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Newton said that all frame of reference are equally valid

  1. Jun 2, 2008 #1
    I'm not sure if either Einstein or Newton said that all frame of reference are equally valid. That from my point of view, the laws of physics is the same from your point of view. All frame reference can be put on an equal footing. I was wondering, can one consider the geocentric view that the earth as the center of the universe to be equally valid as the sun as the center? That the astronomical model that Ptolemy with epicycles can be viewed on a equal footing as the commonly accepted model where the sun is in the center of the solar system.

    I know that it's tempting to say that because the sun center model is simpler and hence more elegant and "better". But I think that it is possible to construct an entire astronomical paradigm having the earth as the origin of reference point. Please let me know what you think.
     
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  3. Jun 2, 2008 #2

    dst

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    I wondered this once. The thing is that the Earth and Sun and indeed, any two binary systems will orbit around their common center of mass. Given that the mass of the sun vastly outmeasures the mass of the Earth, this barycenter is for all intents and purposes, inside the Sun. Saying the planets orbit around the sun is only a slight approximation.

    Yes, all reference frames are equally valid and likewise, you can take either Earth or Sun to be the orbital center. However, Jupiter does not orbit the Earth as an approximation, nor do the other 6 planets (sorry Pluto :frown:). In effect we have a multiple body system in which one of those is, to put it concisely, bloody well huge.

    If I remember correctly, the method used to manipulate scenarios like this is Lagrangian mechanics which doesn't change for anything and the information you can get is limited anyway.
     
    Last edited: Jun 2, 2008
  4. Jun 4, 2008 #3
    I'm not sure about Einstein, but in the Newtonian scheme, the laws of physics must have the same form in all *inertial* frames. With the solar system, we immediately have a complication - the planets are all accelerating and so are *not* inertial frames. Nonetheless, the Ptolemic model was used successfully for many centuries, so the fact that Earth is a non inertial frame can't have made too much difference(?)
    As to which of the two views is better, I suppose it comes down to which gives the best answers to our questions, i.e., which best describes our observations. It is a fact that the planets orbit the sun, and that the rest of the solar system does *not* orbit the Earth. Given this, surely we have to conclude that the Heliocentric view is 'better'.
     
  5. Jun 4, 2008 #4
    Oooh, I cant remember what the answer to this "common objection" is, someone enlighten me.

    If all frames are equally valid, can I not spin around on the spot, declare the sun the be doing the spinning, and thus make it travel faster than light?

    k
     
  6. Jun 4, 2008 #5


    Here's a quote which explains is quite nicely, got it from:
    http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#14

     
  7. Jun 4, 2008 #6
    I've read all your replies and went over it with a friend, and he said that it seems like each of us are answering slightly different questions and presupposing different things. Maybe if we each explain our positions a little further, we can get somewhere.
    I'm sorry I do not have much background in this area. This question just popped into my mind out of nowhere one day. I guess this question has more to do with perspectives than anything else. I've read somewhere that there is no such thing as an privileged observer and that all perspective should be treated equally. As a consequence, do we treat the sun's perspective as the same as the earth's. I think in modern Astronomy, we just assume that the earth is not the center and base everything on that.
     
  8. Jun 4, 2008 #7

    russ_watters

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    That is true. That's called the "principle of relativity".
    That's not exactly what the principle of relativity means. The principle of relativity does allow you to treat objects as stationary or moving depending on your frame of reference. Ie, if you are driving in a car and you pass another car, you can treat the earth as stationary and say you are moving at 65 mph while the person you passed was moving at 60mph. Or you could say the person you passed was stationary and you moved past him at 5 mph.

    Now you could use Newton's law of gravity to construct a model of the earth-sun system using earth as a stationary reference frame. But you'd find the model to be needlessly complicated. More importantly, that model wouldn't make much sense to someone on Mars or someone observing our solar system from the outside (and stationary wrt the stars). Depending on the purpose and precision of the calculations, you'd tend toward the conclusion that either the sun should be considered stationary or the barycenter of the system stationary. The calculations are simpler that way.
    Ptolmey's model of the solar system was an ad hoc mathematical model only. It had to be custom-built for each new object detected (and built over time, through observation of entire cycles) and couldn't make any predictions (it basically just played-back pre-recorded motions) and thus was not a real theory of gravity - it was just a model. Contrasting that with Newton's theory, you can take the current position and motion vector (or two positions and the time between them) and model the entire motion of the system (for an object much smaller than the sun). You can also do things like observe the orbit and calculate the mass of the objects involved. Newton's theory has real predictive power. It is a real theory.
     
  9. Jun 5, 2008 #8
    Newton explained the orbits.


    Newton gave an explanation for Keplers equations in terms of his gravitationsl force law and
    thereby negated the epicycles upon epicycle earth centered model.
     
  10. Jun 6, 2008 #9
    If one considered the earth stationary it would no negate any of the laws of gravity, ultimately one would just get equations of motions for each celestial body in the new non-inertial reference frame. The equivilance of reference frames would not allow you to recover epicycles and such as a predictive model.
     
  11. Jun 6, 2008 #10
    To restate in a perhaps more informative way: To say that the 'sun' or the 'earth' is the center of the solar system is quite arbitrary. The physically important feature is that the 'center of mass' is the most natural reference frame and that as long as the math is done correctly one can change the governing relations through a coordinate transformation to place your 'center' wherever you want. This however does not change anything or validate in any way the geocentric belief (which is not predictive).
     
  12. Jun 6, 2008 #11
    I'm not allowed to post URLs to other sites since I haven't made 15 posts yet. This is from the Conservapedia.com on the talk page of Geocentric Theory.


    I think this section stands in contrast to what have been said thus far.
     
  13. Jun 7, 2008 #12
    It depends on the problem...

    The thing is there is no true centre, so if we feel obliged to use some kind of co-ordinate system we are free to define the origin as we wish. It just so happens that if you are looking at the orbits of all the planets, the mathematics is easier if we consider the sun to be at the origin in the sense that the number of terms in the equations of motion are reduced.

    However, if you wanted to look at the path of an aeroplane flying from London to New York, it would be easiest to choose a different co-ordinate system - in fact a non-inertial, geocentric framework would be easiest (non-inertial because it spins in a geo-stationary sense).

    So to summarize, although any co-ordinate system is equally valid, the best one to use is the one which keeps the mathematics the simplest, which depends entirely on the problem..
     
  14. Jun 7, 2008 #13

    D H

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    A heliocentric frame is an accelerating (i.e., non-inertial) frame. Looking at the orbits of the planets is easiest in a barycentric frame. A barycentric frame is of course accelerating, but toward the center of the galaxy. The gradient of this acceleration is negligible over reasonably short distances (the span of the solar system) and thus can be safely ignored over reasonably short periods of time (thousands to millions of years).

    That said, any reference frame can be used. Newton's second law is not valid in an accelerating frame, but it can be made to appear valid by introducing pseudo-forces called "third body effects". Sometimes the use of these pseudo-forces makes "more sense" than working in an inertial frame. For example, the space agencies inevitably use a geocentric frame to model the orbit of an artificial satellite around the Earth.
     
  15. Jun 7, 2008 #14
    "Numquam ponenda est pluralitas sine necessitate"
     
    Last edited: Jun 7, 2008
  16. Jun 7, 2008 #15

    russ_watters

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    Except for a couple of points, it fits pretty closely with what I said:
    I said roughly the same thing. In any case, the reason no one does that is because it is a pointless thing to do.
    He's mixing a few things here: Ptolmey's model wasn't based on Newton's gravity, so it isn't correct to say that you'd be using "epicycles" to build a geocentric, Newtonian model. Ptolmey's model had no predictive power beyond what each invidividual, custom model could do.
    That's correct.

    One other thing that may or may not be minor. I spent some time at BadAstronomy and there were some Geocentrists there. A distinction was made between a capital and lower case "g" in "geocentrism". A lower-case "g" was used for a "weak" form of geocentrism that simply fit with the idea that any point could be used as a reference. A capital "G" was used to denote the earth being the one and only universally valid frame of reference, ie, Biblical Geocentrism.

    What you were trying to link wasn't a valild reference anyway, so we would have deleted the link. Conservapedia is a site with an adjenda and what you quoted was an ok part of a talk page (not the actual article on geocentrism) who'se purpose was to push the quite incorrect Biblical type of Geocentrism.

    Their biggest, most obvious flaw is the desire to have it both ways: they argue Relativity's version when it suits them (it supports the "weak" version - that any point can be considered the center), then try to discredit Relativity.

    I hope you didn't come here to try to bring that discussion here...
     
    Last edited: Jun 7, 2008
  17. Jun 8, 2008 #16
    Don't worry, I'm not here to argue Biblical type of Geocentrism or the weak version. However, I think I should reapproach the problem a little differently.

    I have a little text book called the "Introduction to Tensor Calculus, Relativity and Cosmology" and it said at the beginning of its preface that "The revolt against the ancient world view of a universe centered upon the earth, which was initiated by Copernicus and further developed by Kepler, Galileo and Newton, reached its natural termination in Einstein's theories of relativity. Starting from the concept that there exists a unique privileged observer of the cosmos, namely man himself, natural philosophy has journeyed to the opposite pole and now accepts as a fundamental principle that all observers are equivalent."

    The work of Einstein, Newton, Kepler, and Galileo was built around the Copernicus Principle, that the universe does not center around the Earth. There is a natural progression of scientific ideas starting with "the revolt" against the Earth-centered model to Newton's theories, and then terminating with Einstein's theories. Galileo (I think) was the one that said that the Earth was not the center, Kepler then provided the empirical equations of the movement of the planets with the sun as the center. Newton arrived at his three laws using Galileo's and Kepler's work. The three laws of Newton are in accordance with the fundamental principle upon which the special theory of relativity is based. The physics that they used relies on the philosophical assumption that the earth is the center. So my using Einstein's fundamental principle that all observers are equivalent hence the Earth-centered perspective is valid as the sun-centered one is not going to prove my case, so to speak, because Einstein's work assumes the Copernicus Principle to be correct (I used Einstein principle incorrectly anyway). It seems that I am using an argument to prove something in which the fundamental principle of that argument is already in contradiction with what I am trying to prove.

    So anyway, I think I will start anew again. To further weaken my case, measurements of the effects of the cosmic microwave background radiation in the dynamics of distant astrophysical systems in 2000 proved the Copernican principle on a cosmological scale. However, there are some good news for the people against the Copernicus princple. I got this article on Wikipedia on the Copernican principle.
    There is a slight chance that an experimental error occurred, but I'm betting that this is a genuine piece of evidence against the Copernican model.

    After talking to a friend about the problem again, what he had to say was that you do not lose any information switching from the complicated model to the more "elegant" model. Sure, with the Copernican principle as the fundamental cornerstone, you get Newton's laws and the theory of relativities. And yes, Ptolmey was only a mathematical model that can only playback recorded movement and have no predictive power. But I feel that we haven't developed that unused paradigm. I'm not actually arguing for Geocentrism, all I'm saying is that it is wrong to point at a model and discard it just because people developed an entire science based on an alternative principle. I think that people are afraid of the philosophical implications on science and our place in the world.
     
  18. Jun 8, 2008 #17

    D H

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    Wikipedia can be a great source. However, it must be taken with a grain of salt. Ideologues and crackpots are drawn to wikipedia like flies to balsamic vinegar. This article appears to be one such case. Look at the discussion for this page: http://en.wikipedia.org/wiki/Talk:Copernican_principle#Relevant_Information_Prejudicially_Removed:
    It helps to go to the source. The paragraph in the edge.org article following the paragraph quoted in the wikipedia article reads
     
  19. Jun 8, 2008 #18

    russ_watters

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    Could you please state explicitly what it is you are trying to prove? You're making a lot of arguments, but I haven't seen anywhere where you have said exactly what it is your point is.
    If that's your entire point, then it really is wrong. The problem is exactly what I said before: any model that will correctly predict the motions of the planets is necessarily reduceable to Newton's (or Einstein's, if your required precision is higher) theory. So not only do you not gain anything by doing geocentric calculations, you lose a lot in simplicity. It really is pointless - so the question is: why would you want to do that?
     
    Last edited: Jun 8, 2008
  20. Jun 8, 2008 #19
    You are right on the mark. As William of Ockham said, "Numquam ponenda est pluralitas sine necessitate"
     
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