Physicists at Washington State University have created atoms that temporarily have negative mass, meaning they accelerate backward when pushed forward.
It’s a discovery that could lead to better understanding of super-dense stars in far-flung corners of the universe, as well as nuclear fission, said Michael Forbes, an assistant professor of physics at WSU who did theoretical analysis for the research.
How does it work? Think of a normal physical object: a car, or a person on roller skates. If you push it, it will move away from you.
That’s how just about everything in the universe behaves. It’s a basic law of motion in classical physics, dating all the way back to Newton.
Negative mass occurs in the realm of quantum physics, where matter can exist both as particles and waves. At WSU, researchers start with rubidium atoms and vaporize them, then cool them “to some of the coolest temperatures anywhere in the universe,” Forbes said.
How cold is that? It’s about a billionth of a degree above absolute zero, which is -459.7 degrees Fahrenheit: the temperature at which atoms stop moving.
At that point, the atoms follow the laws of quantum mechanics and form what’s called a superfluid. The atoms move together, synchronized in a wave without losing energy.
Forbes compares it to a cup of coffee.
“If you stir a cup of coffee you get one big vortex in the center,” he said. But in a superfluid, you’d see a series of small vortices, all repelling each other to form a lattice.
At that point, the atoms’ mass is still positive, and lasers hold the rubidium in place inside the experimental chamber.
Picture a group of atoms being held in an invisible bowl. If you break the bowl, in this case by removing the lasers holding them in place, you’d expect the atoms to expand, like marbles rolling away.
Here’s where it gets weird. With a second set of lasers, researchers can change the way the atoms spin, temporarily giving them a negative mass. So when they’re free to expand, they don’t behave normally.
“The force is trying to push them to the right but the atoms are accelerating to the left,” Forbes said. The overall effect is that the cloud of atoms slows down, held in place by its own negative mass.
The team published a paper describing the research earlier this month in Physical Review Letters.
This isn’t the first time physicists have created objects with temporary negative mass in lab conditions, but Forbes said it’s the first time it’s been done with this degree of control over the system.
In other experiments, researchers have seen negative mass as an incidental part of other things happening in a system. That makes it hard to tell whether the negative mass or something else is responsible for the atoms’ behavior.
“The problem with those systems is you can’t really change it … you can’t control it,” Forbes said.
Forbes is quick to explain that this research isn’t likely to lead to faster-than-light travel or wormholes anytime soon.
“There’s some people who took this story and ran with it as if we can create warp drives,” he said. The difference is in the type of mass being manipulated in the experiment. WSU researchers changed what’s called the inertial mass for atoms, a measure of how the object responds to external force.
There’s a second type of mass – gravitational mass – that measures the force of gravity on an object. Those masses are basically the same thing for physical objects. But the WSU researchers didn’t change the gravitational mass. If they had, it would mean the atoms would be pushing other objects away from them because of the force of gravity.
“The negative gravitational mass is what you need to do crazy things like open wormholes and power warp drives,” Forbes said. To be clear: that’s not something physicists know how to do (yet).
But the research has other applications outside the realm of science-fiction wish fulfillment. Forbes’ interest is in nuclear physics, and he thinks working with negative mass could help researchers better understand and replicate nuclear fission.
Fission involves splitting atoms to create energy, but to be controlled, a reaction has to be trapped in some way so it doesn’t balloon out of control. Forbes is hopeful the way atoms with negative mass self-trap, rather than expanding, might hold some clues to better understanding fission.
Playing with atoms in extremely cold temperatures has other benefits too, including refining the measurement devices that power clocks and phones. And cold atom systems behave similarly to the super-dense neutron stars thousands of light-years away, making it possible for physicists to learn about space through manipulating atoms on Earth.
“They’re extremely far away. We cannot make this matter on earth for obvious reasons and there’s very few signals we can get from them,” Forbes said.
For now, the WSU team hopes to refine its work with negative mass.
“We’re playing with this system now trying to control this a little bit better,” Forbes said.
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