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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 ?
Amaterasu21 said:General relativity doesn't replace gravitational force, it explains the origin of that force as curved space-time.
Amaterasu21 said: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"ronric said: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?
ronric said: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?
Drakkith said: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
PeterDonis said: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.
Amaterasu21 said: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."
Amaterasu21 said: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.Drakkith said: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.
GrrrrrrDrakkith said:Thanks, Peter. I actually meant to explain some of that, but somehow forgot all about it. Can I blame Phinds somehow?
phinds said:Grrrrrr
I eat vets for breakfast and !Drakkith said: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.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.DaleSpam said: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.
Wes Tausend said:The modified definition seems ad hoc to me
All definitions are ad hoc.Wes Tausend said: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.Wes Tausend said: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?harrylin said:Ronrit referred to the formulation of "Einstein's theory", which on face value means the theory as originally formulated by Einstein.
Wes Tausend said:In Newtonian physics, the word "acceleration" means what, in GR, is called "coordinate acceleration" (with the intended implication that it is not "really" acceleration).
Wes Tausend said: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).DaleSpam said:Who is Ronrit? [..]
stevendaryl said: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.
stevendaryl said: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.DaleSpam said: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.
Wes Tausend said:It seems, with the right coordinate system, we could basically eventually get rid of at least one separate fundamental force, that of gravity
MattRob said:If I recall correctly, I think the Newtonian approximations are so precise that space agencies use them for interplanetary missions. Funny story; one big kicker that convinced me to switch my major from Mechanical Engineering (planning to go into astronautical) to Physics-Astronomy was when I learned that I wouldn't be formally studying General Relativity as an Engineering major.
PeterDonis said:If you're talking about "fundamental forces", as in strong, weak, EM, and gravity, that's really a quantum field theory concept, not a GR concept. "Interaction" is a better term in this context, since not all of the manifestations of these things, from a QFT perspective, look like "forces" in the usual layman's sense. In QFT terms, gravity is believed to be an interaction just like the others, with an exchange particle, the graviton, just like the others (gluon, W and Z bosons, and photon). The property of gravity that singles it out from a GR perspective (that an object moving solely under gravity feels zero proper acceleration) is, from a QFT perspective, just a consequence of the fact that the graviton is spin-2 while the other exchange particles are spin-1. None of this depends on your choice of coordinates...
Wes Tausend said:It is my student layman's understanding that Einstein once sought to unify three of these quantum forces with GR (tidal?) gravity (aka Unified Field Theory) in the past.
Wes Tausend said:It was also my understanding that the strong, weak and EM have been more recently(?) associated (reconciled) in QM, but that gravity alone yet remained elusive to this united family.
Wes Tausend said:So now I see that gravitons have a spin-2. Does this directly affect GR vs QM?
Wes Tausend said:It also seemed to me that Einstein's so-named United Field Theory is identical to the Theory of Everything that Steven Hawking still seeks.
Wes Tausend said:from what you have just stated, all four fundamental forces appear already entirely reconciled by QM