  I propose to resolve the controversy over the speed of collapse of
quantum-mechanical wavefunctions by means of an experimental test with a
modified symmetric Mach-Zehnder atom interferometer, with non-intersecting,
parallel, widely separated final beams. According to the conventional collapse
scenario, the coherent twin-peak atomic wavefunction in the beams of the
interferometer suffers an instantaneous collapse, at infinite speed, when the
atom is captured by one of the two detectors at the ends of the beams, but it
remains coherent until that instant. In contrast, according to the
Hellwig-Kraus relativistic collapse scenario, the wavefunction collapses at the
speed of light, backward in time along the past light cone of each detector.
This leads to a premature collapse, or pre-collapse, which for a beam-to-beam
separation of 3 m extends over a time span of 10 ns before arrival at the
detectors. Within this time span the paired wavepackets in the two beams will
be incoherent. The difference between the coherent wavepackets of the
conventional scenario and the incoherent wavepackets of the relativistic
scenario can be tested by probing the atomic beams with a transverse laser beam
crossing them near the detectors. If the paired atomic wavepackets in the two
beams are coherent, the light scattered by the two beams will also be coherent
and generate a standing light wave in the space between the beams, with
detectable interference fringes. If the paired wavepackets are incoherent, no
such interference fringes will be generated, and the distribution of the
scattered light will be that of two independent dipoles. The design parameters
for this test seem to lie within reach of the techniques of atom
interferometry.
