TRIANGULATION: An artist's impression of the LISA observatory as originally designed.
Image: NASA
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The Best Science Writing Online 2012
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Ripples in the fabric of spacetime regularly zip across the universe from titanic cosmic events, such as the mergers of supermassive black holes millions to billions of times the mass of the sun. These so-called gravitational waves ought to be ubiquitous but faint, and no experiment has yet registered the disturbance caused by a passing wave. The Laser Interferometer Space Antenna was supposed to do just that. The spaceborne observatory, also known as LISA, was to be a joint mission between NASA and the European Space Agency (ESA) to detect gravitational waves and give scientists a whole new window through which to look on the universe and understand its underpinnings.
Cost overruns concerning the next-generation James Webb Space Telescope apparently helped doom the ambitious joint mission—NASA and ESA dissolved their decadelong LISA partnership in March 2011. Reports of its death may have been greatly exaggerated, however, as researchers are still fighting hard toward launch. Even scaled-back versions of the project might still have a good chance of making revolutionary discoveries, the scientists maintain.
As originally planned, LISA would have involved three identical spacecraft trailing Earth in an orbit around the sun. Each spacecraft would have targeted the other two with lasers, forming a triangle of light with sides five million kilometers long. Over the five-year mission, the laser beams would have helped detect subtle disturbances in the arrangement of the spacecraft caused by the passage of gravitational waves.
Once NASA and ESA stopped working together on LISA, the project fell off the radar. "It's probably fair to say that many people, even astronomers, think LISA was canceled," says astrophysicist Robin Stebbins of the NASA Goddard Space Flight Center, who is heading the agency's gravitational-wave mission concept study.
But each agency is actually investigating going it alone with cheaper, stripped-down missions. "The partnership may be dead, but the concept and the community and the enthusiasm is not dead," says astrophysicist Tyson Littenberg of the University of Maryland, College Park.
Moreover, improvements in our understanding of how galaxies and black holes evolve suggest these successors might only see a bit less than LISA. "In an extremely short timescale, the LISA community has really come together with a lot of studies as to what we might be able to accomplish at lower cost," says astrophysicist Sean McWilliams of Princeton University. "No one's giving up."
One scenario would scale down LISA's triangle, reducing each side to only one million kilometers in length. A smaller triangle means less propellant to set the satellites in place, saving money. Such a move would change the kinds of gravitational waves the satellites could detect—smaller sides mean sensitivity only to smaller wavelengths from smaller objects.
A downsized triangle would still be sensitive to waves from intermediate-mass black holes—those 10,000 to 100,000 times the sun's mass—which are the building blocks of the supermassive black holes seen at the heart of virtually every large galaxy. Recent astrophysics research suggests most black hole mergers involve those of intermediate-mass. So a smaller triangle could shed much light on the mysteries of how galaxies and supermassive black holes formed, according to findings McWilliams detailed January 9 at a meeting of the American Astronomical Society in Austin, Texas.
A more drastic change to the mission's architecture would be to cut off one of the legs, changing the formation from a triangle to a V-shape. Such a mission could still detect gravitational waves, but without the extra information that a third leg would provide the observatory would be significantly worse at pinpointing the location of gravitational wave sources and determining their properties. One less leg means less hardware and thus smaller satellites, which can lead to cost savings with launch.
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36 Comments
Add CommentI can't do the math, but I suspect that gravity waves from distant events are unlikely to penetrate spacetime that is locally curved by strong gravitational fields. I think they'd be much more likely to be detected in intergalactic space...
Reply | Report Abuse | Link to thisAre gravity waves been proven to be ripples in space and not the force of more cosmic rays coming from the opposite direction?
Reply | Report Abuse | Link to thisBaldEgalitarian, I'm not sure I understand what you mean by "the force of more cosmic rays coming from the opposite direction", but gravitational waves are a completely different beast to cosmic radiation.
Reply | Report Abuse | Link to thisAnd as to whether they've been "proven" to be ripples in space: that's what they are by definition -- theoretical consequences of Einstein's theory of gravity. The question is rather whether these theoretical consequences really occur in nature. We have very strong -indirect- evidence that they do; what we're looking for is -direct- evidence.
jtdwyer, I'm not sure "penetrate"is the right word to use; GWs are -very- weakly interacting -- they essentially penetrate everything. However, like light, their path will be -bent- by regions of high curvature (due to large concentrations of matter). Relatively speaking, the Earth sits in a region of pretty low curvature. Perhaps intergalactic space would be better still, but it would take much longer to get anything out there.
Reply | Report Abuse | Link to thisIf wondered if gravity might be a push instead of a pull, for example, the radiation (energymass) hitting the moon would re-radiate spherically, which might result in less radiation hitting the earth from what was coming directly from behind the moon. Sort of like being behind a rock in a river.
Reply | Report Abuse | Link to thisGravity is unipolar. The wave will be seen as a small periodic pull. It will coast right through stronger gravitational fields like a ripple runs over a larger wave, but the ripple will be bent by the local distortion of masses in its way, such as lensing or diffraction.
Reply | Report Abuse | Link to thisIt certainly seems that gravity waves could be detected in close proximity to massive objects in decaying orbits - the question is: why have they never been directly detected here on Earth? I'm sure that the existing Earth bound detectors were built (at great expense) with the expectation that they would be successful. However, if I were to build a radio telescope I would not chose a site near near equipment generating a strong magnetic field...
Reply | Report Abuse | Link to thisGravitational waves don't penetrate through spacetime. They are actually ripples in spacetime itself. They are propagated by spacetime and follow its curvature, whatever that may be. Also, the earth's (and for that matter the sun's in the region of the earth) gravitational fields are quite weak in absolute terms, producing quite minimal spacetime curvature compared to really massive objects like black holes.
Reply | Report Abuse | Link to thisYes, they are actually ripples in spacetime itself.
Reply | Report Abuse | Link to thisjtdwyer, mankind has no way of putting anything much farther away. Voyager 1 is only just out of the solar system, and it was launched in 1977. Getting to the LISA orbit, and maintaining a communications link, will be difficult and expensive enough.
Reply | Report Abuse | Link to thisEarth-bound detectors have been built (LIGO in the U.S., Virgo & GEO in Europe) and are running. But there are two problems with GW detection (assuming you have no problem with the underlying theory): (1) GWs are fantastically weak; (2) the population of GW sources (neutron-star binaries, low-mass black holes, etc.) is very uncertain. So if we haven't seen anything there yet, one explanation is that GWs don't exist (at least in the way we expect from general relativity); another is that there aren't enough visible, strong sources in the volume of space the detectors can reach. LIGO's being upgraded now, and the result should be greater sensitivity (i.e. less instrumental and environmental noise), which translates to a much larger volume of detectable space.
Like the amplitude of water waves propagating outward on the surface of a pond, the amplitude of gravitational waves decrease the farther away they travel from their source. So the reason gravitational waves have not yet been detected is that we are (fortunately) very far from their likely sources. (We wouldn't want to be close!)
Reply | Report Abuse | Link to thisNot true. Gravity transcends space time, at least according to String Theory, but even if not local curvatures are not severe enough to pose a real problem in detection. Around a black hole, yes maybe, but not around our local galactic cluster and space in general.
Reply | Report Abuse | Link to thisI was studying physics at the University of MD circa 1970 when Joseph Weber was building large suspended metal cylinders to detect gravity waves. The detection effort sure has moved a long way whereas the results apparently have not.
Reply | Report Abuse | Link to thisHowever, the gravitational effects produced by distant black holes are not detectable within Earth's locally much stronger gravitational field. The local energy density of Earth's curved spacetime might prevent any induced oscillation by the now exceedingly low amplitude gravitational waves produced by distant events, somewhat analogous to the inability of light to propagate through dense opaque matter.
Reply | Report Abuse | Link to thisWhile there's been a great deal of research into the theoretical local generation of gravity waves, the conditions necessary for their remote detection seem to be less well known.
Weisberg & Taylor, (2004), "Relativistic Binary Pulsar B1913+16: Thirty Years of Observations and Analysis", http://arxiv.org/abs/astro-ph/0407149
states:
"Accuracy of the test for gravitational radiation damping is now dominated by the uncertainty in the galactic acceleration term. Work now underway should lead to improved accuracy of the pulsar proper motion, and the Sun’s galactocentric distance may be better known in the future. However, we see little prospect for a significant improvement in knowledge of the pulsar distance. Consequently, it seems unlikely that this test of relativistic gravity will be improved significantly in the foreseeable future."
[I hadn't seen your second response earlier]
Reply | Report Abuse | Link to thisI agree - we probably wouldn't be close for very long!
Weber's efforts were just as effective as more recent efforts - and I suspect much less expensive!
Reply | Report Abuse | Link to thisLike Einstein I doubt that anyone really believes in probability waves and an assumed a priori spacetime continuum is just as elusive. Light transmits as a discrete series of pulses consistent with the Planck quantum of action. This is powerful evidence that matter, space and time are discontinuous, synchronously oscillating between still atomic space frames and timeless and spatially indeterminate quantum frames. Atoms are projected as a very rapid series of still space frames linked up by light in a cosmic movie.
Reply | Report Abuse | Link to thisAtoms are particles and waves at the same time because one oscillation defines one primary interval of time. Light can only travel a limited distance with respect to each atom in each space frame so its speed is universal. It defines external linear space with respect to the internal spherical space of an atom. There are no other universal measuring rods out there. All relative particulate motion occurs as relative quantum jumps between successive space frames so position and momentum cannot possibly be known at the same time. Space and time are quantized.
The quantum jumps introduce relative space frame skipping consistent with relativity effects. On a cosmic scale where cyclic motions predominate, the integrated fabric of space-time becomes warped similar to the assumed curvatures of Einstein’s continuum but with different cosmological implications. This requires that there are both universal and particular aspects to atomic matter as indicated by de Broglie’s pilot wave and Bohm’s quantum potential.
In this scenario Gravity is implicitly associated with the primary projection of matter, space frame by space frame from a unified quantum energy field. It is not transmitted as a wave motion via a spacetime continuum, because there is no continuum. It is not transmitted via space and time at all, because it is associated with defining the nature of space and time. See http://www.cosmic-mindreach.com/Gravity.html.
If gravity waves are a distortion of space-time itself how can detectors which are also a part of the distorted space-time work since they and all the measuring equipment facilities and media will distort simultaneously and identically. Everything would be transparent to gravity waves, and so unable to register them.
Reply | Report Abuse | Link to thisThe basic idea of these types of detectors is that gravity waves would minutely increase the distance between split laser beams and their target reflectors. Depending on the specific detector design, the change in beam propagation distance can produce a detectable interference between beams. In the case of a split beam separated by a 90 degree angle, the propagation time of one or both of the two beams will be affected varyingly depending on the gravity wave source direction and polarization.
Reply | Report Abuse | Link to thisPlease see: http://en.wikipedia.org/wiki/LIGO#Operation
The reference you provide is by the winners of the 1993 Nobel Prize in Physics, who discovered a new type of binary pulsar with a decaying orbit which could be explained by the radiation of gravitational energy in the form of gravitational waves. This is the strongest evidence to date for the existence of gravitational waves, albeit indirect. The reference does not concern the difficulty in detecting gravitational waves in the Earth's gravitational field, but rather the difficulty in placing tighter constraints on the measured rate of change of the orbital period, which currently agrees with the predictions of general relativity to within about 0.2%.
Reply | Report Abuse | Link to thisI don't think there is any intrinsic difficulty in detecting gravitational waves on or near Earth due to the Earth's gravitational field. The Earth is not radiating its own gravitational energy, and any gravitational waves arriving at Earth would simply follow the Earth-induced curvature of spacetime to the gravitational wave detector.
I quoted the paper presenting the first indirect evidence of gravitational waves because it described the unknown factors effecting the detection of gravitational waves on Earth, which includes precise relativistic parameters describing the emitter's positional relation to the Earth. I think this includes the relativistic effects of Earth's gravity. Again, the paper states: "However, we see little prospect for a significant improvement in knowledge of the pulsar distance. Consequently, it seems unlikely that this test of relativistic gravity will be improved significantly in the foreseeable future."
Reply | Report Abuse | Link to thisApparently few others think the Earth's field should interfere with the propagation of gravitational waves and their detection on Earth, but then they are not being detected, are they?
I'm no physicist, but in simple terms I can comprehend, gravitational waves carry oscillating energy, momentum and angular momentum away from a gravitational system. While the effects of gravitation are explained in general relativity in terms of spacetime curvature, the curvature of spacetime in turn imparts primarily energy and momentum to material objects. That is why the Earth's gravitation holds us down to its surface - it imparts momentum generally directed to the center of mass.
So, following this simple line of reasoning, I suspect that the directed momentum imparted by Earth's gravity may dampen the exceedingly low amplitude oscillating, differentially directed momentum imparted by gravitational waves. Conversely, I suspect that gravitational waves would be best detected in a location that is not effected by strong gravitational fields.
The Earth would not be radiating oscillating gravitational waves, but it undoubtedly produces significant gravitational effects, especially at its surface.
Please see: http://en.wikipedia.org/wiki/Gravitational_waves#Energy.2C_momentum.2C_and_angular_momentum_carried_by_gravitational_waves
Put another (simple) way, at least some of the exceedingly low amplitude oscillating momentum of gravitational waves would be imparted to and absorbed by the relatively dense molecules in Earth's atmosphere prior to their reaching any Earth-bound detector, even if Earth's gravitation had no other effect on them.
Reply | Report Abuse | Link to thisIt seems incredible that billions should be invested in experiments to establish the existence of gravity waves when basic contradictions remain in the spacetime continuum assumptions of General Relativity. There is no physical evidence whatever that such a thing as a continuum exists. In 1858 Richard Dedekind showed basic contradictions in the notion of continuous space.
Reply | Report Abuse | Link to thisGeneral Relativity assumes that inertial mass and gravitational mass are identical but inertial velocity is clearly distinct from gravitational attraction as evidenced by Foucaults pendulum, the gyro compass and related phenomena that are independent of the earth’s rotation. Relativity has no explanation for why these phenomena synchronously relate to the relatively fixed position of stars thousands of light years distant.
Where are the universal measuring rods of the spacetime continuum of general relativity? Are there clocks and yardsticks out there somewhere that we cannot see? Can measurements of space and time derived a posterior from physical observations on Earth be raised to a priori status to explain their own creation? This obvious bootstrapping is known and ignored.
Why is the speed of light universally constant? Why are atomic particles waves and particles at the same time when they are in rapid relative motion? There is no evidence of this when they are stationary.
Whatever gravity wave experiments do or most likely do not turn up these and other fundamental questions about GR will remain. They will prove nothing. Late in life Einstein himself doubted that physics could be based on the continuous field concept and repeatedly pointed out that singularities are not consistent with the assumptions of General Relativity. This problem also relates to extremely dense rapidly rotating pulsars. In a synchronous discontinuous universe these nagging questions find natural explanations. There is no mathematician out there with yard sticks calculating the orbs within orbs of celestial dynamics. It works beautifully all by itself. See http://www.cosmic-mindreach.com/Gravity.html.
I don't see anything in the uncertainties the paper mentions which have to do with the earth's gravitational field. The uncertainties are in the pulsar distance and in "poorly known galactic constants".
Reply | Report Abuse | Link to thisThe reason why there has not yet been direct evidence detected of gravitational waves is their weakness due to the great distance of earth from putative sources, rather than confounding effects of Earth's gravitational field.
Renew and broaden the partnership to include not only ESA and NASA, but also Roskosmos, JAXA, and, not least, the CNSA. This is an endeavour in the interests of all mankind - all mankind should be invited to participate in it....
Reply | Report Abuse | Link to thisHenri
I don't, but perhaps you fully understand the relativistic effects Earth's gravitation, rotation, orbit around the Sun and the Solar system's rotation within the galaxy in evaluating gravity wave test results? Why would the now Nobel laureate authors, Weisberg & Taylor, be concerned with a "galactic acceleration term" and Sun's "galactocentric distance" in the detection of gravity waves? Perhaps these factors are important in determining the distance of the gravity wave emission source, which they apparently think will prevent necessary improvements in "this test of relativistic gravity".
Reply | Report Abuse | Link to thisIf I've misunderstood, please explain my error to me - I'd be most appreciative!
Again, I refer to this source as evidence that all of the factors contributing to the evaluation of potential gravity waves may not be fully known. Obviously, a large investment in facilities, equipment and highly qualified staff has already been made - without successful results. This fact alone is evidence that there are unknown or poorly understood factors involved in gravity wave detection.
If the "reason why there has not yet been direct evidence detected of gravitational waves is their weakness due to the great distance of earth from putative sources" as you say, why was that not fully understood before making such large investments in now unsuccessful detectors? Perhaps there were some unknown or poorly understood contributing factors?
Obviously in the paper on that binary they're referring to the accuracy of their measurements - and consequent indirect measurements of gravitational waves - where the distance and change in relative velocities between the Solar system and the binary in question put limits on the accuracy of their measurements.
Reply | Report Abuse | Link to thisImagine you're trying to measure how high a distant mountain is. You measure with great accuracy the angle between level and the top of the mountain, and then you measure the angle extended by a measuring stick on top of the mountain. The ratio between the two will give you the height - but the accuracy is limited by how well you can measure the exact angle of the small stick on the top, whether it really is on the top, or on a ridge extending down towards you, and how well you know the length and orientation of that stick, not by how well you can measure the angle between the top and level.
For orbits, you need to measure the period - with very great accuracy! But the measured period depends not only on the real local periodicity of the pulsar, but also on the Doppler shift introduced by the relative motions of you and the pulsar. When your results are limited by the accuracy with which you can determine those motions - accelerations in particular - you can't improve on your experiment by measuring the periods more accurately. That's the problem stated in the referenced paper. They don't know exactly where the pulsar is and how it changes velocity relative to the Solar system. This change directly influences the observed changes in periodicity, and puts limits on the accuracy they can achieve in this indirect measurement of gravitational radiation.
Now simply put, gravitational waves are to gravitational fields, what light (electromagnetic waves) is to electric and magnetic fields.
Even if we were to suppose that the Earths gravitational field was locally very strong, which it isn't, that's only relevant to the design of the experiment (it's hard to make a very long baseline on the surface). It won't affect gravitational waves any more than a bar magnet distorts light passing by, and as long as the Earth doesn't collapse into a singularity while the experiment is running, the pretty much constant local gravitational field will have no effect on observations of gravitational waves. (Not taking any subtle GR effects into account, but they don't change the basic premise.)
Indeed the magnetic fields of the Earth and Sun don't prevent measurements of starlight either. Same thing.
jimfromcanada touches on a point which troubles some who thought they'd understood a little about GR: doesn't the gravitational wave not only produce a linear contraction but also a time dilation, and the two will cancel out, making the wave undetectable?
Reply | Report Abuse | Link to thisI remember Webb's aluminum cylinder and subsequent much more refined attempts (LIGO), all returning zero up till now. Can a point be reached where the failure to detect these waves becomes embarrassing?
Thanks very much for your kindly considered explanation of the binary pulsar indirect gravitational wave detection issue. I understand now that the issue they raise applies only to the indirect detection of gravity waves.
Reply | Report Abuse | Link to thisStating that the Earth's gravitational field is not very strong requires a standard of comparison: any gravitational waves that have reached the Earth are apparently orders of magnitude weaker than the Earth's gravitational field.
As I understand, while bar magnets may not deflect light, even intense laser beams can be deflected by stronger magnetic fields.
While it's expected that gravitational waves can pass through interstellar dust, for example, without significant dispersal or energy absorption, I suspect this is the case for denser matter. While the wind can easily produce waves in a puddle of water, it has very little effect on a denser puddle of mud.
I hope there is something about gravitational wave detection that is not yet fully understood, otherwise I'd have to think that the whatever millions of dollars already invested in detectors has been knowingly wasted.
While any gravitational waves affecting laser interferometer detector equipment would change the reflected distance traversed by the laser beam, wouldn't the time dilation not effect the beam traversal time (at the speed of light over varying distances)?
Reply | Report Abuse | Link to thisAt the very least, existing gravity wave detectors seem to have been knowingly built without sufficient sensitivity to detect any known potential gravity wave source. Personally, I'd like a refund...
jtdwyer: LIGO is apparently an excellent detector of traffic moving some miles off....there's something for your bucks!
Reply | Report Abuse | Link to thisThe paper you cite doesn't have anything to do with direct detection of gravitational waves. The authors did not win their Nobel Prize for directly detecting gravitational waves. They received it for discovering a new type of binary pulsar, i.e., two objects each with about the mass of the sun (probably neutron stars) orbiting each other at a distance of about that of the earth to the moon. By making precision measurements they showed that the speed of the mutual orbiting is gradually increasing, at a rate consistent to within about 0.2% of what would be expected if they were radiating away energy in the form of gravitational waves. The waves were inferred, not directly detected.
Reply | Report Abuse | Link to thisThe direct gravitational wave detectors that have so far been built were recognized by their builders as probably insufficient to detect gravitational waves unless from an extremely strong source fairly close (in astronomical terms) to the earth. They were basically hoping for a minor miracle: for example, two hitherto unsuspected black holes orbiting each other which merged while the detector was up and running. Even if they couldn't detect any waves, building them was a useful exercise in learning how to build these types of instruments, and in putting lower bound constraints on what was out there to find.
Reply | Report Abuse | Link to thisThanks - that's what I said in comment #29.
Reply | Report Abuse | Link to thisTwo relevant points: Scientists have determined that gravity waves are not evident in the massive Crab nebula pulsar.
Reply | Report Abuse | Link to thisTwo objects with massive internal magnetism under a light magnetic shell exist: SGR 0418 and Swift J 1822.3-1606. Swift is claimed to evidence a slowing rotation but not SGR 0418 monitored for over a year with no evidence of slowing. One might well suspect that magnetism trumps gravity.
The Algol system: ancient records show careful record keeping indicated a change in orbital time. Has this been claimed to be evidence of gravity waves? Of course, it being a 3 star system changes things perhaps in the way Centauri A & B with the inclusion of Proxima might.
It's difficult to assess what you're referring to without specific references. As I understand, it is the rotation of proximal massive binaries that are thought to lose orbital momentum through rapidly rotating disparate distributions of mass, like a propeller in water, generating gravitational waves.
Reply | Report Abuse | Link to thisNehgative results (see below) from a search for gravitational waves that might speculatively be produced by possible magnetars undergoing possible surface fractures possibly producing non-spherically symmetrical effects can hardly be taken as a definitive evidence that gravitational waves don't exist. Any gravitational waves that might be produced would seem to be miniscule in comparison to those continuously produced by rapidly rotating binary masses.
As for evidence that "magnetism trumps gravity" there seems to be too many presumptions being made to determine what might be occurring, but in any event the effects of magnetism seems to have severely constrained distance limitations while gravity seems to have none.
The remote detection of gravitational waves would require that they produce oscillations in the local 'curvature' of spacetime produced by Earth's gravitational field, for example, and all other intervening gravitational fields. It would seem to me that producing those oscillations of spacetime curvature must also diminish the magnitude of gravitational waves, eventually diminishing their effects and making their detection ever more difficult.
"Soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are thought to be magnetars: neutron stars powered by extreme magnetic fields. These rare objects are characterized by repeated and sometimes spectacular gamma-ray bursts. The burst mechanism might involve crustal fractures and excitation of non-radial modes which would emit gravitational waves"
"SEARCH FOR GRAVITATIONAL WAVE BURSTS FROM SIX MAGNETARS", doi:10.1088/2041-8205/734/2/L35, http://iopscience.iop.org/2041-8205/734/2/L35/pdf/2041-8205_734_2_L35.pdf