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Light has been a favorite subject of study since
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pretty much forever and it still has more amazing hidden properties waiting to be discovered.
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Recently researchers found that under the right conditions, light can behave like a liquid with particles that flow in unison.
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That’s right—liquid light. Well, that’s not quite right.
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“Liquid” is a useful analogy…but it implies the light is in a liquid state,
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when really it’s in a more exotic state of matter, namely a Bose-Einstein condensate, or BEC.
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Sometimes called the 5th state of matter after solids, liquids, gases, and plasma,
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a BEC is created when a group of particles acts as one giant superparticle.
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This can be done using entire atoms like rubidium and potassium cooled to near absolute zero.
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Like photons, atoms behave like both waves and particles,
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and as they cool to extreme temperatures they lose momentum and their wavelengths blur together.
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Pioneering Indian physicist Satyendra Nath Bose and Albert Einstein predicted they could exist nearly a century ago.
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Because they require such low temperatures, Einstein thought it would be impossible to actually observe one.
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Turns out the impossible became possible in 1995, when the first Bose-Einstein condensate was created using roughly 2,000 rubidium atoms.
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A different technique for creating Bose-Einstein condensates out of light was discovered in 2010,
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and interestingly, it involves much more reasonable temperatures.
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When cooled, photons have a tendency to vanish into the walls of whatever is containing them.
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It’s sort of hard to make several particles behave in concert when they keep running off and hiding…
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so the key is to keep the photons bouncing back and forth off of two curved mirrors placed just over a micrometer apart.
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In between the mirrors is a liquid dye that repeatedly absorbs and re-emits the photons, cooling them to room temperature in the process.
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The mirrors aren’t perfect so some photons still get lost,
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but by keeping their numbers topped up with more photons from a laser, a Bose-Einstein condensate can be created and maintained.
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The same team that cracked this process is still tinkering with manipulating light,
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and in April of 2021 announced they had observed a phase change in a BEC,
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meaning they’ve seen two distinct phases of this exotic state of matter.
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Then in October, another research team announced they had created light that acted like a liquid and demonstrated what they called “social behavior.”
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The scientists sent their cooled photons through a structure known as a Mach-Zehnder interferometer.
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The interferometer splits into two paths that then rejoin and split again.
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A photon can travel down both paths simultaneously,
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but one path can be heated to change its length
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and make it so the photons are out of sync when they meet up again, creating an interference pattern.
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The researchers experimented with multiple interferometers that either had both exit paths open, both closed, or one open and one closed.
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Interestingly with that last interferometer, the photons seemed to “decide” which path to take.
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They went down the closed path, which the researchers postulated was a way for more of the condensate photons to stick together.
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This is why they called it “social behavior.”
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Research like these two recent studies is useful for studying properties of Bose-Einstein condensates.
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What scientists learn may also one day play a part in the encryption of quantum communications.
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Even if these studies have no immediate application, it’s nice to know that light still has plenty of surprises for us in store.
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If you liked this episode on liquid light, you might also like this one about how scientists discovered liquid glass.
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Make sure to hit that subscribe button and keep coming back to Seeker for the latest news on new phases of light and matter and all that good stuff.
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Thanks for watching and I’ll see you next time.