Updating the Gravitational Wave Hunt

by Paul Gilster on November 16, 2011

The Laser Interferometer Space Antenna mission (LISA), slated for launch later this decade, will go about testing one of Einstein’s key predictions, that gravitational waves should emanate from exotic objects like black holes. Detectors like the Laser Interferometer Gravitational Wave Observatory (LIGO) have operated on Earth’s surface but are subject to seismic noise that disturbs the observations in some of the key frequency ranges. The hope is that the space-based LISA will be able to go after the low-frequency gravitational wave spectrum to detect such things as the collision of black holes, the merger of galaxies or the interactions of neutron star binary systems.

The good news out of the European Space Agency is that the LISA Pathfinder precursor mission, planned for a 2014 launch, is showing the accuracy needed to demonstrate what the more sophisticated LISA mission should be able to do. LISA Pathfinder is a small spacecraft unable to make a direct detection of gravitational waves, but if all goes well it should be able to test whether objects moving through space without external influences trace out a slight curve — a geodesic — because of the effect of gravity upon spacetime. That would take us one step closer to the actual detection of gravitational waves using even more sensitive technologies in the full LISA mission.

Image: LISA Pathfinder will pave the way for the LISA mission by testing in flight the very concept of the gravitational wave detection. It will put two test masses in a near-perfect gravitational free-fall and control and measure their motion with unprecedented accuracy. This is achieved through state-of-the-art technology comprising the inertial sensors, the laser metrology system, the drag-free control system and an ultra-precise micro-propulsion system. Credit: Astrium.

The early results on LISA Pathfinder are promising. Detecting ripples in spacetime requires measurements with the accuracy of 10 billionths of a degree, yet thermal vacuum tests are revealing the spacecraft’s subsystems can produce this kind of performance. Under space-like test conditions with an almost complete spacecraft, LISA Pathfinder has been demonstrating a 2 picometer accuracy — a picometer is about a hundredth the size of an atom — and expectations are that once in space the observatory will be able to do even better.

Aboard the spacecraft will be two 4.6 cm3 test masses that will float within two chambers 35 cm apart in the core of the observatory. Their relative positions are to be tracked by a laser measuring system. Any gravitational disturbances should cause minuscule changes in their positions. Bengt Johlander, payload engineer for LISA Pathfinder, describes the system:

“We are seeking to keep the geodesic motion of our test masses as pure as possible. The masses are free to move – and if our experiment is set up correctly they should move together in concert – while the surrounding spacecraft is guided by their motion to follow in the same direction. It is impossible to have zero perturbations, but we have fixed a precise budget for disturbances we will stay within.”

The perturbations that must be avoided are vanishingly small, but any of them could compromise the experiment. They include solar radiation pressure, slight magnetic influences or vibrations from the body of the spacecraft and, remarkably, the internal gravitational pull of the spacecraft structure itself. LISA Pathfinder is to operate 1.5 million kilometers from the Earth at the L1 Lagrange point. The success of its systems paves the way for the later LISA mission, which will involve a trio of spacecraft flying 5 million kilometers part while linked by laser beams. ESA offers a useful backgrounder on LISA Pathfinder and the subsequent LISA mission. And be aware of this graduate level Web-based course on gravitational waves that Caltech makes available.

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Evan November 16, 2011 at 22:23

What if they don’t find gravitational waves? What does that mean about the nature of our universe?

Colin Weaver November 17, 2011 at 0:53

@Evan – someone left them out of the simulation?

Alex Tolley November 17, 2011 at 10:32

Shouldn’t LIGO confirm the presence of gravitational waves before we do LISA?
LIGO has been in operation long enough that if there were some hints of waves, they would have shown up. But AFAIK, the results have been negative.
Is the claim for doing LISA that LIGO is inadequate to detect gravitational waves?

Paul Gilster November 17, 2011 at 10:59

Evan, gravitational waves follow naturally from Einstein’s General Theory of Relativity, so it would be a surprise not to find them at some point. A failure would make us ask what aspects of our views of spacetime are wrong and make our understanding of gravity seem even more incomplete than it is.

Paul Gilster November 17, 2011 at 11:02

Alex Tolley writes:

Is the claim for doing LISA that LIGO is inadequate to detect gravitational waves?

That’s my understanding, Alex — LISA should be able to detect gravitational waves that LIGO is unable to trace because of seismic perturbations, etc. In other words, the benefits of a space-based mission come to the fore.

Erik Anderson November 17, 2011 at 11:35

“A failure would make us ask what aspects of our views of spacetime are wrong…”

Looking forward to that. It’ll be 1887 all over again.

Ron S November 17, 2011 at 15:08

These doubts about LIGO keep persisting though the fact that there is no confirmed detection of gravitational radiation is expected. In its configurations up to this time the probability of an event rising into positive SNR (signal to noise) range is small. LIGO is an active project where over time its sensitivity improves (lower noise floor). LIGO data is being actively mined to probe for “signatures” of expected astrophysical processes that have negative SNR. This isn’t easy. It is also why there has been so much activity in recent years to refine models of extreme astrophysical processes, to develop candidate signatures to search for. This is critical information when it comes to improving successful outcomes from negative-SNR signal processing (in general, not just gravitational radiation).

Also, LIGO is not about “proving” the existence of gravitational radiation and/or general relativity. Gravitational radiation has already been observed by its secondary effects. If LIGO (and LISA) fail to detect gravitational radiation it is more a problem for astronomers and astrophysicists rather than physicists since it would indicate that our understanding of astrophysical objects and their expected behavior and interactions are wrong.

To stretch an analogy, this would be a bit like building a sound recording device that fails to detect music. It is not that the theory of sound is wrong but rather the models predicting the type, quantity and distance of musical instruments are suspect.

LIGO/LISA are exciting because when they do make detections we will learn far more about those targeted astrophysical objects and processes. Either way, general relativity is looking pretty safe.

Paul Gilster November 17, 2011 at 15:22

Ron S writes:

If LIGO (and LISA) fail to detect gravitational radiation it is more a problem for astronomers and astrophysicists rather than physicists since it would indicate that our understanding of astrophysical objects and their expected behavior and interactions are wrong.

Excellent point and much more focused than what I said earlier. Thanks, Ron.

Paul Gilster November 17, 2011 at 15:30

Tatiana Covington passes along this interesting paper on superconductors and the gravitational wave hunt:

http://arxiv.org/abs/1111.2655

Keith Cooper November 17, 2011 at 17:33

@ Alex Tolley and Paul

LIGO and LISA will actually be complementary – they’ll detect gravitational waves of different frequencies. A space-based detector like LISA will be able to detect frequencies all the way down to a millihertz, produced by black hole mergers in the centres of colliding galaxies. Short frequencies have long wavelengths, which is why the three LISA satellites of the full mission will have a huge baseline, millions of kilometres long. Such a huge baseline isn’t suitable however for picking up high frequency, short wavelength gravitational waves from millisecond pulsars or binary neutron stars, for example. So that’s where the smaller scale ground-based detectors come in. LIGO is sensitive to gravitational waves of about 10Hz and upwards (terrestrial noise means they can’t get much lower than 10Hz on the ground), but this covers a region that space-based detectors will not. So in summary, space and ground based detectors are both required to cover the full spectrum of gravitational waves.

Ron S November 18, 2011 at 0:58

Paul: “Tatiana Covington passes along this interesting paper on superconductors and the gravitational wave hunt:
http://arxiv.org/abs/1111.2655

Interesting. I imagine there would have to be more technical challenges to overcome than they list in the paper. Even so it is an intriguing concept.

Erik Anderson November 18, 2011 at 12:29

“These doubts about LIGO keep persisting…”

And so do the apologies. ;-)

“Gravitational radiation has already been observed by its secondary effects.”

The loss of orbital momentum between two closely paired massive bodies (e.g., binary neutron stars) is indeed well-recorded. GR predicts this. So far so good. But does it necessarily follow that the lost energy is “radiated” in a form that is recoverable by a Michelson-Morley experimental apparatus? This is still questionable. I’m not even sure that Einstein himself held a consistent view on this point.

Perhaps a null-result from LISA will renew the debate.

Ron S November 18, 2011 at 15:28

“And so do the apologies. ;-)”

What apologies, Erik? All I see is the correction of misperceptions of the project.

“…does it necessarily follow that the lost energy is “radiated” in a form that is recoverable by a Michelson-Morley experimental apparatus?”

Um, so where do you think it went? Whatever you propose it had better provide a better prediction than general relativity.

“I’m not even sure that Einstein himself held a consistent view on this point.”

Even if true, which from my reading is that it is not true, what does it matter? Einstein was wrong about many things, even regarding the predictions of the theories he developed.

–Perhaps a null-result from LISA will renew the debate.–

There will be debate though perhaps not the one you would prefer.

jkittle November 19, 2011 at 16:27

I think lisa should fly precisely because it CAN fail to detect these waves. Erick is righ,t until they are detected GW should be held as a strong likelihood with a healthy dose of skepticism. And frankly the builders of LIGO we NOT expecting the null results they have found so far. The only reason Gravity Waves are part of a scientific theory is because the are testable and the theory falsifiable. On the other hand, if they are found then there is a whole new area of astronomy /cosmology to explore. My only strong prediction is that we wil get some real surprises.. the kind I live to see!
So we are now waiting on
1) another, more definitive neutrino experiment
2) finding the Higgs or excluding it
3) Gravity waves
4) tyche or no Tyche
5) a nearby brown dwarf ( or three or four!)

all in the next 2 to 5 years!

mortimerzilch December 31, 2011 at 22:57

12/31/11/ just checked in here to see if the results of Ligo were still null. It DOES remind one of the Mickelson-Morely null result – which showed something else instead, that light was a constant. Maybe the null Ligo result is showing us something else too…. hmm…like maybe gravity is not propagated through regular space, but through a sub-space of some kind. Dark Matter may be involved in this, and the whole thing is being overlooked. What about that experiment that showed gravity is not emitted curvilineraly as is light? The combined gravity of an solar eclipse was only strongest when the moon moved in front of the actual position of the sun rather than its apparent position. Ligo cannot detect frequencies or disturbances smaller than the Plank minimum, but that’s where gravity waves may propagate….

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