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

B Can gravitational waves interfere with each other?

  1. Aug 5, 2017 #1
    Just a thought I had...
     
  2. jcsd
  3. Aug 5, 2017 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Theoretically: Yes. They have energy, so the have an effect on each other. In practice: Completely negligible. Tens of orders of magnitude too weak to be relevant.
     
  4. Aug 5, 2017 #3
    Energy is irrelevant. Light gives the same interference pattern no matter how dim it is.
    What matters is whether we can detect it.
    For the common source of gravity waves - inspiralling black holes - are there any directions of space into which gravity waves are not emitted, for reasons of symmetry?
     
  5. Aug 5, 2017 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Ah, I interpreted the question as interaction. The regular wave interference is still there at any intensity, of course.
    Parallel to the axis of orbital angular momentum the intensity is tiny.
     
  6. Aug 6, 2017 #5
    What would it look/feel like? Would there be gravity changes / oscilations?
     
  7. Aug 6, 2017 #6

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Unless you are extremely close to black holes in the merging process, you don't feel anything. The events detected by LIGO changed the lengths of the arms by 1 part in 10-21. If you are extremely close, it might feel like you are pushed/pulled a bit, but "from within".
     
  8. Aug 6, 2017 #7
    How much is a man stretched by 1 decibel sound at 250 Hz?
     
  9. Aug 6, 2017 #8

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    1 decibel is an odd value to choose, but whatever. It corresponds to about 20 µPa. No idea about Young's modulus for a human as a whole. Pick your favorite number. With the 14 GPa for the human bone listed there, we get 10-12 length changes. Most parts of the human are significantly softer and will deform more.
     
  10. Aug 6, 2017 #9
    Well, that´s about the hearing threshold for healthy human ear. Wasn´t sure whether it is 0 db or 1 db that is quietest sound which can be heard.
    The 10-21 stretch of gravity waves sounds like a small number, but the 10-12 stretch of 1 db sound also sounds like a small number.
    Certainly 1 db sound at 250 Hz does not feel like "being pushed/pulled" - it is felt by ears alone.
    So could sufficiently strong gravitational waves be perceived directly by naked ear as a quiet sound?
     
  11. Aug 7, 2017 #10

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    10-12 is the stretch in bones, everything else will be stretched more, especially the eardrum.
    Maybe.
     
  12. Aug 7, 2017 #11
    Good question and the interesting thing about gravitational waves is that they are a distortion of their medium so not only do they interfere by addition and subtraction but also by multiplication and exponentiation - amplitude and frequency modulation: two interacting gravitational waves will modulate each other's phases and their rates of change of phases.
     
  13. Aug 7, 2017 #12
    How do gravitational waves interact with (weak, test) electromagnetic waves?
     
  14. Aug 8, 2017 #13
    I do not believe it works that way. If you stretch body parts using a force then the rigidity matters. The gravity wave is stretching space time. You would not feel anything. Soft tissues change in exactly the same way that hard tissues change.
     
  15. Aug 8, 2017 #14

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    The body parts would expand/shrink to their original size, assuming the frequency is low enough to make that possible. Different body parts would follow at different speed.
     
  16. Aug 8, 2017 #15
    If you ride the vomit comet you perceive micro-gravity through one part of the cycle. At high altitude acceleration of earth's gravity is lower than at low altitude. The distortion of time-space changes but you feel the same micro-gravity. The bicep, humorous bone, and a measuring tape wrapped around the arm are all effected by dilation in the same way. [Tidal forces are different. Might make this example bad]

    My understanding is that the laser light in the LIGO apparatus is the same frequency in both arms and at any point in the apparatus. The photons are arriving at the detector at different times. The arrival time depends on which leg they traveled and the amplitude of the gravity wave passing through the leg. The wavelength of the light is the same if measured at any point. So interactions between the light and matter will be the same. Particle-particle interactions are also the same anywhere in the LIGO apparatus. The observation of when two distant events occurred is effected by the gravity wave. The events themselves are not effected.
     
  17. Aug 8, 2017 #16

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    LIGO is so long that the mirrors are effectively floating in space relative to each other. Your human body is not (at the frequencies LIGO is interested in).
     
  18. Aug 8, 2017 #17
    https://en.wikipedia.org/wiki/Gravitational_wave

    I am not an expert on this topic. Gravity waves passing through the galaxy would dissipate if energy was doing work on the molecules in objects.

    A diagram of LIGO looks like a scaled up Michelson-Morley detector. Did I miss something.
     
  19. Aug 9, 2017 #18
    Does gravitational wave cause the shape of Earth to change, the way the change of direction to Moon does?
     
  20. Aug 9, 2017 #19

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Gravitational waves - gravity waves are something different.
    Gravitational waves are doing an incredibly tiny amount of work on molecules.
    What you cited is the LIGO situation - independent objects.
    A human body doesn't consist of independent objects. If the distances within a solid object decrease in one direction and increase in another, it induces stress in the object.

    @snorkack: That is a different type of change.
     
  21. Aug 9, 2017 #20
    If a 1 meter iron bar is clamped and tightened to 0.99 meters it is under stress. If the length of the space in the clamp and length of the iron bar are equal before, during, and after an event then there was no stress.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Can gravitational waves interfere with each other?
Loading...