黑洞碰撞:Spartan FPGAs 助力 LIGO 发现引力波
最近很热很热的引力波的发现与Spartan FPGAs有着密不可分的关系。不买关子了，我也是不小心发现的，但是是英文的。转给大家看看。
When Black Holes Collide: Gravitational Waves detected by Advanced LIGO with some timely help from Spartan FPGAs.
It's been a long, long wait.
Einstein's theory of general relativity (GR)—first presented in November, 1915—predicted the existence of gravitational waves, which are ripples in the fabric of spacetime that propagate at the speed of light. After 100 years, gravitational waves were the last consequence of Einstein’s GR awaiting experimental verification. Other GR consequences including time dilation, gravitationally-induced frequency shifting of light, gravitational light lensing, and the orbital precession and orbital decay of celestial bodies have all been directly observed. Although the existence of gravitational waves was indirectly confirmed by noting that the observed orbital decay of a binary pulsar was consistent with GR’s prediction of energy loss through gravitational radiation (which causes gravitational waves), direct observation of gravitational waves has eluded scientists… until today’s announcement in Washington, DC made by the LIGO Scientific Collaboration.
Here’s an 8-minute video of the announcement that visually illustrates what the scientists think happened to the black holes:http://www.openhw.org/data/image/9/9b7/01.png
（点击图片进行视频观看）
(The full-length announcement video is here and the actual announcement starts at 27:14.)
The experimental search for gravitational waves to directly verify the last remaining unconfirmed GR prediction started more than half a century ago, during the 1960s. Researchers working on the Advanced LIGO (Laser Interferometer Gravitational-wave Observatory) team announced today that the recently upgraded Advanced LIGO experiment detected the gravitational waves resulting from the cataclysmic collision of two closely orbiting black holes to create one bigger black hole about 1.3 billion light years distant from Earth. Advanced LIGO, called in today’s press conference announcing the findings “the most precise measuring device ever built,” uses a precision timing-distribution system based on Xilinx Spartan-3E FPGAs.
The LIGO project started in 1992 with funding from the US National Science Foundation. Two LIGO observatories were built—one in Hanford, Washington and the other in Livingston, Louisiana. These sites are separated by 3,002 kilometers (1,865 miles)—roughly ten light milliseconds—and that’s important to the story because GR predicts that gravitational waves should travel at the speed of light. So a gravitational wave passing through one LIGO site should arrive at the other site in something like a few milliseconds.
Each LIGO observatory consists of an L-shaped tube. Each tubular leg of the “L” is 4km long and the tubes are evacuated—they’re in vacuum. The tubes house laser interferometers capable of detecting slight length differences between the two legs of the L-shaped tube. The theory is that a gravitational wave will alter the fabric of spacetime enveloping the observatory, shrinking one leg of the tube while stretching the other in a cyclic manner as the wave passes. (We’re talking about spacetime distortions predicted to be on the order of 10-18m—which is smaller than the width of a proton.) LIGO’s laser interferometer is designed to detect these variations in spacetime, allowing direct observation of gravitational waves.
LIGO operated for eight years, from 2002 to 2010, without observing any gravitational waves. Then the observatories were shut down for a 5-year retrofit to improve the interferometers’ sensitivity by as much as an order of magnitude. The retrofit was finished in mid-September, 2015 and the interferometers went on line with 3x improved sensitivity—10x improvement is the ultimate goal.
Today’s announcement by the LIGO team said that the refurbished LIGO experiments observed gravitational waves on September 14, 2015. First, the LIGO facility in Louisiana detected a gravitational wave. Seven milliseconds later, the Washington facility detected a wave with the same signature, thus observationally confirming the last remaining GR prediction and ending a 100-year search.
As you can see from this very brief description, time and precise time measurement play an important role in the LIGO experiment. The absolute time and the relative arrival times of a passing gravitational wave at the interferometers located at the two observatories are crucial for determining the direction of the celestial source generating the detected gravitational waves. In addition, timing at different parts of each interferometer must be closely synchronized, which is complicated by the large size of the gravitational-wave detectors (4km per leg).
The upgraded Advanced LIGO interferometers use a timing-distribution system that’s slaved to GPS time. A collection of master/slave modules distributes UTC-synchronized timing information over an optical fiber network configured in a star topology that runs throughout each facility with enough precision to satisfy experimental needs. Events are time-stamped with 15nsec of overall precision. The timing system also synchronizes ADCs, DACs, and processors in the interferometer detection subsystems. According to published literature, the timing modules are based on Xilinx Spartan-3E FPGAs. For more information about the design of these timing modules, see:
-The Advanced LIGO timing system
-Timing Synchronization for Advanced LIGO Timing System
Now it may seem strange to be discussing Spartan-3E FPGAs, which were introduced way back in 2005 based on “absolutely ancient” 90nm IC process technology, but it’s an important aspect of the experimental equipment design when you’re discussing a long-term scientific project initiated in 1992 with a 5-year upgrade that started in 2010. The second paper referenced above is from 2007. So this small aspect of the LIGO project speaks to the long lifespan of Xilinx semiconductor products and to their long-term support, something that’s pretty important to many Xilinx customers like the LIGO Scientific Collaboration, and to Xilinx.
Personal note: The Advanced LIGO interferometer owes a lot to the Michelson Interferometer developed by Albert Michelson in 1881 and the improved interferometer developed by Michelson and Edward Morley in 1887 to detect luminiferous aether, theorized at the time to be the medium that propagated electromagnetic waves through space. No such medium was detected by these experiments, conducted at the Case School in Cleveland (which subsequently became the Case Institute of Technology and is now Case Western Reserve University or CWRU, my alma mater), or by subsequent aether experiments conducted as recently as 2009 using masers, lasers, and optical resonators. (Einstein’s GR neither requires nor precludes aether.) However, these negative findings still don’t prove the non-existence of luminiferous aether, just as a half century of unsuccessful searching didn’t prove the non-existence of gravitational waves. As of today, we know that they do exist. Einstein proves right once again.
And just because he’s cool, because he explains how an interferometer works, and because he says that the Michelson-Morley experiment transformed experimental science, here’s Neil deGrasse Tyson to explain the experiment:http://www.openhw.org/data/image/c/8c2/01.png
（点击图片进行观看）
本文转自 Xcell Daily Blog
本文转自：http://www.openhw.org/blog-544
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