Xenon's Weblog

Between the Idea and the Reality between the Motion and the Act falls the Shadow T.S. Eliot

Repulsive quantum effect finally measured

18:00 07 January 2009 by Stephen Battersby
For similar stories, visit the Nanotechnology and Quantum World Topic Guides
A quantum effect that causes objects to repel one another – first predicted almost 50 years ago – has at last been seen in the lab.

According to Harvard physicist Federico Capasso, a member of the group who measured the effect, it could be used to lubricate future nanomachines.

The team detected the weak repulsive force when they brought together a thin sheet of silica and a small gold-plated bead, about half the diameter of a human hair.

The force is an example of the Casimir effect, generated by all-pervasive quantum fluctuations.

Strange attraction

The simplest way to imagine the Casimir force in action is to place two parallel metal plates in a vacuum. Thanks to the odd quantum phenomenon, these become attracted to one another.

It happens because even a vacuum is actually fizzing with a quantum field of particles, constantly popping in and out of existence. They can even fleetingly interact with and push on the plates.

However, the small space between the two plates restricts the kind of particles that can appear, so the pressure from behind the plates overwhelms that from between them. The result is an attractive force that gums up nanoscale machines. (To learn more about the Casimir force see Under pressure from quantum foam.)

Capasso says that the Casimir force needn’t be an enemy. “Micromechanics at some point will have to contend with these forces – or make use of them.”

Reverse buoyancy

In 1961, Russian theorists calculated that in certain circumstances, the Casimir effect could cause objects to repel one another – a scenario Capasso’s team have finally created experimentally. The team achieved this by adding a fluid, bromobenzene, to the setup.

The Casimir attraction between the liquid and the silica plate is stronger than that between the gold bead and the silica, so the fluid forces its way around the bead, pushing it away from the plate.

The effect is akin to the buoyancy we experience in the macro world – where objects less dense than water are held up by the liquid around them. But in this case the bromobenzene is less dense than the solid bead. “You could call it quantum buoyancy,” Capasso told New Scientist.

The force he measured was feeble – amounting to just a few tens of piconewtons – but that is still enough to buoy up nanoscale objects.

Quantum bearings

“The next experiment we want to do is use a TV camera to track the motion of one of these spheres, then we should be able to see easily whether you have levitation.”

Harnessing the repulsive Casimir force could provide a kind of lubrication to solve the problem of nanomachines becoming gummed up by the better-known attractive version, says Capasso.

In theory you could instead use a liquid denser than the components to buoy them up, but that wouldn’t be practical. “These gizmos are usually made of metal, so you would have to use mercury,” he explains.

Quantum buoyancy bearings could be used to build delicate sensors, such as a floating “nanocompass” to detect small-scale magnetic fields.

Journal reference: Nature (DOI: 10.1038/nature07610)

17/01/2009 Posted by | science | | Leave a comment

Zeroing in on Hubble’s Constant

Pasadena, CA In the early part of the 20th Century, Carnegie astronomer Edwin Hubble discovered that the universe is expanding. The rate of expansion is known as the Hubble constant. Its precise value has been hotly debated for all of the 80 intervening years. The value of the Hubble constant is a key ingredient in determining the age and size of the universe. In 2001, as part of the Hubble Space Telescope Key Project, a team of astronomers led by Carnegie’s Wendy Freedman determined precision distances to individual far-off galaxies and used them to determine that the universe is expanding at the rate of 72 kilometers per second per megaparsec. While the debate had previously raged over a factor-of-two uncertainty in the Hubble constant, Freedman and her team cut that uncertainty down to just 10%. And now that number is about to be decreased to 3% with the new Carnegie Hubble Program (CHP) using NASA’s space-based Spitzer telescope. Freedman, who is director of the Observatories of the Carnegie Institution, will lead the effort, which includes Carnegie staff members Barry Madore and Eric Persson, and Carnegie Spitzer Fellow, Jane Rigby.

 The Carnegie Hubble proposal was just selected by the Spitzer Science Center on behalf of NASA as a Cycle-6 Exploration Science Program using Spitzer. This space telescope currently takes images and spectra—chemical fingerprints—of objects by detecting their heat, or infrared (IR) energy, between wavelengths of 3 and 180 microns (a micron equals one-millionth of a meter). Most infrared radiation is blocked by the Earth’s atmosphere and thus it has to be detected from space. The Hubble Key Project observed distant objects primarily at optical wavelengths. In its post-cryogenic phase beginning in April 2009 Spitzer will have exhausted its liquid helium coolant but it will still be able to operate two of its imaging detectors that are sensitive to the near-infrared. This portion of the electromagnetic spectrum has numerous advantages, especially when observing Cepheid variable stars, the so-called “standard candles” that are used to determine distances to distant galaxies.

 “The power of Spitzer,” explained Freedman, “is that it will allow us to virtually eliminate the dimming and obscuring effects of dust. It offers us the ability to make the most precise measurements of Cepheid distances that have ever been made, and to bring the uncertainty in the Hubble constant down to the few percent level.”

 Cepheids are extremely bright, pulsating stars. Their pulsation periods are directly related to their intrinsic luminosities. So, by measuring their periods and apparent brightnesses their individual distances and therefore the distance to their parent galaxies can be determined. By considering the rate at which more distant galaxies are measured to be moving faster away from us in the universe we can calculate the Hubble constant and from that determine the size and the age of the universe.

 One of the largest uncertainties plaguing past measurements of the Hubble constant involved the distance to the Large Magellanic Cloud (LMC), a relatively nearby galaxy, orbiting the Milky Way. Freedman and colleagues will begin their 700 hours of observations refining the distance to the LMC using Cepheids newly calibrated based on new Spitzer observations of similar stars in our own Milky Way. They will then measure Cepheid distances to all of the nearest galaxies previously observed from the ground over the past century and by the Key Project, acquiring distances to galaxies in our Local Group and beyond. The Local Group, our galactic neighborhood, is comprised of some 40 galaxies. The team will be able to correct for lingering uncertainties again by observing in the near-IR. Systematic errors such as whether chemical composition differences among Cepheids might affect the period-luminosity relation, will be examined using the infrared data. Spitzer will begin to execute the Carnegie Hubble Program in June 2009 and continue for at least the next two years.

 “In the age of precision cosmology one of the key factors in securing the fundamental numbers that describe the time evolution and make-up of our universe is the Hubble constant. Ten percent is simply not good enough. Cosmologists need to know the expansion rate of the universe to as high a precision and as great an accuracy as we can deliver,” remarked Carnegie co-investigator, Barry Madore.


The Spitzer Space Telescope was launched in August 2003. It detects energy from celestial objects in the infrared part of the spectrum, which is able to penetrate areas in space not visible in the optical spectrum such as dense clouds of gas and dust where stars form, new extrasolar planetary systems, and galactic centers. NASA’s Jet Propulsion Laboratory, Pasadena, CA, manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the SpitzerScience Center at the California Institute of Technology. Caltech manages JPL for NASA. Seehttp://www.spitzer.caltech.edu

Source: http://www.ciw.edu/news/zeroing_hubble_s_constant

10/01/2009 Posted by | science | Leave a comment

Lorentz Invariance Abnormality Could Overturn Building Block Of Einstein’s Relativity, Say Physicists

Physicists at Indiana University have developed a promising new way to identify a possible abnormality in a fundamental building block of Einstein’s theory of relativity known as “Lorentz invariance.” If confirmed, the abnormality would disprove the basic tenet that the laws of physics remain the same for any two objects traveling at a constant speed or rotated relative to one another.

IU distinguished physics professor Alan Kostelecky and graduate student Jay Tasson take on the long-held notion of the exact symmetry promulgated in Einstein’s 1905 theory and show in a paper to be published in the Jan. 9 issue of Physical Review Letters that there may be unexpected violations of Lorentz invariance that can be detected in specialized experiments.

“It is surprising and delightful that comparatively large relativity violations could still be awaiting discovery despite a century of precision testing,” said Kostelecky. “Discovering them would be like finding a camel in a haystack instead of a needle.”

If the findings help reveal the first evidence of Lorentz violations, it would prove relativity is not exact. Space-time would not look the same in all directions and there would be measurable relativity violations, however minuscule.

The violations can be understood as preferred directions in empty space-time caused by a mesh-like vacuum of background fields. These would be separate from the entirety of known particles and forces, which are explained by a theory called the Standard Model that includes Einstein’s theory of relativity.

The background fields are predicted by a generalization of this theory called the Standard Model Extension, developed by Kostelecky to describe all hypothetical relativity violations.

Hard to detect, each background field offers its own universal standard for determining whether or not an object is moving, or in which direction it is going. If a field interacts with certain particles, then the behavior of those particles changes and can reveal the relativity violations caused by the field. Gravity distorts the fields, and this produces particle behaviors that can reveal otherwise hidden violations.

The new violations change the gravitational properties of objects depending on their motion and composition. Objects on the Earth are always moving differently in different seasons because the Earth revolves around the Sun, so apples could fall faster in some seasons than others. Also, different objects like apples and oranges may fall differently.

“No dedicated experiment has yet sought a seasonal variation of the rate of an object’s fall in the Earth’s gravity,” said Kostelecky. “Since Newton’s time over 300 years ago, apples have been assumed to fall at the same rate in the summer and the winter.”

Spotting these minute variances is another matter as the differences in rate of fall would be tiny because gravity is a weak force. The new paper catalogues possible experiments that could detect the effects. Among them are ones studying gravitational properties of matter on the Earth and in space.

The Standard Model Extension predicts that a particle and an antiparticle would interact differently with the background fields, which means matter and antimatter would feel gravity differently. So, an apple and an anti-apple could fall at different rates, too.

“The gravitational properties of antimatter remain largely unexplored,” said Kostelecky. “If an apple and an anti-apple were dropped simultaneously from the leaning Tower of Pisa, nobody knows whether they would hit the ground at the same or different times.”



10/01/2009 Posted by | science | 2 Comments