Gravity ficticuous force because it's relative to mass?

1. Sep 12, 2007

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 eletrical 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 dialation is fixed, while in the case of acceleration, the time dialation 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 dialation between gravity and acceleration.

2. Sep 12, 2007

pmb_phy

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
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.
John A. Peacock - From Cosmological Physics, by John A. Peacock, Cambridge University Press, (1999), page 6-7
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.
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 existance, and a lot of people have a hard time dealing with things that don't have an absolute existance and end up calling them "fictitious."

Pete

3. Sep 12, 2007

pervect

Staff Emeritus
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:

Last edited: Sep 12, 2007
4. Sep 13, 2007

rcgldr

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.

5. Sep 13, 2007

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

6. Sep 13, 2007

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.