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Author Topic: Does gravity attract masses in space, or does it curve space between them?  (Read 5025 times)

Offline PmbPhy

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Quote from: Alan McDougall
"You know this Pete and your comment was uncalled for"!
This is supposed to be a friendly discussion and that's what I was posting into. Please don't make assertions such as "your comment was uncalled for" because it implies that I was intentionally being a perverse, because I most certainly wasn't (I know what I really meant).

Alan; I now consider you a friend and I never make rude comments to a friend of mine and I'm never sarcastic either. So I object to your (untrue) claim that I was reading into your statements something that is not there and then you claim that I know this which I most certainly didn't know. How do you know what I said was troublesome?

The statement you made that I commented on is this one - The answer to this question is much more complex than meets the eye?  When someone starts off a sentence with The answer to this question is ... which I understood, correctly or incorrectly[/i], to mean that you knew the answer. Just because a sentence ends with a question mark it doesn't mean that the person who wrote it was asking a question. There are uses of the question mark on sentences which are in reality statements and that's what I thought that you meant it to mean.

All you had to do was say that it wasn't what I made a mistake. After all I'm not the kind of person who can't admit that they made a mistake. So lets put this bed, okay?
 

Offline Alan McDougall

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Quote from: Alan McDougall
"You know this Pete and your comment was uncalled for"!
This is supposed to be a friendly discussion and that's what I was posting into. Please don't make assertions such as "your comment was uncalled for" because it implies that I was intentionally being a perverse, because I most certainly wasn't (I know what I really meant).

Alan; I now consider you a friend and I never make rude comments to a friend of mine and I'm never sarcastic either. So I object to your (untrue) claim that I was reading into your statements something that is not there and then you claim that I know this which I most certainly didn't know. How do you know what I said was troublesome?

The statement you made that I commented on is this one - The answer to this question is much more complex than meets the eye?  When someone starts off a sentence with The answer to this question is ... which I understood, correctly or incorrectly[/i], to mean that you knew the answer. Just because a sentence ends with a question mark it doesn't mean that the person who wrote it was asking a question. There are uses of the question mark on sentences which are in reality statements and that's what I thought that you meant it to mean.

All you had to do was say that it wasn't what I made a mistake. After all I'm not the kind of person who can't admit that they made a mistake. So lets put this bed, okay?

Noted Pete!! you are the last person I want as my enemy because I look up at you as an expert from which I can learn, as your insightful posts have proved many times.

If I can address the question posed in this thread.

We know that two objects like asteroids when they get close to each other, out there in space, will drift toward each other due to their mutual gravity fields, In that I do not see any bending of space, but a simple attraction due to mass and gravity.

Gravity this scenario looks more like a force such as magnetism although much, much weaker, but the effect it the same?

What is causing them to drift toward each other a force or the bending of space or both?
 

Offline IAMREALITY

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What is causing them to drift toward each other a force or the bending of space or both?

Both.  The 'force' of gravity, which is a product of the the curvature of spacetime caused by any given mass.
 

Offline PhysBang

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What is causing them to drift toward each other a force or the bending of space or both?
One can describe spacetime in such a way that the objects, at rest, just approach each other with no force on the objects. This is part of general relativity.
 
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Offline Alan McDougall

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What is causing them to drift toward each other a force or the bending of space or both?
One can describe spacetime in such a way that the objects, at rest, just approach each other with no force on the objects. This is part of general relativity.

Therefore Gravity is not a force?
« Last Edit: 01/07/2016 17:52:53 by Alan McDougall »
 

Offline JohnDuffield

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It's not a force in the Newtonian sense, in that no work is done on a falling body. Gravity converts potential energy into kinetic energy, and when the latter is dissipated, you're left with a mass deficit. However if you look on the Einstein digital papers there's plenty of mentions of gravitational force, so IMHO one should be pedantic about this.

As for how gravity actually works, I think it's fairly straightforward. See this post where I attempted to explain it in easy-reading terms. 
 

Offline PhysBang

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However if you look on the Einstein digital papers there's plenty of mentions of gravitational force, so IMHO one should be pedantic about this.
It is not pedantic to answer the question. It is however, dishonest to present as a citation a search result that might appear to support the point that Einstein thought of gravity as a force when the specific search results do not support that conclusion.

The search results of Mr. Duffield's citation include the words of Einstein's translators, descriptions of the works of people other than Einstein, and Einstein speaking of other theories that General Relativity, including Newtonian mechanics. They do not include Einstein describing gravity in General Relativity.

Newtonian physics works extremely well for gravity, just not quite as well as General Relativity. Many people, including Einstein, will use Newtonian physics to set up a first approximation to a result and then use relativistic methods to finish a problem or application. This does not, however, commit them to the metaphysics of Newtonian physics. And the question of the OP is about the metaphysical content of gravitational theory. In General Relativity, gravity is not a force, it is part of the natural motion of particles.
Quote
As for how gravity actually works, I think it's fairly straightforward. See this post where I attempted to explain it in easy-reading terms.
Given the penchant of Mr. Duffield for providing, at the very least, misleading information about physics, I would recommend against following that link.
 

Offline Alan McDougall

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To simplify the question!

Are objects in space attracted towards each other due to their receptive masses bending the fabric of space?.

Or is gravity a force acting on objects causing them to drift towards each other if they get close enough something like magnetism?

Or is it both?

Is gravity a fundamental force of nature , like electromagnetism? the Graviton particle?

End of my comments (Alan)

http://www.qrg.northwestern.edu/projects/vss/docs/space-environment/1-what-is-gravity.html

What is gravity according to this article?

Gravity is a force pulling together all matter (which is anything you can physically touch). The more matter, the more gravity, so things that have a lot of matter such as planets and moons and stars pull more strongly.

Mass is how we measure the amount of matter in something. The more massive something is, the more of a gravitational pull it exerts. As we walk on the surface of the Earth, it pulls on us, and we pull back. But since the Earth is so much more massive than we are, the pull from us is not strong enough to move the Earth, while the pull from the Earth can make us fall flat on our faces.

In contrast Albert Einstein said

https://www.thestar.com/news/insight/2012/08/18/einstein_discovered_that_gravity_is_not_a_force_but_a_curvature.html

The classical theory of gravity had been a huge breakthrough in its time, and it still provides a good means of predicting the motion of objects. But it is wrong, in part because Newton misunderstood what gravity is.

Newton considered gravity to be a force that objects exert upon each other.

But, in a burst of brilliance, Einstein realized that no such force is required and in fact no such force exists.
« Last Edit: 02/07/2016 19:40:55 by Alan McDougall »
 

Offline JohnDuffield

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It is not pedantic to answer the question. It is however, dishonest to present as a citation a search result that might appear to support the point that Einstein thought of gravity as a force...
It's not dishonest to refer to the Einstein digital papers. Einstein said what he said.

All: please take what PhysBang says with a pinch of salt. He's a stalker and a troll. He doesn't answer any of the questions, he just badmouths the people who do.
 

Offline PhysBang

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It is not pedantic to answer the question. It is however, dishonest to present as a citation a search result that might appear to support the point that Einstein thought of gravity as a force...
It's not dishonest to refer to the Einstein digital papers. Einstein said what he said.
It is dishonest to claim that there is a force of gravity because you can find it in a search of a set of documents. You clearly were trying to make an argument from authority to Einstein. It is important to counter your dishonesty by noting that the search results were not to the words of Einstein or were not about Einstein's own theory.
Quote
All: please take what PhysBang says with a pinch of salt. He's a stalker and a troll. He doesn't answer any of the questions, he just badmouths the people who do.
I also urge you to take everything I say with a pinch of salt. People on the internet are often dishonest. Mr. Duffield has given us a few examples of this.
 

Offline JohnDuffield

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All: there's been no dishonesty from me. I stand by my references. Read them for yourself.
 

Offline puppypower

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If gravity was a force, there should be an exothermic output; release of energy potential, similar to the way the other three forces give off energy when they lower potential. Maybe dark energy is nothing more than the exothermic output from gravity. Neither the energy output from gravity or dark energy have been seen in the lab. However, both can be inferred from affect, apart from direct lab evidence.

For example, when the EM force lowers potential, the photons given off can cause the EM force to increase elsewhere, to create what appears to be an anti-EM force affect. When mass collapses due to gravity, a rotation will often appear. The rotation generates centrifugal force, which generates a force vector opposite gravity. This cause and affect is consistent with an opposing force being generated.

There are examples of spiral galaxies which more turns than should occur based on the age of the galaxy. This can be explained with the exothermic output profile due to gravity; winds from the center.   
 

Offline PhysBang

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What is causing them to drift toward each other a force or the bending of space or both?
One can describe spacetime in such a way that the objects, at rest, just approach each other with no force on the objects. This is part of general relativity.

Therefore Gravity is not a force?
If one accepts General Relativity, then gravity is not a force.
 

Offline PhysBang

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All: there's been no dishonesty from me. I stand by my references. Read them for yourself.
Mr. Duffield, I urge you of all people to read your own references. Please.
 

Offline AndroidNeox

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Gravity curves spacetime because it stretches spacetime. A gravity well is "stretched", or "dilated", spacetime. Matter exists within spacetime. Light travels through spacetime. Some property of mass causes spacetime to stretch. I think the coupling between mass (or energy, because E=mc˛ has something to do with the Higgs field, but I don't understand Higgs).

The commonly used rubber sheet model is very good in that it shows the stretching as well as, conveniently, letting one roll a marble around the simulated gravity well, simulating an object in orbit. The amount of spacetime dilation is proportional to the difference in gravitational potential energy between two points. The delay in the travel time of a beam of light traveling through a gravity well due to the increase in path because space is stretched is called the Shapiro Delay. For example, between the Earth and the Sun, space is stretched about 60 km, due to the difference in gravitational potential energy.

Ideally, the rubber sheet can stretch infinitely, is perfectly slippery, and stretches vertically with an ideal spring constant (the depth of the well is directly proportional to the mass in the middle). If the mass is fluid, e.g. mercury or BBs, then the sheet will follow a correct curve even under the surface of the fluid mass, curving more toward horizontal instead of getting progressively steeper. The maximum spacetime stretch will be at the center point, when the sheet is stretched deepest. Note, in the center of gravity of an object, although spacetime is maximally dilated and time goes slowest, there is no net gravitational vector so the observer, there,  wouldn't feel any gravity, they'll be in freefall, but their clock will run slower than for an observer at higher gravitational potential energy.

The rubber sheet model is also excellent because the stretch to space is entirely radial about the mass. For example, if the sun were compressed to a black hole, the orbital circumference of the Earth would remain unchanged but the measurable distance to the center of our orbit would approach infinite. The Shapiro delay to any event horizon will be infinite.

But, don't get hung up on the idea that spacetime is a thing. It's just an imaginary framework we've established for showing how motion in different frames relate to each other. Each observer has a "local" stationary, the local inertial frame. Whether you are experiencing acceleration or not, you can define an inertial frame. For example, for an observer in 1 g of acceleration, an inertial frame would fall downward at ~32 ft/sec/sec. For an astronaut floating freely in free fall, she is occupying her inertial frame, whether they are far from any masses or in vacuum in orbit around a planet.
« Last Edit: 15/07/2016 02:22:47 by AndroidNeox »
 

Offline PmbPhy

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Alan - The curvature of spacetime is just another name for tidal gradients. That's all that it is. The curvature of space refers to the fact that if you take measurements of spatial distances between points in space then you'd find that they don't behave like that measurements you'd expect from taking measurements between points in flat space. The distances change and they have the properties of a curved space.

There's nothing wrong with referring to gravity as a force. Einstein did. What Einstein meant was that the force of gravity is an inertial force. Before Einstein inertial forces, like the Coriolis force and centrifugal forces, were thought of as fictitious, i.e. being due to the wrong choice of a frame of reference. Einstein argued that since the gravitational force behaves like an inertial force and since he considered the gravitational force as being "real" he asserted that inertial forces are also "real."  You can read what Einstein and other contemporary physicists wrote on this point at my website at: http://www.newenglandphysics.org/physics_world/gr/inertial_force.htm

Here is the derivation for the expression for the inertial force in general relativity:
http://www.newenglandphysics.org/physics_world/gr/grav_force.htm
 

Offline Alan McDougall

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Alan - The curvature of spacetime is just another name for tidal gradients. That's all that it is. The curvature of space refers to the fact that if you take measurements of spatial distances between points in space then you'd find that they don't behave like that measurements you'd expect from taking measurements between points in flat space. The distances change and they have the properties of a curved space.

There's nothing wrong with referring to gravity as a force. Einstein did. What Einstein meant was that the force of gravity is an inertial force. Before Einstein inertial forces, like the Coriolis force and centrifugal forces, were thought of as fictitious, i.e. being due to the wrong choice of a frame of reference. Einstein argued that since the gravitational force behaves like an inertial force and since he considered the gravitational force as being "real" he asserted that inertial forces are also "real."  You can read what Einstein and other contemporary physicists wrote on this point at my website at: http://www.newenglandphysics.org/physics_world/gr/inertial_force.htm

Here is the derivation for the expression for the inertial force in general relativity:
http://www.newenglandphysics.org/physics_world/gr/grav_force.htm

Thank you, Pete, I went to both links ,the maths  a bit hard for me to follow?

Here, however , is an excerpt from one of the links

'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''

Here is the derivation for the expression for the inertial force in general relativity:

http://www.newenglandphysics.org/physics_world/gr/grav_force.htm


I could not copy and paste the equations

In what follows it must be kept in mind that “no G’s mean no gravitational force.” This should not be confused with ”G means gravitational force”. The former is always true while the later may or may not be true.

In general relativity, gravitational effects manifest themselves through the metric tensor gmn. According to Einstein's Equivalence Principle an accelerating frame of reference is locally equivalent to a gravitational field.

This means that in the absence of a 4-force on particle, the only force acting on the particle will be equivalent to an inertial force, the force resulting entirely from observing the particles motion from a frame of reference accelerating with respect to an inertial frame. Since the first term on the right side of Eq. (4) is proportional to the 4-force it follows that the second term on the right side of Eq. (4) represents the gravitational force.

"If I understand it correct the word "Force" is not a "No-No", when describing gravity?

Alan



 

Offline PmbPhy

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Quote from: Alan McDougall
Thank you, Pete, I went to both links ,the maths  a bit hard for me to follow?
It's not important to be able to follow the derivation. All that you need to know is the result which is Eq. (8a) and the fact that the value for the gravitational force equals the time rate of change of momentum of a particle in a gravitational field. Notice the value of it, i.e.

Gk = m(components of the gravitational field)x(velocity terms)

Here m is the relativistic mass of the particle. Notice how this compares with the Lorentz force, Eq (9) (the force on a charged particle in an electromagnetic field) in that its charge x field x velocity terms. That's why I like relativistic mass, i.e. its like a gravitational charge.

Quote from: Alan McDougall
Here, however , is an excerpt from one of the links
....
I could not copy and paste the equations
That's because the only way I knew of, at least the easiest way, was to write the page in MS Word using its equation editor, the do a screen capture and the use that image of the equation in the webpage. So its actually a GIF file, not something you can cut and past.

Quote from: Alan McDougall
"If I understand it correct the word "Force" is not a "No-No", when describing gravity?
That's a matter of opinion. I know of several GR experts who think of it as a force. I actually showed you a page of quotations from some physicists on this. Just keep in mind that the gravitational force in GR is an inertial force. Now recall the page I posted before, i.e.
http://www.newenglandphysics.org/physics_world/gr/inertial_force.htm

Take a look at it again. Read this quote
Quote
From Introducing Einstein's Relativity, by Ray D'Inverno, Oxord/Clarendon Press, (1992) page 122   

Notice that all inertial forces have the mass as a constant of proportionality in them. The status of inertial forces is again a controversial one. One school of thought describes them as apparent or fictitious which arise in non-inertial frames of reference (and which can be eliminated mathematically by putting the terms back on the right hand side). We shall adopt the attitude that if you judge them by their effects then they are very real forces.
Here the author is saying that the gravitational force is a real force.

Now read the quote from Einstein.
 

Offline PmbPhy

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A good example of how the gravitational force is mentioned in general relativity textbooks can be seen by downloading the following text, which is quite well known and whose author was a renown expert in general relativity

Theory of Relativity (International Series of Monographs on Physics) by C. Moller
http://book4you.org/book/860357/c15966

You'll have to first register for the site and then download and install a STDU reader. In this text you'll find that the author provides a derivation for the gravitational force too. His seems to be the same. I didn't follow a text to get my result but mine is the same as his as, of course, it must be.

Using the STDU reader do a search in this text on the term gravitational force and you'll get a list of all the instances where Moller uses it. If you'd like I can teach you general relativity in the NEP forum. It's not as difficult as you might think. Trust me. :)
 

Offline PhysBang

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Einstein argued that since the gravitational force behaves like an inertial force and since he considered the gravitational force as being "real" he asserted that inertial forces are also "real."  You can read what Einstein and other contemporary physicists wrote on this point at my website at: http://www.newenglandphysics.org/physics_world/gr/inertial_force.htm
That Einstein used dubious metaphysics is not an argument for adopting dubious metaphysics.
 

Offline Jack Qwek

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To me is both things. When you drop a stone in the water, are the waves caused by the stone, or are the waves circular because of the stone? It's both, the waves are caused by the stone and their shape is circular. In the same way, gravity attracts and bend everything, including light. But space itself is not straight nor bent, space has no shape at all. But these things are so obvious that there is no need to quote Einstein, even Newton was aware of this.
 

Offline Alan McDougall

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To me is both things. When you drop a stone in the water, are the waves caused by the stone, or are the waves circular because of the stone? It's both, the waves are caused by the stone and their shape is circular. In the same way, gravity attracts and bend everything, including light. But space itself is not straight nor bent, space has no shape at all. But these things are so obvious that there is no need to quote Einstein, even Newton was aware of this.

A warm welcome to the forum if you have not already been welcomed by some other member!

Space is said to be like a fabric that can bend twist and contort, under the influence of gravity.
 

Offline PhysBang

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But space itself is not straight nor bent, space has no shape at all. But these things are so obvious that there is no need to quote Einstein, even Newton was aware of this.
Sadly, what is obvious to one person is not to others. There is a lot of literature on the shape of space and applications that rely on this knowledge.
 

Offline IAMREALITY

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To me is both things. When you drop a stone in the water, are the waves caused by the stone, or are the waves circular because of the stone? It's both, the waves are caused by the stone and their shape is circular. In the same way, gravity attracts and bend everything, including light. But space itself is not straight nor bent, space has no shape at all. But these things are so obvious that there is no need to quote Einstein, even Newton was aware of this.

A warm welcome to the forum if you have not already been welcomed by some other member!

Space is said to be like a fabric that can bend twist and contort, under the influence of gravity.

Actually I believe the preferred position nowadays of most versed physicists is to not think of spacetime like a fabric at all, and they wince at it being portrayed as such.  But I might be wrong.
 

Offline PmbPhy

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Quote from: PhysBang
That Einstein used dubious metaphysics is not an argument for adopting dubious metaphysics.
And where,  pray tell, in this forum did I make such an argument? All I said is that there are physicists, including Einstein, who though of it that way. And it's far from being dubious in any way, shape of form whatsoever. And this isn't dubious anyway. Just because that's your opinion it doesn't mean that it reflects anybody else's opinion. Far from it in fact.

In any case what Einstein thought on points like this should never be dismissed or ignored. It was those kinds of thoughts which helped lead him to his general theory of relativity. Why do you think Einstein is quoted so often such as in articles which try to convince physicists to stop teaching relativistic mass. In nearly all of those articles they quote his statement against velocity dependent mass. However, little did they know he wasn't speaking of relativistic mass in general.

Next time (or at least until the ignore list function is working again) please make an effort for present a cogent argument of your beliefs rather than merely stating assertions such as "dubious metaphysics". Merely making assertions by stating your opinion helps nobody.
« Last Edit: 15/07/2016 16:42:34 by PmbPhy »
 
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