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Scope of Gravitation!

  1. Aug 26, 2013 #1

    ShayanJ

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    I don't know how famous are Iranian physicists,but anyway.Once I was watching one of the videos of the Reza Mansouri's lectures on GR,it was the first or the second lecture in the series I guess and so he was just talking about general aspects of Gravitation.When he was explaining about the quest for a theory of QG,he said maybe Gravitation is not a fundamental force.Well,that wasn't new for me but the thing he said that I haven't heard before,was about the length scales in which gravitation is proved to exist and said there are experiments that demonstrate Gravitation exists down to the micro-scales but in length scales smaller than that,we can't be sure that gravity exists or not!He says other interesting things too but they are irrelevant to my question!
    My question is,what are those experiments?Can someone give a reference to one of them?or explain about them?
    Thanks
     
  2. jcsd
  3. Aug 26, 2013 #2

    jtbell

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    What does he consider to be micro-scale specifically (range of lengths)?
     
  4. Aug 26, 2013 #3

    ShayanJ

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    He doesn't explain much,so the exact meaning of his words isn't very clear to me!
    But he makes an example that may clarify a little bit...he says no one has measured the gravitational force between two protons and we don't know that such a thing really exist or not...we just assume its reality and calculate it!
    And then uses it to say that maybe gravity is an emergent phenomenon like entropy and isn't fundamental and maybe the fact that we don't have a successful theory of QG simply means Gravity does not exist in the quantum realm and so it can't be quantized!
     
  5. Aug 26, 2013 #4
    Measuring gravity at small scales and low masses is difficult because gravity is such a weak 'force'. It is not part of the Standard Model of particle physics...we don't know how to model it there yet and gravity is really,really weak, compared to say the electromagnetic force [as between a proton and an electron]. So in everyday situations between particles the minor effects of gravity can often be ignored.

    After Einstein proposed general relativity which predicted the bending of light [photons, no rest mass, no charge] by gravity, somebody decided to see if a really big mass [our sun] could do it and Arthur Eddington confirmed the effect. Gravity affects light and everybody believes it also affects matter particles, like electrons.

    One aspect of small scales is that at the Planck scale, about 10-33 cm, much smaller than we can currently detect, everything becomes jumbled together in what is called quantum foam....that means forces, particles and even space and time seem to lose their characteristics...get blurred. At these tiny, tiny scales, one needs really short wavelength [high energy] experimental probes to 'see' something so tiny...even more powerful than an electron microscope by factors of billions or trillions. But all that energy disrupts the observable conditions and 'blinds' us....we can't see what we are looking for...roughly like poking around in fog with plain old eyesight instead of high energy radar.
     
  6. Aug 26, 2013 #5
    It could be that he is referring to the active gravitational mass of individual particles. If that is the case then he is at least correct in one respect; It is just assumed that all particles, such as leptons, generate a gravitational field proportional to their inertial mass. It's a reasonable assumption because if they did not then the equivalence principle and Newton's third law of motion would be violated. And it's for this reason that there is very little interest in doing experiments to measure it.
     
  7. Aug 26, 2013 #6

    gneill

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    @Shyan: Do a web search on "short range measurements of gravitation". The The Eöt-Wash Group is doing interesting work.
     
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