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Equivalence Principle Test Kits

  1. Feb 9, 2015 #1
    I'd like to see for myself the Equivalence Principle. Would there be a commercially available test kit like perhaps a small iron and cotton enclosed in vacuum tubes and they are to fall at same time and hit the bottom at same time with indicator lights to tell if they hit at same time or at bit different time. I'd like to test this in different environments too like inside caves or mountains.

    Also is EP a Lorentz symmetry or Poincare symmetry or what symmetry does EP fall under and why is that?
     
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  3. Feb 9, 2015 #2

    phinds

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    I think you misunderstand what the equivalence principle is. It has nothing to do with the fact that objects in a vacuum but in a gravitational field fall at the same rate regardless of mass, it is ... well look it up for yourself.
     
  4. Feb 9, 2015 #3
    But it's related. See Wikipedia:

    Equivalence principle
    In the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.

    Development of gravitation theory[edit]
    Something like the equivalence principle emerged in the late 16th and early 17th centuries, when Galileo expressed experimentally that the acceleration of a test mass due to gravitation is independent of the amount of mass being accelerated. These findings led to gravitational theory, in which the inertial and gravitational masses are identical.

    The equivalence principle was properly introduced by Albert Einstein in 1907, when he observed that the acceleration of bodies towards the center of the Earth at a rate of 1g (g = 9.81 m/s2 being a standard reference of gravitational acceleration at the Earth's surface) is equivalent to the acceleration of an inertially moving body that would be observed on a rocket in free space being accelerated at a rate of 1g. Einstein stated it thus

    -----
    So Phinds. The consequence of it is the iron and cotton falling at same time. If this is not exactly called EP. Then what term do you use to describe it?
     
  5. Feb 9, 2015 #4

    Simon Bridge

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    The same time-to-fall for iron or cotton, afaik, has no formal name - it just is itself. It is not usually thought of as a consequence of the Einstein equivalence principle.
    The Einstein equivalence principle is how come you feel heavier when riding an accelerating lift upwards... just as if you were under higher gravity.

    The equal independence of gravity on the mass of an object was famously verified experimentally by Galileo.
    Galileo made his own equipment and so should you - it's not expensive or difficult.
    So make a hollow ball that you can put weights in and time how long it take to fall with different weights... or look up how Galileo did it.

    The short answer to your question is that there is no commercially available test rig for what you are asking because the equipment is universally available and cheap.
     
  6. Feb 9, 2015 #5
    An easier test would be to construct two pendulums (i.e., tie two things to pieces of string). Make the string lengths equal but let the weights at the bottom have different masses, or the same mass but different materials, or whatever. Time the period of each pendulum, or let them swing next to each other so you can visually compare their periods. If inertial mass is exactly proportional to gravitational mass, both pendulums should have the same period of oscillation.

    Probably you want both weights to be relatively heavy to minimize the effects of air resistance and the weight of the string.

    Newton wrote about performing such an experiment: http://www.mathpages.com/home/kmath582/kmath582.htm
     
    Last edited: Feb 9, 2015
  7. Feb 9, 2015 #6

    Simon Bridge

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    Pendulums are easy to mess up - heavier weights can get a longer period due to stretching the string or a shorter period due to being taller along the string (raising the center of mass) ... you also have rotational motion creeping in due to non-ideal conditions.
    To do this properly, you need Borda and Cassini's experiment... much more complicated than just dropping stuff or running weights down ramps.
     
  8. Feb 9, 2015 #7
    My impressions of the connection between the test and EP came from the so called Eotvos experiment in such (wiki):

    "The Eötvös experiment was a famous physics experiment that measured the correlation between inertial mass and gravitational mass, demonstrating that the two were one and the same, something that had long been suspected but never demonstrated with the same accuracy. The earliest experiments were done by Isaac Newton (1642–1727) and improved upon by Friedrich Wilhelm Bessel (1784–1846).[1] A much more accurate experiment using a torsion balance was carried out by Loránd Eötvös starting around 1885, with further improvements in a lengthy run between 1906 and 1909. Eötvös's team followed this with a series of similar but more accurate experiments, as well as experiments with different types of materials and in different locations around the Earth, all of which demonstrated the same equivalence in mass. In turn, these experiments led to the modern understanding of the equivalence principle encoded in general relativity, which states that the gravitational and inertial masses are the same."

    But Simon. If the Equivalence Principle didn't occur in nature. The iron and cotton won't fall at same time, would it? so they are kinda related...
     
  9. Feb 9, 2015 #8

    Simon Bridge

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    The words "the equivalence principle" has a special meaning in physics - it refers (usually) to the principle in general relativity where gravitation is equivalent to an accelerating reference frame. It is not usually understood to mean the equivalence between inertial and gravitational mass. Sometimes you do find lectures etc talking about the equivalence of inertial and gravitational mass as the weak equivalence principle. Notice that the passage you quoted is careful to distinguish them?

    It is easy to see how someone could get confused.

    That passage also points out that the modern understanding of gravitational effects, like how come gravitational and inertial mass are the same, is now understood in terms of the equivalence principle. It does not say that these experiments demonstrate the Einstein equivalence principle - which is what you [seemed to be saying] you wanted the apparatus to do.

    (Note: the Eotvos experiment does not demonstrate that different masses fall in the same time.)

    Of course the fall-time and the Einstein equivalence principle are related ... they are both about gravity.
    But demonstrating the first does not demonstrate the second. Otherwise GR would have been invented much earlier.

    I believe your question has been answered.
    You can purchase commercial Borda Pendulum apparatus if you are keen - or construct two of them cheaply.
    You can reproduce the Galileo experiments (ramps and balls, not leaning towers) cheaply.
    The usual high-school demo is just to drop stuff off high places and time the fall.
    There are other cheaper experiments you can do.
     
    Last edited: Feb 9, 2015
  10. Feb 9, 2015 #9

    PeterDonis

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    NASA sent a "test kit" to the Moon and verified that objects of very different masses (a feather and a hammer) fall at the same rate there; you can watch the video here:

    http://science.nasa.gov/science-news/science-at-nasa/2007/18may_equivalenceprinciple/

    Note that NASA attributes this to the "equivalence principle", illustrating (as has already been remarked on in this thread) that that term has several different possible meanings. Whether you call it that or not, though, this NASA experiment tests what you said you wanted to test in the OP.
     
  11. Feb 9, 2015 #10

    Simon Bridge

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    Although the NASA test kit (hammer and feather) are commercially available (purchase separately), the lab-space may be difficult to access without some more substantial financial outlay. But I suppose if someone were really really keen...

    MIT has a setup too:
    http://video.mit.edu/watch/feather-and-coin-in-a-vacuum-6407/
    ... again, all the apparatus is commercially available but I don't think they are sold as kits - but by the look of the one in the vid, maybe.
     
  12. Feb 9, 2015 #11

    PeterDonis

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    Good point. :D

    Cool, I hadn't realized they had this.
     
  13. Feb 9, 2015 #12

    Simon Bridge

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    ... yah. The expensive part will be the pump - I wonder how good-a vacuum you need for a good effect like that?
     
  14. Feb 9, 2015 #13
    Thanks. I learnt the distinction between the weak EP and the Einstein EP.. but there is a third one called the Strong EP.. Wiki defined or differentiates them as:

    Einstein EP
    The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.
    Strong EP
    The outcome of any local experiment (gravitational or not) in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.

    The difference between the above is the word "non-gravitational" in the former and "gravitational or not" in the latter meaning the latter is valid to all wheras the former is only valid to non-gravitational.

    What does it really mean?
     
  15. Feb 9, 2015 #14

    Vanadium 50

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    Universality of free fall.
     
  16. Feb 9, 2015 #15

    Simon Bridge

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    Wikipedia is not a recommended source.

    You've probably seen the discussion:

    http://en.wikipedia.org/wiki/Equivalence_principle
    - notice there is a "cleanup needed" notice on this article. The "weak" section, in particular, seems to mix up bits of each of the other sections.
    I think the wikipedia article is trying to distinguish the case where gravity is treated an extra force (so it needs a special rule to cope) vs being an effect of geometry (which dosn't) but it is difficult to be sure. Look elsewhere for clarification.

    Also have a look at:
    http://www.mathpages.com/home/kmath629/kmath629.htm
    ... takes a historical perspective, points out that the definitions change with time and context. It's more consistent than Wikipedia.

    Cultural history perspective, closer examination:
    http://einstein.stanford.edu/STEP/information/data/gravityhist2.html

    An example of an introductory lesson on the same:
    https://briankoberlein.com/2013/09/07/equivalent-principles/

    TLDR: http://www.reference.com/browse/equivalence+principle [Broken]
    http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/grel.html

    @Vanadium50: thanks, I just found that out in the histories above.
    That's the neat thing about answering these sorts of questions...
     
    Last edited by a moderator: May 7, 2017
  17. Feb 10, 2015 #16
    Thanks for the references above. They have alternatives ideas of gravitons instead of geometry as cause of gravity. But the iron ball should have more gravitons interchange with earth versus the cotton and yet they are attracted to earth the same degree. Why do they fall together at same time in the theory of the gravitons? Do you know what are these people reasonings?
     
    Last edited by a moderator: May 7, 2017
  18. Feb 10, 2015 #17

    Vanadium 50

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    A. You're hijacking your own thread.
    B. This "graviton theory" of yours is not the graviton theory of everybody else. I don't know where you got these ideas, but they are totally wrong.
     
  19. Feb 10, 2015 #18
    I wrote this thread because I'm interested in violations of Lorentz invariance CPT, EP and there are many tests that search for them. I originally
    saw this:

    http://www.physics.indiana.edu/~kostelec/faq.html

    I thought Equivalence Principle violation is part of Lorentz violations. What kind of spacetime symmetry does EP fall under.. is it Lorentz symmetry? Whatever. I read Lorentz violation is related to search for gravitons. And Im trying to understand how it is related to gravitons. If we have gravitons.. does it mean the geometry is not needed anymore? and gravitons artificaly created the geometry in the computations? In wiki:

    "In physics, the graviton is a hypothetical elementary particle that mediates the force of gravitation in the framework of quantum field theory. If it exists, the graviton is expected to be massless (because the gravitational force appears to have unlimited range) and must be a spin-2 boson. The spin follows from the fact that the source of gravitation is the stress–energy tensor, a second-rank tensor (compared to electromagnetism's spin-1 photon, the source of which is the four-current, a first-rank tensor). Additionally, it can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field must couple to (interact with) the stress–energy tensor in the same way that the gravitational field does. Seeing as the graviton is hypothetical, its discovery would unite quantum theory with gravity.[4] This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton, so that the only experimental verification needed for the graviton may simply be the discovery of a massless spin-2 particle.[5]"
     
  20. Feb 10, 2015 #19

    A.T.

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    And in an accelerating reference frame, all free falling object experience the same coordinate acceleration, which is exactly what the drop experiment demonstrates.
     
  21. Feb 10, 2015 #20

    Vanadium 50

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