In a Letter in the journal Physics Review, the team from the
University of Leicester, University of Cambridge, University of Arizona
and University of New South Wales highlight the potential impact of
their research.
“The research opens up new possibilities for searching for exotic
"scalar fields", forms of energy that often appear in theories of
physics that seek to combine the Standard Model of particle physics with
Einstein’s general theory of relativity,” said Professor Barstow.
The electromagnetic force is one of four fundamental forces that
shape the universe. Its strength is given in the Standard Model by a
pure number, with no units, known as the "fine-structure constant",
denoted by the Greek letter alpha (α). Combining the speed of light, the
electric charge of an electron, and Planck's constant, alpha has always
been measured on earth to have the same value, approximately 1/137.
A key question is whether alpha changes in different parts of the
Universe or in strong gravity fields. Recent observations of light from
distant quasars (luminous sources thought to be caused by material
heating up as it swirls around a black hole) hint that the
fine-structure constant varies over the sky at large distances. But as
yet there is no independent check for this result.
Some theories predict that alpha will vary in the presence of exotic
scalar fields like those that are invoked to help unite the Standard
Model with Einstein’s theory of general relativity that describes
gravitation.
White dwarf stars, the compact remnants left behind when stars like
the Sun reach the end of their lives are an ideal natural laboratory to
test this idea. With a lot of matter packed into a sphere about the size
of the Earth, they have strong gravitational fields.
By measuring the value of alpha near a white dwarf, and comparing it
with its value here and now in the laboratory, astronomers can
indirectly probe whether these alpha-changing scalar fields actually
exist.
The team measured alpha for the first time the white dwarf star
G191-B2B using iron and nickel ions (atoms where electrons are added or
removed to give them a net electrical charge) trapped in the atmosphere
of the white dwarf. Despite the strong gravitational field of the white
dwarf - nearly 100,000 times that on Earth - the ions stay above the
surface because they are continually pushed up by the strong radiation
from the star.
The ions absorb some of the light from the white dwarf, making an
"absorption spectrum" that was observed using the Hubble Space
Telescope. That absorption spectrum allows the scientists to probe the
value of alpha in the atmosphere of the white dwarf with high accuracy.
By comparing the positions of the absorption lines measured by the
telescope with the positions measured in the laboratory, they can tell
whether alpha is different near the white dwarf.
Professor Barstow adds: “We found that any difference between the
value of alpha on Earth and that measured in the strong gravitational
field of the white dwarf must be smaller than a part in ten thousand,
which means that any scalar fields must only weakly affect the
electromagnetic force.
‘Unfortunately, our work was limited by the need to use very old
laboratory measurements from the 1970s. In the future, with better
laboratory data to complement the high-precision astronomical data, we
should be able to measure the change in alpha down to one part per
million. At that level we would be able to place strong restrictions on
whether alpha is a true constant of nature.”
Science contacts
Prof. Martin Barstow (at the NAM conference till 5 July)
University of Leicester
Mob: +44 (0)7766 233 362
mab@star.le.ac.uk
University of Leicester
Mob: +44 (0)7766 233 362
mab@star.le.ac.uk
Dr Julian Berengut
University of New South Wales
Tel: +61 (2)9385 7637
Mob: +61 (0)423 115 365
jcb@phys.unsw.edu.au
University of New South Wales
Tel: +61 (2)9385 7637
Mob: +61 (0)423 115 365
jcb@phys.unsw.edu.au
Media contacts
Ms Emma Shea
Head of Development Communications
University of St Andrews
Tel: +44 (0)1334 462 167
Mob: +44 (0)785 090 0352
emma.shea@st-andrews.ac.uk
Head of Development Communications
University of St Andrews
Tel: +44 (0)1334 462 167
Mob: +44 (0)785 090 0352
emma.shea@st-andrews.ac.uk
Mr Ather Mirza
University of Leicester
Tel: +44 (0) 116 252 2415
Mob: +44 (0)7711 927821
pressoffice@le.ac.uk
University of Leicester
Tel: +44 (0) 116 252 2415
Mob: +44 (0)7711 927821
pressoffice@le.ac.uk
Deborah Smith
Landline numbers in NAM 2013 press room (available from 9 a.m. to 5 p.m. from 1-4 July, 9 a.m. to 3 p.m. 5 July): Tel: +44 (0)1334 462231, +44 (0)1334 46 2232
Image and caption
A diagram that shows how absorption line spectra will change if the
electromagnetic force is affected by gravity is available at
Credit: Dr Julian Berengut, University of New South Wales.
Further information
The research group consisted of Professor Martin Barstow, Simon
Preval (both at the University of Leicester), Dr Julian Berengut,
Professors Victor Flambaum and John Webb and Mr Andrew Ong from (all at
University of New South Wales), Prof. John Barrow from the University of
Cambridge and Prof. Jay Holberg of the University of Arizona.
Their work appears in “Limits on variations of the fine-structure
constant with gravitational potential from white-dwarf spectra”,
Physical Review Letters, J. C. Berengut, V. V. Flambaum, A. Ong, J. K.
Webb, John D. Barrow, M. A. Barstow, S. P. Preval, J. B. Holberg, in
press. A preprint of the paper can be seen at http://arxiv.org/pdf/1305.1337.pdf
Notes for editors
Bringing together more than 600 astronomers and space scientists, the
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2013 at the University of St Andrews, Scotland. The conference is held
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University of St Andrews and will form part of the on-going programme to
celebrate the University’s 600th anniversary.
Meeting arrangements and a full and up to date schedule of the scientific programme can be found on the official website at http://www.nam2013.co.uk
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The University is currently celebrating its 600th anniversary and
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For further information go to: www.st-andrews.ac.uk/600/
