###### In a double slit experiment, a screen position x could be endowed with two probabilities: the probability of finding particles which passed through slit A and arrived in x, c1; and the probability of finding particles which passed through slit B and arrived in x, c2.

###### If the probability of finding particles in x when the two slits opened simultaneously was p12, then, p12 = c1 + c2 held. This meant that when the two slits opened simultaneously, the probability of ”finding particles in x” would increase rapidly.

###### For example, if c1 = 0.45 and c2 = 0.5, then when only one slit opened, the event of ”finding particles in x” would be uncertain. However, when the two slits opened simultaneously, p12 = 0.95 would hold meaning that the event of ”finding particles in x” would be almost certain.

###### Thus, in a double slit experiment, as long as the time that the two slits opened simultaneously was long enough, the particle interference phenomenon in x would occur with certainty.