Deceptive-looking vortex line in superfluid led to twice-mistaken identity

September 30, 2014 by Emily Conover, University of Chicago

Researchers produced this image in a computer simulation of an unexpected phenomenon found in an experiment involving ultracold superfluids. This image shows a three-dimensional view of a vortex line (red) as it forms from a decaying vortex ring in a superfluid. Credit: Peter Scherpelz
(Phys.org) —So long, solitons: University of Chicago physicists have shown that a group of scientists were incorrect when they concluded that a mysterious effect found in superfluids indicated the presence of solitons—exotic, solitary waves. Instead, they explain, the result was due to more pedestrian, whirlpool-like structures in the fluid. They published their explanation in the Sept. 19 issue of Physical Review Letters.

The debate began in July 2013, when a group of scientists from the Massachusetts Institute of Technology published results in Nature showing a long-lived structure in a —a liquid cooled until it flows without friction. The researchers created the structure in a superfluid made of ultra-cold lithium atoms, by hitting half of the fluid with a laser, so that the lithium particles would be in different quantum-mechanical configurations in the two halves.

When they imaged the result, the researchers observed a dark line cutting across the cigar-shaped volume of superfluid, indicating a region where the density of particles in the fluid was lower. This, they concluded, was a soliton, which behaves like a sparsely populated wall between two halves of the fluid, separating the particles found in the two different states. This wall persisted for a long time, and oscillated back and forth across the fluid.

The appearance of the soliton wall was a surprising conclusion, because it didn't fit in with the accepted theories about the behavior of such systems.

"If it were a wall, that would mean that there's some very unusual physics that theorists did not know about going on, so it of course attracted a huge amount of attention," said Peter Scherpelz, a postdoctoral scientist in physics and lead author of the paper.

Ensuing saga

Viewed from the side, a computer simulation clearly shows the winding path that a vortex takes through a superfluid over time. Such a view was inaccessible in the original experiment that scientists sought to simulate. Credit: Peter Scherpelz

A scientific saga ensued, in which multiple groups from different institutions attempted to understand the result. But the UChicago group—led by Kathryn Levin, professor in physics—was the first to present the correct explanation.

Levin's group tried to reproduce the puzzling result with a computer simulation of a superfluid. The group had developed the simulation thanks to collaboration with Argonne National Laboratory. Meanwhile, other groups tried their hands at simulations as well. Some concluded that the region of lower density in the fluid was the result not of a soliton but of a vortex ring—a swirling, donut-shaped structure, around which particles circulate. A smoke ring is a well-known example of a vortex ring.

But Levin's group couldn't reproduce these results in their simulation. Instead, they found that a vortex ring was briefly established, but quickly decayed to a simple vortex line, akin to a tornado or whirlpool stretching across the fluid.

Shortly after Levin's group posted their results on the preprint server arXiv, the MIT researchers released their new results in a preprint, explaining that what they had seen were simple vortices—validating the UChicago theory.

"We swam upstream in a way," said Levin. "Not too often theory anticipates experiment, and not too often theory's bold enough to say 'Wait a minute. We don't agree with what the going story is. We think it had to be something else.'"

Symmetry problems

The problems with the earlier simulations came down to symmetry. Much like a cigar looks the same if you rotate it around its long axis, other teams had assumed in their simulations that the behavior in the was symmetric—an approximation that made it easier for structures like rings to persist, but which didn't account for imperfections that are inevitable in real-world experiments.

The original MIT experiment had also assumed an incorrect symmetry to come to their original conclusion. They measured only a two-dimensional projection of their experiment, meaning that they couldn't distinguish between the three possible structures, because a ring or a wall viewed from the side looks just like a line. The MIT group had incorrectly assumed that the feature was symmetric, and that it sliced all the way through the cigar to form a soliton wall.

Physicists are intrigued by the physics of superfluids in part because they are related to superconductors, which have a multitude of technological applications due to their ability to conduct electricity without any resistance. Superfluids, however, often are an easier system to study. The materials are so similar that the simulation code used by the group was originally developed for superconductors, and modified for superfluids.

Another reason physicists want to understand this system is to study physics out of equilibrium, in which the material hasn't reached a balanced, comfortable state. After the superfluid is hit with the laser, half of the atoms are in a different state than the other half, and they want to return to the same state. Vortices form as the superfluid moves toward equilibrium.

"Everything we know about physics is sort of confined to equilibrium and we're trying really hard to test ourselves and learn what goes on out of equilibrium, because that's a lot of the real world," Levin said.

Explore further: Solving a physics mystery: Those 'solitons' are really vortex rings

More information: "Phase Imprinting in Equilibrating Fermi Gases: The Transience of Vortex Rings and Other Defects," by Peter Scherpelz, Karmela Padavić, Adam Rançon, Andreas Glatz, Igor S. Aranson, and K. Levin, Physical Review Letters, Vol. 113, Issue 12, Sept. 19, 2014. DOI: 10.1103/PhysRevLett.113.125301.

Related Stories

Breaking nature's superfluid symmetry

September 6, 2013

Superfluids are an exotic state of matter in which particles flow without experiencing viscosity. Hiroki Ikegami and colleagues from the RIKEN Low Temperature Physics Laboratory in Wako have now observed another remarkable ...

Superfluids: Observation of 'second sound' in a quantum gas

May 15, 2013

Second sound is a quantum mechanical phenomenon, which has been observed only in superfluid helium. Physicists from the University of Innsbruck, Austria, in collaboration with colleagues from the University of Trento, Italy, ...

Hunt for an 'unidentified electron object'

March 24, 2014

New research sheds light on the nature of 'unidentified electron objects'—a mysterious class of objects that exists in superfluid helium at low temperature.

Recommended for you

Researchers study interactions in molecules using AI

October 19, 2018

Researchers from the University of Luxembourg, Technische Universität Berlin, and the Fritz Haber Institute of the Max Planck Society have combined machine learning and quantum mechanics to predict the dynamics and atomic ...

Pushing the extra cold frontiers of superconducting science

October 18, 2018

Measuring the properties of superconducting materials in magnetic fields at close to absolute zero temperatures is difficult, but necessary to understand their quantum properties. How cold? Lower than 0.05 Kelvin (-272°C).

The big problem of small data: A new approach

October 18, 2018

Big Data is all the rage today, but Small Data matters too! Drawing reliable conclusions from small datasets, like those from clinical trials for rare diseases or in studies of endangered species, remains one of the trickiest ...

8 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

tonybudz
1 / 5 (4) Sep 30, 2014
"indicating a region where the density of particles in the fluid was lower"

And they know the density is lower how when they don't even understand its 3 dimensional principal upon a 2 dimensional surface.

Sometimes when you pull something apart such as a molecules molecular cohesion eg Lithium, its modeled pathway leaves an absethenic void when stressed and improperly observed.

They are so close to dark matter now they can almost taste it.
thingumbobesquire
not rated yet Sep 30, 2014
Well, at least dolphin like vortex rings...and magnetic fields. https://www.youtu...ENGq7AZk
Goika
Sep 30, 2014
This comment has been removed by a moderator.
Captain Stumpy
3 / 5 (4) Sep 30, 2014
analogy of photons in AWT
@goika-Zephir
awt is a pseudoscience and has been debunked: see: http://exphy.uni-...2009.pdf

for more details... or you can also read this: http://arxiv.org/...1284.pdf

awt is not a theory, nor is it science as it is debunked as well as proven wrong
therefore your post is off-topic and irrelevant, as well as pseudoscience
Goika
Sep 30, 2014
This comment has been removed by a moderator.
Captain Stumpy
4 / 5 (4) Sep 30, 2014
awt is a pseudoscience and has been debunked
AWT is not mentioned in your links at all.
@zeph
these studies have been linked through PO as well as other sites in articles that state how it applies to your precious dead religion

in fact, one even proves your precious religion dead to a VERY high degree... which was the last one linked here on PO: http://arxiv.org/...1284.pdf

but your post is simply your denial and fear of being found out. You are sunk so now you will simply state that the study does not call out a specific word to save face: not working.

for those who understand physics (and apparently that is NOT you) the study stands as a testament to how science killed your precious religion with a high degree of accuracy

feel free to offer ANY empirical evidence from a reputable peer reviewed source with impact that the study is not valid... i have all the time in the world
Goika
Sep 30, 2014
This comment has been removed by a moderator.
Captain Stumpy
4 / 5 (4) Sep 30, 2014
Which you didn't.
@Goika/eseta/zephir
actually, I provided a PLETHORA of evidence that you are wrong, and that awt is dead

it is just that you cannot understand it, and you live in denial as well as in a delusion state that makes you the only authority you can trust
IMO the vortex rings can be stabilized...
your opinion means nothing in the face of empirical evidence, which is thus: http://arxiv.org/...1284.pdf

your religion is dead. read it and weep...

I noticed that you cannot even bring ANY empirical evidence for your opinion (what is also known as personal conjecture without evidence)

the argument already died. you lost. aether is dead. you are only proving what I said.

you- no evidence and bs pseudoscience double-talk with what you think are big words with important sounding events

Me- evidence, studies, reputable info with a physics impact proving you wrong

you- http://sci-ence.o...-flags2/

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.