Think about an instable particle with exakt rotational symmetry, e.g. a neutral pion π°; let's measure its gravity on a very large sphere with the particle located at the center. Of course the initial gravitational field and the whole configuration has spherical symmetry. Describing the pion decay π° → 2γ by (quantum) fields it's obvious the the final 2γ-state has spherical symmetry, too. b/c no gravitational monopole radiation does exist, the initial and the final gravitational field must be identical; total energy and invariant mass of both initial and final state are identical as well (for a very large sphere we can define energy and invariant mass even in GR b/c we have a asymptotically flat field configuration). Now let's modify the apparatus and let's detect the two photons on the large sphere. Detecting them of course breaks rotational invariance, may cause higher gravitational multipoles and therefore gravitational radiation. That means that the "collaps of the 2γ wave function" has a measurable effect, namely gravitational radiation. What goes wrong?