[Paleopsych] NS: New twist in wrangle over changing physical constant
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Sat Apr 23 09:02:40 UTC 2005
New twist in wrangle over changing physical constant
* 15:33 19 April 2005
* Maggie McKee
A new study of distant galaxies is adding a fresh perspective to the
debate over whether a fundamental physical constant has actually
changed over time. The work suggests the number has not varied in the
last 7 billion years, but more observations are still needed to settle
The controversy centres on the fine-structure constant, also called
alpha, which governs how electrons and light interact. Alpha is an
amalgam of other constants, including the speed of light. So any
change in alpha implies a change in the speed of light - and indeed in
the entire standard model of physics - with string theories touting
extra spatial dimensions stepping in to fill the breach.
So it caused a sensation in 2001 when a team led by John Webb, an
astrophysicist at the University of New South Wales in Australia,
announced the constant had changed by about one part in 100,000 over
12 billion years. Webb's team studied about 140 clouds of gas and dust
that absorb light from distant, bright quasars. Subtle changes in the
relative position of absorption lines from elements in the clouds'
spectra suggested alpha had changed over time.
But another team using higher-quality quasar absorption data, though
fewer observations, failed to find any change in 2004. And different
researchers studying a natural nuclear reactor in Oklo, Gabon in
Africa have also come to opposing conclusions about alpha's constancy.
John Bahcall, an astrophysicist at the Institute for Advanced Study in
Princeton, New Jersey, US, helped pioneer quasar absorption studies in
the 1960s, but he says this method can be slippery to interpret.
"The absorption line data is subject to misidentification of the lines
and to the introduction of theoretical assumptions which may not be
correct," he told New Scientist. He adds that the spectral lines may
be faint and can overlap, making it difficult to tell the lines'
source, and that astronomers must assume that all of the clouds share
the same basic composition. Similarly, he says, interpreting the Oklo
data involves making assumptions about other physical constants that
might be subject to change.
Now, astronomers have used another method Bahcall developed to probe
alpha. Rather than using absorption lines from clouds in space, a team
led by Jeffrey Newman of Lawrence Berkeley National Laboratory in
California, US, has focused directly on a pair of emission lines from
ionised oxygen in the galaxies themselves.
"With emission lines, you can choose systems that have a very strong,
characteristic appearance and are isolated to be sure you have the
right objects," says Bahcall. And because the average wavelength of
the light emitted by the oxygen ions is a direct measurement of alpha,
he says, "the fine-structure constant pops out without any
Clean and simple
However, the new study cannot yet claim the precision of Webb's
result. Newman's group used observations of 300 galaxies, lying
between 4 billion and 7 billion light years away, to find that alpha
has changed by no more than one part in 30,000 - the resolution limit
of the data - in 7 billion years.
But Newman says the method is superior to the absorption line
alternative, telling New Scientist: "It's simpler, it's cleaner, and
to date people haven't gotten contradictory results from it."
Bahcall says the team has done an "absolutely superb job" and adds
that more observations - including more distant galaxies - will
improve the precision of the measurement still further.
Newman presented the results, from a survey of distant galaxies called
DEEP2, at a meeting of the American Physical Society in Tampa,
Florida, on Monday.
* 13 things that do not make sense
* 19 March 2005
* Speed of light may have changed recently
* 30 June 2004
* Disputed 'building block' of physics is constant
* 02 April 2004
* DEEP2 survey, UC Berkeley
* John Bahcall, Institute for Advanced Study
* American Physical Society meeting
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