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That's a very common sentiment, but a mistaken one. Theories do not get accepted only if they match the predictions of the previous theories. People value other

by effie 11y ago

That's a very common sentiment, but a mistaken one. Theories do not get accepted only if they match the predictions of the previous theories. People value other features besides accuracy of predictions, like simplicity and explanatory power. Just recall how Kopernik's theory of solar system got accepted. It had worse predictions than Ptolemy's scheme at the time it was introduced; Ptolemy's scheme was way better in accuracy, but utterly complex and explained little.

pdonis 11y ago
You're missing the point. I agree that matching the predictions of experiments (not previous theories--I'm talking about experimental results that match the predictions of GR, not just those predictions themselves) is not a sufficient condition for a theory to be accepted (which is what you are saying); but it is certainly a necessary condition (which is what I was saying). > Just recall how Kopernik's theory of solar system got accepted. It had worse predictions than Ptolemy's scheme at the time it was introduced Yes, and it wasn't accepted at the time it was introduced. Actually, Copernicus' theory in its original form was never really "accepted"; what was accepted was Kepler's reformulation using elliptical orbits, based on Brahe's more accurate observations. Kepler's model was more accurate than Ptolemy's, and that was a key factor in its acceptance.
- 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.)
        11y ago

by effie 11y ago

pdonis 11y ago

You're missing the point. I agree that matching the predictions of experiments (not previous theories--I'm talking about experimental results that match the predictions of GR, not just those predictions themselves) is not a sufficient condition for a theory to be accepted (which is what you are saying); but it is certainly a necessary condition (which is what I was saying). > Just recall how Kopernik's theory of solar system got accepted. It had worse predictions than Ptolemy's scheme at the time it was introduced Yes, and it wasn't accepted at the time it was introduced. Actually, Copernicus' theory in its original form was never really "accepted"; what was accepted was Kepler's reformulation using elliptical orbits, based on Brahe's more accurate observations. Kepler's model was more accurate than Ptolemy's, and that was a key factor in its acceptance.

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.)
      - 11y ago