Gravitational Wave Detectors: How They Work

It’s official: this Thursday, February 11, at 10:30 EST, there will be parallel press conferences at the National Press Club in Washington, D.C., in Hannover, Germany, and near Pisa in Italy. Not officially confirmed, but highly probable, is that people running the LIGO gravitational wave detectors will announce the first direct detection of a gravitational wave. The first direct detection of minute distortions of spacetime, travelling at the speed of light, first postulated by Albert Einstein almost exactly 100 years ago. Nobel prize time.

Time to brush up on your gravitational wave basics, if you haven’t done so! In Gravitational waves and how they distort space, I had a look at what gravitational waves do. Now, on to the next step: How can we measure what they do? How do gravitational wave detectors such as LIGO work?

Recall that this is how a gravitational wave will change the distances between particles, floating freely in a circular formation in empty space: The wave is moving at right angles to the screen, towards you. I’ve greatly exaggerated the distance changes. For a realistic wave, even the giant distance between the Earth and the Sun would only change by a fraction of the diameter of a hydrogen atom. Tiny changes indeed.

How to detect something like this?

The first unsuccessful attempts to detect gravitational waves in the 1960s tried to measure how they make aluminum cylinders ring like a very soft bell. (Tragic story; Joe Weber [1919-2000], the pioneering physicist behind this, was sure he had detected gravitational waves in this way; after thorough analysis and replication attempts, community consensus emerged that he hadn’t.)

Afterwards, physicists came up with alternative scheme. Imagine that you are replacing the black point in the center of the previous animation with a detector, and the rightmost red particle with a laser light source. Now you send light pulses (represented here by fast red dots) from the light source to the detector; let’s first look at this with the gravitational wave switched off:

Every time a light pulse reaches the detector, an indicator light flashes yellow. The pulses are sent out regularly, they all travel at the same speed, hence they also reach the detector in regular intervals.

If a gravitational wave passes through this system, again from the back and coming towards you, distances will change. Let us keep our camera trained on the detector, so the detector remains where it is. The changing distance to the light source, and also the changing distances between the light pulses, and some of the changes in distance between light pulses and detector or source, are due to the gravitational wave. Here is what that would look like (again, hugely exaggerated):

Keep your eye on the blinking light, and you will see that its blinking is not so regular any more. Sometimes, the light blinks faster, sometimes slower. This is an effect of the gravitational wave. An effect by which we can hope to detect the gravitational wave.

“We” in this case are the radio astronomers working on what are known as Pulsar Timing Arrays. The sender of regular pulses are pulsars, rotating neutron stars sweeping a radio beam across our antennas like a cosmic lighthouse. The detectors are radio telescopes here on Earth. Detection is anything but easy. With a single pulsar, you’d need to track pulse arrival times with an accuracy of a few billionths of a second over half a year, and make sure you are not being fooled by various other sources of timing variations. So far, no gravitational waves have been detected in this way, although the radio astronomers are keeping at it.

To see how gravitational wave detectors like LIGO work, we need to make things a little more complex.

Interferometric gravitational wave detectors: the set-up

Here is the basic set-up: Two mirrors, a receiver (or “light detector”), a light source and what is known as a beamsplitter:

Light is sent into the detector from the (laser) light source LS to the beamsplitter B which, true to its name, sends half of the light on to the mirror M1 and lets the other half through to the mirror M2. At M1 and M2, respectively, the light is reflected back to the beam splitter. There, the light arriving from M1 (or M2) is split again, with half going towards the light detector LD, the other half back in the direction of the light source LS. We will ignore the latter half and pretend, for the sake of our simplified explanation, that all the light reaching B from M1 or M2 goes on to the light detector LD.

(To avoid confusion, I will always refer to LD as the “light detector” and take the unqualified word “detector” to mean the whole setup.)

This setup, by the way, is called a Michelson Interferometer. We’ll see below why it is a good setup for gravitational wave detectors.

In what follows, we will assume that the mirrors and the beam splitter, shown as being suspended, react to the gravitational wave in the same way freely floating particles would react. The key effects are between the mirrors and the beam splitter in what are called the two arms of the detector. Arm length is huge in today’s detectors, running to a few kilometers. In comparison, light source and light detector are very close to the beamsplitter; changes of the distances between these three do not signify.

Light pulses in a gravitational wave detector

Next, let us see how light pulses run through this detector. Here is the same setup, seen from above: Light source LS, the two mirrors M1 and M2, the beamsplitter B and the light detector LD: all present and accounted for.

Next, we let the light source emit light pulses. For greater clarity, I will make two artificial and unrealistic changes. I will send red and green pulses into the detector, representing the light that goes into the horizontal and the vertical arm, respectively. In reality, there is no distinction, just light apportioned at the beamsplitter. Light running towards M1 will be offset a little to the left, light coming back from M1 to the right, for better clarity. Same goes for M2. This, too, is different in a real detector. That said, here come the light pulses: Light starts at the light source to the left. Light that has left the source together, travels together (so green and red pulses are side by side) until the beam splitter. The beam splitter then sends the green pulses on their upward journey and lets the red pulses pass on their way towards the mirror on the right. All the particles that arrive back at the beamsplitter after reflection at M1 or M2. At the beamsplitter, they are directed towards the light detector at the bottom.

In this setup, the horizontal arm is slightly longer than the vertical arm. Red particles have to cover some extra distance. That is why they arrive at the detector a bit later, and we get an alternating rhythm: green, red, green, red, with equal distances in between. This will become important later on.

Here is a diagram, a kind of registration strip, which shows the arrival times for red and green pulses at the light detector (time is measured in “animation frames”): The pattern is clear: red and green pulses arrive evenly spaced, one after the other.

Bring on the gravitational wave!

Next, let’s switch on our standard gravitational wave (exaggerated, passing through the screen towards you, and so on). Here is the result: We have trained our camera on the beamsplitter (so in our image, the beamsplitter doesn’t move). We ignore any slight changes in distance between beamsplitter and light source/light detector. Instead, we focus on the mirrors M1 and M2, which change their distance from the beamsplitter just as we would expect from the earlier animations.

Look at the way the pulses arrive at our light detector: sometimes red and green are almost evenly spaced, sometimes they close together. That is caused by the gravitational wave. Without the wave, we had strict regularity.

Here is the corresponding “registration strip” diagram. You can see that at some times, the light pulses of each color are closer together, at others, farther apart:

At the time I have marked with a hand-drawn arrow, red and green pulses arrive almost in unison!

The pattern is markedly different from the scenario without a gravitational wave. Detect this change in the pattern, and you have detected the gravitational wave.

Running interference

If you’ve wondered why detectors like LIGO are called interferometric gravitational wave detectors, we will need to think about waves a bit more. If not, let me just state that detectors like LIGO use the wave properties of light to measure the changes in pulse arrival rate you have seen in the last animation. To skip the details, feel free to jump ahead to the last section, “…and now for something a thousand times more complicated.”

Light is a wave, with crests and troughs corresponding to maxima and minima of the electric and of the magnetic field. While the animations I have shown you track the propagation of light pulses, they can also be used to understand what happens to a light wave in the interferometer. Just assume that each of the moving red and green dots in the detector marks the position of a wave crest.

Particles just add up. Take 2 particle and add 2 particles, and you will end up with 4 particles. But if you add up (combine, superimpose) waves, it depends. Sometimes, one wave plus another wave is indeed a bigger wave. Sometimes, it’s a smaller wave, or no wave at all. And sometimes it’s complicated.

When two waves are in perfect sync, the crests of the one aligning with the crests of the other, and the troughs aligning, too, you indeed get a bigger wave. The following diagram shows at which times the different parts of two light waves arrive at the light detector, and how they add up. (I’ve placed a dot on top of each crest; that is what the dots where meant to signify, after all.) On top, the green wave, perfectly aligned with the red wave (which, for clarity, is shown directly below the green wave). Add the two waves up, and you will get the (markedly stronger) blue wave in the bottom panel.

Not so if the two waves are maximally misaligned, the crests of each aligned with the troughs of the other. A crest and a trough cancel each other out. The sum of a wave and a maximally misaligned wave of equal strength is: no wave at all. Here is the corresponding diagram: Recall that this was exactly the setup for our gravitational wave detector in the absence of gravitational waves: Red and green pulses with equal spacing; troughs of the one wave perfectly aligned with the crests of the other. The result: No light at the light detector. (For realistic gravitational wave detectors, that is almost true.)

When a gravitational wave passes through the detector, the situation changes. Here is the corresponding pattern of pulse/wave crest arrival times for the animation above: The blue pattern, which is the sum of the red and the green, is complex. But it is not a flat line. There is light at the light detector where there was no light before, and the cause of the change is the gravitational wave passing through.

All in all, this makes a (highly simplified) version of how gravitational wave detectors such as LIGO work. Whatever the scientists will report this Thursday, it is based on light signals at the exit of such an interferometric detector.

And now for something a thousand times more complicated

Real gravitational wave detectors are, of course, much more complicated than that. I haven’t even started talking about the many disturbances scientists need to take into account – and to suppress as far as possible. How do you suspend the mirrors so that (at least for certain gravitational waves) they will indeed be influenced as if they were freely floating particles? How do you prevent seismic noise, cars or trains in the wider neighborhood and so on from moving your mirrors a tiny little bit (either by vibrations or by their own gravity)? What about fluctuations of the laser light?

Gravitational wave hunting is largely a hunt for noise, and for ways of suppressing that noise. The LIGO gravitational wave detectors and their kin are highly complex machines, with hundreds of control circuits, highly elaborate mirror suspensions, the most stable lasers known to physics (and some of the most high-powered). The technology has been contributed by numerous group from all over the world.

But all this is taking us too far, and I refer you to the pages of the detectors and collaborations for additional information:

LIGO pages at Caltech

Pages of the LIGO Scientific Collaboration

GEO 600 pages

VIRGO / EGO pages

You can find some further information about gravitational waves on the Einstein Online website:

Einstein Online: Spotlights on gravitational waves

Gravitational Waves and How They Distort Space

It’s official: on February 11, 10:30 EST, there will be a big press conference about gravitational waves by the people running the gravitational wave detector LIGO. It’s a fair bet that they will announce the first direct detection of gravitational waves, predicted by Albert Einstein 100 years ago. If all goes as the scientists hope, this will be the kick-off for an era of gravitational wave astronomy: for learning about some of the most extreme and violent events in the cosmos by measuring the tiny ripples of space distortions that emanate from them.

Time to brush up on your gravitational wave knowledge, if you haven’t already done so! Here’s a visualization to help you – and we’ll go step by step to see what it means:

Einstein’s distorted spacetime

In the words of the eminent relativist John Wheeler, Einstein’s theory of general relativity can be summarized in two statements: Matter tells space and time how to curve. And (curved) space and time tell matter how to move. (Here is a slightly longer version on Einstein Online.)

Einstein published the final form of his theory in November 1915. By spring 1916, he had realized another consequence of distorting space and time: general relativity allows for gravitational waves, rhythmic distortions which propagate through space at the speed of light.

For quite some time, physicists weren’t sure whether these gravitational waves were real or a mathematical artifact within Einstein’s theory. (For more about this controversy, see Daniel Kennefick’s book “Traveling at the Speed of Thought and  this article.) But since the 1980s, there has been indirect evidence for these waves (which earned its discoverers a Nobel prize, no less, in 1993).

Gravitational waves are emitted by orbiting bodies and certain other accelerated masses. Right now, major international efforts are underway to detect gravitational waves directly. Once detection is possible, the scientists hope to use gravitational waves to “listen” to some of the most violent processes in the universe: merging black holes and/or neutron stars, or the core region of supernova explosions.

Just as regular astronomy uses light and other forms of electromagnetic radiation to learn about distant objects, gravitational wave astronomy will decipher the information contained within gravitational waves. And if you go by recent rumors, gravitational wave astronomy might already have kicked off in mid-September 2015.

What do gravitational waves do?

But what do gravitational waves do? For that, let us look at a simplified, entirely hypothetical situation. (The following are variations on images and animations originally published here on Einstein Online.) Consider particles drifting in space, far from any sources of gravity. Imagine that the particles (red) are arranged in a circle around a center (marked in black):

If a simple gravitational wave were to pass through this image, coming directly at the reader, distances between these particles would change rhythmically as follows:

Note the distinctive pattern: When the circle is stretched in the vertical direction, it is compressed in the horizontal direction, and vice versa. That’s typical for gravitational waves (“quadrupole distortion”).

It’s important to keep in mind that this animation, and the ones that will follow, exaggerate the gravitational wave’s effect quite considerably. The gravitational waves detectors such as aLIGO hope to measure are much, much weaker. If our hypothetical circle of particles were as large as the Earth’s orbit around the Sun, a realistic gravitational wave would distort it by less than the diameter of a hydrogen atom.

Gravitational waves moving through space

The animation above shows what could be called a “gravitational oscillation.” To see the whole wave, we need to consider the third dimension.

We talk about a wave when oscillations propagate through space. Consider a water wave: At each point of the surface, we have an oscillation, with the surface rising and falling rhythmically. But it’s only the fact that this oscillation propagates, and that we can see a crest moving over the surface, that makes this into a wave.

It’s the same with gravitational waves. To see that, we will look not at a single circle of freely floating particles, but at many such circles, stacked one behind the other, forming the surface of a cylinder:

In this image, it’s hard to see which points are in front and which in the back. Let us join each particle to its nearest neighbors with a blue line, and let us also fill out the area between those lines. That way, the geometry is much more obvious:

Just remember that neither the lines nor the whitish surface is physical. On the contrary, if we want the particles to be maximally susceptible to the effect of the gravitational wave, we should make sure they are truly floating freely, and certainly they shouldn’t be linked in any way!

Now, let us see what the same gravitational wave we saw before does to this assembly of particles. From this perspective, the wave is passing from the right-hand side in the back towards the left-hand side on the front: As you can see, the wave is propagating through space. For instance, the point where the vertical distances within the circle of particles is maximal is moving towards the observer. The wave nature can be seen even more clearly if we look at this cylinder directly from the side:

What the animations show is just one kind of simple gravitational wave (“linearly polarized”). Here is another kind (“circularly polarized”):

This, then, is what the gravitational wave hunters are looking for. Except that they do not have particles floating in free space. Instead, their detectors contain test masses (notably large mirrors) elaborately suspended here on Earth, with laser light to detect the minute distance changes caused by gravitational waves.

More realistic gravitational wave signals, which contain information about merging black holes or the bulk motion of matter inside a supernova explosion, are more complicated still. They combine many simple waves of different frequencies, and the strength of such waves (their amplitude) will change over time in a characteristic fashion.

In these animations, gravitational waves look a bit like wriggling space worms. But these space worms could become the astronomers’ best friends, carrying information about the cosmos that is hard or even impossible to obtain in any other way.

[Don’t miss the sequel: Gravitational wave detectors: how they work]

Update: Gravitational Waves Detected

Weekly Space Hangout – Oct 2, 2015: Water on Mars, Blood Moon Eclipses, and More Pluto!

Host: Fraser Cain (@fcain)

Guests:

Morgan Rehnberg (cosmicchatter.org / @MorganRehnberg )
Pamela Gay (cosmoquest.org / @cosmoquestx / @starstryder)
Kimberly Cartier (@AstroKimCartier )
Brian Koberlein (@briankoberlein / briankoberlein.com)
Alessondra Springmann (@sondy)
Continue reading “Weekly Space Hangout – Oct 2, 2015: Water on Mars, Blood Moon Eclipses, and More Pluto!”

Watch: New Documentary Follows the Hunt for Gravitational Waves

A newly released documentary brings you behind the scenes in the hunt for gravitational waves. The 20-minute film, called “LIGO, A Passion for Understanding,” follows the scientists working to create one of the most powerful scientific tools ever made: the Laser Interferometer Gravitational-Wave Observatories (LIGO). You can watch the documentary above.
Continue reading “Watch: New Documentary Follows the Hunt for Gravitational Waves”

The Search for Gravitational Waves: New Documentary About LIGO Premieres Soon

What happens when stars or black holes collide? Scientists have theorized that the energy released would disturb the very fabric of the space-time continuum, much like ripples in a pond. These ripples are called gravitational waves, and while proving the existence of these waves has been difficult, their detection would open a brand new window on our understanding of the Universe.

The Laser Interferometer Gravitational-wave Observatories (LIGO) have been searching for these elusive waves. A new documentary about LIGO will be available soon here on Universe Today, and it documents the science and people behind the unprecedented astronomical tool designed to catch sight of violent cosmic events trillions of miles from our planet.

The new documentary titled, “LIGO, A Passion for Understanding,” follows scientists working with LIGO. It is produced by filmmaker Kai Staats, and this will actually be the first installment to a multi-video series, in fact. Watch the trailer, above.

“A Passion for Understanding” brings to life one of the most important astronomical tools of our time while telling the human story of creativity, passion, and drive to understand the very fabric of the Universe in which we live.
Operated by teams from the California Institute of Technology and Massachusetts Institute of Technology, LIGO’s observatories use 4 km laser beams to hunt for gravitational waves. The LIGO scientific collaboration consists of hundreds of scientists from around the world.

LIGO’s enhanced run ended in 2010, but the Advanced LIGO project featuring newly upgraded instruments is set to begin its run in late 2015. Advanced LIGO will probe deeper into the universe in search of gravitational waves.

Find out more about the documentary on the film’s Facebook page, at the LIGO collaboration website, and on Space.com.

Extreme Telescopes: Unique Observatories Around the World

In 1888, astronomer Simon Newcomb uttered now infamous words, stating that “We are probably nearing the limit of all we can know about astronomy.” This was an age just prior to identifying faint nebulae as separate galaxies, Einstein’s theory of special and general relativity, and an era when a hypothetical substance called the aether was said to permeate the cosmos.

Newcomb would scarcely recognize astronomy today. Modern observatories span the electromagnetic spectrum and are unlocking the secrets of a universe both weird and wonderful. Modern day astronomers rarely peer through an eyepiece, were it even possible to do so with such bizarre instruments. What follows are some of the most unique professional ground-based observatories in operation today that are pushing back our understanding of the universe we inhabit.

VERITAS: Based at the Fred Lawrence Whipple Observatory in southern Arizona, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an observatory designed to observe high energy gamma-rays. Its array consists of four 12-metre aperture reflectors each comprised of 350 mirror scintillators. Each VERITAS array has a 3.5° degree field of view and the array has been fully operational since 2007. VERITAS has been used to study active galactic nuclei, gamma-ray bursts, and the Crab Nebula pulsar.

IceCube: Not the rapper, IceCube is a neutrino detector in based at the Amundsen-Scott South Pole Station in Antarctica. IceCube watches for neutrino interactions by use of thousands of photomultipliers suspended up to 2.45 kilometres down into the Antarctic ice sheet. With a total of 86 detector strings completed in 2011, IceCube is currently the world’s largest neutrino observatory and is part of the worldwide Supernova Early Warning System. IceCube will also complement WMAP and Planck data and can actually “see” the shadowing effect of the Moon blocking cosmic ray muons.

Liquid Mirror Telescopes: One of the more bizarre optical designs out there in the world of astronomy, liquid mirror telescopes employ a large rotating dish of mercury to form a parabolic mirror. The design is cost effective but does have the slight drawback of having to aim directly at the zenith while a swath of sky passes over head. NASA employed a 3-metre liquid mirror telescope as part of its Orbital Debris observatory based near Cloudcroft, New Mexico from 1995-2002. The largest one in the world (and the 18th largest optical telescope overall) is the 6-metre Large Zenith Telescope in the University of British Columbia’s Malcolm Knapp Research Forest.

LIGO: Designed to detect incoming gravity waves caused by pulsar-black hole mergers, the Laser Interferometer Gravitational-Wave Observatory (LIGO) is comprised of a pair of facilities with one based in Hanford, Washington and another in Livingston, Louisiana. Each detector is consists of a pair of 2 kilometre Fabry-Pérot arms and measures a laser beam shot through them with ultra-high precision.  Two geographically separate interferometers are needed to isolate out terrestrial interference as well as give a direction of an incoming gravity wave on the celestial sphere. To date, no gravity waves have been detected by LIGO, but said detection is expected to open up a whole new field of astronomy.

The Very Long Baseline Array: A series of 10 radio telescopes with a resolution the size of a continent, the Very Long Baseline Array (VLBA) employs observatories across the continental United States, Saint Croix in the U.S. Virgin Islands, and Mauna Kea, Hawaii. This is effectively the longest radio interferometer in the world with a baseline of over 8,600 kilometres and a resolution of under one milliarcseconds at 4 to 0.7 centimetre wavelengths. The VLBA has been used to study H2O megamasers in Active Galactic Nuclei and measure ultra-precise positions and proper motions of stars and galaxies.

LOFAR: Located just north of the town of Exloo in the Netherlands,  The LOw Frequency Radio Array is a phased array 25,000 antennas with an effective collection area of 300,000 square metres. This makes LOFAR one of the largest single connected radio telescopes in existence. LOFAR is also a proof on concept for its eventual successor, the Square Kilometre Array to be built jointly in South Africa, Australia & New Zealand. Key projects involving LOFAR include extragalactic surveys, research into the nature of cosmic rays and studies of space weather.

The Pierre Auger Observatory: A cosmic ray observatory located in Malargüe, Argentina, the Pierre Auger Observatory was completed in 2008. This unique instrument consists of 1600 water tank Cherenkov radiation detectors spaced out over 3,000 square kilometres along with four complimenting fluorescence detectors.  Results from Pierre Auger have thus far included discovery of a possible link between some of the highest energy events observed and active galactic nuclei.

GONG: Keeping an eye on the Sun is the goal of the Global Oscillation Network Group, a worldwide network of six solar telescopes. Established from an initial survey of 15 sites in 1991, GONG provides real-time data that compliments space-based efforts to monitor the Sun by the SDO, SHO, and STEREO A & B spacecraft. GONG scientists can even monitor the solar farside by use of helioseismology!

The Allen Telescope Array: Located at Hat Creek 470 kilometres northeast of San Francisco, this array will eventually consist of 350 Gregorian focus radio antennas that will support SETI’s search for extraterrestrial intelligence. 42 antennas were made operational in 2007, and a 2011 budget shortfall put the status of the array in limbo until a preliminary financing goal of \$200,000 was met in August 2011.

The YBJ Cosmic Ray Observatory: Located high on the Tibetan plateau, Yangbajing International Cosmic Ray Observatory is a joint Japanese-Chinese effort. Much like Pierre-Auger, the YBJ Cosmic Ray Observatory employs scintillators spread out along with high speed cameras to watch for cosmic ray interactions. YBJ observes the sky in cosmic rays continuously and has captured sources from the Crab nebula pulsar and found a correlation between solar & interplanetary magnetic fields and the Sun’s own “cosmic ray shadow”. The KOSMA 3-metre radio telescope is also being moved from Switzerland to the YBJ observatory in Tibet.

Pulsar Jackpot Scours Old Data for New Discoveries

Chalk another one up for Citizen Science.  Earlier this month, researchers announced the discovery of 24 new pulsars. To date, thousands of pulsars have been discovered, but what’s truly fascinating about this month’s discovery is that came from culling through old data using a new method.

A pulsar is a dense, highly magnetized, swiftly rotating remnant of a supernova explosion. Pulsars where first discovered by Jocelyn Bell Burnell and Antony Hewish in 1967. The discovery of a precisely timed radio beacon initially suggested to some that they were the product of an artificial intelligence. In fact, for a very brief time, pulsars were known as LGM’s, for “Little Green Men.” Today, we know that pulsars are the product of the natural death of massive stars.

The data set used for the discovery comes from the Parkes 64-metre radio observatory based out of New South Wales, Australia. The installation was the first to receive telemetry from the Apollo 11 astronauts on the Moon and was made famous in the movie The Dish.  The Parkes Multi-Beam Pulsar Survey (PMPS) was conducted in the late 1990’s, making thousands of 35-minute recordings across the plane of the Milky Way galaxy. This survey turned up over 800 pulsars and generated 4 terabytes of data. (Just think of how large 4 terabytes was in the 90’s!)

The nature of these discoveries presented theoretical astrophysicists with a dilemma. Namely, the number of short period and binary pulsars was lower than expected. Clearly, there were more pulsars in the data waiting to be found.

Enter Citizen Science. Using a program known as Einstein@Home, researchers were able to sift though the recordings using innovative modeling techniques to tease out 24 new pulsars from the data.

“The method… is only possible with the computing resources provided by Einstein@Home” Benjamin Knispel of the Max Planck Institute for Gravitational Physics told the MIT Technology Review in a recent interview. The study utilized over 17,000 CPU core years to complete.

Einstein@Home is a program uniquely adapted to accomplish this feat. Begun in 2005, Einstein@Home is a distributed computing project which utilizes computing power while machines are idling to search through downloaded data packets. Similar to the original distributed computing program SETI@Home which searches for extraterrestrial signals, Einstein@Home culls through data from the LIGO (Laser Interferometer Gravitational Wave Observatory) looking for gravity waves. In 2009, the Einstein@Home survey was expanded to include radio astronomy data from the Arecibo radio telescope and later the Parkes observatory.

Among the discoveries were some rare finds. For example, PSR J1748-3009 Has the highest known dispersion measure of any millisecond pulsar (The dispersion measure is the density of free electrons observed moving towards the viewer). Another find, J1750-2531 is thought to belong to a class of intermediate-mass binary pulsars. 6 of the 24 pulsars discovered were part of binary systems.

These discoveries also have implications for the ongoing hunt for gravity waves by such projects as LIGO. Specifically, a through census of binary pulsars in the galaxy will give scientists a model for the predicted rate of binary pulsar mergers. Unlike radio surveys, LIGO seeks to detect these events via the copious amount of gravity waves such mergers should generate. Begun in 2002, LIGO consists of two gravity wave observatories, one in Hanford Washington and one in Livingston Louisiana just outside of Baton Rouge. Each LIGO detector consists of two 2 kilometre Fabry-Pérot arms in an “L” configuration which allow for ultra-precise measurements of a 200 watt laser beam shot through them.  Two detectors are required to pin-point the direction of an incoming gravity wave on the celestial sphere. You can see the orientation of the “L’s” on the display on the Einstein@Home screensaver. Two geographically separate detectors are also required to rule out local interference. A gravity wave from a galactic source would ripple straight through the Earth.

Such a movement would be tiny, on the order of 1/1,000th the diameter of a proton, unnoticed by all except the LIGO detectors. To date, LIGO has yet to detect gravity waves, although there have been some false alarms. Scientists regularly interject test signals into the data to see if system catches them. The lack of detection of gravity waves by LIGO has put some constraints on certain events. For example, LIGO reported a non-detection of gravity waves during the February 2007 short gamma-ray burst event GRB 070201. The event arrived from the direction of the Andromeda Galaxy, and thus was thought to have been relatively nearby in the universe. Such bursts are thought to be caused by neutron star and/or black holes mergers. The lack of detection by LIGO suggests a more distant event. LIGO should be able to detect a gravitational wave event out to 70 million light years, and Advanced LIGO (AdLIGO) is set to go online in 2014 and will increase its sensitivity tenfold.

Knowledge of where these potential pulsar mergers are by such discoveries as the Parkes radio survey will also give LIGO researchers clues of targets to focus on. “The search for pulsars isn’t easy, especially for these “quiet” ones that aren’t doing the equivalent of “screaming” for our attention,” Says LIGO Livingston Data Analysis and EPO Scientist Amber Stuver. The LIGO consortium developed the data analysis technique used by Einstein@Home. The direct detection of gravitational waves by LIGO or AdLIGO would be an announcement perhaps on par with CERN’s discovery of the Higgs Boson last year. This would also open up a whole new field of gravitational wave astronomy and perhaps give new stimulus to the European Space Agencies’ proposed Laser Interferometer Space Antenna (LISA) space-based gravity wave detector. Congrats to the team at Parkes on their discovery… perhaps we’ll have the first gravity wave detection announcement out of LIGO as well in years to come!

-Read the original paper on the discovery of 24 new pulsars here.

-Amber Stuver blogs about Einstein@Home & the spin-off applications of gravity wave technology at Living LIGO.

-Parkes radio telescope image is copyrighted and used with the permission of CSIRO Operations Scientist John Sarkissian.

-For a fascinating read on the hunt for gravity waves, check out Gravity’s Ghost.