Dispute over method to detect gravitational redshift

10 June 2011 | Source: Classical and Quantum Gravity

A constructive dispute — concerning the work of a research team including the US Secretary of Energy Steven Chu — has broken out over a method used to measure the effect of gravitational redshift.

In a study published on Thursday 9 June in IOP Publishing’s journal Classical and Quantum Gravity, a team of researchers, including Claude Cohen Tannoudji (an emeritus professor at the Collège de France, who shared the 1997 Physics Nobel Prize with Steven Chu), question the conclusions of research undertaken by Professor Chu and his team.

Professor Chu and his research colleagues suggested that atom interferometry can be used to detect gravitational redshift in research published last year, but new research now suggests that the complex experiments undertaken may have been measuring a different phenomenon.

Gravitational redshift—a tenet extrapolated from Einstein’s theory of general relativity—explains why it is that clock rates change with gravitational potential, as a result of spacetime being bent by objects of large mass.

The effect has been tested on numerous occasions using atomic clocks; a clock positioned close to an object of large mass, at a high gravitational potential, will tick slower than a clock positioned further away from the object of mass, at a lower gravitational potential.

As one of the most basic building blocks of the modern understanding of spacetime and gravitation, there has been a growing effort to try and measure this effect as accurately as possible.

In the paper published last year, Chu and his colleagues claimed that the atom interferometry devices could detect gravitational redshift with a precision several orders of magnitude greater than current, and future planned, clock tests.

This technique involved using three lasers to position an atom in a stable state, push the atom back faster towards earth, and then measure the difference.

This new study has disagreed with these findings, stating that the atom interferometer is in fact an accelerometer that measures the acceleration of freely falling atoms with respect to the instrument that is at rest in the laboratory.

Lead author Professor Peter Wolf, of the Observatoire de Paris, stated that if the whole measuring instrument was put into a free-falling environment, the acceleration and the experimental signal, would vanish. 

Conversely an atomic clock that is due to be flown into space in 2013 by the European Space Agency will be able to deliver signals, even in the free falling environment of the International Space Station, and will provide a measurement of the gravitational redshift rather than of the acceleration of the freely falling atoms..

Professor Wolf said, “These ongoing discussions will allow researchers to identify the best ways of testing some of the most fundamental building blocks of our understanding of gravitation.”

The authors of the publication are members of the Laboratoire LNE-SYRTE (unité mixte UMR 8630 du CNRS, de l’Observatoire de Paris et de l’UPMC), of the GRECO group at Institut d’Astrophysique de Paris (unité mixte UMR 7095 du CNRS et de l’UPMC), and Laboratoire Kastler Brossel (unité mixte UMR 8552 du CNRS, de l’ENS et de l’UPMC).