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Dark matter and string theory?

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Dark matter and string theory? 17.04.2011

Super-cold neutrons could provide the answer.
A new technique developed by neutron scientists is so sensitive that it could be used to measure the quantum effects of gravity. Minute deviations from Newtonian laws could prove whether dark matter or string theory’s extra dimensions exist.

Particle physicists from the Vienna University of Technology and Institut Laue-Langevin (ILL) have developed a new technique named Gravity Resonance Spectroscopy that bounces ultra-cold neutrons along a mirror to observe their various quantum energy states. By vibrating the mirror at particular frequencies, researchers are able to boost the neutrons to higher quantum energy states.

This is the first resonance-spectroscopy method, which does not use electromagnetic forces, fields or potentials to drive the transitions. Their success is an important step towards modelling gravitational interactions at very short distances and looking for predicted tiny deviations from pure Newtonian gravity.
The research, reported in Nature Physics, could also test the equivalence principle, a 16th century law that states gravity accelerates all objects equally, regardless of their mass. In 1971, it was famously demonstrated on the moon by Apollo astronaut Dave Scott who dropped a hammer and feather, which millions watching at home saw land simultaneously. Researchers hope to use this new technique to test the principle’s accuracy at the atomic level for the first time.

The visible effects of gravity are usually seen only at large scales, governing the motion of stars and planets whilst quantum mechanics is primarily evident at the atomic scale.
“In this tiny world, the gravitational force is so weak that it is difficult to observe its quantum effects,” explains Prof Hartmut Abele, from Vienna’s Institute of Atomic and Subatomic Physics. “Using atoms to measure these effects is extremely difficult as they are disturbed by short range electric forces such as Van der Waals or Casimir forces. However, using ILL’s ultra-cold neutrons, which are chargeless, very slow moving and extremely resistant to electrical disturbance we can measure these effects extremely precisely”.

Prof Abele, Tobias Jenke and scientists from ILL took a high precision spectroscopy technique measuring resonance, which is usually applied to electromagnetic interactions, and for the first time used it to measure gravity. Their work improves on research using ultra-cold neutrons and mirrors to observe quantum gravitation interactions, first devised at the world leading neutron facility in 2002[1]. In this new research Prof Abele and his team mechanically induced transitions between these different energy states through the introduction of an oscillating field, achieved by vibrating the underlying mirror at a particular frequency. Using this technique future research will be able to calculate more precisely the energies behind the various quantum states of a neutron in the Earth's gravitational field.

Dr. Peter Geltenbort, Physicist at ILL and collaborator on this research: “In 2002, ultra-cold neutrons allowed us for the first time to observe the different quantum energy states of the neutron under the Earth’s gravity. Now with this technique we can attribute incredibly precise energies to each of these states. It’s a major breakthrough for those looking to understand the fundamental nature of gravity and bring together the physics of the atomic world with that of our own”.

Some physicists believe that more accurate measurement of these energies will reveal a slight divergence with those calculated using Newtonian laws of gravity. They predict that detecting and quantifying this disparity will provide evidence of dark matter particles known as axions or the extra dimensions suggested by string theory.

Prof Andrew Harrison, Science Director at ILL: “Whilst neutrons provide a valuable resource for applied science, Professor Abele’s research demonstrates their worth in uncovering the most basic truths in nature. Through our ongoing Millennium upgrade programme ILL continues to improve its capabilities to support world class fundamental and applied science.”

Re.:    Nature Physics online, 17 April 2011

Contact: James Romero +44 845 680 1866

Notes to editors

1.    At ILL in 2002, scientists placed a neutron absorber above a horizontal mirror with a small gap in between to fire through their ultra-cold neutrons at an upwards angle. The absorber removed neutrons that crashed into it above a certain vertical velocity whilst those of less energy were trapped by the reflecting surface below and the downward force of gravity.

This ‘neutron trap’ created discrete energy pockets meaning that only neutrons with a vertical velocity corresponding to one of its particular quantum gravitational energy states made it through to the detector at the end. By adjusting the height of the absorber above the mirror, the researchers observed stepped increases in the number of detections, corresponding to the various gravitational quantum energy states of the neutron.   Nature, Vol 415, 2002, p297-299

2.    About ILL – The Institut Laue-Langevin (ILL) is an international research centre based in Grenoble, France. It has led the world in neutron-scattering science and technology for almost 40 years, since experiments began in 1972. ILL operates one of the most intense neutron sources in the world, feeding beams of neutrons to a suite of 40 high-performance instruments that are constantly upgraded. Each year 1,200 researchers from over 40 countries visit ILL to conduct research into condensed matter physics, (green) chemistry, biology, nuclear physics, and materials science. The UK, along with France and Germany is an associate and major funder of the ILL. The remaining part of the budget is provided by 11 other European countries and by India.

3.    About Vienna’s Institute of Atomic and Subatomic Physics – The Institute of Atomic and Subatomic Physics was founded in 1958 as an inter-university institute for all Austrian universities and started operation in 1962, when the TRIGA Mark II research reactor of the institute was officially opened. It was dedicated to research and training in the areas of atomic, nuclear and reactor physics, radiation physics, radiation protection, environmental analytics and radiochemistry, nuclear measurement technology and solid state physics. Since then quantum physics, quantum optics, low temperature physics and superconductivity have developed as additional focus points of research. Right from the beginning the Vienna University of Technology was chosen as the administrative body of the institute, which however maintained its status as an independent legal entity for over three decades. Today, the Atomic Institute is one of four physics institutes at the Vienna University of Technology and forms the Physics Faculty of our University together with the Institutes for Theoretical, General and Solid State Physics

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