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Laboratory Gravity

  1. Jan 4, 2009 #1
    What is the reason for not attempting to both generate and detect gravity waves in a laboratory? I mean if Eotvos could measure the gravitational force from from dissimilar materials with some accuracy, then why not place two more sensitive, modern gravity measuring devices some distance apart; rotate a a heavy object nearby; then look for the expected slight delay in sensing from the two gravitometers? Certainly modern electronics can measure time differences in signals on the order of a few picoseconds. Are gravity measuring devices not yet responsive enough?
    -Harry Wertmuller
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  3. Jan 4, 2009 #2


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    The signal to noise ratio is the problem. If you spin anything reasonably heavy to a high enough rate, the mechanism generates noise with harmonics right on the frequencies you are trying to measure. The same problem plagued the early attempts to detect GW with large lumps of metal suspended in the basements of physics departments ( eg Reading 1972). If a truck or a train moved an inch within several miles of the basement it swamped any extra-terrestrial source.
  4. Jan 4, 2009 #3
    "If a truck or train moved...... swamped any extra-terrestrial source" seems to say that gravity waves from terrestrial sources have been detected. So gravity waves exist?
  5. Jan 4, 2009 #4


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    As far as I know, gravitational-wave detectors function by (trying to) detect extremely tiny mechanical vibrations in large objects. Mechanical vibrations can easily be produced by terrestrial sources that have nothing to do with gravitational waves. I can feel them every night when a train passes by on the railroad track that runs a few hundred yards from my house.
  6. Jan 4, 2009 #5


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    Yes, it's unwanted vibrations that mask any that might be produced by GWs. I am agnostic on whether gravitational waves can be detected or not. I'm told that the change in the rotation rates of binary pulsars fits very well to the radiating model, but there may be another explanation for that. Maybe waves are being radiated, but that does not mean we can detect them in principle.
  7. Jan 4, 2009 #6
    "Unwanted vibrations .... mask .... GWs." So GWs are strongly supported by astronomy but cannot be directly detected from either terrestrial or extra-terrestrial sources?
  8. Jan 4, 2009 #7


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    Gravitational wave detectors like LIGO can detect sources. You can distinguish them from vibrations by detecting the same event at two different instruments at the same time.
    I don't think they have had any unambiguous results so far.
  9. Jan 4, 2009 #8


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    Going back to the theme of 'gravity in the laboratory' - it may be feasible to detect frame dragging, because a non-conducting flywheel on magnetic bearings can be spun up to speed before the power is turned off, leaving a very 'quiet' system.
  10. Jan 4, 2009 #9

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    That sounds hard. It's hard enough to see gravity on meter scale experiments. Frame dragging has to be smaller by at least a factor J/m: that's going to be of order 10-6 or less. So it's a part per million perturbation on top of gravity. I should point out that G isn't even known to a part per million - it's known to 0.1%.

    If any stray charge ends up on your flywheel, you've now generated a magnetic field which can produce forces billions of times larger than gravity.

    If you're interested in some of the challenges in precision measurements of gravity - including some with rotation - I recommend looking at the papers of Eric Adelberger of the University of Washington.
  11. Jan 5, 2009 #10


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    you're right, it's not practicable. Thanks for the ref.
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