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Now a days physicists have succeeded in discovering gravitational waves. Like a recent one with merging two large black holes billions of light years far away

Well it is quite big discovery and interesting

My question is rather simple : Gravitational waves are characterised with ripples in spacetime.

Now consider a point say one light year away from point of interaction :

  1. What will be the situation there?
  2. Will the spacetime be "rippled" in the same way as some blanket does on being given a jerk to create wave.

It rather seems quite unresponsive as there is nothing from where the spacetime blanket is to be compared that it is experiencing a ripple.

  1. Then is word "ripple" appropriate? How can we detect a ripple in the same pond (spacetime) where ripple is ongoing (you can take it to be concept of reference frame) .
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  • $\begingroup$ "It rather seems quite unresponsive as there is nothing from where the spacetime blanket is to be compared that it is experiencing a ripple" This is why LIGO tries to see the difference in timing of light in perpendicular directions. It compares the space in 2 directions on each other. $\endgroup$
    – A. C. A. C.
    Commented Sep 28, 2017 at 16:31
  • $\begingroup$ The events that cause visible ripples are on the order of a second. their travel time is that of light. do they red-shift? An omnidirectional sound wave also travels radially from the source. a gravity wave has similar omnidirectional and physical properties, a loud gravity signal will subside in a few seconds, and it travels billions of light years. when sound arrives at a 2d measuring device, it makes a 2d wave. the shape of the gravity wave effect is not the most complicated shape, it is just similar to a pressure wave from a sound generated by massive cosmic spheres. $\endgroup$
    – bandybabboon
    Commented Oct 17, 2017 at 9:45

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Let's start with your picture of ripples in a pond. How can you say there are ripples? Well suppose you have one detector at a point that itself oscillates with the wave. Then you can't. But if you put multiple detectors that measure each other's relative positions, then when the wave passes through, you can see an oscillating reading (because of the phase difference at different places). Hence you can detect them.

Ligo works in a similar way. Due to a particular nature of gravitational waves(which for technical minded is called quadrupole moment) you cannot put only two detectors at two points and detect them. Because they'll be streched the same amount that light signal passing between them will also be stretched. But if you have 3 detectors at equal initial distance and check the difference in time for light to reach detector 1 from 2 and 3, then you'll see it's nonzero if a wave is passing by.

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Ripples is appropriate for the observer far away from the massive source, where the metric around here can be saw as an flat background plus n perturbation. Similar like the ripples on the sea level, where here the sea level is the flat background.

As for the place near the massive source, perturbation theory breaks down, how spacetime change is unclear, but refer gravitational waves as ripples is not appropriate definitly.

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