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Merging Black Holes May Be Detectable by 2017


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LIGO gravitational wave observatory

The LIGO site in Hanford, Wash., with one of the two interferometer arms stretching into the distance. Credit: LIGO Laboratory

In his indispensable 1994 book Black Holes and Time Warps, physicist Kip Thorne wrote of the tantalizing discoveries that awaited in the coming century. In particular, the existence of gravitational waves—ripples in the very fabric of space and time caused by the motion, and especially the collision, of extremely massive objects—might soon graduate from theoretical prediction to known fact. And those waves could carry all-important hints about their origins. “Gravitational-wave detectors will soon bring us observational maps of black holes, and the symphonic sounds of black holes colliding—symphonies filled with rich, new information about how warped spacetime behaves when wildly vibrating,” Thorne wrote.

That time is nearly upon us, he now believes. The California Institute of Technology theorist writes in the August 3 issue of Science that in five years’ time, ongoing upgrades to the world’s leading gravitational-wave detectors will make those instruments sensitive enough to detect gravitational waves from colliding and merging black holes, which would provide yet another major experimental confirmation of Albert Einstein’s theory of general relativity. The detection would also open up a new regime for studying black holes, those cosmic gluttons whose gravitational pull is so strong that it forms a one-way funnel into the black hole’s maw. Even light cannot escape once it has crossed the event horizon, a black hole’s point of no return.

As of now, astrophysicists can only infer the presence of a black hole by monitoring the environs around the putative object. In the case of Sagittarius A*, in the center of our own Milky Way galaxy, for instance, astronomers can see flares of radiation emanating from the black hole’s location, caused by infalling material heating up and radiating outside the event horizon. Stars at the galactic center betray the presence of Sagittarius A* as well—their orbits point to the existence of a nearby compact object with the mass of four million suns.

The strong gravitational wave signature expected from merging black holes, in comparison, would carry a wealth of information both about the objects involved and about their cataclysmic interaction—the geometrodynamics, or “stormy behavior,” as Thorne puts it, of black holes and other extremely curved regions of spacetime. “The gravitational waveforms from a merging black-hole binary carry detailed information about the initial black holes (their masses, spins, and orbit), the final merged black hole (its mass and spin), and the geometrodynamics of the merger,” he writes in Science.

Two major gravitational-wave detector projects have been on the lookout for these spacetime ripples, but so far the search has not produced any results. Both the LIGO and Virgo observatories are L-shaped instruments with extremely long arms—four kilometers for the two LIGO facilities in Washington and Louisiana, and three kilometers for Italy’s Virgo. They rely on long-baseline interferometry, firing lasers down the perpendicular arms to see if one direction has been stretched or compressed relative to the other by a passing gravitational wave.

The fact that neither has detected gravitational waves is no great cause for concern, writes Thorne, who helped spearhead the development of LIGO in the 1980s. “The initial LIGO and Virgo interferometers (with sensitivities at which it is plausible but not likely to see waves) were designed to give the experimental teams enough experience to perfect the techniques and design for advanced interferometers—that will have sensitivities at which they are likely to see lots of waves,” he writes. “The advanced LIGO and advanced Virgo interferometers are now being installed and by 2017 should reach sensitivities at which black-hole mergers are observed.” Sounds like the race is on to detect gravitational waves, one of the biggest prizes in physics.

About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. jtdwyer 7:20 am 08/3/2012

    I’m not a physicist, just a retired information systems analyst. However, from my perspective general relativity seems to very successfully describe the effects of gravity in terms of an imaginary system of spacetime coordinates without describing any physical basis for the existence of any geometry of spacetime.

    Given the success of general relativity, there’s little reason to argue that the described geometric properties of some undescribed physical element are fundamentally incorrect, but the theory in inherently incomplete.

    Without understanding the physical elements that underlie GR’s described geometry of spacetime, it seems impossible to describe any properties of hypothesized gravitational waves in anything but imaginary imaginary terms.

    While I can’t reasonably asses it, Superfluid vacuum theory (or the theory of Bose–Einstein condensate (BEC) vacuum) is an attempt to describe the physical properties of spacetime that produce the characteristics required of it in GR.

    Please see
    http://en.wikipedia.org/wiki/Superfluid_vacuum
    - particularly
    http://en.wikipedia.org/wiki/Superfluid_vacuum#Curved_space-time
    http://en.wikipedia.org/wiki/Superfluid_vacuum#Gravitational_waves_and_gravitons
    “Though, SVT does not a priori forbid an existence of the non-localized wave-like excitations of the superfluid background which might be responsible for the astrophysical phenomena which are currently being attributed to gravitational waves, such as the Hulse-Taylor binary. However, such excitations cannot be correctly described within the framework of a fully relativistic theory.”

    While those who have complete faith in general relativity may by extension presume the existence of gravitational waves, to the extent that the successfully presumed spacetime geometry of GR has no physical foundation (and is therefore incomplete), there remains the possibility that gravitational waves may not exist.

    Link to this
  2. 2. jtdwyer 7:45 am 08/3/2012

    BTW, the August 3 issue of Science is a special issue on Black Holes. The introduction with links to all related content can be freely accessed at
    http://www.sciencemag.org/content/337/6094/535.full

    While the articles are unfortunately pay-per-view, abstracts, supplementary data and podcast interviews are freely accessible. For example, the tempting cover story can be found at:
    http://www.sciencemag.org/content/337/6094/497.full

    Link to this
  3. 3. hybrid 6:18 am 08/4/2012

    Why are they so convinced about the existence of gravitational waves since no trace has appeared yet, despite 32 years of successive experiments with more and more expensive equipment. Every failure is attributed to the apparatus not being sensitive enough. If nothing can escape the black hole why should it make waves? Are we looking at Kip Thorne’s cash cow?

    Link to this
  4. 4. Jonathan Reynolds 4:12 pm 08/4/2012

    As a rank amateur with a long interest in these questions, wouldn’t the confirmation of gravity waves mean there are gravity particles – gravitons?

    Link to this
  5. 5. jtdwyer 8:07 am 08/10/2012

    Jonathan Reynolds – I’m no physicist, but I think that gravitational waves are though to propagate as small fluctuations in the curvature of spacetime, not as waves propagating through a media of graviton particles (as in water waves).

    Also note that (counter-intuitively) the terms gravitational waves and gravity waves are not interchangeable, complicating things immensely: please see:
    http://en.wikipedia.org/wiki/Gravity_wave
    http://en.wikipedia.org/wiki/Gravitational_wave

    Also, it is thought that gravitational waves can propagate unimpeded through any gravitational field. I may be wrong, but it seems to me that spacetime being dynamically curved by the presence of some proximal mass (such as the Earth or Sun) would be less likely to propagate weak, gravitational waves arriving from some diminishingly great distance than would a weaker gravitational field. In that case, it might be very difficult to detect even once extraordinarily strong gravitational waves (i.e., from merging black holes) within the Earth’s gravitational field.

    Link to this

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