# Double slit with observer?

Apologize if this is a dumb question, I'm just a kid trying to make sense of something that I'm new to.

It shows the wave interference when you send a classical wave through in the second example, where two essentially "new" waves are created at the beginning of each slit at the same time, so adding them together results in the classic two slit pattern. But at the same time, when they send a quantum object through with an observer, the wavefunction collapses and the particle only goes through one slit - but then a new probability wave forms past the observer on the other side of the slit, and as expected the particle's final location follows this probability wave.

My question is, why doesn't interference occur with the observer here? Aren't there still probability waves between the quantum objects going through each slit, and shouldn't these waves interfere and create fringes of some sort?

My question is, why doesn't interference occur with the observer here? Aren't there still probability waves between the quantum objects going through each slit, and shouldn't these waves interfere and create fringes of some sort?

The results of any experiment when modeled with mathematics is absolutely dependent on the boundary conditions, which pick up the correct solution for the case under study.

One can think of the double slit experiment as "the solution that nature gives for these specific boundary conditions".

When there is no observer , the light (or paricle) beam has a coherence, i.e. when written as a combination of sines and cosines the phases are fixed on all fronts. This is what creates the interference patterns, being in step.

An observer in quantum mechanics, means an interaction . Each interaction changes the boundary conditions of the problem and a new quantum mechanical solution appears. Nature says that in this case the distribution appearing on the screen is a classical particle distribution. What has happened is that the phases are destroyed between the probability wave fronts,( in this case in front of the slits), and no interference can appear.

Actually this experiment demonstrates the effect of boundary conditions on the distributions.

With an observer there are no longer two probability waves travelling through each slit. There is a particle exiting one slit, which can be described by having some probability of where it will hit on the other side but there is no "second wave" from the other slit for it to interfere with.

Another way to make it a little easier. If you imagine the particles are shot in a straight line, the exact way every time, then you won't have any dispersion at all in the classical case or the observed case. You will only have two locations of detection opposite each of the slits. In this the quantum object, once observed, is like a classical particle whose trajectory is deterministic.