A disappearing act in ultra-cold conditions shows how lithium atoms can be made to pass through other particles of matter.

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.

Researchers from Rice University in Houston, Texas created BECs in certain states called solitons. These clusters of a few hundred thousand lithium atoms were chilled to just one-millionth of a degree of absolute zero. The structures were created by arranging the groups in a formation that balanced quantum pressures, pushing the clusters apart, with attractive forces between the atoms.

Solitons were believed to have the property that they could pass through each other without warping or slowing down. When researchers attempted to carry out this experiment, they found the bodies maintained a minimum distance from each other, never occupying the same location, before traveling away from the epicenter. The solitons passed through each other, without ever occupying the same location.

"It happens because of 'wave packet' interference. 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," Randy Hulet, physicist at Rice University and leader of the research, said.

Single Bose Einstein condensates were created by the group, then split apart using a sheet of light. Each half now a soliton approached when the sheet was removed, passing through each other without ever occupying the same spot.

Thousands of collisions were carried out in the experiments. In some trials, the solitons appeared to bounce off one another, while the miniscule groups passed through each other in other runs.

Solitons posses a frequency, phase and amplitude. The phase of the wave is the one factor which cannot be controlled by researchers.

"In the out-of-phase case, the one with the gap, where it appeared that they had been bouncing off of each other, we still saw the gap but we also saw the larger soliton emerge unfazed on the other side of the gap. In other words, it jumped through the gap," Hulet told the press.

Temperatures of the Bose-Einsten condensate groups used in the experiment were a million times colder than outer space. This observed effect does not occur at normal temperatures, because the clusters of gas cannot be made to act as a single matter wave.

Solitons passing through each other is shown in a video available on the Rice University YouTube channel.

Study of soliton collisions was published in the journal Nature Physics.