  The collapse of a spatial probability distribution is triggered by a
measurement at a given spacetime point. It is customarily assumed that this
collapse occurs along an equal-time hypersurface, say, t = 0. However, such a
na\"ive instantaneous collapse process is inconsistent with relativity, because
the equal-time hypersurfaces of different inertial reference frames are
different. The attempts at implementation of instantaneous collapse in several
different reference frames then lead to violations of probability conservation
and violations of the scalar character of the probability contained in given
volume elements. This problem affects not only the Copenhagen interpretation of
quantum mechanics, but also other interpretations in which it is still
necessary to specify what changes in probabilities occur when and where in a
manner consistent with relativistic spacetime geometry. In the 1980s Schlieder
and Hellwig and Kraus proposed that collapse of the probability distribution
along the past light cone of the measurement point avoids these difficulties
and leads to a Lorentz-invariant collapse scenario. Their proposal received
little attention and some negative criticisms. In this paper I argue that the
proposed past-light cone collapse is not only reasonable, but is compelled by
Lorentz invariance of probability conservation, and is equally valid for the
spatial probability distributions in quantum mechanics and for those in a game
of chance, for instance, the probability distribution for a game with playing
cards scattered over some spatial region. I examine the objections that have
been made to the past-light-cone collapse scenario and show that these
objections are not valid. Finally, I propose two possible interferometer
experiments that can serve as direct tests of past-light-cone collapse, one
with an atom interferometer, and the other with a light interferometer.

Quantum Physics