Newton's LAW vs Einstein's THEORY

  1. Hello

    I was just thinking about the Newton's law of gravitation and Einstein's theory of relativity. Newton's law of gravitation was inferred by Newton , probably based on some earlier experiments and Einstein's theory of relativity takes its inspiration from Michelson Morley experiment. Both have been verified experimentally to a great extent. So why one is called a law and other is called a theory. What are the differences ?

    thanks
     
  2. jcsd
  3. jtbell

    Staff: Mentor

    A "law" is a single (mathematical) statement: Newton's Law of Gravitation; Ohm's Law; Snell's Law (of refraction); etc. A "theory" is a coherent collection of (mathematical) statements.
     
  4. Matterwave

    Matterwave 3,774
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    The Einstein's theory of gravitation, the theory of general relativity, is more accurate than Newton's law of gravitation. Experimentally, Newton's law of gravitation has been falsified due to for example the anomalous perihelion precession of Mercury, which matches instead with Einstein's theory of gravitation.

    The names are historical. It used to be, during the time of Newton, popular to ascribe physical "laws" to the world (see e.g. Kepler's "laws"). By the time of Einstein, it was no longer popular to do so, and instead we look at physical "theories".

    Usually the names are just historical in nature.
     
  5. Oh... so the term "Law" has become outdated it seems... I am doing Teacher's diploma. Faculty are from department of education. My professor insisted that Newton's theory should be called a "Law" , while Einstein's theory is just a theory. I guess these educationists are probably ignorant about historical issues in physics
     
  6. SteamKing

    SteamKing 8,286
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    IMO, saying that Newton's law of gravitation has been 'falsified' by Einstein is misleading. Newton's laws are still studied and used in the proper settings today. Einstein's theories of relativity were able to explain certain observed astronomical phenomena which classical Newtonian mechanics could not, so in that sense, relativity is a refinement, rather than a total replacement, as say, the Copernican heliocentric theory was a replacement of the Ptolemaic geocentric theory of the heavens.

    http://en.wikipedia.org/wiki/Tests_of_general_relativity

    Newton's laws are still used to plan the paths of spacecraft thru the solar system and track the motion of the planets. As is sometimes said, no one ever used the theory of relativity to design a bridge.
     
  7. I would say Einstein's theory is a theory while Newton's law is just a law. In natural sciences a law is a generalization of experimental observations while a theory additionally put it down to more basic principles.
     
  8. The phrasing "just a theory" sounds worryingly reminiscent of creationist attacks on evolution. If your DoE friends feel that Newtonian gravitation is somehow superior to, or more certain than, relativity, they are mistaken.
     
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  9. Matterwave

    Matterwave 3,774
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    A theory whose predictions do not match observation has been, according to the scientific method, falsified has it not? Not only does Newtonian gravity give the wrong predictions for certain classes of problems (perihelion precession, orbit decay due to gravitational wave emissions, deflection of light, gravitational redshift, etc), but the core paradigm of absolute time in all of Newtonian mechanics has been completely revised by relativity. If one can not call this a "falsified theory", I wonder what the criterion is used to call a theory "falsified"?
     
  10. Nugatory

    Staff: Mentor

    A theory also has a domain of applicability.

    Newtonian mechanics has not been falsified within its domain of applicability. Instead, there is a wide range of situations in which relativity and Newtonian mechanics make the same predictions to the limits of observational accuracy, and within that range only a masochist would choose to use relativity instead of classical mechanics to make predictions.

    You could reasonably argue that the two theories will always be distinguishable with sufficiently precise measurements, so in that sense Newtonian mechanics is (could be, if we really felt that we needed to make such measurements) falsified everywhere. But that's a very different and much weaker sort of falsification than what happened to, for example, theories of a flat earth.

    I'm reminded of Asimov's classic essay on the relativity of wrong here. Some falsified theories are more falsified than others.
     
  11. Matterwave

    Matterwave 3,774
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    This sounds like a practical way of thinking about the problem, but only a practical way. There are very many theories which we would certainly call falsified that nevertheless have SOME domain of applicability (see e.g. Nordstrom's theory of gravitation which actually gives the correct gravitational redshift, or the Bohr atom which gives the correct spectra for Hydrogen, or even the caloric theory of heat which was used by Carnot to develop his engine).

    You can certainly say Newton's theory has a domain of applicability. But so does a lot of falsified theories. The only reason we still hang on to Newtonian (gravitational) theory is that it is significantly simpler to work with than Einstein's Relativity, while still giving reasonably good results, in addition to historical reasons.

    I don't see why there's so much discontent with calling Newtonian gravity falsified.
     
  12. Nugatory

    Staff: Mentor

    In my case, it's because I'm hypersensitive after having heard too much "Evolution is just a theory - Einstein proved that Newton was wrong, so we should give Dr. Ima Craik Pott her chance to prove Darwin wrong". Without the nuance of Newton's relative wrongness, this is a tough argument to counter.
     
  13. SteamKing

    SteamKing 8,286
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    Well, what would you say to those who study classical mechanics at university? Are the introductory engineering courses in statics and dynamics filled with students who are being indoctrinated irresponsibly in a tired dogma?

    Like it or not, classical mechanics still has wide applicability, even for those, ugh, practical reasons. Nobody hits the ground studying relativity without having first studied and understood classical mechanics. Knowing only Newtonian mechanics, Henry Cavendish pointed out in 1784 that light passing by a massive object like a star should be deflected due to gravity. At about the same time, another natural philosopher pointed out to Cavendish that the density of a body could become so great that its escape velocity would exceed the speed of light:

    http://en.wikipedia.org/wiki/Black_hole

    When you claim a theory has been 'falsified', with no other modification, it implies that any result predicted by that theory is unreliable and should be discarded. This is clearly not the case with Newtonian mechanics.
     
  14. micromass

    micromass 18,430
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    I don't think that is the case at all. For example, I don't think anybody will contest that the geocentric model by Tycho Brahe has been falsified, but it still yields many results that are quite reliable and accurate. Of course, now we have a theory that is even more accurate.

    Newtonian mechanics claimed to be universally true. That has been falsified. On a certain domain (for example, low speeds), the theory still makes amazingly reliable and accurate predictions, hence it is still used. But the original framework did not provide such a domain.
     
  15. Matterwave

    Matterwave 3,774
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    Would you contend that the Bohr atom has not been "falsified"? The Bohr atom would be another example of a theory of physics which we still teach, which still has its area of applicability, but which has been falsified. I did not make any such statements that you just made in my original post. I didn't say that we should not teach Newtonian mechanics, nor did I say that it had no areas of applicability. I did not call it a "tired dogma". I only said that it was experimentally falsified.


    @Nurgatory: I do see where you're coming from. I suppose it wouldn't hurt, for the reasons you stated, for me to put some more caveats in my original statement. But as stated, I stand by it as correct.
     
  16. You could say Newton's law is a part of Newton's theory of gravity. You could also throw in there Newton's laws of motion and call it "theory of gravity and motion". Only a single equation can be a law, theory is a collection of equations.

    I think what you really wanted to know is why none of Einstein's equations are called a "law". And why indeed. Possibly because Newton's laws actually make sense and could be tested more directly. I don't see any other difference.
     
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