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Thread: is gravity a force?

  1. #1 is gravity a force? 
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    every article i read or show i watch on the science channel describes gravity differently. some question whether its a force at all...where free falling objects are travelling through curved space at constant velocity and the people standing on the earth are the ones who are accelerating as the surface of the earth applies a force to us. i know no one has discovered the graviton yet. and people question why the force is so much weaker than the others. i have to wonder...do we know for sure what gravity is...or is it still theory? thanks.


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    You more or less cover the bases with your question. Gravity ( General Relativity) and how it relates to Quantum theory is THE big question in physics. Dark energy has thrown a wrench into everything we think we know. Nobody knows with any certainty what gravity is...it can be measured but that is not the same as knowing what it is.

    I find that lectures on Youtube are quite informative for getting a understanding of where 'gravity' is in current physics. Any theory that defines gravity to the satisfaction of cosmologists, particle physicists, string theorists, etc. will earn a Nobel Prize with a cherry on top.


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    Quote Originally Posted by mrjc99 View Post
    i have to wonder...do we know for sure what gravity is...or is it still theory?
    In Newtonian physics, gravity is a force. This is good enough for many applications.

    In general relativity, gravity is the curvature of space-time.

    These are both theories. GR is a more accurate theory than Newton.
    ei incumbit probatio qui dicit, non qui negat
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    It is more accurate to state that Gravity is not a force, but rather the curvature in space-time. The curvature created by Gravity creates the action-reaction phenomenon when there is an external force applied such as the e-m force. Inertia (or pseudo Gravity) is experienced when the the acceleration rate created by e.g., e-m force is different than the free fall rate. This is the very basis of the Equivalence Principle.
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    Quote Originally Posted by mrjc99 View Post
    every article i read or show i watch on the science channel describes gravity differently. some question whether its a force at all...where free falling objects are travelling through curved space at constant velocity and the people standing on the earth are the ones who are accelerating as the surface of the earth applies a force to us. i know no one has discovered the graviton yet. and people question why the force is so much weaker than the others. i have to wonder...do we know for sure what gravity is...or is it still theory? thanks.
    It's a good question. It brings a lot of queries with it in relation to what is a force?, can the notion of a 'force' be expressed in different ways? and at a deep level, are there really any 'forces' at all?

    At the current time, our best theory about gravity is described through general relativity where as Strange has mentioned, gravity 'IS' spacetime curvature. The question then for understanding gravity is what causes spacetime curvature, and that is where the research activity is at today. We know that the mass-energy content of a defined region under investigation is essential to how spacetime 'curves'. The Einstein equation (though complicated) does just that. You define a region of investigation and input the mass-energy distribution and on the other side of the equation, the spacetime curvature expression (or geometry) related to that mass-energy density will be described. You may have heard the 'Wheeler' expression of "mass tells spacetime how to curve and spacetime tells mass how to move". This is effectively where things curently stand in understanding gravity but active research is seeking to unify these concepts in search for an underlying cause.

    In trying to move forward in our understanding you need to view the region under which this curvature is expressed (eg. a gravitational field) and be aware of the context within which your 'test' particle is being observed. Where 'forces' are needed is when you simply narrowly focus on an individual particles properties such as it's mass or charge etc. in an inert background and account for it's mechanics in isolation or in comparison to other seperate 'things'. When viewed from this perspective you need to account for 'invisible things' that exist between the test particles to explain their behaviour. No mention is made of the background as this is assumed to be an inert frame of reference to simply give co-ordinates to measure these things. These 'invisible things' then get categorized as forces in our mindset as what happened in Newton's day. For example the way iron filings are oriented in a magnetic field gives the notion that something 'invisible' is actually doing this. We give a name to this force as electromagnetism.

    An alternate and perhaps more robust way of looking at it however is to give a defined region of spacetime actual properties and describe these spacetime properties as a field. That way you can then place a myriad of test particles into this framework and accurately predict their mechanics. This is far easier that attributing properties to each particle in the experiment and then explaining the mechanics between them. This is where the 'brilliance' of Einstein emerged. Lot's of folk pay trivial respect to Einstein in a sweeping statement that he just extended Newtons Mechanics, but IMO he truly possessed a brilliance in his insight. I get goosebumps just thinking about that incredible mind in action. Yes, others paved the way for Einstein such as Leibnitz, Galileo, Lorentz and Mach.....but he was exceptional in my opinion.

    In Einstein's GR, his fully fledged theory of mechanics was used to predict the mechanics of 'things' in the presence of a gravitational field and which furthermore incorporates the electrodynamics of moving bodies (SR) into its framework. The context within which GR is based are the properties of spacetime itself which influences the way 'things' can move through it. In turn 'some of those things' (such as mass) affect the shape of spacetime itself which allow spacetime to expand or contract (curve) in a rhythmic relativistic counterplay. Other things such as charge can move freely through it at c where they are just bound to follow the simplest route, namely the curvature. There are two forms of movement here, the things moving in spacetime and the movement of spacetime itself. This dynamic media is called spacetime or 'the field'. What this is saying is that a volume of spacetime actually has properties which are different to Newtons notion of an inert background and is further different to the notion of 'nothing' or no spacetime at all.

    If we can disregard an inert background and treat spacetime as having properties then the concept of notions such as the 'equivalence principle' can be more easily explained and it avoids any potential misconceptions such as the idea that acceleration might actually 'be' gravity as opposed to simply a special 'local' condition where uniform acceleration of a test particle in the absence of a gravitational field is simply an equivalent condition to an 'inertial' test particle in the presence of a gravitational field. The uniform acceleration is a key concept here as opposed to different non-uniform types of acceleration. The uniform acceleration allows for an equivalence to be drawn between gravitational mass and inertial mass as a means of representing the smooth curvature of spacetime being equivalent to a thing uniformly accelerating in the absence of a gravitational field. The 'locality' is required in the equivalence principle to correctly frame the context of the thought experiment (define a local region where this equivalence can be demonstrated through simple smooth curvature as opposed to confusing matters by choosing any arbitrary region which may have seperate gravitational influences to account for and hence more complex curvature). Once these concepts are understood you can then leap to the conclusion that free-fall and inertial motion in a gravitational field are physically equivalent. Remember the curvature of spacetime is actually affected by all mass-energy in the universe as evidenced by its long range nature.

    EDIT: Imagine you and another falling object beside you are in freefall in a gravitational field. Note that you and your falling object may exert a small gravitational field themselves but consider that this field is negligible compared to the overall gravitational field of the experiment (eg. the earth's gravitational field). You can locally 'transform' away gravity by removing the gravitational field and simply apply uniform acceleration of 1g to yourself and the falling object beside you.....no matter the actual masses of the falling objects involved (provided their own contribution to the overall gravitational field is negligible). This is different to say two different charges in the presence of an electromagnetic field. If the falling 'things' were charges, they would fall more or less quickly than each other dependent on the amount of charge held in each thing. Furthermore there is no way that you could transform away these effects by choosing an acceleration reference frame for both things. However, gravity affects all masses the same way. This is where the intellectual leap occurred to Einstein....that gravity does not depend on the 'properties' of matter and there is no force involved. Rather gravity is an actual property of spacetime itself, namely its curvature. An alternative way to look at this is as an observer in freefall, you experience no forces. Under uniform acceleration, you as an observer feel a force. If a force can disappear because the point of view of the observer changes it has no objective reality and it is not a real force.

    Given that GR gives physical substance to spacetime by virtue of the properties attributed to it (warning.......note that I am not implying a traditional aether here), you can then clearly see why all things under inertial motion follow the simplest paths (geodesics) in this complex curved spacetime manifold. When looking at the entirety of the system under investigation, mechanics is best described through Langrangrian methods as opposed to Newtonian Mechanics. Without this context, you need a notion of force to describe the mechanics (pushes and pulls) going on.

    ..anyway, enough of the rambling.....but where things get exciting is when at the level of QM we theoretically surmise that things 'pop' into existence from the surrounding field inder an accelerating reference frame such as the Unruh Effect. Quite clearly this demonstrates the relativistic physicality of the spacetime medium.
    Last edited by Implicate Order; December 22nd, 2013 at 11:39 AM. Reason: Lot's of changes to avoid traditional non-relativistic aether arguments
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