Gravity ficticuous force because it's relative to mass?

[rcgldr]

I've read that in GR, gravity isn't considered to be a force, or that it's considered to be a fictitious force because its force is relative to the mass of objects, similar to the reaction forces of objects due to acceleration when the force is applied by contact from another object.

How is this different than the nuclear strong and weak forces, or how is it significantly different than electrical forces which are relative to charge? The common factor I see here is that no contact is involved with these forces.

Also even in GR, gravity is different than acceleration with respect to time dialation. In the case of gravity, the time dilation is fixed, while in the case of acceleration, the time dilation changes with respect to changes in speed.

Say there are two clocks, one on the earth, and one in a rocket, both initially at the same altitude from the Earth's center of gravity. The rocket takes off, and then "hovers" (staying in line with Earth's orbital path around the sun is close enough). An observer will then note that the clock in the rocket is moving faster than the clock on the Earth because of the difference in gravity (difference in intensity of the field to be techincal). Now the rocket starts to accelerate at 1 g in an orbital path around the earth, maintaining a constant distance from the Earth's center of gravity. As the rocket increases speed, an observer will note that the rocket clock is slowing, while the Earth clock remains running at a constant rate. From this, the observer can see that there is a difference in time dilation between gravity and acceleration.

 

[pmb_phy]

I've read that in GR, gravity isn't considered to be a force,...

I disagree with anyone who makes that claim. And I'm in very good company when I say this.

A.P. French - Inertial force is defined as the force on a body that results solely from observing the motion of the body from a non-inertial frame of reference. This in addressed in Newtonian Mechanics, by A.P. French, The M.I.T. Introductory Physics Series,W.W. Norton Pub. , (1971) , page 499. After describing the inertial force as seen from an accelerating frame of reference French writes

From the standpoint of an observer in the accelerating frame, the inertial force is actually present. If one took steps to keep an object "at rest" in S', by tying it down with springs, these springs would be observed to elongate or contract in such a way as to provide a counteracting force to balance the inertial force. To describe such force as "fictitious" is therefore somewhat misleading. One would like to have some convenient label that distinguishes inertial forces from forces that arise from true physical interactions, and the term "psuedo-force" is often used. Even this, however, does not do justice to such forces experienced by someone who is actually in the accelerating frame of reference. Probably the original, strictly technical name, "inertial force," which is free of any questionable overtones, remains the best description.

Cornelius Lanczos - The subject of inertial force is also addressed in The Variational Principles of Mechanics - 4th Ed., by Cornelius Lanczos, Dover Pub., page 98.

Whenever the motion of the reference system generates a force which has to be added to the relative force of inertia I’, measured in that system, we call that force an “apparent force.” The name is well chosen, inasmuch as that force does not exist in the absolute system. The name is misleading, however, if it is interpreted as a force which is not as “real” as any given physical force. In the moving reference system the apparent force is a perfectly real force which is not distinguishable in its nature from any other impressed force. Let us suppose that the observer is not aware of the fact that his reference system is in accelerated motion. Then purely mechanical observations cannot reveal to him that fact.

John A. Peacock - From Cosmological Physics, by John A. Peacock, Cambridge University Press, (1999), page 6-7

The 'weak equivalence principle' is a statement only about space and time. It says that in any gravitational field, however strong, a freely falling observer will experience no gravitational effects - with the important exception of tidal force in non-uniform fields. [...] It may seem that we have actually returned to something like the Newtonian viewpoint: gravitation is merely an artifact of looking at things from the 'wrong' point of view. This is really not so; rather, the important aspects of gravitation are not so much first order effects as second order tidal forces: They cannot be transformed away and are the true signature of gravitating mass. However, it is certainly true in one sense to say that gravity is not a 'real' force, the gravitational acceleration is not derived from a 4-force and transforms differently.

There are two types of forces found in nature. One that can be represented by a non-vanishing 4-vector and one that is known as an inertial force, which can always be transformed away. The term inertial force is defined above.

.. or that it's considered to be a fictitious force because its force is relative to the mass of objects, similar to the reaction forces of objects due to acceleration when the force is applied by contact from another object.

The reason some people claim that the gravitation force is a "fictitious force" is that it can always be transformed away. Therefore it does not have an absolute existence, and a lot of people have a hard time dealing with things that don't have an absolute existence and end up calling them "fictitious."

Pete

 

[pervect]

Gravity is certainly a force in Newtonian mechanics. Which is what two of the three references appear to be talking about, so it's not surprising that they state that gravity is a force - in Newtonian mechanics, it is. The title of French's books states that it is about Newtonian mechanics, and it seems highly likely that Lanzcos is also concerned with classical Newtonian mechanics (though it is possible to formulate GR as an action theory using the Einstein-Hilbert action, I would expect a book on variational principles to be a book about classical mechanics).

Thus two of the three quotes offered to support this rather surprising position are apparently not talking about GR at all.

I'd personally generally agree with Jeff's remarks that gravity isn't really a force in GR, though I don't think I agree with everything he said. I'd also agree with Peacock's remarks (the one author above that appears to be actually talking about GR) that tidal force is the true signature of gravitating mass. Note that Peacock added:

However, it is certainly true in one sense to say that gravity is not a 'real' force, the gravitational acceleration is not derived from a 4-force and transforms differently.

 

[rcgldr]

I'd personally generally agree with Jeff's remarks that gravity isn't really a force in GR.

My point was that even in GR gravity is a real force, just because it shares one quality of reaction forces, that it is relative to the amount of mass doesn't mean it is the same as reaction forces (I prefer the term "reaction force" to "fictitious force").

Reiterating my point, gravity is relative to the amount of mass, while electrical forces are relative to the amount of charge. I don't see enough difference here to classify gravity as a non force or as a fictitious force anymore that I would term electrical forces.

 

[Bob Walance]

Jeff,

There is no external force unless you're somehow opposing the curvature of space-time. What device would you use to measure this supposed force?

Newton used the concept of force in his writings about gravity because he didn't know what we now know. What he thought was force would be real, for example, if planets were connected to the sun with an elastic band AND there was no curvature in the geometry of space-time. However, space-time geometry DOES get curved so there is no need for forces to keep the planets orbiting around the sun.

Here are some ways to create forces and oppose the curvature of space-time:

Holding on to a lit rocket.
Rubbing against an atmosphere.
Standing on a planet.
Electrically charging two objects and bringing them close to each other.
Having two objects hold on to magnets and bring them close to each other.
(and many more, I suppose).

Bob

 

[rcgldr]

So you're basically stating that it takes a force to oppose a non-force? Just because gravity curves space time doesn't prove that it isn't a force. These are independent qualities in my opinion. Call it a non-force if you want, but if there's something that causes an object with mass to accelerate, I prefer to call that a force, GR or not.