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Clues to 'Constant' Change
Some astronomical and geological studies suggest there might have been very small changes in the values of fundamental constants over billions of years, eventhough the results have been inconsistent and controversial. If fundamental constants are changing, the present-day rates of change are too small to be measured using conventional methods. However, a new comparison of NIST's cesium fountain and mercury ion clocks, scheduled to appear in this week's issue of Physical Review Letters,* has narrowed the range in which one of them-the "fine-structure constant"- possibly could be changing by a factor of 20. Widely used in physical theory and experiments, the fine-structure constant, represents the strength of the interaction between electrons and photons.
Astronomers and geologists have attempted to detect changes in natural constants by examining phenomena dating back billions of years. The NIST experiments attained the same level of precision by comparing the relative drifts in the "ticks" of an experimental mercury ion clock, which operates at optical frequencies, and NIST-F1, the national standard cesium clock, which operates at lower microwave frequencies. These data can be plugged into equations to obtain upper limits for possible rates of change of the fine structure constant in recent times.
A second study, based on seven years of comparisons of cesium and hydrogen clocks at NIST and in Europe,** achieved record limits on Local Position Invariance, the principle that two clocks based on natural frequencies of different atoms should undergo proportional frequency shifts when subjected to the same changes in gravitational field. The new experiments lowered the upper limit for a possible violation of LPI, by more than 20 times.
Changes in physical constants such as the fine structure constant or the gravitational constant would violate Albert Einstein's original theory of general relativity. Such violations are predicted in recent theories aimed at unifying gravitation and quantum mechanics. NIST researchers now plan an all-optical-frequency comparison of the mercury ion clock with an aluminum ion atomic clock, which could increase measurement precision further, offering a more stringent test of the theoretically predicted changes. Conducting such tests with a number of different types of atomic clocks offers the best chance of eliminating extraneous factors to clearly identify which, if any, of the fundamental "constants" are changing over time.
Partial support for staff and equipment was provided by Los Alamos National Laboratory.
* T.M. Fortier, N. Ashby, J.C. Bergquist, M.J. Delaney, S.A. Diddams, T.P. Heavner, L. Hollberg, W.M. Itano, S.R. Jefferts, K. Kim, F. Levi, L. Lorini, W.H. Oskay, T.E. Parker, J. Shirley and J.E. Stalnaker. Precision atomic spectroscopy for improved limits on variation of the fine structure constant and local position invariance. Physical Review Letters. Feb. 16, 2007.
** N. Ashby, T. P. Heavner, S. R. Jefferts, T. E. Parker, A. G. Radnaev and Y. O. Dudin. Testing local position invariance with four cesium-fountain primary frequency standards and four NIST hydrogen masers. Physical Review Letters. Feb. 16, 2007.
Media Contact: Laura Ost, laura.ost@nist.gov, (303) 497-4880.
NIST Report on U.S. Innovation Tags Measurement Challenges.
A new report from the National Institute of Standards and Technology (NIST), An Assessment of the United States Measurement System: Addressing Measurement Barriers to Accelerate Innovation, details results of the agency's first-ever assessment of the capacity of the nation's measurement infrastructure-a large, diverse collection of private and public-sector organizations-to sustain U.S. innovation at a world-leading pace.
Innovation is vital to the long-term health of the U.S. economy: the better we do in conceiving, developing and applying new technology, the brighter our nation's future. Innovation has helped the United States sustain the world's most productive workforce, raise our standard of living and open new avenues of opportunity that inspire a continuing quest to discover, invent and be first to market with new products and services.
In all, more than 1,000 people from industry, academia and government contributed to the wide-ranging NIST assessment of the state of the nation's measurement system and its impact on innovation. The result is a snapshot appraisal that was formed by surveying measurement needs across 11 industrial sectors and technology areas. These ranged from materials to software and from building and construction to nanotechnology. Altogether, more than 700 measurement-related barriers to innovation were identified and reviewed.
Measurement challenges distilled in the report were identified in 15 specially convened workshops, reviews of more than 160 technology "roadmaps" produced by private and public sector organizations, and interviews. Examples include the need for versatile, high-accuracy methods to measure the three-dimensional geometry of manufactured products and the need for tools for measuring the properties of nanodevices and materials.
For its part, the report says, NIST will use this assessment to focus its own work in support of U.S. innovation and competitiveness. The report's results and findings, along with input gathered in follow-up activities, will inform NIST's strategic planning decisions. NIST also plans to work with other organizations in both the private and public sectors to raise awareness of the important role that advances in measurement science and technology play in boosting innovation.
An Assessment of the United States Measurement System: Addressing Measurement Barriers to Accelerate Innovation is available on the NIST Web site at: http://usms.nist.gov/.
Posted by: Beverly Source