My wife has this really stupid (or brilliant?) question - if during the collision one solar mass turned into gravitational waves is it possible to create mass from gravitational waves?

 In principle, yes. The Einstein field equations are time symmetric, so you could reverse the situation by pumping in gravitational waves from a great distance in a spherically symmetric way, and have them converge in some central region in such a way as to increase the mass of a black hole at the center, or have it split up into two more massive black holes.
 Isn't it in principle equally difficult as generating concentrial waves on the surface of a swimming pool in such way as to eject a swimmer back on the podium where he jumped in from?
 Like this? ;)
 Flow Wave's tribute to LIGO: https://www.youtube.com/watch?v=qvU5ytghMdQThat's an impressive swimming pool ;-)
 So you could theoretically build a black whole generator by building a sphere that could emit gravitational waves with precision?
 You'd need the sphere to be more than massive enough to collapse under its own gravity, into a black hole; such a structure wouldn't be hypothetically possible. Building black holes with anything other than stars or giant gas clouds (in the early universe) turns out to be hard; your black hole generators inevitably keep collapsing into black holes or at least neutron stars.
 > You'd need the sphere to be more than massive enough to collapse under its own gravity, into a black hole; such a structure wouldn't be hypothetically possible.I don't think that's technically true. You could build a bunch of catapults on the edge of the sphere, and when they all launch rocks at the center of the sphere, they would eventually form a black hole. The catapults could be arbitrarily far from each other as the radius of the sphere increases, such that they would not really do much to each other gravitationally. You'd just have to wait a real long time for the rocks to hit the middle.> Building black holes with anything other than stars or giant gas clouds (in the early universe) turns out to be hard;Well yes, galaxy-sized intelligently designed structures don't really happen.
 I think by "Black hole generator" the other person meant a device which could create black holes remotely through some kind of process, not just a mass that collapses under its own gravity. In that sense, if you could find an old, spun-down neutron star (and man wouldn't that be a fun search! Massive, but tiny and dim...) then as you say, you could just keep adding mass until crush.
 Any black hole generator that doesn't itself become a black hole is essentially throwing part of itself in the black hole (Yay conservation laws!). You can replace the catapults with lasers or plasma guns or whatever.
 Sure, but I think the original commentator was imaging a massive sphere that could emit such powerful and precise gravitational waves, that you could create more black holes as a result. My point was just that any mass capable of achieving that, even hypothetically, would have long since collapses into a black hole.I take your point however, that you could coordinate in some way separated masses, but at some point you'd probably run into issues with the aforementioned galactic-scale of engineering.
 Some A+ structural integrity fields I suppose.
 Oh yeah, with daily baryon sweeps to keep the tribbles out!
 You don't need to emit gravitational waves, you can emit light that converges to a point.
 This is amazing. How did you find this?
 > How did you find this?I found it extremely amazing as well! :p
 Yes, it's almost an exactly analogous situation. The difficulty is that you're going against the second law of thermodynamics in both cases.
 ...Which is at least one going notion as to why the arrow of time exists at all.
 That analogy was great. It does an awesome job at explaining the disproportionate amount of energy that needs to be focused at a point in space.
 No. It's in principle as easy as using array of antennas designed to reinforce each others signals.It's used in normal FM radio.
 I think the practical limitation here is generating and concentrating/focusing the gravitational waves.EDIT: And there would probably have to be some control of the relative phase of each component wave as well.
 Hm, that's interesting. That means that a black hole could spontaneously disappear (or split into two) depending on its internal state. Observing that a black hole does not do this would then give you information about its internal state. This would be a counter-example to the no-hair conjecture, which would be a major breakthrough. So something must be wrong here. It can't be that simple.
 No-hair is about stable black holes in a steady state. When (e.g.) two black holes merge to form a third, its transient state may be more complicated; I guess the "hair" decays exponentially.(Disclaimer: Not an actual physicist.)
 Yes, that's right. When two black holes first merge, the resulting black hole has a sort of peanut shape which contains lots of information (i.e., hair). Such a black hole is not stable, however, and emits lots of gravitational waves that carry all this information away (a process known as balding).
 Thanks! But why then is that not a general solution to the black hole information paradox?
 I've never bothered to really look into the BH information paradox (ie I might be missing something that's obvious to all those legions of theoreticians that have), but just from a cursory examination, it never made sense to me. The no-hair conjecture really only applies to stationary BHs, so once you introduce Hawking radiation, the conjecture evaporates right besides its subject.The analogy I've come up with to illustrate my point is the 'thermodynamic information paradox':An isolated system will tend towards a stationary equilibrium state, uniquely described by just a few macroscopic parameters. This is our version of the no-hair conjecture.Now, instead of a completely isolated system, we allow interaction via absorption and emission of radiation. We assume that no matter the incoming radiation, outgoing radiation will obey totally probabilistic thermal laws as there are no hairs. Now, if we were to radiate away all energy (eventually reaching zero temperature), information will have been irretrievably lost.However, this is clearly nonsense as it tries to apply conclusions drawn under idealized conditions to non-ideal situations: Real thermodynamic systems fluctuate and have hairs. In fact, outgoing thermal radiation necessarily disrupts equilibrium, so the whole question is ill-posed.
 Wow, thank you for that awesome explanation! I wish I could give you 100 upvotes for that.P.S. How do you know all this? Are you a physicist?
 So can "specially crafted" gravity waves converge into a point of negative mass?
 It won't be difficult once we understand how quantum gravity works. :)
 Doesn't seem like a dumb question to me! AFAIK, our models of physics work forwards, as well as backwards. So in theory: yesIANA physicist, and have 0 training in physics, so someone please tell me if I'm wrong
 The answer is actually yes, at least in principle. In practice this would be extremely difficult (read impossible) due to the weakness of gravitational wave interactions.
 i.e. probabilities.