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General Relativity, is gravity a force? 
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#1
Feb2710, 12:40 AM

P: 124

In general relativity, gravity is modeled as a curvature in spacetime. So the earth moves in a 'straight' path in a curved space. At least, this is how it has been explained to me in the past. The sun isn't actually 'pulling' on the earth, the earth is just moving around in the 'gravitational well' of the sun. So in this sense, there is no 'force' of gravity is there? It's just a dynamic of spacetime.
I don't understand how gravity can be one of the four forces of nature if it's just curved spacetime. How is gravity still a force in general relativity? 


#2
Feb2710, 02:40 AM

P: 9

gravitation is a force. space time curvature in general relativity is just a tools to explain in another way



#3
Feb2710, 02:57 AM

P: 43

in every era, human beings have different understanding of the nature.
may be we can treat gravitational force as the MOTHER of concept of curved spacetime or in literal way, curved spacetime is the "improvement" of the concept of gravitational force at least curve spacetime concept can describe the happen of the invariance of the speed of light.but not in the concept of gravitational force, of course curve spacetime concept requires more complex formulation 


#4
Feb2710, 03:33 AM

P: 4,512

General Relativity, is gravity a force?
As you guessed, gravity is not a force in general relativity.
This is a change from Newtonian physics. An object that falls toward a gravitating body in Newtonian physics is subjected to the force of gravity. In general relativity no force acts upon the object. It is said to be 'freely falling'. Beware, there is more than one way the word 'force' is used in physics. The four fundamental forces include the strong force, the weak force, the electromagnetic force and gravitational force. 


#5
Feb2710, 04:00 AM

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#6
Feb2710, 04:52 AM

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#7
Feb2710, 07:56 AM

P: 58

The acceleration of a mass towards another body is a byproduct of the curvature of spacetime.
If people argue this, then they don't understand it. Which will be common since four dimensions is one dimension bigger than our brain evolved to naturally conceptualise. For simplicity of calculations, we conform to the standardized F=mg g=F/m What you are saying it that you think gravity should be changed to units of kg.m/s/s and not the m/s/s that everyone else uses. In which case we would call it a force, and not gravity. 


#8
Feb2710, 08:35 AM

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Gravity, experimentally, is neither a force nor curvature. Experimentally it is simply a phenomena. Now, when you ask a question like "is gravity a force", you must always pose that question within a conceptual framework ('theory') that describes this phenomena. Newton's original theory of gravity is one such framework, and in that theory gravity is a force. Einstein's general relativity is another theory that describes the phenomenon, and in that theory gravity is represented in the formalism as curvature of spacetime.
Theoretical notions like 'force' are not properties that exist in nature, but ideas in a conceptual framework that is used to describe nature. For example, Newtonian mechanics can be reformulated in terms of the so called 'principle of least action', and the description of motion in that theory does not involve the idea of 'force', even though the two descriptions are completely equivalent. 


#9
Feb2710, 09:06 AM

P: 4,194

GR did not remove the "force status" from the "force of gravity". It just moved it to the "inertial forces", saying that if you are observing a "force of gravity" then you are not in an inertial frame.



#10
Feb2710, 12:46 PM

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P: 2,193

The above two posts do a good job uncovering some of the ambiguity of the original question, so I won't talk about that. What I do want to do is go into the GR description of the situation a little further. Like dx says, this is merely the interpretation within one framework.
In GR, the equation of motion for a free particle (i.e. no forces. Note: Gravity does not even exist in GR. So it makes no sense to speak of it as a force.) is the following: [tex]\frac{d^2 x^\alpha}{d \tau ^2} =  \Gamma^\alpha_{\beta \gamma} \frac{d x^\beta}{d \tau} \frac{ d x^\gamma}{d \tau} [/tex] In the case of a flat spacetime, this reduces to: [tex]\frac{d^2 x^\alpha}{d \tau ^2} =0 [/tex] Which is precisely Newton's 2nd law for a free particle. However, if we imagine our particle to be confined to the surface of a sphere, say, then it does NOT reduce to Newton's 2nd law. In this case, there are no identifiable forces (if we look at this classically, we do not even observe gravity!), and yet there is a deviation from the newtonian motion. All that changes when we introduce mass into the picture is that the [tex]\Gamma^\alpha_{\beta \gamma}[/tex] change in a predictable way. No mention of the word gravity is ever needed. This is the way I see it from GR, at least. 


#11
Feb2810, 02:57 AM

P: 58

http://www.youtube.com/watch?v=dII3mcnwycs
Good vid. 33:30 onwards talks a bit about this, and up to around 40:00, specifically the point regarding gravity and it's affect on light. (i.e if gravity were a force, it would not cause light to bend since light has zero mass) 


#12
Feb2810, 03:52 AM

P: 665

AB 


#13
Feb2810, 01:29 PM

P: 96

Would you, please, comment on how you see this phenomena. 


#14
Feb2810, 02:35 PM

Sci Advisor
P: 2,193

[tex]G_{\mu \nu} = 8 \pi T_{\mu \nu}[/tex] Where [tex]T_{\mu \nu}[/tex] is the stressenergymomentum tensor. When this is zero, the EE reduce to the vacuum Einstein equations, one solution to which is flat (minkowski) spacetime (Other solutions exist, which involve dirac delta functions. For example, a schwarzschild black hole). This is what I mean by spacetime being flat: the absense of any contribution from the stressenergymomentum tensor. In this scenario, Newton's 2nd law is followed for free particles. The introduction of anything that makes the stressenergymomentum tensor nonvanishing necessarily curves spacetime, changing the christoffel symbols, and thus the EOM for the particle. Please notice, I have not mentioned the word gravity even once. The word is not necessary in GR. Do not attempt to introduce it, and refer to the preface in my previous post. 


#15
Feb2810, 03:49 PM

P: 665

1 In GR, every mass that curves spacetime has a gravitational nature but in general the tricky word "mass" can replace the widely used 'stuff' from a physical point of view which has nothing to do with the curvature of spacetime unless we spacify whatever happens between these masses and the fabric of spacetime. You're just putting a cap on the name "gravity"! 2 The stressenergymomentum tensor describes matter (density, pressure and stress), radiation and nongravitational "force fields"! All these attributes belong to "gravitational field" (which exerts a gravitational force on the things around) in the Einstein's field equations and this field in turn may be inspired by the existence of "mass" as in the theory of Newtonian gravity. AB 


#16
Feb2810, 11:37 PM

Sci Advisor
P: 2,193

I get the feeling we're arguing over semantics, or else I'm not understanding you clearly. My point is as follows: within the theory of GR (independent from newtonian mechanics!) one never need mention a mysterious "gravity". All you need to mention is the stressenergymomentum tensor.
I've reread your post a dozen or so times and I really have a hard time understanding, I'm sorry. Could you, or perhaps someone else who understands, try and phrase it differently? 


#17
Mar110, 03:25 AM

P: 58

As others have stated, gravity does not exist in an Einsteinian world. If you are going to using gravity to explain things, and treat it as a force, then shows us some equations on how gravity affects zeromass objects. (it doesn't work) THe video I quoted shows at low speed, Newtonian mechanics is the same as Einstein Mechanics. The effect of gravity on an object with mass can be used to explain lowspeed phenomena. It also shows at the beginning (if you watch the whole video) how the Newton equations are derived from the (1+[tex]\epsilon[/tex])^{n} example 


#18
Mar110, 06:34 AM

P: 665

AB 


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