Question about gravity
in [Physics]
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From: Bill Hobba on 15 Mar 2010 22:23 On 15/03/2010 4:42 PM, 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). It is just terminology to say it is a fundamental force. That's how it shows its effects but as far as we can tell today it is really space-time curvature. > > 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? As far as we can tell today it is space-time curvature. > > 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? Actually it is a bit more complicated than either view. In a rather interesting addition to general relativity Einstein did with some other coworkers whose names escape me off the top of my head he showed it is actually only space-time curvature - that's all that is really required. The equations that govern that fully determine an objects motion. The idea particles follow 'straight lines' is not required. That they do is an excellent approximation to what really happens but evidently exceptions do exist. An intreresting consequence of this is that particles follow 'straight lines' in curved space time is really a generalization of the principle of inertia that particles left to themselves either remain at rest of move at constant velocity. So space-time curvature with nothing else added explains inertia - neat hey? > > 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? It comes when you try to combine gravity and quantum mechanics. No one has been able to do it successfully valid to all energy levels. However if you impose a cutoff of energy at about the plank scale (a very very large amount of energy well beyond anything we can even get close to toady) then its actually a pretty good theory. Check out: http://arxiv.org/abs/grqc/9512024 Actually all our most powerful quantum theories such as QED only really make sense if we impose some kind of cutoff to avoid infinities. Its just they are what is called renormalizeable, which basically means all observable quantities don't actually depend on that cutoff. But really to make sense of such theories and sweep the infinities that tend to plague them under the rug you need to (as Zee puts it in his excellent book QFT in a Nutshell) cutoff your ignorance. The thing with gravity is its not like that - it is not renormalizeable - observable stuff does depend on the cutoff chosen. This is not good. > > References to published papers (accessible online) would be much > appreciated! > > Henry Norman If you want to delve into the technicalities check out: http://preposterousuniverse.com/grnotes/ Thanks Bill
From: mpc755 on 15 Mar 2010 22:29 On Mar 15, 10:23 pm, Bill Hobba <bho...(a)yahoo.com> wrote: > On 15/03/2010 4:42 PM, 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). > > It is just terminology to say it is a fundamental force. That's how it > shows its effects but as far as we can tell today it is really > space-time curvature. > > > > > 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? > > As far as we can tell today it is space-time curvature. > > > > > 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? > > Actually it is a bit more complicated than either view. In a rather > interesting addition to general relativity Einstein did with some other > coworkers whose names escape me off the top of my head he showed it is > actually only space-time curvature - that's all that is really required. > The equations that govern that fully determine an objects motion. The > idea particles follow 'straight lines' is not required. That they do is > an excellent approximation to what really happens but evidently > exceptions do exist. An intreresting consequence of this is that > particles follow 'straight lines' in curved space time is really a > generalization of the principle of inertia that particles left to > themselves either remain at rest of move at constant velocity. So > space-time curvature with nothing else added explains inertia - neat hey? > > > > > 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? > > It comes when you try to combine gravity and quantum mechanics. No one > has been able to do it successfully valid to all energy levels. However > if you impose a cutoff of energy at about the plank scale (a very very > large amount of energy well beyond anything we can even get close to > toady) then its actually a pretty good theory. Check out:http://arxiv.org/abs/grqc/9512024 > > Actually all our most powerful quantum theories such as QED only really > make sense if we impose some kind of cutoff to avoid infinities. Its > just they are what is called renormalizeable, which basically means all > observable quantities don't actually depend on that cutoff. But really > to make sense of such theories and sweep the infinities that tend to > plague them under the rug you need to (as Zee puts it in his excellent > book QFT in a Nutshell) cutoff your ignorance. The thing with gravity > is its not like that - it is not renormalizeable - observable stuff does > depend on the cutoff chosen. This is not good. > > > > > References to published papers (accessible online) would be much > > appreciated! > > > Henry Norman > > If you want to delve into the technicalities check out:http://preposterousuniverse.com/grnotes/ > > Thanks > Bill Or the pressure associated with the aether displaced by massive objects is gravity.
From: Bill Hobba on 15 Mar 2010 22:43 On 16/03/2010 12:23 PM, Bill Hobba wrote: > On 15/03/2010 4:42 PM, 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). > > It is just terminology to say it is a fundamental force. That's how it > shows its effects but as far as we can tell today it is really > space-time curvature. > >> >> 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? > > As far as we can tell today it is space-time curvature. > >> >> 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? > > Actually it is a bit more complicated than either view. In a rather > interesting addition to general relativity Einstein did with some other > coworkers whose names escape me off the top of my head he showed it is > actually only space-time curvature - that's all that is really required. > The equations that govern that fully determine an objects motion. The > idea particles follow 'straight lines' is not required. That they do is > an excellent approximation to what really happens but evidently > exceptions do exist. An intreresting consequence of this is that > particles follow 'straight lines' in curved space time is really a > generalization of the principle of inertia that particles left to > themselves either remain at rest of move at constant velocity. So > space-time curvature with nothing else added explains inertia - neat hey? > >> >> 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? > > It comes when you try to combine gravity and quantum mechanics. No one > has been able to do it successfully valid to all energy levels. However > if you impose a cutoff of energy at about the plank scale (a very very > large amount of energy well beyond anything we can even get close to > toady) then its actually a pretty good theory. Check out: > http://arxiv.org/abs/grqc/9512024 > > Actually all our most powerful quantum theories such as QED only really > make sense if we impose some kind of cutoff to avoid infinities. Its > just they are what is called renormalizeable, which basically means all > observable quantities don't actually depend on that cutoff. But really > to make sense of such theories and sweep the infinities that tend to > plague them under the rug you need to (as Zee puts it in his excellent > book QFT in a Nutshell) cutoff your ignorance. The thing with gravity is > its not like that - it is not renormalizeable - observable stuff does > depend on the cutoff chosen. This is not good. > >> >> References to published papers (accessible online) would be much >> appreciated! >> >> Henry Norman > > If you want to delve into the technicalities check out: > http://preposterousuniverse.com/grnotes/ > > Thanks > Bill That the field equations all by themselves imply how particles move is a very startling characteristic of General Relativity. Normally field theories have equations that govern how fields behave and how objects respond to those fields. That GR is not like that is rather interesting. Because of that I thought I would dig up a bit more about it. Check out: http://www.mathpages.com/rr/s5-08/5-08.htm 'Once the field equations have been solved and the metric coefficients have been determined, we then compute the paths of objects by means of the equations of motion. It was originally taken as an axiom that the equations of motion are the geodesic equations of the manifold, but in a series of papers from 1927 to 1949 Einstein and others showed that if particles are treated as singularities in the field, then they must propagate along geodesic paths. Therefore, it is not necessary to make an independent assumption about the equations of motion. This is one of the most remarkable features of Einstein's field equations, and is only possible because of the non-linear nature of the equations. Of course, the hypothesis that particles can be treated as field singularities may seem no more intuitively obvious than the geodesic hypothesis itself. Indeed Einstein himself was usually very opposed to admitting any singularities, so it is somewhat ironic that he took this approach to deriving the equations of motion. On the other hand, in 1939 Fock showed that the field equations imply geodesic paths for any sufficiently small bodies with negligible self-gravity, not treating them as singularities in the field. This approach also suggests that more massive bodies would deviate from geodesics, and it relies on representing matter by the stress-energy tensor, which Einstein always viewed with suspicion.' Thanks Bill
From: mpc755 on 15 Mar 2010 23:40 On Mar 15, 10:43 pm, Bill Hobba <bho...(a)yahoo.com> wrote: > On 16/03/2010 12:23 PM, Bill Hobba wrote: > > > > > On 15/03/2010 4:42 PM, 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). > > > It is just terminology to say it is a fundamental force. That's how it > > shows its effects but as far as we can tell today it is really > > space-time curvature. > > >> 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? > > > As far as we can tell today it is space-time curvature. > > >> 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? > > > Actually it is a bit more complicated than either view. In a rather > > interesting addition to general relativity Einstein did with some other > > coworkers whose names escape me off the top of my head he showed it is > > actually only space-time curvature - that's all that is really required.. > > The equations that govern that fully determine an objects motion. The > > idea particles follow 'straight lines' is not required. That they do is > > an excellent approximation to what really happens but evidently > > exceptions do exist. An intreresting consequence of this is that > > particles follow 'straight lines' in curved space time is really a > > generalization of the principle of inertia that particles left to > > themselves either remain at rest of move at constant velocity. So > > space-time curvature with nothing else added explains inertia - neat hey? > > >> 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? > > > It comes when you try to combine gravity and quantum mechanics. No one > > has been able to do it successfully valid to all energy levels. However > > if you impose a cutoff of energy at about the plank scale (a very very > > large amount of energy well beyond anything we can even get close to > > toady) then its actually a pretty good theory. Check out: > >http://arxiv.org/abs/grqc/9512024 > > > Actually all our most powerful quantum theories such as QED only really > > make sense if we impose some kind of cutoff to avoid infinities. Its > > just they are what is called renormalizeable, which basically means all > > observable quantities don't actually depend on that cutoff. But really > > to make sense of such theories and sweep the infinities that tend to > > plague them under the rug you need to (as Zee puts it in his excellent > > book QFT in a Nutshell) cutoff your ignorance. The thing with gravity is > > its not like that - it is not renormalizeable - observable stuff does > > depend on the cutoff chosen. This is not good. > > >> References to published papers (accessible online) would be much > >> appreciated! > > >> Henry Norman > > > If you want to delve into the technicalities check out: > >http://preposterousuniverse.com/grnotes/ > > > Thanks > > Bill > > That the field equations all by themselves imply how particles move is a > very startling characteristic of General Relativity. Normally field > theories have equations that govern how fields behave and how objects > respond to those fields. That GR is not like that is rather > interesting. Because of that I thought I would dig up a bit more about > it. Check out:http://www.mathpages.com/rr/s5-08/5-08.htm > 'Once the field equations have been solved and the metric coefficients > have been determined, we then compute the paths of objects by means of > the equations of motion. It was originally taken as an axiom that the > equations of motion are the geodesic equations of the manifold, but in a > series of papers from 1927 to 1949 Einstein and others showed that if > particles are treated as singularities in the field, then they must > propagate along geodesic paths. Therefore, it is not necessary to make > an independent assumption about the equations of motion. This is one of > the most remarkable features of Einstein's field equations, and is only > possible because of the non-linear nature of the equations. Of course, > the hypothesis that particles can be treated as field singularities may > seem no more intuitively obvious than the geodesic hypothesis itself. > Indeed Einstein himself was usually very opposed to admitting any > singularities, so it is somewhat ironic that he took this approach to > deriving the equations of motion. On the other hand, in 1939 Fock > showed that the field equations imply geodesic paths for any > sufficiently small bodies with negligible self-gravity, not treating > them as singularities in the field. This approach also suggests that > more massive bodies would deviate from geodesics, and it relies on > representing matter by the stress-energy tensor, which Einstein always > viewed with suspicion.' > > Thanks > Bill Or the pressure associated with the aether displaced by massive objects is gravity.
From: BURT on 16 Mar 2010 00:02
On Mar 15, 8:40 pm, mpc755 <mpc...(a)gmail.com> wrote: > On Mar 15, 10:43 pm, Bill Hobba <bho...(a)yahoo.com> wrote: > > > > > > > On 16/03/2010 12:23 PM, Bill Hobba wrote: > > > > On 15/03/2010 4:42 PM, 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). > > > > It is just terminology to say it is a fundamental force. That's how it > > > shows its effects but as far as we can tell today it is really > > > space-time curvature. > > > >> 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? > > > > As far as we can tell today it is space-time curvature. > > > >> 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? > > > > Actually it is a bit more complicated than either view. In a rather > > > interesting addition to general relativity Einstein did with some other > > > coworkers whose names escape me off the top of my head he showed it is > > > actually only space-time curvature - that's all that is really required. > > > The equations that govern that fully determine an objects motion. The > > > idea particles follow 'straight lines' is not required. That they do is > > > an excellent approximation to what really happens but evidently > > > exceptions do exist. An intreresting consequence of this is that > > > particles follow 'straight lines' in curved space time is really a > > > generalization of the principle of inertia that particles left to > > > themselves either remain at rest of move at constant velocity. So > > > space-time curvature with nothing else added explains inertia - neat hey? > > > >> 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? > > > > It comes when you try to combine gravity and quantum mechanics. No one > > > has been able to do it successfully valid to all energy levels. However > > > if you impose a cutoff of energy at about the plank scale (a very very > > > large amount of energy well beyond anything we can even get close to > > > toady) then its actually a pretty good theory. Check out: > > >http://arxiv.org/abs/grqc/9512024 > > > > Actually all our most powerful quantum theories such as QED only really > > > make sense if we impose some kind of cutoff to avoid infinities. Its > > > just they are what is called renormalizeable, which basically means all > > > observable quantities don't actually depend on that cutoff. But really > > > to make sense of such theories and sweep the infinities that tend to > > > plague them under the rug you need to (as Zee puts it in his excellent > > > book QFT in a Nutshell) cutoff your ignorance. The thing with gravity is > > > its not like that - it is not renormalizeable - observable stuff does > > > depend on the cutoff chosen. This is not good. > > > >> References to published papers (accessible online) would be much > > >> appreciated! > > > >> Henry Norman > > > > If you want to delve into the technicalities check out: > > >http://preposterousuniverse.com/grnotes/ > > > > Thanks > > > Bill > > > That the field equations all by themselves imply how particles move is a > > very startling characteristic of General Relativity. Normally field > > theories have equations that govern how fields behave and how objects > > respond to those fields. That GR is not like that is rather > > interesting. Because of that I thought I would dig up a bit more about > > it. Check out:http://www.mathpages.com/rr/s5-08/5-08.htm > > 'Once the field equations have been solved and the metric coefficients > > have been determined, we then compute the paths of objects by means of > > the equations of motion. It was originally taken as an axiom that the > > equations of motion are the geodesic equations of the manifold, but in a > > series of papers from 1927 to 1949 Einstein and others showed that if > > particles are treated as singularities in the field, then they must > > propagate along geodesic paths. Therefore, it is not necessary to make > > an independent assumption about the equations of motion. This is one of > > the most remarkable features of Einstein's field equations, and is only > > possible because of the non-linear nature of the equations. Of course, > > the hypothesis that particles can be treated as field singularities may > > seem no more intuitively obvious than the geodesic hypothesis itself. > > Indeed Einstein himself was usually very opposed to admitting any > > singularities, so it is somewhat ironic that he took this approach to > > deriving the equations of motion. On the other hand, in 1939 Fock > > showed that the field equations imply geodesic paths for any > > sufficiently small bodies with negligible self-gravity, not treating > > them as singularities in the field. This approach also suggests that > > more massive bodies would deviate from geodesics, and it relies on > > representing matter by the stress-energy tensor, which Einstein always > > viewed with suspicion.' > > > Thanks > > Bill > > Or the pressure associated with the aether displaced by massive > objects is gravity.- Hide quoted text - > > - Show quoted text - The strength of gravity pushes energy faster not the curve. Acceleration gives more motion while curve simply changes the direction the energy is flowing in. There is both a round immaterial curve; and the strength of gravity accelerating/decelerating energy. Mitch Raemsch |