Is Gravity a force, or not a force?

[KirkDamborg]

I am not a student and am simply interested in physics and the like, do studying on my own and just trying to learn about this fascinating subject. While reading or watching videos i get 1000's of questions but i have nobody to ask so i figured id join here... i hope my questions aren't too stupid lol.

Anyways, my question is, so is gravity a FORCE, or is it a cause from an effect?

I thought that there was no 'invisible force' called gravity like what has been believed for so long, and that what was actually going on was that things were merely following a straight line through a curved space/time fabric... which would cause an object to 'attract' to another due to its curving of the fabric.

But in the lecture I'm watching, he says that Gravity IS a force, and its message carrier is the Graviton, and that obviously its one of the major forces of our universe.

So which one is right or are they both right? I know the space/time fabric is right, but is gravity then still a force?

 

[haushofer]

A force is something which results in acceleration. However, due to the equivalence principle this idea for gravity doesn't make sense anymore.

See for instance Carroll's treatment

http://nedwww.ipac.caltech.edu/level5/March01/Carroll3/Carroll4.html

and particularly

It is the EEP which implies (or at least suggests) that we should attribute the action of gravity to the curvature of spacetime. Remember that in special relativity a prominent role is played by inertial frames - while it was not possible to single out some frame of reference as uniquely "at rest", it was possible to single out a family of frames which were "unaccelerated" (inertial). The acceleration of a charged particle in an electromagnetic field was therefore uniquely defined with respect to these frames. The EEP, on the other hand, implies that gravity is inescapable - there is no such thing as a "gravitationally neutral object" with respect to which we can measure the acceleration due to gravity. It follows that "the acceleration due to gravity" is not something which can be reliably defined, and therefore is of little use.

Instead, it makes more sense to define "unaccelerated" as "freely falling," and that is what we shall do. This point of view is the origin of the idea that gravity is not a "force" - a force is something which leads to acceleration, and our definition of zero acceleration is "moving freely in the presence of whatever gravitational field happens to be around."

I think this section is not very technical, so with a little background you should be able to follow ;)

 

[Mentz114]

"the acceleration due to gravity" is not something which can be reliably defined, and therefore is of little use.

If a test particle is released from rest ( wrt to the source of the field), it experiences an acceleration that can easily be defined.

 

[Troponin]

Anyways, my question is, so is gravity a FORCE, or is it a cause from an effect?


But in the lecture I'm watching, he says that Gravity IS a force, and its message carrier is the Graviton, and that obviously its one of the major forces of our universe.

So which one is right or are they both right? I know the space/time fabric is right, but is gravity then still a force?

It depends on what theory you're using.
General Relativity doesn't consider gravity as a force. Massive objects create a space-time geometry with a "curvature" described by the curvature tensor. There is no actual "force" of gravity, just the curvature of space-time that is created by gravity.


The idea of a graviton comes from quantum field theory. In QFT, the "force" of gravity is mediated by the graviton with the stress-energy tensor as the source.


If the lectures are for Newtonian gravity, then gravity is also considered a force.

so is gravity a cause from an effect?

GR considers gravity as the "cause." The "effect" is the geometry of space-time.
I suppose you could say that GR presents gravity as mass. Anything with mass will cause the curved geometry of space-time.

It is this mass (energy) that generates the curvature of space, which causes other objects to accelerate in response to the curvature. When a human accelerates from this curvature, it feels like there is a force that is causing him or her to accelerate. Being vocal creatures, we gave this sensation the name "gravity."

 

[bcrowell]

"the acceleration due to gravity" is not something which can be reliably defined, and therefore is of little use.

If a test particle is released from rest ( wrt to the source of the field), it experiences an acceleration that can easily be defined.

What if the field has multiple sources, all in different states of motion? What if you have a cosmology that's a vacuum solution to the field equations, so there are all kinds of gravitational waves running around, but they were never created by sources? Acceleration is easy to define, but in GR that definition is completely dependent on the choice of coordinates, so the definition is arbitrary.

 

[Mentz114]

I regret saying 'wrt to the source' now. We can define sets of worldlines in any spacetime and define frames where coordinate velocity is zero, which is what I mean (probably) 'from rest'.

What if the field has multiple sources, all in different states of motion?

The definition I had in mind depends on the Christoffel symbols ( I didn't say it was coordinate independent ) and as long as they exist the details of the solution don't matter.

What if you have a cosmology that's a vacuum solution to the field equations, so there are all kinds of gravitational waves running around, but they were never created by sources?

A vacuum solution may still have sources (?). Presumably a spacetime with gravitational waves will still have EOMs. Please put me right if this is incorrect.

For example, a test body released in a standard FLRW expanding dust cosmology feels nothing. The same thing in a Schwarzschild spacetime will begin moving towards the source. The 'sticky-bead' argument says that a test body moves in a gravitational wave.

I need to redefine what I was thinking of in manifold concepts.

 

[KirkDamborg]

Thank you all for your responses... its refreshing actually being able to discuss such things with someone.

It depends on what theory you're using.


General Relativity doesn't consider gravity as a force. Massive objects create a space-time geometry with a "curvature" described by the curvature tensor. There is no actual "force" of gravity, just the curvature of space-time that is created by gravity.


The idea of a graviton comes from quantum field theory. In QFT, the "force" of gravity is mediated by the graviton with the stress-energy tensor as the source.


If the lectures are for Newtonian gravity, then gravity is also considered a force.


GR considers gravity as the "cause." The "effect" is the geometry of space-time.
I suppose you could say that GR presents gravity as mass. Anything with mass will cause the curved geometry of space-time.

It is this mass (energy) that generates the curvature of space, which causes other objects to accelerate in response to the curvature. When a human accelerates from this curvature, it feels like there is a force that is causing him or her to accelerate. Being vocal creatures, we gave this sensation the name "gravity."

Is it understandable that this bothers me?
This puts the two theories against each other and one of them must be flat out wrong in this case...
I rented the 'great courses' lectures on String Theory by Jim Gates, and whole lectures pertain to particle physics and the forces of the universe, in which the graviton gets considerable time, and also on how string theory actually predicts these message carriers to be.

It bothers me then, that this would be in direct opposition to G.R...


At least now i know that its for sure that these two are in opposition and that i wasnt just totally confused.

Thank you for your answer

 

[Passionflower]

I think it is mostly a terminology issue, it depends on how you define a force.

Gravitation causes inertial accelerations and rotations, from that perspective you can say it is not a force in the traditional sense, however gravitation also causes tidal effects and these effects certainly look like a force in the traditional sense.

 

[yenchin]

Thank you all for your responses... its refreshing actually being able to discuss such things with someone.





Is it understandable that this bothers me?
This puts the two theories against each other and one of them must be flat out wrong in this case...
I rented the 'great courses' lectures on String Theory by Jim Gates, and whole lectures pertain to particle physics and the forces of the universe, in which the graviton gets considerable time, and also on how string theory actually predicts these message carriers to be.

It bothers me then, that this would be in direct opposition to G.R...


At least now i know that its for sure that these two are in opposition and that i wasnt just totally confused.

Thank you for your answer

My view is that Science is not about truth, it is about building a model to better understand how the universe works. While in Newtonian description it is considered a force, in GR it is the curvature of spacetime, and so on in other theories, they are all different ways to look at things as mathematical models, but they don't really say much about what gravity *really* is.

 

[Troponin]

Thank you all for your responses... its refreshing actually being able to discuss such things with someone.





Is it understandable that this bothers me?
This puts the two theories against each other and one of them must be flat out wrong in this case...
I rented the 'great courses' lectures on String Theory by Jim Gates, and whole lectures pertain to particle physics and the forces of the universe, in which the graviton gets considerable time, and also on how string theory actually predicts these message carriers to be.

It bothers me then, that this would be in direct opposition to G.R...


At least now i know that its for sure that these two are in opposition and that i wasnt just totally confused.

Thank you for your answer

It probably should bother you that the major theories of the world have definite areas where they can't agree.
But don't let the word "force" be too much of a factor in that.
Force is just the word given to describe a particular mathematical act.

One theory has gravity being sent out as a force (by gravitons) that act on other objects.
The other theory has gravity changing the geometry of the space which those other objects "live." This change in geometry makes those objects feel something like a force.

Whether the Earth is sending gravitons to "force" you from floating away or the Earth is changing the geometry of space in order to "force" you from floating away...you're still stuck on earth.
Just be glad that the atmosphere is feeling the same thing...It's hard to think about physics when your brain isn't getting oxygen.

 

[miske50]

I think gravity cannot be a force in a traditional sense. Namely, it could not affect light if it was a force, as light (photons) have no mass. Gravity bends space-time around it so light, choosing the shortest route (in space-time!), seemingly bends around mass (gravitational lensing).
Another thing. Great Feynman, in one of his lectures has drawn attention to an astounding fact and that is that the gravity pull beween two elctrones is 1/ (4.17 *10^42) in comaprison to the intensity (force) of their electromagnetic repulsion! In other words the two "forces" are a whole universe away!. If you put 10^42 protons in the universe next to one another they would add up to a diameter of the known universe!
So, I think that gravity is in no way connected as a "force" to electromagnetic, strong or weak nuclear force. Gravity is an inherent characteristic of the structure and the fabric of the universe and it has yet to be understood. I don't believe in the "unified" theory at all and it is inherently impossible to find a magic mathematical formula to unite all "four forces".

 

[A.T.]

Newton's model : Gravity is an interaction force
Einsteins's model : Gravity is an inertial force

For the difference see: http://en.wikipedia.org/wiki/Fictitious_force

 

[A.T.]

Gravitation causes inertial accelerations and rotations, from that perspective you can say it is not a force in the traditional sense, however gravitation also causes tidal effects and these effects certainly look like a force in the traditional sense.

Why? The centrifugal force, that appears in a rotating frame, also causes non-uniform acceleration. But it's still just an inertial force.

 

[ApplePion]

"Namely, it could not affect light if it was a force, as light (photons) have no mass."

Photons do not have rest mass, but they do have mass. Anything with energy has mass--you just divide the energy by the square of c to calculate that mass. It might seem that I am engaging in a semantical quibble but I am not--photons generate gravitational fields, a characteristic of things with "mass".

"I think gravity cannot be a force in a traditional sense"

Even if photons did not have mass, their deviation in a gravitational field in General Relativity is not suggestive that gravity in General Relativity is not a force. For example, in the Newtonian regime the force on a body is mg where g is the local gravitational field strength, and the acceleration is thus (mg)/m. For the case of m going to zero, (mg)/m does not at all vanish!

 

[ApplePion]

"Newton's model : Gravity is an interaction force
Einsteins's model : Gravity is an inertial force"

Actually, the Einstein perspective is that gravity is an interaction force--he certainly believed the Sun exerted a force on the Earth-- but that it is also an inertial force. That was the beauty of General Relativity!

It is important to remember that General Relativity is a physical theory. It most certainly was about "forces".

 

[Naty1]

This puts the two theories against each other and one of them must be flat out wrong in this case...

The theories are different but both are approximations rather than 'flat out wrong'. All our
models are approximations. Some to twenty or more decimal places, others not so good! Quantum gravity is the incomplete theory, a work in progress, that it is hoped may someday be completed and combine quantum gravity and general relativity.

We thought we were so 'smart' about mass and energy, and gravity, in the universe until maybe 20 or 25 years ago, I think when Vera Rubin's observations about galaxy rotations led to more widespread acceptance of dark matter [and dark energy] that Fritz Zwicky had postulated in the 1930's. Suddenly all we 'understood' was about 4% of the mass and energy in the universe instead of the virtually 100% previously thought!

Dark energy supposedly has negative pressure, a form of repulsive gravity I think comes from general relativity; anybody know if there is a corresponding view from quantum mechanics??

 

[A.T.]

he certainly believed the Sun exerted a force on the Earth-- but that it is also an inertial force.

Inertial forces are per definition not exerted by some source object. That's what sets them apart from interaction forces.

 

[Dale]

Namely, it could not affect light if it was a force, as light (photons) have no mass.

In Newtonian physics, how much force does it take to produce a finite acceleration in an object with 0 mass?

 

[ApplePion]

"Dark energy supposedly has negative pressure, a form of repulsive gravity I think comes from general relativity"

It does indeed come from GR. There are two major cosmological equations. One indicates that the deceleration of the expansion is proportional to the mass densitiity plus 3 times the pressure. If, as the observational data is, as believed to indicate, thatt the Universe's expansion is accelerating then either the mass density or the pressure (or both) must be negative. But the other equation implies the mass density is positive. So we are stuck with the strange implication of negative pressure if the Universe's expansion really is accelerating.

 

[ApplePion]

"Inertial forces are per definition not exerted by some source object. That's what sets them apart from interaction forces"

Earlier you wrote:

"Newton's model : Gravity is an interaction force
Einsteins's model : Gravity is an inertial force"

Taking both statements together you seem to be implying things like that the Sun is not a source of a force exerted on the Earth. I don't think you really mean to imply that.

 

[A.T.]

I think gravity cannot be a force in a traditional sense. Namely, it could not affect light if it was a force, as light (photons) have no mass.

The acceleration caused by the traditional Newtonian force of gravity does not depend on the mass of the accelerated object.

 

[A.T.]

"Inertial forces are per definition not exerted by some source object. That's what sets them apart from interaction forces"

Earlier you wrote:

"Newton's model : Gravity is an interaction force
Einsteins's model : Gravity is an inertial force"

Taking both statements together you seem to be implying things like that the Sun is not a source of a force exerted on the Earth. I don't think you really mean to imply that.

I do mean to imply that. In GR there is no direct force interaction between Sun and Earth. They interact only indirectly via space-time geometry. The locally experienced force of gravity is an inertial force, due to using an accelerating reference frame.

In GR the Sun is a source of "gravitation", but it's not a source of "the force of gravity". The "force of gravity" in GR is caused by the coordinate choice, not by the Sun.

 

[ApplePion]

"In GR there is no direct force interaction between Sun and Earth."

Well, yeah, it is action at a distance. In electromagnetism a charge interacts with another charge not "directly" but through a field, too.

" They interact only indirectly via space-time geometry."

But a quantity in that geometry, the affine connection, is the forces. You have forces--they are geometric quantities. The two are not mutually exclusive. GRn is a physics theory about physical quantities.

"In GR the Sun is a source of 'gravitation', but it's not a source of 'the force of gravity'." The "force of gravity" in GR is caused by the coordinate choice, not by the Sun."

Obviously coordinate choices can turn gravitational fields on and off locally, but that misses the big picture--the Sun really is acting on the Earth. If the Earth were made to stand still, and then released, it would fall into the Sun and crash into it. The Earth would melt. If the Sun had no mass, no coordinate choice could cause that event to occur.

 

[ApplePion]

Me: "The Earth would melt."

Well, actually, more than just melt, of course.

 

[A.T.]

Obviously coordinate choices can turn gravitational fields on and off locally

"Field" is too vague. The topic is "interaction force". The only way to create a "force of gravity" in GR is to chose a non-inertial reference frame. But that is an "inertial force" then, not an "interaction force".

Sun really is acting on the Earth.

"Acting" is to vague. The topic is "interaction force". To clarify what I mean:

"interaction force" : causes proper acceleration
"inertial force" : causes only coordinate acceleration

Based on this definitions there is no "interaction force of gravity" in GR. Just an "inertial force of gravity".

 

[Dale]

I am with A.T. on this. The only effects of gravity that cannot be transformed away are tidal effects. Other than that, the force of gravity is an inertial force in GR. Inertial forces are related to the Christoffel symbols in a given coordinate system and can always be set to 0 at any event in spacetime and along any geodesic by judicious choice of coordinate system.

The fact that two objects collide is not sufficient to assert a non-inertial interaction force. After all, two non-interacting inertial objects can collide as well. You can even design coordinate systems where they were initially "at rest" wrt each other and accelerated towards each other due to inertial forces.

 

[Naty1]

Kirk: You might find some value in this discussion...at least first and last section [Where are we]...This offers some interesting perspectives on some fundamental theories in physics...broader than gravity, but nevertheless inmsightful. Unfinished revolution
Introductive chapter of a book on Quantum Gravity, edited by Daniele Oriti,
to appear with Cambridge University Press
Carlo Rovelli

http://arxiv.org/pdf/gr-qc/0604045v2.pdf
excerpt:

...In fact, our present understanding of the physical world at the fundamental level is in a state of
great confusion. The present knowledge of the elementary dynamical laws of physics is given by the
application of QM to fields, namely quantum field theory (QFT), by the particle–physics Standard
Model (SM), and by GR. This set of fundamental theories has obtained an empirical success nearly
unique in the history of science: so far there isn’t any clear evidence of observed phenomena that
clearly escape or contradict this set of theories —or a minor modification of the same, such as a
neutrino mass or a cosmological constant.1 But, the theories in this set are based on badly selfcontradictory
assumptions. In GR the gravitational field is assumed to be a classical deterministic
dynamical field, identified with the (pseudo) riemannian metric of spacetime: but with QM we have
understood that all dynamical fields have quantum properties. The other way around, conventional
QFT relies heavily on global Poincar´e invariance and on the existence of a non–dynamical background
spacetime metric: but with GR we have understood that there is no such non–dynamical background
spacetime metric in nature.
In spite of their empirical success, GR and QM offer a schizophrenic and confused understanding
of the physical world.

Many here will, I suspect, strongly disagree with the above comments...but that's one of the things that makes science especially valuable.

 

[Dale]

Many here will, I suspect, strongly disagree with the above comments.

I disagree with the use of the word "selfcontradictory". GR does not contradict itself. QFT does not contradict itself. So neither is self-contradictory.

They each contradict the other, not themselves, so I would have said "mutually-contradictory" rather than "self-contradictory". As such, I think that he is over stating it, but overlooking the hyperbole the content of his comments seems valid.

 

[PeterDonis]

Many here will, I suspect, strongly disagree with the above comments...

Not disagree, exactly, but the comments do leave something out. The argument that GR and QFT are mutually contradictory, as it stands, only has force if one or both of them are supposed to be a "final theory". If both are just approximate "effective theories" valid in limited domains, then there is no issue other than the obvious one of trying to find what lies underneath the effective theories. Rovelli's comments could just as well be interpreted as trying to figure out something about what the underlying theory might be like. (Which is not to say that that's how he meant them to be interpreted.)

 

[pervect]

If a test particle is released from rest ( wrt to the source of the field), it experiences an acceleration that can easily be defined.

I don't think I agree with this. Maybe what you are saying is that if you have a static gravitational field, you can define a co-located static observer, and that you can then define the relative acceleration of the test particle relative to the static observer. That I would agree with, unfortunately it's only a special case and not a general definition. You need some notion to replace the static observer to measure your acceleration relative to, and it's not clear what to replace it with.

 

[haushofer]

Newton's model : Gravity is an interaction force
Einsteins's model : Gravity is an inertial force

For the difference see: http://en.wikipedia.org/wiki/Fictitious_force

I would say that Newtonian gravity is also an inertial force, i can always go to a frame to make it vanish, just like e.g. the cCoriolis force. Why do you make a distinction?

Edit it is a matter of definition, i see.

 

[Mentz114]

If a test particle is released from rest ( wrt to the source of the field), it experiences an acceleration that can easily be defined.

I don't think I agree with this. Maybe what you are saying is that if you have a static gravitational field, you can define a co-located static observer, and that you can then define the relative acceleration of the test particle relative to the static observer. That I would agree with, unfortunately it's only a special case and not a general definition. You need some notion to replace the static observer to measure your acceleration relative to, and it's not clear what to replace it with.

Can you give an example where it's not possible to define a static observer ? This is not a challenge, but I'd like to know because I can't think of any that don't involve null coordinates.

 

[pervect]

Can you give an example where it's not possible to define a static observer ? This is not a challenge, but I'd like to know because I can't think of any that don't involve null coordinates.

Well, the universe as a whole doesn't have a static observer - the metric doesn't have a timelike killing vector.

A binary star would be another example, again, no timelike killing vector. To demonstrate that the metric is a function of time when you don't have a full GR solution, consider the Newtonian approximation where you have two equal mass stars, and ask if the Newtonian potential U and/or the tidal forces are constant. Consider a test object when the binaries and the test object are all inline

() () x

and another when they aren't

()
x
()

 

[DrGreg]

Can you give an example where it's not possible to define a static observer ? This is not a challenge, but I'd like to know because I can't think of any that don't involve null coordinates.

What about inside the event horizon of a Schwarzschild black hole?

 

[Mentz114]

What about inside the event horizon of a Schwarzschild black hole?

That is scraping the barrel !