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According to this article, http://www.sciencemag.org/news/2016/02/gravitational-waves-einsteins-ripples-spacetime-spotted-first-time, the search for gravitational waves was beset by continual failures. Even the LIGO took a very long time to put up a positive result. They even had to upgrade the LIGO, which was presumably already quite sensitive.

Why did it take so long? Are gravitational waves smaller than expected? Or is there some other difficulty with detecting them that was unanticipated? If so, how did physicists fail to predict these things?

I'm not asking why gravitational waves are hard to detect, but rather why they were harder to detect than expected.

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    $\begingroup$ Overcoming technological limitations and peeling back the layers of the onion of noise takes time. Often, an initial instrument design is more to understand limitations, which may be poorly modelled initially, in order to develop better models and to develop new approaches and technologies for the next 'advanced' design. Surely, part of this process in the case of LIGO is the relatively recent ability to numerically simulate binary black hole mergers in order to generate the templates that are used to pull the signal out of the still much higher noise. What answer is it you're looking for? $\endgroup$ – Alfred Centauri Jun 27 '17 at 0:20
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    $\begingroup$ Every bit of engineering I've ever had anything to do with failed first time. Most of it failed the second time too. I don't even want to talk about the third time. We humans aren't as smart as we like to think we are; particularly when it comes to the mundane and everyday, our foresight for problems that arise is woeful. $\endgroup$ – WetSavannaAnimal Jun 27 '17 at 0:32
  • $\begingroup$ What you have said is quite helpful. Would I be correct to interpret you as saying that the early LIGO was less a tool for detecting gravitational waves, and more a tool for figuring out how to detect said waves? $\endgroup$ – Zachary Goodsell Jun 27 '17 at 0:37
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    $\begingroup$ The other comments are correct, but I want to add that this was actually intentional. There were (and are) large uncertainties in the rate and intensity of GW signals, and the upgrades to LIGO require a lot of time and money. So right from the early days (a couple decades ago), the funding agency (NSF) told them to go for the lower sensitivity first, and if that worked, they would be funded to go for higher sensitivity. This was the plan all along. $\endgroup$ – Mike Jun 27 '17 at 0:37
  • $\begingroup$ Moreover, the first attempts at detection by Weber using the Weber bar were eventually unsuccessful - there was not a lot of thought on what might be the source of gravitational waves that it might detect, and the initially thought waves from large bodies going around each other were possible to detect. It needed neutron stars or black holes to be strong enough, and then the wavelengths were kilometers, which things like Weber bars were too small for. Some people thought they were undetectable, and there was a lot of confusion. Feynmann famously explained how they have detectable energy $\endgroup$ – Bob Bee Jun 27 '17 at 0:49
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Gravitational waves were actually just about as difficult to detect as expected. Nobody expected any detections before the first one because of the sensitivity required (detecting mirror motion less than the diameter or a proton). The upgrades were always planned; they were not in response to "failures." All of what you call failures previously were trial runs to root out system bugs and unexpected sources of vibrations (including a ringing telephone in the tunnel at one point). The only way we would have detected a signal sooner was if a black hole merger happened much closer than average, like in our own local group of galaxies. This once-in-10-lifetimes event would have had a much larger, easier to detect signal. As it happens, all of the black holes detected so far have been billions of light-years away.

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