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Why do the space time get curved around a massive object?

  1. Aug 26, 2014 #1
    As far as I have the knowledge of GTR that a mass bends the space time around it.But why does this bend occur?The example from real life that when a mass is placed on a net then the net bends but it us very difficult for me to visualise the situation of bending of spacetime due to a mass.What is actually happening?What is the physical basis behind this bend?I want to say that the books which I have studied till now just say that the space time "just bends" around an object and even my teacher told me that it just bends without any explanation so I just want to know the reason for the occurrence of this bend.What is the need for spacetime to bend around an object?What physical problem we can encounter if we do not consider the bending of space time.
     
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  3. Aug 26, 2014 #2

    Nugatory

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    Put that example out of your mind, please. You'll see it a lot in pop-sci presentations, but it's neither accurate not especially helpful.

    There isn't any good answer to this "why?" question. We know the curvature is there because we can observe its effects, and we know that the Einstein field equations of general relativity allow us to accurately calculate it and its effects. But WHY does mass curve space? That we don't know and may never know.

    But please do note that we have exactly the same problem with ordinary classical gravity. Newton discovered that two masses attract each other with a force given by ##F=Gm_1m_2/r^2##, but not why such a force should exist nor how the presence of the masses leads to those forces.

    We would not have a theory of gravity that worked properly everywhere:
    - People wondered why the orbit of Mercury does not precisely obey the predictions of Newtonian gravity for more than a century before Einstein came up with the more accurate theory of general relativity to explain it.
    - We wouldn't be able to build the GPS system and make it work without general relativity.
    - and many more examples....

    Of course, we could imagine that someday someone might come up with some new theory of gravity that doesn't depend on space-time curvature and that still works as well as GR. So far, however, no one has succeeded in doing this.
     
  4. Aug 26, 2014 #3

    Simon Bridge

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    Welcome to PF;
    There is no "why" to how the Universe behaves - it just is.
    It's like asking why spheres are round. What we do with physics is try to describe the way the Universe works.
    But we cannot tell you why it works that way or why a particular model for a class of phenomenon is a better fit than other models - in any deep way - that is just how it turns out.
    We can answer in a shallow way - in which we use some framework of things that happen ... but coming up with that framework is what physics tries to do and the best framework for understanding gravity and the related phenomenon that we have right now is general relativity. Why is that framework the best? Because it best fits the experimental evidence and the things we see in Nature. Why? Well - because that's the way it turns out.
    See?

    But you do manage to ask some easier questions.

    The "bend" in space-time is sort-of in the maths - it is to do with the relationship between measurements made in different points of view and has no non-GR equivalent. While you are getting used to it, you can think of the "curvature" as the name given to the kind of maths you need to use ... it is very similar to the kind of maths needed to navigate between places far apart on the surface of the earth.

    It is difficult to visualize - I agree. 4-D thinking is hard. To make matter trickier, the common "rubber sheet" or "stretchy net" picture you are usually given when you start trying to learn about these things can be very misleading so you need to take that with a grain of salt. It's supposed to help you feel more comfortable but if it doesn't, then don't bother trying to make it understandable. You are just going to have to get used to it.

    General Relativity solves a bunch of physics problems that are not so easily handled otherwise. One of these is working gravity into special relativity, which people wanted to do because special relativity was already very successful at the time.

    General relativity has been very successful too. We use GR, and thus curvature of space-time, to make GPS navigation work. This is similar to using the fact the surface of the Earth is curved in order to do very precise navigation.

    See also:
    http://en.wikipedia.org/wiki/Tests_of_general_relativity
    ... for other examples of problems that don't work out without assuming space-time curvature.

    Richard Feynman on the difficulty of "why" questions.

    Transcript: http://lesswrong.com/lw/99c/transcript_richard_feynman_on_why_questions/
    ... it is in the context of magnetic force but has general relevance.
     
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