# B Gravity -- a force or a cause of a force?

#### zanick

Summary
Gravity has always been considered a force.... more accurately , an force of attraction. recently, it has been redefined to be more associated with einsteinium physics, causing bending of space-time. Is there an official definition that we can use and a citation to show consensus.
When i get into discussions regarding "Gravity" i say that it is a force that is caused by gravity. that the acceleration is due to gravity. there are some, like George Musser who have recently said, "Gravity is not a force, but can be thought of as a force". i think what he means is that there is a force , or force of attraction caused by gravity . is this not a good way to think about it. After all, we all know a magnet is not a force, but it causes a force. gravity causes a force due to two masses being attracted to each other. this force (or attraction) is said to be proportional to the two masses and inversely proportional to the distance between them. F(g) = Gm1m2/d2 . if this is true, is there an actual citation i can use for proof of this interpretation of gravity to anyone that asks...........or is gravity truly a force as would then magnetism be a force.

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#### A.T.

... is gravity truly a force ...
Gravity is an observed phenomenon that can be modeled as an interaction force or space-time geometry.

#### zanick

so it's not a force, its a phenomenon or a force caused by interaction?........ George Musser said " it could be thought of as a force". i think he is right.... i thnk it causes a force, like a magnet or other force causes, create a force.

#### jbriggs444

Homework Helper
so it's not a force, its a phenomenon or a force caused by interaction?........ George Musser said " it could be thought of as a force". i think he is right.... i thnk it causes a force, like a magnet or other force causes, create a force.
As I understand it, you want to use the word "gravity" to refer to the phenomenon of gravitation and you want to keep that distinct from the "force of gravity" which is what two massive objects appear to exert on each other.

However, in that case using the word "cause" seems inappropriate. If you model gravitation as a force law that follows $F=G\frac{m_1m_2}{r^2}$ then gravitation does not cause the force of gravity. In that model, gravitation is the force of gravity.

Edit: On the other hand, it's all just a word game. Can you describe a physical experiment that could distinguish between "causes" and "is" in this case? If not, this is philosophy, not science.

#### Dale

Mentor
so it's not a force, its a phenomenon or a force caused by interaction?........ George Musser said " it could be thought of as a force". i think he is right.... i thnk it causes a force, like a magnet or other force causes, create a force.
If you write a sentence talking about the force then your sentence should be clearly written so that the readers understand that you are referring to the force.

If you write a sentence talking about the interaction then your sentence should be clearly written so that the readers understand that you are referring to the interaction.

If you write a sentence talking about some other aspect of gravitation then your sentence should be clearly written so that the readers understand that you are referring to the other aspect.

If all of the above is fulfilled, then the rest doesn’t matter.

#### Drakkith

Staff Emeritus
2018 Award
Gravity has always been considered a force.... more accurately , an force of attraction. recently, it has been redefined to be more associated with einsteinium physics, causing bending of space-time. Is there an official definition that we can use and a citation to show consensus.
The official definition depends on which model and theory you are using. The most accurate definition of obviously one where we treat gravity as a manifestation of spacetime curvature. The less accurate definition is one of gravity being a force between two masses.

We commonly associate 'truth' with which theory is more accurate. But I find that usually treating things as varying levels of accuracy is more productive than saying things are true or false, right or wrong.

#### sophiecentaur

Gold Member
On the other hand, it's all just a word game. Can you describe a physical experiment that could distinguish between "causes" and "is" in this case? If not, this is philosophy, not science.
Exactly. The word "is" should always be used carefully and, I would say, never be used to suggest an absolute truth. Where possible "behaves like" etc. should be used, except when the context is within an axiomatic train of thought. (e.g.Maths procedures)

#### PeroK

Homework Helper
Gold Member
2018 Award
Summary: Gravity has always been considered a force.... more accurately , an force of attraction. recently, it has been redefined to be more associated with einsteinium physics, causing bending of space-time. Is there an official definition that we can use and a citation to show consensus.

When i get into discussions regarding "Gravity" i say that it is a force that is caused by gravity. that the acceleration is due to gravity. there are some, like George Musser who have recently said, "Gravity is not a force, but can be thought of as a force". i think what he means is that there is a force , or force of attraction caused by gravity . is this not a good way to think about it. After all, we all know a magnet is not a force, but it causes a force. gravity causes a force due to two masses being attracted to each other. this force (or attraction) is said to be proportional to the two masses and inversely proportional to the distance between them. F(g) = Gm1m2/d2 . if this is true, is there an actual citation i can use for proof of this interpretation of gravity to anyone that asks...........or is gravity truly a force as would then magnetism be a force.
"Gravity" is a word of 7 letters and "force" is a word of 5 letters. What they represent in physics is what they are defined to represent.

No amount of philosophical wordplay is going to produce an absolute answer.

#### anorlunda

Mentor
Gold Member
[Alert: This post is marginally off-topic.]

The word "is" should always be used carefully and, I would say, never be used to suggest an absolute truth.
Bingo @sophiecentaur that's right. But it is a bug in the English language, not just the speaker's fault. The verb "to be" always expresses absolute truth albeit incorrectly.

Linguists sometimes call use of the verb "to be" as "God Mode" speaking. "Gravity is a force" is so final. It allows no nuance, no contradiction, that it might have been the voice of God coming down from the sky. Use of "is" in "gravity is a force" is a linguistic trap expressing absolute truth when it might not be intended.

E-Prime is a dialect of English where nearly all uses of "to be" (including am, is, are, was, were, have been, am being) are eliminated. If we could all learn E-Prime, life would be simpler.

For example, all the above translated to E-Prime:

Bingo @sophiecentaur that would correct the problem. But we should attribute it to a bug in the English language. The verb "to be" always expresses absolute truth albeit incorrectly.

Linguists sometimes call use of the verb "to be" as "God Mode" speaking. "Gravity is a force" sounds so final. It allows no nuance, no contradiction, that it resembles the voice of God coming down from the sky. Use of "is" in "gravity is a force" traps the speaker into expressing absolute truth when he didn't intend that.

A dialect of English called E-Prime eliminates nearly all uses of "to be" (including am, is, are, was, were, have been, am being). If we could all learn E-Prime, we would make life simpler.

#### vanhees71

Gold Member
Well, you can describe one phenomenon at different levels, and if a physicist says "garvity is a force", of course he means that he chooses to describe it as a force. I don't think we should make life in physics even more difficult by using unsensical linguistic experiments. However, of course, one should be precise, and sometimes a lot of physicists' jargon tends to confusing beginning physics students.

First of all, I think, only within Newtonian physics the notion of force makes sense, since it's a concept based on a picture where instantaneous interactions take place. Today we know that this is an approximation valid for Newtonian mechanics only, which implies that the bodies should move at speeds much less than light and the time it takes for interactions to be transferred from one body to the other via fields is small. In the case of gravity and electromagnetism the speed of signal propagation is $c$, the vacuum speed of light. Thus the speeds of the particles must be much less than the speed of light.

#### zanick

does anyone have a citation or something official that says gravity is a force or causes a force.... or something to the effect of "the force due to gravity"? I have a side bet that this is true , but I need an official quote. thanks!

#### A.T.

I have a side bet that this is true...
It's neither true nor false. It's a naming convention.

#### zanick

Great answer.... i understand and appreciate that.

#### GreatestPhysician99

In newtonian mechanics gravity is a force , it is the attraction of two objects with mass.

#### Paul Colby

Gold Member
I think it's interesting that in Newtonian physics in a space far from masses like stars and planets objects move along straight lines at a constant velocity unless acted upon by a force. In making the transition to special relativity these very same constant velocity trajectories become geodesics, extremal paths in space-time. Again, bodies only deviate from them when acted upon by an external force. In passing to general relativity geodesics become quite a bit more complex, and yet bodies still follow geodesics unless acted upon by an external force. Basically, $F=ma$ is alive and well in general relativity though much modified in details of what $a$ is within the theory. In the modern view, as others have said, Newton was able to describe the modifications to geodesics caused by masses by introducing a force, his universal gravitation force we all know and love.

#### vanhees71

Gold Member
You mean General Relativity. Free fall, i.e., the motion of a point particle in space time is a force-free motion along geodesics in spacetime. Inertial frames exist only locally.

"Gravity -- a force or a cause of a force?"

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