Scientists at Houston’s Rice University learned that lithium atoms in near absolute zero conditions can pass through other particles of matter. The research team, led by physicist Randy Hulet team documented the strange phenomenon in a new study published online this week in the journal Nature Physics.
Bose Einstein condensates (BECs) are a state of matter that form at temperatures near absolute zero. In these ultra-frigid conditions, atoms and subatomic particles can merge into single quantum mechanical entities – a “matter wave” of energy. Solitons are waves that do not diminish, flatten out or change shape as they move through space. To form solitons, Hulet’s team coaxed the BECs into a configuration where the attractive forces between lithium atoms perfectly balance the quantum pressure that tends to spread them out.
The researchers expected to observe the property that a pair of colliding solitons would pass though one another without slowing down or changing shape. However, they found that in certain collisions, the solitons approached one another, maintained a minimum gap between themselves, and then appeared to bounce away from the collision. The team carried out thousands of soliton collisions with mixed results.
To create solitons, Hulet and postdoctoral research associate Jason Nguyen, the study’s lead author, balanced the forces of attraction and repulsion in the BECs.
“You never see them together,” said Hulet, Rice’s Fayez Sarofim Professor of Physics and Astronomy. “There is always a hole, a gap that they must jump over. They pass through one another, but they never occupy the same space while they’re doing that.
“It happens because of ‘wave packet’ interference,” he said. “Think of them as waves that can have a positive or negative amplitude. One of the solitons is positive and the other is negative, so they cancel one another. The probability of them being in the spot where they meet is zero. They pass through that spot, but you never see them there.”
Hulet’s team specializes in experiments on BECs and other ultracold matter. They use lasers to both trap and cool clouds of lithium gas to temperatures that are so cold that the matter’s behavior is dictated by fundamental forces of nature that aren’t observable at higher temperatures.