Gravity-Wave Hunters Find Nothing—and Make a Big Discovery

By Eliza Strickland | August 25, 2009 2:23 pm

LIGOPhysicists in Washington State and Louisiana recently spent two years hunting for the mysterious gravitational waves first predicted by Einstein, but detected nothing: zilch, zero, nada, nary a ripple. But that “null result” is itself of great value, researchers say, because it tells them where to look for the waves next. The findings are a nice reminder that scientific progress isn’t always about the dramatic discovery; it’s often a long, careful process of testing hypotheses, analyzing results, and heading back to the drawing board.

Einstein’s theory of general relativity states that every time mass accelerates — even when you rise up out of your chair — the curvature of space-time changes, and ripples are produced. However, the gravitational waves produced by one person are so small as to be negligible. The waves produced by large masses, though, such as the collision of two black holes or a large supernova explosion, could be large enough to be detected [SPACE.com].

Beyond those large disturbances, the universe is thought to be filled with small disturbances left over from the rapid period of expansion that followed the Big Bang, in a phenomenon known as the stochastic (meaning randomly distributed) gravitational wave background. If the expansion of the newborn universe had produced strong gravity waves, the physicists working at the two Laser Interferometer Gravitational-wave Observatory (LIGO) centers would have detected them. Since they found nothing, researchers have determined that smaller waves were produced, which they’ll need more sensitive instruments to detect. Says study coauthor Vuk Mandic: “We now know a bit more about parameters that describe the evolution of the universe when it was less than one minute old” [Sky & Telescope].

The waves are difficult to detect because the ripples are extraordinarily small. Over the distance between Earth and the star Alpha Centauri — 4.3 light years — a gravitational wave would warp space by as little as the thickness of a human hair [The Times].

To catch a glimpse of the waves, LIGO’s two detection centers (as well as a similar detector in Italy named Virgo) rely on laser interferometry. The technique involves splitting a single laser beam so that it travels down two paths, bounces off a mirror at the end of each path, and returns to a detector where the beams are recombined to form an interference pattern. Very small changes in the distance traveled by either beam will appear as relatively large changes in the interference pattern. A passing gravity wave will distort spacetime along each beam’s path, resulting in different path lengths, and therefore interference pattern changes [Ars Technica].

The study of LIGO’s experiments from 2005 to 2007, published in Nature, establishes a new upper limit to the gravitational wave background, and increases the excitement over the coming upgrades to both LIGO and Virgo, which will make the two detectors more than a thousand times more sensitive by 2014. That ought to be sensitive enough to prove Einstein’s predictions, says Mandic: “If Advanced LIGO doesn’t see gravitational waves I think people will be very surprised…. It is likely such a situation would require revision of General Relativity” [SPACE.com].

Related Content:
80beats: World’s Biggest Telescope Will Provide “Baby Pictures” of the Universe
80beats: Mysterious “Dark Flow” Is Tugging Galaxies Beyond the Universe’s Horizon
80beats: Neutron Stars Prove Einstein Right (Again)
DISCOVER: Works in Progress tells the story of LIGO
DISCOVER: Testing the Limits of Einstein’s Theories

Image: LIGO Laboratory

CATEGORIZED UNDER: Feature, Physics & Math
  • CW

    “Einstein’s theory of general relativity states that every time mass accelerates — even when you rise up out of your chair — the curvature of space-time changes, and ripples are produced.”

    This sounds very interesting, but what are the consequences of these ripples in the universe?

  • scribbler

    Personally, I think this type of looking is like trying to spot waves while submerged under the water…

    ;-)

  • http://discovermagazine.com Barry

    Where does this leave relatively if gravity waves are not detected? I have my doubts that they will.

  • alexander

    to CW: the relatively infantesimal size and mass(and therefore gravitational effects) of our human bodies would create space-time distortions so insignificant and so near to our physical mass that it would not even warrant study. the universe would not even notice.

  • Mark

    Gravity Waves do not exist.
    Gravitational waves must comply with all the rules governing behaviour of any physical phenomena called a wave.
    They must crate interference patterns in the Universe and therefore in some regions of the Universe they presence will be amplified (amplitude increase) and in the other diminished in the form of knots. It is debatable if this pattern will change in time due to the expansion(?) of the Universe, but this is not important.
    This pattern of their behaviour would destroy creation of a stable form of mass in the very early Universe (if one believes in the Big Bang) and it is disputable that the influence of gravity waves would allow to crate galaxies as we know them today.
    The gravity waves should be extremely common to observe due to countless sources of their creation and the subsequent multiple interference patterns, however to date nothing like that was observed.
    Furthermore, the interference patterns should slowly (due the length and the nature of “gravity waves”) influence pulsars very precise electromagnetic beam emissions and their directions as well as behaviour of quasars.
    Despite numerous observations dedicated solely to pulsars and quasars, nothing like that was observed, ever – the gravity waves are simply a myth.

  • linda

    Sounds like something from Marvin The Martian.

  • Varun

    To Mark:
    Gravitational Waves do comply with all the rules governing behavior of any physical phenomena called a wave.
    They do create interference patterns, and the reason this pattern does not destroy creation of a stable form of mass is because they simply expand and contract the space between and within masses (the molecules expand and contract within themselves, as do that atoms)
    Furthermore, gravitational waves propagate at the speed of light and therefore this expansion and contraction is only for fractions of seconds. In addition, gravitational waves tend to experience intense damping at a rate of 1/R where R is the distance away from the source.
    Lastly, the force of gravity does not interact in any way with the electromagnetic force except due to gravitational bending, and this is not a property of gravity but a property of the electromagnetic force. Therefore, the gravitational waves have absolutely no effect on electromagnetic beam emissions and their directions.
    Whether or not they exist is in debate, but your reasons are inaccurate.

  • Tom

    Actually, the change in space time intensity should fall off ~ 1/(R^2), generally assumed to be produced by a change in mass and it’s location, a nova or supernova. The Doppler red shift is proof that space time changes, and gravitational lensing is proof of General Relativity, among other experimental examples. All EMF emissions we measure whether they be here on earth, or in space travel through 4D space time, so the direction is changed, thus gravitational lensing.

    As for gravity wave detection, of the 4 forces, electromagnetic, weak, strong and gravity, gravity is the weakest, so it should take something cataclysmic and nearby to be detected, yet still far enough away to make detection useful. That would be the amplitude of the change in space time. As for the wave length, I couldn’t tell you, you’d have to ask Steven Hawkins. I always figured a gravity wave would travel at the speed of space time what ever that is, but apparently for reasons I don’t know I’m wrong.

    Take for example the galaxy nearest the milky way undergoing total gravitational collapse, and there’s nothing saying that couldn’t happen, well it’s likely that mankind, if he’s still around, will be having too many problems of his own to worry about detecting a gravity wave, since most of the stuff nearest us is about the same age.

    Image one of those shopping mall maps that exclaims “YOU ARE HERE” but instead of pointing at the little square that’s the mall map location it points to our current location in space time, and voila, everything is about the same age.

    There might be 2 black holes rotating around one another in near perfect orbit radiating gravity waves the way Cepheid stars do, but you’d have to ask Steven Hawkins.

  • Tom

    I’m not sure who this Steven Hawkins guy is, so I’ll recommend you redirect your questions to:

    http://www.hawking.org.uk/

    instead. ;)

  • Fred Rounds

    Seems that a GW would oscillate everything synchronously. Therefore, the detector would be in sync with the wave and therefore not detect it. To detect a GW you’d need to be outside the universe in which the wave appeared.

  • jgmitzen

    >But that “null result” is itself of great value, researchers say, because it tells

    >them where to look for the waves next.

    Or it should tell them, after all these years of looking and finding nothing, that they don’t exist.

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