Transcript:

The single most important experiment ever done in physics is the double-slit experiment. The double slit experiment opened the door for this bizarre thing about wave-particle duality and all this other stuff. 

So, the way the double-slit experiment works is, if you have an elementary particle, let's say an electron or a photon, that you send one by one, and there are two slits, and you're sending these things one by one. If you don't look at the slit on the other side of the screen, you would see an interference pattern as if it's a wave, even though the particles are going one by one. If you look at the slit, simply looking at the slit changes that interference pattern to two bands, like there is no interference. 

And this is the most bizarre thing, but it gets even more weird. You can design now a double slit experiment using the slits, which are themselves quantum in nature. So now you've got quantum slits interacting with quantum particles, and the result is insane. 

I'll give you one example. Maybe I'll give you the example in the next reel; it's getting too long. So follow the double slit experiment for the next reel.