That general relativity is either incomplete or wrong. It would mean that gravity either propagates far more rapidly or far more slowly than we think.
Given however that c is considered and so far demonstrably the maximum velocity of information propagation in this spacetime, instantaneous propagation seems unlikely, particularly given the Mercury orbital confirmation.
That all said, we may be looking at things naively.
It could also mean that there are no objects large enough to produce gravity waves we can detect.
Black holes for example dilate time, and may either not exist (since they take infinite time to form), or may produce gravity waves that are too time dilated (i.e. slow) to detect.
That too. It could also mean that something unknown to us (dark matter halo?) dampens gravitational waves on earth - which is what ESA's eLISA would get around, if it is the case.
This is wrong, and based on not taking into account that in GR, simultaneity is not only a relative thing, but also a local thing. There is much more detail in several great answers to this Stack Exchange question:
Did you actually read the answers you linked to? They say exactly the same thing: A distant observer never sees a blackhole form, but infalling matter does. Because GR has no such concept of simultaneity the two observers can never agree on when the black hole formed.
But since we are here on Earth we care about how it looks to a distant observer, and from our point of view black holes never form. And what the infalling matter sees is irrelevant to us.
NOTE: You do not need black holes to make gravity waves, neutron stars will also make them. But time is still enormously dilated by neutron stars, and I'm wondering how that affects the gravity waves they make.
In the original comment I replied to, the point was "black holes cannot form because we cannot observe it, so black holes don't exist". This is a misunderstanding, and is what I was pointing out.
Saying that something objectively "doesn't happen" because an observer infinitely far away can't see it is ridiculous.
> This is a misunderstanding, and is what I was pointing out.
No. You are misunderstanding things.
> because an observer infinitely far away can't see it
No, it's not that the observer can't see it (too hard to see or something like that). It never occurs in the timeline of the observer.
From the point of view of the observer it never actually exists. That it exists from the point of view of someone else is irrelevant. It's not a semantic game about I can see it you can't.
It literally and actually simply does not exist.
(Not to mention the context is gravity waves, so if it never exists from the point of view of the observer it also can't make gravity waves.)
Pardon me if I am being obtuse, but the point of the original comment was "black holes do not exist in our universe, since time dilation at the event horizon is infinite", yes? This is what I am arguing against.
The event horizon is within the future null infinity of your observer; i.e. the observer may choose to travel towards to black hole, and will asymptotically fall into it and thus undeniably observe it. Hence it does actually exist even for this observer. Moreover, the observer can observe the shadow, effect on nearby masses and the gravitational lensing produced by the black hole, thus indirectly observing the black hole, even though he cannot see the actual event horizon.
This experiment not detecting gravity waves might not be too big a deal -- "Oh, there just aren't as many sources of gravity waves floating around as we thought."
Eventually, though, a conclusion that gravity waves don't exist would be enormous. It would completely upend mainstream theories of gravity, which all posit gravity waves (of some sort.)
In fact, I'd say the only reason we're looking for gravity waves is to tick a box -- we don't even consider the possibility that they don't occur, we just think it would be cool to have seen one.
Regarding reasons for doing this, there's the hope that more sensitive systems could function as "gravity telescopes", enabling charting the locations of massive bodies that would emit gravity waves. This could potentially allow for confirmed observations of physical regimes not accessible by experiment on Earth, much like other astronomical observatories.
How can you do that? There is no way to block gravity, so you can not make directional observations, the waves you see are the linear sum of all the waves in the universe.
Although I do wonder if an array of detectors might be able to use time differences in the arrival of the waves to differently located detectors, and some math to get some directional hints.
Time differences: Exactly that. Which is why having 3 or 4 synchronized detectors is necessary for narrowing down the direction of a source. (3 minimum, 4 to reduce noise.)
It means that at this point, ground based detectors probably won't work due to the massive amount of background noise (activities from people, geological activity, etc). There are plans for detectors in space that would not suffer the same problems.
There's little reason, almost none at all, to think they don't exist. The standard model clearly predicts they should exist, and non-direct evidence has been seen already.
It would be a spectacular conflict with the Hulse-Taylor binary pulsar system, whose orbital decay is exactly in line with the prediction for energy loss through gravitational waves.
