We have significant evidence to believe that gravitational waves do exist. By measuring the energy of Hulse-Taylor Binaries, we can show that the expected energy loss is there. This was the basis for the 1993 Nobel in physics.

I'd say more than significant evidence. They're predicted by General Relativity. If they don't exist, it would mean our understanding of the universe is seriously flawed. Not many theories of gravity can exist without gravity waves. I think the only one people take seriously is string theory. Though, string theory can exist with or without gravitational waves as it's one of the 'tunable' parameters.

"will still be there in twenty years time"

I'm impatient with space exploration and research obviously because the scales involved make it likely that many great achievements won't occur during my lifetime.

So that and your line remind me of:

"The best time to plant a tree is twenty years ago. The second best time is now."

>Maybe it'd be a good idea to make one hundred percent sure that they actually exist (or else that they're definitely below the limits of LIGO) before building another expensive instrument.

Hmm. I'm not too familiar with that research area. Isn't the point of detectors like LIGO is that we don't know how big they are typically?

How do we make one hundred percent sure that gravity waves exist? What do you propose as a way that would convince ourselves that they do (or don't), and how do we estimate their size? Has the research community done this already?

experiment -> theory

rinse, repeat.

It's not surprising that LIGO and VIRGO (the European analogue) haven't seen signals, given the expected sources. Both observatories are currently being upgraded to more sensitive versions which will begin operations ~2015. The Advanced versions should see sources.

However, LIGO/VIRGO aren't a replacement for LISA, as they look at gravity waves of different frequencies: the sources they'd expect to see are different.