Ever hear of the double slit experiment? It’s one of the most bizarre 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.

It gets weirder: As an episode of PBS’s Space Time shows, just by observing the double slit experiment, the behavior of the photons changes.

The Double Slit Experiment Explained

The idea behind the double slit experiment is 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 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.

Yes, 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.

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But here’s the weirdest part: It still doesn’t work, regardless of when that detection happens. Even if the second photon is detected after the first photon hits the screen, it ruins the interference pattern. This means 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.

A New, Time-Based Double Slit Experiment

New research published in Nature Physics in April 2023 has demonstrated that the double slit experiment also holds true regarding time and not just space.

The original double slit experiment had light waves pass through narrow gaps in physical space. Meanwhile, this new experiment passed light waves through “slits in time” with similar outcomes.

In this experiment, an infrared laser was shone at a typically non-reflective material—layered gold and glass coated in indium tin oxide, commonly used in smartphone screens. A second laser was pointed at the material’s surface, and the material’s properties changed, allowing it to reflect the first laser beam. Now, similar to the original double slit experiment, when the second laser was pulsed twice in quick succession, the reflected (and first) laser beam’s wavelengths became “more complex” and created an interference pattern. However, when the second laser only pulsed once, the reflected laser’s wavelengths stayed monochromatic.

Similar time-based double slit experiments have also been conducted on water waves and electromagnetic waves, and the researchers of this study are hoping to conduct their experiment on sound waves next.

According to Nature, this kind of “temporal interference” technology could be put to good use in a variety of applications, from consolidating 6G antennas to making time crystals, to creating photon-based quantum computers.

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Avery Thompson
twitter.com/physicallyavery
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Jessica Coulon
Service and News Editor

When she’s not out riding her mountain bike, Jessica is an editor for Popular Mechanics. She was previously an editor for Bicycling magazine.