<aside> 💡 You can read or listen Don Hoffman and Rob Reid conversation explaining the experiment and its conclusions. Below is a shortened version of the transcript. To listen to it start at at 38:40.
"This is an experiment it (...) has been carried out probably thousands of times now. (...) Highly, highly, consistent from experiment to experiment. This is an utterly non-controversial statement about the way reality works. Specifically, light, electrons and certain other things sometimes behave like a bunch of particles and at other times behave like waves. (...)
So first to visualize the particular nature of light. Imagine you have one of those machines that shoots tennis balls, and they're all soaked in blue paint. And you're standing in front of a wall that has two large vertical slits in it, and behind the wall is a canvas, like an artist's canvas, a couple feet behind the wall. And you start firing these tennis balls. What'll happen is that most of the tennis balls are just gonna bounce off the wall, but a certain set of them are gonna go sailing on some angle through the left slit, and some of them are gonna go sailing on another set of angles through the right slit. After a certain amount of time, if you go in and you see your artwork that's resulted, you're going to see a cluster of blue spots where the tennis balls came through the left slit, and another cluster where they came through the right slit. And that is basically what would happen if light were behaving like a particle.
So in a sense, these tennis balls are behaving very particle-like, very much like lone photons should behave, photons being the tiniest indivisible units of light. Particles flying through two slits should behave like our tennis balls and create two clusters of dots. With photons we use photographic film or a digital sensor to record where they land rather than a canvas.
Now to visualize what happens when light acts like a wave. Imagine we have a huge deep pan of blue paint. It's so deep we can lower the wall with the two openings halfway into it. So now it's like we have two arched doorways on this little sea of paint instead of two slits. And the pan is filled right to the rim. One more drop of paint and it's overflowing. And the far edge of the pan is touching a fresh canvas. So we're gonna make so more art.
Now on our side of the pan across from the canvas, we start dropping rocks into the paint at regular intervals to make a series of waves. Those waves radiate out in the form of expanding semicircles toward the wall with the two arched doorways. Now when a wave hits, most of it's stopped by the wall. But the parts that go through the doorways become two mini waves, which themselves start radiating out as semicircles. Now when these two new sets of waves meet, they're going to interfere with each other. Sometimes the crest of a left side wave will meet with the crest of a right side wave, and they'll join up and become a higher crest. Other times a crest will meet a trough, and they'll cancel out. At that point of the wave front, there will be no wave. Eventually these much more complicated wave come to the far end of the pan, and they splash over the side. Where two crests have teamed up, they'll make a higher and deeper mark. Where a crest and a trough meet, there's nothing, so they'll make no mark at all. And the result will be a banded pattern of paint on our canvas, light dark, light dark, light dark, or a banded pattern of light on our sensor if we're using light instead of paint. And here our light is acting like waves.
Okay, now imagine you're a grad student replicating the famous double slit experiment for the thousandth time. If you just shine a steady light at the two slits, on the far side you're going to get the classic banding pattern of waves. So how do you get the two shotgun patterns? Well a logical answer might be to start firing the light just one photon at a time, because then you're only sending over solitary packets. And you'd think a solitary packet has to either go through the left slit or the right one. Once you've fired enough single photons to create a discernible pattern on the sensor, it's gonna be two shotgun clusters, right? Well, wrong. You actually get the banded pattern again. It's like the photons, despite going through one slit or the other one at a time, somehow choreographed themselves. They coordinated their landings on the sensor so that instead of producing the dual shotgun pattern of random particles, they made this very specific pattern which should only be made by waves of light. (...) It's like a conspiracy of photons that are ganging up to play a trick on you.
So to figure out what's going on, you put a detector on each slit, which will detect whether or not a photon goes through it. This way, you'll know which side each photon goes through before making its contribution to the banding pattern. But now, the photons suddenly start acting like particles, like tennis balls, and they obediently create the two shotgun patterns rather than the banded interference pattern. It's like they know they're being watched, or measured to be more precise. And sure enough, if you turn off the detectors, the photons go back to making the banded patterns of interfering waves. Now we could go on for days about what might be causing this, but the cliff notes are that many decades after discovering this, thousands of the brightest minds on earth have no idea. (...)