1632: "Double Slit Experiment at the Quantum Level"

0:00A single photon heads towards a barrier with two narrow openings, and somehow it ends up acting like it went through both. People first saw something like this in the early 1800s when Thomas Young passed light through two slits, and a striped pattern appeared on screen. Bright bands, dark bands, the kind you only get when waves overlap and cancel. So, you run again, but slower. One photon at a time, space far enough apart that nothing can interact with anything else. At that point you would expect each photon to go through one slit or the other and build two simple clusters. Instead, the same striped pattern builds up, one dot at a time. And that's the part that most people hear about. The real test comes when you start taking things away. In 2025, researchers at MIT pushed this all the way down to the quantum level by stripping the experiment to the smallest version that could still run.

1:06They removed the parts that normally let waves build and travel, the extra structure, the background interactions, anything that could carry a classical explanation. And by the time that they finished, there was nothing left in the setup that should produce a wave pattern at all. And it still did. So the pattern is not coming from the equipment, and it's not coming from photons interacting with each other. What moves through the slits is not a tiny object picking one path. On the way through, it behaves like a spread of possible paths that spread goes through both openings, overlaps with itself, and shapes where a photon is likely to land. Where those possibilities reinforce, you see a hit, where they cancel out, you don't see anything, you see nothing. Then, at the screen, it all shows up as one point, one event. If you try to check which slit it went through, the pattern actually disappears.

2:11The moment you force it to take one path, the wave behavior is gone. A strange part is not that it can look like a wave. It's that even after you remove every normal reason for it to behave like a wave, it still does. That leaves you with one single photon moving towards two slits, carrying more than one possible path until the moment it lands. These are interesting things with J.C.