You learn about three or four phases of matter in school — solids, which have a shape and volume, liquids, which have a volume only, and gases and plasmas, which have neither a definite shape nor volume. But using the laws of physics, you can create incredible substances that behave nothing like the ones you learn about in chemistry. That includes a substance that behaves like both a solid crystal and a frictionless, perfectly-flowing liquid at the same time.
Image: ETH Zurich / Julian Léonard
Now, groups of American and Swiss researchers have both created this strange new "supersolid" in two different ways. It's not like they have created something you can hold in your hand — these are highly-engineered materials that exist in ultracold vacuum chambers. But there's been a sort of race to create supersolids, which will help us understand the nature of matter itself.
"Our goal is to discover new materials with new properites, ones that people don't even know are possible," Wolfgang Ketterle, physics professor at MIT, told Gizmodo. "We want to make materials that have never existed on Earth."
Ketterle's experimental setup (Image: MIT)
Each team created their supersolid differently, but both groups started by turning atoms into a "Bose-Einstein condensate", a hyper cold gas made from atoms with even numbers of electrons. Having even numbers of electrons (or the same number of electrons as protons) means the atoms that have a whole number spin value, a quantum mechanical property that can either assume half or integer values.
Atoms with whole number spin values are called bosons, which the laws of physics says can occupy the same space. These cold gases, therefore, begin to show the weird effects of quantum mechanics on a macroscopic scale, like flowing without any resistance. It's a field Ketterle would know quite a bit about; he created one of the first Bose-Einstein condensates, and won the Nobel Prize in physics for it back in 2001.
How would a substance that flows like a liquid also be considered a solid? Well, the structure would keep a regular, rigid shape like a solid. At the same time, any change in the crystal, like a missing atom, would flow right through the shape without any resistance, explains Rice University physicist Kaden Hazzard in a comment for Nature.
Each team's goal, then, was to take their Bose-Einstein condensate and impart it with the rigid properties of a true solid. The MIT team used lasers to alter the value of the spin of half of the atoms in their material, which was made of sodium, creating two different Bose-Einstein condensates at the same time. They observed the density of their solid manifest itself in stripes, and when they shined a light on their material, it bounced off of it as if it had hit a grating.
This convinced Ketterle's team that they'd created their coveted new material, and they published their result Wednesday in the journal Nature.
ETH Zurich's team's experimental setup (Image: ETH Zurich)
The group at ETH Zurich in Switzerland used a different approach to impart the rigid properties of a solid. They kept their condensate, of rubidium atoms, in a cavity between pairs of mirrors with light particles, photons, bouncing back and forth. This caused the light to scatter in between atoms, which eventually formed the regular crystalline pattern. They published their result in Nature the same day.
These aren't solids you can hold in your hands by any means, Ketterle warned. They're highly-engineered materials that don't show their "solid" characteristics in every dimension. That makes them even stranger, if you think about it. "Our material ... is blurring what people learn in high school about the three phases of matter. It combines properties of a gas, solid and a liquid."
Other physicists were impressed by the groups' creations. "It is an amazing effect," Jeff Steinhauer, physicist at the The Technion — Israel Institute of Technology in Israel, told Gizmodo in an email. "It might help shed light on the physics of solid helium."
Katterle was excited that both groups had released their discoveries at the same time — it means there's a lot of buzz about the materials in the field.
There's no purpose for making these weird substances aside from basic research — it's not like someone's going to find a use for a vat of frigid crystalline liquid helium any time soon. But forms of matter like these demonstrate just how much more we have to understand about the way our universe works.
"What motivates us is that once it's possible, then people know the laws of nature allow us to realise such materials," said Ketterle. "We hope that 10 to 20 years down the road it influences materials designers to go further, and maybe create a supersolid that exists outside a vacuum chamber."