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Old 05-12-2018, 03:41 AM   #1
Default LIGO, how do they know?

With LIGO, how do they know the direction and what object the waves are coming from? And isn't it highly coincidental they detected merging black holes just right after they turned the instrument on?

Forgive me, I love telescopes and all, but I'm a little skeptical of the claims coming from LIGO.
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Old 05-12-2018, 04:37 AM   #2

They know the direction by timing the delays between the signal arriving at different detectors (the two LIGO detectors as well as other gravitational wave observatories elsewhere). For the first detection they had only the two LIGO detectors, so the direction was fairly uncertain. Subsequent events have been picked up by other GW observatories, so confidence as to direction is much higher.

As to what objects are generating the waves, they know because General Relativity makes fairly precise predictions of what a pair of inspiraling black holes or neutron stars will sound like, and in the case of the neutron star merger that was detected, because optical and gamma ray signals were detected from the same direction at the same time (the optical signal was actually delayed by about 11 hours, but the same sort of delay happens with supernovae, where visible light arrives hours after more penetrating radiation).
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Old 05-12-2018, 05:07 AM   #3

I see. thanks so much!
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Old 06-28-2018, 09:01 PM   #4

They actually have no idea where the signals came from except for the certainty of the last gravitational lensing (if this effect is valid), which of course, could be completely in the opposite direction.... think about it
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Old 06-29-2018, 05:44 AM   #5
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Also, they did not detect black holes right after turning the instrument on. There was also a story before LIGO. The GEO600 detector was pretty important in that phase, since the technologies to detect gravity waves got improved a lot on that small one. LIGO was just the one with the right size and technology to detect it. (There is a plot on the GEO600 homepage showing how the sensitivity got improved over the years)
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Old 06-29-2018, 06:10 AM   #6
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how do they know the direction and what object the waves are coming from?
'know' is a strong word. We don't 'know' there's anything like a black hole, though it's probable.

With plenty of measurements, it really works like this - you start out with a theory of how things are. That theory allows you to compute what signals your detector ought to observe if your theory is true.

Then you switch the detector on - and if you see what the theory says, you tend to believe in the theory more than before.

Of course, every now and then it happens that someone else comes with a different theory that explains the same signals. In which case people try to come up with a different experiment that ought to distinguish between the cases.

Generally it's an ongoing process in which theories are strengthened or undermined and after a few decades, some turn out to survive. The whole thing is usually mis-represented in popular science or press announcements, speaking with a certainty that really isn't there.

People believe in black holes because General Relativity predicts them, and General Relativity has predicted quite a number of other observations correctly - though there are alternative theories to GR around as well (in which there might not be black holes at all).

So LIGO found that particular signal because it's what people were expecting based on the theory, and if the real signal of black hole mergers would be any different, they actually might have missed it on the grounds of not looking for it (this is actually a real concern at particle accelerator experiments - they can't simply 'find' things - if something is not in the search raster, it gets files as noise and discarded). And the 'knew' it'd be black holes because the theory said so before.
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Old 06-29-2018, 09:21 AM   #7
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You can find detailed information on the scientific and technical aspects of LIGO by watching the CaJAGWR seminar series on their YouTube channel. I hope they will publish more CaJAGWR seminars in the future. one of my favorites:

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Old 05-03-2019, 08:53 PM   #8
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Scientists working to detect gravitational waves switched on their instruments for a third time at the beginning of April and immediately began to register events that could be interpreted as cosmic collisions. All five trigger events still need confirmation. The BBC's Roland Pease examines how telescopes worldwide are helping.
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