National conference to see why less is more

Logo for the Special Research Centre for the Subatomic Structure of Matter.

Logo for the Special Research Centre for the Subatomic Structure of Matter.
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Professor Tony Thomas, Director of the Special Research Centre for the Subatomic Structure of Matter and winner of the Harrie Massey Medal.

Professor Tony Thomas, Director of the Special Research Centre for the Subatomic Structure of Matter and winner of the Harrie Massey Medal.
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Wednesday, 29 November 2000

It used to seemed so simple, once. Atoms were the smallest particles of matter. An atom was indivisible; and then they divided it. Splitting the atom unleashed much more than an atomic explosion. It rewrote Newton's fundamental laws of physics, and changed the very ways in which we look at matter and energy.

Physics was once the science of tangible, visible matter and how it behaves, but a whole new branch of physics now examines matter at the subatomic level, and the process of dissecting subatomic particles seems to the novice a never-ending one.

Take an atom apart, and negative electrons are found surrounding a nucleus of positive protons and neutral neutrons. But these nuclear particles can be broken down further, into quarks and gluons.

Such particles are hard to visualise. Conventionally they are drawn as a small solar system, with electrons whirling like tiny planets around a central sun-like nucleus. As you struggle with this model, however, it becomes less satisfactory the further you go, for electrons can behave like particles but also like waves. Tiny objects may not have precise positions, and they can tunnel through walls. Empty space is not empty, and some events do not have a cause.

These are some of the postulates of quantum theory, and they seem constantly at variance with the reliable laws that govern the macroworld that we know. In fact, dealing with the subatomic world requires a bit of radical rethinking.

All observable physical phenomena are now thought to be understandable in terms of four fundamental forces; strong, weak, electromagnetic and gravitational. Strong interactions bind atomic nuclei, provide the fuel for stars, giving us the nuclei of chemical elements and ultimately living things.

"Clap your hands together," says Professor Tony Thomas, from Adelaide University's Special Research Centre for the Subatomic Structure of Matter. "They don't pass through each other because of the electromagnetic interactions, without which life could not exist. Yet atoms also need nuclei, and it is the strong interaction which binds different combinations of protons and neutrons into the stable nuclei of the different elements," he explains. "Stars, including our sun, burn small nuclei to produce bigger nuclei. The mass that is lost in the process becomes the energy that is released."

The electromagnetic interaction gives us light, radio and microwaves among others, while the weak interaction is responsible for radioactive decay. It is also responsible for the means by which supernovae give off their energy as neutrinos, their cores collapsing into neutron stars and black holes.

"In some ways, the weak interaction is the most enigmatic of the forces," says Professor Thomas. "It is responsible for many forms of radioactive decay of unstable nuclei. It lets one kind of nucleus turn into another, and plays a crucial role in the stellar cycle."

"When all the nuclei in a star eventually combine to form the biggest nucleus, iron, the star's fuel is exhausted," says Professor Thomas. "Gravity plays a role here, for without it, stars would blow apart and cease to burn," he says. "Gravitational interaction also determines the motion of tangible objects, planets, solar systems and the universe itself."

Ideally, all of the four forces would arise from a single 'theory of everything.' As yet, there is no such candidate. The organising principle for almost all our understanding of modern physics is the Standard Model. In one elegant framework it combines the first three of these four fundamental physical interactions.

All three can be analysed by the sophisticated mathematics of quantum field theory, especially gauge field theories, but there remain tantalising holes in the understanding of this physical realm.

There is, for example, no generally accepted gauge field theory for gravity, and while the theoretical understanding of weak couplings is profound, it is incomplete when it comes to the strong couplings. It is this area that is of special interest to the Special Research Centre.

The Centre's researchers are leading an attack on some of these problems through a multi-pronged approach, from mathematical modelling and cooperation with experimenters at the world's major particle accelerators, to the use of advanced supercomputers.

The latter involves a technique known as Lattice Gauge Theory. Space-time is represented by a four-dimensional lattice. The computer extrapolates to infinitesimal lattice spacings and infinite lattice volumes, allowing the extraction of quantities of physical interest. These extrapolations require calculations involving ever-increasing numbers of lattice points; prompting the need for a powerful computer.

Adelaide University is now home to the National Computing Facility for Lattice Gauge Theory, with the Orion supercomputer capable of 110 billion calculations per second. All current lattice gauge theory groups in the country are represented in the Centre, which competes in the forefront of international research to advance an understanding of the 'strong' force.

Just as the simple planetary model of an atom has become complicated by the requirements of quantum theory, so the once simple realm of physics has required some reappraisal.

"Physics is an all-enveloping discipline which attempts to explain all observable phenomena in terms of the fewest fundamental constants and with the simplest mathematical framework," says Professor Thomas. "All sciences are subsets of some of these phenomena."

"When the study of systems of a particular type becomes sufficiently specialised, we associate it with some other science," he explains. "For example, chemistry is at heart, the study of how atoms behave when their electron clouds interact. In a certain sense, biology is to chemistry what chemistry is to physics. Yet physicists still have a vital role to play in these related fields, as well as in engineering, because of their capacity for solving problems from first principles."

As the subatomic aspect of physics moves more deeply into the realm of mathematics, it relies increasingly on the interplay between physical insight and the brute force of modern supercomputers. "It is the need to use ever-larger grids in order to better approximate the real universe that drives our unquenchable thirst for larger and larger supercomputers!" says Professor Thomas.

The challenging area of subatomic physics will be among several areas of physics to be debated in December, when Adelaide University hosts the 14th National Congress of the Institute of Physics. The program runs from 10-15 December. During the Congress, the prestigious Harrie Massey Medal will be presented to Professor Thomas for his outstanding research
contributions.

 

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