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Post Info TOPIC: Gravity Waves
Blobrana


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RE: Gravity Waves
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UWM researchers, backed by considerable funding from the National Science Foundation, are taking a leadership role in the quest for gravitational waves in space. Such a find would literally change what we know about the cosmos.
Using new tools to look at the universe, says Patrick Brady, often has led to discoveries that change the course of science. History is full of examples.

Galileo was the first person to use the telescope to view the cosmos. His observations with the new technology led to the discovery of moons orbiting Jupiter and lent support to the heliocentric model of the solar system - Patrick Brady, UWM professor of physics.

Just such an opportunity exists today with a unique observatory that is scanning the skies, searching for one of Einsteins greatest predictions gravitational waves.
Gravitational waves are produced when massive objects in space move violently. The waves carry the imprint of the events that cause them. Scientists already have indirect evidence that gravitational waves exist, but have not directly detected them.
UWM researchers, backed by considerable funding from the National Science Foundation, are taking a leadership role in the quest.
It is an epic undertaking involving about 500 scientists worldwide, including Brady and other members of UWMs Center for Cosmology and Gravitation: associate professors Alan Wiseman and Jolien Creighton, and assistant professor Xavier Siemens.

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Blobrana


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Scientists have attempted to disprove Albert Einstein's theory of general relativity for the better part of a century. After testing and confirming Einstein's prediction in 2002 that gravity moves at the speed of light, a professor at the University of Missouri-Columbia has spent the past five years defending the result, as well as his own innovative experimental techniques for measuring the speed of propagation of the tiny ripples of space-time known as gravitational waves.

Sergei Kopeikin, associate professor of physics and astronomy in the College of Arts and Science, believes that his latest article, "Gravimagnetism, causality, and aberration of gravity in the gravitational light-ray deflection experiments" published along with Edward Fomalont from the National Radio Astronomical Observatory, arrives at a consensus in the continuing debate that has divided the scientific community.
An experiment conducted by Fomalont and Kopeikin five years ago found that the gravity force of Jupiter and light travel at the same speed, which validates Einstein's suggestion that gravity and electromagnetic field properties, are governed by the same principle of special relativity with a single fundamental speed. In observing the gravitational deflection of light caused by motion of Jupiter in space, Kopeikin concluded that mass currents cause non-stationary gravimagnetic fields to form in accordance with Einstein's point of view. The research paper that discusses the gravimagnetic field appears in the October edition of Journal of General Relativity and Gravitation.
Einstein believed that in order to measure any property of gravity, one has to use test particles.

By observing the motion of the particles under influence of the gravity force, one can then extract properties of the gravitational field. Particles without mass such as photons are particularly useful because they always propagate with constant speed of light irrespectively of the reference frame used for observations - Sergei Kopeikin.

The property of gravity tested in the experiment with Jupiter also is called causality. Causality denotes the relationship between one event (cause) and another event (effect), which is the consequence (result) of the first. In the case of the speed of gravity experiment, the cause is the event of the gravitational perturbation of photon by Jupiter, and the effect is the event of detection of this gravitational perturbation by an observer. The two events are separated by a certain interval of time which can be measured as Jupiter moves, and compared with an independently-measured interval of time taken by photon to propagate from Jupiter to the observer. The experiment found that two intervals of time for gravity and light coincide up to 20 percent. Therefore, the gravitational field cannot act faster than light propagates.

Other physicists argue that the Fomalont-Kopeikin experiment measured nothing else but the speed of light.

This point of view stems from the belief that the time-dependent perturbation of the gravitational field of a uniformly moving Jupiter is too small to detect. However, our research article clearly demonstrates that this belief is based on insufficient mathematical exploration of the rich nature of the Einstein field equations and a misunderstanding of the physical laws of interaction of light and gravity in curved space-time - Sergei Kopeikin.

Source University of Missouri-Columbia

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Blobrana


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Title: Dark Gravity
Authors: Frederic Henry-Couannier
(Version v17)

Adopting a non geometrical point of view, we are led to an alternative theory of the order two and symmetric gravitational tensor field of GR. The field is no more interpreted as the metric of our space-time. The true metric is globally Minkowskian and describes a flat manifold, a context which justifies a genuine rehabilitation of the global discrete space-time symmetries involved in the structure of the Lorentz group along with their 'problematic' representations: the negative energy and tachyonic ones.

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Blobrana


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A new way of looking at a previously abandoned mathematical model might help astronomers study and accurately identify an exotic clan of gravitational waves.
The waves in question come from small black holes or neutron stars in extremely elongated orbits around vastly larger black holes.

"This reopens an area of research that was closed several years ago" - Dr. Lior Burko, an assistant physics professor at The University of Alabama in Huntsville (UAH).

The results of this research by Burko and Dr. Gaurav Khanna at the University of Massachusetts at Dartmouth are published on-line by EuroPhysics Letters.
The exotic gravitational waves are generated (as predicted by general relativity theory) when an orbiting compact object changes speed, accelerating as it approaches the larger black hole and slowing as it moves away.

"Just as an accelerating electric charge emits electromagnetic waves, so mass emits gravitational waves as the speed changes" - Dr. Lior Burko.

The time-domain formulas that Burko and Khanna studied had been abandoned because they produced error rates of ten percent or more, compared to the more accurate frequency-domain formulae.
The problem  is that the frequency-domain math doesnt work especially well with objects in extremely elliptic orbits without burning up tons of computer time (and still getting large errors). And objects in elliptic or parabolic orbits are reasonably common in astronomy, as stars and small black holes wander or are pulled into orbits around massive black holes.

What to do, what to do?
The solution started with a change of perspective. To save computer time, early time-domain experiments looked at gravity waves inside a relatively small grid only 100 times the radius of the large black hole.
Burko and Khanna, however, found that the larger you make the grid, the smaller your error bar shrinks. At 500 radii the error had dropped from ten percent to one percent. At 1,500 radii the error shrinks to 0.1 percent. The goal is an error of not more than 0.01 percent.
The tradeoff, of course, is that the larger grid means substantially more grid points and longer computing times. With 40 data points per radius, enlarging the grid from 100 to 1,500 radii increases the number of grid points from 320 million to 72 billion. And each grid point requires hundreds of calculations to simulate how gravity waves might form and evolve. Thats a lot of calculations, even for a super computer. A single run using the new model can take at least two weeks on a fast workstation.
The second big step had to do with how you calculate the size of the small black hole or neutron star. Mathematically, it is considered a point source compared to the much larger black hole. Making calculations based on a single point in the grid introduces new errors so astronomers use a Gaussian formula to simulate the gravity well of the orbiting object.
Burko and Khanna found that how big you make that Gaussian spot also influences how big your error will be.

"Too small and you under sample. Too big and you start bringing in finite size effects. We found a sweet spot that would give you the least errors" - Dr. Lior Burko

The goal is to accurately model what these special gravity waves look like when they leave their home galaxies. Astronomers use that data to calculate (and estimate) how those waves might look after they travel a few billion light years to Earth and various gravity wave detectors in the U.S. or in orbit following the Earth around the Sun. Knowing what to expect will help scientists recognise the "signal" from these gravitational waves.

University of Alabama Huntsville

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Blobrana


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Gravitomagnetism
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Albert Einstein's theory of general relativity has fascinated physicists and generated debate about the origin of the universe and the structure of objects like black holes and complex stars called quasars. A major focus has been on confirming the existence of the gravitomagnetic field, as well as gravitational waves. A physicist at the University of Missouri-Columbia recently argued in a paper that the interpretation of the results of Lunar Laser Ranging (LLR), which is being used to detect the gravitomagnetic field, is incorrect because LLR is not currently sensitive to gravitomagnetism and not effective in measuring it.
The theory of general relativity includes two different fields: static and non-static fields. The gravitomagnetic field is a non-static field that is important for the understanding of general relativity and the universe.

"If the existence of the gravitomagnetic field is confirmed, then our understanding of general relativity is correct and can be used to explain things such as quasar jets and accretion disks in black holes. General relativity explains the origin of the universe, and that's important for all humankind, irrespective of religion or creed. We all live in the same world, and we must understand this place in which we live" - Sergei Kopeikin, associate professor of physics in MU's College of Arts and Science.

Kopeikin said there are four techniques used to test for the gravitomagnetic field. The first, called Gravity Probe B, used a gyroscope in orbit around the earth to measure for the field. It is supported by NASA and took nearly 40 years to develop; scientists recently conducted the experiment and are now analyzing the results. A second experiment involved satellites called Lageos and detected a gravitomagnetic field with a precision not exceeding 15 percent. A third experiment was developed by Kopeikin and other scientists in 2001 and used Very Long Baseline Interferometry (VLBI) to test for the gravitomagnetic field of Jupiter; this experiment detected the field with approximately 20 percent precision.
LLR is a recent testing technique. It involves shooting a laser beam at mirrors called retroflectors, which are located on the moon, and then measuring the roundtrip light travel time of the beam. In a response to a paper about LLR, Kopeikin argued in a letter published in Physical Review Letters that the interpretation of LLR results is flawed. He said analyses of his own and other scientists' research reveal that this approach to the LLR technique does not measure what it claims.
The LLR technique involves processing data with two sets of mathematical equations, one related to the motion of the moon around the earth, and the other related to the propagation of the beam from earth to the moon. These equations can be written in different ways based on "gauge freedom," the idea that arbitrary coordinates can be used to describe gravitational physics. Kopeikin analysed the gauge freedom of the LLR technique and showed that the manipulation of the mathematical equations is causing scientists to derive results that are not apparent in the data itself.

"According to Einstein's theory, only coordinate-independent quantities are measurable. The effect the LLR scientists claimed as detectable doesn't exist, as it vanishes in the observer's frame. The equations add up to zero, having nothing to do with the real data. The results appear this way because of insufficient analytic control of the coordinate effects in the sophisticated computer code used for numerical LLR data processing. We need to focus on the real physical effects of gravity, not the mathematical effects depending exclusively on the choice of coordinates" - Sergei Kopeikin.

A reply from the scientists who support LLR also has published in Physical Review Letters and argues that there are aspects of the technique that cause them to believe it merits worth.

Source University of Missouri

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Blobrana


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Gravitational wave observatories to join forces
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Detecting ripples in space-time is a step closer to reality now that the world's most sensitive observatories have joined forces. The collaboration boosts the chances that gravitational waves could be detected in the next four years.
Gravitational waves are ripples in space-time that expand outwards at the speed of light from violent events like supernovae and mergers of pairs of black holes and neutron stars.

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Blobrana


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GEO-600
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One of the great scientific experiments of our age is now fully underway.

The British-German laser interferometric gravitational wave detector GEO-600, with arm length of 600 meters, built near Hanover, Germany, is in a continuous observational mode. The lab is trying to detect the ripples created in the fabric of space-time.

The Cardiff Relativity Group is one of the participating institutions in this pioneering experiment.
Gravitational waves should be created when massive objects, such as black holes or neutron stars in astronomical binaries interact and spiral-in towards, and eventually collide with, each other emitting a strong burst of gravitational radiation or when a star, at the end of its long evolutionary phase, collapses due to its own gravity resulting in a supernova with the core forming a neutron star or a black hole. Rapidly rotating neutron stars or pulsars with tiny deformities in their spherical shape, and newly formed neutron stars, are continuous emitters of the radiation. There should also be background "noise" made up from a population of such events and, possibly, phase transitions in the early Universe and the echoes of the Big Bang itself.

Gravity waves regularly pass through the Earth unnoticed.

"As gravity waves pass through, they contract or expand by tiny amounts in a plane perpendicular to the direction they are moving, usually too small to notice. If we split a laser signal and send it off in perpendicular directions before bouncing the light back off test masses and recombining it, we can measure whether the light has travelled the same distance in each direction. If a gravity wave has interacted with the system, it will have changed the relative distance between the test masses forming the two perpendicular arms." - Dr Chris Castelli, Birmingham University.

The detector was operated continuously for 17 days from 28 December 2001 to 14 January. The experiment is working alongside a project known as Ligo (Laser Interferometer Gravitational Wave Observatory).

Source

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Blobrana


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Parkes Pulsar Timing Array
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Title: The Parkes Pulsar Timing Array
Authors: R N Manchester

Given sufficient sensitivity, pulsar timing observations can make a direct detection of gravitational waves passing over the Earth. Pulsar timing is most sensitive to gravitational waves with frequencies in the nanoHertz region, with the most likely astronomical sources being binary super-massive black holes in galaxy cores.
The Parkes Pulsar Timing Array project uses the Parkes 64-m radio telescope to make precision timing observations of a sample of about 20 millisecond pulsars with a principal goal of making a direct detection of gravitational waves. Observations commenced about one year ago and so far sub-microsecond timing residuals have been achieved for more than half of these pulsars.
New receiver and software systems are being developed with the aim of reducing these residuals to the level believed necessary for a positive detection of gravitational waves.

Galactic disk MSPs with periods less than 20 ms plotted in celestial coordinates. The size of the circle is inversely related to the pulsar period and for stronger pulsars the circle is filled. The dashed line is the northern declination limit of the Parkes telescope. Pulsars chosen for the PPTA are marked by a star.

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Blobrana


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Gravity Waves
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In the corner of a giant field of beet on an unremarkable patch of wasteland, two 2,000ft-long corrugated steel pipes emerge at right angles from a grey cabin. In the shadow of a row of electricity pylons and surrounded by nettles, there is little about the scene to detain the casual observer.

It is here, however, at the end of a dirt track at the edge of a sleepy village in northern Germany, that scientists are close to the culmination of a 40-year quest for the holy grail of physics.

The shed in Ruthe, near Hanover, contains the heart of the Anglo-German GEO600 interferometer - an instrument so sensitive it can detect an object moving one million billionth of a millimetre. The team believes it is just months away from mankind's first detection of gravitational waves - shifts in space and time caused by the movement of massive astronomical bodies.
If they are correct the discovery will open up a whole new way of observing events across the cosmos, confirm Einstein's general theory of relativity and potentially give astronomers an unprecedented view of the birth of the universe 13.7 billion years ago.
Colleagues across the Atlantic yesterday switched on their detector at a parallel site at Hanford in Washington state that will act in partnership with GEO600.

"Up until now we have been able to learn a great deal about the universe by what we can see. The ability to detect and read gravitational waves will give us as much extra information about the universe as being suddenly given the ability to hear" - Prof Bernard Schutz, of the University of Wales in Cardiff , leading member of GEO600.

In his general theory of relativity set out in 1916, Albert Einstein proposed that bodies of mass such as stars cause distortions in the fabric of space - in a similar way to the effect of placing a ball on a piece of elasticated material.
Gravity, created by the presence of mass, bends space-time, and determines that a body travelling through space past, for example a star, will follow a "curved" path.

Assuming Einstein was right, whenever a mass accelerates, gravitational waves are sent out across the universe causing shudders in time and space.
No one has been able to observe and record a wave because of the fractional changes involved. Even the violent disruption of a super-dense astronomical bodies, such as a black hole, is thought to produce ripples that are equivalent to the distance between the Earth and the Moon changing by one 100 billionth of the width of the thickest human hair. Since they began their search in the 1960s, scientists have developed ever more sensitive equipment. The GEO600 detector, the most sensitive built, will next month begin an 18-month run of readings at the same time as detectors in Washington and Louisiana.

To do so, a laser beam is split into two branches that are sent down two identical 2000ft-long tubes to suspended mirrors and back again. The beams are recombined and, assuming the two arms remain exactly the same distance, cancel each other out.
But if the beams create an interference pattern when recombined, this means the length of the branches has been altered and a gravitational wave has been detected. A detection would only be accepted if picked up by more than one interferometer.
If the GEO600 team make the detection, it will be seen as one of the greatest coups in the history of science and should put its leading members in the running for a Nobel prize for physics.

Prof Jim Hough, a physicist at Glasgow University, has been on the gravitational wave quest for over 30 years.
"Given what we now know about the frequency of events that cause the emission of powerful gravitational waves and the sensitivity of the equipment we now have, I am confident that we will see things during this session" - Prof Jim Hough.

Source

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