There's a great Dr. Quantum youtube video on this which I will post underneath.
The basic idea of the double slit experiment is that electrons and other microscopic things will act different because we are observing it, in other words your computer may not even be a computer when no one is observing it...creepy right?
So how did they arrive to this conclusion?
When you shoot marbles randomly at a wall with one slit in it, it will hit whatever's behind the slit in 1 straight line right? Directly behind the slit. What if you put two slits there? Same thing. 2 straight lines behind the 2 slits.
However when you send out waves instead: light sound etc; 1 slit produces the same result, one line where the force of impact is greatest; put 2 slits though; there becomes an interference pattern because the multiple waves hit each other as they're leaving the slit.
Now to the quantum world: If you shoot electrons through one slit, what would you expect? Yep one line is formed.
If there are two slits, what do you expect? Two lines right? Like marbles because obviously electrons aren't waves.
Incorrect. An interference pattern emerges. Scientists concluded that the multiple electrons must be hitting each other. Therefore they send them one at a time so that it was impossible for them to hit each other. Still an interference pattern emerged. So why was that? The only conclusion they could reach was that the electron was splitting in half and interfering with itself.
So which slit were they actually going through? Scientists then put a sensor to see which slit the electron was going through but the quantum world baffles us once again...Two lines were formed instead of the interference pattern. *Electronns