There have been recent questions about the vacuum. In my simplified knowledge the vacuum is like a ground state energy level, and also that there might even exist other lower energy levels than the vacuum we find ourselves in. The sea is a soup of created and annihilated pairs of virtual particles with virtual energies and momenta.
In a normal sea on earth, which also represents a ground state of water, energy can be taken out of random waves by the clever construction of valves that allow only one way motion of water. Is it conceivable that a gadget of similar function could be found for the vacuum sea, or is it forbidden by conservation laws?
My intuition tells me that it might be possible if GR is taken into account, but my physics knowledge does not stretch to support this.
Edit : An explanation of why I am asking this question:
Let me expand on the example of the sea. The energy from the waves comes from either tides, i.e. gravitational forces, or wind (temperature differentials). If these were missing the oceans would be like glass representing a unique ground state of the gravitational well of the earth.
In an analogy, a gravitational wave going through the vacuum would be supplying energy to the vacuum sea.
I have been thinking of this analogy ever since cold fusion surfaced and refuses to die out, the most recent one being discussed here too. Approached from nuclear physics orders of magnitude the claims seem preposterous. There are people though who believe they have results of extra energy over input energy, much more than chemical reactions could supply.
This set me thinking on vacuum energy and the analogy with getting energy from the sea. A crystal is a prime candidate for any exploration of such concepts and in all cold fusion "successful" results crystals have been used. Now if the effect depended on the vacuum and how much distorted it was by a passage of a gravitational wave at the time of the experiment, or the exact orientation of the crystal, or the type of impurities in the crystal ( F centers etc) one would expect to get haphazard results, and non repeatable by other experimenters.
Of course this would be the first experimental evidence of gravitational waves :).
Edit 20/7/12
Maybe I should clarify that an acceptable answer in the negative would be one based on conservation laws. I believe that data trumps theory, and next in line are conservation laws,because they are the distillation of an enormous amount of data. Some people seem to think that theoretical definitions can substitute for proof in physics, but physical theories change, solid data do not, and this is physics, not axiomatic mathematics.
Best Answer
Well, let us be honest here. This question was not supposed to be answered from the very beggining. First of all we don't know how to mix quantum mechanics and gravity. There is no good consistent theory for that. Another thing is that today "the Dirac sea" analogy is not considered to be a very good thing. It is a pre-QFT naiive picture. Finally we are supposed to talk about "waves" in this "sea"... While the "sea" itself is an obsolete analogy... And all that is in a context of non-existing theory... Come on...
Now. There is actually a formal way to answer the question, because the question is about the "QFT vaccuum". And the QFT vacuum have a precise definition. Which basically says that it is "something you cannot take energy from". Actually, we start from that definition to build QFT. So the answer is: "you cannot take energy out of the QFT vacuum by definition".
Maybe we are wrong to start from this definition. Maybe for quantum gravity we need another starting point. But then it wouldn't be a QFT -- it would be a new theory which will have QFT as a limiting case. And in the range of validity of QFT that "formal" answer will hold.