The double slit experiment a famous some would say infamous experiment in. The experiment requires shooting light or other subatomic particles such as electrons at a screen with two slits in it. Warning things are about to get strange.
When a double slit experiment is perfumed using pure waves the result is an interference pattern. When pure particles like bullets are used they produce two lines behind the slits. But subatomic particles like photons and electrons don’t act like bullets they produce a wave like interference pattern, even when they are shot one at a time. It seems that each of the particles goes through both slits at the same time.
However the strangeness dose not stop there because when detectors are put in place to see which slit each particle goes through their behavior suddenly changes and they produce only the two lines like bullets would.
Even stranger yet when the detectors are on but the no data is taken the result is a wave like interference pattern suggesting it is the conscience act of observing that actually changes the result.
Double Slit Experiment - YouTube