5 ms·

I started this comment thread, so I'm quite confident I know what it's about. It's specifically about the fact that the article makes no mention of a simple, i

I started this comment thread, so I'm quite confident I know what it's about. It's specifically about the fact that the article makes no mention of a simple, important aspect of the underlying reality it's modelling. It'd take only a sentence or two to clarify and provide a stronger foundation for those learning. As evidenced by responses here, it's obvious this is needed. We all should be keenly aware that models only approximate reality, they always get it wrong and the matter is the magnitude of this error. That's important. A student starting out with the idea that the path of one body in a coupled rotation around another is a perfect ellipse or halves of a cone is a wildly idealised version of an underlying reality that's practically different. Emphasising the reality, that there is a natural, built-in, constant pull or push toward entropy and dissolution of the shape of an "orbit" should be essential. It's that part the student will most likely spend their career fighting against.

pdonis 3y ago
> the reality, that there is a natural, built-in, constant pull or push toward entropy and dissolution of the shape of an "orbit" should be essential Again, what are you basing this on? What theory or evidence tells you that "the reality" is what you say it is?
- rhn_mk1 3y ago
  I think the "constant" is throwing off the message here. I think the parent is trying to summarize all the unreliable, occasional, and probablilistic factors that models ignore (because they are hard to model), but which still influence practical outcomes of every situation.
  - pdonis 3y ago
    No, that won't do. Model builders are well aware that there are factors that aren't included in the models. But in many domains, and orbital mechanics is one of them, the accuracy of our experimental knowledge is high enough that we can be sure that whatever factors are not in our models are negligible. So just waving one's hands and muttering "entropy" or "dissolution" or "something something unreliable, occasional, and probabilistic factors" doesn't cut it in this domain. detrites needs to back up the very strong claim he made, that I originally responded to, with something much more specific than that.
    - detrites 3y ago
      The parent is correct, and maybe we're somewhat at cross-purposes here. I'm not criticising the model, I'm criticising the way it's presented. I find it's common in older texts to present a model as if it is reality. And why, ever, do this? It's only bad. As you state, model creators are well aware of the limitations. But, this text is targeted at learners. They're just beginning such a journey and may not at all be so keenly aware of such. I can't supply what you seek of something within the model that violates the model. Which is fine, as it's beside the point and target of my criticism, which isn't of the model but of one aspect of the style used to present it.
      - pdonis 3y ago
        > I can't supply what you seek of something within the model that violates the model. That's not what I asked for. I asked you what you are basing this claim on: "All "orbits" are in reality, spirals. Away, or toward things they're orbiting." Waving your hands and saying "entropy" or "dissolution" or "I don't like the way the models are presented" does not answer that question. Here's an example of a valid answer to the question, but for a particular case, not for "all" orbits: the "orbit" of the Moon around the Earth is in fact not a closed ellipse but a spiral, slowly spiraling away from Earth. Why? Because the action of the Moon's gravity on the tidal bulge in the Earth caused by the Moon causes the Earth's rate of spin to slowly decrease and the Moon's orbital energy and angular momentum (which determine its perigee and apogee) to slowly increase. But note that in this particular case, the effect has nothing to do with "entropy" or "dissolution", and the model that includes it is commonly presented in textbooks and is not a mystery at all. Here's another example for a different particular case: the Hulse-Taylor binary pulsar. The "orbits" of the two pulsars around each other are not closed ellipses: the pulsars are slowly spiraling towards each other as they emit gravitational waves, which carry away energy and angular momentum. Here, again, the effect has nothing to do with "entropy" or "dissolution", and the model that includes it, while generally only presented to students majoring in physics, is again not a mystery at all. But these are just two particular cases. A valid answer from you to my question about your claim, which claimed to cover all cases ("all" orbits), would be something along the same lines as above, but citing an effect which is known to be present in all orbits.

pdonis 3y ago

> the reality, that there is a natural, built-in, constant pull or push toward entropy and dissolution of the shape of an "orbit" should be essential Again, what are you basing this on? What theory or evidence tells you that "the reality" is what you say it is?

rhn_mk1 3y ago
I think the "constant" is throwing off the message here. I think the parent is trying to summarize all the unreliable, occasional, and probablilistic factors that models ignore (because they are hard to model), but which still influence practical outcomes of every situation.
- pdonis 3y ago
  No, that won't do. Model builders are well aware that there are factors that aren't included in the models. But in many domains, and orbital mechanics is one of them, the accuracy of our experimental knowledge is high enough that we can be sure that whatever factors are not in our models are negligible. So just waving one's hands and muttering "entropy" or "dissolution" or "something something unreliable, occasional, and probabilistic factors" doesn't cut it in this domain. detrites needs to back up the very strong claim he made, that I originally responded to, with something much more specific than that.
  - detrites 3y ago
    The parent is correct, and maybe we're somewhat at cross-purposes here. I'm not criticising the model, I'm criticising the way it's presented. I find it's common in older texts to present a model as if it is reality. And why, ever, do this? It's only bad. As you state, model creators are well aware of the limitations. But, this text is targeted at learners. They're just beginning such a journey and may not at all be so keenly aware of such. I can't supply what you seek of something within the model that violates the model. Which is fine, as it's beside the point and target of my criticism, which isn't of the model but of one aspect of the style used to present it.
    - pdonis 3y ago
      > I can't supply what you seek of something within the model that violates the model. That's not what I asked for. I asked you what you are basing this claim on: "All "orbits" are in reality, spirals. Away, or toward things they're orbiting." Waving your hands and saying "entropy" or "dissolution" or "I don't like the way the models are presented" does not answer that question. Here's an example of a valid answer to the question, but for a particular case, not for "all" orbits: the "orbit" of the Moon around the Earth is in fact not a closed ellipse but a spiral, slowly spiraling away from Earth. Why? Because the action of the Moon's gravity on the tidal bulge in the Earth caused by the Moon causes the Earth's rate of spin to slowly decrease and the Moon's orbital energy and angular momentum (which determine its perigee and apogee) to slowly increase. But note that in this particular case, the effect has nothing to do with "entropy" or "dissolution", and the model that includes it is commonly presented in textbooks and is not a mystery at all. Here's another example for a different particular case: the Hulse-Taylor binary pulsar. The "orbits" of the two pulsars around each other are not closed ellipses: the pulsars are slowly spiraling towards each other as they emit gravitational waves, which carry away energy and angular momentum. Here, again, the effect has nothing to do with "entropy" or "dissolution", and the model that includes it, while generally only presented to students majoring in physics, is again not a mystery at all. But these are just two particular cases. A valid answer from you to my question about your claim, which claimed to cover all cases ("all" orbits), would be something along the same lines as above, but citing an effect which is known to be present in all orbits.