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Theory of gravity

  1. Nov 9, 2011 #1
    Hello, all this talk about gravity and what exactly this is really interests me, but I don't seem to be able to understand exactly what about it is that is the hard part to solve, there seem to be so many factors involved that I'm losing track of the entire picture.

    So, could you tell me what exactly a good theory of gravity should be able to describe or explain? Mainly which experimental results it should explain, and which theoretic constructs that follow from other experimental results it should be compatible with. Also, could you tell me what the main theory's about gravity say, where they fall short, and why they can or can't be combined if applicable.
    Perhaps a list like that could help me and other curious but not too knowledgeable people understand what the problem is about. If anyone could help with that then it would also be of great help if statements about it could be put in not too technical terms that also avoid formulas as much as possible, it's not going to help much in my understanding otherwise at least (but if it can't be put in a way a 5 year old would understand then the proper explanation would be great of course).

    Thank you for your time.
     
  2. jcsd
  3. Nov 9, 2011 #2

    Drakkith

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    You're asking for way too much info for a forum thread. I suggest buying a book about gravity, I'd bet there are plenty available online. Also, since relativity is the main theory that describes gravity, I suggest looking for a book on General Relativity.
     
  4. Nov 9, 2011 #3
    I hope you are joking. If you wish to read a book about gravity, please do so - it's either a really simple topic (F = Gm1m2/r2 and that's all there is to say) or a really complicated topic that goes way beyond your head into lots and lots of very complex equations and concepts. You can find books covering this whole spectrum.
     
  5. Nov 10, 2011 #4
    Right, I see, I was hoping for a few key points and that that shouldn't take too much time or effort but I guess I was wrong. Well, I'd be happy with just an answer to one question if that doesn't take too much effort: why does it seem impossible to mix quantized gravity with the gravity in relativity?
     
  6. Nov 10, 2011 #5
    But which one describes how gravity really works?
     
  7. Nov 10, 2011 #6

    chroot

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    Gravity might seem to be a simple phenomenon, because we human beings experience a very calm environment here on Earth. We're not subjected to strong gravitational fields, and we travel at very slow speeds, so a simple theory is adequate to describe all the gravitational phenomena we directly experience. One simple equation--Newton's Law of Universal Gravitation--is sufficient to explain all of the gravitational phenomena you're likely to experience on Earth, and it's all that's needed to send robotic vehicles to distant planets.

    There are many subtler phenomena that a proper theory of gravitation must explain, though. Here's a list of experimental tests of general relativity, compiled by Dr. Baez. Any suitable theory of gravity must explain all the results of these experiments, plus the results of many, many more experiments yet to be conceived.

    http://www.desy.de/user/projects/Physics/Relativity/SR/experiments.html

    - Warren
     
  8. Nov 10, 2011 #7

    DaveC426913

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    In a nutshell (and thus highly over simplified).

    Relativistic gravity assumes a field. A field means that every single point in the universe has a value at that location. A field is a continuum.

    This is antithetical to quantized gravity, which, by definition, is ... well ... quantized.

    The Heisenberg Principle (part of QM) implies that, the smaller the gap between two points, the larger the uncertainty in the energy level. So, if we apply that to a GR's field (where any two points are infinitely close together) the energy level can be infinite.

    i.e.: when you try to apply one set of formulae to the other, you get infinities popping up.
     
    Last edited: Nov 10, 2011
  9. Nov 10, 2011 #8
    Newton's law predicts what the effect of gravity will do but does not explain how. So far no theory has without introducing some undiscovered ingrediant or phenomena.
     
  10. Nov 10, 2011 #9

    Drakkith

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    What undiscovered phenomena are you referring to? And do you mean that the theory predicts phenomena that are not known at the time of its introduction? If so, what's wrong with that?
     
  11. Nov 10, 2011 #10
    Newton's law predicts (calculates) what gravity will do as in the orbits of planets. It may predict certain phenomena but that still has nothing to do with how gravity really works.
     
  12. Nov 10, 2011 #11

    Drakkith

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    "How gravity really works" is exactly what it has to do with. It turns out that until you require a certain accuracy, newton's laws are just as valid as relativity. The entire idea of "how X really works" is meaningless, as there will always be questions we cannot answer at the time.
     
  13. Nov 10, 2011 #12
    As far as I know Newton's laws are as valid as anything. But even Newton said he didn't know how gravity actually worked. How do you explain a force that is suppose to reach out in ever direction to ever point in the universe and grabs hold of things and then pulls them back? Can you physically discribe this process? No one else can.
     
  14. Nov 11, 2011 #13
  15. Nov 11, 2011 #14
    To me it's not a question worth spending much time on, because it veers into philosophy. "How" does it work is answered by the fact that two bodies having mass will attract each other because of their mass, according to equations that are adequate to describe every situation we have encountered and have direct knowledge of (i.e. let's ignore the big bang, black holes, planck scales, etc.). One could ask the same question about electric charge - "how" do charged particles attract or repel each other? "Why" do protons and neutrons like to bind together in more-or-less equal numbers in nucleii? "Why" does Schroedinger's equation appear to be accurate within it's realm of applicability? Etc. Maybe there are deeper, more profound ways to describe these things, but if they are to be useful beyond philosophy they must either make testable predictions that are not now predicted, or must lead to some other theory that does.
     
  16. Nov 11, 2011 #15

    chroot

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    Bill, your concern is a common one. We are accustomed to being able to explain large-scale (human-sized) phenomena in terms of smaller-scale ones. We can explain the behavior of gases with a model of tiny billiard balls banging against each other, and the walls, imparting pressure. That could be considered a description of how gases "work," but is it really? It's just a description of a higher-level phenomenon (pressure) in terms of a lower-level one (inelastic collisions). What then explains how inelastic collisions "work?"

    It's intuitive to think about how our theories "work" by casting them in terms of lower-level theories, but that might not always be an option. There are two possibilities:

    1. The universe becomes progressively more complicated as you look at it more closely, forever. Every physical theory is supported by some theory below it, and our models never stop growing in complexity. Physicists sigh in collective relief that there will always be jobs.

    2. The universe has some finite level of complexity. Eventually, we will reach a theory that can predict the result of every experiment that anyone can ever devise to test it. It will describe the universe perfectly, and it will be possible to prove that it is entirely self-consistent, with no troublesome inconsistencies that might be inklings of any deeper theory.

    If we really live in a universe like (2), it is reasonable to expect the final, innermost layer of Nature's secrets to be a simple list of seemingly arbitrary rules, with no greater, deeper reality in which to imagine them. That might be an unsettling thought, but I think the universe in (1) is even more unsettling!

    Every physicist in the world is aware that our current theory of gravity is flawed, and incomplete. General relativity will one day be superseded by a better theory, and it is possible that that theory will be more descriptive -- more intuitive in some way -- than general relativity. At this time, general relativity is the best horse in the race, so it rightfully gets the most attention. It's still paying dividends in experimental successes, a hundred years later! Experiment is the ultimate arbiter of success in science, and our understanding of gravitation will advance quickly as soon as someone, somewhere, comes up with an [practical, technologically feasible] experiment that general relativity cannot explain.

    - Warren
     
    Last edited: Nov 11, 2011
  17. Nov 11, 2011 #16
    Newton's law of gravity says nothing about bending space. But even Einstein’s analogy of bending space does not describe what the actual force is?? If I indent the top of a trampoline and throw a ball onto the surface, the ball will roll down and around the curvature no problem but did the curvature of the trampoline cause it to go to the bottom or did gravity? So what is gravity?
     
  18. Nov 11, 2011 #17
    Of course things like this are worth spending time on. How gravity actually works as well as the other subjects you mentioned are extremely importantant not to mention interesting and will change the world as we know it.
     
  19. Nov 11, 2011 #18

    chroot

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    bill alsept, there are many people who prefer to deal with the harder-edged topics in physical sciences, those with specific, attainable answers. That's okay!

    This post is admittedly in the wrong forum, though. We have another forum that is devoted to the philosophy of science.

    - Warren
     
  20. Nov 11, 2011 #19
    Einstein and Newton's gravitational descriptions and calculations are ingenious but have nothing to do with what makes gravity do what it does. It only calculates how it will respond. Figuring out how gravity works is very important and not philosophical. As soon as we know everything will change.
     
  21. Nov 11, 2011 #20
    Not sure what your trying to say but I think gravity is a specific and hard edged topic not to mention very attainable.
     
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