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Gravity: why, how and ehhh?

  1. Sep 3, 2011 #1
    First of all, despite my choice of title I have some understanding of it (or atleast acceptance of it's laws and effects), what trips me up is my mind asks what the various implications of our current understanding of gravity are (bad mind, sit, lie down) and I realize that my and our understanding of gravity are not one and the same, in that my understanding is anecdotal and basic.

    I usually can't get to grips with a concept at all unless 1 of the following statements are true.

    I have some grasp of the concepts that underpin a concept.
    I believe (in error) that I have grasped of the concepts that underpin a concept.

    As a result, you might imagine that my 'understanding' of physics is full of bloody great big gaping holes, around which I must reconcile specific laws/phenomena. I find that to make any progress I must work backwards, filling in the blanks and correcting bad assumptions if I am to make progress in accepting new information.

    Ok, so in that spirit I find myself trying to get my head around a few questions concerning gravity, I am probably not alone in that particular endeavour. The question that is really tripping me up which I expect is something that is very clear to the initiated is the following.

    Does a body of mass exert a force of gravity on other masses, or is the it the case that space, being warped by the presence of a massive object, itself is exerting the force. That is to say, that 2 bodies of mass in space (lets call em planets because why not) do not directly influence eachother, but do so indirectly due to the warping of space. There may not even be a distinction between the two statements.

    My second question is this, is there such a thing as an absolute position in space relative to... space? It sounds intuitive enough to say yes, but now and then I read or hear statements to the contrary or atleast to say that the question is inherently meaningless. I accept what I know of relativity as a given, but there is so much I have not understood ie the speed of light as a 'speed limit' I imagine to mean speed relative to absolute space, but again I am often having cause to question my interpretation.

    At this point I will leave it at that, the more I write the more I confuse myself and this post has the potential to end up being an indecipherable mess of words, halfway down I had to struggle to remember what it was I actually wanted to ask. I will close by saying this, I accept the possibility that some of my questions may border on the philosophical, but I am hoping that science has a clear stance on them. Thanks, and apologies for the wordcount it would seem I enjoy hearing myself type!
     
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  3. Sep 3, 2011 #2
    Much of what you ask, if not all of it, is explained at Wikipedia. The contemporary scientific description of gravitation is based on Einstein's Theory of General Relativity and field equations. He didn't know that the Universe was expanding (thought it was static), but he did recognize that objects which possess mass somehow bend the thing called, spacetime. The flea and the supermassive black hole each bend this fabric. However, the amount of curvature introduced by the supermassive black hole swamps the little divot introduced by the flea's mass. So, although everything that is "heavier" than light creates a gravitational field, it is not a directly radiating force. It is an indirect force as it is created by the curvature that is created by the mass.

    As the great Wiki explains, it may be illuminating to consider that all objects which tend to fall to the Earth's surface, due to its gravitational spacetime curvature, are accelerated at the same rate (to the ground), regardless of their individual mass, once the drag created by wind resistance is eliminated (such as by conducting the test in a vacuum). However, if you were to hold a planet that was twice as massive as the Earth in your hand and let go, the ground (of Earth) would tend to rise up to the more massive planet's surface. They'd each fall together somewhat, but the king of the mountain (the one who moves less) would be the one (not with the larger spherical diameter, but the one with) the greater mass (e.g., infinitessimal supermassive black hole).



    -Mesmer8
     
  4. Sep 3, 2011 #3
    The concept of both bodies "falling towards each other" is not intuitive at all, when was the last time the Earth came up to greet the ball instead of the ball falling to the ground? But that's only because the Earth is far more massive.
     
  5. Sep 3, 2011 #4
    It's my own fault for bubble wrapping my question with a whole lot of rambling.

    My specific questions were:
    Is gravity exerted via a force or is the bending of space alone what causes gravitational attraction? (The suggestion of a graviton tends to suggest the former, for example and this is something I cannot reconcile)
    Is there such a thing as an absolute point in space, or must all points in space be considered as relative to a body of mass?
     
  6. Sep 3, 2011 #5
    First question: This is something that's currently being investigated in modern physics. There is no concrete answer as to WHY objects are attracted to eachother through gravitation. We have the theories (Einstein's for example) that attempt to explain the mechanism (warping of spacetime etc) but we don't know WHY this is true, WHY is it that things with mass warp spacetime? It isn't a question we can answer right now with 100% certainty.

    Second: No, there is no absolute reference point. Everything is relative.
     
  7. Sep 3, 2011 #6

    phinds

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    I agree w/ this statement but I don't think it quite addresses the spirit of the OP's post which, I believe, really boils down to the underlying issue you address with a simple statement. What I mean is, I think once he accepts your answer, it still doesn't QUITE answer his question, which morphs into "yes, but why can't you consider a point in space to be the thing that the object is referenced to."

    Diezehl, my apologies for putting words in your mouth. My answer to MY restatement of your question (or extension of it) is that it's a good question but seems, if I understand things correctly, to boil down to the fundamental fact that you can't define a point in space except as it relates to an object and thus you can't use a "point in space" as the reference point because it leads to a circular definition. We only define the position of an object as being relative to another OBJECT, not to something that itself would have to be referenced either to another object or back to the original object. I realize it's also possible that all this became clear to you with Clever-Name's statement but I thought I'd throw in my rambling just in case.
     
    Last edited: Sep 3, 2011
  8. Sep 3, 2011 #7
    Aha glad to see I am not alone in my frustration!

    As for no absolute points in space, I get it, I do, but that being the case I find I am struggling as a result.

    If I am traveling at half the speed of light, and shine a torch infront of me, the light does not travel 1.5x the speed of light, it travels at the speed of light (this is what I have read time and time again unless I am mistaken) is this assertion based purely on the presence of multiple observers? What does this statement actually mean?

    I get that the idea of accelerating mass beyond the speed of light is impossible due to requiring infinite energy, but is the speed considered relative from my starting point?

    It's possible most of my difficulty is bourne of a futile need to visualise everything, but in this case I have gone around nodding my head at everything general and special relativity says without actually checking if I have gleaned the true meaning. I am feeling as though I just missed something important.

    edit> @ phinds, thankyou for clearing that up, I was confused as to whether science insists that there are no absolute points in space or if it is simply the case that we do not define absolute points in space for the reasons you mentioned. I found the former possibility hard to reconcile, and I am glad that is not the case.
     
    Last edited: Sep 3, 2011
  9. Sep 3, 2011 #8

    phinds

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    Light travels at the "universal speed limit" and this is true for all frames of reference. A light beam travels at the same speed regardless of the frame of reference of the emitting object. By the way, I was interested to learn, after I joined this forum, that the "speed of light" is NOT based on just light, it is, as I quoted above, the "universal speed limit" and applies to all massless objects. Light is just weird this way (well, I should say "massless objects are ... ")

    As stated above, the speed of light (that is, the "universal speed limit") has no reference that is any any way limited to YOUR frame of reference and you shouldn't think of it that way. I can't give a very good answer to the first sentence about mass; that may be the reason or it may be more fundamental, or we may just not know. *I* don't know, anyway.

    Uh, dude, in that case do NOT get into quantum mechanics. It will make your head explode.
     
  10. Sep 3, 2011 #9
    Interestingly enough some concepts from quantum mechanics have proved slightly less harsh on my noggin simply because often there is nothing to visualise (whereas in classical physics it's hard not to) I have unlearned the vision of the atom from school textbooks for example as a result of looking in to quantum mechanics, the problem is without a willingness to learn the maths I am often left just nodding my head :D

    As for light speed being the same from every point of reference, this I have somewhat understood and accepted, the question relates more to the idea of matter moving fast, and how exactly the 'speed limit' manifests, that is to say, if I am travelling at 99.9999etc% the speed of light relative to an arbitrary body or observer, and fire a really really (really!) fast missile ahead of me, I know that relativity states the missile will not achieve light speed, and while I can understand -why- this is the case (in so much as the energy required to accelerate it further), I do not understand what the point of reference is.

    If someone else does the same thing in the opposite direction to me that leaves 4 points of reference, one of which is traveling towards me, one behind and away from me, and one ahead of me. If you completely ignore the laws of physics for a moment and substitute the laws of intuitive visualisation you are left with 2 bodies moving away from me at over the speed of light and 1 body towards me and close to twice the speed of light, each missile is moving away from the other at over twice the speed of light relative to eachother. Ok so now we put in the laws of relativity and here is where I have trouble visualising how from every single point of reference nothing is breaking the 'speed barrier' relative to anything else.

    This is a random, crude and convoluted example, and making it more complicated does not magically change the physics or break any laws simply because I do not have adequate imagination to reconcile it, but I am having a hard time understanding what I should imagine happens beyond simply accepting it.

    Let me go one step further to describe my confusion, if there are two bodies of mass A and B about to pass eachother very quickly, lets say half the speed of light relative to eachother, and I am traveling toward the point where they will pass at 99%light speed from a direction that would put me 'behind' A's trajectory and 'infront of' B's trajectory respectively, and I would arrive at that point in space when A would, where would B be?

    I cannot approach B at 124% light speed, so do I slow down relative to A simply due to the presence of B? (I am not suggesting this happens, it is a visual aid to explain my confusion!)

    Again apologies for the lengthy read.
     
  11. Sep 3, 2011 #10

    phinds

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    I had, and to some extent still have, the same confusion. The big thing that helped me see at least some of it is that when you do the math you HAVE to use relativity here because classical mechanics just is not applicable at relativistic speeds and the math makes it clear that velocities do NOT add, they sum towards a limit, and that limit is, of course, the universal speed limit. There's actually nothing weird about it once you get it out of your head that you can even CONSIDER classical mechanics at relativistic speeds.

    There is also the whole thing about the same event NOT appearing the same to different observers when relativistic speeds are involved. I found that this also, for me at least, further complicated the issue. I read about it and thought about it until my head hurt and I THINK I finally internalized it, but I still cannot spew back out to you a good coherent argument that demonstrates it, so it's clear to me that it's somehow just not sticking to me fully. BUT ... I got clear enough on it at one point that it no longer bothers me. I am now pretty much past the point where I run off screaming obscenities every time I read the next impossible thing in cosmology or QM. I'm sure you'll get to that point reasonably soon.
     
    Last edited: Sep 3, 2011
  12. Sep 3, 2011 #11

    pervect

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    In classical relativity, the viewpoint is closer to the later. To whit, Wheeler's famous quote:

    Now, if you're doing quantum gravity, you might take a different view. But since you're here in the GR forum, you would probably do best to use around Wheeler's viewpoint. If at some other time you do quantum gravity you might try the other viewpoint, and I suppose if you manage both you can then think about whether they agree 100% in their predictions or not. If they do, they're equivalent theories, if not then you've got something to test.

    If you try to assume both at the same time, you'll most likely not get anywhere...

    I'm afraid I don't understand your second question at all. Philosophically, all you need to know is that events occur in space-time, and that any given observer can measure the spatial distance between events and the time difference. You'll also need to know about the Lorentz interval to get very far - different observers might assign different spatial and time separations to events, but they'll all agree on the difference of the squares of the space interval and the time interval.
     
  13. Sep 3, 2011 #12
    Thankyou all for the discussion, I am far happier with gravity now and despite royally confusing myself with my own follow up questions I am happy to take these points and ponder further without as many niggling doubts.

    As far as absolute points in space, the question is I suppose purely philosophical, I hear/read about fields, ripples in space etc and can't help a small part of my mind go to that place that wonders if in some way space has substance/characteristics (if space bends what is bending) I do not actively seek out such trains of thought, and consider such questions to be mostly meaningless, but it does lead me in a roundabout way to express them every so often in the form of an equally meaningless question :)

    Again, truly I am grateful for the responses.
     
  14. Sep 3, 2011 #13

    pervect

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    Space does have some properties - for example, curvature. But the curvature properties of space can all be derived from the knowledge of the distances between events - or, in GR, the invariant interval between events.

    http://www.eftaylor.com/pub/chapter2.pdf has a good discussion of curvature, it's an online chapter of one of his books.
     
  15. Sep 3, 2011 #14

    Dale

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    What is the difference? I.e. what would make you think that the descriptions are in any way incompatible?

    Nature does not provide us with a coordinate system.
     
  16. Sep 4, 2011 #15

    zonde

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    This statement means that we pick our coordinate systems in such a way so that speed of light is isotropic and the same in all our "legal" coordinate systems.

    Not exactly. Accelerating mass to the speed of light is impossible.

    Maybe if you will read about http://en.wikipedia.org/wiki/Relativity_of_simultaneity" [Broken] you will understand better how SR coordinate systems are picked out of all theoretically possible coordinate systems.

    And I think it's fair to say that preferred coordinate system is superfluous for mathematical representation of SR but without it (even if we pick it arbitrarily) visualization is hard if not impossible.
     
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  17. Sep 4, 2011 #16

    Mentz114

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    This statement is frequently stated and is untrue. In fact it's not even wrong unless absolute velocity is accepted.

    A constantly accelerating mass, which leaves behind a marker when it begins accelerating will eventually lose causal contact with the marker. It will go behind an horizon called the Rindler horizon.

    Don't take my word for it, the situation is explained here

    http://gregegan.customer.netspace.net.au/SCIENCE/Rindler/RindlerHorizon.html
     
  18. Sep 4, 2011 #17

    Mentz114

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    This statement is frequently stated and is untrue. In fact it's not even wrong unless absolute velocity is assumed to be a valid concept.

    A constantly accelerating mass, which leaves behind a marker when it begins accelerating will eventually lose causal contact with the marker. It will go behind an horizon called the Rindler horizon.

    Don't take my word for it, the situation is explained here

    http://gregegan.customer.netspace.net.au/SCIENCE/Rindler/RindlerHorizon.html
     
    Last edited: Sep 4, 2011
  19. Sep 4, 2011 #18

    Dale

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    It is true in any inertial frame.
     
  20. Sep 4, 2011 #19

    DrGreg

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    I don't understand how your argument is supposed to prove your proposition. In your example, which mass do you think has been "accelerated to the speed of light" and in what sense?
     
  21. Sep 4, 2011 #20

    Mentz114

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    Considering the continuously accelerating observer and the one left behind, I'm equating 'going behind the horizon' ( losing causal contact) with a relative velocity of c between them. Special relativity seems to play the role of cosmic censor and prevents any observer from seeing the accelerating one achieve the speed of c in their frame.

    So, the accelerating observer has been accelerated by their own locomotion apparatus to a speed equal to c, wrt to the observer behind the horizon.
     
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