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About Gravitational Waves

  1. Feb 29, 2016 #1
    G-Waves is a buzzword recently :)

    At the beginning I thought G-waves as the propagation of the changes of the curvature caused by a mass when the status of the mass (e.g. value or location) changes......But moment ago, I was told that G-waves are different from the waves that transmitting the change of curvature caused by mass.

    More specifically, when the distance between earth and moon changes, there are TWO kinds of curvature related WAVES are emitted: 1) the change of curvature due to the gravity change as the distance changes; 2) the Gravitational Waves.

    Both are predicted by General Relativity. For the example of Earth and Moon, more specifically I was told that near the Moon (or Earth), the first type of waves are bigger than Gravitational Waves, but beyond certain distance from the Moon (or Earth), the first type of Waves die out quickly.

    Since I am not a physics professional, even though I trust very much the professional qualification of the person who told me the above info, I still would like to get it confirmed from another physics professional because no one on the internet is saying so even though many are talking about Gravitational Waves these days.

    Thanks in advance!
  2. jcsd
  3. Feb 29, 2016 #2

    Simon Bridge

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    Most sources won't try to tell you about different kinds of gravitational wave because it's hard. For eg.

    But I have a feeling the authority figure you mention may be talking about the different multipole components.
    Impossible to tell without the exact phrasing, and even then, this person knew they were talking to a layman right? So there is no knowing what sort of hand-waving they did in the hope you'd get some sort of idea in the right direction.
  4. Mar 1, 2016 #3

    Thanks a lot for your very kind response. I like the link very much which is helpful. However, I did not get explanation from that link about what I mentioned in my first post.

    Here is the context of the conversation that I mentioned in last post:

    MY POST:

    As a non-physics professional, what I am more interested is not GW but what we can learn from GW about the nature of spacetime. Now we have already learned that spacetime can be curved by a mass...we also know that the change of curvature caused by a mass could be propagating at a finite speed of light....

    Now if there is a Frequency of a GW and the frequency could be independent of any oscillating source like the characteristic frequency of a matter, then it means that the spacetime also could RESIST the change of its curvature created by mass.....but now the following answer from Jorrie asserts that it is NOT the case:

    That is an important assertive information to me :) Thanks a lot.

    However, I am still having some concern about the following statement:

    as I understand that the spacetime is curved by the presence of a mass....so when the presence of that mass changes (e.g. increase of value or change of location), the curvature in spacetime created by it would change as well...and this change is NOT instantaneously but propagates out at the finite speed of light......Therefore, it seems to me, even if there is just ONE SHOT of change (hypothetically at least) , the change still should be transmitted out at the speed of light, but without any wavy rippling (unlike in a medium of matter like water)......Please correct me if I am wrong.



    Yes, any change of the mass distribution will cause a change in curvature that will spread out at 'c' in all directions. This is gravity that changes, not GWs. A good example is the tidal pull of the Moon on Earth that has a period of somewhat over 12 hours. So it is a 'wave', but not a gravitational wave, which has a specific mathematical definition in relativity.

    In fact, the Earth/Moon system does emit tiny gravitational waves, but only from somewhat outside the orbit of the Moon. An observer there will obviously see the orbiting system as an alternating "thin body" and a "fat body".
    These tiny ripples travel outwards at 'c' and the amplitude fall off with inverse if distance (1/r), while the tidal gravity is a shorter range range phenomenon, because its amplitude falls off with the inverse square of distance (1/r2).

    Tidal gravity is much larger than GWs at smaller distances, but there is a point outside the Moon's orbit where the amplitude of the tidal gravity will become smaller than the amplitude of the GWs. This is more or less where gravitational waves is said to originate.



    I read the link you provided which is a very good link. However, I do NOT see explanation from that link about what Jorrie said as I cited above.

    I would appreciate if Simone or anyone here could confirm that what Jorrie said is correct or not.

    Last edited: Mar 1, 2016
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