Institute of Physics

Do you remember the double-slit experiment? It's one of the weirder experiments in modern physics, and cuts to the heart of the weirdness of quantum mechanics. Basically, waves that pass through two narrow, parallel slits will form an interference pattern on a screen. This is true for all waves, whether they're light waves, water waves, or sound waves.

But light isn't just a wave, it's also a particle called a photon. So what happens if you shoot a single photon at the double slits? Turns out, even though there's only one photon, it still forms an interference pattern. It's as if the photon travels through both slits simultaneously. You can read more about the double slit experiment here.

But wait, because it gets even weirder. As a new episode of PBS's Space Time shows, just by observing the double-slit experiment, the behavior of the photons changes.

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The idea behind the double-slit experiment is that even if the photons are sent through the slits one at a time, there's still a wave present to produce the interference pattern. The wave is a wave of probability, because the experiment is set up so that the scientists don't know which of the two slits any individual photon will pass through.

But if they try to find out by setting up detectors in front of each slit to determine which slit the photon really goes through, the interference pattern doesn't show up at all. This is true even if they try setting up the detectors behind the slits. No matter what the scientists do, if they try anything to observe the photons, the interference pattern fails to emerge.

It gets even weirder than that.

A group of scientists tried a variation on the double slit experiment, called the delayed choice experiment. The scientists placed a special crystal at each slit. The crystal splits any incoming photons into a pair of identical photons. One photon from this pair should go on to create the standard interference pattern, while the other travels to a detector. Perhaps with this setup, physicists might successfully find a way to observe the logic-defying behavior of photons.

But it still doesn't work. And here's the really weird part: It doesn't work regardless of when that detection happens. Even if the second photon is detected after the first photon hits the screen, it still ruins the interference pattern. This means that observing a photon can change events that have already happened.

Scientists are still unsure how exactly this whole thing works. It's one of the greatest mysteries of quantum mechanics. Perhaps someday someone will finally be able to solve it.

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