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Shaping the Future

Scientific Milestones During Israel’s First Half-Century
Shaping the Future

First published in 1997

Statistical Mechanics



It's just a phase they're going through

Nature in its complexity is in no hurry to reveal its secrets. Sometimes, tremendous effort is needed to decipher phenomena which at first glance seem simple. What, for example, happens when matter changes from one state to another? What happens when conductive material turns into a superconductor? What happens when matter becomes magnetized? Transformations such as these, from one fixed state to another, are called phase transitions. Weizmann Institute scientists have made a significant contribution to understanding these basic phenomena.
In many instances, the phase transitions of very different substances exhibit similar characteristics, even though the systems are governed by very different forces and laws. Phase transitions from liquid to gas have similar characteristics to the change from a conductor into a superconductor, and these two transformations are similar to the phase transition of a nonmagnetic substance to a magnetized one. It seems as if nature follows a set of rules or formulas by which it carries out all phase transitions; this is described by renormalization group theory.
Phase transitions in nature are usually accompanied by another phenomenon called symmetry breaking. For example, when a sheet of paper is covered with a very thin layer of elongated iron filings (at a temperature higher than their magnetization threshold), the filings point to all directions in a symmetrical pattern. But when the temperature is lowered below the magnetization threshold, most filings line up in the same direction. Just as there are several types of phase transitions in nature, there are also different types of symmetries.
Weizmann scientists described the connection between the thermodynamic properties of the phase transition and of symmetry breaking. That is to say, they placed symmetry (and its breaking) into the context of renormalization group theory. Later they discovered that phase transitions in magnetic materials contain new multicritical points at which a few types of different phase transitions take place simultaneously.


Another dimension

A group of Weizmann Institute scientists focused on studying phase transitions in two dimensions (such as a layer of a specific material one atom thick). The studies showed that phase transitions taking place in two-dimensional layers are characterized by fixed factors different from those defining phase transitions in clumps of three-dimensional materials. The Institute's scientists contributed significantly to understanding the connection between phase transitions and the number of dimensions in which they occur.


Are traffic jams only natural?

Weizmann Institute scientists developed models explaining the difference between phase transitions occurring in systems in a state of equilibrium (which do not change over time), and those in systems not in equilibrium (which are developing and changing). For example, systems which have a liquid, electric current or heat flowing through them (including plants, animals and humans) lack equilibrium. The same could be said of a developing city where there is movement of vehicles along the roads, and so on.
The varied application of these models includes describing traffic on a network of roads. This gives an understanding of the conditions that might cause natural traffic jams (excluding those that occur as a result of traffic accidents, roadwork, or a police barrier). Weizmann scientists also contributed to the development of computational methods based on mechanical statistics for other disciplines, such as learning processes in neural networks.


Minuscule magnetism

Weizmann Institute scientists developed a theory that explains how changes in a magnetic field influence changes in the direction of minuscule magnetic particles found in the field. This theory explains, among other things, how recording and erasing is implemented on magnetic tape used in computers or tape recorders.


Composite materials: more than the sum of their parts?

Artificial composite materials (composed of fibers of one substance, filling a matrix made out of another substance) are used today to construct airplane wings, parts of satellites, bicycle helmets and even innovative tools and kitchen utensils. Many scientists around the world are investigating and developing composite materials for various purposes. One of the main questions puzzling them is how the properties of composite materials are affected by the properties of its individual components.
Weizmann Institute scientists discovered the upper and lower bounds for the range of possible properties of the composite material, based on the known properties of the substances of which it is composed. This discovery assists in planning composite materials with defined properties.


Measuring recoil-free absorption and emission

Weizmann Institute scientists planned and constructed a device for measuring the Mossbauer effect, which describes the phenomenon of recoil-free absorption and emission of radiation. These measurements are important in ascertaining the properties of magnetic materials and understanding different phase transitions in them. Weizmann scientists who used this device were able to verify calibration theories of phase transitions in magnetic materials.
Based on Weizmann designs, large numbers of these devices were built and sold to many scientists and technicians throughout the world. They were a cornerstone product of one of Israel's first advanced industries.


The structure of liquid crystals

Weizmann Institute scientists contributed to understanding the "Blue Phase" phenomenon. It refers to the state of a liquid crystal in a cubic crystalline array; that is, its basic structural unit is much larger than the single molecule that composes it. This knowledge provided tremendous assistance in studying liquid crystals in particular, and researching phase transitions in general.