Gravitational Waves Time taken to reach Earth


I have few basic questions about Gravitational waves (GWs)

  1. As I understand that GWs take billions of light years to reach earth (depending upon the collision). For example, if event GW200322_091133 has been detected at UTC 2020-03-22 09:11 and its luminosity distance (DL) is 3.6 (+7.0/−2.0) GPc. Could you please help me to understand, when this event could have happened in terms of UTC?

  2. There could be a collision in the southern hemisphere or northern hemisphere, but the direction of stretch and compression of the earth might differ depending upon the source origin (in my understanding, it should vary depending upon the source). Hence, my question here is, how do the current instruments detect the GW signals irrespective of the origin of the GWs andhow it is being done? Could you please help me to understand this?


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Hi @gopistudy Thank you for the question!

  1. GW’s travel at the speed of light. A parsec (pc) is around 3.3 light-years, so a Gpc is around 3.3 billion light-years. For the source you mention, the distance of 3.6 Gpc would take around 11 Billion years to travel to earth.
    (3.6 Gpc X 3.3 ly/Gpc = 11.9 Gly)

  2. LIGO can detect GW’s from most directions, but not all. When a GW passes through the detector, it can either squeeze or stretch each arm, depending on the direction of travel. You can see a video that describes how this works.

As long as one arm gets stretched white the other gets squeezed, the signal is potentially detectable. If you like, you can see a map showing the directions where the detectors are more sensitive or less sensitive:

Good luck!

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A small additional note. The light travel time in cosmology is not just the luminosity distance divided by the speed of light, as Jonah computed. For this source’s median redshift of 0.6, the light travel time is only 5.87 Gyr, as computed using Ned Wright’s cosmology calculator and the standard Planck cosmology parameters used in the LVK analyses.

See, e.g., here for some discussion of the different distance measures in cosmology.


Oh, right! That makes sense. Thanks, Nathan.

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