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Insights "Classical Physics Is Wrong" Fallacy - Comments

  1. Mar 15, 2018 #1

    ZapperZ

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  2. jcsd
  3. Mar 15, 2018 #2

    Ibix

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  4. Mar 15, 2018 #3
    - "Classical physics is used in an overwhelming majority of situations in our lives." so what? do examples prove that a theory is correct? this argument makes no sense at all.
    - "any new theory or description must somehow converge and look like the classical physics description under such ordinary conditions.". Who said this "must" be true? The point here is that SR and QM deal with different concepts of reality which are not accounted in classical mechanics. We are looking at the old theories with our "present" eyes. We can't interpret the past this way. The assumptions are totally diferent.
    - "They have identical form!". Again: having the identical equation form proves what? that they have the same assumptions? that they work on the same space with the same properties? of course not!

    It does not matter if you derivade classical physics from modern theories, they have different assumptions! they do not talk about the same thing!
     
  5. Mar 15, 2018 #4

    jbriggs444

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    If a new theory does not produce approximately same answers as the classical theory in the realm where the classical theory has been tested experimentally and demonstrated to be accurate then that new theory must not be correct.

    Experiment is the final arbiter of correctness.
     
  6. Mar 15, 2018 #5
    Another great Insight, thanks Zapperz!
     
  7. Mar 15, 2018 #6
    Of course the new theory will have to explain the old stuff, but it does not mean that it will "converge" at some point to the old one. there is no connection between the two things.

    Apart from that:
    - Newton considered space as an R³ euclidean and time as absolute.
    - Mass and energy were not connected the way we know now.
    - How would we explain the double slit experiment?

    The assumptions can't be ignored.
     
  8. Mar 15, 2018 #7

    Ibix

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    I think you are rather missing the point. If you are travelling at 30mph down the road and a car passes you going 30mph faster, you can calculate its road speed using Newtonian or Einsteinian relativity. You will get the same answer to the precision you can plausibly measure. You need to take into account the velocity variation from flies crashing into the front of the car long before you need to care about Einstein. So simplifying the maths and using Newton isn't wrong. How can it be? Unless you're saying it's wrong to neglect the flies. And if you believe that then you will get nothing done ever.
     
  9. Mar 15, 2018 #8
    I never said that Newtonian mechanics was wrong. never said that. I said that there is no such a thing as a convergence from a new theory to an old one based on some arbitrary limit as they start from different assumptions.
     
  10. Mar 15, 2018 #9

    jbriggs444

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    So your objection is to the term "convergence", taken as an indication that the new theory must, in the appropriate limit, be precisely identical to the old theory and not merely identical to within current experimental uncertainties.

    Yes, that is certainly true. [Though precise agreement in the limit is what we have come to expect]
     
  11. Mar 15, 2018 #10

    ZapperZ

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    In Helen Quinn's essay in Physics Today, she emphasized the same issue:

    There is an under-emphasis of the significance and importance of the claim that "it works!". Nothing is more powerful than having experimental evidence, and having it being verified and used every single day. Sure, as an experimentalist, I am probably very bias towards experimental evidence. However, this is what clearly separates science from non-science, and what makes acceptance in physics extremely difficult due to the requirement of having clear and convincing empirical evidence.

    Zz.
     
  12. Mar 15, 2018 #11

    Dale

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    Obtaining examples where a theory works or doesn’t work is the whole purpose of experiments in the scientific method. We don’t usually use the word “prove”, but examples do verify or falsify theories.
     
  13. Mar 15, 2018 #12
    Thank you for an enjoyable, edifying read.
     
  14. Mar 15, 2018 #13

    Dale

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    @ZapperZ great article! This is why, more than 100 years after Einstein, students still learn Newton. I have had discussions with people who are convinced that because Einstein proved Newton wrong someone will eventually prove Einstein wrong. But future generations of students will still need to learn relativity even if a Lorentz violating theory is eventually verified.
     
  15. Mar 16, 2018 #14
    Well obviously Newtonian mechanics is "wrong" since it does include relativity or quantum mechanics but that does not imply that it "shouldn't be used". Newtonian mechanics is "wrong" in the sense that it is not the real Universe but that does not imply that it is wrong for you to use it because usually the error intrinsic to Newtonian mechanics is small enough that you can safely ignore it.
     
  16. Mar 16, 2018 #15

    Orodruin

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    I am sorry, but this is just ignorant and the limit is not arbitrary. Obviously you will not recover classical mechanics in the ultra-relativistic limit, you will recover it in the classical limit where speeds are much smaller than the speed of light.

    In physics and any empirical science, a theory is nothing else than the sum of its predictions. This is why discussions on quantum mechanics interpretations tend to degenerate and people either leaving upset or agreeing to disagree.

    The first of these follows directly in the classical limit of SR. The second also falls out of the theory as separate conservation of mass and kinetic energy at small velocities.
    When it comes to light it is very well explained using Maxwell's theory. When it comes to particles such as electrons, it is not a prediction of the classical theory but your logic is completely flawed. You have taken one of the shortcomings of the classical theory that inspired people to QM and presented it as a counter argument to the quantum theory having to reduce to the classical theory in the limits where we know that the classical theory works. We already know that the classical theory does not work in this limit and so your argument is empty. Essentially you argument to the statement "A must give the same predictions as B when C is true" is to say "but it does not give the same predictions when C is not true". We come back to your assertion that the limit is arbitrary which, again, is not the case.
     
  17. Mar 16, 2018 #16

    Dale

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    Obviously it is not wrong. It has been experimentally verified by a large number of experiments.

    People tend to think of experimental falsification as much stronger than experimental verification. But it is not. The fact that a theory later produces incorrect results in some new domain does not change the fact that it does produce correct results in the previous domains.

    So “wrong” would be a description for a theory that is never correct in any domain. Other than that theories are “applicable” or not to a given domain, or they are more or less “general” than another theory.
     
  18. Mar 16, 2018 #17

    anorlunda

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    Great article @ZapperZ . I especially liked the part where you talk about the thought processes of the public as compared to physicists (I would use the broader term "technically trained" instead of physicist) .

    We usually discourage talk about philosophy here on PF. But in the meaning of the word, "thinking about thought" it goes the heart of your point. I believe that you are correct; that fundamentally different thought processes forever divide the technical and liberal views of our world.
     
  19. Mar 16, 2018 #18
    What difference does it make if you get the old theory from an upper limit or any other arbitrary direction? My point was that SR is considering space and time as completly different things comparing to old mechanics. This does not make any sense. If I say that the assumptions are: time is relative and space is Riemaniann and you say: time is absolute and space is euclidian, how can we be talking about the same theory?

    No, you are wrong. What I said was that the assumptions are different! If you look at how we treat all the experiments and particles / waves in quantum world, we see that space, time, energy and whatever are all different from what we knew from classical mechanics, which means, we cannot start to say that there exists a derivation from one to another, as they were talking, ever, about the same thing. Again, if you start with different assumptions how can you end up with theories converging at a limit that has been chosen to find a connection between them?

    I am not saying that Newtonian Mechanics is wrong. this has nothing to do with being right or wrong. I'm talking about how do we explain an theory evolution without wanting to find convergencies that were created just to make understanding easier.
     
    Last edited: Mar 16, 2018
  20. Mar 16, 2018 #19

    ZapperZ

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    Actually, it does! That's the whole point of my writing the article, that we frequently have members here claiming that Newtonian mechanics is wrong, and questioned why we continue to use it! If you make no such claim, then the article wasn't meant for you.

    You also read WAY too much philosophical implication to what the article had said. First of all, I never said anything about any "proofs" that one theory equals the other. All I did was show that, in certain limits or situation, one theory can reproduce the mathematical form of the other. None of what I had shown should be a surprise, because we all saw this in undergraduate physics courses!

    Furthermore, we do not have to go to wide extreme to see this. Even within classical mechanics itself, the Newtonian mechanics and the Hamiltonian/Lagrangian mechanics already have philosophically different approach and "world view". And yet, they both arrive at the same mathematical form in describing the kinematics of a system.

    Being able to show that something can be derived into a familiar form is a powerful and extremely useful argument. This is done in mathematics all the time. The ability to reformulate a differential equation into something that we know the solution of is done often. So it is no different here in physics. It says that the new idea can reproduce all of the results of the old ideas, and also shows why and where the old ideas may fail or no longer accurate enough.

    Zz.
     
  21. Mar 16, 2018 #20

    Orodruin

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    Uhmmm ... Are you serious or just trolling? Obviously it matters that you recover the old theory in the limit where the old theory is known to be applicable or if you do so in the opposite limit.

    We are not. We are only talking about the limit of one of the theories. In the classical limit you do recover the very same things as the classical theory. That SR is applicable to a larger set of situations is not the issue here.

    This is just wrong. As already illustrated in the actual Insight. You can start from different assumptions, but in the end what it boils down to is to make identifications of what concepts in a theory that correspond to the concepts of the previous. A theory is not its underlying assumptions - which can never be tested, it is its predictions.

    I am sorry but you are not making any sense here. The point is that the "old" theory typically has made a large number of verified predictions that are well studied enough that we know how experiments behave in a certain range - at least to within experimental uncertainty. Under the same type of conditions, the "new" theory must therefore reproduce exactly the same results up to corrections that are smaller than the experimental uncertainty. This is what it means having the "old" theory as a limiting behaviour. It has absolutely nothing to do with what "basic assumptions" have been made, just about predictions.
     
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