like demanding an explanation of Newton's theory that doesn't mention the inverse square law or Newton's laws.

I don't have a problem with "curvature"; that's a commonly understood word, and can generally be related to, but "space-time curvature", what does that look like?

Every theory contains axioms (statements that you don't try to explain), so this is not just a problem with general relativity. It's a problem that all theories have.

Is your point that nothing explains the axiom? Again, that's a problem all theories have. The only thing that could explain it would be some other theory, but those theories would have axioms too.

I understand, but the theories of relativity, or our understanding of gravity, the way you explain it, seems like one entire axiom.