Question about gravity
in [Physics]
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From: "Juan R." González-Álvarez on 15 Mar 2010 07:38 MicroTech wrote on Sun, 14 Mar 2010 23:42:09 -0700: > Can someone in this forum please help me sort out a confusing issue? > > Many scientists (including Einstein) claim that gravity is not a force, > but the effect of mass on the "fabric of spacetime". Many other > scientists refer to gravity as one of the four fundamental interactions > (three, if one considers the unification of the weak and electromagnetic > interactions, the "electroweak" force). > > Adding to the confusion, some scientists use both concepts with no > apparent difficulty: > Stephen Hawking (in his "A Brief History Of Time") first says that > gravity is not a force, but "simply" the effect of mass on the > "spacetime fabric" (making it "curve"). However, later in the book, he > refers to gravity as a fundamental force, carried by the graviton. Unfortunately, those are two models of gravity often confounded. The first model is a geo(metric) model named GR. The second is a field model named field theory of gravity. People so smart as Feynman or Weinberg confounded both. Einstein also. The source of the confusion is based in a pair of incorrect demonstrations about the identity between both formulations. Thus textbooks will say you that general relativity can be also understood as a field theory for a spin-2 field. The quanta for that field is the hypotetical graviton. Textbooks are wrong. Those demonstrations were flawed as showed in recent papers. Example see Field Theory of Gravitation: Desire and Reality 1999: arXiv:gr-qc/9912003v1. Baryshev, Yurij, V. See also the next analysis done in "From Gravitons to Gravity: Myths and Reality." http://www.worldscinet.com/ijmpd/17/1703n04/S0218271808012085.html The difference between metric and field theories is analized in the section 14 of the next report http://www.canonicalscience.org/publications/canonicalsciencereports/20092.html > So what is gravity, "really"? Does anybody really know? Or do we just > know its effects? It depends. In his more fundamental formulation, Gravity is an action at a distance (AAAD) interaction. See Classical Relativistic Many-Body Dynamics 1999: Springer. Trump, Matthew A.; Schieve, William C. The Landscape of Theoretical Physics: A Global View; From Point Particles to the Brane World and Beyond, in Search of a Unifying Principle 2006: arXiv:gr-qc/0610061v2. Pavsic, Matej. and specially http://www.canonicalscience.org/publications/canonicalsciencereports/20092.html In a local limit, gravity can be approximated by a relativistic field theory, using gravitons. The field potentials h_ab(r,t) are obtained from the more general AAAD potentials h_ab(R(t)) in the limit. Gravitons are the quanta of those fields. From the field formulation we can next obtain the metric formulation of general relativity as another approximation. This is when the effective metric g_ab = n_ab + h_ab is used and EMT are retained only for energy-matter fields. > Is it an attractive force "mutually pulling" the Earth towards the Sun > (and vice versa), "causing" the Earth to "fall" towards the Sun? And due > to the "forward motion" of the Earth, exactly matching the > "gravitational pull", it stays in orbit (just like any other satellite, > man-made or not); OR In the AAAD picture, this is right. > Is it the mass of the Sun that "curves spacetime", so no force is > interacting with the Earth, it is just moving in a "straight line" along > a "curved spacetime" geodesic? In the metric formulation this is right picture. For the system Earth-Sun the differences are negigible and you can solve problems using both pictures. > At my current level of understanding, gravity should be one or the > other, and not both... No. Phenomena can be explained using different models and pictures. E.g. geometric optics is an approximation obtained from physical optics. However, in there exist a limit where both formalisms are indistinguisable and you can use either. You can also re-geometrise GR and obtain another kind of metric theory, named torsion theory, where gravitational phenomena is not due to spacetime curvature but due to spacetime torsion. > If Einstein's concept of "curved spacetime" is "correct," where does the > (hypothetical?) "graviton" (and/or "gravitino") enter the picture? Theories are not correct or incorrect, but applicable or inaplicable. Nowhere in GR enters the graviton, really, if one is rigorous. In GR, gravity is not due to the existence of gravitational fields but due to spacetime curvature. In GR gravity is not a real force, because a change of coordinates eliminates the deviation from flatness. Thus terms as phi(r,t) are not physical potentials but metric potentials. And partial derivatives of those metric potentials are not physical. This explains why, in GR, gravity cannot be considered a force. @phi/@r /= Force In other theories gravity is, however a real force, which is unified with the other interactions. Details and more info given in http://www.canonicalscience.org/publications/canonicalsciencereports/20092.html -- http://www.canonicalscience.org/ BLOG: http://www.canonicalscience.org/publications/canonicalsciencetoday/canonicalsciencetoday.html
From: Sam Wormley on 15 Mar 2010 08:56 On 3/15/10 1:42 AM, MicroTech wrote: > Can someone in this forum please help me sort out a confusing issue? > > Many scientists (including Einstein) claim that gravity is not a > force, but the effect of mass on the "fabric of spacetime". Many other > scientists refer to gravity as one of the four fundamental > interactions (three, if one considers the unification of the weak and > electromagnetic interactions, the "electroweak" force). > Can you accept there is more than one way to look an a specific phenomenon?
From: Huang on 15 Mar 2010 09:16 > > Can someone in this forum please help me sort out a confusing issue? > > > Many scientists (including Einstein) claim that gravity is not a > > force, but the effect of mass on the "fabric of spacetime". Many other > > scientists refer to gravity as one of the four fundamental > > interactions (three, if one considers the unification of the weak and > > electromagnetic interactions, the "electroweak" force). Einstein is correct, that gravity can be regarded as curvature or deformation of space. Other views are equivalent to this model, such as the view that gravity is a force. These two approaches are equivalent in my opinion. The pieces which are missing in this puzzle: [a] Explaining strong force in terms of spacetime curvature, [b] Explaining weak force in terms of spacetime curvature, [c] Explaining electromagnetism in terms of spacetime curvature. When a,b,c have been explained in terms of spacetime deformation, and are shown to be equivalent to the particle explanations of a,b,c, then you will be very close to grand unification.
From: kenseto on 15 Mar 2010 10:04 On Mar 15, 6:43 am, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au> wrote: > Not a whole lot to add to what Inertial in particular said. > > In GR, gravity is a virtual force in a similar way to centrifugal force in > Newton. In both cases its really an acceleration, and the force is just the > product (literally) of this acceleration and the mass of the object. > > Einstein in GR gave a geometric interpretation of what gravity is. This is > very appealing, because it provides a mechanism for force at a distance. Wrong it provides no such physical mechanism. It merely assumes the existence of a physical entity caLLED the fabric of spacetime for the interacting object to follow. The problem with such assumption is: What is the fabric of spacetime physically? This question is relevant because SR/GR deny the existence of physical space. Ken Seto > I > might point out that the central concept of Newton and all other models of > gravity is really acceleration not force, and acceleration is the second > derivative of position, so this suggests a self-contained geometric > interpretation. > > GR AFAIK gives no explanation for EM action at a distance. > > I'm not very knowledgeable about QM, but as I understand it the mechanisms > for force exchange are different, specifically the exchange of virtual > particles for which their are real equivalents. Charged particles > repel/attract through the exchange of virtual photons. This actually > produces the same results - you can consider charge to be fundamental and > photons the derived concept, or have photons as fundamental and derive > charge. This model works also for the weak and strong forces. > > So by analogy if QM is to explain gravity, it must do so through the > exchange of particles, hence gravitons. > > "MicroTech" <henry.ko.nor...(a)gmail.com> wrote in message > > news:3a9b00f7-065e-42a2-b6f2-769bb9d63582(a)s36g2000prh.googlegroups.com... > > > > > Can someone in this forum please help me sort out a confusing issue? > > > Many scientists (including Einstein) claim that gravity is not a > > force, but the effect of mass on the "fabric of spacetime". Many other > > scientists refer to gravity as one of the four fundamental > > interactions (three, if one considers the unification of the weak and > > electromagnetic interactions, the "electroweak" force). > > > Adding to the confusion, some scientists use both concepts with no > > apparent difficulty: > > Stephen Hawking (in his "A Brief History Of Time") first says that > > gravity is not a force, but "simply" the effect of mass on the > > "spacetime fabric" (making it "curve"). However, later in the book, he > > refers to gravity as a fundamental force, carried by the graviton. > > > So what is gravity, "really"? Does anybody really know? Or do we just > > know its effects? > > > Is it an attractive force "mutually pulling" the Earth towards the Sun > > (and vice versa), "causing" the Earth to "fall" towards the Sun? And > > due to the "forward motion" of the Earth, exactly matching the > > "gravitational pull", it stays in orbit (just like any other > > satellite, man-made or not); OR > > > Is it the mass of the Sun that "curves spacetime", so no force is > > interacting with the Earth, it is just moving in a "straight line" > > along a "curved spacetime" geodesic? > > > At my current level of understanding, gravity should be one or the > > other, and not both... > > > If Einstein's concept of "curved spacetime" is "correct," where does > > the (hypothetical?) "graviton" (and/or "gravitino") enter the picture? > > > References to published papers (accessible online) would be much > > appreciated! > > > Henry Norman- Hide quoted text - > > - Show quoted text -
From: dlzc on 15 Mar 2010 10:08
Dear MicroTech: On Mar 14, 11:42 pm, MicroTech <henry.ko.nor...(a)gmail.com> wrote: > Can someone in this forum please help me sort > out a confusing issue? As usual, you have received some pearls and some jokers so far. Let's just add that with "reasonable" simplifications, Einstein's Relativity (gravity =/= force) reduces to Newton's gravitation (gravity = force). > References to published papers (accessible > online) would be much appreciated! Everybody seemed to ignore this request. You aren't going to find much for the layman on the internet, that isn't going to require more study. But we can get the easy stuff out of the way: http://www.physics.adelaide.edu.au/~dkoks/Faq/ .... down to the general General Relativity section .... and the recommended books If you are OK with math, I'd recommend this: http://www.motionmountain.net/contents.html .... down to relativity And John Baez does a pretty good job (not for the faint of heart): http://math.ucr.edu/home/baez/gr/ Good luck, and good hunting! David A. Smith |