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Why does mass warp spacetime?

  1. Jan 11, 2013 #1
    I know that mass warps spacetime and that's basically the force of gravity (warped spacetime), but why does mass warp spacetime? Is it because mass implies energy and energy warps spacetime? If so, then why does energy warp spacetime?
     
  2. jcsd
  3. Jan 11, 2013 #2
    Because it does. Unfortunately, any theories about why mass warps spacetime aren't going to result in any predictions we can test (in part because they'll be tailor-made to predict GR.)
     
  4. Jan 11, 2013 #3

    jtbell

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    It's turtles all the way down from there... :wink:
     
  5. Jan 11, 2013 #4
    Oh, I love that Hawking book :D
    But that's kind of disappointing though about not knowing why mass warps spacetime....
     
  6. Jan 12, 2013 #5

    Dale

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    This is a fundamental limitation of the scientific method, it is rather poor at answering "why" questions. The problem is basically what kinds of answers the questioner would like to their "why" question.

    Sometimes the asker of a "why" question wants a scientific answer, which means that the question must be in terms of one scientific theory and the answer must be in terms of another, more fundamental, scientific theory. This means that you can get answers to "why" questions about Newtonian gravity by explaining GR, since GR is a more fundamental theory than Newtonian gravity. However, this also means that it is inherently impossible to answer why questions about fundamental theories. We use our fundamental theories to explain other theories, we don't have an explanation for them other than they fit the data.

    More often the asker of a "why" question wants some kind of philosophical answer, which doesn't belong here anyway.
     
  7. Jan 12, 2013 #6
    I was actually looking for an answer that would say that a property of spacetime is that it reacts to mass (or something like that, I don't know), but yeah, what you're saying makes sense.
     
  8. Jan 12, 2013 #7

    pervect

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    The only other time I can think of where you can answer a "why" question is when you have a theory that's more fundamental that underlies the original theory.

    Basically, you can always ask "why", and when you get an answer you can ask "why" again.

    At some point you have to stop because you've reached the most fundamental known reason "why".

    However, there isn't (as of yet) any more fundamental theory that GR is based on, so there isn't any answer of this type.
     
  9. Jan 12, 2013 #8

    Dale

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    To the great amusement of your average 3-year-old and the consternation of their parents :smile:
     
  10. Jan 12, 2013 #9

    pervect

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    Yep, exactly :-).
     
  11. Jan 12, 2013 #10
    Okay I have no clue what I am talking about, but I am trying to understand this. I think each element only appears warped from the perspective of other elements of the superposition of the wavefunction of each structure within the system. So if you consider things from the perspective of the mass structures, the spacetime elements appear warped. I am sure if you looked at it from the other perspectives, mass would appear warped. Don't listen to me I have no idea what I am talking about. I am sure I am at least an order of magnitude or a dimension away from being able to grasp it. :)

    Edit: I think a more accurate reason is that gravity and acceleration can be considered the same, in accordance to what we can measure and observe in spacetime, from the frames of reference of knowing that our theories have proven experimentally verifiable in those frames. I think the train of logic is something along that path. There are probably frames in which mass doesn't warp spacetime but something else would have to give and we probably never get to observe from those frames.
     
    Last edited: Jan 12, 2013
  12. Jan 12, 2013 #11

    Drakkith

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    I'm sorry, your universe is in another castle!
     
  13. Jan 12, 2013 #12
    There is a speculation (not a theory) that the spacetime warping is the same thing as the quantum (de Broglie) waves. This hypothesis might get attention if we discover a graviton. With graviton, this is definitely true. For example, the frequency of a graviton's de Broglie wave calculated from QM and the frequency of tidal waves generated by the graviton calculated from GR are the same. This has yet to be confirmed by experiment, but if it turns out true, then it would be natural to generalize it to other particles.
     
  14. Jan 12, 2013 #13
    I think I have a better answer. Consider mass as one manifestation of the energy of a body in spacetime. A body at rest moves through time at the speed of light. An body in motion has some of it's energy diverted away from the time dimension, into the spatial dimensions, causing warps. I'm sure I'm on the right track..
     
  15. Jan 12, 2013 #14

    Vanadium 50

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    Sorry, but we don't allow posting of speculative or personal theories here.

    If you want to ask questions about conventional physics, go ahead. However, "Here's my theory, what's wrong with it?" can't be one of them.
     
  16. Jan 12, 2013 #15

    Drakkith

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    Nope. I suggest you read up on what energy is and what an inertial frame is.
    And as Vanadium said, please don't try to come up with your own reasons for things happening. This forum is just for mainstream scientific theories.
     
  17. Jan 13, 2013 #16
    we could use the principle of equivalence saying gravitation can be reproduced by a local inertial frame and get hence a locally induced space contraction time dilation. But this doesnt explain why a mass create gravitation and in some sense the question remains
     
  18. Jan 20, 2013 #17
    Although phrased as his own idea, this is not. I've heard Einstein attributed to this sort of interpretation on some of those science channel shows, like Through the Wormhole (or one of those). I've never been able to find a technical discussion of what this interpretation comes from, or if Einstein ever said anything like it, but I just wanted to throw it out there that this reasoning has worked it's way into the non-technical science mainstream. It was described more-or-less as above: every single object has a "total velocity" of c through space and time. Photons move entirely along the space axis, and everything else has a vector with components in both space and time, changing in proportion according to relativity, but always maintaining a magnitude of c.

    Perhaps we can dismiss RotatingFrame's delivery and treat it as a question: is there any validity to that interpretation?
     
  19. Jan 20, 2013 #18

    Dale

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    This is why the rules of the forum specify mainstream scientific references, not pop-sci references. Do a search for "Brian Greene" on this forum and you will get a taste of the headache that pop-sci treatments cause.
     
  20. Jan 20, 2013 #19
    Although there is no real scientific answer to your question, string theory offers some interesting insights. [That mass and its equivalent, gravity, can affect space and as well the relative passage of time is one of most profound findings of all time. It's downright 'crazy' based on our everyday intuitions.]

    In string theory, fundamental components of particles, strings, are vibrating energy modes. It turns out that these interact with the degrees of freedom, or geometrical dimensions, in which we all find ourselves. Different sizes and shapes of additional dimensions can be mathematically associated with different characteristics of strings: varying vibration patterns correspond to things like particle size, charge, spin that we observe macroscopically.

    So strings and geometry, spacetime, interact analogous to mass/energy in general relativity. This offers some insights, perhaps, why not only mass, but also energy and momentum density warps spacetime.
     
  21. Jan 20, 2013 #20
    I've got Brian Greene's book FABRIC OF THE COSMOS where he explains acceleration using the above concepts. I found it useful as ONE perspective, but it seems unpopular here among some. I found it helpful when approaching light cones for the first time....I don't see much difference...

    I find Greene's above description along the lines of the 'rubber sheet' analogy for gravity...[which Greene discuss right after in his book] or the 'balloon analogy' for cosmology, useful as a perspective, but they all come with limitations.
     
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