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The old Newtonian gravity

  1. Mar 30, 2015 #1
    Just a small newbie question. If Newtonian gravity is outdated and therefore inaccurate, why is it still taught?

    Surely the fact that Einstein's gravity is right takes priority over the fact that Newton's method is easier.

    Am I being ridiculous?

  2. jcsd
  3. Mar 30, 2015 #2


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    Staff: Mentor

    Start with Asimov's classic essay http://chem.tufts.edu/answersinscience/relativityofwrong.htm
  4. Mar 30, 2015 #3
    On scales comparable to a human being, Newton's theory is accurate enough.
    GR only becomes necessary when describing objects on very large scales and very high speeds..
    It makes no practical difference in the case of ,for example, civil engineering projects, or building vehicles.
    It would just make things vastly more complicated and produce no useful improvement.
    In some cases though we do need to use GR, such as in synchronisation of GPS satellites.
  5. Mar 30, 2015 #4


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    Staff Emeritus
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    In this case, yes.

    Newton's laws are still taught because, on the whole for many applications, they give reasonably correct answers. It is only in extreme cases where the discrepancy between the Newtonian and the Einsteinian theories can be measured with any accuracy. Often in pedagogy, it is desirable to introduce concepts gradually, starting with simple models and then progressing to more complex topics as needed, rather than starting with the most complex concepts.
  6. Mar 30, 2015 #5
    Or look at this way.
    If I want to design a weighing machine suitable for a shopkeeper selling vegetables, I can do this using Newton's laws and a make a highly accurate machine for the purpose.
    I don't need to consider the possibility of billion ton vegetables moving close to light speed.
  7. Mar 30, 2015 #6


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    The Apollo program used Newtonian gravity, not general relativity.
  8. Mar 30, 2015 #7


    Staff: Mentor

    Hmm, I think this may be symptomatic of a misunderstanding of what science does. Science is about finding accurate models. Newtonian gravity has been experimentally confirmed to be a very accurate model for many situations. In those situations it is every bit as "right" as GR.
  9. Mar 30, 2015 #8
    In physics, you use the simplest model that suits your purpose. Whichever error there may is included in the uncertainty.
  10. Mar 31, 2015 #9


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    If you want to understand the dynamics of a huge gravitational source as accurately as possible, use GR. If you want to be able to calculate any one of the millions of things that involve gravity but don't require the accuracy you can get with GR, use newtonian gravity. It's about 10,000 times simpler. Remember that many people who learn science don't become researchers, but engineers, who don't care what the most accurate theory is. They only care they have something that works well enough and is simple enough to use in their jobs.
  11. Mar 31, 2015 #10


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    The correspondence principle implies that Newtonian gravity is not wrong; it's just not as complete as GR. This means that GR can be trusted up to higher energy scales than Newton. But GR itself is also not complete; we suspect that for certain energy scales GR will break down too.

    If I want to describe a cannonball flying through the air, according to your reasoning I would have to use string theory or loop quantum gravity instead of classical physics to describe its motion. Good luck with that.
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