Soliton visualized along a pendulum. Image: pe.soliton.free.fr/
Ghosts are real. This is the case, at least, in a Rice University laboratory, where physicist Randy Hulet and a team of researchers have demonstrated a bizarre, paradoxical phenomenon in which matter is able to pass through other matter unimpeded. The effect in some cases is as if each piece of matter had completely disappeared relative to the other, sharing space but not.Hulet's work is described in today's edition of Nature Physics.
Clearly, we're not talking about regular everyday matter here. The Rice team was experimenting with a strange substance known as Bose-Einstein Condensate, which is a state in which all of the atoms making up a chunk of material start behaving in perfect coordination, all sharing the same quantum state together. The result is (relatively) macroscopic matter, hunks made up of several hundred thousand lithium atoms, that behaves like a single particle. The quantum world invades the classical.These lithium clumps are cooled to one-millionth of a degree above absolute zero, at which point they begin acting like single "matter waves." As we would describe a quantum object or system in terms of its wavefunction—as possibilities or probabilities rather than actualities—so too would we describe one of these BEC clumps, known solitons.
"Studies of matter-wave solitons have mainly examined properties of single solitons," the study notes, "with the study of soliton interactions limited to those occurring in soliton trains and collisions between multiple solitons …"It's the wave-like nature of solitons that allows for our ghost matter. It's possible to tweak waves—confining them to one-dimensional wave guides—such that they have opposite or non-interfering amplitudes. For example, we can imagine one of the two solitons being at 1 and the other at -1 as they pass. The waves become complementary and, as such, they both travel through the same space at the same time, yet don't interact.
"It happens because of 'wave packet' interference," Hulet explained in a statement. "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."The Rice physicists actually found something even stranger in their work. While non-interfering solitons are an old concept, Hulet and his team noted that, on occasion, the solitons simply refused to pass through each other. Instead, they bounced away.The soliton "bounce," however, isn't quite what we might imagine."When we saw the initial data we said, 'This doesn't make sense, because solitons are always supposed to pass through one another and these look like they're bouncing instead,'" Hulet said. "So we began thinking about how we could tag one of the solitons to make it distinct so that we could follow its trajectory in time and see what it did."The Rice physicists succeeded in "tagging" solitons simply by making one bigger than the other. With differently sized solitons, the researchers then set about creating collisions between matter-waves with different phases and, thus, different degrees of interference. "We did that experiment over and over for many different relative phases, and we looked for two cases, one where the relative phase was zero, or in-phase, and another where it was 180 degrees, or completely out-of-phase," Hulet said.As expected, the in-phase solitons slipped by each other as ghosts, while the out-of-phase solitons bounced. Yet, bouncing for solitons means something rather different."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," Hulet said. "In other words, it jumped through the gap!"That's the disappearance. That clumps of matter can pass through each other unimpeded is strange enough, but solitons are capable of being more than mere ghosts and instead blinking out of existence for just long enough to not collide. Indeed, strange things happen when the quantum world is given life in the realm of large-scale, classical physics.