Assessing the dependency of the fine structure constant on gravity using hot white dwarfs


White dwarf stars have recently found a new use in the fundamental physics community, because they potentially allow us to directly observe variations in fundamental constants at locations of high gravitational potential.
Many prospective theories of the fundamental interactions of nature allow traditional constants to vary in some way. In case of the fine structure constant, α, such a change would result in a small shift in wavelength centroid of atomic transitions. This effect is larger for metals with more protons (higher atomic number) and higher ionization energy.
Measurements of this shift have previously been conducted using optical spectroscopy of Quasars, with the implication that α was smaller in the past (Webb et al. 2001). Magueijo et al. (2002) showed how varying constants, like α, can occur in the presence of strong gravitational fields, being proportional to the dimensionless potential GM/Rc2, and hence the compactness (M/R) of an object of mass M and radius R. This means that compact objects with high mass, and small radius - like white dwarfs - could exhibit detectable variations of α.
A first attempt to measure shifts in the wavelengths of highly ionized Fe and Ni lines in the high-resolution STIS spectra of the DA white dwarf G191-B2B was performed by Berengut et al (2013), demonstrating the validity of the method. We are now extending this work by (1) using new (high precision) laboratory wavelengths, (2) refining the analysis methodology (incorporating robust techniques from previous studies towards quasars), and (3) enlarging the sample size by studying other hot white dwarf, but also hot sub-dwarfs (this allows us to cover a broader compactness range).
A successful detection would be the first direct measurement of a gravitational field effect on a bare constant of nature.



Leverhulme Trust
This project is founded by a Leverhulme Trust Research Project Grant.
The Leverhulme Trust was established by the Will of William Hesketh Lever,
the founder of Lever Brothers. Since 1925 the Trust has provided grants and
scholarships for research and education. Today, it is one of the largest
all-subject providers of research funding in the UK, distributing
approximately 80M a year. For more information about the Trust,
please visit www.leverhulme.ac.uk.