Quantum Optics Could Help Detect Gravitational Waves

21. December 2010

By Patricia Daukantas

Can you do quantum mechanics with everyday-sized objects? And could such macroscopic quantum objects help detect the most elusive predictions of Einstein’s theory of general relativity?

Yes and yes, according to Nergis Mavalvala, a physics professor at the Massachusetts Institute of Technology (U.S.A.). She and her colleagues are using lasers for cooling and trapping gram-sized and even kilogram-sized interferometer mirrors--just like optical traps for cooling atoms. In a few years, instruments operating at the standard quantum limit will start hunting for gravitational waves.

Mavalvala, one of two OSA members who won a John D. and Catherine T. MacArthur Foundation “genius grant” in 2010, recently gave a public lecture about her work on the Laser Interferometer Gravitational-Wave Observatory (LIGO) project at the Arlington, Va., headquarters of the U.S. National Science Foundation.

“If you could do astrophysics with a gravitational wave, it’s like turning on a new sense,” Mavalvala said. “You’ve had eyes all along, and suddenly you have ears and you turn on hearing. It’s bound to provide some very different information.”

Albert Einstein’s theory of general relativity predicted the existence of gravity waves traveling at the speed of light. Such waves, if they exist, would stretch and squeeze spacetime transverse to the direction of propagation. The amplitude of a gravitational wave, also known as strain or h, is a dimensionless quantity defined as the change in length per length, similar to tidal forces. In other words, gravitational stretch and squeeze spacetime by fractional amounts proportional to the distance between two objects.

Gravitational waves, if they exist, would have frequencies of 10 kHz or less and would interact only weakly with matter. Einstein was morose over his own calculations when he realized the difficulty of detecting such waves. Two decades after his death, however, radio astronomers provided the first clue to their existence.

In 1974, University of Massachusetts (U.S.A.) researchers Russell Hulse and Joseph Taylor Jr. found a binary pulsar consisting of two neutron stars orbiting each other at 0.15 percent of light speed. Since the pulsars contain more than the mass of the sun packed within a 10-km radius, they have extremely strong gravitational fields.

Taylor and Hulse showed that their orbits are shrinking at exactly the rate that Einstein’s theory would predict for the emission of gravitational waves from the system. Further observations up to the present day continue to confirm Einstein’s theory. The binary pulsar’s energy loss is widely accepted as evidence for gravitational waves, and led to the 1993 Nobel Prize in Physics for Hulse and Taylor, Mavalvala said.

Direct detection of gravitational waves on Earth, however, is incredibly difficult because the strain on an interferometer would be on the order of 10–21. So the two interferometers of the LIGO project, located in Hanford, Wash., and Livingston, La. (U.S.A.), have 4-km-long arms.

Scientists could measure gravitational waves by measuring the phase shifts of light in an interferometer. However, external forces also push the mirrors around much more than a gravitational wave can push them around, and laser light has fluctuations in phase and amplitude, so both sources of noise must be reduced. The LIGO team designed their interferometers with optical cavities in each arm to increase the instruments’ sensitivity to mirror displacement and thus to gravitational waves.

Advanced LIGO: More sensitivity, more issues

The first-generation detectors known as Initial LIGO, which are now being removed, led to much interesting astrophysical research, but no positive detections of gravitational waves (yet). Construction of the next-generation Advanced LIGO detectors, designed to be 10 times more sensitive than their predecessors, began in October.

But with Advanced LIGO, there’s a catch: The detectors will bump up against the standard quantum limit. According to Mavalvala, the team has to get around the quantum limitations by injecting squeezed states of light, with more precise measurements of phase at the expense of knowledge about the amplitude of the light, in order to reduce the quantum shot noise limit.

To deal with the other type of quantum noise--radiation pressure--the Advanced LIGO scientists are using optomechanical coupling to trap and cool macroscopic mirrors down to very low quantum states--the way lasers trap and cool atoms.

Initially Mavalvala and colleagues tested a 1-gram mirror suspended from a pair of specially made glass fibers (designed with fewer impurities and flaws than commercial optical fibers). Starting at room temperature, they shifted the mirror’s oscillator frequency from 10 Hz in the mechanical regime to 500 Hz, and cooled it to just under 1 mK. In other words, the cooled mirror has 35,000 quanta, instead of 109 quanta in its normal state.

“We are not yet in the quantum regime, a factor of 5 or 35,000 quanta is not yet quantum, but it’s getting close,” Mavalvala said.

In the most recent experiment, the team took one of the 2.7-kg mirrors from the Initial LIGO experiment, with a resonant frequency of just under 1 Hz at room temperature (and containing about 1,026 atoms), and shifted its resonance out to about 150 Hz and cooled it town to 1.4 μK--corresponding to only 200 quanta. (When that number equals 1, the kg-scale object will have reached its quantum ground state.)

Advanced LIGO, slated to begin around 2014, will operate at the standard quantum limit. Scientists expect that the project will detect signals that could be gravitational waves several times per year, Mavalvala said.

Astronomy, Astrophysics , , , , , , , , , , , , ,

Lasers Widen Telescopes’ Clear Field of View

6. August 2010

By Patricia Daukantas

 

For the past decade, astronomers have used laser guide star (LGS) adaptive-optics systems to remove the blurring caused by atmospheric turbulence above ground-based telescopes. Such systems, however, have always faced one restriction: an extremely narrow field of sharp viewing.

                          

A team from the University of Arizona (Tucson, U.S.A.) has managed to widen that sharp field by developing a five-laser guiding system for the MMT telescope on Mount Hopkins in southern Arizona. The astronomers report on their system in the August 5 issue of Nature.

 

Michael Hart, of the university’s Steward Observatory, and colleagues wanted to study aging star clusters at near-infrared wavelengths (1.25 to 2.2 μm). One such cluster, dubbed M3, nearly fills the 110-arcsecond-wide field of the MMT’s infrared camera.

 

The researchers arranged five 4-W, 532-nm pulsed lasers in a pentagon and projected them from a small telescope behind the MMT’s adaptive secondary mirror. A combination of three sensors detects the aberrations in the Rayleigh-backscattered light coming back to the telescope, estimates the aberration from ground-level turbulence and directs the secondary mirror to correct the aberrations.

 

On a night when the native “seeing,” or point-spread-function diameter of stellar images, at the MMT was only 0.7 arcseconds, the astronomers improved it to 0.3 arcseconds over a 2-arcminute-wide field of view – roughly the same as the Hubble Space Telescope gets with its most recent upgrades, but with a bigger aperture to gather more light.

 

Astronomers are now developing a similar system for the Large Binocular Telescope on Arizona’s Mount Graham.

2010-08 August, Astronomy, Lasers , , ,

Happy (Belated) 95th Birthday, Charles Townes!

5. August 2010

By Patricia Daukantas

 

Last week, Charles H. Townes passed yet another milestone: he turned 95 years old.

 

Townes, now of the University of California at Berkeley (U.S.A.), is of course most famous in the optics community for his fundamental contributions to laser theory:

 

  • The development of the first maser with James Gordon and Herbert Zeiger in 1953, as Gordon recounted in a recent OPN feature article; and
  • The principles behind the optical maser, or laser, published by Townes and his brother-in-law, Arthur Schawlow, in December 1958.

Along the way, he’s worked at Bell Labs and three prominent universities, served on various U.S. government committees and think tanks, held a Guggenheim Fellowship and a Fulbright Scholarship, and won the Templeton Prize for contributing to the understanding of religion.

 

As we’ve noted in past OPN blog posts, Townes is still active in astrophysical research. So far in 2010, he and his colleagues have published two articles relating to Berkeley’s Infrared Spatial Interferometer, a three-telescope system with high spectral resolution. This year, which is the 50th anniversary of the first working laser, he’s been invited to speak at many scientific conferences, including a special historical symposium at CLEO/QELS 2010.

 

We should also note that 2010 marks two other milestones for Townes. It was 40 years ago, in 1970, that Townes was named an OSA Honorary Member. And it was 50 years ago that Townes, along with 14 other physicists, chemists, engineers and physicians, was named a representative of “U.S. Scientists” for Time magazine’s 1960 “Men of the Year” (now "Person of the Year") issue. Townes and bubble-chamber inventor Donald A. Glaser are the two surviving members of that august ensemble.

 

Townes and his wife of 69 years, Frances, have four daughters. We wish him a Happy Belated Birthday and much joy with his family.

2010-08 August, Astronomy, Astrophysics, Lasers, Optics history , , , ,

Two OSA Award Winners Among Eight Kavli Prize Honorees

3. June 2010

By Patricia Daukantas

 

Two telescope-building astronomers who have won OSA awards for optical engineering are among this year’s winners of the Kavli Prize for Astrophysics.

 

The Kavli Prizes, worth $1 million each, are bestowed every two years in the fields of astrophysics, neuroscience and nanoscience – areas that didn’t really exist when the Nobel Prizes were founded.

 

Jerry Nelson, J. Roger P. Angel and Raymond N. Wilson shared the astrophysics prize for their contributions to the technology behind some of the world’s largest telescopes.

 

Nelson, of the Center for Adaptive Optics at the University of California at Santa Cruz (U.S.A.), served as project scientist for the twin 10-m-aperture Keck Telescopes on Hawaii’s Mauna Kea. These telescopes use his design of lightweight hexagonal mirror segments with active controls to keep the optics perfectly aligned. His pioneering design is being used in other large telescopes now under construction. An OSA member, Nelson received the 1996 Joseph Fraunhofer Award/Robert M. Burley Prize from OSA for his contributions to optical engineering.

 

Angel, director of the Steward Observatory Mirror Lab at the University of Arizona, took a different approach to the design of large, lightweight telescope mirrors: casting them as a single unit in a giant spinning furnace that cools slowly. The resulting mirrors have a near-parabolic top surface and a honeycomb structure underneath. He received OSA’s Fraunhofer Award/Burley Prize in 2007 for his body of work, which includes fiber-fed spectroscopy and solar photovoltaic technology.

 

The third winner of the astrophysics Kavli Prize, Raymond N. Wilson, formerly of the European Southern Observatory in Germany and Imperial College London in England, developed the computer-controlled actuation system for active optics, which is used in many of the world’s largest observatories.

 

Five scientists from U.S. universities and industrial research centers shared the Kavli Prizes in nanoscience and neuroscience. The Norwegian Academy of Science and Letters made the prize announcement this morning. Funding for the prizes comes from comes from the Kavli Foundation.

2010-06 June, Astronomy, Astrophysics , , , , , , ,

Hubble Enters Third Decade of Service to Astronomy

23. April 2010

By Patricia Daukantas

 

First, it was an expensive dream. Next, it was the butt of worldwide jokes – nearsighted and rhyming with “trouble.” Today, though, the Hubble Space Telescope (HST) is an indispensable instrument in the toolboxes of astronomers, and they are celebrating the 20th anniversary of its ride into Earth orbit.

 

NASA’s space shuttle Discovery carried Hubble into orbit on April 24, 1990. Already by then, the telescope idea had been on a long and complex path to realization.

 

Princeton University astrophysicist Lyman Spitzer Jr. (1914-1997) had been pointing out the advantages of an extraterrestrial observatory as far back as 1946 – more than a decade before the launch of the first space satellite. The astronomical community fought back against budget cuts for years until construction funding was approved in the late 1970s. After cost overruns, Hubble was nearly ready to go by January 1986, when the shuttle Challenger‘s accident grounded the remaining shuttles and forced a long postponement of Hubble’s launch.

 

Once Hubble got off the ground, of course, reports of the primary mirror’s spherical aberration made headlines all over the planet. (These reports surfaced in late May and early June – I’ll bet that nobody at NASA will be commemorating that 20th anniversary.) Fortunately, some of the brightest minds in optical science – including more than a few OSA members – got to work on the Independent Optics Review Panel and set to work on designing a package of corrective optics for the orbiting telescope. Installing the corrective instrument, dubbed COSTAR, was the top priority of the first Hubble servicing mission in the fall of 1993.

 

OPN covered this story with several articles. We devoted much of the November 1993 issue to the Hubble rescue effort, with such articles as “Engineering the COSTAR” and “Optical Testing and Verification on HST.” In the August 1994 issue, representatives of a NASA subcontractor, Tinsley Laboratories, wrote about their production of the corrective optics that were then integrated into the instrument package built by Ball Aerospace.

 

Astronomers cheered when they saw the first crisp, clear Hubble images of a nearby galaxy following that first servicing mission. The Hubble discoveries made in the last 16 years are way too numerous to summarize in any blog post.

 

This weekend, you can celebrate Hubble’s 20th anniversary by visiting NASA’s commemorative page or the Hubble public information site. You can join in the Hubble pop culture contest, “friend” Hubble on Facebook, send a congratulatory message on Twitter.com with the hashtag #hst20 or even help astronomers classify galaxies found on real Hubble images with the Galaxy Zoo: Hubble project.

 

Happy 20th Birthday, Hubble, and we hope you’re still active to celebrate your 25th!

2010-04 April, Astronomy, Astrophysics , , , ,

Is the “Citizen Sky” Mystery Solved?

14. April 2010

By Patricia Daukantas

 

For the past year, “citizen scientists” have been observing the behavior of an unusual double star to gain insight into its mysterious eclipses. Recently, a team of professional astronomers used optical interferometry to catch the first glimpse of the dimmer star that eclipses its brighter companion every 27.1 years.

 

As we’ve blogged in the past, the star system Epsilon Aurigae has been the subject of an intensive observing campaign this year. Several of the professional astronomers behind the “citizen sky” project have now seen that the eclipsing object is a large, slightly tilted disk surrounding a single, hot star. The team reported its results in the April 8 issue of Nature.

 

It turns out that the central star of the pair is a F-type star, slightly hotter than our Sun and with up to three times its mass, and its eclipsing companion is an extremely hot B-type star, shrouded in dust warmed to about 550 K. But how do Robert Stencel (University of Denver, Colo., U.S.A.) and his colleagues know that?

 

As it turns out, they imaged the Epsilon Aurigae system with the CHARA array of six telescopes on Mount Wilson in California. (CHARA stands for the Center for High Angular Resolution Astronomy, located at Georgia State University in Atlanta.)

 

With apertures of only 1 m, the six CHARA scopes are tiny by professional standards. However, they are positioned over the mountainside to provide the resolving capability of a much larger single telescope – up to 331 m wide (that’s the array’s longest baseline). Radio astronomers may have been the first to use interferometry to boost the amount of detail we can see in the heavens, but optical (visible and infrared) astronomical interferometry has come of age in recent years – see this post about another interferometer on Mount Wilson, used by OSA Honorary Member Charles Townes and colleagues to study the size of the red giant star Betelgeuse.

 

For more information on CHARA and optical interferometry, see this PDF presentation or the CHARA website. For additional coverage of Epsilon Aurigae, see the U.S. National Science Foundation announcement or this article from Sky & Telescope magazine.

 

And don’t forget … the Citizen Sky website is still collecting observations of Epsilon Aurigae, and anyone can make them regardless of equipment (or the lack thereof). This makes a great class project.

2010-04 April, Astronomy, Astrophysics , , , , ,

Can You See the Stars?

5. March 2010

By Patricia Daukantas

 

Once again, the folks who run the GLOBE at Night project are inviting people from all over the world to measure the brightness of the night sky – no special equipment required.

 

As we described last year, this project takes place during March because that’s when the prominent constellation Orion is high in the sky fairly early in the evening. (In the Northern Hemisphere autumn, one must be a night owl or a before-dawn riser to catch a glimpse of “the Hunter.”)

 

You don’t even have to be savvy about the apparent-magnitude system that professionals use. You don’t even need a telescope. Just carry out these five steps on a clear night between now and Tuesday, March 16:

 

  • Find your latitude and longitude with a Global Positioning System device or online tools.
  • Find Orion in the clear evening sky (simple pattern recognition).
  • Match your view of the constellation to one of the magnitude charts developed by GLOBE at Night – how many stars do you see?
  • Record your observation on the Web site.
  • Compare what you saw to others’ views.

 

Your effort will help GLOBE at Night track the pervasiveness and spread of light pollution. As the organizers of this project said in a press release, “With half of the world’s population now living in cities, many urban dwellers have never experienced -- and maybe never will -- the wonderment of pristinely dark skies.” The more awareness of the problem of light pollution, the greater the chance to stop its spread.

2010-03 March, Astronomy , , , , , ,

A Peek at Past and Future Space Missions

30. October 2008

By Patricia Daukantas

During his plenary speech at last week’s Frontiers in Optics (FiO) meeting, John C. Mather, one of the 2006 Nobel Prize winners in physics, described the optical systems that will go into the James Webb Space Telescope (JWST), which is scheduled for launch in 2013. Mather, a senior astrophysicist at NASA’s Goddard Space Flight Center (USA), spent 15 years taking the Cosmic Background Explorer (COBE) satellite, which measured the faint afterglow from the Big Bang that created the universe.

Mather spent 15 years taking COBE from the proposal stage to its 1989 launch. He was the principal investigator for the COBE instrument called the Far Infrared Absolute Spectrophotometer (FIRAS), which measured the black-body spectrum of the cosmic background radiation from the Big Bang to unprecedented accuracy. (When he first showed his experiment-matches-theory result to an astronomers’ meeting in January 1990, his colleagues gave him a standing ovation.)

Mather’s current project, JWST, involves almost every section of NASA. The planned 6.5-m space telescope looks a little like a solar energy concentrator in the desert, he said, because it needs to have a multilayered sunshade in order to operate at a temperature of 40 K. JWST will be launched on an unmanned Ariane booster rocket and will fly to the L2 Lagrangian point, a stable position about 1 million miles on the opposite side of Earth from the Sun. After showing an animation of the way the telescope is supposed to unfold by itself once it reaches L2, Mather quipped, “If you’re a mechanical engineer, this is either terrifying or thrilling.”

JWST will use a three-mirror anastigmat optical design for a wide field of view. (In addition to the primary and secondary mirrors, it will have a “fine steering mirror” at the Cassegrain focus.) The scientists selected a beryllium mirror because of its superior cryogenic properties—it undergoes much less thermal distortion than the ultra-low-expansion glass used in many ground-based telescopes.

Among JWST’s many cameras will be NIRSpec, a near-infrared imaging spectrograph that can photograph 100 galaxies at once. “If it takes two weeks to get an exposure, we don’t want to be limited to just one galaxy,” Mather said.

Since data from the Hubble Space Telescope showed that galaxies were formed longer ago than scientists had thought, JWST is likely to study the origins of galaxies and the earliest populations of stars. JWST could also study Earth-like transiting planets, or planets that pass between their parent star and the observer. Astrophysicists over the years have proposed a number of schemes for orbiting planet-hunting interferometers, with instruments mounted on widely spaced satellites, but tight budgets at the space agency may mean that not all of those projects will fly.

2008-10 October, Astronomy , , , ,

Space and Other Topics at FiO 2008

22. October 2008

By Patricia Daukantas

Days 3 and 4 (Tuesday and Wednesday) of Frontiers in Optics, OSA’s annual meeting, have been the chilliest and rainiest so far. Inside the Rochester Convention Center, however, the atmosphere has been sunny, as OSA members have been making and renewing friendships and learning from each other in a spirit of collegiality.

Tuesday’s highlight was a day-long symposium honoring the founding of NASA 50 years ago this month. A couple of the invited speakers were unable to attend because they’re working hard on the latest, non-optical troubles facing the Hubble Space Telescope (see my previous blog post here). The other speakers described the original 1993 remedy for Hubble’s defective primary mirror – a masterpiece of technological detective work if there every was one – as well as the upcoming James Webb Space Telescope (JWST) and planet-seeking coronagraphs that may or may not ever leave the NASA drawing boards. Several folks from the JWST team had to leave after the symposium for a telescope team meeting in San Francisco; it was awesome that they were able to give us their time on the way there.

At Tuesday’s OSA Member Reception, President Rod Alferness reminded the gathering that this is the 50th consecutive Annual Meeting at which the University of Rochester’s own Emil Wolf is presenting a paper. Besides his invited talk on Wednesday morning, Wolf is a co-author on six other papers. Truly indefatigable. (Incidentally, in future years the OSA Foundation will hold a Student Paper Competition dedicated to Emil Wolf.)

The reception ended with dozens of young members piling on the dais and posing for group photos in front of the “Welcome to OSA Student Chapters” banner. Undoubtedly, these smiling grad students will in 20 years become the entrepreneurs and professors who will mentor a whole other generation of optical scientists.

Wednesday’s highlights include a symposium on polarized light, the Minorities and Women in OSA luncheon, and postdeadline papers in the evening.

2008-10 October, Astronomy , , , , ,

Hubble Repair Mission Postponed Until 2009

30. September 2008

By Patricia Daukantas

No sooner did we report on the progress toward the next Hubble Space Telescope repair mission than we have to note that the repair mission has been postponed for several months.

NASA officials, including Preston Burch, the Hubble program manager at NASA’s Goddard Space Flight Center, today announced the delay after a serious communications glitch developed within the orbiting telescope. The shuttle mission, known as STS-125, may launch next February.

The glitch – which has nothing to do with Hubble’s optical systems – developed over the past weekend in the orbiting observatory’s science instrument command and data handling unit. An undetermined error caused “side A” of the unit to put itself into safe mode, thus interrupting the flow of science data from the optical instruments to researchers on Earth.

The Hubble team will try to activate “side B” of the data handling unit, which, like “side A,” has been in orbit since the telescope’s launch in 1990. Over the next few days, the team will assess whether that activation will pose any large risk to the telescope, and test out their theories using the Vehicle Electrical System Test (VEST), a copy of the Hubble support system bay that resides in the huge NASA Goddard clean room in Greenbelt, Maryland, USA. If “side B” can turn itself on, astronomers can resume observations with the existing Hubble instruments.

According to Burch, NASA has a backup copy of the dual-sided data handling unit here on the ground, and Goddard researchers will start testing it for flight qualification. If the unit passes the severe vibration, thermal vacuum and acoustical tests, the fresh unit will be added to the STS-125 payload and the spacewalking astronauts will have to make time (up to two hours) to install it on the telescope. Of course, the backup unit is as old as the one in Earth orbit, but the space agency expects that tests will show that it still works.

Leaving only “side B” on the telescope would leave Hubble with several potential single points of failure, while the complete replacement of the data handling unit will bring Hubble’s communications systems back to full redundancy, said Ed Weiler, associate administrator of NASA’s science mission directorate.


The VEST unit inside NASA Goddard’s clean room. (Photo by Patricia Daukantas)

2008-09 September, Astronomy , , , ,