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According to Einstein's theory it is the bending of space that keeps the earth moving on its orbit around the sun and not any invisible force. Why they still teach gravitational force in school ?
General relativity doesn't replace gravitational force, it explains the origin of that force as curved space-time.
as Isaac Asimov said, scientific theories are not so much wrong as incomplete.
So, do you think that if you place two magnets close together in outer space, not touching but close together, that they will not attract/repel each other? That's the clear implication of your statement "force can't travel in vacuum"Correct me if I'm wrong.Since the force can't travel in vacuum like for instance the gap between the sun and the earth. Isn't it more practical if we just say gravity only instead of gravitational force?
Correct me if I'm wrong.Since the force can't travel in vacuum like for instance the gap between the sun and the earth. Isn't it more practical if we just say gravity only instead of gravitational force?
In classical field theory, there is a value associated at every position in space that tells us what the magnitude and direction of a force exerted by the field will be. This holds true for both EM and Gravitational fields. A change in the field will move at c in the form of an EM or Gravitational wave. The 'gravitational force' is the force exerted by the gravitational field
In Newtonian physics, the word "acceleration" means what, in GR, is called "coordinate acceleration" (with the intended implication that it is not "really" acceleration). This works in Newtonian physics because Newtonian physics has absolute space and absolute time, so there are "privileged" systems of coordinates in which acceleration "really is" acceleration. In such a system of coordinates, a rock freely falling towards Earth is "really" accelerating, and we attribute this to the action of a "force", namely gravity.
In GR, however, the word "acceleration" is properly used to denote "proper acceleration" (pun intended ;) ). Proper acceleration is a direct observable: you measure it with an accelerometer (your bathroom scale is an example of an accelerometer). A "force" in GR is then something which produces proper acceleration. Gravity does not do this: an object moving solely under the influence of gravity, like the rock falling, is in free fall, with zero proper acceleration, and therefore is not being subjected to any force. So in GR, gravity is not a force--period.
the word "force" is also used in a rather loose sense by particle physicists to mean "interaction" in general, e.g. "the four fundamental forces."
Outside of the context of Newtonian mechanics "force" is a pretty imprecise term!
Yes, indeed - and that is also the way Einstein used used those words in his theory. In contrast, Peterdonis is referring to modern GR.No. Forces don't 'travel'. In classical field theory, there is a value associated at every position in space that tells us what the magnitude and direction of a force exerted by the field will be. This holds true for both EM and Gravitational fields. A change in the field will move at c in the form of an EM or Gravitational wave. The 'gravitational force' is the force exerted by the gravitational field and 'gravity' is the name of the whole concept, similar to how 'electromagnetism' is the name of the whole concept of electrical charges, magnetic fields, etc.
GrrrrrrThanks, Peter. I actually meant to explain some of that, but somehow forgot all about it. Can I blame Phinds somehow?
Grrrrrr
I eat vets for breakfast and !Down boy, else your geodesic will come to an abrupt halt at a singularity. Whose name is Roger. He's the local vet. ;)
Einstein's theory and modern GR are the same theory. Some minor evolution in terminology and a better understanding of the math hardly makes it a new theory.Yes, indeed - and that is also the way Einstein used used those words in his theory. In contrast, Peterdonis is referring to modern GR.
I took it that harrylin is referring to the relatively modern (new) language sometimes used to describe the same original GR rather than a new theory of GR.Einstein's theory and modern GR are the same theory. Some minor evolution in terminology and a better understanding of the math hardly makes it a new theory.
The modified definition seems ad hoc to me
All definitions are ad hoc.The modified definition seems ad hoc¹ to me
Yes indeed. Ronrit referred to the formulation of "Einstein's theory", which on face value means the theory as originally formulated by Einstein. In that formulation, gravitation is interpreted as a field (so that gravity appears as a local field force*), while in Newton's theory it was modeled (by lack of better) as a mysterious action at a distance. While it is certainly a matter of interpretation, the issue of calling gravity a force does not directly depend on the theory. IMHO it's a philosophical issue and not really physics.harrylin said: ↑
"Yes, indeed - and that is also the way Einstein used used those words in his theory. In contrast, Peterdonis is referring to modern GR."
I took it that harrylin is referring to the relatively modern (new) language sometimes used to describe the same original GR rather than a new theory of GR.
[..]...
Who is Ronrit?Ronrit referred to the formulation of "Einstein's theory", which on face value means the theory as originally formulated by Einstein.
In Newtonian physics, the word "acceleration" means what, in GR, is called "coordinate acceleration" (with the intended implication that it is not "really" acceleration).
As an example, the term "force" (gravitational) is now used (redefined, #12) in a way that did not exist in Newtons day to try to circumvent some awkwardness in our description of GR.
I meant the OP, ronric (sorry for the typo).Who is Ronrit? [..]
This kind of pairing of forces does not apply to "fictitious" forces such as centrifugal force. So mere coordinate acceleration does not imply a force, in Newton's sense.
So coordinate acceleration doesn't work for Newton's physics, either, except in the special case of inertial, cartesian coordinates.
That is a particularily good insight, Dale, and I thank you for that. It has always bothered me that so many learned commentators still often casually refer to gravity as a "force" and I now better understand why.All definitions are ad hoc.
However, PeterDonis is correct to point out that the issue addressed by the change in definition is not unique to GR but was inherent in Newtonian gravity from the beginning. Basically, even in pre-relativistic mechanics we use the word "force" to refer both to real forces, like the EM force, and also fictitious forces like the centrifugal force. All fictitious forces have the mathematical characteristic that they are proportional to mass and the related experimental characteristic that they cannot be detected by accelerometers. Gravity has those same characteristics.
The change with GR is to classify the force of gravity as a fictitious force like the centrifugal force. But the different meanings of the word "force" were already there, and the reclassification does not change any predicted experimental outcomes.