"In order to find the overall probability amplitude for a given process, then, one adds up, or integrates, the amplitude of postulate 3 over the space of all possible histories of the system in between the initial and final states, including histories that are absurd by classical standards."
So, for a photon that goes trough one or the other slit, you integrate over both, and you end up with the interference pattern in your calculation. Also, the same photon goes to the end of the universe, splits into a pair of two cars (one car, one anti-car) that merge back to a photon, then it goes back to your detector behind slits. However, this contributes very little to the end result. However you add all those probabilities.
But from photon's perspective, the universe is contracted to the length of zero in the direction it travels, so, it gets there in zero time, as if its starting point and end point are one and the same.
Can somebody smart connect those two views?