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Is gravity a force or not?

  1. May 11, 2015 #1
    I read somewhere that time and gravity are interrelated which may reflect Gravity as a byproduct of time. In other words, the force we perceive as gravity could be the movement created by the differential of two points of time in space. And sorry for not citing the source - I can't remember.
     
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  3. May 11, 2015 #2

    A.T.

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    Newton modeled gravity as an interaction force. Einstein as a coordinate effect:

     
    Last edited: May 11, 2015
  4. May 11, 2015 #3

    jbriggs444

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    In general relativity, gravity is not a force. It is instead, seen as a manifestation of how four dimensional space-time is curved. Objects in free fall are viewed as falling on straight lines known as "geodesics" in a curved space-time. Objects subject to forces other than gravity deviate from these geodesic paths.

    In order to appreciate a curved space-time you should learn at least some non-Euclidean geometry. If you are after an intuitive understanding rather than a formal mathematical understanding of non-Euclidean geometry then you could do worse than the classic "Sphereland" by Edwin A. Abbot. However, that book will just give you a taste. Things get way deeper. Calling gravity a "byproduct of time" is an oversimplification of monstrous proportions.
     
  5. May 11, 2015 #4

    pervect

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    Welcome to PF.

    Time and gravity are related, the most obvious manifestation of this in my opinion is gravitational time dilation. Only gravity directly causes time dilation - other forces do not cause time dilation, except insofar as they may additionally cause gravity, which in turn causes the time dilation. Note that the time dilation is not directly related to the "force" of gravity, time dilation is approximately (in the weak field) proportional to the gravitational potential energy, which is the product of force * distance.

    I am not sure what this means, so I can't really comment sensibly on it. Gravity can be regarded as curved space-time, though. One can find a reference to this in Wiki, see for instance http://en.wikipedia.org/w/index.php?title=General_relativity&oldid=661864735. General relativity is basically a geometric theory, as the Wiki article mentions.


    One can also regard gravity as a necessary consequence of gravitational time dilation, given some other assumptions (such as what's commonly called the principle of maximal aging). This seems enough similar to what you might have meant, that it's worth mentioning, but it's hard to be sure whether or not it's what you had in mind.

    While general relativity is most commonly regarded as a geometric theory, sometimes people do regard gravity as a force, so it's not necessarily totally wrong to regard it as a force. The principle of equivalence suggests, though, that gravity and inertia are closely related. This implies the "force" of gravity is more like the fictitious force one finds in an accelerating elevator or car, i.e. "Einstein's elevator" - it's a "force" that's really just a manifestation of matter's inertial. Newtonian theory regards gravity differently - it regards it as a real force, but Newtonian theory is unable to explain why the gravitational mass should have exactly the same value as the inertial mass. The equality of gravitational and inertial mass (one form of the principle of equivalence) suggests that gravity is more closely related to a fictitious force than a real force.
     
  6. May 13, 2015 #5
    Pervect,

    Yes, gravitational time dilation. Wouldn't this dilation cause movement? In other words, the difference between two dilated times would cause movement from low to high energy toward the center of mass. If true, wouldn't this be the essence of gravity? Any charts around showing the ratio of gravity and dilated time? Curious about the extent of dilated time in deep space devoid of large masses.
     
  7. May 13, 2015 #6
    I hope my oversimplification does not offend anyone as I am not as well equipped with technical knowledge as the rest of you.
     
  8. May 13, 2015 #7
    Also, from what I understand. - if time dilation did not exist, movement through space would be impossible. Is this correct?
     
  9. May 13, 2015 #8

    PeterDonis

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    You have this backwards. Time dilation is not logically prior to motion. Time dilation is a side effect of motion (among other things). Time dilation is not a fundamental concept; it's a derived concept.
     
  10. May 14, 2015 #9
    Yes I inderstand that time dilation is not prior to motion. It coincides with motion. But without time dilation, objects in space could not move - correct? In other words, time dilation allows for movement. However, in terms of there being increased time dilation from the center of a mass, that would in effect create movement - ie gravity (right?). I'm visuslizing this, but I'm not quite sure how to explain what I'm seeing. And like you said, it is likely I am making an error in how I'm understanding this. I'm inquiring if time dilation is actually the "force" we call gravity because an object caught in different dilated times would have to move to resolve the difference.
     
  11. May 14, 2015 #10

    A.T.

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    You are confusing two different types of time dialtion:
    - Kinetic time dilation due to relative movement.
    - Gravitational time dilation between different positions, at different potentials in a gravitational field.

    Gravitational "attraction" is related to the gradient (change) of gravitational time dilation. Free falling objects always tend towards areas of slower time passage (lower potential). This is simplified in the top picture here:

    http://www.physics.ucla.edu/demoweb..._and_general_relativity/curved_spacetime.html

    It's more like a rolling axle with two wheels, that will deviate towards the side with greater rolling resistance. The slower wheel represents slower advance in time on one side.
     
    Last edited: May 14, 2015
  12. May 14, 2015 #11

    PeterDonis

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    Not always. See A.T.'s post.

    No. Once again, you have the causality backwards. Objects follow different worldlines in spacetime, and this causes differences in the elapsed proper time between selected events, which we call "time dilation". It makes no sense to ask whether objects could or could not move "without time dilation", because the concept of "time dilation" only makes sense once you have the concept of objects following different worldlines, and comparing their elapsed proper times.
     
  13. May 14, 2015 #12
    But I thought the concept of curved space time was just a model for better calculations. In other words, curved space time is a metaphor for what's actually occurring, not a direct representation. So this would make world lines a part of that model which does not explain the "essence" of gravity, but it explains the mathamatical formulations of gravity. So if we're asking "What is Gravity?", we must find where the force originates. I'm questioning if gravity (the force as described using space time calculations) is actually a "force" originating with time itself and not derived from particles or energies within mass. So what I'm visualizing is the real gravitational time dilation as the smoking gun for revealing the essence of gravity by way of an object having to resolve the time dilation it experiences within its boundaries. In other words, time is warped therefore an object must move along its path yielding what is observed as gravity.

    Ok, I'm done and I completely accept that the way I'm seeing this is probably skewed, but had to get it out. And thanks for humoring my thoughts.
     
  14. May 14, 2015 #13

    A.T.

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    All of physics is. Physics is about quantitative predictions, not about some "essence".
     
  15. May 14, 2015 #14

    PeterDonis

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    That doesn't make sense in view of this statement further down in your post:

    If time is warped, then space must be warped as well, because time and space are interrelated; you can't separate the two. So spacetime must be warped. So it looks like you believe that curved spacetime is not just "a model for better calculations" but "the way things really are"; otherwise your statement about time being warped makes no sense.

    In other words, if you agree that time is warped and you think that's what causes gravity, then you are basically agreeing with the GR model of gravity as being due to spacetime curvature.
     
  16. May 14, 2015 #15
    I know I'm contradicting myself because I lack the necessary vocabulary. So here's an article explaining that despite knowing the mathematical formulations of gravity, we really don't know what it is: http://www.universetoday.com/75705/where-does-gravity-come-from/ I think it explains what I'm trying to say from when I say essence of gravity. So I'm saying rather than gravity being a wave or a particle, it may be an artifact of time. - now after reading a few more things may be staying the obvious especially in terms of curvature of time, but does time dilation occur at the quantum level and can this account for the strange behavior of gravity? In other words time being on a giant sliding scale from quantum to galactic. And the difference between time being the unifying theory?

    You guys keep sucking me in and I have a feeling I have done no better at explaining myself. - Or I have and I'm just not making any sense.
     
  17. May 14, 2015 #16

    PeterDonis

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    Sure we do. It's spacetime curvature. But whenever you get an answer like this in any science, you can always ask another question. In this case, that next question is: how is spacetime curvature "built" out of whatever quantum theory of gravity underlies it?

    (Bear in mind that the article you linked to is a pop science article, not a scientific paper or textbook. It's never a good idea to try to actually understand science from pop science articles. Even when they're written by scientists, they end up doing more harm than good to your understanding.)

    What does "an artifact of time" mean? Based on what you've described, I think it's just another way of saying "spacetime curvature".

    At the quantum gravity level, there isn't any such thing as "spacetime"; that's the whole point. The challenge of quantum gravity is to build spacetime out of something that isn't spacetime--some underlying quantum theory that has spacetime as an emergent property. At that underlying level, there won't be any "time dilation" because there won't be any spacetime to begin with.
     
  18. May 14, 2015 #17

    pervect

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    A few comments: I'd say it's an overstatement to say that physics does or can us "what" gravity is. But good physics will tell us how gravity acts. If two different models of gravity give the same predictions, physics doesn't have any definite way of choosing which model to use. It typically doesn't make any direct difference, but it may make a difference when it comes time to expand the theory to cover more and broader situations. It's generally impossible to tell in advance which form of a theory will be the most amenable to expansion, however.

    I believe "Exploring Black Holes" would be the best source for explaining how gravity comes out of the principle of maximal aging. I don't have a copy of the book, unfortunately, to give any quotes, but there are a few tidbits on Taylor's website, http://www.eftaylor.com/leastaction.html. This is the closest I can think of to what I would guess the OP may be asking, though I suspect it's not a really great match. It's close enough to be interesting to him, perhaps, but I'm not sure if there is a good elementary treatment of it out there.

    There are some other interesting models of gravity that don't initially involve geometry at all, such as Straumann's "Reflections on gravity". Interestingly, geometry comes out in the end. I suspect the paper is too advanced for the OP, however. I'll give the link anyway - http://arxiv.org/abs/astro-ph/0006423.

    My personal view is that trying to reduce gravity to "just a force" winds winds up a lot like the fabled Procrustes, who would make people fit an iron bed by stretching their bodies in a rack, or cutting their legs down to size, rather than adjusting the size of the bed.
     
  19. May 15, 2015 #18
    Ok cool. I feel like I'm getting somewhere - which just happens to be back to space time. Without having a formal undetstanding if this, you helped me realize that this is pretty much what I was thinking all along.

    So gravity really isn't a force, it's s
     
  20. May 15, 2015 #19
    I'm still re-reading it, but is interesting. "Conceptual Foundations of Quantum Field Theory". The differences between classical and quantum field theory seems to be too much to say there is spacetime, and gravity changes it's geometry.

    Of course within the theory of General Relativity the "answer" is gravity is a geometric effect on spacetime.

    That link is the first result in a google search of "why is there a presumption of spacetime" lol

    Also this http://philsci-archive.pitt.edu/9055/, it's titled Substantivalist and Relationalist Approaches to Spacetime
     
    Last edited: May 15, 2015
  21. May 18, 2015 #20
    I can not prove my comment theoretically or experimentally. So, do not ask. But I observed some strange similarities between gravity and certain phenomena in nature.
    From those observations I'm leaning towards this conclusion: Origin of gravity may not be physical.
    Einstein's GR may be a hint. But it is not exactly what's happening in nature.
    My 2 cents.
     
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