Chicago, IL, USA. New experiments show that common scientific rules can apply to significantly different phenomena operating on vastly different scales.

The findings, from the field of physics, raise the possibility of making discoveries pertaining to phenomena that would be too small, large or impractical to recreate in the laboratory.

The related math and physics have been under intense scrutiny for centuries. With biology now grounded on scientific procedure and rigorous experiment, questions of breadth and scale have become very important.

The biosciences are interested in modeling from pre-cellular protein formation through cells to organs and complete anatomical structure. Such multiscale modeling efforts are necessary to better understand structural and functional changes associated with unexpected genomic and genetic changes as well as the potential associations with disease. However, many of the necessary computational tools do not yet exist.



Cloud of Cesium Atoms. The Chin Lab explores the quantum world at the lowest temperatures achieved so far (a billionth of a degree Kelvin above absolute zero)

In the image above, a cloud of cesium atoms (shown as red balls) are confined on a horizontal plane and cooled to nano-Kelvin temperatures.

Click Pic for DetailsThe effects of scale are important in a range that extends from the infinitesimal (where objects are so small that they can are neither visible nor measureable) to a size with no known bound or end (ie., infinity). The near-parallel rise of both relativity and quantum mechanics has raised serious questions about the universality of scientic laws and theories.

Imagining the unimaginable

The new research results raise the possibility of making discoveries pertaining to phenomena across a broad range. The computational methods have potential applicability large scale analysis of diverse populations.

Cheng Chin is associate professor in physics and the James Franck Institute at the University of Chicago. Chin and associates Chen-Lung Hung, Xibo Zhang and Nathan Gemelke published their results in the journal Nature.

Chin wants to simulate the impossibly hot conditions that followed the big bang, during the earliest moments of the universe. He uses an ultracold vacuum chamber in his laboratory. The experiments demonstrate the validity of two widely discussed topics in the physics community today: scale invariance and universality. “It’s fascinating to think about all these connections,” Chin says.

Theoretical physicist Lev Pitaevskii had predicted that scale invariance would apply to a two-dimensional, cold-atom gas in 1997. Scale invariance means that the properties of a given phenomenon will remain the same, no matter how much its size is expanded or contracted. This is a sharp contrast with the three-dimensional world of everyday life, where dynamics change dramatically.

Tantalizing possibilities

“There’s a strong reason to believe that this kind of scale invariance can be extrapolated and on a more fundamental level can be mapped to other types of two-dimensional systems,” Chin said.

“The bigger question is whether our observation can shed light on other complex phenomena in nature. So our next step will be to explore going beyond two-dimensional systems.”

For comparison, the smallest objects that the unaided human eye can see are about 0.1 millimeter long, or 100 microns. Under ideal conditions, an observer can see either a human or a paramecium without magnification.

A light microscope is useful to see smaller cells but power is limited by the wavelength of visible light, which is still enough to resolve bacteria (but not viruses). Anything smaller than 500 nm requires an electron microscope, capable of resolving some molecules and even individual atoms.In the biological world, for example, scale invariance does not apply to complex organisms like humans, but exists in simple biological structures like nautilus shells, ferns and even broccoli. In physics, special cases also exist that exhibit scale invariance. Fractal structures have been observed in nature, which manifest similar structures whether magnified 10, 1,000 or a million times.

“There are only a few systems in nature that can display this kind of scale invariance, and we have shown that our two-dimensional system belongs to this very special class,” Chin explained. “Once you identify these special cases and see how they are all linked together, then you can bring all these physical phenomena under the same umbrella,” Chin said. “Now they can be fully described using the same language.”

Exotic transformation

The universality concept applies to matter that undergoes smooth phase transitions. In the physics of everyday life, a phase transition occurs when water freezes to ice on a cold winter day. The phase transition in the experiment reported on here is more exotic: cesium atoms transform from a gas to a superfluid, a form of matter that exists only at temperatures of hundreds of degrees below zero.

Theoretical physicists in the early 1970s predicted that weakly interacting two-dimensional gases would exhibit similar behaviors under a variety of conditions as they neared the critical point of phase transition. Their prediction has remained unverified until now.

In their experiment, the researchers super-cooled thousands of cesium atoms to 10 nano-Kelvin, billionths of a degree above absolute zero (-273.15°C or -459.67°F), then loaded them into a pancake-like laser trap. The trap simulated a two-dimensional system by restricting the atoms’ motion vertically but allowed a significant degree of horizontal freedom.

Chin’s team was able to control the properties of this cold-atom gas system to make it non-interacting, weakly interacting or strongly interacting and then compared the results. “At the same time, we can prepare the two-dimensional system at different sizes and also at different temperatures,” Chin said.

They could adjust the size parameters from 10 to 100 microns (a human hair is approximately 50 microns in diameter), and the temperature parameters from 10 to 100 nano-Kelvin.

Their experiment showed that no matter how they changed these three parameters, just one general description could characterize the resulting dynamics.

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