Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Logical Error of Cavendish Experiment

  1. Jan 30, 2009 #1
    Following Newton's new law of universal gravitation an experiment was devised to measure the faint gravitational attraction between two lead balls Wikipedia Cavendish experiment. This was a way of 'weighing the world'. The mass of the Sun could then be calculated from the value of the mass of the Earth. The mass of any star could then be measured by comparing it to the mass of the Sun. But there was an initial logical assumption made; that the nature of the matter comprising the entirety of the Earth is the same as that of the lead balls. This is not necessarily the case. The concept of entropy was unknown in the past. It is possible that the matter at the center of the Earth has a lower entropy compared to surface baryonic matter, for example. This would imply that the calculated density of Earth, 5.5 times that of water, is an overestimate. The mass of the Sun could therefore be an overestimate etc. Is it possible that there simply isn't a Missing Mass Problem after all? Wikipedia Dark Matter.
     
  2. jcsd
  3. Jan 30, 2009 #2
    I am not sure where you are going with this. Is this a homework assignment?
     
  4. Jan 30, 2009 #3

    jtbell

    User Avatar

    Staff: Mentor

    How does that follow? I don't see the connection.

    There have been a number of experiments ("Eötvös experiments") comparing the gravitational force on different substances. As far as I know, all of them found no difference, sometimes with very high precision. Of course, we haven't compared all possible pairs of substances.
     
  5. Jan 30, 2009 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2015 Award

    Gravitational attraction depends on mass, not entropy. Why do you think Cavendish made an error?
     
  6. Jan 30, 2009 #5
    Let's take Cavendish's bent on things. He wanted NOT to determine G, rather he wanted an estimate on the mass of the Earth.
    If you were in Cavendish's time, then you may not know Gauss's theorems yet (since Gauss came later). What you could do, is assume that the Earth is a uniform sphere. Since the entire Earth attracts anything beyond it integrated envelope (the surface) with an inverse square force, we can estimate the mass of the Earth by comparing the force exerted on a test mass by a known mass. The ratio brings you close. Knowing the size of the Earth gives you an estimate on density.

    Remember, although Cavendish probably knew that the Earth's composition was not uniform, this experiment is okay since it assumes a uniform density (he had to). Now that we have Gauss's law, we can okay this assertion.

    I think what you are implying is that some matter may possess gravity that does not roll off like the inverse square. IF this were the case and exotic matter is at the core, than we have not seen it. Furthermore, you can check for yourself to see what would happen if you added higher order terms to a static inverse square force.

    Have fun, hope this helps.
     
  7. Jan 30, 2009 #6
    This is more like it. This is what I'm talking about. Newton's law of universal gravitation is essentially an assumption which works very well on planetary scales. But there is another possible solution. The fundamental particle is not necessarily a point object which radiates gravity uniformly. A string shape of the fundamental particle would give an asymmetric gravitational field. The order of the particles is then a factor which determines an object's gravity field an how it reacts within the gravity field of another object. Entropy is an essential part of chemistry. I don't understand why so many people have a problem with considering this possibility when applied to matter.

    The assertion of Newton's law of universal gravitation has led to the assumption that there isn't exotic matter at the center of the Earth. It would have been an easy mistake to make, considering that all objects of different weights were found to accelerate at the same rate in freefall.

    Perhaps the forthcoming LHC results will lend credibility to this simple idea.
     
  8. Jan 30, 2009 #7

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Would you care to then explain why the most accurate measurements of gravity at the micron scale out of, say the U. of Washington, still verify the validity of Newtonian gravity?

    It appears that if we go by purely empirical data alone, that there's more support for Newtonian gravity than your scenario, which has no empirical evidence at all.

    Zz.
     
  9. Jan 30, 2009 #8
    I think we should stop the thread here. The comment I made started with a capital "IF." This implies that you would go an compute the force roll off using
    1/r^2 + beta/r^3 + gamma/r^4 etc.... (It's easy with pad and pen. Start with the taking derivatives.)
    Such deviation would be measurable over years of experiments. But we don't see them on a large number of scales. Therefore, the model of Cavendish's experiment and the models of gravity do not need extra factors. In lesser words, Cavendish's experiment was fine.

    And keep in mind entropy affects energy distribution. Mass can be thought of as the localization of energy.
     
  10. Jan 30, 2009 #9

    russ_watters

    User Avatar

    Staff: Mentor

    Science operates on positive evidence, not negative evidence. Currently, there is no evidence that there isn't an invisible purple dragon in my garage, but that does not make it ok to assume that there is an invisible purple dragon in my garage. The same goes for this "exotic matter" in the center of the earth. We need a reason to believe it exists: an absence of evidence (some might quibble with that anyway...) that it doesn't doesn't qualify.
     
  11. Jan 31, 2009 #10
    There is positive evidence that Newton's law of universal gravitation doesn't fit with the world of quantum mechanics. If this wasn't the case then I wouldn't be suggesting the idea of exotic matter. It is a very subtle concept which needs a great deal of contemplation. If there was exotic matter at the center of the Earth, it wouldn't be easily detectable. The shape of the Earth's tidal earth bulge would be slightly 'shield shaped', having an added central acceleration. There is some scientific work in this area which is of course very controversial. See 'gravity shielding' experiments Wikipedia Gravitational Shielding

    There are serious considerations by some members of the scientific community for the need of a fundamental review of gravitational theory. This paper for example: General Theory of Relativity: Will it survive the next decade?
     
    Last edited: Jan 31, 2009
  12. Jan 31, 2009 #11

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    You say there's positive evidence, but cited NONE. Your link about "gravity shielding experiments" refers to Wikipedia in which the first sentence said "...hypothetical process of shielding an object from the influence of a gravitational field...." This is NOT evidence! There are none! The Podkletnov effect, which is the closest thing one might claim to be a gravitational shield, has been discredited and has not been duplicated independently.

    Furthermore, gravitational shielding has nothing to do with "error" in newtonian laws! It just means that there are ways to generate "negative" gravity.

    Again, and this is the last time I will ask this, please provide peer-reviewed citations to back up what I asked earlier, keeping in mind the PF Rules that you agreed to upon joining this forum.

    Zz.
     
  13. Jan 31, 2009 #12
    I believe I am being reasonable. The 2006 paper General Theory of Relativity: Will it survive the next decade? states:

     
    Last edited: Jan 31, 2009
  14. Jan 31, 2009 #13

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    You seem to not know the difference between theoretical predictions or expectations, versus PHYSICAL OR EMPIRICAL EVIDENCE. There are no experimental evidence so far to indicate the failure of the current gravitational description. None!

    There has been a lot of searches for such a violation, especially in terms of extra dimensions. I can cite for you a series of them. I know quite a bit about them. But there hasn't been one single experimental verification yet! So how come you think you can claim of a "logical error" in any of them when the physical evidence isn't there? If all we care about is to accept something as valid is some theoretical hypothesis, why do you think we spend billions of dollars to build the LHC when the Higgs is "logically" predicted to exist?

    Zz.
     
  15. Jan 31, 2009 #14
    I agree with what you are saying; there is no direct evidence to suggest that GR is wrong. There is scientific investigation into this idea though. It isn't a taboo subject anymore.

    Note that one of the authors of the given paper works for NASA JPL Slava G. Turyshev
     
  16. Jan 31, 2009 #15

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Where the authors work has no relevance to this thread. And no one is saying this is a taboo subject, that is also irrelevant to this thread. In physics, nothing is taboo.. Our main job IS to study things that are not known, and to push the boundary of the validity of our knowledge. That's why most of us, including me, get employed! But you somehow are putting the cart before the horse by a series of misunderstanding, and proclaiming that something is logically incorrect and thus, proven to be wrong simply by a series of unverified theoretical predictions.

    You should re-examine the process you use to draw up your conclusions.

    While you are at it, read one of the latest tests on Newtonian gravity up to the 10 micro scale[1]. You may want to dial back a bit of the certainty you wrote in Msg. #6.

    Zz.

    [1] A.A. Geraci et al. Phys. Rev. Lett. v.78, p.022002 (2008).
     
    Last edited: Jan 31, 2009
  17. Jan 31, 2009 #16
    I agree that the thread title may be considered controversial or even misleading. I am still of the persuasion that there is an implicit assumption that the whole Earth is made of matter which is gravitationally equivalent to 'everyday' matter.
     
  18. Jan 31, 2009 #17

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I have no idea what that means, and since you have no valid support to make such a decision, there's no way either I or even you can distinguish that with our current models. In other words, what in the world gave you the idea of such a thing, when you have no valid evidence to support it. How did "cavendish experiment", which has been shown to be VERY valid up to the micro scale, lead to you coming up with what's in the center of our earth, especially now when it has been shown that your starting premise (i.e. Cavendish expt. being wrong) has been shown to be false?

    Zz.
     
  19. Jan 31, 2009 #18
    I'm not saying that the Cavendish experiment is wrong. It's that Newton's law of universal gravitation isn't necessarily correct i.e. 'that all objects attract one another equally, relative to their mass'.
     
  20. Jan 31, 2009 #19

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    But the Cavendish experiment WAS testing Newtonian gravitation! All the more sophisticated tests done by the Adelberger group[1] at U. of Washington is a more glorified Cavendish experiment!

    So what you're saying here is self-contradicting.

    Zz.

    [1] E G Adelberger et al. Ann. Rev. Nucl. Part. Phys. 53 77 (2003).
     
  21. Jan 31, 2009 #20

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2015 Award

    What made you think it ever was?

    I find it hard to believe that you read the paper of Bertolami et al. The very first page discusses the fact that the community "witnessed an impressive progress in this area [tests of GR] over the last two decades." That means that it has been being tested for the last two decades - hardly taboo.

    Figure 1 - the very first one - covers tests over the last 30 years. References in the paper discuss measurements from the 90's, the 80's, the 70's, the 60's and the 50's. When exactly was this period where tests of GR were taboo again?

    Furthermore, the very ideas you are pushing here - deviations from inverse square and deviations from uniform free fall are discussed in the paper, and the extremely stringent limits on these effects are shown. ZapperZ mentioned Adelberger's work - he is referenced a half dozen times in the Bertolami. How can you then be ignorant of what he has done? I don't understand this at all!

    As far as the specific claim you are raising about Cavendish, how can you reconcile that with Figure 2, which shows the strength of possible new forces? (or, equivalently, deviations from Newtonian gravity) It shows the second best measured range is roughly earth-sized, and we know that there are no deviations from Newton in that range 100x better than we know G itself.

    I don't know how anyone who read that paper could write what you write - your own reference disputes what you are saying.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Logical Error of Cavendish Experiment
  1. Logical Errors (Replies: 21)

  2. 1798 - Henry Cavendish (Replies: 8)

Loading...