[BMS Field (OP)]

Gravity has for a long time been forced into the category of a fundamental force of nature but does not really seem to fit.

The basic reason for its acceptance seems to be that without gravity the world or universe would be different place.

However this could be said the same for friction, think what the world would be like without friction.

Friction is not itself a fundamental force but arises from intermolecular forces within the material used.

If a ruler is placed over the edge of a desk and you push down on the open end with your finger and place a small sledge on the ruler. The sledge will slide down to you finger at a speed defined by the laws of friction. But it's the forces that holds the ruler in the curved shape that are described as the fundamental forces

If we replace your finger with a star and the ruler with spacetime are we not looking at the same issues. It is the curve of spacetime and the movement across or through spacetime that we see as gravity.

Should we be looking for the forces that holds and defines spacetime and let gravity be demoted to a secondary force?   

[PmbPhy]

Welcome to the forum, BMS!

Gravity has for a long time been forced into the category of a fundamental force of nature but does not really seem to fit.

What you didn't say is why you believe that the gravitational force "does not really seem to fit." Why?

The basic reason for its acceptance seems to be that without gravity the world or universe would be different place.

What do you mean by "its acceptance"? If you're referring to it being accepted as one of the forces of nature then it's accepted as such because it truly is, in every sense of the term, a fundamental force of nature. There's no deception or trickery to it.

However this could be said the same for friction, think what the world would be like without friction.

Yeah, but friction is closer to being an electromagnetic effect than a gravitational once since at its foundations, friction is due to electromagnetism.

If we replace your finger with a star and the ruler with spacetime are we not looking at the same issues. It is the curve of spacetime and the movement across or through spacetime that we see as gravity.

That's a very common misconception in general relativity. The curvature of spacetime is what we see as tidal forces, not gravitational forces per se. We can have gravity without spacetime curvature. For example; consider a uniform gravitational field. The spacetime curvature of such a field is zero yet the gravitational forces on a particle in such a field is non-zero. For details and the derivation please see
http://home.comcast.net/~peter.m.brown/gr/uniform_force.htm

Should we be looking for the forces that holds and defines spacetime and let gravity be demoted to a secondary force?

No. We should, instead, be teaching students and the layman the difference between gravity and tidal forces.

Einstein himself was against the notion of gravity as a curvature in spacetime. You can read about it in a paper I wrote on the subject at Einstein's Gravitational Field at
http://xxx.lanl.gov/abs/physics/0204044

In that paper you'll see Einstein's comment as follows

... what characterizes the existence of a gravitational field from the empirical standpoint is the non-vanishing of the components of the affine connection], not the vanishing of the [components of the Riemann tensor]. If one does not think in such intuitive (anschaulich) ways, one cannot grasp why something like curvature should have anything at all to do with gravitation. In any case, no rational person would have hit upon anything otherwise. The key to the understanding of the equality of gravitational mass and inertial mass would have been missing.

If you're not familiar with the language of differential geometry let me rephrase that for you

... what characterizes the existence of a gravitational field from the empirical standpoint is the components of the gravitational force 3-vector and not tidal forces.

[jeffreyH]

Take note of what Pete says and read his references because he is right. To help here are 2 wikipedia links that give more on this. Take note of why the tidal forces arise.

http://en.wikipedia.org/wiki/Tidal_force
http://en.wikipedia.org/wiki/Tidal_locking

[PmbPhy]

Take note of what Pete says and read his references because he is right. To help here are 2 wikipedia links that give more on this. Take note of why the tidal forces arise.

http://en.wikipedia.org/wiki/Tidal_force
http://en.wikipedia.org/wiki/Tidal_locking

You're too kinds my good man.

There's more on tidal forces at http://home.comcast.net/~peter.m.brown/mech/tidal_force_tensor.htm

[evan_au]

Field Gravity has for a long time been forced into the category of a fundamental force of nature

For a long time, gravity was the only known force of nature, known to the ancients, but not really measured until Galileo and Newton.

The others were not directly observable:

• Electricity and magnetism were not really measurable as forces until about the time of Michael Faraday*
  
• The strong nuclear force and weak nuclear force were not known until atom smashers became available in the 1900s
  
• *Light is electromagnetic radiation, but the force exerted by it is so slight that it is effectively undetectable when there are air currents around. Researchers are still trying to produce a solar sail with useful acceleration.

Friction is not itself a fundamental force

To be a fundamental force, it would have to exist in the absence of other fundamental forces.

As you say, Friction is not fundamental, because it relies on electromagnetic interactions (in solids) and statistical interactions to produce viscosity (for fluids). However:

• Electromagnetism is fundamental, because it works through a vacuum, between charged and low-mass objects. It has an infinite range, and we have detected its carrier (the photon).
  
• Gravity is fundamental, because it works through a vacuum, between massive, uncharged objects.  It has an infinite range, but we may never be able to detect an individual graviton.
  
• The Strong force is fundamental, because uncharged neutrons respond to it, and it continues to attract protons despite the powerful electric repulsion within an atomic nucleus. It has very short range, which allows it to be easily separated from Gravity & Electromagnetism. It's carrier particle (the gluon) has not been directly detected, but its effects have been seen in particle accelerators.
  
• The Weak force is also fundamental, because it applies to particles which ignore the strong force. Its carriers (W & Z bosons) have been detected in particle accelerators. Its behaviour violates some of the symmetries followed by the strong nuclear force, so it cannot be the same force.
   

[BMS Field (OP)]

Einstein himself was against the notion of gravity as a curvature in spacetime. You can read about it in a paper I wrote on the subject at Einstein's Gravitational Field at
newbielink:http://xxx.lanl.gov/abs/physics/0204044 [nonactive]

In that paper you'll see Einstein's comment as follows

... what characterizes the existence of a gravitational field from the empirical standpoint is the non-vanishing of the components of the affine connection], not the vanishing of the [components of the Riemann tensor]. If one does not think in such intuitive (anschaulich) ways, one cannot grasp why something like curvature should have anything at all to do with gravitation. In any case, no rational person would have hit upon anything otherwise. The key to the understanding of the equality of gravitational mass and inertial mass would have been missing.

If you're not familiar with the language of differential geometry let me rephrase that for you

... what characterizes the existence of a gravitational field from the empirical standpoint is the components of the gravitational force 3-vector and not tidal forces.

I have read this publication (which was helpful many thanks !), and although I do not assume to fully understand all of it, I accept my understanding is the result of the teaching referred to as EGR (Einstein’s General Relativity) and not as MGR (Modern General Relativity).

I can see that under MGR spacetime curvature is seen as a product of gravity not the other way around.

However my reason for starting this new topic was to question what is gravity and what creates it? It was not to question what is a fundamental force or to promote friction.

I have always felt that the curvature of spacetime didn't feel the right answer which your paper has shown that I am not the only one getting confused.

It would therefore be helpful to me and other that maybe confused to provide a simple definition of what you see from a MGR viewpoint the answers to these questions are and why you feel that gravity still seem to be different in terms of size and nature from the other fundamental forces.

[JohnDuffield]

I think gravity is quite easy to understand provided you've read what Einstein said. A concentration of energy, usually in the guise of matter in the guise of a massive planet, "conditions" the surrounding space, the effect of this diminishing with distance. See Einstein's Leyden Address for that. Then imagine you could place optical clocks at various locations throughout an equatorial slice through the Earth and surrounding space. Clocks go slower when they're lower, so when you plot all your clock rates, what you get is a plot like this: 


GNUFDL image by Johnstone, see Wikipedia 

That's a depiction of Riemann curvature. It's basically a picture of curved spacetime. But note that space isn't curved. See the Baez website and note "not the curvature of space, but of spacetime". Curved spacetime isn't curvature of space and curvature of time. It's a curvature in your plot of measurements of motion through space over time. It's a curvature of "the metric", metric being to do with measurement. And note that the force of gravity depends on the slope of the plot at some location, not on the curvature. Like Pmb said, that's to do with the tidal force. Also note this on the Baez website:

"Einstein talked about the speed of light changing in his new theory.  In his 1920 book "Relativity: the special and general theory" he wrote: "... according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [...] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity [Einstein means speed here] of propagation of light varies with position."  This difference in speeds is precisely that referred to above by ceiling and floor observers."

Those clocks I referred to are optical clocks. And they go slower when they're lower because light goes slower when it's lower. See the Shapiro quote:

"The proposed experiment was designed to verify the prediction that the speed of propagation of a light ray decreases as it passes through a region of decreasing gravitational potential."

People refer to this speed of light as the "coordinate" speed of light. The locally-measured speed of light is the speed that's constant, because of a tautology wherein we define the second and the metre using the motion of light. Anyway, like Einstein said, light curves because the speed of light varies with position. There's a nice gif showing this on Ned Wright's deflection and delay article where you can read this: "In a very real sense, the delay experienced by light passing a massive object is responsible for the deflection of the light."



Light doesn't curve because spacetime is curved. It curves because the speed of light varies with position. It's a bit like the way a car veers when it encounters mud at the side of the road. Once you know that and you know about the wave nature of matter, it's quite easy to work out why matter falls down. Think of an electron as a wave going round and round, then simplify it to a wave going round a square path. Then imagine the horizontal sections curve down a little, like the picture below. The electron falls down.



Maybe somebody could improve on this description, but I think the gist of it will be the same. Gravity doesn't work via gravitons flitting back and forth. As for whether gravity is fundamental, I'd say that depends on your definitions, but that it doesn't much matter anyway. What does matter is understanding how gravity works.

[PmbPhy]

I have read this publication (which was helpful many thanks !), and although I do not assume to fully understand all of it, I accept my understanding is the result of the teaching referred to as EGR (Einstein’s General Relativity) and not as MGR (Modern General Relativity).

I am so happy to hear that this paper is being read and that you found it helpful. Thank you very much for the feedback.

..what is gravity and what creates it?

Nobody knows the answer to those questions. People often make the mistake that GR answers those questions, but that's not true. Eddington knew that and wrote about it

The purpose of Einstein’s new theory has often been misunderstood, and it has been criticized as an attempt to explain gravitation. The theory does not offer any explanation of gravitation; that lies outside its scope, and it does not even hint at a possible mechanism. It is true that we have introduced a definite hypothesis as to the relation between gravitation and a distortion of space; but if that explains anything, it explains not gravitation, but space, i.e. the scaffolding constructed for our measures. - Gravitation and the Principle of Relativity by A.S. Eddington, Nature, March 14, 1918, page 36

[BMS Field (OP)]

Thanks JohnDuffield that helps

So if I understand this correctly, please correct me if I am wrong

We can define Gravity as a force that deforms spacetime by creating a field around a mass in which time slows closure to the mass, presumably the gradient of this field increase with the size of the mass

A particle or object that passes through this field will interact with this field. Basically the part of a electron path closest to the mass is in a slower time frame than the part of the electrons path that is further away. This slower time frame could be seen a break being applied to this part of the electrons path have the result of turning the particle or object toward the mass. This is more commonly seen as a force (the force of gravity) being applied to this particle or object.

So this commonly held view of the force of gravity is actually the interaction of a field around a mass and the electron within a particle or object. This I would suggest is not a fundamental force

The force that creates the field around a mass could be defined as a fundamental force depending of how its created and the definition of a fundamental force ??

[evan_au]

The force that creates the field around a mass could be defined as a fundamental force

As I understand it, Mass is what creates the gravitational field around the Mass. But Mass itself is not a force.
- Mass creates the gravitational field
- The gravitational field tells (electrically neutral) masses how to move through space.

[JohnDuffield]

We can define Gravity as a force that deforms spacetime by creating a field around a mass in which time slows closure to the mass, presumably the gradient of this field increase with the size of the mass

It doesn't so much deform spacetime as alters space, and it's the coordinate speed of light that slows down rather than time. Yes, you get a steeper gradient when the mass is large but the size is small.   

A particle or object that passes through this field will interact with this field. Basically the part of a electron path closest to the mass is in a slower time frame than the part of the electrons path that is further away. This slower time frame could be seen a break being applied to this part of the electrons path have the result of turning the particle or object toward the mass. This is more commonly seen as a force (the force of gravity) being applied to this particle or object.

Something like that. Albrecht Giese gives a similar explanation here. There are some things he says that I don't like, but I think the gist of it is right.

 

So this commonly held view of the force of gravity is actually the interaction of a field around a mass and the electron within a particle or object. This I would suggest is not a fundamental force.

This view isn't common. And yes, you could say gravity wasn't fundamental and instead is a "trace" force. But IMHO you just get into arguments about fundamentals and fields and forces.   

The force that creates the field around a mass could be defined as a fundamental force depending of how its created and the definition of a fundamental force ??

That doesn't sound right. Generally speaking, forces are the result of field interactions. And something like the electron is "just field". In QFT it's described as a "field excitation". Remember that we can diffract electrons -  the wave nature of matter is not in doubt. I think it's best to think of the electron as a wave in a closed path that looks like a standing wave, and then say there isn't much difference between a standing wave and a standing field. However you slice it and dice it, there is no billiard-ball thing in the middle, just as there is nothing solid in the middle of a cyclone.

[PmbPhy]

As I understand it, Mass is what creates the gravitational field around the Mass. But Mass itself is not a force.
- Mass creates the gravitational field
- The gravitational field tells (electrically neutral) masses how to move through space.

That's correct. To be specific:

Active Gravitational Mass: The source of gravity

Passive Gravitational Mass: The mass that responds to gravity

Inertial Mass: What gives a body a resistance to changes in their quantity of motion, aka momentum

[jeffreyH]

As I understand it, Mass is what creates the gravitational field around the Mass. But Mass itself is not a force.
- Mass creates the gravitational field
- The gravitational field tells (electrically neutral) masses how to move through space.

That's correct. To be specific:

Active Gravitational Mass: The source of gravity

Passive Gravitational Mass: The mass that responds to gravity

Inertial Mass: What gives a body a resistance to changes in their quantity of motion, aka momentum

I find it amusing when people post how they understand gravitation and have 'found the answer'. The fact that they think they understand it means they have no appreciation of the different aspects and how complex it really is.

[PmbPhy]

As I understand it, Mass is what creates the gravitational field around the Mass. But Mass itself is not a force.
- Mass creates the gravitational field
- The gravitational field tells (electrically neutral) masses how to move through space.

That's correct. To be specific:

Active Gravitational Mass: The source of gravity

Passive Gravitational Mass: The mass that responds to gravity

Inertial Mass: What gives a body a resistance to changes in their quantity of motion, aka momentum

I find it amusing when people post how they understand gravitation and have 'found the answer'. The fact that they think they understand it means they have no appreciation of the different aspects and how complex it really is.

I agree. Most people have never heard of the three different types mass that I just mentioned. Most people have also never heard of the term gravitation mass (a shorthand term for either active or passive gravitational mass) referred to as gravitational charge. However if one does a Google search they'd find the term listed many times. I recommend doing so.

For example:

Dark Matter Is an Illusion, New Antigravity Theory Says
http://news.nationalgeographic.com/news/2011/08/110831-dark-matter-proof-gravity-quantum-theory-cern-space-science/

"Mainstream physics assumes that there is only one gravitational charge, while I have assumed that there are two gravitational charges," Hajdukovic said.

According to his idea, outlined in the current issue of the journal Astrophysics and Space Science, matter has a positive gravitational charge and antimatter a negative one.
...
Hajdukovic suggests that a similar phenomenon happens with gravity. If virtual matter and antimatter particles have different gravitational charges, then randomly oriented gravitational dipoles would be generated in space.
...
Doser is a member of CERN's AEgIS experiment, which aims to measure the gravitational charge of antihydrogen, the antimatter version of hydrogen, and thus would prove whether matter and antimatter are gravitationally repulsive.

By "one gravitational charge" he's referring to charges with only one sign.

Gauge Invariance and the Quantization of Mass (of Gravitational Charge) by L. Motz, Nuovo Cimento B
11 Dicembre 1972, Volume 12, Issue 2, pp 239-255
http://link.springer.com/article/10.1007%2FBF02822633

The Quantization of Gravitational Charge and the Value of the Fine-Structure Constant by L. Motz, Nuovo Cimento B, Vol. 37, 1977

[yor_on]

What would he mean by "According to his idea, outlined in the current issue of the journal Astrophysics and Space Science, matter has a positive gravitational charge and antimatter a negative one." Pete? Can't be 'anti gravity' at least, as it wants to explain Dark Matter.  Would they coexist then, in a same patch of 'space'?
=

I'll be flabbergasted
He wants it to become some sort of fluctuating 'virtual particles' it seems?

[JohnDuffield]

It's junk. A concentration of energy causes gravity. It doesn't matter whether you've got two 511keV photons in a mirror box, or a 511keV/c² electron and a 511keV/c² positron, or perm any two from four, the gravity is the same.

[PmbPhy]

What would he mean by "According to his idea, outlined in the current issue of the journal Astrophysics and Space Science, matter has a positive gravitational charge and antimatter a negative one." Pete? Can't be 'anti gravity' at least, as it wants to explain Dark Matter.  Would they coexist then, in a same patch of 'space'?

I didn't read it since I'm not interested in it. I only mentioned it to show that the phrase "gravitational charge" is used in physics. However I see no reason to assume that antimatter has negative mass.

[yor_on]

yeah, I agree
I have enough difficulties trying to comprehend the idea of gravity. Don't need two signs to it, please.