Even Huge Molecules Follow the Quantum World's Bizarre Rules

Magnify a speck of dirt a thousand times, and suddenly it no longer seems to play by the same rules. Its outline, for example, won’t look well-defined most of the time and will resemble a diffuse, sprawling cloud. That’s the bizarre realm of quantum mechanics. “In some books, you’ll find they say a particle is in various places at once,” says physicist Markus Arndt of the University of Vienna in Austria. “Whether that really happens is a matter of interpretation.”Another way of putting it: Quantum particles sometimes act like waves, spread out in space. They can slosh into each other and even back onto themselves. But if you poke at this wave-like object with certain instruments, or if the object interacts in specific ways with nearby particles, it loses its wavelike properties and starts acting like a discrete point—a particle. Physicists have observed atoms, electrons, and other minutiae transitioning between wave-like and particle-like states for decades.

But at what size do quantum effects no longer apply? How big can something be and still behave like both a particle and a wave? Physicists have struggled to answer that question because the experiments have been nearly impossible to design.

Now, Arndt and his team have circumvented those challenges and observed quantum wave-like properties in the largest objects to date—molecules composed of 2,000 atoms, the size of some proteins. The size of these molecules beats the previous record by two and a half times. To see this, they injected the molecules into a 5-meter-long tube. When the particles hit a target at the end, they didn’t just land as randomly scattered points. Instead, they formed an interference pattern, a striped pattern of dark and light stripes that suggests waves colliding and combining with each other. They published the work today in Nature Physics.

Physicists at the University of Vienna keep the inside of their instrument in vacuum and stabilize its exterior so it never moves more than about 10 nanometers.

Photograph: Quantum Nanophysics Group at University of Vienna
“It’s surprising that this works in the first place,” says Timothy Kovachy of Northwestern University, who was not involved in the experiment. It’s an extremely difficult experiment to pull off, he says, because quantum objects are delicate, transitioning suddenly from their wavelike state to their particle-like one via interactions with their environment. The larger the object, the more likely it is to knock into something, heat up, or even break apart, which triggers these transitions. To maintain the molecules in a wave-like state, the team clears a narrow path for them through the tube, like police cordoning off a parade route. They keep the tube in a vacuum and prevent the entire instrument from wobbling even the slightest bit using a system of springs and brakes. The physicists then had to carefully control the molecules’ speed, so they don’t heat up too much. “It’s really impressive,” says Kovachy.