Chapter 2 – Wave functions and uncertainty

Observation

We saw how evolving probability distributions gives an interference pattern but that only works if we don’t know which slot the electron went through. The next experiment adds a detector to observe the electron and surprisingly the interference pattern vanishes if we track exactly where the electron goes!

Observations changing results is one or the core features of quantum mechanics that makes it different from the world we experience. So what is happening here?

A good analogy goes back to our traveling friend. We had these distributions for where they could be after 1/2 hour and 1 hour:

Now imagine calling the friend on their cell phone after 30 minutes and finding out where they are. This changes the distribution because you now know the current position with certainty. It also changes the probabilities for where they could be after 1 hour of driving.

The same happens with the electron’s probability distribution. The probability distribution at time 4 gave the electron equal probability of going through each slot. However, when we saw the electron go through the top slot it changes the probability distribution at time 5 and later. This is equivalent to us finding out where the friend is. The interference pattern vanishes when the electron no longer has the possibility of going through either slot.

This change to the probability distribution explains why the interference pattern vanished. The next section will look at whare the electron was before it was observed.