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I agree with you that if a new theory was to replace the old one for making specific set of predictions, it should give predictions of similar or better accurac

by effie 11y ago

I agree with you that if a new theory was to replace the old one for making specific set of predictions, it should give predictions of similar or better accuracy. But I do not think that replacement is necessary for the new theory to compete or be accepted; it is the new benefit it brings, whatever its nature may be, that is crucial. The two can temporarily both be accepted to coexist, if both have their strengths. For example, quantum theory does not make the same predictions as classical theory when it comes to classical experiments (mechanics, basic EM phenomena) and is largely useless in that domain. It only gives probabilities of results of specified experiments of certain kinds; it does not reproduce the old predictions (like definite trajectories, Moon phases or solar eclipses), but provides new results (like resonance frequencies of atoms and molecules and their bond energies). Similar thing can happen with a new theory of gravity; it may not give the same prediction for Mercury perihelion precession, but it may be able to explain other things, like why the inverse square law, why no repulsive gravity or why the mutual gravity force between electrons is so much lower than the mutual EM force. Explanation for oddities in Mercury motion could then wait for further data and repetition of calculations. It is natural to expect of any new theory to bring new results, but demanding that it reproduces all the old ones along is too much. That happens rarely and such expectation only prevents any new ideas from being considered.

pdonis 11y ago
> quantum theory does not make the same predictions as classical theory when it comes to classical experiments (mechanics, basic EM phenomena) Yes, it does. Do you know how the classical limit of quantum theory works? That limit is what allows us to use classical physics in the domain where it works. If that limit didn't work, we would have a serious problem with consistency. > It only gives probabilities of results of specified experiments of certain kinds; it does not reproduce the old predictions (like definite trajectories, Moon phases or solar eclipses) Are you aware that all of those "old predictions" can indeed be derived from quantum theory, using the classical limit I described above? The reason that works is that, in the classical limit, quantum theory predicts a probability of 1 for one result--the classical result. > It is natural to expect of any new theory to bring new results, but demanding that it reproduces all the old ones along is too much. You appear to have a mistaken understanding of how new theories get accepted. New theories that don't reproduce all of the predictions of the theory they replace, in the domains where the old theory is verified by experiment, are not accepted. If general relativity had not reproduced all of the predictions of Newtonian gravity in the weak field, slow motion limit, it would not have been accepted. And if quantum theory had not reproduced all of the predictions of classical physics in the classical limit, it would not have been accepted.
- effie 11y ago
  As I wrote above, I agree with you on the requirements for replacement theory. My point is that a new theory of a phenomenon does not need to replace and reproduce all the results of the old theory to be considered worthwile, competing, acceptable.
  - pdonis 11y ago
    Can you give an example of a new theory that was considered worthwhile even though it didn't replace and reproduce all the results of the old theory? I'm not aware of any. (The Copernicus example given upthread is not a valid example, as I said in response to that post.)
    - effie 11y ago
      Schroedinger's theory of hydrogen atom and his wave mechanics (1926). It explained positions of emission lines of excited hydrogen, but it didn't explain how the atoms lose excitation energy as there is no c and no spontaneous emission in that theory. Larmor's older theory (1897) explained how the energy is lost - by EM radiation - and gave formula connecting acceleration and losses that is used to this day. Joseph Larmor, LXIII, On the theory of the Magnetic Influence on Spectra ; and on the Radiation from moving Ions, Philosophical Magazine Series 5 Vol. 44, Iss. 271, 1897 Erwin Schrodinger, Quantisierung als Eigenwertproblem. Annalen der Phys. 384 (4) (1926)
      - pdonis 11y ago
        > Schroedinger's theory of hydrogen atom and his wave mechanics (1926). This was not a "new theory" that was competing with any "old theories". It was a tentative model in a regime where no previous theory existed, and it was never claimed to cover anything outside that limited regime. It wasn't competing with any other theories, because there were no other theories to compete with. The question of whether or not Schrodinger's model reproduced the predictions of the "old" theory never arose, because there was no "old" theory. (Technically, there was a sort of "old" theory of the hydrogen atom--Bohr's model--but Schrodinger's model did reproduce all of its correct predictions, plus it added more correct predictions of things that the Bohr model got wrong.) The position with regard to gravitational waves is very different; we already have a comprehensive, fundamental theory--General Relativity--that explains them. Any alternative theory that only explained GWs, and didn't also explain all the other experimental results that GR explains, would be a nonstarter. > Larmor's older theory (1897) This wasn't a separate "theory" at all; it was just a derivation of a particular formula using an already known theory, Maxwell's Equations.

by effie 11y ago

pdonis 11y ago

> quantum theory does not make the same predictions as classical theory when it comes to classical experiments (mechanics, basic EM phenomena) Yes, it does. Do you know how the classical limit of quantum theory works? That limit is what allows us to use classical physics in the domain where it works. If that limit didn't work, we would have a serious problem with consistency. > It only gives probabilities of results of specified experiments of certain kinds; it does not reproduce the old predictions (like definite trajectories, Moon phases or solar eclipses) Are you aware that all of those "old predictions" can indeed be derived from quantum theory, using the classical limit I described above? The reason that works is that, in the classical limit, quantum theory predicts a probability of 1 for one result--the classical result. > It is natural to expect of any new theory to bring new results, but demanding that it reproduces all the old ones along is too much. You appear to have a mistaken understanding of how new theories get accepted. New theories that don't reproduce all of the predictions of the theory they replace, in the domains where the old theory is verified by experiment, are not accepted. If general relativity had not reproduced all of the predictions of Newtonian gravity in the weak field, slow motion limit, it would not have been accepted. And if quantum theory had not reproduced all of the predictions of classical physics in the classical limit, it would not have been accepted.

effie 11y ago
As I wrote above, I agree with you on the requirements for replacement theory. My point is that a new theory of a phenomenon does not need to replace and reproduce all the results of the old theory to be considered worthwile, competing, acceptable.
- pdonis 11y ago
  Can you give an example of a new theory that was considered worthwhile even though it didn't replace and reproduce all the results of the old theory? I'm not aware of any. (The Copernicus example given upthread is not a valid example, as I said in response to that post.)
  - effie 11y ago
    Schroedinger's theory of hydrogen atom and his wave mechanics (1926). It explained positions of emission lines of excited hydrogen, but it didn't explain how the atoms lose excitation energy as there is no c and no spontaneous emission in that theory. Larmor's older theory (1897) explained how the energy is lost - by EM radiation - and gave formula connecting acceleration and losses that is used to this day. Joseph Larmor, LXIII, On the theory of the Magnetic Influence on Spectra ; and on the Radiation from moving Ions, Philosophical Magazine Series 5 Vol. 44, Iss. 271, 1897 Erwin Schrodinger, Quantisierung als Eigenwertproblem. Annalen der Phys. 384 (4) (1926)
    - pdonis 11y ago
      > Schroedinger's theory of hydrogen atom and his wave mechanics (1926). This was not a "new theory" that was competing with any "old theories". It was a tentative model in a regime where no previous theory existed, and it was never claimed to cover anything outside that limited regime. It wasn't competing with any other theories, because there were no other theories to compete with. The question of whether or not Schrodinger's model reproduced the predictions of the "old" theory never arose, because there was no "old" theory. (Technically, there was a sort of "old" theory of the hydrogen atom--Bohr's model--but Schrodinger's model did reproduce all of its correct predictions, plus it added more correct predictions of things that the Bohr model got wrong.) The position with regard to gravitational waves is very different; we already have a comprehensive, fundamental theory--General Relativity--that explains them. Any alternative theory that only explained GWs, and didn't also explain all the other experimental results that GR explains, would be a nonstarter. > Larmor's older theory (1897) This wasn't a separate "theory" at all; it was just a derivation of a particular formula using an already known theory, Maxwell's Equations.