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

Question concerning Composition Dependent Gravity

  1. Apr 5, 2009 #1
    I have been able to find lots of information about experiments that have been done to determine how different compositions of matter respond in a gravitaional field. However, I have not been able to find any information about experiments done to determine any difference in strength of the gravitational field produced by different compositions of matter. Does anyone know of any experiments along those lines, most likely involving the torsion balance?

    Since I have been unable to find any evidence of such experiments, I am lead to beleive that there may be some reason for it. If so, does anyone know the reason? Please note that I am speaking of very small differences. Obviously if there were any large differences it would have been noticed a long time ago. I think that both classical and GR both postulate that all mass attracts equally. But where are the experiments?
  2. jcsd
  3. Apr 5, 2009 #2
    You might find this in your school library, or contact the author.
    Annual Review of Nuclear and Particle Science
    Vol. 53: 77-121 (Volume publication date December 2003)


    E.G. Adelberger, B.R. Heckel, and A.E. Nelson

    Department of Physics, University of Washington, Seattle, Washington 98195-1560; email: eric@gluon.npl.washington.edu heckel@phys.washington.edu anelson@phys.washington.edu

    ▪ Abstract We review recent experimental tests of the gravitational inverse-square law and the wide variety of theoretical considerations that suggest the law may break down in experimentally accessible regions.
  4. Apr 5, 2009 #3
    Thank you for your speedy reply. I appreciate it very much. However, what I seek is not information on tests of the gravitational inverse-square law. Variations in gravitational force produced by various compositions of matter (if it exists) would not violate the inverse-square law. It is generally assumed or theorized (at least as far as I can tell, since I cannot find experimental evidence) that all matter, regardless of it's make-up, produces the same strength gravitational force based on mass alone. I am looking for experimental tests to determine if one form of matter produces the same strength gravitational field as another form, when both forms have the same inertial mass - such as copper and lead for example. It should, but has it ever been tested? There have been lots and lots of torsion balance experiments to determine if different forms of matter respond differently in the presence of an external gravitational field, but none (that I know of) that tests for any differences in the force of gravity produced by different forms of matter.
    Last edited: Apr 5, 2009
  5. Apr 6, 2009 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I think you're looking for the Eötvös Experiment, which you can Google.
  6. Apr 6, 2009 #5
    They also review lead vs. gold vs. copper etc. Do neutrons and protons both obey the same gravitational force?
    Last edited: Apr 6, 2009
  7. Apr 6, 2009 #6
    The Eotvos experiments, to the best of my knowledge, relied on the Earths gravitational field and did not directly measure or compare the strength of the gravitational field produced by the test samples.

    Would the experiments still attempt to answer this question if you changed obey to produce?

    All of the experiments I have investigated test how different samples react in an external gravitational field produced by a much larger body of unknown composition. To better clarify what I am looking for I will describe an example experiment. But the actual experiment could be anything that accomplishes the same objective.

    1. Obtain two objects of different composition.

    2. Form the two objects so that they are the same size and shape. (This is so that the shape of the gravitational field produced by the objects are the same - important for the torsion balance)

    3. Using a balance, measure the difference (as a ratio) between the inertial masses of the two objects.

    4. Using a torsion balance, measure the difference (as a ratio) between the gravitational force produced by the two objects.

    Result: The ratio obtained in step 3 should equal the ratio obtained in step 4 (allowing for equipment tolerances and errors) for any and all samples tested. If not, then we have found composition dependent gravity.

    Since I have been unable to find such experiments, I have concluded that either something is wrong with my line of thinking, or experiments for this objective have never been performed. I find the latter hard to believe. So I am also searching for the fault in my thinking.
  8. Apr 6, 2009 #7
    Just an update to say that I have found a paper describing an experiment of the type I am looking for. You can read it here:

    http://cos.cumt.edu.cn/jpkc/dxwl/zl/zl1/Physical%20Review%20Classics/gravity/032.pdf [Broken]

    The key words I should have used in my original post was passive and active gravitational mass. But I'm still learning (and it's good to know my line of thinking is not that far off). I would like to thank those who responded, and if anyone knows of any other experiments that test the equivalence of passive and active mass you can post them here.
    Last edited by a moderator: May 4, 2017
  9. Apr 6, 2009 #8


    User Avatar

    Staff: Mentor

    Yes, there are at several problems with the premise itself:

    1. "different compositions of matter" - There is no such thing on an atomic level. All atoms are made of exactly the same stuff.
    2. Your use of "matter" "mass" and "inertia" - they are different words for the same thing (or the implications of that thing), so you can't separate them in the way you are attempting. Ie, copper and lead have different mass/inertia because they contain different amounts of matter, not because they have different types of matter in them.
    3. There is no need to test on separate objects independent of earth to find the gravitational field strength. Due to #2, you are trying to separate things that are the same. An object that earth is pulling on is also pulling on the earth with the same force. Gravitational force is a function of the mass of both objects.
  10. Apr 6, 2009 #9
    Even if they're made of the same stuff, it doesn't necessarily mean they are the same. I didn't invent the term. Lots of experiments have been done involving different compositions of matter. See: http://www.npl.washington.edu/eotwash/intro/intro.html [Broken]

    Yes, I know about the Equivalence Principle. But this principle has been, and continues to be, tested all the time.

    I'm not sure what you mean. Are you referring to step #2 in my example experiment?

    Ok, I see you're refering to your number 2. Well, in the case of trying to test the Equivalence Principle, as it applies to passive and active gravitational mass, you do have to measure an objects gravitational strength independent of Earth. One of the functions of a torsion balance is to cancel out the Earths gravitational pull.
    Last edited by a moderator: May 4, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook