Recently LIGO discovered gravitational waves caused by two black holes that were orbiting each other, and then collapsed into one black hole. A few months later, we find out that this actually happens quite often. However, these collisions are far enough away from Earth that no one can feel the gravitational waves, and it actually takes very precise measurement to even determine that they happen. So my question is, how close would a black hole collision have to be for us to feel the gravitational waves pass through Earth?

Secondly, is this even possible? As in, would the distance have to be so short that we'd get swallowed up before the black holes collided? And, if we were to feel such gravitational waves, what would they feel like?

EDIT: Let's say that the black holes have the same mass as Earth. So two Earth-sized black holes, ~18 mm in diameter (the size of a dime), that are stuck in an orbit finally collapse is the scenario here.

  • $\begingroup$ Welcome to the site Cameron. I am no expert in astrophysics (there's a couple around here somewhere) but I would hazard a guess that the range would be dependent on the size of the black holes in question, so you will probably need to add that information to your question. $\endgroup$
    – James
    Jun 23, 2016 at 20:52
  • $\begingroup$ For more info on the site and how it functions check out help center and feel free to join us in Worldbuilding Chat once you hit 20 rep. $\endgroup$
    – James
    Jun 23, 2016 at 20:53
  • 3
    $\begingroup$ Similar question on astronomy se astronomy.stackexchange.com/questions/13764/… $\endgroup$
    – James K
    Jun 23, 2016 at 22:11
  • 5
    $\begingroup$ I'm voting to close this question as off-topic because we really don't know, it's not about wb, and it would be appropriate to ask on Astronomy. $\endgroup$
    – JDługosz
    Jun 24, 2016 at 2:40
  • 2
    $\begingroup$ We have done many questions on planets/black holes and alignments. This is quite literally world building scale conversation and is absolutely ON TOPIC $\endgroup$
    – James
    Jun 24, 2016 at 14:40

1 Answer 1


The strain observed in the https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves is a maximum of 10^-21. Assuming that a displacement of 10^-3 ( a 1m long object gets made 1mm longer) and that the strain is inversely proportional to distance, then the event would need to be 10^18 times closer to be noticeable. As the original signal was 3 billion light years away and a light year is 10^16 m then the max noticeable distance is 3*10-9 ly =3*10^7m under a tenth of the distance to the moon.


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