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