[PmbPhy]

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

[Alan McDougall (OP)]

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

[PmbPhy]

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.

G_k = 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.

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.

"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

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.

[PmbPhy]

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.

[PhysBang]

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.

[Jack Qwek]

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.

[Alan McDougall (OP)]

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.

[PhysBang]

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.

[IAMREALITY]

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.

[PmbPhy]

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.

[Jack Qwek]

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.

Hi there, thank you for the welcome, very kind of you.

[Jack Qwek]

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.

Yes, I was suggesting that the shape is given by something else, mass, gravity, etc. Otherwise space has no shape. Like in the example of the stone in the water, the waves have circular shapes, but we would never say that the water is bent.

[PmbPhy]

Yes, I was suggesting that the shape is given by something else, mass, gravity, etc. Otherwise space has no shape. Like in the example of the stone in the water, the waves have circular shapes, but we would never say that the water is bent.

Terms like "shape", "fabric" and "curvature" are all terms which are defined in analogy to physical objects and geometric shapes. When physicists say that space is curved they're referring to the distance relationships between points in space.

[Jack Qwek]

Yes, I was suggesting that the shape is given by something else, mass, gravity, etc. Otherwise space has no shape. Like in the example of the stone in the water, the waves have circular shapes, but we would never say that the water is bent.

Terms like "shape", "fabric" and "curvature" are all terms which are defined in analogy to physical objects and geometric shapes. When physicists say that space is curved they're referring to the distance relationships between points in space.

Yes, and in absence of matter, the relationship between points in space is absent.

[jeffreyH]

Yes, I was suggesting that the shape is given by something else, mass, gravity, etc. Otherwise space has no shape. Like in the example of the stone in the water, the waves have circular shapes, but we would never say that the water is bent.

Terms like "shape", "fabric" and "curvature" are all terms which are defined in analogy to physical objects and geometric shapes. When physicists say that space is curved they're referring to the distance relationships between points in space.

Can you repost that capitalized and in red Pete. It would make it so much easier if people understood this.

[AndroidNeox]

Be honest... Do you just search quora for topics to post? Lol

There was a day awhile back that I noticed at least 3 of Alan's topics were straight copy pastes from quora. I don't visit quora enough to know for sure but if I noticed that many the actual number must be significant. Other question and answer sites could also be providing material.

I often post the same question on other sites because I almost never get an answer, here.

[Professor Mega-Mind]

.........Gravity Force Illusion .
*Updated Metaphor pg.5 , link @ Reply 56  connects to thread : How does the expansion of space work ?
________
》Imagine an infinitesimally small sphere .  A hypodermic needle comes along , and begins injecting strong but stretchy rubber into the sphere . The sphere grows & grows & grows , like a maniac .  Being rubber , it is strongly internally connected , like a solid rubber ball .  Being under great internal pressure , this ball continues to swell and swell . This rubber is subject to phase change and spot-collapse , similar to say , hydrogen . The resulting dense spot is still connected to the rest of the ball , so it pulls on the rubber around it much more strongly than when it was large and diffuse . This in turn , causes more rubber to collapse , in an ongoing chain-reaction . The end result is similar to rising bread .  The ball is full of low-density  voids , and dense walls . The rubber that is part of high-density sections , experiences more pull than rubber that is part of low-density sections . This pull is called "Gravity" , but is more of a mechanical effect than is normally realized .  Lorentz & Relativistic effects are direct consequences of this "Mechanical Connection of Everything ." .
This also explains why gravity is omni-directional , and has a limited velocity .  Thus "gravity" does NOT curve space , it merely affects the components of the Universe in a manner that gives that appearance  . Einstein himself acknowledged this fundamental physics reality .
The question remaining is still "Is the substance of Space still being added to our universe , or has the injection stopped ?" .
Professor Megamind

*Updated Metaphor on pg.5

[yor_on]

Gravity isn't about densities Mr Megamind. To be so you will need too make it into a 'medium', which it is not.

[yor_on]

Let's take a force. In my mind that is connected to 'action and reaction'.
Let's take a 'gravitational wave', where is the action and reaction as it passes you?

[Bill S]

By way of a slight diversion: Carlo Rovelli:  https://www.theguardian.com/books/2018/apr/14/elastic-concept-order-of-time-carlo-rovelli links time dilation to gravitational attraction in what seems to be a causative way.

If things fall, it is due to this slowing down of time. Where time passes uniformly, in interplanetary space, things do not fall. They float. Here on the surface of our planet, on the other hand, the movement of things inclines naturally towards where time passes more slowly…….. Things fall downwards because, down there, time is slowed by the Earth.

Wouldn’t it be equally reasonable to argue that things fall in the direction in which the strength of the gravitational field increases? 

The fact that time also slows in the same direction is not necessarily a causative factor.