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(Phys) Fundamental Constants

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PostPosted: Sat Apr 29, 2006 9:13 am    Post subject: (Phys) Fundamental Constants Reply with quote

National Institute of Standards and Technology (NIST)

Measurements may help show if constants are changing

Physicists at JILA have performed the first-ever precision measurements using ultracold molecules, in work that may help solve a long-standing scientific mystery--whether so-called constants of nature have changed since the dawn of the universe.
The research, reported in the April 14 issue of Physical Review Letters,* involved measuring two phenomena simultaneously--electron motion, and rotating and vibrating nuclei--in highly reactive molecules containing one oxygen atom and one hydrogen atom. The researchers greatly improved the precision of these microwave frequency measurements by using electric fields to slow down the molecules, providing more time for interaction and analysis. JILA is a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.

Compared to the previous record, set more than 30 years ago, the JILA team improved the precision of one frequency measurement 25-fold and another 10-fold. This was achieved by producing pulses of cold molecules at various speeds, hitting each group with a microwave pulse of a selected frequency, and then measuring how many molecules were in particular energy states. The apparatus and approach were similar to those used in the NIST-F1 cesium atomic fountain clock, the nation's primary time standard, raising the possibility of designing a clock that keeps time with molecules, instead of atoms.

The JILA team's ability to make two molecular measurements at once enables scientists to apply mathematical calculations to probe the evolution over time of fundamental natural properties such as the fine structure constant, which is widely used in research to represent the strength of electromagnetic interactions. Another research group at the National Radio Astronomy Observatory plans to make similar frequency measurements soon of the same molecules produced in distant galaxies, which are so far from Earth that they represent a window into ancient history. By comparing precision values for the fine structure constant on Earth and in distant parts of the universe, scientists hope to determine whether this constant has changed over 10 billion years. Because the fine structure constant is used in so many fields of physics, these measurements are a way to test the consistency of existing theories. The JILA measurements could enable any change in the fine structure constant over time to be determined with a precision of one part per million.

The work at JILA is supported by the National Science Foundation, NIST, the Department of Energy, and the Keck Foundation.

*E.R. Hudson, H.J. Lewandowski, B.C. Sawyer, and J.Ye. 2006. Cold molecule spectroscopy for constraining the evolution of the fine structure constant. Physical Review Letters. April 14 (Vol. 96, 143004).


Questions to explore further this topic:

What are fundamental constants?

How many fundamental constants are there?

A Booklet on Fundamental Constants

Fundamental Constants (2002 Values)

Are Fundamental Constants constant?

Are the laws of nature changing with time?






What is SI (International System of Units)?

The Units

The Prefixes

Non-SI units



What is the metric system?

A Metric Pyramid

A Metric Activity for Children

Household Weights and Measures

The Metric Kitchen

The Olympics and the Metric System (Sports)

How is uncertainty expressed in measurements?

The Nobel Prize in Physics 2005

Universal Constants

Characteristic impedance of vacuum

Permittivity of vaccum

Permeability of vacuum

Gravitational constant

Planck's constant

The Speed of Light in vacuum


Bohr magneton

Elementary charge

Nuclear magneton

Atomic and Nuclear

Bohr radius

The Electron

The Proton

The Neutron

Fine-structure constant

Rydberg constant


Avogadro's number

Boltzmann's constant

Faraday constant

The Ideal Gas constant

Adopted Values

Non-SI Units

Frequently Used Constants


Last edited by adedios on Sat Jan 27, 2007 4:18 pm; edited 2 times in total
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PostPosted: Wed Aug 16, 2006 7:38 am    Post subject: Atoms looser than expected Reply with quote

American Physical Society
15 August 2006

Atoms looser than expected

Single-electron merry-go-round measures universal atomic force
All the atoms in the universe just got looser, at least in the eyes of humans. No, the laws of physics didn't change overnight, but our knowledge of how strong atoms are held together did have to be readjusted a bit in light of a new experiment conducted at Harvard University.

By studying how a single electron behaves inside an electronic bottle, Gerald Gabrielse and his colleagues at Harvard were able to calculate a new value for a number six times more precise than the previous measurements called the fine structure constant, which specifies the strength of the electromagnetic force, which holds electrons inside atoms, governs the nature of light and provides all electric and magnetic effects we know, from a flash of lightning to a magnet on a refrigerator. Knowledge of these fundamentals helps scientists and engineers design new kinds of electronic devices–and obtain more profound details on the workings of the universe.

Gabrielse sums up the experiment this way: "Little did we know that the binding energies of all the atoms in the universe were smaller by a millionth of a percent--a lot of energy given the huge number of atoms in the universe."

Electrons are the outermost part of every atom. When detached from their home atoms, electrons constitute the electricity that flows through all powered machines.

By studying an individual electron in isolation from any other particle, scientists can eliminate complications of measuring a single object too small to see with even the most powerful microscopes. The Harvard scientists achieved extraordinary conditions of isolation for their individual electron. First of all, the inside of their trap apparatus is pumped free of almost all other particles, establishing a vacuum comparable to that in interplanetary space. And it's ultra-frigid inside: the apparatus is chilled to millionths of a degree above absolute zero, a temperature far colder than the surface of Pluto.

The lone electron and its surrounding cage constitute a sort of gigantic atom. Combined electric and magnetic forces in the trap keep the electron in its circular orbit. In addition to this circular motion, the electron wobbles up and down in the vertical direction, the direction of the magnetic field. It's like a giant merry-go-round, with an electromagnetic trap as the carousel and the electron as the lone horse.

The circuitry used to activate the electrodes keeping the electron pretty much centered in the trap is so sensitive that the system knows when the electron is bobbing upwards and approaching one of the electrodes. A feedback effect using the combined electric and magnetic forces, supplied by electrodes and coils, restricts the motion of the electron. This allows the electron's energy to be measured with great precision.

By measuring the electron's properties so meticulously, physicists could improve their calculation of the fine structure constant, the number that determines the strength of the electromagnetic forces that hold all atoms together. The new value for the constant is slightly smaller than the best previous value (revealing atoms to be just a tiny bit looser) and six times more accurate.

The Harvard work with the special electron trap has taken more than twenty years and has produced more than a half dozen PhD theses, all centering on a single electron.

These results appear in two papers in the July 21, 2006 issue of the journal Physical Review Letters (
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PostPosted: Fri Feb 16, 2007 7:53 am    Post subject: Clock comparison yields clues to 'constant' change Reply with quote

National Institute of Standards and Technology (NIST)
15 February 2007

Clock comparison yields clues to 'constant' change

Years of comparisons among the world's best atomic clocks—based on different atoms—have established the most precise limits ever achieved in the laboratory for detecting possible changes in so-called "constants" of nature. The comparisons at the National Institute of Standards and Technology (NIST) may help scientists test the latest theories in physics and develop a more complete understanding of the history of the universe.

Some astronomical and geological studies suggest there might have been very small changes in the values of fundamental constants over billions of years, although 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 scientists 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 many 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.
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PostPosted: Thu Jun 14, 2007 9:29 am    Post subject: Australia weighs in to make the perfect kilogram Reply with quote

Australia weighs in to make the perfect kilogram
Reference: 07/108

Australian scientists and optical engineers will be making a perfect sphere that may one day re-define the kilogram – and tomorrow they’re taking delivery of the cylinder of silicon from which it will be made.

14 June 2007

The kilogram is one of seven base units in the International System (SI) used in science, commerce and everyday life.

However, it is the only one still defined by a physical object – a lump of metal, known as the International Prototype, sitting in a vault in France. All the others have moved with the scientific times and are defined in terms of a fundamental constant of nature so anyone anywhere can reproduce them and they do not change over time.

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PostPosted: Sat Sep 22, 2007 9:58 am    Post subject: A better definition for the kilogram? Scientists propose a p Reply with quote

Georgia Institute of Technology Research News
21 September 2007

A better definition for the kilogram? Scientists propose a precise number of carbon atoms
How much is a kilogram?

It turns out that nobody can say for sure, at least not in a way that won’t change ever so slightly over time. The official kilogram – a cylinder cast 118 years ago from platinum and iridium and known as the International Prototype Kilogram or “Le Gran K” – has been losing mass, about 50 micrograms at last check. The change is occurring despite careful storage at a facility near Paris.

That’s not so good for a standard the world depends on to define mass.

Now, two U.S. professors – a physicist and mathematician – say it’s time to define the kilogram in a new and more elegant way that will be the same today, tomorrow and 118 years from now. They’ve launched a campaign aimed at redefining the kilogram as the mass of a very large – but precisely-specified – number of carbon-12 atoms.

“Our standard would eliminate the need for a physical artifact to define what a kilogram is,” said Ronald F. Fox, a Regents’ Professor Emeritus in the School of Physics at the Georgia Institute of Technology. “We want something that is logically very simple to understand.”

Their proposal is that the gram – 1/1000th of a kilogram – would henceforth be defined as the mass of exactly 18 x 14074481 (cubed) carbon-12 atoms.

The proposal, made by Fox and Theodore P. Hill – a Professor Emeritus in the Georgia Tech School of Mathematics – first assigns a specific value to Avogadro’s constant. Proposed in the 1800s by Italian scientist Amedeo Avogadro, the constant represents the number of atoms or molecules in one mole of a pure material – for instance, the number of carbon-12 atoms in 12 grams of the element. However, Avogadro’s constant isn’t a specific number; it’s a range of values that can be determined experimentally, but not with enough precision to be a single number.

Spurred by Hill’s half-serious question about whether Avogadro’s constant was an even or odd number, in the fall of 2006 Fox and Hill submitted a paper to Physics Archives in which they proposed assigning a specific number to the constant – one of about 10 possible values within the experimental range. The authors pointed out that a precise Avogadro’s constant could also precisely redefine the measure of mass, the kilogram.

Their proposal drew attention from the editors of American Scientist, who asked for a longer article published in March 2007. The proposal has so far drawn five letters, including one from Paul J. Karol, chair of the Committee on Nomenclature, Terminology and Symbols of the American Chemical Society. Karol added his endorsement to the proposal and suggested making the number divisible by 12 – which Fox and Hill did in an addendum by changing their number’s final digit from 8 to 6. So the new proposal for Avogadro’s constant became 84446886 (cubed), still within the range of accepted values.

Fast-forward to September 2007, when Fox read an Associated Press article on the Web site about the mass disappearing from the International Prototype Kilogram. While the AP said the missing mass amounted to no more than “the weight of a fingerprint,” Fox argues that the amount could be significant in a world that is measuring time in ultra-sub-nanoseconds and length in ultra-sub-nanometers.

So Fox and Hill fired off another article to Physics Archive, this one proposing to redefine the gram as 1/12th the mass of a mole of carbon 12 – a mole long being defined as Avogrado’s number of atoms. They now hope to generate more interest in their idea for what may turn out to be a competition of standards proposals leading up to a 2011 meeting of the International Committee for Weights and Measures.

At least two other proposals for redefining the kilogram are under discussion. They include replacing the platinum-iridium cylinder with a sphere of pure silicon atoms, and using a device known as the “watt balance” to define the kilogram using electromagnetic energy. Both would offer an improvement over the existing standard – but not be as simple as what Fox and Hill have proposed, nor be exact, they say.

“Using a perfect numerical cube to define these constants yields the same level of significance – eight or nine digits – as in those integers that define the second and the speed of light,” Hill said. “A purely mathematical definition of the kilogram is experimentally neutral – researchers may then use any laboratory method they want to approximate exact masses.”

The kilogram is the last major standard defined by a physical artifact rather than a fundamental physical property. In 1983, for instance, the distance represented by a meter was redefined by how far light travels in 1/299,792,458 seconds – replacing a metal stick with two marks on it.

“We suspect that there will be some public debate about this issue,” Fox said. “We want scientists and science teachers and others to think about this problem because we think they can have an impact. Public discussion may play an important role in determining how one of the world’s basic physical constants is defined.”

How important is this issue to the world’s future technological development"

“When you make physical and chemical measurements, it’s important to have as high a precision as possible, and these standards really define the limits of precision,” Fox said. “The lack of an accurate standard leaves some inconsistency in how you state results. Having a unique standard could eliminate that.”

While the new definition would do away with the need for a physical representation of mass, Fox says people who want a physical artifact could still have one – though carbon can’t actually form a perfect cube with the right number of atoms. And building one might take some time.

“You could imagine having a lump of matter that actually had exactly the right number of atoms in it,” Fox noted. “If you could build it by some kind of self-assembly process – as opposed to building it atom-by-atom, which would take a few billion years – you could have new kilogram artifact made of carbon. But there’s really no need for that. Even if you built a perfect kilogram, it would immediately be inaccurate as soon as a single atom was sloughed off or absorbed.”
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