In the double slit experiment I know that the electron fires as a particle one at a time then splits goes through both slits and recombined and interferes with itself and hit the wall creating a interference pattern.

 [![double slit electron interference pattern][1]][1]    

But I don’t understand how it can just change just by observing it. Like say you’re in the middle of a experiment and the interference pattern is building up and say if an ant sees the experiment how does that just make the electrons behave as a particle and rewrite the past and get rid of the interference pattern it had been building up. 


   [1]:     https://i.sstatic.net/kQN2z.png