Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does the existance of relativity prove that gravity's wrong?

  1. Mar 4, 2009 #1
    If Einstein's work proves that Newton's gravity is wrong, why do physicists still use gravity as though it's a sound scientific theory? When Drake successfully circumnavigated the Earth in the 16th century, scientists realized that Aristotle's 'flat Earth' was wrong and this opened the way for Newton to imagine gravity. If gravity has already failed the test of time as proven by relativity, are physicists of today merely hanging on to an incorrect idea because they're incapable of thinking outside the 'box' created by Newton?
  2. jcsd
  3. Mar 4, 2009 #2


    User Avatar
    Homework Helper

    While strictly speaking the Newtonian theory of gravity is wrong, the theory that superceded it, General Relativity, still reduces to it in the appropriate limits (weak gravitational field/small curvature of spacetime). Strictly speaking the theory of Newtonian mechanics (F = ma) is wrong too, but at speeds much less than the speed of light we can't detect the difference. The same goes for the theory of gravity: anytime we use the Newtonian theory of gravity it's because its predicitions are nearly indistinguishable from the full theory's predicitions. This shouldn't be surprising since if the original theories didn't make correct predictions they never would have been accepted.

    One place we do have to use GR instead of regular Newtonian gravity is GPS systems: due to time dialation effects GR is needed to give accurate locations, etc. So we use GR when we need it and Newtonian gravity when we don't need the full power of GR.
  4. Mar 4, 2009 #3


    User Avatar
    Science Advisor

    In the same way that we still use the flat earth theory - for small regions of the earth, like Singapore.
  5. Mar 4, 2009 #4
    Another important thing to notice is that Newton didn't have a full theory in the sense that he had no mechanism to explain why to masses attract each other. He even said essentially, I can't figure out why this happens so sooner or later someone is going to come along with a better theory that provides a mechanism that shows why things attract (GR)
  6. Mar 4, 2009 #5


    User Avatar
    Gold Member

    GR does not invalidate Newton's gravity. The latter is a special case and works well enough in weak fields. It's possible to start with GR, make some simplifying assumptions and derive Newton's gravity.

    "If gravity has already failed the test of time as proven by relativity, are physicists of today merely hanging on to an incorrect idea because they're incapable of thinking outside the 'box' created by Newton?"

    It hasn't failed and no.
  7. Mar 4, 2009 #6
    GR does not explain WHY mass bends space :)
  8. Mar 4, 2009 #7
    The point I'm trying to get across is that gravity might be wrong, completely and utterly WRONG!! Much of our current understanding(Big Bang, black holes, gravitons, etc.) is based on the idea that mass generates gravity. If this concept is wrong, then every one of those ideas that's based on the presumption that gravity is infallible is also wrong. This would include Einstein's own relativity which deals with objects with 'mass' when they near the near the speed of light and with 'massive' bodies. Physicists seem to be trying to make their observations and understanding 'fit' into their thoughts of gravity and relativity. If some aspect of reality doesn't seem to 'fit' into their preconcieved notion of how things should work, they merely ignore it or move on to something else. Take galactic cores for example. Physicists can explain how rings are formed on planets and why planets revolve around the sun in a thin plane. They can explain how stars are drawn into the thin disc of a galaxy but they ignore the question of how billions of stars can orbit around a galactic core without bumping into each other and growing into a black hole. Some of them suggest that a black hole is at the heart of every galaxy but if this were the case, logic dictates that this black hole would absorb stars out of the core and grow larger. Once larger, it would absorb more stars until ultimately it would swallow up all the stars in the core and begin on the stars in the disc. Of the billions of galaxies that have been catalogued and observed, not a single one exists as a 'donut' shape with a dark center.

    In fact, if you ask any physicist, "How does mass generate gravity?", they all answer the same way; I don't know!! I think that before another tax dollar is spent on researching some aspect of physics that's in any way relative to mass, all efforts should be put into answering the questions that have been ignored for hundreds of years. We should be absolutely certain that we are on the right path and that Einstein's work isn't merely an alarm telling us that Newton's work is entirely incorrect.

    I'm sorry if I offend any physicists with my opinions but I'm in search of the truth. If anyone on Earth can explain how mass generates gravity I would love to hear it.
  9. Mar 4, 2009 #8
    Black hole quickly 'eats out' all the stars which orbit close enough. All other stars orbit safely around the black hole like they would orbit around any other body of the same mass (you are victim of a popular misconception about the black holes). Finally, the probability that star collide and bump into each other is so low that such collisions are rare even when the whole galaxies collide. And it was well known a long time ago.

    It is a pity I did not find the famous "General relativity is just a theory" in the post above :)
  10. Mar 4, 2009 #9
    Most of your post #7 is simplistic and silly. Your original question is entirely reasonable.

    But why pick on gravity? You don't know what mass is and neither does anybody else. Nor time, nor space, nor energy...Where do the free constants in the "standard model" come from?...why do we use those?...Simple: all those are the best theories/values we have and in most cases theory, however imperfect or incomplete, matches experimental results.

    Science usually makes progress in baby steps since formulating theories is very hard work. Did science still assume a static universe after Hubble discovered recession of distant galaxies? (well, likely some did.) But each NEW theoretical or experimental result usually means something unusual, something previously not understood. So sometimes it takes even brilliant people time to comphrend the meaning of new results and sometimes they argue for decades....different versions of quantum mechanics versus each other and relativity for example.
  11. Mar 4, 2009 #10


    User Avatar
    Science Advisor

    For final truths, try religion.
    Any answer to that will create the next 'how' or 'why' question. Any 4 year old kid can create an infinite chain of 'why' questions. :smile:
  12. Mar 4, 2009 #11
    What is mass :rofl: Be it the Higgs or some other theory I think you need to know what mass is before you can say how mass generates gravity. If I'm not mistaken the LHC should amoung other things look for the Higgs boson which would be the source of mass if found(corrrect me if i'm wrong). There is a lot that goes into a theory like that. And I don't think physcists blindly follow in Einstien's path. In fact I think most of them realize that either Einstein, QM, or both are incorrect in some sense because we you but them together they don't work...

    I'm a little confused by what you mean when you say Gravity is wrong. Do you mean that something else is pulling my book to the floor when i drop it? Physicists aren't absolutely certain. Thats why we have lots of other theories. They are relatively sure that something along the lines of GR will do the trick
  13. Mar 4, 2009 #12
    Nor was it intended to. Your taking it out of context. The point of it is to explain gravity not bending space :tongue:
  14. Mar 4, 2009 #13
    jefswat, my point was that the question WHY is usually does not have any sense when applied to the fundamental things. Like, why 5 is a prime number? yes, I know that there are no such numbers that A*B=5, but WHY?
  15. Mar 4, 2009 #14
    Newtonian theory works well for the vast majority of practical problems.Engineers,for example would not use relativity to work out the forces acting on a bridge it would be like using a sledgehammer to crack a nut.
  16. Mar 4, 2009 #15
    Yes, something else is pulling your book to the floor, electromagnetism. Don't bother explaining that your book isn't made of metal or magnets, I understand that. It is made of atoms though and each and every atom is acted on by the electromagnetic field of the Earth. If I rub a balloon(not metal or magnet) on my head, hair or small pieces of paper(not metal or magnet) will cling to the balloon. This is exactly what happens to your book when you release it, it's acted on by the Earth's electromagnetic field and is pulled to the field until it can become 'grounded' once again. Physicists should have clued into the reality of what's going on in 1805 when Coulomb's law turned out to have the identical formula as Newton's law of gravity. They are one and the same force only one of them is real and one of them exists only within the minds of humans as a means of explaining the workings of the universe.
  17. Mar 4, 2009 #16
    The similarity is only due to the number of dimensions of our space (it gives r^2 in the denominator). In fact, in GR it is not exactly the same formula for the strong gravitational fields. And for electomagnetism it is valid for charges in rest only. Also, in GR gravitation force is not a force at all. Electromagnetism is similar to the weak force which is very short range nand not /r^2 at all.

    I can not blame you for thinking this way, even Eistein was trying to unify gravity and electromagnetism, while he should start from the weak force. So saying about the "identical formula" is like very old and naive classifications of the plants based just on their appearance, like "trees", "grass" made in the middle ages.

    Finally, as the formula is NOT identical it is a total BS. Just a spagetti of the popular books.
  18. Mar 4, 2009 #17
    So then how come the needle on my compass points north and not down? And how does EM explain Time dilation. Also I'm fairly certain the EM equations you are thinking of have also been proven out dated and better equations have been found(QED I think?)

    But Newton's equations are wrong. They fail to even predict the orbits of all the planets(to a high degree of accuracy) where GR does. Physicists are trying to merge gravity with the strong, weak, and electromagnetic forces and so far have not been successful. I'm not trying to say GR is right I'm trying to say its better than Newton's. Regardless it is a model not the the universe actually works for sure.
  19. Mar 4, 2009 #18
    I know I was just poking fun. Your concern is a good one. As someone else said, solving a problem just raises 2 more "whys"
  20. Mar 4, 2009 #19

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Both of these statements are in a sense wrong. Newton's equations are not so much wrong as they are inaccurate. They do an amazingly accurate job of predicting the orbits of the planets, even Mercury's orbit, over centuries. General relativity is a pretty small effect, even on Mercury. General relativity results in a small precession of Mercury's perihelion angle: 43 seconds of arc per century, or 3 million years per 360 degrees. The time scale on which general relativistic effects become apparent (unless you are looking for meter-level accuracy) is much, much greater than that for all but Mercury.

    That 3 million years is about the same as the solar system's Lyapunov time. Beyond this time scale any predictions are pretty much garbage, even those with a full relativistic model.
  21. Mar 4, 2009 #20
    I think that general relativity proved that gravity is correct. Consider a Mossbauer effect experiment, with a Mossbauer source on top of a 100 foot tall tower, and the Mossbauer detector at the base of the tower (and vice versa). Experiments show that the photons gain energy (just like Newton's apple) while falling from the tower, so either the source or the detector has to be moved vertically to compensate for the energy gained by the photons. Harvard did this experiment using the Fe^57 source mounted on a loudspeaker coil.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook