I understand how unbelievably lucky the discoverers were to catch the wave produced billions of years ago by an event that happens so rarely one hour into a test run of their equipment. But one thing is still not clear to me – how did they know what exactly caused the spike? Was that merely a conclusion that this must have been two black holes colliding or there was any additional astronomical observation done that spotted the collision?


This question is similar, and one of the answers mentions that LIGO observation can not be interpreted as evidence of WHAT the actual event was, but it also never directly tells me if any proven-to-work ways of observation parallely detected the black hole merger.

Because if only LIGO was involved in detection, than how do we know WHEN and WHERE the merger happen? As far as I understand, LIGO technology does not communicate any space-time coordinates of the event. Or am I wrong?

This and other articles insist that event took place 1.3 billion years ago.


marked as duplicate by Rob Jeffries, DilithiumMatrix, Yashas, John Rennie black-holes Mar 9 '17 at 7:18

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.


To kick things off, there's a couple of misappreciations in your initial statement:

how unbelievably lucky the discoverers were to catch the wave produced billions of years ago by an event that happens so rarely one hour into a test run of their equipment.

This event is rare, but it isn't particularly rare: each pair of black holes will collide and emit waves once, for a few seconds, out of many millions of years orbiting, but space is big and there are lots and lots of black holes in the many millions of galaxies that are within 1Gly of Earth.

Ultimately, it comes down to (i) estimating the strength of the signal of a given event at a given distance, (ii) estimating the number of events that will happen within a given volume, and (iii) building a detector that is sensitive enough that its detection volume will include enough events to be worthwhile. All of these calculations were done way before LIGO was built, as part of building the case that it was a smart thing to build in the first place.

On another track, the claim that the detection happened within an hour of the first test run is inaccurate. The detection happened two days into the engineering run that preceded the first science run of Advanced LIGO, and there had been plenty of previous runtime on the standard and Enhanced configurations. The two-days time is pretty lucky, indeed, but not unreasonably so; the second detection, GW151226, happened within three months of the first one.

On to your main question, then. For many years running up to the science runs, there was a strong effort from the numerical-GR community to explore all known possible sources of gravitational waves and predict how the signal would look on Earth, how strong that signal would be, and how it would depend on the characteristics of the source.

The initial source, GW150914, was easy to pick out, because it has a very characteristic shape, which essentially seals it as a black-hole merger. Moreover, the waveform has a very characteristic duration, frequency, and chirp, and all of those can be directly related to the characteristics of the source collision.

Thus, these aspects of the shape of the pulse let us infer what the source was, including in particular the masses of the black holes and their orbit. This, in turn, tells us the amount of energy that was emitted, and from that we can infer the absolute 'luminosity' of the source, which we can then compare with the observed intensity of the waveform to obtain the distance to the source. This tells us 'where' the signal comes from, at least in terms of the distance.

The direction the signal came from, on the other hand, is worked out in a more mundane fashion, by looking at the relative delay in the observation of the two sites, which provides one constraint. In addition, since the waves are polarized and the two detectors point in different directions, the relative intensity of the two detections can provide additional information about the direction (but not as much). Thus, if you look at the area of the sky the signal could have come from, you get a pretty wide patch.

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    $\begingroup$ Thank you for clearing things up. I will summarize: No additional astronomical detection was made at the same time. Where, when and what were inferred from shape of the measured curve based on calculated predictions. $\endgroup$ – Division by Zero Mar 8 '17 at 21:55
  • $\begingroup$ Yes, precisely. $\endgroup$ – Emilio Pisanty Mar 8 '17 at 22:31

People ran computer simulations that told them what wave pattern would be observed for various cosmological events.

Each even has its own "fingerprint" than can be used to distinguish the various events.

As far as I know there were no additional astronomical observations that gave definite results on the origin of the wave. This is due to the bad resolution of ligo. People don't know where exactly they should search for the origin of the wave.

REQUESTED EDIT: I can answer your "where" question but I don't know enough about the subject to answer the "when" question.

Gravitational waves stretch spacetime, which is exactly what has been observed via the interferometer. The way in which this stretching happens is not arbitrary and tells you something about the direction in which the signal is traveling. As I mentioned before the angular resolution is not very good such that an exact pinpoint of the origin is impossible (for now)

  • $\begingroup$ Thank you for the answer, I've done an edit to my question as was asked by Rob Jeffries here to clarify some points. Would you care to expand your answer to include these? $\endgroup$ – Division by Zero Mar 8 '17 at 14:14
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    $\begingroup$ I won't include the "when" in my answer as I am not quite sure about it, but id guess that the computers simulations not only told what "fingerprint" a given event (e.g. black hole merge) would give but also its intensity. Comparing this with the measured intensity than gives a rough estimate of how far away the event took place. We also know that gravity waves travel at light speed (or at least very close to it) from which we can than derive when the event took place... At least, this would be my first try, LIGO probably used a better approach but the main scheme was probably this. $\endgroup$ – gertian Mar 8 '17 at 14:59

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