‘Gravity’s rainbow’ calls to mind a novel by Thomas Pynchon, but in this case I’m thinking less in literary terms than scientific ones. Let’s talk about the full spectrum of views on the subject of gravity itself. It’s always a pertinent question because we can make sense out of the universe, up to a point, using Einstein’s understanding of gravity. But when we get down to the quantum level, we have no insights into what happens at the atomic level and below. Thus the search for a ‘quantum theory of gravity,’ one we’re likely to be a long time establishing.
In that context, two quotes caught my eye over the weekend. The first is from Freeman Dyson, from a short piece that’s now published in his new collection of essays called The Scientist as Rebel (an unfortunate title in this context, and one I suspect a marketer rather than Dyson chose).
Dyson had been discussing “…those who build grand castles in the air and those who prefer to lay one brick at a time on solid ground,” and he goes on to say:
“As a conservative, I do not agree that a division of physics into separate theories for large and small is unacceptable. I am happy with the situation in which we have lived for the last 80 years, with separate theories for the classical world of stars and planets and the quantum world of atoms and electrons.”
You see why I dislike the title of the new book. As George Johnson points out in his New York Times review, Dyson defies categorization, and in any case tends to identify himself with hard science done with new tools. He was reviewing Brian Greene’s The Fabric of the Cosmos when he wrote the above, giving us an idea what else we’ll find in the book about string theory and its penchant for sketching mathematical castles in the air.
And now this quote from Dimitrios Psaltis (University of Arizona), on the reason for continuing the quest for a deeper understanding of gravity:
“First, new ideas are challenging our previous notions of how the gravitational force works and pervades spacetime itself. And second, it is astonishing to realize that even though most of these ideas were unheard of a mere decade ago, they can be tested using present-day astronomical and cosmological observations. It is this exciting interplay of new theoretical ideas and new experimental tests that has ignited new interest in this field.”
Centauri Dreams is much in favor of experimental tests when it comes to new ideas, especially when they offer some hope of uniting those ideas with the universe we actually observe. Here we turn to the study of dark energy, that mysterious field that seems to account for three-quarters of the energy in the universe, and on another level, a very deep question indeed — why is gravity so much weaker than other fundamental forces?
Now ponder this, also from Psaltis:
“New ideas born from different branches of physics — high energy physics and cosmology, for example — and many new experiments that involve very different physical systems and techniques that include table-top experiments, laser ranging with the moon, and gravity wave and other cosmological observations provide an unprecedented opportunity to test and understand the fundamental aspects of Einstein’s theory of gravity.”
We are moving into an exciting era indeed. Papers on topics ranging from LISA, a mission to study gravitational waves from supermassive black holes, to the analysis of gravity at sub-millimeter distances will be presented in Tucson on January 22-24 at a conference called “Rethinking Gravity: From the Planck Scale to the Size of the Universe.” All of which should make for fascinating discussion.
And I suspect most conference goers will agree in principle with what Dyson is talking about. As we work toward better solutions, an incomplete understanding of gravity’s dilemmas is preferable to shoehorning facts into any theory, no matter how elegant the mathematics. Onward with solid, experimental science.
The New Science of Gravitational Waves
Authors: Craig J. Hogan
(Submitted on 5 Sep 2007)
Abstract: A brief survey is presented of new science that will emerge during the decades ahead from direct detection of gravitational radiation.
Interferometers on earth and in space will probe the universe in an entirely new way by directly sensing motions of distant matter over a range of more than a million in frequency. The most powerful sources of gravitational (or indeed any form of) energy in the universe are inspiralling and merging binary black holes; with LISA data, they will become the most distant, most completely and precisely modeled, and most accurately measured systems in astronomy outside the solar system.
Other sources range from already known and named nearby Galactic binary stars, to compact objects being swallowed by massive black holes, to possible effects of new physics: phase transitions and superstrings from the early universe, or holographic noise from quantum fluctuations of local spacetime.
Comments: 10 pages, LaTeX, to appear in “Frontiers of Astrophysics: A Celebration of NRAO’s 50th Anniversary”, eds. A.H.Bridle, J.J.Condon and G.C.Hunt
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0709.0608v1 [astro-ph]
Submission history
From: Craig J. Hogan [view email]
[v1] Wed, 5 Sep 2007 10:11:46 GMT (19kb)
http://arxiv.org/abs/0709.0608
Relativistic versus Newtonian orbitography: the Relativistic Motion Integrator (RMI) software. Illustration with the LISA mission
Authors: S. Pireaux (Observatoire Royal de Belgique, Department 1, Brussels, Belgium), B. Chauvineau (Observatoire de la Cote d’Azur, Department ARTEMIS, Grasse, France)
(Submitted on 23 Jan 2008)
Abstract: The Relativistic Motion Integrator (RMI) consists in integrating numerically the EXACT relativistic equations of motion, with respect to the appropriate gravitational metric, instead of Newtonian equations plus relativistic corrections. The aim of the present paper is to validate the method, and to illustrate how RMI can be used for space missions to produce relativistic ephemerides of satellites. Indeed, nowadays, relativistic effects have to be taken into account, and comparing a RMI ephemeris with a classical keplerian one helps to quantify such effects.
LISA is a relevant example to use RMI. This mission is an interferometer formed by three spacecraft which aims at the detection of gravitational waves. Precise ephemerides of LISA spacecraft are needed not only for the sake of the orbitography but also to compute the photon flight time in laser links between spacecraft, required in LISA data pre-processing in order to reach the gravitational wave detection level.
Relativistic effects in LISA orbitography needed to be considered and quantified. Using RMI, we show that the numerical classical model for LISA orbits in the gravitational field of a non-rotating spherical Sun without planets can be wrong, with respect to the numerical relativisitic version of the same model, by as much as about 9 km in radial distance during a year and up to 59 km in along track distance after a year… with consequences on estimated photon flight times.
We validated RMI numerical results with an analytical developpement. Finally, the RMI relativistic numerical approach is soon more efficient than the analytical development. Moreover, RMI can be applied to other space missions.
Comments: 26 pages, 16 eps figures, 0 table, submitted to Celestial Mechanics
Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph)
Cite as: arXiv:0801.3637v1 [gr-qc]
Submission history
From: Sophie Pireaux Dr [view email]
[v1] Wed, 23 Jan 2008 18:16:06 GMT (374kb)
http://arxiv.org/abs/0801.3637
A new gravitational wave background from the Big Bang
Authors: Juan Garcia-Bellido, Daniel G. Figueroa
(Submitted on 27 Jan 2008)
Abstract: The reheating of the universe after hybrid inflation proceeds through the nucleation and subsequent collision of large concentrations of energy density in the form of bubble-like structures moving at relativistic speeds. This generates a significant fraction of energy in the form of a stochastic background of gravitational waves, whose time evolution is determined by the successive stages of reheating: First, tachyonic preheating makes the amplitude of gravity waves grow exponentially fast. Second, bubble collisions add a new burst of gravitational radiation. Third, turbulent motions finally sets the end of gravitational waves production. From then on, these waves propagate unimpeded to us.
We find that the fraction of energy density today in these primordial gravitational waves could be significant for GUT scale models of inflation, although well beyond the frequency range sensitivity of gravitational wave observatories like LIGO, LISA or BBO. However, low-scale models could still produce a detectable signal at frequencies accessible to BBO or DECIGO. For comparison, we have also computed the analogous background from some chaotic inflation models and obtained similar results to those of other groups.
The discovery of such a background would open a new observational window into the very early universe, where the details of the process of reheating could be explored. Thus, it could also serve as a new experimental tool for testing the Inflationary Paradigm.
Comments: 20 pages, 8 figures, to appear in the Proceedings of JGRG17, Nagoya (Japan), 3-7 December 2007
Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph); High Energy Physics – Phenomenology (hep-ph)
Report number: IFT-UAM/CSIC-08-05
Cite as: arXiv:0801.4109v1 [gr-qc]
Submission history
From: Juan Garcia-Bellido [view email]
[v1] Sun, 27 Jan 2008 01:46:00 GMT (139kb)
http://arxiv.org/abs/0801.4109
Is there potential complementarity between LISA and pulsar timing?
Authors: Matthew Pitkin, James Clark, Martin A. Hendry, Ik Siong Heng, Chris Messenger, Jennifer Toher, Graham Woan
(Submitted on 18 Feb 2008)
Abstract: We open the discussion into how the Laser Interferometer Space Antenna (LISA) observations of supermassive black-hole (SMBH) mergers (in the mass range ~10^6-10^8 Msun) may be complementary to pulsar timing-based gravitational wave searches. We consider the toy model of determining pulsar distances by exploiting the fact that LISA SMBH inspiral observations can place tight parameter constraints on the signal present in pulsar timing observations. We also suggest, as a future path of research, the use of LISA ring-down observations from the most massive (greater than ~ a few 10^7 Msun) black-hole mergers, for which the inspiral stage will lie outside the LISA band, as both a trigger and constraint on searches within pulsar timing data for the inspiral stage of the merger.
Comments: 5 pages, 2 figures, submitted for the proceedings of the Amaldi 7 conference
Subjects: Astrophysics (astro-ph); General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:0802.2460v1 [astro-ph]
Submission history
From: Matthew Pitkin [view email]
[v1] Mon, 18 Feb 2008 11:52:52 GMT (35kb)
http://arxiv.org/abs/0802.2460
First joint search for gravitational-wave bursts in LIGO and GEO600 data
Authors: LIGO Scientific Collaboration: B. Abbott, et al
(Submitted on 17 Jul 2008)
Abstract: We present the results of the first joint search for gravitational-wave bursts by the LIGO and GEO600 detectors. We search for bursts with characteristic central frequencies in the band 768 to 2048 Hz in the data acquired between the 22nd of February and the 23rd of March, 2005 (fourth LSC Science Run – S4). We discuss the inclusion of the GEO600 data in the Waveburst-CorrPower pipeline that first searches for coincident excess power events without taking into account differences in the antenna responses or strain sensitivities of the various detectors.
We compare the performance of this pipeline to that of the coherent Waveburst pipeline based on the maximum likelihood statistic. This likelihood statistic is derived from a coherent sum of the detector data streams that takes into account the antenna patterns and sensitivities of the different detectors in the network. We find that the coherentWaveburst pipeline is sensitive to signals of amplitude 30 – 50% smaller than the Waveburst-CorrPower pipeline. We perform a search for gravitational-wave bursts using both pipelines and find no detection candidates in the S4 data set when all four instruments were operating stably.
Comments: 30 pages, 8 figures
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Report number: LIGO-P080008-A-Z
Cite as: arXiv:0807.2834v1 [gr-qc]
Submission history
From: Ik Siong Heng [view email]
[v1] Thu, 17 Jul 2008 16:58:25 GMT (297kb)
http://arxiv.org/abs/0807.2834
Gravitational Radiation from Neutron Stars and Black Holes
Authors: Carlos F. Sopuerta
(Submitted on 4 Aug 2008)
Abstract: Gravitational Wave Astronomy is becoming a reality as Earth-based interferometric gravitational-wave detectors reach the design sensitivities and move towards advanced configurations that may lead to gravitational-wave detections in the immediate future.
In this contribution, I briefly summarize the basic characteristics of this new area, the discovery prospects and the potential for fundamental physics. Then, I present results of some investigations of two different sources of gravitational waves that are potential targets for present and future planned observatories.
First, I will discuss the generation of gravitational radiation by non-linear effects arising from the coupling between radial and non-radial oscillations of neutron stars, which may produce distinctive gravitational-wave signatures. The gravitational radiation emitted by these sources is in the frequency band of Earth-based detectors. And second, I will discuss the gravitational-wave emission during the inspiral of extreme-mass-ratio compact binaries. In this case, the gravitational waves have low frequencies, inside the frequency band of space observatories like LISA.
Comments: 14 pages, 3 figures. Proceedings of the conference “Supernovae: lights in the darkness” (XXIII Trobades Cientifiques de la Mediterrania), October 3-5, 2007, Mao, Menorca (Spain). To appear in Proceedings of Science
Subjects: Astrophysics (astro-ph)
Journal reference: PoS(SUPERNOVA)026 (2008)
Cite as: arXiv:0808.0389v1 [astro-ph]
Submission history
From: Carlos F. Sopuerta [view email]
[v1] Mon, 4 Aug 2008 07:39:52 GMT (156kb,D)
http://arxiv.org/abs/0808.0389
http://www.fas.org/blog/secrecy/2008/12/jason_study.html
Dec 15, 2008
JASON Study Debunks Gravitational Wave “Threat”
The elite JASON defense science advisory panel dismissed claims that high frequency gravitational waves (HFGW) could pose any kind of national security threat.
In a study (pdf) prepared for the Office of the Director of National Intelligence, the JASONs concluded that “No foreign threat in HFGW is credible, including: communication by means of HFGW; object detection or imaging (by HFGW radar or tomography); vehicle propulsion by HFGW; or any other practical use of HFGW.”
Gravitational waves were predicted by Einstein’s general theory of relativity and their existence has been indirectly confirmed by experiment. But up to now they have never been directly measured.
“Unfortunately, relativity and gravitation theory have, over the last century, been the subject of a great deal of pseudo-science, in addition to real science. Therefore, in evaluating ambitious claims about gravitational applications, one must consider the possibility that the claims are misguided and wrong,” the JASONs advised. “There is no substitute for seeking expert scientific and technical opinion in such matters.”
A copy of the new JASON report was obtained by Secrecy News. See “High Frequency Gravitational Waves,” October 2008.
http://arxivblog.com/?p=1271
Were gravitational waves first detected in 1987?
March 4th, 2009 | by KFC |
In 1987, Joe Weber, a physicist at the University of Maryland, claimed to have detected gravitational waves at exactly the same moment that other astronomers witnessed the famous supernova of that year, SN1987A.
His equipment consisted of several massive aluminium bars that were designed to vibrate in a unique way when a large enough gravitational wave passed by.
His claims were ignored largely because other physicists calculated that gravitational waves ought to be several orders of magnitude too weak to be picked up by this kind of gear. (And he’d made several similar claims throughout the 60s and 70s that others had failed to repeat.)
But Weber’s claims may have to be re-examined, says Asghar Qadir, a physicist at the National University of Sciences and Technology in Rawalpindi, Pakistan. He points out that predicting the strength of a gravitational wave is by no means easy and until recently, only first order effects have been considered.
He and colleagues have now worked out that in certain circumstances, second order effects can enhance the waves. But this only happens when there is a certain kind of assymetry in the event that created the waves.
But get this: the assymetry can enhance the waves by a factor of 10^4.
He also points out that SN1987A is aspherical in exactly the way that might create this enhancement. So if SN1987A generated gravitational waves, Weber would have been perfectly able to detect them.
Qadir concludes: “The claim of Weber to have observed gravitational waves from [SN1987A] needs to be re-assessed”.
By all accounts, Weber was a careful experimenter who got something of a rough deal for his efforts (the most comprehensive telling of the tale is in a book called Gravity’s Shadow by Harry Collins) .
Weber died in 2000 but it wouldn’t do any harm to re-examine his work in the light of this development.
Ref: http://arxiv.org/abs/0903.0252: Gravitational Wave Sources May Be “Closer” Than We Think
http://arxivblog.com/?p=1279
How superconducting sheets could reflect gravitational waves
March 6th, 2009 | by KFC |
Gravitational waves are the elusive distortions in spacetime created by the universe’s most violent events–collisions between black holes, stars exploding and even the big bang itself.
Nobody has bagged a confirmed sighting of these waves but that may change thanks to an intriguing idea from Raymond Chiao and pals at the University of California, Merced. They propose the existence of a new kind of mirror that reflects gravitational waves and may even convert them into electromagnetic waves.
First some background. Theoretical physicists have long noticed that in certain circumstances, Einstein’s equations of general relativity, which predict the existence of gravitatonal waves, bear a remarkable similarity to Maxwell’s equations that describe the behaviour of electromagnetic radiation. That’s an important clue for understanding how gravitational waves behave, says Chiao.
He points to the specific case in which a thin superconducting film reflects em waves. If that works for em waves, then the mathematics indicates that it must also work for gravitational waves.
Here’s the thinking. A gravitational wave stretches and squeezes space as it moves through the universe. Any object in its way will appear to be squashed and stretched in the same way, the particles within this object will move with the distorted space in a specific trajectory (called geodesic motion).
The new idea comes from considering what happens to a superconducting sheet when a gravitational wave passes by. The Cooper pairs within the sheet are quantum objects governed by the uncertainty principle and so cannot have specific trajectory: they are entirely delocalised. On the other hand, the ions that make up the crystal structure of the superconductor are not delocalised and so can move along a geodesic trajectory when a gravitational wave passes.
This is the basis on which a gravitational wave can interact with a superconducting sheet. “Quantum delocalization causes the Cooper pairs of a superconductor to undergo non-geodesic motion relative to the geodesic motion of its ionic lattice,” says Chiao and buddies.
They speculate that this difference in motion causes the sheet to absorb energy from the gravitational wave and then re-radiate it as gravitational wave travelling in the opposite direction–in other words specular reflection.
That’s an extraordinary claim which needs some further investigation, not least because there’s a fair amount of disagreement over the GR-Maxwell link in the first place.
Nevertheless, Chiao and co go even further by ending their paper with this:
“This implies that two charged, levitated superconducing spheres in static mechanical equilibrium, such that their Coulombic repulsion balances their Newtonian attraction, should be an efficient transducer for converting EM waves into GR waves and vice versa. A Hertz-like experiment in which a transmitter and receiver of GR microwaves are constructed using two such transducers should therefore be practical to perform.“
So a pair of levitating, superconducting spheres would act as an antenna for gravitational waves and convert them into electromagnetic waves.
Why wait for LIGO? What’s the betting that superconducting spheres can make the detection first?
Ref: http://arxiv.org/abs/0903.0661: Do Mirrors for Gravitational Waves Exist?