Sigma Xi, The Scientific Research Society

Colossal ComputationsAuthor(s): Mike MaySource: American Scientist, Vol. 82, No. 4 (JULY-AUGUST 1994), pp. 323-324Published by: Sigma Xi, The Scientific Research SocietyStable URL: http://www.jstor.org/stable/29775231 .

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New Scientist, the University of Pennsylvania is

seeking to patent the technique. The application filed with the European Patent Office includes de?

scriptions of how the technique could be used in arurnals, including some applications "wherein said ardmal is human." In Walters's opinion, Brinster's

technique would not be appropriate for human use. The mention of human uses, he proposes, was

probably made at the suggestion of patent attor?

neys trying to anticipate every possible use of the

technology. It is also unlikely that the patent will be

granted as is. New Scientist quoted Christian

Gugerell, director of the European Patent Office in Munich, as saying it would be "highly doubtful whether something like this could be patented."

Whether or not this patent is granted, advances such as Brinster's promise that someday, the argu?

ment surrounding germ-line gene therapy will focus on ethical rather than technical issues. And these will be less easy to resolve. In a paper published last

year in Science, Walters and Nelson Wivel, director of NIH's Office of Recombinant DNA Activities, enu? merate points on both sides of the argument. They suggest that the health professions are morally oblig? ed to use the best available treatments, and that par

ents should have the freedom to decide to use avail? able technologies for the purposes of having a

healthy child. Germ-line gene therapies, they say, will also likely be more efficient and cost-effective than somatic-cell gene therapy. Finally, pursuing re? search in germ-line therapy will enhance the scien? tific enterprise in general and add to the existing body of knowledge. To these points Verma adds a

compelling one. There are some cases where genetic disorders threaten to wipe out certain ethnic groups, and germ-line gene therapy may provide an effec? tive way to save those groups from extinction.

Equally compelling are the arguments against germ-line intervention. Walters and Wivel note that the technique will always involve unavoid? able risks and irreversible mistakes. It would be difficult to advocate the use of risky technology, they say, when safer alternative therapies exist. Furthermore, it is inevitable that people will, at some point, seek to use the technology for en?

hancement, rather than for the intended purpose of treating serious genetic disease. Ultimately, say the authors, the technology may or may not be deemed too dangerous to undertake. The de? bate is only beginning.?Michelle Hoffman

1 Colossal Computations'^ An alert parent has no trouble recognizing

a phase change in a child?the onset of the Terrible Twos, say, or the beginning

of puberty. The underlying process of change may remain obscure, but there are often clear be?

ginning and end points to the changes that mark the development of a child's personality. So it

goes, too, in the physical world. Phase changes in materials can be dramatic and

can seem sudden?solid becomes liquid, magnet becomes nonmagnet. They are known to happen in multiple steps over time, but many of these sta?

ges are as obscure as the steps that bring about a child's developmental leaps. Producing a phase change is relatively easy. But it is another matter

entirely to explain the microscopic shifts that gen? erate it. Now David Landau, a research professor of physics at the University of Georgia, and Kun Chen, a postdoctoral fellow at the Oak Ridge Na? tional Laboratory, have simulated those steps.

On a fairly crude scale of time and space, it is

possible to measure the static behavior of a phase transition?for instance, to record temperature changes in a beaker of water in an ice bath, from one second to the next, as it begins to freeze. But investi?

gators have known since the turn of the century that phase transitions involve long-lived fluctua? tions at the microscopic level. In other words, the water does not become consistently colder through? out the beaker; rather, its temperature decreases in

steps that last for nanoseconds and extend over micro? meters. Such fluctuations are characteristic of dy? namic critical behavior, the short-time changes dur

Counting calculations with nickels, Landau and Chen's simulation would generate a stack that extends through the Earth and into outer space.

1994 July-August 323

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ing a phase transition. These changes can be ex?

plored experimentally, but according to Landau, "This is very difficult, very time-consuming and

very expensive, with the emphasis on difficult." In

fact, only a few experiments have provided ade?

quate data on dynamic critical behavior. Landau began conducting experiments on phase

transitions as a graduate student. The difficulty, time and expense of such experiments suggested to him that a better approach must exist. So he turned to simulations. In a simulation one describes a material

numerically, inserts a stimulus (such as temperature change) into the program and then, using equations expressing physical laws, calculates the effect.

Landau and Chen, working at the University of

Georgia's Center for Simulational Physics, simulat? ed a phase transition in a ferromagnetic-like mater? ial. Such a material is not magnetic at high temper? atures because the magnetic moments of the atoms are oriented randomly. If the temperature decreases, however, the magnetic moments begin to align at the temperature, called the critical point, that marks the phase transition. Reducing the temperature be? low the critical point leads to more alignment, but at a slower rate, a process called critical slowing down. The time course of such slowing down is one of Landau and Chen's major interests.

To simulate a ferromagnetic material, they built a numerical lattice of cubes. The simulated ferromag net did not mimic any specific material, but it re? sembled europium oxide. Each cube had an ion at its center, and the ion interacted with ions in neighbor? ing cubes. Using Newton's laws, it is possible to

write equations for the magnetic moment of each ion. As Landau explains, "Then you know what the force, or in this case the torque, is on an ion's mag? netic moment due to its nearest neighbors. The prob? lem is: As a magnetic moment that you're looking at

moves under this torque, its neighbors are also mov?

ing. So you really need to calculate the change in all of the neighbors, the neighbors of their neighbors, and so on and so on." In a large system, with 128,000 microscopic moments or cubes, there are 128,000 equations, each a vector equation, resulting in 384,000 coupled differential equations."

But the computational nightmare has only be?

gun. To explore dynamic behavior, the simulation

must follow the material through time as the tem?

perature decreases below the critical point. The sim? ulation begins at a high temperature, and all of the

equations are solved. Then time is advanced slight? ly, and the equations are solved again, a process re?

peated through as many as 20,000 time steps, re?

quiring the program to solve more than 7.5 billion

equations. The process is carried out on a variety of different initial arrangements of the magnetic mo?

ments, sometimes as many as 500. That drives the number of solved equations to nearly four trillion. "It is absolutely overwhelming," says Landau.

Landau and Chen ran their simulation on a

Cray C-90 vector supercomputer at the Pittsburgh Supercomputing Center. Studying a single type of lattice, at a variety of sizes and initial condi? tions, required hundreds of hours of run time on the Cray, even though the program was written to take advantage of a C-90's special capabilities.

From all of these calculations, Landau and Chen became the first to determine numerically the dynamic critical exponent, a measure of the time course of dynamic behavior during a phase change. Their result is nearly identical to the val? ue determined from phase-transition theory. In another material, an

anti-ferromagnet, however,

preliminary simulations indicate a different time course than the one expected from theory.

One day, Landau and Chen may model phase transitions in realistic materials, using perhaps the actual structure of europium oxide. "To do a prop? er job of that," Landau warns, "means more-dis? tant neighbors interacting, more complexities in the interactions; you may have to include small amounts of impurities, and you may need anoth? er factor of 10 in computing time."

Landau believes that simulations surpass theo? retical approaches to understanding phase transi? tions. "Very often," he explains, "theoretical tech?

niques that work on very simple systems cannot be used on a more complex one. It just becomes in? tractable. Simulations, though, can work on more

complex systems with very little modification. It

may require more computer time, but generally speaking most of the techniques follow through."

If, that is, you can make it through the Terrible Two Trillions.?Mike May

B The Personality of Physicists^4easurIdB The only personal ad I ever answered was

one placed by a physicist. There was a sim?

ple reason for this. As a science writer and

editor, I had had the opportunity to work with sev? eral hundred scientists, most of them men, and

usually under circumstances that guaranteed I would learn how aggressive they were. Physicists, for the most part, hadn't tried to push me around.

I was therefore amused to read in the April 1994 issue of Physics World, a new review maga? zine published in Britain, that psychometric test

ing has confirmed my assessment. Among other

things male physicists tend to be less pushy, less

competitive and less confrontational than others, tend to enjoy being on their own, are just a little

more flexible than the typical male, are more em

pathic than other people, more sensitive and car?

ing, and more inclined to be socially responsible and reliable. They are also less tidy than average.

The profile is based on a survey of a randomly chosen sample of 600 members of Britain's Institute of Physics, 40 percent of whom responded. The sur

324 American Scientist, Volume 82

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