Ultra-cool experiments test fundamental physics

An Auckland scientist has tested one of the most fundamental theories in physics to such accuracy that theoretical physicists will now have to race to catch up with the experimental data.

The research by Dr Maarten Hoogerland from The University of Auckland, working as part of an international team at the Free University in Amsterdam, has been published in the journal Science.

“Quantum electrodynamics is the best theory we have to describe how atoms are put together and, since all matter is made of atoms, it is fundamental to our understanding of how everything around us works,” says Dr Hoogerland.

“Like all theories it must be tested experimentally to see whether or not its predictions hold true. Our findings were in agreement with the theory, but went so far beyond what it can predict that the theoretical physicists will now have some work to do catching up with what we’ve found.”

The research team tested the theory by comparing predicted versus experimentally determined characteristics of one of the simplest atoms - helium.

“One of the predictions that can be made with the theory is the exact colour of light that will be absorbed by helium atoms. We wanted to test this experimentally, but in order to do so we had to stop the atoms from moving around as they would at room temperature. We therefore cooled a cloud of helium atoms to a temperature one millionth of a degree above absolute zero, to bring them to a standstill.”

“We captured the gas in the focus of a laser beam, and illuminated it with a second beam of known colour and were able to measure the absorption of the coloured light to twelve-digit accuracy. This was more accurate, by a factor of thousand, than the theory itself could predict.”

The study also allowed the size of the nucleus of a helium atom to be determined. The scientists measured the absorption of two isotopes, helium-3 and helium-4, knowing that the nucleus of helium-3 has one fewer neutrons than helium-4. The difference in the measurements between the two isotopes allowed the size of the helium-3 nucleus to be determined to an accuracy of 4 attometre (one attometre is one billionth of one billionth of a metre).

Most tests of fundamental physics are done in large particle accelerators, such as at CERN in Switzerland, but recent developments in ultra-stable lasers and accurate atom clocks have allowed small scale-experiments of extreme accuracy, of the kind done by Dr Hoogerland and colleagues. In fact, the determination of the size of the helium nucleus was more accurate than would have been possible with a particle accelerator.