The simplest answer is that gravity is the field created by a mass distribution in the spacetime around it; gravitational force is the force exterted by the field on a test mass in the field. The difference is that gravity (the field) is there in absence of a test mass, the gravitational force "exists" only if there is a test mass at the point. From a general-relativistic point of view, "gravity" is the space-time curvature induced by an energy-mass distribution, the "gravitational force" then does not exist, since a test mass will follow a geodetic in the curved space-time.
The simplest answer is that gravity is the field created by a mass distribution in the spacetime around it; gravitational force is the force exterted by the field on a test mass in the field. The difference is that gravity (the field) is there in absence of a test mass, the gravitational force "exists" only if there is a test mass at the point. From a general-relativistic point of view, "gravity" is the space-time curvature induced by an energy-mass distribution, the "gravitational force" then does not exist, since a test mass will follow a geodetic in the curved space-time.
In the formula f=Gm1*m2/r*r f is the force and G is the gravitational strength constant. The latter is useful for comparison with the other 3 fundamental farces.
It all depends on how you define them to be which.
Gravity is the force exerted on a mass due to gravitational attraction. Gravitational force is the force exerted on a mass due to acceleration due to gravity.
It's simple, both are the same.
They are both forces but with a difference:
Gravity(or force due to gravity) is termed as the force of attraction by earth and gravitational force(also called gravitation) is the gravitational force between any two objects.
Krishnananda,
Antonio has the best answer so far.
I'll paraphrase my take on it - but his answer is worth studying in detail - including the vector algebra reference. One doesn't get far without it!
Gravity is the phenmomenon by which objects with mass are drawn together. But one can measure many aspects of gravity, such as the acceleration that it delivers to an object (dimensions of length time^-2), and as Antonio pointed out the gradient of a potential is a force - and so one could also measure the second derivative with respect to distance, etc.
Gravity is "special". It is an absolutely democratic interaction: it accelerates in same way all the bodies; more precisely, gravitational and inertial mass "accidentally" coincide (this does not happen for any of the other fundamental interactions, electro-weak and strong). Then, if you are in free fall, you wont measure any acceleration on you or in bodies close to you. In free fall, locally there is no gravity (that's why things in Space Station appear weightless). If you are in a closed box in free fall, you cant determine that you are free falling in a gravitational field with any experiment (till the moment you splash on the body's surface, of course :-) ). This simple observation has far reaching consequences: the einstein theory of gravitation.
In this picture, gravity is space-time curvature induced by a space-time mass-energy distribution. Gravity is therefore a tensor. It is described by the 2nd rank einstein's tensor, which is proportional to the energy-momentum tensor of the mass-energy distribution.
As a consequence, a test particle will move "freely" in the curved space-time following the space-time geodetic between initial and final positions. This is defined by the 2nd order covariant derivative with respect to the proper time of 4-position to be zero. For an observer not moving with the test mass (eg not free falling), the motion will appear deviating from rectilinear, uniform motion (just as if you are in a rotating frame) because of curvature, that is he will invoke an acceleration.
Matter tells the spacetime how to curve, spacetime tells the matter how to move.
In the case of "weak" gravitational fields (as earth's or sun's field), one can restore from GR field equations the classical formalism of the newtonian theory (eg lagrangian or hamiltonian). On the other hand, in strong fields as around compact astrophysical objects (neutron stars, black holes, say the schwarschild solution) the newton theory fails. As if you try to describe the hubble's law or the acceleration of the universe's expansion (the cosmological constant).
Gravity is much more than a simple potential. Imho :-)
The best answer is given by Mr Keshe. Seeing his patent
and explanation.
1 http://www.youtube.com/watch?v=98HwpzWEuGQ&feature=relmfu
2 http://www.youtube.com/watch?v=Hd1-f5eR4PY&feature=relmfu
3- http://www.youtube.com/watch?v=_J4aOVwW5wM&feature=relmfu
4- http://www.youtube.com/watch?v=74gxlb8A59s&feature=relmfu
5- http://www.youtube.com/watch?v=NBeVCsndiMk&feature=relmfu
gravitational patent. Mr Keshe
http://open-source-energy.org/rwg42985/geert8550/EP1770717A1.pdf
Chis,
I'm sure you don't mean
Fg = GMm / (R + r)²
but,
mg = GMm / (R + r)²
the lower case m's cancel and we have an equation for g in terms of the gravitating mass "M" and the involved radii.
Eric,
It will take more time than I have on my hands to watch all the video references you provided.
As far as the gravitational patent. How can you patent gravity? Also, I was not aware that there exists a theory which allows magnetic fields to create gravitational force phenomena. Electromagnetic forces are many orders of magnitude stronger than gravitational force. They are not the same. Magnetic forces are caused by movement of electric charges. Gravity exists because of mass.
There is gravity and the gravitational force. The second of these comes from Newton's laws of gravitaion where the effect of gravity is characterised as a force meaning the induction of an acceleration on a given mass determined by another mass and dependent on the inverse of the distance between the two masses. This is a purely Newtonian idea which was later exporessed through a gradient in potential.
Later on Einstein came up with general relativity which also describes gravity but not through the idea of force but rather through the geometry of space-time. In this case it is the way that the combination space-time curves that determines whether a massive particle feels a "graviational force" on it. In other words gravity effects everything which posseses mass-energy, such things must be directed by the space-time curvature since they lie within it. This is the famous rubber sheet idea where you put a heavy object like a bowling ball in the centre of a rubber sheet clamped at its edges and you get a hollow area in the middle near the ball. then you roll another ball, like a tennis ball al,ong the now curved rubber sheet. You note that the tennis ball follows the curve of the sheet as it must. This curve is the force of gravity experienced by a massive particle.
So, gravity may be explained in all sorts of ways without refering to force.
The analogy is the usual. Although, I wish I could visualize it in four dimensions.
I think a good way of visualizing the non-existence of a "force" of gravity is to imagine a test particle -- let's say a ball bearing -- in a container on a spacecraft orbiting the earth. Although the craft can be seen to circle the earth from a reference frame here on the surface, and so to "accelerate" along a curved path, the test particle will show no evidence of an acceleration, or a force, of any kind. Gravitation is just warped geometry, unassociated with force unless it is resisted, as when be are prevented from gravitating toward the center of the earth by the solidity of the surface.
Emanuele, I voted up your answer for its clarity and correctness.
However, I believe most followers of this discussion don't master enough mathematics to understand statements like "Gravity is therefore a tensor. It is described by the 2nd rank einstein's tensor, which is proportional to the energy-momentum tensor of the mass-energy distribution" and "This is defined by the 2nd order covariant derivative with respect to the proper time of 4-position to be zero."
http://www.youtube.com/feed/UCP1CJKRkg6ImllD6N2qIKHg
Really, this is the best explanation I've read in my life.
Mr Keshe is right.
really worth watching their videos-gravity, light, universe
Hi
There are so many explanations for gravity that it is important to look at facts, not only theories.
A confirmed fact is that a ray of light will block some gravitational force. The theory of space time curvature is based on mass that curves space time. The bigger the mass, the bigger the curvature, the bigger the force.
But light ray does not have mass and cannot curve space time. It is not suppose to affect space time curvature or not suppose to block some gravitational force. But it does do that.
How can we accept both the theory and the confirmed facts when they are in direct opposition. Eisntein said that if a fact disprove a theory, we should keep the facts and change the theory.
If someone is not convinced that light can block the force of gravity, just do the simple experiment yourself using a torsion pendulum, and a light source.
If one sucks the air of a plastic bottle, the walls of the bottle come close to one another.
Simple explanation: the walls are attracted to one another.
Another explanation: the walls are pushed toward one another by the outside pressure.
Almost every one will accept the second explanation.
Simple explanation for the earth revolving around the sun: the sun attracts the earth.
Another explanaiton: the pressure between the earth and the sun is lower than the external pressure and the result is that the earth is pushed toward the sun/.
Which one is correct?
We have to keep an open mind and looks at facts, not only at pretty complex theories.
Louis
Matts, I agree that the answer was quite technical. But afterall, we are supposed to be in a serious science forum :-).
GR is a theory that connects the energy-mass density in a volume of spacetime to the geometry of the spacetime (and viceversa). In this context, space and time are not simply the stage on which matter and energy move around but space and time are strectched and deformed by the energy-mass distribution itself as a plastic, elastic material is deformed by stresses acting on it (this is an analogy not to be taken too literally). With this 3D analogy in mind, to describe the deformations on a small volume around a point we need to know 3 stress vectors: normal stress (ie pressure) and 2 shear stresses. These 3 stress vectors can be combined in a unique object, the stress tensor. In GR, the role of the stress tensor is played by the energy-momentum tensor while the "deformations" are described by the metric tensor (and all in 4D).
GR is simple in principle but its mathematics is terribly complicated :-)
Louis,
I'm not familiar with the fact you reference: that "light will block some gravitational force." Please provide a source.
The paper was published in Dec 2011 in Physics Essays. The editor has the copyrights.
A simplified version was posted on Gravityforces.com {light blocks gravity} on May 2012.
Louis
Louis, a copyright doesn't prevent you from explaining yourself. The abstract of your paper in Physics Essays states:
"It is shown in the first part that when a 30° laminar beam of 1.5 W red laser light passes between a freely moving mass on a torsion pendulum and a fixed mass, there is an increase of the attraction between the two masses. In the second part, when a laminar light beam passes close to a freely moving single mass on a torsion pendulum, the mass will move toward the light beam."
You are claiming that light increases an attraction between two masses. If light "blocks" gravitation, wouldn't light decrease the attraction? And in any case, given that it is extremely difficult to measure variations in the effects of the earth's gravitational field, how can you possibly expect to detect variations between two bodies in a laboratory? And what makes you think the effects you describe are gravitational? I'm perplexed.
Gravity is a phenomenon while gravitational force is a force in nature !!!!!!
Hi.
Answer to your questions. When light passes between the 2 masses, the mobile mass went closer to the fixed mass as if there was an increase of the attraction between the 2 masses. That effect was caused by the presence of light in between. At first, I thought that light had caused an increase of attraction. But it was not possible to explain with present physics theories. I then used only the torsion pendulum without a fixed mass and when light was placed on the north side of one mass, that mass went towards the north. When the ligth was moved to the south side, the mass went towards the south side as if light was attracting.. but I could not accept it. After much thought, I realize that the forces on the moving mass was greater on the side where there was no light. That is why the mobile mass is pushed toward the light. Light does not attract the mass, It only blocks some of the pushing force on that side. That means there is forces pushing in every directions on the mobile mass. Light does blocks some of that pushing force and that explains why the mobile mass acts like this. Since we call that force gravity, that means light can block some of that gravity, not all. That means also that there must be something in common between the nature of light and the nature of what causes pushing gravity. Maybe light of many wavelenghts causes some of that force and the rest comes from all the tiny things going at the speed of light in space around us.
The torsion pendulum uses a ribbon as a support in its middle. That makes if very sensitive and it has to be enclosed in a box so air movement will not affect it. Even passing near the box will causse a movemen of the masses. They move almost constantly from one side to the other, about one mm. Using a small mirror at the middle and a fien point laser, it is possible to measure the movement.
Another interesting conclusion is that if gravity is a space time curvature caused by masses and if it depends on the quantity of masses, then light which has no mass should not change that curvature. If light changes what we call gravitational force, then the theory of space time curvature is not valid any more.
Louis
A torsion pendulum is such a simple mechanical device so that it will not teach modern physics anything new. All of your observational results must be due to poor control of the experimental setup and systematic errors.
Louis,
Eötvös was only able with great difficulty to measure miniscule variations in the entire earth's gravitational field. You are claiming significant variations between the gravitational fields of masses small enough to be contained in a laboratory.
It is easy for you to say it is not possible; why don't you build one yourself and you will see it can measure a very small forces. My results were confirmed by over 1000 hours of experiments. How many hours have you put to test it to confirm what your wrote
Louis
If variations in the gravitational effects of the mass of the earth are difficult to discern, I don't need an experiment to confirm that variations for masses in a laboratory are going to be imperceptible.
Louis,
Thanks for describing your experiment. Presuming that it's properly controlled and conducted, the result might also be explained in the context of GR. As I understand, according to the principle of mass–energy equivalence even a propagating energy wave should exhibit equivalent mass-energy effects, such as curving as it obliquely traverses a gravitational field of sufficient amplitude.
In the case of gravitational lensing, it is light that is detectably curved by spherically symmetrical masses producing a regularly curved gravitational field and otherwise distorted by an irregular distribution of massive objects, such as elliptical galaxies.
Perhaps when a small body is suspended within a gravitational field of regularly curved spacetime, the linear distortion of spacetime produced by a proximal propagating wave produces a disruption within the Earth's field of curved spacetime sufficient to produce a slight, but detectable, imbalance in the otherwise precisely balanced gravitational effects imparted to the suspended object.
IMO, GR describes only the effects of gravity in terms of dimensional coordinates of an abstract geometry of spacetime - it does not describe a mechanism or aspect of spacetime that physically produces the very accurately predicted effects.
To me, your experiment seems to be reminiscent of experiments that demonstrate the Casmir effect, thought to be related to quantum fluctuations of virtual particles (energy) within the vacuum of spacetime...
The Casimir effect is good only for very small distances, not distances bigger than a millimeter. I still have the impression that space is full of fast moving something that can push slightly on nucleus, thus causing gravity push. No experience yet has proven that it is impossible to be like that...
Have a good week
Louis
Good point, but in your experiment there is relativistic velocity and a delicately balanced mass involved, not just two metal plates. I think your experiment may be a much more sensitive test. In your test the fast moving something might be the distortion of spacetime produced by relativistic wave packets.
Well Louis, if dark matter and nuclei interact, yes. But they are only known to interact gravitationally, too weakly to have been seen in the laboratory. In galaxies and supergalaxies they do interact gravitationally. If they didn't, those objects would not exist.
Louis, your explanation of the experimental setup is somewhat vague. However, if you see some effect when you turn on your "light" (laser beam?) and see some motion, there are two possible sources: thermodynamic and electrodynamic. You might be creating a small temperature gradient and/or creating polarization or ionization effects. You should look for plausible, natural explanations for your experiment rather than claiming that your experiment changes known physical laws :).
Matts,
I have to ask - what might dark matter possibly have to do with Louis' experiment?
Louis,
I suggest that an insulator (perhaps a manilla folder) placed between the laser and the torsion pendulum - this should eliminate the possibility that EM effects are being mediated between the light and pendulum and, depending on the proximity of the light and pendulum, may also provide sufficient thermal isolation. It should also eliminate the possibility that airflow is producing the observed pendulum motion. Certainly others can suggest better experimental constraints.
BTW, have you done any sensitivity analysis of the distance between the laser and pendulum and observed effects?
James,
nothing, I suppose. But Louis thought "that space is full of fast moving something that can push slightly on nucleus". This fits the description of dark matter.
Matts,
Well, if Louis' experiment has revealed the pressure of cold dark matter, he should be expecting a Nobel prize, don't you think?
Fast moving implies heat, or energy, and IMO the configuration of inferred dark matter in the Bullet Cluster provides clear evidence that it cannot interact much (having interacted no more than the merging clusters' sparse galaxies), or produce any significant pressure.
Of course, that the weak gravitational lensing effects inferring dark matter are coincident with merged clusters' separate galaxies might also simply indicate that it is actually the presence of the galaxies that produces all of the indicated lensing effects. This could occur either the aggregated galactic mass has been underestimated, or the minute lensing effects have been overestimated, or both.
To illustrate that point, the configuration of dark matter inferred for the Train Wreck Cluster by statistically derived weak gravitational lensing effects imparted to thousands of background galaxies has been dramatically revised in the past year. Please see http://www.eurekalert.org/pub_releases/2012-11/ou-cm112912.php#.
Certainly cold dark matter could not produce any pressure that might be affected by a laser to move a proximal pendulum as in Louis' experiment... I was confused by the association in your comment...
Some of my comments are just sarcastic side remarks, don't take me (nor the comments I address) too seriously.
So you didn't really think "that space is full of fast moving something that can push slightly on nucleus" fits the description of dark matter?
I think you're pulling my leg right now! Whether and how much dark matter interacts with itself seems to be a contentious issue that is critical for those who think that dark matter particles annihilate, producing detectable gamma rays an positrons, etc. To the extent that they interact, they can produce pressure...
Dark matter appears to be pressureless, there is no indication to the contrary. But nothing is absolute. If it has pressure it is below detection level, for instance it plays no detectable role in galaxy formation.
If what is called dark matter is really something going at the speed of light and has an interacting zone close to the Planks limit size, then that would be possible that it interact just enough to cause what we call gravity push force. By itself, it would have no mass because when it condense into a system called particle, that system has the property of mass when it interact with other system. When electron and positron interact and become a gamma ray, there is no real annihilation but instead, a reorganization of what space is made of. Does that make sense.
I noted these answers, but I want to ask another question. is there no difference between them if progress of time at the same rate of place shift in the spacetime curvature ?
Every instance of inferred dark matter involves many billions of massive objects, at cosmological scales of distance. Massive particles cannot inherently propagate at relativistic velocities - external energy must be applied to accelerate them, as demonstrated by protons accelerated in LHC experiments.
Among two sufficiently separated spherically symmetrical objects of mass, Newton's simple universal law of gravitation describes very well their gravitational interactions without any missing mass. I think that this indicates that dark matter, or compensatory mass, is not necessary to describe the gravitational interactions between two point masses, even within the Milky Way.
Only when simplifying methods of approximation are used to extend standard gravitational models to large scale compound objects consisting of many billions of discrete massive objects (violating requirements for spherical symmetry and discrete point masses) do the results conflict with observations, seeming requiring either modifications to gravitational equations or compensatory mass.
The periphery of a spiral galaxy disk, composed of billions of self gravitating masses, should not be expected to behave as an isolated peripheral planet in the Solar system, decelerating as a function of radial distance, whose only significant source of gravitational potential is the Sun. Each peripheral star within a galaxy disk interacts with many billions of neighboring stellar scale masses, much more than with any composite mass at the distant galactic center.
I certainly don't think there's any documented requirement for compensatory mass provided by some exotic form of unidentified dark matter to be involved in any gravitational interaction within the Solar system, at least.
IMO, the yet unidentified mechanism that physically produces the effects of gravitation may well involve a 'pushing' pressure provided by vacuum energy.
James, you are right about no motivation for dark matter within the solar system. DM is measurable on galactic and super-galactic scales, not in our minuscule solar system. In fact, all theoretical ideas about dark matter, dark energy, and modified gravitation must meet very stringent requirements of non-observation set by the dynamics of the solar system.
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 addition to depending on the amount of mass, gravity also depends on how far you are from something. This is why we are stuck to the surface of the Earth instead of being pulled off into the Sun, which has many more times the gravity of the Earth.
Is there any difference between gravity and gravitational force.
In this form the question is too foggy because gravitational force may be viewed as a part of gravity. The more clear form of the same question is following: are there difference between geometrical understanding of the gravity phenomenon (general relativity and many its modifications) and relativistic quantum field understanding gravity as a fundamental force like all other fundamental physical interactions (strong, weak, electromagnetic), so understanding gravity as a physical exchange of quantum particles (virtual and real gravitons).
For this question there is a very clear answer: YES it does.
Though the both theories are very similar (but not identical) at the weak field conditions, so that all really observed classical relativistic gravity effects have the same values (but other interpretation in terms of physical gravitational interaction), however there are important conceptual and observational differences which can be tested by crucial forthcoming experiments and observations. Among such differences between geometrical and field gravity approaches are the physical definition of the energy-momentum of the gravitational field, the free fall of the rotating bodies, the tensor and scalar gravitational waves, the pulsating character of the massive star collapse, cosmological gravitational redshift and others.
One may see a history of such Field Gravity approach (Poincare, Thirring, Feynman) and its comparison with GR in papers:
http://arxiv.org/abs/0809.2323
http://arxiv.org/abs/0809.2328
and the book
http://link.springer.com/book/10.1007/978-94-007-2379-5/page/1
Quote from "Space-Time Structure" by Erwin Schrodinger:
"At any rate the very foundation of the theory, viz. the basic principle of equivalence of acceleration and any gravitational field, clearly means that there is no room for any kind of "force" to produce acceleration save gravitation, which however is not to be regarded as a force but resides on the geometry of space-time. Thus in fact, though not always in the wording, the mystic concept of force is wholly abandoned."
Murod Abdukhakimov,
Good quotation - the only issue left to resolve is what is meant by "resides on the geometry of space-time"! What physical aspect of the vacuum is described by the abstract geometric coordinates of dimensional spacetime? While those geometric coordinates may describe the length dilation of a ruler, what physically causes the length to change?
While I agree that gravitation cannot be the product of an exchange of a material force, since it has been shown to produce dimensional effects imparted to the vacuum of spacetime - in the immediate absence of matter.
I think there are two fundamental difficulties with comprehending gravitation: One is the (evident) impossibility of conceiving spacetime as something that is immaterial but has a structure, and dimensions, and can be warped in the presence of mass. It is so unlike any THING in our experience that we can only acknowledge its manifestations, and let go the need to fully comprehend it.
The other difficulty comes from the common association of gravitation-as-geometry with gravitation-as-force. I'll have to plug my paper "Gravitation, force, and energy" here at Researchgate in order to discuss the reason geometry, by itself, can't account for the full range of gravitational phenomena, which makes it difficult to accept the geometric explanation:
https://www.researchgate.net/publication/225029730_Gravitation_force_and_energy?ev=prf_pub
There's an example in the paper where I show that gravitation is only combined with force when gravitation is resisted. Once this is recognized, the question arises: Why does gravitation continue to press against resistance, which of course can't be explained by geometry. I've only intimated in the conclusion to the above paper what I discuss more fully in another paper, published in The European Scientific Journal, but linked, along with the abstract, at:
https://www.researchgate.net/publication/236577283_An_advancing_time_hypothesis?ev=prf_pub
The implication of both papers is that I believe it can be shown that motion in time accounts for the energy associated with gravitation when it is resisted, as it is at the earth's surface. Time is the dynamic aspect of spacetime, and as Minkowski pointed out with his discovery of the significance of spacetime as a continuum, time is motion in (or rather across, and perpendicular to) space. Gravitation can in this way be understood as the product of motion of time in space, which can be warped, and sometimes obstructed, by the presence of mass -- but (as we know about time), it is persistent, and relentless, which is why we continue to gravitate against the earth's surface as our motion in spacetime is directed toward the center of mass due to the warping of spacetime.
Data An advancing time hypothesis
If space time curvature is caused by mass, how do we explain that something without mass, like a ray of light, can decrease gravitational force?. Nothing in the theory predicts that fact. Is it because the theory is incomplete or completely wrong?
I agree with Louis, geometrical gravity miss many physical phenomena, which means that General Relativity is an approximate theory of gravity and valid only in restricted region of physical experiments (by the way as ALL real physical theories).
Dear Murod, your claim about absolute validity of the equivalence principle demonstrate that you missed the whole professional discussions about internal contradictions within different non-equivalent formulations of the equivalence principle. E.g. the well-known question is:
Will free falling charge be radiate electromagnetic energy? - can not be answered.
First, according to EP the free fall system behaves as an inertial system in which the charge not radiate. Second, the free falling system is under acceleration and charge has to radiate.
Actually all existing tests of the EP have mechanical origin, while it should be tested in all physical conditions.
Your reference to Schrodinger is not convincing, because great physicists sometime make great mistakes. As for Schrodinger he proved in 1918 that general relativity has no physical concept of the energy of the gravitational field so geometrical gravity and EP has restricted applications, especially GR is not a quantum theory. For further examples see e.g.:
http://link.springer.com/book/10.1007/978-94-007-2379-5/page/1
Louis,
Space time curvature is caused by the energy-momentum tensor which contains other terms than just rest mass.
I have a question for anyone interested in a complex computation. If gravitation is a force, how much energy would it take for the sun to accelerate it through one complete orbit? I suspect that the answer would suggest that the sun would have to radiate so much energy that it would have burned out long ago.
In GR *any* form of energy will gravitate. Mass density is just one aspect that energy can assume. A gas of massless particles with a non zero pressure will gravitate. It means it will be source of gravity and will undergo to gravitational interactions.
Charged particles in free fall: is there any experiment on this paradox?
Gravity force can be understood as an exchange of virtual gravitons in the same way as for all other fundamental physical forces. E.g. static electrical force between charged particles is a consequence of exchange of virtual photons which do not lead to the loss of the mass-energy (they radiated and returned through vacuum). Around a charged particle exist the electric energy density e_el = (grad Fi)^2/8pi which is included in the total mass of the particle. The same effect we have for the gravity force – where around mass M exists the energy density of gravitational field e_g = (grad Fi_g)^2 / 8piG, which give very small (but measurable) additton to the rest mass M_o of the ordinary bodies. By the way from integration of this energy density (from a minimal radius R up to infinity) one get the total energy of the gravity field around the mass within the field itself which is E_field = GM^2/ 2R , so there is a minimum radius of any mass when the energy of the field became larger then measured rest mass of the body, and this is R_m = GM / 2c^2. This means that black holes cannot exist in the field gravity theory. This approach is analogous to the derivation of classic radius of electron R_e = e^2 / mc^2.
Yurij,
You're speaking hypothetically, since gravitons are not known to exist, correct?
Also, isn't static electricity produced by an actual exchange of electrons?
As Emanuele described in 'Popular Answers' above,
"In this picture, gravity is space-time curvature induced by a space-time mass-energy distribution. Gravity is therefore a tensor. It is described by the 2nd rank einstein's tensor, which is proportional to the energy-momentum tensor of the mass-energy distribution."
If the geometric spacetime curvature described by the 2nd rank Einstein tensor actually represents a geodesic contraction of external kinetic energy contained within the vacuum of space, then couldn't the abstract geometry of spacetime actually describe a physical energy density directed by and to localized mass-energy - not a force of matter but of space? In this case gravity would not be a material exchange of force but a boundary interaction between potential mass-energy and some vacuum energy.
It seems that in such a scenario would be consistent the effects of general relativity, but rather than the abstract geometry of spacetime 'telling matter where to go', the kinetic energy of spacetime, locally contracted by aggregated potential mass-energy, would act upon material objects to accelerate them...
Physics is not a philosophical conversation about love and hate, space and matter and so on.
Physics is based on firm experiments which can test an alternative mathematical models having different predictions for crucial experiments.
In modern physics ALL fundamental forces (strong, weak, electromagnetic) are explained by the exchange of virtual and real bosons (integer spin particles) between fermions (half-integer spin particles – matter). As Richard Feynman emphasized in his book devoted to field approach to gravitation (Richard Phillips Feynman, Fernando B. Morinigo, William G. Wagner, Brian Hatfield, "Feynman Lectures on Gravitation (Frontiers in Physics)" , 2002 | ISBN: 0813340381 | 232 pages ) the geometrical approach is not necessary for gravity physics.
Field gravity approach is based on Lagrangian formalism of the modern relativistic quantum theory and describe gravitational potential as the second rank symmetric tensor field in Minkowski space-time, which guaranty the conservation of the energy-momentum tensor of the gravitational field (which is absent in general relativity). Intriguingly .all classic relativistic gravity effects, which really tested by experiments/observations has the same values in both geometrical and field approaches. However there are several crucial experiments which can distinguish between these approaches (see papers http://arxiv.org/abs/0809.2323 and http://arxiv.org/abs/0809.2328).
"Physics is based on firm experiments which can test an alternative mathematical models having different predictions for crucial experiments."
It used to be that physics used experiments to test conceptual models, and then formalized them, if necessary, with mathematics. Einstein's special relativity, for example, was first of all a physical intuition.
Physics has become a branch of mathematics, and no experiment can shake a belief that is based on a mathematical paradigm.
The idea of radiant gravitation waves was born of a mathematical analogy between the radiation of electromagnetic waves when a charge oscillates in two-dimensions along an antenna and the two-dimensional appearance on the lens of a telescope of a distant three-dimensional force-free binary star system. Conceptually, physically, there is no analogy. But mathematically, there is no need of concepts or actual physics. And so, vast sums of money are spent searching for gravitational waves.
I've tried to show here with simple physical thought experiments that there is no evidence of a gravitational force, no force associated with gravitation, except when it is being resisted. If I have to put it mathematically to be heard, let me just say that 0 = 0.
Dear James Arnold
I agree with you that in modern physics there are too many abstract mathematical theories without real physical experiments, this is not the physics.
Real physicists in modern time should uses physical conceptual basis, experiments and mathematical theories all together. Discovery of Lorentz invariance, electromagnetic waves, quantum states of atoms, became physical instruments for practical physics and prediction for new experiments just via exact mathematical models of corresponding physical phenomena.
James Arnold
You are wrong, there is massive experimental evidence of a gravitational force.
It would hurt, because I'd be in the way of the heavy object that's moving uniformly along its geodesic in spacetime -- I would prevent its gravitation.
Here's a really heavy reference:
http://en.wikipedia.org/wiki/Gravitational_force#Einstein.27s_solution
"These objections were explained by Einstein's theory of general relativity, in which gravitation is an attribute of curved spacetime instead of being due to a force propagated between bodies. In Einstein's theory, energy and momentum distort spacetime in their vicinity, and other particles move in trajectories determined by the geometry of spacetime. This allowed a description of the motions of light and mass that was consistent with all available observations. In general relativity, the gravitational force is a fictitious force due to the curvature of spacetime, because the gravitational acceleration of a body in free fall is due to its world line being a geodesic of spacetime."
I think this is generally consistent with all introductory material relating to gravitation - but, unlike some experts, I could be wrong! Maybe gravitation really is a material force, and Einstein was wrong...
True, James, I should not have used the term "gravitational force" yesterday, it is really an obsolete and fictitious concept, replaced in GR by the curvature of spacetime,
There is very important physical experiment, the free fall of neutrons, which demonstrate that gravity is the force.
This experiment has principal meaning for quantum physics and unification of fundamental physical interactions, because it is the first demonstration that gravitation acts as other forces (not as geometry). In the paper Nesvizhevsky et al., Nature, Volume 415, Issue 6869, pp. 297-299 (2002), Quantum states of neutrons in the Earth's gravitational field ( http://adsabs.harvard.edu/abs/2002Natur.415..297N ) they presented results of an experiment for free fall of ultra-cold neutrons, where quantum states of falling neutrons were measured, exactly as quantum states of an electron in the Coulomb field. So they demonstrate that the nature of gravity force is the same as in other fundamental forces, i.e. exchange of virtual particles, including virtual gravitons
Yurij,
suppose an astronomical object is emitting neutrons which we can detect after they have been lensed by a galaxy or cluster on their way. Their path is along a geodetic curve, thus it is determined by geometry. One could also state that they deviated from a straight line path because they fell some distance towards the lensing mass.
There is no contradiction between the two descriptions, right? The ultracold neutrons you refer to were falling straight towards a detector following a geodetic
line in the local spacetime, and the curving of the local spacetime could not be observed because that effect was much too weak.
Yurij, the experiment you mention is highly interpretive, and a quantum-level oddity hardly undermines nearly a century of evidence in support of GR.
Place a test body in the center of a box in a force-free enviroment (space), apply a force to the box or the test body, and the test body will move toward a side of the box, then press against it for as long as the force is applied. Bring the box near a planetary body and the test body will not move -- NO FORCE.
I want to expalin this by being a student of 8th class:-
Due to gravity there is gravitatonal force.
Any how no gravity there will be gravitational force.
gravity is only an acceleration of earth unit m/s2
And by Newtonian Mechanics Gravitational Force {Fg = mass (m) x Acceleration due to gravity (g)}
Yurij,
Isn't this experiment directly analogous to light curving as it obliquely traverses a gravitational field of sufficient magnitude (i.e., the Sun) - except for the momentum of light and therefore the magnitude of gravity required to affect it?
Can't the results be explained in the context of GR?
Gravity is a word to explain a certain phenomenon and gravitational force is calculated on or by a certain mass.
I, think the whole universe is completed by all 4 forces including the gravitationa force. So that gravity is basic nessicities to hold up the entire Universe. Conclusion is that if universe is exiested so that gravity is also vice versa.
General Relativity is our best description of gravity on all scales, microscopical to cosmological, but it is so far well tested only in the weak limit. Note that in that limit GR takes the form of Newton's gravitational law. The concepts of gravity and gravitational force refer to the same physics, the only difference being that one is used to refer to a force in the Newtonian limit. GR does not need that concept, there is nothing in Einstein's equations that needs to be termed "force". This is the complete answer to the question What is the difference between gravity and gravitational force.
Gravity facts
Fact : objects are moved by what we call gravity. Maybe objects are pushed by gravitational forces or maybe objects are attracted by gravitational forces.
Fact: The bigger the object and the bigger the influence called gravity. Bigger objects have more atoms. No atoms, no gravity.
Fact: A strong light blocks some of the gravitational force, not all, but enough to be observed in real physical experiment.
Fact: Light bends a little only when passing near a big object with lots of atoms. Maybe there is an effect of gravity from all the atoms on the rays of light or maybe around a big object, the index of refraction is enough to bend the light ray.
Fact: To make an electron, we have to shine a light ray of the right frequency and intensity on a fine sheet of heavy material, like lead or gold. Electrons and positrons appear on the other side of the sheet. Maybe the electrons and positrons were forced out of the nucleus of the heavy atoms or in classical view, the light energy was transformed in mass energy called electron or positron. If there is a transfer of energy to mass, what is really going on there. Nobody knows. Some just accept it and some question it.
Fact: to make protons and neutrons, we just need a higher frequency of light. Using the formula ENERGY IN JOULES = MASS x (SPEED OF LIGHT)2 AND EQUALS FREQUENCY OF LIHGT x ( VALUE OF h ).
Fact: light is emitted by objects made of atoms. Atoms can be made from light. Atoms give off light. Is the stuff making atoms the same as the stuff making light ? Maybe. If yes, we can explain many strange facts.
Fact: A plastic comb going through hair have an excess of electrons compared to the number of its protons. If the comb is moved back and forth, it sends light at the same frequency as the movement of the comb. If it was possible to vibrate the comb at a very high frequency, our eyes would call it color.
Fact: A small object placed between the moving comb and the detector blocks something that was going in straight line. At what speed? Probably at the speed of light because it is light of a low frequency, usually call radio wave.
Fact: in a lab experiement, a 500 watt fluorescent lamp enclosed in aluminum foil will affect a moving mass on a rotational pendulum almost as much as a one kg brass cylinder placed near the moving mass. If light blocks some gravity as much as the cylinder, does that mean that the cylinder also blocks gravityt that was going horizontally on the moving mass. Probably. That would also mean that what causes gravitational force is going through space in all directions, at the speed of light and light can interact feebly with it, changing its direction. Light is really shielding some of the gravity like the 1 kg mass was shielding some of the gravity also.
Fact: To change the movement of an object, a force has to be applied to the object. That force can come from another moving object. If gravity can move objects, it is possible that something is feebly interacting with atoms of the object and gives a small push to change its direction. On our planet there is no stationary objects because our planet is moving through space. If an object falls to the ground, its real trajectory is not a straight line down but a curved line in space as the object also move with the planet. Gravity change its path and the object gets pushed towards down.
Possibility: What we call gravitational force might just be all the radiations, particles, neutrinos flying in all directions through space and interracting feebly with atoms of the objects, giving the push that we call gravity. That means also that it is possible that what we call matter is simply complex systems made of the same material as light, same material as what electrons are sending in space, same material as what causes gravity. We do not need imaginary virtual particles. What is now called basic particles is probalbly very complex systems made of that same stuff.
Uhm, although historically both terms refer to the same phenomenom, I tend nowadays to restrict my ideas to new ways of thinking physiscs, not using the term "forces", because I really suscribe the alternate, more realistic concept of "interaction". So, gravity can be addressed to the inter-attractive interaction between two existant bodies (particles, planets, fotons, etc...), a large scale interaction, with the interchange of the proper amount of the pertinent virtual particles ( "bosons" always) which in gravity case are "THE GRAVITONS" never seen yet (too thin to be detected brothers, huhu). Please, read the profitable articles about a historical approach of the unification of theories of "forces" written in Scientific American in year 1978 or so, research that publications and enjoy the newly, fresh approaches in terms of interactions better than forces. Greetings from Mendoza, Argentine. Gustavo.
By the way, Gustavo, I greatly enjoy the Malbecs from Mendoza. Mis complimentos!
Well, at last Matts, we fully agree, good enough for a chemist / materials res. / nanomaterials scientific worker like me. Anyway, I tend to answer all sort of questions in Sci. and Tech. that I can rationalize. Greets again and "skoll". jeje. Gustavo.
An alternative view; Entire space, outside basic 3D matter-particles, is filled with an all-encompassing universal medium, structured by quanta of matter. See; http://vixra.org/abs/1007.0042 . Due to its structure, universal medium is inherently under compression. A 3D matter-particle, in the universal medium, experiences compression from universal medium. This property of universal medium is gravitation. Force/pressure exerted by gravitation is so enormous that it can convert lower spatial dimensional matter to 3D matter.
Magnitude of gravitation corresponds to extent of universal medium that exerts the pressure. Gravitational pressure on outer sides of two 3D matter-particles is always less than gravitational pressure on them from inner sides. This difference produces ab resultant tendency to move these 3D matter-particles towards each other. This motion/tendency to move is attributed to an apparent force - gravitational attraction - between the 3D matter-particles. Gravitational attraction (gravity) is the resultant (relatively a minor by-product) of separate gravitational actions on two 3D matter-particles. See: http://vixra.org/abs/1206.0056 'MATTER (Re-examined)' www.matterdoc.info
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