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A Gravitational Wave Observatory in Space

Posted March 18, 2016 9:42 AM by Bayes

Given the recent success of LIGO detecting gravitational waves, it shouldn't be surprising there is renewed interest in the press regarding other plans for building gravitational detectors. I found the recent article in Scientific American on the subject interesting...

Test Marks Milestone for Deep-Space Gravitational Wave Observatory

Scientists have long dreamed of launching a constellation of detectors into space to detect gravitational waves - ripples in space-time first predicted by Albert Einstein and observed for the first time earlier this month. That dream is now a step closer to reality. Researchers working on a €400 million (US$440 million) mission to try out the necessary technology in space for the first time-involving firing lasers between metal cubes in freefall - have told Nature that the initial test-drive is performing just as well as they had hoped.

"I think we can now say that the principle has worked," says Paul McNamara, project scientist for the LISA Pathfinder mission, which launched in December. "We believe that we now are in a good shape to look to the future and look to the next generation." "Everything works as we designed it. It's sort of magical and you rarely see that in your career as an experimentalist," says Stefano Vitale, a physicist at the University of Trento in Italy, and a principal investigator for the Pathfinder mission. The European Space Agency financed the test, and hopes ultimately to launch a €1-billion mission to hunt for gravitational waves. That would bounce lasers between three spacecraft, set millions of kilometres apart. Each craft would contain a test mass (a metal cube) which would be placed in freefall, protected from any forces except that of gravity. Because gravitational waves stretch and compress space-time, the observatory hopes to be able to see passing waves by using the lasers to detect minute changes in the distance between the freefalling cubes.

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Re: A Gravitational Wave Observatory in Space

03/18/2016 10:54 AM

The first engineering problem I see in this project is getting the blocks in a "free fall" state. Particularly with such a relatively short resonant cavity length of just 38 cm. LIGO used interferometers with 4 kilometer lengths to detect spatial anomalies at a resolution "1/10,000th the width of a proton!" This arm is 10,000 times shorter than LIGO. Any release mechanism to "free fall" these cubes will certainly jostle the cubes in a random fashion much more than 1/(10,000*10,000) of the width of a proton.

I realize the mission of this satellite based instrument is not to directly measure gravity wave distortions of space only 38 cm long but to hopefully codify most of the error mechanisms a network of satellites will produce when arms longer than the Earth are attempted. I wonder if they plan on running LIGO while this experiment runs. It will be interesting to see if any of the results correlate outside of nominal statistical probabilities.

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Re: A Gravitational Wave Observatory in Space

03/21/2016 9:38 AM

I was also thinking 38 cm is a pretty short reference length for such an experiment. Maybe with them moving the spacecraft around the cube(s) to get both cubes perfectly normal to the laser beam, they know something about this we do not?

I do not really get it, how any motion of the cubes is really cancelled out to the point where they will rest in a static position until a gravitational disturbance comes by.

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Re: A Gravitational Wave Observatory in Space

03/22/2016 1:26 PM

Oh, no. The sides of this will be massive...5000 km. Here's an article that does a better job explaining the experiment - http://www.nature.com/news/2008/080305/full/452018a.html

Unaided earthly eyes will never actually see the Laser Interferometer Space Antenna (LISA), as this artificial constellation is to be named. Its three component spacecraft will be too small, and the light with which they shine will be invisible infrared. Unseen as it may be, though, it will still be humanity's largest ever creation - 5 million kilometres on a side.

...

Using freely falling objects to spot gravitational waves in this way, as LISA is intended to do, is a three-step process. First you set up a situation in which masses can fall freely along their geodesics without being disturbed by magnetic fields or other spurious forces. Then you must measure with extraordinary precision how the distance between their geodesics changes when passing gravitational waves distort the local curvature of space. The last step is analysing these changes to determine the exact shape, frequency and intensity of the distortions to the curvature of space, so as to learn about the nature of distant events producing them.

To provide Vitale's geodesic-joining rays of light, LISA uses neodymium-YAG lasers, which shine at a wavelength of a little more than a micrometre, a wavelength they stick to with extreme precision. These will illuminate cubes four kilograms in mass but just four centimetres on a side - polished 'test masses' of gold-coated gold-platinum alloy as beautiful as fine jewellery and much more costly. The test masses respond to gravity and not much else. "The gold-platinum alloy has a magnetic susceptibility almost as low as glass," explains Paul McNamara, an ESA project scientist in Noordwijk, the Netherlands. Once in space, the test masses' sole purpose is to follow their own paths, each falling freely along its geodesic within one of the LISA spacecraft while reflecting the laser light with which the other spacecraft illuminate it.

LISA will be arranged so that these geodesics are 5 million kilometres apart, with the spacecraft falling around the Sun 20┬░ behind the Earth in the same orbit. That will put them about 50 million kilometres from their planet of origin. Once during every annual orbit the triangle will 'breathe', its sides taking turns to grow and shrink by about 50,000 kilometres.

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