  An experiment proposed by Karl Popper to test the standard interpretation of
quantum mechanics was realized by Kim and Shih. We use a quantum mechanical
calculation to analyze Popper's proposal, and find a surprising result for the
location of the virtual slit. We also analyze Kim and Shih's experiment, and
demonstrate that although it ingeniously overcomes the problem of temporal
spreading of the wave-packet, it is inconclusive about Popper's test. We point
out that another experiment which (unknowingly) implements Popper's test in a
conclusive way, has actually been carried out. Its results are in contradiction
with Popper's prediction, and agree with our analysis.
