I understand that in recent discovery gravitational waves have been detected. But knowing space time is not static it probably changes constantly in many places in the universe due to changes in distribution of massive bodies. For example, could the detected wave just be a slight oscillation in Gravity due to Jupiter fly by, or moon, or something similar?


It couldn't be Jupiter or the moon because

  1. The frequency of the detected waves is too fast. The detector found something that rotated several hundred times a second, and then stopped.

  2. The amplitude is too big. Even though Jupiter is close, the gravitational waves it produces in its orbit are extraordinarily weak. Far too weak for us to detect them (good thing too, because if it was producing powerful gravitational radiation, it would start to spiral into the sun)

We know one thing that can produce a chirp of this frequency and amplitude, and that is a black hole merger. In the last few moments before merging, these two black holes converted about three solar masses into gravitational radiation: That is a colossal amount of energy. About $$500,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000$$Joules. That is why for one moment it was the "brightest" point in the gravitational sky

(Note. standard form is for wimps)

  • $\begingroup$ So are these different waves already calculated theoretically and are now confirmed experimentally? Or were they just trying to detect something, whatever distortion and then looking what might have caused it. Also, is this signal still coming and this chirping frequency is still moving the instruments as we speak? $\endgroup$ – Ska Feb 13 '16 at 10:19
  • $\begingroup$ They had predicted what a black hole merger would be like, and were looking for it. Look for the #chirpforligo hashtag. $\endgroup$ – James K Feb 13 '16 at 16:21
  • $\begingroup$ Ok, that's cute. But how much does it match what they have predicted? And is the signal still coming? $\endgroup$ – Ska Feb 14 '16 at 18:11
  • $\begingroup$ The signal lasted for less than a second. And it matched excellently ligo.caltech.edu/image/ligo20160211a $\endgroup$ – James K Feb 14 '16 at 20:35
  • $\begingroup$ - but did they actually know when it will come for this particular merger, at least approximately? - did Einstein calculate the signal for this specific merger or in general for similar events? All this is beyond mind blowing. $\endgroup$ – Ska Feb 14 '16 at 20:49

The signal found was quasi-periodic, with a rapidly increasing frequency - characteristic of a merging binary system (a so-called "chirp").

The characteristic frequency of the GWs of a "binary" system are at twice the orbital period. So the frequency "emitted" by the Jupiter-Sun system would be wayyyy to low to be detected by LIGO, which is sensitive down to around 10 Hz (ie orbital periods of 0.2 s or shorter).

These sorts of periods are only found in binaries involving stellar-sized black holes and neutron stars just before they merge.

The interesting thing about the power produced by a binary is that it goes as mass to the power of 5. Another point is that we detect the wave amplitude, which only falls as the inverse distance to the source.

Thus LIGO is most likely to see the merger of the rarest, most massive black hole systems at very large distances.


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