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Gravitational Waves and their Limits

Sometimes what you don’t detect tells a scientific story just as important as what you do. In the case of LIGO (Laser Interferometer Gravitational-Wave Observatory) and the VIRGO Collaboration, we’re talking about setting limits to the amount of gravitational waves that would have been produced by the Big Bang. Those waves, predicted by Albert Einstein in 1916 and consistent with his theory of General Relativity, should be traceable and quite valuable to us, carrying as they do information about the earliest stages of the universe.


Image: Modeling gravitational wave complexity. Laser interferometers should be able to detect the gravitational waves produced by the most violent astrophysical events, such as the merging of two black holes. Credit: MPI for Gravitational Physics/W.Benger-ZIB.

The gravitational waves ought to be out there (General Relativity predicts that all accelerating objects should produce them) but they have yet to be observed directly. In fact, the so-called ‘stochastic background’ — this Caltech news release likens it to the superposition of numerous waves of varying strengths going in different directions on the surface of a pond — is what LIGO and the VIRGO interferometer are probing, looking into the universe as it was in the first minute of its existence. Lee Samuel Finn (Penn State) has a more colorful description:

“Space-time is the living stage upon which the drama of the universe plays out. The primordial stochastic gravitational waves are the warps, twists, and bends in space-time that were laid down as the universe expanded from its earliest moments to the present. The observations we report in this paper are the closest direct examination of the framework of the living, breathing universe.”

What we find in the new work, drawing on data from 2005 to 2007 and just published in Nature, is that the stochastic background of gravitational waves has yet to be discovered. But the data are precise enough that that fact is itself significant. It allows us to set upper limits on the phenomenon. Vuk Mandic (University of Minnesota) explains:

“Since we have not observed the stochastic background, some of these early-universe models that predict a relatively large stochastic background have been ruled out. We now know a bit more about parameters that describe the evolution of the universe when it was less than one minute old. We also know that if cosmic strings or superstrings exist, their properties must conform with the measurements we made — that is, their properties, such as string tension, are more constrained than before.”

The VIRGO Collaboration makes use of a three-kilometer long interferometer in Cascina, Italy and works with LIGO’s 700 scientists in jointly analyzing data gathered by various instruments, which include the GEO600 interferometer near Hannover, Germany. LIGO itself is composed of detectors in Hanford, WA and Livingston, LA, each using a laser beam split into two beams that travel down the interferometer arms.

The difference between the lengths of those arms should tell the tale, for General Relativity predicts that a passing gravitational wave should stretch one arm slightly while compressing the other. We’re talking about detecting a change of less than a thousandth the diameter of an atomic nucleus in the relative lengths of the arms. That’s enough to get us to today’s result, but Advanced LIGO, which comes online in 2014, is to be ten times more sensitive still, allowing us to probe various models of the early universe including those involving cosmic strings.

To place all this in perspective, recall that the Cosmic Microwave Background that has been so valuable in our studies of the early cosmos can take us back to about 380,000 years after the Big Bang. We’re now pushing into the investigation of early universe models that can take us back to the cosmos’ first minute. The paper, a joint work of the LIGO Scientific Collaboration and the Virgo Collaboration, is “An upper limit on the amplitude of stochastic gravitational wave background of cosmological origin,” Nature 460 (20 August 2009), pp. 990-994 (abstract).


Comments on this entry are closed.

  • george scaglione August 20, 2009, 14:05

    guys,forgive me for speaking before i really know anything but i do have a nagging feeling that – gravity drives for star ships will be a FANTASTIC source of propulsion someday. after all ,what makes things move better than good old gravity and it is everywhere! we “just” need to figure out how to do it! apologies again,lol,that statement does pretty much apply to alot of the things we have looked into here.(?!) respectfully to all your friend george

  • James M. Essig August 20, 2009, 16:14

    Hi George;

    I am greatly inspired by your above comments. Gravity is the stuff of space time curvature and it is indeed everywhere. I have always enjoyed muzing on the concept of anti-gravity negative drive whereby the force of repulsion would aomehow grow essentially without limit between the space craft and the gravitational mass it pushes off against.

    Even plain old antigravity would be an interstellar travel God send.


    Your Friend Jim

  • Adam August 20, 2009, 17:37

    Hi George

    Everything we learn about gravity makes a gravity-drive that much closer – if it’s possible. I’m not sure exactly how such a system is supposed to work, but by keeping to the path of learning as much as we can about how gravity behaves, then the clearer a ‘drive’ becomes. The closest analogue to a gravity drive I can think of is the Alcubierre metric, at least in its sub-light form. Ignoring the difficulties of FTL motion, the Alcubierre metric allows arbitary acceleration without subjecting the ship itself to anything more than flat space. Of course just how we envelop a ship in a hemisphere of expanding space joined to a hemisphere of collapsing space is presently a big unknown.

  • J. R. August 20, 2009, 20:29

    I wonder that as the results of these gravitational waves come out, how much could it support of disprove the theory about time-space waves as mentioned in the previous article.

  • george scaglione August 21, 2009, 13:15

    jim, yes sir i can always count on you to take a good idea and stretch it out to get more,no pun intended “mileage” ! to somehow amplify gravity drive.that will get us where we want to go. adam,you seem to be concerning yourself with the finer points of warp drive.also not the most easy thing to do in fact you brought out a concept which i had never thought of myself ! j.r. agreed again.space/time is indeed the place to look i have NO DOUBT in order to find the best means of starship propulsion in the future! thoughts of this nature occurred to me ,perhaps for the first time,the first time i read about the zero point field about, maybe,errr ahhh – eight years ago.god that now seems like a long time! actually had to run downstairs and have a look at a book i have in order to make a proper “guesstimate”! respect to one and all your friend george

  • James M. Essig August 22, 2009, 20:55

    Hi George and other Folks;

    Interesting comments!

    Imagine a plot of an equation for a time dependent acceleration profile such as a = f(t) = mt + b and let m equal say 1,000,000 and b = 1,000,000. At t = + 1 second, the acceleration equals 2,000,000 meter/(second EXP 2), thus the realm of special relativity is approached very rapidly.

    If we assume that the rate of change of acceleration of the space craft relative to the space craft reference frame is constant at da/dt = m = 1,000,000 meter/(second EXP 3), then after 1 million seconds Earth time, a will equal 1,000,001,000,000 meter/(second EXP 2) ship’s reference frame. After, one trillion seconds Earth time, a will equal 1,000,000,000,001,000,000 meter/(second EXP 2) ship’s reference frame. After 10 EXP 15 seconds Earth time, a will equal 1,000,000,000,000,001,000,000 meter/(second EXP 2).

    Now scientific theoretical exploration and science fiction are no strangers to the concept of negative drive by which a space craft would accelerate at an ever increasing rate as it traveled away from its point of origin. The highly speculative mechanisms of how such drives could or might work are probably numerous, however, we can see, that for linear time dependence of Jerk = da/dt on time and the slope of the line of the plot of such linearly dependent Jerk verses time, i.e.,for linearly dependant Jerk vs time, the slope can be made arbitrarily great, for lexicographical consideration, even to values equal to an arbitrary ensemble, an arbitrary infinity scrapper, or even a slope of infinity, in which case, infinite acceleration would be reached after one infinitesimal time step, Earth time. Thus you can see may fascination with negative drive concepts.

    In theory, for a(t) = b(t EXP N); J1 = N(b)[t EXP(N-1)]; J2 = d(J1)/dt = N(N-1)(b)[t EXP(N-2)]; J3 = d(J2)/dt = N(N-1)(N-2)(b)[t EXP(N-3)] and so on. For a(t) = b(t EXP N); JN = N!(b), J(N-M) = [N!/(M!)](b)(t EXP M) where N is greater than or equal to one, and M is greater than or equal to zero, but not greater than N and where N! denotes the factorial operation.

    In consideration that special relativity deals with inertial reference frames, general relativity deals with accelerated reference frames which are indistinguishable from the effects of gravity, perhaps extreme values of Jerk 1 or J1, Jerk 2 or J2, Jerk 3 or J3, each have their respective or associated phenomenon or mathematical frame work that becomes highly manifest at certain values or thresholds of Ji. I would not expect any new physics even for say J1 beyond that of general relativity unless extreme values of J1 were produced and maintained.

    Since gravitation is the stuff of general relativity, anything we learn about the strength of gravity waves including any descrepancies between measured and predicted particular waveforms and wave amplitudes might help us in physics beyond general relativity. LIGO and VIRGO can and hopefully will be of help here

  • andy August 23, 2009, 13:20

    My intuitive feeling is that funky tricks with gravity/spacewarping etc. are going to be wildly impractical for use in actual space travel, because of the weakness of gravity and consequently the large amounts of mass/energy needed. For example, the mass required to achieve a Schwarzschild radius of one metre is about 18% greater than the mass of Saturn, which does not bode well for useful wormholes.

  • J. R. August 25, 2009, 20:17

    I’ve been thinking about a concept for a while and the possibility of being able to measure gravity waves brought it back. Just what is the cause and effect where it is theorized that acceleration produces gravity waves? Does the acceleration actually compress or warp space/time in front of the object and the measureable effect is gravity waves? Any ideas?

  • James M. Essig August 26, 2009, 12:31

    Hi J.R.;

    An accelerating mass produces gravity waves accordingg to general relativity, however, at ordinary every day phenomenon scales, the gravity waves would be increadibly weak, and undetectable by current laboratory apparatus. This does not mean that some kinematical effects might not be produced or bootstrapped into higher levels of space time distortion via technologically manipulated ordinary every day phenomenon scaled gravity waves. The secret here would be unlocking such bootstrapping effects in a useful, controlled and safe manner.

    Acceleration would indeed produce time dilation, and as an object falls into a black hole, the degree of time dilation that an outsider would observe, of say a radio beacon falling into the blackhole would progress for all practical purposes to infinite degrees of time dilation as the object was observed to effectively reach the black holes event horizon from the outsiders point of view.

    I believe gravitational time dilation has even been measured between two reference points, one at the base of a tall building and one at the top of a tall building via highly accurate clocks which I presume where some sort of atomic clocks.

    You might have noticed some of the talk about the possible production on mini-blackholes at the LHC when it resumes operation at the eventual planned 14 TeV collision energies that were the subject of some doom and gloom scenarios last September. The production of such black holes is a theoretical possibility as a result of the increadible acceleration rates that colliding particles would undergo by virtue of the fact that such tremendous accelerations would be indistinguishable from gravity. If such black holes are produced, they are thankfully, expected to decay almost instantly via Hawking Radiation Emission and so are extremely unlikely to pose any threat to Earth nor to any of the LHC infrastructure nor to its operators .

  • J. R. August 27, 2009, 13:46

    My question is that if the extreme accelerations and speeds of newly formed objects shortly after the big bang actually compressed space-time in front of the objects, wouldn’t it be possible for these objects to appear to the outside observer standing in normal space that the objects were exceeding the speed of light? If so, would gravitational waves be the only measureable remnant indicating that this may have happened?

  • Richard Benish August 29, 2009, 23:09

    Sure would be nice to have some empirical evidence to support any of the many hypotheses mentioned above.

    Imagine a massive sphere with a hole through its center far away from any other large masses. Drop a test object into the hole. Everybody “knows” the test object is supposed to oscillate through the hole. But we have no empirical evidence. We spend billions on the exotic speculative fringes but have yet to look, in effect, right under our noses.

  • ljk September 14, 2009, 13:02

    Accelerating the Universe with Gravitational Waves

    Authors: I. A. Brown, L. Schrempp, K. Ananda

    (Submitted on 10 Sep 2009)

    Abstract: Inflation generically produces primordial gravitational waves with a red spectral tilt. In this paper we calculate the backreaction produced by these gravitational waves on the expansion of the universe.

    We find that in radiation domination the backreaction acts as a relativistic fluid, while in matter domination a small dark energy emerges with an equation of state w=-8/9.

    Comments: 11 pages, 4 figures

    Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO)

    Report number: HD-THEP-09-19

    Cite as: arXiv:0909.1922v1 [gr-qc]

    Submission history

    From: Iain Brown [view email]

    [v1] Thu, 10 Sep 2009 15:23:23 GMT (32kb)


  • ljk October 4, 2009, 23:07

    Detection of IMBHs with ground-based gravitational wave observatories: A biography of a binary of black holes, from birth to death

    Authors: Pau Amaro-Seoane, Lucia Santamaria

    (Submitted on 1 Oct 2009)

    Abstract: Even though the existence of intermediate-mass black holes has not yet been corroborated observationally, these objects are of high interest for astrophysics. Our understanding of formation and evolution of supermassive black holes (SMBHs), as well as galaxy evolution modeling and cosmography would dramatically change if an IMBH was observed.

    The prospect of detection and, possibly, observation and characterization of an IMBH has good chances in lower-frequency gravitational-wave (GW) astrophysics with ground-based detectors such as LIGO, Virgo and the future Einstein Telescope (ET).

    We present an analysis of the signal of a system of a binary of IMBHs based on a waveform model obtained with numerical relativity simulations coupled with post-Newtonian calculations at the highest available order so as to extend the waveform to lower frequencies.

    We find that initial LIGO and Virgo are in the position of detecting IMBHs with a signal-to-noise ratio (SNR) of $\sim 10$ for systems with total mass between 100 and $500 M_{\odot}$ situated at a distance of 100 Mpc. Nevertheless, the event rate is too low and the possibility that these signals are mistaken with a glitch is, unfortunately, non-negligible.

    When going to second- and third-generation detectors, such as Advanced LIGO or the proposed ET, the event rate becomes much more promising (tens per year for the first and thousands per year for the latter) and the SNR at 100 Mpc is as high as 100 — 1000 and 1000 — $10^{5}$ respectively.

    The prospects for IMBH detection and characterization with ground-based GW observatories would not only provide us with a robust test of general relativity, but would also corroborate the existence of these systems. Such detections would be a probe to the stellar environments of IMBHs and their formation.

    Comments: Submitted to ApJ; abstract abridged, figure 1 has a lower resolution

    Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Galaxy Astrophysics (astro-ph.GA)

    Cite as: arXiv:0910.0254v1 [astro-ph.CO]

    Submission history

    From: Pau Amaro-Seoane [view email]

    [v1] Thu, 1 Oct 2009 20:21:15 GMT (1749kb)


  • fousia June 29, 2012, 2:22

    how can we understand about the source from the gravitational wave we detected directly in the interferometer