Ultima Thule

In ancient times the northernmost region of the habitable world - hence, any distant, unknown or mysterious land.

Sunday, December 03, 2006

Colourful calculations

By Aussiegirl

The author begins her second paragraph thus: Understanding how the universe works at the most fundamental scale is often likened to peeling away the layers of an onion. How handy a metaphor the onion is! Here are some onion quotations that I found:

The onion and its satin wrappings is among the most beautiful of vegetables and is the only one that represents the essence of things. It can be said to have a soul. (Charles Dudley Warner)

Why is it that the poet tells
So little of the sense of smell?
These are the odors I love well:
The smell of coffee freshly ground;
Or rich plum pudding, holly crowned;
Or onions fried and deeply browned.
(Christopher Morley)

Mine eyes smell onions: I shall weep anon. (Shakespeare)

Colourful calculations (December 2006) - Physics World - PhysicsWeb

The formidable computational power of lattice QCD is finally allowing researchers to make solid predictions about the force that binds quarks inside protons and neutrons, describes Christine Davies.

Understanding how the universe works at the most fundamental scale is often likened to peeling away the layers of an onion. The outermost layer of the onion represents atoms, and we have known about these for a century or so. The next layer of structure, which was revealed by Rutherford in 1911, is the atomic nucleus – a much smaller object that contains almost all of the atomic mass. Some 20 years after that discovery, physicists realized that the nucleus is composed of more fundamental objects called protons and neutrons. However, peeling back the next layer of the onion has turned out to be much more of a challenge.

It is now universally accepted that protons and neutrons are made up of fractionally charged particles called quarks: two "up" quarks and a "down" quark in a proton, and two downs and an up in a neutron. There are six types of quarks in total, but none of them has ever been observed as a free particle. Smashing protons together at enormous energies in particle accelerators, for instance, reveals not single quarks but yet more particles made of quarks. Such particles are called hadrons, and there are hundreds of them: some are "baryons", which contain three quarks, while the rest are "mesons" made up of quark-antiquark pairs. It might therefore seem, as indeed it did to particle physicists in the 1960s, that the core of the onion is forever hidden.

The only way we can understand the properties of quarks is to compare experimental measurements of hadrons with calculations based on quantum chromodynamics or QCD: the theory of the "strong force" that binds quarks together. Despite being around for over 30 years, however, the equations of QCD have proved eye-wateringly difficult to solve. Indeed, to the immense frustration of particle physicists, it has been impossible to calculate properties of hadrons with an accuracy of better than 10%.

In the December issue of Physics World, Christine Davies shows how a the technique "lattice QCD" is allowing these equations to be solved much more accurately by representing space-time as a 4D lattice – which, she says, could reveal what the final layer of the onion looks like.


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